Current Knowledge on the Effect of Technology and Sterilization on the Structure, Properties and Longevity of UHMWPE in Total Joint Replacement
Abstract
Ultrahigh molecular weight polyethylene (UHMWPE) is the most frequently used bearing surface in currently used total joint replacements (TJR). According to the literature available, UHMWPE is the best polymer material, in terms of biocompatibility, mechanical properties and wear resistance, for this application.
In spite of this fact, UHMWPE wear (i.e., release of microscopic particles from the polymer surface) remains one of the main reasons of TJR failures. Consequently, the wear of UHMWPE is a subject of intensive study by both materials scientists and orthopaedic surgeons.
The structure and properties of UHMWPE strongly depend on the way of processing and post-processing modifications. The processing includes polymer resin preparation (microparticles about 100 pm in size) and resin consolidation (forming bulk material).
Post-processing modifications aim at increasing wear resistance and oxidation stability which are regarded as major factors involved in TJR failure. In order to maintain high purity materials for medical application, it is not allowed to use additional chemicals during the modification processes.
The only exception is the use of vitamin E, a natural stabilizer and antioxidant. Considering all the above mentioned facts, the modifications can be based on (I) ionizing radiation such as gamma rays or accelerated electrons, (II) thermal modification, (Ill) additional stabilization with vitamin E, and (IV) sterilization.
According to the modifications, we usually differentiate three generations of UHMWPE. The 1st generation UHMWPE is not modified except for obligatory sterilization.
The sterilization procedures based on chemical procedures (formaldehyde vapours, hot water) have lately been forbidden, abandoned and replaced by gamma-irradiation with doses of 25-45 kGy. In the course of time, sterilization by means of gamma-irradiation showed to be unsuitable due to oxidative degradation of UHMWPE, which resulted in lower wear resistance, worse material properties and a decrease in longevity of joint replacements.
The 2nd generation UHMWPE or highly cross-linked UHMWPE is material treated with ionizing radiation (gamma-rays or accelerated electrons) at higher doses (usually 50-100 kGy). The irradiation is followed by thermal treatment in order to eliminate residual radicals and limit oxidative degradation that occurs in the 1st generation UHMWPE types.
Finally, the material is sterilized usually by modern procedures using ethylene oxide or gas plasma neither of which causes oxidative degradation (as opposed to gamma-irradiation sterilization). The 3rd generation UHMWPE is basically the same as that of the 2nd generation.
The main difference consists in slightly better mechanical properties (strength, fatigue) and further stabilization to oxidative degradation. This is achieved by special procedures, some of which are either controversial (sequential irradiation) or well-accepted by a great majority of the orthopaedic community (vitamin E stabilization).
Only the future will show whether the difference between the 2nd and 3rd generations of UHMWPE will play any role in increasing TJR longevity and therefore the quality of its performance. The final objective of all of UHMWPE modifications is, as already mentioned, to increase wear resistance, oxidative resistance and mechanical performance of the polymer.
These changes, especially an increase in wear resistance, have been confirmed to prolong the longevity of total joint replacements based on UHMWPE.