UMEM Educational Pearls

Category: Visual Diagnosis

Title: What's the Diagnosis? By Michael Allison

Posted: 2/5/2012 by Haney Mallemat, MD (Emailed: 2/6/2012) (Updated: 8/28/2014)
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28 y.o. male felt his left knee "pop" after landing from a jump while playing basketball. Knee exam revealed limited knee extension. X-ray is shown. Diagnosis?


Answer: Patella Alta secondary to Patellar tendon rupture

  • 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 below)
    • This ratio is the patellar length (PL) (in cm) divided by the distance between inferior pole of the patella and tibial tuberosity (PT)
    • Insall-Salvatti ratio PL/PT =1 is normal; <0.8 is predictive of tendon rupture


  • 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.e3-261.e

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