Patella luxations are dislocations of the kneecap (patella) due to injuries or developmental causes. The patella is a bone found within the tendon for the quadriceps muscles in the front of the leg. This tendon crosses the knee joint and attaches to the tibia. The patella fits in a groove on the lower end of the femur to allow the leg and muscles to stay properly aligned. The quadriceps muscles, the patella and the tendon all work together to allow the patient to stand on the leg (extensor support). Without this functional unit your pet cannot push off on the leg or jump or run. Patella luxations occur when the patella slips out of the groove on the end of the femur. Patella luxations occur most commonly in small breeds and the luxations usually occur toward the inside of the leg.

Causes: Although trauma can cause the patella to luxate, most cases of patella luxation are due to congenital defects. The cause isn’t known but it is felt to be due to the interaction of multiple genes (polygenic inheritance). In normal dogs and cats, the femur, patella, and tendon are aligned in a straight line. In many small dogs, the femur is rotated outward and the tibia rotates inward. The tendon and patella are pulled to the inside of the leg and the patella slips out of the groove. The deformity tends to worsen with time.

As with many abnormalities of the body, there are degrees of patellar luxation that can vary from mild to severe. We grade patella luxations from 1-4.

Grade 1 Patella Luxation: In a standing position the patella is in the groove. The patella can be displaced from the groove with digital pressure but it returns to its normal position when released.

Grade 2 Patella Luxation: In a standing position the patella is in the groove. Upon flexion or rotation of the knee, the patella spontaneously slips out of the groove. The patella returns to the groove when the leg is straightened.

Grade 3 Patella Luxation: In a standing position the patella is out of the groove and positioned on the inside of the knee (medial luxation). The patella remains outside of the groove with both flexion and extension of the knee and leg. The patella can be placed into the groove with digital pressure but it will not stay in the groove once it is released.

Grade 4 Patella Luxation: In a standing position the patella resides on the inside of the knee (medial luxation). The patella cannot be moved into the groove with digital pressure because of the chronic scarring and muscle contraction on the inside of the knee and deformities of the femur and tibia. The patient has now lost normal standing strength in the affected leg.

Diagnosis: A patella luxation is diagnosed by physical examination and palpation of the knee. Radiographs are usually not diagnostic of any specific problem unless the patella luxation is a grade 4 in which deformities of the tibia and femur will be seen.

Treatment: Animals with a patella luxation that is a grade 1 or 2 luxation and no clinical signs (i.e. any pain or lameness due to the luxation) may require no treatment. Patients with a grade 2, grade 3 or a grade 4 luxation that have lameness, discomfort, or gait abnormalities need surgery to correct the patella displacement. Patients with grade 2 luxations are at risk for serious ligament injury from the patella slipping out of the groove while running. Grade 3 and grade 4 patella luxations are at risk for permanent disability/arthritis of the knee because of irreversible changes to the bones and muscles of the leg. There is no medical treatment for patella luxation. Some dogs with grade 1 patella luxations can improve with physical therapy.

Surgery: The goal of surgery is to place the patella in the groove such that it does not slip out with normal movements of the knee while also allowing normal function of the knee. Surgery may include any or all of the following procedures:

  • Making the patellar groove deeper
  • Correcting the inner pull of the patellar tendon by cutting the bone at the tendon’s insertion at the tibia and moving the bone and tendon laterally 2-4 mm. The bone and tendon are reattached with a pin.
  • The outer (lateral) portion of the knee called the joint capsule is incised and tightened to aid in pulling the patella into the groove.
  • A permanent stabilizing suture is placed from the outside of the knee to around the patella to help prevent displacement.
  • An incision on the inside of the knee is made to relieve contracture and scarring of the inner joint capsule and muscles. This procedure is called a medial release and it is commonly required for grade 4 patella luxations.

Post-operative Care: Patients are not bandaged after surgery. Typically, the patient will begin to put weight on the surgically treated leg in 4-6 weeks. Physical therapy that includes flexing and extending the leg is started 3 weeks after surgery. All patients are restricted to walks on a leash and no jumping for 8 weeks.

Long Term Prognosis: 90% of patients who undergo surgery do not develop arthritis at a later date from the surgery. Patients have normal function in their leg within 4 months after surgery. A small percentage of patients with grade 3 or grade 4 patella luxations will have a relapse of the luxations because of the leg deformities that make it very difficult to prevent the patella from moving to the inside of the leg.