Surface Topography and Radiographic Spinal Damage in Radiographic Axial Spondyloarthritis

Radiographic axial spondyloarthritis (r-axSpA) causes progressive structural damage of the spine, conventionally quantified on lateral cervical and lumbar radiographs using the modified Stoke Ankylosing Spondylitis Spine Score (mSASSS). Repeated radiography entails cumulative radiation exposure, substantial inter-reader variability and low sensitivity in early disease. DIERS Formetric 4D (four-dimensional) surface topography (DIERS International GmbH, Schlangenbad, Germany) provides radiation-free three-dimensional modelling of the spine and pelvis. The association between sagittal balance and structural damage has been demonstrated with a radiation-based biplanar low-dose radiography system (EOS imaging, Paris, France), but whether a radiation-free topographic method reflects the same association has not been investigated. This single-centre, cross-sectional observational study will enrol 53 adults with r-axSpA fulfilling the Assessment of SpondyloArthritis international Society (ASAS) criteria with radiographic sacroiliitis according to the modified New York criteria. At a single study visit, static DIERS Formetric 4D measurements, chest expansion, the Bath Ankylosing Spondylitis Metrology Index (BASMI), the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), the Bath Ankylosing Spondylitis Functional Index (BASFI) and the Ankylosing Spondylitis Disease Activity Score with C-reactive protein (ASDAS-CRP) are obtained. mSASSS is scored by a single reader, blinded to clinical and topographic data, on existing lateral cervical and lumbar radiographs obtained within the preceding 12 months; no additional radiographs are performed for the study. The primary analysis is the correlation between total mSASSS and the DIERS flèche cervicale (cervical sagittal anterior displacement).

Inclusion Criteria: * Age 18 to 65 years * Diagnosis of radiographic axial spondyloarthritis fulfilling the Assessment of SpondyloArthritis international Society (ASAS) criteria, confirmed by radiographic sacroiliitis according to the modified New York criteria * Able to provide written informed consent and to complete the assessments * Available lateral cervical and lumbar radiographs obtained within the 12 months preceding the DIERS measurement Exclusion Criteria: * Neurological disease affecting balance, posture or gait (for example Parkinson disease, amyotrophic lateral sclerosis, cerebellar or vestibular disorders, history of stroke) * History of lower extremity joint arthroplasty (hip or knee) * Orthopaedic conditions affecting posture, balance or gait (ununited fracture, previous spinal or lower extremity surgery) * Advanced cardiopulmonary disease, active infection or malignancy * Pregnancy