Development of a Radiographic Scoring Tool for Ankylosing Spondylitis Only Based on Bone Formation: Addition of the Thoracic Spine Improves Sensitivity to Change

Objective. The modified Stokes Ankylosing Spondylitis Spinal Score (mSASSS) quantifies radiographic changes in the cervical spine (C-spine) and the lumbar spine (L-spine), but not in the thoracic spine (T-spine). Our objective was to study the contribution of the lower part of the T-spine to structural damage in patients with ankylosing spondylitis (AS). Methods. Radiographs of 80 AS patients obtained at baseline and after 2 years were scored by 2 readers using the mSASSS. In addition, changes in the lower T-spine (T10-T12) were quantified. On this basis, a new scoring tool was developed: the Radiographic Ankylosing Spondylitis Spinal Score (RASSS). The RASSS includes 2 changes: no scoring of erosions in order to confine the scoring to new bone formation, and no scoring of squaring in the C-spine for anatomic and feasibility reasons. Results. The mean +/- SD change was 0.9 +/- 2.5 units using the mSASSS and 1.6 +/- 2.8 units using the RASSS (P < 0.001). Although the mSASSS identified new syndesmophytes in mean +/- SD 1.4 +/- 2.9 vertebral edges over 2 years, an additional 0.6 +/- 1.2 vertebral edges were seen in the lower T-spine. New syndesmophytes or ankylosis were found in 15 patients (21.4%; 95% confidence interval [95% CI] 13.1-32.4%) in the C-spine/L-spine and in 6 patients (8.6%; 95% CI 3.8-17.2%) in the T-spine alone. The reliability of the RASSS and the agreement between readers was excellent. Conclusion. The lower T-spine improves the sensitivity to change of scoring radiographic progression in AS. The tool developed in this study, the RASSS, showed better face and content validity than the mSASSS and was proven to be superior in the quantification of new bone formation in AS.