*Corresponding author. Tel.: #1-201-934-4358; fax: #1-201-934-4443.

E-mail address: edidin@warwick.net (A.A. Edidin).

Biomaterials 21 (2000) 1451}1460

Degradation of mechanical behavior in UHMWPE after naturaland accelerated aging

A.A. Edidin! ,*, C.W. Jewett", A. Kalinowski", K. Kwarteng!, S.M. Kurtz"!Stryker Howmedica Osteonics, 49 Route 17, Allendale, NJ 07401, USA

"Exponent Failure Analysis Associates, 2300 Chestnut Street, Suite 150, Philadelphia, PA 19103, USA

Received 27 October 1999; accepted 18 January 2000

Abstract

Ultra-high molecular weight polyethylene (UHMWPE) is known to degrade during natural (shelf) aging following gammairradiation in air, but the mechanical signature of degradation remains poorly understood. Accelerated aging methods have beendeveloped to reproduce the natural aging process as well as to precondition total joint replacement components prior to jointsimulator wear testing. In this study, we compared the mechanical behavior of naturally (shelf) aged and accelerated aged tibial insertsusing a previously validated miniature specimen testing technique known as the small punch test. Tibial inserts made of GUR 1120and sterilized with 25 to 40 kGy of gamma radiation (in air) in 1988, 1993, and 1997 were obtained; a subset of the 1997 implants weresubjected to 4 weeks of accelerated aging in air at 803C. To determine the spatial variation of mechanical properties within each insert,miniature disk shaped specimens were machined from the surface and subsurface regions of the inserts. Analysis of variance of the testdata showed that aging signi"cantly a!ected the small punch test measures of elastic modulus, initial load, ultimate load, ultimatedisplacement, and work to failure. The accelerated aging protocol was unable to reproduce the spatial mechanical pro"le seen in shelfaged components, but it did mechanically degrade the surface of GUR 1120 tibial components to an extent comparable to that seenafter 10 years of natural aging. Test specimens showed a fracture morphology consistent with the decreased ductility and toughnesswhich was corroborated by the small punch test metrics of this study. Our data support the hypothesis that UHMWPE undergoesa spatially nonuniform change towards a less ductile (more brittle) mechanical behavior after gamma irradiation in air and shelfaging. ( 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Ultra-high molecular weight polyethylene; UHMWPE; Mechanical behavior; Oxidation; Degradation

1. Introduction

Degradation of ultra-high molecular weight polyethy-lene (UHMWPE) following gamma sterilization in airhas been associated with decreased longevity of sometotal joint replacement components [1,2]. It has beenestimated that in the United States alone, between 1980and 1995, up to 4 million patients may have beenimplanted with UHMWPE acetabular and tibial compo-nents that were gamma irradiated in air [1]. While con-temporary sterilization practices minimize the likelihoodof degradation of UHMWPE implants, the sequelae of

gamma irradiation in air will remain clinically relevantthroughout the "rst decade of the 21st century.

Because oxidation of UHMWPE takes months oryears to reach appreciable levels at ambient or bodytemperature, thermal aging techniques have been de-veloped to accelerate the oxidation of UHMWPE, withthe expectation that the mechanical behavior after accel-erated aging will be comparable to naturally aged mate-rial [3}5]. Predicated upon this assumption, acceleratedthermal aging of UHMWPE has been widely used bymembers of the orthopaedic community to test the resis-tance to aging of modi"ed UHMWPE materials [6,7], aswell as to precondition components prior to fatigue [8]or hip simulator wear testing [4,9,10]. The physical andchemical properties of UHMWPE following gammasterilization have been studied extensively after naturaland accelerated aging [1,2]. However, the e!ect of agingon UHMWPE mechanical behavior as derived from tests

0142-9612/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved.PII: S 0 1 4 2 - 9 6 1 2 ( 0 0 ) 0 0 0 2 1 - 1

on clinically relevant hip or knee components has notbeen well documented, mainly due to the di$culty withusing conventional, relatively large mechanical test speci-mens for such characterization. The development andvalidation of miniature specimen testing techniques, suchas the small punch test, now permits direct measurementof UHMWPE mechanical properties as a function ofaging in clinically relevant components.

Thermally accelerated oxidation of UHMWPE wasinitially validated by comparing the density and infraredabsorbance spectra through the thickness of thermallyaged and naturally aged components [3}5]. Sun et al.have argued that for gamma air-irradiated UHMWPE,23 days of thermal aging at 803C, with an initial heatingrate of 0.63C/min or slower, is equivalent to 10 years ofnatural shelf aging [11]. However, recent research hashighlighted certain limitations of thermal aging tech-niques. For example, Greer and colleagues have referredto thermal aging of UHMWPE as a &severe oxidativechallenge' to distinguish the process from natural aging[9]. Another limitation is that there are substantial di!er-ences between the crystalline microstructure of thermallyaged and naturally aged air-irradiated UHMWPE [12].For example, the stacking of crystalline lamellae ob-served in thermally aged UHMWPE may be consistentwith an annealing process, and di!ers from the twistedlamellae observed in naturally aged UHMWPE [12].

The mechanical behavior of UHMWPE evolves dur-ing natural (shelf) aging after gamma irradiation in air,but the kinetics and characteristics of mechanical degra-dation remain poorly understood, due largely to previousemphasis on indirect measurement techniques. Further-more, while it is recognized that aging at elevated tem-peratures will accelerate the oxidation of air-irradiatedUHMWPE, the clinical relevance of such a thermallydegraded material remains uncertain, particularly iffatigue or joint simulator testing is to be performed afteraging. Recent advances in miniature specimen testingmethods [13}20], such as the small punch test [13}16],have enabled direct mechanical characterization of or-thopaedic component sized samples. Consequently, thepurpose of this study was to compare the mechanicalbehavior of UHMWPE tibial components after 5 and 10years of natural shelf aging with tibial components thathad been subjected to accelerated aging. This study alsoaddressed the following research questions: (a) whatquanti"able changes in mechanical behavior are asso-ciated with the degradation of UHMWPE after gammairradiation in air? and (b) how does the mechanical be-havior of UHMWPE after accelerated aging comparewith clinically relevant UHMWPE that has been nat-urally aged for 5}10 years? To address these researchquestions, miniature disk-shaped specimens 0.5 mmthick were prepared from the surface and subsurfaceregions of naturally aged and thermally aged compo-nents. The specimens were then tested in equibiaxial

tension using the small punch (disk-bend) testing tech-nique to characterize the linear elastic and nonlinearlarge deformation mechanical behavior of theUHMWPE. Di!erential scanning calorimetry (DSC) andFourier transform infrared spectroscopy (FTIR) werealso conducted to complement the mechanical testingresults. The long-term goal of this research is to eluci-date the mechanisms of mechanical degradation inUHMWPE and the impact on long-term clinical perfor-mance of orthopaedic joint replacement bearings.

2. Methods and materials

We studied eight knee implants of the same commer-cially available design and manufacture (Omni"t; StrykerHowmedica Osteonics, Allendale, NJ). All of the im-plants studied were manufactured from compressionmolded GUR 1120 (RCH 1000 grade) sheet stock usingthe same machinery and programming normally used toproduce commercially available inserts. The implantswere sterilized with a standard dose (25}40 kGy) ofgamma radiation (in air) in 1988 (n"2), 1993 (n"2),and 1997 (n"4), respectively.

Components that were irradiated in 1997 were precon-ditioned at room temperature for two months prior toaccelerated thermal aging. Two of the components irra-diated in 1997 were then subjected to 4 weeks of acceler-ated aging in a digitally controlled, air circulating oven at803C and at atmospheric pressure. The initial heatingrate was 0.13C/min. Characterization of the aged andunaged 1997 irradiated implants was performed in 1998,less than a month after the aging experiment. The nat-urally aged 1993 and 1988 components were also testedin 1998, having attained shelf ages of 5 and 10 years,respectively. Thus, four groups of gamma air-irradiatedGUR 1120 tibial components were examined: unagedcontrol (n"2), control with accelerated aging (n"2),5-year shelf age (n"2), and 10-year shelf age (n"2).

The mechanical behavior of the UHMWPE was dir-ectly measured using the small punch testing technique[15,16,21]. Cylindrical cores of 7 mm diameter weretaken perpendicular to the articulating surface from onecondyle of each implant and machined to prepare500 lm-thick, 6.4mm-diameter disk-shaped specimens atnear-surface and subsurface locations. The contralateralcondyle was reserved for further characterization usingDSC and FTIR. Small punch test specimens from thenear-surface location started within 25 lm of the articu-lating surface (n"23), whereas specimens from the sub-surface location were obtained from a depth of1.5}2.0 mm (n"24) corresponding to the previously re-ported location of maximum subsurface degradation inair-irradiated and shelf aged components [22,23]. Thus,a total of 47 small punch tests of control and shelf agedUHMWPE were conducted in this investigation.

1452 A.A. Edidin et al. / Biomaterials 21 (2000) 1451}1460

Fig. 1. Schematic view of the small punch test apparatus.

Fig. 2. Features of the small punch test load displacement curve.

The miniature UHMWPE specimens were placed ina custom built punch and die "xture and tested inequibiaxial tension using the previously described smallpunch testing method at room temperature and ata punch displacement rate of 0.5 mm/min (Fig. 1). Theload}displacement curve from each small punch test wascharacterized by an initial sti!ness, initial peak load, anultimate load, and ultimate displacement (Fig. 2). Basedon previous analytical studies using the "nite elementmethod, the initial sti!ness of the small punch load}dis-placement curve was related to the elastic modulus of

UHMWPE [15]

E"13.5k, (1)

where the initial sti!ness (k) is in N/mm and the elasticmodulus (E) is in MPa. The work to failure, calculated asthe area under the load}displacement curve, provideda measure of toughness. Finally, the shape of theload}displacement curve, which in UHMWPE is sensitiveto resin, irradiation, crosslink density, and oxidativedegradation [13,15,16,21,24], was interpreted in order toprovide insight into the large-strain plastic deformationresponse of the UHMWPE. Analysis of variance wasperformed using SAS Version 7 (SAS Institute, Cary, NC)to evaluate the following factors: location (near-surfaceand subsurface), age (unaged, arti"cially aged, aged5 years, aged 10 years), and age-location interaction. A P-value of 0.05 was taken to indicate statistical signi"cance.

Representative small punch specimens were examinedafter testing and characterized using a variable pressurescanning electron microscope operating at 2.4 kV (LEO435 VP: Leo Scanning Electron Microscopes, Ltd., Cam-bridge, England). Images at 2.4 kV were obtained underhigh vacuum conditions using secondary electrons. Itwas not necessary to sputter coat the specimens witha conductive layer of material (e.g., gold) when imagingunder these conditions.

Surface di!erential scanning calorimetry (DSC) andthrough-thickness Fourier transform infrared (FTIR)spectroscopy was also conducted on two 5-year and two10-year shelf aged GUR 1120 tibial components to char-acterize the degree of crystallinity and oxidation of theimplants to complement mechanical testing results.The crystallinity measurements were performed using

A.A. Edidin et al. / Biomaterials 21 (2000) 1451}1460 1453

Fig. 3. Near-surface vs. subsurface load}displacement curves for (a) control UHMWPE tibial inserts; and (b) control UHMWPE tibial inserts afteraccelerated aging.

a Perkin-Elmer DSC-6 di!erential scanning calorimeter.Three 5}10 mg samples were taken from the surface of thecomponents (six measurements total for each age group)and heated from 25 to 2003C at a rate of 103C/min innitrogen. After reaching 2003C, the samples were cooled at103C/min and then reheated to 2003C. The degree ofcrystallinity was calculated by dividing the change in en-thalpy for the sample by 289.5 J/g, which is the theoreticalheat of fusion for perfectly crystalline polyethylene [25].

Oxidation of the 5- and 10-year shelf aged tibial insertswas quanti"ed from the FTIR spectra obtained throughthe thickness of the components. A thin slice ofUHMWPE was microtomed normal to the articulatingsurface and placed on the motorized stage of a Nicolet

FTIR equipped with a microscope attachment. By ad-justing the motorized stage, FTIR spectra were collectedin 250 lm increments from the articulating surface. Foreach spectrum, the oxidation index was calculated bynormalizing the area under the carbonyl absorbancepeak by the reference peak area at 1468 cm [26].

3. Results

The small punch load}displacement (mechanical) be-havior for the UHMWPE from the control group dis-played an initial peak load (during initial bending of thedisk-shaped specimen), followed by a drawing phase un-der equibiaxial tension (Fig. 3a). Both the surface and

1454 A.A. Edidin et al. / Biomaterials 21 (2000) 1451}1460

Fig. 4. Near-surface vs. subsurface load}displacement curves for UHMWPE tibial inserts after 5 years of natural aging.

Fig. 5. Near-surface vs. subsurface load}displacement curves for UHMWPE tibial inserts after 10 years of natural aging.

subsurface locations of the control implants displayedvery similar behavior (Fig. 3a). Compared with the con-trol material response, accelerated aging resulted ina change in the material small punch load}displacementbehavior at the surface, characterized by an increase ininitial sti!ness (corresponding to an increase in elasticmodulus), a reduction in initial peak load, and an indica-tion of relatively brittle behavior by the elimination of theductile drawing phase of the load}displacement curveafter the initial peak load (Fig. 3b). The subsurface defor-mation behavior of the UHMWPE, on the other hand,

was not appreciably a!ected by accelerated aging whencompared with the control material response (Fig. 3b).

In contrast with the accelerated aging e!ect that wasrestricted to the surface, degradation of mechanical be-havior for the naturally aged materials (including a re-duction in the ductile drawing phase) was evident at bothsurface and subsurface locations (Figs. 4 and 5). Whereasthe "ve year components still showed persistent evidenceof ductile drawing during the small punch test (Fig. 4), thecomponents with 10 years of natural aging showed noevidence of a signi"cant ductile drawing phase (Fig. 5). In

A.A. Edidin et al. / Biomaterials 21 (2000) 1451}1460 1455

Fig. 6. E!ect of accelerated and natural aging on surface vs. subsurface (a) elastic modulus; (b) initial peak load; (c) ultimate load; (d) ultimatedisplacement; (e) work to failure.

addition to the loss of ductility, the naturally agedUHMWPE also showed a substantial increase in initialsti!ness and decrease in the ultimate load when com-pared with the controls (Figs. 4 and 5). At a depth of1.5}2.0 mm, the average elastic modulus after 10 years ofnatural aging was 152% higher than the unaged controls(Fig. 6a); the average initial peak load 47% lower thanthe unaged controls (Fig. 6b); the ultimate load was 85%lower (Fig. 6c); the ultimate displacement was 53% lower(Fig. 6d); and the work to failure was 78% lower (Fig. 6e).

Aging signi"cantly a!ected the small punch testmeasures of elastic modulus, initial load, ultimate load,ultimate displacement, and work to failure for the im-plants (Table 1, Fig. 6a}e, P(0.007). Furthermore, a sig-

ni"cant age-location interaction was observed for theelastic modulus (P"0.0004, Fig. 6a), initial load(P(0.0001, Fig. 6b), ultimate load (P(0.0001, Fig. 6c),ultimate displacement (P(0.0002, Fig. 6d), and work tofailure (P(0.0001, Fig. 6e). Location signi"cantly a!ec-ted the elastic modulus (P(0.03), initial load (P(0.03),and work to failure (P"0.009) of the UHMWPE afteraccelerated aging, but not after 5 and 10 years of naturalaging (P'0.09, Fig. 6a). Location was not signi"cant foreither group (P'0.053) for ultimate displacement, andwas signi"cant for both groups for ultimate load(P(0.02).

Examination of the deformed control specimens afterthe small punch test showed evidence of homogeneous,

1456 A.A. Edidin et al. / Biomaterials 21 (2000) 1451}1460

Table 1Summary of small punch testing results (mean$SD)

Shelf age whentested (years)

Specimenlocation

Number ofspecimenstested

Elasticmodulus(MPa)

Initial peakload (N)

Ultimateload (N)

Ultimatedispl. (mm)

Work tofailure (mJ)

0 (control) Surface 3 485$85 64.6$2.9 55.1$2.1 4.11$0.04 191$80 (control) Subsurface 5 589$121 64.9$3.1 60.9$3.4 4.07$0.08 196$13

AA! Surface 5 1281$363 62.5$8.5 17.7$12.1 2.20$1.57 78$71AA! Subsurface 4 672$188 70.3$1.0 65.4$1.8 4.06$0.04 207$5

5 Surface 8 1100$129 72.6$1.5 56.7$6.2 4.13$0.20 214$175 Subsurface 8 1056$134 69.7$1.3 39.0$3.7 4.11$0.15 190$5

10 Surface 7 1326$313 58.1$7.7 22.5$16.9 2.64$0.32 99$2910 Subsurface 7 1487$150 34.7$1.7 8.9$0.0 1.91$0.12 43$4

!AA: control components, subjected to 4 weeks of accelerated aging at 803C.

large-scale plastic deformation (Fig. 7a), characteristicof undegraded UHMWPE [15,16]. The control speci-mens displayed a single ductile tear or rupture. Afteraccelerated aging, the surface (but not the subsurface)specimens displayed multiple &brittle' radial cracks with-out evidence of large-scale plastic deformation (Fig. 7b),consistent with the apparently reduced (from control)material ductility and toughness manifested as a decreasein small punch ultimate load (Fig. 6c), ultimatedisplacement (Fig. 6d), and work to failure (Fig. 6e). After10 years of natural aging, the surface and subsurfacespecimens also displayed characteristics of &brittle' frac-ture, with evidence of both radial and circumferentialcracking (Fig. 7c).

The average degree of crystallinity at the surface of thenaturally aged UHMWPE components after 5 and 10years was 56$1 and 65$4%, respectively, during the"rst heat (Fig. 8a). The relative increase in crystallinity asa function of shelf age was con"rmed by the reheatingcycle (Fig. 8a). The peak melting temperature of theUHMWPE was insensitive to the shelf age (Fig. 8b).

The oxidation pro"les of the naturally aged compo-nents showed a subsurface maximum around 1.5 mmfrom the articulating surface (Fig. 9). The gradient of thesubsurface oxidation peak was very steep, but the oxida-tion decreased to undetectable levels in the center of thecomponents. Nearly an order of magnitude increase inoxidation was measured at the surface and subsurfacelocations of the naturally aged components. Forexample, in the 10-year shelf aged insert, the averageoxidation index was 0.032$0.022 across a depth rangeof 0 and 0.5 mm (corresponding to the location of thenear-surface small punch specimens), and was 0.232$0.022 across a depth range of 1.5}2.0 mm (correspondingto the location of the subsurface small punch specimens).

4. Discussion

Post-irradiation aging increases the elastic modulusand decreases the ductility, ultimate strength and tough-ness of UHMWPE after gamma irradiation in air, consis-tent with a progressive embrittlement process. After10 years of natural aging, the elastic modulus did notvary signi"cantly as a function of depth, but the smallpunch test metrics of initial peak load, ultimate load,ultimate displacement, and work to failure displayedprominent depth dependence. The accelerated aging em-ployed in this study mechanically degraded the surface(but not the subsurface) regions of tibial components.Based on a comparison with results of the mechanicaltesting of naturally aged components, the surface embrit-tlement produced by the accelerated aging was compara-ble to 10 years of natural aging. Although the acceleratedaging e!ectively embrittled the surface, the depth of pen-etration was not consistent with the naturally aged com-ponents. Consequently, the accelerated aging protocolemployed in this study may not be appropriate for repro-ducing damage mechanisms that extend below the articu-lating surface, such as the pitting or delaminationobserved in tibial components. The applicability of suchan arti"cial technique to reproducing damage mecha-nisms observed in hips is more controversial. On the onehand, it may be argued that means of rapidly degradingthe surface of a hip insert provides an excellent way toperform a worst case study in vitro. On the other hand,since the subsurface degradation features we observed inknee inserts in this study have also been observed in hipinserts by other authors, it is not clear that an arti"cialaging method that exclusively ages the surface of theinsert is appropriate. This is even more so in view of thefact that total hip arthroplasties do not exhibit increasing

A.A. Edidin et al. / Biomaterials 21 (2000) 1451}1460 1457

Fig. 7. Scanning electron micrographs of (a) near-surface control smallpunch test specimen; (b) near-surface control small punch test specimenafter accelerated aging; (c) subsurface small punch test specimen from10-year naturally aged component.

wear rates with time, nor is the wear rate well correlatedwith the shelf aging time. For example, a review of 126consecutive modular acetabular components whose acet-

abular inserts were sterilized in air performed at a meanfollow-up of 9.5 years showed that the wear rate de-creased from about 0.25 mm/year at three years to0.15 mm/year at seven years [37]. Such an observationsuggests that accelerated aging of hip inserts may notre#ect either the chemical or clinical pathway actuallytaken by such inserts.

It is now well established that gamma-irradiation inair, followed by aging under ambient conditions, resultsin oxidative degradation of UHMWPE, as summarizedin recent review articles [1,2]. However, many previousstudies of degradation in UHMWPE have focused onchanges to the chemical or physical properties of thepolymer, or on inferring changes in mechanical behaviorbased on measurements of density or crystallinity[22,23,27}36]. The data collected in this study illustratethe di$culty in predicting changes in mechanical behav-ior based on indirect measurement techniques. Forexample, according to the oxidation index measurements(Fig. 9), the subsurface region of the tibial inserts after5 years was more heavily oxidized than the near surfaceregions after 10 years of aging. However, our small punchmeasurements contradict that ranking (Figs. 4 and 5),suggesting that the surface regions of the 10 year agedimplants were substantially less ductile than the subsur-face regions of the 5-year implants. The interaction be-tween aging and location observed in this study providesa potential impediment to relating long-term, depth-dependent changes in physical and chemical propertiesto mechanical behavior.

Direct measurements from retrieved components arenecessary to relate mechanical degradation to in vivoperformance. By conducting miniature uniaxial tensiletests on retrieved acetabular and tibial inserts, re-searchers have previously shown a reduction of ultimateproperties (i.e., ultimate tensile strength and elongationto failure) for components that were gamma irradiated inair [17}20]. In this study, we elucidated the changes inmechanical behavior, including both the initial linearelastic behavior, as well as the nonlinear plastic #owresponse, as a function of natural and accelerated aging.Mechanical degradation of UHMWPE has importantclinical implications, especially for highly stressed tibialcomponents, which may be subjected to locally largedeformations during in vivo loading and may exhibitfatigue damage, such as pitting or delamination. In a pre-vious study of retrieved UHMWPE tibial inserts, astatistical relationship was observed between the totaldamage score and the small punch test metrics of initialpeak load, ultimate load, and work to failure [24]. In thecurrent study, natural aging of up to 10 years was asso-ciated with signi"cant reductions in initial peak load,ultimate load, and work to failure. We thus hypothesizethat the heterogeneous, post-irradiation embrittlement ofUHMWPE may be an important factor in the surfacedamage mechanisms observed in retrieved tibial inserts.

1458 A.A. Edidin et al. / Biomaterials 21 (2000) 1451}1460

Fig. 8. (a) Degree of crystallinity and (b) peak melting temperaturedetermined at the articulating surface after 5 and 10 years of naturalaging using di!erential scanning calorimetry.

Fig. 9. Oxidation index vs. depth after 5 and 10 years of natural aging.The oxidation index was calculated by dividing the area of the carbonylpeak by the area of the reference peak at 1468 cm~1.

All of the tibial components in this study were fab-ricated from compression molded GUR 1120. The aver-age molecular weight of this resin is reported to bearound 3.5 million g/mol based on intrinsic viscosity [1].Calcium stearate, a processing agent, was also added tothe resin [1]. Because orthopaedic components may befabricated from resins with di!erent molecular weights(e.g., GUR 1120 vs. 1150), calcium stearate concentra-tions (e.g., GUR 1120 vs. 1020), consolidation techniques(e.g., ram extrusion vs. compression molding), and steril-ization methods (e.g., gamma irradiation in a low oxygenpackage, gas plasma, or ethylene oxide) one cannot atthis time extrapolate the "ndings from this study to othertypes of UHMWPE. However, preliminary work per-formed by the authors on shelf aged GUR 1020 andGUR 1150 hip and knee components suggests that theextent of mechanical degradation following sterilizationin air is comparable in both magnitude and location tothat observed in the GUR 1120 tibial inserts. The resultsof this research suggest the hypothesis that severe degra-dation after gamma irradiation in air is accompanied bya change toward less ductile (more brittle) mechanical

behavior in UHMWPE. However, further research ofnaturally aged components fabricated using di!erentprocessing and sterilization techniques will be necessaryto fully de"ne the conditions necessary for the develop-ment of brittle mechanical behavior in UHMWPE.

The small punch test method, which has been exten-sively validated for wear-tested components and short-term retrieved implants [13}16,24], was found to bea reproducible method for directly measuring the mech-anical behavior of the severely degraded UHMWPEeven after 10 years of shelf aging. Because the small testtechnique deforms UHMWPE under multiaxial loadingconditions, it potentially provides a more clinically rel-evant state of deformation than uniaxial deformationstypically achieved by standard bulk testing techniques.This study establishes miniature specimen mechanicaltesting techniques as an invaluable and particularly sen-sitive tool for validation of proposed accelerated agingmethods for UHMWPE.

Acknowledgements

This work is supported by research grants fromStryker Howmedica Osteonics. Special thanks to JudeFoulds, Exponent Inc., for many helpful discussions.

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