Similar to the human knee, a horse's stifle joints are like hinges—some of the largest in its skeletal system. Occasionally, a stifle joint becomes locked due to overstraining or genetic joint problems and predisposing conformation. When this happens, its hind leg appears to be stuck in extension, often causing alarm. But don't stress—locked stifles are relatively common. This condition can happen to any breed of horse or pony, but those with upright hind legs and straighter angles on their hock and stifle joints may be more susceptible. Mild cases may only cause slight lameness, and there are ways to make your horse sound again (often without invasive procedures). Sometimes, simply backing the horse will release the locked stifle. However, if left untreated, horses that display regular locking stifles may be unsafe to ride and require surgery.
What Is a Locked Stifle Joint?
A locked stifle joint happens when one of the horse's ligaments remains hooked over a ridge on the head of its femur bone during movement. Stifle joints help the horse bend its rear legs and stabilize itself, and these joints are actually meant to lock when standing. This important function is part of a horse's stay apparatus (an arrangement of muscles, tendons, and ligaments in the horse's hind limbs that work together so it can remain standing using very little muscular effort).
This action comes into play when horses sleep, keeping them upright on their feet. Normally, the horse can flex the joint with little effort to unlock it. However, in some horses, unlocking becomes delayed. A small delay will cause mild symptoms, but long delays result in more severe symptoms. Medically, this condition is called "upward fixation of the patella."
Symptoms of Locked Stifle Joints in Horses
A horse with a locked stifle may develop a slight lameness in the hindquarters which, when mild, can be easily overlooked. In more serious cases, locked stifle joints can cause the horse to struggle to move normally or even lock the leg fully. You may notice the following symptoms:
A simple hesitancy can result as the horse responds to discomfort and stiffness, especially after standing still for long periods of time. The horse can stumble or knuckle in the back end, especially on downward transitions like a trail ride with a descending slope. A locked stifle may also cause a horse to short step and display difficulty changing leads at the canter. Or, it can make your horse canter on different leads in the front and the back or hop slightly, flinging its back leg. If you notice your horse having problems working in a circle or dragging its toes, don't write it off as bad behavior or a bad habit. Consult your vet to diagnose the issue so that treatment can begin.
In severe cases, a horse's hind leg will appear obviously locked, making it hard to overlook. It may stretch its leg out behind it when trying to walk, or it may kick backward and step oddly to get the stifle joint to release. Your horse may not be able to alleviate its locked joint on its own and may drag its leg behind. Then, for no visible reason, the leg may snap back into a normal position (you may even hear a click). If this happens—even if everything appears fine—get it checked by a professional.
What Causes Locked Stifle Joints in Horses?
Locked stifles are most common in ponies, foals, and horses that are unfit, although the exact cause remains unknown. However, there are a few theories that may explain why your horse's joint becomes locked in place.
Rapid Growth Spurts
There is speculation that young horses may develop this condition due to rapid growth spurts when the bones grow faster than the muscles develop. Even a small growth spurt can change the angle of a horse's joint, causing inappropriate functioning.
Lack of Muscle Tone
Another theory suggests that an unfit horse (or one that has suddenly lost muscle tone) may develop this condition for similar reasons—the joint angle suddenly becomes compromised and, therefore, locks.
How Do Vets Diagnose Locked Stifle Joints in Horses?
In severe cases, a locked stifle will be very obvious. The horse will have difficulty moving its leg normally. However, a locked stifle could also be mistaken for stringhalt—a neurological disease that causes exaggerated and uncontrollable movement, sometimes making your horse jerk its hind leg up high while stepping. In any case, a veterinarian will need to examine your horse and manipulate its stifle joint to see if they can manually induce the unlocking mechanism. After that, a radiograph may be performed to see if the lameness is caused by something else, such as a bony abnormality. Lastly, your vet may administer a local nerve block to determine if the lameness is actually caused by pain or if it is truly a mechanical issue.
How to Treat Locked Stifle Joints
For mild locked stifle cases, exercise and a balanced hoof trim may help your horse. Lack of fitness causes weak muscles and ligaments, so simply conditioning your horse can sometimes help solve the stifle problem. For severe locking, ask your farrier to "rocker," or roll, the toe of the hoof. They can also fit your horse with corrective shoes and pads to help the hoof break over before the locking occurs.
Certain cases that do not respond to conditioning or corrective farrier work may call for surgery performed by a veterinary surgeon called a medial patellar desmotomy. During the surgery, a vet makes small incisions in the patellar ligament while the horse is sedated and standing. This frees the strain of the ligament, allowing for ample movement within the joint (and alleviating the catching). However, since freeing the ligament often causes instability in the patella itself, the downside to this surgical treatment is the potential development of both arthritis and bone spurs.
Prognosis for Horses with Locked Stifle Joints
Locked stifle joints are often mild to moderate problems that can be remedied through exercise, corrective shoeing, and hoof trims that allow the horse to unlock its joint through natural movements. Horses dealing with severely locked stifle joints can still live happy, healthy lives after being treated by a veterinarian or undergoing surgery. Owners of horses that have medial patellar desmotomy surgeries should also be mindful of any future developments that may indicate arthritis or bone spurs. Seek veterinary help if any signs of these issues begin to appear.
How to Prevent Locked Stifle Joints
Some horses are more likely to experience locked stifle joints than others, but owners can take preventative care steps to keep their muscles strong and healthy. All horses can benefit from regular exercise on a schedule that is catered to each horse's specific needs:
Since the lack of muscle tone may contribute to locked stifles, horses should be exercised gradually. Activities like trail riding—while increasing the distance and speed very slowly and over several weeks—will help a horse achieve its ideal fitness level in a safe way.
Strengthening Specific Muscles
Try cavaletti training over small wooden jumps or pole work that makes your horse pick up its feet. Lunging your horse on a line or riding it on a slight incline (so that it drives with its hindquarters) also makes for safe training when done in small, slightly increased intervals. It's important to start slowly, avoid overworking your horse, and thoroughly discuss your strategy with a veterinarian before beginning any training regimen if the horse has already experienced locked stifles.
Locking Stifles in Horses. Kentucky Equine Research, 14 May 2014
Disorders Of The Stifle In Horses. Veterinary Manual, October 2020
Locking Stifles. What Does It Mean?. Darling Downs Vets, 2020
Intermittent Upward Fixation Of The Patella And Delayed Patella Release In Horses. Veterinary Manual, June 2016