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What is Osteoarthritis?

Osteoarthritis is the most common type of arthritis, especially among older people. Sometimes it is called degenerative joint disease or osteoarthrosis. Osteoarthritis is a joint disease that mostly affects the cartilage.

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

Most joints--the place where two moving bones come together--are designed to allow smooth movement between the bones and to absorb shock from movements like walking or repetitive movements. The joint is made up of:

  • Cartilage : a hard but slippery coating on the end of each bone. Cartilage, which breaks down and wears away in osteoarthritis.
  • Joint capsule : a tough membrane sac that holds all the bones and other joint parts together.
  • Synovium: a thin membrane inside the joint capsule.
  • Synovial fluid: a fluid that lubricates the joint and keeps the cartilage smooth and healthy.
  • Ligaments, tendons, and muscles : tissues that keep the bones stable and allow the joint 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 contract to produce movement when stimulated by nerves.

Cartilage is the slippery tissue that covers the ends of bones in 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, bone spurs--small growths called osteophytes--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 limited movement. Unlike some other forms of arthritis, osteoarthritis affects only joints and not internal organs. For example, rheumatoid arthritis--the second most common form of arthritis--affects other parts of the body besides the joints. It begins at a younger age than osteoarthritis, causes swelling and redness in joints, and may make people feel sick, tired, and (uncommonly) feverish.

Osteoarthritis is one of the most frequent causes of physical disability among adults. More than 20 million people in the United States have the disease. By 2030, 20 percent of Americans--about 70 million people--will have passed their 65th birthday and will be at risk for osteoarthritis. Some younger people get osteoarthritis from joint injuries, but osteoarthritis most often occurs in older people. In fact, more than half of the population age 65 or older would show x-ray evidence of osteoarthritis in at least one joint. Both men and women have the disease. Before age 45, more men than women have osteoarthritis, whereas after age 45, it is more common in women.

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.

 

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