Hypothesis ThreeÑARTHRITIS ARTHRITIS associated with the stifle joint. Canine rheumatoid arthritis seen in small and toy breeds, intermittent lameness, localized to a few joints, as seen here in stifle and tail vertebrae. Instability develops over a few weeks to months. Pathophysiology-immune mediated progressive poly-arthritis, an unknown antigen stimulates Ig-M and results in the deposition of immune complexes in the joint. Complement attach 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. ry 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 anti-nuclear antibodies. Note: Arthritis may also occur secondary to any inflammatory or infectious disease.