Biomechanical Study of Horizontal Screw-screw Crosslink in C1–2 Pedicle Screw-rod Fixation

Study Design: This is a biomechanical study in vitro. Objective: To investigate the biomechanical differences between horizontal rod-rod crosslink (hR-R CL) and the horizontal screw-screw crosslink (hS-S CL) implementation in C1–2 pedicle screw-rod (C1–2 PSR) fixation. Summary of Background: To improve internal fixation stability, transverse connector (TC) is used in C1-2 PSR to increase torsional stiffness. The connection mode of horizontal connection includes hR-R CL and hS-S CL. hS-S CL adopted in C1–2 PSR was rarely reported and its biomechanics are still unclear. Materials and Methods: Six fresh cadaveric cervical spine specimens were each tested as an Intact model, then modified and tested as an Instability model (unstable odontoid fractures), and then as 3 internal fixation models, including C1–2 PSR, C1–2 pedicle screw-rod+horizontal rod-rod crosslink (C1–2 PSR+ hR-R CL), C1–2 pedicle screw-rod+horizontal screw-screw crosslink (C1–2 PSR+ hS-S CL). The ROM of the C1–2 segments was measured by applying 1.5 nm load in 6 loading conditions, including flexion-extension (FE), left and right lateral bending (LB), and left and right axial rotation (AR). Results: The C1–2 PSR+hR-R CL and C1–2 PSR+hS-S CL models, respectively, showed 60% and 75% lower ROM than the C1–2 PSR model in LB and AR conditions ( P <0.05). ROM was comparable between the C1–2 PSR+hR-R CL and the C1–2 PSR+hS-S CL models in all loading conditions ( P >0.05). Conclusion: Both types of crosslinks showed superior C1–2 stability under LB and AR conditions than PSR without crosslinks. The C1–2 segment stability was comparable between the 2 types of crosslinks themselves.