Finite Element Analysis to Determine Pull-Out Strength of Fixation Around Large Defect Site in Femur Reconstruction Surgery

Abstract

During the process of bone fracture fixation and reconstruction surgery, bone screws and or fixation plates are used by surgeons. Preliminary experiment showed that the micro-motion of the bone screw leads to loosening and causes the implant to dislocate from its location. Several experimental procedures such as push-in, pull-out, screwing torque and bending tests are used to evaluate the strength of the screw fixation. Although pull-out tests are widely used by researchers, the interaction between the screw fixation and bone surface is largely unexplored until now. This paper aimed at assessing the screw pull-out using homogenized finite element analysis. Owing to the need for longer computational time, screw threads were not considered in most of the finite element analyses done until now. So, it is unclear how different screw types affect the screw–bone interaction. In this paper, finite element analyses were used to compare different types of the screw–bone interfaces. Analyses were performed using buttress and reverse buttress screws, and then the resulting stress distributions around the bone–screw interfaces were analysed. To resemble the actual scenario, screws were pulled out from the bone considering the physiological loading conditions. The obtained results showed that the influence of screw type on stress distribution in the bone-implant interface is significant. As a conclusion, the use of longer screw rods could provide an increased anchoring effect to the fixation device. However, increasing the number of screws potentially cause the stress concentration on the rod which should be take into account. The developed model was also extended to a three-dimensional case study by considering the bone plate, bone screws and the cortical bone. The result obtained from the three-dimensional finite element analysis showed only about 7% error from the experimental counterpart under same conditions.