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

Anatomical and biomechanical data have suggested that pedicle screw fixation at the sacrum is optimum in the anteromedial direction into the S1 vertebral body, yet the possibility of posterior iliac crest interference with this screw pathway has been considered but not defined. This study aimed to determine if the anteromedial direction of screw placement into the vertebral 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 patients. Analysis using a digitizer allowed characterization of an ideal screw pathway with variable screw and screw head diameters in an anteromedial direction into the S 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 wider than the sacral spinal canal, a straight-ahead screw direction will often achieve placement into the S1 vertebral body, if the starting point for the screw allows screw placement adjacent to the medial border of the S1 pedicle with only 1.5 mm of cortical bone separating the canal and the screw. The space between the posterior iliac crest and the lateral aspect of the screw corridor ranges from a maximum of 52.4 mm to a minimum of 12.8, 6.2, and 0 mm for the 7-, 10-, and 12.5-mm screw corridors. On only three occasions (1.5%) was the ideal screw corridor not possible because of posterior iliac crest overlap. In each case, this occurred only unilaterally and when the widest of the screw corridors (12.5 mm) was used. Both the distance between the posterior iliac crests and the space available for optimum screw placement are greater in females than males.