Rheumatoid arthritis is a chronic inflammatory disease that causes redness, pain, swelling or a hot (or warm) feeling in the lining of a joint. This inflammation slowly destroys cartilage, bone, tendons and ligaments. Rheumatoid arthritis can also affect other parts of the body, such as the eyes, lungs or heart. Researchers believe that the inflammation is triggered by the body's immune system, which fights infection and heals injuries by producing masses of white blood cells at the injured site. Normally, the inflammation is controlled and the process is self-limiting, but in people with chronic rheumatoid arthritis, the process keeps going. The damage becomes worse because the immune system's attack does not stop and eventually causes destruction of cartilage, bone, tendons and ligaments.
Osteoarthritis, the most common form of arthritis, is caused by the breakdown of cartilage. Cartilage is a hard but slippery elastic material that covers and protects the ends of bones. Osteoarthritis is often referred to as "wear and tear" of the joints as cartilage begins to crack and break. Small fragments of the cartilage can get into the joint space and may irritate soft tissue and muscles. Sometimes the pain associated with osteoarthritis is a result of cartilage damage. This damage causes the muscles and tissues involved in joint movements to function in an abnormal way. Eventually, cartilage erosion may leave bones rubbing against bones. Osteoarthritis generally occurs after age 45, but it has been known to occur at any age.