Background/Purpose: In axial SpA (axSpA), the computed tomography (CT) syndesmophyte score (CTSS) has shown to capture more spinal structural damage progression and to better correlate with spinal mobility than the modified Stoke Ankylosis Spondylitis Spine Score (mSASSS, the current gold standard) (Stal R et al, RMD Open 2023). Recently, a combined score of syndesmophyte and facet joint ankylosis (FJA) development has been proposed as a more sensitive instrument in capturing damage progression. We aimed to assess whether this combined score retains good correlation with spinal mobility compared to current measurement instruments.

Methods: Data from the Sensitive Imaging in Ankylosing Spondylitis cohort were used, including axSpA patients fulfilling the modified New York criteria and with ≥1 syndesmophyte. Conventional radiography (CR) and low-dose computed tomography (ldCT) of the spine were performed at baseline (BL) and scored by 3 (CR) or 2 (ldCT) central readers, blinded for time order. Syndesmophytes were assessed on CR and ldCT by the mSASSS and CTSS, respectively. FJA was evaluated on ldCT with the facet ankylosis score (FAS) on 19 facet joints (levels C5-T2 excluded due to poor visibility, range: 0-38). Three different versions of a combined score of FJA and syndesmophyte development (the CT facet ankylosis syndesmophyte spine score, CTFASSS) were proposed and computed by adding the CTSS and FAS while giving equal weight to both components (CTFASSS-A = CTSS + FAS), 50% more weight to FAS (CTFASSS-B = CTSS + 1.5xFAS) or 100% more weight to FAS (CTFASSS-C = CTSS + 2xFAS). At BL, the following spinal mobility measures were evaluated: occiput-to-wall distance (OWD), lateral spinal flexion (LSF), modified Schober test (mSchober), cervical rotation and chest expansion. The composite score BASMI was calculated. Correlations between each spinal structural damage score and spinal mobility were assessed at BL with Spearman’s rank correlation coefficients and compared. A cumulative probability plot was created to better illustrate correlations between CTFASSS and BASMI.

Results: Forty-six patients had data available for all structural damage scores and spinal mobility measures at BL: mean (SD) age of 47 (9.6) years, 87% male, 83% HLA-B27+. All versions of the CTFASSS showed substantial correlations with OWD (CTFASSS-A: rs 0.65; CTFASSS-B and CTFASSS-C: rs 0.66) and LSF (CTFASSS-A and CTFASSS-B: rs -0.72; CTFASSS-C: rs -0.73) and moderate correlations with the other measures, to a similar degree to that observed for CTSS, but better than mSASSS (Table 1). Notably, compared to mSASSS, FAS alone correlated better with OWD (rs 0.64 vs 0.50) and cervical rotation (rs -0.62 vs -0.49). However, no additional benefit was obtained with a CTFASSS score attributing more weight to FAS compared to the simple sum of the components (i.e. CTFASSS-A). Although substantial correlations were observed between all imaging scores and BASMI, better results were seen with CTSS and CTFASSS (Figure 1).

Conclusion: A composite score assessing syndesmophytes and FJA correlates with spinal mobility similarly well as CTSS but better than mSASSS, supporting the validity of the combined score in assessing spinal damage progression in axSpA.

Table 1. Spearman’s correlation between spinal structural damage scores and spinal mobility (at baseline, n=46)*

Figure 1. Cumulative probability plot representing the relationship between BASMI and CTFASSS-A

(BASMI: Bath Ankylosing Spondylitis Metrology Index; CTFASSS-A: Computed Tomography Facet Ankylosis Syndesmophyte Spine Score version A; CTSS: Computed Tomography Syndesmophyte Score; FAS: Facet Ankylosis Score)

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/a-combined-score-of-facet-joint-ankylosis-and-syndesmophyte-development-correlates-well-with-spinal-mobility-in-axial-spondyloarthritis/