Biomechanical comparison of seven occipital-cervical fixation constructs using finite element modeling

This was a basic science study.

The objective of this study was to determine which occipital-cervical (OC) fixation construct best balances construct stiffness with minimization of occipital screw pullout forces.

Finite element analysis was used to simulate seven OC construct variations spanning across C1 with fixation in C2 and C3. Variations included the use of occiput rods, keel plates with varying width, two-hole clamps, two-hole clamps in addition to medial or lateral one-hole clamps, and occiput screw diameter. Modeling was in accordance with ASTM F2706/ISO 12189 guidelines. Materials were in accordance with ASTM F1295/ASTM F1839/ISO 10243 guidelines. Outcomes included bending stiffness, axial screw pullout reaction forces, and von Mises stress distribution to identify stress localization and predicted failure modes. Constructs were ranked using multiattribute utility theory (MAUT).

Construct E (comprised two-hole clamps with added one-hole medial clamps) demonstrated the highest bending stiffness (820 N.mm/deg) and the highest MAUT utility score (0.79), representing the optimal balance of stiffness and minimized screw pullout forces. Construct F (comprised two-hole clamps alone) demonstrated the lowest bending stiffness (509 N.mm/deg).

A construct composed of medially fixed one-hole and two-hole clamps provided the best balance of bending stiffness and minimized occipital screw pullout reaction forces. Aligning screws near the median plane, using bilateral clamp fixation, and avoiding components that mechanically join the two rods improved construct performance. Increasing occipital screw diameter from 4.5 mm to 5.0 mm had little biomechanical impact. Stress localization at occipital component-rod interfaces suggests a potential weak point with risk of notching and fatigue failure.