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Unilateral Sacroiliitis: Differential Diagnosis Between Infectious Sacroiliitis and Spondyloarthritis Based on MRI Findings.

OBJECTIVE
The purpose of this study was to identify the MRI features that aid in the differentiation between infectious sacroiliitis and unilateral sacroiliitis associated with spondyloarthritis.


MATERIALS AND METHODS
The MR images of 54 patients who received a diagnosis unilateral sacroiliitis between August 2001 and August 2013 were reviewed. MR images were evaluated for bone lesions (extent and distribution of bone marrow edema and presence and size of bone erosions), soft-tissue lesions (capsulitis, extracapsular fluid collections, and periarticular muscle edema), and joint space enhancement. The Fisher exact test was used for comparison of categoric data, and multivariate stepwise logistic regression analysis was performed.


RESULTS
Thick capsulitis, extracapsular fluid collection, and periarticular muscle edema were all more frequently observed in infectious sacroiliitis (p < 0.001). Iliac-dominant bone marrow edema and joint space enhancement were statistically significantly more common in spondyloarthritis (p < 0.001 and p = 0.014, respectively). The presence of periarticular muscle edema was the only independently differentiating variable on multivariate stepwise logistic regression analysis. When periarticular muscle edema was the sole predictor, unilateral sacroiliitis in spondyloarthritis was correctly identified in 77.3% of cases, and infectious sacroiliitis was correctly identified in 90.6% of cases. The overall accuracy was 85.2%.


CONCLUSION
MRI features of the bone lesions, soft-tissue lesions, and joint space enhancement in unilateral sacroiliitis aid in the differential diagnosis between infection and spondyloarthritis. Among various findings, periarticular muscle edema was the single most important predictor of infectious sacroiliitis.