Upward Fixation of the Patella in Horses
Have you ever seen a horse with one hind leg extended out behind him, seemingly stuck? If so, it could be due to upward fixation of the patella.

Veterinarians most commonly see this condition in young horses and ponies. It occurs when the medial patellar ligament (which connects the patella, in the stifle joint, to the tibia below) gets ‘stuck’ on the femur during limb extension. The signs can be quite variable in severity and frequency.

In cases of complete fixation, the stifle and hock become locked in extension, and the horse might hop and drag his toe behind him to move. When the ligament releases, it appears as a jerking movement that some owners confuse with stringhalt (a neurologic disease that causes uncontrollable, exaggerated hind-limb movement).

With the severe form the leg can be stuck behind the horse momentarily, for a few minutes, or even permanently. In more mild, intermittent cases the release of the ligament is delayed and the horse has a jerky gait during limb flexion. Walking down a hill, backing, and walking in tight circles make these signs more noticeable.

The cause of upward fixation of the patella is not well-understood, although researchers have identified some predisposing factors. These might include:

  • Straight hind-limb conformation;
  • Upright medial (inner) hoof walls and elongated toes;
  • Weak quadraceps (a group of muscles that controls the stifle’s position and serves to extend the joint) and poor muscling in young horses;
  • Prolonged periods of stall rest or time off from work due to other injuries; and
  • Breed, due to a hereditary component (Shetlands, specifically, are affected).

During your veterinarian’s initial examination, it is prudent that he or she evaluate the horse’s gait and radiograph (X ray) and potentially ultrasound the stifle(s) to ensure the horse doesn’t have concurrent stifle disease, as this can change the course of treatment.

A conditioning program is key for young or weak horses with intermittent upward fixation. Conditioning should focus on strengthening the quadriceps. This is best done by incorporating hill work in which the horse is asked to walk up and down hills several times per day. Limiting time in the stall is also beneficial. Working with your farrier to make sure the horse has a balanced foot can help, as well.

If conservative management is unsuccessful, your veterinarian might perform counterirritant injections (blistering). This involves injecting iodine around the medial and middle patellar ligaments to cause inflammation and subsequent tightening. Veterinarians often have to repeat these injections, and the treatment produces mixed results.

With any medical therapy, upward fixation of the patella can recur. ­Therefore, veterinarians do not suggest complete stall rest.

When medical therapy is unsuccessful or for moderate-to-severe or recurrent cases, the current surgical treatment of choice is medial patellar ligament desmoplasty (splitting). Surgeons complete this procedure with the horse either under brief general anesthesia or standing with local anesthesia. They use a blade or needle (sometimes guided by ultrasound) to make small incisions in the medial patellar ligament to cause it to tighten and become inflamed. The horse can go back to light work the following day. Exercise is important post-desmoplasty because the resulting inflammation helps tighten that ligament.

The overall success rate for this procedure varies, but researchers have reported it to be 70-90%. In cases that do not respond to medical management and splitting, or when the patella remains persistently locked, medial patellar ligament desmotomy (cutting the ligament completely to release the locked patella) is the surgical treatment of choice. Surgeons perform this under standing sedation.

Although this is a straightforward resolution to the problem, it is not without complications, including development of secondary osteoarthritis of the stifle. Therefore, desmotomy should only be performed when necessary. This procedure also requires that the horse have more time off from work.

Take-Home Message

Upward fixation of the patella is one cause of stifle-related gait abnormalities. Conditioning resolves many of these cases. Surgery is only indicated when the horse is unresponsive to more conservative therapies.