As a part of the passive stay apparatus, the horse is able to 'lock' its patella by hooking it over the medial trochlear ridge of the femur. For movement, the patella must be unlocked - a process that normally takes no longer than a fraction of a second. If the time taken for a horse to unlock its patella is increased, or it remains permanently fixed, this is a pathological condition known as a habitual or permanent upward fixation of the patella.