Numerical study of a screw's pullout failure and the influence of its stiffness

There are indications in the literature that the mechanical behaviour of a screw is influenced by the difference between the screw stiffness and the respective stiffness of its hosting material. In addition it is known that bone remodelling around orthopaedic implants is greatly influenced by the intensity and the uniformity of the stress field developed. In this context an experimentally validated finite element (FEmodel of a screw and its hosting material was employed for the study of the stress field developed inside the hosting material and the estimation of the screw's pullout strength. Special attention was paid to the accurate simulation of the hosting material failure. The results indicated that when the screw and its hosting material have comparable moduli of elasticity (e.g. titanium threaded bars inserted into marble, ETi/EMarble∼100 most of the pullout load is carried by the first threads and therefore the hosting material in the vicinity of these threads is more likely to fail first. On the contrary when the screw is significantly stiffer than the hosting material (e.g. titanium screws inserted into bone ETi/ECancelous Bone∼103most of the load is carried by the deepest threads. Additionally the analysis revealed that there is an optimum screw-hosting material modulus of elasticity ratio for which the stresses are more evenly distributed into the hosting material increasing this way the screw's pullout strength.