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Excerpt from Knee, Extensor Mechanism Injuries (MRI)Synonyms, Key Words, and Related Terms: patellar malalignment, patellar dysplasia, excessive lateral pressure syndrome, ELPS, overuse injuries, patellar fractures, patellar dislocation, osteochondral injuries, osteochondritis dissecans, OCD, patellar tendinopathy, patellar tendon tears, quadriceps tendinopathy, quadriceps tendon tear, myotendinous strain, prepatellar bursitis, housemaid's knee, pes anserinus bursitis, iliotibial band friction syndrome, synovial plica syndrome, fibrosis, Osgood-Schlatter condition, Sinding-Larsen-Johansson syndrome, bipartite patella Please click here to view the full topic text: Knee, Extensor Mechanism Injuries (MRI)Background The extensor mechanism of the knee consists of the quadriceps muscle group, quadriceps tendon, patella, patellar retinaculum, patellar ligament, and adjacent soft tissues. Injuries to the extensor mechanism are common and consist of chronic degenerative injuries, overuse injuries, and acute trauma. For excellent patient education resources, visit eMedicine's Breaks, Fractures, and Dislocations Center. Also, see eMedicine's patient education articles Knee Injury and Magnetic Resonance Imaging (MRI). Normal anatomy of the knee extensor compartment An understanding of normal anatomy and biomechanics of the knee extensor mechanism is necessary to comprehend the imaging of extensor mechanism injuries. The extensor mechanism of the knee begins above the hip with the origin of the rectus femoris muscle on the anterior inferior iliac spine. The remainder of the quadriceps muscle group, vastus lateralis, vastus intermedius, and vastus medialis originate on the femoral shaft. The quadriceps tendon represents the confluence of the 4 muscle tendon units and inserts on the superior pole of the patella. The quadriceps tendon has 3 laminae, including (1) the most superficial lamina, which is formed by the rectus femoris; (2) the intermediate lamina, which is formed by the vastus lateralis and vastus medialis; and (3) the deep lamina, which is formed by the vastus intermedius. Laterally, the iliotibial band supports the extensor mechanism and is an important lateral stabilizer of the patellofemoral joint. It originates above the hip joint as a wide fascial band, originating from the gluteal muscles, tensor fascia lata, and vastus lateralis. Distally, the iliotibial band consists of two tracts. The iliotibial tract inserts on Gerdy tubercle of the lateral tibial plateau. Fibers connecting the iliotibial band to the patella are referred to as the iliopatellar band. The patellar retinaculum is an important soft tissue stabilizer of the patellofemoral joint. It is composed of a medial and lateral component. The thicker lateral retinaculum comprises a distinct, thick deep layer and a thin superficial layer. The deep layer is a confluence of several fibrous structures consisting of the lateral patellofemoral ligament and iliopatellar band. Deep to the medial patellar retinaculum, there are 3 focal capsular thickenings. These occasionally are referred to as the medial patellofemoral, patellomeniscal, and patellotibial ligaments. The patella, the largest sesamoid bone in the body, possesses the thickest articular cartilage. The articular surface, which can have a variable contour, articulates with the trochlear groove of the femur. Most patellae possess a median ridge that divides the proximal patella into a medial and lateral facet; the medial facet usually is the smaller of the two. The patellar tendon, occasionally termed the patellar ligament, originates at the inferior pole of the patella and inserts onto the tibial tuberosity. The patellar tendon is invested in a paratenon of loose fibrillar tissue. Additional soft tissue structures of the knee extensor compartment consist of the infrapatellar fat pad and pretibial and prepatellar bursae. Injuries to these structures are associated with extensor mechanism injuries and can result in anterior knee compartment pain. Normal biomechanics of the knee extensor compartment The quadriceps muscle group functions as a knee extensor when the leg is elevated. When the foot is on the ground, contraction of the quadriceps stabilizes the knee, functioning as a decelerator. The patella provides a significant mechanical advantage to the knee extensor mechanism, allowing the knee to extend with a smaller contractile force of the quadriceps. In addition, the patella redirects the force exerted by the quadriceps, resulting in a large compressive stress on the patellofemoral joint. The magnitude of this stress usually is at a maximum with the knee flexed 90° and the foot planted, such as that occurring when one stands from a sitting position. Only a portion of the patellar cartilage articulates with the femoral trochlea at any one time. With the knee extended 0°, the patella rides laterally within the trochlear groove and is not in direct contact with the trochlear cartilage. With knee flexion, the patella moves medially, and the degree of surface contact of the patellofemoral joint increases. Bone and cartilage contours of the patella and trochlear groove determine stability of the patellofemoral joint. Patellar subluxation (lateral displacement of the patella from the trochlear groove) is associated with a flat articular surface of the patella and a shallow trochlear groove. Additional soft tissue structures provide both dynamic and static stabilization of the patellofemoral joint. The vastus medialis obliquus (VMO) is an important dynamic medial stabilizer of the patellofemoral joint. The VMO originates in the adductor tubercle of the distal medial femur and inserts on the medial retinaculum and superomedial patella. The iliotibial band provides dynamic lateral stabilization of the patella through the iliopatellar band. Additional dynamic stabilization is provided by insertion of fibers from the vastus medialis and lateralis onto the patellar retinacula. Static stabilizers consist of the medial and lateral retinaculum and the joint capsule. Pathophysiology of injury Most chronic degenerative injuries of the extensor mechanism are related to patellar malalignment. Abnormal tracking of the patella within the trochlear groove can produce focal areas of increased stress on the patellofemoral joint. The shallow trochlear groove seen with trochlear dysplasia allows the patella to sublux laterally with knee flexion. Abnormal lateral patellar tilt can be seen with a tight lateral retinaculum and is associated with excessive lateral pressure syndrome (ELPS). Over time, the abnormal stresses applied to the patellofemoral joint can result in focal cartilage degeneration, patellofemoral osteoarthritis, and damage to the soft tissue stabilizers of the patellofemoral joint. Overuse injuries typically are seen in high performance athletes, most commonly in sports requiring long distance running or jumping. Cumulative microtrauma produced by repetitive knee flexion and extension results in focal inflammation. Acute traumatic injuries occur secondary to direct blunt trauma or excessive tension applied to the extensor mechanism through quadriceps contraction. Patellar fractures are the most common traumatic injury of the knee extensor mechanism. Frequency In the United States, knee injuries occur in more than 3 million people per year. Chronic anterior knee pain is common and often is associated with patellar malalignment and abnormal patellar tracking. Mortality and morbidity Blunt anterior knee trauma and abnormal patellar tracking is associated with increased incidence of patellofemoral osteoarthritis. Failure to treat complete tears of the knee extensor mechanism can result in quadriceps atrophy and chronic weakness with knee extension. Race No significant racial predilection exists. Sex Patellofemoral arthralgia most commonly occurs in adolescent females. Painful swelling of the tibial tuberosity (Osgood-Schlatter syndrome) and the distal pole of the patella (Sinding-Larsen-Johansson syndrome) occur most commonly in adolescent males. Age Anterior knee pain is most common in physically active adolescents. Based on findings by Goodfellow and coworkers, basal degeneration of patellar cartilage is more common in younger patients while superficial cartilage lesions are prevalent in older patients. The region of the extensor mechanism susceptible to disruption correlates with the patient's age. The older the patient, the more proximal the area ruptured. Disruption of the quadriceps tendon occurs more often in older patients while more distal tears of the patellar tendon and the tibial tubercle occur in younger patients. Differentials Knee, Anterior Cruciate Ligament Injuries (MRI) Knee, Posterior Cruciate Ligament Injuries (MRI) Please click here to view the full topic text: Knee, Extensor Mechanism Injuries (MRI) |
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