Surgical Treatment for Delayed Patellar Release Studied

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.

Photo: The Horse Staff

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, discussed the efficacy of this procedure during the 2014 American Association of Equine Practitioners Convention, held Dec. 6-10 in Salt Lake City, Utah. She said the technique's purpose is to create scar tissue that thickens the ligament so it releases more easily from the femur's trochlear ridge.

During the medial patellar ligament splitting procedure, the surgeon makes small scalpel incisions at various spots along the ligament. Because practitioners can perform this procedure in the standing horse, the surgery costs and risks are less than one involving general anesthesia. She noted that there is no need for ultrasound guidance to perform the procedure and post-operative complications (e.g., infection, rupture of the ligament) are rare. Additionally, the procedure does not compromise the joint's stability.

In James' study, she reviewed the medical records of 64 horses with an average age of 8 that had undergone medial patellar ligament splitting surgery. She found that delayed patellar release resolved completely in 37 (58%) of those horses; improved in 20 (31%); and was not corrected in seven (11%). Of the 37 horses with complete resolution, six needed additional treatment, such as intramuscular estrogen injections (which reportedly relax the ligament so it can more easily move off the trochlear ridge without becoming entrapped), a second splitting effort, or injection of an irritant (to scar-in the tissues to reinforce joint stability). Of the 20 improved horses, four underwent additional treatment without further improvement while six had rehabilitation factors that might have contributed to the lack of full resolution.

Peters obtained an average of 4 ½ years of follow-up information on these horses. Of the horses that had the surgery, 63% experienced clinical improvement in 30-60 days. Seventy-three percent went on to perform at the owner's desired level of performance. For those horses not able to return to previous performance levels, six went on to be ridden in different athletic disciplines, making this musculoskeletal abnormality more of a performance-limiting rather than performance-ending issue. Five of these horses had another significant condition or lameness that kept the owners from pursuing further athletic pursuits.

“In general, 89% of horses benefited from this medial patellar ligament splitting procedure,” James said, which goes to show that it can be an effective surgery for long-term success and return to performance.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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