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Syndesmophytes are ossifications of spinal ligaments and annulus fibrosus that bridge adjacent vertebral bodies.

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They appear as vertical, thin, or bulky bony outgrowths extending from the margins of vertebral bodies.
Seen in inflammatory arthritides, especially seronegative spondyloarthropathies.

Pathogenesis

Chronic inflammation at the entheses (sites where annulus fibrosus and spinal ligaments attach to vertebrae).
Repair → ossification and bony outgrowth.
Leads to stiffness and eventual ankylosis of the spine.

Classification

Type	Key features	Association
Marginal Syndesmophytes	• Thin, delicate, vertically oriented ossifications.
• Originate from the outer fibers of annulus fibrosus.
• Symmetric, continuous → Bamboo spine in advanced disease.	• ‣
Non-marginal (Parasyndesmophytes)	• Coarse, bulky, asymmetrical bony projections.
• Extend from vertebral margins obliquely, not strictly vertical.
• Do not involve the annulus directly.	• ‣
• ‣
Flowing Syndesmophytes	• Continuous ossification of anterior longitudinal ligament (ALL) along multiple vertebrae.
• Smooth, undulating “candle-wax” appearance.	• ‣

Radiology

X-ray spine (AP, lateral):
- Marginal: Thin vertical lines bridging vertebral corners (AS).
- Non-marginal: Coarse, asymmetric, “skip” distribution (PsA, ReA).
- Flowing: Continuous ALL ossification (DISH).
MRI: Shows early enthesitis and bone marrow edema before ossification.
CT: Useful for subtle bony bridging and surgical planning.

![a, b 48-year-old man with primary ankylosing spondylitis. a Coronal multiplanar reformatted CT image of the thoraco-lumbar spine and b sagittal multiplanar reformatted CT image of the lumbo-sacral junction show symmetric and delicate syndesmophytes (arrows in a, e, b) and ossification of the anterior longitudinal ligament (thin arrows in b)

Paparo, F., Revelli, M., Semprini, A. et al. Seronegative spondyloarthropathies: what radiologists should know. Radiol med 119, 156–163 (2014). https://doi.org/10.1007/s11547-013-0316-5](attachment:5c74a2c8-8196-4be8-97ff-cbc223f3cf67:image.png)

a, b 48-year-old man with primary ankylosing spondylitis. a Coronal multiplanar reformatted CT image of the thoraco-lumbar spine and b sagittal multiplanar reformatted CT image of the lumbo-sacral junction show symmetric and delicate syndesmophytes (arrows in a, e, b) and ossification of the anterior longitudinal ligament (thin arrows in b)

Paparo, F., Revelli, M., Semprini, A. et al. Seronegative spondyloarthropathies: what radiologists should know. Radiol med 119, 156–163 (2014). https://doi.org/10.1007/s11547-013-0316-5

![Syndesmophytes and spondylophytes in a patient with established ankylosing spondylitis. Syndesmophytes are marked with *, showing bone growth <45° to the horizontal axis. Spondylophytes are marked with O, showing bone growth >45° to the horizontal axis.

Image courtesy of Xenofon Baraliakos. Leah, E. A new angle on the relationship between DISH and AS. Nat Rev Rheumatol 8, 4 (2012). https://doi.org/10.1038/nrrheum.2011.183](attachment:72dc3476-7994-40cf-9b31-02373fe9fa1c:image.png)

Syndesmophytes and spondylophytes in a patient with established ankylosing spondylitis. Syndesmophytes are marked with *, showing bone growth <45° to the horizontal axis. Spondylophytes are marked with O, showing bone growth >45° to the horizontal axis.

Image courtesy of Xenofon Baraliakos. Leah, E. A new angle on the relationship between DISH and AS. Nat Rev Rheumatol 8, 4 (2012). https://doi.org/10.1038/nrrheum.2011.183

Clinical Significance

Ankylosing spondylitis: Thin marginal syndesmophytes → rigid “bamboo spine” → prone to fractures.