Arthritis is joint inflammation. Most causes for arthritis are known and most are very treatable. The most common form of arthritis, osteoarthritis, is a chronic, degenerative joint disease that affects mostly middle-aged and older adults. It is characterized by the breakdown of joint cartilage. According to the Centers for Disease Control and Prevention, an estimated 50 million U.S adults (about 1 in 5) report doctor-diagnosed arthritis. Arthritis, which literally means inflammation of a joint, refers to more than 100 different diseases that cause pain, stiffness, and swelling in joints or other supportive body structures, such as muscles, tendons, ligaments, and bones. In addition to osteoarthritis, other prevalent forms of arthritis are fibromyalgia, a chronic, widespread pain in muscles and soft tissues surrounding the joints throughout the body, and rheumatoid arthritis, an inflammatory disease that involves the lining of the joint. Joints are the areas where two bones meet. Most joints are mobile, allowing the bones to move. Although osteoarthritis can occur in any joint, it usually affects the hands, knees, hips, or spine. Cartilage is a durable, resilient connective tissue that covers the surface of a bone at a joint. It helps reduce the friction of movement within a joint. In osteoarthritis, the cartilage in the joint wears down. As a result, the bone ends may thicken, forming bony growths or spurs that interfere with joint movement. In addition, small fragments of bone and cartilage may float within the joint space, and fluid-filled cysts may form in the bone, limiting joint movement. Osteoarthritis occurs in most people as they age, but it also may occur in young people as a result of injury or overuse. It can be classified as primary or secondary. Primary osteoarthritis has an unknown cause, while secondary osteoarthritis is caused by another disease, infection, injury, or deformity. Several risk factors are associated with osteoarthritis, including the following: Heredity: Slight joint defects or genetic defects may contribute to the development of osteoarthritis. Obesity: Excessive weight can put undue stress on such joints as the knees. Injury/overuse: Significant injury to a joint, such as the knee, can later result in osteoarthritis. Injury may also result from repeated overuse or misuse. In addition to a complete medical history and physical exam, the most common way a doctor diagnoses osteoarthritis in a joint is by examining X-ray films of the affected joints. The goals of treatment are to reduce joint pain and stiffness and to improve joint movement. Treatment may include aerobic and strengthening exercises, heat treatment, physical and occupational therapy, weight maintenance, medication, or joint surgery.