Rupture of Peroneus Tertius
The peroneus tertius is a strong muscular band of tissue that lies between the long digital extensor and the tibialis cranialis muscle of the rear limb. Rupture of Peroneus Tertius is usually due to overextension of the hock joint.
Peroneus Tertius originates from the extensor fossa of the distal lateral femur and inserts distally as a tendinous band to the third Metatarsal bone and laterally on the fourth metatarsal bone. It is an important part of the reciprocal apparatus, mechanically flexing the hock when the stifle joint is flexed. The muscle or tendon can rupture anywhere along its course and can result in an avulsion fracture at its origin in the extensor fossa. When this muscle is ruptured, the stifle flexes but the hock does not.
Etiology
Rupture of the peroneus tertius is usually due to overextension of the hock joint. This may occur if the limb is entrapped and the horse struggles violently to free its limb. Rupture also may occur during the exertion of a fast start, when tremendous power is transferred to the limb, causing overextension, such as in jumping. It can also occur after a full-limb cast is applied to the hindlimb.
Clinical signs
- Signs of rupture of the peroneus tertius are well defined.
- The stifle joint flexes as the limb advances and the hock joint is carried forward with very little flexion.
- That portion of the limb below the hock tends to hang limp, giving the appearance of being fractured as it is carried forward.
- When the foot is put down the horse has no trouble bearing weight and shows little pain.
- As the horse walks, however, it is noted that there is a dimpling in the Achilles tendon. If the limb is lifted from the ground, a dimpling can easily be produced in the Achilles tendon by extending the hock.
- It is noted that the hock can be extended without extending the stifle; this cannot be done in the normal limb.
- If the origin of the peroneus tertius fractures from the femur, femoropatellar effusion is a prominent feature, and the gait deficit is similar.
Diagnosis
- Classical clinical signs
- Radiography, if the injury is more proximal
- Ultrasonography for confirmatory diagnosis
Treatment
Complete rest is the best treatment.
The horse should be placed in a box stall and kept quiet for at least 4 to 6 weeks, and then limited exercise should be given for the next 2 months.
Most cases heal and show normal limb action, and if properly conditioned, most horses can return to normal work.
Surgical intervention is not recommended. Hand walking is advisable when exercise is first begun to help control the horse and prevent re-injury.