Rheumatoid arthritis is a condition that can cause pain, swelling and stiffness in joints.

It is what is known as an auto-immune condition. This means that the immune system, which is the body’s natural self-defence system, gets confused and starts to attack your body’s healthy tissues. In rheumatoid arthritis, the main way it does this is with inflammation in your joints.

Rheumatoid arthritis affects around 400,000 adults aged 16 and over in the UK. It can affect anyone of any age. It can get worse quickly, so early diagnosis and intensive treatment are important. The sooner you start treatment, the more effective it’s likely to be.

How does a normal joint work?

A joint is where two bones meet. Most of our joints are designed to allow the bones to move in certain directions and within certain limits.

For example, the knee is the largest joint in the body and one of the most complicated. It must be strong enough to take our weight and must lock into position, so we can stand upright.

It also has to act as a hinge, so we can walk, and needs to twist and turn when we run or play sports.

The end of each bone is covered with cartilage that has a very smooth, slippery surface. The cartilage allows the ends of the bones to move against each other, almost without rubbing.

The joint is held in place by the synovium, which contains thick fluid to protect the bones and joint. The synovium has a tough outer layer that holds the joint in place and stops the bones moving too far. Strong cords called tendons anchor the muscles to the bones.

What happens in a joint affected by rheumatoid arthritis?

If you have rheumatoid arthritis, your immune system can cause inflammation inside a joint or a number of joints. Inflammation is normally an important part of how your immune system works. It allows the body to send extra fluid and blood to a part of the body under attack from an infection. For example, if you have a cut that gets infected, the skin around it can become swollen and a different colour.

However, in the case of rheumatoid arthritis, this inflammation in the joint is unnecessary and causes problems.

When the inflammation goes down, the capsule around the synovium remains stretched and can’t hold the joint in its proper position. This can cause the joint to become unstable and move into unusual positions.