The Trick Knee

By Mark Hale DVM

Bolivar, MO

            The medial luxating patella, commonly called a "trick knee," is an extremely common problem in toy breed dogs. The owner typically notices a little skip in the dog's gait. Sometimes the dog may even run on three legs, and then suddenly start back to using four legs as if nothing has happened.

            What has happened is that the patella, or "kneecap", has slipped out of the smooth groove in which it usually rides up and down in. It usually slips medially, which means toward the midline. With the patella luxated, or "dislocated", medially, the knee joint cannot flex properly and stays bent. Hopefully, the pet will be able to slip the kneecap back into the groove where it belongs and return to normal in only a few steps. For some dogs however, getting it back in place is not possible. Approximately 50% of affected dogs have both knees involved, while the other 50% have only one knee affected. The condition is most often congenital (inherited) and associated with varying degrees of skeletal deformity of the femur and tibia. The short term effects of this may be minimal and short-lived, but the long term effects, arthritis and degeneration, can be painful and debilitating.

            Diagnosis of this condition can usually be made at an early age such as at 8 weeks when most purebred puppies are purchased. The grade (severity) of luxation may change as the puppy becomes a few months older, but by 6 months of age changes would be minimal. The pet owner and their veterinarian should decide early if surgery should be done to correct the condition. Early surgical correction, before degeneration begins, allows the joint to remain healthy.

            There are several surgical techniques used to correct a luxating patella. Depending on its grade and the degree of skeletal deformity, one or a combination of these bone reconstruction and soft tissue correction techniques may be required. The main technique involves removing a wedge of bone to deepen the groove the patella should ride in. Many of the affected dogs do not form a significant groove at all, and in fact the femur may be completely flat where the groove should be.

            The prognosis for pets that receive early surgical correction is considered good. All animals with patellar luxation must be excluded from breeding to prevent this condition in future generations.

            If you are purchasing a toy breed puppy I would highly recommend that you have it examined early to check for problems such as this. If you have questions about this or other pet health topics please make an appointment with your veterinarian.