Percutaneous fixation with iliosacral screws has been shown to be a safe and reproducible method for the management of certain posterior pelvic injuries. However, the method is contraindicated in patients with sacral anatomical variations and dysmorphism. The incidence and the pattern of S1 anatomical variations were evaluated in 61 volunteers (35 women and 26 men) using MRI scans of the sacrum. S1 dimensions (12 parameters) in both the transverse and coronal planes were recorded and evaluated. Individuals were divided in four groups based on the S1 body size and the asymmetry of dimensions on the transverse and coronal planes. In 48 (78.6%) patients, dimensions in both planes were symmetrical despite the varying size of the S1 body. In 9 (14.8%) patients, coronal plane dimensions were disproportionally smaller compared to those of the transverse plane with a varying size of S1 body making effective iliosacral screw insertion a difficult task. In 2 (3.3%) patients there was a combination of large transverse plane and small coronal plane dimensions, with large S1 body size. A preoperative imaging study of S1 body size and coronal plane dimensions and an intraoperative fluoroscopic control of S1 dimensions on the coronal plane are suggested for safe iliosacral screw fixation