“Bone spavin” (sometimes called “jack spavin”) is a term with roots in the antique veterinary vocabulary, and because it was difficult to treat, its development was one of the lamenesses that 19th century horse owners feared most. Today, the term is used in a very general sense to mean any abnormal ossification involving the hock bones and/or the hind cannons or splint bones, either on the back, front, or lateral side near the lower part of the hock joint. The lumpy exostosis which develops to weld the bones together looks exactly the same as in splints or ringbone, and it develops from a similar cause, namely bending or twisting forces applied to the ligament fibers which bind together and stabilize the small bones of the hock. As in the forelimbs, the development of exostosis is characterized during the acute phase by swelling, heat and pain from the proliferation of small blood vessels.
The diagnosis of bone spavin is readily made from x-ray (next page), and its appearance in the skeleton is unmistakable. Of our four horses, Rolf shows some co-ossification of three of the lower bones of the hock. None of the performance horses have it, and particularly Ethan Allen, who has been reported to show this lesion, can now be confirmed to have been entirely free of it.
These images from an old veterinary textbook show “jack spavin.”