Evaluation of sacroiliac joint anatomic variations with magnetic resonance imaging

Aim: To study the prevalence of anatomic variations of Sacroiliac joint (SIJ) on Magnetic Resonance Imaging (MRI), to describe their appearances and to differentiate the findings that mimic the sacroiliitis and emphasize MRI features which can create difficulties in differential diagnosis. Materials and Methods: The sequential MRI images for sacroiliac joints at our hospital between january 2009- january 2022 were retrospectively studied. Eight anatomic variations, presence of lumbosacral transitional vertebra (LSTV) and edematous and structural changes accompanying the variations were evaluated. Results: Study consisted of 840 patients between the ages of 18-79, of which 516 were females (%61,4) and 324 (%38,6) were males. In 334 patients, at least 1 anatomic variation was identified and in 94 patients LSTV was present. The incidence of anatomic variations in order was: 1. Accessory SIJ (128 patients, %36,2), 2. Iliosacral complex (114 patients, %32,2), 3. Semicircular defect (44 patients, %12,4), 4. Cresentric defect (26 patients, %7,3), 5. Bipartite complex (24 patients, %6,8), 6. Dysmorphic changes (18 patients, %5,1). Isolated synostosis and persistant ossification center variations were not detected. The variations were more frequent in females and more common with increasing age; and these results were statistically significant (p<0,001). Accessory SIJ and dysmorphic changes were associated with bone marrow edema and structural changes. Iliosacral complex and semicircular defect variations featured evident vascular structures stretching along the bone surfaces. In patients with LSTV, sacroiliitis was identified, and the difference was statistically significant (p=0,034). Conclusion: SIJ variations are common in females and their occurrence increases with advancing age. Special care must be taken when evaluating SIJ MR images, since these variations particularly accessory SIJ and dysmorphic changes variations, may demonstrate some edematous and structural signal intensity changes which can mimic sacroiliitis. Keywords: Sacroiliitis, Magnetic Resonance Imaging, Sacroiliac Joint Variations, Bone Marrow Edema