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.
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