This is a common condition, usually seen in unconditioned and young horses. It appears to owners as a hind limb that is suddenly “stuck out behind” the horse. The fetlock folds over and the hoof may drag. The horse has difficulty walking, as the hind limb remains stiff and straight and cannot be brought forward.
In many cases, suddenly the limb will pop loose and the horse will move a few steps, only to have it “lock up” again. Locking stifle is a dangerous condition in the horse being ridden at speed, as it can cause a horse to fall.
This condition is caused by too much laxity or slack in the patellar ligaments that hold the kneecap (patella) in place. In the normal standing horse, the patella rests upon a ridge of bone on the lower femur. When it is in this position, the limb is temporarily locked into position. When a normal horse steps forward, there is a reflex tightening of the quadriceps muscle group which lifts the patella off of this ridge and allows it back into its moving position. If the muscles are weak or the ligaments too lax or stretched, this may not happen, resulting in locking patella.
Think of a folding table. When the legs are straightened and locked into position, they become weight bearing. The legs must become unlocked in order to fold or bend again. This normal mechanism allows the horse to rest without expending muscle effort.
Locking patella can also be subtle or intermittent. Intermittent upward fixation of the patella is a problem that can affect the performance horse and can sometimes appear as lameness, poor performance or stringhalt-like gait.
In most cases, horses having Upward Fixation of the Patella are not lame. In most cases, this is not a painful condition but causes the horse distress in not being mobile.
Diagnosis requires clinical veterinary exam. In most cases, the diagnosis is obvious to the trained eye. The key is trying to identify the underlying reason for the problem, and manage that.
Treatment starts with improved fitness and muscular strength of the hindquarters, achieved through specific exercise regimens. Injection of ligaments and certain surgical procedures are used for more persistent cases.
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Prognosis & Relevant Factors
Horses of certain conformation are predisposed to the condition. Miniature horses and some other breeds are more predisposed to the condition. Genetics thus plays a role in the development of the condition.
One of the most important predisposing factors is periods of inactivity. We often see this condition in horses that are stabled without adequate exercise. It follows that any disease causing muscular weakness of the hind limbs also makes this more likely. If a horse is rehabilitating from another injury, they become weak and this condition arises.
The prognosis for locking patella is usually good with treatment, but this will depend on the underlying cause being remedied.
Horses that have had their medial patellar ligaments cut to treat locking patella may end up with degenerative joint disease in the stifle.
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