Upward Fixation of the Patella in Horses

The stifle joint of the horse is akin to the knee in humans. Horses have a patella (or knee cap) that will slide along the far aspect of the thigh bone amid inflection of the joint. The patellar ligaments hook over a notch on the femur, which enables the horse to secure his hind legs in the standing position. Should one of these ligaments stay in the angled position, the horse will not be able to bend its back legs, a condition called upward fixation of the patella. A horse with upward fixation of the patella can experience a small delay in the forward motion of his leg, through the complete locking of the leg for an extended time period.

Upward fixation of the patella is when one of the patellar ligaments that hook over an indention on the femur of a horse stay in the hooked position, causing the horse to be unable to bend his hind legs.

Should your horse experience the condition of upward fixation of the patella, you may notice the following:

• His gait is jerky when moving from stifle extension to inflection
• His leg will release either suddenly or as a result of help with a jolt or snap 
• The delayed release is periodic and is typically most noticed when the horse begins to move
• He has an inability to flex the stifle and tarsus without help

In most cases the condition is in both pelvic limbs, though the horse will initially show clinical signs in one limb first; once that limb is treated successfully, the condition will show itself in the other limb.

Types 

Upward fixation of the patella can be mild, where conservative treatment (for example strengthening the horse’s surrounding muscles) can resolve the condition. In some more mild cases of the condition, the horse may even outgrow the problem. Less frequently, the condition is more severe, requiring surgical intervention for resolution.

The cause of upward fixation of the patella is not clear, though there are some similarities in horses that have the condition, to include:

• Straight hind limb conformation
• Atypical structure of the stifle joint
• Swelling and pain in the stifle joints as a result of conditions like osteochondrosis or traumatic cartilage injuries
• Poor conditioning or not in shape
• Sudden management or exercise changes

Should the limb of your horse be locked in extension, as in a severe case, diagnosis of upward fixation of the patella will be simple. The majority of cases, however, will be mild which makes diagnosis more of a challenge. Your veterinarian will conduct a physical examination of your horse as well as ask you what symptoms you have noticed in your horse, when you noticed them and if there have been any changes. In order to help with diagnosis, your veterinarian may put your horse in one or more circumstances where pelvic limb extension over a longer period of time is necessary. This may include walking up and down hills, the transition from a trot to a canter, and backing up. Your veterinarian will then be able to observe how your horse responds; he may show upward patellar fixation through temporarily locking the pelvic limb or swing the limbs outward to avoid pelvic limb extension.

Ultrasound and radiography may be utilized in making the diagnosis as well. Other causes of hind leg gait anomalies like shivers, stringhalt, and fibrotic myopathy should be ruled out by your veterinarian while he confirms the diagnosis of upward patellar fixation.

Conservative treatment for the condition will include a conditioning program where exercising will be focused on strengthening and toning the muscles that surround the patella. A nutritional program will also be recommended in order to improve your horse’s health. In addition, your veterinarian may recommend corrective shoeing.

Should conservative treatment not lead to improvement in your horse, your veterinarian may recommend injections or surgery to remove a section or split the medial patella ligament (called desmotomy). As these treatments do not always correct the problem, they will typically be considered when other options have not been successful.

Most horses will respond to conservative treatment. In fact, many horses will grow out of the condition. In cases where surgery is necessary, the prognosis is fair and there may be some limits to the gait of the leg that has been affected.

It is important to follow the recommendations of your veterinarian to ensure the best outcome for your horse. Should strength and conditioning exercises along with dietary changes be recommended, it is important that you implement them to improve the overall health of your horse and to ensure the best possible outcome for him. Should your horse undergo surgery, he will likely be restricted to his stable or stall for about two months of rest in order to minimize the chances of complications. It is important that you attend follow up appointments as requested in order to confirm that your horse is healing well and not experiencing any complications from the procedure.