Investigation of fixation screw pull-out strength on human spine

For the purpose of fixation and stabilizing the spine, the pull-out strength of the screw is one of the most important factors to be considered. The material properties of the bone, coupled with the principal dimensions of the screw such as major diameter, minor diameter, pitch and purchase length, may affect the pull-out strength of the fixation screw. In this study, the effects of various factors on the bone screw pull-out strength were studied using finite element method of analysis. A three-dimensional finite element model simulating the threaded connection of bone and surgical screw was constructed. The behavior of both the bone and the screw during screw pull-out were analyzed and discussed. The effect of the screw parameters on the screw pull-out strength was also investigated. The results showed that failure of the connection was due to bone shearing which occurred along a cylindrical surface determined by the outer perimeter of the screw and penetrated length. The distribution of the shear stress along the threaded length in the bone was nearly uniform. The effect of the major diameter on the screw pull-out strength was more significant than those of the minor diameter and the pitch. The minor diameter hardly affected the pull-out strength.