title: Fat infiltration on magnetic resonance imaging of the sacroiliac joints has limited diagnostic utility in nonradiographic axial spondyloarthritis.
abstract: OBJECTIVE
  To explore whether morphological features of fat infiltration (FI) on sacroiliac joint (SIJ) magnetic resonance imaging (MRI) contribute to diagnostic utility in 2 inception cohorts of patients with nonradiographic axial spondyloarthritis (nr-axSpA).
  
  
  METHODS
  Four blinded readers assessed SIJ MRI in 2 cohorts (A/B) of 157 consecutive patients with back pain who were ≤ 50 years old, and in 20 healthy controls. Patients were classified according to clinical examination and pelvic radiography as having nr-axSpA (n = 51), ankylosing spondylitis (n = 34), or nonspecific back pain (n = 72). Readers recorded FI, bone marrow edema (BME), and erosion, predefined morphological features of FI (distinct border, homogeneity, subchondral location), and anatomical distribution of SIJ FI. The proportion of SIJ quadrants affected by FI and frequencies of various SIJ FI features were analyzed descriptively. We calculated positive/negative likelihood ratios (LR) to estimate the diagnostic utility of various features of FI, with and without associated BME, and erosion.
  
  
  RESULTS
  Of the patients with nr-axSpA in cohorts A/B, 45.0%/48.4% had FI in ≥ 2 SIJ quadrants. Of those, 25.0%/22.6% and 20.0%/25.8% showed FI with distinct border or homogeneous pattern, respectively, and 50% to 100% of those patients displayed concomitant BME or erosion. FI per se in ≥ 2 SIJ quadrants had no diagnostic utility (LR+ 1.62/1.91). FI with distinct border (LR+ 8.29/2.13) or homogeneity (LR+ 6.24/3.78) demonstrated small to moderate diagnostic utility.
  
  
  CONCLUSIONS
  SIJ FI per se was not of clinical utility in recognition of nr-axSpA. Distinct border or homogeneity of FI on SIJ MRI showed small to moderate diagnostic utility in nr-axSpA, but were strongly associated with concomitant BME or erosion, highlighting the contextual interpretation of SIJ MRI.
authors.affiliation: From the Department of Rheumatology and the Department of Radiology, Balgrist University Hospital, Zurich, Switzerland; Copenhagen Center for Arthritis Research, Center for Rheumatology and Spinal Diseases, University of Copenhagen, Copenhagen, Denmark; Rufibach rePROstat, Basel, Switzerland; Department of Ophthalmology, the Department of Radiology and Diagnostic Imaging, and the Department of Medicine, University of Alberta, Edmonton, Alberta, Canada.
authors.forename: Ulrich
authors.initials: U
authors.lastname: Weber
authors.forename: Susanne J
authors.initials: SJ
authors.lastname: Pedersen
authors.forename: Veronika
authors.initials: V
authors.lastname: Zubler
authors.forename: Kaspar
authors.initials: K
authors.lastname: Rufibach
authors.forename: Stanley M
authors.initials: SM
authors.lastname: Chan
authors.forename: Robert G W
authors.initials: RG
authors.lastname: Lambert
authors.forename: Mikkel
authors.initials: M
authors.lastname: Østergaard
authors.forename: Walter P
authors.initials: WP
authors.lastname: Maksymowych
mesh_terms: D000273:Adipose Tissue / Q000473:pathology*; D000293:Adolescent; D000328:Adult; D001416:Back Pain / Q000175:diagnosis* / Q000473:pathology; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D012446:Sacroiliac Joint / Q000473:pathology*; D012680:Sensitivity and Specificity; D025241:Spondylarthritis / Q000175:diagnosis* / Q000473:pathology; D013167:Spondylitis, Ankylosing / Q000473:pathology; D055815:Young Adult
keywords: ANKYLOSING SPONDYLITIS; FAT INFILTRATION; MAGNETIC RESONANCE IMAGING; SACROILIAC JOINT; SPONDYLOARTHRITIS