title: Backfill is a specific sign of axial spondyloarthritis seen on MRI.
abstract: OBJECTIVE
  To summarize the characteristics of backfill in patients with axial spondyloarthritis (SpA) and patients with non-specific back pain (NSBP) and healthy controls, and to assess the value of backfill in diagnosing axial SpA.
  
  
  METHODS
  Three readers blinded recorded backfill seen on T1SE MRI scans from 647 subjects: 297 patients with ankylosing spondylitis (AS), 126 patients with non-radiographic axial SpA (nr-axSpA), 147 patients with NSBP, and 77 healthy controls. The SPARCC SIJ Structural Score (SSS) method was used to assess backfill. The changes of backfill were evaluated by the follow-up MRI scans from 157 patients. We summarized the characteristics of backfill and calculated its sensitivity and specificity for diagnosing axial SpA.
  
  
  RESULTS
  Backfill was recorded in 78.8% AS patients, 11.1% nr-axSpA patients, 1.8% patients with NSBP, and no healthy control. Backfill affected more frequently at ilium bone, lower half of sacroiliac joints in axial SpA (both P<0.05). The SSS score of backfill was much higher in axial SpA than in patients with NSBP (both P<0.01) and it did not correlate with demographics and BASDAI, BASFI, and CRP (all P>0.05). The score of backfill only positively correlated with symptom duration in AS (r=0.251, P<0.01) and in nr-axSpA (r=0.743, P<0.01) patients. Only 8.9% patients had the change of backfill in an average follow-up time of 1.09 years. Backfill had high specificity (0.98) and moderate sensitivity (0.59) for diagnosing axial SpA.
  
  
  CONCLUSIONS
  We summarized the characteristics of backfill and found that backfill is a specific sign of axial SpA seen on T1SE MRI.
authors.affiliation: Department of rheumatology, the third affiliated hospital, Sun Yat-Sen university, 600, Tianhe Road, 510630 Guangzhou, China.
authors.forename: Zaiying
authors.initials: Z
authors.lastname: Hu
authors.affiliation: Department of radiology, the third affiliated hospital, Sun Yat-Sen university, 600, Tianhe Road, 510630 Guangzhou, China.
authors.forename: Xiaohong
authors.initials: X
authors.lastname: Wang
authors.affiliation: Department of rheumatology, the third affiliated hospital, Sun Yat-Sen university, 600, Tianhe Road, 510630 Guangzhou, China.
authors.forename: Jun
authors.initials: J
authors.lastname: Qi
authors.affiliation: Department of radiology, the third affiliated hospital, Sun Yat-Sen university, 600, Tianhe Road, 510630 Guangzhou, China.
authors.forename: QingCong
authors.initials: Q
authors.lastname: Kong
authors.affiliation: Department of rheumatology, the third affiliated hospital, Sun Yat-Sen university, 600, Tianhe Road, 510630 Guangzhou, China.
authors.forename: Minjing
authors.initials: M
authors.lastname: Zhao
authors.affiliation: Department of rheumatology, the third affiliated hospital, Sun Yat-Sen university, 600, Tianhe Road, 510630 Guangzhou, China. Electronic address: gujieruo@163.com.
authors.forename: Jieruo
authors.initials: J
authors.lastname: Gu
mesh_terms: D000293:Adolescent; D000328:Adult; D001416:Back Pain / Q000000981:diagnostic imaging* / Q000209:etiology; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D011446:Prospective Studies; D025241:Spondylarthritis / Q000175:diagnosis / Q000000981:diagnostic imaging*; D055815:Young Adult
keywords: MRI; Spondyloarthritis