1.Caracterizarea Particulelor de Uzură Generate Din Aliaj CoCrMo Sub Alunecare

8/12/2019 1.Caracterizarea Particulelor de Uzur Generate Din Aliaj CoCrMo Sub Alunecare

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Wear 271 (2011) 165816 66

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Wear

j o u rna l h om e page : www.e l sev i e r. com/ loca t e /wea r

Characterization of wear particles generated from CoCrMo alloy under slidingwear conditions

R. Pourzal a , , I. Catelas b , R. Theissmann c , C. Kaddick d , A. Fischer aa University Duisburg-Essen, Materials Science and Engineering, Lotharstr. 1, 47057 Duisburg, Germanyb University of Ottawa, Department of Mechanical Engineering, 161Louis Pasteur, Ottawa, Ontario, K1N 6N5, Canadac University Duisburg-Essen, Faculty of Engineering and CeNIDE, Lotharstr. 1, 47057 Duisburg, Germanyd ENDOLAB GmbH, Seb.-Tiefenthaler Str. 13, 83101 Thansau/Rosenheim, Germany

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Article history:Received 15 September 2010Received in revised form27 December 2010Accepted 28 December 2010Available online 2 April 2011

Keywords:Wear particlesHip jointCoCrMoWearTEM

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Biological effects of wear products (particles and metal ions) generated by metal-on-metal (MoM) hipreplacements made of CoCrMo alloy remain a major cause of concern. Periprosthetic osteolysis, poten-tial hypersensitivity response and pseudotumour formation are possible reactions that can lead to earlyrevisions. To accurately analyse the biological response to wear particles from MoM implants, the exactnature of these particles needs to be characterized. Most previous studies used energy-dispersive X-rayspectroscopy (EDS) analysis for characterization. The present study used energy ltered transmissionelectron microscopy (TEM) and electron diffraction pattern analysis to allow for a more precise deter-mination of the chemical composition and to gain knowledge of the crystalline structure of the wearparticles.

Particles were retrieved from two different test rigs: a reciprocating sliding wear tribometer (CoCrMocylinder vs. bar) and a hip simulator according to ISO 14242-1 (CoCrMo head vs. CoCrMo cup). All testswere conducted in new born calf serum (30 g/l protein content). Particles were retrieved from the testmedium using a previously published enzymatic digestion protocol.

Particles isolated from tribometer samples had asize of 100500 nm. Diffraction pattern analysis clearly

revealed

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lattice

structure

of

strain

induced

hcp

-martensite.

Hip

simulator

samples

revealed

numer-ous particles of 1530 nm and 3080 nm size. Most of the larger particles appeared to be only partiallyoxidized and exhibited cobalt locally. The smallest particles were Cr2 O3 with no trace of cobalt. It opticallyappeared that these Cr2 O3 particles were aking off the surface of larger particles that depicted a veryhigh intensity of oxygen, as well as chromium, and only background noise of cobalt. The particle size dif-ference between the two test rigs is likely related to the conditions of the two tribosystems, in particularthe difference in the sample geometry and in the type of sliding (reciprocating vs. multidirectional).

Results suggest that there may be a critical particle size at which chromium oxidation and cobalt ioniza-tion are accelerated. Since earlier studies have shown that wear particles are covered by organic residuewhich may act as a passive layer inhibiting further oxidation, it would suggest that this organic layermay be removed during the particle isolation process, resulting in a change of the particle chemical com-position due to their pyrophoric properties. However, prior to being isolated from the serum lubricant,particles remain within the contact area of head and cup as a third-body. It is therefore possible thatduring that time, particles may undergo signicant transformation and changes in chemical compositionin the contact area of the head and cup within the tribological interface due to mechanical interactionwith surface asperities.

2011 Elsevier B.V. All rights reserved.

1. Introduction

Total hip replacements (THR) made from CoCrMo alloy areused since more than 50 years [1,2] . Due to the great success of

Corresponding author. Tel.: +49 203379 1261; fax: +49203 379 4374.E-mail address: [email protected] (R. Pourzal).

metal-on-polyethylene (MoP)articulationsin the 1970s and 1980s,the number of metal-on-metal (MoM) implants declined strongly.However, since osteolysis triggered by polyethylene wear debriswas determined to be a major cause of implant loosening andfailure, MoM bearings gained renewed interest as an alternativebearing material because of their lower volumetric wear rate [3] .The overall good clinical performance of early MoM articulationsin the 1960s has proven CoCrMo to be a competent bearing mate-rial for orthopaedic applications with studies reporting little to

0043-1648/$ see front matter 2011 Elsevier B.V. All rights reserved.

doi: 10.1016/j.wear.2010.12.045
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no osteolysis around MoM implants [2,4,5] . During the running-in phase, the linear wear rate has been determined to be as lowas 2535 m/yr, decreasing to 15 m/yr during the steady-statephase [6] . The volumetric wear rate after running-in phase hasbeenreported to be 0.3 mm 3 /yr, which is approximately 60 times lowerthan in conventional MoP couples [6,7] . However, the total num-ber of particles generated within 1 year in MoM THRs lies between6.7 10 12 and2.5 10 14 , which is 13500-foldgreater than in MoPTHRs [8] .

Therehasbeenagreatconcernregardingthepotentialbiologicaleffects of metal nanoparticles and ions [9,10] . Although no appar-ent toxic effectwas determined, it was reported that large amountsof wear particlesgenerated in MoM THRmigrate to theliver, spleenand abdominal lymph nodes [11] . A recent in vitro study has shownthatCoCrMo nanoparticles may cause damageto DNAand chromo-somes [12] . The tested particles, however, were generated underdifferent conditions than in a THR and may differ in shape andcomposition from those generated by MoM implants in vivo . Otherstudies showeda potentialhypersensitivity response to metal wearproducts [13,14] . Finally, recent reports on so-calledpseudotumorshavegenerateda growing concern [15,16] . Thesepseudotumorsaredescribed as soft tissue masses or uid-lled bursa leading to earlyimplantfailures.Althoughtheir causesare stillnot wellunderstood,they could be due to excessive wear, metal hypersensitivity or acombination of the two [17] .

Many studies have been conducted to determine the exact size,morphology and chemical composition of MoM wear particlesgen-eratedin MoM hip implants [8,1821] . There is a general agreementthat the particle size ranges from 30 to 70nm. Some authors alsoreported larger particles linked to the occurrence of agglomera-tions [8] . The shape of wear particles appears to slightly vary fromstudy to study, including round, oval and needle. Regarding thechemical composition, most studies reported that the majority of particles were chromium-oxides, whereas a minority appeared tobe of CoCrMo alloy or possibly carbide origin [8,19] . Since thealloy consists mostly of cobalt ( 64%), this suggests that Co woulddissolve and leave the body through the urine, whereas mainlyoxidized chromium would remain within the organism. Anotherpossibilitymay be thatthe chromiumoxideparticles originatefromthe passive layer initially covering the implant surface. Finally, it isalsopossiblethat thechromium-oxideparticles stillexhibita cobaltcore that would not have been detected in previous studies usingonlyenergydispersive X-rayspectroscopy (EDS)analysis for chem-ical characterization. Indeed, EDS is limited for the quantitativeanalysis of light elements like oxygen or carbon. Hence, thepresentstudyused energyltered transmission electron microscopy(TEM)andelectrondiffraction pattern analysis to allow for a more precisedetermination of the chemical composition and to gain knowledgeof the crystalline structure of the wear particles.

Regarding the generation of particles from in vitro studies andthe biological response to these particles, it is essential to precisely

mimic the tribological conditions as they occur in the articial joint in vivo . Changes to the operating variables as well as in theenvironment may have signicant effects on the size and chemicalcomposition of the generated wear particles. It is therefore nec-essary to show that particles isolated from testing uid are reallyfrom the same nature than those occurring in vivo . The presentstudy aimed to characterize and compare the composition of wearparticles generated by two different tribosystems (a reciprocatingsliding wear tribometer and a hip simulator).

2. Materials and methods

2.1. Samples

Three groups of particle samples were available for analysis.

Fig.1. (a)Test setup of cylinder-on-bar tribometer; (b)schematic setup of cylindersand bar sample.

Fig. 2. Bar and cylinder samples made from hc CoCrMo.

Table 1Operating variables for cylinder-on-bar tribometer test procedure.

Normal force 25 NNominal surface pressure 135 MPaRelative v elocity ( frequency) 6 Hz, t riangle s ignalStroke 3 mmTemperature 37 CMotion Reciprocating slidingTest duration 0.12 million cycles (MC)Test medium Bovine serum test uid (30 g/l protein)

Particles in the rst group were generated in a reciprocatingsliding wear test rig with cylinder-on-bar conguration ( Fig. 1).

All samples were made from high carbon wrought CoCrMo alloy.A bar (80 mm 10mm 10mm) reciprocated on a vertical axiswhile two cylinders ( 10mm 15mm length) applied horizon-tally the normal force ( Fig. 2 ). A triangle signal (6Hz) was usedin order to minimize stick slip. The test was performed in newborn calf serum (30g/l protein content) at 37 C. Even thoughthe unidirectional reciprocating sliding motion does not repre-sent the circumstances in a hip joint, it allows for a similarnominal contact pressure. The normal and friction force wererecorded during the test. Furthermore, the cylinders and thebar were electrically isolated from the sample holders in orderto prevent the formation of a galvanic element in the test-ing uid. Both bar and cylinders, were polished ( Ra = 0.04). Theoperating variables are given in Table 1 . Wear particles were

retrieved from the test period of 0.11 million cycles. Particles


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from this group are referred to as CoB (cylinder-on-bar) parti-cles.

Particles in the second group were generated in a hip simulatoraccording to ISO 14242-1. The tests were conducted at EndolabGmbH (Thansau/Rosenheim, Germany). The tested samples wereMoM hip joints made of high carbon CoCrMo wrought alloy (headandcup) with a 28mm head diameter.Particles were only availablefrom the 0.51 million cycles test period. The tests were conductedin newborn calf serum (30g/l protein content) at 37 C. Particlesfrom this group are referred to as HS (hip simulator) particles.

The third group consisted of commercially available chromiumoxide-particles (Sigma-Aldrich, St. Louis, USA) which were used asreference samples. The particles were available in powder formwitha nominal average sizeof 60nm. The powderhad a mattgreencolor as it is typical for this type of chromium oxide (Cr 2 O3 ).

2.2. Particle isolation

The testing uid (bovine serum) containing the wear par-ticles was stored at 20 C until needed for further analysis.Proteins disturb the image contrast, increase the degree of agglomeration and inhibit the identication of single particles.Therefore, several different protocols have been described in theliterature [8,1822] to isolate particles from the protein richenvironment of the testing uid. Compared to polyethylene par-ticles, the isolation of metallic wear particles requires a protocolusing enzymatic digestion to remove organic components, espe-cially proteins, without damaging the particles that are subjectto oxidation. Particles in the present study were isolated fol-lowing the protocol developed by Catelas et al. [18,19,23] , asbriey described below, with minor changes. This protocol wasshown to minimize particle damage [18,23] . It consists of sev-eral washing as well as incubation steps with two differentenzymes, papain and proteinase K. Papain is a cysteine proteaseenzyme. Its main mechanism of action is through breaking of peptide bonds. Proteinase K is a broad spectrum serine protease.This enzyme is known for its protein denaturing abilities, espe-cially in presence of reagents like etylenediaminetetraacetic acid(EDTA).

Summary of particle isolation procedure (see [19] f or moredetails): the amount of testing uid retrieved from the labora-tory tests ranged from 60ml (hip simulator tests) up to 300ml(sliding wear test rig). Fluid samples were aliquoted into 40mltubes and centrifuged at 18,000 g for 15min using a Sorvall RC-5Bsuperspeed centrifuge (particles from the different aliquots of thesametesting uidsample wererecombinedafter the rstovernightincubation). All subsequent centrifugations were performed for10min at 18,000 g . Supernatants were discarded except 1.5 mlin order to resuspend the pellets and transfer them in micro-centrifuge tubes. Pellets were then resuspended in 2.5% SodiumDodecyl Sulfate (SDS) (v/v distilled water) and boiled for 10min.

This was followed by one wash in 80% acetone and three washes in1 ml of 250mM sodium phosphate buffer solution (PBS) contain-ing 25mM EDTA at pH 7.4. After 30s of sonication, papain (3.2Uin 1 ml PBS/EDTA) was added. The samples were then incubatedovernight on an Eppendorf thermo-mixer at 65 C under slightmotion. After the incubation, pellets from aliquots of the same ini-tial uid sample were combined in a single microcentrifuge tube.Pellets were then resuspended in 2.5% SDS, boiled again for 10minand then washed twice in 1ml of 50mM TrisHCl pH 7.6. Beforeadding proteinase K (3U in 1ml TrisHCl), the pellets were son-icated for 30s. A second overnight incubation was conducted at55 C under slight motion. After the incubation, the samples wereboiled again in 2.5% SDS. The pellets were then washed once in1 ml of 50mM TrisHCl, once in 3% SDS (v/v 80% acetone) and

once in distilled, deionized water. At the end of the isolation pro-

tocol, the pelleted particles were stored in 100% ethanol at 4 C.In a few cases, the pellets still contained denatured organic com-ponents. For those cases, an additional overnight incubation withproteinase K was repeated. This may have been due to the use of 40ml initial aliquots instead of 15ml as previously described in[19] .

Particles were then embedded in epoxy resin for TEM anal-ysis as described by Catelas et al. [18,19] , with minor changes.Prior to embedding, the particles were centrifuged for 20min at18,000 g . The supernatant was removed and the particles wereresuspended in a solution of 0.5ml acetone and 0.5ml liquidepoxy resin (Epoxy 3000, Cloeren Technology GmbH, Wegberg,Germany). Microcentrifuge tubes were then placed on a ther-momixer overnight under slight motion at room temperature toallow pellet inltration by the resin and particle individualization(for better spreading on TEM sections). The tubes were furthercentrifuged for 20min at 18,000 g and vacuumed for 1 h to evap-orate the acetone. One ml of epoxy resin was added and the tubeswere placed in a vacuum chamber for 1h to remove potentialair bubbles and trace of remaining acetone. The samples werethen placed in an oven at 60 C for 1h to harden the epoxy resin.Once hardened, the transparent epoxy cone at the tip of the tubesshowed the embedded particles. Approximately 100nm thick sec-tions were cut using a diamond knife and placed on carbon lmcoatedcopper grid nets (formvar carbon-lm S162-4, Plano GmbH,Wetzlar, Germany) for TEM analysis. It was mandatory to placethe samples on carbon lm grid nets in order to provide sufcientstability under high TEM voltage. Particles placed on pure cop-per grid nets were not stable under an electron beam higher than120kV.

The purchasedparticles wereonly suspendedin ethanol (i.e.,notsubmitted to the whole isolation procedure). After centrifugationthey were embedded in epoxy resin and prepared for TEM analysisas described in [18,19] and above.

2.3. Microscopy

Particles were visualized under TEM. Two different TEMs wereemployed: An EM 400 (Philips, Eindhoven Netherlands) with avoltage of 120 kV and a Tecnai F20 (FEI, Eindhoven, Netherlands)with a voltage of 200kV. The latter also allowed for electron dis-persive X-ray spectroscopy (EDS) and energy ltered TEM mode(EFTEM). EDS analysis was used to determine the chemical com-position of the particles. If the sections were thin enough, EFTEMmapping could be employed to visualize the element distribu-tion. Electron diffraction pattern analysis was used in order to gaininformation about the particle lattice structures. The number of analyzed particles for each section varied from 10 to 50. Eight sec-tions were viewed with TEM for both CoB and HS samples, andtwo sections for the commercially available chromium oxide par-ticles.

For comparison, TEM analysis was also performed in thesubsurface zone of the bar samples prior to and after testing.The necessary cross section preparation method is described in[24,25] .

3. Results

3.1. Particles from the cylinder-on-bar tribometer (Group 1)

The wear mechanisms observed on bar samples tested in thereciprocatingsliding weartribometer differedfromthosedescribedin literature for retrieved MoM hip joints [26] . Grooves in slidingdirectionwere a resultof three-body abrasivewearduring running-

in caused by carbides ( Fig. 3). TEM images demonstrated a nano-


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Fig. 3. Grooves in sliding directionon thebar sample after testing; dark spots referto subsurface carbides.

crystalline (nc) subsurface zone prior to testing as it was reportedto form in MoM hip joint in situ [26,27] . After 1 million cycles of testing, a nano-crystalline zone could be observed as well ( Fig. 4).Itappearedthat, similar toMoM hipjoints,this nc-zone was aresultof ongoing grain renement and fcc hcp phase transformation.

The CoB particles had a long shape with a length of 300800 nmand a width of 100300nm ( Fig. 5). The result of the electrondiffraction pattern analysis revealed clearly the hexagonal closedpacked -martensite. The samelattice structureand chemical com-position was observed as in the subsurface zone of the articulating

Fig. 6. Brighteld image of singletype I and type II HS particles. EDS revealed highpeaks forchromiumandoxygenin bothtypes. Type I particlesadditionally exhibitedpeaks of cobalt and molybdenum.

surfaces. The thickness of these particles was too large to perform

an accurate EFTEM analysis. However, the lattice structure clearlyindicateda CoCrMosolid solution. Thus, extensive oxidation canbeexcluded.

3.2. Particles from hip simulator (Group 2)

Three different types of HS particles were identied. The rsttype (type I) consisted mostly of oxygen and chromium, but

Fig. 4. Nano-crystalline subsurface zone of CoCrMo bar (a) prior to and (b) after testing in cylinder-on-bar tribometer.

Fig. 5. Bright eld images and electron diffraction pattern of CoB particles; electron diffraction pattern analysis revealed hcp lattice of -martensite.


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Fig. 7. EFTEM-mappingof theelementschromium (Cr), cobalt (Co), oxygen(O) andcarbon (C)on type I andII HS particles.

exhibited cobalt locally. The second type (type II) exhibited onlychromium and oxygen. Both types I and II, had a particle size of 3080 nm. Type I and type II particles are shown in Fig. 6 , locatednext toeach other.EFTEMmappingnot only displayedthe chemicalcomposition butalsothe exact location of singleelements( Fig.7 ). Itcan be seen that the type IIparticles had a veryhigh intensity in theoxygen map. A strong signal was also received from chromium,butthe cobalt map showed only background noise. The type I particlesexhibited similar appearances on one side, whereas on the otherside the cobalt map showed a relatively high intensity. Hence, thecobalt did not form a core but appeared localized at one side of the

type I particles. An attempt of taking a diffraction pattern imageby TEM failed. Only an amorphous pattern occurred ( Fig. 8 ). How-ever, crystalline structures could be observed in thecobalt rich areawhen using high resolution mode ( Fig. 8). Judging from the brighteld image, it appeared that smaller particles were aking off thesurface of the type II particles, generating a third type of particles(type III).

These type III particles appeared to be predominant. Consistedmostly of chromium and oxygen and were smaller in size, rang-ing from 5 to 15nm ( Fig. 9). Accumulations of these particles werefound in every epoxy section. The diffraction pattern of type IIIpar-

Fig. 8. (a) Electron diffraction pattern of type I HS particle exhibited no crystalline structure, (b) high resolution imaging revealed crystalline structures in the cobalt rich

areas of type I HS particles.


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Fig. 9. Brighteld (BF) image and EFTEM-mappingof theelementschromium cobalt and oxygen on type IIIHS particles.

ticles ( Fig. 10 ) revealed the lattice structureof Cr 2 O3 . Finally, brighteld images visualized defects in the lattice structure, suggestingthat these type III particles had been exposed to high shear stress.

3.3. Purchased particles (Group 3)

Numerousparticlescould be visualizedby TEM.The particlesizevaried largely. Even though the majority of particles appeared tohave a size of 4080nm, many particles exhibited dimensions of 100200nm ( Fig. 11 ). The diffraction pattern of theseparticlesgavea marked ring pattern which pointed clearly to the structure of Cr2 O3 . So, the structure was identical to those observed with thetype III HS particles. Bright eld images showed that the particle

surface of the commercially availableparticleswas muchsmootherthan that of type III HS particles retrieved from the testing uid.This can be explained by the generation process of these particles.In contrast to particles generated during wear tests, commerciallyavailable chromium-oxide particles did not undergo mechanicalload and tribological stresses. Therefore, the surface exhibited nodefects and the particle structure remained defect-free.

4. Discussion

Particles generated in a given tribosystem are mostly suspectedto have the same structure and chemical composition as the mate-rial close to the worn surfaces. This was veried for CoB particles.

Fig. 10. bright eld (BF) anddark eld image (DF) of type IIIHS particlesand thecorrespondingdiffraction pattern.


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Fig. 11. (a and b) Brighteld imagesand (c)electrondiffractionpattern of purchased Cr 2 O3 -particles.

Even though the thickness of these particles was too large to per-form an accurate analysis of the chemical composition, the resultsof theelectrondiffractionpattern analysis conrmthe same crystalstructure as the deformed CoCrMo alloy. Thus, the particles werenot oxidized although a thin oxide lm of 12 nm had most likelyformed on the surface but could not be visualized. The fact thatthe particle size was signicantly larger than the crystal size inthe subsurface zone of the bar sample suggests that different wearmechanisms must have been active as it is the case for MoM hip joints [28,29] . This is mainly linked to the test conditions (recip-rocating vs. multidirectional sliding) and the fact that body andcounter body had non-conforming surfaces.

Different results can however be expected for wear particlesgenerated from tribosystems operating under ultra-low wear con-ditions such as MoM hipjoints. In such systems, particleshave beenshown to be generated from an in situ formed tribolayer ratherthan the underlying deformed subsurface zones [2931] . Thus, thewear particle material is altered by strain, mechanical-mixing andtribo-chemical reactions prior to its detachment from the surface.

This tribolayer has been well described in the literature [27,28] .Bscher et al. reported the in situ formed nc-zone in MoM hip joints as key factor for the generation of wear particles within thenm-range [26] . Interestingly, the grain size in that zone stronglycorrelates with the observed size of type I and II HS particles inthe present study. Wimmer et al. further pointed out the impor-tance of denatured proteins forming a tribolm upon the bearingsurfaces [24,30] . Due to mechanical-mixing, carbon-rich materialfrom organic origin would get incorporated between the crystalsof the nc-zone forming a metallo-organic composite (tribolayer)[25,32] . In addition, plastic ow due to the displacement and/orrolling of entire nano-size grains would allow for low wear undernormal sliding conditions. Fig. 12 depicts how single particles canbe embedded within the carbon rich material. However, the exact

mechanism of particle detachment is yet unclear. Also the surfacestate of such particles is unknown. A recent study has shown par-ticles that were not completely oxidized and still exhibited bothcobalt and chromium in the joint capsule [33] . Phosphate and car-bon groups were found to bond to the surface of these particles.Furthermore, it was shown that organic tribolms on bulk surfacesprovide a type ofpassivation, butnot bymeansof an oxidelm [34] .Thus, wear particles might be protected by organic lms againstchemical alteration.

With regards to the analysis of HS particles isolated in thepresent study, it should be underlined that only serum samplesfrom simulator tests within the transition from the running-into the steady state regime were available (0.51 million cycles).Hence, the amount of particles was signicantly lower than itwould have been expected for samples from early running-in wear

Fig.12. Mechanical-mixed subsurface zoneof a MoM hipjoint femoral heads bear-ing surface:single grains embedded in carbon rich zone.

(00.5 million cycles). The majority of the particles appeared to bechromium oxide, which corroborates with most literature studies[19,22,35,36] . However, particles containing cobalt (type I parti-cles) were also found, similar to those described by Catelas et al.[19] , who also stated that the number of such particles was rela-tively low understeady-stateconditions. Interestingly, results fromthe present study suggest that cobalt would not form the core of type I HS particles but instead, it would only be present locally,on one side of the particles. Indeed, the type I particles exhibiteda crystalline structure only in this specic area whereas the restof the particles was amorphous. Hence, these results ruled out thepossibility that the chromium oxideparticles described in previousstudies using only EDS exhibited a cobalt core. The type II parti-cles showed a comparable appearance and size but consisted of

chromium and oxygen with only trace of cobalt (background noiseon EFTEM mapping) and were completely amorphous. The surfaceof these particles appeared to be torn with particles aking off giv-ing rise to a third type of HS particles (type III) which depicted thecrystalline structure of Cr 2 O3 . Earlier studies suggested that parti-cles consisting only of chromium and oxygen may originate fromthe passive oxide lm on the bearing surface [19,36] , which wouldget disrupted during sliding. However, after running-in wear, theoxide lm, which most likely exhibits a thickness of 2 to maximum10nm [37] , may be too thin to be at the origin of the tribologicallygenerated type III particles. Commercially available Cr 2 O3 particleswere completely crystalline and showed no lattice defects in con-trast to type III HS particles. This indicates that type III HS particleswere likely subjected to high mechanical and chemical reactions

enabled by strain and protein interaction.


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Under bulk conditions, one would expect the surface of CoCrMoto form a protective oxide lm inhibiting chemical alteration. Thisappeared to be the case for the rather large CoB particles as noalteration of the lattice structure was observed compared to thenano-crystalline subsurfacezone of headand cup.Thus,the particlesurface must have been covered with a thin oxide lm preventingalteration of the lattice structure and the chemical composition.However, nanoparticles are known to possess unique propertieslike the decrease of the melting temperature [38] or reversibletransition to the amorphous state under load [31,39] . Usually, so-called size effects are well known for particles being smaller than20nm. Even though type I and II HS particles were considerablylarger than 20nm, several parameters may have interfered. First,the surface/volume ratio was very high compared to larger parti-cles (>100nm) as, for example, the observed CoB particles. Second,originating from the nc-zone, the particles were exposed to highshear stress as depicted by a high density of lattice defects in typeIII HS particles, and thus, had a larger inner energy and were highlyreactive. Also the transformation to the amorphous state in type Iand II HS particles may be related to this effect. Third, due to thecurvatureof theparticles,therewould bemore vacanciesat thesur-face, as well as a higher degree of lattice strain, both contributingto a high diffusion rate of elements like oxygen and cobalt. Takingallthese facts into account, it appears that particles generatedfromthe nc-zone could likely exhibit pyrophoric properties.

One may question if the particle isolation procedure may alterthe particle shape (through the centrifugation steps) or chemicalcomposition (through the elimination of the denatured proteinscovering the particle surface). Because of the small size of the par-ticles, the applied pressure on the particles under 18,000 g isnegligible. Therefore, it is unlikely that the particle morphologywould be altered due to the centrifugation process. On the otherhand, it is possible that through the removal of the organic com-ponents to isolate the particles from the new born calf serum, theprotective organic lmthat initially covers the particles would alsobe removed, hence possibly leading to a change in the chemicalcomposition of the particles. However, when looking at the effectsof theisolation protocol on wear particlesisolatedfrom 95%bovineserum (compared to 30 g/l protein content newborn calf serumused inthe present study), Catelas etal. didnot show anysignicantlevels of Co ions released in solution after applying the enzymatictreatments on the particles, and also showed that organic serumcomponents adhering to the particles did not seem to be removedduring the isolation procedure [23] . Hence, the origin of the largeamount of chromium oxide particles still remains uncertain. Analternative scenario of particle chemical alteration may be linkedto their dwell period within the contact area of the bearing sur-faces. Acting as third-body, such particles are too small in size tocause damage to the surface. However, before particles exit the tri-bological interface, they may be exposed to high mechanical loadwhen trapped between surface asperities of head and cup during

contact. A protective lmof denaturedproteins could be partlydis-rupted allowing for oxidation. This, however, contrasts with therecent observations in [33] , showing particles with a high amountof cobalt within the tissue of the joint capsule. Hence, at this point,the exact pathways of particle generation and detachment into the joint environment still remain unclear.

5. Summary and outlook

Wear particles generated from CoCrMo alloy in two differenttribometers were analyzed. Particles retrieved from a cylinder-on-bar-tribometer were about 10 times larger than particles retrievedfrom a hip simulator. They exhibited the same chemical com-

position and lattice structure as the near-surface material. These

particles would detach from below the nc-zone due to the moresevere tribological conditions of the test setup.

The hip simulator particles had a size range from 5 to 70nm.Three different types of particles were identied by means of size,chemical composition and lattice structure. Such particles wouldoriginate from the nc-zone or, more precisely, the mechanicallymixed zone. A possible pathway would be that single grains withinthe nc-zone are transported due to the plastic ow of entire grainsto the mechanically mixed zone where carbon-rich denaturedorganic material as well as phosphates settle at the grain bound-aries. During ongoing plastic ow, such grains could be transportedto the uppermost surface where they would stick in a lm of dena-tured proteins. Eventually, the grains would be pushed out fromthe tribological interface and form single particles. At this point,the particles would still exhibit a relatively high amount of cobaltas well as chromium, and be protected by a shell of organic mate-rial [33,40] . In that case, it would suggest that the particle isolationprotocol may have removed the organic protective lmon the par-ticle surface, resulting in the activation of size effects that wouldlead to an increased pyrophoric behavior of the particles. However,at this point, it is still unclear which alterations the wear particlesgo through between their generation and their detachement in the joint environment. It is also possible that particles undergo alreadysignicant transformation and changes in chemical composition inthe contact area of the head and cup within the tribological inter-face leading to the disruption of the organic protective lm andthus particle oxidation. This remains to be further investigated.

In order to gain more precise information about the nature of wearparticlesgenerated in MoM hipjoints, it is essentialto analyzeparticles from periprosthetic tissues using TEM. In addition,EFTEMmapping and EELS (electron energy loss spectroscopy) need to beapplied for more accurate determination of the particle chemicalcomposition. Finally, in order to determine whether particles arecrystalline or not and to gain knowledge of the occurring latticestructures, electron diffraction pattern analysis needs to be appliedas well.

Acknowledgements

The authors would like to acknowledge Ronnie Seeger, BirgitGleising, Marco Joeken and Priska Stemmer for their support andexcellent sample preparation. Further, we would like to thank Prof.W. Dudzinski (Technical University, Wroclaw, Poland) for prepar-ing microtomefoils, Prof. Ralf Kppers andHeike Heise (InstitutfrZellforschung, Universittsklinikum Essen, Germany) for providingthe high performance centrifuge and Prof. Michael Farle (Faculty of Physics, AG Farle, University Duisburg-Essen, Germany) for pro-viding access to the high resolution TEM. A part of this work wassponsored by NIH under grant RC2 AR058993-01.

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1.Caracterizarea Particulelor de Uzură Generate Din Aliaj CoCrMo Sub Alunecare

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