Contact mechanics of metal-on-metal hip implants employing a metallic cup with a UHMWPE backing

Abstract

The contact mechanics in metal-on-metal hip implants employing a cobalt chromium acetabular cup with an ultra-high molecular weight polyethylene (UHMWPE) backing were analysed in the present study using the finite element method. A general modelling methodology was developed to examine the effects of the interfacial boundary conditions between the UHMWPE backing and a titanium shell for cementless fixation, the coefficient of friction and the loading angle on the predicted contact pressure distribution at the articulating surfaces. It was found that the contact mechanics at the bearing surfaces were significantly affected by the UHMWPE backing. Consequently, a relatively constant pressure distribution was predicted within the contact conjunction, and the maximum contact pressure occurred towards the edge of the contact. On the other hand, the interfacial boundary condition between the UHMWPE backing and the titanium shell, the coefficient of friction and the loading angle were found to have a negligible effect on the contact mechanics at the bearing surfaces. Overall, the magnitude of the contact pressure was significantly reduced, compared with a similar cup without the UHMWPE backing. The importance of the UHMWPE backing on the tribological performance of metal-on-metal hip implants is discussed.

Effect of microseparation on contact mechanics in metal‐on‐metal hip replacements—A finite element analysis

“…35 Both the cup and head version angles were chosen to be zero to match the corresponding set‐up in a simulator test20 for microseparation with a focus on cup inclination only. With this simplification, a half solid geometry can be used for the computational model25 by making use of the symmetry about the medial‐lateral plane (Figure 2), which also allows the FE model to have a substantially finer mesh with a relatively low computational time. As the corresponding symmetric boundary, for all the nodes on the symmetry plane (Figure 2, OXY plane), the degree of freedom in the direction perpendicular to the symmetry plane was constrained; for the load, only a half magnitude was required and applied to the head centre.…”

“…The FE models were meshed with 8‐node brick and 6‐node triangular prism elements in NX I‐deas 6.125 (Siemens PLM Software, TX). The materials were assumed to be elastic‐perfectly plastic with the yield strength of 840 MPa38 for the CoCr head and cup bearings, and linearly elastic for the cement, with the Young's moduli of 230 and 3.3 GPa and Poisson's ratios of 0.3 and 0.35,39 for the metal bearing and cement, respectively.…”

“…The head‐cup rim contact was modeled with the finite sliding algorithm using ABAQUS/Implicit40(Version 6.11‐1; Dassault Systemes Simulia Corporation, Providence, RI). The effect of friction on the contact stress was considered as negligible25 in the static analysis. The contact computation for each magnitude of the vertical loads in the range of 0.5∼3kN was individually carried out without a loading history.…”

“…FE using commercial software has been extensively developed for contact mechanics of artificial joints 25, 26, 27, 28, 29, 30. Scifert et al29 developed a contact model to study factors that can influence hip dislocation; Elkin et al30, 31, 32 extended the model to investigate the effect of subluxation and impingement for MoM bearings and fracture for CoC implants; Mak et al18, 24 calculated contact stress for edge loading due to microsepartion in CoC hip joints; Sariali et al33 also carried out similar predictions for CoC bearings.…”

Contact mechanics and elastohydrodynamic lubrication in a novel metal-on-metal hip implant with an aspherical bearing surface

“…Diameter and diametral clearance of hip bearings have been theoretically and experimentally found to be the key parameters of enhancing lubrication and minimizing wear (Medley et al, 2001;Smith et al, 2001;Dowson et al, 2004;Rieker et al, 2005;Jin, 2006;Liu et al, 2006b). Other factors such as structural supports and the wall thickness of the cup can also affect the contact pressure and lubrication performance in hip implants (Besong et al, 2001;Jagatia and Jin, 2002;Liu et al, 2003;Liu et al, 2004;Liu et al, 2006b). …”