Analysis of the S2 Alar-iliac Screw As Compared with the Traditional Iliac Screw: Does It Increase Stability with Sacroiliac Fixation of the Spine?

Background Context: Arthrodesis of the lumbosacral junction continues to be a challenge in pediatric and adult spinal deformity surgery.

Purpose: To evaluate the biomechanical rigidity of two types of lumbosacral fixation. Our hypothesis was that the use of S2 alar-iliac (S2AI) fixation will result in statistically similar biomechanical fixation as compared with use of an iliac screw with a 95% confidence interval.

Study Setting: Controlled biomechanical laboratory METHODS: Ten human cadaveric lumbosacral specimens were separated into two test groups: (1) S2AI (n=5) and (2) iliac screw (n=5). S2AI and iliac screws were placed according to current clinical practice techniques. Specimens were mounted in an unconstrained dual leg stance configuration for testing in flexion, extension, lateral bending, and axial rotation. These loads were induced by moving the offset loading arm 10 mm in the respective direction from the point of neutral motion with displacement control up to a 10 N-m moment, except axial rotation which used a 4 N-m moment. Optical tracking was used to monitor motion of the vertebra, pelvis, and fixation instrumentation during testing. Specimens were tested in intact and instrumented states. The stiffness values between S2AI and iliac screw configurations were compared.

Disclosure: The present study received external research support (>$50,000 -<$75,000) from Stryker Spine (Allendale, NJ, USA).

Results: There was a consistent trend of increased construct stiffness for all S2AI samples compared with the iliac screw group. However, none of the groups tested reached statistical significance for a 95% confidence interval.

Conclusions: S2AI screws are just as stable as iliac screws with biomechanical testing in flexion, extension, rotation, lateral bending, and axial rotation. Given the similarities of biomechanical testing to human movements, these findings support S2AI screws as a viable option for lumbosacral fixation.