Robotic-guided Sacro-Pelvic Fixation using S2 Alar-Iliac Screws: Feasibility and Accuracy

Presented at SMISS Annual Forum 2016
By Isador Lieberman MD, MBA, FRCSC
With Xiaobang Hu PhD,

Disclosures: Isador Lieberman MD, MBA, FRCSC A; Mazor Robotics. B; Mazor Robotics, Stryker Spine, Globus, Misonix, Safe Orthopaedics, Medtronic, SI Bone. D; Mazor Robotics. F; Mazor Robotics Xiaobang Hu PhD ,

Introduction

In an effort to minimize the risk of pseudoarthrosis at L5S1 and lessen the strain on the S1 pedicle screws, surgeons have more often integrated multiple pelvic fixation techniques. The S2-alar iliac (S2AI) technique is a recently described method that involves fixation from the S2 level across the sacroiliac joint into the ilium. Due to the complex and variable anatomy of the sacral-pelvic region and intervening sacroiliac joint, optimal screw positioning is difficult even with biplane fluoroscopic guidance.

Aims/Objectives

To review and report our experience with robotic guided S2AI screw placement.

Methods

We retrospectively reviewed patients who underwent S2AI fixation with robotic guidance. Screw placement and deviation from the preoperative plan were assessed by fusing preoperative CT (with the planned S2AI screws) to postoperative CT. The software’s measurement tool was used to compare the planned vs. actual screw placements in axial and lateral views, at entry point to the S2 pedicle and at a 30mm depth at the screws’ mid-shaft, in a resolution of 0.1 millimeters. Medical charts were reviewed for technical issues and intra-operative complications.

Results

35 S2AI screws were reviewed in 18 patients (10 female) operated between 2012 and 2014. The patients’ mean age was 60 years (range 34-81) and mean BMI was 26.6 (range 20.6-33.3). No intra-operative complications that related to the placement of S2AI screws were reported and robotic guidance was successful in all 35 screws. Post-operative CT scans showed that all trajectories were accurate. No violations of the iliac cortex or breaches of the anterior sacrum were noted. At the entry point, the screw deviated from the pre-operative plan by 3.0±2.2 mm in the axial plane and 1.8±1.6 mm in the lateral plane. At the 30 mm depth, the screw deviated from the pre-operative plan by 2.1±1.3 mm in the axial plane and 1.2±1.1 mm in the lateral plane.

Conclusions

No screw malpositions or complications that related to the placement of S2AI screws occurred in this series. The observed deviation in entry point may be attributed to ploughing of the screw during the correction maneuver or skiving of the drill bit on the irregular surface of the sacrum. Despite the deviation, all screws were clinically acceptable. Larger studies are needed to assess the long-term clinical outcomes of robotic guided sacral-pelvic fixation.