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 abnormalities and joint injuries. In this disorder, an affected joint experiences a progressive loss of cartilage, the slippery material that cushions the ends of bones.

As a result, the bone beneath the cartilage undergoes changes that lead to bony overgrowth. The tissue that lines the joint can become inflamed, the ligaments can loosen, and the associated muscles can weaken. The patient experiences pain when using the joint. In addition to humans, nearly all vertebrates suffer from osteoarthritis, including porpoises and whales, as did long-extinct terrestrial travelers such as dinosaurs.

Joints

Joints are designed to provide flexibility, support, stability, and protection. These functions, essential for normal and painless movement, are primarily supplied by specific parts of the joint: the synovium and cartilage.

Synovium. The synovium is a membrane that surrounds the entire joint. It is filled with synovial fluid, 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 components:

• Chondrocytes. Chondrocytes, the basic cartilage cells, are critical for balance and function.
• Water. Cartilage contains a high percentage of water, although it decreases with age. About 85% of cartilage is water in young people, and about 70% is water in older individuals.
• Proteoglycans. These are large molecules that help make up cartilage. Their important value is their capacity to bond to water, which ensures the high-fluid content in cartilage.
• Collagen. This is the critical protein in cartilage. It 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, including 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. The synovial fluid lubricates and provides oxygen and nutrients to the bloodless cartilage.

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 the disease the surface of the cartilage, or even the synovium in some people, becomes inflamed and swollen. There is a loss of proteoglycan molecules and other tissue components that cause water loss. Fissures and pits appear in the cartilage.
• As the disease progresses and more tissue is lost, the cartilage loses elasticity and fluid. It becomes increasingly prone to damage due to repetitive use and injury.
• Eventually large amounts of cartilage are destroyed, leaving the ends of the bone within the joint unprotected.

To compound the process, bone around arthritic joints is not structurally normal. 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.
• Fluid pockets may also form within the bone marrow itself, causing swelling. The marrow, which runs up through the center of bone, is rich in nerve fibers, and such injuries may be an important source 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) proliferate and grow abnormally.

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

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