Comparison of Cross-Linked Polyethylene Materials for Orthopaedic Applications

Abstract

Cross-linked polyethylenes are being marketed by orthopaedic manufacturers to address the problem of osteolysis caused by polyethylene particulate wear debris. Wear testing of these cross-linked polyethylenes in hip simulators has shown dramatic reduction in wear rate compared with standard ultrahigh molecular weight polyethylene, either gamma irradiated in air or nitrogen - or ethylene oxide-sterilized. However, this reduction in wear rate is not without cost. The cross-linking processes can result in materials with lower mechanical properties than standard ultrahigh molecular weight polyethylene. To evaluate the effect of the various cross-linking processes on physical and mechanical properties of ultrahigh molecular weight polyethylene, commercially available cross-linked polyethylenes from six orthopaedic manufacturers were tested. This study was the culmination of collaboration with these manufacturers, who provided cross-linked polyethylene for this study, wear characteristics of the material they provided, and review of the physical and mechanical properties measure for their polyethylene. Cross-linked materials were evaluated as received and after an accelerated aging protocol. Free radical identity and concentration, oxidation, crystallinity, melt temperature, ultimate tensile strength, elongation at break, tensile stress at yield, and toughness are reported for each material. By comparing these physical and mechanical properties, surgeons can evaluate the trade-off that results from developing materials with substantially lower wear rates.

Do First-generation Highly Crosslinked Polyethylenes Oxidize In Vivo?

“…This increase was not seen at either the backside or the rim, suggesting a mechanism involving mechanical loading. Previous studies of retrieved and unimplanted [5,30] remelted liners have shown little to no oxidation; however, the retrieval results were based on a comparatively short implantation time. Although we could detect an increase in OI over time, the magnitude was below 1 (even at implantation times close to a decade); this level is not anticipated to negatively influence the mechanical behavior [7,17].…”

“…Remelted polyethylene, with reportedly undetectable free radical concentrations [27], would likewise be considered relatively oxidatively stable. First-generation annealed and highly crosslinked polyethylene, which contain a higher concentration of macroradicals as compared with gammasterilized conventional polyethylene liners [5], would be expected to exhibit greater in vivo oxidation than uncrosslinked gas-sterilized liners and remelted highly crosslinked liners. At elevated levels, oxidation may compromise the ultimate strength and ductility of the polyethylene, leading to the complete loss of mechanical integrity [8,17,19].…”

“…However, in the case of a malfunctioning hip such as chronic instability with dislocation, rim oxidation may contribute to localized delamination or rim fracture [18]. Previous studies of remelted liners, including shortterm retrievals, demonstrated low levels of oxidation [5,30]. Additional longer-term studies are necessary to monitor the oxidative stability of these materials as well as to assess the potential impact of in vivo oxidation on wear and mechanical behavior.…”

A fully functional drug-eluting joint implant

“…The mechanical properties (yield strength, UTS, IS, EAB) and wear rate of VPE was all within the limit of clinically used UHMWPE [29] (Figure 3e–f, Supplementary Figure 5–7). and were superior to those of 11 % vancomycin-loaded BC (Figure 3e and 3f).…”

Femoral Head Size and Wear of Highly Cross-linked Polyethylene at 5 to 8 Years

“…These include various cross-linking radiation doses (50-100 kGy) and techniques (gamma or electron beam), thermal treatment (melted or annealed) used to remove free radicals, and terminal sterilization method (gas plasma, ethylene oxide, or gamma radiation in nitrogen) [17]. These differences influence the mechanical properties, crystallinity, and preaging and postaging oxidation levels of the various components [6]. Thus, currently available XLPE acetabular liners are not all equivalent, and their performances in vivo may differ [8-11, 18, 27].…”