Characteristic Spine Features of Ankylosing Spondylitis
Ankylosing spondylitis is characterized by sacroiliitis, enthesitis, and a marked propensity for sacroiliac joint and spinal fusion, with universal involvement of sacroiliac joint inflammation or fusion and prevalent spinal ankylosis distinguishing it from other spondyloarthropathies. 1
Structural Radiographic Features
The spine in AS demonstrates several distinctive structural changes that develop over time:
Syndesmophytes: These are vertical bony bridges that form between vertebral bodies, representing ossification of the outer fibers of the annulus fibrosus 1. New syndesmophytes develop in approximately one-third of patients over 2 years, with male sex, elevated C-reactive protein, and preexisting syndesmophytes being consistent predictors of progression 2.
Vertebral body squaring: Loss of the normal anterior concavity of vertebral bodies occurs due to erosion and new bone formation at the anterior corners 1.
Shiny corners: These represent erosions at the vertebral body corners visible on radiographs 1.
Spinal ankylosis: Progressive fusion of vertebral segments leads to the classic "bamboo spine" appearance, resulting from extensive ligamentous ossification and vertebral joint fusion 1, 3.
Erosions: Destructive changes at vertebral endplates and facet joints 1.
Inflammatory MRI Features
MRI reveals active inflammatory changes that precede structural damage:
Corner inflammatory lesions (spondylitis): The presence of two or more corner inflammatory lesions has 69% sensitivity and 94% specificity for AS 1. The Assessment of SpondyloArthritis international Society (ASAS) considers spine MRI positive for axial spondyloarthritis if three or more sites of inflammatory spondylitis are present 1.
Spondylodiscitis: Inflammatory changes at the discovertebral junction 1.
Lateral vertebral inflammatory lesions: These demonstrate high specificity of 97% for AS 1.
Fatty corner lesions: Multiple fatty corner lesions have 98% specificity and can be useful even in the absence of active inflammatory lesions 1. The presence of at least 5 inflammatory lesions or 5 fatty lesions in the spine has 95% specificity for axial spondyloarthritis 1.
Enthesitis: Inflammation at vertebral ligamentous attachments, including costovertebral joints, costotransverse joints, and facet joints 1.
Distribution Patterns
Sacroiliac joint involvement: Universal in AS, forming the core of the modified New York classification criteria 1.
Spinal involvement patterns: In active axial spondyloarthritis, 99% of patients have active inflammatory lesions in the axial skeleton, with 52% having isolated sacroiliac joint changes, 5% having isolated spine changes, and 41% having both 1.
Early disease: In early axial spondyloarthritis, 28.3% of patients show inflammation involving both sacroiliac joints and spine 1.
Pathological Consequences
The extensive remodeling of the spinal axis creates several clinical complications:
Osteoporosis: Develops alongside new bone formation, creating a paradoxical weakened vertebral column 3.
Kyphotic deformities: Progressive spinal fusion leads to fixed kyphosis, particularly affecting the cervical and thoracic spine 4, 3.
Increased fracture susceptibility: The ankylosed spine becomes highly susceptible to fractures even with minor trauma, with fractures being highly unstable and frequently missed on initial imaging 3.
Loss of spinal flexibility: Progressive ankylosis eliminates normal spinal motion 1.
Important Clinical Pitfalls
Degenerative mimics: Both inflammatory and fatty lesions can occur in degenerative changes, so MRI findings must be interpreted with attention to associated morphologic findings such as disc degeneration or osteophytes that favor degenerative rather than inflammatory etiology 1.
Contrast enhancement: While contrast-enhanced MRI can demonstrate active inflammation, multiple studies show noncontrast and contrast-enhanced MRI have overall similar diagnostic utility for axial spondyloarthritis 1.
Correlation with symptoms: There is disagreement about the extent to which inflammatory changes in the axial skeleton correspond with symptoms, though imaging the spine in the clinical region of interest is beneficial when referable symptoms are present 1.
Time to radiographic changes: Radiographic features may take years to develop, which can exclude patients early in the disease course from meeting classification criteria 1.