Biomechanical differences between occipital plate and modified C1 lateral mass screw in the treatment of complex craniocervical malformations: a finite element analysis.

Abstract

Complex craniocervical malformations pose significant challenges to surgical fixation. The biomechanical advantage of occipital plate fixation versus short-lever modified C1 lateral mass screw fixation remains controversial, and finite element analysis (FEA) is a reliable tool for implant performance evaluation. To compare biomechanical characteristics of occipital plate fixation and modified C1 lateral mass screw fixation in AOZ-BI and AOZ-AAD models via FEA, and guide surgical decision-making. A validated healthy occipito-atlantoaxial (C0-C2) FEA model was established using CT data. Two pathological models were constructed: AOZ-BI (Group A, atlantoaxial distance [ADI] < 5 mm) and AOZ-AAD (Group B, ADI ≥ 5 mm with transverse ligament dysfunction), each divided into occipital plate and modified C1 lateral mass screw subgroups. Static loads (40 N preload + 1.5 N·m torque) simulated flexion (Fe), extension (Ex), lateral bending (LB), and axial rotation (AR). C1-C2 range of motion (ROM) and screw-rod peak Von Mises stress (PVMS) were measured. Modified C1 lateral mass screw fixation reduced C1-C2 ROM by 19.67% (flexion-extension) to 48.51% (lateral bending) compared with occipital plate fixation. In flexion/extension/axial rotation, C1 lateral mass screw fixation increased screw-rod peak Von Mises stress (PVMS) by 51.17%-131.37% in the AOZ-BI group and 36.51%-56.02% in the AOZ-AAD group; in lateral bending, PVMS decreased by 19.46% in the AOZ-BI group but increased by 5.24% in the AOZ-AAD group. occipital plate fixation consistently had higher ROM (Group B highest) but lower PVMS. Modified C1 lateral mass screw fixation provides superior C1-C2 stability for AOZ-associated BI-AAD but increases implant stress in Fe/Ex/AR. Occipital plate fixation is less stable but reduces stress. Clinically, C1 lateral mass screw is preferred for AOZ-BI; AOZ-AAD requires balancing stability and stress risk. Occipital plate suits patients with severe C1 lateral mass hypoplasia. FEA effectively evaluates craniocervical fixation biomechanics.