Lateral luxation of the patella is a rare, inherited condition in foals caused by a recessive gene. Luxation of the patella in adult horses is unusual and likely to be traumatic in origin. Lateral luxation is more common than medial luxation and may be more likely in horses or foals with hypoplasia of the lateral trochlear ridge of the femur. The condition may be unilateral or bilateral and varies in severity from intermittent luxation that readily reduces to persistent luxation that cannot be reduced.
Severely affected foals are unable to extend the stifle and so adopt a characteristic crouching position. If the condition is less severe, foals or horses may be reluctant to flex the stifle and thus demonstrate a stiff hindlimb gait. Diagnosis can be confirmed by radiography.
Although a number of surgical treatments have been reported, the prognosis in adult horses and in horses with concurrent osteoarthritis is poor. The prognosis for athletic function in foals may be slightly better.