Some musculoskeletal disorders affect primarily the joints, causing arthritis. Others affect primarily the bones (eg, fractures , Paget disease of bone , tumors ), muscles (eg, myositis ), peripheral nerves (eg, ulnar nerve entrapment ), or other periarticular soft tissues (eg, bursitis , tendinitis , sprains , ligament injuries). 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 ( monoarthritis ) or multiple joints ( polyarthritis ) in a symmetric or asymmetric manner, with or without involving the spine.