Hypothesis 1
Torn Ligament: could be cruciate, collateral or femoropatellar ligament
A torn ligament in the joint would be immediately painful with inflammatory signs. Collagen fibers would be ruptured with hematoma formation.  This would result in acute pain and swelling followed by fibrin deposition at the site of rupture.  This would provide some restoration of joint stability, therefore, decreasing pain at this point in time. The joint would not be as stable as it was originally.  The loss of stability will lead to cartilage destruction, subchondral bone sclerosis, osteophyte production and joint effusion.  This may be a chronic sprain which would cause soft tissue changes lameness, but no inflammation.  The pain would be caused by pressure on the joint capsule from the abnormal location of the patella.  In addition the ruptured ligament would cause laxity in the joint leading to osteoarthritis.