The Journal of Arthroplasty Vol. 23 No. 4 2008

A Highly Crosslinked UHMWPE for CR and PSTotal Knee Arthroplasties

Aiguo Wang, PhD,* Shi-Shen Yau, PhD,* Aaron Essner, MS,* Lizeth Herrera, BS,*Michael Manley, PhD,y and John Dumbleton, PhDz

Abstract: X3 is a highly crosslinked ultrahigh molecular weight polyethylene(UHMWPE) produced by a sequential irradiation and annealing process. Thesequential process results in a material with a free radical content that is 1% that ofconventional UHMWPE gamma sterilized in nitrogen resulting in an oxidationresistance similar to that of virgin UHMWPE. Yield strength and ultimate tensilestrength exceed American Society for Testing and Materials minimum rates forUHMWPE. Simulator testing of contemporary cruciate retaining (CR) and posterior-stabilized knee inserts (Triathlon) manufactured by the sequential process demon-strated 68% and 64% less wear, respectively, compared to conventionally processedinserts. The wear and mechanical integrity of sequentially processed posterior-stabilized inserts was unaffected by accelerated aging, whereas conventionalUHMWPE exhibited increased wear, cracking, and delamination. Key words: highlycrosslinked, polyethylene, wear reduction, knee, CR, PS.© 2008 Elsevier Inc. All rights reserved.

Highly crosslinked ultrahigh molecular weight poly-ethylene (XLPE) materials were introduced as totalhip bearing surfaces starting in 1998 [1] based ondemonstrated reduced wear in hip simulator studies[2-5]. These materials have been widely accepted,and reduced wear has been demonstrated comparedto ultrahigh molecular weight polyethylene(UHMWPE) without high crosslinking levels [6-9].Highly crosslinked polyethylene materials differ

in radiation dose and methodology of irradiation, inthe thermal treatment used to form crosslinks andextinguish free radicals, and in sterilization pro-cesses. One particular difference is between materi-

From the *Stryker Orthopaedics, Mahwah, New Jersey; yHomerStryker MD Center, Mahwah, New Jersey; and zConsultancy in MedicalDevices, Ridgewood, New Jersey.

Submitted August 30, 2006; accepted May 5, 2007.This study received support from Stryker Orthopaedics.The authors are employees or consultants of Stryker.Reprint requests: Michael Manley, PhD, Homer Stryker MD

Center, 325A Corporate Dr, Mahwah, NJ 07430.© 2008 Elsevier Inc. All rights reserved.0883-5403/08/2304-0013$34.00/0doi:10.1016/j.arth.2007.05.007

559

als that are heated above the melting temperature(remelted XLPE materials) and those heated to justbelow the melting temperature (annealed XLPEmaterials). For radiation doses at or near 100 kGy,the dose commonly chosen, remelted XLPE materi-als appear to have reduced mechanical and fatigueproperties compared to conventional UHMWPEand to annealed XLPE materials [10-14]. Concernsover this potential shortcoming have limited theuse of remelted XLPE for knee devices [1]. How-ever, annealed XLPE materials contain a higherlevel of free radicals [10,15] than their remeltedcounterparts, and may have more oxidation poten-tial with possible reduced mechanical and fatigueproperties over time [16-18]. This has also limitedthe use of annealed XLPE materials for knee jointarthroplasty devices.

The level of free radicals is also influenced bythe type of terminal sterilization. In particular,ionizing sterilization greatly increases the levelof free radicals. The free radical concentration is 4times higher for Crossfire XLPE (Stryker Orthopae-dics, Mahwah, NJ) than for UHMWPE gamma

560 The Journal of Arthroplasty Vol. 23 No. 4 June 2008

sterilized in nitrogen, although both receive the samesterilization radiation dose [15]. This appears to bedue to the constraints of crosslinks in the XLPE thatreducemolecularmobility and hinder the eliminationof free radicals via crosslinking or other mechanisms.This findingwas the basis for the sequential process ofcrosslinking described later. The hypothesis was thatannealing after a relatively low radiation dose wouldspace the crosslinks so that free radicals could beeliminated by annealing. The process could berepeated to achieve a high level of crosslinkingwithout a high concentration of free radicals.Researchers are now investigating methods of

eliminating the remelting step while maintainingresistance to oxidation. One approach is the use ofplastic deformation at temperatures below remeltingafter crosslinking. This process is said to extinguishmost free radicals [19,20]. Another approach is touse vitamin E in conjunction with radiation andannealing [21]. In the present study, we describe asequential irradiation and annealing process thatresults in an XLPEmaterial with oxidation resistancesimilar to that of virgin UHMWPE and withretention of mechanical strength. In knee simulatortesting, the sequentially irradiated and annealedXLPE reduces wear for both cruciate retaining (CR)and posterior-stabilized (PS) knees compared toconventional UHMWPE.

Materials and Methods

Materials

All studies were carried out using compressionmoldedGUR1020UHMWPE blocks certified tomeetAmerican Society for Testing and Materials (ASTM)standard F648-00e1. Crosslinkingwas achievedwiththe sequential irradiation and annealing process.This comprised 3 cycles, each cycle consisting ofgamma irradiation in air to 30 kGy followed byannealing in air at 130°C for 8 hours. The radiationdose was a total of 90 kGy. After completion of thesequential process, the oxidized outer surface of theblockswas removed (1-3mm). Components or otherspecimens were machined from blocks of the cross-linked material. Sterilization was by gas plasma. Theresulting XLPE is branded as X3 (Stryker Orthopae-dics, Mahwah, NJ). Controls were either GUR 1020irradiated to 30 kGy in nitrogen (conventionalUHMWPE) or virgin UHMWPE, as appropriate.

Methods

The free radical concentration of the treatedmaterial and controls was measured using electron

spin resonance spectroscopy with a Bruker Instru-ments, Model EMX spectrometer (Dr S Jahan,University of Memphis, Memphis, Tenn). The instru-ment was calibrated with a ruby standard containing6 × 1016 spins/g for determination of absoluteconcentrations (National Bureau of Standards, Elec-tron Paramagnetic Resonance Intensity Standard:SRM 2601). The specimens were 3 mm in diameterand 10 mm in length and were machined from thecenter of UHMWPE blocks and vacuum packaged inair impermeable aluminum foil tominimize the effectof oxidation during transit. Six replicates were usedper condition.

To evaluate the resistance to oxidation, 90× 20× 10mm specimens were subjected to accelerated agingfollowing ASTM F2003-02. The aging protocol was asfollows: 14 days at 70°C in 5 atm of oxygen. The area(intensity) of carbonyl absorptions centered near1720 cm−1 was measured on 150-μm-thick sectionsprepared by microtoming perpendicular to the longaxis of the sample from the front to the back surface.ANicolet 750 Magna-Infrared (IR) spectrometer(Thermo Electron Corporation, Waltham, Mass)with microscope attachment was used to obtainFourier transform infrared spectra based on theaverage of 128 scans for each thickness level. Theoxidation index was determined according to ASTMF2102-01e1 as the ratio of the peak areas between1650 and 1850 cm−1 and the area between 1330 and1396 cm−1. Five specimens were aged and measuredfor each condition.

Tensile tests were conducted according to ASTMD638-03, using type IV specimens on an Instron 4505test machine at a crosshead speed of 5.08 cm/min.Eight specimens per condition were tested to obtainyield strength, ultimate tensile strength, and elonga-tion. The tensile modulus was determined accordingto ASTM D638-03 on an Instron Model 1137 testingmachine at a crosshead speed of 0.5 cm/min usingtype I tensile specimens. Ten specimens were testedper condition. Crystallinity was determined by using aPerkin-Elmer (Perkin Elmer, Inc., Wellesley, Mass)7-Series Thermal Analysis System according to ASTMD3417-99. Heating and cooling runs were made at10°C/min. Crystallinity was calculated as the heat offusion divided by 287.3 J/g (the heat of fusion forperfectly crystalline polyethylene). Ten specimenswere measured per condition. Tensile and crystallinitydeterminations were also carried out after oxidativechallenge according to ASTM F2003-02.

The wear testing of Triathlon CR tibial inserts (size4, 9 mm nominal thickness) articulating againstcobalt-chromium alloy Triathlon femoral compo-nents (Minnesota Testing Systems, Eden Prairie,Minn) was conducted on an MTS motion-controlled

Table 1. Crystallinity Values for Virgin UHMWPE,Conventional UHMWPE, and X3 Before and After Aging

Aging Crystallinity (%) P

Virgin UHMWPE No 59.2 ± 0.7Yes 59.3 ± 0.3 .683

Conventional UHMWPE No 61.3 ± 0.8yes 70.8 ± 0.8 .001

X3 No 61.7 ± 0.6Yes 61.0 ± 0.5 .060

Virgin UHMWPE vsconventional UHMWPE

No – .001

Virgin UHMWPE vs X3 No – .001Conventional UHMWPE vs X3 No – .222

Table 2. Comparison of the Tensile Properties ofConventional UHMWPE, X3, and Both Sets Before

and After Aging

ensile Properties Before Aging After Aging P

onventional UHMWPEensile modulus (MPa) 862 ± 16 – –ield strength (MPa) 23.2 ± 0.4 27.9 ± 0.4 .001ltimate tensile strength MPa) 54.8 ± 2.5 29.9 ± 1.2 .001longation (%) 363 ± 10 143 ± 14 .001

3ensile modulus (MPa) 876 ± 8 – –ield strength (MPa) 23.5 ± 0.3 23.6 ± 0.2 .446ltimate tensile strength MPa) 56.7 ± 2.1 56.3 ± 2.3 .722longation (%) 267 ± 7 266 ± 9 .808

onventional UHMWPE and X3ensile modulus (MPa) 862 ± 16 876 ± 8 .084ield strength (MPa) 23.2 ± 0.4 23.5 ± 0.3 .112ltimate tensile strength MPa) 54.8 ± 2.5 56.7 ± 2.1 .122longation (%) 363 ± 10 267 ± 7 .001

P values indicate the level of statistical significance ofthe differences.

A Highly Crosslinked UHMWPE for CR and PS Total Knee Arthroplasties � Wang et al 561

knee joint simulator using 3 inserts processedaccording to the X3 method and 3 conventionalUHMWPE inserts. All inserts were mounted oncobalt-chromium alloy tibial trays. The lockingmechanism was modified to allow disassembly forperiodic wear measurement. Level walking wassimulated with motion and loading waveformsfollowing ISO 14243-3 [22]. The lubricant wasAlpha Calf Fraction serum (Hyclone Laboratories,Logan, Utah) diluted to 50% using a pH-balanced20 mmol solution of deionized water and EDTA toobtain a physiologic protein level (≈20 g/L) [23].EDTA was added to retard serum decomposition andthe solution was passed through a 0.2-μm pore filterbefore use. The lubricant was replaced every 0.5 mil-lion cycles and the inserts were weighed at the sametime after ASTM F2025 with correction for fluidabsorption via soak control inserts that wereunloaded. Testing was conducted to 5 million cycles.Visual and scanning electron microscope (SEM)observations were made on the tibial inserts aftertesting. Wear debris was collected each time thelubricant was changed and pooled for final examina-tion at the end of the test. The isolation methodologywas according to Scott et al [24]. Briefly, debris wascollected on 0.2-μm polycarbonate filters and dried,sputter-coated with gold, and examined under SEM.Multiple random fields were analyzed for multiplesamples measuring the length and width of theparticles and calculating the equivalent circle dia-meter (ECD).The wear testing of Triathlon PS tibial inserts (size

4) was conducted by using a machine similar to thatused for the CR knee wear evaluations. However,for the PS component evaluation, stair climbingkinematics were applied to ensure stress transfer tothe polyethylene post. Additional simulator fixtur-ing allowing an anterior-posterior–oriented springforce limited translation was incorporated. Thismechanism was included to simulate soft tissuestructures around the knee. This model has beendescribed elsewhere [25]. An analysis of the

motions and implant geometry indicated that themaximum force on the PS post was approximately900 N and occurred on the back of the post in ananterior direction at flexion of 56° (about one thirddown from the top of the post) with further postcontact occurring at 95° of flexion (near the base ofthe post). Components were tested to 5 millioncycles. As before, visual and SEM observations weremade. Weight loss of the tibial inserts was measuredat 0.5-million cycle increments. In a second study onsize 4 Triathlon PS knees, 9-mm-thick tibial insertsof X3 and conventional UHMWPE were agedaccording to ASTM F2003-02 before testing understair climbing conditions.

Results

The free radical concentration for conventionalUHMWPE was (1550 ± 32) × 1014 spins/g and thatof X3 was (14 ± 2) × 1014 spins/g. Virgin UHMWPEdid not have detectable free radicals (b5 × 1013

spins/g). X3 has only 1% of the number of freeradicals compared to conventional barrier-packagedUHMWPE. After accelerated aging, conventionalUHMWPE exhibited an oxidation index peak of1.42 ± 0.03, about 1 mm below the surface with anassociated white band on sectioning. The oxidationindex for both virgin UHMWPE and X3 had a diffusemaximum near the middle of the thickness of thespecimen with a value of 0.05 ± 0.01, and wassignificantly lower than for conventional UHMWPE(P = .001). The crystallinity values for X3, con-ventional UHMWPE, and virgin UHMWPE before

T

CTYUE

XTYUE

CTYUE

Fig. 1. Volumetric wear of X3 and conventional UHMWPE as a function of number of cycles for Triathlon CR size 4, 9-mm-thick components.

able 3. Wear Particle Dimensions (Length, Width, andCD) for X3 and Conventional UHMWPE from Triathlon

CR Tibial Components

HMWPEaterial

Particle Length(μm)

Particle Width(μm) ECD (μm)

onventionalUHMWPE

0.326 ± 0.14 0.23 ± 0.09 0.31 ± 0.12

3 0.312 ± 0.13 0.22 ± 0.08 0.29 ± 0.11

562 The Journal of Arthroplasty Vol. 23 No. 4 June 2008

and after aging are given in Table 1. The crystal-linity values of X3 and conventional UHMWPEwere not significantly different from each other,but both materials had higher crystallinity thanvirgin UHMWPE. The crystallinity rates of X3 andvirgin UHMWPE were not significantly increasedby aging.Tensile data are given in Table 2 for X3 and

conventional UHMWPE. There was no significantdifference in tensile modulus, yield strength, orultimate tensile strength between X3 and conven-tional UHMWPE. The elongation of X3 was 26%lower than that of conventional UHMWPE. Thisreduction was statistically significant (P = .001).Tensile properties were unchanged for X3 afteroxidative challenge. However, the yield strengthincreased, and the tensile strength and elongationdecreased for conventional UHMWPE.Fig. 1 shows the volumetric wear as a function of

the number of cycles for the Triathlon CR kneesimulator “walking” tests. The total volume loss at5 million cycles was 98.6 ± 6.1 mm3 for conven-tional UHMWPE and 26.8 ± 1.3 mm3 for X3representing a 68% decrease (P = .0002). The wearrates were 17.7 ± 2.2 and 5.7 ± 1.5 mm3/millioncycles for conventional UHMWPE and X3, respec-tively, representing a 68% decrease compared toconventional UHMWPE (P = .002).

The contact regions of the articular surfacesshowed polishing or burnishing, deformation,striations, and scratches typical of mild wear.These areas were slightly larger for control inserts,reflecting the higher wear values. No surface orsubsurface cracks, pits, or delamination wereobserved for either X3 or conventional inserts.The back surfaces of the inserts showed stipplingreflecting the surface pattern/finish of the tray butmachine marks were still visible.

The average length, width, and ECD of the wearparticles are given in Table 3. A total of 544conventional UHMWPE particles and 502 X3 parti-cles were identified. There are no statistical differ-ences in length, width, or ECD for the 2 materials(P = .10, P = .11, P = .7, respectively).

The volumetric wear vs number of cycles for theTriathlon PS “stair climbing” knee study is shown in

TE

UM

C

X

Fig. 2. Volumetric wear of X3 and conventional UHMWPEas a function of number of cycles for Triathlon PS size 4,9-mm-thick components.

A Highly Crosslinked UHMWPE for CR and PS Total Knee Arthroplasties � Wang et al 563

Fig. 2. The total volume loss at 5 million cycles was20.0 ± 3.0 mm3 for a wear rate by linear regressionof 4.1 ± 0.7 mm3/million cycles for conventionalUHMWPE. The corresponding values for X3 were6.4 ± 2.6 mm3, a decrease of 68% compared toconventional UHMWPE (P = .0042), and 1.5 ±0.5 mm3/million cycles, a decrease of 64% com-pared to conventional UHMWPE (P = .008). Con-dylar surfaces of both conventional UHMWPE andX3 inserts showed wear scars that included burnish-ing or polishing, deformation, striations, and somescratches. Scanning electron micrographs of con-dylar and post surfaces for both conventionalUHMWPE and X3 inserts generally displayed sur-face smearing and rippling, typical of adhesive wear[26,27]. Wear occurred on the posts of bothmaterials as a result of posterior femur contactguiding the articulation. The wear mechanisms weresimilar to those seen on articular surfaces and wereindicative of mild wear. No surface or subsurfacecracks, pits, or delamination were observed onarticular surfaces or posts for either material.Knee simulator testing after accelerated aging

showed a nonstatistically significant reduction inwear rate for X3 (1.5 ± 0.5 mm3/million cyclesbefore aging and 1.0 ± 0.2 mm3/million cycles afteraging; P = .2). There was no surface appearancedifference between aged and unaged X3 compo-nents after testing. Condylar surfaces and posts forthe X3 components showed burnishing, deforma-tion, striations, and scratching; no cracks or delami-nation were observed (Fig. 3A and C). Agedconventional UHMWPE components exhibitedcracking and severe delamination (Fig. 3B and D).The wear rate increased from 4.1 ± 0.7 to 25.1 ±7.8 mm3/million cycles after aging for conventionalUHMWPE components.

Discussion

The incidence of failure after total knee arthro-plasty is low [28]. The revision rate in the UnitedStates has remained stable at 8.2% between 1990and 2002 [29]. However, the number of total kneearthroplasty procedures tripled in that period andthere were 35,000 revision procedures in 2002. Therevision rates in other countries are comparable tothose in the United States [29]. A retrospectivereview of 212 revision knee surgeries over a 3-yearperiod indicated 25% of revisions were due topolyethylene wear [30]. A classification of failuredue to wear was made if the components exhibited“gross wear or damage,” hence including plasticdeformation and fatigue wear as well as surfacewear. Wear can occur at the tibial or the patellarcomponents. For the tibial component, wear canoriginate at the articular [31] or back surfaces [32]and at the post in a PS knee [33]. Concern overosteolysis due to wear has prompted the introduc-tion of highly crosslinked UHMWPE for CR knees[34], and at present 2 remelted highly crosslinkedmaterials, differing mainly in radiation dose, areused [1]. Clinical results for highly crosslinkedpolyethylene tibial components have not beenreported but limited information is available fromknee simulator studies [35,36] and on retrievedtibial components [37].

The absence of detectable free radicals has beenhighlighted as the primary reason for the choice ofremelting for crosslinked UHMWPE processing [35].However, the remelting process causes changes inthe morphology and lowers the mechanical andfatigue properties [10,11]. Crack propagation resis-tance is lower for remelted than for annealed XLPEmaterials [38]. Surface cracking on retrieved acet-abular components from a remelted highly cross-linked polyethylene has been reported [39].Mechanical and fatigue behavior are even moreimportant at the knee than at the hip because of thehigh contact stresses associated with activities ofdaily living [40]. This may lead to some questionsabout the appropriateness of remelted XLPE forthis application.

The result of the sequential irradiation andannealing process is a low concentration of freeradicals. The molecular weight of GUR 1020 is4 million d and the free radical concentration of 14 ×1014 spins/g in X3 represents 1 free radical for every100 molecules. The low radical count likely con-tributes to the oxidation resistance of X3 processedmaterial. The oxidation resistance of X3 is demon-strated not only by chemical measurements, but alsoby the constancy of crystallinity and tensile

Fig. 3.Macroscopic and SEM views of accelerated aged X3 and conventional UHMWPE PS components after stair climbingtests. Macroscopic view of X3 post (A); macroscopic view of conventional UHMWPE post (B); SEM of X3 post at contactarea (C); SEM of conventional UHMWPE post at contact area (D).

564 The Journal of Arthroplasty Vol. 23 No. 4 June 2008

properties after oxidation challenge. Acceleratedaging also did not increase the condylar wear ofthe Triathlon PS knee in the simulator studies.The yield and tensile strength of X3 exceed the

ASTM F648-00e1 properties specified for UHMWPEby 12% and 62%, respectively, and are superior tothose reported for remelted XLPE materials [10-12].This appears to be a reflection of the highercrystallinity of X3, which is similar to that ofconventional polyethylene.For the Triathlon CR knee, the wear rate of X3

was 68% lower than that of conventionalUHMWPE. This is in agreement with the predic-tions based on the effect of radiation dose on wearrate [31-43] and is also consistent with findings onthe same sequentially crosslinked and annealedmaterial in a different knee design [25,44]. Thosestudies found 77% to 79% reductions for PS andCR devices over conventional UHMWPE, respec-tively. The wear mechanisms appeared similar forthe 2 materials; the particle size distributions werealso similar for X3 and conventional UHMWPE. Noinformation on articulating vs backside wear was

determined in this study. However, visual inspec-tion of the backside of the unlocked insertssuggests that little wear occurred at that interface.For the Triathlon PS knee, the wear rate of X3 was64% lower than that of conventional polyethy-lene. This may be considered an aggressive testbecause the entire 5 million cycles represented stairclimbing with post contact twice during flexion.Other researchers have used a ratio of 70:1 forlevel walking to stair climbing in knee simulatorevaluation [45]. A person with a knee jointarthroplasty who completed 2 million steps (cycles)in 1 year would only engage in some 28,000 stairclimbing steps.

The wear rate for the PS knee in stair climbing wasonly one third that of the CR knee in level walking.Measurement of the articular surface contact areasshowed that the A-P translation with the PS kneewas only half that with the CR knee, showing theeffect of the post and the guided motion it provides.It has been shown that A-P translation increasesknee wear [46]. The decrease in wear for the PSknee is also likely due to the change in kinematics. A

A Highly Crosslinked UHMWPE for CR and PS Total Knee Arthroplasties � Wang et al 565

modest addition of stair climbing to normal gaitkinematics has been shown to decrease wearcompared to normal gait alone [47].Similarwearmechanismswere observed for unaged

X3 and unaged conventional UHMWPE as evidencedby the similar morphological features on the worncondylar surfaces and posts. The surface features weretypical of mild adhesive wear [26,27]. The wear rateand wear mechanisms were unchanged for X3 afteraging. However, conventional UHMWPE exhibitedcracking and delamination due to oxidative degrada-tion and high contact stress.The excellent mechanical and wear properties of

X3 combined with oxidation resistance equivalentto that of virgin polyethylene, suggest that thismaterial is suitable for knee joint arthroplastycomponents. Knee simulator studies indicate thatX3 can be used not only for CR but also for PS tibialcomponents. Careful clinical evaluation of thismaterial will demonstrate whether this promiseis fulfilled.

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