The place where two bones meet is called a joint. Joints allow movement and flexibility of various parts of the body. The movement of the bones is caused by muscles which pull on tendons that are attached to bone.
Cartilage covers the end of bones. Between the cartilage of two bones which form a joint there is a small amount of thick fluid called synovial fluid. This fluid ‘lubricates’ the joint, which allows smooth movement between the bones.
The synovial fluid is made by the synovium. This is the tissue that surrounds the joint. The outer part of the synovium is called the capsule. This is tough, gives the joint stability, and stops the bones from moving ‘out of joint’. Surrounding ligaments and muscles also help to give support and stability to joints.
About 1 person in 10 with psoriasis develops psoriatic arthritis. (About 2 in 100 people develop psoriasis at some stage in their lives.)
In most cases, the arthritis develops after the psoriasis - most commonly within 10 years after the psoriasis first develops. However, in some cases the arthritis develops much later. In a small number of cases the arthritis develops first, sometimes months or even years before the psoriasis develops. Men and women are equally affected.
Psoriasis most commonly first occurs between the ages of 15 and 25 and psoriatic arthritis most commonly develops between the ages of 25 and 50. However, both psoriasis and psoriatic arthritis can occur at any age, including in childhood.
Note: people with psoriasis also have the same chance as everyone else of developing other types of arthritis such as rheumatoid arthritis and osteoarthritis. Psoriatic arthritis is different, and is a particular type of arthritis that occurs only in some people with psoriasis.