This shot shows a horses hock being extended straight while the stifles flexed, which can only be done if the peroneus tertius is ruptured.

Athletic horses, as with human athletes, have a high incidence of orthopedic problems. Different endeavors with the horse cause a different ratio or incidence of certain problems. Racehorses tend to suffer primarily from injuries to the front legs, while cutting and roping horses have equal distribution in incidence of injuries to the hind and front limbs. It seems to me this phenomenon is just a reflection of the stress of their jobs.

The most common sites of problems in the hind legs of roping horses are the hock, stifle, suspensory ligament or pelvis. This month, I’m going to describe a fairly unusual case that came up recently. It was a rupture of the peroneus tertius caused by the horse kicking over a fence. The peroneus tertius is a tendon that runs from its attachment at the end of the femur to the top of the cannon bone. Its function is to automatically flex the hock joint when the stifle joint flexes.

What you will see clinically when this happens is a horse that has trouble extending the leg forward when moving. Depending on the severity, this motion problem is exaggerated from the walk to the trot. The hock does not flex and come forward normally when the stifle flexes. There will be a noticeable flaccidness of the tendon above the point of the hock posteriorly.

If the injury has just happened, there will be swelling and soreness to palpation in the front of the leg, between the stifle and hock. The diagnostic test is that you can extend the hock manually while the stifle is flexed. (See photo.) This position can’t be accomplished unless the peroneus tertius is ruptured.

The good news is that most all horses will recover fully if kept confined in a stall for a couple of months. Since there is usually no damage to any joint and the function of this structure is simply a mechanical one, no long-term problems usually result.