Because dimensions of the upper sacral cortexes vary greatly among individuals, preoperative computed tomographic (CT) evaluation of individual sacrum may help surgeons choose sacral screw insertion techniques. Axial CT scans were performed on 40 dry sacrum specimens to quantitatively evaluate the internal structure of the lateral sacral mass in the first and second segments. The results showed that the greatest cortical thickness in the S1 vertebra was found in the anterior cortex (3.4+/-0.9 mm), followed by the anterolateral (3.2+/-1.2 mm), and anteromedial (2.9+/-1 mm). The greatest cortical thickness in the S2 region was noted in the anteromedial cortex (2.4+/-0.5 mm), followed by the anterior and anterolateral (2.2+/-0.9 mm). The mean percentage of the anterior cortex thickness versus the lateral sacral mass depth was 12.8+/-3.7 for S1 and 11.1+/-2.8 for S2. Bicortical screw placement is recommended to achieve stronger fixation, but care should be taken not to violate the vital structures anterior to the sacrum.