Patella tendon ruptures are less common than quadriceps tendon ruptures. They most commonly occur in patients younger than 40 years; whereas, quadriceps tendon ruptures tend to occur in individuals older than 40 years. Risk factors for patellar tendon ruptures include rheumatoid arthritis, diabetes mellitus, systemic lupus erythematosus, chronic renal failure, systemic corticosteroid therapy, local steroid injection, and chronic patellar tendonitis. The greatest forces to the patellar tendon occur at 60 degrees of knee flexion. Forces through the patellar tendon are approximately 3 times body weight while climbing stairs.
Many patella tendon ruptures are associated with pre-existing degenerative changes. The average thickness of the patellar tendon is 4 mm, although it widens to 5-6 mm at the tibial tubercle. The patellar tendon merges with the medial and lateral retinacula. It is comprised of 90% Type I collagen.
The blood supply to the posterior aspect of the patellar tendon is supplied by the fat pad vessels via the inferior medial and lateral geniculate arteries. The anterior portion of the tendon is supplied by retinacular vessels via the inferior medial geniculate and recurrent tibial arteries. The proximal and distal insertion areas are relatively avascular and subsequently are a common site of rupture. Ruptures most commonly occur at the inferior pole of the patella.
There is no widely accepted classification system. The ruptures can be categorized by location of tear (proximal insertion most common), partial versus complete, and length of time between injury and presentation. Tears are considered acute if a repair is done within 2 weeks from the time of injury.
Patients often report a forceful quadriceps contraction with a flexed knee. They may recall an audible "pop" at the time of injury. The patient may demonstrate a quadriceps avoidance gait. The patient may be unable extend the knee against gravity.
On physical examination, there may be a palpable defect. A hemarthrosis is evident if the injury is acute. Passive knee flexion may elicit pain. There is a partial or complete loss of active extension. There may be atrophy of the quadriceps with chronic injury.
Radiographs, including AP and lateral views of the knee, are helpful in making the diagnosis and ruling out fractures. Patella alta will be seen on the lateral view, as defined by the patella residing superior to Blumensaat's line in 30 degrees of knee flexion or an Insall-Salvati ratio less than 0.8. Ultrasonography may show tendon discontinuity, but it is operator and reader dependent. MRI is a highly effective at diagnosing patella tendon injury and often used to confirm the diagnosis. The advantage of MRI is its ability to detect other intraarticular or soft tissue injuries that may be present. However, its disadvantage is its cost.
The goal of treatment is to restore the extensor mechanism. Nonoperative treatment is reserved for partial tears in which the patient is able to fully extend the knee. Treatment is immobilization in full knee extension for 3 to 6 weeks (i.e. cylinder cast).
All other patients are best managed with surgical repair. Repairs are categorized as early or delayed. The outcome of early repairs are far superior to delayed repairs. For early repairs, a midline incision is used, the ruptured patellar tendon and retinacular tears are exposed, the hematoma is evacuated, the frayed edges are debrided, and nonabsorbable sutures are used to repair the tendon to the patella. The suture should be placed in a locking manner, such as the Krackow technique. The sutures are passed through longitudinal, parallel tunnels drilled in the patella and tied at the superior pole of the patella. Retinacular tears should be repaired, and the repair should be assessed intraoperatively with knee flexion prior to closing the skin incision.
Post-operatively, a locked, hinged knee brace is used in slight flexion. Immediate isometric quadriceps exercises can be done. Active flexion with passive extension exercises can be started at 2 weeks. The range of motion allowed depends on the intra-operative assessment, but in general, is 0 to 45 degrees in the first week and advanced 30 degrees each subsequent week. Active extension is allowed after 3 weeks. Initial toe-touch weight-bearing is gradually increased as motion improves. All restrictions are removed once the patient has regained full range of motion and 80-90% of the contralateral quadriceps strength. Most patients require approximately 4 to 6 months to fully recover.
Repairs done after 6 weeks from injury are considered delayed. The results of delayed repairs are poor compared to acute repairs. Quadriceps contracture and patellar migration are commonly encountered. Adhesions may have formed between the patella and femur. Options for delayed repairs include autograft or allograft augmentation of a primary repair or reconstruction. The autografts or allografts used include hamstring, fascia lata, and Achilles tendon. Post-operative management after a delayed repair is more conservative, typically a cylinder cast for 6 weeks. Active range of motion is allowed once the cast is removed.
Complications from patellar tendon rupture include:
- knee stiffness
- persistent quadriceps weakness
- patella baja.