The horse is examined by a vet, and it is determined that the horse has an “upward fixation of the patella” – in simple words this means that the patella, or knee cap, slides out of position. This condition is seen in horses that have very upright or straight stifles, as you can see in this picture. In horses with straight stifles, the stifle is very straight, so it is easier for the patella to slide up and catch on the femur. This problem is also seen when horses have time off, as the muscles that hold the stifle in place become weak, leading to reduced tension on the patellar ligaments, and they become slack resulting in the sliding of the patella.