Asian Spine J.  2022 Dec;16(6):874-881. 10.31616/asj.2021.0268.

Three-Dimensional Analysis of the Ideal Entry Point for Sacral Alar Iliac Screws

• Watanabe N
• Takigawa T
• Uotani K
• Oda Y
• Misawa H
• Tanaka M
• Ozaki T

• KMID: 2537258
• DOI: http://doi.org/10.31616/asj.2021.0268

Abstract

Study Design: This is a virtual three-dimensional (3D) imaging study examining computed tomography (CT) data to investigate instrumentation placement. Purpose: In this study, we aim to clarify the ideal entry point and trajectory of the sacral alar iliac (SAI) screw in relationship to the dorsal foramen at S1 and the respective nerve root. Overview of Literature: To the best of our knowledge, there is yet no detailed 3D imaging study on the ideal entry point of the SAI screw. Despite the evidence suggesting that the dorsal foramen at S1 is a landmark on the sacrum, the S1 nerve root disruption is a general concern during the insertion of SAI screws. No other study has been published examining the nerve root location at the S1and SAI screw insertions.
Methods
Preoperative CT data from 26 patients pertaining to adult spinal deformities were investigated in this study. We applied a 3D image processing method for a detailed investigation. Virtual cylinders were used to mimic SAI screws. These were placed to penetrate the sacral iliac joint without violating the other cortex. We then assessed the trajectory of the longest SAI screw and the ideal entry point of SAI using a color mapping method on the surface of the sacrum. We measured the location of the nerve root at S1 in relation to the foramen at S1 and the sacral surface.
Results
As per the results of our color mapping, it was determined that areas that received high scores are located medially and caudally to the dorsal foramen of S1. The mean angle between a horizontal line and a line connecting the medial edge of the foramen and nerve root at S1 was 93.5°. The mean distances from the dorsal medial edge of the foramen and sacral surface to S1 nerve root were 21.8 mm and 13.9 mm, respectively.
Conclusions
The ideal entry point of the SAI screw is located medially and caudally to the S1 dorsal foramen based on 3D digital mapping. It is also shown that this entry point spares the S1 nerve root from possible iatrogenic injuries.

Keyword

S1 nerve root; S1 dorsal foramen; Adult spinal deformity; Sacral alar iliac screw; Ideal entry point