Ankylosing spondylitis is a complicated type of arthritis that often targets the spine. It's a condition that can baffle even physicians, sometimes taking years to be diagnosed.

See Types of Arthritis

Watch the award-winning Ankylosing Spondylitis Video

To help you understand this tricky condition, we have an award-winning video about ankylosing spondylitis that explains the progression of this condition in clear and straightforward language and illustrations.


How ankylosing spondylitis starts

Ankylosing spondylitis mostly affects the joints of the spine. It also affects the ligaments and tendons surrounding the spine, causing inflammation. These soft tissues help support our spines and offer flexibility.

A close up view of an illustrated spine with the supraspinal and intraspinal ligaments labeled

The spot where the ligaments or tendons attach to the bone is called the enthesis. When the enthesis becomes inflamed, it’s called enthesitis. This inflammation is the main process that drives the progression of ankylosing spondylitis.

See Ankylosing Spondylitis Causes and Risk Factors

A close up view of an illustrated spine showing two vertebrae and a disc. The connection point between them is called the entheses and it is labeled.

The inflammation triggers bone erosion at the enthesis site, which the body then tries to repair by growing new bone. This process is known as calcification.

See Ankylosing Spondylitis Symptoms

People with ankylosing spondylitis often experience the first symptoms of the disease in the sacroiliac (SI) joints, where the base of the spine meets the pelvis. This causes pain in the buttocks that may radiate to the lower back or the back of the thighs.

See Sacroiliac Joint Pain and Inflammation

An illustrated view of the lower back and legs. The sacroiliac joint is labeled

A cause of pain and disability as it progresses

The cycle of inflammation and calcification can eventually lead to the sacroiliac joint to fuse into place. This can also occur throughout the spine, including at the facet joints, the joints between the spine and ribs, and the spinal discs between the vertebrae.

See Progression of Ankylosing Spondylitis in the Spine

A close up view of two vertebrae showing fibrosus calcification between the discs

As more and more of these joints fuse, it can severely limit mobility. The spine can be immobile and fixed in position. Because the spine is less able to cushion movement, stress fractures can occur—especially where new bone has grown. This can cause extreme pain at the point of fracture.

See 5 Unusual Ankylosing Spondylitis Symptoms

A close up view of a stress fracture of a vertebrae of the spine.

When these stress fractures occur in the thoracic, or middle, portion of the spine, they can shorten the front of the spinal column and cause a hunched-forward posture. In extreme cases, this spinal distortion can crowd internal organs.

See Complications of Severe Ankylosing Spondylitis

A close up illustration showing fractures in a vertebrae

Ankylosing spondylitis can also affect the eyes and other joints in the body, including the hips, knees, feet, or shoulders.

Learn more:

Ankylosing Spondylitis Medications

Ankylosing Spondylitis Physical Therapy and Exercise