Radiation Cross-linked Ultrahigh Molecular Weight Research Articles

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. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1343-1347, 2017.

High vitamin E content, impact resistant UHMWPE blend without loss of wear resistance.

Antioxidant stabilization of radiation cross-linked ultrahigh molecular weight polyethylene (UHMWPE) has been introduced to improve the oxidative stability of total joint implant bearing surfaces. Blending of antioxidants (most commonly vitamin E) with UHMWPE resin powder followed by consolidation and uniform radiation cross-linking is currently available for use in both total hips and total knees. It was previously shown that the fatigue resistance of vitamin E-blended and irradiated UHMWPEs could be further improved by spatially manipulating the vitamin E concentration throughout the implant and limiting cross-linking to the surface of the implant where it is necessary for wear resistance. This was possible by designing a low concentration of vitamin E on the surface and higher concentration in the bulk of the implant because cross-linking is hindered in UHMWPE as a function of increasing vitamin E concentration. In this study, we hypothesized that such a surface cross-linked UHMWPE with low wear rate and high fatigue strength could be obtained by limiting the penetration of radiation into UHMWPE with uniform vitamin E concentration. Our hypothesis tested positive; we were able to obtain control of the surface cross-linked region by manipulating the energy of the irradiation, resulting in extremely low wear, and high impact strength. In addition, we discussed alternatives of improving the oxidation resistance of such a material by using additional vitamin E reservoirs. These results are significant because this material may allow increased use of antioxidant-stabilized, cross-linked UHMWPEs in high stress applications and in more active patients.

Novel active stabilization technology in highly crosslinked UHMWPEs for superior stability

Abstract Radiation cross-linked ultrahigh molecular weight polyethylene (UHMWPE) is the bearing of choice in joint arthroplasty. The demands on the longevity of this polymer are likely to increase with the recently advancing deterioration of the performance of alternative metal-on-metal implants. Vitamin E-stabilized, cross-linked UHMWPEs are considered the next generation of improved UHMWPE bearing surfaces for improving the oxidation resistance of the polymer. It was recently discovered that in the absence of radiation-induced free radicals, lipids absorbed into UHMWPE from the synovial fluid can initiate oxidation and result in new free radical-mediated oxidation mechanisms. In the presence of radiation-induced free radicals, it is possible for the polymer to oxidize through both existing free radicals at the time of implantation and through newly formed free radicals in vivo. Thus, we showed that reducing the radiation-induced free radicals in vitamin E-stabilized UHMWPE would increase its oxidative stability and presumably lead to improved longevity. We describe mechanical annealing and warm irradiation of irradiated vitamin E blends as novel methods to eliminate 99% of radiation-induced free radicals without sacrificing crystallinity. These are significant improvements in the processing of highly cross-linked UHMWPE for joint implants with improved longevity.

The effect of an additional phosphite stabilizer on the properties of radiation cross‐linked vitamin E blends of UHMWPE

Antioxidant stabilization of radiation cross-linked ultrahigh molecular weight polyethylene (UHMWPE) has been introduced to improve the oxidative stability of total joint implant bearing surfaces. Blending of an antioxidant with UHMWPE resin powder followed by consolidation and radiation cross-linking has been cleared by the FDA for use in both total hips and total knees for designs incorporating two antioxidants, namely vitamin E and Covernox™ (a medical grade version of Irganox™ 1010). The antioxidants in the polymer are expected to protect the polymer during consolidation, during radiation cross-linking, on the shelf before implantation, and in vivo after implantation. To maximize the protection of the polymer afforded by the antioxidant in vivo, a novel approach may be the use of multiple antioxidants, especially to protect the primary antioxidant for a longer period of time. We hypothesized that the addition of a phosphite stabilizer (Irgafos 168™) commonly used in conjunction with hindered phenolic antioxidants in polymer processing could improve the oxidative stability of radiation cross-linked blends of vitamin E. To test our hypothesis, we prepared UHMWPE blends with 0.05 wt% Irgafos and 0.05 wt% vitamin E and compared its cross-link density, wear resistance, tensile properties, and impact strength to control blends containing only vitamin E. Our hypothesis was not supported; the cross-link density of UHMWPE was significantly decreased by the additive without additional benefit to oxidative stability. To our knowledge, this was the first attempt at using multiple stabilizers in medical grade UHMWPE.

Structure, mechanical and tribological properties of radiation cross-linked ultrahigh molecular weight polyethylene and composite materials based on it

• Effect of irradiation, oriented drawing, and filling on the structure and properties of UHMWPE was studied. • Radiation cross-linking leads to an increase in the melting temperature of UHMWPE. • The optimal irradiation dose is found to be 20 Mrad. • Strength of UHMWPE can be improved by a combination of irradiation, orientation, and filling with nanotubes. The effect of irradiation with electrons, oriented drawing, and reinforcement with multi-walled carbon nanotubes (MWCNT) on the structure, physico-mechanical and tribological properties of ultrahigh molecular weight polyethylene (UHMWPE) is studied. It is shown that the radiation cross-linking leads to the melting temperature of UHMWPE nearly linear increases with the dose of radiation. The optimal irradiation dose with respect to the mechanical characteristics is found to be 20 Mrad. It is shown that the strength characteristics of UHMWPE can be improved most efficiently by a combination of irradiation, oriented drawing, and reinforcement with nanotubes, and the second and the third factors have a stronger effect than the first one. A combined effect of three factors enabled us to enhance the yield strength of material by almost four times without a detrimental effect on its plasticity.

Comparative oxidative stability of α‐tocopherol blended and diffused UHMWPEs at 3 years of real‐time aging

Vitamin E (α-tocopherol) is a free-radical stabilizing antioxidant used to maintain oxidative stability in radiation cross-linked ultra-high molecular weight polyethylene (UHMWPE) used in total joint replacements. We investigated the oxidative resistance of vitamin E-stabilized UHMWPE in (i) postirradiation vitamin E-diffused UHMWPE, (ii) vitamin E blended and irradiated UHMWPE, and (iii) unstabilized, irradiated UHMWPE after accelerated aging and real-time aging in an aqueous environment at 40°C for 36 months. Unstabilized samples exhibited substantial oxidation throughout the surface and bulk with both types of aging. While vitamin E-stabilized, radiation cross-linked UHMWPEs were all superior to unstabilized samples, irradiated blends showed surface oxidation and subsurface oxidation potential beginning at ten months in real-time aging. In contrast, postirradiation vitamin E-diffused UHMWPEs showed no detectable oxidation and no increase in oxidation potential despite elution of some vitamin E. We also showed that current thermal accelerated aging methods were unable to differentiate among the oxidative stability of vitamin E-stabilized, radiation cross-linked UHMWPEs prepared by different processes.

In Vivo Biological Response to Vitamin E and Vitamin-E-Doped Polyethylene

Cross-linking has decreased the wear of ultra-high molecular weight polyethylene, a cause of osteolysis leading to total joint replacement failure. Compared with melting or annealing, doping cross-linked ultra-high molecular weight polyethylene with vitamin E stabilizes free radicals from irradiation while maintaining mechanical properties and wear resistance. This study was done to determine the local tissue effects of free vitamin E and vitamin E eluted from ultra-high molecular weight polyethylene implants in the joint space. Three studies were performed. First, pure vitamin E and solubilized vitamin E were injected into rabbit knees to simulate vitamin-E elution from radiation cross-linked ultra-high molecular weight polyethylene; second, vitamin-E-doped, irradiated ultra-high molecular weight polyethylene plugs were implanted into dorsal subcutaneous pouches of rabbits to determine the local effects of vitamin-E elution from radiation cross-linked ultra-high molecular weight polyethylene; and, third, two groups of vitamin-E-doped, irradiated acetabular liners (high surface and uniform vitamin-E concentration profiles) were compared with undoped, control ultra-high molecular weight polyethylene liners in a canine model of total hip replacement to determine the effect of possible vitamin-E elution on bone ingrowth and the local tissue response to it in a load-bearing environment. Injection of solubilized vitamin E resulted in histologically normal surrounding soft tissue at both two and twelve-week follow-up intervals, while injection of pure vitamin E resulted in acute and chronic inflammation at the time of the two-week follow-up. Both control and vitamin-E-doped subcutaneous plugs showed inflammation associated with surgery at two weeks of follow-up, but showed stable fibrous encapsulation without inflammation at twelve weeks of follow-up. In the canine total hip replacement model, there was no qualitative difference in local tissue appearance and no significant difference in the percent bone ingrowth and the percent bone density between the control and vitamin-E groups. These investigations showed that vitamin-E-doped ultra-high molecular weight polyethylene plugs and total hip replacement components are well tolerated in both a small and a large-animal model with no observed adverse effects on the surrounding tissues at twelve weeks of follow-up.

Highly Cross-Linked Ultrahigh Molecular Weight Polyethylene With Improved Fatigue Resistance for Total Joint Arthroplasty: Recipient of the 2006 Hap Paul Award

Eliminating postirradiation melting and stabilizing the residual free radicals of radiation cross-linked ultrahigh molecular weight polyethylene (UHMWPE) with vitamin E resulted in improved fatigue crack propagation resistance without compromising wear resistance. We designed a cantilever postbending test to determine the bending fatigue resistance of α-tocopherol–doped, irradiated UHMWPE ( α-TPE) in comparison to conventional UHMWPE. The bending fatigue behavior of α-TPE was comparable to conventional UHMWPE. Upon accelerated aging, the fatigue resistance of α-TPE was substantially better than that of conventional UHMWPE. α-TPE has shown improved wear and oxidation resistance, migration stability of vitamin E, and improved mechanical properties. The use of this material may be beneficial in total knee arthroplasty where its improved fatigue properties may be an advantage under high stresses.

The effect of α-tocopherol on the oxidation and free radical decay in irradiated UHMWPE

We developed a radiation cross-linked ultra-high molecular weight polyethylene (UHMWPE) stabilized with alpha-tocopherol (Vitamin E) as a bearing material in total joint replacements. The stabilizing effect of alpha-tocopherol on free radical reactions in UHMWPE is not well understood. We investigated the effect of alpha-tocopherol on the oxidation and transformation of residual free radicals during real-time aging of alpha-tocopherol-doped, irradiated UHMWPE (alphaTPE) and irradiated UHMWPE (control). Samples were aged at 22 degrees C (room temperature) in air, at 40 degrees C in air and at 40 degrees C in water for 7 months. During the first month, alphaTPE showed some oxidation at the surface, which stayed constant thereafter. Control exhibited substantial oxidation in the subsurface region, which increased with time. The alkyl/allyl free radicals transformed to oxygen centered ones in both materials; this transformation occurred faster in alpha-TPE. In summary, the real-time oxidation behavior of alpha-TPE was consistent with that observed using accelerated aging methods. This new UHMWPE is oxidation resistant and is expected to maintain its properties in the long term.

Radiation-induced Crosslinking of Plastics for Reduction of Wear in Textile Machine Applications

Plastic materials are now increasingly used in tribological systems, as their specific properties may be utilised to increase the lifetime and reliability of all system elements. This paper addresses the reduction of wear as the result of the radiation-induced crosslinking of plastics. As a typical example of sliding wear, studies were performed to determine the wear on radiation-crosslinked ultrahigh molecular weight polyethylene used in textile machines and the limitations of this modified material determined. The specific objective was the development of appropriate thermoplastics for use as counterparts to steel components in the traversing unit in a winding machine with the aim of increasing its lifetime. High wear resistance on the surface, low moisture absorption under changing ambient conditions plus high impact strength were found to be the fundamental properties for the fulfilment of the specific demands for this application. To summarise, it is established that radiation-induced crosslinking leads to a significant increase in surface strength, thermal resistance and, consequently, wear resistance in the application in question.