Similar to the human knee, a horse's stifle joints are like hinges—some of the largest in a horse's skeletal system. Occasionally, a stifle joint becomes locked due to overstraining or genetic joint problems. When this happens, its back leg appears to be stuck in extension, often causing alarm. But don't stress—locked stifles are relatively common. And while mild cases may appear vague (with only a slight lameness), there are ways to make your horse sound again, often without invasive procedures. However, if left untreated, horses that display a regular locking stifle may be unsafe to ride and may require surgery.
What Is a Locked Stifle Joint?
Stifle joints help the horse bend its rear legs and stabilize itself, as 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 hind limbs that work together so that it can remain standing using very little muscular effort). This action comes into play when sleeping, keeping a horse upright on its feet. A locked stifle joint occurs when one of the ligaments remains hooked over a ridge on the head of the femur bone. 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 a long delay will 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. A simple hesitancy can result as the horse responds to the discomfort and stiffness, especially after standing still for long periods of time. The horse can appear to 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, a horse may try to 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 look 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 when this happens). If this happens—and even if everything appears fine—get it checked by a professional.
Causes of Locked Stifle Joints
Locked stifles are most common in ponies, foals, and horses that are unfit, although the exact cause remains unknown. 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. 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.
Diagnosing Locked Stifle Joints
In severe cases, a locked stifle will be very obvious. The horse will have difficulty moving its leg normally. However, don't mistake a locked stifle 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 she 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.
For mild locked stifle cases, exercise and a balanced hoof trim may help your horse. Lack of fitness may cause 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. Or have him 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 a surgery performed by a veterinary surgeon called 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.
How to Prevent Locked Stifle Joints
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 fitness level in a safe way. Also, 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.
Locking Stifles In Horses. Kentucky Equine Research
Disorders Of The Stifle In Horses. Veterinary Manual
Locking Stifles. What Does It Mean?. Darling Downs Vets, 2020