To understand how rheumatoid arthritis affects the joints, it is first helpful to understand the structure of a joint and how it normally works.

A joint is where two bones meet. The adjacent ends of the bones are each covered in a cushioning layer of a slippery tissue called cartilage. The area just surrounding the joint is enclosed in a protective sac called the joint capsule. The lining of the joint capsule, the synovium, produces synovial fluid, a clear liquid that lubricates the cartilage and bones. Synovial fluid also supplies oxygen and nutrients and provides supportive cushioning. In a healthy joint, all of these elements work together to produce smooth, painless movement.

Rheumatoid arthritis causes the body's immune system to attack the cells of the synovium. White blood cells are dispatched to the area, and the cells of the synovial lining begin to multiply and the composition of the synovial fluid begins to change. The amount of fluid increases. The joint feels warm, tender and swollen.

Eventually the abnormal synovial fluid forms a destructive tissue called pannus. Joint tissues are no longer nourished and protected; instead, pannus attacks and destroys them with digestive enzymes.

In an untreated patient, pannus first destroys the cartilage at the ends of the bones, which narrows the space between the bones. As cartilage is destroyed the body dispatches more white cells, which further inflame the area and accelerate the destructive process. The lack of protective cartilage makes bone tissue vulnerable to attack, and it too suffers irreversible damage.

Doctors now believe that bone erosion takes place early in rheumatoid arthritis, most likely during the first or second year of the disease. This is why early diagnosis and treatment of RA are critical. Disease modifying drugs can arrest the destructive process before irreversible joint damage and deformities occur.

Rheumatoid arthritis can attack any joint in the body with a synovial lining. RA characteristically attacks joints of the body in a symmetrical fashion. For example, if the right wrist is affected, so is the left one.

References

First published September 1, 1999
Last updated December 16, 2002
Copyright © 1999 Accordant Health Services, Inc. All Rights Reserved.

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