The horses stifle is the same joint as the human knee. Just like the human knee there is a knee cap, the patella, a small flat bone that rides back and forth in the front of the femur. The patella connects the large muscles in the front of the upper leg with the long bone of the gaskin using three large ligaments. When this muscle contracts it pulls up on the patella and helps to straighten the leg out, ...image.

Unlike the human knee, the horse is able to hang the medial (inside) patellar ligament over a bony prominence of the femur, intentionally. By doing so the leg is locked in extension and the horse can rest and sleep standing up. Some horses have trouble with unintentional interference or even have the leg lock up and unable to flex normally. Even worse, the traditional surgical treatments for this condition have been found to cause permanent low grade lameness. This article consists of diagnosis, treatment, rehabilitation, and prognosis for this condition.

Upward fixation of the patella (UFP) occurs when the medial patellar ligament (MPL) fails to disengage the notch of the medial ridge of the femoral trochlea as the horse begins to bring his leg forward. Clinical signs can vary in severity and frequency:

• The severe form is when the catching of the patella is complete so the leg cannot flex and the horses remains extended out behind him. He may require help to become unstuck. Some horses become repeatedly stuck and must stop and work the leg to get it unstuck.
• The moderate presentation is when there is a partial and intermittent locking of the patella and a palpable and sometimes audible click as the patella is released.
• The mildest form is a subtle delayed release of the patella, which can be seen vibrating or jerky, especially as the horse slows. These horses may present as lameness or performance problems.

Many low grade lameness problems of the rear legs, that are difficult to localize, may get blamed on UFP. In these cases it is important to identify the vibration set up in the stifle just as the leg begins its forward motion.

Though the problem often starts in one leg, both are frequently affected in time. This occurs more commonly in young horses and ponies. Though it occurs in all breeds, Shetland ponies are most commonly affected.