ACL/Cartilage

Allografts

Allografts have been advocated as a viable option for ACL reconstruction and have been used extensively during the past 20 to 30 years. There is purported to be an increase in interest in these grafts in the past several years, although they are used relatively infrequently compared to auto- grafts.30 Decreased morbidity, preservation of the extensor or flexor mechanisms, provision of a source of graft mate- rial when the patient has none, decreased operative time, availability of larger graft sources, lower incidence of arthrofibrosis, and improved cosmetic appearance are all enticing advantages that have been attributed to the use of allografts.29,112,144,290 To offset these advantages, the risk of infection, slow and incomplete incorporation and remodeling of the graft, higher costs, availability, tunnel enlargement, alteration of graft structural properties by sterilization and storage procedures, and the immunologic response have all been reported as problems.††† The effects that immunologic response and delayed revascularization and remodeling have on the ultimate clinical outcomes of ACL allograft reconstructions are probably quite different, and perhaps detrimental, from autograft reconstruction. Thus, more investigation is necessary to establish the ulti- mate place for allografts in the reconstruction of the ACL. At the present time, there are no RCTs comparing the outcomes of ACL reconstructions using allografts versus autografts. Numerous authors have compared case series of these 2 methods; however, these studies suffer from con- founding and bias.‡‡‡ In general, these authors report no difference in the outcomes in spite of the wide variation in follow-up intervals, surgical techniques used, and graft types (different material, sterilization and storage proce- dures).190 Only Shino et al263 concluded that allografts cor- rected anterior laxity better than autografts in knees undergoing ACL reconstruction, but they only evaluated the patients in both groups that were described as clini- cally successful at the time of follow-up. Another author evaluating a series of allografts compared with autografts concluded that anterior laxity increased in the allograft group.290 Several authors alluded to the failure of allo- grafts to maintain normal anterior laxity, but in humans, at least, the body of evidence does not appear to support this opinion.208 Much of the work demonstrating deterio- ration of results after ACL allograft reconstruction was based on animal experiments.123 One of the major proposed advantages of allografts ver- sus autografts has been that of perioperative and postop- erative morbidity.278 Although most of the articles did not carefully evaluate morbidity, Saddemi et al245 were unable to find a difference in morbidity between autograft and allograft ACL reconstruction at any time in the first 2 years after surgery. There is no proof in the present litera- ture that there is less morbidity after allograft ACL recon- struction.190 At the current time, there certainly is a place for ACL allografts, but their routine use for primary ACL recon- structions probably cannot be justified by the present information. The risk of infection, although low, cannot be entirely eliminated, and the added cost of these procedures is hard to justify. Allografts are a viable option when the patient has no remaining graft source (multiple ligament injuries or failed ACL reconstructions). Surgeons who interpret the information presently available on the efficacy of allograft reconstruction differently may propose and use other indications than those stated here. Well-designed RCTs comparing allograft ACL reconstruction with auto- graft ACL reconstruction are needed to establish the place of the allograft in our surgical armamentarium.

GRAFT HEALING

Although it has been reported that BPTB autografts used to reconstruct the ACL in the rabbit model undergo bio- logical remodeling and incorporation after implantation (a process termed ligamentization6), the fully incorporated graft never replicates the normal ACL, and it appears to function as a check rein of organized scar tissue.28 Oaks and associates87 have performed quantitative ultrastruc- tural morphometric analysis of collagen fibril populations in the ACL and patellar tendon grafts using the goat model. The graft remodeling process was found to change the ultrastructural profile of the original patellar tendon at the time of harvest to one containing a larger number of small-diameter fibrils (<100 nm). A rapid decrease in the number of large-diameter collagen fibers (>100 nm) was found after 12 weeks of healing. Remodeling was found to begin from the outside of the graft and then move toward the graft center as the remodeling progressed over time. Oaks et al87 found this remodeling behavior to be consis- tent with revascularization of the graft from synovium, demonstrating the importance of not only investigating the surface and central portions of the graft but also studying different regions along the length of the graft. Remodeling was found to continue for as long as 52 weeks after reconstruction. Arnoczky et al9 evaluated the tempo- ral revascularization behavior of the patellar tendon auto- graft using a canine model. They demonstrated that revas- cularization of the patellar tendon graft came from the bone tunnels and progressed from the proximal and distal regions to the central portion of the graft. After 5 months of healing, revascularization was complete. The choice of graft material and the method of fixation affect the healing and remodeling response at the graft-bone tunnel interface, and this relationship has been studied in various animal models. Rodeo et al94 used the canine model to study the tensile failure properties of digital extensor tendons fixed in bone tunnels. After 2, 4, and 8 weeks, graft failure occurred by pullout from the bone tunnels; after 12 and 26 weeks, the graft failed at its midsubstance. Grana et al46 studied the healing response of a hamstring autograft used to reconstruct the ACL in rabbits. They found that hamstring grafts healed by for- mation of a fibrous insertion to bone, and the fixation strength of the bone-graft composite in the bone tunnel exceeded the intra-articular portion of the autograft strength early in the postoperative period. At 3 weeks, failure of the bone-hamstring graft-bone construct occurred at the intra-articular portion of the graft and not by pullout from the bone tunnels.46 In a subsequent study of hamstring graft insertion-site healing in rabbits reported by the same group, the formation of the fibrous insertion was found to be complete after 26 weeks of healing.21 Tomita and colleagues105 compared intraosseous graft healing between a doubled flexor tendon graft and a BPTB graft used to reconstruct the ACL in canines. Tensile fail- ure testing revealed that the weakest site of the doubled flexor tendon graft was the graft-tunnel wall interface at 3 weeks and the intraosseously grafted tendon at 6 weeks. For the BPTB graft, the weakest site was the graft-tunnel wall interface at 3 weeks and the proximal site in the bone plug at 6 weeks.105 After 3 weeks, the ultimate failure strength of the doubled flexor tendon graft was 45% of the BPTB graft, and this value had increased to 85% at 6 weeks of healing.105 It is important to point out that experimental studies using the animal knee joint as a model are limited with respect to the lack of similarity to the human knee joint and that animals have an uncontrolled postoperative rehabilitation regimen. Although animal investigations have provided insight into graft remodeling and biome- chanical behavior during graft healing, direct application of these results to clinical practice must be made with cau- tion. For example, BPTB grafts used to reconstruct the ACL may not undergo the dramatic decrease in structural properties that have been reported from animal studies.17 This opinion is supported by a case study of a patient who underwent ACL reconstruction with a central third BPTB autograft.17 Eight months after surgery, the linear stiff- ness and ultimate failure load values of the graft approached those of the contralateral, normal ACL, whereas laxity of the injured knee was greater than the normal knee. More recently, Delay et al30 described a case study of a central third BPTB autograft after 18 months of healing. They found complete osseous union of both tibial and femoral bone blocks, although deep areas of necrosis asso- ciated with bone graft remodeling were observed. The deep and superficial regions of the proximal half of the soft tis- sue graft had become revascularized, whereas the deep portion of the distal half of the graft had persistent regions of necrosis that were acellular and avascular. These find- ings are supported by the work of Rougraff and Shelbourne,96 who performed second-look arthroscopy and biopsy of patellar tendon grafts on 9 subjects after 3 to 8 weeks of healing. Although all specimens showed regions of acellularity and degeneration, the researchers observed that graft vascularity was present at 3 weeks and contin- ued to increase over the 8-week sample interval. Petersen and Laprell90 obtained biopsy specimens of BPTB and hamstring grafts from patients undergoing revision sur- gery. Both graft materials healed within the femoral and tibial tunnels, but the insertions were different. The inser- tion of the BPTB grafts to the bone tunnels healed by bone plug incorporation and resembled the chondral insertion of the normal ACL, whereas the hamstring grafts healed by the fibrils of the graft penetrating the bone directly and resulted in a fibrous insertion of the tendon, not the nor- mal chondral insertion of the ACL to the tibia or femur. The temporal change in the structural and material properties of different autografts used for ACL reconstruc- tion has been investigated using primate, canine, goat, sheep, and rabbit models.81 Animal studies that have investigated the iliotibial tract (ITT) autograft for as long as 1 year indicate that the ultimate failure load values of this graft ranged between 23% and 40% of the control ACL, whereas the graft stiffness was 45% of the average uninjured ACL.81 Studies in animals that have investigated the healing response of patellar tendon autografts a year or more after reconstruction have reported ultimate fail- ure load values ranging between 30% and 45% of the con- trol ACL, whereas the stiffness has been reported to range between 35% and 57% of the normal ACL.81 Hunt et al55 used the superficial digital flexor tendon as an autograft to simulate hamstring reconstruction of the ACL in sheep. After 1 year, grafts fixed anatomically with interference screws in the tibia and femur had an ultimate failure load value that was 45% of the normal ACL. It is not enough to evaluate the structural and material properties of an ACL graft; in addition to having adequate strength and stiffness, an ACL graft must also control anterior translation of the tibia relative to the femur and, to a lesser degree, internal-external and varus-valgus lax- ity of the knee joint. Butler25 used the canine model to investigate the A-P displacement response of the knee joint at 4 time intervals after a combined ACL reconstruc- tion using the fascia lata and lateral one third of the patel- lar tendon. At implantation, the A-P laxity of the operated knee was 154% of the control limb; after 4 weeks of heal- ing, this ratio increased to 306%. After 12 and 26 weeks of healing, the A-P laxity had decreased to 209% and 153% of the control limb, respectively. Animal studies of ACL allografts have shown a slower rate of biological incorporation, a greater decrease in struc- tural properties, and a prolonged inflammatory response compared to ACL autografts.56 Human retrieval studies have shown that remodeling of ACL allografts is slow.52,74 After 2 years of transplantation, the central portions of the allografts were found to remain acellular. Complete remodeling and cellular replacement of the entire graft were seen in grafts studied 3 years after transplantation. The healing of BPTB and hamstring grafts is different both temporally and histologically. Thus, aggressive reha- bilitation within the first 6 weeks after ACL reconstruc- tion with a hamstring graft may cause greater anterior knee laxity compared to reconstruction with a BPTB graft. Attempts to improve healing by treating ACL autografts with growth factors and gene therapy are in the develop- ment stages, and although these approaches hold great promise,109,113 there are no proven clinical applications for these procedures at the present time.

NONOPERATIVE TREATMENT OF ACL TEARS IN ADULTS

Although there is a definite place for nonoperative treat- ment for patients with complete tears of the ACL, it is very difficult to determine from the extensive literature evalu- ating this subject exactly what its role should be. In a sur- vey of American Orthopaedic Society for Sports Medicine members in 1999, the vast majority of the 742 respondents treated their ACL-injured patients with surgery rather than by nonoperative means.67 Of these surgeons, 70% used nonoperative treatment on 25% or less of their patients. Only 5.7% of the respondents reported that they used nonoperative treatment for between 75% and 100% of their ACL-deficient patients. Several case series reveal that a nonoperative approach to the torn ACL can be effective for the majority of patients willing to avoid high-risk activities.45,49,57,86,87,249 Other case series have implied that even with return to high-risk activities, many patients do well.53,242 Many case series of ACL injuries in children and adolescents reveal that non- operative treatment very frequently leads to reinjuries, meniscal tears, and arthritis.†† In one cross-sectional study of patients choosing nonoperative therapy versus surgical reconstruction of the ACL, the Lysholm, Cincinnati, and Orthopadische Arbeitsgruppe Knie outcome scores were significantly superior in those who underwent reconstruc- tion.294 The problems with noncontrolled case series, compar- isons of case series, or cross-sectional studies include both confounding and bias. Susceptibility (selection) and per- formance biases are not avoided, which may lead to inap- propriate conclusions. Patients choosing to undergo non- operative therapy rather than surgical reconstruction probably have very different goals from those opting for reconstruction. This factor makes the results of the above- mentioned studies difficult to interpret and potentially unreliable. Variations in the inclusion or exclusion of comorbid problems, the lack of information concerning the rehabilitation of patients treated without surgery, varia- tion in the definition of high-risk activities, as well as sev- eral other variables, make comparison of case series impractical, if not impossible. We also found 4 RCTs concerning nonoperative ACL treatment versus ACL repair or repair with augmentation that had the advantage of minimizing the biases (suscep- tibility, performance, detection, and transfer) less elegant studies cannot avoid.14,15,219,247 One of these investigations demonstrated no differences in outcomes between repair of the ACL compared with nonoperative treatment.247 These findings were used in Sweden in the late 1980s to suppress the utilization of ACL reconstructions because the out- comes of this study were erroneously generalized by some authorities to include all ACL reconstruction procedures. In 3 other Swedish RCTs, superior results were achieved in patients with ACL repairs with augmentation using the iliotibial band, compared with those with repair alone or nonoperative treatment.14,15,220 No RCTs have been reported comparing bone-patellar tendon-bone or multistrand hamstring autograft ACL reconstructions with nonopera- tive treatment. Despite the uncertainties of expected outcomes after ACL reconstructions reported in the literature, many investigators conclude that patients who desire to return to high-risk activities or who have other significant knee injuries (meniscal tears, disruptions of other ligaments, articular cartilage damage, as well as marked anterior lax- ity) are probably best served by an ACL reconstruc- tion.57,86,87,124,294,307 Conversely, patients with low-risk activity levels, isolated ACL injuries, and mild pathologic laxity may be successfully treated without surgery.141,204 It appears that the patient's age has little effect on the respective results.83 Each patient must be evaluated indi- vidually and be fully informed of the advantages and risks of any treatment method proposed, based on the individ- ual surgeon's interpretation of the present body of evi- dence available concerning the efficacy of nonoperative treatment of ACL injuries.

Reinold MM, Wilk KE, Macrina LC, Dugas JR, Cain EL. Current concepts in the rehabilitation following articular cartilage repair procedures in the knee. J of Orthop. & Sports Med 2006; 36:774-794.

Due to its avascular nature, articular cartilage has very limited healing properties, and injuries to this tissue will often result in persistent and recurrent symptoms if managed conservatively. Traditionally, these injures have been managed using nonoperative treatment and lavage, both of which have led to unsuccessful results. There are several principles presented in this article that are of importance when considering the development of a rehab program for articular cartilage injuries. The first principle is individualization and stresses the importance of developing an individualized approach to each patient scenario. Every patient has a unique quality of articular cartilage as well as other factors that effect healing potential that should be considered. A second important principle is the creation of a healing environment that is primarily achieved through implementing controlled weight bearing and range of motion (ROM) with a gradual progression. Another important principle of rehabilitation following articular cartilage repair includes knowledge of biomechanics of the knee. Ranges of motion that apply pressure to the exact location of cartilaginous injury should be avoided and it is crucial to consider the joint arthrokinematics in progressing rehab and avoiding excess stress on healing tissue. Reduction of pain and effusion is also key when rehabilitating articular cartilage lesions. Several authors have noted a reduction in volitional quadriceps activity with the presence of pain and effusion; therefore, it is crucial to minimize pain and swelling in order to restore quad strength and control. Another important principle is the restoration of soft tissue balance. In order to avoid athrofibrosis, it is crucial for the knee joints to regain full passive ROM as early as possible through mobilization, stretching, and ROM exercises. Restoring muscle function is paramount in articular cartilage rehab. Exercises that strengthen the entire lower extremity should be included and electrical stimulation while performing isometrics is effective in restoring quad function immediately post op. Another important principle is enhancement of proprioception and neuromuscular control in order to promote dynamic stability and reduce stress placed on the articular cartilage. Controlling the application of loads is also vital to remember during the rehabilitation process. The stress applied to the injured knee must be gradually increased, which will provide healthy stimulation for the healing tissue without damaging it in the process. A final principle presented in this article is team communication. The cooperation of a team of medical practitioners in developing the optimal rehabilitation program and progression guidelines is important for achieving the best outcome for the patient. There are four biological maturation phases of cartilage on which the rehabilitation progression is based. The first phase is the proliferation phase, lasting 4-6 weeks and focusing on decreasing swelling, gradually restoring PROM and weight bearing, and enhancing volitional control of the quads. The second phase is the transitional phase, lasting 4-12 weeks with a focus on progressing from partial to full weight bearing while full ROM and soft tissue flexibility is achieved. The third phase is the remodeling phase, lasting 3-6 months and focusing on integrating more functional activities and should have full ROM. The fourth and final phase is the maturation phase, -6 months and focuses on gradually returning to full premorbid activities as tolerated. It is at this phase that the tissue reaches its full maturation. The following is a brief description of several of the most common surgical techniques used to treat articular cartilage injuries. Detailed descriptions of specific postoperative guidelines are provided in the article. Debridement and chondroplasty is a procedure in which the degenerative tissue is arthroscopically cleaned out and the subchondral bone is abraded in order to cause a release of bone marrow and facilitate production of fibrocartilage. This procedure aims at facilitating tissue healing as opposed to creating repair tissue. The Microfracture procedure also promotes tissue healing by impacting an awl arthroscopically into the subchondral bone in order to release bone marrow and stimulate production of fibrocartilage. The OATs (osteochondral autograft transplantation) procedure uses bone plugs with overlying articular cartilage that are harvested from non-weight bearing areas of the knee and transplanted to areas of injury. An ACI (autologous chondrocyte implantation) procedure uses biotechnology to facilitate the growth of new chondrocytes in a laboratory that are then transplanted onto the area of injury with a layer of periosteum over the cells to ensure they remain in that area. The post-operative rehabilitation process is vital to the success of the surgical management of articular cartilage injuries and requires a great amount of thought and consideration of multiple principles mentioned above.

NEUROMUSCULAR IMBALANCES

Sex-related neuromuscular imbalances are often observed in female athletes.30 Neuromuscular imbalances that women may demonstrate include ligament dominance, quadriceps domi- nance, and leg dominance. Ligament dominance occurs when an athlete allows the knee ligaments, rather than the lower extremity musculature, to absorb a significant portion of the ground reaction force during sports maneuvers.31 Andrews and Axe32 first introduced the concept of cruciate ligament domi- nance in their classical analysis of knee ligament instability. Hewett et al31 expanded the concept with their description of ligament dominance during sports activities. Ligament domi- nance, visually evidenced by increased medial knee motion during sports maneuvers, can result in high valgus knee moments and high ground reaction forces.22,33,34 Typically during single-leg landing, pivoting, or deceleration, all common ACL injury mechanisms, the female athlete allows the ground re- action force to control the direction of motion of the lower extremity joints. This motion is especially evident at the knee, which is not only influenced by direct external moments but also by the ankle and hip internal moments.35 This lack of coordinated muscular control of the lower extremity may lead to high forces and potentially irrevocable loads on the knee ligaments (Figure 1). Neuromuscular imbalances that repeat- edly put athletes near ''the position of no return'' may increase the risk for ligament injury.36 Several authors22,33,37,38 have demonstrated this sex-related tendency toward imbalanced lig- ament dominance, as evidenced by increased knee-valgus an- gle, coronal-plane knee-valgus motion, and net knee-valgus torques in female athletes compared with males. Another imbalance frequently observed in female athletes is quadriceps dominance. Quadriceps dominance is an imbal- ance between the quadriceps and hamstring recruitment pat- terns. With quadriceps dominance, female athletes tend to preferentially increase their knee extensor moments over their knee flexor moments when performing sport movements that generate high lower extremity joint torques.22 This overreli- ance on the quadriceps muscles is hypothesized to lead to im- balances in strength and coordination between the quadriceps and the knee flexor musculature. Hewett et al22 reported that peak flexor moments at the knee were 3-fold higher in male high school athletes than in female athletes landing from a volleyball block. They also demonstrated that peak hamstring torques measured by dynamometry were significantly lower in female athletes than in male athletes. Quadriceps dominance has been observed in elite female collegiate athletes.24 Female athletes reacted to a forward translation of the tibia primarily with a muscular activation of the quadriceps muscles, whereas male athletes relied on their hamstring muscles to counteract the anterior tibial displacement. Malinzak et al39 examined men and women during the sport-specific tasks of running, cross-cutting, and side cutting. Women used less knee flexion, increased quadriceps activation, and decreased hamstring ac- tivation compared with men when performing the tasks. This tendency to land with a straighter knee during high-intensity tasks could be exacerbated with premature activation of quad- riceps (or delayed activation of hamstring) during weight-bear- ing stance.40 Chappell et al37 concluded that the increased an- terior shear force demonstrated by the female athletes was potentially due to the combination of increased quadriceps force, decreased hamstring force, and decreased knee flexion. Landing with the knee near full extension is a common mechanism of ACL injury.41 At low knee-flexion angles (0 to 30 of knee flexion), quadriceps contractions pull the tibia for- ward and increase stress on the ACL, especially without bal- anced knee-flexor (hamstring and gastrocnemius) cocontrac- tion to decrease strain on the ligament. At knee-flexion angles less than 45, the quadriceps is an antagonist of the ACL. At knee-flexion angles beyond 45, the quadriceps translates the tibia in the opposite (posterior) direction, which reduces strain on the ligament.27 Athletes who demonstrate quadriceps dom- inance may increase their risk for ACL injury when they cut and land with low knee-flexion angles. Another neuromuscular imbalance that female athletes dem- onstrate is leg dominance. Leg dominance is the imbalance between muscular strength and joint kinematics in contralat- eral lower extremity measures. Female athletes have been re- ported to generate lower hamstring torques in the nondominant than in the dominant leg.22 Ford et al42 showed that adolescent female athletes had significant side-to-side differences in max- imum knee-valgus angle compared with male athletes during a box-drop vertical jump. Side-to-side imbalances in muscular strength, flexibility, and coordination have been shown to be important predictors of increased injury risk.23,43,44 Knapik et al44 demonstrated that side-to-side balance in strength and flexibility is important for the prevention of injuries, and when imbalances are present, athletes are more injury prone. Baum- hauer et al43 also found that individuals with muscle-strength imbalances exhibited a higher incidence of injury. Hewett et al23 developed a model to predict ACL injury risk with high sensitivity and specificity. Half of the factors in the predictive model were side-to-side differences in lower extremity kine- matics and kinetics. Side-to-side imbalances may increase the risk for both limbs. Overreliance on the dominant limb can place greater stress and torques on that knee, whereas the weaker limb is at risk because the musculature cannot effec- tively absorb the high forces associated with sporting activi- ties. Neuromuscular imbalances may not be the only factors un- derlying the sex differences in knee injury rates, but neuro- muscular control may be the greatest contributor to dynamic knee stability and offer the greatest potential for intervention.29 Female athletes may especially benefit from neuromus- cular training, because they often display decreased baseline levels of strength and power compared with their male coun- terparts. Comprehensive neuromuscular training programs de- signed for young women can significantly increase power, strength, and neuromuscular control and decrease sex differ- ences in these measures.45,46 Dynamic neuromuscular training also reduces sex-related differences in force absorption, active joint stabilization, muscle imbalances, and functional biome- chanics while increasing the strength of structural tissues (bones, ligaments, and tendons).22,47-50 Most importantly, in- creasing evidence suggests that different types of neuromus- cular training can prevent injuries, specifically ACL inju- ries.51-53

Fixation of Osteochondral Fractures/ Osteochondritis Dissecans Lesions

Symptomatic focal chondral lesions are often associated with a specific traumatic event resulting in a true osteo- chondral fracture. Decision making regarding fixation of osteochondral fragments will depend on the condition and quantity of articular cartilage, the size of the associated subchondral bone, as well as the shape, thickness, viability (extent of necrosis and measure of chronicity), and site of the lesion in the knee. We attempt repair of a long-stand- ing osteochondritis dissecans (OCD) lesion only if it becomes truly symptomatic or becomes traumatically dis- placed. In cases involving chronic symptomatic fragments, fibrous tissue is often interposed under the fragment, which can impede anatomical reduction and healing. If the fragment is severely comminuted, avascular, deformed, or otherwise irreparable, it may require removal. If at all pos- sible, however, articular surface fractures or symptomatic OCD fragments should be reduced, stabilized, and bone grafted if required.87,92,97 Osteochondritis dissecans has long been recognized to occur in the capitellum,67 wrist,27 distal tibia,7 talus,4 femoral head,71 and patella85 but is most commonly found on the femoral condyle,2 and there- fore we will focus on the surgical techniques of fixation on the femoral condyle. First, nonviable or necrotic debris under the fragment is removed with a shaver, rasp, or curette. Fixation is advised for symptomatic unstable OCD fragments with adequate subchondral bone as seen on plain radiographs (Figure 1) that are detached or mobile when probed or that have evi- dence on T2-weighted images of synovial fluid dissection through or behind the base of the lesion (Figures 2 A and B). For lesions located at readily accessible surfaces of the femoral condyle, arthroscopic reduction and fixation are often possible. Less common patella and tibial plateau lesions are more challenging locations; an arthrotomy is usually required to ensure adequate exposure, reduction, and secure fixation. For arthroscopic fixation, accessory portals should be used if needed to ensure that the fixation device is inserted perpendicular to the fracture plane. Often, 2 or more points of fixation are required to provide rotational stability. Drilling an OCD lesion is thought to create a biologic stimulus for healing. To promote the local biologic response, a 0.062-inch-diameter smooth Kirschner wire may be used with either a retrograde or antegrade technique.1 Retrograde drilling necessitates care to avoid the open physis, and antegrade requires care to avoid pen- etrating normal or intact articular cartilage.3 Alternatively, a microfracture awl is used to violate the subchondral bone and to induce bleeding and egress of marrow ele- ments. Cancellous bone graft may be obtained from the Gerdy tubercle or the intercondylar femoral notch by using osteochondral autograft harvest devices. Postoperative care includes nonweightbearing for up to 6 to 8 weeks and early range of motion, including continuous passive motion (CPM), if available to the patient. For fixation, we recommend headless metallic cannulated screws with a differential thread pitch, which provide a lag effect, compressing the fragment into its native bed. These screws are easily countersunk below the articular surface, and although they often provide optimal fixation, they may require staged arthroscopic removal. If metallic fixation is used, then the hardware is removed after clinical and radiographic signs of healing at up to 3 months post- operatively. After this technique, success rates of 80% to 90% have been reported, with poorer results in lateral femoral condyle lesions.2,23,97 Metallic staple fixation is not recommended, as these staples are associated with low (50%) healing rates and a significant rate (30%) of staple breakage.54 Bioabsorbable devices are available for smaller lesions or if the lesion consists mostly of cartilage with scant subchondral bone available for fixation. Advantages of bioabsorbable fixation devices are a lower profile with smaller bore perforation of the articular surface and unlikely need for staged removal of hardware. The disad- vantages of these devices include potentially inferior fixa- tion strength, less interfragmentary compression com- pared to metallic devices, higher implant cost, and the potential for osteolysis in the case of rapid polymer break- down causing localized lactic acid overload.94 Series with small numbers and short follow-up cautiously recommend bioresorbable fixation as first-line treatment of nondis- placed fragments, with metallic fixation reserved for pri- mary failures or more unstable fragments in the femoral condyles.96 Successful biologic fixation of femoral condyle OCD with autologous osteochondral plugs has also been reported.9,68 Reports of OCD fixation in the patella,80 including tech- niques for retrograde fixation of OCD lesions of the patella using fluoroscopic guidance, are present in the literature, but outcomes of such an uncommon entity remain unclear.81 Series with small numbers demonstrate encour- aging results after arthroscopic fixation of OCD of the capitellum.18,58,76 There are currently no randomized clinical trials exam- ining the treatment of OCD of the talar dome. One recent comprehensive review by Verhagen et al identified 39 studies comparing the options of nonoperative treatment; excision; excision and curettage; excision, curettag,e and drilling; bone graft after excision and curettage; osteo- chondral transplantation; or fixation and retrograde drilling. In that review, excision, curettage, and drilling had the best subjective outcome of 86% good to excellent at a minimum of 2 years, but a definitive conclusion could not be drawn from the data available.93 Finally, OCD of the capitellum has also been reported with fixation leading to successful symptom resolution.48

DIAGNOSIS/EVALUATION

The first step in evaluating a cartilage restoration patient is to obtain a careful history, which includes the mecha- nism of injury, onset and pattern of symptoms, prior treat- ments, and the response to treatment, as well as a thor- ough review of previous operative reports, arthroscopic images, and videos. In one study, the average patient pre- senting for cartilage restoration had 2.1 previous treat- ments,101 usually with a different physician. In this set- ting, direct verbal or written communication with the pre- viously treating surgeons is extremely helpful. One goal of the physical examination of a patient with chondral injury is to reveal the relative contribution of coexisting abnormalities. In addition to the sites of point tenderness, crepitus, and catching, the examination should carefully assess for the ligamentous stability of the joint, patellofemoral tracking, and the mechanical alignment of the lower extremity. In addition, the condition of the menisci and opposing articular surfaces, particularly in the symptomatic compartment, is critical. Other mechani- cal issues of obesity and gait patterns may exclude a patient from certain treatments because of a potential inability to comply with often extensive rehabilitation pro- tocols. Radiographic evaluation should include standing AP, lateral, patellar skyline (Merchant), and 45° flexion PA weightbearing views, as well as full-length alignment films. The PA weightbearing 45° flexion (skiers) view is crucial, as it brings the posterior femoral condyle into a tangential position relative to the tibial plateau. A normal- appearing joint in a standing AP radiograph may reveal severe articular cartilage damage to the posterior femoral condyle when viewed with the knee in 45° of flexion. Recent advancements in cartilage-specific MRI technol- ogy permit precise diagnosis and measurement of articu-lar cartilage abnormality. High-resolution fast spin echo sequence techniques can determine location, size, and depth of cartilage lesions,102 and fat-saturation protocols combined with ionic gadolinium diethylene triamine penta-acetic acid (Gd-DTPA) contrast24,80 can describe bio- mechanical and biochemical changes associated with matrix degeneration. These advancements provide preop- erative information and may allow for a postoperative assessment of actual glycosaminoglycan content of repaired or replaced tissue. Animal studies have suggested the utility of ultrasound technology in the evaluation of articular surfaces,59 but there is no evidence of its utility in human studies. Nuclear medicine studies are not recommended to evalu- ate focal chondral defects of traumatic causes because of the nonspecific nature of the information they provide. In the evaluation of osteochondritis dissecans, however, a bone scan can be helpful to describe the biologic activity of the lesion fragments. An examination under anesthesia will allow for an assessment of comorbidities that may need to be addressed. A thorough arthroscopic evaluation is valuable in determining the location, topical geography, surface area, and depth of a defect. In addition, arthroscopy allows for a formal assessment of comorbidities, such as the con- dition of the opposing articular surface, ligament and meniscus status, and other unsuspected cartilage defects. Grading of articular cartilage lesions depends on direct visual assessment and has interobserver and intraobserver variability. In addition to the rating systems of Outerbridge,96 Insall,55 Bauer and Jackson,10 and Noyes and Stabler,92 which are frequently cited in the literature, the International Cartilage Repair Society has offered a grading system to be used as a universal language when surgeons are communicating about cartilage lesions.17 Verbal or written grading of articular surfaces should specify which grading system is being used and should be accompanied by a written and diagrammatic description of the lesion. Direct arthroscopic evaluation of the menisci will allow for an assessment of the quality of remaining meniscal tissue in the setting of a previous meniscectomy and aid in the decision to include a meniscal transplant in the comprehensive surgical plan. Despite the availability of several techniques for the past 3 decades, patient evaluation and treatment selection remain challenging. This is in part owing to the fact that the natural history of commonly found asymptomatic lesions is unclear. Although it is widely believed that a symptomatic cartilage lesion is likely to persist or worsen without treatment,67,82,116 the likelihood of a cartilage lesion detected incidentally on MRI or at arthroscopy to become symptomatic likely depends on its location, depth, geographic pattern, the demands of the patient, as well as the presence of associated comorbidities. Preexisting liga- mentous instability, meniscal deficiency, or malalignment of the tibiofemoral or patellofemoral joints may cause some lesions to become more rapidly symptomatic than others. In addition, articular cartilage responds to injury with a disordered and often incomplete repair response,which adds to the highly variable pattern of symptoms seen after cartilage injury.75,111

STUDIES OF LIGAMENT STRAIN IN VIVO

The rehabilitation program after ACL reconstruction reg- ulates the strain environment of the graft while preventing muscle atrophy. Autogenous grafts have a viable cell popu- lation at the time of implantation that respond to mechan- ical strain. Although strain stimulates healing, excessive strains could permanently stretch out or fail the tissue. The failure strain of the ACL is approximately 15%, whereas that of a patellar tendon graft is 20% less (4). Unfortunately, the failure strain and strength of the tissue are significantly re- duced once the graft is implanted. The magnitude, optimal frequency, and duration of strain required to optimize healing remain unknown. Direct measurements of ACL strains have been performed in humans to gain insight into the strains applied to the healing ACL graft when rehabilitation exercises are per- formed (2,8,9). For these studies, the ACL serves as a surro- gate for the graft because it is not possible for the patients to perform these tasks at the time of their reconstruction. ACL strains were measured in subjects undergoing diagnostic ar- throscopy for minor meniscal lesions or chondral débride- ment with the use of local anesthesia (2,8,9), or spinal anesthesia (7) using an implantable strain transducer (differ-ential variable reluctance transducer (DVRT)) (2). ACL strains were measured in response to activation of selected muscles (2,7), tibiofemoral compressive loading (8,9), and various OKC and CKC exercises (2,8). Using the DVRT, Beynnon et al. (2) determined that ACL strains were dependent on the knee flexion angle and exten- sion torques applied during isometric quadriceps contractions (2). At 30 Nm of extension torque, ACL strain values pro- duced at 15° of knee flexion were significantly greater than those produced at 30°, whereas no strain was produced at 60 and 90° (Table 1). For isometric hamstrings contractions, the ACL strains were found to be independent of flexion torque or knee position; hamstrings contractions did not strain the ACL at any knee angle tested (Table 1). The strain values produced by cocontraction of the quadriceps and hamstrings at 15 and 30° were less than those produced during isolated isometric quadriceps contractions (Table 1). Because the proximal tendons of the gastrocnemius span the tibial plateau and insert on the posterior- distal aspect of the femur, its contraction could potentially strain the ACL by forcing the tibial plateau anterior. Using the in vivo strain measurement approach, the ACL strains produced by gas- trocnemius contractions were determined (7). These patients underwent spinal anesthesia, and the contractions were in- duced using electrical stimulation to isolate the muscle con- tractions. With the knee at 5 and 15° of flexion, contractions of the gastrocnemius increased ACL strains relative to the relaxed state to levels close to that of an isolated quadriceps contraction (Fig. 3). With the knee in greater flexion (30 and 45°), gastrocnemius contraction did not strain the ACL (Fig. 3). Furthermore, it was demonstrated that cocontraction of the hamstrings or gastrocnemius with the quadriceps did not significantly reduce the ACL strains when the knee was near extension. Tibiofemoral compressive loads have been shown to in- crease joint stiffness and decrease anterior displacement of the tibia, and are therefore thought to protect the healing ACL graft. The effects of applying an external compressive load to the knee, such as that produced by body weight or the leg-press exercise, were assessed with the knee at 20° of flexion (9). The ACL was strained as the knee transitioned from no compressive load (the OKC condition) to a com- pressive load equal to 40% of body weight (the CKC condi- tion) (Fig. 4). The strain is most likely produced by the anterior neutral shift of the tibia that has been observed in ACL-deficient patients as they undergo the transition be- tween nonweight bearing and weight bearing (3). Beynnon et al. also reported that the maximum ACL strains produced during a simple squat (90° to 10°), a closed- kinetic chain exercise, were similar to those produced during active extension of the knee (90° to 10°), an open-kinetic chain exercise (Fig. 5) (2). It was noted, however, that an increase in resistance during an OKC exercise (active exten-sion vs active extension against 44 N of resistance) increased ACL strains, whereas this did not occur during CKC exer- cises (squatting vs squatting with Sport Cord). Other CKC exercises, such as stationary bicycling, also did not exhibit the increase in strain with an increase in resistance. To systematically evaluate the effects of increasing resis- tance and external compressive load during exercise, ACL strains were measured while subjects performed flexor and extensor exercises against increasing resistance with and without a compressive load applied to a foot in an effort to simulate the CKC and OKC conditions, respectively (Fig. 6) (8). During the extensor exercise (quadriceps dominant), a significant increase in ACL strain was observed with an increase in resistance when no external compressive load was applied to the foot (OKC), whereas no significant increase in ACL strain was observed with increased resistance when the external compressive load was applied (CKC). The increase in strain from 2.3% to 3.8%, which occurred during the OKC simulation from 0 and 24 Nm of resistance torque, respec- tively, was equal to that produced when an anterior shear load of 150 N was applied directly to the proximal tibia when the knee was at 30° of flexion (i.e., the Lachman test) (Fig. 7). Although the increase in strain was significant between 0 and 24 Nm of torque for the OKC condition, there was no statistical difference between the mean peak strains of the OKC and CKC conditions with the 24-Nm resistance ap- plied, a relatively high load for early rehabilitation of the knee. Direct measurements of ACL strain have provided insight into the healing environment of the ACL. However, several limitations of this approach should be noted. First, the strain measurements were performed on the intact ACL. However, Beynnon et al. (1) have shown that the strain patterns produced in the patellar tendon graft were similar to those of the ACL during passive knee motion. Thus, it is reasonable to assume that exercises inducing high strains on the ACL would produce high strains on an ACL graft during dynamic activities. Second, subjects were undergoing arthroscopic partial meniscectomy or chondral débridement, which could alter knee kinematics. However, there was no evidence of ligamentous damage. Third, the measurements were per- formed under intraarticular anesthesia, which could poten- tially alter the way the muscles function. Finally, the DVRT is only capable of measuring the strain response of the an- teromedial bundle of the ACL in vivo, and not the entire strain distribution of the ligament. This bundle comprises 65% of the total cross-sectional area of the ligament.

Osteochondral Autograft Transfer

This technique involves transfer of an osteochondral plug from a relatively nonweightbearing region of the knee to restore a damaged articular surface. The application of the technique is limited by the amount of donor tissue avail- able in the knee. Ideal indications include symptomatic, distal femoral condyle articular cartilage lesions with intact menisci and tibial cartilage in a nondegenerative joint with proper mechanical alignment. Although large lesions have been treated with this technique, we believe the ideal lesion size is 1 to 2 cm in diameter. Lesions up to 3 to 4 cm in diameter can be treated, although graft limi- tations tend to limit optimal indications to treating smaller lesions. The treatment of patella or tibial surface lesions as well as intact but loose International Cartilage Repair Society (ICRS) grade II OCD lesions would be relative indications.46 The risk of donor site morbidity increases as more tissue is harvested. The typical site of harvest is the femoral intercondylar notch and the periphery of the lateral femur just proximal to the sulcus terminalis. Simonian et al86 evaluated these 2 typical sites of harvest and found that they demonstrated significant contact pressure, although the clinical relevance is unknown. Garretson et al31 described low contact pressures on the medial trochlea and relatively low contact forces at the distal lateral trochlear ridge, near the sulcus terminalis, identifying this as a possible harvest location on the lateral trochlea. Topographic mapping of the articular surface may allow for selection of donor sites to create plugs whose contour matches the recipient locations.6 All stages of the procedure, including graft plug harvest, recipient tunnel preparation, and plug insertion, can be done through a small arthrotomy or arthroscopically, depending on the location of the lesion. There are several commercially available systems to perform this procedure. A sizer is used to determine the number and size of grafts that will be needed. The properly sized graft harvester with collared pin is introduced perpendicular to the donor site. It is lightly tapped into bone to a depth of approxi- mately 12 to 15 mm (Figure 3). For removal, the harvester is twisted abruptly 90° clockwise and counterclockwise with a parallel pull to remove the donor plug. The har- vester has a plunger that will push the donor plug into the recipient hole. The recipient hole is created at a depth of 2 mm less than the donor graft just harvested and extracted in the same manner as the donor core. It is important to main- tain a perpendicular relationship with the articular sur- face to create well-defined vertical walls in the recipient hole (Figure 4), which will facilitate congruent plug place- ment. This requires a constant knee flexion angle and often multiple accessory portals; implanting the graft plug immediately after harvesting will facilitate maintaining the proper insertion angle. The donor tube harvester is then placed over the recipient site, taking care to maintain perpendicular orientation, and the donor plug is gently advanced atraumatically into the defect, often leaving the plug slightly proud (Figure 5A). Premature advancement of the plug before it is well seated in the recipient tunnel may result in loss of control of the plug and may require plug collection using loose body retrieval techniques. The final seating of the plug can be done with an oversized tamp, taking care not to damage the articular cartilage on the surface of the plug graft (Figure 5B). The final plug position should be flush with the surrounding articular cartilage (Figure 5C). Graft congruence is key to minimiz- ing shear. Studies indicate that tunnel depth should equal plug length precisely. Supported grafts heal well, but unsupported grafts tend to subside, eventually becoming covered by fibrous tissue.72,83 If performing several transfers in a single lesion, the location of all plugs should be planned before placing the first one to minimize risk of tunnel confluence or tunnel wall fracture. Beginning at the periphery of a lesion, loca- tion and depth of the recipient tunnels and donor plugs are selected to create a convexity to match the surrounding joint surface. Postoperatively, passive and active range of motion is encouraged. The patient is kept on protective weightbear- ing for up to 6 weeks. Graft healing is assessed both clini- cally and by plain radiographs to evaluate bone plug posi- tion and integration. Cartilage-specific MRI scans areelpful at 3-month intervals. After evidence of bone heal- ing on radiographs at 6 to 8 weeks, the patient is advanced to full weightbearing as tolerated. Closed chain strength- ening exercises only are allowed for 3 months to prevent undo shear on the articular surface. The greatest amount of shear is seen at the interface of the donor plug and the recipient bed of cartilage, and a slightly prominent plug causes more shear than does a slightly recessed plug.19 Animal studies have demonstrated chondrocyte loss in areas of high shear at the edges of implanted plugs.50 Perhaps combining microfracture and autologous plug transfer would provide a fibrocartilage interface for improved graft integration, reduced shear, and improved graft strength. Pull-out strength of properly placed press-fit plugs averages 93 N and correlates with plug length. It has been demonstrated that the force required to dislodge the graft is reduced by half with graft reinsertion or levering at the time of harvest.25 The advantages of using autologous plug transfer to treat focal chondral lesions are graft availability (ie, there is no ordering or waiting for grafts), the absence of disease transmission risk, and the relatively low cost of a single- stage procedure. The disadvantages include donor site morbidity and limited available graft volume.25,45,46 In addition, it is technically difficult to position the plugs to re-create the contour of curved surfaces. Despite these lim- itations, in small- and medium-size lesions, this technique has been reported to result in 91% good to excellent results after more than 3 years for femoral condyle lesions that are isolated,25,42,45 if combined with ACL reconstruction,12 or as treatment of OCD.13,47 Overall, autologous osteo- chondral plug transfer has been shown to result in a greater percentage of good to excellent results for femoral condyle lesions (92%) than for tibial plateau (87%) or the patellofemoral joint surface (79%).45 Autologous osteochondral transfer is often used to treat focal lesions in other joints as well. Encouraging results have been reported in treatment of OCD of the talus47 at up to 7 years after treatment. In treating traumatic focal chondral lesions of the talus, autologous plug transfer resulted in a favorable outcome in 94% of patients when compared to marrow-stimulation techniques (ie, microfrac- ture).44 In addition, reports of the use of osteochondral transfer to treat focal chondral lesions in the femoral head45 and elbow OCD13 demonstrate encouraging results. Further refinements of this technique will minimize donor site morbidity by careful selection of the harvest site. In the future, a refined technique may avail autolo- gous hyaline cartilage plugs to treat focal chondral defects in any synovial joint with hyaline cartilage. Just as the iliac crest has become the utility source of autogenous bone graft, so too may the relatively nonweightbearing surfaces of the femoral condyle provide osteochondral plugs for routine transfer to other joints.

Rehabilitation Braces

Three prospective RCTs have compared rehabilitation with and without the use of a rehabilitation brace.23,48,78 A prospective RCT of rehabilitation after ACL recon- struction with a central third BPTB graft was performed by Harilainen et al.48 Subjects were randomized by their birth year to either the braced group (rehabilitation with a gradual increase in weightbearing and the use of a brace for 12 weeks postoperatively) or the unbraced group (reha- bilitation with crutch use for 2 weeks postoperatively fol- lowed by weightbearing as tolerated but without use of a brace). The number of surgeons involved with the investi- gation and the specific details of the rehabilitation pro- gram were not presented. The 1- and 2-year follow-up intervals included 100% and 93% of subjects enrolled, respectively, and these follow-ups revealed no differences between the treatments in activity level, joint laxity, and isokinetic thigh muscle strength. Moller et al78 performed a prospective RCT that focused on the use of a rehabilitation brace during the first 6 weeks after ACL reconstruction with a BPTB graft. The rehabilitation program included early weightbearing and lasted 6 months. Four surgeons were involved in the study (2 performed a single-incision procedure, 2 performed a 2- incision procedure). Subjects were randomized to undergo rehabilitation without a brace or with a brace during the initial 6 weeks of healing (the first 2 weeks during the day and night, the subsequent 4 weeks during the day). Ninety-eight percent of the subjects were evaluated at 6- month follow-up, and those who did not receive a rehabili- tation brace had a better Tegner activity score. However, at the 2-year follow-up, which included 90% of subjects enrolled, there were no differences between the treat- ments in subjective outcome (Lysholm score and visual analog scale measurements of pain, discomfort, and insta- bility), range of knee motion, functional performance (1- legged hop test), isokinetic strength, and A-P knee laxity. A prospective RCT focusing on the use of a rehabilita- tion brace was also performed by Brandsson et al.23 After single-incision ACL reconstruction with a central third BPTB graft, patients were randomized to undergo 6 months of rehabilitation without a brace or the same 6- month program with a rehabilitation brace during the ini- tial 3 weeks of healing. Follow-up measurements were per- formed by an independent investigator through a 2-year follow-up interval; the results included 92% and 80% of subjects in the braced and unbraced groups, respectively. Rehabilitation with a brace resulted in fewer problems with swelling, a lower prevalence of hemarthrosis and wound drainage, and less pain throughout the early recovery period compared to rehabilitation without a brace. At the 2-year follow-up, no differences were found between the treat- ments in terms of activity level (Tegner scale), IKDC rat- ing, function (1-legged hop test and isokinetic strength), and knee laxity (KT-1000 arthrometer measurement). n the 3 randomized trials described above, only Harilainen et al48 described their method of randomiza- tion; only Brandsson et al23 indicated that follow-up meas- urements were performed by an independent investigator, and it was unclear if this examiner was blinded to the treatment groups. All studies revealed a minimal loss of patients to follow-up. There appears to be a consensus among investigators that, during the early phase of recovery, the use of a reha- bilitation brace results in fewer problems with swelling, lower prevalence of hemarthrosis and wound drainage, and less pain compared to rehabilitation without a brace; however, at longer-term follow-up, rehabilitation bracing does not appear to have an effect on clinical outcome, range of knee motion, subjective outcome, A-P knee laxity, activity level, or function (thigh muscle strength and 1- legged hop test). It should be mentioned that one of the primary reasons for using rehabilitation braces is to help prevent flexion contractures by maintaining full knee extension during the early phase of healing.

OATs

While marrow stimulation techniques (chondroplasty and microfracture) often have initial success in symptom reduction and the restoration of function, several studies have shown a gradual dete- rioration of the fibrocartilagenous tissue over time.8,9,28,36 Other procedures have been designed to attempt to repair cartilage defects with type II hyaline articular cartilage similar to that of the native carti- lage. Theoretically, this type of repair should re- semble the normal biomechanical strength of the original cartilage and thus be more resilient to deterioration.4,5,22,42,43 The OATs procedure involves the transplantation of plugs of bone with overlying articular cartilage that are harvested from non- weight-bearing areas of the knee (such as the proxi- mal lateral trochlea or the intercondylar notch area).1 These plugs are round and range in size to match the size and shape of the defect. The har- vested plugs are then implanted into holes drilled to receive the specifically sized grafts within the defect. Several plugs of similar or varying size can be utilized to fill the defect as much as possible, which is why this procedure is also referred to as mosaicplasty. Due to the extent of the procedure, this surgery is done through an open incision. Rehabilitation following OATs procedures requires the avoidance of early deleterious forces to avoid disrupting the healing transplanted bone plugs Table 3. Currently we alter the pace of the rehabilita- tion program following OATS procedures based not only on the size of the lesion, but also the amount of transplanted bone plugs. We progress more cautiously when numerous bone plugs are used due to the potential for a less congruent surface. The early proliferation phase lasts until the eighth week postop- eratively. A 44% reduction in the push-in and pull-out strength of the transplanted bone plugs has been observed at 1 week postoperatively,61 emphasizing the need for strict non-weight bearing early after surgery. We typically begin partial weight bearing 2 to 4 weeks following surgery, based on the size of the lesion and the number of transplanted bone plugs utilized. Although the original hyaline cartilage remains intact and viable,39 the strength of the bone plugs is the limiting factor when designing the postoperative re- habilitation program. By 4 weeks, the cancellous bone plugs are united22 and by 6 weeks there is full subchondral integration and 29% of grafts have shown bonding between the articular cartilage of the bone plugs and the sur- rounding articular cartilage.39 Although integration has occurred, a 63% decrease in graft stiffness is still observed at 6 weeks postoperative.39 During this time, weight bearing is gradually progressed as the strength of the repair increases. At 8 weeks postoperative, fibrocartilage has been observed to grow to the surface and seal the recipient and donor site hyaline cartilage, and progression is made to full weight bearing. Immediate weight bearing is initiated for patellofemoral lesions with the patient using a drop- lock knee brace and progressed to full weight bearing without the brace at approximately 6 to 8 weeks postoperatively. ROM during the early protective phase is gradually progressed to assure that adhesion formation and loss of motion is avoided. Due to the large incision and invasive nature of the procedure, motion is pro- gressed gradually to assure that effusion formation is minimized. Exercises are progressed from non-weight-bearing exercises, such as quadriceps sets and multi-angle straight leg raises, to gentle weight-bearing exercises after week 6. During the transition phase, full ROM and weight bearing are achieved, typically between weeks 8 and 10, although patients with larger lesions may need to further delay the progression to full weight bearing for up to 12 to 14 weeks. At this point the strengthen- ing program is progress to include weight bearing and machine exercises. Again, it is during this phase that patients return to low-impact functional activities. During the remodeling and maturation phases, strength, proprioception, and neuromuscular control are enhanced while gradually applying impact-loading stresses as tolerated without an increase in symptoms. Patients may return to various sport activities as the progression in rehabilitation and cartilage healing allows. Generally, low-impact sports such as skating, rollerblading, and cycling are permitted at about 6 to 8 months. Higher-impact sports, such as jogging, running, and aerobics, may be performed at 8 to 10 months. High-impact sports, such as tennis, basket- ball, and baseball, are allowed at 12 to 18 months.

...

chemical staining. In 10% to 15% of cases, the biopsy site demonstrated an exaggerated healing response in the notch, resulting in discomfort and catching, which may occur between 3 and 9 months. This routinely responded well to simple arthroscopic debridement. To study the long-term durability of ACI, Peterson et al followed 61 patients for a mean of 7.4 years after ACI. Good or excellent subjective results were found in 81% at 2 years and 83% at 5- to 11-year evaluation. The total fail- ure rate was 16%, all of which occurred in the first 2 years. In this series, patients with the longer outcome were early patients who underwent ACI before full maturation of the surgical technique. As all failures occurred before 2 years, this study illustrated the durability of results at 2 years.73 Cole et al22 reported on a series of 103 defects in 83 patients who had been evaluated prospectively after their ACI procedures. Cincinnati, International Knee Documentation Committee, Tegner, Lysholm, Knee Injury and Osteoarthritis Outcome, and SF-12 physical scores all showed significant (P < .05) subjective improvement in 30 patients evaluated at a minimum of 2 years compared to preoperative ratings. Although patients were not pain free, nearly all reported approximately a 50% reduction in pain. In this series, patient satisfaction was high; 79.3% stated that they were completely satisfied, and 92.9% stated that they would have the surgery again, given similar circum- stances. To compare microfracture to ACI, Knutsen et al55 ran- domized 80 patients with single focal chondral defects in stable, nonarthritic knees with proper mechanical align- ment to receive either ACI or microfracture as a primary treatment. At 2 years, objective arthroscopic evaluation and biopsy combined with subjective clinical evaluation using Tegner, Lysholm, ICRS, and SF-36 demonstrated significant improvement in both groups, with statistically significantly greater improvement in the microfracture group than in the ACI group (P = .004). Younger and active patients did better in both groups. In this series, both groups of patients were allowed immediate partial weight- bearing (up to 50 lb), which may have been disruptive for the fragile ACI patch. Horas et al49 compared ACI to osteochondral autograft transplantation at 2 years in 40 patients with a single femoral condyle chondral defect. Both treatments decreased symp- toms, but the subjective improvement provided by ACI lagged behind that provided by the osteochondral auto- graft transplant. Objective histologic data revealed that the ACI tissue was primarily fibrocartilage, whereas the osteochondral transplants retained their hyaline charac- ter. There was a persistent gap and lack of integration between the bone plugs and the surrounding articular car- tilage. This study had a small number of patients in each group, a relatively short follow-up, and no control group. To compare mosaicplasty to ACI, Bentley et al8 random- ized 100 patients with a mean age of 31 years with isolated traumatic focal chondral defects to receive either ACI or mosaicplasty. Modified Cincinnati scores and clinical assessment measures showed subjective good to excellent results in 88% of ACI patients and only 69% of the mosaic- plasty patients. Objective arthroscopic visualization at year demonstrated 82% healing among ACI patients but only 34% healing among mosaicplasty patients. This is the only prospective, randomized, controlled comparison of ACI and mosaicplasty and appears to demonstrate the superiority of ACI over small-plug autologous mosaicplasty. Autologous Chondrocyte Implantation in Other Joints. Although ACI technology has traditionally been applied to treat focal chondral lesions in the knee, it has recently been used to treat lesions on other joint surfaces. Unpublished reports of resurfacing a femoral head defect (Lars Peterson, J.W.A. personal communication, December 7, 2003) and published reports of using ACI to treat elbow lesions13,15 are emerging. The senior author recently reported the use of ACI to treat a young athlete with a large full-thickness 6-cm2 articular cartilage defect of the proximal humerus.78 In addition to the shoulder, ACI technology has recently been applied to treating osteochondral defects of the talus. Because of the traumatic nature of these lesions, the osteo- chondral fragment of the talus is carefully evaluated dur- ing an initial ankle arthroscopy, and a decision whether to attempt fixation is made. Successful results of fixation efforts overall are reported to be between 36% and 81% and are more thoroughly discussed elsewhere.5,53,79 After completion of the ankle arthroscopy, an arthroscopic artic- ular cartilage biopsy is obtained from the ipsilateral knee. In evaluating the chondral lesion, important variables to consider include the size and dimensions of the lesion, the location of the lesion, and whether an osteotomy would be required for an open exposure for an ACI procedure. The need for bone grafting via the "sandwich technique" can also be predicted and is determined by the depth of the talar lesion. The need for osteotomy of the medial or lateral malleoli at the time of cell implantation depends on both the size and location of the defect. Comorbidities of the ankle including instability or frac- tures requiring fixation must be carefully evaluated, and the decision to perform a concomitant ankle stabilization procedure should be predetermined and incorporated into the surgical plan. If the patient would benefit from lateral ligament reconstruction, then this is carried out in con- junction with the ACI procedure in a single stage. If the patient has an OCD in the anterolateral aspect of the talus, the lateral ligament reconstruction can be per- formed at the completion of the ACI implantation via the same anterolateral incision. Treating an OCD lesion at a second location on the talus will likely require an osteotomy.61 Once surgical exposure and a plan for treating comorbid conditions are established, lesion preparation, periosteal harvest, graft suturing, and cell implantation are per- formed according to standard ACI protocol as described above. Postoperatively, the patient is placed in a hinged ankle brace that allows for a 20° arc of dorsiflexion and plantar flexion. Continuous passive motion is initiated 8 hours postoperatively. The patient is permitted toe-touch weight- bearing during the first 2 weeks and is progressively increased to 75% weightbearing at the end of 6 weeks and to full weightbearing by 8 weeks. At 6 months, a gradual return to jogging and sport-specific training may be initi- ated. To date, there are 3 published series of reported subjec- tive improvement after ACI as treatment of chondral lesions of the talus. Giannini et al34 reported the outcomes of 8 patients who were treated with ACI as a second-line treatment, and in their series, American Orthopaedic Foot and Ankle Society scores improved from 32 points preop- eratively to 91 points at 2 years after implantation. In 2002, Koulalis et al57 reported on 8 patients at a mean of 17 months who also universally reported subjective improvement after failing other treatments, despite no objective evidence of type II collagen at follow-up biopsy. Mandelbaum et al61 recently reported a series of 14 patients at 32 months with 79% good and 21% poor out- comes; half of the patients in this series required arthro- scopic debridement for periosteal patch hypertrophy. Future Directions for ACI. In the future, techniques using minimally invasive implantation will spare the patient the morbidity of an open arthrotomy. All-arthro- scopic techniques have been reported but are not current- ly implemented in the United States.26 The technique is based on implanting a 2-mm-thick polymer fleece preloaded with autologous chondrocytes in a fibrin gel that is anchored to the condyle arthroscopically. Other techniques have implemented in vitro culturing of a chondrocyte- laden scaffold before implantation.59 In a canine model, Lee et al59 evaluated full-thickness focal chondral defects without bone involvement 15 weeks after implantation of an autologous articular chondrocyte-laden type II collagen scaffold that had been cultured in vitro before implanta- tion. In these cultured scaffolds, the reparative tissue formed from the scaffolds filled 88% of the cross-sectional area of the original defect, with hyaline cartilage account- ing for a mean of 42% (range, 7%-67%) of the defect area. Further work is necessary to identify the specific culture and cell density parameters needed to maximize this advantage of in vitro scaffold culture before final implan- tation compared to the results of noncultured implanta- tion.14,74 In the future, allogenic sources of cells or single- stage biologic techniques may offer the added advantage of eliminating the need for biopsy before implantation.

Allograft Use and Processing

A total of 154 tissue banks were identified in a January 2001 report issued by the Office of the Inspector General Department of Health and Human Services. In the mid- 1990s, the yearly number of organ donors increased more than 3-fold, from 6000 in 1994 to more than 20 000 in 1999. This remarkable increase in donor availability cor- relates with increases in the yearly distribution of 750 000 allografts by 1999.33 In 1992, the most commonly distrib- uted tissues from tissue banks were bone-patellar ten- don-bone (95%), Achilles tendon (90%), fascia lata (86%), and meniscus (33%), with very little osteochondral allo- graft use.123 Allograft tissue-processing techniques have been advancing rapidly over the past decade.4,32,76,78,105,112 Data from detailed donor medical and social history and serology testing are used before graft procurement. The grafts are procured within 12 hours of death, and the tis- sue may be harvested with the use of sterile technique or may be procured and processed in a clean room environ- ment. Thorough lavage removes marrow components, which are the main source of disease transmission and immune reaction. They are transferred to an antibiotic solution for a day at 37°C to kill microorganisms and sub- sequently stored at 4°C until used, but low temperatures may have an effect on chondrocyte viability.128 The virucidal dose of radiation required to eliminate viral DNA is 30 kGy, which not only kills chondrocytes but also affects mechanical properties and therefore is not used for fresh osteochondral allografts.105 Currently, most osteochondral allografts are transplanted fresh, to preserve both cartilage cells and matrix. The suc- cess of an osteochondral graft implantation is directly related to the percentage of viable chondrocytes that remain after implantation.128 The grafts are preserved in either lactated Ringer's solution or a physiologic culture medium to maximize the viability of the chondrocytes. Viable chondrocytes can be maintained in lactated Ringer's solution cooled to 4°C for 7 to 14 days. Recent data demonstrate a detectable decrease in the percentage of viable cells after 24 hours and a gradual decrease in chon- drocyte viability at 7 days after the donor's death when grafts are stored in lactated Ringer's solution or after 14 days when grafts are stored in a physiologic culture medi- um.8 After 14 days of storage, fresh human osteochondral allografts undergo significant decreases in chondrocyte viability, viable cell density, and metabolic activity. Although tissue glycosaminoglycan content and biome- chanical properties of cartilage matrix are preserved dur- ing storage for 28 days, the chondrocytes necessary to maintain the matrix demonstrate decreased viability dur- ing that storage period, with the most abrupt drop occur- ring at 15 days.129 Bone marrow elements are the primary source of allo- graft immunogenic cells, and these are dramatically reduced during lavage at procurement. Host-donor match- ing of the major histocompatibility complex of chondrocyte surface antigens has further reduced the immunogenic load. Friedlaender et al39 compared immunologic response and clinical outcome at 10 years after implantation of mas- sive osteochondral allografts in 29 patients. In that series, 8 patients (28%) had anticlass II human leukocyte antigen responses, but of those, 5 (63%) had good to excellent results. Of the 21 without an immune reaction, 18 (86%) had satisfactory outcomes. They concluded that immune reactions found with even massive grafts were self-limited and did not preclude a satisfactory result. Since the work of Langer and Gross61 in 1974, we have learned that although free chondrocytes are immunogenic, if the carti- lage matrix remains intact, sensitization does not occur. The dense matrix in which the chondrocytes are embedded acts as a barrier that limits antigen exposure. Cartilage surface deterioration allows the chondrocytes to beexposed, leading to sensitization. The use of immunosup- pressants is another way to decrease the host response to an allograft, but it is generally thought the morbidity of this treatment greatly outweighs the potential benefit, and their use is not recommended in the setting of carti- lage restoration surgery.107

Home- Versus Clinic-Based Rehabilitation

Three RCTs have compared home-based rehabilitation (limited supervision by a physical therapist during heal- ing) to clinic-based rehabilitation (supervision by a physi- cal therapist throughout the entire rehabilitation pro- gram), and there appears to be reasonable consensus that home-based programs produce similar outcomes compared to clinic-based programs.14,40,97 Schenck et al97 studied rehabilitation after 2-incision ACL reconstruction with a central third BPTB graft per- formed by the same surgeon. Subjects were randomized after surgery via a lottery drawing to rehabilitation with either a clinic-based program (mean of 14.2 visits to phys- ical therapy) or a home-based program that was monitored by a physical therapist (mean of 2.9 visits to physical ther- apy). All patients were examined at a minimum of 1 year after surgery. There were no differences in functional or subjective outcomes between the clinic- and home-based rehabilitation programs. Fischer and colleagues40 performed an RCT of rehabili- tation after ACL reconstruction with a BPTB graft per- formed with a 2-incision technique. Patients were ran- domized to undergo rehabilitation with either a home- based program that included a mean of 5 physical therapy visits (range, 3-7 visits) or a clinic-based program that included 20 physical therapy visits (range, 10-28 visits). Compliance with the rehabilitation programs was docu- mented with a training log. More recently, Beard and Dodd14 reported the results of rehabilitation after ACL reconstruction with a central third BPTB graft performed by the same surgeon. During the first month after surgery, all patients participated in the same regimen. They were subsequently randomized to either home-based rehabilitation (attending physical ther- apy only for education, assessment, and monitoring of the treatment plan) or the same home-based program with supervision (attending physical therapy twice weekly). Randomization was performed with a computer-based random-number generator, the study subjects and investi- gator responsible for making the follow-up measurements were blinded to the treatment groups, and follow-up at 12 and 24 weeks was performed on 86% and 81% of subjects in the home and supervised programs, correspondingly. No differences were found between the treatments in terms of activity level, IKDC rating, function, muscle strength, and knee laxity. These findings led the authors to conclude that "supervised exercises, in addition to a home program, has minimal extra benefit for patients that have undergone ACL reconstruction."14(p134) In the 3 randomized trials reviewed above, both Schenck et al97 and Beard and Dodd14 described their methods of randomization; only Beard and Dodd14 stated that the study subjects and investigators were blinded to the treat- ments that were studied. However, all 3 studies revealed a minimal loss of subjects at follow-up. It is important to point out that the studies we reviewed compared rehabilitation programs with different amounts of supervision and that none of the programs was com- pletely unsupervised. These reports suggest that rehabili- tation after ACL reconstruction need not be monitored by a physical therapist in a continuous manner, but attending physical therapy for the purpose of education, assessment, and monitoring of the treatment plan remains a critical aspect of a safe and effective rehabilitation program. Details were not presented regarding the specific activities and restrictions associated with both programs; instead a goal-based approach was used, with the following temporal sequence of activities: restoration of range of motion, beginning strengthening, advanced strengthening, and improvement of agility and speed. All of the subjects in the home-based program and 96% of those in the clinic-based program (1 patient was excluded because of unanticipated foot surgery) were fol- lowed up for 6 months. There were no differences between the treatments with regard to clinical examinations (range of motion, thigh atrophy, knee laxity, and pivot-shift examinations), 1-legged hop test, and the health status questionnaire.

EFFECTS OF SEX, AGE, AND ACTIVITY LEVEL ON THE OUTCOME OF ACL RECONSTRUCTION

here are many studies on the outcome of ACL recon- struction in the literature. Unfortunately, few of these articles have addressed the potential differences between male and female patients. There are no prospective, con- trolled studies directly comparing the outcomes on men and women after ACL reconstruction. Barber-Westin and colleagues12 published one of the first studies comparing the outcome of ACL reconstruction with BPTB grafts between male and female patients. Their report showed that sex alone should not be the basis for selection regard- ing surgical intervention. Failure rates were low, and out- comes were similar between male and female subjects. Ferrari et al38 also studied the use of autogenous BPTB grafts for ACL reconstruction in men and women. Their retrospective review showed no significant difference between the sexes with regard to clinical and functional outcomes; they reported that equal functional scores and overall high subjective satisfaction levels can be expected after ACL reconstruction in both men and women. Ott and associates88 also looked at the results of ACL reconstruc- tion with a BPTB graft between male and female patients and concluded that ACL reconstruction was equally suc- cessful in similar populations of men and women. Wiger et al110 compared the results of ACL reconstruction in male and female competitive athletes. All athletes had a preinjury Tegner activity level ≥7 and a normal contralat- eral knee. No differences were seen in Lysholm score, Tegner activity level, IKDC score, A-P knee laxity (KT- 1000 arthrometer measurement), anterior knee pain, and subjective evaluation between the 2 groups. The main con- clusion of the study was that the overall results were com- parable for male and female athletes 2 to 5 years after ACL reconstruction. In an earlier study by Aglietti et al,1 it was reported that patellofemoral problems were more frequent after BPTB ACL reconstruction in female sub- jects, although gender comparisons were not a specific focus of that study. When evaluating hamstring ACL reconstructions, Aglietti et al2 and Maeda et al73 used quadruple hamstring tendon autografts fixed outside the bone tunnels with posts or staples and found no significant differences between male and female patients with regard to the prevalence of anterior knee pain and clinical out- come. In contrast, a study by Noojin and colleagues83 com- paring the outcome of autogenous semitendinosus and gracilis tendon grafts in men and women showed increased laxity for female subjects, as measured by KT- 1000 arthrometer, Lachman, and pivot-shift tests. They reported more pain and lower rates of return to preinjury levels of activity for female patients, as judged by the Tegner activity score after ACL reconstruction. Female subjects also had a significantly higher rate of graft failure after ACL reconstruction in this study. In reviewing the available literature, it appears that ACL reconstruction with a BPTB autograft is equally suc- cessful in male and female patients. Whether there is a dif- ference when the ACL is reconstructed with an autoge- nous hamstring graft has not been answered in the litera- ture to date. Three studies27,43,83 have reported possible increased laxity and poorer outcomes when hamstring ACL grafts are used in female subjects, but those studies were not prospective, controlled investigations, and such a study needs to be conducted. The effect of age and activity level on outcomes after ACL reconstruction is more difficult to determine, as most studies we reviewed were not designed to investigate these variables. A study by Barber et al11 investigated outcomes of ACL reconstructions in patients both younger and older than 40 years. At a mean follow-up of 21 months, there were no differences between the 2 groups in activity level (Tegner score), A-P knee laxity (KT-1000 arthrometer measurement), clinical outcome (Lachman and pivot-shift tests), and complications. Based on the current literature age alone should not determine whether a person is a good candidate for ACL reconstruction. No study in the literature has looked directly at the issue of activity level on ACL reconstruction outcomes. Clearly, activity level before ACL injury should be consid- ered before advising patients to undergo ACL reconstruc- tion, but the literature does not currently show that one group of patients will have a poorer outcome based on pre- operative activity levels alone.

Single-Incision Versus 2-Incision ACL Reconstruction

n recent years, arthroscopically assisted ACL reconstruc- tion has become the procedure of choice.18 Initially, arthroscopic techniques required 2 incisions for outside-in drilling of bone tunnels, but there has been a trend toward using a single-incision arthroscopic approach with inside- out drilling of the femoral tunnel. This technique has also been called the 1-incision, endoscopic, or all-inside arthro- scopic reconstruction of the ACL. Advocates of the 1-incision technique have claimed that morbidity is less, the cosmetic appearance is better, the postoperative pain is less, the hospital stays are shorter, the operative time is less, the technique potentially improves proprioception, the tech- nique allows earlier rehabilitation, and the costs are decreased.31,36,205 Those who advocate the 2-incision tech- nique state that they do so primarily because of the more demanding technique required for the single-incision pro- cedure and believe that the 2-incision procedure makes accurate tunnel placement easier.23,114,146 Hess et al stated, "The single-incision technique is more difficult, has a steeper learning curve, and perhaps a higher incidence of complications. Therefore, before a single-incision tech- nique is recommended, it should be proven not only equal to, but superior to, the established 2-incision proce- dures."114 Other problems attributed to the single-incision technique include a greater potential to break out the pos- terior wall, screw divergence, and mismatch between the length of the bone-patellar tendon-bone graft and the overall length of both bone tunnels.47,104,110,114,157,229 In our review of the recent literature to validate the claims made by advocates of the 1-incision or 2-incision ACL reconstruction procedures, we found 4 RCTs compar- ing the 1-incision with the 2-incision techniques and 7 case series comparing the 2 techniques. The 4 RCTs have low case numbers, short follow-ups, vague descriptions of the randomization procedures, and multiple surgeons. They make little or no mention of the number of patients lost to follow-up, and half did not mention who performed the outcome evaluations.47,96,114,239 Although the vast majority of items compared between the 2 groups revealed no sig- nificant differences in outcome measures, not all studies evaluated the same outcomes. There were no differences in early pain and swelling, KT-1000 arthrometer measure- ments, IKDC and Lysholm scores, 1-legged hop test results, isokinetic strength, and complications. Brandsson et al47 found that sick leave was significantly longer for the 1-incision technique than for the 2-incision technique. They gave no explanation for this difference, which was, on average, 5 weeks longer for the 1-incision compared with the 2-incision procedure. Gerich et al96 found no difference in pain after 1-incision versus 2-incision ACL reconstruc- tions during the first 6 postoperative days. In their RCT, Reat and Lintner239 reported no difference in postopera- tive pain or swelling at 1, 3, and 6 weeks after 1-incision versus 2-incision ACL reconstructions. The 7 articles that compared case series of 1-incision versus 2-incision ACL reconstructions reported no differ- ence in KT-1000 arthrometer measurements at the time of final outcome.§§ In 5 of the studies using this technique, IKDC outcomes were no different: 1 study by Howell and Duetsch117 revealed that there was a significant decrease in the functional IKDC score in the endoscopic group. Gerich et al96 reported Lysholm outcome scores were no different, whereas Karlsson et al142 found a significantly higher Lysholm score in their 1-incision group. Two authors found a higher rate of complications in the 1- incision group,17,142 whereas 2 others found no difference in the complication rate.229,250 Two of the case series eval- uated tunnel position by radiographs.110,229 Harner110 found no difference in tunnel placement using the 2 tech- niques, whereas Panni et al229 found that the inside-out tunnels were significantly more vertical in the 1-incision procedure. They offered no statement as to the significance of this finding. Howell and Duetsch117 reported a higher incidence (21% vs 12%) of second surgery after the ACL reconstruction in the single-incision group, but the major- ity of these were for removal of hardware. Four of the case series articles found an increase in divergence of the inter- ference screws used to fix the bone block in the femoral tunnel in the 1-incision procedures.18,142,229,250 Lemos et al157 found the same result in another study. None of these authors reported any evidence that screw divergence leads to worse results overall. In reviewing the literature presented above concerning the outcomes of the 1-incision versus 2-incision procedures for ACL reconstruction, there is no evidence that one method is superior to the other. It is interesting to observe that only 2 of the 11 authors evaluated tunnel position. This finding implies that it is probably difficult to ascer- tain the exact position of the tunnels. If there is a signifi- cant difference in tunnel placement between the single- incision and 2-incision techniques, the outcome measures used in the studies reviewed are not sensitive enough to demonstrate a difference in results. Delay et al67 recently published a large survey that found that 85% of surgeons use an endoscopic single-incision technique, 12% use an arthroscopic 2-incision technique, and 3% use mini-open techniques.

Biologic Techniques

Autogenous Chondrocyte Implantation. Autogenous chondrocyte implantation (ACI) is a 2-stage procedure in which an arthroscopic biopsy of normal hyaline cartilage is cultured in vitro, and the resulting chondrocytes are then reimplanted into a cartilage defect beneath an autologous periosteal patch. Animal studies began in the 1980s and led to the clinical application of this procedure after revealing the formation of hyaline-like cartilage.15,43 In 1994, Brittberg et al14 first reported ACI in humans, and it has grown in popularity since then. Articular chondrocytes are embedded in the hyaline car- tilage matrix, where they maintain the homeostasis of matrix proteins that are necessary for tissue matrix struc- ture. Individual chondrocytes can be released by enzymatic digestion and expanded in culture.44 During the expan- sion, the cells gradually dedifferentiate and lose type II collagen expression, but they are able to reexpress their phenotype when cultured in agarose gels.12 Culture- expanded chondrocytes demonstrate phenotypic plasticity in their ability to form cartilage in pellet mass cultures, adipose cells in dense monolayer cultures, or a calcium- rich matrix in an osteogenic assay. In contrast with mes- enchymal stem cells, chondrocytes formed cartilage only (and not bone) in the in vivo osteochondrogenic assay. These results suggest that within articular cartilage, there is a subpopulation of chondrogenic cells that exhibit a level of phenotypic plasticity that is comparable with that of mesenchymal stem cells.120 When chondrocytes grow in culture, there is a linear relationship between their biosynthetic activity and the number of seeded chondro- cytes. For this reason, the number of cells in the initial biopsy is undoubtedly important,27 but the precise number of cells required for successful clinical implantation of the chondrocytes either as a suspension or in a scaffold has not been studied sufficiently. LeBaron and Athanasiou62 noted that polylactide-polyglycolide scaffolds seeded with a den- sity of <10 million cells/mL resulted in the formation of very little cartilage. They concluded that seeding at high cell density seemed desirable.61 Puelacher et al104 observed that seeding scaffolds at a cell density ranging from 20 to 100 million cells/mL resulted in the formation of cartilage when the scaffold was implanted subcutaneously into nude mice. In the clinical setting today, the aim is to trans- plant at a cell density of 30 × 106 cells/mL. In the future, techniques using minimally invasive implantation will spare the patient the morbidity of an open arthrotomy. All arthroscopic techniques have been reported but are not currently implemented in the United States.34 The all-arthroscopic technique is based on implanting a 2-mm-thick polymer fleece preloaded with autologous chondrocytes in a fibrin gel that is anchored to the condyle arthroscopically. Lee et al63 implemented in vitro culturing of a chondrocyte-laden scaffold before implantation. In a canine model, they evaluated full-thickness focal chondral defects without bone involvement 15 weeks after implantation of an autologous articular chondrocyte-laden type II collagen scaffold that had been cultured in vitro before implantation.63 In these cultured scaffolds, the reparative tissue formed from the scaffolds filled 88% ± 6% of the cross-sectional area of the original defect, with hyaline cartilage accounting for 42% ± 10% (range, 7%-67%) of the defect area. Further work is neces- sary to identify the specific culture and cell density param- eters needed to maximize this advantage of in vitro scaf- fold culture before final implantation compared to the results of noncultured implantation.15,100 In the future, allogenic sources of cells or single-stage biologic tech- niques may offer the added advantage of eliminating the need for biopsy before implantation. As ACI technology becomes more mainstream and techniques improve, it will likely be used more routinely to treat other joint surfaces as well as the knee. Recently, ACI has been used to treat shallow chondral defects in the shoulder109 and hip (L. Peterson, J. W. A. personal communication, December 7, 2003) as well. Meniscal Transplant. The limb-sparing reconstructions performed almost a century ago represent the first menis- cal allograft transplantations that were combined with complete knee transplantation.83 In 1989, Milachowski et al84 performed the first isolated meniscal allograft procedure. Today, fresh meniscal allografts are custom fashioned from tibial hemi-plateaus and are implanted using arthroscopic techniques. Adequate function of the meniscal transplant relies on secure bone fixation of the anterior and posterior horns.74,118,124 This is commonly accomplished using either bone plugs or a slot/bone bridge technique. The vast major- ity of menisci are transplanted into the knee to treat iso- lated meniscal deficiency or in conjunction with other knee abnormalities. Recently, however, the senior author (B. J. C.) used meniscal allografts in the shoulder as a biologic inter- position in young patients with relatively localized articu- lar cartilage disease of the glenohumeral joint.

TRAINING GROUNDWORK

Injury-prevention techniques for the ACL have previously been demonstrated.51-53 Selective incorporation and integra- tion of these techniques could be used to address specific neu- romuscular imbalances in female athletes.21 As Griffin report- ed,29 Henning identified 3 potentially dangerous maneuvers in sport that should be modified through training to prevent ACL injury. He suggested that athletes land in a more bent-knee position and decelerate before a cutting maneuver. Preliminary work implementing these techniques in a pilot sample of ath- letes suggested a decrease in injury rate in trained versus un- trained subjects.29 Subsequently, Hewett et al52 developed a training program, based on a thorough review of the literature and prior athletic training experience, that included an initial phase devoted to correcting jump and landing techniques in female athletes. Four basic techniques were stressed: (1) cor- rect posture (ie, chest over knees) throughout the jump, (2) jumping straight up with no excessive side-to-side or forward- backward movement, (3) soft landings, including toe-to-heel rocking and bent knees, and (4) instant recoil preparation for the next jump. Using the training program, women in the in- tervention group were able to significantly reduce noncontact ACL injuries compared with controls. Both studies highlight the importance of incorporating dynamic, biomechanically correct movements into training protocols aimed at injury pre- vention. Caraffa et al51 prospectively evaluated the effect of balance- board exercises on noncontact ACL injury rates in elite male soccer players. The training consisted of 20 minutes of bal- ance-board exercises divided into 5 phases. Athletes who par- ticipated in proprioceptive training before their competitive season had a significantly decreased rate of knee injuries. Con- versely, others have examined the effects of similar progres- sive balance-board training and found no similar reduction in ACL injuries in female athletes.54 Subsequently, Myklebust et al53 examined the effects of a more comprehensive and dy- namic neuromuscular training program on female athletes. Their program elaborated on the balance-board protocol of Caraffa et al51 and the techniques of Hewett et al22 by adding a focus to improve awareness and knee control during stand- ing, cutting, jumping, and landing. They were able to reduce the incidence of ACL injury in women's elite handball players over 2 competitive seasons. These studies demonstrate the ability of a neuromuscular/balance component to reduce knee- injury risk in female athletes when incorporated into an injury- prevention protocol. Education and public awareness of the high rate of occurrence and mechanisms of ACL injury have also been shown to decrease injuries in a group of Vermont ski instructors.55 Ski instructors viewed videotapes of ACL injuries and were encouraged to formulate their own preventive strategies. Pilot data demonstrated a significant reduction in ACL injuries by greater than 50% with this technique. Elements from the Ver- mont study should be applied to other sports. It is important to teach athletes to avoid biomechanically disadvantageous and dangerous positions in any sport. Hewett et al22 expanded the concept, using clinician verbal and visualization cues to provide feedback and awareness to an athlete during training. Each athlete was encouraged to perform as many jumps as possible using the proper technique. As the athletes became fatigued, they were encouraged to stop if they could not ex- ecute each jump with correct biomechanics. Similarly Myk- lebust et al53 used partner training to provide the critical feed- back. Partners encouraged each other to focus on the quality of their movements, specifically on the knee-over-the-toe po- sition. Both Hewett et al22 and Myklebust et al53 recognized the critical analysis as a contributor to the successful reduction of ACL injuries in their studies. Laboratory experiments also provide support for the efficacy of analysis and feedback in reducing dangerous biomechanics. Onate et al56 and Prapa- vessis and McNair57 reduced peak vertical ground reaction forces using verbal and visual feedback. These groups sug- gested the inclusion of critical analysis and feedback into in- jury-prevention models for athletes who participate in sports that require jump landings. Cumulatively, prior studies dem- onstrated the importance of analyzing the biomechanics of sports movements and providing constant and consistent feed- back to the athlete regarding proper technique. Scientific evidence combined with experience-based empir- ical evidence allowed us to develop a neuromuscular training rationale. Dynamic neuromuscular analysis training is a syn- thesis of the most important findings derived from existing research studies and prevention techniques developed through more recent empirical and analytical evaluations of neuro- muscular training and on-field play. In contrast to the work of previous authors, dynamic neuromuscular analysis training is not a protocol. Rather, it is a rationalized approach to address- ing specific risk factors that may be evident in at-risk athletes. The training principles and techniques provide a general framework to clinicians who wish to design and administer injury-prevention programs targeted to this population. The 3 essential components of a comprehensive training protocol are dynamic, biomechanically correct movement skills; neuromuscular patterning based on the identification of underlying neuromuscular imbalances; and constant biome- chanical analysis by the instructor with feedback to athletes both during and after training. When training to prevent injury, athletes and clinicians must interact. This interactive form of movement training requires intense instructor-to-athlete tech- nique analysis and immediate and consistent feedback, with the goal of programming the neuromuscular system to perform athletic maneuvers in a powerful, efficient, and safe manner.

CONCLUSION

The complex attachment of the normal ACL and ACL graft requires 3 dimensions to fully describe; very little is known, however, about the relationship between the 3- dimensional position of an ACL graft and clinical out- comes. Indeed, almost everything that is known has been based on 2-dimensional measurements obtained from pla- nar radiographs. The footprint of the normal ACL attach- ment to bone or to the Blumensaat line has been used as a coordinate system to describe the intra-articular location of the femoral bone tunnel. Placement of the tibial tunnel is important to prevent graft impingement against the roof of the femoral notch as the knee is extended. The center of the femoral attachment of an ACL graft should be located along a line parallel to the Blumensaat line just posterior to the center of the normal ACL's insertion to bone, and at either the 2 o'clock position (left knee) or the 10 o'clock position (right knee) when observed through the femoral notch. Patients with a femoral insertion of the ACL graft that is greater then 2 mm anterior to the center of the nor- mal ACL insertion along a line parallel to the Blumensaat line have greater A-P knee laxity at follow-up than patients with posterior positioned tunnels along a line par- allel to the Blumensaat line. The tibial tunnel should be placed to avoid impingement of the graft against the roof of the femoral notch as the knee is brought into extension. The tension applied to an ACL graft at the time of fixa- tion and the knee position during the tensioning procedure have a direct effect on knee biomechanics and clinical out- come. With regard to the 4-strand hamstring graft, there is a significant correlation between the tension applied to the graft at the time of fixation and A-P knee laxity measure- ments after healing is complete. Patients receiving 80 N of tension with the knee in full extension during graft fixa- tion have A-P knee laxity values that are closer to normal than those receiving 20 N and 40 N of tension. Likewise, for the central third BPTB graft, patients receiving 90 N of tension with the knee in full extension during graft fixa- tion have A-P knee laxity values that are close to normal at 2-year follow-up, whereas those receiving 45 N have increased laxity values. Fixation of BPTB grafts with interference screws is ade- quate for the loads produced by current rehabilitation pro- grams; however, the frequent publication of new tech- niques to fix free-tendon grafts, such as the 4-strand ham- string graft, suggests that an optimal technique of fixation has yet to be identified. BPTB grafts heal in the bone tun- nels through bone plug incorporation and resemble the chondral insertion of the normal ACL, whereas free- tendon grafts heal more slowly by the fibrils of the graft penetrating the bone directly and result in a fibrous inser- tion of the tendon. This characteristic implies that aggres- sive rehabilitation, including immediate weightbearing, open chain quadriceps contraction near extension, and early return to sport, may be a concern during the first few weeks after ACL reconstruction with free-tendon grafts. Bone tunnel enlargement occurs more frequently and is greater after ACL reconstruction with hamstring grafts compared to BPTB grafts; however, tunnel widening does not appear to have an adverse effect on the outcome of ACL reconstruction, although it can certainly make revi- sion reconstruction more difficult. The use of a rehabilitation brace results in fewer prob- lems with swelling and wound drainage and less pain com- pared to rehabilitation without the use of a brace; at longer follow-up intervals, however, rehabilitation bracing has no effect on outcome. Similarly, the use of a functional knee brace after ACL reconstruction does not appear to have an effect on outcome. The literature on rehabilitation after ACL reconstruc- tion, derived from RCTs, provides information on how much loading and motion a knee with a healing BPTB graft can sustain without permanently stretching the graft (as evidenced by abnormal increases of anterior knee laxity), disrupting the graft, or causing failure of graft fix- ation. In contrast, there is little information derived from RCTs on the effect of rehabilitation on other graft materi- als (such as the 4-strand hamstring graft) or on other structures that are commonly injured at the time of an ACL tear (such as the articular cartilage and meniscus). After ACL replacement with a BPTB graft, it is clear that immobilization of the knee, or restricted motion without muscle contraction, leads to undesired outcomes for the ligamentous, articular, and muscular structures that sur- round the joint. Rehabilitation that incorporates early joint motion is beneficial for reducing pain, minimizing capsular contractions, and decreasing scar formation that can limit joint motion, as well as beneficial for articular cartilage. Immediately after ACL reconstruction with a BPTB graft, weightbearing is possible without producing an unwanted increase in anterior knee laxity; weightbear- ing is beneficial because it lowers the incidence of anterior knee pain. Recent studies of healing BPTB grafts indicate that accelerated rehabilitation (a 19-week program allow- ing unrestricted weightbearing after 1 week, no brace use after 2 weeks, open kinetic chain exercises involving con- traction of the quadriceps muscle group with the knee near extension after 4 weeks, and return to preinjury activity at 24 weeks) produces the same clinical, functional, and patient-oriented outcomes compared to nonaccelerated rehabilitation (a 32-week program that includes unre- stricted weightbearing after 3 weeks, no brace use after 4 weeks, open kinetic chain exercises involving contraction of the dominant quadriceps muscles at 12 weeks, and return to preinjury activity at 32 weeks). Relatively few studies have been published that advocate return to sport between 4 and 6 months after an ACL reconstruc-tion.53,98,99 Six months after ACL reconstruction, patients walk with normal kinematics but do so with dramatic alterations in torque and power about the hip and knee joints, which has the potential to affect both the graft and articular cartilage metabolism.31 Despite these observa- tions, many recent articles recommend return to rigorous sports 6 months after surgery,8,13,33,36,37,58,91,98 whereas oth- ers recommend return at 9 months or later.26,27,67,89 From these perspectives, there is little evidence in the literature that early return to high-risk sports involving activities such as jumping, planting and pivoting, or planting and cutting is safe and effective. It appears that ACL reconstruction with a BPTB graft is equally successful in male and female patients; however, the effect of sex on the outcome of reconstruction of the ACL with a hamstring graft has not been studied ade- quately. Age and activity level have not been shown to be determinants for successful ACL reconstruction.

ACI

Another procedure designed to generate hyaline cartilage is ACI. Rather than using large transplanted bone plugs from within the patient's own knee, this procedure involves biotechnology to facilitate the growth of chondrocytes in a laboratory from a small piece of cartilage harvested from the patient's own knee.18 The harvest of the cartilage is carried out arthroscopically. Several weeks are necessary for the cells to be adequately isolated and multiplied. The implantation of chondrocytes is accomplished via a second surgery that requires an open arthrotomy of the knee. During this second surgery, a patch of periosteum, sized to cover the defect, is harvested from the medial border of the midshaft of the tibia. The periosteum is sewn over the defect using sutures. Once in place, a fibrin glue is used to seal the edges of the patch to create a watertight seal and the chondrocytes are injected beneath the patch. The injected chondrocytes gradually mature and form tissue that is mostly composed of type II hyaline-like cartilage.42,43 The rehabilitation program following chondrocyte implantation is vital for success and long-term out- comes of the procedure (Table 4). Early, controlled ROM and weight bearing are necessary to stimulate chondrocyte development; although caution is placed on overaggressive activities that may result in cell damage or graft delamination. Rehabilitation may begin as early as 4 hours postoperatively in the form of CPM. At this time, the chondrocytes are aligned and attach to the underly- ing surface of the defect.55 It is imperative that the patient be appropriately positioned to allow for the effect of gravity to evenly distribute the chondrocytes on the base of the defect during these first 4 hours as the cells adhere to the surface. A recent study by Sohn et al55 has shown that the defect orientation during these first 4 hours can be an important factor in the uniformity of cell distribution. For example, patients with patellofemoral lesions are positioned in the prone position for the first 4 hours postopera- tively to ensure that the chondrocyte distribution is aligned on the base of the lesion rather than along the periosteal patch, if positioned in the traditional supine position. Proliferation of the chondrocytes occurs in the first 6 weeks following cell implantation. During the first 24 hours after cell implantation, the cells line the base of the lesion and multiply several times to produce a matrix that will fill the defect with a soft repair tissue up to the level of the periosteal cover.20,43 this time, PROM and controlled partial weight bearing will help to promote cellular nutrition through synovial fluid diffusion as well as provide the proper stimulus for the cells to produce specific matrix markers. During this initial phase, controlled PROM and a gradual weight-bearing progression are 2 of the most important components to the rehabili- tation process. Immediate toe-touch weight bearing is performed on smaller lesions, progressing to 25% body weight at weeks 2 to 4, 50% body weight at weeks 5 to 6, and finally full weight bearing at week 8. This progression may be delayed approximately 2 weeks, with 2 weeks of non-weight bearing, if the lesion is large, deep, or uncontained. For lesions within the patella or trochlea, the patient is allowed to weight bear as tolerated immediately after surgery with a brace locked in full extension. ROM is progressed cau- tiously to avoid swelling, with a goal of 90° of flexion at week 1, 105° at weeks 2 and 3, 115° at week 4, and 125° at week 6. Early strength and proprioceptive exercises are performed within the patient's weight- bearing status. During the transition phase, which includes weeks 7 through 12, the repair tissue at this point is spongy and compressible with little resistance. Upon arthroscopic examination the tissue may in fact have a wave-like motion to it when sliding a probe over its surface.20,43 During this phase the patient achieves full ROM and progresses from partial weight bearing to full weight bearing. Continued maturation of the repair tissue is fostered through higher-level func- tional and motion exercises. Weight-bearing exercises, such as front lunges, step-ups, and wall squats, are performed as well as machine exercises for the entire lower extremity. Again, caution should be placed on exercises that produce shear forces in patients with patellofemoral lesions. The remodeling phase occurs from 12 through 32 weeks postoperatively. During this phase, there is a continuous production of matrix, with further remod- eling into a more organized structural tissue. The tissue at this point has the consistency of soft plastic upon probing.20,43 As the tissue becomes more firm and integrated, it allows for more functional training activities to be performed, as well as elliptical, bicycle, and a gradual walking program. The final maturation phase can last up to 15 to 18 months, depending upon the size and location of the lesion. By the end of this phase, the stiffness of the cartilage resembles that of the surrounding tissue.20,43 The duration of this phase varies based on the several factors such as lesion size and location. Basic science studies have shown that it may take up to 6 months for the graft site to become firm, and at least 9 months to become as durable as the surrounding healthy articular cartilage.20,43 Thus, low-impact activi- ties are initiated by months 5 to 6 and progressed to moderate-impact activities from months 7 to 9 as tolerated.

INDICATIONS FOR TREATMENT

Based on the present knowledge, it is clear that not all ACL-deficit patients require surgical reconstruction. The rationale for surgical treatment includes the assumption that the ACL is vital for knee function, that ACL-deficient knees frequently degenerate, and that surgical reconstruc- tions can succeed in restoring normal function. The ration- ale for nonsurgical treatment assumes that the ACL- deficient knee may function reasonably well under certain circumstances and that reconstructions do not necessarily prevent the untoward sequela of osteoarthritis. Because we do not know the true natural history of the ACL- deficient knee, and, for that matter, the true ultimate out- come of ACL reconstructions, no rigid criteria for patient selection for surgery exist. A nearly universally accepted indication for an ACL reconstruction is a high-risk lifestyle requiring heavy work, sports, or recreational activities.64,83 Likewise, repeated episodes of giving way (pivot shift) in spite of rehabilitation are considered a strong indication for ACL reconstruction.83 Age, by itself, is not thought to be a significant indicator, but, of course, younger patients tend to be more active.28,83,267 Meniscal tears, ACL tears associated with severe injuries to other ligamentous structures in the joint, generalized ligamen- tous laxity, and recurrent instability with activities of daily living have all been factors supportive of surgical reconstruction. A marked increase in side-to-side differ- ences in anterior laxity as measured by arthrometers has been advocated by several authors as an important indica- tor for reconstruction.64,83 In contrast, several authors have found no correlation between the magnitude of ante- rior knee laxity and symptoms of instability or physical performance measures, and thus, they do not feel that increased laxity as measured by an arthrometer is a strong indicator for surgical intervention.34,49,73,158,252 Patients who may be satisfactorily treated with nonsurgi- cal intervention, despite a totally disrupted ACL, include those who have little exposure to high-risk activities such as sports and heavy work activities, those who are willing to avoid high-risk activities, those who are more than 40 years of age, those who are successful in prolonged coping or adapting to an ACL insufficiency, those who have advanced arthritis of the involved joint, and those who are unable or unwilling to comply with postreconstruction rehabilitation.288 Each patient must be considered individ- ually, for there may be circumstances that differ from the above general rules that may indicate a reason to move ahead with surgical intervention or to treat the patient without a reconstruction. One absolute contraindication for a surgical reconstruction would be a patient who has acute intra-articular sepsis.83,288 For patients with open physes, an intrasubstance tear of the ACL presents a significant challenge. It has been well established that treatment of children and adolescents with complete ACL disruptions by modification of activi- ties and bracing is often ineffective and can lead to menis- cal tears and arthritic changes.21,100,140,185 Thus, young patients who cannot or will not modify their level of activ- ity until growth is completed may require ACL recon- struction. In animal models, in which soft tissue was placed across open epiphyses to observe the effect on growth, conflicting conclusions have resulted.74,106,274 Thus, Stanitski,275 in 1995, advocated very exact establishment of each patient's skeletal maturity when considering the timing and surgical method to be used for an ACL recon- struction in a growing child. The clinical experience with soft tissue grafts placed through physial drill holes has encouraged some to advo- cate this type of an intervention, even in patients with wide-open growth plates. Fixation of bone blocks or metal devices spanning the physes have resulted in growth dis- turbances.145,147,164 Several articles have presented satis- factory results with reconstruction of the ACL using soft tissue grafts passed through transphysial drill holes with minimal problems with growth disturbance.7,16,24,93,164,165 However, other articles have been published that report growth disturbances in the patients undergoing ACL reconstruction.16,145,147,164 Because of this finding, several authors have concluded that drilling across the physes in very young patients is potentially dangerous,21,35,81,187,228 although there appears to be an assumption on the part of many surgeons that violation of the physes and filling the holes with a soft tissue graft are relatively safe. here is no compelling evidence that indicates that this procedure avoids irreversible damage to the growth plate. Many of the articles that advocate such surgical interventions in ACL-deficient children include patients who are nearing the end of their growth. In these situations, there is prob- ably little risk of producing a growth disturbance by a physial-violating procedure as long as the tunnel is filled with soft tissue. The information provided above has led several authors to advocate surgical interventions that are nonanatomical but physial sparing in those patients who cannot avoid activities that lead to reinjury.# A recent article described an anatomical reconstruction of the ACL in very young children that avoids placing tunnels across the tibial and femoral physes.10 Also, several investigators have suggested that in very young children, attempts should be made to postpone surgery by having patients avoid sports and other vigorous activities in an attempt to prevent reinjury events.16,147,185,236,300 In very young children with wide-open physes, there appears to be no literature that can prove that drill holes through the epiphysial plates are safe. Thus, the topic of reconstruction of the ACL in very young children remains controversial, and surgeons violating the growth plate must do so with the understanding that there is at least a potential risk for producing growth disturbances. Certainly, the nonoperative treatment of ACL injuries in very young children is often unsuccessful. Thus, more research is necessary to define the best methods to resolve this very perplexing problem. Partial tears of the ACL present another situation in which decisions have to be made whether to treat the patient nonoperatively, with the assumption that the remaining ligament will be adequate to allow normal activities, or to assume that because of significant damage to the ACL, future activities are likely to complete the tear and eventually lead to damage of other structures unless the ACL is reconstructed. What constitutes a partial tear of the ACL that one can expect to return to normal activi- ties is not established. Daniel and Fritschy63 suggested that if the arthrometer laxity difference between the injured and uninjured sides was less than 3 mm, no recon- struction was necessary. Noyes et al216 reported that if three fourths of the ACL remained intact, infrequent pro- gression toward complete injury was observed. If half the ACL was intact, 50% progressed, and if only a fourth of the ACL was intact, 86% progressed.216 The problem is to ascertain, by direct observation, the degree of injury to the ACL. Even if a portion of it appears to be clinically intact and to be quite bulky, there is no way to determine the structural status of the remaining ligament. Lintner et al163 stated that arthroscopic probing was best to establish the grade of ACL injuries, but even this method is not fool- proof. There is a place for surgical intervention in ACL-deficient knees when no intention to reconstruct the ACL is consid- ered. In nonoperative candidates in whom a portion of the stump has flipped anteriorly, causing impingement, per- sistent effusion, loss of a few degrees of extension, or pain with hyperextension at the inferior pole of the patella, arthroscopic removal of the stump of the ACL can alleviate symptoms.56,289 Likewise, patients who have coped with an ACL insuffi- ciency satisfactorily for a long time may develop a menis- cal tear that requires partial meniscectomy or even can be considered for repair. If a repair is possible, an ACL recon- struction performed at the same time as the meniscal repair is the most appropriate way to treat these patients. The ACL reconstruction may have to be staged in these circumstances if the knee is markedly swollen and there is marked loss of motion at the time of surgery, that is, a locked bucket-handle tear. Another challenging problem for the orthopaedic sur- geon is ACL insufficiency with concurrent osteoarthritis. Persistent or recurrent pain and effusion, without insta- bility, probably would not be helped by an ACL recon- struction. It has been noted that ACL reconstruction alone may not relieve the symptoms of aching, swelling, and even giving way resulting from arthrosis.21 However, if instability causes a significant increase in pain and swelling, then ACL reconstruction may be indicated. Reports of improvement after ACL reconstruction alone in arthritic knees have been published.207,258,259 In some cases, a high tibial osteotomy (HTO) in conjunction with ACL reconstruction can help patients with pathologic varus alignment and arthritic changes.** Others have reported that in situations in which osteoarthritis, patho- logic varus alignment, and ACL insufficiency coexist, an osteotomy alone can lead to satisfactory results that make it unnecessary to perform an ACL reconstruc- tion.21,155,217,292 It is difficult to determine which method or combination of methods results in the most satisfactory results because of the small number of cases, short follow-ups, variation in surgical techniques and outcome measures, and the lack of RCTs. Williams et al,292 when comparing 2 case series, con- cluded that the best results occurred with simultaneous HTO and ACL reconstructions rather than osteotomy alone.292 Noyes et al217 found similar functional outcomes and symptoms in ACL-deficient patients with varus defor- mity and medial compartment arthritis in 3 groups treated with an HTO alone, an HTO and an extra-articular ili- otibial band tenodesis, or an HTO and intra-articular allo- graft reconstruction. Lattermann and Jakob155 likewise found no differences in functional scores, pain, or knee lax- ity when comparing results of those treated with an HTO alone, those treated with an HTO and a staged patellar tendon autograft, or those treated with an HTO and a simultaneous patellar tendon autograft. These authors did express concern about the high complication rate (37%) that they encountered. They thus advised that an HTO be attempted first in isolation, and if giving-way episodes continue, then an ACL reconstruction should be performed later. Dejour et al66 and Noyes et al217 reported complica- tion rates that were much less prevalent.

Graft Tension at the Time of Fixation

Most surgeons would agree that the initial tension applied to an ACL graft at the time of fixation has a direct effect on outcome; an undertensioned graft may result in abnor- mal knee laxity and an unstable knee, and an overten- sioned graft may lead to graft failure, fixation failure, or restricted range of knee motion. Our review revealed 4 prospective RCTs that evaluated the effect of this critical surgical variable on clinical and functional outcomes. In an investigation by Yasuda et al,114 subjects were randomized into 3 groups with initial graft tensions of 20, 40, and 80 N applied to a 4-strand hamstring graft connected in series with polyester tape and fixed extra-articularly with double staples. Follow-up measurements were made at a mean interval of 2.5 years (range, 2.0-3.5 years) and included 94% of the subjects. The 6% of patients who were lost to follow-up were evenly divided between the 20-N and 40-N tension groups; all of the subjects in the 80-N initial tension group returned for follow-up. A significant correla- tion was found between the initial tension applied to the graft at the time of fixation and A-P knee laxity measure- ments after healing was complete. Subjects in the high- tension group had anterior laxity values closer to normal compared with similar values in the low-tension group. There were, however, no differences between the initial tension groups with regard to knee motion and clinical outcome (Noyes scale85). The same surgeon performed all reconstruction procedures, yet the method of randomiza- tion was not described, A-P knee laxity was not measured immediately after graft fixation to establish baseline laxi- ty values for each treatment group, and there was no description of whether the examiner responsible for mak- ing the follow-up measurements was blinded to the treat- ment groups.114 A prospective RCT was performed by van Kampen et al107 to determine the effect of tensioning BPTB grafts at 20 and 40 N with the knee in 20° of flexion. The details of how the randomization was performed were not pre- sented. Surgery was performed by 2 surgeons using the same single-incision procedure, and all subjects followed the same rehabilitation program, which included immedi- ate full range of motion, weightbearing as tolerated, and return to sport 9 months after surgery. Follow-up meas- urements were made at 1 year and included all study par- ticipants; there was, however, no description of whether the patients and the researcher making the follow-up measurements were blinded to the treatment groups. Immediately after surgery (baseline) and at the 1-year follow-up interval, there was no difference in A-P laxity between the 2 treatment groups. It may be that the differ- ence in the 2 tension levels was not of sufficient magnitude to create differences in knee laxity at baseline or that a large proportion of the tension applied to the bone block was lost at the bone block-tunnel interface. From this perpective, the observation of similar knee laxity between the 2 initial graft tension groups at the 1-year follow-up may be attributed, at least in part, to the fact that the ini- tial tension groups had similar knee laxity values at base- line.107 Yoshiya et al115 performed a similar prospective RCT comparing 2 groups in which initial tensions of 25 and 50 N were applied to BPTB grafts with the knee in full exten- sion. Follow-ups were performed immediately after sur- gery (KT-1000 arthrometer measurement) and at 3, 6, 12, and 24 months. Among the 50 subjects enrolled in the study, 88% of those in the 25-N initial tension group and 84% of those in the 50-N initial tension group returned for all follow-up visits. Immediately after surgery and at the 2-year follow-up, there was no difference in A-P knee laxity between the 2 groups. Similarly, no differences were found between the treatment groups in terms of knee motion, isokinetic thigh muscle strength, and International Knee Documentation Committee (IKDC) rating. Two surgeons performed the procedures; however, the method used to perform the randomization was not presented, and there was no description of whether the patients and examiners were blinded to the treatment groups. As with the earlier study by van Kampen et al,107 the observation of similar knee laxity values between the 2 initial tension groups after healing may be explained by the fact that laxity values for the 2 groups were the same at baseline.115 Nicholas et al82 performed a prospective, randomized, double-blind clinical trail that compared 2 groups in which initial tension loads of 45 and 90 N were applied by the same surgeon to central third BPTB grafts with the knee in full extension. Knee motion and A-P knee laxity were measured before surgery and at 1 week and 20 months after surgery. A total of 49 subjects enrolled in the study; 100% of those in the 45-N initial tension group and 82% of those in the 90-N initial tension group returned for all follow-up visits. Subjects receiving the 45-N tension load had increased anterior knee laxity, whereas patients receiving the 90-N tension load had laxity values similar to normal. At follow-up, 23% of the subjects in the 45-N ini- tial tension group had side-to-side differences in knee lax- ity greater than 5 mm, whereas none of the subjects in the 90-N initial tension group showed such abnormal changes. Subjects in both groups had similar range of motion. Our review revealed that 1 RCT of ACL reconstruction with a BPTB graft compared the effect of preloading (application of a 39-N tensile load for 10 minutes before graft implantation) to no preloading before graft implan- tation.32 Two years after surgery, there were no differences between the treatment groups with regard to activity level, clinical outcome (IKDC grade), and A-P knee laxity. Graft tensioning at the time of fixation involves consid- eration of the knee's position during the tensioning proce- dure (eg, the flexion angle and internal-external rotational position of the tibia relative to the femur) and, of course, the magnitude of tension applied to the graft at the time of its fixation to bone. Both variables interact and have a direct effect on knee biomechanics. It was difficult to com- pare the RCTs that were reviewed and to develop a con-sensus because different tensioning procedures and graft materials were used. If a BPTB graft is used and the ten- sioning procedure is performed with the knee in extension, application of high tension (90 N) appears to produce more normal A-P knee laxity values compared to application of low tension (45 N). Similarly, if a 4-strand hamstring graft is used, application of high tension (80 N) appears to pro- duce A-P knee laxity values similar to normal, whereas low tension (40 N and less) results in increased anterior knee laxity. The effect that graft tensioning at the time of fixation has on the contact stress distribution of healing articular cartilage about the tibiofemoral joint is currently unknown and requires study.

Closed Versus Open Kinetic Chain Rehabilitation

Our review revealed 3 randomized trials comparing the use of closed versus open kinetic chain exercises during ACL rehabilitation. Bynum et al26 performed a prospective RCT comparing open versus closed kinetic chain rehabilitation after ACL reconstruction with a central third BPTB autograft. Immediately after surgery, patients' knees were placed in a rehabilitation brace that was adjusted to allow 0° through 90° of motion, and continuous passive motion from 0° to 60° of flexion was started. Rehabilitation was begun on the first postoperative day and for all patients included passive and active motions of the knee without external resistance. Partial weightbearing with the use of crutches was permitted, and subjects progressed to full weightbearing as tolerated. Patients were then random- ized via a computer-generated list of random numbers to either open or closed kinetic chain rehabilitation groups. Subjective and objective follow-up measurements were taken on 66% of patients 1 year after surgery; the examiner was blinded to the patients' rehabilitation program. The subjects in the closed kinetic chain group had KT-1000 arthrometer measurements that were closer to normal, in addition to less anterior knee pain, earlier return to nor- mal daily activities, and greater satisfaction compared to the subjects in the open kinetic chain group. The authors attributed the decreased anterior knee pain in patients from the closed kinetic chain group to reduced patellofemoral reaction forces associated with exercises performed with the knee near extension, in contrast to the open kinetic chain treatment exercises, which were per- formed with the knee in a more flexed position.26 Mikkelsen et al77 reported the results of an RCT that compared closed kinetic chain to combined closed and open kinetic chain rehabilitation programs initiated 6 weeks after single-incision BPTB reconstruction of the ACL per- formed by 1 of 3 surgeons. The randomization procedure appeared to be designed to match patients with regard to age, gender, and type and level of physical activity, although the details of how this design was accomplished were not presented, and it is unclear whether the follow- up measurements were made with the investigator blinded to the treatment groups. Assessment at 6 months after surgery revealed that the addition of open kinetic chain exercises produced a significant improvement in quadri- ceps strength (evidenced by moderate improvements in extension torque), an earlier return to sport at the prein- jury level, and no effect on KT-1000 arthrometer measure- ments of A-P knee laxity. Hooper et al51 reported the results of a prospective RCT comparing closed to open kinetic chain rehabilitation exer- cises during the early phase of healing. Reconstruction was performed by 3 surgeons: 1 surgeon reconstructed the ACL with a ligament augmentation device, and the other 2 surgeons used a central third BPTB graft. Patients were assigned to the treatment groups using a block random- ization scheme; there were no details given with regard to whether the follow-up measurements were made with blinding of the examiner to the treatment groups. Two weeks after ACL surgery, the patients underwent a base- line gait analysis and were then randomly assigned to either closed or open kinetic chain rehabilitation exercises administered 3 times per week during a 4-week interval. At the 6-week follow-up, the patients underwent a second gait analysis. At this early stage of healing, no differences were found between the closed and open kinetic chain pro- grams in the gait variables associated with level walking, ascending stairs, and descending stairs; however, subjects in both groups had functional deficits in the involved side compared to their contralateral, normal side. Subsequent investigation of the same patients revealed no differences in anterior knee pain or knee laxity between treatment groups.79,80 In the 3 RCTs reviewed above, different open and closed kinetic chain rehabilitation programs were compared over different times, and it is therefore impossible to come to a consensus regarding the effectiveness of one approach compared to another. he report by Bynum and col- leagues26 had the longest follow-up interval. This study indicated that rehabilitation with closed kinetic chain exercises is more effective in terms of patient satisfaction, reduced anterior knee pain, and earlier return to daily activities compared to programs that include open kinetic chain exercises.

SUMMARY

Our review revealed that the ACL is the most frequently totally torn knee ligament, that this injury is common among athletes, and that the incidence rate is higher among female athletes compared to male athletes partici- pating in the same sport. Many different risk factors have been implicated in an attempt to identify those persons at risk for ACL injury. There is, however, very little known about how potential anatomical, biomechanical, neuro- muscular, and hormonal risk factors combine to increase a person's risk of sustaining an ACL tear. The most impor- tant reason to identify these risk factors is to develop pre- ventive interventions targeted toward persons who are at increased risk of sustaining an ACL injury, and more research in this area is needed. Trauma to the ACL is asso- ciated with other ligamentous, meniscal, articular carti- lage, and bone injuries. The complete natural history of ACL injury has not been characterized by a prospective study; however, it is clear that ACL disruptions are func- tionally disabling, they predispose the knee to subsequent injuries such as tears of the menisci, and they are associ- ated with the early onset of osteoarthritis. Because the true natural history of the ACL-deficient knee and the ultimate outcome of ACL reconstruction are unknown, rigid criteria for patient selection for surgical versus non- surgical reconstruction have not been established. The rationale for nonsurgical treatment assumes that the ACL-deficient knee may function reasonably well under certain circumstances and that reconstruction does not necessarily prevent the untoward sequela of osteoarthri- tis. Patients with a totally disrupted ACL that may be treated satisfactorily without surgery include those who have minimal exposure to high-risk activities, prolonged successful coping with ACL insufficiency, or advanced arthritis of the involved knee. The rationale for surgical treatment is based on the observation that the ACL is vital for knee function, that ACL-deficient knees frequently degenerate, and that surgical reconstruction of the ACL can succeed in restoring normal function. A nearly univer- sally accepted indication for ACL reconstruction is a high- risk lifestyle requiring heavy work, sports, or recreational activity and repeated episodes of giving way (pivot-shift episodes) despite rehabilitation. Reconstruction of the ACL in very young children by placing soft tissue grafts through bone tunnels that cross open physes has not yet been proven to be safe. If surgical reconstruction of the ACL is chosen, the time interval from ACL injury to recon- struction is not as important as the condition of the knee at the time of surgery. Before reconstruction, the knee should have full range of motion with minimal effusion, and patients should have minimal pain and be mentally prepared for the reconstruction and for the rehabilitation required after surgery. In reviewing the literature compar- ing the outcome of 2-incision versus single-incision proce- dures for ACL reconstruction, there was no evidence that one method is superior to the other. Likewise, although double-tunnel reconstruction may restore normal joint kinematics better in cadaveric models, there is presently no proof that these more complex procedures result in bet- ter outcomes than do standard single-bundle procedures. There is a limited role for extra-articular procedures in the treatment of ACL insufficiency. Intra-articular reconstruc- tion of the ACL with a 2-strand hamstrings graft results in worse clinical outcome in comparison with reconstruction with a bone-patellar tendon-bone graft. In contrast, reconstruction with the 4-strand hamstrings graft results in similar clinical and functional outcomes compared with reconstruction with a bone-patellar tendon-bone graft. The safety and efficacy of reconstruction of the ACL with allograft material have not been substantiated through RCTs with adequate follow-up intervals, and therefore, it is difficult to justify this material as a routine graft source for primary reconstruction of the ACL. Reconstruction of the ACL with synthetic material, whether for total, per- manent replacement, a scaffold for ingrowth of host tissue, or a stent to protect an allograft or autograft as it heals, has not proven to be satisfactory for treatment of a torn ACL.

Intensity and Duration of Rehabilitation

Shelbourne and colleagues were one of the first groups to report that rehabilitation with immediate walking and full weightbearing, combined with early return to sport, was effective and safe.99,100,101 Shelbourne et al100 studied the effect of rehabilitation on subjects undergoing ACL recon- struction with a BPTB graft. Follow-up at a mean interval of 4 years (range, 2-9 years) included 81% of patients treated. Patients were able to return to sport-specific activities at a mean of 6.2 weeks (range, 1-13 weeks) and to athletic competition at full capacity at a mean of 6.2 months (range, 2-18 months). This achievement was accomplished while only 2.6% of the subjects retore their ACL graft at a mean interval of 2.5 years postoperatively (range, 4-78 months). The duration and intensity of rehabilitation after ACL reconstruction have been evaluated in 3 RCTs.18,34,42 Ekstrand34 performed a prospective RCT of soccer ath- letes after ACL reconstruction and compared standard rehabilitation (a 6-month program) to extended rehabilita- tion (an 8-month program). At baseline and 1-year follow- up, there were no differences between the groups. There was, however, a trend for subjects in the extended treat- ment group to have more normal knee laxity compared to those in the standard group.34 Using the criteria of 90% quadriceps strength in comparison to the contralateral side and full range of joint motion, the subjects who par- ticipated in the standard program were allowed to return to sports at 6 months, whereas those in the extended pro- gram returned at 9 months. Frosch et al42 carried out an RCT comparing prolonged rehabilitation (2.5-hour sessions performed 3-5 times/wk) to a standard rehabilitation program (30-minute sessions performed 2-3 times/wk). Baseline data were not reported, and at the 1-year follow-up, subjects receiving the pro- longed program had better joint position sense and Lysholm scores and returned to work earlier than those receiving the standard program. Recently, our group reported the results of a prospective RCT comparing accelerated versus delayed rehabilita- tion.18 Patients who had ACL reconstruction with a BPTB graft were randomized to either accelerated rehabilitation (a 19-week program that allowed unrestricted weightbear- ing after 1 week, no brace use after 2 weeks, open kinetic chain exercises involving contraction of the quadriceps muscle group with the knee near extension [0°-45°] after 4 weeks, and return to preinjury activity at 24 weeks) or nonaccelerated rehabilitation (a 32-week program that included the same exercises prescribed over a delayed time interval, including unrestricted weightbearing after 3 weeks, no brace use after 4 weeks, open kinetic chain exer- cises involving contraction of the quadriceps muscles [0°- 45°] at 12 weeks, and return to preinjury activity at 32 weeks). Compliance with the rehabilitation programs was monitored with training logs. At the time of surgery, and then 3, 6, 12, and 24 months later, measurements of A-P knee laxity (KT-1000 arthrometer), clinical assessment (IKDC evaluation), patient satisfaction (Knee Osteoarthritis Outcome Score), function (1-legged hop test), and cartilage metabolism (synovial fluid-based bio- markers of synthesis and cleavage of type II collagen, and turnover of aggrecan) were completed. At 2-year follow-up, there was no difference in the increase of anterior knee laxity between the 2 groups (a 2.2-mm vs 1.8-mm increase relative to the normal knee for the nonaccelerated and accelerated programs, respectively). The treatments were also similar in terms of clinical assessment, patient satis- faction, activity level, function, and the response of the synovial fluid biomarkers of articular cartilage metabo- lism. There was concern that the biomarkers from subjects in both groups remained elevated over periods consider- ably longer than modern rehabilitation programs and sub- stantially greater than the interval after which most peo- ple attempt to return to preinjury activities. Soon after injury and just before surgery, the levels of cleavage and synthesis of type II collagen and turnover of aggrecan were elevated compared to normal values. After 12 months of healing, cleavage of type II collagen returned to normal values, whereas synthesis of collagen and turnover of aggrecan remained elevated. Synthesis of type II collagen remained elevated at 24-month follow-up, whereas the turnover of aggrecan approached normal limits. In the 3 RCTs reviewed above, only Beynnon et al18 ade- quately described their method of randomization, both Beynnon et al18 and Frosch et al42 stated that the investi- gators were blinded at follow-up, and all authors revealed a minimal loss of patients to follow-up. Our review of the ACL rehabilitation literature revealed several concerns with the quality of the studies. Although most studies claimed to be based on prospective RCT designs, many reports did not describe how the random- ization was performed, it was often unclear if the assessors and study subjects were blinded to the treatment groups, in most reports the follow-up intervals were quite short, and in some studies the proportion of subjects lost to follow-up was not presented. Most of the randomized stud- ies that were reviewed clearly described the activities (and restrictions) a subject was advised to perform and the time hat the activities were recommended. Little information, however, was presented regarding the frequency and dura- tion of the activities and how well subjects complied with the prescribed program. Many of the articles delineated when subjects were allowed to return to sports, but few reports provided data describing whether subjects actually returned to sport and if so, at what level. Furthermore, no consensus was given on what primary outcome measure should be used to determine if a rehabilitation program is both safe and effective. A common outcome for most of the investigations we reviewed was A-P knee laxity. Although most orthopaedic surgeons would agree that an increase in anterior laxity of more than 3 mm in the index knee com- pared to the normal knee is a concern from a biomechani- cal perspective,29 it remains unclear what magnitude of an increase in A-P laxity is a concern from a biological per- spective. It may be that increases of knee laxity that are within certain limits of normal do not result in altered metabolism of the articular cartilage, damage to the meniscus, or additional intra-articular injury. However, we do not know the relationship between increased anterior knee laxity and metabolism of the menisci and articular cartilage, and therefore, any increased laxity has the potential to lead to adverse changes within the joint over time. There also appears to be a place for determining the control of rotational laxity by modern reconstruction pro- cedures. Single-bundle procedures, especially involving femoral tunnel placement high in the notch (11 o'clock to 12 o'clock), may well be unable to prevent abnormal rota- tional kinematics of the tibia relative to the femur. Our interpretation of the ACL rehabilitation literature is that there is some information available from prospec- tive RCTs regarding how much loading and motion a knee with a healing BPTB graft can sustain without perma- nently stretching the graft (as evidenced by abnormal increases of anterior knee laxity), disrupting the graft, or creating failure of graft fixation. In contrast, there is very little information available about the effect of rehabilita- tion on other graft materials, such as the 4-strand ham- string graft. As well, there is little information available about the effect of rehabilitation on the healing response of an ACL graft with combined injuries to the articular cartilage and meniscus.

Graft Fixation

Substitutes for the ACL can be prepared as either bone- tendon-bone grafts or tendon grafts. Depending on the graft material that is selected for ACL reconstruction, the bony or soft tissue portions of these constructs can be fixed either within bone tunnels or externally to cortical bone. From a biomechanical perspective, fixation represents the weak link during the early stages of healing. The long- term goal is to obtain biological incorporation of the graft at the anatomical attachment site of the ACL and to restore the transition from soft tissue to fibrocartilage, to calcified fibrocartilage, and to bone. This review did not focus on the techniques and associ- ated biomechanical characteristics of the wide array of fix- ation devices used in cruciate ligament surgery; this topic has been covered in outstanding review articles by Brand et al22 and Wilson et al.111 Instead, we focused on the 3 prospec- tive RCTs of ACL graft fixation that have been published. Aglietti et al,3 performing an RCT of ACL reconstruction with a central third BPTB graft, compared 2 different types of tibial fixation. Graft fixations in the tibial tunnel were performed on 2 groups of subjects, with an interfer- ence screw placed either at the level of the tibial plateau (aperture fixation) or distal to the plateau. The same fixa- tion device was used in the femoral tunnels for both groups. The ACL reconstruction was performed by the same surgeon, randomization was performed using an alternating scheme, all subjects underwent the same reha- bilitation program, and follow-up was performed by an independent examiner at a mean interval of 18 months. There were no differences between the treatments in range of motion, A-P laxity, symptoms, and subjective eval- uation of outcome. Tibial tunnel enlargement was less fre- quent in the group that underwent graft fixation at the level of the tibial plateau (23% vs 43%); however, tibial tuberosity pain, attributed to harvesting a longer tibial bone block, was more frequent in this group. Presenting an RCT of ACL reconstruction with a central third BPTB graft, Fink and colleagues39 compared 2 dif- ferent types of femoral fixation. Subjects were randomized to undergo graft fixation in the femoral tunnel with either a titanium interference screw or a polyglyconate (a copoly- mer of polyglycolic acid and trimethylene carbonate) bioabsorbable interference screw. The same metal interfer- ence fit fixation was used in the tibial tunnels of both groups. Two surgeons performed the single-incision proce- dures, all subjects observed the same rehabilitation proto- col, and follow-up evaluations were made over a 2-year period. Graft fixation with the bioabsorbable screw pro- duced the same clinical outcome and A-P knee laxity val- ues compared to graft fixation with the metal screw. Complete degradation of the bioabsorbable screw was apparent at 1 year, and replacement of the screw with bone occurred by 3 years. A similar investigation by the same group revealed that fixation of BPTB grafts with a polyglyconate interference screw was safe and effective compared to fixation with a titanium interference screw.15 Our review of the literature on ACL graft fixation revealed that the fixation of BPTB grafts with interference screws is adequate for the loads produced by current reha- bilitation programs. In contrast, the frequent publication of new techniques to fix free-tendon grafts, such as the 4- strand hamstring graft, suggests that an optimal tech- nique of fixation has yet to be identified, particularly when rehabilitation includes immediate weightbearing, early use of the quadriceps muscles with the knee near exten- sion, and early return to sport.

Beynnon BD, Johnson RJ, Abate JA, Fleming BC, Nichols CE. Treatment of anterior cruciate ligament injuries, Part I. American J of Sports Med. 2005; 33:1579-1602.

The ACL is a biomechanically complex structure that acts as a primary restraint to anterior displacement of the tibia relative to the femur and acts as a restraint to internal/external rotation, varus/valgus angulation, and a combination of these. This ligament is very commonly injured in athletes through both contact and non-contact mechanisms. The ACL endures various stresses from the body as well as external forces. It has been shown that the tension placed on the patellar tendon with contraction of the quadriceps increases proximal tibial anterior displacement forces, while contraction of the hamstrings act to protect the ACL by applying a force to the proximal tibia. The highest amount of strain placed on the ACL by the quadriceps is between full extension and 50 degrees of flexion. For this reason, co-contraction of the hamstrings and the quads is important to decrease the stress on the ACL. The highest strain values occur during isometric contraction of the quadriceps while the knee is in 15 degrees of flexion. Compressive loads through the tibiofemoral joint as produced by weight bearing have, in the past, been considered protective of the ACL due to the belief that the increased joint stiffness created by the body weight decreased anterior displacement of the tibia. However, it has been proposed that compressive loads produced by body weight do not strain-shield the ACL graft and actually place an anterior displacing force on the ACL. There is a higher rate of ACL injuries among women (between 2.4-9.7 times greater) but a greater rate of reconstructions performed on males, most likely due to their propensity to participate in more at-risk sports. Patients with an ACL deficient knee are more prone to experience meniscal tears and osteoarthritis of the knee. Injuries to the ACL are rarely isolate and possible accompanying injuries to other structures include MCL, meniscus, articular cartilage, and bone bruises. These injuries can often complicate the rehabilitation process and should be carefully considered when choosing intervention strategies. Risk factors for ACL injury include the a previously reconstructed ACL, dry weather conditions, greater number of cleats and associated higher torsional resistance at the foot/turf interface, small intercondylar notch, generalized joint laxity, higher body mass index, and KT-2000 arthrometer measurements of A-P knee laxity greater than 1 standard deviation above the mean. Not all ACL injuries require surgical intervention. Some indications for reconstruction include: high-risk lifestyle, repeated episodes of giving way, associated meniscal tear, generalized ligamentous laxity, and recurrent instability. Indications for non-surgical intervention include: little exposure to high risk activities, more than 40 years old, those who are successfully coping and adapting to ACL insufficiency, advanced arthritis, and patients unwilling/unable to comply with postreconstruction rehabilitation. For young children with ACL injuries, it has yet to be established whether a reconstruction is a safe intervention. It is important to consider each patient as an individual case and consider all factors when deciding between surgical and nonsurgical intervention. Timing of ACL reconstruction has been a topic studied in the literature, however, there is no consensus of opinion and it has been proposed that it is more important to consider the condition of the knee at the time of surgery (pain, effusion, ROM), in order to determine risk for arthrofibrosis and avoid it as much as possible. The newest trend in surgical technique proposed is the single-incision arthroscopic approach over the 2 incision procedure with the potential advantages of less morbidity, improved cosmesis, shorter hospital stays, decreased postoperative pain, shorter operative time, improved proprioception, earlier rehab, and decreased costs. However, it is not recommended to begin using this technique on a regular basis until it has been shown superior, seeing that evidence thus far has shown neither to be superior. The double tunnel technique, as opposed to the single tunnel technique has begun to be recommended for more closely reestablishing more accurate anatomical imitation of the original ACL, however, until it is proven to be beneficial, it is proposed that the increased complications of this surgery are not worthwhile unless there is a significant benefit. Additionally, extra-articular reconstruction has been shown to provide limited benefit. When comparing different graft options, the use of bone-patellar tendon-bone autograph grafts has been shown in the literature to result in better outcomes than the use of 2 strand-hamstrings autograph. However, 4 strand hamstrings autographs have been shown to be equally as effective as bone-patellar tendon-bone autographs. The primary advantage of using hamstrings tendon autograph is a decreased number of patients with patellofemoral crepitus and less extension loss. Allografts are still used relatively infrequently, but they offer the benefits of decreased morbidity, preservation of extensor/flexor mechanism, provision of outside source of graft material, decreased operative time, larger graft source, lower incidence of arthrofibrosis, and improved cosmesis. However, it poses the disadvantages of risk of infection, slow and incomplete remodeling of graft, higher cost, limited availability, tunnel enlargement, alteration of graft structural properties by sterilization and storage procedures, and immunological response. For these reasons, although this type of graft offers potential benefits, there needs to be more research before allograft use is recommended for routine use.

Phase 4. Maturation Phase

The final phase begins in a range of 4 to 6 months and can last up to 15 to 18 months post- surgery.4,5,13,20,22,39,42,43 It is during this phase that the repair tissue reaches its full maturation. The duration of this phase varies based on several factors such as lesion size and location, and the specific surgical procedure performed. The patient will gradually return to full premorbid activities as toler- ated. Impact-loading activities are gradually intro- duced. Although such procedures as OATS and ACI are designed to restore function rather than return to high-impact athletic activities, a return to athletic activities is determined based on the unique presenta- tion of each patient. A return to competitive athletics has been documented for microfracture,57 OATS,25 and ACI37,38 procedures.

Microfracture

The microfracture procedure is a form of marrow stimulation, similar in concept to the chondroplasty procedure.57 A sharp awl is used arthroscopically through 1 of the portals and a mallet is used to impact the awl into the subchondral bone and thus generate bleeding from the bone. This procedure is also referred to as ''picking,'' due to its nature. Holes are created at regular intervals until the entire defect has been addressed. The penetration of the sub- chondral bone eventually creates fibrocartilagenous tissue that covers the cartilage lesion.57 The rehabilitation following a microfracture procedure progresses more cautiously than that of adebridement or chondroplasty (Table 2). The proliferation phase begins immediately following surger yand lasts until the fourth week postoperatively. During this time, defects have been shown to beginfilling with a fibrin clot, although no fibrocartilage ispresent. A period of non-weight bearing is used forthe first 2 to 4 weeks postoperatively for most lesions. A recent study by Marder et al32 compared the results of patients with small focal lesions of less than 2.0 cm2 utilizing 2 postoperative rehabilitation programs. Group 1 utilized touchdown weight bearing and a CPM machine for 6 to 8 hours a day for 6 weeks. Group 2 weight bearing as tolerated immediately following surgery with active-assisted heel slides for ROM (without the use of a CPM). The authors reported significant improvements in both groups and no significant differences in the subjective or objective outcomes of both groups with a minimum of 2 years follow-up. Thus, it appears that it may be possible to begin early controlled weight bearing for small, focal lesions without applying deleterious forces to the repair site. We begin initial, controlled, toe touch weight bearing for lesions that are localized and smaller than 2.0 cm2 in patients with good tissue quality. For patients with patellofemoral lesions, immediate weight bearing is performed due to the lack of lesion articular contact during weight bearing; however, a drop-locked knee brace is utilized to keep the knee in full extension and avoid deleterious shear forces to the healing repair site. Due to the arthroscopic nature of the procedure, PROM is performed immediately without restrictions. Full PROM is achieved within weeks 3 to 4, often with little difficulty. The transition phase begins at week 4 and lasts until week 8. It is during this time that the patient may progress to full weight bearing and more func- tional weight-bearing exercises. At 6 weeks postopera- tively, a thin layer of tissue covers the base of the lesion.17 Although the repair is still incomplete, fibrocartilagenous tissue is present and by 8 weeks some tissue with hyaline-like characteristics has been detected.13 By 12 weeks the defect is completely filled and the quality of cartilaginous tissue improves sig- nificantly.17 Weight bearing is thus progressed to full at week 8 for most lesions, when the strength of the repair tissue is increasing. However, the progression to more advanced exercises including impact loading is de- layed until the end of the remodeling phase, when the defect is completely filled. The patient may gradually begin to return to former activities during the maturation phase between months 4 to 6; how- ever, larger lesions require delaying the progression to high-impact activities for up to 8 months.

Create a Healing Environment

The next principle of articular cartilage rehabilita- tion involves creating an environment that facilitates the healing process while avoiding potentially delete- rious forces to the repair site. Through animal studies, as well as closely monitoring the maturation of repaired tissue in human patients via arthroscopic examination, the biological phases of maturation have been identified following several articular carti- lage repair procedures.4,5,20,42,43 Knowledge of the healing and maturation process following these pro- cedures will assure that the repair tissue is gradually loaded and that excessive forces are not introduced too early in the healing process. Two of the most important aspects of rehabilitation of articular cartilage procedures are weight-bearing restrictions and range-of-motion (ROM) limitations. Unloading and immobilization have been shown to be deleterious to healing articular cartilage, resulting in proteoglycan loss and gradual weakening.2,21,59 Therefore, controlled weight bearing and ROM are essential to facilitate healing and prevent degenera- tion. This gradual progression has been shown to stimulate matrix production and improve the tissue's mechanical properties.6,7,60 Controlled compression and decompression forces observed during weight bearing may nourish the articular cartilage and provide the necessary signals to the repair tissue to produce a matrix that will match the environmental forces.2,21,59 A progression of partial weight bearing with crutches is used to gradually increase the amount of load applied to the weight-bearing surfaces of the joint. The use of a pool or aquatic therapy may also be beneficial to initiate gait training and lower extremity weight- bearing exercises. The buoyancy of the water de- creases the amount of weight-bearing forces to approximately 25% of the individual's body weight when submerged to the level of the axilla, and 50% of the individual's body weight when submerged to the level of the waist.23 Commercially available de- vices to unload the patient's body weight during treadmill ambulation may also be useful. A force platform is another useful tool during the early phases of rehabilitation when weight bearing is limited. This can be used to monitor the percentage of weight bearing on each extremity during exercises such as weight shifts, mini-squats, and leg presses (Figure 1). The pool and force platforms may be used during early phases of rehabilitation to perform limited weight-bearing activities designed to facilitate a nor- mal gait pattern and enhance strength, propriocep- tion, and balance. The authors believe that beginning controlled weight-bearing activities during the early protective stage of healing is a critical component to the overall rehabilitation process. Although the re- turn to functional activities will differ for each patient, it is our opinion that early initiation of controlled exercise enables the individual to return to functional activities sooner than those that are immobilized and non-weight bearing. Passive range of motion (PROM) activities, such as continuous passive motion (CPM) machines or manual PROM performed by a rehabilitation special- ist, are also performed immediately after surgery in a limited ROM to nourish the healing articular carti- lage and prevent the formation of adhesions.48-50,62 Motion exercises may assist in creating a smooth low frictional surface by sliding within the joint's articular surface, and may be an essential component to cartilage repair.48,51 It is the authors' opinion that PROM is a safe and effective exercise to perform immediately postoperatively, with minimal disadvanta- geous shear or compressive forces, if performed with patient relaxation. This assures that muscular contrac- tion does not create deleterious compressive pres- sures or shearing forces. The use of CPM has been shown to enhance cartilage healing and long-term outcomes following articular cartilage procedures.47,48 Comparing the outcomes of patients following microfracture proce- dures, Rodrigo et al47 reported an 85% satisfactory outcome in patients utilizing a CPM machine for 6 to 8 hours per day for 8 weeks, as compared to 55% satisfactory outcome in those patients who did not utilize a CPM machine. PROM can also be performed on an isokinetic device (Biodex Corporation, Shirley, NY) in the passive mode or using a bike with adjustable pedals that can alter the available ROM (Unicam Corporation, Ramsey, NJ) (Figure 2).

Marrow-Stimulating Techniques

Abrasion Arthroplasty. Abrasion arthroplasty is tradi- tionally performed arthroscopically with a shaver or bur, with recommendations to remove 1 to 2 mm of exposed sclerotic bone down to the vasculature of the subchondral plate.10 This results in a fibrin clot that later develops into fibrocartilage. Although abrasion arthroplasty is based on sound biologic principles, results comparing simple debridement with the addition of abrasion arthroplasty indicate that for both groups, roughly half of the patients improved, but 33% of the abrasion arthroplasty group reported a worse Hospital for Special Surgery (HSS) knee score than before surgery.10 Abrasion arthroplasty appears to be technique sensitive, and minimizing the amount of subchondral bone destruction remains challenging. Microfracture. The microfracture technique uses the same sound biologic principles as the abrasion arthroplasty without systematic bone removal. Arthroscopically, angled awls are used to perforate the subchondral bone of focal articular cartilage surface lesions.11,35,36,90 By creating per- forations without power drilling, the potential risk fo thermal necrosis is eliminated, and a more controlled and precise subchondral bone perforation depth and location can be obtained. The perforations should access the under- lying cancellous bone, resulting in release of blood and mesenchymal cells, leading to reparative tissue formation. Under protected loading conditions and CPM, the cells in the resulting "superclot" proliferate and differentiate into a fibrous or fibrocartilage mosaic repair tissue.28 Ideal indications for microfracture treatment include focal grade III or IV articular surface lesions without bone loss that are surrounded by normal articular cartilage in a young patient. Contraindications include significant sub- chondral bone loss, mechanical axis malalignment, bipolar lesions, or a high risk of noncompliance with postoperative rehabilitation protocols. A disadvantage of this technique is that, at best, repair tissue will be composed of predomi- nantly type I collagen-rich fibrocartilage, which does not resist compression and shear loads as predictably as hya- line cartilage does and is likely less durable over time. The first step in this procedure is critical to its success and involves creating precise perpendicular edges of the lesion at the transition zone adjacent to the healthy artic- ular cartilage. Thus, a "well-shouldered" lesion will improve the local mechanical environment by reducing shear and compression on the lesion, thereby allowing the formation of fibrocartilage. All unstable cartilage should be removed. Animal studies suggest that removing the cal- cified cartilage with a curette greatly enhances the per- centage and quality of defect fill.29 A surgical awl is then used to create holes placed 2 to 3 mm apart beginning first at the periphery of the lesion. Great care should be taken to prevent confluence of the holes, as this will cause unsta- ble bone fragments that may break free from between the holes. When fat droplets can be seen coming from the mar- row cavity, the approximate depth (2-4 mm) has been reached.89 The arthroscopy fluid inflow is then clamped to allow visual confirmation that blood and marrow fat droplets are emerging from each hole. The success of the procedure depends as much on patient compliance with the rehabilitation protocol as it does on proper surgical technique. For lesions of the weightbearing surfaces of the femoral condyle and tibial plateau, the patient remains strictly nonweightbearing for 6 weeks and on protected weightbearing for an additional 2 weeks. Early passive motion is implemented. For lesions in the patellofemoral joint, the patient is braced with a flexion stop of 30° to 40° to limit patellofemoral contact. The brace is removed only for therapist-supervised range of motion and strengthening. Surgeon adherence to and patient compliance with these postoperative limitations are paramount to the success of this procedure. Following proper technique and postoperative protocol, this relatively benign, inexpensive outpatient procedure can provide symptomatic relief and functional improve- ment in properly selected patients without eliminating further treatment options should the microfracture fail. Optimal outcome has been noted in younger patients with smaller lesions and a well-defined history of trauma.38 One explanation for this finding is that the marrow of younger patients has a greater number of mesenchymal cells, and, with increasing age, the pluripotential cell count drops off precipitously. In addition, a traumatic lesion, in contrast to a degenerative one, is likely to be surrounded by normal cartilage outside the zone of impact, which allows for the creation of "shoulders" at the periphery of the lesion. Steadman et al recently reported a series of 72 patients with a mean 11-year follow-up who demonstrated significant subjective improvement in the Tegner, Western Ontario and McMaster Universities Osteoarthritis Index, Lysholm, and Short Form-36 (SF-36) scores. At 7 years, 80% of patients reported themselves as improved. Younger age (<45 years) at surgery was correlated with a better out- come.88

BASIC SCIENCE Form and Function

Articular cartilage is a viscoelastic material and therefore has variable load-bearing properties associated with dif- ferent positions and activities. This vital characteristic and its role of minimizing surface friction on articular sur- faces are a function of its ultrastructure composition and complex organization. Hyaline cartilage comprises an extracellular matrix that makes up approximately 95% of the tissue by volume, with sparsely distributed chondro- cytes. The matrix is principally composed of type II colla- gen, but types V, VI, IX, X, XI, XII, and XIV are also pres- ent in smaller amounts. Sulfated proteoglycan macromolecules constitute 12% of articular cartilage weight. Carboxyl and sulfate groups (keratin sulfate and chondroitin sulfate) on the gly- cosaminoglycans carry a negative charge. The negative harge creates a high affinity for water that helps carti- lage resist compressive loads and causes the aggrecans to repel one another, resulting in maximal volume expansion. The flow of water through charged regions of the proteo- glycan-rich matrix generates piezoelectric charges that further modulate the rate of water flow contributing to the viscoelastic behavior of articular cartilage.124 In addition, there is evidence that electric and electromagnetic fields can produce a sustained upregulation of growth factors in articular cartilage.1 Chondrocytes are of mesenchymal stem cell origin and are responsible for synthesizing the matrix. In the hypoxic environment of articular cartilage, chondrocytes are mainly anaerobic. Their low turnover rate and sparse distribution allow for little cell-to-cell contact.21 Chondrocytes constitute just 2% of the total volume of adult articular cartilage. Chondrocyte survival depends on the proper chemical and mechanical environment, including growth factors, mechanical loads, hydrostatic pressures, and piezoelectric forces.20 Local paracrine effects have been demonstrated to drive chondrogenic processes.70 Healthy chondrocytes are integral to articular cartilage survival, as they synthesize the extracellular matrix and contribute to the various zones of hyaline cartilage. Each zone of hyaline cartilage has a characteristic com- position and architecture consisting of chondrocytes, colla- gen, aggrecan, and fluid dynamics that relate directly to that zone's function (Figure 1).21 The superficial zone con- sists of a "lamina splendens" layer of tightly packed colla- gen fibers parallel to the articular surface and a cellular layer of flattened chondrocytes. Preservation of this super- ficial layer is critical to protect the deeper zones. Type IX collagen is found in this layer between type II bundles that provide resistance to shear. It is thought that this layer limits passage of large molecules between synovial fluid and cartilage. The transitional layer, or intermediate zone, is composed of spherical chondrocytes, proteoglycans, and obliquely oriented collagen fibers that primarily resist compressive forces but also serve as a transition between the shearing forces on the surface and the compressive forces placed on the deeper layers. The deep zone consists of collagen fibers and chondrocytes oriented perpendicular to the articular surface, which resist compressive loads. The calcified layer consists of the tidemark that separates subchondral bone from the calcified cartilage and provides complex adhesive properties of the cartilage to bone. Collectively, these highly specialized layers produce the superior loading and minimal friction characteristics of hyaline cartilage that make it particularly difficult to restore or duplicate once it is damaged or lost. Injury to any part of this complex system can disrupt the normal biomechanical properties of articular cartilage, leading to further degeneration. In contrast, meniscal tissue is composed of cells that are either elongated on the surface or ovoid in deeper layers. These cells are equipped with few mitochondria, suggest- ing anaerobic metabolism.113 The extracellular matrix of menisci is 74% water by weight. Type I collagen composes about 65% of the dry weight, and glycosaminoglycans make up 2% of the dry weight. With this structure, the menis-cus is able to resist tension, compression, and shear. Other collagens (II, III, V, and VI) make up about 5% of the dry weight. There are other noncollagenous proteins including elastin, fibronectin, and thromboplastin that probably assist in organizing the matrix by binding molecules. The blood supply to the meniscus is derived from the inferior medial and lateral geniculate arteries that form a plexus encompassing 10% to 30% the width of the medial menis- cus and 10% to 25% the width of the lateral meniscus.5 There is a 1- to 3-mm cuff of vascular synovium on the peripheral femoral and tibial surfaces. This complex blood supply is key to successful meniscal repair or transplanta- tion. In addition, there is a network of micropores that per- mits synovial fluid to pump through the meniscal tissue with normal cyclical joint compression. This synovial fluid circulation is important for articular cartilage health.87 The structure of meniscal tissue allows it to behave as a fiber-reinforced, porous-permeable composite material containing both solid (matrix proteins) and fluid (water) components.36,37 The function of a meniscus is to transmit load across the tibiofemoral joint, improve joint congruency, increase the surface area of joint contact, and assist in syn- ovial fluid circulation. An intact meniscus converts joint loading forces to radial-directed hoop stresses that lead to tensile stress on circumferential collagen fibers. As a result, the menisci transmit 50% of the joint load when the knee is in extension and 90% when the knee is in flex- ion.126 In vitro animal studies have demonstrated that loss of just 20% of a meniscus can lead to a 350% increase in contact forces.113 The importance of an intact meniscus is of primary importance in the setting of articular cartilage restoration procedures. Compared to total meniscectomy, meniscal transplantation has been demonstrated to improve contact forces, thereby protecting articular carti- lage, provided that the posterior and anterior horns of the meniscus transplant are adequately anchored to bone.2 It is well understood that the posterior horn of the medial meniscus acts as a secondary restraint to posterior-anteri- or translation of the tibia on the femur.64,117 Untreated prior medial meniscectomy or incompetence of the medial posterior horn has been associated with joint instability in the anteroposterior plane, even in the setting of a proper- ly reconstructed ACL.115 This stability is a requirement for cartilage restoration surgery. In addition to load transmis- sion and joint stability, an intact meniscus diminishes fric- tion in the knee; the coefficient of friction in a meniscec- tomized knee is increased by at least 20%.72 An intact meniscus disperses synovial fluid across the articular sur- faces via micropores; the fluid provides chondrocytes with nutrition. The compression of the menisci with normal joint mechanics causes extrusion of the fluid out of the menisci, bathing the articular cartilage with nutrients.87 For these reasons, it is often reasonable to consider a meniscal transplant in the setting of other articular carti- lage restoration procedures in a meniscus-deficient knee.

Rationale and Clinical Techniques for Anterior Cruciate Ligament Injury Prevention Among Female Athletes

terior cruciate ligament (ACL) research has resulted Ain more than 2000 scientific articles outlining injury incidence, mechanism, surgical repair techniques, and rehabilitation of this important stabilizing knee ligament.1 However, despite the many scientific advances in the treatment of ACL injury, osteoarthritis occurs at a 10 times greater rate in individuals with ACL injury regardless of the treatment (conservative management versus surgical treatment).2 Epi- demiologic research has demonstrated that female athletes have a 4- to 6-fold increased risk for ACL injury compared with their male counterparts playing at similar levels in the same sports.3,4 The increased ACL injury risk coupled with increased sports participation by young women over the last 30 years (9-fold increase in high school5 and 5-fold increase in collegiate sports6) has increased public awareness and fu- eled many sex-specific mechanistic and interventional inves- tigations. This paradigm shift of isolated research focus away from treatment and rehabilitation is warranted, especially to injury mechanism and prevention, because an estimated 38 000 ACL injuries occur in young female athletes per year.7 At a cost per ACL injury of approximately $17 000,8 surgical and rehabilitative costs total approximately $646 000 000 an- nually in the United States. This is in addition to the traumatic effect to these individuals of potential loss of entire seasons of sports participation and possible scholarship funding, sig- nificantly lowered academic performance,9 long-term disabil- ity, and up to 105 times greater risk for radiographically di- agnosed osteoarthritis in the future.10 Contrary to what is observed in adolescent athletes, a thor- ough analysis of the published literature demonstrates no ev- idence that a difference in ACL injury rates exists in prepu- bescent athletes.11-14 Knee injuries do occur in pediatric athletes, as evidenced by 63% of the sport-related injuries in children aged 6-12 years being classified as joint sprains, most of which occur at the knee.14 However, ACL sprains are more rare in prepubescent children than in adolescents, and ruptures do not present at significantly different rates in boys and girls before puberty.11-13 Although equal numbers of ligament sprains occur in girls and boys before adolescence, girls have higher rates immediately after their growth spurt and into ma- turity.15 The anthropometric measures of growth and development show very similar trends between the sexes, but male and fe- male force-production capabilities diverge significantly during and after puberty. Men demonstrate a neuromuscular spurt, whereas women, on average, exhibit little change throughout puberty.16,17 The neuromuscular spurt is defined as increased power, strength, and coordination that occur with increasing chronologic age and maturation stage in adolescent boys.16,17 No similar correlations among height, weight, and neuromus- cular performance have been demonstrated in pubescent girls. For example, vertical jump height (a measure of whole-body power) increases steadily in boys during puberty but not in girls.16-19 No sex differences in peak leg power were noted before age 14, but boys have significantly greater power after that age.20 A plateau in the peak power of girls occurred around 16 years of age.20 In the absence of appropriate neu- romuscular adaptation, the musculoskeletal growth during pu- berty may increase neuromuscular imbalances. We define neu- romuscular imbalances as muscle strength or activation patterns that lead to increased joint load. Female athletes may demonstrate one or more neuromuscular imbalances that in- crease lower extremity joint loads during sports activities.21,22 With ligament dominance, the neuromuscular and ligamentous control of the joint is unbalanced, as demonstrated by an in- ability to control dynamic knee valgus when landing and cut- ting.23 Quadriceps dominance relates to an imbalance between knee extensor and flexor strength, recruitment, and coordina- tion.22 With leg dominance, the 2 lower extremities are un- balanced in strength and coordination.21,23 These developmen- tal imbalances, if left unchecked, may continue through adolescence into maturity. Huston and Wojtys24 showed that college-aged elite female athletes, well past their developmen- tal years, demonstrated neuromuscular recruitment and leg- strength imbalances when compared with male athletes and nonathletic controls. Neuromuscular imbalances may be important contributors to ACL injury, which occurs under conditions of high dynamic loading of the knee joint, when active muscular restraints do not adequately compensate and dampen joint loads.25 De- creased neuromuscular control of the joint may stress the pas- sive ligament structures, exceeding the failure strength of the ligament.26,27 High levels of neuromuscular control are nec- essary to create dynamic knee stability.26,28 Any neuromus- cular imbalances that limit the effectiveness of the active mus- cular control system in working synergistically with the passive joint restraints to create dynamic knee stability may increase the risk for an ACL injury. Identifying these imbal- ances may offer the greatest potential for interventional de- velopment and application in high-risk populations.29 ; Our pur- pose is to present the theory and rationale of methods that may be helpful in identifying and correcting neuromuscular imbalances in female athletes.

Autologous Chondrocyte Implantation

utologous chondrocyte implantation (ACI) is employed when traditional first-line treatments fail to improve on the patient's clinical presentation after adequate time for recovery and response. It is ideal for symptomatic, unipo- lar, well-contained chondral or shallow osteochondral defects measuring roughly 2 to 10 cm2. Commonly, patients have failed previous treatments. It is traditionally indicated for treatment of focal defects in the knee, but its "off-label" use has recently been expanded to include the treatment of chondral defects in the ankle,34,56 shoulder,78 elbow,13 wrist,70 and hip.51 In the knee, off-label use for the patella and tibia has also met with success rates that par- allel those for the femoral condyle and trochlea. Bipolar lesions (greater than grade II change on the opposing sur- face) are a relative contraindication to ACI. As already dis- cussed, malalignment, ligament instability, and meniscus deficiency are not considered absolute contraindications to ACI as long as they are addressed concomitantly or in a staged fashion. The first stage involves an arthroscopic evaluation of the focal chondral lesion to assess containment, depth, and potential bone loss (Figure 6). Biopsy of normal hyaline cartilage is performed from either the superomedial edge of the trochlea64 or our preferred site, the lateral edge of the intercondylar notch (ie, where bone is removed for an ACL notchplasty) using a curved bone graft harvesting gouge (Figure 7). If the biopsy is obtained from the trochlear ridge, it is recommended that an open-ring curette be used to allow for visualization of the biopsy process. The total volume of the biopsy should be approxi- mately 200 to 300 mg, preferably in 3 "Tic-Tac-sized" frag- ments. The prepared shipping container has a collection vial that is clearly marked to indicate adequate biopsy volume (Figure 8). As when performing an ACL notchplasty, it is important not to violate weightbearing articular car- tilage. We send the biopsy to Genzyme Biosurgery Corporation (Cambridge, Mass) for processing and cellular expansion. The second stage of the procedure is cell implantation, which typically takes place between 6 weeks and 18 months after the biopsy, although the cells can be cryopre- served for up to 4 years. A tourniquet is typically used until after the defect is prepared and the periosteal patch is harvested. The surgical exposure depends on defect loca- tion. Patellofemoral lesions are approached through a mid- line incision, allowing a simultaneously performed tibial tubercle osteotomy. We prefer to access patellofemoral lesions through a lateral retinacular release without com- pletely everting the patella. We also avoid disruption of the fat pad and dissection around the patellar tendon to reduce potential for postoperative stiffness. A tibial tuber- cle osteotomy affords some increased patellar mobility, facilitating access to the defect, but we intentionally avoid complete elevation and "flipping" of the tibial tubercle to minimize trauma to the fat pad and patellar tendon. Femoral condyle lesions are addressed through limited parapatellar arthrotomies. For medial defects, we use a imited sub-vastus medialis approach that has, in our experience, reduced the magnitude of postoperative pain, allowing earlier and more complete return of motion. Lateral defects are approached through a limited lateral retinacular release. We then use a separate 3-cm incision beneath the pes anserine tendon insertion to harvest the periosteal patch. These modifications have greatly reduced postoperative pain and have allowed us to perform the majority of our ACI procedures on an outpatient basis. Defect preparation involves removing any existing fibro- cartilage covering the lesion, as well as loose articular car- tilage flaps, leaving healthy surrounding hyaline cartilage to form stable vertical walls shouldering the lesion. Circular or oval-shaped prepared defects are biomechani- cally more stable.24 A No. 15 scalpel and sharp-ring curettes are used to incise the defect border to but not through the level of the subchondral bone (Figures 9 A and B). Hemostasis is controlled with the use of neuropatties soaked with a dilute 1:1000 epinephrine solution. The periosteal patch is harvested through a 3-cm inci- sion on the proximal medial tibia, 4 fingerbreadths distal to the pes anserine tendon attachments. More distal and anteromedial locations tend to provide the best source for the periosteal patch. If a simultaneous tibial tubercle osteotomy is performed, we use a single extensile incision and harvest the periosteum before performing the osteoto- my. Superficial subcutaneous fat is carefully removed with sharp dissection from the periosteum on the anteromedial ibia to avoid inadvertent penetration. Smokers tend to have a poor-quality periosteum, and obese patients have a larger amount of adherent adipose tissue to separate from the periosteum and will require extra care. In addition, older patients tend to have a thin periosteum. A patch that is at least 2 mm larger than the defect is harvested to allow for slight shrinkage after detachment. The patch edges are scored to bone with a No. 15 scalpel and elevated with a sharp, curved periosteal elevator beginning distally and moving toward the inferior edge of the pes and over- lying sartorius fascia (Figure 10). The character of the periosteum will change as the sartorius fascia fibers are encountered. It is recommended that the fat and small blood vessels found on the periosteum be dissected off after the periosteum is safely elevated from the bone but before detaching the final superior edge. The outer surface is marked to distinguish it from the inner cambium layer. Additional sources for periosteum, if necessary, are the dis- tal femur, which is thicker and more vascular than the periosteum on the proximal tibia, and the contralateral tibia, which carries the disadvantage of a second surgical site. In extreme cases, 2 periosteal patches may be sewn together, taking care to minimize suture bulk at the seam. After defect preparation and periosteal harvest, the tourniquet is deflated and meticulous hemostasis is obtained. The patch is then sewn onto the cartilage so as to remain taut over the defect with the cambium layer fac- ing the defect base. The periosteum is secured with a 6-0 absorbable Vicryl suture (Ethicon Inc, Johnson & Johnson, Somerville, NJ) on a P-1 cutting needle. The suture is passed first through the periosteum patch and then through the articular cartilage. The goal is to anchor the periosteu m flush with the surrounding articular cartilage surface. A gap should be maintained between the final sutures to allow for chondrocyte implantation with an angiocatheter. If small holes are inadvertently created in the patch, they may be carefully repaired with a single 6-0 Vicryl suture. If the surrounding cartilage is unable to hold suture, micro-anchors loaded with absorbable suture may be used. At the edge of an articular surface, bone tun- nels may be created with a 0.45 Kirschner wire to pass transosseous 6-0 sutures. Watertightness testing is performed with a nonantibiot- ic saline-filled tuberculin syringe and 18-gauge catheter. After the saline is injected for the watertightness test, it should be removed completely. Additional sutures are placed at leakage locations, and after gently drying the cartilage surrounding the patch, the edges of the patch are sealed with fibrin glue (Tisseel, Baxter Healthcare Corp, Glendale, Calif) and a second watertightness test is per- formed as previously described. The chondrocytes are delivered and stored in vials that should remain upright at all times. Meticulous attention to sterile technique is paramount during this step, as the vial's exterior is not sterile. The vials are held in a vertical position without disturbing the pellet of cells in the bottom of the vial. An 18-gauge angiocatheter is inserted into the vial and advanced so the tip is submerged in the fluid but above the pellet of cells in the bottom of the vial to allow repetitive gentle aspirations and reinjections of the fluid to atraumatically suspend the chondrocytes (Figure 11). The total volume of the homogeneous suspension in the vial is then drawn into the syringe. A new sterile angiocatheter tip is used for the implantation step. To implant the cells into the prepared defect, the catheter is placed through an opening at the top of the periosteal patch and advanced to the distal end of the defect. The cells are slowly injected into the bed of the defect to ensure even dispersal while the catheter is slowly withdrawn. The opening is then closed with additional sutures and sealed with fibrin glue (Figure 12). Technical Considerations. Most defects are easily acces- sible on the weightbearing surface of the femoral condyle through a standard parapatellar arthrotomy. However, far posterior condylar lesions or focal cartilage defects of the tibial plateau may require additional strategies for expo- sure, including an open submeniscal approach or even en bloc osteotomy of the collateral ligaments. Traditionally, ACI has been applied to treat relatively shallow articular cartilage lesions with minimal involve- ment of the subchondral bone. For osteochondral defects of more than 8 to 10 mm in depth, bone grafting is recom- mended. The bone graft may be performed at the time of biopsy and the implantation delayed to allow for bone graft consolidation. Alternatively, the "sandwich tech- nique" has been used to graft and resurface the defect in a single step. A layer of periosteum is sealed against the grafted defect with the cambium layer facing outward toward the joint and fixed with 6-0 Vicryl sutures (Ethicon Inc) and fibrin glue. A second periosteal patch is placed with the cambium layer facing into the defect, creating a cambium-lined chamber overlying the bone graft. The chondrocytes are then injected between the 2 layers of periosteum. A complete description of the procedure is reported elsewhere.15 It is commonly believed that for all of these techniques, realignment osteotomy should be performed as an adjunct procedure if the lesion is in a compartment under more than physiological compression.42 Outcome data clearly indicate that poorer results are expected if mechanical axis or patellofemoral joint malalignment is left uncor- rected at the time of the cartilage restoration procedure.15 The rehabilitation protocol for ACI in the knee is based on the 3 phases of the natural maturation process of the chondrocytes.37,65,66 The proliferative phase occurs soon after the cells are implanted, followed by the matrix pro- duction phase, during which the tissue becomes incorpo- rated and integrated into the host. To assist cellular orien- tation and to prevent adhesions, early passive motion is crucial. The graft must be protected from mechanical over- load; closed chain strengthening exercises are initiated to allow for a functional gait. Continuous passive motion for 6 to 8 hours per day at 1 cycle/min and restricted weight- bearing are required until 4 to 6 weeks, when progression to full weightbearing is allowed. The third recovery phase is the maturation phase, which results in stiffness closely resembling the surrounding articular cartilage. During this extended phase, various impact loading activities are phased in with increased strength work. Return to normal activities of daily living and light sporting activity is con- sidered at 4 to 6 months. Outcomes of ACI in the Knee. It is estimated that ACI has been performed on 10 000 patients worldwide.15 Micheli et al63 reported on 50 patients who were followed for a minimum of 36 months and demonstrated a signifi- cant subjective improvement of 5 points on the modified Cincinnati scale measuring overall knee function (10-point scale). Eighty-four percent had an improvement in their conditions, 2% were unchanged, and 13% deteriorated. One third of these patients had failed a previous marrow- stimulation procedure. Peterson et al75 published their results on 94 patients with 2- to 9-year follow-up. The results varied considerably based on defect location. The results of ACI when treating the patella initially were only 62% good to excellent. However, later in the series, antero- medialization tibial tubercle osteotomies were performed simultaneously when treating patellar lesions, and results improved to 85% good or excellent. Twenty-four of the 25 isolated femoral condyle lesions were graded as having good to excellent results with a 92% success rate. In the OCD group, 16 of 18 patients were rated good to excellent, representing an 89% success rate. The majority of follow- up biopsies revealed objective evidence of hyaline-like tis- sue that demonstrated type II collagen on immunohisto-

TRAINING RATIONAL

Dynamic neuromuscular analysis training should address the neuromuscular imbalances present in the population to be trained. Special attention should be given to the female athlete who may display one or more neuromuscular imbalances. Dy- namic, multiplanar, sport-specific movements that are a challenge to the proprioceptive system are a required component of neuromuscular analysis training. The exercises selected must challenge the dynamic joint restraints (muscle-tendon units) that maintain limb and joint position in response to changing loads. Dynamic sport-specific training should pro- vide female athletes with an effective means for facilitating the desired adaptations to the proprioceptive function of the knee joint. The dynamic component progresses them to high- risk, sport-specific maneuvers that can be performed in a safe and controlled manner. Properly trained athletes are better pre- pared both to handle the high joint forces generated during athletic competition in order to reduce the risk of injury and to achieve peak performance. The neuromuscular component of this training is a balance between challenging an athlete's proprioceptive abilities and exposing her to movement patterns that generate greater dy- namic knee control. This proprioceptive stress may aid the development of protective spinal reflexes and multijoint neu- romuscular engrams that more effectively manage the ground reaction forces encountered during the midstance phase of high-risk maneuvers. However, assuming that the spinal stretch reflex occurs in 50 to 70 milliseconds, with an addi- tional time lag due to the electromechanical delay, an injury may occur before the reflex response begins to generate sig- nificant muscle force at initial contact.58 Neuromuscular train- ing that teaches athletes to better develop joint-stabilization patterns that employ feed-forward mechanisms (muscular preactivation patterns) may preset muscular contraction to in- crease knee stability at initial contact.59 This enhanced neu- romuscular control may help decrease joint motion and protect an athlete's ACL from high impulse loading.58,60 The analysis component of dynamic neuromuscular-analysis training involves exercises that provide instructors with the tools to analyze imperfections in technique. The training should focus on perfecting the technique of each training ex- ercise, especially early in the training cycle. If athletes are allowed to perform the exercise maneuvers improperly, then the training will reinforce improper techniques. Clinicians should give continuous and immediate feedback both during and after each exercise bout to make athletes aware of proper form and technique, as well as undesirable and potentially dan- gerous positions.57 Additionally, athletes should receive visual feedback via a video camera and television monitor or via exercise in front of a mirror to help make them cognizant of landings with visually identifiable poor biomechanics.56 Visual and verbal feedback helps athletes to match their perceived techniques to their actual techniques. Selected protocol times are general guidelines that provide an expected and attainable goal. Not every athlete will be at a level of muscular strength, coordination, skill, or desire to achieve the selected exercise duration. Clinicians should be skilled in recognizing the de- sired technique for a given exercise and should learn to en- courage athletes to maintain perfect technique for as long as possible. If an athlete fatigues such that she can no longer perform the exercise perfectly and displays a sharp decline in proficiency, then she should be instructed to stop. The duration of each completed exercise should be noted, and the goal of the next training session must be to continue to improve tech- nique and to increase volume (number of repetitions) or in- tensity (difficulty). The goal of increasing the quantity or in- tensity of exercises while maintaining the quality of exercises is critical in achieving successful outcomes from the training. Although prior research demonstrates the benefits of injuryprevention training among a wide variety of athletes, those who demonstrate neuromuscular imbalances as evidenced by poor dynamic knee stability might benefit the most from train- ing.23 The following discussion will address the theoretic basis for the design of a dynamic neuromuscular-analysis training program aimed at correcting specific neuromuscular imbalanc- es in female athletes. The goal of the discussion is to provide clinicians and coaches with specific methods to assess neuro- muscular imbalances and to subsequently provide training techniques targeted to correct an athlete's specific imbalances.

STUDIES OF INTERSEGMENTAL KINEMATICS/KINETICS OF THE KNEE

Measurements of anterior tibial displacement in an ACL- deficient knee during different rehabilitation exercises have been routinely used to infer the strain environment of the healing ACL graft (12). These techniques are based on the premise that the ACL is the primary restraint to anterior translation of the tibia. An increase in anterior tibial dis- placement in the ACL-deficient knee relative to the con- tralateral ACL-intact knee suggests that the ACL, or ACL graft, would be strained to a greater extent. Several studies have reported significantly greater anterior tibial translations in the ACL-deficient knee during OKC exercises when compared against CKC exercises. Jonsson et al. reported a 1.9-mm increase in the average anterior directed tibial displacement in the ACL-deficient knee (rel- ative to the ACL-intact knee) during the active knee exten- sion exercise (OKC) when the knee was near extension (15° to 10°), whereas no differences were found when the knee was extended during the step-up exercise (CKC) (11). Kvist and Gillquist compared anterior tibial translation during three squatting exercises (CKC) and active knee extension exercises against three different resistances (OKC) with those produced when a 90-N anterior-directed shear load was applied directly to the tibia relative to the immobilized thigh (i.e., a "Lachman" test) (12). In the ACL-injured knees, all exercises (except for active knee extension against the highest resistance (8 kg)) produced similar peak anterior tibial displace- ments that equaled those produced during the Lachman test. On average, the peak translations for all of the exercises oc- curred when the knee was near 20° of flexion. The OKC exercises with 8 kg of resistance produced displacements exceed- ing those produced by the Lachman test by 20% (12). Analytical models have also been developed to predict the intersegmental forces in the knee, and the forces generated in the ACL, when subjects perform OKC and CKC activities (6). Inverse dynamic models, which incorporate limb geom- etry, kinematics, and the externally applied forces (i.e., ground reaction force) as inputs, are used to predict the net intersegmental loads at the knee. To maintain dynamic equi- librium, the net intersegmental loads produced at the knee are balanced by the ligaments, contact surface geometry, and the musculature. Equilibrium models are then used to esti- mate the tibiofemoral compressive forces and cruciate liga- ment forces from the intersegmental resultant loads using EMG, and estimates of muscle, ligament, and contact surface geometry and properties. Using this approach, Escamilla et al. determined that the mean peak force on the ACL was 158 N during the OKC exercise when the knee was at 15° of flexion (leg extension against 78 kg), whereas it was not loaded during the CKC exercises (leg press and squatting against 146-kg weights) in experienced weight lifters (6). Analytical modeling provides an indirect and noninvasive means to predict the force on a healing ACL graft. Unfortunately, many assumptions are required when constructing the models. The complex geometry of the articular surface and soft tissue structures of the knee are generally ignored, and the interactions between the ligaments, bony geometry, and the menisci must be considered. An accurate forward dynamic model that incorporates the 3-dimensional mor- phometry of the knee with the appropriate representation of the ligament, menisci, and articular surface geometry and their material properties or a direct measurement approach is needed to accurately establish the loading environment on the ACL graft.

Phase 1. Proliferation Phase

The first phase of cartilage healing requires protec- tion of the repair and typically involves the first 4 to 6 weeks following surgery.4,5,13,20,22,39,42,43 During this phase, the initial healing process begins and it is imperative to decrease swelling, gradually restore PROM and weight bearing, and enhance volitional Gradual PROM and controlled partial weight bear- ing will help to nurture the cartilage through diffu- sion of synovial fluid, as well as provide the proper stimulus for the cells to produce specific matrix markers.2,6,7,21,59,60 Individuals begin with partial weight-bearing activities using crutches and progressive-loading exercises to gradually increase the amount of load applied to the weight-bearing sur- faces of the joint. The use of a pool or aquatic therapy may be beneficial for gait training and lower extremity exercises once the incisions are well healed. PROM exercises, performed by a rehabilitation specialist or CPM machines, are also performed immediately after surgery to nourish the healing articular cartilage and prevent the formation of adhesions.47,48,62 The use of a CPM typically begins 6 to 8 hours following surgery and is performed for at least 2 to 3 weeks, with recommended use up to 6 to 8 weeks.44,47 A CPM should be used throughout the day for a total time of 6 to 8 hours.44,47 The patient is also instructed to perform active-assisted ROM fre- quently throughout the day. Patellar mobilization, soft tissue mobilization, and soft tissue flexibility exercises are also performed to minimize scar tissue formation and avoid loss of motion. Early strengthening exercises are performed to restore volitional quadriceps control and neuromuscular control, through the use of concomi- tant electrical stimulation.11,54 Exercises performed during this phase are limited based on the specific weight-bearing status of each patient. These typically include quadriceps sets, straight leg raises, and basic proprioception exercises such as weight shifting.

Debridement and Chondroplasty

The rehabilitation following an arthroscopic debridement or chondroplasty is fairly simple due to the nature and goal of the procedure to facilitate tissue healing rather than to create repair tissue. The surgical procedure is performed arthroscopically and involves the use of a mechanical shaver or burr that is used to clean out the degenerative tissue and abrade the subchondral bone, thus facilitating the bone marrow elements to exude out onto the subchondral bone, allowing growth factors and undif- ferentiated cells to produce fibrocartilagenous tissue. This fibrocartilagenous tissue has been shown to deteriorate over time due to the inferior mechanical properties in comparison to normal hyaline articular cartilage.8,9,28,36 Initial weight bearing is limited for the first 3 to 5 days using axillary crutches, although weight bearing is permitted as tolerated. PROM exercises are pro- gressed as tolerated immediately, with no postopera- tive limitations in motion. Full passive motion is typically achieved in 2 to 3 weeks. Non-weight- bearing exercises are performed initially, although weight-bearing strengthening exercises and bicycle riding are normally incorporated by the end of the first week. The patient is allowed to return to full functional activities and begin progressing to moderate-impact activities, such as light jogging and sports, beginning at week 4. The progression of impact loading may be delayed if significant degenerative changes are present within the knee or if symptoms of pain and/or effusion persist.

CONCLUSION

The rehabilitation process following articular carti- lage repair procedures is vital to the long-term success and functional outcome of these patients. The rehabilitation programs discussed are based on our current understanding of articular cartilage and the natural healing response observed following ar- ticular cartilage repair procedures. Rehabilitation is based on several key principles used to facilitate the repair process by creating a healing environment, 16. while avoiding deleterious forces that may overload exercise in strengthening thigh musculature after ante- rior cruciate ligament surgery. Phys Ther. 1988;68:660- 663. the healing tissue. It must also consider any concomi- tant surgery performed. The basic principles outlined in this paper may be applied and integrated as our understanding and clinical use of the next generation of procedures, such as collagen-covered ACI and matrix-induced ACI, evolve.

Phase 3. Remodeling Phase

This phase generally takes place from 3 to 6 months postoperatively.4,5,13,20,22,39,42,43 During this phase there is a continuous remodeling of tissue into a more organized structure4,5,13,20,22,39,42,43 that is increasing in strength and durability. As the tissue becomes firmer and integrated, it allows for more functional training activities to be performed. At this point, the patient typically notes improvement of symptoms and has normal ROM. The patient is encouraged to continue with the rehabilitation pro- gram independently to maximize strength and flex- ibility. Low- to moderate-impact activities, such as bicycle riding, golfing, and recreational walking, may often be gradually incorporated.

Phase 2. Transition Phase

This phase typically consists of weeks 4 through 12 postsurgery.4,5,13,20,22,39,42,43 The repair tissue at this point is gaining strength, which will allow for the progression of rehabilitation exercises. During this phase the patient progresses from partial to full weight bearing while full ROM and soft tissue flexibil- ity is achieved. Continued maturation of the repair tissue is fostered through higher level functional and motion exercises. It is during this phase that patients typically resume most normal activities of daily living. The rehabilitation program will gradually progress strengthening activities to include machine weights and weight-bearing exercises, such as leg press, front lunges, wall slides, and lateral step-ups, as the pa- tient's weight-bearing status returns to normal. The progression of weight-bearing activities and ROM restoration involves the gradual advancement of activities to facilitate healing and avoid postsurgical complications. Common complications include mo- tion restrictions and scar tissue formation. Further more, an overaggressive approach early within the rehabilitation program may result in increased pain, inflammation, or effusion, as well as graft damage. Progression is controlled for strengthening exercises, proprioception training, neuromuscular control drills, and functional drills. For example, exercises such as weight shifts and lunges are progressed from straight- plane anterior-posterior or medial-lateral directions to involve multiplane and rotational movements. Exer- cises using 2 lower extremities, such as leg press and balance activities, are progressed to single lower extremity exercises. Thus the postoperative rehabilita- tion program involves a gradual progression of ap- plied and functional stresses to provide a healthy stimulus for healing tissues without causing damage.

Fleming BC, Oksendahl H, Beynnon BD. Open- or closed-kinetic chain exercises after anterior cruciate ligament reconstruction?. Exerc. Sport Sci. Review 2005:33;134- 140.

Traditionally, most post surgical rehabilitation programs for ACL reconstruction have incorporated closed kinetic chain (CKC) exercises and avoided open kinetic chain (OKC) exercises. The belief behind this practice is that open kinetic chain exercises pose an excessive amount of strain on the healing graft while closed kinetic chain exercises are much safer and protect of the healing graft. In the article, open kinetic chain exercises are defined as "those in which the foot is not in contact with a solid surface." Closed kinetic chain exercises are defined as "those in which the foot is in contact with a solid surface." Though it is important to avoid excess strain the healing tissue, it is important to allow some level of strain in order to promote healing. In a study conducted by Beynnon et al. (1998), the findings revealed that there is no difference in the maximum ACL strain produced during active knee extension (OKC exercise) and simple squats (CKC exercise). However, it was found that there was a significantly greater amount of strain produced during the OKC exercise when resistance was increased, compared to the strain produced during the CKC exercise. The optimal parameters of ACL strain that maximize the healing effect while minimizing injury to the tissue is unknown. The amount of strain transmitted to the healing ACL graft during isometric exercises is dependent upon the angle of knee flexion and the extension torques. A hallmark study conducted by Mikkelsen et al. (2000) compared the outcomes patients status post ACL reconstruction performing only CKC exercises to a group that performed a combination of CKC and OKC exercises. The study revealed that the group performing a combination of CKC and OKC exercises demonstrated higher quadriceps torque, and a greater proportion of the patients returned to sport at preinjury level. Hooper and Morrissey (2001) conducted a study comparing a CKC exercise based rehabilitation program and an OKC exercise based rehabilitation program and found no difference in knee laxity values and no clinically significant differences in functional improvement. This article demonstrated that though there may be increased strain on the healing ACL with the use of OKC exercises with increased resistance, there is not a significant difference when lower resistance is used. This important finding, in combination with the improved outcome shown with the implementation of a program combining both OKC and CKC exercises, suggests that it is important to include OKC exercises in our rehab protocols status post ACL reconstruction. Caution should be taken to ensure that resistance is not high enough to impose excess strain when using OKC exercises, and discretion should be used in implementing OKC exercises in the early stages of rehab.

preparation, graft suturing, and cell implantation require subluxation or eversion of the patella and should be per- formed before establishing rigid fixation of the tubercle osteotomy distally. Ligamentous Instability. Uncorrected ligamentous insta- bility is a contraindication to cartilage restoration proce- dures. Methods of ligament reconstruction are well estab- lished and will not be reviewed here. It is important to carefully plan a sequence of treatment options for each patient and to consider staging procedures if needed. If a single stage will incorporate ligament reconstruction with cartilage restoration or meniscus transplant, a preopera- tive plan that allows for a stepwise incorporation of both procedures is crucial. The type of concomitant procedure may affect the graft selection for ACL reconstruction. For example, when performing an ACL reconstruction in the setting of an ACI, periosteal patch harvest would occur before hamstring harvest or tibial drilling. If treating patellar defect with ACI and distal realignment, ham- string autograft or an allograft graft source would be required, as the osteotomized patellar tendon insertion would be unavailable as autograft. If an opening medial high tibial osteotomy is required in association with an ACL reconstruction, the osteotomy can be tailored to reduce the posterior slope of the tibial plateau, which will protect the ACL reconstruction and increase anterior stability. Medialization of the tibial tun- nel by rotation of the guide will limit the possibility of inadvertent communication with the osteotomy. In the set- ting of a high tibial osteotomy, a hamstring graft for ACL reconstruction may be considered, as it may allow for smaller tunnel diameter, thereby reducing the risk of com- municating with the osteotomy. Whichever graft is selected, interference screws for tibial fixation should be used with caution, as they will create hoop stresses in the tibial tun- nel aperture near the osteotomy. If a meniscus transplant is combined with either pri- mary or revision ACL, there are several issues to consider related to the 3-dimensional relationship of tunnels in the tibial metaphysis. Prior tunnel expansion and position, intended locations of new tunnels, ACL graft selection, and meniscus anchor method offer variability to address the needs of each particular patient. For example, the ideal entry of the ACL tunnel is at the level of the posterior edge of the anterior horn of the lateral meniscus; if performing a concomitant ACL reconstruction and lateral meniscal transplant, a slot and bone bridge technique will allow for adequate fixation even if the bone bridge is partially vio- lated by the tibial tunnel,21 and a hamstring ACL graft will allow for narrower tunnels in the tibia. If necessary, procedures should be staged and bone graft applied to metaphysial defects. A guideline of 2 procedures per operation and staging subsequent procedures should be followed. Generally, osteotomies should be performed first, with a 6-month interval between procedures to allow for complete bone healing and remodeling. Subsequent hardware removal should be planned, and a strategy for dealing with resulting stress risers from residual empty screw holes should be devised. FUTURE DIRECTIONS

Current research efforts are paving the way for future advances to improve results and further limit the morbid- ity associated with cartilage restoration. Advances in engi- neered matrix tissue scaffolds and bioadhesives40 have combined with greater controlled manipulation of gene- modified tissues.39 A greater understanding of the scope of pluripotential cells' differentiation and the most efficient pulse sequence of growth factor manipulation with bone morphogenic proteins82 will enhance our ability to process and expand tissue ex vivo in tissue-loading bioreactors. Advancing toward a common goal of improving the clinical outcomes of cartilage restoration will require a coordinat- ed comprehensive approach incorporating the basic sci- ence of genetic engineering and biochemistry with out- come analyses and surgical decision making and tech- niques developed by clinicians.

REHABILITATION AFTER ACL RECONSTRUCTION

Although most clinicians would agree that the strains applied to an ACL graft by body weight, muscle activity, and joint motion affect its healing response, there is little consensus on how these factors influence the biomechani- cal behavior of the healing graft and, in turn, how this behavior modulates the healing response of the graft, car- tilage, and knee. Our review of the literature did not identify a consensus regarding which variables should be used to characterize a rehabilitation program, nor did it reveal how this infor- mation should be used to compare different programs. Although it is clear that rehabilitation after ACL recon- struction includes characteristics such as the series of activities (or restrictions) that a subject is directed to per- form, the time when the activities are recommended, the duration of the activities (eg, the number or sets and rep- etitions per day, week, month), the overall length of the program, and the time when a subject is advised to return to sport-specific training and subsequently to sport, it is unclear how a single term such as "accelerated rehabilita- tion" can be used to provide insight into these characteris- tics. As a consequence, there is little consensus in the lit- erature about what composes an accelerated versus a more conservative rehabilitation program or an aggressive versus a nonaggressive approach to rehabilitation. Our review identified a substantial number of RCTs that have focused on rehabilitation after ACL reconstruction, but very few of these reports described the rehabilitation pro- tocol adequately or provided details with regard to subject compliance. These concerns made it difficult to arrive at a consensus with regard to optimal rehabilitation programs after ACL reconstruction. Our review of RCTs of rehabilitation programs after ACL reconstruction was categorized according to the use of the following approaches: cold therapy, immediate versus delayed motion, immediate versus delayed weightbearing, closed versus open kinetic chain exercises, bracing, home- versus clinic-based rehabilitation, neuromuscular electri- cal stimulation versus voluntary muscle contraction, spe- cific exercise programs, and intensity and duration of rehabilitation.

Individualization

An individualized approach for each patient is one of the most important principles involving the reha- bilitation following articular cartilage repair proce- dures. There are several variables to consider when developing the unique rehabilitation progression for each patient. These include specifics regarding the patient, lesion, and surgery (Table 1). The quality of each individual's articular cartilage is the result of several factors, including age, body mass index, general health, nutrition, and history of previous injuries. The composition of articular carti- lage undergoes a gradual degeneration that results in a breakdown of tissue matrix and a reduction in the load-bearing capacity of the cartilage.10 The specific factors that contribute to this deterioration remain controversial, but it appears that age, obesity, poor nutrition, joint malalignment (such as a varus knee), history of injury (such as ligamentous or meniscal injury), and a history of repetitive-impact loading (through work or sport activities) may result in osteoarthritic changes.10 Thus, younger patients with isolated defects and relatively healthy surrounding articular cartilage oftentimes are able to progress their rehabilitation more rapidly than older individu- als with more degenerative changes and less dense cartilage structure. Furthermore, the patient's motiva- tion and previous activity levels must be considered when determining the rehabilitation approach to assure that the goals of each patient are addressed. The rehabilitation program should be individualized to the specific demands of each patient's activities of daily living, work, and/or sport activities. There are also several variables to consider in regard to the lesion that may have a dramatic effect on the rehabilitation process. Most importantly is the exact location of the lesion. Rehabilitation of lesions on a weight-bearing surface of a femoral condyle must attempt to avoid deleterious compressive forces and require a different approach than for lesions located within the trochlea or retrosurface of the patella, where excessive shear forces should be mini- mized. The size, depth, and containment of each lesion must also be considered. Lesions that are large, deep, or poorly contained within healthy surrounding articular cartilage may require a slower rehabilitation progression than smaller, shallower, or well-contained lesions. Lastly, the specifics of each surgical procedure will also influence the rehabilitation process. It is the authors' opinion that arthroscopic procedures, such as chondroplasty or microfracture, may progress at a different pace than procedures with larger incisions and greater tissue involvement, such as OATS or ACI, which require a slower rehabilitation process to protect the healing structures. Each specific surgical procedure also has different biological healing re- sponses postoperatively. Finally, any concomitant procedures to address alignment, stability, or meniscal function may also alter the rehabilitation program because of the need to protect other healing tissues.

Treatment of Anterior Cruciate Ligament Injuries, Part 2

Anterior cruciate ligament tears are common among ath- letes, and although their true natural history remains unclear, these injuries are functionally disabling; they pre- dispose the knee to subsequent injuries and the early onset of osteoarthritis. This article, the second of a 2-part series, was initiated with the use of the PubMed database (http://www.nlm.nih.gov) and a comprehensive search of publications that appeared between January 1994 and the present, with anterior cruciate ligament as keywords. A total of 3810 citations were identified and reviewed to determine the current state of knowledge regarding the treatment of ACL injuries. Part 1 focused on studies that pertained to the biomechanical behavior of the ACL, the prevalence of and risk factors for ACL injuries, the natural history of the ACL-deficient knee, injuries associated with ACL disruption, indications for treatment of ACL injuries, and nonoperative and operative treatments. Part 2 includes technical aspects of ACL surgery, bone tunnel widening, graft healing, rehabilitation after ACL recon- struction, and the effects of sex, age, and activity level on the outcome of ACL surgery. Our approach was to build on prior outstanding reviews41,59,102 and to provide an overview of the literature for each of the aforementioned areas of study by summarizing the highest level of scien- tific evidence available. For the areas that required a descriptive approach to research, we focused on the prospective studies that were available; for the areas that required an experimental approach, we focused on the prospective, randomized, controlled trials (RCTs) and when otherwise necessary, the highest level of evidence available on the subjects presented. As with part 1, we were pleasantly surprised to learn that considerable advances were made during the past decade with regard to the treatment of this devastating injury.

SUMMARY

Articular cartilage damage is a problem that has posed a seemingly insurmountable challenge to the medical com- munity for centuries. Recent advancements have allowed aggressive repair in properly selected patients, often with combined procedures. Of critical importance are issues of careful patient selection, patient counseling, and preoper- ative planning, especially when combined procedures are planned.

DIFFERENCES IN ELECTROMYOGRAPHIC (EMG) ACTIVATION LEVELS BETWEEN MALE AND FEMALE ATHLETES Proximal

Asymmetry of proximal muscle activation may alter the position of the knee during landing and cutting in female athletes. Decreased activation of the trunk and hip muscu- lature may lead to lower extremity malalignment. Decreased activation of proximal stabilising muscles may lower load bearing capacity of the knee joint. Lephart et al16 report that female subjects have increased hip internal rotation during landing. Increased hip internal rotation and valgus may increase strain on the ACL.2 34 Zazulak et al8 report lower gluteal EMG activity in women than men during landing (figs 1 and 2). The proximal stabilising muscles, specifically the gluteals, control lower limb position, energy absorption, and function as powerful extensors, external rotators, and abductors of the hip during landing.8 35 Chimera et al36 evaluated the effects of plyometric training on muscle activation patterns during jump exercise and reported increased firing of the hip adductor muscles during the pre-landing phase. The experimental group showed greater preparatory adductor to abductor muscle activation. Hewett et al31 showed significant decreases in abduction/ adduction moments after plyometric training. These findings delineate the role of hip muscle activation in dynamic restraint and control of lower extremity alignment.

IDENTIFICATION OF QUADRICEPS DOMINANCE

Athletes can be screened for indicators of quadriceps dom- inance using relatively common measurement techniques such as isokinetic dynamometry or perhaps even simple leg-curl and leg-extension machines. Huston and Wojtys24 demonstrat- ed that female athletes had increased quadriceps dominance compared with males and nonathletic controls. If an athlete exhibits a high level of quadriceps strength, a low level of hamstring strength, or a low hamstring-to-quadriceps ratio in one or both limbs, quadriceps dominance may be present. Hamstring-to-quadriceps strength ratios of less than 55% may indicate a quadriceps-dominant athlete who may demonstrate inappropriate or decreased hamstring recruitment patterns dur- ing dynamic tasks.68 If objective measures are not available, then an athlete can be tested on her ability to hop and hold a single-leg stance in deep knee flexion. To maintain upright posture with deep knee flexion (90) requires relatively more recruitment of hamstrings than quadriceps.69 Inability to main- tain stance with deep knee flexion may indicate some level of quadriceps dominance.

RISK FACTORS FOR ACL INJURY

Because of the high morbidity and loss of function associ- ated with ACL trauma, many studies have been performed on the diagnosis and treatment of this complex injury; however, very little is known about the risk factors that predispose a person to an ACL tear. An understanding of ACL injury risk factors is useful for the development of intervention strategies and for identifying who is at increased risk of sustaining this injury, thus potentially allowing an intervention to be targeted at specific subjects rather than the entire population at risk. Orchard and colleagues253 studied athletes participating in Australian football and found that a history of ACL reconstruction and weather conditions that were charac- terized by high evaporation and low rainfall before matches were risk factors for repeated ACL injury.225 In a study of American football, Lambson et al152 found that athletes with a greater number of cleats and an associated higher torsional resistance at the foot-turf interface were at increased risk for sustaining an ACL tear. In a study of high school athletes, Souryal and Freeman270 reported that athletes with a small intercondylar notch width index (the ratio of the width of the anterior outlet of the inter- condylar femoral notch divided by the total condylar width at the level of the popliteal groove) were at significantly increased risk for sustaining an ACL injury. LaPrade and Burnett153 studied collegiate athletes and reported similar findings. Uhorchak et al285 performed a comprehensive study of military academy cadets and reported that several risk factors predisposed them to noncontact ACL injury. Significant risk factors included a small femoral notch width, generalized joint laxity, and, in women, higher than normal body mass index and KT-2000 arthrometer meas- urements of A-P knee laxity that were 1 standard devia- tion or more above the mean.285 In contrast, Lombardo et al169 studied professional male basketball players and reported that intercondylar femoral notch width could not be used to identify an athlete at risk for an ACL tear. Undoubtedly, the risk factors that predispose an athlete to an ACL tear are multifactorial and differ between male and female athletes. For example, after adjusting for body weight, the ACL is smaller in females compared with males, and therefore, if females and males generate simi- lar intersegmental forces across the knee during an at-risk activity, not only would the stress distribution in the ACL be greater for females, but it may also be closer to the ulti- mate failure stress.11 Several studies have been performed to determine if ACL injuries in females occur randomly or correlate with a specific phase of the menstrual cycle; however, there is little consensus.266,295,296 Three studies reported the risk of sustaining a noncontact ACL injury is greater at or near the onset of menstruation,194,200,266 2 studies found a sig- nificantly larger proportion of injuries occurred during the ovulatory phase of the menstrual cycle,295,296 and 1 reported that the least number of injuries occurred during ovu- lation.20 Our review of the literature revealed that only the work of Uhorchak et al285 has investigated multiple risk factors for ACL injury. The other studies have either focused on isolated potential risk factors (eg, anatomical, biomechan- ical, neuromuscular, environmental, or hormonal risk fac- tors) or have not included females (the group that appears to be at increased risk for this injury). Very little is known about whether risk factors act in combination to increase an athlete's risk of sustaining a knee ligament injury, which is important because it may be that each con- tributes in a unique manner to increase the risk of injury. It also remains unclear whether risk factors intrinsic to an athlete are different between male and female athletes and whether they are dependent on the type of sport in which an athlete participates.

TREATMENT OF LIGAMENT DOMINANCE

Before being taught the dynamic exercises, athletes should be shown proper athletic position (Figure 2). The athletic po- sition is a functionally stable position with the knees com- fortably flexed, shoulders back, eyes up, feet approximately shoulderwidth apart, and body mass balanced over the balls of the feet. The knees should be over the balls of the feet and the chest over the knees.22 This is the athlete-ready position and should be the starting and finishing position for most train- ing exercises. The wall-jump exercise can be used initially to treat liga- ment dominance (Figure 3). This low- to moderate-intensity jump allows clinicians to begin analyzing an athlete's level of ligament dominance or valgus-varus motion in the knee. Dur- ing wall jumps, athletes do not go through deep knee-flexion angles; most of the vertical movement is provided by active ankle plantar flexion. The relatively straight knee makes even slight amounts of medial knee motion easy to identify visually. When medial knee motion is observed, clinicians should begin to give verbal feedback cues (eg, ''keep knees square,'' ''no inward knee motion,'' ''keep knees aligned straight,'' ''keep knees apart and use your knee like a hinge, not a ball and socket''). This feedback allows athletes to cognitively process the proper knee motion required to perform the exercise; ath- letes also find this feedback easier to process because the ex- ercise does not require maximal effort. Finally, control of me- dial knee motion is critical when landing with knee angles close to full extension. Direct ACL load is high during forceful quadriceps contraction with dynamic valgus positioning at small knee-flexion angles.63 Thus, the initial technical goal for a ligament-dominant athlete is to keep the knees apart when landing and to err on the side of varus knee positioning, which produces decreased ACL load in the knee-flexion angles used in this jump.63 Another useful exercise to target ligament-dominant athletes is the tuck jump (Figure 4). The tuck jump is on the opposite end of the intensity spectrum from the wall jump and requires a high level of effort. Initially, athletes focus most of their cognitive efforts solely on the performance of this difficult jump. Clinicians can quickly identify those athletes who may demonstrate abnormal levels of coronal-plane knee displace- ment during jumping and landing, because such an athlete usu- ally devotes minimal attention to technique in the first few repetitions. In addition, tuck jumps can be used to assess im- provements in lower extremity biomechanics. If an athlete can improve her neuromuscular control and biomechanics during this difficult jumping and landing sequence, then she is gaining dynamic neuromuscular control of the knee joint and may be- gin to develop a learned skill that might be transferred to com- petitive play. The wall jump and the tuck jump provide clinicians with an analysis of an athlete's movements primarily in the coronal plane. The broad jump and hold (Figure 5) allows clinicians to assess an athlete's knee motion while progressing forward in the sagittal plane. Gaining dynamic knee control while per- forming multiplanar tasks is critical to changing biomechanics that can transfer to on-the-field play. Some athletes may dis- play active valgus, a position of hip adduction and knee ab- duction, which is a result of muscular contraction rather than ground reaction forces. Active valgus occurs when taking off from a jump rather than landing and should be corrected dur- ing training. The broad jump is a moderate-intensity exercise that offers clinicians another opportunity to provide feedback on technique and to assist athletes to perfect their technique during each jump. In addition, proper execution of the broad jump requires athletes to hold the landing for 3 to 5 seconds, which forces them to gain and maintain dynamic knee control over a more prolonged period of time. By stabilizing her knees over a prolonged period of time, an athlete learns to recognize proper foot positioning and knee control, thereby enhancing proprioceptive and kinesthetic abilities. Recognition of proper positioning and technique is a primary goal of training; how ever, more reflexive motor patterns are desired for the ultimate success of neuromuscular reprogramming. The 180 jump (Figure 6) can be incorporated into training to teach dynamic body and lower extremity control in the transverse plane. The 180 jump creates rotational force in the transverse plane, which must be absorbed and immediately redirected in the opposite direction. This movement is impor- tant for teaching an athlete to recognize and control dangerous rotational forces, which if combined with knee abduction or anterior load, produce additive ACL loads.63 '' Additionally, in- creased body awareness and control can improve movement patterns, which will reduce injury risk and also improve mea- sures of performance.22,62 Once an athlete has learned to jump, land, and hold the broad jump in bipedal stance, the single-leg hop-and-hold ex- ercise (Figure 7) can be incorporated into the training. Proper progression into the single-leg hop and hold is critical to en- sure athlete safety during training. This point is salient for clinicians, because ACL injury-prevention techniques should not introduce the inappropriate risk for injury during training. Most noncontact ACL injuries occur when landing or decel- erating on a single limb.41 The single-leg hop-and-hold exer- cise roughly mimics a mechanism of ACL injury during com- petitive play. When initiating the single-leg hop-and-hold, an athlete should be instructed to jump only a few inches and to land with deep knee flexion. As she masters the low-intensity jumps, the distance can be increased, as long as she can con- tinue to maintain deep knee flexion when landing and control the coronal-plane motion at the knee. Athletes must be given continuous feedback (with cues to ''use deep knee flexion,'' ''sit down deep,'' ''soft, silent landings,'' ''light as a feather,'' ''knee square,'' ''knee bent'') on both her medial-lateral knee motion control and her ability to absorb the landing in an appropriate manner through ankle, knee, and hip flexion. The final level of training used to target ligament dominance is unanticipated cutting movements. Before being taught un- anticipated cutting, athletes should first be able to proficiently attain the proper athletic position (see Figure 2). This athlete- ready position is the goal position to achieve before initiating a directional cut. Adding the directional cues to the unantici- pated part of training can be as simple as pointing or as sport specific as using partner-mimic or ball-retrieval drills. Single-faceted sagittal-plane training and conditioning pro- tocols that do not incorporate cutting maneuvers will not pro- vide levels of external varus-valgus or rotational loads similar to those seen during sport-specific cutting maneuvers.64 Train- ing programs that incorporate safe levels of varus-valgus stress may induce more muscle-dominant neuromuscular adapta- tions.61 Such adaptations can better prepare an athlete for more multidirectional sport activities, which can improve perfor- mance and reduce the risk of lower extremity injury.22,52,65 Female athletes perform cutting techniques with decreased knee flexion and increased valgus angles.39 Knee-valgus loads can double when an athlete performs unanticipated cutting ma- neuvers similar to those used in sport.66 Thus, the endpoint of training designed to reduce ACL loading via valgus torques can be achieved by teaching athletes to use movement tech- niques that produce the low abduction knee-joint moments.61 Recent evidence demonstrates that training incorporating un- anticipated movements can reduce knee-joint loads.62 Addi-tionally, training individuals to preactivate their musculature before ground contact may facilitate appropriate kinematic ad- justments, and ACL loads may be reduced.66,67 Training an athlete to employ safe cutting techniques in unanticipated sport situations may instill technique adaptations that will more readily transfer onto the field of play. A ligament-dominant athlete may become muscle dominant, reducing her future risk of ACL injury.22,52

DIFFERENCES IN ELECTROMYOGRAPHIC (EMG) ACTIVATION LEVELS BETWEEN MALE AND FEMALE ATHLETES distal

Calf and ankle muscle recruitment may play a role in dynamic stabilisation of the knee joint. Besier et al10 reported selective activation of medial knee musculature, including the medial gastrocnemius, during sidestep tasks with valgus and external rotation moments. Nyland et al43 concluded that the gastrocnemius provided synergistic and compensatory dynamic knee stabilisation with quadriceps fatigue. ACL deficient female subjects showed decreased preactivation of the lateral gastrocnemius.44 Shultz et al45 evaluated the protective neuromuscular response and activation patterns to an imposed perturbation during weight bearing stance—that is, a sudden forward and either internal or external rotation moment of the trunk and femur. The gastrocnemius fired faster than the hamstring, which fired faster than the quadriceps. This activation pattern is similar to the postural response reported by Nashner,46 specifically a distal to proximal firing pattern, with the distal muscles preceding the proximal muscles by 10-15 milliseconds.

TREATMENT OPTIONS OVERVIEW

Careful patient evaluation is essential in selecting the proper treatment plan. It is important to identify both the characteristics of the cartilage lesion and associated comorbidities. Untreated mechanical malalignment, liga- mentous laxity, and deficient menisci are contraindica- tions to articular cartilage restoration. Whether corrected in a staged or concomitant fashion, a comprehensive plan to address each feature of the patient's joint abnormality must be devised and discussed at length with the patient before proceeding. In the knee, ligament reconstruction, corrective osteotomies, or meniscal transplants are fre- quently required in addition to the articular cartilage resurfacing procedure chosen to provide a symbiosis of 2 or more mutually beneficial procedures. It is important to avoid "linear reasoning" while evalu- ating a particular patient; for a specific patient at a par- ticular point in time, there may be several viable treat- ment plans. A central tenet of cartilage restoration is that each treatment must allow for further treatments should they prove necessary. This paradigm of not "burning bridges" is especially important in the relatively young population, who often require more than one procedure. We conceptualize treatment options in categories of clin- ical utility with considerable overlap depending on the clinical scenario (Figure 2). These categories range from those considered palliative (debridement/lavage), intended to reduce mechanical irritation and inflammatory media- tors; to reparative (marrow stimulation techniques, ie, microfracture), designed to recruit pluripotential cells from marrow stromal cells to proliferate fibrocartilage repair tissue; to restorative (osteochondral grafting), designed to replace articular cartilage and subchondral bone as a single unit. Autologous chondrocyte implanta- tion crosses the biologic boundary between reparative and restorative options. The goal of each treatment option is to provide the patient with the greatest chance for symptom reduction and a return to a productive level of function, while allowing for future treatment options, should they become necessary.

Team Communication

Communication between the surgeon and therapist is essential to determine the accurate rate of progres- sion based on the location of the lesion, size of the lesion, tissue quality of the patient, and the addition of concomitant surgical procedures. Also, communi- cation between the medical team and patient is essential to provide the patient with education re- garding the avoidance of deleterious forces, as well as improving the patient's compliance with precautions. Often times a preoperative physical therapy evalua- tion may be useful to mentally and physically prepare the patient for the articular cartilage procedure and postoperative rehabilitatio

SUMMARY AND CONCLUSIONS

Differences are observed in male and female EMG firing patterns. Decreased neuromuscular control of the trunk and lower extremity in women may increase the potential for valgus lower extremity position and increased ACL injury risk. Identification of these neuromuscular imbalances has potential for both screening of high risk athletes and targeting interventions to specific deficits. Dynamic neuro- muscular training can increase active knee stabilisation and decrease the incidence of ACL injury in the female athletic population.26 31 50 51 Training may facilitate neuromuscular adaptations that provide increased joint stabilisation and muscular preactivation and reactive patterns that protect the athlete's ACL from increased loading.50 52 53 In conclusion, there is evidence that neuromuscular training alters muscle firing patterns as it decreases landing forces, improves balance, and reduces ACL injury incidence in female athletes. Future approaches could be to use EMG analysis to assess the relative efficacy of these interventions in order to achieve the optimal effect in the most efficient manner possible. Selective combination of neuromuscular training components may provide additive effects, further reducing the risk of ACL injuries in female athletes. Additional research directions include the assessment of relative injury risk using mass neuromuscular screening. The development of screening and intervention protocols may lead to the reduction of ACL injury incidence in female athletes through the identification of the high risk female athlete subgroup that displays decreased hip and increased quadriceps muscle firing, and the correction of these neuromuscular control deficits.

Restoring Muscle Function

Due to the inhibition of the quadriceps muscle secondary to pain and effusion electrical muscle stimulation and biofeedback are often incorporated with therapeutic exercises to facilitate the active contraction of the quadriceps musculature in the acute stage of rehabilitation (Figure 5). The use of electrical stimulation and biofeedback appears to facilitate the return of muscle activation.11,54 Clini- cally, we use electrical stimulation immediately follow- ing surgery while performing isometric and isotonic exercises such as quadriceps sets, straight leg raises, hip adduction and abduction, and knee extensions (Figure 5). Electrical stimulation is used when the patient presents acutely with the inability to activate the quadriceps in an attempt to recruit a maximum amount of muscle fibers during active contraction and may also be used throughout the rehabilitation process. Once independent muscle activation is present, biofeedback may be utilized to facilitate further neuromuscular activation of the quadriceps. Exercises that strengthen the entire lower extrem- ity, such as machine weights and weight-bearing exercises, may be included as the patient progresses to more advanced phases of rehabilitation. It is important not to concentrate solely on the quadriceps. Furthermore, the importance of incorpo- rating core stability exercises cannot be overlooked. Training of the trunk, hip, and ankle musculature is emphasized to assist in controlling the production and dissipation of forces in the knee.

TECHNIQUES OF ACL RECONSTRUCTION

During the past 30 years, there has been an extensive evo- lution of the surgical procedures for repair or reconstruc- tion of the ACL. There was considerable controversy about the functional status of the ACL until the 1970s. Thus, simply excising the disrupted ACL was common before that time. In the 1970s, a trend toward repair of the ACL was started by surgeons who thought the ACL performed an important function.14,15,219,247 These procedures were not universally successful.220,221,222 Repair of the ACL with augmentation using autografts was supplanted in the 1970s and early 1980s by the development of ACL recon- structions completely replacing the torn ligament with autografts and occasionally allografts.14,15,219 Until the development of arthroscopic techniques, the procedures were performed by arthrotomy.

TIMING OF MUSCLE FIRING

EMG studies show sex related differences in the timing of muscle activation during athletic movement.6 8-10 37 47 48 Zazulak et al8 reported increased peak quadriceps activity in female subjects during the pre-contact phase of landing. Greater rectus femoris activity was observed in female than male subjects (fig 3). This increased activation of rectus femoris may increase strain on the ACL during landing. Increased quadriceps activity combined with decreased hamstring activity may decrease kinetic energy absorption during landing and may increase ground reaction forces and torques associated with ACL injury. Wojtys et al37 reported that female athletes have a slower response of hamstring activation to anterior stress on the ACL. Cowling and Steele48 reported sex differences in muscle activation strategies in the hamstrings musculature that contradict the findings of Wojtys et al. Male subjects were found to activate their semi-membranosis muscle later than female subjects in the pre-landing phase and reach peak activity sooner.48 Besier et al10 examined a sidestep cut at two different angles under both preplanned and unanticipated conditions. They found increased varus/valgus and internal/ external knee moments during unanticipated movements and suggested that there may be increased potential for non- contact knee injuries during unanticipated sport movements. Lower extremity muscle activation during cutting may be different between preplanned and unanticipated conditions.47 Besier et al47 also reported that activation patterns during cutting manoeuvres are preplanned to counter loading in response to varus/valgus and internal/external rotation moments at the knee. The unanticipated sidestep condition was reported to increase muscle activation 10-25%, with the greatest increase before initial contact.47 ACL injuries occur too quickly for reflexive or voluntary muscular activation. However, preactivation may reduce the probability of injuries caused by unexpected perturbations. The lower extremity musculature may be 40-80% activated at the time that the foot touches the ground.49

Arthroscopic Lavage and Debridement

Efforts to debride friable inflammatory tissue began 6 decades ago when Magnusson73 popularized this as a method of reducing mechanical symptoms. Without debridement, arthroscopic joint lavage alone provides short-term benefits in 50% to 70% of patients.11 When combined with lavage and debridement of friable tissue, marrow stimulation appears to improve results and pro- vide a more durable outcome.52,56,81 Arthroscopic debride- ment and lavage alone have shown to have no significant lasting benefit in arthritic knees without specific localized mechanical symptoms,86 but in carefully selected patients with a specific history of low-energy trauma, mechanical symptoms, minimal malalignment, stable ligaments, and low body mass index, arthroscopic debridement may be of some use.49 In 1987, Rudd et al110 completed a canine model investi- gating humeral chondral defects prepared with and with- out beveling of the margins of focal chondral lesions at 16 weeks after defect creation. The authors identified a greater number of defects with beveled edges that pro- gressed, compared to those created with vertical, "well- shouldered" margins. In addition, chondral damage to the glenoid surface occurred more frequently opposite beveled defects compared to those opposing defects with vertical walls.110

CONCLUSIONS

Female athletes may demonstrate one or more neuromus- cular imbalances of ligament dominance, quadriceps domi- nance, and leg dominance. Dynamic neuromuscular analysis training provides a method to specifically address and correct these neuromuscular imbalances. Correction of neuromusculaimbalances is important for both optimal biomechanics of ath- letic movements and reduction in knee injuries. Objective, standardized jump-landing risk assessments that can be quick- ly and easily administered by clinicians should be encouraged for preseason screening of potentially at-risk female athletes. Further study on the effects of neuromuscular retraining on biomechanical performance and knee-injury incidence is im- portant to advance injury-prevention programs and women's athletics. Although significant strides have been made, contin- ued advancements in sports injury-risk screening, including the incorporation of high-risk sports movements and matura- tional assessment into the screening tests, are necessary. Fur- thermore, research is warranted to determine the most effective timing for interventional training in young female athletes, with the goals of maximizing efficiency and precluding them from high-risk competitive seasons.

DIFFERENCES IN ELECTROMYOGRAPHIC (EMG) ACTIVATION LEVELS BETWEEN MALE AND FEMALE ATHLETES anterior/posterior

Female athletes show increased activation of the quadriceps relative to the antagonistic hamstring musculature.14 31 37 This disproportional recruitment of the vastus musculature increases anterior shear force at the low knee flexion angles that occur during high risk landing and pivoting movements.9 34 The quadriceps, through the anterior pull of the patellar tendon on the tibia, contribute to ACL loading when knee flexion is less than 30 ̊.34 38 Muscular co- contraction compresses the joint, due in part to the concavity of the medial tibial plateau, which may protect the ACL against anterior drawer.39 Zazulak et al8 reported greater peak quadriceps activity in female than male subjects (fig 3). Decreased balance in strength and recruitment of the flexor relative to the extensor musculature may put the ACL at greater risk.31 Adequate co- contraction of the knee flexors is needed to balance contraction of the quadriceps, compress the joint, and control high knee extension and abduction torques.31 Appropriate hamstrings recruitment may prevent the critical loading necessary to rupture the ACL during manoeuvres that place the athlete at risk of an injury.

TREATMENT ALGORITHM

For isolated focal chondral defects of articular cartilage, success or failure of first-line treatments guides future treatment options, all of which leave options for further restoration treatments or arthroplasty (Figure 18). The location of the lesion, assuming minimal bone loss, will determine other necessary evaluations of the appropriate mechanical alignment, ligament stability, or meniscal defi- ciency. The size of the lesion and the degree to which it is contained, or surrounded by healthy cartilage, will further influence treatment selections. In addition, the success or failure of first-line treatments will influence future treat- ment selections, as will the relative demands placed on the knee by the patient. For patellofemoral lesions, an antero- medialization tibial tubercle osteotomy is generally recom- mended concomitantly. Palliative techniques, including arthroscopic lavage/ debridement and in some cases marrow stimulation, are typ- ically implemented as a first line of treatment with the intention of temporarily, if not permanently, reducing the symptoms associated with chondral lesions. Secondary procedures include reparative techniques of ACI and in some cases osteochondral grafts, which strive to restore true hyaline cartilage and are associated with greater morbidity because they usually require an open exposure. Autologous chondrocyte implantation is best used in rela- tively young patients with focal-contained shallow lesions. The preferable size is approximately 2 to 10 cm2. Larger, deeper lesions with bone loss in an older patient may require a restorative procedure using an osteochondral implant, either autograft or allograft, depending on the lesion's size. Deficient menisci often require transplanta- tion concomitantly with an articular cartilage procedure. Other comorbidities that require assessment and often simultaneous treatment include ligamentous instability and malalignment of the mechanical limb axis or the patellofemoral joint. With these guidelines in mind, we have constructed a conceptual treatment algorithm based on lesion size, patient age, and success or failure of previ- ous treatments. In this conceptual diagram, first-line treatments are selected based on lesion size and location in the context of other patient characteristics.

TREATMENT OF LEG DOMINANCE

In order to correct for leg dominance, training must pro- gressively emphasize double-leg and then single-leg move- ments. Equal leg-to-leg strength, balance, and foot placement are stressed throughout the program. For example, to perform tuck jumps (see Figure 3), a leg-dominant athlete may repeat- edly place the weaker limb under greater stress to maintain symmetry throughout the performance of the double-leg jump. If she does not produce equal force output in each leg, then she will have difficulty maintaining proper position in jumping and landing and will have increased difficulty maintaining an upright vertical tuck jump. Thus, to perform the tuck jump, the weaker leg must work harder to maintain an equal position with the stronger leg. In turn, greater neuromuscular adapta- tion may be evoked in the weaker limb. An important training point for clinicians is to pay special attention to foot placement during landing and jumping. Athletes should not be allowed o drop one leg posterior to the other leg when performing double-leg jumps. This position will only facilitate leg-to-leg imbalances further by overloading the stronger (posteriorly po- sitioned) leg, while unloading the stress on the weaker (ante- riorly positioned) leg. An athlete who continuously exhibits the problem of dropping a leg or overusing the stronger leg can reprogram this dangerous pattern with single-leg exercises such as the hop-and-hold exercise mentioned earlier or with the single-leg balance exercise on an unstable surface (Figure 10). By forcing each limb to work independently of the other limb, no compensation can be provided by the contralateral limb. Similar amounts of work force greater muscular adap- tations in the weaker limb and may decrease imbalances that may have been displayed before training. Bounding exercises (Figure 11) can be incorporated into training to aid in the correction of lower extremity imbalances and to help an athlete learn to coordinate multiplanar move- ments. To correctly perform the bounding exercise, an athlete should jump with maximum distance in both the vertical and horizontal planes. This power jump incorporates high-intensity jumps and landings on a single limb. This type of high-inten- sity, single-leg movement can accelerate the correction of im- balances between the lower extremities. During this exercise, the weaker leg is repeatedly and quickly exposed to the forces generated by the other leg. The neuromuscular stress placed on the less coordinated leg to individually dampen and quickly redirect the forces generated by the dominant limb during this exercise may help to normalize any bilateral strength or co- ordination discrepancies between the lower extremities. Correction of leg dominance requires an athlete to learn to coordinate multijoint actions and multiplanar movements into power skill movements that can be used during competitive play. The endpoint of training should incorporate exercises that force an athlete to use maximum effort with perfect tech- nique. The jump, jump, jump, vertical jump (Figure 12) can be used to evaluate an athlete's potential ability to transfer proper techniques into sport-related tasks. This exercise is per- formed with maximum effort for horizontal momentum, which must quickly and efficiently be transferred into vertical move- ment. Execution of this movement is similar to that during competitive play. Examples may include a soccer player who must rapidly stop and jump to head the ball, a basketball play- er performing a jump-stop layup, or a volleyball player who uses the approach to gain momentum for maximum height during the attack. An improper or unbalanced limb in any step of the jump sequence makes the jump difficult to execute. The reward of a more easily accomplished task pushes an athlete to learn bilateral limb control during this jump. In summary, the progression from double-leg to single-leg power maneuvers is a requirement for correcting leg-to-leg imbalances. In addition, the incorporation of multiplanar movements that equally recruit both lower extremities for op- timal performance is necessary. More complex movement pat- terns require greater synchronization and coordination in side- to-side performance, which leads to greater balance in side-to-side muscle recruitment and equalization of leg-to-leg coordination and power.

INJURIES ASSOCIATED WITH ACL DISRUPTIO

Injuries of the ACL rarely occur in isolation. The effects of other injuries, including other ligament sprains, meniscal tears, articular cartilage injuries, and bone bruises, com- plicate the treatment and eventual outcomes of ACL dis- ruptions. There can be no doubt that the addition of 1 or more of these associated injuries adversely affects the out- comes of treatment, but it is very difficult to quantitate or predict exactly how they will alter the results.

Concomitant Knee Abnormality

It has been well established that regardless of the tech- nology employed, cartilage restoration procedures have better outcomes when comorbidities are corrected. This is particularly true when treating lesions in the knee, but the same principles apply to other joints in which cartilage restoration technology is used. When performing a carti- lage restoration procedure, the surgeon must identify and correct a deficient meniscus, ligamentous instability, or malalignment of the mechanical limb axis or patellofemoral joint. Often, the most difficult step in cor- recting these associated abnormalities is identifying them. As "the eye sees only what the mind knows" (Rene Descartes, 1642), we are at risk of not properly identifying and correcting comorbid conditions unless we are looking carefully for them at the time of initial patient evaluation. Left uncorrected, comorbidities represent a relative con- traindication to cartilage resurfacing. The following sec- tion will discuss the issues of meniscal transplant, liga- ment reconstruction, and corrective osteotomies in the set- ting of cartilage restoration procedures. Meniscus Transplant. In part 1, we discussed the role the meniscus plays in protecting the articular surface through load transmission. A step-by-step technique of meniscal transplants has been described else- where.21,52,77,84,95 Attaining bone anchorage of the anterior and posterior horns is essential to providing a functional meniscus transplant. Although securing the graft with soft tissue alone is technically easier, load transmission is superior when the graft is secured with bone.21,91 Either bone plugs or a bone bridge in the form of a "trough," "slot," or "keyhole" is used to anchor the anterior and posterior horns. In the setting of a combined cartilage restoration and meniscal transplant, it may be necessary to use a bone plug technique, even on the lateral meniscus, to allow greater versatility and exposure of the articular cartilage. When combining cartilage restoration with a meniscal transplantation in the same compartment, it is important to plan the exact sequence of events in a detailed preoper- ative plan. For example, implanting an osteochondral allo- graft and performing a meniscal transplant to treat a deep articular cartilage defect on the lateral femoral condyle in a previously lateral meniscectomized knee will require that the posterior horn anchor be established before preparing the articular cartilage defect and implanting the osteochondral allograft plug. The bone plug and ante- rior horn of the meniscal allograft are gently retracted out of harm's way during implantation of the osteochondral graft and inserted in a blind tunnel at the anatomical site of the anterior horn after the osteochondral graft implan- tation is completed. Corrective Osteotomies. It is commonly believed that for all of these techniques, realignment osteotomy should be performed as an adjunct procedure if the lesion is in a com- partment under more than physiological compression.41 Patellofemoral joint realignment with a tibial tubercle osteotomy is a familiar procedure and has been in main- stream orthopaedics for decades.30 We recommend that most cartilage restoration procedures performed on the patellofemoral joint be combined with a distal realignment procedure that anteriorizes the patella to unload the newly resurfaced patellofemoral joint. Medially based patellofemoral chondral lesions may be an exception to this generalization. Subtleties of the patellar tracking problems must be appreciated to plan the correct osteotomy. Anteriorizing the patella will unload the patellofemoral joint, whereas medializing patellar tracking may help cor- rect lateral instability associated with a pathologic Q angle. Flatter angles will medialize more than anteriorize, and steeper angles will provide more anteriorization than medialization. There are commercially available surgical instruments to make the procedure technically easier and more precise (Tracker AMZ guide, Mitek, Norwood, Mass). A high tibial osteotomy is required to correct the varus angulation of the lower limb mechanical axis when per- forming a cartilage restoration procedure in the medial compartment of a varus knee. Unlike standard high tibial osteotomy for isolated medial compartment osteoarthritis, in which the aim is to correct the mechanical axis lateral- ly to 66% of the width of the tibial plateau in the lateral compartment,60 high tibial osteotomies combined with car- tilage restoration in the medial compartment should cor- rect the mechanical axis to just beyond neutral. Commercially available instrumentation (Arthrex) allows for a technically simple, rapidly performed opening medial osteotomy with precision and rigid fixation. Although the opening medial osteotomy allows exposure to the medial tibia for ACL reconstruction or meniscus transplantation, there is no absolute reason to choose opening medial over closing lateral osteotomies, as long as the goals of realign- ment are met and the osteotomy heals without sequelae. For valgus angulation of a knee joint with lateral com- partment disease, a distal femoral osteotomy is required to restore a normal mechanical axis. As with a high tibial osteotomy to treat varus disease, the goal with a distal femoral osteotomy is to correct the mechanical axis to neu- tral. Care must be taken to avoid overcorrection, which creates a varus alignment. Generally, we recommend an opening lateral distal femoral osteotomy with rigid plate fixation, although other techniques and fixation methods have been described, including a percutaneous dome osteotomy combined with temporary external fixation and intramedullary nail fixation.43 When performing a corrective osteotomy combined with a cartilage restoration procedure, it is critical to establish a preoperative plan that allows for a stepwise incorpora- tion of both procedures. For example, when performing an ACI of the patellofemoral joint with a combined distal realignment, the periosteal patch must be harvested from the anteromedial tibia before making the osteotomy of the tubercle through that area. Articular cartilage lesion preparation, graft suturing, and cell implantation require subluxation or eversion of the patella and should be per- formed before establishing rigid fixation of the tubercle osteotomy distally. Ligamentous Instability. Uncorrected ligamentous insta- bility is a contraindication to cartilage restoration proce- dures. Methods of ligament reconstruction are well estab- lished and will not be reviewed here. It is important to carefully plan a sequence of treatment options for each patient and to consider staging procedures if needed. If a single stage will incorporate ligament reconstruction with cartilage restoration or meniscus transplant, a preopera- tive plan that allows for a stepwise incorporation of both procedures is crucial. The type of concomitant procedure may affect the graft selection for ACL reconstruction. For example, when performing an ACL reconstruction in the setting of an ACI, periosteal patch harvest would occur before hamstring harvest or tibial drilling. If treating patellar defect with ACI and distal realignment, ham- string autograft or an allograft graft source would be required, as the osteotomized patellar tendon insertion would be unavailable as autograft. If an opening medial high tibial osteotomy is required in association with an ACL reconstruction, the osteotomy can be tailored to reduce the posterior slope of the tibial plateau, which will protect the ACL reconstruction and increase anterior stability. Medialization of the tibial tun- nel by rotation of the guide will limit the possibility of inadvertent communication with the osteotomy. In the set- ting of a high tibial osteotomy, a hamstring graft for ACL reconstruction may be considered, as it may allow for smaller tunnel diameter, thereby reducing the risk of com- municating with the osteotomy. Whichever graft is selected, interference screws for tibial fixation should be used with caution, as they will create hoop stresses in the tibial tun- nel aperture near the osteotomy. If a meniscus transplant is combined with either pri- mary or revision ACL, there are several issues to consider related to the 3-dimensional relationship of tunnels in the tibial metaphysis. Prior tunnel expansion and position, intended locations of new tunnels, ACL graft selection, and meniscus anchor method offer variability to address the needs of each particular patient. For example, the ideal entry of the ACL tunnel is at the level of the posterior edge of the anterior horn of the lateral meniscus; if performing a concomitant ACL reconstruction and lateral meniscal transplant, a slot and bone bridge technique will allow for adequate fixation even if the bone bridge is partially vio- lated by the tibial tunnel,21 and a hamstring ACL graft will allow for narrower tunnels in the tibia. If necessary, procedures should be staged and bone graft applied to metaphysial defects. A guideline of 2 procedures per operation and staging subsequent procedures should be followed. Generally, osteotomies should be performed first, with a 6-month interval between procedures to allow for complete bone healing and remodeling. Subsequent hardware removal should be planned, and a strategy for dealing with resulting stress risers from residual empty screw holes should be devised.

DIFFERENCES IN ELECTROMYOGRAPHIC (EMG) ACTIVATION LEVELS BETWEEN MALE AND FEMALE ATHLETES medial/lateral

Joint compression through muscular co-contraction allows valgus load to be carried by articular contact forces, protecting the ligaments. Decreased medial joint compression may limit passive resistance to dynamic knee valgus, predisposing the female knee to medial femoral condylar lift off and increased loads on the ACL.12 40 41 Rozzi et al6 reported that female athletes show a disproportionate (4 times greater) firing of their lateral hamstrings during landing. Myer et al9 showed a decreased ratio of medial to lateral quadriceps recruitment in female subjects (fig 4). The decreased ratio combined with unbalanced medial ham- strings recruitment may decrease control of coronal plane forces at the knee6 34 40. Markolf et al34 showed that muscular contraction can decrease both the valgus and varus laxity of the knee threefold. A low ratio of medial to lateral quadriceps recruitment combined with increased lateral hamstring firing may compress the lateral joint, open the medial joint, and increase anterior shear force, which directly loads the ACL.6 34 4

Biomechanics of the Knee

Knowledge of the biomechanics of the tibiofemoral and patellofemoral joints is essential to appropriately design rehabilitation programs following articular car- tilage repair procedures to assure that exercises are selected and performed in a manner that does not cause deleterious forces to the repair site. Articulation between the femoral condyle and tibial plateau is constant throughout knee ROM. Near-full knee extension the anterior surface of each femoral condyle is in articulation with the middle aspect of the tibial plateau. With weight bearing, as the knee moves into greater flexion, the femoral condyles progressively roll posteriorly and slide anteriorly, caus- ing the articulation to shift posteriorly on the femoral condyles and tibial plateaus.27,33 The articulation between the inferior margin of the patella and the trochlea begins at approximately 10° to 20° of knee flexion depending on the size of the patella and the length of the patellar tendon.26 With knee flexion, the contact area of the patellofemoral moves proximally. At 30°, the area of patellofemoral contact (inferior facets) is approximately 2.0 cm2.26 The area of contact gradually increases as the knee is flexed. At 60° of knee flexion, the middle facet of the patella articulates with the trochlea. At 90° of knee flexion, contact area increases up to 6.0 cm2,26 and the superior facets are in contact with the femoral condyles. Using this knowledge of joint arthrokinematics, the rate of weight bearing, PROM, and exercise progres- sion may be based on the exact location of the lesion (Figure 3).3,14,15,19,34 For example, a patient with a lesion on the anterior aspect of the femoral condyle may perform exercises into deeper flexion without causing articulation at the repair site. Conversely, lesions on the posterior condyle may require the avoidance of exercise in deep knee flexion due to the rolling-and-sliding component of the articulation dur- ing deeper knee flexion. Furthermore, the rehabilita- tion program for lesions on the trochlea may include immediate partial weight bearing with a brace locked in full knee extension because the patella is not in contact with the trochlea in this position. Rehabilitation exercises are also altered based on the biomechanics of the knee to avoid excessive compressive or shearing forces. While the exact ROM that articulation of the lesion occurs is the most important factor to consider when designing the rehabilitation program, the amount of compressive and shear forces obser ved at the joint also var y throughout the ROM. Exercises, such as seated knee extension, are commonly performed from 90° to 40° of knee flexion. This ROM provides the lowest amount of patellofemoral joint reaction forces while exhibiting the greatest amount of patellofemoral contact area.25,26,58 Weight-bearing exercises, such as the leg press, vertical squats, lateral step-ups, and wall squats are performed initially from 0° to 30°, where tibiofemoral and patellofemoral joint reaction forces are lower.25,26,58 Clinically, we begin these exercises using a leg press machine, rather than the vertical mini-squat, due to the better ability to control the amount of weight applied to the lower extremities. As the repair site heals and patient symptoms subside, the ROM in which exercises are performed is pro- gressed to allow greater muscle strengthening over a greater range of movement. Exercises are progressed based on the patient's subjective reports of symptoms (pain, clicking, etc) and the clinical assessment of increased swelling and crepitation.

IDENTIFICATION OF LEG DOMINANCE

Leg dominance can be assessed using a dynamometer or leg-curl and leg-extension machines. A difference in strength or power of 20% or more between limbs indicates a neuro- muscular imbalance that may underlie significant injury risk.44 Another indicator of bilateral imbalances is an athlete's ability to perform a single-leg balanced stance on an unbalanced plat- form that can objectively quantify postural sway (ie, a stabi- lometer). Women demonstrate poor scores on stabilometry measures taken on unaffected limbs after ACL injury.31 These pilot data may indicate some relationship between ACL injury risk and poor stabilometric scores. Tropp and Odenrick77 re- ported that athletes who could not demonstrate postural bal- ance within 2 standard deviations of normal had a significantly higher risk for an ankle injury. Increased proficiency in bipedal and single-leg balance can be gained through balance-board training.78 Tropp and Odenrick77 were able to reduce injury rates in athletes returning to sport from prior injury with 10 weeks of balance-board training. Finally, field exercises such as X hops (Figure 9) can be used as a measure to grade bi- lateral differences in single-limb performance. Identifying limb imbalances will assist clinicians and researchers in inter- vening with athletes who need dynamic neuromuscular anal- ysis training.

Ligament Injuries

Ligament injuries varying from a mild sprain of 1 other ligament to complete tears of all major ligaments (dislo- cated knee) are often associated with torn ACLs. In many cases, the associated ligament injuries require no surgical treatment. This outcome is especially true for grade I and grade II tears of the medial collateral ligament (MCL). Many surgeons now advocate nonsurgical management, even when grade III tears of the MCL exist in association with ACL disruptions.212,255,257 Noyes and Barber-Westin212 believe that complete tears of the MCL do not require sur- gical intervention when associated with ACL tears; but when the oblique fibers of the MCL and the posterior cap- sule are also torn, allowing gross medial opening, then MCL and capsule repairs are necessary. The existing liter- ature does not clearly define when it is necessary or unnec- essary to surgically repair or reconstruct a complete tear of the MCL in combination with ACL injuries.201,255,265 Care must be taken to avoid the assumption that the lit- erature supporting nonoperative care of isolated complete tears of the MCL can be applied to cases in which both the ACL and MCL are completely ruptured. The difficulty with interpretation of the present literature concerning combined ACL and MCL injuries is that the definition of a complete tear of the MCL is unclear. A complete tear of the superficial MCL, with minimal separation of the 2 ends, usually heals adequately. When the ends of the torn MCL are markedly separated and associated with complete dis- ruption of the posterior oblique ligament as defined by Hughston,118 extensive posterior capsule damage and avulsion of meniscal attachments, it is unlikely that non- operative treatment of the medial complex injury will result in a successful outcome if only the ACL is recon- structed. Fortunately, disruption of the posterolateral and lateral structures of the knee is not commonly associated with rupture of the ACL, but when they are present and not rec- ognized, an ACL reconstruction will often fail.8,97,111,130,209 Thus, ligament and capsular injuries involving the pos- terolateral complex must be recognized and surgically treated (acute repair or chronic reconstruction) at the time of ACL reconstruction or preceding it. No outcome data on such combined procedures exist, however. It is beyond the scope of this review to discuss treatment of dislocated knees.

Assessment of MR Imaging and Definitions

Magnetic resonance images were evaluated independently from each other and from physical examination data by 2 radiologists (DV and SB). In case of disagreement, consen- sus was reached. Bone bruise was identified as an area of abnormal high- signal intensity in the subchondral bone or marrow on the T2-weighted fat suppressed images. Menisci were classi- fied as intact or torn. Cruciate ligaments and collateral ligaments were dichotomized as normal or partial/total rupture. Effusion was dichotomized as nonsevere (absent or without bulging of the knee capsule) or severe (bulging of the knee capsule). Osteoarthritis of the knee was defined as absent, that is, no abnormalities or minimal osteophytes of dubious significance, or present, that is, osteophytes and/or joint space narrowing, possibly associated with scle- rosis of the subchondral bone. Workload was assessed and classified according to the baseline questionnaire. We identified 4 categories: no occu- pation; sedentary occupation, for example, office jobs; occu- pation in which the patient had to stand nearly all day, for example, hairdressers; and heavy physical work, for exam- ple, construction workers. Sports load was also assessed and classified according to the baseline questionnaire: no sporting activity, <4 hours of sporting activity per week, and ≥4 hours of sporting activity per week. We assumed that a patient returned to his/her baseline occupation and sporting activity as soon as possible.

Meniscal and Articular Cartilage Damage

Meniscal damage frequently occurs at the time of an ACL injury, and the incidence increases in patients who do not undergo reconstruction. Reports of meniscal injury associ- ated with acute ACL disruption range from 15% to 40% and become much higher with chronic ACL deficiency.159 Most reports concerning patients who had meniscal injuries at the time of the ACL reconstruction do not show an increased incidence of arthrosis until 5 years after sur-gery.71,116,128,148,226 In patients observed from 7 to 17 years after ACL reconstruction, those who had a damaged meniscus at the time of the ACL reconstruction had a much higher incidence of arthrosis than did those who had no meniscal damage.¶ Fink et al85 noted that the amount of arthrosis observed 10 years after ACL reconstruction was related to the amount of meniscus removed at the time of reconstruction. The effects of chondral lesions observed at the time of ACL reconstruction are difficult to ascertain from the cur- rent literature. The definition of the grade and extent of cartilage injury at the time of the index surgery described in the literature is quite variable, which makes compar- isons of outcomes difficult. Shelbourne et al254 reported that patients with grade III and grade IV chondral lesions that had a mean diameter of 1.7 cm compared with a sim- ilar group with no such lesions observed for 6 years after ACL reconstruction had significantly worse subjective International Knee Documentation Committee (IKDC) scores (94 vs 95.2 for cartilage lesions located in the medial compartment, and 92.8 vs 95.9 for cartilage lesions locat- ed in the lateral compartment), but no difference in IKDC radiographic ratings was observed. On the other hand, Wu et al301 found no significant relationship between articular cartilage injury and functional outcome in a 10-year follow- up of ACL reconstruction. Drogset and Grøntvedt,71 in an 8-year follow-up of patients who underwent ACL recon- struction, reported that articular cartilage lesions were an important predisposition for the development of arthrosis, whereas no such correlation could be found with associat- ed meniscal injuries. In 2000, Shelbourne and Gray253 reported in a 5-year to 15-year follow-up of patients who underwent ACL reconstruction that those with more meniscal and articular cartilage damage at the index sur- gery had more radiographic evidence of arthrosis and sub- jective symptoms at follow-up. Although most would agree that reconstruction of the ACL improves function and decreases the risk of second- ary meniscal tears, there is evidence that reconstruction may not decrease the likelihood of suffering posttraumatic osteoarthritis.65 In a meta-analysis involving 33 clinical follow-up studies, the efficacy of ACL repair or reconstruc- tion in retarding the progression of osteoarthritis was not substantiated.168 In a 7-year prospective study of patients who had an acute ACL reconstruction, 66% of those with a concomitant meniscectomy developed radiographic evi- dence of osteoarthritis, whereas only 11% of those with normal menisci were so effected.135 In another group that underwent a chronic ACL reconstruction, 100% of those with a meniscectomy had radiographic evidence of osteoarthritis, whereas 50% of those with normal menisci at the time of reconstruction had osteoarthritis at the 7- year follow-up.135 Therefore, it appears that the patients undergoing ACL reconstruction who have significant articular cartilage injuries or meniscal damage, or both, will eventually develop signs and symptoms of arthrosis. The problem is that at the present time, 10% to 20% of patients who have isolated ACL injuries also develop arthrosis after ACL reconstruction.98,135,253 There is cer- tainly no convincing evidence that arthrosis can be pre- vented by ACL reconstruction.64,84,234 It is probable that "successful" ACL reconstructions allow improved short- term function.92 It is possible that if ACL function is restored, the menisci can be preserved, articular cartilage damage can be avoided, and the development of arthrosis can be minimized.85,98,136,168 No studies presently exist that confirm this contention.

RANDOMIZED CLINICAL TRIALS COMPARING OKC AND CKC EXERCISES

The effects of OKC and CKC exercises on functional outcome have been evaluated in three independent prospective randomized clinical trials (5,10,14). Bynum et al. per- formed a clinical trial comparing outcomes after ACL recon- struction with patellar tendon grafts after 19 months of healing (5). Patients were randomized to rehabilitation pro- grams that consisted of either OKC or CKC exercises. They found that the mean side-to-side difference in knee laxity of the OKC group (3.3 mm) was significantly greater than that of the CKC group (1.6 mm). In addition, the CKC group had a faster return to sport. At 9 months, patellofemoral pain was reported in 15% of the CKC group (compared with 38% in the OKC group), although there was no difference at 19 months. They also reported no significant differences in Lysholm score, Tegner activity score, overall subjective rat- ing of the knee (Lachman and pivot shift test), or ranges of knee motion. However, when comparing the two rehabilita- tion protocols, there were differences in the levels of resis- tance and the progression of exercise between the groups. The OKC group performed cocontraction isometrics, ham- string concentric and eccentric isotonics, and single-leg raises at 30° of flexion in the first 6 wk; the CKC group performed double one-third knee bends, seated leg presses, and ham- string curls in the first 6 wk. The CKC group was also allowed to begin jogging against Sport Cord resistance at 8 wk, and sport-specific exercises at 16 wk, whereas the OKC began isotonic quadriceps exercises at 6 wk, progressing to isoki- netic at 24 wk. The OKC group did not begin jogging until 16 wk, and sport-specific exercises were initiated at 7 to 8 months. These differences may account for the faster return to previous level of activity of the CKC group. Mikkelsen et al. measured anterior knee laxity, isokinetic muscle torque, and the time to return to sports after 6 months of healing in patients who underwent ACL reconstruction (13). Subjects were randomized to one of two rehabilitation programs; one used CKC exercises for a 24-wk period, the other used the same CKC rehabilitation program with the addition of OKC exercises from weeks 6 to 24. The OKC exercises consisted of isokinetic quad strengthening between 90° and 40° at 6 wk, and progressed to between 90° and 10° by 12 wk. The treatment group using both exercise types had significantly higher quadriceps torque, and a greater propor- tion of the patients returned to sports at their preinjury level. No comments relating to patellofemoral complications in either group were made. This study indicates that the addi- tion of the limited range of motion OKC exercises in week 6 increasing to near-full extension by week 12 may benefit subjects. However, the improvement may be because of the addition of exercises, and not dependent on the type of exercise added. Nonetheless, the addition of the OKC exer- cises in this time frame did not produce a negative outcome. Morrissey and Hooper studied the effects of prescribing OKC versus CKC hip and knee extensor muscle exercises after surgery (10,14,15). In both treatment groups, the reha- bilitation program was initiated 2 wk after surgery and com- pleted after week 6. In designing the two programs, they attempted to control for training velocity, ROM (90° to 0°), and the number of exercise repetitions. The knee-laxity values obtained at the conclusion of the rehabilitation period using OKC or CKC exercises were not significantly different (OKC 10.3 mm vs CKC 10.0 mm; P 0.32) (14). No differences were found for knee pain (15). Gait analysis was also performed in these patients to examine differences in joint kinematics and kinetics during level walking, stair as- cent, and stair descent (10). Patients also assessed their level of disability using the Hughston Clinic visual analog scales. The only gait variable affected by the rehabilitation program was the knee flexion angle at contact during step-up. This kinematic parameter was improved by an average of 2° in the patients who performed the OKC exercises, and is probably not clinically significant. The effects of the OKC and CKC exercise programs relative to all other parameters of knee kinetics and kinematics were not significantly different. The authors concluded that there are no "clinically significant differences in the functional improvement resulting from the choice of OKC and CKC exercises in the early period of rehabilitation" (10). These findings may be limited because of the short period of supervised rehabilitation (2-6 wk).

Restore Soft Tissue Balance

One of the most important aspects of articular cartilage rehabilitation involves the avoidance of arthrofibrosis, particularly with the OATS and ACI procedures, due to the large open incision and extensive soft tissue trauma. This is achieved through the restoration of full passive knee extension, patellar mobility, and soft tissue flexibility of the knee and entire lower extremity. The inability to fully extend the knee results in abnormal joint arthrokinematics and subsequent increases in patellofemoral andtibiofemoral joint contact pressure, increased strain on the quadriceps muscle, and muscular fatigue.41 Therefore, a drop-lock postoperative knee brace locked into 0° of extension is used during ambulation and PROM out of the brace is performed immediately following surgery. The goal is to achieve at least 0° of knee extension within the first few days following surgery. Specific exercises to be performed include manual PROM exercises applied by the rehabilitation specialist, su- pine hamstring stretches with a wedge under the heel, and gastrocnemius stretching with a towel. Overpressure of 2.7 to 5.4 kg (6-12 lb) may be used for a low-load long-duration stretch as needed to achieve full extension. Modalities such as moist heat and ultrasound may also be applied to facilitate greater ROM improvements before and/or during these stretching techniques.31,4 Soft tissue flexibility and pliability are also impor- tant for the entire lower extremity. Soft tissue mobili- zation and scar management is performed to prevent the development of adhesions around the anterior, medial, and lateral aspects of the knee. In addition, flexibility exercises are performed for the entire lower extremity, including the hamstrings, hip, and calf musculature. As ROM improves and the lesion begins to heal, quadriceps stretching may also be performed as tolerated by the patient.

Cartilage Replacement Techniques

Osteochondral Autograft. Osteochondral autografts involve the transfer of intact hyaline cartilage and sub- chondral bone,60 and they heal to the surrounding recipi- ent tissue.46 The key to this technique is chondrocyte via- bility because only living chondrocytes can produce and maintain the extracellular matrix of proper load-bearing capacity.19 Osteochondral autografts are small bone plugs covered with normal hyaline articular cartilage that are removed from a relatively nonweightbearing surface and transferred in a single stage to the chondral defect. In 1985, the first results of autogenous osteochondral grafts for the treatment of osteochondritis dissecans lesions were published.131 The first arthroscopic treatment using auto- grafts was reported in 1993.79 Many studies have been published since that have investigated the ideal donor site and plug size.13,45,47,48,95 Complex contact pressures of the patellofemoral joint41 make this a particularly challenging region with respect to osteochondral plug size, articular surface contour, and implantation technique. Mechanical studies of autograft plugs have demonstrated that the pull-out strength of press-fit plugs using current- ly available systems is directly related to the length and diameter of the plug; 15-mm-long plugs had a mean pull- out of 93 N, and, of those, 11-mm-wide grafts were signifi- cantly stronger (92 N) than were 8-mm-wide grafts (41 N). These pull-out strengths were reduced by half with graft reinsertion or levering at the time of harvest.34 In another study, fixation strength of mosaic autografts decreased 44%, from 135.7 N to 75.5 N, over a 7-day period while soaked in a physiologic solution in vitro, suggesting that there is substantial deterioration of short-term fixation strength of mosaicplasty grafts in the immediate postop- erative period.127 In the case of graft-length mismatch, mechanical studies have demonstrated that a plug that is .5-mm proud has poorer mechanical effects and more shear than a .5-mm sunk plug. Therefore, although the mosaic bed of plugs should be constructed to match the local contour, care must be taken not to overcontour the graft construct.25 In animal studies, grafts that were 2-mm proud demonstrated graft micromotion and fissuring, which prevented proper graft integration and function. In addition, these studies emphasized the importance of fully seating the graft in a well-supported recipient site. Supported grafts heal well, but unsupported grafts tend to subside and become cov- ered by fibrous tissue.99 It is the periphery of these mosaic reconstructions that experiences the highest shear, which may lead to progres- sion of the lesion or failure of resurfacing efforts. At the edge of prepared cartilage lesions, there is a considerable loss of chondrocytes, but these fewer number of chondro- cytes are able to upgrade their metabolism to produce an equal amount of proteoglycan.54 In the future, perhaps the combination of marrow stimulation and autologous plug transfer will provide a fibrocartilage interface for better integration between plugs and intact surrounding carti- lage to reduce shear at this interface. This would concep- tually integrate the strategy of reconstruction and repair, possibly providing improved histology and biomechanical stability at the periphery of the lesions after restoration. Physiologic pressure on the donor sites is thought to be responsible for a significant amount of morbidity after autologous plug transfers. In one study, 10 of 10 donor sites' pressure films demonstrated a significant exposure to pressure with physiologic range of motion.96 Recent cadaveric studies have shown that contact pressures are lowest along the medial trochlea and decrease distally along the lateral trochlea.41 The topography of variou s regions of articular cartilage must be taken into account when matching a donor site with a recipient lesion. Topographic mapping has demon- strated that the articular cartilage of the lateral and medial femoral trochlea matches the weightbearing portions of medial and lateral femoral condyles better than the carti- lage from the central intercondylar notch does.9 Although originally developed to treat chondral lesions in the knee, autologous plugs are now being used with good early results to treat chondral lesions in other joints as well.3,57 A more complete description of this technique and outcomes will be presented in part 2. Osteochondral Allograft. Fresh osteochondral allografts provide larger constructs of subchondral bone and viable cartilage from cadaveric donors. Osteochondral allografts were first used to restore the articular surface in 1908 by Lexer,66 who reported a 50% success rate with adequate function of the allograft and incorporation into host bone.65 In the 1940s and 1950s, it was recognized that allografts could represent a biologic alternative to knee replacement in young patients with focal articular cartilage damage.38 Cryopreserved osteochondral allografts were used for limb salvage after resection of bone tumors in the 1970s,98,125 and several investigators reported moderate success rates with problems related to the massive size and limited via- bility of frozen chondrocytes. Currently, osteochondral allograft implantation is con- traindicated in lesions caused by diffuse disease processes, such as osteoarthritis and inflammatory arthropathies, and diffuse avascular necrosis. If avascular necrosis is localized and the surrounding bone is healthy, allograft implantation may be considered. Defects limited to one joint surface (unipolar) have better results than do lesions on opposing joint surfaces (bipolar or kissing lesions). As with other cartilage restoration procedures, an intact meniscus, ligamentous stability, and proper angular align- ment of the limb are required for allograft implantation. The comorbidities of deficient or absent meniscus, liga- mentous instability, and mechanical axis malalignment are treatable, however, and must be corrected before or concomitantly with allograft implantation. The upper limit of patient age for these procedures remains an area of con- troversy. Although the majority of investigators recom- mend an age limit of 40 to 45 years, others have extended this to 60 years of age in healthy, active individuals.69,83,132 Concerns related to frozen chondrocyte viability108 led to the routine use of fresh osteochondral allografts when treating isolated articular cartilage defects. It is generally recommended that fresh articular cartilage allograft be transplanted within days of harvest, with the understand- ing that the longer the wait, the greater the death of car- tilage cells. The urgent nature of using osteochondral grafts as they become available creates logistical chal- lenges of obtaining the correct size graft at a time and place that the patient is available for surgery. Some cen- ters have expanded the use of osteochondral allografts to include total replacement of the entire tibiotalar joint with carefully size-matched fresh cadaveric joints.121 The tech- nique and outcomes of osteochondral allograft implanta- tion to treat focal chondral defects will be presented in part 2. Periosteal and Perichondral Grafting. In the 1970s106 and 1980s,58 early encouraging results from perichondri- um transplantations to articular cartilage defects in ani- mals demonstrated that the transplanted tissue was his- tologically similar to articular cartilage with 74% type II collagen.51,94 Only performed in a limited number of cen- ters, this procedure works best in younger patients.114 Because of the limited use of this procedure, there are few reported outcomes that widely endorse its use

Immediate Versus Delayed Motion

Our review identified 5 RCTs comparing immediate to delayed knee motion during the initial stages of rehabili- tation, and there appears to be reasonable consensus that immediate motion is beneficial for the healing ACL graft and soft tissue structures that span the knee.47,50,86,92,95 Haggmark and Eriksson were among the first to per- form a prospective RCT of rehabilitation after ACL recon- struction with a patellar tendon graft.35,47 Patients were treated with a dorsal plaster splint during the first week after surgery and were then randomly assigned to continue rehabilitation during the following 4 weeks while wearing either a hinged cast that allowed knee motion or an ordi- nary cylinder cast that prevented knee motion. All of the patients were followed up during a 1-year interval; those treated with standard cast immobilization had significant atrophy of the slow-twitch muscle fibers of the vastus lat- eralis, whereas those treated with the hinged cast and early motion demonstrated no changes in the cross-sec- tional area of the slow- or fast-twitch fibers. Haggmark and Eriksson47(p55) noted that "there appeared to be no dif- ference in the end result of the surgical procedure" and that treatment with the hinged cast "facilitated an early return to sports." A prospective RCT that compared immediate to delayed range of motion after ACL reconstruction was carried out by Noyes et al.86 Subjects in the immediate motion pro- gram began continuous passive motion of the knee on the second postoperative day, whereas those in the delayed motion group had their knees placed in a brace at 10° of flexion and began continuous passive motion on the sev- enth postoperative day. Subjects in both rehabilitation pro- grams reported similar rates of joint effusion, hemarthro- sis, soft tissue swelling, flexion and extension limits of the knee, use of pain medications, and time of stay in the hos- pital. Continuous passive knee motion immediately after ACL reconstruction did not lead to an increase in anterior knee laxity during healing. Rosen et al95 carried out a prospective RCT of rehabili- tation after arthroscopically assisted ACL reconstruction with a central third BPTB autograft performed by the same surgeon. After surgery, subjects were randomized via a lottery system to 1 of 3 programs: early active motion, continuous passive motion, or a combination of both. This work extended the research of Noyes et al86 by showing that continuous passive motion during the first month after ACL reconstruction, compared with early active motion, produced similar range of joint motion and KT- 1000 arthrometer measurements of A-P knee laxity. Richmond et al92 reported the results of a prospective RCT that compared the effects of continuous passive knee motion for 4 to 14 days after arthroscopically assisted ACL reconstruction with a BPTB autograft. They found similar values for knee range of motion and lower limb girth between treatment groups. More recently, Henriksson et al50 described a prospective RCT of rehabilitation after ACL reconstruction with a BPTB graft performed by 1 of 4 surgeons using the same technique. After surgery, subjects were randomly assigned to rehabilitation protocols consisting of cast immobiliza- tion or early range of motion training with a brace. Subjects in both groups underwent similar supervised rehabilitation, and during the first 5 weeks, all rehabilita- tion exercises, with the exception of range of motion exer- cises, were the same for both treatments. Follow-up meas- urements made after 2 years included 88% and 92% of subjects in the brace and plaster cast treatment groups, respectively. The researchers found that rehabilitation with the use of a brace and early range of motion training after ACL reconstruction produced equivalent knee laxity, knee motion, subjective knee function, and activity level in comparison to rehabilitation with plaster cast immobiliza- tion for 5 weeks. There were, however, differences in terms of strength. At 2-year follow-up, subjects in the brace group had a larger strength deficit of the knee flexors (5.9% loss compared to the contralateral, normal side) in comparison to subjects in the plaster cast group (0.9% loss). As well, there was a strong trend for subjects in the brace group to have a strength deficit of the knee exten- sors (11.1% decrease compared to the contralateral side) in comparison to patients in the plaster cast group (3.8% decrease). Of the 5 RCTs reviewed above, only Rosen et al95 ade- quately described their method of randomization, and only Haggmark and Eriksson47 and Henriksson et al50 had min- imal loss of patients at follow-up; no author stated whether the investigators were blinded at follow-up. After ACL reconstruction, it is clear that extended immo- bilization of the knee, or limited motion without muscle activity, is detrimental (inferior structural and material properties) to the structures that surround the knee (liga- ments, cartilage, bone, and musculature).4,10,62-65,70,84,112 There is little doubt that early joint motion after ACL reconstruction is beneficial; it leads to a reduction in pain, lessens adverse changes in articular cartilage, and helps prevent the formation of scar and capsular contractions that have the potential to limit joint motion

GRAFT OPTIONS

Our review of the literature revealed that graft material has received the greatest attention in terms of the relative proportion of prospective RCTs that have focused on sur- gical variables. These studies have compared bone-patellar tendon-bone with hamstrings tendon grafts (used as 2- strand or 4-strand constructs).

NATURAL HISTORY OF THE ACL-DEFICIENT KNEE

Our review of the literature revealed that the true natural history of the ACL-deficient knee has not been character- ized by a well-designed prospective cohort study, and con- sequently, the complete natural history of this complex injury remains unclear. Our review of the available retro- spective studies revealed a common consensus that ACL injury is immediately problematic because of functional instability and that it is the source of long-term complica- tions such as meniscal injuries, failure of secondary stabi- lizers, and the early onset of osteoarthritis. Over time, ACL-deficient patients may experience giving-way episodes and are more likely to develop further intra-articular damage, including meniscal tears,48,58,113,214,215 and osteoarthritis of the knee.133,139 According to Levy and Meier,159 the incidence of meniscal tears in patients with unreconstructed ACL injuries is 40% at year 1, 60% at 5 years, and 80% by 10 years after the initial ACL disrup- tion. Disruption of the ACL, isolated or combined with damage to the meniscus, leads to radiographic changes of the knee that suggest osteoarthritis in 60% to 90% of sub- jects 10 to 15 years after the index injury.ll Subjects with ACL injury and posttraumatic osteoarthritis are, on aver- age, 15 to 20 years younger than patients with primary osteoarthritis when they seek medical advice for their symptoms and when their joints show radiographic evi- dence of osteoarthritis.241

ACL Reconstruction With Bone-Patellar Tendon-Bone Versus 2-Strand Hamstrings Autograft

Our review revealed 3 prospective RCTs that compared ACL reconstruction with a bone-patellar tendon-bone graft versus the 2-strand hamstrings graft.12,40,223 All 3 studies provided an adequate description of the methods used to perform the randomization, had an adequate follow- up interval of at least 2 years, used the same standardized rehabilitation for all patients, and had minimal loss of patients at the time of follow-up. There appears to be a consensus among these studies that reconstruction of the ACL with the 2-strand hamstrings graft results in worse clinical outcome in comparison to reconstruction with a bone-patellar tendon-bone graft after 2 years of healing (Table 2). In all 3 studies, subjects receiving the bone-patellar tendon-bone graft had A-P laxity values that were closer to normal in comparison to those under- going reconstruction with a 2-bundle hamstrings graft.

Reduction of Pain and Effusion

Patients often exhibit significant pain and effusion following articular cartilage repair procedures, specifi- cally surgical procedures that require a large incision and soft tissue trauma, such as ACI and OATs. Numerous authors24,40,56,61 have reported a progres- sive decrease in volitional quadriceps activity as the knee exhibits increased pain and distention. There- fore, the reduction in knee joint pain and swelling is crucial to minimize this reflex inhibition and restore normal quadriceps activity. Furthermore, any increase in intra-articular joint temperature has been shown to stimulate proteoglytic enzyme activity, which has a detrimental effect on articular cartilage.24,40 Treatment options for swelling reduction include cryotherapy, elevation, high-voltage stimulation, and joint compression through the use of a knee sleeve or compression wrap (Figure 4). Patients presenting with chronic joint effusion may also benefit from a knee sleeve or compression wrap to apply constant pressure while performing everyday activities. Pain can be reduced through the use of cryotherapy, transcutaneous electrical nerve stimula- tion, and analgesic medication. Immediately following injury or surgery, the use of a commercial cold wrap can be extremely beneficial. PROM may also provide neuromodulation of pain during acute or exacer- bated conditions.50

Enhance Proprioception and Neuromuscular Control

Proprioceptive and neuromuscular control drills of the lower extremities should be included to restore dynamic stabilization of the knee joint postopera- tively. Proprioceptive deficits have been noted in the injured and postoperative knee.12,45 Specific drills initially include weight shifting side-to-side, weight shifting diagonally, mini-squats, and mini-squats on an unstable surface such as a tilt or balance board (Figure 6). Perturbations can be added to challenge the neuromuscular system, as can additional exer- cises, including lunges, step-ups, and balance onto unstable surfaces

ACL Prostheses

Prosthetic replacement of the ACL with synthetic material, whether for total, permanent replacement (prosthesis), a scaffold for ingrowth of host tissue, or a stent (ligament augmentation device) to protect an allograft or autograft as it heals, has not been proven to be a satisfactory solu- tion to the ACL-deficient knee.§§§ Although claims of acceptable results with short-term follow-ups have been published, at the present time, we are unaware of any investigation that proves that these devices have any place in the treatment of ACL injuries.105

PRINCIPLES OF ARTICULAR CARTILAGE REHABILITATION

Several principles exist that must be considered when designing a rehabilitation program following articular cartilage repair procedures. These key principles have been designed based on our understanding of the basic science and mechanics of articular cartilage. We will briefly de- scribe each one below.

Clinical Consequences of Posttraumatic Bone Bruise in the Knee

Since the introduction of magnetic resonance (MR) imaging, many studies have described the occurrence of so-called bone bruises in posttraumatic knees. These lesions are seen on MR imaging as areas of signal abnormality in the sub- chondral bone and marrow, particularly as a high-intensity signal on T2-weighted fat suppressed images. There is little histological evidence of such bone lesions; after trauma, they are presumably the result of microfracture of medullar trabeculae.13,24 In histological studies, hemorrhage and edema are found in the region of signal abnormalities.14,16,17 One might assume that bone bruises cause pain, even in the absence of other substantial soft tissue injury. Little is known, however, about the clinical consequences of these lesions. In the few studies that give information on the sub- ject, it is often not possible to distinguish between symptoms caused by bone bruise and symptoms caused by other lesions, such as ligament ruptures or meniscal tears. Moreover, most studies that do give clinical information use combined symptom scores, for example, the Noyes symptom score, which assesses pain, swelling, instability, and lock- ing19,20,23; thus, because no isolated pain scores are given, no conclusions can be made about a possible relation between pain and bone bruise. The purpose of our study was to investigate the possiblerelationship between pain severity (as scored on a numeric pain scale) and the presence of bone bruise in patients with sustained knee injury.

Neuromuscular Electrical Stimulation Versus Voluntary Muscle Contraction

Snyder-Mackler and colleagues104 performed an RCT of rehabilitation after ACL reconstruction with either a semitendinosus tendon combined with a ligament aug- mentation device or a central third BPTB preparation. After surgery, patients were randomized to undergo reha- bilitation with neuromuscular electrical stimulation and volitional exercises or with volitional exercises alone. All subjects documented compliance with the use of a training log; follow-up measurements of gait and thigh muscle strength were made at 8 weeks. Patients who underwent rehabilitation incorporating combined volitional exercises and neuromuscular electrical stimulation had more nor- mal gait parameters and stronger quadriceps muscles compared to patients who underwent volitional exercises alone. Subsequent to this effort, Snyder-Mackler et al103 reported the results of a multicenter RCT of rehabilitation after ACL reconstruction with a variety of graft materials and surgical techniques. All patients participated in the same rehabilitation program 3 times per week for the first 6 weeks and were then randomized to have additional treatments of either high-intensity neuromuscular electri- cal stimulation, high-level volitional exercises, low-intensity neuromuscular electrical stimulation, or combined high- and low-intensity neuromuscular electrical stimulation. Rehabilitation was monitored with training logs, and the investigators making the follow-up measurements were blinded to the treatment groups. The 4-week follow-up revealed that high-intensity neuromuscular electrical stimulation combined with volitional exercises was better at restoring extensor strength compared to volitional exer- cises alone. The methods used to perform the randomization were not presented, and the proportion of subjects who were fol- lowed up was not described in either of the above-mentioned studies. In the more recent study,103 the investigators mak- ing the follow-up measurements were blinded to the treat- ment groups that the subjects were assigned to, whereas there was no mention of whether this procedure was fol- lowed in the earlier study.104 There appears to be consen- sus that rehabilitation with volitional exercises combined with neuromuscular electrical stimulation results in more normal gait parameters and better restoration of extensor strength compared to rehabilitation with volitional exer- cises alone.

Marrow Stimulation Techniques

Soon after Magnusson described open debridement of chondral injuries, Pridie103 described drilling of denuded areas of articular cartilage to stimulate reparative carti- lage formation. In 1976, Mitchell and Shepard85 demon- strated that such treatment resulted in repair tissue but that the early repairs deteriorated after 1 year in a rabbit model. In the early 1980s, Johnson59 introduced abrasion arthroplasty, which used a motorized instrument to arthroscopically remove 1 to 3 mm of subchondral bone. In contrast to these techniques, the contemporary microfrac- ture technique is a relatively reproducible and atraumatic method of exposing the defect to pluripotential marrow stem cells without bone removal or the risk of thermal necrosis. This technique, popularized by Steadman et al119 in 1997, uses arthroscopic picks to penetrate the subchon- dral bone in a controlled pattern within a carefully pre- pared lesion. A more complete description of this technique and outcomes will be presented in part 2. Techniques designed to stimulate marrow rely on the differentiation of primitive mesenchymal cells to produce fibrocartilage, which is repair cartilage.26 Unlike hyaline cartilage, which contains primarily type II collagen, fibro- cartilage is primarily composed of type I collagen, with marked differences in biomechanical and structural prop- erties.7,22 After these techniques (drilling, abrasion arthro- plasty, microfracture), the extent of fill is rarely more than 75% of the total volume of the chondral defect, and the bio- mechanical properties of the repair fibrocartilage are infe- rior to those of hyaline cartilage.23

Graft Position (Single-Tunnel Technique)

The ACL has a complex, 3-dimensional attachment to bone. The femoral insertion of the ACL does not insert on a flat area that is aligned in an anatomical plane, as many publications have suggested; rather, it is located on a curved surface, with the wall of the femoral notch becom- ing the roof of the femoral notch. Our review of the litera- ture revealed that different approaches for characterizing the location of tunnel position in relation to anatomical landmarks (eg, the location of bone tunnels, or an ACL graft fixed within bone tunnels, in relation to anatomical landmarks) were a source of confusion. These different approaches stem in part from the fact that arthroscopic visualization of the knee occurs when it is flexed, whereas standard anatomical nomenclature is referenced to the fully extended knee.7 Although most orthopaedic surgeons would agree that graft position is a critical variable affecting the outcome of ACL reconstruction, our review found no prospective RCTs that studied the superiority of one position as opposed to another. This finding is not surprising because it would be unethical to perform a superiority or equivalency trial involving the placement of a graft in any location other than where it is considered optimal. Consequently, the effect of graft position on clinical outcome has been, and probably will remain, based on prospective studies that are descriptive in design. Our review of the literature revealed 2 prospective stud- ies that evaluated the relationship between ACL graft insertion site and outcome.45,66 In the well-designed study by Good et al45 of bone-patellar tendon-bone (BPTB) ACL reconstruction, the center of the femoral tunnel was meas- ured from a lateral radiograph of the knee, was expressed as a percentage of the total condylar depth measured along a line constructed parallel to the Blumensaat line, and was then referenced to the center of the anatomical insertion of the ACL.44 Two years after surgery, subjects with an ACL graft insertion greater than 2 mm anterior to the center of the anatomical femoral insertion had signifi- cantly greater anterior-posterior (A-P) knee laxity values than subjects with centrally or posteriorly positioned femoral tunnels. Khalfayan et al66 built on the earlier work of Good et al45 by measuring the tibial and femoral tunnel positions of central third BPTB grafts with the use of lat- eral and anteroposterior view radiographs; they also reported that graft tunnel position has a direct effect on clinical outcome. A femoral tunnel placed too far anteriorly was found to limit knee flexion or produce graft attrition. Position of the tibial tunnel is also important. Anterior placement causes graft impingement against the roof of the femoral notch when the knee is extended, with per- sistent flexion contracture or graft attrition and subse- quent failure. It is important for us to point out that most of what is known about the effect of graft impingement on outcome has been derived from retrospective studies, and this issue is a concern because when making a diagnosis of graft impingement after failure of ACL reconstruction, one must consider that anterior subluxation of the tibia rela- tive to the femur may give a false impression of impinge- ment.5 Howell and Clark53 performed a retrospective study with a 6-month follow-up period of BPTB grafts and reported that knee stability and extension were signifi- cantly better when the center of the tibial tunnel was 2 to 3 mm posterior to the center of the normal ACL insertion on the tibia. In a subsequent retrospective study, Howell et al54 reported that a tibial tunnel angle of 75° or more in the coronal plane was associated with greater loss of flex- ion and increased anterior knee laxity. A tibial tunnel angle between 65° and 70° was recommended. Our group evaluated the relationship between the elon- gation behavior of central third BPTB grafts produced by flexion-extension of the knee at the time of surgery (a measurement that is dependent on the location of the graft's insertion to bone) and clinical outcome.16 Graft elongation values produced by knee flexion-extension motion, at the time of surgery, outside the 95% confidence limits of the normal ACL resulted in significant increases in anterior knee laxity at 5-year follow-up, whereas grafts with elongation values similar to the normal ACL had A-P knee laxity values similar to the normal knee.19 This find- ing suggests that it is not only important to restore A-P laxity to within normal limits at the time of surgery, it is also important to consider the biomechanical behavior of the graft at the time of surgery. Arthroscopic visualization, combined with modern drill guides, allows experienced surgeons to identify where they want to locate bone tunnels relative to landmarks such as the ACL "footprint," the "over-the-top" position, and the clock face (eg, 2 o'clock) positions. Even with the use of these advanced tools, placing the tibial and femoral bone tunnels in desired locations without creating impingement of the graft against the femoral notch is a challenging task. This point is well emphasized by the work of Kohn et al,68 who studied a series of cadaveric knees after endo- scopic ACL reconstruction with a BPTB graft. Only 17% of the knees were considered to have correct tunnel place- ment without graft impingement. Femoral tunnel place- ment was considered excellent, acceptable, or unaccept- able in 17%, 33%, and 50% of the knees, respectively. Similarly, tibial tunnel placement was classified as excel- lent, acceptable, or unacceptable in 42%, 33%, and 25% of the knees, correspondingly. The position of an ACL graft is the most critical surgical variable because it has a direct effect on knee biomechan- ics and, ultimately, on clinical outcome. Our review of the literature revealed that the position of an ACL graft has been measured with 2-dimensional, radiographically based approaches. When the radiographic technique is obtained in a standardized manner—for example, with the posterior aspects of the femoral condyles superimposed in a lateral view—and measurements are made relative to reproducible bone-fixed coordinate systems, this approach has the advantage of characterizing graft position in a manner that is clinically relevant and that can be repro- duced in biomechanical studies. However, an accurate descrip- tion of an ACL graft's position requires a 3-dimensional technique that is applied to make measurements relative to standardized bone-fixed coordinate systems. Although such technology has recently become available in the form of electromagnetic and video-based position tracking sys- tems, there has been no clinical study reported that has determined the relationship between the 3-dimensional position of an ACL graft and clinical outcome. Currently, there are limited data available from prospective studies that identify the optimal intra-articular position of an ACL graft on the femur and tibia. There is, however, con- siderable lower-level evidence that derives from retrospec- tive studies. Our assessment of the literature is that the center of the femoral attachment of an ACL graft should be located along a line parallel to the Blumensaat line just posterior to the center of the normal ACL's insertion to bone and at either the 2 o'clock position (left knee) or the 10 o'clock position (right knee) when observed through the femoral notch. The tibial tunnel should be placed to avoid impingement of the graft against the roof of the femoral notch as the knee is brought into extension.

EPIDEMIOLOGY: ACL INJURIES IN THE GENERAL PUBLIC AND THE GENDER BIAS ASSOCIATED WITH ACL INJURY

The ACL is the most frequently, totally disrupted ligament in the knee,196 and although this injury is relatively uncommon in the general population,64 it occurs frequently in athletics, particularly among female athletes.19,20,120 The only study on the prevalence of ACL injuries in the general population has estimated the annual incidence rate as 1 injury for every 3500 people, resulting in approximately 95 000 new ACL disruptions per year in the United States.64,191 This estimate is low because more than 100 000 ACL reconstructions are performed per year in the United States.227 Although the incidence rate of ACL tears for female ath- letes ranges between 2.4 and 9.7 times greater than that of male athletes competing in similar activities (Table 1),§ overall, there are more ACL reconstructions performed on males in the United States because more males partici- pate in at-risk sports, for example, American football.227

Double-Tunnel ACL Reconstructions

The complex anatomy and the biomechanical function of the ACL have been well described by numerous authors. Woo et al297 believe that some of the failures after ACL reconstruction occur because ACL grafts, which are placed through a single femoral and tibial tunnel, are insufficient to control combined internal and valgus torques applied to the knee. Several investigators, using human cadaveric knees, have reported improved ability to restore ACL func- tion after ACL reconstructions in which 2 femoral attach- ment sites are used to more nearly replicate the normal ACL anatomy.llll In spite of these findings, Radford et al238 concluded that 2 femoral attachment sites were not supe- rior to a single site in a sheep model of ACL reconstruc- tion, probably because of the more complex surgery. Likewise, Hamada et al,109 comparing 2 case series, found no evidence that using a bisocket ACL reconstruction in humans improved results compared with a single-socket technique. In an RCT, Adachi and colleagues1 also found no evidence that a 2-tunnel hamstring autograft restored more normal knee laxity than a single-tunnel procedure. Even so, it stands to reason that more closely reestablish- ing the complex anatomy of the ACL may be advanta- geous. However, care must be taken when converting to 2- socket techniques at the present time because the added complexity of the surgery may negate the theoretical advantages. Carefully designed prospective RCTs are nec- essary to establish the efficacy of these complex proce- dures.

SUMMARY

The complex and highly specialized composition of normal articular cartilage makes it a formidable challenge to replace or repair once damaged or lost. Asymptomatic lesions have an unclear incidence or likelihood to progress to symptomatic defects, but after careful patient evalua- tion that identifies associated abnormalities, various sur- gical treatment options for symptomatic focal chondral defects can lead to improved function and decreased symp- toms. In part 2 of this "Current Concepts" article, we will discuss the specific techniques and outcomes of these var- ious methods of cartilage restoration.

Response to Injury

The complex structure and function of normal articular cartilage can be disrupted by even minor injuries. The response to the injury depends on the severity and depth of the injury. Low-energy, seemingly trivial superficial injuries may disrupt or damage cells and matrix and initi- ate a cascade toward degeneration in the absence of visi- ble changes to the surface. Larger macrodisruption injuries may result in visible chondral fissures or partial- thickness loss. Full-thickness injuries result when the sub- chondral bone is violated, often resulting in an osteochon- dral fracture. The highly specific microscopic anatomy and interde- pendent physiology of articular cartilage can be disrupted by small, superficial injuries, even without immediate car- tilage loss. Superficial damage will injure chondrocytes, limit their metabolic capacity for repair, and lead to decreased proteoglycan concentration, increased hydra- tion, and altered fibrillar organization of collagen.71,75,77,89 Proteoglycan loss, increased water content, decreased car- tilage stiffness, and increased hydraulic permeability lead to increased force transmission to the underlying sub- chondral bone, which increases its stiffness and, in turn, causes impact loads to be more readily transmitted to the partially damaged cartilage. This vicious cycle is thought to contribute to the progression of partial-thickness artic- ular cartilage injuries.88 After autologous osteochondral plug transfer, there is less stiffness of the transferred car- tilage at 6 weeks, but this stiffness returns at 12 weeks.90 The avascular nature of articular cartilage means that pure cartilage injuries do not cause hemorrhage or fibrin- clot formation or provoke an immediate inflammatory response. The chondrocytes respond by proliferating and increasing the synthesis of matrix macromolecules near the injury site, but the new matrix and proliferating cells cannot restore the surface.75 A full-thickness injury to articular cartilage that pene- trates subchondral bone provides access to cells, blood sup- ply, and, theoretically, a higher capacity for repair.42 Localized bleeding initiates a cascade beginning with hematoma formation, stem cell migration, and synthesis of type I cartilage, resulting in fibrocartilage rather than the hyaline cartilage produced by the chondrocyte.40 This repair tissue has inferior stiffness, inferior resilience, and poorer wear characteristics than does normal hyaline or hyaline-like articular cartilage.91 After a successful microfracture procedure (discussed in part 2), the result- ing fibrocartilage covering must be protected with com- plete compliance with postoperative limitations to achieve optimal outcomes. Forces applied to articular cartilage restoration tissue create a challenging mechanical envi- ronment for an appropriate healing response, but studies show that without exposure to some joint motion and physiologic load, chondrocytes will atrophy.18 A variety of growth factors (eg, transforming growth factor-β [TGF-β], bone morphogenic proteins, insulin-like growth factor [IGF], fibroblast growth factor [FGF], and platelet-derived growth factor) influence chondrocyte and other mesenchymal cell functions such as cell migration, proliferation, matrix synthesis, and differentiation. Basic FGF (B-FGF), IGF-I, and TGF-β have been shown to stim- ulate matrix synthesis in vivo. Some growth factors poten- tiate the metabolic effects of other growth factors. For example, TGF-β can potentiate the mitogenic effects of B- FGF or IGF-I, and IGF-I and B-FGF act synergistically to increase matrix synthesis. Further work is required to identify the most effective factors or combination of fac- tors, the optimal doses and methods of delivery, and the best methods of maintaining and releasing them at the site of cartilage injury.23 A thorough understanding of this complex response to injury has led to the development of gene transfer tech- nology as novel treatment avenues for damaged articular cartilage. Several cDNAs have been cloned that could stimulate cartilage healing by inducing chondrocyte mito- sis and matrix synthesis, inducing chondrogenesis by mes- enchymal progenitor cells, or inhibiting cellular responses to inflammatory stimuli that damage articular cartilage. This technology is being applied to deliver a vector to a cartilage defect or through the synthesis of cartilaginous implants. The basic science behind this technology is encouraging, and in the future, perhaps it will be used to guide biological processes toward both accelerated and improved articular cartilage repair. Currently, however, there are no clinical applications to this technology available.122

Functional Braces

The effect of a functional brace on the knee and ACL graft is determined by the brace attachment technique, brace design parameters, the brace-limb attachment interface, and the loading environment to which the braced knee is exposed. Our review revealed 2 RCTs that studied the effect of functional bracing on healing after ACL recon- struction with a BPTB graft. McDevitt and associates75 presented a prospective RCT comparing rehabilitation using functional bracing for 1 year to rehabilitation without bracing after ACL recon- struction with a BPTB graft. The patients were random- ized by random numbers or a coin toss. Both groups of 50 patients were treated for the first 3 weeks after surgery with a rehabilitation brace locked in extension. The brace was removed 2 to 3 times per day for range of motion activ- ities. In the functional brace group, the knee was mobilized gradually from 3 to 6 weeks with the rehabilitation brace used intermittently. Then, the patient was fitted for a func- tional brace at 6 weeks and was allowed full range of motion. The brace was worn full-time for the following 6 months and thereafter during all rigorous activities until 1 year after surgery. In the nonbraced group, bracing was discontinued after 3 weeks. Other details of the rehabili- tation protocol were not provided. Ninety-five percent of the patients were followed up for a minimum of 2 years (mean, 29 months). At the time of final follow-up, no dif- ferences were revealed between the groups in terms of A- P knee laxity, 1-legged hop distance, IKDC and Lysholm scores, range of motion, and isokinetic strength. Two braced subjects and 3 nonbraced subjects sustained rein- juries to their ACL graft. The authors concluded that there were no significant differences between the braced and nonbraced treatment groups. In a prospective RCT, Risberg et al93 compared rehabili- tation with functional bracing to rehabilitation without bracing after ACL reconstruction with BPTB grafts in 60 patients. The number of surgeons was not provided; the patients were randomized into braced and nonbraced groups by an unspecified method. The braced group was protected by a rehabilitation brace for 2 weeks, and a func- tional brace was used nearly full-time for the following 10 weeks. Thereafter, the functional brace was used as needed for sports. The nonbraced group had no brace at any time postoperatively. Otherwise, both groups followed the same postoperative rehabilitation protocol, which was described in detail. Ninety-three percent of the patients were fol- lowed up for 2 years. The authors found no evidence that bracing had an effect on knee joint laxity, range of motion, strength, functional knee tests, patient satisfaction, or pain at final follow-up. No evidence that bracing reduced the risk of new injury was observed. The 2 studies reviewed in this section do not identify a compelling reason to use functional braces after ACL reconstruction.

Incidence and Natural History of Chondral Injuries

The exact incidence of symptomatic high-grade chondral injuries is poorly defined. It has been reported that between 5% and 10% of young, active patients who present with a hemarthrosis of the knee after a specific traumatic event will have a focal chondral injury.93 Curl et al31 retro- spectively reviewed 31 516 knee arthroscopies of patients in all age groups and reported chondral lesions in 19 827 (63%) of patients, with a mean of 2.7 articular cartilage injuries per knee. The incidence of grade III lesions was 41% and grade IV lesions was 19%. In the younger popu- lation (younger than 40 years), however, there was an inci- dence of unipolar grade IV lesions of the femoral condyle of only 5%. A review of 1000 arthroscopies by Hjelle et al50 also reported an incidence of 5% grade III and IV chondral lesions. Many of these lesions are clinically silent at the time of detection. In a review of 993 knee arthroscopies in patients with a mean age of 35 years, there was an 11% incidence of full-thickness lesions (International Cartilage Repair Society grade III or IV)17 that could have benefited from surgical treatment.6 The incidence of these asympto- matic lesions in the general population can only be inferred from these limited data. Although the precise likelihood of a lesion becoming symp- tomatic with time is unknown, chondral lesions have been shown to further degenerate within the knee.67,82 In a series by Shelbourne et al, 123 incidental chondral lesions discovered at the time of more than 2700 ACL reconstruc- tions caused patients to report lower (P < .05) Noyes sub- jective scores than did controls with normal articular car- tilage after a mean of 8.7 years. Lateral chondral lesions caused worse subjective scores than did medial chondral lesions, despite the absence of changes on radiographs.116 Radiographic evidence of progression of untreated focal chondral defects exists, however. Recent studies following unipolar, unicompartmental full-thickness articular carti- lage lesions after simple debridement have shown pro- gression to joint space narrowing as shown on radi- ographs.82 Once early changes occur on radiographs, pro- gression toward osteoarthritis is likely.130 Studies using newer cartilage-specific MRI protocols demonstrate a close correlation with chondral defects, clinical symptoms, and a likelihood of symptom progression.68 After partial menis- cectomy, up to 6.5% volumetric loss of articular cartilage per year has been demonstrated, implicating menisci as having a protective role.29 Even if associated ligamentous instability is successfully treated, untreated focal chondral lesions may progress; small lesions may remain asympto- matic,35,74 but larger lesions (>2 cm) that are not "well shouldered," meaning that the periphery of the lesion has a clearly identifiable edge with vertical walls, are likely to progress and become more symptomatic with time.110,118

HISTORICAL PERSPECTIVE AND BASIC SCIENCE CONSIDERATIONS OF TREATMENT OPTIONS

The first arthroscopic treatment of chondral injuries was to debride the cartilage to reduce mechanical symptoms and inflammation that may arise from inflammatory mediators. Early cartilage repair techniques penetrated the subchondral bone to recruit pluripotential mesenchy- mal marrow stem cells that would differentiate and form fibrocartilage.19 Recently, autograft and allograft osteo- chondral plugs with true hyaline cartilage and subchon- dral bone have become popular. Biologic replacement with autologous chondrocyte implantation has led to more advanced biologically derived solutions to cartilage restora- tion. Future directions will likely involve synthetic implants and single-stage biologically active carriers or matrices.

IDENTIFICATION OF LIGAMENT DOMINANCE

The measurement of neuromuscular imbalances allows in- dividuals at potential risk for ACL injury to be identified.23 An athlete may display techniques that place her at risk be- cause of one or more imbalances. During single-leg landing, pivoting, or deceleration, the motion of a ligament-dominant knee in an athlete may be directed by the external ground reaction forces, rather than by her musculature.22 Initial eval- uation of an athlete's level of ligament dominance can be ac- complished using a 31-cm box-drop test combined with a maximum-effort vertical leap.33,49 When landing from a box, a ligament-dominant athlete may display substantial medial knee motion in the coronal plane that can be visually identified. Medial knee motion may be related to an overall dynamic knee valgus (femoral adduction, femoral internal rotation in relation to the hip, tibial external rotation in relation to the femur with or without foot pronation). Movement patterns that place an athlete in positions of high ACL load (excessive ex- ternal knee-abduction moments) combined with a low knee- flexion angle may increase the risk for ligament injury or fail- ure.61 Landing in the valgus position also correlates with increased impact ground reaction force.22 Thus, a female ath- lete who lands with excessive medial knee motion may be classified as ligament dominant. To address ligament dominance, an athlete should be taught to control dynamic knee motion, especially unwanted motions in the coronal plane. She should be shown how to use the knee as a single-plane hinge joint allowing flexion and exten- sion, not valgus and varus. Education for dynamic control of knee motion in the sagittal plane may be achieved through progressive exercises that challenge the neuromuscular system. The first step to addressing ligament dominance is to make an athlete aware of proper form and technique as well as unde- sirable and potentially dangerous positions. Athletes can be videotaped or placed in front of a mirror to make them aware that they are landing with visually identifiable medial knee motion. Second, clinicians must critically evaluate the jumping and landing sequence and provide constant, technique-oriented eedback. This feedback is similar to the coaching required to teach a specific skill required for a sport.57 Clinicians can use verbal feedback such as ''on your toes,'' ''soft-silent land- ings,'' and ''straight as an arrow'' as athletes jump; ''light as a feather'' as they land; and ''knees square,'' ''knees bent,'' ''shoulders back,'' ''head up,'' and ''touch and go.'' These cues should be repeated often to help promote a strong foun- dation of proper technique.22,62 Sports medicine clinicians should attempt to continuously bridge the gap in an athlete's perceived technique and actual technical performance. Clini- cians must be diligent in providing adequate feedback for cor- rect technical performance to facilitate the desirable neuro- muscular alterations. If the feedback is inadequate or inappropriate, then an athlete may be reinforcing improper techniques with the neuromuscular training.

Controlling the Application of Loads

The next principle of rehabilitation involves gradu- ally increasing the amount of stress applied to the injured knee as the patient returns to functional activities. This progression is used to provide a healthy stimulus for healing cartilage tissues, while assuring that forces are gradually applied without causing damage. Common clinical signs that a patient may be progressing too quickly and overloading the healing tissue are joint line pain and effusion. This should be monitored throughout the rehabilitation process. Additionally, patients may benefit from the use of orthotics, insoles, and bracing to alter the applied loads on the articular cartilage during functional activities. These devices are used to avoid excessive forces by unloading the area of the knee where the lesion is located. Unloading braces are often used for patients with subtle uncorrected abnormal alignments (such as genu varum), large or uncontained lesions, as well as in the presence of concomitant osteotomies and meniscal allografts (Figure 9).

Open- or Closed-Kinetic Chain Exercises After Anterior Cruciate Ligament Reconstruction?

The optimal rehabilitation program after anterior cruciate ligament (ACL) reconstruction has changed considerably over the past 20 yrs. Accelerated rehabilitation programs, which permit early ROM, immediate weight-bearing, and early return to sport, have become the accepted standard. The trend toward accelerated rehabilitation, however, has been based primarily on clinical perception, retrospective observations, and the patients' desire to return to full activity quickly—not on prospective randomized controlled trials. The optimal rehabilitation program after ACL reconstruc- tion remains undetermined. One of the goals of postoperative rehabilitation is to re- store range of knee motion and muscle strength to the injured knee, while protecting the healing graft from forces that could permanently deform it. It is generally thought that the biomechanical environment of the healing graft can be op- timized by prescribing "closed kinetic chain" (CKC) exer- cises and avoiding "open kinetic chain" (OKC) exercises early in the rehabilitation program. CKC exercises have been justified for early rehabilitation, in part, because they: 1) reduce the anterior-directed intersegmental forces that act on the tibia relative to the femur (2,5,6,8,9,12); 2) increase tibiofemoral compressive forces (5,6,8,9); 3) increase cocon- traction of the hamstrings (2,7,12); 4) mimic functional activities more closely than OKC exercises (6,10); and 5) reduce the incidence of patellofemoral complications (5,6,10). Despite the frequent use and acceptance of the OKC and CKC terminology, a variety of definitions can be found in the literature. For the purpose of this article, we defined OKC exercises as those in which the foot is not in contact with a solid surface. The resistive loads are applied to the tibia and transferred directly to the knee (Fig. 1). Only the muscles spanning the knee are required to per- form the exercise. Leg extension exercises and kicking are examples of OKC exercises. We defined CKC exercises as those in which the foot is in contact with a solid surface. The foot is opposed by a ground reaction force, which is transmitted to all of the joints in the lower extremity (Fig. 1). Muscles spanning all of the joints of the lower extrem- ity are used. Examples of CKC exercises are the squat, leg press, and lunge. In this brief review article, we explore the hypothesis that OKC and CKC for the rehabilitation of the ACL-recon- structed knee do not differ in their effects on graft healing, postoperative knee function, and patient satisfaction (Fig. 2). The article focuses on the OKC and CKC exercises involving knee flexion-extension. The review uses relevant biome- chanical and clinical studies to assess the potential effects that these exercises may have on graft healing. These include studies evaluating the intersegmental kinematics/kinetics of the knee, ligament strains, and clinical outcome through prospective randomized clinical trials.

REHABILITATION FOLLOWING ARTICULAR CARTILAGE REPAIR PROCEDURES

The rehabilitation progression is designed based on the 4 biological phases of cartilage ion.4,5,13,20,22,39,42,43 The duration of each phase will vary depending on the lesion, patient, and the specifics of the surgery discussed previously; however, the concepts of each phase are consistent. The following is an overview of the general rehabilitation process during each of the 4 phases and may be applied to a variety of articular cartilage repair procedures.maturation: proliferation, transitional, remodeling, and matura-

CONCLUSIONS

The review supports our hypothesis that controlled OKC and CKC exercises for rehabilitation of the ACL-recon- structed knee should not differ in their effects on graft heal- ing, postoperative knee function, and patient satisfaction. Although noninvasive biomechanical studies suggest that OKC and CKC exercises produce different loads at the knee, the direct ACL strain measurements comparing leg extension exercises up to 24-Nm resistance with squatting exercises indicate that the differences may not be clinically significant. Most prospective randomized clinical trials, although somewhat limited, suggest that both exercise types, in combina- tion, may be important for ACL rehabilitation. Additional prospective randomized clinical trials must be performed to determine the optimal time to introduce these exercises.

Extra-Articular Reconstruction in the ACL-Deficient Knee

There are many methods of tenodesing the lateral aspect of the tibia to the femur to prevent anterior lateral rota- tion of the tibia and minimize anterior translation of the tibia relative to the femur, either as an isolated procedure or as a backup for intra-articular reconstruction. Only 1 of the studies published that we evaluated was based on an RCT.11 All others reported here were based on uncontrolled case series that suffer from bias and confounding vari- ables. This factor, coupled with variations in the surgical techniques, graft materials used, and outcome measures at follow-up, makes it difficult to establish the efficacy of these procedures. Several human cadaveric investigations reveal that all of the lateral tenodeses studied overconstrained lateral tibial motion, which may eliminate the pivot shift and may adversely alter normal joint kinematics.70,76,77,150,182 These alterations prevent the normal screw home mechanism of the knee, lead to increased contact stress within the joint, and may result in the eventual failure of the tenodesis and the development of arthritis. Despite these findings, Durkan et al72 and Frank and Jackson91 reported accept- able results when a lateral extra-articular tenodesis is used alone. Several other groups have not been satisfied with the results of these procedures and do not advise their continued routine use.61,95,154,240,286 Several authors report satisfactory outcomes when an intra-articular reconstruction was used in combination with a lateral extra-articular tenodesis.2,27,134,137,173 Three investigations that compared uncontrolled case series evaluating the results of intra-articular reconstructions alone with those receiving the same procedure and a lateral tenodesis were unable to demonstrate any benefits to the lateral tenode- sis.32,244,280 Noyes and Barber211 did report improved out- comes with a lateral tenodesis when an allograft patellar tendon was used to reconstruct the ACL. In comments on another investigation, Noyes206 reported that there is no need for an extra-articular tenodesis in 90% of ACL recon- structions using autografts. Consensually validated statements resulting from an American Orthopaedic Society for Sports Medicine- sponsored workshop evaluating the role of extra-articular reconstruction in the ACL-deficient knee were published in 1992.231 The 32 panelists concluded that there is a very limited application of these procedures in the treatment of ACL injuries. Some of the consensus statements include he following: "In the skeletally immature ACL-deficient patient, the extraarticular reconstruction has little or no role as either a substitute for the injured ACL or a back-up for intraarticular reconstruction."231 "In adults, there is a limited role for extraarticular procedures used as the pri- mary means of treatment of ACL insufficiency." 231 "In the chronic anterior cruciate deficient knee, short- and long- term results of intraarticular surgery were clearly superior to those of extraarticular procedures." 231

Specific Exercise Programs

There have been 4 prospective RCTs focusing on the effect of specific exercises on thigh muscle strength after ACL reconstruction.20,49,72,76 The effect of adding isokinetic strength training to reha- bilitation programs after augmented repair or reconstruc- tion of the ACL was studied by Hehl et al.49 Significant improvements in muscle strength were found with the addition of isokinetic strength training between the sev- enth and ninth weeks after ACL reconstruction; at the 6- month follow-up, there was no difference in knee joint laxity. Blanpied and associates20 carried out an RCT of reha- bilitation after ACL reconstruction with a central third BPTB graft performed by the same surgeon. After surgery, subjects were randomized to a home-based rehabilitation program that included lateral slide exercises or the same home-based program without lateral slide exercises. All of the study subjects were followed up at 8- and 14-week intervals; results revealed that the group rehabilitated with the lateral slide exercises showed significant improvement in knee extension strength compared to patients rehabilitated without lateral slide exercises. Meyers and colleagues76 performed an RCT of rehabili- tation after ACL reconstruction with a central third BPTB graft performed by the same surgeon. All patients followed the same aggressive rehabilitation program for 4 weeks after surgery, at which time patients were randomized to undergo either 8 weeks of rehabilitation with stair climb- ing or 8 weeks of rehabilitation with cycling. Twelve weeks after surgery, there were no differences between the reha- bilitation programs with regard to isokinetic thigh muscle strength. An RCT of rehabilitation after ACL reconstruction with a semitendinosus tendon was presented by Liu-Ambrose et al.72 After surgery, patients underwent 12 weeks of rehabilitation with either isotonic strength training or proprioceptive training. Compliance with these programs was monitored through direct supervision and the use of training logs, and follow-up measurements were made over the same time interval. Follow-up included all 5 study participants in each group, a sample size that was chosen a priori based on a 10% difference in time to create peak torque between the treatments. At the end of the 12-week training period, subjects in both programs experienced similar improvements in function and subjective scores. Patients who underwent proprioceptive training experi- enced a greater increase in isokinetic torque compared to those who underwent strength training. In the 4 randomized studies reviewed above, no authors adequately described their method of randomization, no authors stated whether the investigators were blinded at follow-up, and only Blanpied et al20 and Liu-Ambrose et al72 revealed that they had minimal loss of study patients at follow-up.

TIMING OF ACL RECONSTRUCTION

There is no consensus of opinion as to the ideal timing for ACL reconstruction. In the early 1980s, it was common for patients to have ACL surgery within 1 week of the injury. Unfortunately, many patients had difficulty regaining full range of motion, and therefore, surgeons suggested that delaying surgery would minimize the possibility of arthrofibrosis. The loss of extension after ACL reconstruc- tion is often functionally worse for patients than their pre- operative instability. Other authors have felt that early ACL reconstruction might lead to more normal knee laxity and less meniscal and cartilage damage. Studies that report good results for both acute and delayed reconstructions of the ACL can be found in the lit- erature.129,233,256 A major problem with the literature is that most studies are retrospective in nature, they suffer from bias and confounding, and there is no accepted defi- nition for the descriptors: acute versus chronic. The defini- tion of acute varies from days to weeks after injury, and subacute may include the first several months after injury. The classification of chronic or late reconstruction varies from 3 months to more than 1 year. This inconsistency makes it very difficult to compare results of acute and delayed ACL reconstructions reported in the literature. Hunter et al119 published a prospective study that com- pared 2 case series that addressed the question of timing of ACL reconstruction on postoperative motion and A-P laxity. Patients were divided into 4 groups: group 1 had reconstruction within 48 hours of the injury; group 2, between 3 and 7 days after the injury; group 3, between 1 and 3 weeks after the injury; and group 4, more than 3 weeks after the injury. Restoration of knee motion and ACL integrity after ACL reconstruction was found to be independent of the timing of surgery; however, a large pro- portion of patients who underwent ACL reconstruction within 3 weeks of the injury had additional surgical inter- vention for complications related to loss of motion, whereas none of the patients who underwent ACL reconstruction more than 3 weeks after the injury required repeat opera- tions for revision surgery or motion problems. This inves- tigation was not randomized, the patients determined the time frame within which they had surgery, and various surgical procedures were used. Other case series on timing of ACL reconstruction have been reported in the literature, but they cannot be directly compared. Some have found that ACL reconstruction per- formed relatively early has an increased risk of loss of motion and arthrofibrosis,‡‡ whereas other studies have not.119,171,174 A review article by Shelbourne and Patel,256 evaluating many of these issues, reported that the timing of ACL reconstruction should not be measured in absolute terms from the onset of the initial trauma. Patients who had excellent range of motion, little swelling, good leg con- trol, and an excellent mental state before the surgery usu- ally had a smooth postoperative course, whether the sur- gery was performed very early or was delayed. They stressed that preoperative range of motion equal to the opposite side, including hyperextension and minimal knee swelling are much more important than whether the reconstruction is performed early or late. In a recent study by Mayr et al183 that retrospectively looked at the occur- rence of arthrofibrosis in more than 223 patients after ACL reconstruction, it was evident that preoperative syn- ovitis of the knee with limited range of motion and pain was highly significant; for 70% of the patients who had a swollen, inflamed knee at the time of reconstruction devel- oped postoperative arthrofibrosis. The interval between knee trauma and ACL reconstruction was not the deter- mining factor, as patients still had an increased risk of arthrofibrosis if they had a stiff, inflamed knee 9 weeks after trauma. These findings concur with Shelbourne and Patel's conclusions. After reviewing the literature on this subject, it appears that the time interval from ACL injury to reconstruction is not as important as the condition of the knee at the time of surgery. The knee should have a full range of motion with minimal effusion; the patient should have minimal pain and be mentally prepared for the reconstruction and rehabilitation after surgery. There are no absolutes as to when ACL reconstruction should be performed, as this procedure is elective. Although some have argued that ACL reconstruction immediately after injury may help prevent additional trauma to the knee caused by giving- way episodes, there is no indication to perform surgery within the first several days or weeks after the injury if the joint is stiff, inflamed, and painful. Nonsteroidal anti- inflammatory drugs, physical therapy, and an elastic wrap may be used to decrease synovitis and improve range of motion as soon as practical after the injury. When the inflamma- tion has subsided, either surgical or nonsurgical methods of treatment can be chosen. It is hoped that adherence to these principles will help prevent the devastating compli- cation of arthrofibrosis after ACL reconstruction.

ACL Reconstruction With Bone-Patellar Tendon-Bone Versus 4-Strand Hamstrings Autograft

Thirteen prospective RCTs have compared ACL recon- struction with a bone-patellar tendon-bone graft versus the 4-strand semitendinosus graft.¶¶ All 13 studies reported that they had minimal loss of patients at the time of follow- up, 12 of the studies had a follow-up interval of at least 2 years, and 11 of the studies mentioned the randomization scheme that was used.## In these 11 studies, rehabilitation after reconstruction was the same for all groups evaluated. In 9 of these investigations, the same surgeon performed the reconstruction. Six different surgeons performed the surgeries in the study reported by Laxdal et al,156 and 8 different surgeons performed the surgeries in the report by Eriksson et al.78 Only the report by Aglietti et al5 stated that the investigators were blinded at the time of follow-up, although half indicated that an independent investiga- tor made the follow-up measurements. Outcomes common to all studies are summarized in Table 3. Carter and Edinger52 performed a study that random- ized subjects to undergo ACL reconstruction with either a central third bone-patellar tendon-bone graft, a semi- tendinosus graft, or a combined semitendinosus-gracilis tendon graft. Follow-up measurements of isokinetic thigh muscle strength were made at 6 months and included 88% of those enrolled. There was no difference in knee exten- sion or flexion strength among the 3 graft sources; however, after 6 months of healing, a substantial proportion of sub- jects in each treatment group had strength deficits of 80% or greater. Aglietti et al5 performed an investigation of single-incision ACL reconstruction that randomized subjects to receive either a bone-patellar tendon-bone graft tensioned to 20 N with the knee extended and then fixed with a transcondylar screw in the femur and interference screw in the tibia or a 4-strand hamstrings graft that was ten- sioned to 20 N with the knee extended and fixed with a transcondylar screw in the femur and a WasherLoc device in the tibia. An investigator who was blinded as to which treatment group the subjects were assigned performed the follow-up measurements, and the 2-year follow-up included all of those enrolled. The 2-year follow-up revealed no dif- ference in A-P knee laxity, the patient's perspective of the outcome as measured with the Knee Injury and Osteoarthritis Outcome score, the IKDC subjective and objective scores, anterior knee pain, muscle strength recovery, and return to sports activities between the 2 treatment groups. Aune et al25 performed an investigation of single-incision ACL reconstruction that randomized subjects to receive either a bone-patellar tendon-bone graft tensioned to 20 lb with the knee extended and then fixed with interference screws in both tunnels or a 4-strand hamstrings graft that was tensioned to 20 lb with the knee extended and fixed with the suspensory EndoButton technique on the femur and an interference screw with a staple backup on the tibia. Independent observers performed the follow-up measurements on 85% of the subjects during a 2-year interval. Each treatment group had an equal proportion of subjects lost to follow-up. Patients receiving the 4-strand hamstrings graft showed a trend toward better subjective results, thigh muscle strength, and function at the 6- month follow-up, but no differences were observed at the 2-year follow-up interval. There was significant weakness of the knee flexors among the patients receiving the 4- strand hamstrings graft, and although anterior knee pain was not different between the treatments, knee discomfort during kneeling was significantly greater for subjects receiving the bone-patellar tendon-bone graft. No differ- ence in A-P laxity values was found between the treat- ments.25 In the study by Beard et al,33 patients were randomized to have ACL reconstruction with either the central third patellar tendon using the modified Jones technique com- bined with interference screw fixation in the tibial and femoral tunnels or a 4-strand hamstrings graft with inter- ference screw fixation in the tibial and femoral tunnels. Follow-up measurements were made at relatively short intervals of 6 and 12 months and included 75% of those enrolled; however, the details of whether the follow-up measurements were made in a blinded manner were not presented. One year after surgery, there were no differ- ences between the treatments in terms of activity level, IKDC grade, and A-P knee laxity. Improvements in isoki- netic thigh muscle strength were observed for patients in both groups; however, there was a trend for patients in the hamstrings group to have decreased flexion strength in comparison with those in the patellar tendon group.33 Eriksson et al78 performed a study in which patients were randomized to have ACL reconstruction with either a central third patellar tendon graft fixed with interference screws in the tibia and femur or a 4-strand hamstrings graft fixed with the EndoButton in the femur and 2 screw- spiked washers in the tibia. Independent observers per- formed follow-up measurements on 94% of the original subjects at a minimum of 24 months. The ACL graft was torn in 2.4% and 3.75% of those receiving the bone-patellar tendon-bone and 4-strand hamstrings grafts, respectively. There were no differences between the treatments in terms of activity level, IKDC grade, A-P knee laxity, ability to perform the 1-legged hop test, visual analog measures of patient satisfaction and knee function, and patellofemoral pain scores. Subjects who underwent reconstruction with a patellar tendon graft had a slight decrease of knee exten- sion in comparison to the contralateral, normal side.78,79 Shaieb et al251 presented a well-controlled study of single- incision ACL reconstruction with subjects randomized to either a bone-patellar tendon-bone graft or a 4-strand hamstrings graft, each of which used the same interfer- ence screw fixation in the tibia and femur. An independent observer performed the 2-year follow-up, which included 85% of the subjects. This study revealed no differences between the treatments in terms of activity level; A-P knee laxity; ability to perform jumping, cutting, and pivoting activities; and return to sports. Ninety-seven percent of the subjects receiving the patellar tendon graft and 100% of those treated with the 4-strand hamstrings graft rated their outcome as good or excellent. In contrast, subjects receiving the patellar tendon graft had more anterior knee pain (42% reported pain) in comparison to those receiving the hamstrings graft (20%), and a significantly greater proportion of subjects in the patellar tendon group (52%) experienced a loss of motion compared to those in the hamstrings group (27%). This difference may not have been clinically relevant because when considering all sub- jects, the loss was only 3.4° and 0.97°, respectively. The patients apparently did not think that the anterior knee pain was very troublesome.251 Ejerhed et al75 also reported the results of single-incision ACL reconstruction. All subjects had the same graft ten- sioning with the knee in hyperextension, using interfer- ence screw fixation of the graft in the tibia and femur. Patients were randomized to have ACL reconstruction with either a bone-patellar tendon-bone graft or ham- strings graft (38% received a 3-strand graft and 62% had a 4-strand graft). Ninety-three percent of the subjects were observed for 2 years by 1 of 2 independent observers. The ACL graft was torn in 5.4% and 2.9% of the subjects in the 4-strand hamstrings and bone-patellar tendon-bone graft groups, respectively. There were no differences between the treatments in terms of activity level, A-P knee laxity, ability to perform the single-legged hop test, IKDC grade of outcome, isokinetic thigh muscle strength, loss of exten- sion and flexion motion, and anterior knee pain during stair walking, sitting, and activity. A significantly greater proportion of subjects receiving the bone-patellar tendon- bone graft (53%) were unable to walk on their knees in comparison to the hamstrings group (23%).75 In the study reported by Jansson et al,127 patients were randomized to have ACL reconstruction with either a 2- incision technique combined with a bone-patellar tendon- bone graft that was fixed with interference screws in the tibia and femur or a single-incision technique that was performed with a 4-strand hamstrings graft that was fixed extra-articularly with a Dacron loop proximally and a screw and spiked washer distally. Follow-up at 2 years included 90% of the subjects and was made by the treating surgeons. Eight percent of the patients were unable to attend the follow-up (all from the bone-patellar tendon-bone group), and 2% were excluded because of graft rupture (all from the hamstrings group). There were no differences between the treatments in terms of activity level, A-P knee laxity, IKDC grade of outcome, isokinetic quadriceps muscle strength, and the Kujala patellofemoral score.127 Webster et al291 performed a study that randomized patients to have a single-incision ACL reconstruction per- formed with either a central third bone-patellar tendon-bone graft or a 4-strand hamstrings graft. The femoral fixation of both graft materials was obtained with the EndoButton combined with polyester tape. The distal end of the bone-patellar tendon-bone graft was fixed with a metal interference screw, whereas the distal end of the hamstrings graft was fixed with suture line tied around a post. Radiographic examination revealed a significant increase in femoral and tibial tunnel widths for patients receiving the hamstrings graft compared with the bone-patellar tendon-bone graft. Although tunnel enlargement was greater and more common with the use of the hamstrings graft, the 2-year follow-up revealed no differences in KT-1000 arthrometer measurements of A-P knee laxity, range of knee motion, and IKDC and Cincinnati scores between the 2 reconstruction proce- dures. Feller and Webster82 performed a study that randomized patients to undergo a single-incision ACL reconstruction with either a central third bone-patellar tendon-bone graft or 4-strand hamstrings graft. The graft was ten- sioned at 70° of knee flexion. Graft fixation was similar for treatments proximally and included a suspensory loop combined with an EndoButton. Distal fixation of the bone-patellar tendon-bone graft was obtained with an interference screw, whereas distal fixation of the ham- strings graft used a suture line tied over a post. A single independent investigator performed follow-up 3 years after surgery, which included 88% of those enrolled. There were no differences between the treatments with regard to the Cincinnati knee score, IKDC grade of outcome, and rates of return to preinjury activity levels. Patients receiv- ing a bone-patellar tendon-bone graft had greater deficits in strength of the extensors at 4-month and 8-month follow- ups, whereas those undergoing the hamstrings procedure had greater deficits in flexion strength at 8 and 12 months after surgery. Of clinical concern was the finding that A-P knee laxity was significantly greater for patients treated with the hamstrings graft in comparison to those receiving the bone-patellar tendon-bone graft. At the 3-year follow- up interval, side-to-side differences of A-P knee laxity were 3 mm or greater in 15% of the subjects in the hamstring group compared with 5% of the bone-patellar tendon-bone group.82 Laxdal et al156 performed a study that randomized patients to have ACL reconstruction with either a central third bone-patellar tendon-bone graft, 3-strand semi- tendinosus graft, or 4-strand semitendinosus-gracilis graft. In all 3 treatment groups, interference screws were used to fix the graft in the tibia and femur. Follow-up was performed by 1 of 3 investigators at 2 years and included 93% of those enrolled. There was no difference between the treatment groups with regard to KT-1000 arthrometer measurements of A-P knee laxity, IKDC evaluation, Lysholm score, and Tegner activity level. Patients receiv- ing the bone-patellar tendon-bone graft reported increased knee discomfort when walking on their knees in comparison to those treated with the hamstrings graft. In contrast to findings of the RCTs that compared ACL reconstruction with the 2-strand hamstrings graft to the bone-patellar tendon-bone graft, ACL reconstruction with a 4-strand hamstrings graft appears to result in similar clinical and functional outcomes compared to reconstruc- tion with a bone-patellar tendon-bone graft during a 2- year follow-up interval (Table 3).*** These findings are supported by the recent systematic review of the literature concerning graft material used to reconstruct the ACL reported by Spindler et al.272 It is important to mention that the 2 studies that have follow-ups of 3 years or more present evidence of increased pathologic knee laxity in patients undergoing ACL reconstruction with a 4-strand hamstrings graft.6,82 It is possible that knee laxity contin- ues to increase at follow-up intervals in excess of 2 years. An explanation that no differences in outcome between bone-patellar tendon-bone and 4-strand graft materials in many previous studies may have been that they did not have an adequate sample size and corresponding statisti- cal power to establish that there was, in fact, no difference in outcomes such as A-P knee laxity. This observation is supported by the meta-analyses published by Yunes et al304 and Goldblatt99 that combined the findings of 4 and 11 randomized, controlled trials, correspondingly. Yunes et al revealed patients undergoing ACL reconstruction with a bone-patellar tendon-bone graft had A-P knee laxity val- ues that were closer to normal than those reconstructed with a 4-strand hamstrings graft. In the recent report by Goldblatt et al, reconstruction with a bone-patellar ten- don-bone graft resulted in a greater proportion of subjects with KT-1000 manual-maximum side-to-side differences of A-P knee laxity less than 3mm and a smaller proportion of subjects with a significant flexion loss compared to those reconstructed with a 4-strand hamstrings graft.99 The pri- mary advantage associated with the use of the hamstrings graft material was a lower proportion of subjects with patellofemoral crepitus and a strong trend for fewer sub- jects to experience an extension loss. Most of the studies of graft material have considered A- P knee laxity as an important primary outcome measure. It is, however, essential for us to point out that it may be equally important to restore the internal-external rota- tional laxity of the reconstructed knee to within the limits of the normal knee. To accomplish this, new techniques must be developed to measure internal-external rotational laxity of the knee, or at least document displacements of the medial and lateral compartments of the tibiofemoral joint.

DISCUSSION

This review highlights specific studies that investigate the five potential differences between OKC and CKC exercises in an effort to address the hypothesis (Fig. 2). The intersegmental forces at the knee indicate that the CKC exercises produce lower anterior shear load on the tibia, increase the tibiofemoral compressive forces, enhance muscle cocontraction, and decrease patellofemoral compressive forces near extension, all factors thought to protect the graft and restore knee function. The in vivo strain data also provide evidence that the ACL is a primary restraint to anterior-directed shear load as demonstrated by the Lachman data (Table 1), and that knee hamstring cocontrac- tions reduce ACL strains relative to isolated contractions of the quadriceps and/or gastrocnemius muscles. Although the strains are reduced, they are not eliminated when the knee is near extension (30°). Application of a compressive load to the tibiofemoral joint, such as that produced by weight bearing, strains the ACL, suggesting that the compressive load does not strain shield the ligament as previously thought. Direct com- parison of the peak ACL strains during OKC exercises were similar to the CKC exercises, although an increase in resistance during the OKC exercise produced increases in strain that did not occur during CKC exercises. Nonetheless, the strains pro- duced during the knee extension exercise against the 24-Nm resistance were similar to those produce during a Lachman test with a 150-N anterior shear load (Table 1), a test that is frequently performed in the early postoperative healing period. The effects of these exercises on graft healing, knee func- tion, and patient satisfaction must be assessed through pro- spective randomized clinical trials. The two studies directly comparing OKC and CKC protocols provide different con- clusions: one suggests an OKC program produces an increase in joint laxity and patellofemoral problems (5), whereas the other does not (10,13,14). Knee function and patient satis- faction were similar between groups in both studies. The study comparing a CKC-based rehabilitation protocol with one that contains CKC and OKC exercises indicates that the latter results in better function and earlier return to sport without increased knee laxity (13). It is well known that muscle strengthening is task specific. In reviewing these data, the combination of exercise types may be necessary to fully rehabilitate ACL-reconstructed patients back to their previ- ous level of function.

TREATMENT OF QUADRICEPS DOMINANCE

To decrease the tendency toward quadriceps dominance, ex- ercises are employed to emphasize cocontraction of the knee flexor-extensor muscles.70 It is difficult to develop a more ap- propriate firing pattern for the knee flexors while performing exercises that also strongly activate the knee extensors. If the hamstrings are adequately activated at the proper time, they can decrease ACL loading. However, at low knee-flexion an- gles, the hamstrings have little ability to protect against ACL loads.64,71,72 Additionally, at angles greater than 45, the quad- riceps resist anterior tibial translation, providing an agonistic role to the ACL.73,74 Therefore, it is important to use deep knee-flexion angles to put the quadriceps into an ACL-agonist position and the hamstrings into an ACL-protective position. Athletes trained with deep knee-flexion jumps can learn to increase the amount of knee flexion at landing and decrease the amount of time spent in the more dangerous straight-leg- ged position. We hypothesize that the repetitive achievement of proper positioning may facilitate increased muscle coacti- vation and possibly lead to reduced ACL loads. When training female athletes with dynamic exercises, especially exercises that use deep knee-flexion angles, clinicians should be aware of the potential introduction of anterior knee pain, which may occur at an increased rate in this population.75 Modifying the jump exercises with decreased knee flexion and pain-free range of motion may be warranted for training athletes to re- duce the potential for patellofemoral pain.76 Squat jumps (Figure 8) and broad jump and holds (see Fig- ure 5) can specifically address the training goal of improving the protective nature of increased sagittal-plane flexor mo- ments. Squat jumps require an athlete to go into deep knee- flexion angles, past 90. Squat jumps may increase the relative recruitment and strength of the flexor musculature, as well as provide a mechanism to learn closed-chain knee control over a large range of motion.69 Additionally, the squat jump can help teach an athlete to land in a more flexed-knee position, which decreases the quadriceps' ability to load the ACL and improves the ability of the hamstrings to offset anterior shear forces due to their line of pull.64,71-74 Although large knee-flexion angle jumps, like squat jumps, may provide increased hamstring activation through continu- ous knee range of motion, the broad jump and hop-and-hold exercises (see Figure 5) are important for training the ham- string cocontraction to provide stabilization in static position. When performing the broad-jump-and-hold exercise, ham- string firing is required early in the landing to prevent the anterior tibial shear force needed to counteract the quadriceps firing during deceleration from a landing and later in the ma- neuver to prevent the dangerous valgus knee motion. Ham- string muscles work to provide resistance against these high force motions, but they must also provide adequate cocontrac- tion to maintain upright posture. Thus, repetitive training with deep knee-flexion hold exercises may improve hamstring strength and recruitment and quadriceps agonistic support and reinforce safe positioning when performing sport maneuvers.

Osteochondral Allograft Transplantation

To treat large lesions (ie, 2.5 cm2) or those with significant bone loss, osteochondral allograft transplantation provides a valuable treatment option. The advantage of osteochon- dral allografts is the ability to provide fully formed articu- lar cartilage without specific limitations with respect to defect size. In addition, there is no concern for donor site morbidity. Potential disadvantages include graft availabil- ity, cell viability at the time of implantation, immuno- genicity, and the potential for disease transmission. A more complete discussion of allograft tissue processing can be found in part 1 of this 2-part "Current Concepts" article. Although there is no specific limitation as to the largest size of a defect that can be treated with an allograft, the minimum size is debated. Although surgeons have used allografts to treat lesions as small as 1 cm2, most reports recommend the lesion be 2 to 3 cm2 or greater.20,32,33,41 The ultimate decision of limits of graft size and patient age will require a careful measure of risks and benefits with respect to each specific patient's needs. Technique. In many cases, a medial or lateral peripatel- lar mini-arthrotomy can be used to expose the lesion. The lesion is then assessed to determine the graft shape that would best fit the defect. Well-contained, centrally located lesions can generally be replaced with a dowel-shaped graft (Figure 13). An instrumentation system (Arthrex Inc, Naples, Fla) is used to size and harvest a cylindrical graft plug from the allograft. Because of the close tolerance between the donor plug and recipient socket that results from this technique, it is usually possible to press fit the graft, eliminating the need for supplemental internal fixa- tion. The diameter of the defect is matched to the sizing cylin- der (range, 12-35 mm) that best incorporates the majority of the defect. The sizing cylinder is held centered and per- pendicular to the defect, and a guide pin is drilled in the center of the lesion to a depth of 2 to 3 cm. While the joint is irrigated with normal saline, the cannulated counter bore is drilled over the pin to create a cylindrical defect to a depth of 8 to 10 mm, and the bottom of the prepared defect is penetrated with a small drill to create vascular access channels (Figure 14). Bone depth is limited to between 8 and 10 mm to facilitate graft implantation and to limit the volume of immunogenic donor bone implanted. Shallower grafts will not achieve adequate press fit. A sterile marking pen is used to mark the 12-o'clock position of the lesion to appropriately orient the donor plug, as the depth of each quadrant of the recipient lesion is measured and used to tailor the exact depth of the final cut of the donor plug. If an entire hemicondyle is made available, it is first sec- tioned to create a flat surface perpendicular to the pro- posed harvest site (Figure 15). The allograft is secured in the allograft workstation. The bushing is secured such that the contour of the donor site matches the contour of the recipient site from a low-angle side view of the work- station, using the sizing cylinder for orientation (Figures 16 A-C). Although matching the location of the defect on the donor condyle is preferred, defects smaller than 2 cm2 are easily matched from most regions of the hemicondyle. The 12-o'clock position of the donor graft is marked. While irrigated with normal saline, the donor graft is then drilled through its entire depth with a harvester, and the graft is extracted. A ruler is used to measure and mark the graft at the depth of the 4 quadrants of the previously measured recipient site. Holding forceps are used to secure the allograft while it is irrigated and cut using an oscillat- ing saw. To facilitate insertion, the edge of the allograft is slightly beveled with a rongeur. Before insertion, pul-satile lavage is used to remove residual blood and bone marrow elements from the allograft, which further reduces the chance of disease transmission and graft immunogenicity. A calibrated dilator is inserted in the recipient socket to dilate the socket an additional 0.5 mm. The graft is press fit into the socket by hand after carefully aligning the 4 quadrants to the recipient site. Further impaction is achieved with gentle use of an oversized tamp, remaining mindful that excessive force will damage chondrocytes. The goal is a secure, well-seated plug that matches the host contour and is flush with the surrounding articular surface (Figures 17 A and B). For large lesions, 2 or more allograft plugs may be placed tangentially. If the implanted allograft is particularly large, fixation may be augmented with bioabsorbable pins or metal screws. When necessary, we prefer a headless screw with differential thread pitch that provides low-profile compression, but it may need to be removed at a later date if not properly recessed. If the lesion is not amenable to a cylindrical graft, a shell graft can be fashioned freehand, typically in a trapezoidal configuration that matches a hand-prepared defect bed using a motorized bur and oscillating saw with cold irriga- tion. Freehand sizing of a graft is more time consuming and usually requires fixation. After osteochondral allograft implantation, restricted weightbearing is recommended for at least 8 weeks to pro- tect the cartilage surface and to minimize the chance for subchondral collapse during the creeping substitution phase of graft healing. Continuous passive motion is used for6to8hoursperdayat1cycle/minforthefirst4to6 weeks. Return to normal activities of daily living and light sporting activity is considered at 4 to 6 months. In general, high-impact sports are not recommended after osteochon- dral allografting for large articular cartilage lesions because of the theoretical risk of graft collapse and poten- tial deterioration in the long-term survival of the graft.69 Outcomes of Osteochondral Allograft Transplantation. Clinical studies have indicated that younger patients with an isolated lesion secondary to trauma or OCD, and with- out other joint abnormality, tend to have more optimal out- comes after osteochondral allografting. In 1985, the University of Toronto reported the subjective outcomes of their first 100 fresh allograft shell grafts used to treat lesions of the femur, tibia, patella, and talus. At a mean of 3.8 years, modified HSS scores were good or excellent in only 56% overall. However, within this group, traumatic lesions had a 75% (36/48) success rate, whereas osteoarthritic lesions had 42% (10/24) success, and lesions from avascular necrosis had only a 27% (3/11) success rate.62 In 1999, Chu et al20 reported the results of 123 patients with a purely traumatic cause to their articular cartilage lesions. In this series, a success rate of 86% was reported at a mean of 7.5 years. There was a survivorship of 95% at 5 years and 71% at 10 years. Early failure was found in patients older than 50 years, those with bipolar lesions or mechanical axis malalignment, or those on workers' compensation. In another series of OCD of the femur, 94% (16/17) of patients treated with 10 dowel grafts and 7 shell grafts were asymptomatic at a mean of 3.5 years.32 Bugbee16 reported on 211 knees at more than 4 years; femoral grafts had a 93% (116/125) success rate, patellofemoral grafts had 76% (35/46) success, but tibiofemoral bipolar lesions had only a 65% (26/40) success rate. Uncorrected ligamentous instability and mechanical limb malalignment were associated with worse outcomes.17 In summary, it is reasonable to expect subjective improvement in 75% to 85% of patients after osteochon- dral allograft implantation treatment of properly selected chondral lesions, provided that the surgeon practices care- ful patient selection and accurately identifies and corrects concomitant knee abnormality.

mmediate Versus Delayed Weightbearing

Two prospective RCTs have compared immediate versus delayed weightbearing rehabilitation programs after ACL reconstruction, and both have reported that immedi- ate weightbearing programs produce similar clinical, patient, and functional outcomes to delayed weightbearing programs.60, Jorgensen et al60 performed a prospective RCT to evalu- ate the effect of weightbearing on the results of ACL recon- struction with the iliotibial band graft. After surgery, sub- jects were randomized to undergo rehabilitation with either immediate weightbearing or nonweightbearing for 5 weeks followed by a gradual return to full weightbearing during the first 9 weeks of healing. Evaluation 2 years after surgery revealed no differences between the groups with regard to A-P knee laxity and patient activity level (evaluated with the Tegner and International Knee Documentation Committee [IKDC] scores). In a subsequent prospective RCT of ACL reconstruction with a central third BPTB autograft, Tyler et al106 com- pared rehabilitation with immediate weightbearing to delayed weightbearing for 2 weeks. Only 2 subjects in each treatment group were lost to follow-up. At a mean follow- up of 7.3 months, there were no differences between the treatments with regard to knee range of motion, vastus medialis oblique function, and A-P knee laxity (clinical examination and KT-1000 arthrometer measurement). However, patients treated with immediate weightbearing had a decreased incidence of anterior knee pain. Authors of these RCTs did not describe their method of randomization, and they did not mention if the subjects or investigators responsible for the follow-up measurements were blinded to the treatments that were studied. The findings from these investigations indicate that immediate weightbearing after ACL reconstruction does not produce excessive loads that permanently deform the graft or its fixation and suggest that immediate weight- bearing may be beneficial because it lowers the incidence of anterior knee pain. After ACL injury and reconstruc- tion, the effect of weightbearing on the healing response of injured articular cartilage or meniscus repair is currently unknown.

Use of Cold Therapy Immediately After ACL Reconstruction

We identified a prospective RCT by Konrath et al69 that focused on the effectiveness of postoperative cold therapy in patients undergoing ACL reconstruction. After ACL reconstruction with a BPTB graft, patients were random-

Bone Bruises

With the advent of MRI, it became apparent that occult osteochondral lesions (bone bruises) are commonly found in association with ACL injuries. Several investigators reported 80% or more of ACL injuries were associated with bone bruises in the lateral compartment.131,243,271,273 The implication of this bony injury, most likely caused by impaction between the posterior aspect of the lateral tibial plateau and the lateral femoral condyle during displace- ment of the joint at the time of injury, is articular cartilage damage. Whether this injury leads to permanent sequelae has not been clearly established. Several investigators have reported that the majority of bone bruises resolve within a few months to 6 years after ACL injury.60,203 However, others have found a high incidence of chondral and subchondral sequelae as determined by MRI per- formed between 2 and 6 years after the bone bruise.60,80,181,243,287 Johnson et al131 performed histologic analysis of bone bruises in humans. They found large areas of degeneration of chondrocytes in the overlying articular cartilage and necrotic osteocytes in subchondral bone, which led them to suggest that MRI evidence of bone bruising indicates significant damage to articular carti- lage homeostasis. Faber et al80 reported MRI changes con- sistent with significant damage to the articular cartilage surface on the lateral femoral condyle in more than half of their patients observed for 6 years. During this interval, they could not find any evidence of alteration in the out- comes as measured by the Mohtadi Quality of Life out- come measurement of ACL injury between those patients who had cartilage thinning compared with those who had apparently normal cartilage. Likewise, Costa-Paz et al60 found no correlation of the MRI residual findings in patients who had articular cartilage thinning and those who did not in an IKDC outcome measure of results of ACL reconstruction with a 2-year follow-up. They found that injuries that did not result in cortical disruption resolved by 2 years, but those that were associated with impaction fracture of the subchondral bone were associated with thinning of the articular cartilage at the time of follow- up. Because of differences in the definition of the bone bruises and the methods of follow-up investigations, the comparison of published studies is difficult. Thus, it will take more extensive analysis and longer follow-up of patients who had bone bruises associated with ACL injury before we actually know the significance of these lesions.

BONE TUNNEL WIDENING

ansson et al57 performed an RCT that studied ACL recon- struction using a 4-strand hamstring graft fixed proximally with an EndoButton and distally with a screw and a spiked washer, compared to reconstruction using a central third BPTB graft fixed with interference screws in the tibia and femur. Subjects were randomized by birth year, given the same rehabilitation protocol, and followed up over 2 years. No details were presented with regard to the number of surgeons who performed the procedures or whether the follow-up measurements were made by an independent examiner. No differences were observed between the groups in A-P knee laxity values, clinical find- ings, and knee scores. At the 2-year follow-up, however, the femoral and tibial tunnel diameters detected on antero- posterior view radiographs for subjects who underwent reconstruction with the 4-strand hamstring graft had increased by means of 33% and 23%, respectively. Although the increases in tunnel diameters were consid- erable, the 2 graft materials and corresponding fixation methods were reported to produce similar outcomes.57 Webster et al108 carried out an RCT, over a 2-year inter- val, that focused on whether tunnel enlargement in ACL reconstruction differed when performed with a central third BPTB or 4-strand hamstring graft. Subjects were randomized via a computer-based random-number gener- ator to undergo ACL reconstruction with either a central third BPTB graft (fixed proximally to the femur with an EndoButton and in the tibial tunnels with a metal inter- ference screw) or a 4-strand hamstring graft (fixed proxi- mally to the femur with an EndoButton and to the tibia with suture tied to a fixation post). All subjects had the same single-incision procedure performed by the same surgeon, they all observed the same postoperative rehabil- itation program, and 94% of the subjects were followed up at 4 months, 1 year, and 2 years. The clinical outcome was similar between the treatments; however, bone tunnel enlargement was more common and greater with the 4- strand hamstring graft. Eleven percent of the subjects who underwent ACL reconstruction with a BPTB graft had tunnel widening greater than 25%, compared to 94% of subjects who received a hamstring graft.108 Although it is clear that bone tunnel enlargement occurs more frequently and is greater after ACL reconstruction with hamstring grafts compared to BPTB grafts, the cause and clinical relevance of tunnel widening remain unclear. One explanation for the increased tunnel widening associ- ated with hamstring grafts secured with EndoButton fixa- tion has been offered by Jorgensen and Thomsen,61 who observed movement of the graft in the proximal two thirds of the tibial tunnel with cinematic MRI. Alternatively, L'Insalata et al71 reported that the tunnel expansion asso- ciated with hamstring grafts may be produced by the greater distance between fixation points, in comparison to BPTB grafts, and the corresponding larger force-moment produced during graft cycling. To date, we are unaware of any study confirming that tunnel widening has an adverse effect on the outcome of ACL reconstruction. Perhaps such an effect will be found as the number of cases identified with this condition and the length of follow-up periods increase. Significantly enlarged bone tunnels, however, make revision ACL reconstruction more difficult.

Cartilage Restoration, Part 2 Techniques, Outcomes, and Future Directions

n part 1 of this 2-part Current Concepts article, we reviewed the basic science of normal articular and menis- cal cartilage and its response to injury. We reviewed the historical perspectives and basic science of various carti- lage restoration methods and presented a rationale for patient evaluation, treatment selection, and timing. In part 2, we review the specific indications for the treatment of chondral injuries and describe the techniques and out- comes of the various treatment options. In addition, we examine specific complex clinical scenarios emphasizing comorbid conditions including ligament instability, menis- cal deficiency, and malalignment. A limited review of the application of these techniques in joints other than the knee is also presented. A conceptual algorithm is devel- oped to assist in clinical decision making. After each technique description, a brief review of out- comes is presented. In these outcomes, there is a prepon- derance of subjective data related to patients' reports of decreased symptoms and increased function. Objective data, including direct arthroscopic visualization, MRI, and biopsies of the treated lesions, are included where avail- able.

Basic Science, Historical Perspective, Patient Evaluation, and Treatment Option

or more than 2 centuries, the medical community has known that articular cartilage damage is a "troublesome thing and once destroyed, it is not repaired."53 Partial- thickness articular cartilage defects do not heal but, fortu- nately, are only rarely associated with significant clinical problems.75 Chondral lesions that involve the subchondral bone may fill with fibrocartilage, which has inferior bio- mechanical and biochemical features compared to hyaline cartilage.16,40,75 Small full-thickness cartilage lesions can fill with fibrocartilage and render a patient asymptomatic, but large osteochondral defects are less likely to benefit from the fibrocartilaginous healing response and more fre- quently result in pain and disability.30,75 Surgical proce- dures supported by basic science principles of cartilage physiology and known responses to injury are evolving to treat these lesions. Selecting the proper treatment algo- rithm for a particular patient depends on careful patient evaluation, including the recognition of comorbidities such as ligamentous instability, deficient menisci, or malalign- ment of the mechanical limb axis or extensor mechanism. These comorbidities may need to be treated in conjunction with symptomatic chondral injuries to provide a mutually beneficial effect. Thus, treatment of chondral injuries is often combined with ligament reconstruction, meniscus transplantation, and realignment osteotomies to achieve maximum benefit. Although cartilage restoration proce- dures are most commonly used to treat lesions in the knee, they are now being applied to other diarthrodial joints as well. A central tenet of cartilage restoration is to leave future treatment options available should they become necessary. In this article (part 1), we review the basic sci- ence of chondral injuries, the historical perspective of the available surgical options, and the present guidelines for patient evaluation and treatment selection. In part 2, sur- gical techniques and outcomes will be presented.

Current Concepts in the Rehabilitation Following Articular Cartilage Repair Procedures in the Knee

rticular cartilage defects of the knee are a common cause of pain and functional disability in orthopedics and sports medicine. The avascular nature of articular cartilage predis- poses the individual to progressive symptoms and degenera-tion due to the extremely slow and often times inability of the cartilage to heal.6-10 Nonoperative rehabilitation and palliative care are frequently unsuccessful, and further treatment is required to alleviate symptoms. This presents a significant challenge for patients, particularly young and more active individuals, that present without gross degenerative changes but rather focal cartilage defects. Traditional methods of treatment, such as nonoperative treatment and lavage, have led to unfavorable results,29,35,52 stimulating the need for newer surgical procedures designed to facilitate the repair or transplantation of autogenous cartilage tissue. Postoperative rehabilitation pro- grams will var y greatly among pa- tients and are individualized based on the characteristics of the lesion, patient, and surger y. Thus, the development of an appropriate re- habilitation program is challenging and must be highly individualized to assure successful outcomes. These programs are designed based upon knowledge of the ba- sic science, anatomy, and biome- chanics of articular cartilage, as well as the biological course of healing following surger y. The goal is to restore full function in each patient as quickly as possible without overloading the healing articular cartilage. In this paper we will discuss the principles of rehabilitation follow- ing articular cartilage repair proce- dures, as well as specific rehabilitation guidelines for de- bridement, abrasion chondro- plasty, microfracture, osteochon- dral autograft transplantation (OATS), and autologous chondro- cyte implantation (ACI).