Total hip replacements with bearings with cross-linked polyethylene

Abstract

Introduction: Polyethylene has excellent properties as a bearing material for total joint replacements, though it also has some shortcomings. Polyethylene wear debris leads to a biological response, osteolysis and aseptic component loosening. Improved processing techniques have been developed to improve wear resistance, including irradiation and heat treatment. Irradiation with β- or γ-rays produces cross linking of ultra-high molecular weight polyethylene (UHMWPE), resulting in high wear resistance but prone to oxidative degradation due to residual free radicals in crystallinic regions. Th is problem can be solved with heat treatment, which compromises the mechanical properties. Characteristics: Cross-linked polyethylene (XLPE) generates less particles, which are smaller, and rounder, yet far more biologically active then those in conventional UHMWPE. Mechanical properties are worse due to postirradiation modifi cations, with a reduction of crystallinity, and as shown by in vitro studies, fractures and cracking produced under stress loading. Clinical results: In vivo performance of XLPE has shown excellent results with a reduction of wear rate of 50-94% compared to conventional UHMWPE. Th e results are still subject to short-term follow-up with a high number of patients lost to follow-up. No study has compared products of diff erent manufacturers. First cases of osteolysis with the use of XLPE liners have been reported. Many, but not all studies have shown that wear rate is not dependent on head size. Conclusions: Th e aim of the second generation XLPE is to retain the same level of wear resistance without trading off mechanical proprieties. New post-processing techniques are being developed, such as addition of trace amounts of vitamin E.