A 64-year-old patient, 2 years after posterior-stabilised total knee replacement, reports an audible clunk and pain when extending the knee from a seated position, occurring at roughly 40 degrees short of full extension. Lateral radiographs are unremarkable. The most likely diagnosis is:
- A Patellar clunk syndrome due to a fibrous nodule catching on the femoral cam ✓
- B Periprosthetic joint infection with synovitis
- C Polyethylene post fracture with mechanical blocking
- D Extensor mechanism rupture with patella baja
Correct answer: A. Patellar clunk syndrome due to a fibrous nodule catching on the femoral cam
Explanation
Patellar clunk syndrome is specific to posterior-stabilised designs. B fibrous nodule forms at the junction of the quadriceps tendon and superior pole of the patella and snaps over the anterior femoral flange and intercondylar cam during extension from deep flexion, typically around 30 to 45 degrees. Arthroscopic or open debridement of the nodule cures it. B normal lateral film and the characteristic arc of symptoms argue against post fracture or extensor rupture.
Reference: Miller's Review of Orthopaedics, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.