Brinda N. Doshi

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.

Fatigue toughness of irradiated vitamin E/UHMWPE blends.

Radiation cross‐linked ultrahigh molecular weight polyethylenes (UHMWPEs) have become the standard‐of‐care in total joint replacements (TJR) in the last decade because of their superior wear resistance in comparison with previously used “conventional” gamma sterilized UHMWPE. Some first generation radiation cross‐linked UHMWPEs were stabilized against oxidation by post‐irradiation melting, which significantly reduced their fatigue crack propagation resistance or fatigue toughness. Second generation cross‐linked UHMWPEs incorporated instead an antioxidant such as vitamin E, eliminating the need for melting. In this study, we investigated the fatigue crack propagation resistance and the impact toughness of vitamin E‐blended and radiation cross‐linked UHMWPEs as a function of vitamin E concentration and radiation dose. Both properties were strongly dependent on the cross‐link density and they showed a good correlation with each other (R2 = 0.89).

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.

Surface cross‐linked UHMWPE using peroxides

Crosslinking of ultra‐high molecular weight polyethylene (UHMWPE) has been successfully used to improve its wear performance. Wear is a surface phenomenon and limiting crosslinking to a layer only on the surface is desirable, as crosslinking of the bulk of the implant reduces its mechanical strength and toughness. We present a novel technique to surface crosslink consolidated UHMWPE/vitamin‐E blends by diffusing an organic peroxide into the polymer at moderate temperatures, followed by heating to above the peroxide decomposition temperature to cause crosslinking on the surface. We characterized the surface crosslink density and wear rate of surface crosslinked UHMWPE/vitamin‐E blends with two different types of peroxides. Both peroxides resulted in surface crosslinking with an increase in wear resistance comparable to the state‐of‐the‐art highly crosslinked UHMWPE used for orthopedic implants. The addition of the antioxidant vitamin‐E led to higher oxidation resistance.

Surface cross-linked ultra high molecular weight polyethylene by emulsified diffusion of dicumyl peroxide

Cross-linking improves the wear resistance of ultrahigh molecular weight polyethylene (UHMWPE) used in hip and knee implants. Free radicals, generated by ionizing radiation or chemically, react to form cross-links. Limiting cross-linking to the articulating surface of the implant is desirable to enable high wear resistance on the surface and higher strength and toughness in the bulk. We investigated the diffusion of emulsified dicumyl peroxide (DCP) into vitamin E-blended UHMWPE (0.1 and 0.3 wt. % vitamin-E) with subsequent thermal decomposition in situ to obtain surface cross-linking with the objective of achieving surface wear rate equivalent or lower than that of current clinically available materials. We diffused emulsified DCP at 100°C followed by in situ decomposition at 150°C. We also assessed the effect of having vitamin-E in the DCP emulsion. The oxidative stability of the treated samples increased with increasing vitamin E concentration in the blend and by incorporating vitamin E into the peroxide emulsion. The impact strength of a surface cross-linked, 0.3 wt% vitamin E blended UHMWPE prepared using this method was superior to a clinically available irradiated and melted highly cross-linked UHMWPE while the wear resistance was comparable. These results showed the feasibility of surface cross-linking using emulsified peroxide diffusion as a method of making tough and wear resistant joint implant bearing surfaces.

High temperature homogenization improves impact toughness of vitamin E‐diffused, irradiated UHMWPE

Diffusion of vitamin E into radiation cross‐linked ultrahigh molecular weight polyethylene (UHMWPE) is used to increase stability against oxidation of total joint implant components. The dispersion of vitamin E throughout implant preforms has been optimized by a two‐step process of doping and homogenization. Both of these steps are performed below the peak melting point of the cross‐linked polymer (<140°C) to avoid loss of crystallinity and strength. Recently, it was discovered that the exposure of UHMWPE to elevated temperatures, around 300°C, for a limited amount of time in nitrogen, could improve the toughness without sacrificing wear resistance. We hypothesized that high temperature homogenization of antioxidant‐doped, radiation cross‐linked UHMWPE could improve its toughness. We found that homogenization at 300°C for 8 h resulted in an increase in the impact toughness (74 kJ/m2 compared to 67 kJ/m2), the ultimate tensile strength (50 MPa compared to 43 MPa) and elongation at break (271% compared to 236%). The high temperature treatment did not compromise the wear resistance or the oxidative stability as measured by oxidation induction time. In addition, the desired homogeneity was achieved at a much shorter duration (8 h compared to >240 h) by using high temperature homogenization.