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Ankylosing spondylitis (AS) is a chronic, rheumatoid, inflammatory joint condition which causes arthritic changes in the sacroiliac joint at the junction of the sacrum (lower spine) and the pelvis.

The human spine is a complex structure responsible for many tasks, including:

• Bearing weight of the upper body
• Keeping the body upright and erect when standing
• Providing structure and support
• Maintaining flexibility of movement of the upper body
• Housing and protecting the spinal cord

The spine is comprised of many components including 24 vertebrae and more than 100 joints. The vertebrae are divided into three sections, namely:

• 7 cervical vertebrae in the neck region - these vertebrae provide the greatest mobility of the spinal vertebrae.
• 12 thoracic vertebrae in the chest region - each of the thoracic vertebrae has a rib attached to it which arches around to the front of the body and encompasses the lungs, heart, and the uppermost part of the abdominal organs.
• 5 lumbar vertebrae which lie behind the abdominal organs.

The sacrum is a triangular bone which sits below the lumbar vertebrae and attaches the spine to the pelvis. The joint that connects the sacrum and the pelvis is called the sacroiliac joint. Inflammation of the sacroiliac joint is the hallmark sign of ankylosing spondylitis and is the place where lower back pain associated with AS typically originates. Involvement of the axial joints, primarily the sacroiliac joint, is the earliest manifestation of ankylosing spondylitis although peripheral joints and extra-articular sites can be involved as well. Older names that have been used in the past to describe ankylosing spondylitis include "poker back", rheumatoid spondylitis, and Marie-Strumpell's spondylitis.