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Can erosions on MRI of the sacroiliac joints be reliably detected in patients with ankylosing spondylitis? - A cross-sectional study

Ulrich Weber12*, Susanne J Pedersen3, Mikkel Østergaard3, Kaspar Rufibach4, Robert GW Lambert5 and Walter P Maksymowych1

Author Affiliations

1 Department of Medicine, Division of Rheumatology, 562 Heritage Medical Research Building, University of Alberta, Edmonton, Alberta, Canada T6G 2S2

2 Department of Rheumatology, Balgrist University Hospital, Forchstrasse 340, CH-8008 Zürich, Switzerland

3 Department of Rheumatology, Copenhagen University Hospital at Glostrup, Nordre Ringvej 57, DK-2600 Glostrup, Denmark

4 Division of Biostatistics, Institute of Social and Preventive Medicine, University of Zürich, Hirschengraben 84, CH-8001 Zürich, Switzerland

5 Department of Radiology and Diagnostic Imaging, University of Alberta, 2A2.41 Walter C. Mackenzie Health Sciences Centre, 8440-122 Street, Edmonton, Alberta, Canada T6G 2B7

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Arthritis Research & Therapy 2012, 14:R124  doi:10.1186/ar3854

Published: 24 May 2012



Erosions of the sacroiliac joints (SIJ) on pelvic radiographs of patients with ankylosing spondylitis (AS) are an important feature of the modified New York classification criteria. However, radiographic SIJ erosions are often difficult to identify. Recent studies have shown that erosions can be detected also on magnetic resonance imaging (MRI) of the SIJ early in the disease course before they can be seen on radiography. The goals of this study were to assess the reproducibility of erosion and related features, namely, extended erosion (EE) and backfill (BF) of excavated erosion, in the SIJ using a standardized MRI methodology.


Four readers independently assessed T1-weighted and short tau inversion recovery sequence (STIR) images of the SIJ from 30 AS patients and 30 controls (15 patients with non-specific back pain and 15 healthy volunteers) ≤45 years old. Erosions, EE, and BF were recorded according to standardized definitions. Reproducibility was assessed by percentage concordance among six possible reader pairs, kappa statistics (erosion as binary variable) and intraclass correlation coefficient (ICC) (erosion as sum score) for all readers jointly.


SIJ erosions were detected in all AS patients and six controls by ≥2 readers. The median number of SIJ quadrants affected by erosion recorded by four readers in 30 AS patients was 8.6 in the iliac and 2.1 in the sacral joint portion (P < 0.0001). For all 60 subjects and for all four readers, the kappa value for erosion was 0.72, 0.73 for EE, and 0.63 for BF. ICC for erosion was 0.79, 0.72 for EE, and 0.55 for BF, respectively. For comparison, the kappa and ICC values for bone marrow edema were 0.61 and 0.93, respectively.


Erosions can be detected on MRI to a comparable degree of reliability as bone marrow edema despite the significant heterogeneity of their appearance on MRI.