Using reformatted axial computed tomography images in isolation will miss narrow S1 transsacral screw corridors.

OBJECTIVE To evaluate S1 transsacral (TS) corridors on reformatted and non-reformatted computed tomography (CT) images to determine which most reliably identifies narrow corridors. DESIGN Retrospective cohort. SETTING Level 1 trauma center. PATIENTS 245 patients with operative pelvic ring injuries. INTERVENTION CT scan review. MAIN OUTCOME MEASUREMENTS Preoperative CT scans were evaluated to determine the width of the S1 TS corridor on standard axial non-reformatted (ANR), axial reformatted (AR), and coronal reformatted (CR) images. Sensitivity and specificity of each format to detect a narrow corridor (<10 mm on AR or CR) was calculated. Patients with S1 TS screws were evaluated to determine the rate of screw breach with narrow corridors. RESULTS The axial width of the S1 TS corridor was consistently smaller on ANR vs. AR images (mean difference 1.4, 95% CI 1.1 to 1.5). The corridor width on ANR images was on average 86% of the AR measurement. ANR images had the highest sensitivity and specificity (100% and 98%) for detecting S1 TS corridors <10 mm. 53 S1 TS screws were placed in corridors ranging 10 to 23 mm on AR images and 7 to 19 mm on ANR images. Four (57%) of the seven screws placed in corridors less than 10 mm in width on ANR images breached sacral cortex. CONCLUSION Using ANR images to measure the S1 TS corridor consistently measured smaller widths than AR images and identified all narrow corridors. A high rate of screw breach was noted with screw placement in narrow corridors. LEVEL OF EVIDENCE Diagnostic Level III. See Instructions for Authors for a complete description of levels of evidence.