Morphologic considerations of the first sacral pedicle for iliosacral screw placement.

Study Design: Morphometric, radiographic, and computed tomographic evaluation of the pedicle of the first sacral vertebra was performed, and the pedicle's spatial relation with the posterior surface of the ilium was defined.

Objectives: To facilitate accurate localization of the entry site of the iliosacral pedicular screw on the posterior surface of the ilium, to provide optimal length and direction of iliosacral screw placement, and to investigate the feasibility of inserting two screws through the first sacral vertebral pedicle for unstable posterior pelvic fixation.

Methods: Anterior and posterior pedicular height, pedicular depth, alar depth, and posterior alar height of S1 vertebrae were measured in 11 body pelves bilaterally. Sacral pedicular height was also measured on the outlet view radiograph as visualized during intraoperative fluoroscopy, and compared with actual anatomic pedicular height. The distance from the posterior limit of the ilium to the S1 ala, pedicle, and pedicle axis, and the distance between the outer table of the ilium and anterior cortex of the sacrum were measured on axial computed tomography scans. Finally, parasagittal sections of the sacral were made to assess the safety zone for placement of two pedicular screws into the vertebral body.

Results: The mean anterior and posterior pedicular heights were 30.2 and 26.1 mm, respectively. The depths of the pedicle and ala were 27.8 and 45.8 mm, respectively. The mean posterior alar height was 28.7 mm. The mean first sacral pedicular height measured on the outlet-view radiographs was 20 mm, which was significantly less (P < 0.0001) than the actual gross anatomic pedicular height. The mean distance from the posterior limit of the ilium to the pedicle axis projection point on axial computed tomography scans was 32.5 mm, and the mean distance from this point to the greater sciatic notch was 38.6 mm. The mean distance between the outer table of the ilium and the anterior cortex of the sacrum was 105.2 mm. The safety margin for two closely inserted pedicular screws was only 4 to 6 mm.

Conclusions: This study suggests that placement of one screw through the S1 pedicle into the vertebral body is safer, and routine placement of two sacral pedicular screws may be difficult. The optimal starting point for placement of single iliosacral screw is 3 to 3.5 cm anterior to the posterior border of the iliac bone in the sagittal plans, and 3.5 to 4 cm cephalad to the greater sciatic notch. The screw should be directed perpendicular to the outer surface of the table from this entry point. The safe length of the iliosacral pedicular screw is up to 80 mm.