A parametric study of cylindrical pedicle screw design implications on the pullout performance using an experimentally validated finite element model

The present study aims to the design of a finite-element model simulating accurately the pullout behaviour of cylindrical pedicle screws and predicting their pullout force. Three commercial pedicle screws, subjected to pure pullout from synthetic bone, were studied experimentally. The results were used for the design, calibration and validation of a finite-element model. Special attention was paid to the accurate simulation of the failure inside the host material. For this purpose a bilinear cohesive zone material model was adopted to control mode-II debonding of neighbouring elements in the vicinity of the screw and simulate this way the failure in shear of the hosting material. Comparison between experimental and numerical results proved that the implementation of this method can significantly enhance the accuracy of the numerical simulation of a screw’s mechanical behaviour under pure pullout loads. The numerical model was used for the parametric study of various factors affecting the pullout performance of a cylindrical pedicle screw. It was concluded that the major parameters influencing the pullout force is the outer radius and purchase length of the screw. Significantly weaker was the dependence of the screw’s pullout force on its thread inclination, depth and pitch.