Pigmented villonodular synovitis: a disease in evolution.

Pigmented villonodular synovitis (PVNS) is a rare, benign, proliferative disease of the synovial membrane of joints, tendon sheaths, and bursas. Joint aspiration typically yields hemorrhagic or xanthochromic/serosanguinous (brown, murky) fluid. We describe a case of PVNS that presented as an acute, painless, nontraumatic right knee effusion with clear synovial fluid on arthrocentesis. Initial magnetic resonance imaging of the knee revealed no evidence for hemosiderin. A diagnostic arthroscopy and surgical arthrotomy revealed a unique case of PVNS evolving from local to diffuse involvement of the synovium.