Upward fixation of the patella is a hindlimb gait abnormality that occurs quite commonly in young horses and sometimes also in older horses.
The horse’s stifle is designed so that the patella (kneecap) can hook over the medial trochlear ridge (the protuberance at the bottom and inside) of the femur (see diagram). This locks the leg in place, allowing the horse to stand using minimal energy and also to sleep while standing up. While this may have been of evolutionary benefit when horses needed to run away from predators, it is perhaps less useful in today’s sport horse — particularly when the system fails.
Upward fixation occurs when the patella does not release quickly and easily from the distal femur.
The condition can range from severe, when the leg is completely stuck and dragged behind the horse, to more mild, where the patella “catches” but the limb does not truly become stuck. This intermittent catching may make the horse look jerky in his movement, particularly during downwards transitions.
The most common reason for developing upward fixation is weakness of the thigh muscles (the quadriceps and biceps femoris) responsible for releasing the patella.
Horses with overly straight/upright hindlimb conformation or poor foot conformation (long toe, low heel and high inside hoof wall), or those that have suffered trauma to the stifle region, may also be predisposed. In older horses, the cause may be loss of muscle mass, perhaps due to rest.
Diagnosis is based on history and observation, or description of the typical gait abnormality.
Radiographs and ultrasonography are not always essential, but may be useful to identify any underlying abnormalities of the stifle joint, particularly in horses which do not respond to conservative treatment.
Improving the muscle mass of the hindlimbs with strengthening exercises, such as hill work and working over poles, is key to conservative management. Working in small circles may exacerbate the problem, so straight lines are best. Long-reining may be useful in young horses not yet broken to saddle.
As much exercise and turnout as possible is a good idea. Where there is an underlying reason for any sudden loss of muscle mass, an unrelated injury or lameness, for example, this must also be treated.
Where the horse is in poor body condition, particularly in a young or growing animal, it should be ensured that the diet is adequate and that he has been wormed appropriately.
Various feed supplements can help build up muscle mass without adding excessive protein. The aim is for the horse to be neither too fat nor too thin, with adequate muscle mass over the quarters.
The majority of young horses will grow out of this condition as they mature and with appropriate management. Depending on breed, use and level of training, this will usually happen by the time a sport horse is six — if it happens at all.
More problematic cases may have an underlying conformational issue. While upright hindlimbs cannot be made less straight, conformation of the foot can be altered to an extent.
Some affected horses may improve after appropriate trimming, with shortening of the toe and medio-lateral imbalance correction. Remedial farriery with shortening of the toe and application of a lateral wedge has helped certain cases in the short term, but the significant medio-lateral imbalance sometimes created may have long-term side-effects, by predisposing the horse to collateral ligament damage.
A horse who still has issues after these points have been addressed, or who frequently becomes stuck and unable to release the patella, will require further intervention.
Iodine injected as an irritant around the medial patellar ligament has been used. The aim of this is to encourage scarring, which results in shortening of the ligament, thus stopping the patella hooking over the top of the medial trochlear ridge.
The results of this procedure are inconsistent. I will admit to having limited experience of injecting counterirritants, as I discourage owners from this approach due to its unreliable results and the potential disastrous complication of inadvertent injection of the irritant solution into the stifle joint itself.
In the small number of horses who do not respond to strengthening work and conservative management, surgical treatment is preferred.
The standard surgical treatment involves cutting the medial patellar ligament close to its insertion on the tibia, which stops any hooking and instantly solves the problem. This remains the treatment of choice in the severely affected, distressed horse requiring instant relief, and where the patella cannot be released by manipulation of the joint.
Until fairly recently this was the only surgical method used. Its popularity in milder cases waned as it became recognised that the procedure can result in long-term complications, most notably fragmentation of the patella and development of osteoarthritis.
A further downside is that the horse will require rest for between three and five months after surgery.
A more recent surgical option is “splitting” the medial patellar ligament. Instead of completely cutting the ligament, small stab incisions are made at the proximal aspect (top) of the ligament.
This leads to localised inflammation and thickening of the ligament (similar to the rationale behind injection of counterirritants, but without the risk of damaging the joint) and stops the ligament hooking over the top of the medial femoral trochlea. The horse starts walking the day after surgery and improvement is usually seen within two weeks.
Advantages of this procedure over the conventional surgery are the rapid return to exercise and an apparent reduction in complications. A recent study from the University of Copenhagen, following up on 85 horses treated in this way over 14 years, reported a resolution rate of 97.6% — with no long- or short-term complications.
Ref: Horse & Hound; 17 March 2016