Arthritis is a joint problem that can reduce mobility and cause pain. Arthritis can be caused by injury, infection, the body’s own immune system, or developmental problems. The most common form of arthritis is called osteoarthritis (osteo = bone; arthr = joint; itis = inflammation) or degenerative joint disease. Normally, joints form smooth connections between bones. Osteoarthritis involves thinning of joint cartilage (a protective cushioning between bones), buildup of fluid within the joint, and the formation of bony growths within the joint. Over time, this can lead to reduced joint mobility as well as pain.