Volume VIII, Number 1 | Spring 2024

A Comparison of S2 Alar-Iliac (S2AI) Screw Trajectories Placed Utilizing Open and Minimally Invasive Techniques on Patients Undergoing Spinal Fusion

1McDermott M, 2Rogers M, 3Prior R, 4Byrne N, 4Patel A
1Ohio University, Athens, Ohio, United states; 2Ross University, Miramar, Florida, United states; 3Fransiscan Health Olympia Fields, Olympia Fields, Illinois, United states; 4Duly Health, Downers Grove, Illinois, United states

Pelvic fixation in long spinal fusions can improve construct stability and reduce L5-S1 pseudoarthrosis rates. Minimally invasive (MIS) S2AI screw placement could offer an alternative to open techniques with less tissue dissection and blood loss. MIS pelvic screw placement is confined by the MIS exposure. In this study, we compare the trajectories of S2AI screws placed utilizing an open and MIS technique.

The purpose of our study is to compare the trajectories of S2AI screws placed using open and minimally invasive techniques.

A retrospective review was conducted on patients undergoing spinal fusion with pelvic fixation between June 2019 and January 2023. Axial views of postoperative lumbar CT scans were used to determine screw trajectories. These were recorded as S2AI, sulcus, or ilium screws. Sulcus screws were defined as pelvic screws which penetrated the sacrum but lacked circumferential purchase or entered through the posterior SIJ ligamentous complex. Trajectory rates were determined by the number of screws coursing along each trajectory over the total number of pelvic screws placed. MIS pelvic screws were placed through the same ipsilateral wiltse incision as required for the lumbar screws. No additional midline incision was made.

Eighty-seven (87) patients (Male:37%, Female:63%) and 181 pelvic screws were included in the study. 95% of the screws were 8.5x90mm. The most common diagnosis was degenerative scoliosis (36%) or L5-S1 spondylolisthesis. Sixty-eight (68) patients (141 pelvic screws) underwent postoperative CT scans. Six (6) of these screws were placed intentionally into the ilium and excluded from analysis. Of the remaining 135 pelvic screws, 67 were MIS screws and 68 were open screws. The average levels fused for MIS and open cases was 3 and 8 respectively. The overall rate of placing a screw along the true S2AI trajectory was 43.7% (Sulcus: 30.41%, Ilium: 25.9%). There was a higher rate of achieving an S2AI trajectory using an open technique (58.2%, n=68) then MIS (28.8%; n=67). Placing a pelvic screw using the MIS technique had a higher rate of sulcus (MIS vs open; 33.4% vs 28.4%) and ilium (37.9% vs 13.4%) trajectories. There was 1 pelvic screw with its tip breaching the border of the pelvis that did not require revision. No neurovascular injuries, visceral injuries, or intraoperative sacral fracture were observed.

MIS and open techniques offer similar methods for pelvic fixation along the S2AI trajectory. While open techniques provide a more reliable outcome to achieve boney purchase along the S2AI trajectory, both methods resulted in increased stability at the distal portion of the constructs.

Image Number: 1

The Journal of the American Osteopathic Academy of Orthopedics

Steven J. Heithoff, DO, FAOAO

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