Some musculoskeletal disorders affect primarily the joints, causing arthritis. Others affect primarily the bones (eg, fractures, Paget disease of bone, tumors), muscles or other extra-articular soft tissues (eg, polymyalgia rheumatica, myositis), or periarticular soft tissues (eg, bursitis, tendinitis, sprain). Arthritis has myriad possible causes, including infection, autoimmune disorders, crystal-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.
