Ankylosing spondylitis typically originates in the sacroiliac (SI) joint, which joins the bottom of the spine with the pelvis. As the disease progresses, the middle and upper spine can be affected.
Initial Pain in the Sacroiliac Joint
The first symptom of ankylosing spondylitis is typically sacroiliitis, the inflammation of the entheses at the sacroiliac joint. This condition may also be referred to as sacroiliac joint dysfunction.
The sacroiliac joint connects the sacrum (the triangular bone at the bottom of the lumbar spine) on both sides to the pelvis’s ilium. The sacrum and the ilium are connected with a powerful network of ligaments.
The sacroiliac joint is highly susceptible to enthesitis and inflammation because:
- It undergoes significant physical stresses
- It has a relatively high concentration of fibrocartilage at the entheses
Sacroiliac joint inflammation can cause radiating pain that travels from the buttock to the thigh or lower back. Continued sacroiliitis and the inflammation-erosion-calcification cycle can eventually lead the bones of the sacroiliac joint to fuse together. While a normal sacroiliac joint has a minimal range of motion measured in just millimeters1, sacroiliac joint fusion and immobility can cause pain as well as difficulty with bending forward, backward, and side-to-side.
Symptoms Progress up the Spine
The inflammation and calcification process can continue moving up the spine, and may affect the following joints and structures:
Facet joints. A pair of facet joints is located on each of the spine’s vertebra. Calcification and fusion of the facet joints can greatly diminish mobility, as they provide for much of the twisting ability of the spine.
Costotransverse and costovertebral joints. The joints where the ribs connect to the thoracic spine (at T2 through T10) can become fused, which can eventually limit lung capacity and impact posture.
Annulus fibrosus. The annulus fibrosus is the tough exterior of a spinal disc that surrounds the nucleus pulposus, the disc’s soft inner core. Ankylosing spondylitis can cause a disc’s annulus fibrosus to become calcified and fuse with adjacent vertebrae. This process may be referred to as fusion of a spinal segment.
In cases of severe ankylosing spondylitis, the spinal column can become one large, inflexible unit, sometimes referred to as bamboo spine
Ankylosing spondylitis cannot be prevented or cured, but its progression can be slowed and its effects can be managed with medication, physical therapy, and lifestyle changes.