Peroxide cross-linked UHMWPE blended with vitamin E.

Radiation crosslinked ultrahigh molecular weight polyethylene (UHMWPE) is the bearing surface material most commonly used in total joint arthroplasty because of its excellent wear resistance. Crosslinking agents such as peroxides can also effectively increase wear resistance but peroxide crosslinked UHMWPE has low oxidative stability. We hypothesized that the addition of an antioxidant to peroxide crosslinked UHMWPE could improve its oxidation resistance and result in mechanical, tribological, and oxidative properties equivalent to currently utilized radiation crosslinked UHMWPEs. Various vitamin E (0.1-1.0 wt % and peroxide concentration (0.5-1.5 wt %) combinations were studied to investigate changes in crosslink density, wear rate, mechanical properties, and oxidative stability in comparison to radiation crosslinked UHMWPE. Peroxide crosslinking was more efficient as compared to radiation crosslinking in the presence of vitamin E with the former resulting in lower wear rate with vitamin E concentrations above 0.3 wt %. The tensile mechanical properties were comparable to and the impact strength was higher than those of the clinically relevant radiation crosslinked controls. We also determined that gamma sterilization of peroxide crosslinked vitamin E blends improved wear resistance further. In summary, peroxide crosslinking of vitamin E-blended UHMWPE may provide a feasible and economical alternative to radiation for achieving clinically relevant properties for total joint implants using UHMWPE. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1379-1389, 2017.

Effects of simulated oxidation on the in vitro wear and mechanical properties of irradiated and melted highly crosslinked UHMWPE.

Radiation crosslinked ultrahigh molecular weight polyethylene (UHMWPE) have reduced the wear rate of the bearing surface in total joint arthroplasty and the incidence of peri-prosthetic bone loss due to wear particles. The oxidation potential afforded to the material by the trapped residual free radicals after irradiation was addressed in first generation crosslinked UHMWPEs by using thermal treatments such as annealing or melting after irradiation. Postirradiation melted crosslinked UHMWPE did not contain detectable free radicals at the time of implantation and was expected to be resistant against oxidation for the lifetime of the implants. Recent analyses of long-term retrievals showed it was possible for irradiated and melted UHMWPEs to oxidize in vivo but studies on the effects of oxidation on these materials have been limited. In this study, we determined the effects of in vitro aging on the wear and mechanical properties of irradiated and melted UHMWPE as a function of radiation dose and found that even small amount of oxidation (oxidation index of 0.1) can have detrimental effects on its mechanical properties. There was a gradual increase in the wear rate below an oxidation index of 1 and a drastic increase thereafter. Therefore, it was shown in a simulated environment that oxidation can have detrimental effects to the clinically relevant properties of irradiated and melted UHMWPEs.

Surface cross-linked UHMWPE can enable the use of larger femoral heads in total joints.

Limiting cross-linking to the articular surfaces of ultrahigh molecular weight polyethylene (UHMWPE) to increase wear resistance while preventing detrimental effects of cross-linking on mechanical strength has been a desirable goal. A surface cross-linked UHMWPE can be achieved by blending UHMWPE with a free radical scavenger, such as vitamin E, consolidating the blend into an implant shape, extracting the vitamin E from the surface, and radiation cross-linking the surface extracted blend. This process results in high cross-link density in the vitamin E-depleted surface region because vitamin E hinders cross-linking during irradiation. In this study, we described the properties of successful extraction media and the manipulation of the wear and mechanical properties of extracted, irradiated blends. We showed that these formulations could have similar wear and significantly improved mechanical properties compared to currently available highly cross-linked UHMWPEs. We believe that these materials can enable thinner implant forms and more anatomical designs in joint arthroplasty and may provide a feasible alternative to metal-on-metal implants.

Increasing irradiation temperature maximizes vitamin E grafting and wear resistance of ultrahigh molecular weight polyethylene.

Vitamin E stabilization of radiation crosslinked ultrahigh molecular weight polyethylene (UHMWPE) for total joint implants can be done by blending of UHMWPE resin powder with vitamin E, followed by consolidation and irradiation of the blend. It is well known that vitamin E prevents crosslinking in UHMWPE during ionizing radiation. We hypothesized that there would also be a significant amount of grafting of vitamin E onto UHMWPE during irradiation. Spectroscopic analysis of radiation crosslinked vitamin E-blended UHMWPE before and after extraction with boiling hexane showed vitamin E grafting in up to 30% of the blended vitamin E. Grafting increased with irradiation temperature. We also discovered that increasing irradiation temperature resulted in better preservation of active vitamin E in the polymer and increased crosslinking efficiency of UHMWPE. As a result, warm-irradiated vitamin E-blended UHMWPEs had significantly less wear than those irradiated at ambient temperature. It may be desirable to graft vitamin E on UHMWPE to decrease the possibility of elution and increase long-term stability. Warm irradiation of vitamin E blends may present an advantage in increasing vitamin E potency, as well as decreasing the wear of UHMWPE, which is crucial in decreasing the incidence of periprosthetic osteolysis in total joint replacement patients.