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

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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 m .....