Arthritis means joint inflammation. In a normal joint, where two bones meet, the ends are coated with cartilage, a smooth, slippery cushion that protects the bone and reduces friction during movement. A tough capsule lined with synovial membrane seals the joint and produces a lubricating fluid. Ligaments surround and support each joint, connecting the bones and preventing excessive movement.
Muscles attach to bone by tendons on each side of a joint.
Inflammation can affect any of these tissues. Inflammation is a complex process that causes swelling, redness, warmth, and pain. It's the body's natural response to injury and plays an important role in healing and fighting infection. Joint injury can be caused by trauma or by the wear and tear of aging. But in many forms of arthritis, injury is caused by the uncontrolled inflammation of autoimmune disease, in which the immune system attacks the body's own tissues.