P143. The "V" sign: a reliable anatomic and radiographic landmark for posterior percutaneous S1 screw placement.

Posterior percutaneous fixation (PPF) can be challenging, particularly in patients with advanced deformity, osteoporosis or prior surgeries. Sacrum S1 pedicle screws breach or misplacement can be related to the anatomic variability of the entry points and the lack of corresponding reliable radiographic landmarks. A reliable intraoperative radiographic description of the S1 pedicle entry point is, therefore, necessary for a safe and reproducible PPF. This study highlights a reproducible anatomical sacral landmark ("V" sign) for the safe localization of the S1 pedicle entry point under fluoroscopy. An anatomic cadaveric study and a retrospective review of patients who had S1 screws placed percutaneously using intraoperative radiographic "V" landmark. Human cadavers (N=14) were dissected for the anatomic description of the "V" landmark. The CT scans of 135 consecutive adult patients were evaluated for the accuracy of S1 pedicle screws percutaneously placed under fluoroscopy and "V" sign guidance. Mean distance between the bottom point of the "V" landmark and the anatomic entry point to the S1 pedicle was measured on all cadavers. Accuracy of percutaneously placed S1 screws using the "V" sign in patients who underwent PPF. Human cadavers were dissected for the anatomic description of the "V" landmark, and its relationship with the anatomic entry point of the S1 pedicle. The spatial "V" landmark was defined medially by the lateral border of the superior facet of S1 and laterally by the posterior projection of the sacral ala. The mean distance between the bottom point of the "V" landmark and the anatomic entry point to the S1 pedicle was measured on all cadavers. Subsequently, a review of CT scans of patients who underwent minimally invasive antepsoas surgery (MIS-ATP) and PPF using the "V" sign as a landmark for S1 pedicle screw placement. Postoperative CT scan S1 screws breaches were collected and graded. In 14 cadavers, the average distance between the bottom point of the "V" landmark and the anatomical entry of the S1 pedicles was 12.0 mm on the left and 11.3 mm on the right. In the medial-lateral axis, all the entry points were within a zone defined +/- 2 mm from the vertical line dropped from the base of the "V". There were no significant differences in this measurement with respect to laterality and sex. Additionally, a retrospective review of 135 patients who underwent percutaneous S1 pedicle screw placement using the "V" landmark showed 8.5% breaches. These were graded as B (5.2%), C (2.2%), D (0.7%), and E (0.4%). Identifying the anatomical entry point to the S1 pedicles under radiographic imaging is crucial for safe MIS surgery. The "V" sign serves as a reliable anatomic and radiographic landmark, situated 12 mm proximal to and on the same vertical line of the anatomical entry of S1 pedicles. This landmark helps the MIS surgeons overcome the radiographic ambiguity of the sacral anatomy, and ultimately mitigates improper S1 pedicle screws placement. S1 Pedicle screws (Approved for this indication) [ABSTRACT FROM AUTHOR]