Ankylosing Spondylitis¶

Definition¶

Ankylosing spondylitis (AS) is a chronic inflammatory spondyloarthropathy that primarily affects the axial skeleton, characteristically beginning at the sacroiliac joints and progressing cranially through the spine. It is the prototypical seronegative spondyloarthropathy, strongly associated with HLA-B27, and predominantly affects young men (male-to-female ratio approximately 3:1) with onset typically between ages 15 and 35.

Pathophysiology¶

AS is characterized by enthesitis — inflammation at the sites where ligaments, tendons, and joint capsules attach to bone. In the spine, this manifests as inflammation at the vertebral body corners (Romanus lesions), the annulus fibrosus attachment to the vertebral endplates, and the interspinous and supraspinous ligaments. Over time, the inflammatory process leads to reactive new bone formation (syndesmophytes), progressive ankylosis, and ultimately a completely fused "bamboo spine."

Imaging Findings¶

Sacroiliac Joints (Earliest Site)¶

MRI — Bone marrow edema on STIR at the sacroiliac joint margins is the earliest imaging finding, often preceding radiographic changes by years. Erosions, joint space widening (early), and eventual fusion (late).
Radiography/CT — Bilateral, symmetric sacroiliitis progressing through stages: pseudo-widening (erosions) → sclerosis → narrowing → complete ankylosis

Spine¶

Romanus lesions — Inflammatory erosions at the anterior vertebral body corners ("shiny corners" on MRI — bright on STIR)
Syndesmophytes — Thin, vertical bridges of bone connecting adjacent vertebral bodies along the annulus fibrosus (marginal syndesmophytes). These are characteristically thin and vertical, unlike the bulky, asymmetric osteophytes of degenerative disease.
Bamboo spine — End-stage complete fusion of the spine through syndesmophytes, posterior element fusion, and interspinous ligament ossification
Andersson lesions — Discovertebral destructive lesions (see Romanus and Andersson Lesions)
Squaring of vertebral bodies — Loss of the normal anterior concavity due to erosion and new bone formation at the corners

CT¶

Best for demonstrating syndesmophytes, facet joint ankylosis, and posterior element fusion
Sacroiliac joint erosions and sclerosis

Clinical Pearl

The fused spine in ankylosing spondylitis is extremely brittle and susceptible to fractures from even minor trauma. Fractures in AS are unstable and often involve all three columns (like a long bone fracture through a fused segment). They most commonly occur at the cervicothoracic junction and carry a high risk of neurological injury. Any AS patient with new-onset neck or back pain after even trivial trauma should be imaged — CT and MRI of the entire spine.

Key Points¶

Bilateral symmetric sacroiliitis is the hallmark and earliest finding
STIR MRI detects sacroiliac and vertebral inflammation before radiographic changes
Thin, marginal syndesmophytes distinguish AS from degenerative osteophytes
End-stage bamboo spine is completely fused and extremely fracture-prone
HLA-B27 positive in ~90% of patients
Young men aged 15–35 are most commonly affected

References¶

Wenker KJ, Quint JM. Ankylosing Spondylitis. In: StatPearls. Treasure Island (FL): StatPearls Publishing; updated 2023 Jun 20. https://www.ncbi.nlm.nih.gov/books/NBK470173/
Ankylosing spondylitis. Radiopaedia.org. https://radiopaedia.org/articles/ankylosing-spondylitis
Rudwaleit M, van der Heijde D, Landewé R, et al. The development of Assessment of SpondyloArthritis international Society classification criteria for axial spondyloarthritis (part II): validation and final selection. Ann Rheum Dis. 2009;68(6):777-783. https://pubmed.ncbi.nlm.nih.gov/19297344/
van der Linden S, Valkenburg HA, Cats A. Evaluation of diagnostic criteria for ankylosing spondylitis. A proposal for modification of the New York criteria. Arthritis Rheum. 1984;27(4):361-368. https://pubmed.ncbi.nlm.nih.gov/6231933/
Hermann KGA, Althoff CE, Schneider U, et al. Spinal changes in patients with spondyloarthritis: comparison of MR imaging and radiographic appearances. RadioGraphics. 2005;25(3):559-569. https://pubmed.ncbi.nlm.nih.gov/15888608/
Expert Panel on Musculoskeletal Imaging; Czuczman GJ, Mandell JC, Wessell DE, et al. ACR Appropriateness Criteria® Inflammatory Back Pain: Known or Suspected Axial Spondyloarthritis: 2021 Update. J Am Coll Radiol. 2021;18(11S):S340-S360. https://pubmed.ncbi.nlm.nih.gov/34794593/