Osteoarthritis is a very common condition which can affect any joint in the body. It’s most likely to affect the joints that bear most of our weight, such as the knees and feet. Joints that we use a lot in everyday life, such as the joints of the hand, are also commonly affected.

In a healthy joint, a coating of tough but smooth and slippery tissue, called cartilage, covers the surface of the bones and helps the bones to move freely against each other. When a joint develops osteoarthritis, part of the cartilage thins, and the surface becomes rougher. This means the joint doesn’t move as smoothly as it should.

When cartilage becomes worn or damaged, all the tissues within the joint become more active than normal as the body tries to repair the damage. The repair processes may change the structure of the joint but will often allow the joint to work normally and without any pain and stiffness. Almost all of us will develop osteoarthritis in some of our joints as we get older, though we may not even be aware of it.

However, the repair processes don’t always work so well and changes to the joint structure can sometimes cause or contribute to symptoms such as pain, swelling or difficulty in moving the joint normally.

For example:

• Extra bone may form at the edge of the joint. These bony growths are called osteophytes and can sometimes restrict movement or rub against other tissues. In some joints, especially the finger joints, these may be visible as firm, knobbly swellings.
• The lining of the joint capsule (called the synovium) may thicken and produce more fluid than normal, causing the joint to swell.
• Tissues that surround the joint and help to support it may stretch so that after a time the joint becomes less stable.