Pigmented villonodular synovitis.

Pigmented villonodular synovitis, a rare tumor of the synovial lining of a joint, is suggested when aspiration of a chronically swollen and painful single joint (usually the knee) yields a brownish fluid. The diagnosis is confirmed by biopsy, and the preferred treatment is complete excision of all involved synovial tissue. The recurrence rate is directly related to the degree of synovial involvement and to the thoroughness of the synovectomy performed. Although pigmented villonodular synovitis does not become malignant, it can destroy a joint so that an arthroplastic procedure may be necessary.