RESUMEN
Pigmented villonodular synovitis (PVNS) is a rare benign condition that is locally aggressive and may destructively invade the surrounding soft tissues and bone causing functional loss of the joint and the limb. The knee is the most affected joint (range, 28% to 70%) but involvement of the bone is not a common feature seen at this site. We present a rare case of diffuse PVNS of the knee associated with subchondral cyst of the lateral femoral condyle. This posed a diagnostic dilemma because of bone invasion. The radiological image of synovitis was pathognomonic of PVNS but etiology of the osteolytic lesion was confirmed only on histopathology. The large osteochondral defect was eventually managed in a staged manner with bone grafting and osteochondral autograft transfer.