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What Is Causing Your Knee Pain? (Illustrated)

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Updated July 01, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

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Step 3 of 5 - Inflamed Synovium (Rheumatoid Arthritis)
Cut section view of knee joint - rheumatoid arthritis

Cut section view of knee joint - rheumatoid arthritis

Photo © ADAM

In rheumatoid arthritis, the body attacks its own joints. White blood cells, agents of the immune system, travel to the synovium and cause inflammation (synovitis). The inflamed synovium causes warmth, redness, swelling, and pain.

During the inflammatory process, the synovium thickens and causes the joint to swell. As rheumatoid arthritis progresses, abnormal synovial cells invade and erode cartilage and bone within the joint. Surrounding muscles, ligaments, and tendons weaken.

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