The biomechanics are easily described by what we call the bowstring effect , in which the muscle fiber group on one side works better than the other side, and puts tension on the patellar tendon and associated bone structures. This wears away the ridge of that trochlea so the effect is to slip out of the groove more easily as time goes on. Meanwhile, the underside of the patella also abrades, becoming less V-shaped and less likely to stay in the track where it belongs. In the more severe cases, the abnormal strain on the muscles and joint caused by forces being misdirected in this manner results in pain. It is usually considerable and almost always leads to a very noticeable change in gait, such as skipping or perhaps even a refusal to walk at times.
Mild cases can go undiagnosed by a vet and, to some extent, unnoticed by the owner.
Most pups can be diagnosed only after 4 to 6 months of age. If the luxated patellar bone is offset toward the centerline of the dog, the condition is called medial luxation or displacement. If the bone moves on the outside , or away from the centerline, it is called lateral luxation. A method you can use to keep these terms straight might be to think of the median strip in the middle of a divided highway. Medial patellar luxation is almost undoubtedly inherited.