Kinesiology of the knee

Factors Affecting Patellar Tracking (important!) : local and global factors

Why patellar tracking is important : - The large compression forces that naturally occur at the patellofemoral joint are typically well tolerated, provided that the forces are evenly dispersed across the largest possible area of articular surface. - A joint with less than optimal congruity, or one with subtle structural anomalies, will likely experience abnormal "tracking" of the patella. - As a consequence, the patellofemoral joint is exposed to higher joint contact stress, thereby increasing its risk for developing degenerative lesions and pain. - Such a scenario may ultimately lead to patellofemoral pain syndrome or potentially trigger osteoarthritis - Quadriceps muscles - excessive lateral pull (Q-angle) and stabilizing effect of compressive forces Factors that oppose lateral pull of the quadriceps on patella - Local factors (Anything that adds on to the patella) -balance between the following: - Lateral 'bowstringing' forces, e.g. tight ITB, lateral patellar retinacular fibres - the overall line of force of the quadriceps is indicated by the Q-angle. Biomechanically, this net lateral pull of the quadriceps produces a lateral "bowstringing" force on the patella - a larger Q-angle creates a larger lateral bowstringing force.A large lateral bowstringing force has the tendency to pull the patella laterally over a region of reduced contact area, thereby increasing the stress on its articular surfaces and potentially increasing the likelihood of dislocation - The oblique fibers of the vastus medialis (frequently abbreviated as VMO) appear uniquely oriented to help balance the lateral pull exerted on the patella by the quadriceps muscle as a whole - Global factors (anything that adds outside to the patellofemoral joint) (those that resist excessive valgus or extremes of axial rotation of tibiofemoral joint favour optimal patellar tracking of the PF joint) - Excessive genu valgum(e.g. MCL laxity or weak hip abductor resulting in ipsilateral trunk leaning or excessive pronation (subtalar eversion) -> increase Qangle -> bowstringing effect - Excessive external rotation of the knee -> Q-angle -> bowstringing More indepth on GLOBAL FACTORS: - Excessive genu valgum can increase the Q-angle and thereby increase the lateral bowstringing force on the patella - If persistent, the lateral force on the patella can alter its alignment and thereby increase the stress at the patellofemoral joint. - Increased valgus of the knee can occur from laxity of or injury to the MCL of the knee, but also indirectly from a chronic posture of the hip that involves increased adduction of the femur in an upright position. - Weakness of the hip abductor muscles, tightness of the hip adductor muscles, or a condition of coxa vara can allow the femur to slant excessively medially toward the midline during standing, thereby placing excessive tension on the medial structures of the knee—often a precursor to excessive valgus of the knee. - Furthermore, excessive pronation (eversion) of the subtalar joint can, in some cases, create an excessive valgus load and posturing at the knee in a weight-bearing position. - Excessive external rotation of the knee often occurs in conjunction with an excessive valgus load. External rotation of the knee places the tibial tuberosity and attached patellar tendon in a more lateral position relative to the distal femur. - excessive external rotation of the knee can also increase the Q-angle and thereby amplify the lateral bowstringing force on the patella.

What are patellar retinacular fibres?

- Extensions of fibrous connective tissue covering the vastus lateralis, vastus medialis, and iliotibial band - Fibres have attachments to femur, tibia, patella, quadriceps, patellar tendon, collateral ligaments and menisci - Clinical implication of tight retinaculum and surgical indication

"Screw-Home" Rotation of the Knee

Extension : - Full extension requires about 10 degrees of conjunct rotation (external rotation of tibia). - Different from the 'axial rotation' of a flexed knee, coupled to full extension and cannot be independently performed. - Driven by shape of medial femoral condyle, tension in ACL and lateral pull of quadriceps. - Outcome is maximum joint contact. - Function to increase joint congruence and thus stability. (important Flexion : - Reverse occurs. - Driven by popliteus muscles externally rotating femur to initiate femoral-on-tibial flexion or internally rotating tibia to initiate tibial-on-femoral flexion. A similar but less obvious locking mechanism also functions during femoral-on-tibial extension : - When one rises up from a squat position, for example, the knee locks into extension as the femur INTERNALLY rotates relative to the fixed tibia.

Muscles around the Knee

Extensors of the knee : - Quadriceps femoris Flexors-Rotators of the knee : - Hamstrings - Sartorius - Gracilis - Popliteus

"The Evolute" -changing axis of rotation

Flexion and extension at the knee occur about a medial-lateral axis of rotation. The medial-lateral axis of rotation for flexion and extension is not fixed, but migrates within the femoral condyles. The curved path of the axis is known as an "evolute" the migrating axis alters the length of the internal moment arm of the flexor and extensor muscles of the knee. - This fact explains, in part, why maximal-effort internal torque varies across the range of motion. Clinically, this has implications to: 1. Measurement of ROM by goniometer 2. Measurement of isokinetic strength 3. Prescription and rehabilitation following hinged knee orthosis - Many external devices that attach to the knee, such as a goniometer, an isokinetic testing device, or a hinged knee orthosis, rotate about a fixed axis of rotation. - During knee motion, therefore, the external devices may rotate in a slightly dissimilar arc than the leg. - As a consequence, a hinged orthosis, for example, may act as a piston relative to the leg, causing rubbing against and abrasion to the skin. - To minimize this consequence, care must be taken to align the fixed axis of the external device as close as possible to the "average" axis of rotation of the knee, which is close to the lateral epicondyle of the femur.

Control of Tibial-on-Femoral Osteokinematics

Flexor-rotator muscles accelerate or decelerate lower leg during swing phase of walking or running. These muscles produce low-to-moderate forces but at high shortening or lengthening velocities. - These muscles rapidly contract to accelerate knee flexion to shorten functional length of lower limb during swing phase but also quickly eccentrically decelerate knee extension during late swing phase.

Knee extensor mechanism

Knee extensor mechanism formed by: 1. Quadriceps muscle 2. Patella 3. Patellar tendon - Capable of generating up to 6,000 N in trained young males! - A cross-section through the right quadriceps muscle. The arrows depict the approximate line of force of each part of the quadriceps: vastus lateralis (VL),vastus intermedius(VI), rectus femoris(RF),vastus medialis longus(VML),and vastus medialis obliquus(VMO).Much of the vastus medialis and vastus lateralis muscles originate on the posterior side of the femur, at the linea aspera.

Knee Flexor-Rotator Muscles

Knee flexors : 1. Biceps femoris(short and long heads) 2. Semitendinosus 3. Semimembranosus Knee medial (internal) rotators : 1. Sartorius 2. Gracilis 3. Semitendinosus Knee lateral (external) rotators : 1. Biceps femoris 2. Tensor fascia lata

Clinical Application -consider the post-ACL reconstruction surgery (important!)

Lines of force of the quadriceps and hamstring muscles relative to the anterior cruciate ligament(ACL)for a knee in full extension (A) and in 80 degrees of flexion (B). Drawing is based on the mean data from five cadavers, based on the work of Herzog and Read. Note that the change in joint angle significantly alters the line of force of the muscles and the orientation of the ACL. Angles-of-insertion of the muscles are indicated relative to the long axis of the tibia. Angles are approximate, and vectors are not drawn to scale. tendency of the quads is to pull the tibia forward, the ACL stops this. therefore ACL rehab should include training hamstrings (pull tibia back). Avoid knee extension after ACL surgery

femoral-on-tibial extension (closed chain)

femoral-on-tibial extension is shown in (D) to (F).

Hamstrings as ACL

(A) Contraction of the hamstring muscles flexes the knee and slides the tibia posterior relative to the femur. Knee flexion elongates the quadriceps muscle and most of the fibers within the posterior cruciate ligament (PCL). (B) The posterior drawer test evaluates the integrity of the PCL. Tissues pulled taut are indicated by thin black arrows.

interaction between muscle contraction and tension in the anterior cruciate ligaments.

(A) Contraction of the quadriceps muscle extends the knee and slides the tibia anterior relative to the femur. Knee extension also elongates most of the anterior cruciate ligament(ACL) ,posterior capsule, hamstring muscles, collateral ligaments and adjacent capsule (the last two structures not depicted). - Note that the quadriceps and ACL have an antagonistic relationship throughout most of the terminal range of extension.(The angle-of-insertion between the patellar tendon and the tibia is indicated by α.) (B) The anterior drawer test evaluates the integrity of the ACL. Note that spasm in the hamstring muscles places a posterior force on the tibia, which can limit the tension on the ACL. - strengthen the hamstrings because it pulls the tibia back and functions like an ACL Although some fibers of the ACL remain relatively taut throughout the full range of sagittal plane motion, most fibers, especially those within the posterior-lateral bundle, become increasingly taut as the knee approaches and reaches full extension

More on lateral bowstringing of the patella

(A) Neutral alignment of the knee, showing the characteristic lateral bowstringing force acting on the patella. (B) Excessive knee valgus and knee external rotation can increase the Q-angle and thereby increase the lateral bowstringing force on the patella. - Blue arrows indicate bone movement that can increase knee external rotation, and purple arrows indicate an increased valgus load placed on the knee. - Note that the increased external rotation of the knee can occur as a combination of excessive internal rotation of the femur and external rotation of the tibia.This may be due to weak hip external rotators, weak hip abductors, tight hip adductors

Control of Femoral-on-Tibial Osteokinematics when catching ball

(A) Several muscles are shown controlling the rotation of the head, neck, trunk, pelvis, and femur toward the approaching ball. Because the right foot is fixed to the ground, the right knee functions as an important pivot point. (B) Control of axial rotation of the right knee is illustrated from above. The short head of the biceps femoris contracts to accelerate the femur internally (i.e., the knee joint moves into external rotation). - Active force from the pes anserinus muscles in conjunction with a passive force from the stretched medial collateral ligament (MCL) and oblique popliteal ligament (not shown) helps to decelerate, or limit, the external rotation at the knee.

The active arthrokinematics of knee extension

(A) Tibial-on-femoral perspective. (open chain) - During tibial-on-femoral extension, the articular surface of the tibia rolls and slides anteriorly on the femoral condyles. (B) Femoral on-tibial perspective. (closed chain) - During femoral-on-tibial extension, as in standing up from a deep squat position, the femoral condyles simultaneously roll anteriorly and slide posteriorly on the articular surface of the tibia In both (A) and (B), the meniscus is pulled toward the contracting quadriceps. - These "offsetting" arthrokinematics limit the magnitude of anterior translation of the femur on the tibia. - The quadriceps muscle directs the roll of the femoral condyles and stabilizes the menisci against the horizontal shear caused by the sliding femur.

Menisci

- 2 menisci: medial and lateral. - Crescent-shaped and fibrocartilaginous. - Lateral is more mobile than medial menisci. - Anchored to intercondylar regions by ant. and post. horns. - Attached to tibia and adjacent capsule by coronary (or meniscotibial) ligaments, which are loose, allowing movements. - Medial meniscus is attached to MCL and medial capsule; lateral meniscus to lateral capsule. - Transverse ligament links both menisci together. - Some muscles have fibres attached to menisci, e.g. quadriceps and semimembranosus attach to both menisci; popliteus to lateral meniscus (TRY TO REMEMBER). - Peripheral 1/3 receives blood from branches of popliteal artery. - Inner 1/3 is nourished by synovial fluid. - Blood supply to the menisci is greatest near the peripheral (external) border. Blood comes from capillaries located within the adjacent synovial membrane and capsule. - The internal border of the menisci, in contrast, is essentially avascular.

Right distal femur, tibia, and fibula.

- Although the fibula has no direct function at the knee, the slender bone splints the lateral side of the tibia and helps maintain its alignment. The head of the fibula serves as an attachment for the biceps femoris and the lateral collateral ligament. - (A) Anterior view. (B) Posterior view. Proximal attachments of muscles are shown in red, distal attachments in gray. The dashed lines show the attachment of the joint capsule of the knee

Bursae

- As many as 14 bursae - Some bursae are extensions of the synovial membrane - Some have fat pads, e.g. suprapatellar and deep infrapatellar bursae - Function mainly to reduce friction, absorb shock

Axial rotation in the knee

- External rotation range of motion generally exceeds internal rotation by a ratio of nearly 2 : 1. - Once the knee is in full extension, however, axial rotation is maximally restricted, rotation is only able to occur when the knee is slightly flexed - Rotation of the knee is significantly blocked by passive tension in the stretched ligaments, parts of the capsule, and increased bony congruity within the joint. Internal and external (axial) rotation of the right knee. The axes of rotation are shown as small circles near the center of the joint. (A) Tibial-on-femoral (knee) rotation. In this case the direction of the knee rotation (internal or external) is the same as the motion of the tibia; the femur is stationary. (B) Femoral-on-tibial rotation. In this case the tibia is stationary and the femur is rotating (over a partially flexed knee). The direction of the knee rotation (external or internal) is the opposite of the motion of the moving femur: external rotation of the knee occurs by internal rotation of the femur; internal rotation of the knee occurs by external rotation of the femur.

Knee complex

- Femur, tibia and patella - Tibiofemoraljoint is the largest joint in the body Tibiofemoraland patellofemoral joints - 3 articulating surfaces: 1. medial tibiofemoral, lateral 2. tibiofemoral and 3. patellofemoral articulations. - 2 degrees of freedom of motion - flexion-extension, internal-external rotation - Withstand 4-6 times of body weight - Biomechanically can support the body weight in the erect position without muscle activity, i.e. relying solely on ligamentous support - 2/3 of the muscles crossing the knee also cross hip or ankle

Anterior and Posterior Cruciate Ligaments

- Intracapsularligaments, covered by synovial membranes - Poor blood supply (medial genicular artery) - Both are thick, strong and capable of withstanding large forces before rupture (e.g. ACL - 1,800 N) - Together they resist the extremes of all knee movements - But primarily anterior-posterior shear forces between femur and tibia. ACL : - The tension and orientation of the ACL change as the knee flexes and extends. - Although some fibers of the ACL remain relatively taut throughout the full range of sagittal plane motion, most fibers, especially those within the posterior-lateral bundle, become increasingly taut as the knee approaches and reaches full extension. - In addition to most fibers of the ACL, the posterior capsule, parts of the collateral ligaments, and all knee flexor muscles also become relatively taut in extension, which helps stabilize the knee, especially during weight-bearing activities - During the last approximately 50 to 60 degrees of complete knee extension, the active force generated by the contracting quadriceps pulls the tibia anteriorly, thereby powering the anterior slide arthrokinematics. - The resulting tension in the stretched fibers of the ACL helps limit the extent of this anterior slide. PCL : - What is known, however, is that some fibers within the PCL remain taut throughout most of flexion and extension, although the majority of the ligament becomes increasingly taut with greater flexion. - Between full extension and approximately 30 to 40 degrees of flexion, most of the PCL is relatively slackened; tension peaks between 90 and 120 degrees of flexion. - In addition to becoming taut in flexion, the PCL provides a secondary restraint to varus-and-valgus loads, as well as excessive axial rotation. - While a person actively flexes the knee against gravity, such as when lying prone, the knee flexor muscles (such as the hamstrings) actively slide the tibia (along with the fibula) posteriorly relative to the femur. - The extent of the posterior slide arthrokinematics is limited, in part, by passive tension in the PCL Functions: - Provide multiple planar dynamic stability to the knee - Restrains anterior-posterior sliding between tibia and femur - Contribute to proprioception of the knee

Quadriceps femoris

- Large muscle groups 2.8 times greater cross-sectional area than that of hamstrings. - Rectus femoris(20% of total extension torque), vastus lateralis, vastus medialis and vastus intermedius (vastus group 80%). - Function of rectus femoris and vastus muscles? - Mainly extends the knee (vastus) - Extends the knee and flexes the hip (rectus femoris) - All muscles unite to form quadriceps tendon, attached to patellar base and sides, the patellar tendon in turn connects patellar apex to tibial tuberosity.

Osteology of the right patella, articular surfaces of the distal femur, and proximal tibia

- Lateral and medial epicondyles project from each condyle, providing elevated attachment sites for the collateral ligaments. - A large intercondylar notch separates the lateral and medial condyles, forming a passageway for the cruciate ligaments. - A narrower than average notch may increase the likelihood of injury to the anterior cruciate ligament - The intercondylar groove is concave from side to side and slightly convex from front to back. - The sloping sides of the intercondylar groove form lateral and medial facets. The more pronounced lateral facet extends more proximally and anteriorly than the medial facet. - The steeper slope of the lateral facet helps to stabilize the patella within the groove during knee movement. - Lateral and medial grooves are etched faintly in the cartilage that covers much of the articular surface of the femoral condyles. - When the knee is fully extended, the anterior edge of the tibia is aligned with these grooves

Lateral capsule

- Lateral view of the right knee shows many muscles and connective tissues reinforcing the lateral capsule. - The iliotibial band, lateral head of the gastrocnemius, and biceps femoris are cut to better expose the lateral collateral ligament, popliteofibular ligament, popliteus tendon, and lateral meniscus. - The majority of posterolateral knee injuries are caused by a blow to the anteromedial aspect of the knee, a contact or noncontact hyperextension injury, or a varus noncontact injury

Compressive Forces at Patellofemoral Joint

- Maximum patellar contact with trochlear groove 60-90°flexion. - Compression force in the PF joint is also greatest 60-90°flexion. - Compressive forces therefore can rise to very high levels during a descent into a squat or lunge position. - Stress (force/area) is greatest in the PF joint 60-90°flexion. - The stress in the PF joint can only be reduced with greater contact area, which is not the case. - Having the area of joint contact greatest at positions that are generally associated with the largest muscular based compression force naturally protects the joint against stress-induced cartilage degeneration. This mechanism allows most healthy and normally aligned patellofemoral joints to tolerate large compression forces over a lifetime, often with little or no appreciable discomfort or degeneration of the articular cartilage or subchondral bone. - Clinical implication includes cartilage degeneration.

Posterior capsule

- Posterior view of the right knee that emphasizes the major parts of the posterior capsule: the oblique popliteal and arcuate popliteal ligaments. The lateral and medial heads of the gastrocnemius and plantaris muscles are cut to expose the posterior capsule. - Note the popliteus muscle deep in the popliteal fossa, lying partially covered by the fascial extension of the semimembranosus. What structures block hyperextension? (in elbow, it is bony block) - The muscles and posterior capsule limit hyperextension.

Muscle action of quadriceps

- Quadriceps produced extensor torque about 2/3 greater than that produced by hamstrings. - Traditional isokinetic concentric H:Q torque about 0.5 - Isometric activation -stabilizes knee - Eccentric activation - decelerates the descent of body's centre of mass and dampens impact of joint loading, e.g. landing from a jump; consider what happens to a knee in a brace. - Eccentric activation of these muscles also provides shock absorption to the knee. At the heel contact phase of walking, the knee flexes slightly in response to the ground reaction force - Concentric activation -accelerates tibia or femur movements strongly - This action is often used to raise the body's center of mass, such as during running uphill, jumping, or standing from a seated position.

functions of the meniscus

- Reduce compressive stress across tibiofemoral joint - Stabilize joint during movements - Lubricate articular cartilage - Provide proprioception - Guide knee arthrokinematics - Reduce pressure on articular cartilage by tripling joint contact area Consider also the consequences of surgically removing meniscus (menisectomy) - A complete lateral meniscectomy has been shown to increase peak contact pressures at the knee by 230%, which increases the risk of development of stress-related arthritis. - Even a tear or a partial meniscectomy significantly increases local stress, which is strongly believed to cause excessive wear on the articular cartilage

Medial condyle is on average 1.7 cm larger than the lateral condyle in adults. Why?

- The medial condyle is larger to compensate the oblique femoral alignment to the vertical, allowing the distal femur to lie horizontal in the frontal plane.

Patella : what is it and what are its functions?

- The patella is a nearly triangular bone embedded within the quadriceps tendon. - It is the largest sesamoid bone in the body. - The patella has a curved base superiorly and a pointed apex inferiorly. - The thick patellar tendon attaches to and between the apex of the patella and the tibial tuberosity. - In a relaxed standing position, the apex of the patella lies just proximal to the knee joint line. The subcutaneous anterior surface of the patella is convex in all directions. - The posterior articular surface of the patella is covered with articular cartilage up to 4 to 5 mm thick. - Part of this surface articulates with the intercondylar groove of the femur, forming the patellofemoral joint. The thick cartilage helps to disperse the large compression forces that cross the joint. Functions: 1. Improve the efficiency and increase torque of the knee extensors during ROM 2. Centralize the forces of the four quadriceps muscles into one concerted direction of pull 3. Provide a smooth gliding mechanism for the quadriceps muscle and tendon to reduce compression and friction forces during activities such as deep knee bends 4. Contribute to the overall stability of the knee 5. Provide bony protection from direct trauma to the femoral condyles when the knee is flexed.

Patella in full knee extension

- The patella rests completely proximal to the trochlear groove and against the suprapatellar fat pad in full extension. - Only 45% of the contact area occurring at 60 degrees knee flexion is present between patella and trochlear groove in full extension. This reduced fit between patella and femur predisposes subluxation of patella laterally (in line of force of quadriceps). - If quadriceps are relaxed, patella can be moved freely.

Arthrology : genu valgum and genu varum of the femur and proximal tibia

- The shaft of the femur angles slightly medially as it descends toward the knee. This oblique orientation is a result of the natural 125-degree angle of inclination of the proximal femur. - Because the articular surface of the proximal tibia is oriented nearly horizontally, the knee forms an angle on its lateral side of about 170 to 175 degrees. - This normal alignment of the knee within the frontal plane is referred to as genu valgum. - Variation in normal frontal plane alignment at the knee is not uncommon. A lateral angle less than 170 degrees is called excessive genu valgum, or "knock-knee" . - In contrast, a lateral angle that exceeds about 180 degrees is called genu varum, or "bow-leg"

Tibiofemoral Flexion and Extension

- The tibiofemoral joint possesses two degrees of freedom: flexion and extension in the sagittal plane - and provided the knee is at least slightly flexed, internal and external rotation. These motions are shown for tibial-on-femoral and femoral-on tibial situations

summary of quadricep action and extensor lag

- Tibial-on-femoral extension (e.g. open chain leg extension) - External (flexion) torque greatest 45°to 0° - Internal (muscle) torque greatest 45°to 70°flexion - Internal moment arm (leverage) greatest 20°to 60°flexion - Femoral-on-tibial extension (e.g. squat to stand) - External (flexion) torque greatest from 90°to 45° - As the knee moves near extension, the quadriceps's ability to produce force is significantly diminished. A patient who is unable to achieve full active knee extension but has full passive motion into extension has an extensor lag. (tibia slowly falls down) - The patella functions to increase the internal moment arm of the knee extensor mechanism, thereby increasing the potential force produced by quadriceps muscles.

The anterior view of a right knee of a young adult flexed to about 90°. The anterior capsule is excised and the patella turned down to view the joint

- the transverse ligament connects the medial and lateral menisci

ACL injury and the 3 factors associated with it

A drawing of a young healthy woman immediately after landing from a jump. Note the combined and excessive valgus and external rotated position of the right knee (via internal femoral rotation over a fixed tibia). Note that in a weight-bearing position, the positions of the right hip and foot strongly influence the positions of the femur and tibia, respectively. In particular, the right hip is adducted and internally rotated, which strongly contributes to the exaggerated valgus and externally rotated position of the knee. Reduced activation of hip abductors and external rotator muscles could contribute to this position of the hip. The inset on the left shows the increased tension in the ACL and the line of force of the quadriceps muscle. - Note the relative lateral displacement of the patella relative to the trochlear groove of the femur. (Purple arrows depict excessive valgus alignment; blue arrows depict the excessive internal rotation of the femur. Three factors associated with ACL injury: 1. Strong quadriceps contraction in near/full extension 2. Marked valgus force with foot firmly planted 3. Excessive external rotation of tibia (or internal rotation of femur on fixed tibia)

maximal knee extension torque

A plot showing the maximal-effort knee extensor torques produced between about 90 and 5 degrees of flexion. The internal moment arm (leverage) used by the quadriceps is greatest between about 60 and 20 degrees of knee flexion. Knee extensor torques are produced isometrically by maximal effort, with the hip held in extension. Data from 26 healthy males, average age 28 years old. - Rapid decline as knee approaches full extension, 50%-70% reduction.

Q angle

Alignment of the shaft of the femur with the tibia form the Q angle (quadriceps angle). It is the angle created by drawing a line from the anterior superior iliac spine (ASIS) to the center of the patella and extending another intersecting line from the tibial tuberosity to the center of the patella upward. - Men -10-14° - Women -15-23° - Differences may be due to differences in strength, height, etc - Excessive Q angle -> genu valgum or knock knee - Q angle close to 0 -> genu varum or bow legged

Patellofemoral Joint

Between patellar and trochlear groove of femur Factors for stability: - Quadriceps muscle - Articular surfaces - Passive restraint from ligaments and soft tissues Mobility: - Tibial-on-femoral knee flexion - patella slides on fixed trochlear groove of femur (IMPORTANT) ; patella is pulled in the direction of tibia because of patellar tendon - Femoral-on-tibial knee flexion (e.g. squat) -trochlear groove slides relative to fixed patella (held fixed by eccentric quads and strong patellar tendon) Abnormal kinematics and possible instability of the patellofemoral joint are all too common and often are implicated with chronic anterior knee pain and even joint degeneration

Understanding the Biomechanical Interactions between External and Internal Torques

In many upright activities, an external (flexor) torque is acting on the knee. This external torque is equal to the external load being moved or supported, multiplied by its external moment arm. The external flexor torque must often be met or exceeded by an opposing internal (extensor) torque, which is the product of quadriceps force multiplied by its internal moment arm.

Medial and Lateral Collateral Ligaments and their functional considerations

MCL : - Superficial part: 10 cm from femoral medial epicondyle to the medial-proximal tibia; blends with medial patellar retinacular fibres - Deep part: shorter and oblique fibres, distal and posterior to superficial fibres, attached to posterior-medial capsule, medial meniscus and semimembranosus tendon - Because the deeper fibers of the MCL are shorter than the superficial, the deeper fibers experience a greater percentage of stretch when subjected to similar valgus (abduction) strain. - Primarily for this reason, the deeper fibers of the MCL are more frequently injured than the superficial fibers during an excessive valgus-related trauma - The collateral ligaments and adjacent capsule also provide resistance to the extremes of internal and external rotation. - Most notable in this regard are the elongation and subsequent increased passive tension in the superficial fibers of the MCL at the extremes of external rotation of the knee. - Planting the right foot securely on the ground and vigorously rotating the superimposed femur (and body) to the left, for example, may damage the superficial fibers of the right MCL. - This potential for injury increases if the externally rotating knee (i.e., internally rotating femur) is simultaneously experiencing a substantial valgus load. LCL : - Short, cordlike, more vertical, between lateral epicondyle and fibular head; blends distally with the biceps femoris tendon; no attachment to lateral meniscus. - Popliteus tendon runs between lateral meniscus and LCL. The primary function of the collateral ligaments is to limit excessive knee motion within the frontal plane

extension of the knee causes mcl and other structures to be taut, flexion opposite

MCL fibres, being more posterior to the medial-lateral axis of rotation of the knee, become taut in near or full extension. Other structures are also taut. flexion relaxes the structures

Maximal Torque Production of Flexor-Rotator Muscles

Maximal-effort knee flexion torque is generally greatest in the last 20 degrees of knee extension, and then declines steadily as the knee is flexed. A plot showing the maximal-effort knee flexor torques produced between 5 degrees and about 90 degrees of flexion. The internal moment arm (leverage) used by the knee flexors (hamstrings) is greatest between about 50 and 90 degrees of knee flexion. Knee flexor torques are produced isometrically by maximal effort, with the hip held in extension - the hamstrings (and presumably other knee flexors) generate their greatest torque at knee angles that coincide with relative elongated muscle length, rather than high leverage. - this is in slight contrast to the quadriceps, where maximal-effort knee extensor torque partially overlaps the point in the range of motion where leverage is greatest.) Flexing the hip to elongate the hamstrings promotes even greater knee flexion torque. The length-tension relationship appears to be a very influential factor in determining the flexion torque potential of the hamstring muscles

medial capsule

Medial view of the right knee shows many muscles and connective tissues. The tendons of the sartorius and gracilis are cut to better expose the superficial part of the medial collateral ligament and the medial capsule. The anterior 1/3 is a thin layer of fascia reinforced by the medial patellar retinacular fibres. The middle 1/3 includes reinforcement from medial collateral ligaments. The posterior 1/3 is the thickest (called posterior-medial capsule or posterior oblique ligament) and reinforced by muscles (e.g. pes anserinus, semimembranosus

Knee -mobility vsstability

Need for mobility : - Running and walking - Weight bearing activities - Functional activities Need for stability : - Soft tissues e.g. ligaments, joint capsule and menisci - Muscles - Lower limbs are subjected to large amount of forces in weight-bearing activities

Patellofemoral Joint Kinetics

PF joint is exposed to high compressive forces: - Walking on level surfaces (1.3 times body weight) - Straight leg raises (2.6 times) - Stair-climbing (3.3 times) - Deep 'knee bends' or squats (7.8 times) Factors influencing compressive forces experienced by PF joint: - Force within quadriceps muscle - Knee flexion angle The relationship between quadriceps activation, depth of a squat position, and the compression force within the patellofemoral joint is shown. (A) Maintaining a partial squat requires that the quadriceps transmit a force through the quadriceps tendon(QT) and the patellar tendon (PT).The vector addition of QT and PT provides an estimation of the patellofemoral joint compression force(CF). (B) A deeper squat requires greater force from the quadriceps because of the greater external (flexion) torque on the knee. Furthermore, the greater knee flexion (B) decreases the angle between QT and PT and consequently produces a greater joint force between the patella and femur. Increasing knee flexion by descending into a deeper squat significantly raises the force demands throughout the extensor mechanism, and ultimately on the patellofemoral joint. The increased knee flexion associated with the deeper squat also reduces the angle formed by the intersection of force vectors QT and PT. As shown by vector addition, reducing the angle of these force increases the magnitude of the CF directed between the patella and the femur

ACL tension when quads are working alone and when quads and hamstrings are working together

Relationship between the tension in the anterior cruciate ligament (ACL) and the knee joint angle during a submaximal force produced by (1) isolated contraction of the quadriceps and (2) a combined contraction from the quadriceps and hamstrings. The combined muscle force was designed to simulate cocontraction of the two sets of muscles. - The concepts described in the preceding paragraphs provide credence to the advice given to some patients to avoid exercises in the very early phases of post-ACL reconstruction rehabilitation that involve strong and isolated contraction of the quadriceps, specifically in the last 30 to 40 degrees of extension (tibial on femoral) - Exercises that involve femoral-on-tibial (closed kinematic chain) exercises in moderate degrees of knee flexion therefore place relatively low and usually acceptable levels of strain on the ACL. - In addition to their functional nature, these types of exercises are well ingrained into ACL rehabilitation protocols because they require quadriceps activation in a more flexed knee position, and they naturally require coactivation of the quadriceps and hamstrings muscles. - As stated above, tension in the ACL remains at zero when the muscles are coactivated at knee angles greater than 30 degrees of flexion

The kinematics at the patellofemoral joint during active tibial-on femoral extension

The circle depicted in (A) to (C) indicates the point of maximal contact between the patella and the femur. - As the knee extends, the contact point on the patella migrates from its superior pole to its inferior pole. - Note the suprapatellar fat pad deep to the quadriceps. (D) and (E) show the path and contact areas of the patella on the trochlear groove of the femur. The values 135, 90, 60, and 20 degrees indicate flexed positions of the knee. - Between about 90 and 60 degrees of flexion, the patella is usually well engaged within the trochlear groove of the femur. (important for physios) - Within this arc of motion, the contact area between the patella and femur is therefore greatest (D, E) (important for physios). - Even at this maximum, only about one-third of the total surface area of the posterior patella is in contact with femur. Joint pressure (i.e. compression force per unit area) can therefore be very high.

Tibia-on- femoral knee extension (open chain)

The external (flexion) torques are shown imposed on the knee between flexion (90 degrees) and full extension (0 degrees). Tibial-on-femoral extension is shown in (A) to (C), The external torques are equal to the product of body or leg weight times the external moment arm (EMA). (as the external moment arm increases, the external torque increases) The increasing red color of the quadriceps muscle denotes the increasing demand on the muscle and underlying joint, in response to the increasing external torque.

Capsules, its reinforcing ligaments and muscles

The fibrous capsule of the knee encloses the medial and lateral compartments of the tibiofemoral joint and the patellofemoral joint. The capsule is reinforced by ligaments, muscles and fascia. 5 regions of the capsule can be identified: 1. Anterior 2. Lateral 3. Posterior 4. Postero-lateral 5. Medial - Unlike the elbow, the knee has no bony block against hyperextension. The muscles and posterior capsule limit hyperextension.

external torque-angle graph

The graph shows the relationship between the external torque—normalized to a maximum (100%) torque for each method of extending the knee—for selected knee joint angles. (Tibial-on-femoral extension is shown in black; femoral-on-tibial extension is shown in gray.) External torques greater than 70% for each method of extension are shaded in red.

Functional Role of the Patella

The patella acts as a "spacer" between the femur and quadriceps muscle, which increases the internal moment arm of the knee extensor mechanism - Pulley system of the patella and quadriceps. A) With a patella, the lever arm of the quadriceps is larger. B)When the patella is absent, the moment arm of the quadriceps reduces significantly, causing a reduction in potential force provided by the quadriceps.