Patellar Luxation in Dogs
1]. The patella is ovate: the proximal blunt surface is the base and may extend beyond the articular surface, whereas the distal pointed end, the apex, does not (2]. Muscular forces of the vastus medialis and vastus lateralis control medial and lateral movement and stability of the patella. The patella is an essential component of the extensor mechanism, serving to alter the direction of the pull of the quadriceps mechanism, preserving even tension of the extensor mechanism during stifle extension, and acting as a lever arm, increasing the mechanical advantage of the quadriceps muscle group. The patella also protects the tendon of the quadriceps muscle group during movement, provides a greater surface area for the tendon to engage the trochlea of the femur, and provides cranial and rotary stability to the stifle joint in the extensor mechanism. The location and prominence of the tibial tuberosity are important for the mechanical advantage of the extensor mechanism.
Medial Patellar Luxation
Medial patellar luxation is more common in all sizes and breeds of dogs than is lateral patellar luxation (LPL). In a study of 124 dogs that were referred for patellar luxation, the majority of dogs had a congenital form (82%), as opposed to acquired patellar luxation (15%), and the majority (89%) had MPL, as opposed to lateral luxation . MPL accounted for 98% in small breeds (<9.1 kg), 81% in medium breeds (9.1-18.2 kg), 83% in large breeds (18.2-36.4 kg), and 67% in giant breeds (>36.4 kg). In one study of 70 referred large breed dogs, MPL accounted for 97% and LPL accounted for 2.8% . That study also suggested that in large breed dogs MPL occurs more frequently in males (male:female sex ratio of 1.8:1). In contrast, other studies have shown that in small breed dogs females are more frequently affected (male:female sex ratio of 1:1.5) . Although females are more likely to be affected than males, prevalence is similar among spayed females, neutered males, and intact females, with intact males being at a lower risk. Bilateral luxations are significantly more common (65%) than unilateral luxations (35%) [3,8].
Three groups of patients were identified based on clinical pattern by Brinker, Piermattei, and Flo: 1) neonates and older puppies with abnormal hind-leg carriage and function from the time of ambulation; 2) young to mature dogs with intermittent and/or progressively abnormal gait; and 3) older dogs with an acute onset of lameness associated with degenerative changes and cranial cruciate ligament rupture .