An Evaluation of Retrieved UHMWPE Hip Joint Cups

In this study it is demonstrated that the combined chemical and mechanical influences of the implant situation cause property changes of ultra-high-molecular-weight polyethylene (UHMWPE) hip joint cups. Nearly 250 loosened hip cups, retrieved 3 weeks to 14 years after implantation, were investigated. The clinical long-term behavior of various shaped polyethylene hip sockets are statistically analyzed. The main damage features were defined and described. Density measurements show a density increase with implantation time and a dependence of these changes from implant position and loading conditions. The rate of extractable constituents also increases with course of time. An increased in vivo conditioned oxidation of the UHMWPE can be demonstrated by infrared (IR) spectrometry. The density increase can be explained by post-crystallization, which is the result of oxidative chain scission. This leads to a reduction of the average molecular weight of the PE and to an increased extractability of constituents. Since these changes have been recognized as the reasons for aging and failing of UHMWPE, the methods of material characterization used in this study for retrieved implants will help to develop suitable in vitro testing and simulating methods. Characteristic damage features of hip cups allow direct relationships with construction characteristics and their improvement.