Hypothesis one:  
Inflammation secondary to trauma
Inflammation of joint can occur secondary to trauma, which occurred 6 months ago after 
jumping off the bad.  Signs of inflammation would include joint pain, limitation of 
movement, crepitus, occasional effusion, variable degrees of local inflammation 
including heat, swelling, pain.
The signs presented in this case that support inflammation secondary to trauma include 
3/4  non wt. Bearing lameness. Pain on palpation when cranial draw and patella luxation 
on the left leg.  The trauma presented 6 months ago, and resolved in a few weeks.  The 
dog was treated with NSAIDS at first which had no effect.  Signs have recently appeared 
again.