A patellar tendon rupture involves a complete tear of the tendon that runs from the patella's inferior pole to the tibial tubercle. It is typically seen in males in their third or fourth decade of life. It tends to result from an overall weakened tendon placed under high tensile forces. These tendon ruptures are best classified into acute versus chronic tears, depending on the time from rupture. This type of injury requires prompt diagnosis and surgical repair as the patellar tendon is a part of the extensor mechanism. The extensor mechanism of the knee is crucial to the function of the lower extremity, including ambulation. It is solely responsible for extending and straightening the knee as well as resisting knee flexion – a crucial aspect of standing with a flexed knee and, therefore, ambulation. Without a properly functioning knee extensor mechanism, the patient is severely limited functionally. Surgical intervention depends on timing and location of the rupture. Acute ruptures are amenable to primary repair whereas chronic ruptures often require tendon reconstruction.
The knee extensor mechanism is composed of the quadriceps muscle, quadriceps tendon, medial and lateral patellar retinaculum, patella, patellar tendon, and tibial tubercle.
The quadriceps muscle is composed of four separate muscles with different origins but a common insertion point on the patella through the quadriceps tendon.
Quadriceps muscles with origins
- Rectus femoris – anterior superior iliac spine and superior acetabular rim
- Vastus lateralis – greater trochanter and lateral linea aspera
- Vastus intermedius – proximal femoral shaft
- Vastus medialis – intertrochanteric line and medial linea aspera
The medial and lateral patellar retinaculum are on their respective sides of the patella and are continuous with the vastus fascia to the tibia and the patella. They are minor patellar stabilizers and, if intact, can provide knee extension and straight leg raising despite a patellar or quadriceps tendon rupture.
The patella is a sesamoid bone. It functions to increase the moment arm from the knee joint axis, thereby increasing the mechanical advantage and quadriceps pull in extension. The patella begins to engage the trochlea at 20 degrees of flexion and is fully engaged by 40 degrees of flexion. Joint reaction forces in the patellofemoral joint can be up to three times the body weight with stair climbing and seven times the body weight with deep bending.
The patellar tendon, by definition, is a ligament as it connects bone (patella) to bone (tibial tubercle). The patellar tendon is approximately 30 mm wide by 50 mm long, with a thickness of 5 to 7 mm. The origin on the inferior pole of the patella is juxtaposed on the articular cartilage on the deep side and becomes confluent with the periosteum of the patella anteriorly. The tibial insertion is narrower and invests the entirety of the tibial tubercle, connecting the quadriceps muscles to the lower leg.