Ankylosing spondylitis is a form of chronic inflammation of the spine and the sacroiliac joints. The sacroiliac joints are located at the base of the low back where the sacrum (the bone directly above the tailbone) meets the iliac bones (bones on either side of the upper buttocks) of the pelvis. Chronic inflammation in these areas causes pain and stiffness in and around the spine, including the neck, middle back, lower back, and buttocks. Over time, chronic inflammation of the spine (spondylitis) can lead to a complete cementing together (fusion) of the vertebrae, a process referred to as ankylosis. Ankylosis causes loss of mobility of the spine.