The human musculoskeletal system consists of the human skeleton, made by bones attached to other bones with joints, and skeletal muscle attached to the skeleton by tendons.
Some musculoskeletal disorders affect primarily the joints, causing arthritis. Others affect primarily the bones (eg, fractures, Paget's disease, tumors), muscles or other extra-articular soft tissues (eg, polymyalgia rheumatica, fibromyalgia), or periarticular soft tissues (eg, bursitis, tendinitis, sprain). Arthritis has myriad possible causes, including infection, autoimmune disease, crystal-induced inflammation, other kinds of inflammation, and noninflammatory tissue degeneration; systemic diseases may be involved. Arthritis may affect single or multiple joints, in either a symmetric or asymmetric manner.
Pain is the most common symptom of many joint disorders. Usually, pain originating from superficial structures is better localized than pain originating from deeper structures. Pain originating in small distal joints tends to be better localized than pain originating in large proximal joints. Joint pain can be referred from extra-articular structures or from other joints. Arthritis often produces “aching” pain, whereas neuropathies often produce “burning” pain.
“Stiffness” may mean weakness, fatigue, or fixed limitation of motion to patients. Stiffness of importance to rheumatic disease is difficulty in motion that develops when attempting to move a joint after a period of rest. The duration of stiffness after beginning joint motion reflects its severity. Stiffness is more severe in inflammatory joint disorders. Morning stiffness can be an important early symptom of RA.
Fatigue is a desire to rest that reflects exhaustion. It differs from weakness, inability to move, and reluctance to move due to pain with movement.
Instability, or buckling of a joint, may suggest weakness of the ligaments or other structures that stabilize the joint, which are assessed by stress testing. Buckling occurs most often in the knee and most often results from an internal joint derangement.
For musculoskeletal imaging, plain x‑rays may be obtained first, but these are often less accurate than CT or MRI. MRI is the most accurate study for fractures not visible on plain x‑rays, particularly in the hip and pelvis, and for soft tissues and internal derangements of the knee.