Your horse's stifle joints are somewhat similar to the human knee. These joints are like hinges and are the largest joint in your horse's skeletal system. Occasionally, the stifle joint becomes locked. Locked stifles are relatively common. When your horse's stifle joint locks, you might be alarmed to see a back leg appear to be stuck in extension. In mild cases, it may appear as a vague lameness and can be hard to diagnose. Once it is determined that locked stifles are a problem, there are ways to get your horse back into good soundness, often times without any invasive procedures. There are a few reasons why stifles lock. Sometimes horses are born with joint problems that can cause locking. Sometimes it happens because of strain.
A horse with a locked stifle may look slightly lame in the hindquarters. It's easy to overlook a locked stifle problem because the cause isn't really apparent— you may see just a vague lameness or hesitancy as the horse responds to the discomfort and stiffness. The horse can appear to stumble or knuckle under in the back end, especially in downward transitions. I may short step, have difficulty changing leads at the canter, try to canter on different leads in the front and back, hop slightly or fling a back leg out slightly. Your horse may have problems working in a circle and drag its toes. You might mistake some of these symptoms for bad behavior.
In more severe cases, the hind leg will look obviously locked. The horse may stretch the leg out behind him as he tries to walk, kick backward or step oddly to get the stifle joint to unlock. The horse may not be able to stretch out of the locked joint at all and drag the leg behind it. For no visible reason, the leg may snap back into normal position, and you may even hear a click when this happens.
It is normal for a horse to be able to lock its stifle when standing, and that is an important function that works with the stay and check apparatus that keeps a horse on its feet while it sleeps. Locking occurs when one of the ligaments in the stifle joint remains hooked over a ridge in the head of the femur bone. Normally, the horse can flex the joint with little effort to unlock the joint. However, in some horses, this unlocking process becomes delayed. A small delay will cause mild symptoms. A long delay will result in more severe symptoms.
Locked stifles are most common in ponies, foals and horses that are unfit.
In severe cases, a locked stifle will be very obvious. The horse will have difficulty moving the entire leg normally. In milder cases, a veterinarian may need to examine the horse, and perhaps manipulate the joint to see if they can manually induce the unlocking. Radiographs can reveal if vague lameness is caused by something other than a stifle problem. A painkiller like Butazone can be administered to determine if lameness or other symptoms are caused by pain, or if they are bio-mechanical.
For milder cases, exercise and balanced hoof trims may help your horse. Lack of fitness means your horse will have weaker muscles and ligaments, and becoming stronger may help solve the stifle problem. For more severe problems, your farrier may rocker or roll the toe of the hoof, or corrective shoes and pads may be applied to help the hoof break over before the locking occurs. In very severe cases that do not respond to conditioning and corrective farrier work, a surgery called a Medial Patellar Desmotomy may be performed by a veterinarian. During the surgery, the ligament is cut. In medial patellar ligament splitting the veterinarian will make small incisions through the ligament. The downside of surgical treatments is that your horse has a good chance of eventually developing arthritis and bone spurs in the joint.
Since lack of muscle tone may contribute to locked stifles, horses should be brought into fitness gradually. Things that will help your horse become fitter are trail riding while very slowly increasing distance and speed over several weeks, cavelleti and pole work that makes your horse pick up its feet, lunging or riding your horse on a slight incline so that it must drive with its hindquarters. It’s important to start slowly, and not overwork your horse. Discuss your strategy with your veterinarian.