Background Fat metaplasia is detected as bright signal on T1W MRI and has been shown to develop after resolution of inflammation in spine and sacroiliac joints (SIJ). Tissue with bright signal on T1W MRI, termed backfill (BF), may also fill areas of excavated sacral and iliac bone along the joint space and is thought to reflect repair of erosion. Imaging studies in the spine have indicated that fat metaplasia predicts development of new bone.
Objectives To test the hypothesis that ankylosis in the SIJ develops following repair of erosion and that fat metaplasia is a key intermediary step in this pathway.
Methods We used the SPARCC SIJ Structural Score (SSS) method to assess fat metaplasia (FAT), erosion (ER), BF, and ankylosis (ANK). This score relies on the T1W sequence and assesses 5 consecutive coronal slices anteriorly through the cartilaginous portion of the joint from the transitional slice. Lesions are scored dichotomously (present/absent) in SIJ quadrants (fat, erosion) or halves (backfill, ankylosis). Scoring ranges are: FAT (0-40), ER (0-40), BF (0-20), ANK (0-20). Four readers independently assessed 45 pairs of MRI scans blinded to time point (baseline, 2 years) from 45 cases in a prospective cohort receiving either standard (n=22) or anti-TNF (n=23) therapies. In a second study, two readers independently assessed 147 pairs of scans blinded to time point (baseline, 2 years) from cases on standard (n=69) or anti-TNF (n=78) therapies. Correlations between change in SSS scores were analyzed using Pearson chi-square. Univariate and multivariate regression analyses focused on identifying significant MRI predictors of change in ANK scores, adjusted for age, sex, symptom duration, treatment, CRP (baseline and 2-year change), SPARCC SIJ inflammation score (baseline and 2-year change), and baseline SSS scores for FAT, ER, BF, and ANK.
Results Using mean SSS scores for 4 readers in the 45 cases, resolution of ER was significantly correlated with the development of new ANK (p=0.045) at 2 years. Using mean scores of two readers in the 147 cases, resolution of erosion was significantly correlated with the development of fat metaplasia (p<0.0001) and new ANK (p=0.0001) at 2 years. New fat metaplasia was also significantly correlated with development of ankylosis (p=0.0005). Significant predictors of new ankylosis in univariate analyses were: SPARCC SIJ baseline (β=0.04, p=0.03), Baseline BF (β=0.09, p=0.002), Erosion 2-year change (β=-0.19, p<0.0001), Baseline fat (β=0.05, p=0.007), Fat 2-year change (β=0.29, p<0.0001). Significant independent predictors of new ANK in the multivariate model (adjusted R2 =0.28, F ratio =14.4, p<0.0001) were 2-year change in BF score, decreased erosion score and development of new fat metaplasia (Table).
Conclusions Ankylosis in the SIJ develops following repair of erosion and fat metaplasia is a key intermediary step in this pathway.
Disclosure of Interest : None declared