FRI0399 Backfill Is More Important than Erosion and Fat Deposition in Diagnosing Axial Spondyloarthritis as Seen on MRI

Background The structural lesions in patients with axial spondyloarthritis (SpA) seen on magnetic resonance imaging (MRI) include erosion, fat deposition, backfill and ankylosis. Backfill is the new tissue fills in the excavated bone in the sacroiliac joint (SIJ) which can be seen on T1SE MRI. Recent studies have shown that backfill is a key intermediary step in the development of ankylosis in patients with axial SpA. But data comparing the diagnostic value of backfill and other lesions is rare. Objectives To record erosion, fat deposition and backfill seen on T1SE MRI and to evaluate whether backfill is more important than other lesions in diagnosing axial SpA. Methods Three readers investigated erosion, fat deposition and backfill seen on T1SE MRI scans from 423 patients with axial SpA and 224 subjects with non-specific back pain (NSBP) or healthy (classified as control group). The SPARCC SIJ Structural Score (SSS) method was used to assess these three kinds of lesion. The sensitivity, specificity and positive likelihood ratio (LR) of three kinds of lesion for diagnosing axial SpA were calculated. Results 246 axial SpA patients (58.2%) were recorded with backfill, 343 (81.1%) were recorded with erosion, and 372 (88.0%) were recorded with fat deposition. While in healthy controls, only 4 (1.8%) were seen with backfill, 26 (11.6%) were with erosion, and 43 (19.2%) were with fat deposition. The SSS score of backfill, erosion, and fat deposition were all much higher in axial SpA patients than in control group (backfill: 2.8 ± 1.6 vs. 0.1± 0.7; erosion: 18.4 ± 9.7 vs. 4.5 ± 6.3; fat deposition: 25.8 ±17.2 vs. 16.4 ± 18.1; all p Conclusions Backfill is more important than erosion and fat deposition in diagnosing axial spondyloarthritis as seen on MRI. References Weber U, Lambert RGW, Ostergaard M, et al. The diagnostic utility of magnetic resonance imaging in spondylarthritis. An international multicenter evaluation of one hundred eighty-seven subjects. Arthritis Rheum 2010;62:3048–58. Weber U, Pedersen SJ, Ostergaard M, et al. Can erosions on MRI of the sacroiliac joints be reliably detected in patients with ankylosing spondylitis? - A cross-sectional study. Arthritis Res Ther 2012; 14:R124. Maksymowych WP, Wichuk S, Chiowchanwasawakit P, et al. The spondylo arthritis research consortium of Canada MRI sacroiliac joint structural score: a method for reliable detection of structural progression. Arthritis Rheum 2013;65 (suppl): S1239. Maksymowych WP, Wichuk S, Chiowchanwisawakit P, et al. Fat Metaplasia and Backfill are Key Intermediaries in the Development of Sacroiliac Joint Ankylosis in Patients with Ankylosing Spondylitis. Arthritis Rheumatol 2014;66:2958–67. Disclosure of Interest None declared