Fat Metaplasia on MRI of the Sacroiliac joints is a Lead Indicator of Radiographic Progression in the Spine of Patients with Ankylosing Spondylitis

Maksymowych WP, Wichuk S, Chiowchanwisawakit P, Lambert RG, Pedersen SJ

Background/Purpose .  Fat metaplasia in the SIJ on MRI is an early feature of sacroiliitis and occurs both in subchondral bone marrow and also in the excavated area caused by erosion, when it is called backfill. Recent data has shown that these lesions are key intermediaries in the development of SIJ ankylosis.  We aimed to test the hypothesis that these lesions may also be lead indicators of new bone formation in the spine of patients with axial SpA. This could provide an important target for therapeutic intervention.

Methods . Bone marrow fat metaplasia and backfill were scored using the SPARCC MRI SIJ structural score (SSS) by two readers and an adjudicator using pre-specified rules for adjudication. 137 pairs of MRI scans blinded to time point (baseline, 2 years) were assessed from a prospective cohort of AS patients (mean age 40.5years, mean symptom duration16.9 years, 53% on anti-TNF) followed for mean 2.3 years. Two readers and an adjudicator independently scored pairs of radiographs (baseline, 2 years) from the same patients using the mSASSS.  Radiographic progression was compared in patients with and without positive SIJ MRI for fat metaplasia (SSS score ≥2 or <2) and the degree of SIJ fat metaplasia at baseline (absolute SSS score) was compared in patients with and without radiographic progression (mSASSS >0 or =0) using Mann-Whitney and cumulative probability. Multivariate regression analyses included variables significant in univariate analyses (age, sex, symptom duration, CRP, baseline mSASSS) and treatment.

Results . Radiographic progression was significantly greater in those with positive SIJ fat metaplasia (p=0.015) (figure), and especially in patients who only received non-biologic therapy (p=0.023). Baseline SSS SIJ fat metaplasia scores were significantly higher in those who developed radiographic progression compared to those without (1.58 vs 0.65, p=0.008).  Both positive SIJ MRI for fat metaplasia and the degree of fat metaplasia were significantly associated with radiographic progression in multivariate analyses (β=0.38 (p=0.005) and β=0.06 (p<0.0001) respectively). SSS score for backfill was also significantly associated with radiographic progression in multivariate analysis (β=0.04 (p=0.019)).

Conclusion . The appearance of fat metaplasia in SIJ subchondral bone marrow and/or at sites of erosion in the SIJ may identify AS patients at increased risk of radiographic progression in the spine.