Patella luxation is a common orthopedic condition in dogs. It may cause varying degrees of lameness, pain, and progression of osteoarthritis. It is most commonly diagnosed in small breed dogs, however large breed dogs are also affected. Both large and small breed dogs more commonly exhibit medial luxation (MPL) than lateral. Lateral patella luxation (LPL) is rare in small breed dogs, but is seen more frequently in large breed dogs.

Etiology and Pathophysiology

Patella luxation in dogs is rarely the result of trauma. Most commonly it is due to congenital conformational abnormalities, resulting in malalignment of the stifle extensor mechanism. The extensor mechanism consists of the quadriceps muscle group, the patella, and the patellar ligament. The muscles of the quadriceps originate from the proximal femur and the caudal ilium, just cranial to the acetabulum. Extension of the stifle begins with quadriceps contraction. This force is transmitted to the patella, which articulates with the distal femur, and transmits the force to the patellar ligament. The patellar ligament inserts on the tibial tuberosity. During quadriceps contranction, the extensor mechanism must be aligned with the trochlear groove of the distal femur, or patella luxation results.

In addition to extensor mechanism malalignment, dogs with patella luxation may exhibit a shallow trochlear groove, erosion of the medial trochlear ridge, varying degrees of degenerative joint disease and periarticular fibrous tissue proliferation. Most of these changes are probably secondary to extensor malalignment.

Presentation and Physical Exam

Patella luxation is most commonly diagnosed in young to middle aged dogs. They may present for lameness evaluation or luxation may be an incidental finding on physical examination. Physical examination is best initiated with the patient standing. Both left and right stifles can be palpated simultaneously with the dog in a more relaxed position than in lateral recumbency. The patella can be palpated between the thumb and index finger, and its location relative to the femoral condyles and trochlear groove evaluated. If the patella cannot be palpated immediately, it can be located by following the patellar ligament proximally from its distal attachment on the tibial tuberosity.

Patella Luxation is Graded on a scale of I-IV

Grade I: The patella can be luxated with manual pressure, but immediately reduces when pressure is released.

Grade II: The patella can be luxated with manual pressure, and spontaneously luxates during ambulation. It easily reduces by extending the stifle or by manual pressure, and resides in the trochlear groove a majority of the time.

Grade III: The patella resides outside of the trochlear groove a majority of the time. It can be reduced by manual pressure.

Grade IV: The patella resides outside of the trochlear groove continually, and cannot be reduced.

If the patella is found to be luxated, attempt to reduce it by extending the stifle and moving the patella medially or laterally with the thumb or index finger. If the patella is found to be reduced, its stability is best evaluated by extending the stifle and attempting to force the patella medially or laterally. With the patella in its most mediolaterally displaced position, the stifle can now be flexed. The normal patella will track back into the trochlear groove, while a luxating patella will remain deviated. In addition, internally or externally rotating the tibia can aid in luxating or reducing the patella. For example, internal rotation of the tibia facilitates medial patella luxation, while external rotation facilitates reduction of a medial luxation.

The degree of patella luxation is associated with differing degrees of lameness. Dogs with a grade I luxation most often are asymptomatic. Dogs with grade II luxation may be asymptomatic, or may present with a complaint of an intermittent "skipping" lameness. Owners often report that these episodes resolve after the dog "stretches his leg out behind him". Dogs with grade III and IV luxation more typically present with chronic lameness. It may be worse after exercise or inactivity, but unlike grade II dogs, the lameness does not resolve. These dogs often appear bowlegged and may seem to walk in a crouched position, due to their inability to extend the stifle completely.