Computed tomographic evaluation of the internal structure of the lateral sacral mass in the upper sacra

Because dimensions of the upper sacral cortexes vary greatly among individu als, 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 i nternal structure of the lateral sacral mass in the first and second segmen ts. The results showed that the greatest cortical thickness in the S1 verte bra was found in the anterior cortex (3.4+/-0.9 mm), followed by the antero lateral (3.2+/-1.2 mm), and anteromedial (2.9+/-1 mm). The greatest cortica l 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 placeme nt is recommended to achieve stronger fixation, but care should be taken no t to violate the vital structures anterior to the sacrum.