Fat metaplasia on MRI of the sacroiliac joints increases the propensity for disease progression in the spine of patients with spondyloarthritis

OBJECTIVE: We tested the hypothesis that fat metaplasia on MRI of the sacroiliac joints (SIJ) increases the propensity for new bone formation in the spine of patients with spondyloarthritis. METHODS: We assessed baseline T1-weighted and short τ inversion recovery SIJ MRIs from patients in the Follow Up Research Cohort in Ankylosing Spondylitis (FORCAST). Radiographic progression was assessed using the modified Stoke Ankylosing Spondylitis Spine Score (mSASSS). Structural and inflammatory lesions were scored using the Spondyloarthritis Research Consortium of Canada (SPARCC) SIJ structural and SPARCC SIJ inflammation scores, respectively. Radiographic progression was compared in cases with and without definite MRI lesions (score ≥2 or <2) and the extent of MRI lesions at baseline was compared in patients with and without radiographic progression. The predictive capacity of MRI SIJ lesions for radiographic progression in the spine was assessed in univariate and multivariate regression analyses. RESULTS: The extent of MRI structural lesions in the SIJ at baseline was significantly greater in those patients who had spinal radiographic progression on follow-up (p=0.003, 0.02, 0.003 for fat metaplasia, backfill and ankylosis, respectively). Also, radiographic progression was significantly greater in cases with definite baseline SIJ ankylosis (p=0.008). In multivariate regression that included all types of MRI lesions and was adjusted for age, sex, symptom duration, duration of follow-up, CRP, baseline mSASSS and treatment, the extent of SIJ fat metaplasia and ankylosis at baseline were independently associated with radiographic progression. CONCLUSIONS: SIJ ankylosis and fat metaplasia but not inflammatory lesions increase the propensity for radiographic progression in the spine.