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.
Abstract:The occurrence of edge loading in hip joint replacement has been associated with 25 many factors such as prosthetic design, component malposition and activities of daily living. 26The present study aimed to quantify the occurrence of edge loading/contact at the articulating 27 surface and to evaluate the effect of cup angles and edge loading on the contact mechanics of 28 a modular metal-on-polyethylene (MoP) total hip replacement (THR) during different daily 29 activities. A three-dimensional finite element model was developed based on a modular MoP 30 bearing system. Different cup inclination and anteversion angles were modelled and six daily 31 activities were considered. The results showed that edge loading was predicted during normal 32 walking, ascending and descending stairs activities under steep cup inclination conditions 33 (≥ 55°) while no edge loading was observed during standing up, sitting down and knee 34 bending activities. The duration of edge loading increased with increased cup inclination 35 angles and was affected by the cup anteversion angles. Edge loading caused elevated contact 36 pressure at the articulating surface and substantially increased equivalent plastic strain of the 37 polyethylene liner. The present study suggested that correct positioning the component to 38 avoid edge loading that may occur during daily activities is important for MoP THR in 39 clinical practice. 40 41
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