Arthritis is a general term for a group of more than 100 diseases. The
word "arthritis" means "joint inflammation." Inflammation is one of the
body's natural reactions to disease or injury, and includes swelling,
pain and stiffness. Inflammation that lasts for a very long time or
recurs, as in arthritis, can lead to tissue damage.

A joint is where two or more bones come together, such as the hip or
knee.

The bones of a joint are covered with a smooth, spongy material called
cartilage, which cushions the bones and allows the joint to move
without pain. The joint is enclosed in a fibrous casing called the
synovium. The synovium's lining produces a slippery fluid -- called
synovial fluid -- that nourishes the joint and helps limit friction
within. Strong bands of tissue, called ligaments, connect the bones and
help keep the joint stable. Muscles and tendons also support the joints
and enable you to move.

With arthritis, an area in or around a joint becomes inflamed, causing
pain, stiffness and, sometimes, difficulty moving. Some types of
arthritis also affect other parts of the body, such as the skin and
internal organs.

Types of Arthritis

There are more than 100 different types of arthritis. Some of the more
common types include:

Osteoarthritis. This is the most common type of arthritis. It occurs
when the cartilage covering the end of the bones gradually wears away.
Without the protection of the cartilage, the bones begin to rub against
each other and the resulting friction leads to pain and swelling.
Osteoarthritis can occur in any joint, but most often affects the hands
and weight-bearing joints such as the knee, hip and facet joints (in
the spine). Osteoarthritis often occurs as the cartilage breaks down,
or degenerates, with age. For this reason, osteoarthritis is sometimes
called degenerative joint disease.

Rheumatoid arthritis. Rheumatoid arthritis is a long-lasting disease
that can affect joints in any part of the body but most commonly the
hands, wrists, and knees. With rheumatoid arthritis, the immune system
-- the body's defense system against disease -- mistakenly attacks
itself and causes the joint lining to swell. The...

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