Baseline Severity of Sacroiliitis and Extensive Fat Metaplasia Predicts the Progression of Backfill at the Sacroiliac Joint in Patients With Axial Spondyloarthritis.

Objectives: Fat metaplasia in an erosion cavity, also known as backfill, is an essential intermediary in new bone formation in axial spondyloarthritis (axSpA) patients; however, the predictors of backfill progression are unknown. This longitudinal study aimed to assess the predictors of backfill progression in axSpA patients on magnetic resonance imaging (MRI).

Methods: Clinical and MRI data were collected at baseline and follow-up in 52 axSpA patients. Backfill progression was defined as the new or increased T1 hyperintensity within the SI joint space. Logistic regression analyses were performed to identify the predictors of the backfill progression.

Results: A total of 19 patients had "backfill" at baseline and 30 patients exhibited the backfill progression after follow-up. The mean disease duration and SPARCC scores at baseline were significantly different between patients with and without backfill progression (<0.001, =0.003, respectively). Patients with backfill progression had a higher frequency of backfill at baseline, a higher SSS score of fat metaplasia, and a higher SSS score of backfill than those without (=0.001, <0.001, and =0.002, respectively). A higher fat fraction value in the fat metaplasia area at the baseline was more frequent in patients with, than without, backfill progression (=0.019). In the univariate logistic regression analyses, a higher SPARCC score for inflammation and a higher SSS score for fat metaplasia at baseline were associated with backfill progression.

Conclusions: Severity of sacroiliitis and extensive fat metaplasia at baseline are predictors of the backfill progression in axSpA patients.