The Knee

Fibular Collateral Ligament

(lateral collateral ligament) - slants posteriorly from proximal to distal (as it passes from the lateral femoral epicondyle to the fibular head), and it does not attach to the lateral meniscus) * The ligament is extracapsular and can easily be palpated by flexing the knee, externally rotating the thigh, and palpating in the lateral joint line *Function - one of the static stabilizers that limit varus stress

Close Packed position

(position of maximal stability and joint congruence), is in full extension (0 degrees)

Closed Kinetic Chain Mechanisms

(when both ends are relatively fixed, such as occurs during ambulation [foot fixed on floor and body weight maintaining status of the hip/trunk/head, due to gravity], and there is force applied through the joint that helps maintain stability. In this case, the femur moves on a relatively fixed femur.

Knee Ligamentous Testing

done preferentially in a loose-packed state (exception of PCL) * Testing in the closed packed position tests for the integrity of the primary stabilizer of the knee joint, the PCL - If unstable in this position, there is significant ligamentous damage to the knee joint and the PCL is likely ruptured (thus, for the knee, only the PCL lends itself to examination in the close-packed position). * Testing in the loose-packed position permits testing the major ligaments of the knee individually, placing stress on ligaments one at a time with the appropriate varus, valgus, anterior, posterior, or rotational force.

Open Kinetic Chain Mechanisms

for the knee, with the foot free, such as sitting and extending the leg. In this case, without the body weight and resultant forces being expressed through the knee joint, there is less mechanical stability and more stress is put on supporting anatomical structures such as the ACL and PCL. (For patient conditions such as an ACL reconstruction, open kinetic chain movements are initially avoided, since they would tend to put too much stress on healing structures).

Normal Knee AROM

from 0 to 135 degrees of flexion

Patellofemoral Articulation

largest sesmoid bone in the body that also contains the thickest articular cartilage in the body * Working on the belief that structure subserves function, this indicates that the patellofemoral joint is subject to significant stress

Roll and Slide

must occur during flexion and extension, or the femur would roll off of the top of the tibia (also needed since the lengths of the femoral condyles are not equal)

Anterior Cruciate Ligament

named for its attachment on the tibia (anteriorly) * Passes from the anterior tibia backwards to attach to the medial portion of the lateral femoral condyle * Function is to limit the anterior translation of the tibia on the femur

Posterior Cruciate Ligament

named for its attachment on the tibia (posteriorly) * This is the major stabilizer of the knee joint *Passes from the posterior tibia forwards to the lateral portion of the medial femoral condyle * Limits posterior translation of the tibia on the femur

Lateral Meniscus

not attached to the LCL and therefore has more mobility * Circular shaped * Has a slip from the popliteus that attaches to the posterior aspect of the meniscus - this allows retraction of the meniscus during flexion of the knee

The Anterolateral Femorotibial Ligament

not considered by all sources an actual ligament, but it can be an important lateral stabilizer * Actually a portion of the iliotibial tract - fibers extend from the linea aspera of the femoral condyle to Gerdy's tubercle * Deep portions provide passive stability to the knee joint * Tightens with knee flexion - this is another example of a dynamized ligament

Tibiofibular (proximal) Articulation

not normally considered part of the "knee joint", but rather a separate joint that is located in the region of the knee

Forces that resist lateral displacement (medial forces)

o Height of the lateral trochlea (creates a "wall" that blocks lateral displacement" o Resistance of the medial retinaculum: connective tissue on the medial side of the knee that limits lateral movement of the patella (for a lateral dislocation to occur, this medial retinaculum typically has to be torn) o Perpendicular resultant force vector, serves to keep the patella within the trochlea (due to the pull of the quadriceps) o Vastus medialis muscle - resists lateral displacement of the patella (especially the more obliquely oriented fibers of the vastus medialis obliquus).

Forces that promote Lateral Displacement

o Q-angle: angle created from the intersection of two lines. The first line is from the ASIS through the middle of the patella. The second is from the middle of the patella through the tibial tubercle. (Q-angles are typically larger in women, due to the width of the pelvis). o Lateral retinaculum: connective tissue on the lateral side of the knee o Screw-home mechanism: with terminal knee extension, the tibial tubercle externally rotates (screwing home), promoting a stable knee with little muscle activity needed (but this also increases the Q-angle and further promotes lateral displacement of the patella).

Coronary Ligaments

portions of the joint capsule that attach to the medial and lateral meniscus (from the tibia)

Meniscofemoral Ligaments

presence is not consistent. Some individuals have one of these ligaments, some have two, and many individuals have neither ligament. These fibers are an additional attachment from the lateral meniscus to the medial femoral condyle, in the region of the posterior cruciate ligament: * Anterior meniscofemoral ligament (ligament of Humphrey) * Posterior meniscofemoral ligament (ligament of Wrisberg)

Knee in the Frontal Plane

the knee is characterized by valgus, and because of that, the natural direction of pull is lateral (i.e., the shortest distance between two points is a straight line, and when placed under tension, the patella naturally attempts to drift laterally)

Position of the Knee Joint: Close Packed State

the position of maximum congruence, where the joint surfaces are tightly compressed and the ligaments are generally taught. ♦ The position of maximum stability (used during standing, etc) ♦ This position is normally avoided during joint testing, since this is when the majority of joint structures are under maximal tension (preventing identification of isolated injuries, structures) - an exception is PCL testing. ♦Position that a patient will avoid if they have joint effusion.

Oblique Popliteal Ligament

this is a dynamized ligament that is created by fibers from the semimembranosus muscle

Arcuate Ligament

y-shaped ligament that crosses over the popliteus muscle (3 insertion sites) * Inserts on the head of the fibula * Inserts on the posterior intercondylar area of the tibia * Inserts on the lateral femoral condyle

Popliteal Artery Leaving the Popliteal Fossa

• As it leaves the knee distally under the soleus, the artery splits: * Posterior tibial artery - that will further split into the: (1) branch that continues as the posterior tibial artery, and (2) fibular (peroneal) artery (both of these travel in the posterior compartment of the leg, although the fibular artery will have branches that provide blood to the lateral compartment of the leg) * Anterior tibial artery - that supplies the anterior compartment

Blood Supply to the Knee

• Femoral artery - the descending genicular artery arises from the femoral and immediately splits into: (1) articular branch, and (2) saphenous branch. • Descending branch of the lateral circumflex femoral artery • Popliteal artery (direct continuation of the femoral artery) - five branches: * Medial superior genicular artery * Lateral superior genicular artery *Medial inferior genicular artery * Lateral inferior genicular artery * An unpaired middle genicular branch that supplies the intercondylar notch Others: * Posterior tibial recurrent artery * Anastomosis that forms around the knee from the branches outlined above (patellar plexus)

Bursa and Fat Pads of the Knee

• Infrapatellar fat pad - located below the patella, under the patellar ligament (can become inflamed and be a significant source of pain) • Bursae (numerous bursae about the knee - abridged list provided below): *Suprapatellar bursa or pouch - pulled out of the way during knee extension by the articularis genu muscle *Subpopliteal bursa or recess, located between the popliteus muscle and tibia *Subcutaneous pre-patellar bursa - source of "Housemaid's knee" * Subcutaneous and deep infrapatellar bursae - source of "Clergyman's knee" *Many others (pes anserine, around the semimembranosus muscle, at the fibular collateral ligament, etc)

Unique Characteristics of the Knee

• Largest joint in the human body • Some authorities contend that it is the most complex joint in the body • Interposed between the two longest bones • One of the least congruent joints • Shape of the Bones Which Make-up the Knee • Medial side: ♦ Femur sits on tibia like a wheel on a plate ♦ Medial femoral condyle is longer than the lateral - this is an important element in the screw-home mechanism • Lateral side: femur sits on the tibia like a ball on a dome and the condyle orientation between the two condyles is not absolutely parallel

Plantaris

• Proximal attachment: inferior end of the lateral supracondylar line of the femur and the oblique popliteal ligament • Distal attachment: posterior surface of the calcaneus via the calcaneal tendon (on the medial side of the tendon is where this attaches) • Function: plantarflexes the foot (weak) • Nerve: tibial NOTE: This is a muscle that is on occasion referred to as the "fools nerve", since it can easily be mistaken for a nerve during practical examinations, if not careful

Popliteus

• Proximal attachment: lateral surface of the lateral condyle of femur and lateral meniscus • Distal attachment: posterior surface of the tibia, superior to the soleal line • Function: o Unlocks the knee with flexion (resulting in internal rotation of the tibia on the femur) o Serves as a weak knee flexor • Nerve: tibial

The Popliteal Fossa

• This is a quadrilateral space with the following borders: * Proximally - the biceps femoris laterally (posterior-laterally) and the semimembranosus medially (posterior-medially) * Distally - the medial and lateral heads of the gastrocnemius muscle * Floor - popliteal surface of the femur, the oblique popliteal ligament, and the popliteus muscle

Position of the Knee Joint: Loose Packed State

♦ Best position to test joint play movements (movements that have to be available for normal function, but are normally only seen with the application of an external force [eg., testing the medial knee structures with a valgus stress]). ♦ Congruency in this position is diminished, the joint capsule is relaxed, and this is the position that the patient will naturally seek if they have joint effusion. ♦ Injury from an external force (eg., lateral blow) is less likely in this position than in a close-packed state.

Knee Capsular Reinforcement:

♦ The patella and patellar tendon ♦ The patellar retinaculum ♦Fibular and tibial ligaments ♦ The oblique popliteal ligament (this is a dynamized ligament, since it is an extension from the semimembranosus muscle that reinforces the posterior-medial aspect of the knee capsule and provides dynamic input regarding knee position and stress to the posterior capsule) ♦ The arcuate ligament - a triangular shaped ligament that reinforces the posterior-lateral aspect of the knee capsule, passing over a portion of the popliteus muscle ♦ The iliotibial band - due to the orientation of the fibers, this is functionally an important lateral stabilizer of the knee (working with the fibular collateral ligament)

Knee in the Sagittal Plane

In full extension, the patella is free of the trochlea and it articulates with a small supratrochlear fat pad. With flexion, the patella is drawn into the trochlea during the first 20 degrees of flexion. Beyond 30 degrees, the patella is firmly placed within the trochlear groove and stability is rarely a problem (i.e., if a patellar dislocation is going to occur, it typically occurs prior to this point). The contact zone of the patella varies and it moves proximally on the patella, creating a mechanism that maintains relatively constant stress (by having a larger surface area when compressive forces are large, and a smaller contact surface area when compressive forces are not so large). With extreme of knee flexion (> 135 degrees), the odd facet is engaged.

Muscles over Anterior Knee

Quadriceps femoris

Synovial Membrane of Knee

Synovial membrane - attachments closely follow the articular margins, but the synovial membrane does not cover any articular surfaces (it lines the capsule and covers all of the intra-articular structures except for the menisci) * Three structures that are intracapsular but extrasynovial: ♦ Cruciate ligaments (anterior and posterior) ♦ The popliteus muscle ♦ The infrapatallar fat pad

Femoral Articulation

Femoral - tibial joint: hinge joint primarily, but an additional rotatory component is crucial to the normal functioning of the knee * Joint type is a compound synovial, where there is an instantaneous axis of motion for which the radius of motion creates an evolute spiral

Medial Meniscus

attached on its periphery to the MCL * C-shaped (segment of an oval) *Due to attachment to the MCL, this ligament is less mobile - in addition to passive movement, there is limited retraction due to a slip from the semimembranosus muscle to the meniscus

Screw Home Mechanism

dictates that the knee will externally rotate (tibia on a fixed femur), during terminal knee extension - thus, in addition to flexion and extension, rotation is needed for normal movement of the joint. This conjoint rotation is caused by the shape of the bones, not to muscle action. Additionally, when this conjoint rotation is prevented during forced flexion or extension, the menisci are often damaged. During flexion, it is the action of the popliteus that "unlocks" the knee.

Clinical Correlates of the Meniscus

* A normally functioning meniscus changes position and shape (and surface area), with movement of the knee (with flexion and extension). The menisci slide forward with knee extension, and are retracted with knee flexion. * The menisci are often torn by a non-physiologic combination of rotation and forced extension or flexion. * Menisci are penetrated by nerves - therefore, a tear often has acute pain associated with it. * Menisci are avascular, except for the most peripheral zones. Therefore, with a tear, there is typically no bleeding (and consequently, there is also no healing). Experimental procedures continue to be investigated to promote healing. * The "old way" of treating a torn meniscus with a complete meniscectomy resulted in degenerative joint disease in almost all cases, 10+ years out.

Other Meniscal Attachments

* Coronary ligaments - attachments to both menisci * Meniscofemoral ligaments - associated with the lateral meniscus * In association with the knee joint capsule, the patellar retinaculum

Functions of the Patella

* Facilitates extension of the knee, increasing torque by as much as 50%. * Decreases friction of the quadriceps mechanism * Acts as a guide for the quadriceps * Protects the trochlea and the femoral condyles

Functions of Meniscus

* Increase congruence of the joint * Dissipates force - serves to absorb shock * Spreads synovial fluid throughout the joint

Muscles over the posterior/lateral aspect of the knee

* Popliteus * Lateral head of the gastrocnemius * Plantaris * Tensor fascia latae and the gluteus maximus muscle through the iliotibial band

Nerve Supply to the Knee

* Saphenous nerve (from the femoral nerve and its branches to the vasti muscles) * Tibial nerve (the chief supply to the flexors of the knee [L4-S3]) * Obturator nerve * Fibular nerve (common peroneal nerve) NOTE: Some of these nerves also supply the hip - this is one reason why pain is readily referred from a diseased hip to the knee

Muscles over posterior/medial aspect of the knee

* Sartorius * Gracilis *Semitendinosus ♦ Pes anserinus - the "gooses foot", where there is the combined insertion of three tendons (SGT acronym, for sartorious, gracilis and semitendinosus), on the medial aspect of the knee. √ Collectively work to both flex and internally rotate the knee √ Note that all three of these muscles are innervated by different nerves (femoral, obturator and tibial portion of sciatic) * Semimembranosus * Medial head of the gastrocnemius

Contents of the Popliteal Fossa

* The sciatic nerve (tibial nerve and common fibular [peroneal] nerve) * Sural nerve * Small saphenous vein * Posterior cutaneous nerve of the thigh * Deep structures: ♦ Popliteal artery ♦Popliteal vein ♦Superior lateral genicular artery ♦Superior medial genicular artery ♦ Inferior lateral genicular artery ♦Inferior medial genicular artery ♦ Middle genicular artery ♦ Descending branch of the lateral circumflex femoral artery ♦ Descending branch of the genicular artery ♦ Lymph nodes and fat

Facets of the Patellafemoral Joint

*The superior 75% of the undersurface of the patella is covered with articular cartilage (the thickest in the body), and it can readily be divided into two facets: ♦ Lateral facet - largest of the two facets (to determine the side of the patella [right or left], face the apex away and let go of the patella - it will tip to the appropriate side, due to the larger surface area of the lateral facet). ♦ Medial facet - typically further subdivided into two facets, the medial (where most contact with the medial femoral condyle occurs), and the odd facet (which is engaged during extreme amounts of knee flexion [i.e. > 135 degrees of knee flexion]). *The inferior 25% of the undersurface of the patella is non-articular

Medial Collateral Ligament

- slants anteriorly from proximal to distal (as it passes from the medial condyle to the medial aspect of the proximal tibia), and it consists of fibers separated into two portions: * Superficial fibers- external to the joint capsule (up to 8 cm long) * Deep fibers - fibers attach directly to the medial meniscus and functionally work to limit the movement of this meniscus (related to increased injury incidence, compared to the lateral meniscus) * Function - one of the static stabilizers that limit valgus stress