Alternative bearing surfaces: Crosslinked polyethylene

Aseptic loosening resulting from polyethylene wear with impingement and/or particle-mediated osteolysis has been the major cause of failure of total hip arthroplasty over the past four decades. Hard-on-hard bearings have been widely used as an alternative to metal-on-polyethylene couples. Ceramic-ceramic and metal-metal couples have very low wear rates, but both are vulnerable to accelerated wear patterns with superior rim loading if the cup is inserted somewhat too open (abducted). Some of these hard-on-hard bearings do not have the flexibility to provide different liner options (i.e. extended lip, offset and constrained liners). In addition, ceramic-ceramic couples are still tainted with the legacy of ceramic fracture (although contemporary material and manufacturing quality control make this a rare occurrence today), while the biological implications of the generation and dissemination of metallic debris remain a cause for concern. The heritage of polyethylene dictates that the development of a new material with very low wear characteristics will be widely welcomed. However, there have been many false dawns with “new developments in polyethylene”. It has been these failures that have led researchers to study new failure mechanisms and develop new testing methods. As a result of its higher crystallinity, hylamer had a higher concentration of free radicals, which led to a higher rate of oxidation. Only when this mechanism was understood, did artificial ageing become standard practice in hip simulator testing. Clinical failure has – in the past – provided the impetus for appropriate refinement of laboratory testing methods. Any new material may carry intrinsic risks, not yet known or understood. Highly crosslinked polyethylene does not refer to a homogenous group of materials. The various commercially available highly crosslinked polyethylenes are prepared with different doses and types of irradiation, and have differing aftertreatments (some are melted while others are not). It would not be unreasonable to speculate that they might exhibit different behaviour patterns. In fact, several research groups have shown that radiation dosage and aftertreatment have a marked effect on mechanical properties like impact strength and crack propagation rate. It is, therefore, important to note that “historical” materials are rather dissimilar to modern highly crosslinked polyethylenes. Oonishi (1) used a radiation dose in the order of 100Mrad, Wroblewski (2) used chemical crosslinking adding a radical scavenger and Grobbelaar (3) performed crosslinking in the presence of a crosslinking agent. All of them used gamma irradiation for the sterilisation of the final product and none of them performed any form of thermal aftertreatment – either annealing or remelting. It has been noted that highly crosslinked polyethylenes have substantially better wear resistance even in an abrasive environment. While this may well be true, Fisher and his colleagues (4) have reported that these polyethylenes generate a far greater number of biologically active particles than standard polyethylene when exposed to an abrasive environment. This must be a cause for some concern. Highly crosslinked polyethylene is an exciting development. It will no doubt be widely embraced when the dawn of exciting potential has been realised by the day of clinical reckoning.