A Randomized Controlled RSA Study Comparing the 2nd Generation Highly Cross-linked Polyethylene to Conventional Polyethylene in Cemented Total Hip Arthroplasty

Background:

Total hip replacements is a cost effective treatment of a symptomatic degenerative hip disease. Cemented total hip replacements are the most common implants in Sweden (Swedish hip registry, 2009).

Wear of ultra high molecular weight polyethylene has been a issue with regard to implant survival. At the time of the shift of the millennium highly cross-linked polyethylene was introduced to the markets. Experimental studies had shown reduction of wear in hip simulator studies (McKellop,1999). Subsequently the intention is to reduce osteolysis and implant loosening. Single surgeon pilot studies in the 1970 (Wroblevski et al. 1996, Oonishi 1998, Grobelaar 1999) were the first clinical series with good results leading to the development of 2nd generation highly cross-linked polyethylenes which is now the almost exclusively used PE worldwide. So far medium term clinical evaluation and laboratory assessment have showed reduced wear and good mechanical properties (Digas et al. 2003, 2004, 2007, Röhrl et al. 2005, 2007, D´Antonio et al. 2005)

Production of highly crosslinked polyethylene (HXLPE) consists of several steps. First the PE material is crosslinked by being subjected to either gamma irradiation of various intensity or electron beam irradiation. The second step is to remove the free radicals created during the irradiation. This is done either by annealing (i.e. warming up the PE to just under its melting temperature) or melting (i.e. warming up the PE to above its melting temperature). Finally, the implant is sterilized either by gamma irradiation or Ethylene Oxide and packed in vacuum or inert atmosphere.There are some possible drawbacks and challenges in the process of cross-linking polyethylene with regard to preserving the mechanical properties and elimination of free radicals (Kurtz et al. 2006, Currier et al. 2007).

Submelt annealing preserves mechanical properties but on the other hand cannot eliminate all free radicals existing in the PE. Retrieval-studies of some 2nd generation highly cross-linked polyethylenes manufactured in this way have shown signs of oxidation. The in-vivo oxidation has been located in the non articulating area such as the circumference of the acetabular liner. It has been discussed that the femoral head may protect the polyethylene against oxidation, and hence prevent the possible negative consequences of remaining free radicals (Currier et al 2007).