Pigmented villonodular synovitis of the knee presenting as a popliteal cyst.

Pigmented villonodular synovitis is a locally aggressive tumor of the synovium of joints and tendon sheaths. It is commonly seen in the synovial lining of the flexor tendons of the hand and in the synovium of the knee and less commonly in other joints. A case of pigmented villonodular synovitis of the knee in a 60-year-old man, with an intra-articular origin extending extra-articularly, is presented. The interesting point is that the initial diagnosis was a Baker cyst.