The Primary Problem in Ankylosing Spondylitis: Ligamentous Structures
The primary pathological process in ankylosing spondylitis involves the intervertebral ligaments and entheses (ligament-bone junctions), not the intervertebral discs themselves. The disease is fundamentally characterized by ossification of the outer fibers of the annulus fibrosus and spinal ligaments, leading to the formation of syndesmophytes—vertical bony bridges between vertebral bodies 1.
Pathological Mechanism
The hallmark of ankylosing spondylitis is ligamentous ossification that progressively fuses the axial skeleton:
- Syndesmophyte formation occurs through ossification of the outer annulus fibrosus fibers (the ligamentous portion), creating vertical bony bridges between vertebrae 1
- Extensive ligamentous ossification throughout the spine leads to the classic "bamboo spine" appearance, resulting from progressive fusion of vertebral joints and ligamentous structures 1, 2
- The disease process promotes vertebral joint fusion through inflammatory changes at entheses (where ligaments attach to bone), not through primary disc degeneration 3
Why Not the Disc?
While the discovertebral junction can show inflammatory changes (spondylodiscitis) on MRI 1, this is a secondary manifestation rather than the primary pathological process. The disc itself is not the initiating site of disease:
- The inflammatory process begins at entheses and ligamentous attachments to bone 3
- Erosions occur at vertebral corners and endplates where ligaments attach, not within the disc substance 1
- The annulus fibrosus involvement is specifically its outer ligamentous fibers, not the inner disc material 1
Clinical Implications
Understanding this ligamentous pathology explains the key clinical features:
- Progressive spinal ankylosis results from ligamentous ossification eliminating normal spinal motion 1
- The ankylosed spine becomes brittle and susceptible to fractures because the fused ligamentous structures cannot accommodate normal mechanical forces 3, 4
- Sacroiliac joint involvement is universal (present in all AS patients), as these joints are primarily ligamentous structures 1, 2
Important Caveat
The chronic inflammatory process is systemic, affecting multiple structures including synovial joints, entheses, and ligamentous attachments throughout the axial skeleton 5, 3. However, the characteristic radiographic and pathological changes that define ankylosing spondylitis—syndesmophytes, vertebral squaring, and spinal fusion—all result from ligamentous ossification, not disc pathology 1.