Patellar tendon rupture results from forceful contraction of the quadriceps muscles resisted by a flexed knee (eccentric contraction, knee flexion against a fully contracted quadriceps muscle). Patellar tendon rupture usually occurs after chronic degeneration of the tendon has already taken place. It can affect adults of any age but usually occurs in patients younger than 40. It is very rare in the pediatric population. The condition is more common in men than women.

Risk factors include being immunocompromised / systemic disease, taking systemic corticosteroids, and use of anabolic steroids as well as patellar tendinitis, past injury, or degeneration. A history of cortisone injection into a patella tendon is associated with a high rate of tendon rupture.

Patellar tendon rupture presents with pain. There will be a palpable defect in the region of the patellar tendon. Patients may report hearing a "pop" or feeling their knee give way. The patient will have an inability to perform a straight leg raise (raise their heel from the exam table while the knee is fully extended).

The condition can be an acute or chronic (delayed presentation > 6 weeks).

It is a rare but well-described condition to have simultaneous contralateral extensor mechanism injury.