Development of an Accelerated Aging Method for Evaluation of Long-Term Irradiation Effects on Ultrahigh-Molecular-Weight Polyethylene Implants

Abstract

A general scheme for developing an accelerated aging method for irradiated biomaterials is proposed. Using UHMWPE implants as an example, an accelerated thermal diffusion oxidative aging (ATDOA) method has been developed. The method requires an optimum initial heating rate and an optimum aging temperature to accelerate oxidation reactions. Based upon oxidation-induced material property changes (crystallinity by DSC, tensile properties by ASTM D638 tensile test, oxidation index by FTIR, and low molecular weight fraction by GPC), correlations between accelerated aging time and post-radiation shelf aging time were obtained. The new ATDOA method allows a rapid evaluation of long-term irradiation effects on the material properties of UHMWPE implants. Ultra high molecular weight polyethylene (UHMWPE) has been used as an orthopaedic bearing material for more than two decades with satisfactory clinical results. The current research effort is aimed at prolonging its performance in vivo. As a virgin polymer, UHMWPE is chemically inert and stable. However, upon the forming and fabrication process (compression molding, ram extrusion, etc.) and high energy beam radiation ( such as γ-ray, x-ray, or electron beam irradiation, a step required for sterility), free radicals are formed causing the UHMWPE to become reactive.Oxidation can occur during irradiation, post-irradiation shelf-aging, or in vivo in the presence of oxidants. Accompanying the oxidation reaction are slow but progressive material property changes in UHMWPE which have been unobserved in the past, due to the lack of sensitive evaluation tools. In this study, we first evaluated shelf-aged UHMWPE specimens to obtain correlations between material properties and aging time. An accelerated aging method was then developed to simulate the long-term oxidation effects in a short period of time. Finally, material property changes of clinical retrievals reported in the literature were discussed.

Oxidation‐induced dynamic changes in morphology reflected on freeze‐fractured surface of gamma‐irradiated ultra‐high molecular weight polyethylene components

“…[22][23][24][25][26] To assess the oxidation-induced morphological appearance resulting from accelerated aging, and compare it with that induced by oxidative degradation occurring naturally as a consequence of gamma radiation, we used the protocol proposed by Sun et al 23,25 Puck-shaped disc specimens (10-mm thick, 40-mm diameter), which had been directly compression-molded under a pressure of 5 MPa from Hoechst GUR1120, by the implant manufacturer (Nakashima Medical Division, Nakashima Propeller Co., Okayama, Japan), were sterilized with gamma radiation at a nominal 25 KGy, the standard radiation dose for conventional UHMWPE implants, in the presence of air. These were consecutively placed in an oven at a constant temperature of 80°C for 23 days.…”

Ultra High Molecular Weight Polyethylene and its Reinforcement with Carbon Nanotubes in Medical Devices

“…Accelerated aging under method B, i.e. 14 days, is equivalent to approximately 8 years of shelf aging (Sun [1996] A more complex reality arises during in vivo oxidation. When the UHMWPE components are implanted in the human body, they stay permanently surrounded by the body fluids which contain molecular oxygen necessary for in vivo oxidation.…”

Evolution of morphology in UHMWPE following accelerated aging: The effect of heating rates

“…11,[24][25][26][27][28][29][30][31] Thermal aging methods recently have been developed to accelerate the oxidation of gamma-irradiated UHMWPE components. 29,[32][33][34][35] Due to the length of time associated with natural aging, 5,16,36 which typically takes place during the course of months or years, the development and validation of accelerated aging protocols is of tremendous practical importance to the orthopedic community. Accelerated aging protocols are used widely to precondition components prior to testing in wear simulators, 18,19,37,38 in which the increased wear of UHMWPE acetabular components has been attributed to oxidative degradation following accelerated aging.…”

“…Accelerated aging protocols are used widely to precondition components prior to testing in wear simulators, 18,19,37,38 in which the increased wear of UHMWPE acetabular components has been attributed to oxidative degradation following accelerated aging. Although previous studies have compared the oxidation, 29,[32][33][34] density, 35 and crystallinity 32 of UHMWPE following natural and accelerated aging, the extent to which accelerated aging protocols reproduce the morphology of naturally aged components has not been examined in detail.…”

“…Sun and colleagues 29,32 observed that the extent of oxidation in bulk components is sensitive to the initial heating rates, and they hypothesized that a slow initial heating rate increases oxidation by improving the thermal diffusion of oxygen into the material. In the present study, we sought to establish whether a thermal diffusion or an oxidation kinetics mechanism might be responsible for the previously observed sensitivity of long-term artificial aging to initial heating rates.…”