Characterization of UHMWPE hip cups run on joint simulators

The density and crystallinity of UHMWPE-hip cups were investigated as a function of thickness from the inner stressed surface to the unstressed outer surface. The effects of mechanical strain and chemical reactions during simulation tests, and damage of the material due to pretreatments and storage, resulted in changes of the structure, as indicated by variations in the crystallinity. Independent of either the batch of UHMWPE supplied or the manufacturer and the type of simulator used, the individual sample-sets showed a similar characteristic curve of density versus wall thickness. Infrared spectroscopic evaluations indicated the presence of oxidative degradation, and answers the question as to which areas of the polymers are changed by aging and which compounds are newly formed. The characteristic carbonyl groups were also determined. The concentration trend of carbonyl groups versus wall thickness obtained agrees surprisingly well with the locally determined density and crystallinity trend. As these compounds are formed by reactions which produce stable oxidative degradation products and also crosslinking, we have determined the degree of cross-linking. The determination of the soluble constituents after extraction showed lower degree of crosslinking on the surface than in the middle of the material. Hence it follows that on the surfaces oxidative chain scission is prevailing, whereas in the interior mainly crosslinking is developed. These results indicate that the samples used for the simulation tests had distinct differences in characteristics. Generally the results show that wear tests in joint simulators lead to property changes in UHMWPE which differ considerably from test results previously obtained on retrieved hip cups.