Dog lame in right hind leg
Small breed dog wht=11.6lb : small breed dogs are predisposed to certain conditions
Fell off bed a year ago : possible traumatic event leading to injury
Limp with wht bearing lameness (rt hind) : eliminates severe trauma leading to non-wht bearing lameness
Clinically normal w/in month : original insult repaired
Mild lameness recurred 6 mo later : probable secondary changes in jt
Tx with antiinflammatory injection-no response : indicates severe pain or non inflammatory process
Lameness hasn't progressed significantly
Indoor dog
Grade III/IV lameness on rt hind : intermittent non wht bearing lameness
Medial buttressing on rt stifle : medial proliferative compensation for stifle stability
Cranial drawer negative-pain noted : wasn't anesthetized so doesn't rule out cruciate rupture
Patella easy luxated medially-pain noted : indicates laxity in stifle
Pain response with lt hind limb palpatation : possibly bilateral problem

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. 

Hypothesis 2
Fracture involving stifle joint
A fracture resulting in displacement of bone would cause severe inflammation and pain initially.  Initially there would be hematoma followed by callous formation.  This would be followed by formation of new bone and remodeling. Since the fracture was not set/immobilized the bone may not be restored to original form and function.  This will also cause joint instability and may progress to osteoarthritis.  The deformed joint may cause the patella to luxate with minimal force.

Hypothesis 3 
Congenital malformation of trochlear ridge
A congenital malformation of the trochlear ridge can be caused by failure to express normal antiversion of the femur, dysplasia of the quadriceps muscle, or trauma disrupting lateral femoropatella restraints.  Increased force on the medial aspect of the femur slows growth of the medial condyle and accelerates growth of the lateral condyle.  This results in medial displacement of the tibial turbercle and displacement of the patella medially.  The displacement of the patella increases overtime due to soft tissue stretching and increased skeletal deformity. This can be a unilateral or bilateral malformation and can result in varying degrees of luxation and lameness.