
==== Front
Intern Med
Intern Med
Internal Medicine
0918-2918
1349-7235
The Japanese Society of Internal Medicine

38104986
10.2169/internalmedicine.3075-23
Pictures in Clinical Medicine
Unexpected Sacroiliitis Mimicker in Axial Spondyloarthritis
Nakamura Akihiro 123
1 Department of Medicine, Division of Rheumatology, Queen's University, Canada
2 Translational Institute of Medicine, School of Medicine, Queen's University, Canada
3 Spondyloarthritis Program, Rheumatology Clinic, Kingston Health Science Centre, Canada.
Correspondence to Dr.　Akihiro Nakamura, Akihiro.Nakamura@queensu.ca

18 12 2023
15 8 2024
63 16 23452345
18 10 2023
30 10 2023
Copyright © 2024 by The Japanese Society of Internal Medicine
https://creativecommons.org/licenses/by-nc-nd/4.0/ The Internal Medicine is an Open Access journal distributed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. To view the details of this license, please visit (https://creativecommons.org/licenses/by-nc-nd/4.0/).
sacroiliitis
Crohn's disease
axial spondyloarthritis
low back pain
==== Body
pmcA 46-year-old HLA-B27-positive woman with chronic inflammatory back pain, Crohn's disease (CD), and a family history of axial spondyloarthritis (axSpA) was referred to our SpA clinic for possible sacroiliitis based on abdominal magnetic resonance imaging (MRI) (yellow circle, Picture A; T2-weighted image). Although the image strongly suggested sacroiliitis, we subsequently requested more focused MRI of the sacroiliac joint (SIJ). The new image eventually revealed an unexpected finding consistent with fibrous dysplasia, a benign bone tumor (1), as shown in Picture Picture B and C (T2- and T1-weighted STIR images, respectively). Notably, the heterogeneous T2 high-intensity signal was localized in the right iliac bone (Picture B) with no evidence of joint surface erosion or fat metaplasia (Picture C), all of which are extremely uncommon in sacroiliitis (2). This case highlights that conditions such as fibrous dysplasia can mimic sacroiliitis, even in patients with chronic inflammatory back pain, confirmed CD, family history of axSpA, and HLA-B27 positivity. This case serves as a reminder that a dedicated SIJ MRI is essential for an accurate assessment (2) and that abdominal MRI alone is not sufficient for a comprehensive evaluation.

Picture.

Informed written consent was obtained from the patient.

The author states that he has no Conflict of Interest (COI).
==== Refs
1. Anitha N , Sankari SL , Malathi L , Karthick R . Fibrous dysplasia-recent concepts. J Pharm Bioallied Sci 7 : S171-S172, 2015.26015701
2. Baraliakos X , Østergaard M , Lambert RG , et al . MRI lesions of the spine in patients with axial spondyloarthritis: an update of lesion definitions and validation by the ASAS MRI working group. Ann Rheum Dis 81 : 1243-1251, 2022.35609977
