Methods for determining the spatial distribution of oxidation in ultra-high molecular-weight polyethylene prostheses

Abstract

Oxidative degradation is a well-known problem for UHMWPE used in prostheses. The aim of the present study has been to find suitable techniques to study the spatial distribution of this oxidation in 8 retrieved acetabular cups. The techniques used were visual examination using an optical microscope and computer scanner, FTIR mapping, imaging chemiluminescence, and staining with SO 2 and HCl. The staining technique is based on a previous study which showed that by treating oxidized UHMWPE with SO 2 followed by heat treatment, the hydroperoxides present in the sample react with the SO 2 and discolor the sample. The intensity of this discoloring is, at low levels of oxidation, proportional to the amount of hydroperoxides and accordingly to the level of the oxidation. The same study also showed that staining a sample with hot HCl resulted in a brown discoloration which was proportional to the amount of carbonyls. It was found that the staining techniques do not give as much information about the chemical and physical changes in the material as FTIR mapping but have a great advantage in better spatial resolution of the oxidation and are also much quicker and easier to use. Imaging chemiluminescence turned out not to be a suitable method to use, compared to the other two, since it gives less information and is more difficult to interpret. When interpreting the results from the different techniques used, it was found that all cups showed the typical oxidation behavior of gamma sterilized UHMWPE. All cups but one showed substantial wear of the articulating surface but very little backside wear. Examination of the oxidation and whitening profile suggests that at least some of the oxidation must have occurred in vivo.