Pedicle screw placement at the sacrum: Anatomical characterization and limitations at S1

Anatomical and biomechanical data have suggested that pedicle screw fixatio n at the sacrum is optimum in the anteromedial direction into the S1 verteb ral body, yet the possibility of posterior iliac crest interference with th is screw pathway has been considered but not defined. This study aimed to d etermine if the anteromedial direction of screw placement into the vertebra l body is possible in all cases at S1 and to assess the limiting effect of the posterior iliac crest. Computed tomography scans of the upper sacrum at the S1 pedicle parallel to the sacral endplate were examined in 100 patien ts. Analysis using a digitizer allowed characterization of an ideal screw p athway with variable screw and screw head diameters in an anteromedial dire ction into the S1 vertebral body. The effects of the posterior iliac crest upon these pathways were studied. The study demonstrated that anteromedial placement with bicortical fixation at the vertebral body was theoretically possible in almost all (98.5%) cases. Because the sacral body is often wide r than the sacral spinal canal, a straight-ahead screw direction will often achieve placement into the S1 vertebral body, if the starting point for th e screw allows screw placement adjacent to the medial border of the S1 pedi cle with only 1.5 mm of cortical bone separating the canal and the screw. T he space between the posterior iliac crest and the lateral aspect of the sc rew corridor ranges from a maximum of 52.4 mm to a minimum of 12.8, 6.2, an d 0 mm for the 7-, 10-, and 12.5-mm screw corridors. On only three occasion s (1.5%) was the ideal screw corridor not possible because of posterior ili ac crest overlap. Tn each case, this occurred only unilaterally and when th e widest of the screw corridors (12.5 mm) was used. Both the distance betwe en the posterior iliac crests and the space available for optimum screw pla cement are greater in females than males.