Thoracic pedicle screw fixation under axial and perpendicular loadings: A comprehensive numerical analysis

Many studies have assessed the pullout fixation strength of pedicle screws, but only a few investigated the fixation strength under non-axial forces such as the ones applied with modern instrumentation techniques. The purpose is to biomechanically compare the fixation strength of different pedicle screw dimensions, bone engagement, entry point preparation and vertebra dimensions under axial pull-out and perpendicular loadings.

A finite element model of two thoracic vertebrae (T3, T8) with three different cortical bone thickness configurations (5th, 50th and 95th percentile) was used. Two bone engagements, two screw diameters and three entry point enlargement scenarios were numerically tested under an axial and four perpendicular forces (cranial, caudal, medial and lateral) until failure for a total of 180 simulations. Force-displacement responses were analyzed using ANOVA and Pareto charts to determine the individual effects of each parameter.

The screw diameter was the predominant parameter affecting the screw anchorage in all loading directions. The larger screw diameter increased by 35% the initial stiffness and force to failure. Cortical bone removal around the entry point reduced the axial and perpendicular initial stiffness (27% and 17% respectively) and force to failure (20% and 13%). Better screw anchorage was obtained with bicortical bone engagement.

The screw diameter and amount of cortical bone left around the entry point are essential for pedicle screw fixation in all loading scenarios. The proximity of the screw threads to the cortical bone (pedicle fill) has a major role in pedicle screw fixation.