The fibularis (peroneus) tertius is a tendinous structure that originates from the extensor fossa ofthe femur and runs over the craniolateral aspect ofthe tibia to insert on the dorsoproximal aspect ofthe third metatarsal bone, the calcaneus, and the third and fourth tarsal bones. It is part ofthe reciprocal apparatus ofthe hindlimb, which means there is concurrent flexion and extension ofthe hock and stifle. Ruptureofthe fibularis tertius may occur as a result of hyperextension ofthe limb and usually occurs inthe middle ofthe crus, or laceration may occur on the dorsal aspect ofthe tarsus. Avulsion ofthe origin on the fibularis tertius is rare in mature horses but may occur in young animals.
Clinical signs are pathognomonic, because ruptureofthe 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 ofthe soft tissues ofthe crus. At trot, an obvious lameness is usually evident, with delayed protraction ofthe limb due to overextension ofthe 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.
Last full review/revision September 2015 by Jane C. Boswell, MA, VetMB, CertVA, CertES (Orth), DECVS, MRCVS