By Dr Luke Poore

A locking stifle is caused by upward fixation of the patella in the stifle joint of the horse.

The patella

The patella is one of three bones that make up the equine stifle and is the equivalent of our kneecap. One of the most important functions of the patella is its role in the stay apparatus, which allows the stifles to lock in extension. This apparatus allows the horse to rest and sleep standing up, which saves time and energy if the horse needs to take flight from a predator.

On the inside of the patella is a ligament (the medial patella ligament) that runs from the top and inside of the patella down to the tibia. On the inside at the bottom of the femur (thigh bone) is a protuberance, the medial trochlear ridge. The horse can lock his hind leg by moving the medial patellar ligament and patella over this protuberance. When the horse wants to walk off, the quadriceps muscles, attaching to the top of the patella, contract, lifting the patella off the medial trochlear ridge.

Although this evolutionary adaptation is extremely useful and sometimes lifesaving in the wild horse, it can occasionally go wrong, leaving the domestic horse or pony with a locked stifle. A locked stifle or ‘upward fixation of the patella’ occurs when the natural locking mechanism fails to unlock quickly and spontaneously. The condition can range from a severe fixation, with the limb fixed in extension for hours or days, to an intermittent fixation, where a horse will occasionally fixate his patella for several strides before the leg returns to normal. This intermittent fixation may make the horse look jerky as he first starts to walk or during his downward transitions.

Affected horses

Upward fixation of the patella occurs in two main classes of horse:

a) Young horses who are immature and not in regular work.

b) Adult horses who have been in full work and then for reasons of illness are suddenly rested.

The main reason these two types of horses suffer from this condition is a loss of thigh muscle tone, especially in the quadriceps and biceps femoris muscle masses, which are used to lift the patella off the femur.

Horses with straight hind limb conformation often also suffer from this condition, as it is easier for the medial patella ligament and patella to hook over the medial trochlear ridge because the stifle is already slightly extended. Horses in poor condition are also often affected by this condition, as they cannot generate enough force from the quadriceps muscle masses to release the medial patella ligament and patella from their locked positions. Horses with poor foot conformation, such as a long toe and low heel or high inside hoof wall, may also be predisposed to upward fixation of the patella.

Diagnosis

It can be very difficult for vets to diagnose upward fixation of the patella, as the horse needs to demonstrate the abnormal limb movement at the time of the veterinary examination. Turning the horse or pony in tight circles, backing up, and manipulation of the patella can all provoke the abnormality, but sometimes vets have to rely on the clinical history and description of the problem. It is also sensible for vets to perform radiographs (x-rays) and ultrasound examinations of the stifle to rule out other stifle problems such as osteoarthritis or osteochondrosis.

Conservative treatment

Many young or poorly muscled horses will significantly improve or grow out of the condition as they are brought back into work or as their work is increased. Feeding a balanced diet, ensuring good dental care, implementing worming treatments, correcting poor foot balance and sometimes shoeing with a slightly raised heel can lead to resolution of the problem. However, the most important way of managing the problem is through a controlled exercise programme that develops the thigh muscles.

Certain exercises are particularly beneficial for horses suffering from upward fixation of the patella and include:

• Consistent work in an extended, forward-going trot.
• Riding up and down hills to strengthen the hind limbs.
• Working in shallow water to promote correct muscle development of the stifle area.
• Working over poles.
• Avoiding tight circles as this may exacerbate the problem.
• Long reining young horses who are not yet broken to saddle.

As much turnout as possible is useful to maintain the muscle masses developed during ridden exercise, and feed supplements can be used to increase muscle mass without adding excessive protein.

Injection of counterirritants

In the small number of cases where conservative management has failed, it may be possible to inject counterirritants, such as iodine, into the medial and middle patellar ligaments to thicken the ligaments and make them more fibrous. The aim of this treatment is to stop the medial patellar ligament hooking over the medial trochlear ridge and prevent further upward fixation of the patella. The results of this procedure are, however, inconsistent and potential complications include injection into the stifle joints with catastrophic results.

Surgical treatment

In cases where conservative treatment isn’t successful, two surgical options are available to vets.

Option one: Medial patellar ligament desmotomy

The first surgical approach is to cut the medial patellar ligament close to its insertion on the tibia. This surgery is performed with the horse standing, using sedation and local anaesthetic injected over the medial patellar ligament. A small 1cm incision is made at the bottom of the ligament through the skin. A curved knife is inserted on the inside of the ligament and the ligament is cut. The patella is allowed to realign and then, once healed, is unable to lock around the medial trochlear ridge.

This option was, until recently, the main surgical approach for upward fixation of the patella and is still used in severe cases where the horse is distressed. Unfortunately, serious complications have become evident with this procedure in recent years. Approximately 30% of cases treated with this surgery develop fragmentation of the patella and osteoarthritis. This is due to inappropriate movement of the patella within the trochlear groove due to the unopposed force from the lateral patellar ligament. Additionally, horses normally require rest for three months after the surgery.

 Option two: Medial patellar ligament splitting

The second surgical approach is also performed with the horse standing under local anaesthetic. Three rows of interspersed stab incisions are made through the skin and into the medial patellar ligament. This causes localised inflammation and thickening of the ligament of approximately three times its normal size, which prevents it hooking over the medial trochlear ridge. Horses only need two to four weeks’ rest and the procedure has recently been shown to be successful in 97% of cases with no reported complications.

Conclusion

Although upward fixation of the patella is a relatively common condition, the majority of horses and ponies affected by the condition respond to an increase in exercise and an improvement in the quadriceps muscle masses. In the small number of horses who don’t respond, surgical treatment with splitting of the medial patellar ligament can allow a rapid return to exercise and has no long-term complications.