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A study of long-lived free radicals in gamma-irradiated medical grade polyethylene
Radiation Physics and Chemistry 62 (2001) 141–144
A study of long-lived free radicals in gamma-irradiated medical grade polyethylene M.S. Jahana,*, M.C. Kinga, W.O. Haggardb, K.L. Sevob, J.E. Parrb a
Department of Physics, The University of Memphis, Memphis, TN 38152, USA b Wright Medical Technology, Inc., Arlington, TN 38002, USA
Abstract Ultra-high molecular weight polyethylene (UHMWPE), used in orthopaedic implants, was irradiated with gamma rays at room temperature (231C) in air or vacuum, and it was then stored in the same environment for 2 yr at 231C, 371C, or 751C. ESR measurements showed decay as well as changes from alkyl to peroxy and polyenyl radicals in air due to reaction with oxygen. In vacuum, on the other hand, radicals (primarily allyl) decayed without any observable transformation from one type to another. The residual concentrations obtained after 2 yr of post-sterilization storage at 231C, 371C, and 751C were found to be 15–20%, 10–15%, and 2–10%, respectively. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Ultra-high molecular weight polyethylene; Radiation sterilization; Free radicals; Oxidation; Thermal annealing
1. Introduction Ultra-high molecular weight polyethylene (UHMWPE) is routinely used in medical implants to cushion the articulating surfaces in total joint replacements. It has a high resistance to mechanical stress and is relatively chemically inert within the body, making it ideal for use in hip, knee, and finger joint replacements. One of the major concerns over UHMWPE’s performance, however, centers on the polymer’s ability to maintain its properties over a long period of time (Streicher, 1988). Prior to implantation within the body, UHMWPE must be sterilized to prevent contamination. Although several methods of sterilization exist, the most common method of sterilization for UHMWPE is exposure to gamma rays, with an average prescribed dose of 2.5 Mrad (Streicher, 1988). While gamma irradiation is a very clean, convenient, and effective method for sterilization of implant materials, it also causes distinct *Corresponding author. E-mail address: [email protected] (M.S. Jahan).
chemical and structural changes within the polymer matrix (Ohnishi et al., 1960). These changes have largely been reported to weaken or degrade the UHMWPE, thereby reducing the effective life of the medical implants. Specifically, gamma irradiation produces highly reactive free radical species in the core as well as surface of a bulk UHMWPE component (Eyerer, 1987; Jahan et al., 1998). Evidence suggests that the time-dependent changes are a result of long-lived free radicals found within the PE matrix, and it has been shown that these long-lived radicals may be present for many years after irradiation (O’Neill et al., 1999). Thermal annealing of UHMWPE has been used as a means of stabilizing free radicals; however debate continues today over effective annealing temperatures and times. It has been reported that free radical formation can be stabilized by annealing at 501C for 144 h (Sun et al., 1996). Investigation into this claim of stabilization was sought within the scope of the present work. In this study, free radical measurements were employed to determine the effects of storage environment (vacuum or open air) and temperature (231C, 371C or 751C) on aging of gamma-sterilized UHMWPE.
0969-806X/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 1 ) 0 0 4 3 1 - 5
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2. Chemical background UHMWPE is made of extremely long chains of the polyethylene monomer. The molecular weight of UHMWPE tends to be very high, measuring 3 106 g/ mol or higher with a density of 0.94–0.97 g/ml. It is around 30–40% crystalline, and crystalline regions are randomly oriented with respect to each other. Free radicals formed due to irradiation can reside in the amorphous as well as crystalline regions of the polymer. Because of inaccessibility of oxygen in the crystalline region, free radicals in this region remain relatively stable than those in the amorphous region where oxygen can diffuse. The initial free radicals produced during irradiation are primarily a combination of alkyl (aCH2aCHdaCH2a) and allyl (aCH2aCH ¼ CHaCHdaCH2a), although polyenyl (aCH2a(aCH ¼ CHa)nCHdaCH2a) and peroxy (POd2 ) radicals may also be present (Igarashi, 1983; Nausbaum and Rose, 1979; O’Neill et al., 1999; Seguchi and Tamura, 1973).
3. Experiment 3.1. Materials and methods The samples for free radical measurements were machined (2 2 10 mm3 each) from 3-in diameter ram-extruded rods of resin GUR 4150. Prior to sterilization, they were cleaned in ethyl alcohol according to the protocol established by the NSF-IndustryUniversity Cooperative Research Center for Biosurfaces. Each sample in the ‘‘vacuum’’ category was sealed in an evacuated (10 mTorr) suprasil quartz tube (Wilmad Glass). Samples in the ‘‘open-air’’ category were simply placed in a large bag. 3.2. Sterilization All samples were sterilized by Steris Isomedix Services at room temperature, with a dose of approximately 2.5– 3.2 Mrad. The dose rate was maintained at 0.75 Mrad (=1.25 104 rad/min). Before ESR analysis could be conducted on irradiated samples, the damage induced in the quartz tube during sterilization had to be annealed. The time lapse between irradiation and the first (initial) measurement was about 1 week. 3.3. Testing and storage/annealing An X-band ESR spectrometer (Vairan, E-4), fitted with a TE102 cavity, and operating at 9.5 GHz microwave frequency and 100 kHz magnetic-field modulation frequency, was used to test all samples in this study. The amplitude of the microwave power was maintained at
2 mW and the field-modulation amplitude was varied between 2 and 10 G (peak-to-peak) as needed for a better signal-to-noise ratio. While peak-to-peak height or double integration method was used to determine relative free radical concentration (FRC), a ruby standard containing 6 1016 spins (NIST) was used for absolute concentration. After initial measurements, each sample was stored at its prescribed temperature (231C, 371C, or 751C). During subsequent measurements, samples were removed from the ovens, tested at room temperature, and returned to their original storage/annealing temperatures.
4. Results and discussion The results of this long-term study are divided into two sections: (1) dependence of free radical type and (2) dependence of FRC on annealing temperature and environment. The former is generally determined by observing the variation in the structure of the ESR spectrum, and the latter by double integration of the first-derivative spectrum or by measurements of the peak heights of specific spectral lines. Remember that the purpose of this work is not to perform spectral analyses but to determine the fate of the free radicals in a given environment at a given temperature, which may lead to a better understanding of the aging process of gamma-sterilized UHMWPE. For this, the structure of the observed spectra and the changes thereof were compared with the known structural information of PE radicals found in the literature. 4.1. Spectral structure and free radical type The ESR spectra of UHMWPE (GUR 4150), recorded at 231C following post-sterilization aging in vacuum at 231C, 371C, and 751C are shown in Fig. 1(A), (B), and (C), respectively. Note that in each figure, (A), (B), or (C), the aging time of each sample has been indicated. The spectra recorded at 231C, following poststerilization aging of UHMWPE in the presence of oxygen (open air) at 231C, 371C, and 751C, for t ¼ 0 (initial) to t ¼ 2 yr are shown in Fig. 2(A), (B), and (C), respectively. Only representative spectra are shown in these figures. The appearance of the central line in the spectra of Fig. 2 (shown by arrow) is an indication of the growth of oxygen-induced radical (OIR). Evidently, the initial free radicals in the open-air samples differ significantly from those in the vacuumsealed samples. The initial structure seen in the spectrum of vacuum-sealed samples is the common seven-line pattern (the weak lines at the ends are shown by arrows in Fig. 1) with a hyperfine splitting of approximately 13 G. They are primarily allyl radicals (aCH2aCH ¼ CHaCHdaCH2a), although some alkyl
M.S. Jahan et al. / Radiation Physics and Chemistry 62 (2001) 141–144
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Fig. 1. ESR spectra recorded as a function of time following post-sterilization aging in vacuum at 231C (A), 371C (B), and 751C (C), respectively. The arrows indicate presence of weak lines. Magnetic field modulation was maintained at 5 or 10 G.
Fig. 2. ESR spectra recorded as a function of time following post-sterilization aging in the presence of oxygen (open air) at 231C (A), 371C (B), and 751C (C), respectively. The appearance of oxygen-induced radical (OIR) is shown by an arrow at the center of the spectrum. Magnetic field modulation was maintained at 5 or 10 G.
(aCH2aCHdaCH2a) and polyenyl radicals (aCH2a(aCH ¼ CHa)nCHdaCH2a) may be present. The observed spectra of the open-air samples exhibit asymmetric lines with hyperfine splitting of approximately 25 G. They are primarily due to alkyl radicals, although a combination of allyl, polyenyl, and peroxy radicals may also be present as evidenced from the asymmetric features of the spectra. With time, the asymmetric hyperfine lines in the spectra of the open-air samples reduced to a single narrow line of width about 10 G (DHpp ) whereas the total spectral width of the vacuum samples remained constant (DHpp B100 G) and the intensity of the hyperfine lines was reduced. This difference can be explained as follows. In vacuum, allyl radicals recombine to form neutral (nonradical) species without giving
rise to any detectable new radicals (alkyl or peroxy). However, the presence of polyenyl radicals cannot be ruled out, because the broad umbrella (DHpp B100 G) of the final signal (in this study) may contain other singlets (due to polyenyl radicals). Radical recombination or decay was found to depend on annealing/storage temperature. In the open-air samples, on the other hand, radical transformation and decay (reduction in concentration) can occur simultaneously because of reaction of the radicals with oxygen. Since the evolution of the single line in the ESR spectrum can occur only in the open-air samples (i.e., in presence of oxygen), the singlet is attributed by many to peroxy radicals (see, for example, Jahan et al., 1991; O’Neill et al., 1999). However, the presence of oxygen-centered radicals such as aCH2aOd or aCHaOda, cannot be ruled out.
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is stored in an inert environment (vacuum, for example). The reaction in presence of oxygen, however, is associated with structural changes in the polymer radicals. This study further suggests that the OIR is stable at 231C or 371C, but unstable at 751C.
Acknowledgements Work was supported in part by grants through NSFIndustry/University Cooperative Research Center for Biosurfaces and by the University of Memphis.
Fig. 3. Combined FRC recorded, respectively, after 1 and 2 yr of post-sterilization aging in vacuum or air at 231C, 371C, or 751C. In open air at 751C, no radicals were detected after 1 day.
Compared to the broad singlet (DHpp ¼ 100 G) produced by the vacuum samples, the narrow singlets (DH ¼ 10 G) of the open-air samples are evidently produced by the OIR. The OIR was found to remain stable at 231C and 371C for the duration of this study, but disappeared after about one day at 751C (see Fig. 2(C)). 4.2. Free radical concentration The free radicals that remained trapped in the polymer matrix following post-sterilization aging for 1 and 2 yr, respectively, are shown in Fig. 3 as combined concentration (obtained by double-integration method). The initial concentrations (not shown) measured in a large number of samples were in the range between 1016 and 1017 spins/g.
5. Conclusions This study has outlined the long-term behavior of gamma sterilization-induced free radicals in UHMWPE at three temperatures, 231C, 371C, and 751C, in air (presence of oxygen) and in vacuum. One of the most important findings of this study is the presence of residual primary (allyl/alkyl) radicals in UHMWPE 2 yr after post-sterilization storage/annealing at 231C, 371C, or 751C. Evidently, primary radicals cannot be completely annealed at or below 751C as long as UHMWPE
References Eyerer, P., 1987. Property changes of UHMWPE during implantationFfirst hints for the development of an alternative polyethylene. In: Lee, S.M. (Ed.), Advances in Biomaterials. Technomic Publ. Co., Lancaster, PA, pp. 62–68. Igarashi, M., 1983. Free radical identification by ESR in polyethylene and nylon. J. Polym. Sci.: Polym. Chem. 21, 2405–2425. Jahan, M.S., Wang, C., Schwartz, G., Davidson, J.A., 1991. Combined chemical and mechanical effects on free radicals in UHMWPE joints during implantation. J. Biomed. Mater. Res. 25, 1005–1017. Jahan, M.S., Thomas, D.E., Banerjee, K., Trieu, H.H., Haggard, W.O., Parr, J.E., 1998. Effects of radiationsterilization on medical implants. Radiat. Phys. Chem. 51, 593–594. Nausbaum, H.J., Rose, R.M., 1979. The effects of radiation sterilization on the properties of ultra-high molecular weight polyethylene. J. Biomed. Mater. Res. 13, 557–576. Ohnishi, S., Ikeda, Y., Sugimoto, S., Nitta, I., 1960. On the EPR singlet spectra frequently observed in irradiated polymers at a large dose. J. Polym. Sci. 47, 503–507. O’Neill, P., Birkinshaw, C., Leahy, J.J., Barklie, R., 1999. The role of long lived free radicals in the ageing of irradiated ultra high molecular weight polyethylene. Polym. Degrad. Stability 63, 31–39. Seguchi, T., Tamura, N., 1973. Mechanism of decay of alkyl radicals in irradiated polyethylene on exposure to air as studied by electron spin resonance. J. Phys. Chem. 77, 40–44. Streicher, R.M., 1988. Influence of ionizing radiation in air and nitrogen for sterilization of surgical grade polyethylene for implants. Radiat. Phys. Chem. 31, 693–698. Sun, D.C.,Wang, A., Stark, C., 1996. The concept of stabilization in UHMWPE. Transactions of the Fifth World Biomaterial Congress, Vol. 2, p. 195.
A study of long-lived free radicals in gamma-irradiated medical grade polyethylene M.S. Jahana,*, M.C. Kinga, W.O. Haggardb, K.L. Sevob, J.E. Parrb a
Department of Physics, The University of Memphis, Memphis, TN 38152, USA b Wright Medical Technology, Inc., Arlington, TN 38002, USA
Abstract Ultra-high molecular weight polyethylene (UHMWPE), used in orthopaedic implants, was irradiated with gamma rays at room temperature (231C) in air or vacuum, and it was then stored in the same environment for 2 yr at 231C, 371C, or 751C. ESR measurements showed decay as well as changes from alkyl to peroxy and polyenyl radicals in air due to reaction with oxygen. In vacuum, on the other hand, radicals (primarily allyl) decayed without any observable transformation from one type to another. The residual concentrations obtained after 2 yr of post-sterilization storage at 231C, 371C, and 751C were found to be 15–20%, 10–15%, and 2–10%, respectively. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Ultra-high molecular weight polyethylene; Radiation sterilization; Free radicals; Oxidation; Thermal annealing
1. Introduction Ultra-high molecular weight polyethylene (UHMWPE) is routinely used in medical implants to cushion the articulating surfaces in total joint replacements. It has a high resistance to mechanical stress and is relatively chemically inert within the body, making it ideal for use in hip, knee, and finger joint replacements. One of the major concerns over UHMWPE’s performance, however, centers on the polymer’s ability to maintain its properties over a long period of time (Streicher, 1988). Prior to implantation within the body, UHMWPE must be sterilized to prevent contamination. Although several methods of sterilization exist, the most common method of sterilization for UHMWPE is exposure to gamma rays, with an average prescribed dose of 2.5 Mrad (Streicher, 1988). While gamma irradiation is a very clean, convenient, and effective method for sterilization of implant materials, it also causes distinct *Corresponding author. E-mail address: [email protected] (M.S. Jahan).
chemical and structural changes within the polymer matrix (Ohnishi et al., 1960). These changes have largely been reported to weaken or degrade the UHMWPE, thereby reducing the effective life of the medical implants. Specifically, gamma irradiation produces highly reactive free radical species in the core as well as surface of a bulk UHMWPE component (Eyerer, 1987; Jahan et al., 1998). Evidence suggests that the time-dependent changes are a result of long-lived free radicals found within the PE matrix, and it has been shown that these long-lived radicals may be present for many years after irradiation (O’Neill et al., 1999). Thermal annealing of UHMWPE has been used as a means of stabilizing free radicals; however debate continues today over effective annealing temperatures and times. It has been reported that free radical formation can be stabilized by annealing at 501C for 144 h (Sun et al., 1996). Investigation into this claim of stabilization was sought within the scope of the present work. In this study, free radical measurements were employed to determine the effects of storage environment (vacuum or open air) and temperature (231C, 371C or 751C) on aging of gamma-sterilized UHMWPE.
0969-806X/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 1 ) 0 0 4 3 1 - 5
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2. Chemical background UHMWPE is made of extremely long chains of the polyethylene monomer. The molecular weight of UHMWPE tends to be very high, measuring 3 106 g/ mol or higher with a density of 0.94–0.97 g/ml. It is around 30–40% crystalline, and crystalline regions are randomly oriented with respect to each other. Free radicals formed due to irradiation can reside in the amorphous as well as crystalline regions of the polymer. Because of inaccessibility of oxygen in the crystalline region, free radicals in this region remain relatively stable than those in the amorphous region where oxygen can diffuse. The initial free radicals produced during irradiation are primarily a combination of alkyl (aCH2aCHdaCH2a) and allyl (aCH2aCH ¼ CHaCHdaCH2a), although polyenyl (aCH2a(aCH ¼ CHa)nCHdaCH2a) and peroxy (POd2 ) radicals may also be present (Igarashi, 1983; Nausbaum and Rose, 1979; O’Neill et al., 1999; Seguchi and Tamura, 1973).
3. Experiment 3.1. Materials and methods The samples for free radical measurements were machined (2 2 10 mm3 each) from 3-in diameter ram-extruded rods of resin GUR 4150. Prior to sterilization, they were cleaned in ethyl alcohol according to the protocol established by the NSF-IndustryUniversity Cooperative Research Center for Biosurfaces. Each sample in the ‘‘vacuum’’ category was sealed in an evacuated (10 mTorr) suprasil quartz tube (Wilmad Glass). Samples in the ‘‘open-air’’ category were simply placed in a large bag. 3.2. Sterilization All samples were sterilized by Steris Isomedix Services at room temperature, with a dose of approximately 2.5– 3.2 Mrad. The dose rate was maintained at 0.75 Mrad (=1.25 104 rad/min). Before ESR analysis could be conducted on irradiated samples, the damage induced in the quartz tube during sterilization had to be annealed. The time lapse between irradiation and the first (initial) measurement was about 1 week. 3.3. Testing and storage/annealing An X-band ESR spectrometer (Vairan, E-4), fitted with a TE102 cavity, and operating at 9.5 GHz microwave frequency and 100 kHz magnetic-field modulation frequency, was used to test all samples in this study. The amplitude of the microwave power was maintained at
2 mW and the field-modulation amplitude was varied between 2 and 10 G (peak-to-peak) as needed for a better signal-to-noise ratio. While peak-to-peak height or double integration method was used to determine relative free radical concentration (FRC), a ruby standard containing 6 1016 spins (NIST) was used for absolute concentration. After initial measurements, each sample was stored at its prescribed temperature (231C, 371C, or 751C). During subsequent measurements, samples were removed from the ovens, tested at room temperature, and returned to their original storage/annealing temperatures.
4. Results and discussion The results of this long-term study are divided into two sections: (1) dependence of free radical type and (2) dependence of FRC on annealing temperature and environment. The former is generally determined by observing the variation in the structure of the ESR spectrum, and the latter by double integration of the first-derivative spectrum or by measurements of the peak heights of specific spectral lines. Remember that the purpose of this work is not to perform spectral analyses but to determine the fate of the free radicals in a given environment at a given temperature, which may lead to a better understanding of the aging process of gamma-sterilized UHMWPE. For this, the structure of the observed spectra and the changes thereof were compared with the known structural information of PE radicals found in the literature. 4.1. Spectral structure and free radical type The ESR spectra of UHMWPE (GUR 4150), recorded at 231C following post-sterilization aging in vacuum at 231C, 371C, and 751C are shown in Fig. 1(A), (B), and (C), respectively. Note that in each figure, (A), (B), or (C), the aging time of each sample has been indicated. The spectra recorded at 231C, following poststerilization aging of UHMWPE in the presence of oxygen (open air) at 231C, 371C, and 751C, for t ¼ 0 (initial) to t ¼ 2 yr are shown in Fig. 2(A), (B), and (C), respectively. Only representative spectra are shown in these figures. The appearance of the central line in the spectra of Fig. 2 (shown by arrow) is an indication of the growth of oxygen-induced radical (OIR). Evidently, the initial free radicals in the open-air samples differ significantly from those in the vacuumsealed samples. The initial structure seen in the spectrum of vacuum-sealed samples is the common seven-line pattern (the weak lines at the ends are shown by arrows in Fig. 1) with a hyperfine splitting of approximately 13 G. They are primarily allyl radicals (aCH2aCH ¼ CHaCHdaCH2a), although some alkyl
M.S. Jahan et al. / Radiation Physics and Chemistry 62 (2001) 141–144
143
Fig. 1. ESR spectra recorded as a function of time following post-sterilization aging in vacuum at 231C (A), 371C (B), and 751C (C), respectively. The arrows indicate presence of weak lines. Magnetic field modulation was maintained at 5 or 10 G.
Fig. 2. ESR spectra recorded as a function of time following post-sterilization aging in the presence of oxygen (open air) at 231C (A), 371C (B), and 751C (C), respectively. The appearance of oxygen-induced radical (OIR) is shown by an arrow at the center of the spectrum. Magnetic field modulation was maintained at 5 or 10 G.
(aCH2aCHdaCH2a) and polyenyl radicals (aCH2a(aCH ¼ CHa)nCHdaCH2a) may be present. The observed spectra of the open-air samples exhibit asymmetric lines with hyperfine splitting of approximately 25 G. They are primarily due to alkyl radicals, although a combination of allyl, polyenyl, and peroxy radicals may also be present as evidenced from the asymmetric features of the spectra. With time, the asymmetric hyperfine lines in the spectra of the open-air samples reduced to a single narrow line of width about 10 G (DHpp ) whereas the total spectral width of the vacuum samples remained constant (DHpp B100 G) and the intensity of the hyperfine lines was reduced. This difference can be explained as follows. In vacuum, allyl radicals recombine to form neutral (nonradical) species without giving
rise to any detectable new radicals (alkyl or peroxy). However, the presence of polyenyl radicals cannot be ruled out, because the broad umbrella (DHpp B100 G) of the final signal (in this study) may contain other singlets (due to polyenyl radicals). Radical recombination or decay was found to depend on annealing/storage temperature. In the open-air samples, on the other hand, radical transformation and decay (reduction in concentration) can occur simultaneously because of reaction of the radicals with oxygen. Since the evolution of the single line in the ESR spectrum can occur only in the open-air samples (i.e., in presence of oxygen), the singlet is attributed by many to peroxy radicals (see, for example, Jahan et al., 1991; O’Neill et al., 1999). However, the presence of oxygen-centered radicals such as aCH2aOd or aCHaOda, cannot be ruled out.
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is stored in an inert environment (vacuum, for example). The reaction in presence of oxygen, however, is associated with structural changes in the polymer radicals. This study further suggests that the OIR is stable at 231C or 371C, but unstable at 751C.
Acknowledgements Work was supported in part by grants through NSFIndustry/University Cooperative Research Center for Biosurfaces and by the University of Memphis.
Fig. 3. Combined FRC recorded, respectively, after 1 and 2 yr of post-sterilization aging in vacuum or air at 231C, 371C, or 751C. In open air at 751C, no radicals were detected after 1 day.
Compared to the broad singlet (DHpp ¼ 100 G) produced by the vacuum samples, the narrow singlets (DH ¼ 10 G) of the open-air samples are evidently produced by the OIR. The OIR was found to remain stable at 231C and 371C for the duration of this study, but disappeared after about one day at 751C (see Fig. 2(C)). 4.2. Free radical concentration The free radicals that remained trapped in the polymer matrix following post-sterilization aging for 1 and 2 yr, respectively, are shown in Fig. 3 as combined concentration (obtained by double-integration method). The initial concentrations (not shown) measured in a large number of samples were in the range between 1016 and 1017 spins/g.
5. Conclusions This study has outlined the long-term behavior of gamma sterilization-induced free radicals in UHMWPE at three temperatures, 231C, 371C, and 751C, in air (presence of oxygen) and in vacuum. One of the most important findings of this study is the presence of residual primary (allyl/alkyl) radicals in UHMWPE 2 yr after post-sterilization storage/annealing at 231C, 371C, or 751C. Evidently, primary radicals cannot be completely annealed at or below 751C as long as UHMWPE
References Eyerer, P., 1987. Property changes of UHMWPE during implantationFfirst hints for the development of an alternative polyethylene. In: Lee, S.M. (Ed.), Advances in Biomaterials. Technomic Publ. Co., Lancaster, PA, pp. 62–68. Igarashi, M., 1983. Free radical identification by ESR in polyethylene and nylon. J. Polym. Sci.: Polym. Chem. 21, 2405–2425. Jahan, M.S., Wang, C., Schwartz, G., Davidson, J.A., 1991. Combined chemical and mechanical effects on free radicals in UHMWPE joints during implantation. J. Biomed. Mater. Res. 25, 1005–1017. Jahan, M.S., Thomas, D.E., Banerjee, K., Trieu, H.H., Haggard, W.O., Parr, J.E., 1998. Effects of radiationsterilization on medical implants. Radiat. Phys. Chem. 51, 593–594. Nausbaum, H.J., Rose, R.M., 1979. The effects of radiation sterilization on the properties of ultra-high molecular weight polyethylene. J. Biomed. Mater. Res. 13, 557–576. Ohnishi, S., Ikeda, Y., Sugimoto, S., Nitta, I., 1960. On the EPR singlet spectra frequently observed in irradiated polymers at a large dose. J. Polym. Sci. 47, 503–507. O’Neill, P., Birkinshaw, C., Leahy, J.J., Barklie, R., 1999. The role of long lived free radicals in the ageing of irradiated ultra high molecular weight polyethylene. Polym. Degrad. Stability 63, 31–39. Seguchi, T., Tamura, N., 1973. Mechanism of decay of alkyl radicals in irradiated polyethylene on exposure to air as studied by electron spin resonance. J. Phys. Chem. 77, 40–44. Streicher, R.M., 1988. Influence of ionizing radiation in air and nitrogen for sterilization of surgical grade polyethylene for implants. Radiat. Phys. Chem. 31, 693–698. Sun, D.C.,Wang, A., Stark, C., 1996. The concept of stabilization in UHMWPE. Transactions of the Fifth World Biomaterial Congress, Vol. 2, p. 195.