Contact mechanics of modular metal-on-polyethylene total hip replacement under adverse edge loading conditions

Abstract

Edge loading can negatively impact the biomechanics and long-term performance of hip replacements. Although edge loading has been widely investigated for hard-on-hard articulations, limited work has been conducted for hard-on-soft combinations. The aim of the present study was to investigate edge loading and its effect on the contact mechanics of a modular metal-on-polyethylene (MoP) total hip replacement (THR). A three-dimensional finite element model was developed based on a modular MoP bearing. Different cup inclination angles and head lateral microseparation were modelled and their effect on the contact mechanics of the modular MoP hip replacement were examined. The results showed that lateral microseparation caused loading of the head on the rim of the cup, which produced substantial increases in the maximum von Mises stress in the polyethylene liner and the maximum contact pressure on both the articulating surface and backside surface of the liner. Plastic deformation of the liner was observed under both standard conditions and microseparation conditions, however, the maximum equivalent plastic strain in the liner under microseparation conditions of 2000 µm was predicted to be approximately six times that under standard conditions. The study has indicated that correct positioning the components to avoid edge loading is likely to be important clinically even for hard-on-soft bearings for THR.

Keywords: Contact mechanics; Edge loading; Metal-on-polyethylene; Microseparation; Total hip replacement.

Fig. 1

The boundary conditions and components of the finite element model.

Fig. 2

The plastic stress-strain relation for the polyethylene liner (Liu, 2005).

Fig. 3

The definition of cup inclination angles and head lateral microseparation distances in the FE model, four cup inclination angles and 12 microseparation distances were considered in the present study. Only four microseparation distances are shown in this figure.

Fig. 4

The distribution of contact pressures (MPa) on the frontside articulating surface as a function of cup inclination angles and microseparation distances.

Fig. 5

The variation of the maximum stress in the liner against cup inclination angles and microseparation distances: (a) maximum von Mises stress of the liner, (b) maximum contact pressure on the frontside articulating surface, (c) maximum contact pressure on the backside surface of the liner.

Fig. 6

The equivalent plastic strain in the polyethylene liner under standard conditions and microseparation conditions (1000 µm) under cup inclination angle of 45°.

Fig. 7

The variation of the maximum equivalent plastic strain in the liner against cup inclination angles and microseparation distances.

Fig. 8

The variation of the maximum stress and equivalent plastic strain in the liner against loading directions and microseparation distances: (a) maximum von Mises stress of the liner, (b) maximum contact pressure on the articulating surface, (c) maximum equivalent plastic strain in the liner.