Pigmented villonodular synovitis presenting as a large lateral knee mass. Case report and review of the literature.

Pigmented villonodular synovitis is characterized by synovial villonodular lesions with hemosiderin pigmentation and a stromal infiltrate of histiocytes. Analogous lesions may occur in joints, bursae, and tendon sheaths. Although the etiology and natural history of pigmented villonodular synovitis remain unknown, it should be considered a benign entity. It usually presents as either diffuse monoarticular involvement with chronic pain and swelling, or a more localized nodular lesion, typically involving the fingers or knees. Although treatment involves excision of the involved tissue, due to its benign behavior, radical surgical procedures are not indicated. A case report of a large, extra-articular mass is described, with histologic and clinical changes consistent with localized PVS.