Magnetic Resonance Imaging in Axial Spondyloarthritis: Beyond the Sacroiliac Joints—Focus on Facet Joint Involvement

Background: Axial spondyloarthritis (axSpA) is a chronic inflammatory disease of the sacroiliac joints (SIJs) and spine, encompassing non-radiographic axSpA (nr-axSpA) and radiographic axSpA (r-axSpA) or ankylosing spondylitis (AS). Early diagnosis is challenging, particularly in nr-axSpA, due to the absence of radiographic sacroiliitis and overlap with nonspecific back pain. Imaging plays a pivotal role in disease classification and monitoring. Objective: To synthesize current evidence on inflammatory and structural facet joint involvement in axSpA, with particular emphasis on MRI characterization, available scoring approaches, differentiation from degenerative abnormalities, and the current limitations of facet joint assessment as a potential imaging biomarker. Methods: A review of current evidence regarding imaging modalities in axSpA, including conventional radiography, computed tomography (CT), MRI, radionuclide techniques, and musculoskeletal ultrasound (US), was performed. Particular emphasis was placed on ASAS/OMERACT MRI definitions and facet joint assessment. Results: MRI enables early detection of inflammatory lesions such as bone marrow edema (BME), spondylitis, and enthesitis before structural damage becomes visible on radiographs. It also identifies structural changes including fat metaplasia, erosions, syndesmophytes, and ankylosis. Increasing evidence supports the clinical relevance of facet and costovertebral joint inflammation, which correlates with impaired spinal mobility and functional disability. However, inflammatory facet joint changes remain underreported in routine practice despite the development of semiquantitative scoring systems. Ultrasound complements MRI in assessing enthesitis but is limited in evaluating deep axial structures. Conclusions: MRI has become indispensable in axSpA diagnosis and monitoring. Standardized imaging definitions and structured reporting are essential to improve clinical integration and patient outcomes.