An unusual case of knee locking.

Knee locking is an incapacitating condition that requires urgent orthopaedic intervention. The most common cause is meniscal injury, followed by torn anterior cruciate ligament, osteochondral loose bodies and foreign bodies in the joint space. This report describes a patient who had an unusual case of left knee locking. After clinical examination and radiological investigations, the provisional diagnosis was a lateral meniscal tear, which was not visible on magnetic resonance imaging. Diagnostic arthroscopy of the left knee revealed a 1-cm yellowish lump on the medial meniscus, and the histology revealed pigmented villonodular synovitis. The menisci and cruciate ligaments were intact.