Explanation of Facts: Small breed dog specific problems Legg Perthes Disease, avascular necrosis of femoral head, clinical presentation of patient does not support this disease process Fell off bed a year ago Traumatic incident potentially compromised patellar ligaments and tendons of both the right and left rear legs. Weight bearing lameness on right leg Trauma to right leg at time of fall induced the lameness. Perhaps the patellar ligament, lateral collateral ligament and the cranial cruciate ligament were over- extended in fall and the joint capsule was stressed resulting in a mild inflammatory response with minor joint swelling and pain. Subluxation of the joint occurred as the ligaments stretched and this reduced the functional articular surface area. A smaller area of the joint experienced the entire weight of the dog as it landed on the floor therefore there was some cartilage damage. Chondrocytes were damaged and responded by releasing inflammatory mediators that led to mild joint effusion and joint pain (as evidenced by mild lameness). Resolving of right limb lameness after one month due to reabsorption of joint fluid and termination of the mild inflammatory response by chondrocytes. Mild right rear lameness reoccurred 6 months after fall Perhaps the dog retraumatized right stifle through aggressive play or through another fall. Patellar ligament, lateral collateral ligament and the cranial cruciate ligament were once again over extended and the joint capsule was stressed. However this time the joint subluxation was more severe due to previous laxity in the patellar, lateral collateral and the cranial cruciate ligaments. Due to the severe subluxation that occurred there was significant cartilage damage and chondocytes responded with a strong inflammatory response. Treated with anti-inflammatories after lameness reoccurred and saw no improvement Fact that previous vet put dog on anti-inflammatories could indicate that they sensed significant stifle joint inflammation. Lameness has not progressed in last six months Since the lameness has not progressed rapidly in the past six months indicates that it is not likely to be caused by a malignant neoplasia, such as a synovial sarcoma. The fact that lameness has not progressed in the past six months may indicate that the stifle joint has responded by thickening its joint capsule to reduce the risk of further joint subluxation. A thickening of the joint capsule could significantly reduce the strain on the patellar, lateral collateral and cranial cruciate ligaments. A more stabilized joint would be better able to normalize articular joint pressure and minimize chondrocyte damage. She has occasional "allergies" which cause seasonal sneezing Persistent hypersensitivity response to antigens in lungs causes a massive IgE response and the release of inflammatory mediators by mast cells, basophils, eosinophils and neutrophils. Mast cells and basophils are bound to IgE. IgE, mast cells, basophils, eosinophils and neutrophils are present in systemic circulation and could potentially deposit in the joint space. Perhaps IgE that deposits in the joint space is cross reactive with a self antigen and this cross-reactivity results in immune complexes depositing in the joint space. IgE would stimulate the degranulation of bound mast cells and basophils as it formed immune complexes. This would result in chemotaxis of eosinophils and neutrophils. Major basic protein (from eosinophils) and many other inflammatory mediators would initiate necrosis of chondrocytes and damage to cartilage. Dental tartar Potentially a systemic bacterial infection caused by a poor dental hygiene may have localized in the right stifle joint. Bacteria may have released endotoxins that elicited a sepsis in the joint space. Why wouldn't sepsis have occurred in all the joints? Perhaps the right stifle joint capsule was compromised from the previously mentioned trauma and therefore it was more susceptible to joint sepsis? Grade 3 out 4 intermittent none weight bearing lameness on right hind leg Indicates that there is significant joint pain and reluctance to use joint in a normal fashion. Medial buttressing of right stifle Stifle joint is able to move medially because of compromise of the lateral collateral ligament. The lateral collateral ligament was stretched during the subluxation described above hence it permits medially movement it would normally prevent. According to Dr. Martin medial buttressing is also a common response in small dogs to cranial cruciate rupture. In an attempt to improve joint stability the body responds by thickening the joint capsule. In many small breed dogs medial buttressing provides enough support to provide adequate stifle joint stabilization for the rest of the dogs life without surgical intervention. Possible when the dog initially fell it tore its cranial cruciate ligament and that could be supported by the fact that there is medial buttressing. Cranial draw negative on right stifle Cranial draw is highly likely to present for the reasons discussed above. Cranial draw can be hard to detect in a non-anesthetized patient due to quadriceps contracting and reducing joint laxity. Perhaps anesthetizing this dog and then performing the cranial draw test a different result may be concluded. The patella was easily luxated to the medial side of the trochlear groove when the leg is in extension, when flexed patella returned to trochlear groove Through the traumatic incident described and above and the other speculated incident the patellar ligament was over extended and permanently stretched. The stretched patellar ligament is not effective at holding the patella in place. However the patellar ligament is definitely still in tact because when the joint is flexed the patella returns to the trochlear groove. If the patellar ligament was ruptured there would be no forces to pull the patellar back into it appropriate anatomic position. No abnormalities palpated on right hip joints, long bones, muscles or other joints in legs Hip joints appear to not be painful ruling out the presence of Legg Perthes disease. Long bones feel smooth reducing the likelihood of neoplasia. Muscles do not appear atrophied so it is not likely that there is nerve damage or such severe joint pain that there is muscle disuse. Pain response on left leg with cranial drawer when patellar luxation attempted. Probable damage to left stifle during traumatic incidents described above. Likely to discover cranial draw signs and medial buttressing under anesthesia. Since the dog never showed lameness in the left leg throughout the entire traumatic incident could the left stifle problem be caused by a totally different etiology such as the immune mediated response described above? Perhaps the severe lameness in the right stifle over the past 6 months combined with the fact that the dog is 4 pounds overweight led to increased weight bearing stress on the left rear leg and compromise of the left stifle joint. No abnormalities palpated on left hip joints, long bones, muscles or other joints in legs Hip joints appear to not be painful ruling out the presence of Legg Perthes disease. Long bones feel smooth reducing the likelihood of neoplasia. Muscles do not appear atrophied so it is not likely that there is nerve damage or such severe joint pain that there is muscle disuse. A pain response was also elicited on dorsal flexion of the tail. No pain response on direct pressure to the spine and lumbo-sacral joint. Probable direct trauma to tail in the past since there is no palpable pain response in the spine and lumbo-sacral joint. Maybe the dog got it tail caught in a door? Perhaps there was a subluxation of spinal vertebrae from the traumatic incident the spinal cord recovered and but the tail didn't. Perhaps the etiology of this pain in the tail stems from an immune-mediated response described above? The dog is overweight for its breed This dog should only weigh 7 lbs but it weighs 11 lbs. That additional 4 pounds is certainly putting a lot stress on its joints. Could the excessive amount of weight this dog is carrying be leading to degenerative joint changes in the stifle joints or perhaps just the left stifle joint since it is now bearing most of the weight?