• Water. Cartilage is composed mostly of water, which decreases with age. About 85% of cartilage is water in young people. Cartilage in older people is about 70% water.
• Chondrocytes. Chondrocytes, the basic cartilage cells, are critical for joint health.
• Proteoglycans. These large molecules 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 (meniscus) 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 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 can cause pain.
• 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 is less likely to involve many joints around the body or migrate from around one joint to another. Rather, it affects one or a few joints, often joints that have received extra wear. 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 as common in these locations.

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Review Date: 06/16/2010
Reviewed By: Reviewed by: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org)