A Biomechanical Comparison of Alternatives to C2 Pedicle Screws

STUDY DESIGN: Cadaveric biomechanics study. 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. MATERIALS AND 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 in 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 with 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 with the pedicle screw and subfacetal showed an 83% increase in pullout strength compared with 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.