Human versus Robot: A Propensity-Matched Analysis of the Accuracy of Free Hand versus Robotic Guidance for Placement of S2 Alar-Iliac (S2AI) Screws

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Study Design.Retrospective matched cohort analysis.Objective.To compare the accuracy of S2 alar-iliac (S2AI) screw placement by robotic guidance versus free hand technique.Summary of Background Data.Spinopelvic fixation utilizing S2AI screws provides optimal fixation across the lumbosacral junction allowing for solid fusion, especially in long segment fusion constructs. Traditionally, S2AI screw placement has required fluoroscopic guidance for accurate screw placement. Herein, we present the first series comparing a free hand and robotic-guided technique for S2AI screw placement.Methods.Sixty-eight consecutive patients who underwent S2AI screw placement by either a free hand or robotic technique between 2015 and 2016 were reviewed. Propensity score-matching was utilized to control for preoperative characteristic imbalances. Screw position and accuracy were evaluated using three-dimensional manipulation of computed tomography scan reconstructions from intraoperative O-arm imaging.Results.A total of 51 patients (105 screws) were matched, 28 (59 screws) in the free hand group (FHG) and 23 (46 screws) in the robot group (RG). The mean age in the FHG and RG were 57.9 ± 14.6 years and 61.6 ± 12.0 years (P = 0.342), respectively. The average caudal angle in the sagittal plane was significantly larger in the RG (31.0 ± 10.0° vs. 25.7 ± 8.8°, P = 0.005). There was no difference between the FHG and RG in the horizontal angle, measured in the axial plane using the posterior superior iliac spine (PSIS) as a reference (41.1 ± 8.1° vs. 42.8 ± 6.6°, P = 0.225), or the S2AI to S1 screw angle (9.4 ± 7.0° vs. 11.3 ± 9.9°, P = 0.256), respectively. There was no difference in the overall accuracy between FHG and RG (94.9% vs. 97.8%, P = 0.630). Additionally, there were no significant intraoperative neurovascular or visceral complications associated with S2AI screw placement.Conclusion.Free hand and robotic-guided S2AI screw placement both prove to be safe, accurate, and reliable techniques for achieving spinopelvic fixation.Level of Evidence: 3

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