Medial Patellar Luxation

The Quadriceps/Patellar Mechanism
The patella is an ossification in the tendon of insertion of the quadriceps muscle group. The patella articulates with the concave femoral trochlea, bounded medially and laterally by ridges. The patellofemoral articulation functions as a simple pulley to alter the direction of the quadriceps muscle pull to accomplish stifle extension. The quadriceps group converges on the patella and continues as the patellar ligament to insert on the tibial tuberosity. Proper anatomic alignment of the quadriceps/ patellar mechanism with the underlying skeleton fosters proper patellar tracking. The patella is surrounded by attachments of joint capsule, retinaculum and medial/ lateral femoropatellar ligaments whose balanced tension helps maintain normal patellar position.

Pathophysiology of Medial Patellar Luxation (MPL)
MPL is the result of structural abnormality and progressive degradation in the bony and soft tissues of the pelvic limb. In skeletally immature animals with MPL, the displaced quadriceps group creates a restrictive “bowstring effect” causing abnormal angular and torsional femoral growth; the femoral trochlea does not develop normally because of the absence of normal patello-femoral compression. Musculoskeletal abnormalities associated with MPL include: medialization of the tibial crest, shallow/short femoral trochlea, medial displacement of the quadriceps muscle group, hypoplastic or eroded medial trochlear ridge, rotational instability of the stifle, contracture/ scarring of the medial joint capsule and muscular tissues, and tearing/stretching of lateral joint capsule and retinacular tissues. MPL is classified according to severity on a 4-point scale (Table 1). [...]