The stifle joint in a horse’s hind leg corresponds anatomically to the knee joint in the human leg. However, instead of appearing halfway down the limb like the human knee, the horse’s stifle doesn’t even look like a joint because it is hidden within the structure of the horse’s upper hind leg. If you put your hand on the front of the horse’s hind leg where it ties into the flank, you can feel the patella, a small bone that is the anatomic equal of the human kneecap. The patella sits just above the stifle joint where the horse’s femur (upper leg bone that ties into the hip) and the tibia (long bone above the hock) meet.

The medial patellar ligament has the important function of hooking over a notch in the end of the femur when the horse is standing still. This stabilizes the stifle and allows the standing or snoozing horse to bear weight on the hind leg without muscular effort.

Normally, the ligament slides out of the notch when the horse swings its leg forward as it begins to walk. If the ligament gets hung up and doesn’t slip into an unlocked position, the hind leg can’t be flexed forward and the horse has to drag the stiffened limb forward for a few steps before the ligament releases. This is commonly known as a locking or sticking stifle. While veterinarians term the condition “upward fixation of the patella,” old-time horsemen have a simpler descriptive phrase: “That horse is stifled.” They might add, “Back him up a few steps to get it to release,” and this trick often works.  

Locking stifles aren’t limited to one breed or type of horse or pony, but they are somewhat more common in horses that have very upright hindlimb conformation, with overly straight angles of the hock and stifle joints. There isn’t much an owner can do to correct this faulty conformation, but putting the horse in a gradual conditioning program will strengthen the muscles around the stifle and decrease the incidence of locking. The problem of sticking stifles has been relieved in some young horses that gained 55 to 100 lb (25 to 45 kg), possibly because they developed a larger fat pad behind the patella. Any desired weight gain in horses should be the result of a gradual increase in caloric intake, not through drastically increasing the amount of grain given to the horse.

Corrective shoeing helps to eliminate sticking stifles in some horses. The farrier encourages hoof rotation by trimming the inside wall or applying a lateral heel wedge. Better medial breakover can be enhanced by rounding the medial aspect of the toe of the hoof or shoe.

In one study1 that looked at treatments for locking stifles, 40% of horses with locking stifles showed complete recovery, and 20% had marked improvement following corrective shoeing.  Another 10% of affected horses showed improvement when corrective trimming was combined with weight gain and exercise.

If these noninvasive techniques don’t help, veterinarians can use one of several procedures to cause mild scarring of the ligament, decreasing its elasticity. When the ligament is somewhat less flexible, it can be pulled into position more easily rather than stretching and staying locked in place. These procedures eliminate the problem in some horses but are less successful in others.

Sticking stifles are not always a serious problem, and mildly affected horses may be usable as long as the rider takes into account that the horse should not be asked to make smooth, athletic movements as it begins to walk after standing still. These horses may not always show classic locking, but might display more subtle signs such as a shortened stride, difficulty picking up or maintaining a canter lead, or a bit of scrambling while going up or down hills. Horses that regularly display classic locking stifles and don’t achieve a normal gait after a few strides may not be safe to ride.

To detect the problem in a horse being considered for purchase, ride the horse at all gaits and in circles as well as straight lines. Check for problems going up and down hills, and have an experienced horseman or horsewoman watch for regularity of gaits and shortened strides in a hind leg. Avoid buying a horse with obvious conformation faults like crooked or overly straight legs. If you suspect a problem with legs or joints, ask the veterinarian about your concerns during the horse’s prepurchase examination.

1Upward fixation of the patella in horses: prevalence, results of conservative and surgical treatment Michèle Dumoulin UGent, Frederik Pille UGent, Paul Desmet, Ann Martens UGent and Frank Gasthuys UGent (2004) Proc. 12th congress of the European Society of Veterinary Orthopaedics and Traumatology (ESVOT). p.230-230