The peroneus tertius muscle is an important component of the reciprocal apparatus of the hindlimb. This apparatus is necessary to allow only equal movement of the hock and the stifle. Occasionally the peroneus tertius muscle can suffer a trauma-induced rupture. This results in unequal movement, such that the hock can be extended while the stifle is flexed (see image below).
The injury is thought to occur when the horse falls with the leg retracted or when trying to extract a hindleg from entrapment in a gate or fence. Typically, horses can appear to bear weight normally on the affected limb but will have reduced weight bearing during movement. The limb will move forward with normal stifle flexion, but normal hock flexion is delayed and is only moved forward under flexion due to momentum of the distal limb.
The diagnosis and progression during healing is best monitored with ultrasound.
Tags: Musculoskeletal