A Biomechanical Comparison of Alternatives to C2 Pedicle Screws

Introduction: Variations in vascular and pedicle anatomy of the C2 vertebra (high riding vertebral artery (HRVA), narrow pedicle) can make C2 fixation difficult. Two novel trajectories, the medial in-out-in and subfacetal, may be alternatives. This biomechanical study compares C2 fixation methods: pedicle, pars, translaminar, medial in-out-in, and subfacetal screws.

Methods: Polyaxial screws were inserted in the bilateral C1 lateral mass and bilateral C2 vertebrae of 12 fresh frozen human cadaver specimens. Specimen were assigned to one of four test groups based off C2 screw trajectory: pedicle, pars, medial in-out-in, and subfacetal combined with translaminar. Range of motion (ROM) in flexion/extension (FE), lateral bending (LB), and axial rotation (AR) were measured at C1-2. Two ROM tests were performed on each specimen: 1) intact ROM and 2) surgical construct with screw-rod ROM. Following ROM, C2 screws were subjected to pullout testing using a uniaxial MTS machine at 5mm/min.

Results: Average ROM for intact specimens was 11.04° in FE, 3.21° in LB, and 59.43° in AR. A two-way mixed ANOVA showed there was no significant difference in ROM between C2 trajectory groups for FE (p=.786), LB (p=.468), or AR (p=.998). There was a significant difference (p <.001) between intact specimens and screw-rod fixated specimens. Average pullout strength was 559 N, 240 N, 616 N, 438 N, and 338 N for the pedicle, pars, medial in-out-in, subfacetal, and the translaminar group, respectively. Although not significantly different, medial in-out-in showed 10% higher pullout strength compared to the pedicle screw and subfacetal showed an 80% increase in pullout strength compared to the pars screw.

Conclusion : This study suggests that two novel C2 screw trajectories, medial-in-out in and subfacetal screws, have comparable biomechanical fixation strength and ROM to traditional C2 trajectories: pedicle, pars, and translaminar.