The patella, or kneecap, is a small bone that glides in a groove at the front of the knee joint.
In many dogs, and less commonly in cats, the patella will luxate (pop out of place), resulting in lameness or an abnormal gait. This condition can vary in severity from luxating occasionally to being permanently luxated.
Signs vary dramatically with the degree of luxation. In milder cases, the patella primarily remains in place and lameness is evident only when the patella is in the luxated position. The affected limb is carried with the stifle joint flexed, but may be touched to the ground intermittently. Once the patella returns to its normal position, the lameness disappears. Dogs that are more severely affected often exhibit a crouching, bowlegged stance with the feet turned inward and with most of the weight transferred to the front legs.
Although patellar luxation can result from trauma, it is typically a hereditary and congenital condition. Affected dogs develop an abnormally shaped knee, allowing the patella to luxate as they mature. Toy and miniature breeds of dogs have a much higher incidence of this disease than other dogs.
As the patella continues to luxate, it not only causes lameness but also causes abnormal wearing of the cartilage surfaces in the knee, leading to secondary osteoarthritis.
Treatment for patellar luxation involves surgery to reposition and stabilize the patella in the patellar groove. The key to successful treatment lies in early diagnosis and therapy.