Diagnosis

Bone injury to distal femur which healed crooked and led to weakened lateral 
support for 
patella and eventual luxation.
How:  The distortion of the femur from the former injury stretches the medial 
patellar 
ligament, keeping the patella in place when the joint is in flexion.  When the 
joint is in 
extension, there is laxity in the ligament enabling the patella to be luxated 
medially.  If the patella continues to luxate on a regular basis, then 
degenerative joint disease will eventually become apparent.