Hypothesis ThreeÑARTHRITIS ARTHRITIS associated with the stifle joint. Canine rheumatoid arthritis- Manifests initially as a shifting leg lameness, with soft tissue swelling around the involved joints in following weeks or months. Pathophysiology- Normally localized to a few joints as seen here in stifle and tail vertebrae. After localization into individual joints characteristic radiographic changes develop. Earliest radiographic changes show soft tissue swelling and loss of trabecular bone density in area of joint. Lucent, cyst-like areas are frequently seen in the subchondral bone. Prominent lesion is progressive erosion of cartilage and subchondral bone in the area of the synovial attachment which results in loss of articular cartilage and collapse of joint space. Angular deformities often occur and luxation of joint is frequently a sequelae. Deformities are most common in carpal, tarsal and phalangeal joints and less frequent in elbow and stifle. Synovial fluid changes indicate a sterile inflammatory synovitis with increase in total cell count and a high proportion of neutrophils in the synovial cell population. Condition is believed to be due to deposition of immune complexes in the synovium. Antigens stimulate Ig-M and this results in the deposition of immune complexes in the joint. Complement attaches to these complexes which attract neutrophils which cause tissue damage resulting in inflammation. Osteoarthritis is a disease that is very common as a secondary lesion, but it my also be a primary lesion when there is an inherited abnormality of cartilage components. Osteoarthrits is usually subsequent to trauma. It is initiated by damage to the chondrocytes. The damage increases the production of metalloproteases and the release of inflammatory mediators. This causes the breakdown of proteoglyacans and collagen. Free collagen fragments are released into the joint capsule, resulting in thickening of the capsule. As the cartilage is broken down, subchondral bone experiences more stress and subsequently thickens. Once the subchondral bone is affected, the condition is self perpetuating and irreversible. Inflammation usually results in joint effusion due to the increased permeability of synovial vasculature. Systemic Lupus associated arthritis-this is seen with a polyarthritis and dermatitis, and this animal is being treated with Benadryl for skin problems. The clinical signs are episodic, as seen here. There is usually depression, anorexia and fever. It is an immune mediated disease caused by a loss of control over B cell production and activity. The B cells produce anti-nuclear antibodies which form immune complexes that deposit in the synovia to cause arthritis. This disease is usually seen in middle-aged dogs (this patient is 3 years old) and the most common clinical sign is symmetrical nonerosive polyarthritis. To make a diagnosis of SLE, the dog must have 2 clinical signs (this dog has skin lesions and bilateral joint pain, possibly indicative of polyarthritis) and a positive ANA test. Note: Arthritis may also occur secondary to any inflammatory or infectious disease. Septic Arthritis- Frequently associated with bacterial agents such as Staph. Strep. or coliforms. Causes include hematogenous spread or surgery (iatrogenic causes), failure of passive transfer, spread from adjacent osteomyelitis, RMSF, Ehrlichiosis and spirochetes (borreliosis). Clinical signs include: Lameness, swelling, pain in affected joints and systemic signs of fever, malaise, anorexia and stiffness. Radiography may reveal joint effusion in early cases and DJD in chronic conditions. Athrocentesis will reveal WBCÕs, especially neutrophils. Synovial fluid may be grossly purulent