[A finite element biomechanical study of anterior transpedicular root screw plate fixation system in the lower cervical spine].

Abstract

Objective: To establish a two-segment vertebrectomy model using the finite element method, and to measure and compare the biomechanical properties of the lower cervical anterior transpedicular root screw (ATPRS) plate system, lower cervical anterior pedicle screw (ATPS) plate system, and lower cervical anterior cervical locked-plate (ACLP) system on this model.

Methods: CT data of the cervical spine (C₀-T₁) from a 34-year-old healthy adult male volunteer were collected. A nonlinear complete model of the lower cervical spine (C₃-C₇) was established using Mimics 10.01 software, based on which the ATPRS fixation model, ATPS fixation model, and ACLP fixation model were constructed respectively. An axial pressure of 75 N and a pure couple moment of 1.5 N·m were applied to C3 to make the model perform flexion-extension, left-right lateral bending, and left-right rotation movements. The range of motion (ROM) and stress distribution of each model under different working conditions were compared.

Results: The ROM of the C₄-C₇ segments in the ACLP group, ATPS group, and ATPRS group was reduced to 0.65° (-95.2%), 0.58° (-95.7%), and 0.62° (-95.4%) respectively compared with the intact model during flexion-extension movement;during lateral bending movement, it was reduced to 0.58° (-95.2%), 0.51°(-95.8%), and 0.60° (-95.1%) respectively;during rotation movement, it was reduced to 1.17° (-89.6%), 1.26° (-88.8%), and 1.27°(-88.7%) respectively. In terms of the stress on the titanium mesh graft, the ATPS group and ATPRS group had the maximum load during extension and the minimum load during flexion. Compared with the ACLP group, the stress on the titanium mesh graft in ATPS and ATPRS decreased by (-33.7%) and (-15.8%) in flexion, (-29.4%) and (-13.2%) in extension, (-26.2%) and (-23.4%) in lateral bending, and (-18.8%) and (-5.4%) in rotation, respectively. In terms of bone-screw interface stress, the peak bone stress near the C₇ screw in the ACLP group, ATPS group, and ATPRS group increased by 49.2%, 45.0%, and 47.6% respectively compared with the peak bone stress near the C₄ screw during extension. However, during flexion and lateral bending, there was no significant difference in the peak bone stress near the C₄ and C₇ screws. During rotation, the difference between the peak bone stress near the C₄ screw and that near the C₇ screw showed that in the ACLP group, left rotation (37.6%) was similar to right rotation (36.7%), while in the ATPS group and ATPRS group, left rotation was lower than right rotation.

Conclusion: Compared with the ACLP group, the ATPS group and ATPRS group have greater fixation stiffness and more stable fixation. However, in rotational movement, due to the uneven distribution of fixation stiffness, the stress distribution during torsion is uneven, but it is still better than the ACLP group. This indicates that ATPRS, like ATPS, has good primary stability, providing favorable conditions for bone graft fusion.