The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 86, Issue 5

Scientific Articles | May 01, 2004

Sterilization and Polyethylene Wear: Clinical Studies to Support Laboratory Data

Christi J. Sychterz, MS¹; Karl F. Orishimo, MS²; Charles A. Engh, MD²

¹ Ridgewood Orthopaedic Specialists, 2201 Ridgewood Road, Suite 200, Wyomissing, PA 19610. E-mail address: christi@aori.org
² Anderson Orthopaedic Research Institute, P.O. Box 7088, Alexandria, VA 22307

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In support of their research or preparation of this manuscript, one or more of the authors received funding (a general institutional grant) from INOVA Health Care Services, Alexandria, Virginia. In addition, one or more of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. (The senior author [C.A.E.] has a financial affiliation with DePuy, a Johnson and Johnson company. This study was not influenced by this affiliation.) No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Investigation performed at the Anderson Orthopaedic Research Institute, Alexandria, Virginia

The Journal of Bone and Joint Surgery, Incorporated

J Bone Joint Surg Am, 2004 May 01;86(5):1017-1022

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Abstract

Background: Recently, in vitro studies have linked gamma radiation sterilization in air and subsequent oxidation to decreasing mechanical properties and rapid polyethylene wear. As a result of these studies, orthopaedic device manufacturers have developed alternate sterilization strategies designed to limit or to prevent oxidative degradation. We compared how the in vivo wear performance of conventional and highly-crystalline polyethylene acetabular liners was affected by two sterilization strategies (gas plasma sterilization and gamma radiation sterilization in vacuum-barrier packaging) as compared with gamma radiation sterilization in air.

Methods: Using multivariate linear regression analysis, we analyzed radiographic wear data on 385 total hip implants after a mean duration of follow-up of 6.2 years (range, four to eleven years). Two hundred and twenty-seven components had been sterilized with gamma radiation in air, seventy-seven had been sterilized with gas plasma, and eighty-one had been sterilized with gamma radiation in vacuum-barrier packaging. The liners had a mean shelf life of 0.65 ± 0.85 year. Two hundred and fifty-two components were made from conventional ultra-high molecular weight polyethylene (Enduron), and 133 were made from highly crystalline ultra-high molecular weight polyethylene (Hylamer).

Results: In the conventional polyethylene group, the clinical wear performance of the components that had been sterilized with gamma radiation, either in air or in vacuum-barrier packaging, was superior to that of the nonirradiated components that had been sterilized with gas plasma. However, in the highly-crystalline polyethylene group, only the components that had been sterilized with radiation in vacuum-barrier packaging had a reduced rate of in vivo wear; the components that had been sterilized with gamma radiation in air had essentially the same high wear rate as did the liners that had been sterilized with gas plasma.

Conclusions: The radiographic wear data on conventional polyethylene are consistent with the findings of laboratory studies that have demonstrated that radiation-induced cross-linking has a beneficial effect on polyethylene wear resistance. The conventional and highly-crystalline polyethylene liners in the present study, however, responded differently to gamma radiation sterilization in air despite the fact that the implants in both groups had very short shelf lives. This finding probably was due to the difference in the morphological structures of the two polyethylenes and the resulting greater susceptibility of highly-crystalline polyethylene to oxidative degradation.

Clinical Relevance: These findings provide clinical information that will help surgeons to make informed decisions concerning which components to implant. The data on Hylamer polyethylene liners provide insight into the expected performance of liners of this type that have already been implanted as well as potential insight into the performance of new polyethylenes. In particular, this information demonstrates that differences in the structure of polyethylene, as seen with the different highly cross-linked polyethylenes currently on the market, can affect clinical wear behavior.

Level of Evidence: Therapeutic study, Level III-2 (retrospective cohort study). See Instructions to Authors for a complete description of levels of evidence.

Figures in this Article

For the past three decades, gamma radiation has been the most common method of sterilizing ultra-high molecular weight polyethylene (hereafter simply referred to as polyethylene) for use in total hip arthroplasty components^1-3. This method of sterilization, however, causes breakage of chemical bonds and the creation of free radicals within the polymer. Although the creation of free radicals can enhance the properties of the polyethylene through subsequent cross-linking, it leaves the polyethylene vulnerable to oxidation^1,3. If the polyethylene is packaged in an air environment, oxygen that is present in the air during sterilization can react with the free radicals and can affect the properties of the polymer. In vitro studies have linked gamma radiation sterilization in air and subsequent oxidation to decreasing mechanical properties and rapid polyethylene wear following both total knee and total hip arthroplasty^1,3-7. As a result of those studies, orthopaedic device manufacturers have abandoned gamma radiation sterilization in air and have developed alternate sterilization strategies designed to limit or to prevent oxidative degradation and to reduce the wear of polyethylene components.

Currently, orthopaedic surgeons can choose from conventional ultra-high molecular weight polyethylene components sterilized with two fundamentally different methods. The first approach is sterilization without radiation, which includes surface treatments such as ethylene oxide and gas plasma sterilization. These treatments have no apparent effect on the properties of the polymer^1-3. By omitting radiation, the problems of bond breakage and the formation of free radicals are eliminated, as is the potential for oxidation on the shelf and in vivo. However, any potential improvement in wear resulting from radiation-induced cross-linking also is eliminated.

The second approach is sterilization with radiation in a low-oxygen or oxygen-free environment. In this approach, components are packaged in a vacuum with use of packaging that is impermeable to oxygen (so-called vacuum-barrier packaging) or are packaged in an inert gas, such as nitrogen, before and after radiation sterilization^1-3. These methods substantially reduce the potential for shelf oxidation because the component is exposed to little or no oxygen prior to implantation. However, free radicals generated during radiation sterilization remain within the polymer. The in vivo effect of these residual free radicals and the subsequent potential for in vivo oxidation remain unknown.

To help surgeons to make informed decisions regarding the sterilization of conventional polyethylene components, some authors have presented laboratory data about the effect of different sterilization techniques on the wear resistance of polyethylene liners^1-3. Collier et al. reported that radiation sterilization and storage of components while packaged in a vacuum as well as sterilization with gas plasma prevented degradation of polyethylene during shelf storage². In a hip simulator study, McKellop et al. suggested that the in vivo wear rate of cups sterilized with gamma radiation while in low-oxygen packaging would be substantially lower than the in vivo wear rate of cups sterilized without radiation³. However, while in vitro studies can indicate how an implant might behave, the performance of a component once it has been implanted in the body sometimes can be very different. This was historically demonstrated in studies of carbon-fiber reinforced polyethylene and highly-crystalline (Hylamer; DePuy, Warsaw, Indiana) acetabular components^8-11.

We examined the in vivo wear behavior of conventional and highly-crystalline polyethylene acetabular liners that had been sterilized with three methods: gamma radiation in air, gamma radiation in vacuum-barrier packaging, and gas plasma. The purpose of the present study was to compare the in vivo effect of these three sterilization strategies on the performance of two types of polyethylene.

Materials and Methods

Materials and Methods | Results | Discussion | References

We analyzed 385 hip arthroplasties that had been performed at a single institution. The patients who were chosen for this retrospective analysis were those who had had a primary hip replacement with use of a Duraloc acetabular cup (DePuy); an uncemented, porous-coated component (DePuy); and a 28-mm-diameter femoral head (DePuy). Both ceramic and cobalt-chromium femoral heads were included. In order to be considered for inclusion, patients also needed a minimum of four years of follow-up and at least four radiographs (one postoperative radiograph and three annual follow-up radiographs) for wear analysis. The nonconsecutive patient population consisted of 171 men and 214 women who had a mean age of 60.2 years (range, 24.3 to 94.2 years) at the time of surgery. The patients were followed for a mean of 6.2 years (range, four to eleven years). Two hundred and fifty-two of the implanted components were made from conventional polyethylene (Enduron; DePuy), and 133 were made from highly-crystalline polyethylene (Hylamer; DePuy) (Tables I and II).

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TABLE I Demographic Data on Patients Managed with Conventional (Enduron) Polyethylene Liners

	Number of Patients	Age*(yr)	Duration of Follow-up*(yr)	Shelf Life*(mo)	Ratio of Ceramic Heads to Cobalt-Chromium Heads (No. of Patients)	Male Gender (%)
Sterilization method
Gamma radiation in air	165	63.3 ± 11.0	6.7 ± 1.5	11.6 ± 12.6	22:143	43.6
Gas plasma (no radiation)	52	63.0 ± 11.7	4.7 ± 0.6	2.7 ± 2.0	3:49	44.2
Gamma radiation in	35	66.1 ± 9.0	5.7 ± 0.7	2.7 ± 2.5	3:32	42.9
vacuum-barrier packaging
Mean		63.6 ± 10.9	6.1 ± 1.5	8.5 ± 11.1		43.7

The data are given as the mean and the standard deviation.

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TABLE I Demographic Data on Patients Managed with Conventional (Enduron) Polyethylene Liners

	Number of Patients	Age*(yr)	Duration of Follow-up*(yr)	Shelf Life*(mo)	Ratio of Ceramic Heads to Cobalt-Chromium Heads (No. of Patients)	Male Gender (%)
Sterilization method
Gamma radiation in air	165	63.3 ± 11.0	6.7 ± 1.5	11.6 ± 12.6	22:143	43.6
Gas plasma (no radiation)	52	63.0 ± 11.7	4.7 ± 0.6	2.7 ± 2.0	3:49	44.2
Gamma radiation in	35	66.1 ± 9.0	5.7 ± 0.7	2.7 ± 2.5	3:32	42.9
vacuum-barrier packaging
Mean		63.6 ± 10.9	6.1 ± 1.5	8.5 ± 11.1		43.7

The data are given as the mean and the standard deviation.

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TABLE II Demographic Data on Patients Managed with Highly-Crystalline (Hylamer) Polyethylene Liners

	Number of Patients	Age*(yr)	Duration of Follow-up*(yr)	Shelf Life*(mo)	Ratio of Ceramic Heads to Cobalt-Chromium Heads (No. of Patients)	Male Gender (%)
Sterilization method
Gamma radiation in air	62	53.2 ± 10.9	7.0 ± 1.6	8.7 ± 10.5	32:30	37.1
Gas plasma (no radiation)	25	58.6 ± 13.2	4.7 ± 0.7	2.8 ± 2.5	8:17	66.7
Gamma radiation in vacuum-barrier packaging	46	52.2 ± 10.1	6.3 ± 1.0	5.2 ± 4.1	32:14	60.9
Mean		53.9 ± 11.2	6.3 ± 1.5	6.4 ± 7.9		45.9

The data are given as the mean and the standard deviation.

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TABLE II Demographic Data on Patients Managed with Highly-Crystalline (Hylamer) Polyethylene Liners

	Number of Patients	Age*(yr)	Duration of Follow-up*(yr)	Shelf Life*(mo)	Ratio of Ceramic Heads to Cobalt-Chromium Heads (No. of Patients)	Male Gender (%)
Sterilization method
Gamma radiation in air	62	53.2 ± 10.9	7.0 ± 1.6	8.7 ± 10.5	32:30	37.1
Gas plasma (no radiation)	25	58.6 ± 13.2	4.7 ± 0.7	2.8 ± 2.5	8:17	66.7
Gamma radiation in vacuum-barrier packaging	46	52.2 ± 10.1	6.3 ± 1.0	5.2 ± 4.1	32:14	60.9
Mean		53.9 ± 11.2	6.3 ± 1.5	6.4 ± 7.9		45.9

The data are given as the mean and the standard deviation.

Changes in polyethylene sterilization had been implemented by the manufacturer, who subsequently made running changes of components in the hospital's inventory. At the time of implantation, the surgeon used polyethylene that had been sterilized by whichever method the manufacturer was stocking at the time. The polyethylene liners had been sterilized in one of three ways. Two hundred and twenty-seven components had been packaged in air and sterilized with gamma radiation at a dose of between 25 and 39 kGy (Tables I and II). The mean shelf life for the 227 components that had been gamma irradiated in air was 10.8 months (range, 0.26 months to 5.7 years). Seventy-seven components had been sterilized with gas plasma with no radiation. Eighty-one components had been sterilized with gamma radiation in a vacuum, at a dose of between 19 and 40 kGy, and stored in vacuum-barrier packaging that was impermeable to oxygen. Sterilization methods, dates, and doses were confirmed by the manufacturer on the basis of product and lot numbers from each component's box.

Two-dimensional in vivo head penetration into the polyethylene liner was measured on annual follow-up antero-posterior pelvic radiographs with use of two computer-assisted radiographic techniques. Head penetration was calculated as the movement of the center of the prosthetic femoral head relative to the center of the acetabular cup^12,13. To define the centers of the head and cup, the peripheries of the head and cup are first defined. In the present study, wear data were retrieved from our wear database and two observers used two methods to define the peripheries of the prosthetic head and cup. One method (used by a single observer) involved the use of a digitizing tablet to define the peripheries¹³, and the other method (used by both observers) involved the use of a digital edge-detection algorithm¹². In a previous study, we performed a comparative analysis and found no significant difference in error between these measurement methods when used to evaluate good clinical radiographs; both systems were accurate and reproducible¹⁴. Additionally, to ensure the comparability of these methods, we analyzed sixteen random hips in the present study with use of both methods. There was no significant difference between the two methods with regard to either the amount of head penetration (0.846 compared with 0.749 mm; p = 0.48) or the true wear rate (0.102 compared with 0.105 mm/year; p = 0.94). Thus, head penetration measurements made with both methods were included in our analysis. Linear regression analysis was used to model head penetration over time for each patient. As previously reported and defined^15,16, the slope of the regression line was used to represent the true polyethylene wear rate for each patient.

Because the demographic characteristics of the subpopulations in this retrospective analysis were not matched, direct comparisons of wear rates between sterilization groups were not conducted. To control for the confounding effects of age, duration of follow-up, head material, gender distribution, and shelf life for each group, multivariate linear regression was used to analyze the data. Conventional polyethylene and highly-crystalline polyethylene liners were analyzed separately. For each analysis, the dependent variable was the true wear rate and the independent variables were age, duration of follow-up, shelf life, head material, gender, and sterilization method. These variables were not matched among the groups. Patient age, duration of follow-up, and shelf life were treated as continuous independent variables. Dummy coding was used for the categorical independent variables of sterilization (gamma radiation in air, gas plasma, or gamma radiation in vacuum-barrier packaging), head material (ceramic or cobalt-chromium), and gender (male or female).

The beta coefficients generated from the multiple regression analysis quantified how each independent variable affected the true wear rate when the effects of the other independent variables were held constant. For comparison purposes, we used the beta coefficients and the computed regression equation to calculate a baseline true wear rate. The baseline true wear rate was calculated (with use of author-selected data for the independent variables) as the six-year rate for a female patient who was managed, at the age of sixty years, with the implantation of a gamma-in-air-sterilized polyethylene liner with a shelf life of six months that was coupled with a cobalt-chromium femoral head.

Statistical analysis was performed with use of SPSS software (SPSS version 8.0; SPSS, Chicago, Illinois). Probability values of <0.05 were considered to be indicative of significance.

Results

Materials and Methods | Results | Discussion | References

The beta coefficients and the equation constant generated from the multivariate linear regression analyses are given in Table III. For conventional polyethylene liners, the multivariate regression model accounted for 28% of the variation in the true wear rate data (r² = 0.28). Variables that were significantly correlated with increased wear were younger age (p < 0.01), male gender (p < 0.01), and gas plasma sterilization (p < 0.01). For highly-crystalline liners, the multivariate regression model accounted for 19% of the variation in the true wear rate data (r² = 0.19). Younger age was significantly correlated with increased true wear rates (p = 0.02), whereas the use of a ceramic femoral head (p = 0.02) and radiation sterilization in vacuum-barrier packaging (p = 0.01) were significantly correlated with decreased true wear rates (Table III).

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TABLE III Regression Coefficients for Analysis of Conventional and Highly-Crystalline Polyethylene Liners

	Conventional Polyethylene					Highly-Crystalline Polyethylene
Variable	Beta Coefficient	P Value		Beta Coefficient	P Value
Age*(yr)	-0.003	<0.01		-0.002	0.02
Shelf life*(yr)	+0.006	0.23		+0.031	0.07
Duration of follow-up*(yr)	-0.006	0.20		-0.008	0.32
Gas plasma sterilization†	+0.088	<0.01		+0.014	0.69
Radiation sterilization in vacuum-barrier packaging†	-0.034	0.10		-0.064	0.01
Ceramic head†	-0.032	0.10		-0.052	0.02
Male gender†	+0.040	<0.01		-0.004	0.85
Equation constant			0.302			0.356
Baseline true wear rate (mm/yr)			0.09			0.18

The beta coefficient indicates the change in true wear rate per unit increase in this continuous variable as compared with the baseline true wear rate calculated for a female patient who was managed, at the age of sixty years, with the implantation of a gamma-in-air-sterilized polyethylene liner with a shelf life of six months, coupled with a cobalt-chromium femoral head. †The beta coefficient indicates the absolute change in the true wear rate from the baseline true wear rate.

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TABLE III Regression Coefficients for Analysis of Conventional and Highly-Crystalline Polyethylene Liners

	Conventional Polyethylene					Highly-Crystalline Polyethylene
Variable	Beta Coefficient	P Value		Beta Coefficient	P Value
Age*(yr)	-0.003	<0.01		-0.002	0.02
Shelf life*(yr)	+0.006	0.23		+0.031	0.07
Duration of follow-up*(yr)	-0.006	0.20		-0.008	0.32
Gas plasma sterilization†	+0.088	<0.01		+0.014	0.69
Radiation sterilization in vacuum-barrier packaging†	-0.034	0.10		-0.064	0.01
Ceramic head†	-0.032	0.10		-0.052	0.02
Male gender†	+0.040	<0.01		-0.004	0.85
Equation constant			0.302			0.356
Baseline true wear rate (mm/yr)			0.09			0.18

The baseline true wear rate for the conventional acetabular liners that had been sterilized with gamma radiation in air was 0.09 mm/year. With the effects of the other variables held constant, regression analysis demonstrated that gamma radiation sterilization in vacuum-barrier packaging did not significantly change the true wear rate (0.06 mm/year; p = 0.10) whereas gas plasma sterilization significantly increased the true wear rate (0.18 mm/year; p < 0.01) (Tables III and IV). The baseline true wear rate for the highly-crystalline liners that had been sterilized with gamma radiation in air was 0.18 mm/year. With the effects of the other variables held constant, regression analysis demonstrated that gamma radiation sterilization in vacuum-barrier packaging significantly decreased the true wear rate (0.11 mm/year; p = 0.01) whereas gas plasma sterilization did not significantly change the true wear rate (0.19 mm/year; p = 0.69) (Tables III and IV).

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TABLE IV Mean True Wear Rates Calculated with Multivariate Regression Analysis

	Mean True Wear Rate*(mm/yr)
Sterilization Technique	Conventional Polyethylene	Highly-Crystalline Polyethylene
Gamma radiation in air	0.09	0.18
Gas plasma (no radiation)	0.18	0.19
Gamma radiation in vacuum-barrier packaging	0.06	0.11

Mean true wear rates were calculated for a female patient who was managed, at the age of sixty years, with the implantation of a polyethylene liner with a shelf life of six months, coupled with a cobalt-chromium femoral head.

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TABLE IV Mean True Wear Rates Calculated with Multivariate Regression Analysis

	Mean True Wear Rate*(mm/yr)
Sterilization Technique	Conventional Polyethylene	Highly-Crystalline Polyethylene
Gamma radiation in air	0.09	0.18
Gas plasma (no radiation)	0.18	0.19
Gamma radiation in vacuum-barrier packaging	0.06	0.11

Discussion

Materials and Methods | Results | Discussion | References

We examined the effect of three methods of sterilization (gamma radiation sterilization in air, gas plasma sterilization, and gamma radiation sterilization in vacuum-barrier packaging) on the true wear rate of conventional (Enduron) and highly-crystalline (Hylamer) polyethylene acetabular components. The two types of polyethylene liners were used with the same acetabular shell design at the senior author's (C.A.E.'s) institution over the course of nearly ten years. During that period, the manufacturer had sterilized the liners successively with the different techniques. The changes in sterilization techniques presented a unique opportunity to study how different sterilization processes affected the clinical wear performance of two types of ultra-high molecular weight polyethylene liners when coupled with a single type of acetabular shell. Because there was a surgical bias toward placing Hylamer liners into younger, more active patients, the patients in the Hylamer group were an average of ten years younger than those in the Enduron group (Tables I and II) and, therefore, direct comparisons between the true wear rates of Hylamer and Enduron liners were not conducted in this study.

The mean true wear rates that were calculated in the conventional polyethylene group indicated that the components that had been sterilized with gamma radiation had better wear performance than did those that had been sterilized with gas plasma without radiation, at least for the first ten years in vivo (Table IV). This was true for components that had been sterilized with radiation in vacuum-barrier packaging as well as those that had been sterilized with radiation in air. The mean true wear rate for conventional polyethylene components that had been sterilized with gamma radiation in air was 50% less than that for the components that had been sterilized with gas plasma without radiation (Table IV). These clinical data support the hip simulator results of McKellop et al., who reported a 50% reduction in the wear rate in association with gamma radiation sterilization³.

Perhaps surprisingly, the mean true wear rate for the conventional polyethylene liners that had been sterilized with gamma radiation in air was not significantly different from that for the liners that had been sterilized with gamma radiation in vacuum-barrier packaging. Although not significant, the 0.03 mm/year decrease in the mean true wear rate for the liners that had been sterilized with radiation in vacuum-barrier packaging can be considered a clinically relevant difference and worthy of note. The nonsignificant p value could have been due to the number of specimens that were included in the study. Alternatively, the lack of a significant difference between these two groups could have been related to the short preimplantation shelf life of the components that had been sterilized with gamma radiation in air (mean, 0.97 ± 1.05 year); regression analysis demonstrated that the shelf life of these liners was not significantly correlated with increased wear. These data are consistent with those from a previous analysis in which the researchers found no correlation between a shelf life of as long as three years and the wear rate of conventional polyethylene liners¹⁷, indicating that such liners realized the benefits of cross-linking without being affected substantially by preimplantation oxidation.

Similarly, we found that highly-crystalline polyethylene liners that had been sterilized with gamma radiation in vacuum-barrier packaging performed better than did their nonirradiated, gas plasma-sterilized counterparts. Again, the wear rates associated with radiation sterilization were reduced compared with those associated with gas plasma sterilization, although not to the same degree as was noted for the conventional liners. Interestingly, however, this relationship did not hold true for the highly-crystalline liners that had been sterilized with gamma radiation in air, despite the fact that the shelf lives of these components were even shorter than those of the conventional polyethylene liners in this study. The mean true wear rate for the highly-crystalline components that had been sterilized with gamma radiation in air was essentially the same as that for the gas plasma-sterilized components. On the basis of available laboratory data^3,18-20, we hypothesize that the reason for this finding is the increased impact of oxidative degradation on highly-crystalline polyethylene. In an analysis of retrieved conventional and highly-crystalline acetabular liners, Collier et al. demonstrated that, for a given level of oxidation, highly-crystalline polyethylene experienced a greater reduction in mechanical properties than did conventional polyethylene, suggesting a more detrimental impact of oxidation on the latter material¹⁸. Collier et al. explained that oxidation occurs preferentially in the amorphous regions of a polymer. Because of its high crystallinity, Hylamer has fewer amorphous regions and, therefore, oxidation may be more localized and concentrated¹⁸. These findings are especially salient because early laboratory studies demonstrated that highly crystalline (Hylamer) polyethylene was more resistant to oxidation than conventional polyethylene was²¹.

The current study had several limitations. First, the different sterilization groups were not matched with regard to demographic characteristics. Potentially, there could have been a bias for surgeons to implant the different components into specific types of patients, leading to a bias in true wear rates. However, multivariate regression analysis allowed us to examine the effect of the sterilization method on wear while holding constant the effects of the other confounding variables. In this way, we were able to limit the influence of any potential implantation bias. Second, we acknowledge the absence of an assessment of the activity level of these patients. As Schmalzried et al. demonstrated, activity level greatly affects polyethylene wear and varies tremendously among patients who have undergone hip replacement²². As we lacked information on the activity level of the patients, we were only able to account for variations in variables that are somewhat correlated with patient activity, such as age and gender (with younger age and male gender having been previously associated with greater activity levels²³). We recognize that the ability to quantify a patient's activity level and the inclusion of activity level as a confounding variable in any study of polyethylene wear would be optimal. Third, we acknowledge that this was a retrospective analysis. Certainly, the most accurate way to assess the effect of any variable on clinical wear behavior is with a prospective, randomized study. In the absence of such data, however, we believe that the examination of retrospective data by means of multivariate regression analysis produced information that is clinically relevant for the orthopaedic surgeon.

The data presented here support the hypothesis that the cross-linking induced by radiation sterilization is beneficial to the wear resistance of polyethylene, confirming the results of in vitro studies^1-3. These findings provide clinical information that can assist surgeons in choosing which components to implant. Less obvious is the importance of the data demonstrating the susceptibility of highly-crystalline polyethylene (Hylamer) to oxidation and the effect of different sterilization techniques on the wear behavior of Hylamer. One might consider information on the wear performance of Hylamer as irrelevant because this polyethylene is no longer used clinically. However, these data provide insight into the expected performance of Hylamer liners that have already been implanted as well as potential insight into the performance of new polyethylenes. In particular, this information demonstrates that differences in the structure of polyethylene can affect clinical wear behavior, which is directly relevant to newly developed highly cross-linked polyethylenes. Companies manufacture highly cross-linked polyethylene with use of different processes involving differences in heating levels (above or below the melting point), radiation sources (gamma or electron-beam), dose levels, and terminal sterilization methods²⁴. Each of these variables can change the amorphous and crystalline structure of the polymer, ultimately affecting its mechanical properties²⁴. As we have shown, a change in structure can affect in vivo wear performance in unexpected ways. Because they have different mechanical properties, it is unlikely that all of the new highly cross-linked polyethylenes will perform in the same way. Only in vivo wear analysis can demonstrate which polyethylene manufacturing and sterilization processes will truly improve clinical wear behavior.

References

Materials and Methods | Results | Discussion | References

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Topics

gamma rays ; plasma ; sterilization, sexual ; polyethylene ; sterilization for infection control ; shelf life

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Christi J. Sychterz, Karl F. Orishimo, Charles A. Engh; Sterilization and Polyethylene Wear: Clinical Studies to Support Laboratory Data. The Journal of Bone & Joint Surgery. 2004 May;86(5):1017-1022.

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