Pigmented villonodular synovitis presenting as a popliteal cyst

A rare case is described of giant swelling of the soft tissue around the knee joint. A large popliteal cyst was present, the fleshy walls of which penetrated the muscle layers of the calf. The place of arthrography in diagnosis is stressed. The yellow-brown mass was confirmed histologically to be pigmented villonodular synovitis. The mechanism of formation of a popliteal cyst is discussed. The differential diagnosis and treatment are reviewed briefly: our preference is for radical surgery.