A Biomechanical Comparison of Alternatives to C2 Pedicle Screws.

Abstract

Study design: Cadaveric biomechanics study.

Objective: This study compares five C2 fixation methods: pedicle, pars, translaminar, medial in-out-in, and subfacetal screws.

Summary of background data: Variations in vascular and pedicle anatomy of the C2 vertebra can make C2 fixation difficult. Two novel trajectories--the medial in-out-in and subfacetal trajectory--may be alternatives. The medial in-out-in trajectory enables three-point cortical fixation while the subfacetal trajectory avoids the VA.

Methods: Polyaxial screws were inserted into C1 lateral masses and C2 vertebrae in 12 cadaveric specimens. Specimens were assigned to one of four test groups based on C2 screw trajectory: pedicle, pars, medial in-out-in, subfacetal, and translaminar. Range of motion (ROM) in flexion/extension (FE), lateral bending (LB), and axial rotation (AR) were measured at C1-2 using a custom-built six-degree-of-freedom motion simulator and motion analysis software. Two ROM tests were performed on each specimen: 1) intact construct and 2) screw-rod construct. C2 screws were then subjected to pullout testing.

Results: Average ROM for intact constructs was 11.04° in FE, 3.21° in LB, and 59.43° in AR. There was a significant difference of ROM in all three directions (P<.01) among constructs. For the screw-rod construct, average ROM decreased 87% in FE, 86% in LB, and 97% in AR compared to intact. A two-way mixed ANOVA showed there was no significant difference in ROM between test groups for FE (P=0.738), LB (P=0.714), or AR (P=0.996) independent of construct. Medial in-out-in showed 10% higher pullout strength compared to the pedicle screw and subfacetal showed an 83% increase in pullout strength compared to the pars screw.

Conclusions: The medial in-out-in and subfacetal trajectories represent viable alternatives for C1-2 stabilization in anatomically constrained cases, such as a high-riding vertebral artery or a narrow C2 pedicle. These novel techniques may expand surgical options for achieving robust C2 fixation.