Upward Fixation of the Patella Also known as a
, upward fixation occurs when the stifle is locked in
extension as the medial patellar ligament catches on the trochlear ridge of the femur.
Causes of patellar fixation is poor stifle conformation resulting in straight hind limbs, poor muscle tone and medial patellar ligament laxity.
When a horse rests a hind limb, they ‘lock’ the opposite patella on the femur to allow them to conserve energy. The medial patellar ligament is lifted over the medial trochlear ridge of the distal femur and temporarily locks the limb in place. When a horse has fixation, they are unable to lift the patella and medial patellar ligament back up and over the ridge to release the patella causing the horse to be in constant stifle extension. stifles and/or horses with poor condition or muscle tone of the hind quarter muscles. This often occurs in young horses prior to training or conditioned horses put on stall rest for long periods of time. If conformation is the primary contributing factor, fixation can be bilateral. Those predisposed to this condition includes horses with Hoses suffering from constant upward fixation of the patella appear with the hind limb locked in , they often drag the toe as they are unable to flex the stifle to clear their foot off the ground. Some horses may only have intermittent fixation where the patella catches and suddenly releases, resembling stringhalt. When horses are walked down a slope, the hindlimb often buckles slightly at each step. The horse often has to be backed to release the patella, sometimes, manual pressure has to be applied in and down on the patella to help it release. The medial patellar ligament becomes stretched over time which can further exacerbates the condition.
Diagnosis is through signs and symptoms. X-rays are often done to rule out other conditions such as OCD or patellar problems.
Treatment for the horse with poor muscle tone is to increase their activity to increase the muscle tone, especially of the quads. If a horse is on stall rest, hand-walking daily can help avoid or slow the loss of muscle tone. Adding to the hind limbs to alter the stifle angle may also be beneficial. This will help to ease the break over of the limb so that the stifle doesn’t need to extend as much. Irritants may be injected into the medial & middle patellar ligaments to promote fibrosing & ‘tightening’ of the ligament so that it can’t catch on the ridge. Surgery may be indicated in horses with bilateral cases where muscle tone is adequate. The medial patellar ligament is cut so that the patella cannot lock. Successful desmotomies mean horses are considered sound and can be used. Prognosis for athletic return is excellent in horses that responded to increase in exercise. For those that have had patellar desmotomy have fair prognosis for athletic return. Unfortunately, other stifle joint problems may occur as a consequence.
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