Hypothesis 2

In small breed dogs, medial luxation of the patella is a common cause of lameness.  Initially, the lameness is due to lack of stability in the stifle joint.  The luxation is thought to be due to misalignment of the extensor mechanism of the stifle joint (problems with one or more of the following: quadriceps muscle group, patella, a shallow trochlear groove, straight patellar ligament, or rotational instability of the stifle joint).  During the dog's development, these deformities cause abnormal forces on the growth plates.  The forces can cause changes in the cartilage columns in the developing bone, leading to abnormally shaped long bones.  Examples of these secondary effects include lateral torsion of the distal femur, lateral bowing of the distal 1/3 of the femur, medial displacement of the tibial tuberosity, medial bowing of the proximal tibia, and lateral torsion of the distal tibia.  The degree of any of these deformities is variable, based on the age of the patient (activity of the growth plate) and permanancy of the luxation.   Any of these secondary deformities could increase the misalignment, exacerbating the problem.

Misalignment of the joint for any (or multiple) of the above reasons would lead to continued abnormal forces on the joint - rubbing of bone against cartilage with resulting cartilage degeneration and erosion  (and, ultimately,  eburnation), irritation of the joint capsule, and potentially bone remodeling.  Uncorrected medial luxation of the patella can result in degenerative joint disease.  This progression could explain why the lameness hasn't resolved, as we might expect if the lameness were due to trauma.