Uneven loading
Uneven loading causes excessive compression of the cartilage and bone on one side, and strain in the joint capsule and supporting ligaments on the other side. When the joint is repeatedly overloaded on the edge of its surface,
exostoses, or "bone spurs," occur. Strain of the supporting ligaments can cause exostosis around the joint as well.
Contributing factors
There are several conformational defects that contribute to bone spavin. Those that cause uneven loading of the hocks, such as
sickle hocks and
cow hocks, are especially noteworthy. Poor trimming or
shoeing can also contribute to bone spavin in any horse, no matter what their conformation.
Certain types of activities may also contribute to uneven or repeated loading of the lower hock joints, and thus bone spavin. These include sports that require a great deal of hock flexion (
dressage), stress (
jumping), sudden stops or turns (western events, such as
reining), or a great deal of concussion (
Standardbred racing).
"Juvenile spavin" is the occurrence of bone spavin in horses less than 3 years old. It usually occurs before the animal has done much work. While
osteochondrosis lesions are the likely cause in some cases, this condition can also occur secondary to the distortion of the cuboidal bones which can occur in premature or dysmature foals.
Typical signs of bone spavin
Initially, signs of bone spavin may include sporadic and vague hindlimb lameness. Some horses may become uncomfortable in one lead, or may demonstrate stiffness walking downhill.
In many cases lameness worsens, becoming more obvious and consistent. Advanced cases may have a bony swelling on the hock, typically on the inside of the joint. Lameness, although usually worse in one leg, is commonly bilateral.
The affected limb usually lands toe-first, wearing down that foot faster than the other. The affected limb usually has a shorter, lower arc than the other foot, as the horse is trying to reduce the painful flexion of the joint, so the leg appears to drag.
A
flexion test of an affected limb often produces a temporary worsening of the lameness. Such a response to a flexion test would support the diagnosis of bone spavin. A flexion test involves holding the hock in forced
flexion for a period of time before trotting the horse away immediately.