Introduction:

Osteoarthritis, also known as degenerative joint disease, is the most common form of arthritis. Scientists now believe osteoarthritis results from a combination of genetic problems and joint injuries. In this disorder, a joint loses cartilage, the slippery material that cushions the ends of bones, over time.

Osteoarthritis is a chronic disease of the joint cartilage and bone, often thought to result from "wear and tear" on a joint. There are other causes, such as birth defects, trauma, and metabolic disorders. Joints appear larger, are stiff and painful, and usually feel worse with increased use throughout the day.

As a result, the bone beneath the cartilage changes and develops bony overgrowth. The tissue that lines the joint can become inflamed, the ligaments can loosen, and the muscles around the joint can weaken. The patient feels pain when using the joint. Nearly all vertebrates suffer from osteoarthritis, including porpoises and whales, as did dinosaurs.

Joints

Joints provide flexibility, support, stability, and protection. Specific parts of the joint: the synovium and cartilage, provide these functions.

Synovium. The synovium is the fluid-filled lining of a joint. Synovial fluid is a lubricating liquid that supplies nutrients and oxygen to cartilage.

Cartilage. The cartilage is a slippery tissue that coats the ends of the bones. Cartilage is one of the few tissues in the body that does not have its own blood supply. It has a number of essential parts, including:

• Chondrocytes. Chondrocytes, the basic cartilage cells, are critical for balance and function.
• Water. Cartilage contains a lot of water, which decreases with age. About 85% of cartilage is water in young people. Cartilage in older people is about 70% water.
• Proteoglycans. These large molecules help make up cartilage. They are important because they can bond to water, which keeps high amounts of water in cartilage.
• Collagen. This essential protein in cartilage forms a mesh to give the joint support and flexibility. Collagen is the main protein found in all connective tissues of the body, including the muscles, ligaments, and tendons.

The combination of collagen mesh and water forms a strong and slippery pad in the joint. This pad cushions the ends of the bones in the joint during muscle movement.

Osteoarthritis: The Disease Process

Deterioration of Cartilage. Osteoarthritis develops when cartilage in a joint deteriorates. The process is usually slow.

• In the early stages of disease, the surface of the cartilage, or the synovium (joint lining) in some people, becomes inflamed and swollen. The joint loses proteoglycan molecules and other tissues. This joint then begins to lose water. Fissures and pits appear in the cartilage.
• As the disease progresses and more tissue is lost, the cartilage starts to get hard. As a result, it 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.

Complicating the process are abnormalities in the bone around arthritic joints. As the body tries to repair damage to the cartilage, problems can develop:

• Clusters of damaged cells or fluid-filled cysts may form around the bony areas or near the fissures in the cartilage.
• Fluid pockets may also form within the bone marrow itself, causing swelling. The marrow, which runs up through the center of the bone, is rich in nerve fibers. As a result, these injuries may cause a lot of pain in many patients with osteoarthritis.
• Bone cells may respond to damage by multiplying, growing, and forming dense, misshapen plates around exposed areas.
• At the margins of the joint, the bone may produce outcroppings, on which new cartilage cells (chondrocytes) multiply and grow abnormally.

Location of Osteoarthritis

Unlike some other types of arthritis, such as rheumatoid arthritis, osteoarthritis does not spread through the entire body. Rather, it affects one or several joints. Osteoarthritis affects joints differently depending on their location in the body.

• It is common in joints of the fingers, feet, knees, hips, and spine.
• It sometimes occurs in the wrist, elbows, shoulders, and jaw, but it is not common in these locations.