Surface Topography and Radiographic Spinal Damage in Radiographic Axial Spondyloarthritis

Design: single-centre, cross-sectional, non-interventional correlation study in a single patient group; no control group. All participant data are collected in one assessment session lasting approximately 45 to 60 minutes.

Measurements: (a) demographic and anthropometric data; (b) disease duration, age at symptom onset, comorbidities, extra-articular involvement (uveitis, inflammatory bowel disease, psoriasis), HLA-B27 status, current medication category; (c) radiographic sacroiliitis grade according to the modified New York criteria and mSASSS (range 0 to 72), scored independently by two blinded readers with inter-reader agreement reported as the intraclass correlation coefficient; (d) chest expansion; (e) BASDAI, BASFI and ASDAS-CRP, the latter using routine C-reactive protein values with no additional blood sampling; (f) BASMI total score and its five components; (g) DIERS Formetric 4D static parameters: fleche cervicale (cervical sagittal anterior displacement, mm), sagittal imbalance (mm and degrees), coronal imbalance (mm), pelvic obliquity (mm), pelvic torsion (degrees), cervical and lumbar depth distance (mm), kyphotic angle (degrees) and lordotic angle (degrees).

Statistical analysis: distribution is assessed with the Shapiro-Wilk test. The primary analysis is a Pearson or Spearman correlation between total mSASSS and fleche cervicale, reported with effect size and 95% confidence interval, followed by multiple linear regression adjusted for age and disease duration. Secondary analyses comprise correlations of DIERS parameters with clinical indices and an exploratory receiver operating characteristic analysis of the ability of DIERS sagittal parameters to predict prespecified mSASSS thresholds. The Benjamini-Hochberg procedure is applied to correct for multiple comparisons. Significance is set at p<0.05. Analyses are performed with IBM SPSS version 23 or later.

Sample size: calculated with G*Power 3.1.9 for a bivariate normal correlation model, two-tailed, rho H0 = 0, expected effect size r = 0.40, alpha = 0.05 and power = 0.85, giving a required sample of 53 participants (actual power 0.857). The effect size was set conservatively at r = 0.40 rather than the r = 0.61 reported by Wanders et al.

(2005) for occiput-to-wall distance versus mSASSS, because DIERS parameters are not identical to clinical mobility measures.

Reporting follows the STROBE statement.

• DIERS Formetric 4D surface topography (Diagnostic test): Radiation-free three-dimensional assessment of spinal and pelvic surface topography performed once in standing position. No radiation, no contact, no intervention on treatment.

Primary outcomes

• Correlation between total mSASSS and DIERS fleche cervicale [Single assessment session (Day 1); mSASSS scored on radiographs obtained within the 12 months preceding the assessment]: Total modified Stoke Ankylosing Spondylitis Spine Score (mSASSS; range 0 to 72, higher scores indicate greater structural damage), calculated as the mean of two independent blinded readers, correlated with fleche cervicale (cervical sagittal anterior displacement, mm) measured by DIERS Formetric 4D surface topography. Reported as a correlation coefficient with magnitude, direction and 95% confidence interval.

Secondary outcomes

• Correlation between total mSASSS and other DIERS Formetric static parameters [Single assessment session (Day 1); mSASSS scored on radiographs obtained within the 12 months preceding the assessment]: Total mSASSS (range 0 to 72, higher scores indicate greater structural damage) correlated with the remaining DIERS Formetric 4D static parameters: sagittal imbalance (mm and degrees), coronal imbalance (mm), pelvic obliquity (mm), pelvic torsion (degrees), cervical depth distance (mm), lumbar depth distance (mm), kyphotic angle (degrees) and lordotic angle (degrees). Reported as correlation coefficients with 95% confidence intervals.
• Correlation between DIERS sagittal parameters and BASDAI [Single assessment session (Day 1)]: Bath Ankylosing Spondylitis Disease Activity Index (range 0 to 10, higher scores indicate greater disease activity) correlated with DIERS sagittal parameters.
• Correlation between DIERS sagittal parameters and BASFI [Single assessment session (Day 1)]: Bath Ankylosing Spondylitis Functional Index (range 0 to 10, higher scores indicate worse physical function) correlated with DIERS Formetric 4D static parameters: fleche cervicale (mm), sagittal imbalance (mm and degrees), coronal imbalance (mm), pelvic obliquity (mm), pelvic torsion (degrees), cervical and lumbar depth distance (mm), kyphotic angle (degrees) and lordotic angle (degrees).
• Correlation between DIERS sagittal parameters and BASMI [Single assessment session (Day 1)]: Bath Ankylosing Spondylitis Metrology Index (range 0 to 10, higher scores indicate worse spinal mobility) correlated with DIERS Formetric 4D static parameters: fleche cervicale (mm), sagittal imbalance (mm and degrees), coronal imbalance (mm), pelvic obliquity (mm), pelvic torsion (degrees), cervical and lumbar depth distance (mm), kyphotic angle (degrees) and lordotic angle (degrees).
• Correlation between DIERS sagittal parameters and ASDAS-CRP [Single assessment session (Day 1)]: Ankylosing Spondylitis Disease Activity Score with C-reactive protein (higher scores indicate greater disease activity) correlated with DIERS Formetric 4D static parameters: fleche cervicale (mm), sagittal imbalance (mm and degrees), coronal imbalance (mm), pelvic obliquity (mm), pelvic torsion (degrees), cervical and lumbar depth distance (mm), kyphotic angle (degrees) and lordotic angle (degrees).
• Correlation between DIERS sagittal parameters and chest expansion [Single assessment session (Day 1)]: Chest expansion measured in centimetres correlated with DIERS Formetric 4D static parameters: fleche cervicale (mm), sagittal imbalance (mm and degrees), coronal imbalance (mm), pelvic obliquity (mm), pelvic torsion (degrees), cervical and lumbar depth distance (mm), kyphotic angle (degrees) and lordotic angle (degrees).
• Discriminative performance of DIERS sagittal parameters for mSASSS thresholds [Single assessment session (Day 1); mSASSS scored on radiographs obtained within the 12 months preceding the assessment]: Exploratory receiver operating characteristic analysis of DIERS Formetric 4D sagittal parameters for discriminating prespecified total mSASSS thresholds. Reported as area under the curve with 95% confidence interval, sensitivity and specificity.

Sex: All, ages: 18 Years to 65 Years

Inclusion Criteria:

* Age 18 to 65 years
* Diagnosis of radiographic axial spondyloarthritis fulfilling the 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

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• Wanders A, Landewe R, Dougados M, Mielants H, van der Linden S, van der Heijde D. Association between radiographic damage of the spine and spinal mobility for individual patients with ankylosing spondylitis: can assessment of spinal mobility be a proxy for radiographic evaluation? Ann Rheum Dis. 2005 Jul;64(7):988-94. doi: 10.1136/ard.2004.029728. PMID 15958757
• Turk E, Yurdakul FG, Guler T, Bodur H. Posture, balance and gait in axial spondyloarthritis: a case-control study. Rheumatol Int. 2024 Nov;44(11):2527-2538. doi: 10.1007/s00296-024-05710-5. Epub 2024 Sep 4. PMID 39230687

• Ajda Bal, Principal investigator, Professor of Physical Medicine and Rehabilitation