An explanation of what happens with this most common form of arthritis, and how it affects different parts of the body.
Osteoarthritis, also known as degenerative joint disease, is the most common arthritic disease. In addition to man, nearly all vertebrates suffer from osteoarthritis, including porpoises and whales as well as long-extinct terrestrial travelers such as dinosaurs. Osteoarthritis occurs in the joints of the body, when cartilage is damaged and lost and bones undergo abnormal changes.

Joints are designed to provide flexibility, support, stability, and protection. These functions, essential for normal and painless movement, are primarily supplied by cartilage, a slippery tissue that coats the ends of the bones, and the synovium, a membrane that surrounds the entire joint. The synovium is filled with lubricating liquid--the synovial fluid. This fluid also supplies nutrients and oxygen to cartilage, one of the few tissues that does not have its own blood supply. The cartilage itself contains a high percentage of water--85% in young people to about 70% in older individuals. This high content is made possible by water-binding qualities of large molecules called proteoglycans--one of the primary building blocks of cartilage. The other major component of cartilage is collagen, which forms a mesh to give support and flexibility to the joint. (Collagen is the main protein found in all the connective tissues of the body, which also include the muscles, ligaments, and tendons.) The combination of the collagen meshwork and the high water content tightly bound by proteoglycans creates a resilient and slippery pad in the joint, which resists the compression between bones during muscle movement.

When cartilage in a joint deteriorates, osteoarthritis develops. In the early stages of the disease, the surface of the cartilage becomes swollen, and there is a loss of proteoglycans and other tissue components. Fissures and pits appear in the cartilage. In some sufferers, inflammation occurs around the synovium. As the disease progresses and more tissue is lost, the cartilage loses elasticity and becomes increasingly prone to damage from repetitive use and injury. Eventually, large amounts of cartilage are destroyed, leaving the ends of the bone within the joint unprotected.

Other problems occur as the body tries to repair this damage. Clusters of damaged cells or fluid-filled cysts may form around the bony areas or near the fissures. Bone cells may respond to damage by multiplying and growing, forming dense, misshapen plates around the exposed areas. At the margins of the joint, the bone may produce outcroppings on which new cartilage grows.

Unlike some other types of arthritis, such as rheumatoid arthritis, osteoarthritis is not systemic--that is, it does not spread through the entire body, but rather concentrates in one or several joints where deterioration occurs. Osteoarthritis affects joints differently depending on their location in the body. It is commonly found in the joints of the fingers, feet, knees, hips, spine, and, rarely, joints of the wrist, elbows, shoulders, and jaw.

Fingers

Osteoarthritis affects the fingers, where bony knobs form in the joints, most commonly in the first joint below the tips (known as Heberden's nodes) or less commonly in the next joint down (Bouchard's nodes). Gelatinous cysts may also form in the finger joints, which sometimes resolve. Osteoarthritis also frequently damages the base of the thumb. Osteoarthritis of the fingers occurs most often in older women and may be inherited in families.

Knees

Osteoarthritis is particularly debilitating in the weight-bearing joints of the knees. Here, the joint is usually stable until the disease reaches an advanced stage, when the knee becomes enlarged and swollen. Although painful, the arthritic knee usually retains reasonable flexibility.

Hips

Osteoarthritis frequently strikes the weight-bearing joints in one or both hips. Pain develops slowly, usually in the groin and on the outside of the hips, or sometimes in the buttocks. The pain also may radiate to the knee, confusing the diagnosis. Those with osteoarthritis of the hip often walk with a limp, because they rotate the affected leg slightly to reduce pain.

Spine

Osteoarthritis may affect the cartilage in the disks that form cushions between the bones of the spine, the moving joints of the spine itself, or both. In any case, the patient can experience pain, muscle spasms, and diminished mobility. In addition, the nerves may become pinched, causing pain and, in advanced cases, numbness and muscle weakness. Osteoarthritis of the spine is most troublesome when it occurs in the lower back or in the neck, where it may even cause difficulty in swallowing