Rupture of the Peroneus Tertius Tendon
The peroneus tertius tendon (red arrow above) originates in common with the long digital extensor (LDE) tendon from the extensor fossa of the femur (in the hindlimb) and inserts on the dorsal surfaces of the proximal third metatarsal, calcaneal and fourth tarsal bones. The tendon is a critical part of the reciprocal apparatus of the pelvic (hind) limb and is responsible for simultaneous stifle and hock joint flexion in the normal horse. Rupture of the peroneus tertius tendon occurs secondary to hyper- or overextension of the hock joint(s).
Clinical signs associated with rupture of the peroneus tertius tendon are classical in appearance: the hock joint does not flex as the hindlimb moves forward and the distal aspect of the limb appears to hang limp. Flexion of the stifle joint occurs independently of hock joint flexion. Most horses with peroneus tertius tendon rupture are willing to bear full weight on the limb since pain is not a typical characteristic of this problem. Diagnosis is confirmed via manual extension of the hock and simultaneus flexion of the stifle joint (impossible to demonstrate in the normal horse).
While in this position, dimpling and/or excessive laxity of the common calcaneal tendon is evident.
Horses sustaining peroneus tertius tendon rupture are treated with rest. Fibrotic (or scar) tissue forms between the two ends of the fragmented tendon, eventually reestablishing near-normal function of the structure. Most of the scar tissue has formed by 60-90 days after rupture but won't achieve maximum strength until 10-12 months post-injury. See more about tendon injuries HERE.