Ankylosing spondylitis (AS) is a chronic inflammatory spondyloarthropathy that progressively affects the axial skeleton, starting from the sacroiliac joints. It has a strong association with the HLA-B27 antigen (90-95% carriage). The disease starts with sacroiliitis and extends upward to the lumbar, thoracic, and finally cervical spine. Characteristic findings include bilateral symmetric sacroiliitis, syndesmophytes (thin vertical ossification of the annulus fibrosus), bamboo spine (multilevel fusion), squared vertebra (corner erosions following Romanus lesion), and Andersson lesion (disco-vertebral destruction). Chalk-stick fractures occurring with minimal trauma in the ankylosed spine can be life-threatening. Prevalence in the general population is approximately 0.1-0.5%, with a 2-3:1 male predominance in younger patients.
Age Range
15-45
Peak Age
25
Gender
Male predominant
Prevalence
Uncommon
The fundamental pathology of AS begins with enthesitis (inflammation of tendon and ligament bone attachment points). Synovitis and subchondral bone marrow inflammation in the sacroiliac joints is the first stage — on MRI, this appears as bone marrow edema (STIR hyperintensity) around the SI joints because inflammatory infiltrate and edema contain water. Cytokines released during inflammation (TNF-alpha, IL-17, IL-23) lead to osteoblast activation, and the healing process paradoxically causes heterotopic bone formation (syndesmophytes). The Romanus lesion appears as bone marrow edema at vertebral corners due to anterior longitudinal ligament enthesitis — seen on CT as reactive sclerosis (shiny corner) because increased osteoblastic activity raises bone density. Progressive ossification of the annulus fibrosus creates thin vertical bony bridges (syndesmophytes). Confluence of syndesmophytes at multiple levels creates the bamboo spine appearance. Facet joint, costovertebral joint, and interspinous ligament fusion transforms the spine into a rigid structure. This rigid structure loses flexion resistance and becomes prone to transverse fractures (chalk-stick) with minimal trauma — similar to a long bone fracture mechanism.
The most recognized imaging sign of advanced AS. The spine acquires a segmental appearance resembling a bamboo cane due to confluence of syndesmophytes at multiple consecutive levels, longitudinal ligament ossification, and facet joint ankylosis. Most prominent on CT coronal (frontal) reformat. This finding is nearly pathognomonic for AS and is distinguished from DISH by thin symmetric syndesmophytes and facet joint involvement.
Bilateral symmetric bone marrow edema around the sacroiliac joints on STIR sequence. Subchondral hyperintense signal changes on iliac and sacral sides. Indicates active inflammation and is sufficient for MRI-positive sacroiliitis diagnosis according to ASAS criteria (bone marrow edema in at least two consecutive slices or two different SI joint quadrants).
Report Sentence
Bilateral symmetric subchondral bone marrow edema (STIR hyperintense signal) in the sacroiliac joints is identified, consistent with active sacroiliitis; should be evaluated with a preliminary diagnosis of ankylosing spondylitis.
Thin vertical bony bridges (syndesmophytes) developing along the outer fibers of the annulus fibrosus. Marginal syndesmophytes originating from vertebral corners and extending to the adjacent vertebra are characteristic. Distinguished from coarse, flowing anterolateral ossification of DISH by their thin and symmetric structure. First appear at thoracolumbar and lumbosacral junctions.
Report Sentence
Thin symmetric syndesmophytes originating from vertebral corners at multiple levels in the thoracolumbar/lumbar spine are identified, consistent with chronic changes of ankylosing spondylitis.
In advanced AS, confluence of syndesmophytes at multiple consecutive levels, anterior and posterior longitudinal ligament ossification, facet joint ankylosis, and interspinous ligament ossification result in the spine appearing as a single bone piece. CT frontal reformat shows a segmental appearance resembling a bamboo cane. Movement is completely lost.
Report Sentence
Bamboo spine appearance resulting from confluence of syndesmophytes at multiple consecutive levels and facet joint ankylosis in the thoracic/thoracolumbar spine is identified, consistent with advanced ankylosing spondylitis.
Romanus lesion (shiny corner sign): Inflammatory bone marrow edema at the anterosuperior or anteroinferior vertebral corner due to anterior longitudinal ligament enthesitis. In acute phase, hyperintense on MRI STIR, hypointense on T1. In chronic phase, reactive sclerosis (shiny corner — high density on CT) and squared vertebra formation. Loss of normal anterior concavity due to vertebral corner erosions.
Report Sentence
Inflammatory bone marrow edema (STIR hyperintense) / reactive sclerosis consistent with Romanus lesion at lumbar/thoracic vertebral corners is identified, evaluated as an active/chronic ankylosing spondylitis finding.
Transverse fracture occurring after minimal trauma in the ankylosed spine. The fracture line may cross all 3 columns (anterior, middle, posterior) and may be unstable. Risk of epidural hematoma and spinal cord injury is high. CT shows a transverse fracture line in the vertebral body, posterior elements, or both. May occur through disc level or vertebral body.
Report Sentence
A transverse fracture line crossing all 3 columns (chalk-stick fracture) at T___/L___ level in the ankylosed spine is identified, requiring urgent evaluation for instability and spinal cord injury.
Criteria
MRI-positive sacroiliitis present but no structural damage on conventional radiography. Axial spondyloarthropathy can be diagnosed by MRI according to ASAS criteria.
Distinct Features
Bone marrow edema around SI joints on MRI STIR (bilateral symmetric), minimal or no structural erosion. Symptom duration usually <5 years. Early initiation of biologic therapy slows progression.
Criteria
Bamboo spine, facet joint ankylosis, SI joint fusion. Complete loss of spinal mobility. Restrictive ventilatory pattern in thorax.
Distinct Features
High risk of chalk-stick fracture. Unstable fractures, epidural hematoma, and spinal cord injury may occur after minimal trauma. Surgical stabilization is challenging.
Criteria
Inflammatory or mechanical disco-vertebral destruction at disc level in the ankylosed spine. Endplate edema and disc signal changes on MRI, endplate irregularity and sclerosis on CT.
Distinct Features
Pseudoarthrosis-like appearance. Must be distinguished from infectious spondylodiscitis (AS history + stable clinical course + negative infection markers). May develop from stress fracture or active inflammation.
Distinguishing Feature
DISH preserves disc spaces and facet joints, no sacroiliitis, coarse anterior ossification; AS has thin symmetric syndesmophytes, facet joint involvement, bilateral sacroiliitis present
Distinguishing Feature
OPLL has isolated posterior ligament ossification, no sacroiliitis; AS has anterior and posterior ossification, bilateral sacroiliitis, syndesmophytes present
Distinguishing Feature
Spondylolysis has isolated pars defect, young athlete; AS has diffuse spinal involvement, sacroiliitis, syndesmophytes
Urgency
routineManagement
medicalBiopsy
Not NeededFollow-up
6-monthAS is a chronic progressive disease, and early diagnosis with biologic therapy (anti-TNF agents: adalimumab, infliximab; IL-17 inhibitors: secukinumab) slows progression. MRI is the gold standard for early sacroiliitis diagnosis and ASAS criteria are applied. Chalk-stick fractures in the ankylosed spine require urgent evaluation — full-spine CT is recommended even after minimal trauma. Regular rheumatology and radiological follow-up (MRI every 6-12 months) is recommended.
Ankylosing spondylitis is a chronic progressive disease. Early diagnosis and biologic therapy (anti-TNF, IL-17 inhibitors) can slow progression. MRI is the gold standard for early sacroiliitis diagnosis. Chalk-stick fractures occurring with minimal trauma in the ankylosed spine can be life-threatening — requiring urgent CT and neurological evaluation.