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<0.01). The sensitivity and specificity of backfill, erosion, and fat deposition for diagnosing axial SpA were (0.58, 0.98), (0.81, 0.88), and (0.88, 0.81), respectively. The positive LR of using backfill to diagnose axial SpA was quite high (32.6), while it was moderate for erosion (7.0) and fat deposition (4.6).
Conclusions Backfill is more important than erosion and fat deposition in diagnosing axial spondyloarthritis as seen on MRI.
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Disclosure of Interest None declared