Preoperative CT determination of angles for sacral screw placement

Placement of screws in the sacrum is essential for assuring rigid fixation during instrumentation of the lumbosacral spine. However, the safest approach to obtaining sacral fixation has not been clearly delineated and is complicated by significant interpatient variation in sacral morphology. In the present study, CT evaluation of the S1 pedicle and sacral ala was performed in 10 cadavers and established "average" angles for screw placement. Placement of screws at the "averages" was undertaken and verified on CT. Violation, or near violation, of important anatomic landmarks was found in 4 of 10 specimens. As a result of this study, the authors contend that placement of screws in the sacrum at average angles presents unnecessary risks to important structures. The authors propose that each patient should undergo individualized preoperative CT evaluation to determine safe angles to minimize these risks.