What Is It?
Osteoarthritis affects structures throughout the joint. The cartilage, the slippery tissue that covers the ends of bones in a joint, begins to break down and wear away. This allows bones to rub together, eventually causing inflammation and pain.
There are more than 100 kinds of arthritis, which literally means joint inflammation. According to the Centers for Disease Control and Prevention (CDC), about 54.4 million (one in four) Americans are afflicted, and 27 million have osteoarthritis, by far the most common form, especially among older people. Sometimes called degenerative joint disease, osteoarthritis accounts for most of the hip and knee replacement surgeries performed in the United States. As with other types of arthritis, women are at higher risk than men for the condition.
Osteoarthritis affects structures throughout the joint. Among them the cartilage, the slippery tissue that covers the ends of bones in a joint, is usually affected. Healthy cartilage allows bones to glide over one another and absorbs energy from the shock of physical movement. In osteoarthritis, the surface layer of cartilage breaks down and wears away, which allows bones under the cartilage to rub together. This causes bone remodeling and change in boney shape and loss of motion of the joint. Over time, the joint loses its normal shape. Also, bone spurs—small growths called osteophytes—may grow on the edges of the joint. By the time a person has symptoms from osteoarthritis, it is no longer just a disease of cartilage. Bone has changed, muscles across the joint have often weakened and there is occasionally inflammation in the lining of the joint.
Unlike some other forms of arthritis, osteoarthritis only affects joints, not internal organs. Rheumatoid arthritis—another common form of arthritis—affects other parts of the body besides the joints. It causes inflammation and may make people feel sick, tired and sometimes feverish, among other symptoms.
Some younger people get osteoarthritis from a joint injury, but osteoarthritis most often occurs in people over 40. In fact, at least 80 percent of people over age 55 have X-ray evidence of osteoarthritis in at least one joint, but not all of them have symptoms of the disease. Since the number of older Americans is increasing, so is the number of people with osteoarthritis. Before age 55, more men have the condition (often the result of a sports or work injury), while after age 55, osteoarthritis is more common in women.
Researchers suspect that genes play a role in some cases of osteoarthritis. In one rare form of disease starting as early as age 20, affected people may have a defect in the gene that helps the body make collagen, the protein that makes up cartilage. More commonly, minor genetic variations may increase the risk of osteoarthritis. They may affect minor molecules in the joint, changing the metabolism of joint structures, or they may change the shape of a joint during growth, leaving slight defects in the way bones that come together in joints fit together. This may cause cartilage to wear away more quickly than normal. Double-jointedness, an inherited trait where the joints bend farther than usual, may also increase risk for osteoarthritis.