Anatomical Variations of Structures around the Lumbosacral Junction and Determination of the Safe Zone for Bicortical S1 Pedicle Screw Placement: A Cadaveric Study

Background: Pedicle screws are widely used for lumbosacral fixation, but their placement poses a risk to adjacent neurovascular structures, including the abdominal aorta, inferior vena cava, and common iliac vessels. Aims and Objectives: This cadaveric study aims to define the anatomical relationships of these structures, document variations, and establish a safe zone for bicortical S1 pedicle screw placement. Methodology: A cadaveric study was conducted from November 2020 to November 2022, involving 22 fresh or embalmed cadavers. A midline longitudinal incision was used to expose the major vessels, and direct measurements were taken to assess their spatial relationships with the sacral promontory, vertebrae, and disc spaces. Results: No significant sex-based morphometric differences were found. The left common iliac vein (CIV) was 20.36 ± 6.27 mm from the mid-sacral promontory, while the right CIV was 27.09 ± 6.45 mm away. The right common iliac artery (CIA) was 28.05 ± 4.92 mm, and the left CIA was 29.41 ± 4.28 mm from the promontory. The vascular safe zone extended 19.09 ± 4.32 mm on the right and 17.68 ± 4.17 mm on the left. The right and left L5 nerve roots were 37.32 ± 3.0 mm and 37.23 ± 3.6 mm, while the S1 nerve roots were 34.05 ± 3.14 mm and 33.95 ± 3.71 mm, respectively. Conclusion: Venous structures are closer to the mid-sacral promontory than arterial structures, with greater anatomical variations. The left side has a smaller vascular window. S1 pedicle screws should be directed medially and inferiorly to minimize neurovascular injury.