Biomechanical Impact of Splint Rods in Posterior Cervicothoracic Fixation: A Finite Element Analysis

Study Design Basic Science Study. Objective To determine the impact of splint rods, rod material, and rod diameter on cervicothoracic construct load bearing capacity. Methods Finite element analysis was used to simulate 8 construct variations in a C7 vertebrectomy model with pedicle screw fixation in C5-T2. Variations included the material of rods (titanium or cobalt-chrome alloys), presence or absence of splint rods, the diameters of the splint rods (3.5 or 4.5 mm), and the size of lateral mass screws (3.5 mm or 4.0 mm). Boundary conditions replicated ASTM F1717/ISO 12189 standards. Yield load, displacement, stiffness, and stress distributions were analyzed under worst-case loading (2 cm displacement). Results The best configuration was comprised of 3.5 mm titanium primary and splint rods, which achieved the highest load capacity of 107N and stiffness of 29.8 N/mm. The worst configuration was comprised of single 3.5 mm titanium rods, which demonstrated the lowest load capacity of 66N. Conclusions Adding splint rods improves load bearing capacity of cervicothoracic fixation constructs. Increasing construct stiffness through increasing diameter of the rods or screws, or through change in rod alloy to cobalt chrome does not always result in an improvement of the load bearing capacity as it risks earlier failure at the bone-screw interface. Optimal construct design is a careful balance between construct stiffness and load-sharing.