Arthritis is inflammation of the joints.
There are many different types, all with different causes, but osteoarthritis is by far the most common, being present on X-rays in 85 per cent of the elderly population. Other well-known types are rheumatoid arthritis, gout and infectious arthritis. These joints are made up of the ends of the joining bones enclosed by a capsule. The space within the capsule is filled with fluid, which allows the two bone surfaces to move smoothly over each other. The bone surfaces are also covered by a smooth substance called cartilage to aid this smooth movement.