Bilateral pigmented villonodular synovitis in a dog.

Villonodular synovitis is an extremely rare condition of the synovial membrane in the dog. A 10-year-old, neutered crossbreed was presented with bilateral, progressive hindlimb lameness. Periarticular swelling was noted in both stifle joints. No craniocaudal instability was noted. Radiographs showed massive intra-articular soft tissue proliferation in both joints, with no bony involvement. Arthrocentesis was unsuccessful. Exploratory arthrotomy of the left stifle revealed a greatly thickened, florid, proliferative synovial membrane. An incisional biopsy was carried out and the histopathological diagnosis was chronic active villonodular synovitis. A radical synovectomy was carried out in the right stifle joint 10 days later. Corticosteroid treatment was initiated 10 days after the second surgery and continued for six weeks, with a continuous clinical improvement. Eighteen months after discontinuation of the steroid therapy, the owners reported no recurrence of clinical signs although a mild stiffness was still present.