Radicals In Ultra-high Molecular Weight Polyethylene Research Articles

Ex Vivo Stability Loss of Irradiated and Melted Ultra-High Molecular Weight Polyethylene

Radiation crosslinking reduces wear of ultra-high molecular weight polyethylene (UHMWPE), and subsequent annealing or melting increases oxidative stability. Little is known about the oxidative stability of polyethylene total joint components after in vivo service and subsequent shelf storage in air. We analyzed thirty-four surgically retrieved, radiation crosslinked acetabular liners to determine their oxidative stability after in vivo service (range, 0.5 to 84.0 months). Oxidation was determined at the time of explantation. After shelf storage in air (range, 7.0 to 72.0 months), oxidation, crosslink density, and thermal properties were determined. Oxidation of one control liner that was shelf-aged in air (for eighty-four months) was also determined. At the time of explantation, all components showed minimal oxidation; however, oxidation levels increased during shelf storage, with a concomitant decrease in crosslink density and increase in crystallinity. Increasing oxidation, increasing crystallinity, and decreasing crosslink density correlated with the duration of ex vivo storage. The shelf-aged control liner showed no detectable oxidation. The oxidation and loss of crosslink density of the irradiated and melted UHMWPE was surprising. Two potential mechanisms that might alter the oxidative stability of UHMWPE in vivo are cyclic loading and absorption of lipids. Both of these mechanisms can generate new free radicals in UHMWPE and can initiate and propagate its oxidation.

Effect of electron beam irradiation on physical properties of ultrahigh molecular weight polyethylene

AbstractIf a large amount of polymer radicals remain trapped after the irradiation of ultrahigh molecular weight polyethylene (UHMWPE), the radicals may result in a significant alteration of its physical properties during long‐term shelf storage and implantation. An electron spin resonance spectroscopic study was undertaken to investigate the remaining free radicals in UHMWPE after electron beam irradiation up to 500 kGy in air and an N2 environment. Heat treatment was employed at 110 and 145°C for various periods of time to decay the free radicals. The free radicals were rapidly decayed for 1 h and gradually decayed as a function of time with the heat treatment. The decay of the free radicals was completed more rapidly with a heat treatment at 145°C than at 110°C. Therefore, a longer heat treatment time is required to scavenge all the free radicals formed in UHMWPE at 110°C. The oxidation profiles showed that the oxidation index of the heat‐treated UHMWPE was lower than the oxidation index of the non‐heat‐treated UHMWPE. The heat treatment of irradiated UHMWPE can substantially reduce the concentration of free radicals; therefore, UHMWPE has resistance against long‐term oxidative degradation. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 103–116, 2005

Influence of ionizing irradiation in air and nitrogen for sterilization of surgical grade polyethylene for implants

The influence of the atmosphere and the applied dose during ionizing radiation treatment on selected properties of ultra high molecular weight polyethylene (UHMWPE) have been investigated. A linear correlation between extinction coefficient and applied dosis in air from 6 to 125 kGy was found, while oxidation was not linear with irradiation in nitrogen. Bacteria survival rate shows a necessary minimum dose of 15 kGy for assured sterility of the product. Post reaction of latent free radicals in UHMWPE created during irradiation, which react or recombine time- and environment dependent, has also been investigated after storage of UHMWPE-films in air and nitrogen at 21°C and in water at body temperature 37°C for up to nine months. Results show that the properties of UHMWPE after radiation-sterilization change depending on time, the absorbed dose, the atmosphere where irradiation took place and the environment of storage. UHMWPE, which mainly crosslinks during irradiation degrades by an oxidation process after sterilizing when stored in air and even more in water at body temperature. So irradiation and storage in nitrogen before implantation in the human body is beneficial.