Scoring radiographic progression in ankylosing spondylitis: should we use the modified Stoke Ankylosing Spondylitis Spine Score (mSASSS) or the Radiographic Ankylosing Spondylitis Spinal Score (RASSS)?

Introduction: Radiographic damage is one of the core outcomes in axial SpA and is usually assessed with the modified Stoke Ankylosing Spondylitis (AS) Spine Score (mSASSS). Alternatively, the Radiographic AS Spinal Score (RASSS) is proposed, which includes the lower thoracic vertebrae, under the hypothesis that most progression occurs in these segments. We aimed to compare the mSASSS and RASSS with regard to performance. Methods: Two-yearly spinal radiographs from patients followed in the Outcome in AS International Study (OASIS) were used (scored independently by two readers). A total of 195 patients had at least one radiograph (12-year follow-up) to be included. We assessed the accessibility of vertebral corners (VCs) for scoring, as well as status and 2-year progression scores of both scoring methods. To assess the potential additional value of including the thoracic segment in the score, the relative contribution (in %) to the 2-year total RASSS progression of each spinal segment (cervical, thoracic and lumbar) was determined, and compared to the expected contribution, under the assumption that a balanced segmental progression would occur, proportional to the number of sites per segment. Results: The mSASSS could be scored in a total of 809 radiographs and the RASSS in 78% of these. In 58% of the latter, the score was based on one to two available thoracic VCs scores, and the remaining two to three were imputed because they were missing. There were 520 two-year mSASSS intervals available, and in 63% of them RASSS progression could be assessed. The mean (SD) 2-year interval progression score (330 intervals) was 2.0 (3.6) for the mSASSS and 2.4 (4.4) for the RASSS, yielding a similar effect size (mSASSS 0.57 and RASSS 0.55). Exclusive progression of the thoracic segment occurred in only 5% of the cases. There was no significant difference between the observed (14%) and expected (16%) contribution to progression of the thoracic segment (P = 0.70). Conclusions: The determination of RASSS for radiographic damage of the spine is frequently impossible or strongly influenced by non-contributory imputation. In comparison to the mSASSS, the contribution of thoracic VCs in the RASSS method is negligible, and does not justify the additional scoring efforts.