Dependence of the pullout behaviour of pedicle screws on the screw-hosting material relative deformability

Indications exist in literature that a screw’s pullout behaviour is among others influenced by the relative deformability of the screw and its hosting material. In addition it is known that the stress field developed in the vicinity of an orthopaedic implant significantly influences bone remodelling. In this context an experimentally validated finite element model of a screw and its hosting material was employed for the study of the pullout phenomenon. The results indicated that the stress distribution within the screw’s hosting material is strongly influenced by the ratio of the screw’s elastic modulus over the respective one of its hosting material. In addition it is concluded that an optimum value of this ratio exists for which the stresses are more uniformly distributed along the length of the screw improving this way the pullout behaviour, and therefore the overall mechanical response, of the ‘screw-hosting material’ complex.