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What is arthritis?
As many as 36 million people in the United States have some form of arthritis or joint inflammation. It is a major cause of lost work time and serious disability for many people. Although arthritis is mainly a disease of adults, children may also have it.

What is a joint?
A joint is where the ends of two or more bones meet. For example, a bone of the lower leg, called the shin or tibia and the thighbone, called the femur, meet to form the knee joint. The hip is a ball and socket joint. It is formed by the upper end of the thighbone - the ball - fitting into the socket - the part of the pelvis called the acetabulum.

The bone ends of a joint are covered with a smooth material called cartilage. The cartilage cushions the bone and allows the joint to move easily without pain. The joint is enclosed by a fibrous envelope called the synovium which produces a fluid that helps to reduce friction and wear in a joint. Ligaments connect the bones and keep the joint stable. Muscles and tendons power the joint and enable it to move.

What is inflammation?
Inflammation is one of the body's normal reactions to injury or disease. In an injured or diseased joint, this results in swelling, pain, and stiffness. Inflammation is usually temporary, but in arthritic joints, it may cause long-lasting or permanent disability.

Types of arthritis
There are more than 100 different types of arthritis.

What is osteoarthritis?
The most common type of arthritis is osteoarthritis. It is seen in many people as they age, although it may begin when they are younger as a result of injury or overuse. It is often more painful in weightbearing joints such as the knee, hip, and spine than in the wrist, elbow, and shoulder joints. All joints may be more affected if they are used extensively in work or sports, or if they have been damaged from fractures or other injuries.

In osteoarthritis, the cartilage covering the bone ends gradually wears away. In many cases, bone growths called "spurs" can develop in osteoarthritic joints. The joint inflammation causes pain and swelling. Continued use of the joint produces pain. Some relief may be possible through rest or modified activity.

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