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What is osteoarthritis?

Back to Osteoarthritis (OA)

Osteoarthritis is a condition that affects your joints. The surfaces within your joints become damaged so the joint doesn’t move as smoothly as it should (see Figures 1 and 2). The condition is sometimes called arthrosis or osteoarthrosis. Older terms are degenerative joint disease or wear and tear.

When a joint develops osteoarthritis, some of the cartilage covering the ends of the bones gradually roughens and becomes thin, and the bone underneath thickens. All the tissues within the joint become more active than normal – as if your body is trying to repair the damage:

  • The bone at the edge of your joint grows outwards, forming bony spurs called osteophytes.
  • The synovium (the inner layer of the joint capsule which produces synovial fluid) may thicken and make extra fluid. This causes your joint to swell.
  • The capsule and ligaments (tough bands that hold the joint together) slowly thicken and contract as if they were trying to make your joint more stable.

A joint with mild osteoarthritis

A joint that has been deformed by severe osteoarthritis

Sometimes your body’s repairs are quite good and the changes inside your joint won’t cause pain or problems. But in severe osteoarthritis, the cartilage can become so thin that it doesn’t cover the ends of your bones. Your bones rub against each other and start to wear away. The loss of cartilage, the wearing of bone and the bony spurs can change the shape of your joint, forcing your bones out of their normal position.

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