Comparison of the pathological features of rheumatoid arthritis and osteoarthritis. Rheumatoid arthritis is characterized by an inflammatory process mediated by innate, adaptive, and stromal autoimmune responses. The pannus, which is formed by activated synovial cells (“mesenchymal transformation”) invades and degrades the cartilage, while increased osteoclast activation results in bone erosion. In contrast, osteoarthritis is primarily a degenerative disease involving age-related changes in articular cartilage and bone structure and function that lead to subchondral bone sclerosis, ectopic bone (osteophyte) formation and cartilage destruction. Local synovial inflammation is a secondary feature seen in some cases of osteoarthritis
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