The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 82, Issue 12

Articles | December 01, 2000

Effect of Sterilization Method and Other Modifications on the Wear Resistance of Acetabular Cups Made of Ultra-High Molecular Weight Polyethylene : A Hip-Simulator Study*

Harry McKellop, Ph.D.†; Fu-wen Shen, Ph.D.†; Bin Lu, M.S.†; Patricia Campbell, Ph.D.†; Ronald Salovey, Ph.D.‡

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Investigation performed at the J. Vernon Luck Orthopaedic Research Center and the Joint Replacement Institute, Los Angeles Orthopaedic Hospital, and the Departments of Orthopaedics, Biomedical Engineering, and Materials Science, University of Southern California, Los Angeles, California
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Funds were received in total or partial support of the research or clinical study presented in this article. The funding sources were the National Institutes of Health Grant 40996 and the Los Angeles Orthopaedic Hospital Foundation. Materials were donated by DePuy-DuPont Orthopaedics; Howmedica, Incorporated; Intermedics Orthopedics, Incorporated; Poly Hi Solidur, Incorporated; Spire Corporation; and Zimmer, Incorporated.
†The J. Vernon Luck Orthopaedic Research Center (H. McK., F.-W. S., and B. L.) and the Joint Replacement Institute (P. C.), Orthopaedic Hospital, 2400 South Flower Street, Los Angeles, California 90007. E-mail address for H. McKellop: hmckellop@laoh.ucla.edu.
‡Department of Materials Science, University of Southern California, University Park Campus, Los Angeles, California 90089.

J Bone Joint Surg Am, 2000 Dec 01;82(12):1708-1708

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Abstract

Background: Wear of ultra-high molecular weight polyethylene acetabular cups in hip prostheses produces billions of submicrometer wear particles annually that can cause osteolysis and loosening of the components. Thus, substantial improvement of the wear resistance of ultra-high molecular weight polyethylene could extend the clinical life span of total hip prostheses. It has become apparent that the conditions under which ultra-high molecular weight polyethylene cups have been sterilized can markedly affect their long-term wear properties, and new sterilization methods and other modifications have been developed to minimize the negative effects.

Methods: In the present study, a hip-joint simulator was used to assess whether it is preferable to sterilize ultra-high molecular weight polyethylene cups without gamma irraSdiation, to avoid radiation-induced oxidative degradation, or to sterilize with gamma irradiation while the cups are packaged in a suitable low-oxygen atmosphere to minimize oxidation while retaining the increased wear resistance conferred by the radiation-induced cross-linking. Ion-implanted cups and cups made of a highly crystalline polyethylene (Hylamer) also were investigated. Cups made of each material were subjected to wear-testing prior to and after artificial thermal aging to accelerate oxidative degradation.

Results: The results of the present study demonstrated that the cross-linking induced by gamma irradiation improves the wear resistance of ultra-high molecular weight polyethylene, while oxidation reduces it. Without thermal aging, the two types of cups that were sterilized with gamma irradiation while in low-oxygen packaging exhibited about a 50 percent lower rate of wear than did either the nonsterilized cups or the nonirradiated cups sterilized with gas plasma. There was a comparable advantage in the rate of wear after fourteen days of thermal aging. However, after thirty days of aging, the cups sterilized with gamma irradiation in low-oxygen packaging wore several times faster than did the nonirradiated cups. Ion-implanting improved the wear resistance without thermal aging, but after extensive thermal aging the oxidation and wear were greater than those of the controls. Hylamer cups (that is, those that were sterilized with gas plasma) exhibited wear properties very close to those of the nonsterilized ultra-high molecular weight polyethylene cups (the controls) with or without aging.

Conclusions: Sterilizing an ultra-high molecular weight polyethylene acetabular cup without radiation (for example, with ethylene oxide or gas plasma) avoids immediate and long-term oxidative degradation of the implant but does not improve the inherent wear resistance of the polyethylene. Sterilizing with use of gamma irradiation with the implant packaged in a low-oxygen atmosphere avoids immediate oxidation and cross-links the polyethylene, thereby increasing its wear resistance, but long-term oxidation of the residual free radicals may markedly reduce the wear resistance. Ideally, cross-linking with gamma irradiation to reduce wear should be done in a manner that avoids both immediate and long-term oxidation.

Clinical Relevance: The present study demonstrated how the fabrication and sterilization processes influence the resistance to oxidation and wear of the various types of ultra-high molecular weight polyethylene that are currently available. As an exact quantitative relationship between days of thermal aging and years of real-time aging (on the shelf and/or in vivo) has not yet been established, it is not possible to predict precisely when, if ever, the in vivo wear rate of cups sterilized with gamma irradiation while in low-oxygen packaging would exceed that of nonirradiated cups. Nevertheless, the results of these wear tests with use of a hip simulator suggest that, for at least ten years of clinical use, the in vivo wear rate of cups sterilized with gamma irradiation while in low-oxygen packaging will be substantially lower than that of cups sterilized without irradiation. The fundamental interactions among radiation, cross-linking, and oxidation exhibited by the specific materials included in the present study may also apply to acetabular cups of other types of polyethylene. Understanding these fundamental interactions will assist the surgeon in making an informed choice among the materials examined in the present study and among other types of modified polyethylene already in clinical use, including those sterilized with ethylene oxide, those sterilized with gamma irradiation in other forms of low-oxygen packaging, and the various new cross-linked and thermally stabilized polyethylenes.

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Introduction

Introduction | Materials and Methods | Results | Discussion | References

Wear of the polyethylene bearing surfaces of prosthetic hip joints produces billions of submicrometer wear particles annually^7,26,36, often causing a foreign-body response that may lead to bone resorption (osteolysis) and loosening of the components^{1,18,20,24,53,71}. This is of particular concern for young and/or active patients as they may face one or more revisions with cumulative bone loss in their lifetime. Thus, a substantial improvement in the wear resistance of the polyethylene could substantially extend the clinical life span of total hip prostheses.

The majority of ultra-high molecular weight polyethylene components implanted in the past two decades were sterilized by exposure to gamma radiation at doses between 2.5 and 4.0 megarad (twenty-five and forty kilogray), with the components packaged in air. It is now known that the oxygen that is present in the polyethylene when it is irradiated, or that diffuses into the material during shelf storage and/or during in vivo use, reacts with residual free radicals that are generated by the radiation^{13,29,47,48,72}. The resultant chain scission reduces the molecular weight, leading to increased density, stiffness and brittleness, and reduced fracture strength and elongation to failure^{4,8,13,17,49,63}. These changes, in turn, reduce the resistance to wear^50,57.

On the other hand, particularly if little oxygen is present, the free radicals generated during irradiation can form carbon-carbon cross-links between adjacent polyethylene molecules, and a number of laboratory tests have shown that, in an appropriate amount, such cross-linking improves the wear resistance of the polymer^12,22,44,51. Consequently, several manufacturers currently seal their polyethylene components in various types of low-oxygen packaging in order to minimize the detrimental effects of oxidation during sterilization with irradiation and during subsequent shelf storage^2,60,21. Some manufacturers take the additional step of heating acetabular polyethylene cups after irradiation to reduce the level of residual free radicals, thereby reducing subsequent oxidation⁶⁹. Other manufacturers advocate sterilization without irradiation - for example, sterilization with ethylene oxide⁶⁶ or gas plasma¹⁵ - since, even with irradiation in a low-oxygen package, free radicals that can lead to oxidation during prolonged shelf storage and/or in vivo use remain in the polymer. However, while sterilization without irradiation avoids the problem of oxidation of the free radicals entirely, it also eliminates any improvement of the wear resistance that might be conferred by radiation-induced cross-linking.

Since surgeons performing hip arthroplasty presently must choose between these two fundamentally different approaches to sterilization, it is important to assess the relative resistance to wear, including the potential effects of long-term oxidative degradation, of acetabular cups sterilized by each method. In the present study, a hip-joint simulator was used to compare the resistance to wear of acetabular cups prepared by five manufacturers with use of methods that included sterilization without irradiation and sterilization with irradiation in a low-oxygen atmosphere, along with other modifications. The cups were tested prior to and after a thermal aging treatment designed to accelerate oxidative degradation.

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Fig. 1:: Flowchart showing the production of the ultra-high molecular weight polyethylene (UHMWPE) as modified by the orthopaedic implant manufacturers. Beginning with a single lot of extruded bar stock produced from a single batch of GUR 4150 resin, the polyethylene was routed through the five participating manufacturers. The cups that were used as nonsterilized controls and those sterilized with irradiation in air were machined by Zimmer as a service to the study, but they do not represent Zimmer's current product. All of the gamma sterilization was done with the cups in a single container to provide an equal, simultaneous radiation dose.

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Fig. 2:: Graph showing the oxidation levels from the inner bearing surface to the center of the cup wall (left to right) obtained from an ultra-high molecular weight polyethylene cup after seventeen years of shelf-aging and from the nonworn zone of an ultra-high molecular weight polyethylene cup retrieved after thirteen years of use in vivo. Both cups were originally sterilized with gamma irradiation in air. These oxidation levels are among the highest that have been measured in our laboratory for cups sterilized with gamma irradiation in air. (The microtomed sections from the retrieved cup were soaked in hexane for sixteen hours to reduce contaminants prior to Fourier transform infrared analysis.)

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Fig. 3:: Graph showing the oxidation levels from the inner surface to the center of the cup wall (left to right) in the nonworn zones of the cups tested without artificial aging. (The corresponding wear volumes and rates are shown in Figure 4-A and Figure 4-B.) Two cups of each material were analyzed. Since the oxidation profiles were comparable for the two cups in a pair, only the mean oxidation levels are shown for clarity.

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Fig. 4-A:: Graph showing the volumetric wear of the two cups of each material tested without artificial aging. (The corresponding oxidation profiles are shown in Figure 3.) The scale at right indicates the approximate maximum depth of wear - that is, the depth at the center of the contact zone.

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Fig. 4-B:: Graph showing the mean wear rates and the standard deviations for the six different types of polyethylene and for the two cycle intervals without artificial aging.

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Fig. 5:: Graph showing the oxidation levels from the inner surface to the center of the cup wall (left to right) in the nonworn zones of the cups tested after fourteen days of artificial aging at 80 degrees Celsius. (The corresponding wear volumes and rates are shown in Figure 6-A and Figure 6-B.) Since the levels of oxidation were comparable for the two cups in a pair, only the mean oxidation levels are shown for each material for clarity.

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Fig. 6-A:: Graph showing the volumetric wear of the two cups of each material tested after fourteen days of artificial aging at 80 degrees Celsius. (The corresponding oxidation profiles are shown in Figure 5.) The scale at right indicates the approximate maximum depth of wear - that is, the depth at the center of the contact zone.

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Fig. 6-B:: Graph showing the mean wear rates and the standard deviations for the six different types of polyethylene and for the two cycle intervals after fourteen days of artificial aging at 80 degrees Celsius.

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Fig. 7:: Graph showing the oxidation levels from the inner surface to the center of the cup wall (left to right) in the nonworn zones of the cups tested after thirty days of artificial aging at 80 degrees Celsius. (The corresponding wear volumes and rates are shown in Figure 8-A and Figure 8-B.) When the oxidation profiles were comparable for the two cups in a pair, only the mean oxidation levels are shown for clarity. The profiles are shown for both cups made of ion-implanted polyethylene and both made of polyethylene irradiated with an oxygen scavenger since the two cups in each of these pairs exhibited substantially different oxidation levels.

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Fig. 8-A:: Graph showing the volumetric wear of the two cups of each material tested after thirty days of artificial aging at 80 degrees Celsius. (The corresponding oxidation profiles are shown in Figure 7.) The scale at right indicates the approximate maximum depth of wear - that is, the depth at the center of the contact zone.

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Fig. 8-B:: Graph showing the mean wear rates and the standard deviations for the six different types of polyethylene and for the three cycle intervals after thirty days of artificial aging at 80 degrees Celsius.

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TABLE I: Modified Polyethylene as Fabricated by Orthopaedic Manufacturers

*All of the cups sterilized by gamma irradiation were placed in a single container and irradiated to 2.72 megarad (27.2 kilogray) by a commercial sterilizer.†The Abtox Plazlyte sterilization system is manufactured by Abtox, Mundelein, Illinois.

Type of Polyethylene	Group Symbol	Fabrication Method and Source	Sterilization Method*
Nonsterilized controls	C	Machined from extruded bar stock at Zimmer	None
Hylamer, gas plasma	H	Extruded bar stock converted to Hylamer at DePuy-DuPont Orthopaedics; cups machined from bar stock	Gas-plasma sterilized by DePuy-DuPont with use of an Abtox Plazlyte unit†
Irradiated in air	A	Machined from extruded bar stock at Zimmer	Gamma-irradiated in air to 2.72 megarad (27.2 kilogray)
Ion-implanted (IONGUARD III and irradiated in air	I	Machined from extruded bar stock at Zimmer; implanted with nitrogen ions at Spire	Gamma-irradiated in air to 2.72 megarad (27.2 kilogray)
Irradiated in nitrogen and thermally stabilized (Duration)	N	Machined from extruded bar stock at Howmedica	Packaged in nitrogen and gamma- irradiated to 2.72 megarad (27.2 kilogray), then returned to Howmedica for thermal stabilization by heating to 50 degrees Celsius for 6 days while still packaged in nitrogen
Irradiated with oxygen scavenger (Oxygenless Packaging)	O	Machined from extruded bar stock at Intermedics Orthopedics	Packaged in foil with an oxygen scavenger (nontoxic, activated iron particles) in a Tyvek (high-density polyethylene) pouch, and gamma- irradiated to 2.72 megarad (27.2 kilogray)

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TABLE I: Modified Polyethylene as Fabricated by Orthopaedic Manufacturers

Type of Polyethylene	Group Symbol	Fabrication Method and Source	Sterilization Method*
Nonsterilized controls	C	Machined from extruded bar stock at Zimmer	None
Hylamer, gas plasma	H	Extruded bar stock converted to Hylamer at DePuy-DuPont Orthopaedics; cups machined from bar stock	Gas-plasma sterilized by DePuy-DuPont with use of an Abtox Plazlyte unit†
Irradiated in air	A	Machined from extruded bar stock at Zimmer	Gamma-irradiated in air to 2.72 megarad (27.2 kilogray)
Ion-implanted (IONGUARD III and irradiated in air	I	Machined from extruded bar stock at Zimmer; implanted with nitrogen ions at Spire	Gamma-irradiated in air to 2.72 megarad (27.2 kilogray)
Irradiated in nitrogen and thermally stabilized (Duration)	N	Machined from extruded bar stock at Howmedica	Packaged in nitrogen and gamma- irradiated to 2.72 megarad (27.2 kilogray), then returned to Howmedica for thermal stabilization by heating to 50 degrees Celsius for 6 days while still packaged in nitrogen
Irradiated with oxygen scavenger (Oxygenless Packaging)	O	Machined from extruded bar stock at Intermedics Orthopedics	Packaged in foil with an oxygen scavenger (nontoxic, activated iron particles) in a Tyvek (high-density polyethylene) pouch, and gamma- irradiated to 2.72 megarad (27.2 kilogray)

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TABLE II: Wear Rates of Acetabular Cups without Artificial Aging

*The values are given in cubic millimeters per million cycles.

Type of Cup	Cup No.	Between 0.5 and 3.0 Million Cycles*		Between 3.0 and 5.0 Million Cycles*
Wear Rate	Mean and Standard Deviation	Wear Rate	Mean and Standard Deviation
Nonsterilized controls	C1C2	34.936.9	35.9 ± 1.4	40.642.1	41.4 ± 1.0
Hylamer, gas plasma	H1H2	35.034.0	34.5 ± 0.7	36.233.3	34.8 ± 2.1
Irradiated in air	A1A2	37.636.0	36.8 ± 1.1	26.127.3	26.7 ± 0.8
Ion-implanted (IONGUARD III) and irradiated in air	I1I2	25.025.3	25.2 ± 0.2	24.224.1	24.2 ± 0.1
Irradiated in nitrogen and thermally stabilized (Duration)	N1N2	18.615.6	17.1 ± 2.1	23.019.9	21.5 ± 2.2
Irradiated with oxygen scavenger (Oxygenless Packaging)	O1O2	18.616.9	17.8 ± 1.2	18.517.1	17.8 ± 1.0

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TABLE II: Wear Rates of Acetabular Cups without Artificial Aging

*The values are given in cubic millimeters per million cycles.

Type of Cup	Cup No.	Between 0.5 and 3.0 Million Cycles*		Between 3.0 and 5.0 Million Cycles*
Wear Rate	Mean and Standard Deviation	Wear Rate	Mean and Standard Deviation
Nonsterilized controls	C1C2	34.936.9	35.9 ± 1.4	40.642.1	41.4 ± 1.0
Hylamer, gas plasma	H1H2	35.034.0	34.5 ± 0.7	36.233.3	34.8 ± 2.1
Irradiated in air	A1A2	37.636.0	36.8 ± 1.1	26.127.3	26.7 ± 0.8
Ion-implanted (IONGUARD III) and irradiated in air	I1I2	25.025.3	25.2 ± 0.2	24.224.1	24.2 ± 0.1
Irradiated in nitrogen and thermally stabilized (Duration)	N1N2	18.615.6	17.1 ± 2.1	23.019.9	21.5 ± 2.2
Irradiated with oxygen scavenger (Oxygenless Packaging)	O1O2	18.616.9	17.8 ± 1.2	18.517.1	17.8 ± 1.0

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TABLE III: Comparisons of the Wear Rates of Various Materials without Artificial Aging

*C = nonsterilized controls; H = Hylamer, gas plasma; A = irradiated in air; I = ion-implanted and irradiated in air; N = irradiated in nitrogen and thermally stabilized; and O = irradiated with oxygen scavenger.†Power is shown in parentheses.

Compared Groups*	P Value for Difference Between Groups†
At 0.5 to 3.0 million cycles
C > H	0.35 (99.9%)
C < A	0.56
C > I	0.009
C > N	0.009
C > O	0.005
H < A	0.14
H > I	0.003
H > N	0.008
H > O	0.004
A > I	0.005
A > N	0.007
A > O	0.004
I > N	0.03
I > O	0.014
N < O	0.75 (84%)
At 3.0 to 5.0 million cycles
C > H	0.06 (74%)
C > A	0.004
C > I	0.002
C > N	0.007
C > O	0.002
H > A	0.04
H > I	0.02
H > N	0.02
H > O	0.009
A > I	0.05
A > N	0.09
A > O	0.01
I > N	0.22
I > O	0.01
N > O	0.16 (61%)

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TABLE III: Comparisons of the Wear Rates of Various Materials without Artificial Aging

Compared Groups*	P Value for Difference Between Groups†
At 0.5 to 3.0 million cycles
C > H	0.35 (99.9%)
C < A	0.56
C > I	0.009
C > N	0.009
C > O	0.005
H < A	0.14
H > I	0.003
H > N	0.008
H > O	0.004
A > I	0.005
A > N	0.007
A > O	0.004
I > N	0.03
I > O	0.014
N < O	0.75 (84%)
At 3.0 to 5.0 million cycles
C > H	0.06 (74%)
C > A	0.004
C > I	0.002
C > N	0.007
C > O	0.002
H > A	0.04
H > I	0.02
H > N	0.02
H > O	0.009
A > I	0.05
A > N	0.09
A > O	0.01
I > N	0.22
I > O	0.01
N > O	0.16 (61%)

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TABLE IV: Wear Rates of Acetabular Cups After Artificial Aging at 80 Degrees Celsius in Air for Fourteen Days

*The values are given in cubic millimeters per million cycles.

Type of Cup	Cup No.	Between 0.5 and 3.5 Million Cycles*		Between 3.5 and 5.0 Million Cycles*
Wear Rate	Mean and Standard Deviation	Wear Rate	Mean and Standard Deviation
Nonsterilized controls	C3C4	26.124.3	25.2 ± 1.2	29.625.9	27.8 ± 2.6
Hylamer, gas plasma	H3H4	27.529.5	28.5 ± 1.4	31.632.6	32.1 ± 0.7
Irradiated in air	A3A4	81.857.9	69.9 ± 16.9	82.342.3	62.3 ± 28.3
Ion-implanted (IONGUARD III) and irradiated in air	I3I4	28.229.8	29.0 ± 1.1	60.052.9	56.5 ± 5.0
Irradiated in nitrogen and thermally stabilized (Duration)	N3N4	13.413.2	13.3 ± 0.2	19.215.1	17.2 ± 2.9
Irradiated with oxygen scavenger (Oxygenless Packaging)	O3O4	14.314.0	14.2 ± 0.2	13.614.1	13.9 ± 0.4

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TABLE IV: Wear Rates of Acetabular Cups After Artificial Aging at 80 Degrees Celsius in Air for Fourteen Days

*The values are given in cubic millimeters per million cycles.

Type of Cup	Cup No.	Between 0.5 and 3.5 Million Cycles*		Between 3.5 and 5.0 Million Cycles*
Wear Rate	Mean and Standard Deviation	Wear Rate	Mean and Standard Deviation
Nonsterilized controls	C3C4	26.124.3	25.2 ± 1.2	29.625.9	27.8 ± 2.6
Hylamer, gas plasma	H3H4	27.529.5	28.5 ± 1.4	31.632.6	32.1 ± 0.7
Irradiated in air	A3A4	81.857.9	69.9 ± 16.9	82.342.3	62.3 ± 28.3
Ion-implanted (IONGUARD III) and irradiated in air	I3I4	28.229.8	29.0 ± 1.1	60.052.9	56.5 ± 5.0
Irradiated in nitrogen and thermally stabilized (Duration)	N3N4	13.413.2	13.3 ± 0.2	19.215.1	17.2 ± 2.9
Irradiated with oxygen scavenger (Oxygenless Packaging)	O3O4	14.314.0	14.2 ± 0.2	13.614.1	13.9 ± 0.4

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TABLE V: Comparisons of the Wear Rates of the Various Materials After Fourteen Days of Artificial Aging

Compared Groups*	P Value for Difference Between Groups†
At 0.5 to 3.5 million cycles
C < H	0.13 (88%)
C < A	0.07
C < I	0.08
C > N	0.005
C > O	0.006
H < A	0.08
H < I	0.75
H > N	0.004
H > O	0.005
A > I	0.08
A > N	0.04
A > O	0.04
I > N	0.002
I > O	0.003
N < O	0.05 (99.9%)
At 3.5 to 5.0 million cycles
C < H	0.15 (77%)
C < A	0.23
C < I	0.02
C > N	0.06
C > O	0.02
H < A	0.27
H < I	0.02
H > N	0.02
H > O	0.001
A > I	0.8
A > N	0.15
A > O	0.14
I > N	0.01
I > O	0.007
N > O	0.25 (46%)

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TABLE V: Comparisons of the Wear Rates of the Various Materials After Fourteen Days of Artificial Aging

Compared Groups*	P Value for Difference Between Groups†
At 0.5 to 3.5 million cycles
C < H	0.13 (88%)
C < A	0.07
C < I	0.08
C > N	0.005
C > O	0.006
H < A	0.08
H < I	0.75
H > N	0.004
H > O	0.005
A > I	0.08
A > N	0.04
A > O	0.04
I > N	0.002
I > O	0.003
N < O	0.05 (99.9%)
At 3.5 to 5.0 million cycles
C < H	0.15 (77%)
C < A	0.23
C < I	0.02
C > N	0.06
C > O	0.02
H < A	0.27
H < I	0.02
H > N	0.02
H > O	0.001
A > I	0.8
A > N	0.15
A > O	0.14
I > N	0.01
I > O	0.007
N > O	0.25 (46%)

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TABLE VI: Wear Rates of Acetabular Cups After Artificial Aging at 80 Degrees Celsius in Air for Thirty Days

*The values are given in cubic millimeters per million cycles.

Type of Cup	Cup No.	Between 0.5 and 3.0 Million Cycles*		Between 3.0 and 5.0 Million Cycles*		Between 5.0 and 7.5 Million Cycles*
Wear Rate	Mean and Standard Deviation	WearRate	Mean and Standard Deviation	Wear Rate	Mean and Standard Deviation
Nonsterilized controls	C5C6	33.333.0	33.2 ± 0.2	34.9 38.3	36.6 ± 2.4	33.936.9	35.4 ± 2.1
Hylamer, gas plasma	H5H6	45.732.5	39.1 ± 9.4	41.8 37.3	39.6 ± 3.2	36.936.2	36.6 ± 0.5
Irradiated in air	A5A6	82.165.3	73.7 ± 11.8	82.7 68.4	75.6 ± 10.1	61.072.8	66.9 ± 8.3
Ion-implanted (IONGUARD III) and irradiated in air	I5I6	46.351.4	48.9 ± 3.6	105.5112.2	108.9 ± 4.7	83.968.8	76.4 ± 10.7
Irradiated in nitrogen and thermally stabilized (Duration)	N5N6	61.843.6	52.7 ± 12.9	69.0 58.7	63.9 ± 7.3	48.951.6	50.3 ± 1.9
Irradiated with oxygen scavenger (Oxygenless Packaging)	O5O6	49.320.8	35.1 ± 20.2	90.9 31.7	61.3 ± 41.9	90.071.2	80.6 ± 13.3

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TABLE VI: Wear Rates of Acetabular Cups After Artificial Aging at 80 Degrees Celsius in Air for Thirty Days

*The values are given in cubic millimeters per million cycles.

Type of Cup	Cup No.	Between 0.5 and 3.0 Million Cycles*		Between 3.0 and 5.0 Million Cycles*		Between 5.0 and 7.5 Million Cycles*
Wear Rate	Mean and Standard Deviation	WearRate	Mean and Standard Deviation	Wear Rate	Mean and Standard Deviation
Nonsterilized controls	C5C6	33.333.0	33.2 ± 0.2	34.9 38.3	36.6 ± 2.4	33.936.9	35.4 ± 2.1
Hylamer, gas plasma	H5H6	45.732.5	39.1 ± 9.4	41.8 37.3	39.6 ± 3.2	36.936.2	36.6 ± 0.5
Irradiated in air	A5A6	82.165.3	73.7 ± 11.8	82.7 68.4	75.6 ± 10.1	61.072.8	66.9 ± 8.3
Ion-implanted (IONGUARD III) and irradiated in air	I5I6	46.351.4	48.9 ± 3.6	105.5112.2	108.9 ± 4.7	83.968.8	76.4 ± 10.7
Irradiated in nitrogen and thermally stabilized (Duration)	N5N6	61.843.6	52.7 ± 12.9	69.0 58.7	63.9 ± 7.3	48.951.6	50.3 ± 1.9
Irradiated with oxygen scavenger (Oxygenless Packaging)	O5O6	49.320.8	35.1 ± 20.2	90.9 31.7	61.3 ± 41.9	90.071.2	80.6 ± 13.3

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TABLE VII: Comparisons of Wear Rates of the Various Materials After Thirty Days of Artificial Aging

Compared Groups*	P Value for Difference Between Groups†
At 0.5 to 3.0 million cycles
C < H	0.46 (40%)
C < A	0.04
C < I	0.03
C < N	0.17
C < O	0.9
H < A	0.08
H < I	0.30
H < N	0.35
H > O	0.82
A > I	0.10
A > N	0.23
A > O	0.14
I < N	0.72
I > O	0.44
N > O	0.41
At 3.0 to 5.0 million cycles
C < H	0.41 (91%)
C < A	0.03
C < I	0.003
C < N	0.04
C < O	0.49
H < A	0.04
H < I	0.003
H < N	0.05
H < O	0.54
A < I	0.05
A > N	0.32
A > O	0.69
I > N	0.02
I > O	0.25
N > O	0.94
At 5.0 to 7.5 million cycles
C < H	0.53 (99.9%)
C < A	0.04
C < I	0.03
C < N	0.02
C < O	0.04
H < A	0.04
H < I	0.03
H < N	0.01
H < O	0.04
A < I	0.43
A > N	0.11
A < O	0.34
I > N	0.08
I < O	0.76
N < O	0.09

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TABLE VII: Comparisons of Wear Rates of the Various Materials After Thirty Days of Artificial Aging

Compared Groups*	P Value for Difference Between Groups†
At 0.5 to 3.0 million cycles
C < H	0.46 (40%)
C < A	0.04
C < I	0.03
C < N	0.17
C < O	0.9
H < A	0.08
H < I	0.30
H < N	0.35
H > O	0.82
A > I	0.10
A > N	0.23
A > O	0.14
I < N	0.72
I > O	0.44
N > O	0.41
At 3.0 to 5.0 million cycles
C < H	0.41 (91%)
C < A	0.03
C < I	0.003
C < N	0.04
C < O	0.49
H < A	0.04
H < I	0.003
H < N	0.05
H < O	0.54
A < I	0.05
A > N	0.32
A > O	0.69
I > N	0.02
I > O	0.25
N > O	0.94
At 5.0 to 7.5 million cycles
C < H	0.53 (99.9%)
C < A	0.04
C < I	0.03
C < N	0.02
C < O	0.04
H < A	0.04
H < I	0.03
H < N	0.01
H < O	0.04
A < I	0.43
A > N	0.11
A < O	0.34
I > N	0.08
I < O	0.76
N < O	0.09

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TABLE VIII: Results of Wear Debris Analysis

*The values are given as the mean and the standard deviation, with the composition of the debris in parentheses.

Type of Cup	Size and Composition of Debris* (m)
Without Aging			14 Days of Aging			30 Days of Aging
Granules	Fibrils	Flakes	Granules	Fibrils	Flakes	Granules	Fibrils	Flakes
Nonsterilized controls	0.21 ± 0.04 (66%)	1.9 ± 1.1 (26%)	0.34 ± 0.1 (7%)	0.23 ± 0.05 (85%)	2.1 ± 1.1 (14%)	0.36 ± 0.03 (1%)	0.22 ± 0.04 (81%)	2.4 ± 1.2 (6%)	0.34 ± 0.09 (14%)
Hylamer, gas plasma	0.23 ± 0.06 (60%)	1.9 ± 1.1 (30%)	0.41 ± 0.1 (9%)	0.23 ± 0.04 (74%)	2.3 ± 1.2 (23%)	0.38 ± 0.13 (2%)	0.23 ± 0.04 (69%)	2.0 ± 1.1 (15%)	0.39 ± 0.10 (14%)
Irradiated in air	0.20 ± 0.04 (83%)	1.5 ± 0.8 (14%)	0.36 ± 0.1 (3%)	0.22 ± 0.05 (90%)	1.9 ± 1.1 (7%)	0.29 ± 0.12 (5%)	0.21 ± 0.04 (79%)	1.7 ± 0.8 (11%)	0.35 ± 0.08 (9%)
Ion-implanted (IONGUARD III) and irradiated in air	0.21 ± 0.04 (81%)	1.5 ± 1.0 (14%)	0.41 ± 0.1 (4%)	0.23 ± 0.07 (77%)	2.1 ± 1.2 (18%)	0.44 ± 0.09 (5%)	0.23 ± 0.04 (73%)	1.9 ± 0.9 (10%)	0.38 ± 0.11 (18%)
Irradiated in nitrogen and thermally stabilized (Duration)	0.21 ± 0.07 (61%)	2.0 ± 1.5 (30%)	0.56 ± 0.6 (9%)	0.23 ± 0.06 (77%)	1.8 ± 0.8 (18%)	0.43 ± 0.09 (5%)	0.19 ± 0.03 (83%)	1.6 ± 0.6 (5%)	0.29 ± 0.06 (11%)
Irradiated with oxygen scavenger (Oxygenless Packaging)	0.20 ± 0.04 (75%)	1.6 ± 1.0 (17%)	0.44 ± 0.2 (8%)	0.24 ± 0.06 (76%)	2.0 ± 0.8 (20%)	0.39 ± 0.11 (4%)	0.19 ± 0.04 (90%)	1.6 ± 0.8 (4%)	0.34 ± 0.12 (7%)

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TABLE VIII: Results of Wear Debris Analysis

*The values are given as the mean and the standard deviation, with the composition of the debris in parentheses.

Type of Cup	Size and Composition of Debris* (m)
Without Aging			14 Days of Aging			30 Days of Aging
Granules	Fibrils	Flakes	Granules	Fibrils	Flakes	Granules	Fibrils	Flakes
Nonsterilized controls	0.21 ± 0.04 (66%)	1.9 ± 1.1 (26%)	0.34 ± 0.1 (7%)	0.23 ± 0.05 (85%)	2.1 ± 1.1 (14%)	0.36 ± 0.03 (1%)	0.22 ± 0.04 (81%)	2.4 ± 1.2 (6%)	0.34 ± 0.09 (14%)
Hylamer, gas plasma	0.23 ± 0.06 (60%)	1.9 ± 1.1 (30%)	0.41 ± 0.1 (9%)	0.23 ± 0.04 (74%)	2.3 ± 1.2 (23%)	0.38 ± 0.13 (2%)	0.23 ± 0.04 (69%)	2.0 ± 1.1 (15%)	0.39 ± 0.10 (14%)
Irradiated in air	0.20 ± 0.04 (83%)	1.5 ± 0.8 (14%)	0.36 ± 0.1 (3%)	0.22 ± 0.05 (90%)	1.9 ± 1.1 (7%)	0.29 ± 0.12 (5%)	0.21 ± 0.04 (79%)	1.7 ± 0.8 (11%)	0.35 ± 0.08 (9%)
Ion-implanted (IONGUARD III) and irradiated in air	0.21 ± 0.04 (81%)	1.5 ± 1.0 (14%)	0.41 ± 0.1 (4%)	0.23 ± 0.07 (77%)	2.1 ± 1.2 (18%)	0.44 ± 0.09 (5%)	0.23 ± 0.04 (73%)	1.9 ± 0.9 (10%)	0.38 ± 0.11 (18%)
Irradiated in nitrogen and thermally stabilized (Duration)	0.21 ± 0.07 (61%)	2.0 ± 1.5 (30%)	0.56 ± 0.6 (9%)	0.23 ± 0.06 (77%)	1.8 ± 0.8 (18%)	0.43 ± 0.09 (5%)	0.19 ± 0.03 (83%)	1.6 ± 0.6 (5%)	0.29 ± 0.06 (11%)
Irradiated with oxygen scavenger (Oxygenless Packaging)	0.20 ± 0.04 (75%)	1.6 ± 1.0 (17%)	0.44 ± 0.2 (8%)	0.24 ± 0.06 (76%)	2.0 ± 0.8 (20%)	0.39 ± 0.11 (4%)	0.19 ± 0.04 (90%)	1.6 ± 0.8 (4%)	0.34 ± 0.12 (7%)

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

Types of Polyethylene Tested

In order to eliminate variation in the wear rates due to batch-to-batch differences in the original polymer, all of the acetabular cups (Table I) were prepared from a single batch of GUR 4150 ultra-high molecular weight polyethylene (Hoechst Celanese, Houston, Texas) that had been extruded into three-inch (7.6-centimeter) diameter bars in a single run (Poly Hi Solidur, Fort Wayne, Indiana) and had been made available to numerous laboratories as a reference material (S. Li, The Hospital for Special Surgery, New York, N.Y.). Sections of bar stock were forwarded to the five participating manufacturers, who prepared them according to their respective processes (Table I, Fig. 1). The cups were machined to a specified generic shape (a 28.2-millimeter inside diameter and a ten-millimeter wall thickness) suitable for use on the hip simulator. These cups were tested with balls that had a nominal diameter of twenty-eight millimeters. The inside diameter of the cups, which were measured with a dial caliper after all processing steps were complete, ranged from 28.11 to 28.21 millimeters, so the ball-cup clearances averaged 0.2 0.01 millimeter. The materials (Table I) included nonsterilized ultra-high molecular weight polyethylene cups (the controls), Hylamer cups (DePuy-DuPont Orthopaedics, Wilmington, Delaware) sterilized with gas plasma, conventional polyethylene cups sterilized with gamma irradiation in air (as has been done historically), cups implanted with nitrogen ions prior to irradiation in air, and cups irradiated while in one of two types of low-oxygen packages. The nitrogen ions (IONGUARD III) were implanted at Spire Corporation (Bedford, Massachusetts) with use of an ion implanter (model 10-80; Varian, Palo Alto, California) at 160 kiloelectron volts, to a total dose of about 3 l0¹⁵ ions per square centimeter. With use of theoretical modeling software TRIM (Transport of Ions in Matter)⁷³, the depth of penetration was calculated as about 0.5 micrometer.

Since the amount of cross-linking induced in ultra-high molecular weight polyethylene by gamma irradiation markedly affects its wear resistance, the cups that were sterilized with gamma irradiation were placed together in a single container (SteriGenics, Charlotte, North Carolina) to ensure equal and simultaneous doses. Five dosimeters (Howmedica, Rutherford, New Jersey) that were spaced evenly in the container were subsequently analyzed, and they indicated a mean dose of 2.72 0.04 megarad (27.2 0.4 kilogray). After irradiation, the nitrogen-packaged cups (Table I) were returned to the manufacturer (Howmedica) for thermal stabilization to reduce the residual free radicals. This process included heating the cups, while they were still packaged in nitrogen, to 50 degrees Celsius for about six days. The cups were then returned to our laboratory for inclusion in the wear study.

Artificial Aging of the Cups

Three wear tests were carried out: the first was performed without artificial aging, the second was run after thirty days of thermal aging, and the third was conducted after fourteen days of aging. Although all of the cups in a given test were treated identically, because of the availability of the wear simulator the storage times varied among the three tests. For the first test (without thermal aging), the cups that were irradiated in the two types of low-oxygen packaging (Howmedica and Intermedics Orthopedics, Austin, Texas) were removed from their packets about one month after the irradiation and stored in air along with the remaining cups for about three months prior to the start of the wear test. For the second test the cups were stored under the same conditions for about six months prior to the thirty days of thermal aging, and for the third test the cups were similarly stored for about twelve months prior to the fourteen days of thermal aging. For aging, the cups were placed in an oven (inside dimensions, thirty-three by thirty-three by forty-eight centimeters) in ambient air and the temperature was gradually increased by approximately 0.2 degree Celsius per minute to 80 degrees Celsius, maintained for fourteen or thirty days, and then slow-cooled (that is, the heating unit was shut off with the cups still in the oven) to room temperature⁶¹. There was no forced circulation of the air in the oven during aging.

Oxygen that diffuses into the surface layer of polyethylene cups during storage in air tends to increase the amount of oxidation during the subsequent thermal aging⁵⁹. Thus, the prestorage of the cups in air served to reduce the duration of the subsequent oven heating that was required to artificially age the cups. In contrast, most manufacturers currently recommend storage of the cups in their low-oxygen packages, and they recommend against long-term storage on a shelf prior to implantation. Thus, the extended period of storage in air prior to thermal aging used in the present study should be considered an inherent part of the artificial aging process rather than a model of current clinical practice.

The first wear test, performed without artificial aging, was intended to compare the wear characteristics of the various materials that would likely occur if they were implanted shortly after fabrication and sterilization. For the second wear test, the thirty days of thermal aging were sufficient to oxidize the cups that had been irradiated in air to a degree comparable with the greatest degree of oxidation that we have measured in any clinically retrieved polyethylene cups that had been irradiated in air. For example, Figure 2 shows the oxidation profiles of polyethylene cups after extended periods of shelf-aging and/or in vivo implantation. Since the exact composition, processing, and storage history of these cups were unknown, their oxidation profiles are presented only as examples of the highest levels of oxidation that have been observed in cups irradiated in air. The question addressed was: if nonirradiated cups or cups irradiated in low-oxygen packaging were to be subjected to a combination of shelf-aging and in vivo aging that, in the past, has led to high levels of oxidation in cups irradiated in air (Fig. 2), how much lower would the oxidation levels in the cups irradiated in low-oxygen packaging be and how would this amount of oxidation affect their wear resistance?

Since the thirty days of artificial aging used in the second wear test markedly affected the oxidation levels and the wear resistance of the irradiated materials, and this amount of oxidation represented the most severe that we had observed on retrieved implants, the third set of cups was thermally aged for only fourteen days to evaluate the effects of an intermediate amount of oxidative aging.

Method of Wear-Testing

In each of the three wear tests, two cups made of each material, bearing against forged cobalt-chromium-molybdenum balls (Howmedica), were tested on a twelve-station hip-joint simulator (Shore Western Manufacturing, Monrovia, California) under a double-peaked, Paul-type physiological hip load⁴⁵, with a 2000-newton maximum, in bovine blood-serum lubricant (HyClone, Logan, Utah). EDTA was added to a concentration of twenty millimoles to minimize precipitation of calcium phosphate onto the bearing surfaces³⁰, and sodium azide was added to 0.2 weight percent to retard bacterial degradation. The cups were mounted below the balls and were oscillated through a 46-degree biaxial arc at one cycle per second^32,33.

To determine wear, the cups were removed from the simulator, cleaned ultrasonically, vacuum-dried, and then weighed on a microbalance (Mettler AT 261; Mettler-Toledo AG, Greifensee, Switzerland). The absorption of fluid by a worn cup was corrected for by increasing its measured weight loss by the mean weight gain of two cyclically loaded but nonoscillated soak-control cups of the same material. The corrected weight loss was converted to volumetric wear by dividing it by the original density of each cup (that is, 0.927, 0.935, and 0.960 gram per cubic centimeter for the nonsterilized cups, the cups sterilized with gamma irradiation, and the Hylamer polyethylene cups, respectively). While the use of a single value for the density ignored any change in density caused by subsequent oxidation of the polyethylene or other effects, the resultant error in the calculated volumetric wear rates was estimated to be less than 3 percent. The wear rates (Table II, Table III, Table IV, Table V, Table VI, and Table VII) were calculated by linear regression applied to the corrected weight-loss data. Linear regression was applied to separate subintervals during which, from inspection of the wear graphs (Fig. 4-A, Fig. 4-B, Fig. 6-A, Fig. 6-B, Fig. 8-A, and Fig. 8-B), it was apparent that one or more of the materials had exhibited a wear rate substantially different from that in the other intervals in the same test. These intervals are indicated in the tables listing the wear rates.

The p values indicating the levels of significance of the differences between the mean wear rates were calculated with use of a double-sided t test. When the wear behavior of two materials was similar, the power to detect a difference of at least 50 percent (delta) of the mean wear rate at a significance (p value) of 0.05 (alpha) or less was also calculated. This choice for delta was based on the assumption that, in clinical use, a difference of at least 50 percent between the mean wear rates of two types of polyethylene would be necessary for the prevalence of osteolysis to be substantially different³⁷.

The wear of the cups subjected to fourteen days of artificial aging also was assessed by comparing the dimensions of the cups before and after the wear test, with use of a coordinate-measuring machine (BRT 504; Mitutoyo, Aurora, Illinois) fitted with a ruby-tipped touch probe (TP-200; Renishaw, Hoffman Estates, Illinois). (Because the coordinate-measuring machine was not obtained until after the first two wear tests were performed, wear could not be determined by the coordinate-measuring machine for the cups from the other two tests.) Calibration tests indicated that the accuracy was within approximately two micrometers. The coordinate-measuring-machine readings were made over a grid of 301 points evenly distributed over the convex interior of the cup. These data were imported into a computer, and a commercial program (Qualstar 2.0H; ICAMP, Los Alamos, New Mexico) was used to fit a spherical surface to the data with use of a least-squares method. The cups were measured again after the wear test, and the displacement of each measured point from the original reference sphere was calculated, providing a map of the total penetration over the inner surface of the worn cup. When the maximum depth for each was plotted against its corresponding wear volume (that is, as determined from the corrected weight measurement), an approximate linear fit was obtained, with a slope of about 1.5 micrometers per cubic millimeter. On the basis of the assumption that a comparable relationship held for the other two wear tests, this scale factor was used to indicate the approximate corresponding depth of penetration on the plots of wear volume versus cycles (with an estimated accuracy of within approximately twenty-five micrometers).

Measurement of the Levels of Oxidation

Oxidation was measured in each wear-tested cup (that is, two cups for each material and aging condition) with use of transmission Fourier transform infrared spectroscopy. Segments were cut from the cups, embedded in polymethylmethacrylate, and microtomed into 200-micrometer-thick sections. The sections were soaked in hexane for sixteen hours prior to Fourier transform infrared analysis since, in preliminary studies, hexane extraction had reduced the contaminants absorbed from the serum lubricant that can cause erroneously high oxidation readings^25,58,62. Measurements were made with use of a Fourier transform infrared spectroscopy system (Polaris IR 10410; Mattson Instruments, Madison, Wisconsin) with an attached microscope (Spectra-Tech, Shelton, Connecticut) with use of a 0.1 by 0.1-millimeter window, starting at the inner (bearing) surface of the cup and moving across the thickness in 100-micrometer steps, with use of sixty-four scan summations at a resolution of sixteen centimeters^-1 with a mercury cadmium telluride detector. The oxidation ratio^58,72 was calculated as the height of the carbonyl absorption peak at 1717 centimeters^-1 divided by the height of the reference peak at 2022 centimeters^-1. Because the wear process removed much of the oxidized surface layer in the contact zone, the oxidation profiles in the present study were made across the entire thickness in nonworn regions of the cups to provide the complete profiles. The interactive effects of oxidation and cross-linking on the wear resistance for a particular cup then could be assessed by comparing the oxidation ratio at any particular depth to the slope of the wear curve at the same depth. In most cases, the oxidation profiles were similar for the two cups in a pair, and the mean oxidation ratios of the two cups were plotted. For the exceptions (that is, the ion-implanted cups and the cups irradiated with an oxygen scavenger and aged for thirty days), the profiles were plotted for both cups in the pair.

Characterization of Wear Debris

At the end of each wear test, the polyethylene wear debris was isolated from the serum with use of a density-gradient technique5. The particles were examined with a scanning electron microscope (DSM 960; Carl Zeiss, Oberkochen, Germany), and micrographs of representative particles were transferred to a digital image-analysis system (Image One, West Chester, Pennsylvania). For each cup tested, 150 to 400 randomly selected particles were measured and categorized as smooth, round, submicrometer-sized granules; as elongated fibrils; or as irregularly shaped flakes. The level of significance of the differences in the particle morphologies, as indicated by the associated p value, was calculated with use of a double-sided t test.

Results

Introduction | Materials and Methods | Results | Discussion | References

Oxidation and Wear without Artificial Aging

There was little or no oxidation in the nonsterilized control cups or in the gas-plasma-sterilized Hylamer cups (DePuy-DuPont Orthopaedics) (Fig. 3), and the two materials wore at nearly constant and comparable rates throughout five million cycles (Fig. 4-A and Fig. 4-B, Table II and Table III). In contrast, all of the irradiated cups showed some oxidation, with the differences among the materials occurring primarily from the surface to about three millimeters deep. The conventional polyethylene cups irradiated in air showed maximum oxidation at the surface, with a secondary peak about 800 micrometers deep. The corresponding wear rate was initially slightly higher than that for the nonsterilized cups (Fig. 4-A and Fig. 4-B, Table II and Table III), but it decreased with increasing depth from the surface, becoming about 65 percent of that of the nonsterilized cups between three and five million cycles.

Initially, the ion-implanted cups irradiated in air had a markedly yellow-brown coloration, but this was worn off the contact area of the bearing surface during the first 0.25 million cycles. These cups exhibited a peak of oxidation that was comparable with that of the conventional polyethylene cups irradiated in air at the bearing surface, but they showed less oxidation at a depth of greater than approximately 400 micrometers (Fig. 3). Initially, wear was less than that of the conventional polyethylene cups irradiated in air; it decreased further during the middle section of the test, and then it increased to a comparable rate during the final two million cycles (Fig. 4-A and Fig. 4-B, Table II and Table III).

The two sets of cups that were irradiated in a low-oxygen environment had substantially less oxidation in the surface layer than did the cups irradiated in air (Fig. 3). The wear rates also were lower, and they were nearly constant for the duration of the five million cycles (Fig. 4-A and Fig. 4-B), reflecting the relatively constant levels of oxidation (and, therefore, cross-linking) across the thickness. Although the oxidation levels in the two types of cups irradiated in low-oxygen packaging were higher than those in the nonsterilized controls and in the gas-plasma-sterilized Hylamer cups, the wear rates were about 50 percent lower (Table II and Table III), presumably because of the radiation-induced cross-linking.

Oxidation and Wear After Fourteen Days of Artificial Aging

Aging of the nonsterilized cups and the gas-plasma-sterilized Hylamer cups (DePuy-DuPont Orthopaedics) for fourteen days at 80 degrees Celsius produced little or no oxidation through the thickness of the polyethylene (Fig. 5), and the wear rates were again nearly constant over the duration of the test (Fig. 6-A and Fig. 6-B, Table IV and Table V). In contrast, the cups irradiated in air exhibited a substantial oxidation peak at the surface, and there was correspondingly rapid wear during the first 1.5 million cycles. Although the wear rate decreased with increasing depth, the wear rate during the final 1.5 million cycles was still about two and one-quarter times greater than that of the aged nonsterilized controls (Table IV and Table V); this finding reflected the effect of elevated oxidation caused by the aging - that is, the degradation of the cross-linked network.

After fourteen days of aging, the oxidation level at the surface of the ion-implanted cups was substantially lower than that of the conventional polyethylene cups irradiated in air, but it increased to a comparable level at a depth of about 0.4 millimeter (Fig. 5). Consistent with this, the wear rate was initially much lower than that of the cups irradiated in air, but it increased to a comparable level after 3.5 million cycles (Fig. 6-A and Fig. 6-B, Table IV and Table V).

The two sets of cups that had been irradiated in a low-oxygen atmosphere again showed comparable oxidation profiles (Fig. 5) and wear behavior (Fig. 6-A and Fig. 6-B) throughout the five million cycles. Although the oxidation levels were greater than those of the same materials prior to artificial aging (Fig. 3 compared with Fig. 5), they were still substantially lower than those of the cups irradiated in air, and the wear rates were again lower than those of the nonirradiated (that is, the non-cross-linked) cups.

Oxidation and Wear After Thirty Days of Artificial Aging

Thirty days of aging at 80 degrees Celsius produced no measurable oxidation of the nonsterilized cups and only slight oxidation of the gas-plasma-sterilized Hylamer cups (Fig. 7). The corresponding wear rates were nearly constant and were comparable for the two materials throughout the 7.5 million cycles (Fig. 8-A and Fig. 8-B, Table VI and Table VII), again reflecting the lack of oxidation or cross-linking gradients within these cups. In contrast, there was severe oxidation of the surface layer of the cups irradiated in air (Fig. 7), which wore off during the initial 0.5 million cycles. Between three and five million cycles, the mean wear rate of the cups irradiated in air and subjected to thirty days of aging was about 21 percent greater than that of the cups with fourteen days of aging and 183 percent greater than that of the cups without aging (Fig. 8-A and Fig. 8-B, Table II, Table IV, and Table VI).

The oxidation at the surface of the ion-implanted cups after thirty days of aging was again much lower than that of the cups irradiated in air without ion implantation, but both ion-implanted cups exhibited substantial subsurface oxidation peaks (Fig. 7). Consistent with these oxidation profiles, the wear rates of the ion-implanted cups were initially substantially lower than those of the cups irradiated in air - that is, they were comparable with those of the materials irradiated in a low-oxygen environment (Fig. 8-A) - but they increased during the test, becoming about 44 percent greater than those of the cups irradiated in air between three and five million cycles and 14 percent greater between five and 7.5 million cycles (Fig. 8-B, Table VI and Table VII).

Thirty days of aging of the cups that were irradiated in nitrogen and thermally stabilized produced substantially greater surface and subsurface oxidation than was present in these materials after fourteen days of aging (Fig. 7 compared with Fig. 5). Nevertheless, the oxidation level within one millimeter of the surface was again much lower than that of the cups irradiated in air, demonstrating the effect of sterilization in the low-oxygen atmosphere. Throughout the 7.5 million cycles, the mean wear rate of the nitrogen-packaged and thermally stabilized cups was lower than that of the cups irradiated in air but was greater than that of the nonsterilized controls and the Hylamer gas-plasma-sterilized cups (Fig. 8-A and Fig. 8-B, Table VI and Table VII).

In contrast, although both of the cups that were irradiated while packaged with an oxygen scavenger showed low levels of oxidation at the surface (Fig. 7), one of these cups (O5) showed a severe oxidation peak at a depth of about 0.5 millimeter (comparable with the maximum for one of the ion-implanted cups) and the wear rate was several times greater than that of the nonirradiated cups (Fig. 8-A and Fig. 8-B, Table VI and Table VII). The other oxygen-scavenger cup (O6) exhibited much less oxidation (Fig. 7) and underwent the least wear of any cup for the first six million cycles of the test (Fig. 8-A and Fig. 8-B). Of the eighteen pairs of cups in the three wear tests, only this pair exhibited such marked differences between the wear rates of the two cups. Although it was not determined why the oxidation level was much higher in cup O5 than it was in cup O6, additional cups that were subsequently prepared by the manufacturer with use of the same process and then were irradiated and aged exhibited oxidation levels comparable with those of the less oxidized wear-test cup (O6), suggesting the possibility of a packaging defect with cup O5.

The wear debris recovered from the serum lubricant consisted primarily of granules (60 to 90 percent of the particles were granules), with fibrils and flakes making up the remainder (Table VIII), as is typically found in periarticular tissues from hips with ultra-high molecular weight polyethylene cups irradiated in air³⁶. In addition, the distribution of the particle sizes was comparable among the different types of polyethylene tested. However, for most of the materials, the wear debris from the cups that had been artificially aged for thirty days contained a lower percentage of fibrils (p = 0.006) and a higher percentage of granules (p = 0.08) and flakes (p = 0.02) than did those without aging or those after fourteen days of aging (Table VIII).

Discussion

Introduction | Materials and Methods | Results | Discussion | References

With the exceptions noted above, the oxidation profiles and wear patterns were very similar for two cups of a particular material and for different materials that had comparable radiation-cross-linking histories. This repeatability was due in part to the minimum variation in material properties obtained by the use of a single parent batch of ultra-high molecular weight polyethylene and the fact that all of the cups that were sterilized with radiation were exposed together, in the same container, to a single dose. When large systematic differences were found between the mean wear rates of two materials that had been prepared under substantially different conditions (for example, nonsterilized cups compared with those irradiated in a low-oxygen atmosphere [Fig. 4-A and Fig. 4-B]), testing of only two samples of each material was sufficient to provide high levels of statistical reliability, indicating that these differences did not occur by chance selection (Table III, Table V, and Table VII).

In contrast, the power to detect substantial differences between pairs of materials that exhibited similar wear rates within a particular test interval was sometimes low. For example, the power to detect a 50 percent difference between the mean wear rates, during the interval from 3.5 to 5.0 million cycles, of the two sets of cups irradiated in a low-oxygen atmosphere and then subjected to fourteen days of artificial aging (materials N and O) (Fig. 6-A and Fig. 6-B) was 46 percent. However, it should be emphasized that this power calculation applies only to the likelihood of detecting a difference between the wear rates within that specific interval, and it does not take into account the fact that these two materials also showed comparable behavior throughout that entire wear test and throughout the test run without aging (Fig. 4-A and Fig. 4-B). This consistent similarity of the wear behavior increases the reliability of a conclusion that the wear rates are not different in general - that is, the conclusion has greater reliability than that indicated by the power calculation for that interval alone. In addition, the similarity of the wear behavior was consistent with the fact that the cups of the two materials had similar distributions of oxidation and, presumably, cross-linking, at least until they were extensively oxidized (Fig. 8-A and Fig. 8-B). This reasoning applies to the other pairs of materials that showed comparable wear rates (for example, the two types of polyethylene that were not irradiated or cross-linked [materials C and H]) (Fig. 4-A, Fig. 4-B, Fig. 6-A, Fig. 6-B, Fig. 8-A, and Fig. 8-B).

In general, when the cups were heated in air⁶¹ the maximum oxidation was near the surface, whereas the peak is typically below the surface in shelf-aged and/or in vivo aged cups (Fig. 2) and in cups aged under pressurized oxygen¹⁵. While this difference must be taken into account when one predicts the long-term clinical wear behavior of a given material, placement of the oxidized zone nearer the surface of the cup had the practical advantage of shortening the duration of the wear-testing required for the wear to penetrate into the most oxidized (and, therefore, least cross-linked) layer. Thus, the differences among the wear rates of the artificially aged materials in the present study probably represent the maximums that might occur over decades of clinical use.

The shift in the composition of the wear debris away from drawn-out fibrils and toward granules and flakes for each of the materials after thirty days of thermal aging probably reflected the embrittlement of the polyethylene due to oxidative degradation, as has been measured in tensile tests of sections cut from retrieved cups⁶³. Nevertheless, the similar morphology and size distributions of the wear debris from the six types of polyethylene (Table VIII) suggested that the osteolytic reactions to comparable amounts of wear debris would also be similar.

For the cups sterilized with gamma irradiation in air, the higher wear rates after artificial aging were consistent with the marked oxidation (and, therefore, reduced cross-linking) of their surface layers. Because of this effect, most manufacturers no longer sterilize polyethylene components while packaged in air. In contrast, the nonsterilized and gas-plasma-sterilized cups had no free radicals to induce oxidation, and these cups showed comparable wear rates that were relatively immune to artificial aging. Although we did not include cups that had been sterilized with ethylene oxide in the present study, other tests performed in this laboratory⁴¹ and elsewhere⁶⁹ have shown that the wear resistance of such cups is closely comparable with that of nonsterilized controls, as used in the present study. Clinically, the present results indicate that one can expect the wear rates of cups sterilized without irradiation to be lower than that of cups sterilized with gamma irradiation in air, particularly if the latter cups are stored for a substantial period prior to implantation.

The cups prepared by the Hylamer process (DePuy-DuPont Orthopaedics) followed by gas-plasma sterilization exhibited little oxidation, and they had wear rates and particle morphologies comparable with those of the nonsterilized conventional polyethylene, even after extensive artificial aging. Thus, the two materials that were neither cross-linked nor oxidized exhibited nearly identical wear properties. Similarly, in a previous hip-simulator study in our laboratory^34,35, the wear resistance of Hylamer cups that had been sterilized with gamma irradiation in air (but without artificial aging) was very comparable with that of cups made of conventional ultra-high molecular weight polyethylene, produced from the same parent batch of material, and sterilized with gamma irradiation in air. In contrast, with use of a hip simulator comparable with that used in the present study, Wang et al.⁶⁹ found that the wear of Hylamercups sterilized with gamma irradiation in air was about 2.3 times greater than that of cups made of conventional polyethylene and sterilized in the same manner. However, in their study, the cups were not all fabricated from the same parent batch of polyethylene, they were irradiated separately, and they were shelf-stored for different intervals prior to the wear-testing; each of these factors might have affected the relative levels of oxidation and/or cross-linking and the resultant wear rates.

Clinical studies also have shown contradictory results with Hylamer cups sterilized with gamma irradiation in air. For example, in a five-year follow-up study, Sychterz et al.⁶⁵ recently reported that the mean wear rate of Hylamer cups sterilized with gamma irradiation in air was about 25 percent less than that of conventional cups during the first three years and about 15 percent greater during the fifth year. In contrast, other clinical series have shown that the wear rates, and the resultant prevalence and severity of osteolysis, were apparently greater in patients with Hylamercups sterilized with gamma irradiation in air^6,31.

Unfortunately, it is difficult to isolate quantitatively the relative influences of variables, other than the type of polymer, on the relative rate of wear in such clinical studies. Patient age and activity, the mixing of components from different manufacturers, and the likelihood of contamination by third-body abrasive particles are some of the variables that may influence wear^23,36,55. Nevertheless, the results of recent laboratory analyses of retrieved Hylamer implants have suggested that the presence of radiation-induced free radicals, combined with differences in the duration of storage of the Hylamer cups sterilized with gamma irradiation in air prior to wear-testing or clinical implantation, could have been a key factor leading to the differences in the results among the above studies. For example, in an earlier study, we found that the level of oxidation in Hylamer cups sterilized with gamma irradiation in air was comparable with that in conventional ultra-high molecular weight polyethylene cups after 6.5 years of shelf storage, regardless of whether this storage was in air or in water³⁸. However, in tensile tests of sections cut from retrieved implants, even at comparable levels of oxidation, Hylamer sterilized with gamma irradiation in air exhibited substantially greater reductions in ultimate tensile strength and elongation than did conventional polyethylene⁹. If there is a corresponding time-dependent decrease in the wear resistance of Hylamer sterilized with gamma irradiation in air, this could explain the elevated wear rates exhibited by Hylamer both in the laboratory study by Wang et al.⁶⁹ and in some of the clinical series^6,31.

The fact that the yellow-brown discoloration of the ion-implanted cups in our study was worn off early in the wear test was consistent with the very shallow depth of penetration of the nitrogen ions (that is, 0.5 micrometer). For the balance of the wear test, the variation of wear with depth in the ion-implanted cups was consistent with their oxidation-cross-linking profiles. Although the nitrogen ions penetrated to a depth of only about 0.5 micrometer, the differences in the oxidation levels and the wear rates suggested that ion-implanting somehow served to retard oxidation²⁸ at a substantially greater depth in the polymer during subsequent thermal aging (Fig. 5), such that there was less reduction of the wear resistance. However, the mechanism by which the superficial implanting of ions affected the oxidation properties deeper in the polyethylene was unexplained.

Similarly, the cause of the very high subsurface oxidation that occurred after thirty days of aging in one of the ion-implanted cups, but not in the other (Fig. 7), was unexplained. Nevertheless, the variation in wear with depth for these cups relative to conventional cups sterilized with gamma irradiation in air (Fig. 8-A) again reflected the principle that higher oxidation and/or reduced cross-linking resulted in a higher rate of wear.

Packaging the cups in a low-oxygen atmosphere resulted in a lower concentration of oxygen in the surface layer during irradiation, as reflected by the lower oxidation levels in the surface layers (Fig. 3). In a separate study⁴¹, we found that this also results in greater cross-linking of the surface layers, as was reflected in the lower wear rates relative to those of the cups irradiated in air and those of the nonirradiated cups. Nevertheless, the increase in oxidation levels of the cups irradiated in a low-oxygen atmosphere and subjected to thirty days of artificial aging (Fig. 7) indicated that a substantial number of free radicals remained in these cups, even in those that were thermally stabilized after irradiation. The elevated wear rates exhibited on wear-testing after thirty days of artificial aging by the cups irradiated in a low-oxygen atmosphere might have been a direct effect of the oxidative degradation of the material, which may have been due to a lowering of the molecular weight by breaking of the intramolecular carbon-carbon bonds and/or to an indirect effect of a reduction in cross-linking caused by the oxidation^14,41. Regardless of the precise mechanism, the results of the present hip-simulator wear-testing study suggested that, if a comparable amount of oxidation were to occur clinically in cups that had been irradiated in a low-oxygen atmosphere, then the clinical wear rates might eventually exceed those of nonirradiated cups.

Thus, the reliability of using the results of the present hip-simulator study to predict the relative clinical performance of the materials depends on how closely the oxidation levels and other changes produced by artificial aging compare with those occurring during shelf storage and subsequent use in vivo and on how many days of artificial aging are the equivalent of a given number of years of real-time aging. While it is possible to find examples of cups sterilized with irradiation in air, shelf-aged, and clinically retrieved with oxidation levels as high as those generated in the cups irradiated in air and artificially aged for thirty days in the present study (Fig. 2 compared with Fig. 7), much of the oxidation seen in the clinically retrieved cups might have occurred during shelf storage prior to implantation rather than during in vivo use¹⁶. Furthermore, since cups sterilized with gamma irradiation are now almost exclusively irradiated and stored in some type of low-oxygen packaging, and most manufacturers recommend against prolonged shelf storage, such cups are unlikely to reach oxidation levels as high as those generated by the thirty days of artificial aging in the present study.

The range of linear penetration rates exhibited by the acetabular cups in the present study fell within the range of linear penetration rates measured radiographically for cups of comparable materials in vivo. One million cycles on the hip simulator is approximately the equivalent of one year of average clinical use of a hip prosthesis^54,67. On the basis of the coordinate-measuring-machine measurements, the equivalent clinical linear penetration rates for the ultra-high molecular weight polyethylene cups sterilized with gamma irradiation in air and tested in the simulator were about 0.04 millimeter per year (between three and five million cycles [Table II]) without artificial aging, about 0.09 millimeter per year (between 3.5 and five million cycles [Table IV]) after fourteen days of aging, and about 0.10 millimeter per year (between five and 7.5 million cycles [Table VI]) after thirty days of aging. The highest wear rate measured after testing in the simulator, which occurred during the first one million cycles for the cups sterilized with gamma irradiation in air and then subjected to thirty days of aging (Fig. 8-A and Fig. 8-B), was the equivalent of a linear penetration rate of about 0.27 millimeter per year clinically. The latter linear penetration rate is somewhat above the range of 0.1 to 0.2 millimeter per year that has been reported for the mean clinical linear penetration rate of polyethylene sterilized with gamma irradiation in air^10,56,64, but it is well below the maximum clinical linear penetration rates, which can be as high as 0.5 millimeter per year. This finding is consistent with the fact that, in clinical use, factors in addition to oxidative degradation, such as poor initial quality of the polyethylene, a patient with an unusually high body weight and/or activity level, and the entrapment of third-body abrasive particles^{3,11,23,36,46,55}, can combine to accelerate the wear rate, and these latter factors were not included in the simulator model of the present study.

The equivalent clinical wear rates of the other materials in the present study also were somewhat below the mean values reported for polyethylene irradiated in air. For example, after thirty days of aging, the mean wear rates between 5.0 and 7.5 million cycles (Table VI) were the equivalent of about 0.053 millimeter per year for the nonsterilized cups of conventional polyethylene, 0.054 millimeter per year for the gas-plasma-sterilized Hylamer cups, 0.11 millimeter per year for the ion-implanted polyethylene cups irradiated in air, 0.075 millimeter per year for the polyethylene cups irradiated in nitrogen and thermally stabilized, and 0.12 millimeter per year for the polyethylene cups irradiated with an oxygen scavenger.

Another factor that might have contributed to the somewhat lower wear rates of cups tested in the simulator compared with wear rates associated with in vivo use was the use of 90 percent bovine serum as a lubricant. The protein concentration of 90 percent serum is at the high end of the range that has been measured in fluids obtained from hips with artificial joints⁴³, and it has been shown that the wear of a polyethylene acetabular cup in a hip simulator decreases as the concentration of proteins in the test lubricant increases above this range^19,69. The optimum concentration of serum proteins necessary for the simulator wear rate to best approximate that of an acetabular cup in vivo has not yet been established^30,52,70. Fortunately, the type of wear generated in 90 percent serum is closely comparable with that of polyethylene cups in vivo^36,68 so the relative wear rates of different types of polyethylene, as measured in the present study, should also be comparable with those in vivo.

If testing of the cups in the hip simulator in the present study is a reasonably accurate model of in vivo conditions, then the wear rate of an ultra-high molecular weight polyethylene acetabular cup that has been sterilized with gamma radiation while packaged in a suitable low-oxygen environment can be expected to be substantially lower than that of a nonirradiated cup during the first decade of clinical use and possibly longer; however, this advantage eventually might be substantially reduced or eliminated by oxidative degradation of residual free radicals. In view of the present uncertainty of long-term predictions based on accelerated-aging techniques, some surgeons might consider use of an acetabular cup that has been sterilized without radiation in their youngest patients because of the potential for long-term oxidative degradation of the cups irradiated in a low-oxygen environment to eventually offset the benefits of the cross-linking. On the other hand, if a cup without cross-linking fails during the first decade as a result of wear-induced osteolysis, there will be no long-term benefit from its greater resistance to oxidation.

Another approach to minimizing both oxidation and wear is to use radiation to cross-link the polyethylene while it is still in the form of an extruded rod or molded block. The cross-linked material can then be heated above the melt temperature (about 150 degrees Celsius) to extinguish the residual free radicals before it is machined into the final components. In laboratory tests, acetabular cups made of polyethylene that had been cross-linked with radiation and then remelted have demonstrated resistance to oxidation comparable with that of nonirradiated polyethylene and, when cross-linking was sufficient, wear resistance that was superior to that of acetabular cups sterilized with the conventional gamma dose in low-oxygen packaging, as used in the present study^27,39,40,42.

Finally, it should be emphasized that the specific results of the present study might apply only to the type of wear generated in a hip prosthesis. Since oxidation also tends to degrade the fatigue resistance of ultra-high molecular weight polyethylene, the modifications investigated in the present study could have very different relative effects on the wear resistance of some designs of total knee replacements, in which the cyclic contact stresses are substantially greater.

References

Introduction | Materials and Methods | Results | Discussion | References

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Topics

aging ; oxygen ; free radicals ; oxidation ; hip region ; hip joint ; nitrogen ; plasma ; sterilization, sexual ; polyethylene

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Harry McKellop, Fu-wen Shen, Bin Lu, Patricia Campbell, Ronald Salovey; Effect of Sterilization Method and Other Modifications on the Wear Resistance of Acetabular Cups Made of Ultra-High Molecular Weight Polyethylene A Hip-Simulator Study*. The Journal of Bone & Joint Surgery. 2000 Dec;82(12):1708-1708.

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