Locking Stifle Syndrome in the gaited Horse Part 1

A common physical affliction among gaited horses is upward fixation of the patella, otherwise known as locking stifles.The symptoms of this may be so subtle as to cause the horse owner many nights of restless, worried wondering, or so obvious that it seems an otherwise healthy horse has become functionally useless. The good news is that oftentimes these animals – even those with very pronounced symptoms – can be brought back to soundness using basic riding and conditioning techniques.

The stifle joint, equivalent to the knee joint in a human, is mechanically complex. Horses have the unique ability to rotate and lock the knee ‘cap,’ or patella, over a groove in the lower, protruding portion of the femur (large upper bone of the leg). A horse with its leg thus braced in place is able to stand while sleeping or resting. This is a rather handy device for an animal of prey that may not have time to raise its large body from the ground and escape quickly should it suddenly come under attack. This locking mechanism also enables the horse to maintain forward extension of the long, heavy hind limb with minimal effort, as the extended limb is temporarily braced into position.

The ability to temporarily and selectively lock this joint in place is certainly useful. But on occasion this locking mechanism goes haywire, and the patella is snagged, or locked in place, over the protruding portion of the lower femur for an inappropriate length of time. This hinders freedom of motion. Symptoms of this mechanical glitch may be quite diverse, creating a great deal of confusion for both horse owner, and veterinarian. While most vets are acquainted with the problem, few are aware that it is especially prevalent among gaited horses. Therefore, a veterinarian may not immediately consider this syndrome when evaluating a horse with varied and intermittent lameness issues.

Symptoms of locking stifles include: chronic dragging of the toe of one hind foot; the horse seeming to ‘bunch up’ for several strides after the rider asks it to move forward; stiffness in one hind limb, particularly after a period of rest; a pronounced hitch or popping action in the horse’s rear leg; the leg actually locking in place so that it drags behind the horse; the sudden sensation that the horse’s hind leg is giving out from beneath itself; swinging the hind leg in an outward arc, rather than straight under the body; an inability or unwillingness to take up a proper canter lead; shortening of hind stride.

To further confuse matters, any number and combination of these symptoms may be intermittent, seasonal, or subject to particular conditions. Young horses may exhibit dramatic and frightening symptoms, then simply grow out of it a season or two later. Some horses will display distinct lameness after being confined to a stall, or following periods of prolonged rest. Horses in northern climates may demonstrate problems during the first few spring rides, after a long winter layover. It’s not uncommon for a horse to lock up while traveling up or down hills, or at the end of an especially trying ride. The animal that refuses to take a correct canter lead, especially on a circle, might be considered suspect. The truth is this problem is so pervasive among gaited horses – and the symptoms so intermittent and varied – that the smart owner will always include it in their short list of possible causes when they face difficult-to-pin-down hind limb lameness issues.

Have you ever owned a motor vehicle that misfired or otherwise demonstrated problems at inopportune times, but ran like a clock when you took it to a mechanic for repair? This is fairly typical of what happens when a horse with upward fixation of the patella is examined by a veterinarian. Sure-fire diagnosis of this condition can be tricky!

The causes of the syndrome include poor or weak supportive muscling in the horse’s hind leg, too straight – post-legged – hind leg conformation, and lax patellar ligaments. You might wonder why, then, the condition is so much more common among gaited horses?

As I’ve stated in previous articles, gaited horses are ‘quadridextrous’ in nature; i.e.: they can move every leg independently of every other leg. In order to express such free, loose motion, (how often have you heard the term: that horse is a ‘loose mover’?) the animal’s ligaments must possess a high degree of elasticity. People with this kind of tissue elasticity are often termed ‘double jointed.’ The downside to being built this way is that excessively loose ligaments may allow so much freedom in the joint that when stressed, the joint pops right out of place. I learned this lesson first hand as an athletic young person when my shoulders routinely popped out of joint!

The dynamics here are similar, except that in the case of upward fixated patella, the lax ligament allows the patella to pop – and get stuck – over the protruding bottom end of the femur joint at inappropriate times.

An important contributing factor of this syndrome in gaited horses is the poor riding dynamics employed by many gaited horse riders. Given the extra degree of inherent elasticity in the joint ligaments of gaited horses, it becomes especially important that the muscles and tissues surrounding the joint are maintained in strong, dense condition in order to support the joint and eliminate excess laxity in the ligaments. Routinely riding horses in an uncollected, ‘strung out’ form is extremely detrimental to the overall strength and soundness of the horse’s hind leg structures. Yet this form of riding is the norm for gaited horses of all kinds.

A pacing or step-pacing horse, for example, places far too much body weight on each hind leg when the leg is set back behind the horse. This not only stresses the joints and stretches those lax ligaments, but contributes nothing to the strengthening of the supportive muscles.  This makes these gaits – smooth as they might be for the rider – a physically debilitating ‘double whammy’ for the horse.

That’s the bad news – and that’s also the good news! Because just as poor riding form contributes to weakening of the horse’s hind limb structure and the subsequent development of the locking stifle problem, so can good riding and conditioning help to reverse the condition. This is often true even when the symptoms appear quite severe.

We once owned a nice little gaited Appaloosa mare with very straight hind legs whose right hind stifle locked up intermittently the first season she came to live with us. This usually occurred at the end of a challenging ride. After a season of regular use, the condition never cropped out again. The steps we took to return this mare to soundness were simple, practical – and not beyond anyone’s ability to duplicate.

Read Part II: Practical Exercises for the Horse with Locking Stifle Syndrome

Watch Brenda's Video about Locking Stifle Syndrome