Closed posterior superior iliac spine impeding pediculocorporeal S-1 screw insertion

Placement of pedicle screws into S-1 is difficult. In cases in which there is a closed posterior superior iliac spine (PSIS), its medial situation prevents lateral oblique placement of the screw inserter sleeve and directing the screw to the anteromedial aspect of S-1. In the present study, the authors discuss anatomical variations of the PSIS and sacrum, and they describe a safe and effective S-1 screw insertion technique.

The relation of 50 PSISs obtained from 25 dry pelvises (15 male and 10 female cadavers) was examined. The distance from the inferolateral aspect of the S-1 superior articular facet to its promontory was estimated. The relation between the point of anterior penetration of the "screw line" and "safe zone" was analyzed. Penetration of screw lines into the S-1 body was also measured. (An illustrative case of closed PSIS is presented with pre- and postoperative computerized tomography [CT] scan findings.) The authors found that that PSIS was situated in 28% of the specimens. When screws were directed anteromedially, the screw lines failed to penetrate the S-1 body in 24% of the male and in 15% of female specimens. The screw lines deviated from the safe zone anteriorly in 34% of the male and in 20% of the female specimens. When the PSIS was medial to the line that connects the inferolateral aspect of the S-1 superior articular facet to the promontory, a classification of closed PSIS was assigned.

The accuracy of the placement of the screws and their pullout strength are increased when using the present technique. Preoperative CT scanning should be performed to determine the presence of a closed PSIS; in cases in which a closed PSIS is found, the ilium should be resected to enable a greater anteromedial trajectory for placement of S-1 pedicle screws.