Middle-aged or older dogs with a more constant hind limb lameness. Careful questioning of the owner is essential with these patients to determine the course of the lameness. If the luxating patella is responsible for the lameness, there will be a long-term history of intermittent lameness, including the characteristic “skipping gait.” The owner may comment that the dog has had a problem with the leg from time to time for as long as they can remember, but the symptoms seem to be getting worse. This is often due to eburnation of cartilage on the medial trochlear ridge and the underside of the patella (
1,
2,
4). Osteoarthritic changes in the joint may also contribute to lameness in these patients, but this may be of somewhat lesser importance, since the degenerative changes seem to be slower in developing, are of a more minor nature, and are less clinically debilitating than the changes seen in rupture of the cranial cruciate ligament, for example (
3,
7). If questioning of the owner reveals an acute lameness or an acute worsening of an intermittent lameness, the patellar luxation is frequently a “red herring.” More often, rupture of the cranial cruciate ligament or other causes of acute lameness will be present as a more significant clinical entity (
1,
2,
5,
8).
Medial patellar luxation and rupture of the cranial cruciate ligament are intertwined in several ways. It is estimated that at least 15% to 20% of dogs with patellar luxation will eventually rupture their cranial cruciate ligament (
1). This may be due to a combination of 3 factors: First, dogs with significant patellar luxation usually have internal rotation of the tibia, which puts stress on the cranial cruciate ligament. Second, the quadriceps musculature-patella-patellar tendon mechanism normally provides cranial stability to the stifle joint. In the dog with patellar luxation, a good portion of this mechanism is deviated medially, thus offering less resistance to forces that would tend to subluxate the proximal tibia cranially. Third, cartilage erosion and degenerative joint disease may create an environment in the stifle that promotes degeneration of the cranial cruciate ligament (
1,
5).