A Biomechanical Comparison of Alternatives to C2 Pedicle Screws

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.