Characteristic Spine Features of Ankylosing Spondylitis

Structural Radiographic Features

The American College of Radiology notes that ankylosing spondylitis is characterized by the formation of vertical bony bridges, known as syndesmophytes, between vertebral bodies, representing ossification of the outer fibers of the annulus fibrosus 4, 5.
Vertebral body squaring occurs due to erosion and new bone formation at the anterior corners, as reported by the American College of Radiology 4.
The presence of shiny corners, representing erosions at the vertebral body corners, is a characteristic feature of ankylosing spondylitis, according to the American College of Radiology 4.
Spinal ankylosis leads to the classic "bamboo spine" appearance, resulting from extensive ligamentous ossification and vertebral joint fusion, as described by the Arthritis and Rheumatology society 1, 3.
Erosions at vertebral endplates and facet joints are destructive changes that occur in ankylosing spondylitis, as noted by the American College of Radiology 4, 5.

The Assessment of SpondyloArthritis international Society (ASAS) considers spine MRI positive for axial spondyloarthritis if three or more sites of inflammatory spondylitis are present, with a sensitivity of 69% and specificity of 94% for ankylosing spondylitis, as reported by the American College of Radiology 4, 5, 6.
Corner inflammatory lesions (spondylitis) have a high specificity of 94% for ankylosing spondylitis, according to the American College of Radiology 6, 5.
Spondylodiscitis, or inflammatory changes at the discovertebral junction, is a feature of ankylosing spondylitis, as noted by the American College of Radiology 6, 5.
Lateral vertebral inflammatory lesions demonstrate high specificity of 97% for ankylosing spondylitis, as reported by the American College of Radiology 6, 5.
Fatty corner lesions have 98% specificity for ankylosing spondylitis and can be useful even in the absence of active inflammatory lesions, according to the American College of Radiology 4, 6, 5.

Sacroiliac joint involvement is universal in ankylosing spondylitis, forming the core of the modified New York classification criteria, as described by the Arthritis and Rheumatology society 1, 2, 3, 7.
Spinal involvement patterns show that 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, as reported by the American College of Radiology 5.
In early axial spondyloarthritis, 28.3% of patients show inflammation involving both sacroiliac joints and spine, according to the American College of Radiology 6, 5.

Loss of spinal flexibility occurs due to progressive ankylosis, which eliminates normal spinal motion, as described by the Arthritis and Rheumatology society 1, 3.

Degenerative mimics can occur in degenerative changes, so MRI findings must be interpreted with attention to associated morphologic findings, as noted by the American College of Radiology 4, 6, 5.
Contrast enhancement is not necessary for diagnosis, as noncontrast and contrast-enhanced MRI have overall similar diagnostic utility for axial spondyloarthritis, according to the American College of Radiology 4, 6, 5.
Correlation with symptoms is important, as imaging the spine in the clinical region of interest is beneficial when referable symptoms are present, as reported by the American College of Radiology 6, 5.
Time to radiographic changes can take years to develop, which can exclude patients early in the disease course from meeting classification criteria, as described by the Arthritis and Rheumatology society 1, 2, 3, 7.