Rupture of the Peroneus Tertius in Horses

The peroneus tertius in horses is a tendinous structure that originates from the extensor fossa of the femur and spans the craniolateral aspect of the tibia to insert on the dorsoproximal aspect of the third metatarsal bone, the calcaneus, and the third and fourth tarsal bones. It is part of the reciprocal apparatus of the hindlimb, which links the actions of the hock and stifle (so that both joints either extend or flex together).

Hyperextension of the limb can cause the peroneus tertius to rupture anywhere along its length; rupture in the tibial region is the most common. Avulsion of the origin of the peroneus tertius typically carries a worse prognosis for return to athletic function.

Clinical signs of peroneus tertius rupture are pathognomonic, because the rupture allows the hock to be extended while the stifle is flexed. Horses are able to bear weight on the affected limb. At a walk, the gastrocnemius and superficial digital flexor muscles appear rather flaccid, and there is a characteristic dimple on the caudodistal aspect of the soft tissues of the crus. At a trot, an obvious lameness is usually evident, with delayed protraction of the limb due to overextension of the hock.

Diagnosis of peroneus tertius rupture is usually based on clinical signs and can be confirmed with ultrasonography.

Peroneus tertius rupture is treated with strict stall rest for 4–6 weeks, followed by a controlled rehabilitation program and careful ultrasonographic monitoring. The prognosis for return to athletic soundness is generally good.