Written by: Nicole Kitchener

Find out what to do about upward fixation of the patella – the term used to describe a condition in which a ligament hooks over a piece of bone, causing the stifle joint to “lock.”

Thumbnail for Stuck Stifles: Upward Fixation of the Patella

Nicole Kitchener photo

It’s about as close to a literal hitch in a horse’s getalong as you will find. Upward fixation of the patella (UFP) is a condition in which a ligament in the horse’s upper hind leg gets stuck over a piece of bone, making it difficult or impossible for the animal to flex the affected limb.

“It’s primarily a young horse disease based on the conformation of the stifle,” said Dr. Helen Douglas, owner of Valley Veterinary Clinic in Carleton Place, Ontario.
Dr. Douglas explained that UFP is more likely to occur in horses with straight stifles and poorly muscled quadriceps. But to really understand what is going on when a horse has UFP, we first have to take a look at the stifle and how it works.

Peering Into the Stifle

The stifle is the largest of all the horse’s joints. It corresponds to the human knee. Within the stifle, two bones meet – the femur, which is the long, large upper bone of the hind leg between the stifle and the hip joint, and the tibia, which runs from the stifle to the hock.

Another smaller, wing-shaped bone, the patella, is located on the lower end of the femur. Similar to the human kneecap, the patella glides up and down over the front of the femur as the leg extends and flexes. Among the other structures of the stifle, there are three distal patellar ligaments: the lateral, medial and middle ligaments that attach to and provide support to the patella.

Horses are able to lock their stifles to allow them to remain in an upright resting position while either asleep or awake without too much muscular exertion. The patella slides up, the medial ligament shifts and hooks over a notch in the femoral trochlea, which is a knobby structure located at the bottom of the femur. This mechanism, called the passive stay apparatus, keeps the stifle extended. Without it, the quadriceps muscles would always be in use. In effect, the passive stay apparatus enables the horse – a prey animal – to rest on its feet while being able to flee immediately when faced with a threat.

Usually horses have no problem unlocking the stifle joint to achieve forward movement. They flex the hind limb by contracting the quadriceps and other muscles to allow the medial ligament to slide over the femoral trochlea.

But sometimes the medial ligament gets stuck and the patella becomes “fixated.” In mild or moderate cases, this means a hesitation or vibration (even somewhat of a jerking motion) when the horse moves forward as the ligament works its way off the notch in the femoral trochlea. In fact, at its most benign, intermittent UFP can be so subtle that owners instead suspect performance problems or lameness. So, the horse might: take shorter strides with one or both hind legs; stumble in the hind end; drag the toe (wearing of the toe of the hoof or shoe might be evident); swing the affected limb to the outside; have problems cantering on one or both leads; perform awkward transitions and be unwilling to transition to canter from trot; pick up the wrong canter lead or change leads constantly; or be hesitant doing work that was not previously a problem.

This form of UFP is most evident when the horse is first exercised, and quite often the gait will improve through the workout.

Severe UFP

When UFP is at its worst, the hind leg remains extended behind the horse in the locked position. “They can’t bend the stifle or flex the hock. The leg is held slightly behind with the toe dragging,” said Dr. Douglas. “It’s painful, then the muscle contracts, making it even more difficult to unlock. The horse can’t lift its kneecap.”

The situation can prove alarming for both human and horse, especially when the animal isn’t able to unlock the joint himself. He may even try to hop forward on three legs, dragging the affected limb. In such instances, human assistance is required. Backing the horse up may resolve the problem or manually forcing the patella back into place (although that should definitely be left to a qualified veterinarian). Or, Dr. Douglas said another suggestion, which should be attempted with caution, is to stand with the horse on a lead, and have someone startle it from behind. She said the jerking forward movement is often enough to pop the patella back into place.

UFP commonly appears in both hind limbs, but one side will likely be more affected than the other. Once it occurs, the medial ligament stretches, making recurrence likely. Chronic UFP can lead to lameness, as the patella and the femoral trochlea rub together creating inflammation and eventual onset of osteoarthritis and/or permanent ligament damage. The horse may become sore and unwilling to work.

Contributing Factors

As Dr. Douglas noted, UFP is a “young horse disease.” This is largely due to the fact that still-maturing young horses, three to four years old – particularly those in training for sports that require intense use of the hind end such as cutting and reining – don’t have adequately developed quadriceps or other muscles. This causes instability in the patella, which prevents it from unlocking. Also, heavy training of youngsters, like that found in racing, for example, places excessive stress on the joints.

There are some reports of young horses that have exhibited no further signs of the condition after gaining up to 50kg. The belief is that increased fat behind the patella reduces the chance of the medial ligament catching on the femoral trochlea.

But horses that are mature and unfit, in poor condition, on prolonged stall rest, or having sustained stifle, hind end, back or hock injuries can also have poor muscle tone and possibly altered movement, which may also lead to the condition.

Inherent breed traits or familial genetic lines too can play a role. Animals with upright hind leg conformation through the hocks and stifles (so-called post-legged), such as miniature horses and donkeys, are predisposed to UFP because the angle of the stifle brings the patella close to the femoral trochlea making it easier for fixation to take place. “You used to see [UFP] in Clydesdales and other large breeds, but now you see it mostly in Warmbloods and in some ponies,” said Dr. Douglas.

Veterinary Intervention

A veterinarian should examine the horse if UFP is suspected. The condition will be quite obvious to the vet if the hind leg is locked in extension, but if the patella locks intermittently or more subtly, the vet may want to rule out other conditions that resemble UFP such as stringhalt, wobblers and some neurological diseases.

Radiographs, ultrasound, scintigraphy or arthroscopy are also often performed in the diagnostic process to rule out other conditions.

Physical examination includes watching the horse in motion – backing up, turning in tight circles, walking down hills – listening for audible clicks or feeling for a catching sensation as the patella locks. It may also involve manually locking the patella by forcing it upward and outward. The vet will investigate for pain and tenderness in the joints and ligaments as well as heat and/or swelling. If inflammation appears to be present, anti-inflammatories will likely be prescribed.

Most experts advise exercise as a first-line treatment. Improving muscle and ligament tone and strength will help the stifle joint’s internal mechanisms work properly. For young horses, this might be achieved by increasing their work level. But for older horses, a thoughtful exercise regime that encourages the horse to lift his hind feet and flex through the hock and stifle often resolves UFP without further treatment. Prescribed workouts include: walking and trotting up hills; long trotting in straight lines on soft ground; long-lining; trotting poles or cavaletti; working in soft footing. Lunging, working in tight circles and round penning are not recommended.

Beyond Exercise

Some horses can also benefit from corrective shoeing such as raised heels, rolling the toes of the hoof or the shoes, and wedge pads to help limit the degree of stifle joint extension. Combined with improved fitness, this could be all that many horses need to resolve UFP. But other animals will need further intervention.

Some practitioners have found estrogen compound injections every six to eight weeks are helpful. It is believed the presence of the hormone increases the tension of ligaments, which makes it more difficult for UFP to occur.

Another treatment involves injection of a counterirritant (such as an iodine/oil solution or steroid based product) along the three patellar ligaments to create an inflammatory reaction. This form of internal blistering thickens and shortens the ligaments making it less likely that the medial ligament will get stuck.

When UFP does not resolve with use of the above treatments, surgery is the next step. Animals with conformational deficits are more likely to fall into this category than those with purely mechanical issues.

Traditionally, the recommended surgical treatment for severe and chronic UFP is to sever the medial ligament so that it is unable to catch on the femoral trochlea. Called a medial patellar desmotomy, the relatively simple procedure continues to help many horses, although many vets have steered away from it due to possible complications that include permanent lameness from developmental joint disease and fractures of the patella.

A newer, less-invasive surgical option was developed in the early 2000s by Dr. Aziz Tnibar, a researcher at the Equine Clinic of the Veterinary School of Alford in France. The procedure involves splitting the upper third of the medial patellar ligament in multiple sites using a small scalpel or a large-gauge needle. Ultrasound guides the exact location of the cuts. The goal is to cause thickening of the ligament to prevent it from hooking over the femoral trochlea. Over the past decade or more, veterinarians have witnessed success with this procedure or their own variations of it.

The prescribed post-surgical rest period to allow the patella to stabilize can vary, but generally ranges anywhere from around two weeks to three months. Research has shown that, typically, longer durations for both the desmotomy and the ligament splitting procedure results in fewer complications compared to rest periods of less than one month. The recovery time must be monitored by a veterinarian.

Other treatment options owners might find helpful for UFP include acupuncture, chiropractic and equine physiotherapy.
The goal of any treatment is to prevent further joint deterioration, provide relief and get the horse back to work, whether for performance or pleasure.

If you think your horse has a mild to moderate case of UFP, take notes, pictures, perhaps even a video and consult with a vet for an assessment. And, of course, if the leg is locked in extension, it is imperative a vet be called immediately.

Dr. Douglas said UFP is one of those conditions she sees a couple of times a year in her practice. “It’s not common, but it’s not totally uncommon either.” She added that unfortunately, there is no way to prevent UFP. “You have to deal with it when it happens to the horse.”

AN ALTERNATIVE TREATMENT

Dr. Michael Perron, of West Coast Equine Clinic in Surrey, BC, uses acupuncture in his practice to treat a number of ailments, including upward fixation of the patella (UFP).
He explained that acupuncture, a form of Traditional Chinese Medicine in which a practitioner inserts fine, flexible needles into the skin at specific points, “involves stimulation of neurotransmitters, which can act throughout the body and [produce] local effects at the site the acupuncture needle is placed.” It is used to “re-establish physiological balance to the body” and promote healing.

In Dr. Perron’s experience, acupuncture is most effective in mild cases of UFP. He said, “I would consider a mild case a horse who when I push the patella into the fixated position, then ask it to walk forward, moves the patella out the fixated position with a slight jerk felt by my hand, which is placed over the patella.” He added, “These horses’ patellar ligaments are not sore to palpate, and there is no effusion/filling in any of the compartments of the stifle joints.”

In addition to acupuncture, which Dr. Perron said he also uses alongside conventional treatments for more severe cases due to its “synergistic effect,” he performs aqua-puncture (also known as aquapressure) when dealing with UFP. “Aqua-puncture is an acupuncture technique in which sterile solutions are injected into acupuncture points with a hypodermic needle,” said Dr. Perron. “The effect is the same or similar to using traditional acupuncture needles, which is referred to as ‘dry needling.’

“I use vitamin B12 or so-called counter irritants, such as iodine in oil products, which cause an inflammatory reaction. Often, I will use vitamin B12 in younger horses with milder signs of UFP, and counter irritants in horses with more severe signs, or ones that have not responded to B12.

“I will usually only do this once, combined with appropriate conditioning to build strength and coordination. A lot of these cases involve younger, underdeveloped horses, or horses going through growth spurts which are causing them to be higher behind, which can change the biomechanics in the stifle, predisposing them to UFP. In older horses, I may use aqua-puncture initially, but would typically use counter irritants or tendon splitting if refractory to vitamin B12 and conditioning.”

Dr. Perron said he has seen well over 100 UFP cases resolved using acupuncture and aqua-puncture during the course of his practice.
~ Amy Harris