Arthritis: how osteoarthritis and rheumatoid arthritis affect joints
View the 3 joints below to see the effect of arthritis on a joint. Scroll down to compare a normal, healthy joint with joints affected by rheumatoid arthritis or osteoarthritis.
Normal joint
A joint is a place at which bones meet. The muscles surrounding a joint make the bones move by contracting and relaxing. The muscle is joined to the bone by a fibrous cord known as a tendon.
The ends of most bones have a layer of cartilage covering them. The cartilage is smooth and slippery and forms a cushion that absorbs shock and also prevents bones from rubbing against each other.
The joint is surrounded by a membrane called the synovium or synovial membrane. This produces a thick fluid called synovial fluid that protects and lubricates the joint.
The synovium is enclosed by the capsule which holds the joint together with its fibrous bands called ligaments.
Joint affected by osteoarthritis
Osteoarthritis is the most common type of arthritis. Often associated with ageing, osteoarthritis can be caused by excessive wear on joints or by injury.
Osteoarthritis involves the breakdown of the protective cushion of cartilage surrounding the ends of the bones where 2 bones meet to form a joint. A healthy joint has a lining of smooth cartilage and is lubricated by synovial fluid.
In osteoarthritis, the cartilage becomes flaky and rough and small pieces break off to form loose bodies in the synovial fluid. This causes irritation and inflammation of the synovial membrane. The loss of cartilage leaves bones unprotected and vulnerable to damage.
As the roughened cartilage becomes thinner, the bone underneath thickens. The smooth functioning of the joint is then lost and the bone can lose shape and bony spurs (osteophytes) may form on the bone end.
The capsule and ligaments thicken slowly to try to stabilise the joint as it changes shape.
Weight-bearing joints, such as those found in the knees and hips, the neck and the lower back, are the areas most commonly affected by osteoarthritis.
Joint affected by rheumatoid arthritis
Rheumatoid arthritis (RA) results in inflammation of the joints and, in particular, the synovial membrane that covers them. An over-production of synovial fluid occurs which causes joints to swell and the capsule to stretch, which causes pain.
The inflammation in the joints causes damage to the cartilage and sometimes to the bone ends. If this process is not halted, the cartilage damage can result in deformities or destruction of the joint. This is why suppressing inflammation early on is very important in RA.
Rheumatoid arthritis commonly affects joints on both sides of the body at the same time and can involve the small joints found in the fingers, wrists, ankles and toes and also the shoulders and the knees.
Last Reviewed: 29 July 2009
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