Abstract
Introduction
Ultra-high molecular weight polyethylene (UHMWPE) is the sole polymeric material currently used for weight- bearing surfaces in total joint replacement. However, the wear of UHMWPE and the polyethylene wear debris generated in the human body after total joint replacement cause serious clinical and biomechanical reactions.
Therefore, the wear phenomenon of UHMWPE in total joint replacement is now recognized as one of the major factors restricting the longevity of these implants. In order to minimize the wear of UHMWPE and to improve the longevity of artificial joints, it is necessary to clarify the factors influencing the wear mechanism of UHMWPE.
Materials and Methods
The wear and/or failure characteristics of 33 retrieved UHMWPE acetabular cup liners of hip prostheses were examined in this study. The retrieved liners had an average in vivo duration of 193.8 months (75 to 290 months). Several examples of the retrieved liners are shown in Figure 1. The elasto-plastic contact analyses between metal femoral neck and polyethylene liner and between metal femoral head and polyethylene liner using the finite element method (FEM) were also performed in order to investigate the factors influencing the wear and/or failure mechanism of the polyethylene liner in hip prosthesis.
Results
In the examination of the retrieved polyethylene liners, the generation of component impingement was observed in 24 cases of the 33 retrieved liners (72.7%) as shown in Figures 1(a) and (b). In addition, the generation of cold flow into the screw holes in the metal acetabular cup was observed in 27 cases of the 33 retrieved liners (81.8%) as shown in Figures 1(c) and (d). Several examples of the results of the FEM contact analyses are shown in Figure 2. In the simulation of the component impingement, it was found that high contact stresses which exceed the yield stress of UHMWPE and considerable plastic strains occurred in the rim of the polyethylene liner as shown in Figures 2(a) and (b). In the simulation of the cold flow, it was found that the stress concentration near the edge of screw hole has significant influence on the states of contact stresses and plastic strains in the surface and undersurface (backside) of the polyethylene liner as shown in Figures 2(c) and (d).
Discussion and Conclusions
In this study, we focused on the impingement between the metal femoral neck and the polyethylene liner and the cold flow into the screw holes on the backside of the polyethylene liner as the factors influencing the wear and/or failure of the UHMWPE acetabular cup liner in hip prosthesis. The results of these retrieval and analytical studies confirmed that the component impingement and the cold flow into the screw holes contribute to increase of wear and/or failure of the polyethylene liner. Therefore, it is necessary to improve resistance to the component impingement and the cold flow in order to decrease the wear and/or failure of the UHMWPE acetabular cup liner and to increase the longevity of hip prosthesis.
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