Low Profile Pelvic Fixation Anatomic Parameters for Sacral Alar-Iliac Fixation Versus Traditional Iliac Fixation

Study Design. Three-dimensional computed tomography (CT) radiographic analysis. Objective. To describe the parameters for a trajectory through a sacral starting point as a method of pelvic fixation in spinal deformity and to compare this technique with insertion from the posterior superior iliac spine (PSIS). Summary of Background Data. Long anchors projecting into the ilium provide optimal pelvic fixation. The traditional starting point in the PSIS requires muscle dissection and connectors or rod bends. Methods. Twenty pelvic CTs of mature adolescents were analyzed using InSpace, a three-dimensional CT program, by 2 surgeons. Trajectory with maximal length and width through the sacral ala and iliac wing was obtained through CT imaging plane manipulation. Trajectory and starting-point parameters were measured. Parameters were evaluated and compared for insertion from the PSIS. Results. Based on the ideal trajectory, the mean starting point in S2 was 25 mm caudal to the superior endplate of S1 and 22 mm lateral to the sacral midline (S2 alar-iliac [S2AI] path). Maximal mean S2AI distance was 105 mm (range, 74-129 mm; SD = 11 mm). Maximal mean length for PSIS insertion was 118 mm (range, 99-147 mm; SD = 13 mm). Mean angulation was 40 degrees (SD = 6 degrees) laterally in the transverse plane and 39 degrees (SD = 6 degrees) caudally in the sagittal plane. The mean difference between surgeons in selecting the trajectory was 2 degrees and 1 degrees in the transverse and sagittal plane, respectively. The S2AI pathway traversed 35 mm of sacral ala. The narrowest mean width of the ilium along this path was 12 mm (range, 6-18 mm). The starting point for the S2AI was 19 mm deep to the PSIS. The distance from skin for S2AI versus PSIS techniques was 52 and 37 mm, respectively. Conclusion. Iliac fixation through the S2 ala provides a reproducibly chosen starting point in line with S1 pedicle anchors. Implant prominence is minimized because the starting point is 15 mm deeper than the PSIS entry. It is less likely to be affected in cases using iliac crest bone graft harvest because of the more anterior position of the anchor in the ilium.