The use of trans iliac-sacral-iliac bars is an alternative to sacro-iliac screws in the treatment of bilateral lesions of the posterior pelvic ring, and the same biomechanical principles can be applied.Of 20 patients, ten men and ten women, a CT-scan of the pelvis was performed to study the individual and common safe area at the level of S1 and S2. The location and maximal diameter of the individual safe area were studied using a computer-navigation system, displaying images in sagittal, coronal and axial anatomic planes together with a 3-D reconstruction. The common safe area was studied using three points: upper- (UA) and lower anterior corner (DA) of S1 and S2, and the centre of the safe area.It would have been possible to place an iliac-sacral-iliac bar (5 mm or more) in S1 and S2 in all the men, but in the women a bar could only have been inserted in only five in S1 and eight in S2. A statistically significant difference between men and women was found at S1 (P=0.033) but not at S2 (P=0.211). No significant correlation was found between the diameter of the safe area at both levels in men and women and age, height, and weight. Furthermore, no common safe area of 5 mm or more was measured at the same levels.Four patients were treated using trans iliac-sacral-iliac bars. Three were placed under fluoroscopic control in combination with a frame, and in one patient an image-guided system was used. A postoperative CT confirmed the correct position of the bars in each patient.The complexity and individual variability of the sacrum makes complex preoperative planning of the iliac-sacral-iliac path mandatory.