The fibularis (peroneus) tertius is a tendinous structure that originates from the extensor fossa of the femur and runs over 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 means there is concurrent flexion and extension of the hock and stifle. Rupture of the fibularis tertius may occur as a result of hyperextension of the limb and usually occurs in the middle of the crus, or laceration may occur on the dorsal aspect of the tarsus. Avulsion of the origin on the fibularis tertius is rare in mature horses but may occur in young animals.
Clinical signs are pathognomonic, because rupture of the fibularis tertius means horses are able to extend the hock while the stifle is flexed. Horses are able to bear weight on the affected limb. At 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 trot, an obvious lameness is usually evident, with delayed protraction of the limb due to overextension of the hock.
Diagnosis is usually based on clinical signs and can be confirmed with ultrasonography.
Conservative treatment with 3–4 mo of stall rest followed by slow and careful reintroduction to exercise usually results in complete resolution of signs and return to athletic soundness.