Pigmented villonodular synovitis of the hip--association with osteochondromatosis.

Pigmented villonodular synovitis, diffuse or nodular in nature, is most commonly seen in the knee but occasionally involves the hip. The authors reviewed the medical records of seven patients with a pathological diagnosis of unilateral pigmented villonodular synovitis of the hip. Characteristic findings were proliferation of the synovial lining cells and subsynovial invasion of fibroblast or polyhedral mesenchymal cells in a nodular pattern. Multiple initiating factors have been advanced to account for the occurrence of this monoarticular chronic arthritis, but the etiology and pathogenesis still remain obscure. On three occasions the authors found an association with free floating osteochondromas and well-delineated foci of metaplastic cartilage under the synovial lining cells. Two benign lesions may occur simultaneously and are possibly related entities involving the synovial membranes as a reaction to a common etiologic factor. Although the histopathology of pigmented villonodular synovitis of the hip and the knee is identical, localization in the former joint has its typical clinical, radiological, differential diagnostic and therapeutic aspects.