Ankylosing Spondylitis: AI-Powered MRI & X-ray Analysis

The Assessment of SpondyloArthritis International Society (ASAS) definition of active sacroiliitis on MRI requires bone marrow edema (osteitis) on STIR or T1 fat-saturated post-gadolinium sequences in a typical sacroiliac joint distribution — specifically in the subchondral bone of the iliac or sacral side of the joint. A single clearly positive quadrant on two consecutive slices, or two positive quadrants on a single slice, meets the definition. MRI can detect sacroiliitis years before structural damage becomes visible on plain radiographs, enabling early diagnosis in the non-radiographic axial spondyloarthritis (nr-axSpA) phase and initiation of biologic therapy. Structural lesions (fat metaplasia, erosions, sclerosis, ankylosis) represent chronic damage and are better seen on CT.

Spinal involvement in AS progresses from inflammatory erosion of vertebral corner Romanus lesions (shiny corner sign on MRI STIR or T1 gadolinium) to fatty metaplasia (Anderson lesions), then to syndesmophyte formation (vertical bony bridging of the annulus fibrosus) and ultimately complete intervertebral fusion. The bamboo spine describes complete fusion of the vertebral column with continuous vertically oriented syndesmophytes on AP radiograph, accompanied by fusion of the apophyseal joints (trolley-track sign) and posterior spinal ligament ossification. This rigid column is highly susceptible to fracture — even low-energy trauma can cause unstable three-column fractures, most commonly at C5–C7, which carry high risk of neurological deficit and must be evaluated with full-spine CT and MRI regardless of symptom severity.

HLA-B27 is present in 90–95% of patients with AS of Northern European descent, compared with 6–9% of the general population, making it a strong genetic risk factor but not diagnostic alone — approximately 80% of HLA-B27-positive individuals never develop AS. The modified New York criteria require grade 3–4 bilateral or grade 4 unilateral radiographic sacroiliitis plus at least one clinical feature (inflammatory back pain, reduced lumbar mobility, or reduced chest expansion). ASAS criteria for non-radiographic axSpA require sacroiliitis on MRI or HLA-B27 positivity plus two or more spondyloarthritis features. NSAIDs remain first-line for symptom control; continuous use may slow radiographic progression. TNF inhibitors (adalimumab, etanercept, certolizumab) and IL-17A inhibitors (secukinumab, ixekizumab) are recommended for patients with inadequate NSAID response, providing significant improvement in disease activity (BASDAI and ASDAS scores) and MRI inflammation scores.