Arthritis is a term that refers to more than 100 different diseases that affect the joints, according to the Arthritis Foundation.
With osteoarthritis, the cartilage that covers the ends of bones in joints deteriorates over time, causing pain and making joints less flexible as bone rubs against bone. It can be caused by injury, genetic and environmental factors, or by an overloaded joint in someone who is overweight.
Rheumatoid arthritis, which affects mostly women, involves a malfunction of the body's immune system that results in an inflammation of the joint lining. Gout, lupus and fibromyalgia are other forms of arthritis.
About 21 percent of U.S. adults reported that they had arthritis or chronic joint problems in 2002, according to the Arthritis Foundation.
In the past 25 years, there have been significant advances in efforts to treat arthritis beyond taking aspirin or other pain relievers. Artificial hip and knee joints have been developed, and scientists are starting to understand the cells in joint cartilage, said John Hardin, chief scientific officer of the Arthritis Foundation.
For now, there is no cure for osteoarthritis, which tends to affect the hips, hands, knees, lower back and neck in people age 40 and older. It can be controlled with a combination of drugs, rest, exercise, orthotics, joint braces and the use of heating pads and ice packs to reduce pain. Weight loss, for those who are overweight, is a very effective way to treat osteoarthritis.
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Robin Lloyd was a senior editor at Space.com and Live Science from 2007 to 2009. She holds a B.A. degree in sociology from Smith College and a Ph.D. and M.A. degree in sociology from the University of California at Santa Barbara. She is currently a freelance science writer based in New York City and a contributing editor at Scientific American, as well as an adjunct professor at New York University's Science, Health and Environmental Reporting Program.