Normally, when a horse moves one of his hind limbs, the joints higher up in the leg (e.g., the hock, stifle, and hip) all flex or straighten together. But, if the ligament connecting the patella (the kneecap) to the tibia (the long bone between the stifle and hock joint) catches on the rounded knob, the medial trochlear ridge, at the end of the femur, the horse’s stifle locks into an extended position.

In most cases, the horse hyperflexes the limb or can be backed up to release the patella, but some can stay locked up for a while—this is referred to as upward fixation of the patella. Delayed patellar release is more subtle and most often occurs as the horse moves from a standing still position or during a downward transition. These horses typically don’t “lock up” entirely, often showing less obvious signs such as jerky patellar movements in a standing horse, rocking from side to side or moving in a “crouched” position when negotiating uphill inclines, or shortened hind limb motion on downhill slopes.

Delayed patellar release tends to occur more frequently in young horses and ponies, as well as horses with straight hind limb conformation and/or long toes and low heels. Conservative treatment involves exercises to develop muscle tone and coordination. Veterinarians don’t advise surgically cutting (desmotomy) the ligament, as it’s been shown to lead to joint instability and osteoarthritis. Instead, some recommend an alternative surgical method to manage this problem: splitting of the medial patellar ligament.

Sarah James, DVM, Dipl. ABVP, of Steinbeck Country Equine Clinic, in Salina, California, d