Ankylosing spondylitis (AS) is a type of chronic (long-term) arthritis that affects the bones, muscles, and ligaments of the spine.

Anyone can develop ankylosing spondylitis at any time, although it is five times more common in men than it is in women. It usually occurs between 15-35 years of age.

The spine

The spine consists of a column of interlocking bones called vertebrae that run from the coccyx (tailbone) to the top of the neck, where it joins the skull. There are flexible 'discs' of tissue between each vertebral body which help to cushion the load that is carried by the spine.

The vertebrae are supported by muscles and ligaments (tissue that connect bones) and help to control the movements (flexion and rotation) of the spine. The surrounding tissues help to strengthen the spine and hold it in place.

The bones behind each vertebral body form a protective 'canal' for the spinal cord. They also make joints above and below the vertebra, allowing the spine to bend and twist. Behind the spinal canal there are vertebral spines, which can be felt as a series of bumps along the spinal column.

The spine carries the weight of the head, arms, chest, and abdomen. The sacroiliac joints connect the spine to the pelvis. The purpose of the sacroiliac joints is to transmit the weight of the upper body to the pelvis.

How does ankylosing spondylitis occur?

In ankylosing spondylitis, the spinal joints and ligaments, and the sacroiliac joints become inflamed. Inflammation in the spine can cause pain and stiffness in the neck and back. Sacroiliitis (inflammation of the sacroiliac joints) causes pain in the lower back and buttocks.

If not treated, over many years, the inflammation can cause the neck and back to become rigid. This process is called ankylosis. In some people who have severe, long-standing ankylosis, their rib cage (chest) can also become stiff and inflexible.

Ankylosing spondylitis can also lead to arthritis developing in the large joints, such as the hips and knees.