Solid Aneurysmal Bone Cyst of the Distal Metatarsus in a Horse

Abstract

A horse was presented with a slowly growing mass of the distal metatarsal region of the right hind leg. Radiographic examination revealed an eccentric cyst-like lesion with distortion of the lateral margin of the distal metatarsus. The lesion involved approximately half of the metatarsal width, exhibited fine septa, a sclerosed margin towards unaffected bone and a smooth rim of thin compact bone at the periphery. Longitudinal sectioning of the distal metatarsus revealed a solid pale yellow mass with reddish foci and margin. Histologically, the mass consisted of a cell-rich, well vascularized fibrous stroma, containing numerous irregularly shaped trabeculae of woven bone with regular osteocytes. Osteoblasts were observed around the osteoid, and there were foci of numerous osteoclast-like giant cells. The lesion was diagnosed solid variant of an aneurismal bone cyst.