Fat Metaplasia and Backfill Are Key Intermediaries in the Development of Sacroiliac Joint Ankylosis in Patients With Ankylosing Spondylitis

Abstract

Objective. Fat metaplasia in bone marrow on T1-weighted magnetic resonance imaging (MRI) scans may develop after resolution of inflammation in patients with ankylosing spondylitis (AS) and may predict new bone formation in the spine. Similar tissue, termed backfill, may also fill areas of excavated bone in the sacroiliac (SI) joints and may reflect resolution of inflammation and tissue repair at sites of erosions. The purpose of this study was to test our hypothesis that SI joint ankylosis develops following repair of erosions and that tissue characterized by fat metaplasia is a key intermediary step in this pathway.Methods. We used the Spondyloarthritis Research Consortium of Canada (SPARCC) SI structural lesion score (SSS) method to assess fat metaplasia, erosions, backfill, and ankylosis on MRIs of the SI joints in 147 patients with AS monitored for 2 years. Univariate and multivariate regression analyses focused first on identifying significant MRI predictors of new backfill and fat metaplasia. We then assessed the role of backfill and fat metaplasia in the development of new ankylosis. All analyses were adjusted for demographic features, treatment, and baseline and 2-year change in SSS values for parameters of inflammation and MRI structural lesions.Results. Resolution of inflammation and reduction of erosions were each independently associated with the development of new backfill and fat metaplasia at 2 years on multivariate analyses. Multivariate regression analysis that included demographic features, baseline and 2-year change in parameters of inflammation and MRI structural lesion showed that reduction in erosions (P ‫؍‬ 0.0005) and increase in fat metaplasia (P ‫؍‬ 0.002) at 2 years was each independently associated with the development of new ankylosis.Conclusion. Our data support a disease model whereby ankylosis develops following repair of erosions, and fat metaplasia and backfill are key intermediary steps in this pathway.Spondyloarthritis (SpA) is an inflammatory disorder primarily of the axial spine that typically begins with sacroiliitis. The first sign of disease on radiography is the appearance of erosion of the subchondral bone of the ilium, which is seen as a blurred cortical outline. Progression results in more extensive erosion involving the sacral bone and the appearance of pseudowidening. Subsequent features include bone sclerosis and, finally, ankylosis across the joint. Magnetic resonance imaging (MRI) constitutes a major advance in the field through its ability to demonstrate active inflammation on fatsuppressed sequences, such as short tau inversion recovery (STIR), and structural lesions, such as fat metaplasia, erosions, and ankylosis, on T1-weighed sequences (1).

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“…Следует отметить, что помимо существования и вы-раженности отека КМ на прогрессирование влияет ста-дия, в которой подавлено воспаление [6,[27][28][29]. По дан-ным ряда работ, оптимально воздействие на структурные повреждение на самых ранних стадиях -до формирова-ния жирового перерождения и эрозий [29,30].…”

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“…The new SSS method 3 potentially provides a higher sensitivity to change and additionally allows a detailed assessment of the temporal and spatial relation between the individual pathologies. The system has already been used to demonstrate that backfill and fat infiltration are predictors of development of ankylosis 9 and that TNF-inhibitor therapy is an independent predictor of resolution of erosion 10 and development of ankylosis 11 .…”