ATYPICAL VILLONODULAR SYNOVITIS IN A HORSE

A 4-year-old sexually intact male Standardbred trotter was evaluated f or left forelimb lameness. A presumptive diagnosis of severe cartilage damage was made because the horse had a history of infectious arthrit is involving the left metacarpophalangeal joint. Arthroscopic evaluati on revealed what was presumed to be a large villonodular lesion. The m ass was surgically removed, and the horse was treated with procaine pe nicillin G, trimethoprim-sulfamethoxazole, phenylbutazone, and polysul fated glycosaminoglycans and eventually returned to racing, Histologic examination of the mass revealed a bed of granulation tissue covered with keratinized epithelium. We hypothesize that the lesion developed secondarily to implantation of epithelial cells into a reactive villon odular lesion.