No abstract

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.

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.

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. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2551-2556, 2017.

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. The catastrophic periprosthetic consequences of metalon-metal (MOM) hip implants 1 have increased demands for longevity with alternative bearings. MOM benefits were decreased risk of dislocation 2 and increased range of motion, both resulting from the larger femoral head sizes (36-60 mm diam.) that could be accommodated. Conversely, alternative bearings comprised of metal heads and ultrahigh molecular weight polyethylene (UHMWPE) acetabular liners are typically limited to smaller sizes (commonly 32-40 mm) for two reasons.First, current hip joint technology is usually modular with a metal acetabular shell and a UHMWPE liner, making a metal-on-polyethylene (MOP) system thicker than a comparable MOM. Thus, the jump distance for the femoral head to dislocate from the acetabulum is larger in MOM implants (average of 28 mm compared to 16 mm for MOP). Second, the recommended UHMWPE thickness is at least 6 mm, albeit based on a study of conventional gamma-in-air sterilized oxidation-prone UHMWPE, 3 which is not the norm today.4 Highly cross-linked UHMWPEs, 5 developed to provide superior wear and oxidation resistance, 6,7 have significantly reduced fatigue strength. 8,9These materials have decreased wear dramatically at 10 years of clinical use 10 and showed superior oxidation resistance in vivo compared to conventional UHMWPE. At the same time, concerns exist about fatigue-related fracture and failure when using thinner implants, especially at the rim and in locking mechanisms, which are susceptible to damage under adverse loading conditions such as impingement. Therefore, a need exists to develop MOP bearing systems with improved fatigue strength to enable use of thinner UHMWPE bearings with larger femoral heads. This may also enable the use of UHMWPE bearing surfaces with the possibility of fewer dislocations and larger range of motion, perhaps equivalent to the larger ...