Hypothesis 1ÑTrauma to the stifle joint

a. Degenerative changes to the joint

Dog fell off the bed a year ago followed by subsequent right hind 
limb lameness.  It is possible that the patellar ligaments were 
ruptured as this would explain why the patella is 
able to be luxated medially.  Abnormal joint movement can result in 
erosion of articular surfaces and bone remodeling leading to 
osteoarthritic changes.  This would manifest as lameness. 

b. re-injury during healing process

Initial injury to right hind limb due to falling off the bed.  Dog 
was improving but dog plays with other household dog.  Medial 
buttressing is a thickening of the joint capsule from chronic 
instability this may indicate that the original injury never 
finished healing completely.