9 year old West Highland White Terrier

Discussion

Erosive polyarthritis, or rheumatoid arthritis, causes inflammation of the synovium in an autoimmune pathway. The toxic byproducts of this inflammatory tissue attack the cartilage and subchondral bone in the joint. This excessive granulomatous tissue is called pannus and has the capacity to destroy articular cartilage, subchondral bone, and tendons of the affected joint. Proteases, enzymes, and other substances are produced by the invading tissue and are directly responsible for the destruction of the joint. In some cases, the pannus will grow beneath the articular cartilage, resulting in subchondral bone destruction. Articular cartilage loss begins at the joint margins. Radiolucent areas are frequently seen at the juxta-articular attachment of ligaments as well. As the condition progresses, loss of articular cartilage and subchondral bone continues, frequently resulting in a collapse or narrowing of the joint space.
Radiographic signs could include:

• periarticular soft tissue swelling, multiple joints, bilaterally symmetrical (most commonly carpus and tarsus)
• joint capsule effusion
• joint space widening and later joint space narrowing
• loss of subchondral and juxta-articular bone
• subluxation, luxation, ankylosis of the affected joint
• soft tissue atrophy, periarticular soft tissue mineralization
• changes characteristic of degenerative joint disease