Some musculoskeletal disorders affect primarily the joints, causing arthritis. Others affect primarily the bones (eg, fracturesPaget disease of bonetumors), muscles or other extra-articular soft tissues (eg, polymyalgia rheumaticamyositis), or periarticular soft tissues (eg, bursitistendinitissprain). Arthritis has myriad possible causes, including infection, autoimmune disorderscrystal-induced inflammation, and minimally inflammatory cartilage and bone disorders (eg, osteoarthritis). Arthritis may affect single joints (monarthritis) or multiple joints (polyarthritis) in a symmetric or asymmetric manner, with or without involving the spine.

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