28 year-old male felt his left knee "pop" after landing from a jump. He has limited ability to extend his knee. Xray shown. What's the diagnosis?

• Mechanism is knee flexion combined with forceful contraction of the quadriceps
• Swelling, patellar tenderness, and limited extension of knee
• Radiographs are helpful in diagnosis when using the Insall-Salvatti ratio (see references)

Reference

• Insall J, Salvati E. Patella position in the normal knee joint. Radiology 1971;101:101
• Sibley T, Algren D, Ellison S. Bilateral patellar tendon ruptures without predisposing systemic disease or steroid use: a case report and review of the literature. American Journal of Emergency Medicine 2012; 30: 261