Musculus fibularis tertius
The peroneus tertius is entirely tendinous in the horse (unlike in other species). It originates from the extensor fossa of the distal femur (together with the long digital extensor) and inserts by several branches on the tarsal bones and the dorsal surface of the proximal third metatarsus. It is not innervated as a muscle because it has no contractile tissue. Together with the superficial digital flexor tendon, it forms the reciprocal apparatus that forces the stifle and hock to flex and extend in unison. This linkage improves efficiency and is part of the stay apparatus.
Origin: Extensor fossa of the distal femur
Insertion: Dorsal surface of the proximal metatarsus (MT3) and tarsal bones
Action: Mechanically links stifle flexion to hock flexion (reciprocal apparatus)
Innervation: Not innervated (entirely tendinous in horses)
Rupture of the peroneus tertius disrupts the reciprocal apparatus. The classic sign is the ability to extend the hock while the stifle is flexed, which is normally impossible. It typically occurs from trauma such as catching a hindlimb in a fence or gate. Diagnosis is clinical and confirmed with ultrasound. Treatment is usually prolonged rest.
Rupture (from hyperextension injuries, kicks, or getting a leg caught) breaks the reciprocal apparatus, allowing the hock to extend while the stifle is flexed — an anatomically impossible combination
A hindlimb that can be extended at the hock while the stifle remains flexed after trauma needs immediate veterinary evaluation. Do not force the limb into abnormal positions.
Rupture (from hyperextension injuries, kicks, or getting a leg caught) breaks the reciprocal apparatus, allowing the hock to extend while the stifle is flexed — an anatomically impossible combination in normal horses. Diagnostic and dramatic. Signs include: Horse can flex the stifle while the hock remains extended (pathognomonic sign — impossible in a normal horse); hock drops when the stifle is flexed; dimpling of the skin over the front of the hock; often surprisingly little lameness initially; surgical emergency.
Rupture of the peroneus tertius disrupts the reciprocal apparatus. The classic sign is the ability to extend the hock while the stifle is flexed, which is normally impossible. It typically occurs from trauma such as catching a hindlimb in a fence or gate. Diagnosis is clinical and confirmed with ultrasound. Treatment is usually prolonged rest.
A hindlimb that can be extended at the hock while the stifle remains flexed after trauma needs immediate veterinary evaluation. Do not force the limb into abnormal positions.
See the Peroneus Tertius on our interactive 3D horse model. Open 3D Model