The reciprocal apparatus of the hind limb is a normal anatomic linkage between the upper and lower limb that forces the hock and stifle to move in concert. If the stifle flexes, the hock must flex, and vice-versa. If one extends, the other must extend.
The peroneus tertius is the tendinous muscle that lies between hock and stifle on the front, lateral (outside) part of the gaskin. It acts as the critical front component of the reciprocal apparatus of the hind limb. Certain types of trauma can cause rupture of this tendon, disrupting the reciprocal apparatus. This injury typically occurs in horses that have caught the lower limb on an immoveable object, at speed, tearing the tendon.
The result of this injury is a classic abnormality in which the horse can extend its hock while flexing its stifle. This should normally be physically impossible. There are also typical changes observed in the gait of a horse with this injury. Notably, there is a slowed front phase of the stride, and dimpling of the Calcanean tendon area above the point of the hock on the rear of the limb.
Diagnosis requires a clinical lameness exam and can be confirmed by ultrasound. There is often swelling of the area of rupture right after the traumatic injury.
Treatment requires enforced box stall rest for 60-90 days. NSAIDS may be useful for the first week after injury, for pain and swelling.
Other Diagnoses Considered
Treatments May Include
Prognosis & Relevant Factors
The prognosis is good with enforced rest. Most horses can return to their prior level of athletic ability. Healing is much slower if horses are allowed free turnout.
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