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What Is Osteoarthritis?

Jul 8, 2010; 10:40 AM ET

Osteoarthritis (AH-stee-oh-ar-THREYE-tis) is the most common type of arthritis, and is seen especially among older people. Sometimes it is called degenerative joint disease or osteoarthrosis. Osteoarthritis mostly affects cartilage (KAR-til-uj), the hard but slippery tissue that covers the ends of bones where they meet to form a joint. Healthy cartilage allows bones to glide over one another. It also absorbs energy from the shock of physical movement. In osteoarthritis, the surface layer of cartilage breaks down and wears away. This allows bones under the cartilage to rub together, causing pain, swelling, and loss of motion of the joint. Over time, the joint may lose its normal shape. Also, small deposits of bone - called osteophytes or bone spurs - may grow on the edges of the joint. Bits of bone or cartilage can break off and float inside the joint space. This causes more pain and damage. People with osteoarthritis usually have joint pain and some movement limitations. Unlike some other forms of arthritis, such as rheumatoid arthritis, osteoarthritis affects only joint function and does not affect skin tissue, the lungs, the eyes, or the blood vessels. In rheumatoid arthritis, the second most common form of arthritis, the immune system attacks the tissues of the joints, leading to pain, inflammation, and eventually joint damage and malformation. It typically begins at a younger age than osteoarthritis, causes swelling and redness in joints, and may make people feel sick, tired, and uncommonly feverish.

Who Has Osteoarthritis?

Osteoarthritis is by far the most common type of arthritis, and the percentage of people who have it grows higher with age. An estimated 27 million Americans age 25 and older have osteoarthritis. Although osteoarthritis is more common in older people, younger people can develop it - usually as the result of a joint injury, a joint malformation, or a genetic defect in joint cartilage. Both men and women have the disease. Before age 45, more men than women have osteoarthritis; after age 45, it is more common in women. It is also more likely to occur in people who are overweight and in those with jobs that stress particular joints. As the population ages, the number of people with osteoarthritis will only grow. By 2030, 20 percent of Americans - about 72 million people - will have passed their 65th birthday and will be at high risk for the disease.

What Areas Does Osteoarthritis Affect?

Outline of a woman highlighting neck, spine, hips, fingers, and knees to show common locations of osteoarthritis

Osteoarthritis most often occurs in the hands (at the ends of the fingers and thumbs), spine (neck and lower back), knees, and hips.

How Does Osteoarthritis Affect People?

People with osteoarthritis usually experience joint pain and stiffness. The most commonly affected joints are those at the ends of the fingers (closest to the nail), thumbs, neck, lower back, knees, and hips. Osteoarthritis affects different people differently. Although in some people it progresses quickly, in most individuals joint damage develops gradually over years. In some people, osteoarthritis is relatively mild and interferes little with day-to-day-life; in others, it causes significant pain and disability. While osteoarthritis is a disease of the joints, its effects are not just physical. In many people with osteoarthritis, lifestyle and finances also decline.

Lifestyle effects include

-depression

- anxiety

- feelings of helplessness

- limitations on daily activities

- job limitations

- difficulty participating in everyday personal and family joys and responsibilities.

Financial effects include

* the cost of treatment

* wages lost because of disability.

Fortunately, most people with osteoarthritis live active, productive lives despite these limitations. They do so by using treatment strategies such rest and exercise, pain relief medications, education and support programs, learning self-care, and having a "good attitude."

Osteoarthritis Basics: The Joint and Its Parts

A joint is the point where two or more bones are connected. With a few exceptions (in the skull and pelvis, for example), joints are designed to allow movement between the bones and to absorb shock from movements like walking or repetitive motions. These movable joints are made up of the following parts:

Cartilage: a hard but slippery coating on the end of each bone. Cartilage breaks down and wears away in osteoarthritis.

Joint capsule: a tough membrane sac that encloses all the bones and other joint parts.

Synovium (sin-O-vee-um): a thin membrane inside the joint capsule that secretes synovial fluid.

Synovial fluid: a fluid that lubricates the joint and keeps the cartilage smooth and healthy.

In a healthy joint, the ends of bones are encased in smooth cartilage. Together, they are protected by a joint capsule lined with a synovial membrane that produces synovial fluid. The capsule and fluid protect the cartilage, muscles, and connective tissues.

With osteoarthritis, the cartilage becomes worn away. Spurs grow out from the edge of the bone, and synovial fluid increases. Altogether, the joint feels stiff and sore. Ligaments, tendons, and muscles are tissues that surround the bones and joints, and allow the joints to bend and move. Ligaments are tough, cord-like tissues that connect one bone to another. Tendons are tough, fibrous cords that connect muscles to bones. Muscles are bundles of specialized cells that, when stimulated by nerves, either relax or contract to produce movement.

Cartilage: The Key to Healthy Joints

Cartilage is 65 to 80 percent water. The remaining three components - collagen, proteoglycans, and chondrocytes - are described below.

- collagen (KAHL-uh-jen): A family of fibrous proteins, collagens are the building blocks of skin, tendon, bone, and other connective tissues.

- proteoglycans (PRO-tee-uh-GLY-kanz): Made up of proteins and sugars, strands of proteoglycans interweave with collagens and form a mesh-like tissue. This allows cartilage to flex and absorb physical shock.

- chondrocytes (KAHN-druh-sytz): Found throughout the cartilage, chondrocytes are cells that produce cartilage and help it stay healthy as it grows. Sometimes, however, they release substances called enzymes that destroy collagen and other proteins. Researchers are trying to learn more about chondrocytes.