Dinesh Nathwani, Alister Hart, Ravi Shenoy, Herb Schwartz and Matthew Hill review the mechanical properties and advances in the development of polyethylene bearing surfaces for use in knees, discussing a new material with potentially improved mechanical and wear properties
Highly cross-linked polyethylene (HXLPE) has been and continues to be commonly used for liners in total hip arthroplasty (THA) due to its known ability to reduce wear during articulation. Its use in the tibial tray for total knee arthroplasty (TKA), however, has been much more controversial. Specifically, decreased mechanical properties and larger wear particles are concerning. A new micro-composite with hydrophilic properties, known as BioPoly®, may be able to offer an alternative material to offset some of the current concerns regarding HXLPE in total knee replacements (TKR).
History and properties of HXLPE
Before examining the use of HXLPE in the knee, it is important to understand the history and properties of the material as used for hip liners. Ultra-high molecular weight polyethylene (UHMWPE) was first introduced in the 1970s by John Charnley as a bearing material for THA. Although the material was successful, concerns eventually grew in regard to implant loosening due to osteolysis (bone loss surrounding an implant) in the 1990s. Polyethylene wear particles are the primary cause of osteolysis as they induce an autoimmune response that results in bone resorption. Osteolysis has now been shown to be one of the most common causes of revision for primary THA [1]. In response to this problem, HXLPE was developed.
HXLPE has a proven capability both in vitro and in vivo to reduce polyethylene wear in hip liners. A recent literature-based systematic review found weighted average wear rates of 0.042mm/year (range: 0.002–0.08mm/year) and 0.137mm/year for HXPLE and conventional UHMWPE respectively, based on radiographic analysis [2]. For comparison, it has been shown that osteolysis is unlikely to occur at wear rates below 0.1mm/year [3]. Patients with these lower wearing HXLPE hip liners have an 87 per cent less likely incidence of osteolysis when compared with conventional UHMWPE liners [2]. Despite the decreased wear and decreased incidence of osteolysis from HXLPE liners, there are still some questions regarding the clinical benefit of the technology. A recent randomised clinical trial [4] for THA (mean follow-up = 6.8 years) found significantly lower wear for HXLPE when compared with conventional UHMWPE (0.003mm/year vs 0.051mm/year), but clinical outcomes were not significantly different between the two different types of liners. Nevertheless, HXLPE has been able to consistently demonstrate decreased wear when compared with conventional UHMWPE, leading to its widespread use in hip liners for THA.
Although wear is reduced, the processing required for HXLPE components has significant deteriorative effects on the oxidative and mechanical properties of the material, potentially increasing the risk of fracture during articulation. Cross-linking of polyethylene requires the generation of free radicals (via gamma or e-beam irradiation, or chemically), which cause polymer chains to bond together in the amorphous phase of the material. Depending on the amount of cross-linking required, doses between 75 and 100kGy are typically used, with cross-linking density increasing with higher dosages. Free radicals, however, are also produced in the crystalline phase of the material, and these free radicals are unable to react. Therefore, these remaining free radicals are susceptible to oxidation in vivo, resulting in chain breakage and decreased mechanical properties [5] due to premature aging of the materi