The dog fell of the bed suggesting trauma and limped for about 1 month before showing 
improvement. The injury, therefore could have been unresolved ( partial tear, the joint 
used natural mechanisms to stabilize the joint, such as fibrosis of the joint capsule, 
evidenced by the medial buttressing).  Nothing was done to evaluate the progess of the 
injury until months later. There are numerous ways thedog could have reinjured the joint 
area. There is another animal in the house ( we do not know the size or activity  level of 
the other dog). If the initial injury was not completely stabilized, rough housing could 
have easily reinjured any area of the joint.  Often times when there is already a congenital 
predisposition to patellar luxations,( decrease trochlear groove, weak muscles, ligements. 
Etc) there is an initial injury, usually to the cranial cruciate ligament. Months later there 
can be subsequent injury to the meniscus due to the joint instability from the previous 
injury. This results in the second series of clinical lameness.