Assessment of Anatomical Variations in the Sacroiliac Joint Using Magnetic Resonance Imaging: A Retrospective Study of 840 Patients

Background/Objectives: This study aimed to examine the prevalence of anatomical variations in the sacroiliac joints (SIJs) as observed through Magnetic Resonance Imaging (MRI), to characterize their manifestations, and to identify MRI features that may resemble inflammatory alterations. Methods: A retrospective review was conducted on consecutive MRI scans of the SIJ performed from January 2009 to January 2022. Eight anatomical variations, along with associated edematous and structural changes, were assessed. Results: The study encompassed 840 patients, with anatomical variations identified in 39.7% of the cohort, occurring more frequently among female participants. The most prevalent variations were accessory SIJ (36.2%) and the iliosacral complex (32.2%). Notably, isolated synostosis and persistent ossification center variations were absent. The increased frequency of variations in women, as well as their correlation with advancing age, was statistically significant (p = 0.034). Accessory SIJ and dysmorphic alterations were linked to bone marrow edema and structural modifications. In the iliosacral complex and semicircular defect variations, prominent vascular structures were observed extending along the bone surfaces. The number and depth of edema slices in sacroiliitis exceeded those observed in the variation (p < 0.001). Conclusions: Anatomical variations of the SIJ are prevalent among women and tend to increase with advancing age. Given that these variations, particularly accessory SIJ and dysmorphic alterations, may present with edematous and structural signal intensity changes that resemble sacroiliitis, it is crucial to recognize these variations. It is recommended to assess axial and coronal images concurrently and to exercise caution in the interpretation of SIJ MR images.