INDUCED STRESSES AND SHELL/LINER MICROMOTIONS IN THA COMPONENTS DURING DISLOCATION. A FINITE ELEMENT STUDY.-asme/terms-of-use

Abstract

INTRODUCTION During total hip replacement (THR) many supporting structures for the retention of the femoral head within the acetabulum are removed. Previous studies have determined that this lack of supporting structures leads to separation of the femoral head within the UHMWPE liner. During the relocation phase the femoral head impacts with the liner creating high stress conditions that may contribute to premature polyethylene wear and instabilities. The proposed study focuses on the femoral head, liner and acetabular shell when subjected to intermittent loading conditions due to dislocations of the femoral head. The objective of this paper is to address the effect of dislocation and the induced stresses during impact. The parameters studied include the dislocation or gap distance, the impact force duration and the friction between all the components involved. The research highlights the build up of residual stresses and their contribution to the cup/liner and acetabular bone instability. MATERIAL AND METHODS A three-dimensional 3595 element FE model ( A rigid material type was chosen for the titanium components, such as the acetabular shell and the femoral head, and a Linear Elastic Isotropic material for the UHMWPE liner. The material properties of the liner were based on the information obtained in the available literature (E=975 Mpa, µ=0.46) [1]. A linear contact between implant components was modeled using an automatic surface-to-surface contact (ASTS) with an experimental friction coefficient between titanium and UHMWPE of µ=0.038. The acetabular cup was fully constrained; the femoral head was constrained with respect to the rotational degrees of freedom. In order to simulate the fixation grooves of the liner and the acetabular shell, specific nodes placed in the external surface of the liner were fully constrained while the remainders of the liner's nodes were fully un-constrained. The Model was validated with recent literature [1]. Figure1: Fe model According to previous studies by Douglas A. Dennis et al [2], where the separation of the femoral head and the liner was measured using fluoroscopy techniques, the separation values ranged from ∆y=0 mm to ∆y=5.3 mm. Most of the data provided [1] highlighted the axial separation (∆y, STRESS FR