Abstract
Introduction
Wear phenomenon of ultra-high molecular weight polyethylene (UHMWPE) in hip and knee prostheses is one of the major restriction factors on the longevity of these implants. In retrieved hip prostheses with screw holes in the metal acetabular cup for fixation to the pelvis, the generation of cold flow into the screw holes is frequently observed on the backside of the UHMWPE acetabular cup liner. In most retrieved cases, the protruded areas of cold flow on the backside were located on the reverse side of the severely worn and deformed surface of the polyethylene liner. It would appear that the cold flow into screw holes contributes to increase of wear and damages of the polyethylene liner in hip prosthesis.
Methods
In a previous study (Cho et al., 2016), we pointed out the generation of cold flow into the screw holes on the backside of the retrieved UHMWPE acetabular cup liner as shown in Figure 1. The primary purpose of this study was to investigate the influence of the cold flow into the screw holes on the wear of the polyethylene liner in hip prosthesis. In this study, computer simulations of the generation of cold flow were performed using the finite element method (FEM) in order to propose the design criteria about the cold flow of the hip prosthesis for improving the wear resistance of the polyethylene liner. We especially focused on the influence of polyethylene thickness and contact surface conformity on the generation of cold flow into the screw hole.
Results
An example of the results of a series of the FEM simulations performed in this study is shown in Figure 2. This figure shows the distributions of the contact stress in the polyethylene liners. The graphs shown in Figure 3 are the summary of results of a series of the FEM simulations performed in this study. The graph in Figure 3(a) shows the changes in the maximum contact stress in the polyethylene liner with the thickness of polyethylene liner. The graph in Figure 3(b) shows the changes in the maximum contact stress in the polyethylene liner with the radial clearance between the femoral head and the polyethylene liner.
Discussion and Conclusions
It was found that the magnitudes of cold flow and maximum contact stress in the polyethylene liner had a tendency to increase with decreasing the thickness of polyethylene liner. It was also found that the magnitude of cold flow and maximum contact stress in the polyethylene liner had a tendency to increase with increasing the radial clearance between the femoral head and the polyethylene liner. The results of this study suggest that polyethylene thickness and contact surface conformity have a significant influence on the generation of cold flow into the screw holes and wear of the polyethylene liner.
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