MetalIons–AScientificReview
Introduction
M
etal-on-metalbearingsintotalhipreplacement
haveexperiencedaresurgenceofpopularity
inrecenttimes.Thisisduemainlytoasignificant
numberoflong-termclinicalstudieshighlighting
theremarkablylowwearratesofmetal-on-metal
bearingsimplantedoverthepast40years.1-4Clinical
complicationswiththeearlygenerationcomponents
usuallywererelatedtomanufacturingissuessuch
as,impropertolerancesandpoordesign.5Recent
advancementsinmanufacturingtechnologyhave
enabledmoreprecisetolerances,surfacefinish,and
designsthatprovideanincreasedrangeofmotion
(Figure1).Documentedclinicalsuccesswith
moderndeviceshasledtotheFDAgrantingmarket
approvaltosomeorthopediccompaniesintheUS.6,7
Early
RadialClearanceµm
Sphericity=10µm
SurfaceFinishRa=1µm
EarlyMOMHead
120–500
10–20
.1–1
Modern
25–75
<5
<.005
RecentMOMHead
Figure1
Metal-on-metalisparticularlyattractiveinthetotalhipnotonlyduetotheultra-lowwearexhibitedin
numerousclinicalandsimulatorstudies1–8(Figure2),butalsoduetotheflexibilityaffordedbydesigns
basedonthepropertiesofcobaltchromiumalloy.Thisflexibilityenablestheuseoflargediameterheads
inavarietyofacetabularconfigurations.Largermetal-on-metalfemoralheadscanprovideincreased
range-of-motionandstability,aswellasincreasedresistancetodislocation.9-12Theincorporationof
largerdiameterfemoralheadsintoceramicandpolyethylenebearingspresentsdesignchallengesdue
toimpingementconcernswithceramics,andincreasedbrittlenesswiththinpolyethyleneliners,all
leadingtoanincreasedriskofmechanicalfracture.13,14Metal-on-metalcomponentsdonotposearisk
ofmechanicalfracture.Currentmetal-on-metaldesignsallowtheuseofthelarger38mmheadsin
acetabularcomponentsizesassmallas46mm.
LinearWear(µm/y)
In-vivoWearRates
140
120
100
80
60
40
20
0
130
79
73
25
4
Poly
(Head44)
ArCom
®
(Head44)
HXLP
COC
MOM
(Oonishi45) (Hamadouche46) (Schmidt1)
Device
Figure2
Metal-on-metalhasexhibitedultra-lowwearratesnotonlyin
simulatorstudies,butalsothroughpublishedin-vivowear
studiesasshowninthein-vivowearcomparisonabove.
Despiteitslengthyandpositiveclinicalhistory,
therehavebeenconcernsexpressedregardingthe
long-termbiologicalconsequencesofmetal-onmetalweardebris,particularlythereleaseofcobalt
andchromiumionsintothebody.Questionsabout
metalionsandtheirbiologicaleffectsarenot
new.Manystudieshavebeenconductedoverpast
decades.Allhavefailedtoestablishacausallink
betweenionrelease,andcancerorothernegative
systemiceffects.15–22Aswitheachbearingsurface,
certainunknownsstillexist,andinrecenttimes,
fueledbyrumorsandmisinformationaboutthe
metalions,thedebatelingerson.Followingare
fivereasonswhymanyleadingresearchersconsider
themetaliondebatetobeinconsequential.
1
1
Cobaltchromealloyisaknownmaterial.
V
itallium™orcobaltchromiumalloyhasbeeninuse
inorthopedicsformorethan80yearssinceitsfirstuse
byDr.Smith-Petersenintotalhipreplacementsurgeryin
1917.23Thestrengthanddurabilityofcobaltchromehas
beenappliedtoavarietyofbearingsurfacesovertheyears
includingtotalknees,elbows,shouldersandhips.Cobalt
chromewasfirstusedinmetal-on-metalapplicationsas
earlyas1958indesignssuchastheRing,Muller,McKee/
Farrar,andSivashprostheses.Sincethoseearlydays
oforthopedics,hundredsofthousandsofpatientshave
receivedmetal-on-metalimplants,withthemajority
beingimplantedinEurope.Interestingly,40yearsand
hundredsofthousandsofimplantslater,metal-on-metal
ionreleaseremainsanon-issueamongmostEuropean
orthopedicsurgeons.
Cobaltchromehasbeen
usedinawidevarietyof
orthopedicimplantsoverthe
past80yearsincludingthis
hipimplantfromtheearly
20thcentury.
Throughoutthehistoryofmetal-on-metalvariousauthors
haveexaminedtheionreleaseissue.Followingarebrief
summariesofsomeoftheirfindings.
TheRiskofCancerFollowingTotalHiporKneeArthroplasty,
JBJS,May200115
Dr.Tharani,etal.,reportedontheriskofcancer
followingtotalhiporkneearthroplasty.15Ninedifferent
studiesencompassing140,000totalhipandkneecases
werecombinedandstatisticallyevaluatedinorderto
comparetherelativeriskofcancertothatofthegeneral
populationoracontrolgroup.Intheseninestudiesa
totalof25casesofmalignantdiseasewerereported,
and34%ofthecancersoccurredwithinthefirsttwo
yearsofimplantation.Thesefindingsshowed,“the
availabledatadonotsupportacausallinkbetweentotal
hiparthroplastyandthedevelopmentofcancer.”In
addition,noincreasedriskofcancerwithametal-onmetalhipimplantversusapolyethyleneimplantwas
found.Long-termconcernsaboutmetal-on-metalinthe
hipshouldbeweighedagainstthestudydatashowingthat
34%ofthecancersaftertotaljointreplacementoccurred
withinthefirsttwoyears,andwerenotassociatedwith
thetotaljointimplant.
2
Cobaltchromewasfirst
usedinmetal-on-metal
applicationsasearlyas
1958indesignssuchas
theMuller,McKee/Farrar,
SivashandRing(pictured
ontheleft)prostheses.
CancerRiskAfterMetal-on-MetalandPolyethylene-on-MetalTHA,CORR,August199616
Dr.Visuri,etal.,alsostudiedthecancerriskaftermetal-on-metalandpolyethylene-on-metaltotalhip
arthroplasty.16Thestudyinvolved579metal-on-metalpatientsand1,585metal-on-polyethylenepatientswith
upto16yearfollow-up.Theirobjectivewastocomparetheincidenceofcancerafterbothmetal-on-metal
andmetal-on-polyethylenetotalhipreplacementtothatofthegeneralpopulationinFinland.Therewasno
statisticallysignificantincreaseincancerofanytype,at16-yearfollow-up,witheitherametal-on-metalor
metal-on-polyethylenecomparedtotheexpectedcancerrateofthegeneralpopulation.Theauthorsconcluded
that,“theobservedvariationintheincidenceofdifferentcancersamongpatientswhohadtotalhip
arthroplastycomparedwiththegeneralpopulationsuggeststhatfactorsotherthantotalhiparthroplastyplay
amajorroleintheoriginorcancer”(Figure3).
CancerIncidence
140
120
100
80
60
40
MOM
Expected
20
0
1 51015
Years
Figure3
Dr.Visurireportednostatisticallysignificantincreaseincancerofanytypewitheitherametal-on-metalormetal-onpolyethylenehipreplacementcomparedtotheexpectedcancerrateofthegeneralpopulationata16-yearfollow-up.
DistributionofCobaltChromiumWearandCorrosionProductsandBiologicReactions,CORR,August199617
Dr.Merritt,etal.,examinedthedistributionofcobaltchromiumwearandcorrosionproductsandthe
subsequentbiologicreactions.17Theissueofcobaltchromiumwearandcorrosionproductswasevaluated
inanefforttoseehowthismightrelatetothebiologicperformanceofimplantdevices.Thefindingswere
extremelyfavorableasrelatedtoawellfunctioningmetal-on-metalhipimplant.Theauthorsconclude,“itis
clearatthisstageintotaljointreplacement,thatmorereportedadversebiologicresponsesareoccurringto
thepolyethylenethantothemetal.Itistemptingtoaddressthisissuebyeliminatingthepolyethylene.”
3
Metalionreleaseisnotuniquetometal-onmetaldevices.
M
etalionsareabyproductofvirtuallyeverymetallicimplant.Nails,screws,stems,
metalcups,plates,modularheads,cages,andcableswilleachreleaseionsoftheirperspective
metals.ThisisfurtherevidencedinstudiesbyDr.Kriebichwhodemonstratedthatloosecobaltchrome
stemsexhibitequivalentbloodionlevelscomparedtothatofametal-on-metalimplant24(Figure4).
CobaltandChromiumValuesintheSerum
2.5
2.24
MicrogramsperLiter
2
2
1.5
1.29
1
1.09
.9
Cobalt
1.28
Chromium
0.5
0
Kriebich
(Howmedica
PCAStem)
M-PE
Jacobs
(McKee
Farrar)
MOM
Brodner
(Sulzer)
MOM
Figure4
Serumstudiesofmetalionsdemonstratesimilarvaluesbetweenloosecobaltchromestemsandthatofametal-on-metalimplant.
Someceramic-on-ceramicdesignsmayalsoreleasemetal
ionsduetoneckimpingementandfrettingatmodulartaper
junctions.33Therehavebeencasereportsoffemoral
necknotchingwithsomerecentceramic-on-ceramicdesigns
leadingtoexcessivemetaldebris,resultinginmetallosis.25
Clearly,invivotransfersofmetalionsarenotuniqueto
metal-on-metalbearings,sothequestionbecomes:ifwe
can’tavoidmetalionsintotalhiparthroplasty,whatisan
acceptableamount?Researchershavebeentryingtodefine
maximumvaluesforcobaltandchromiumionsforyears.The
Someceramic-on-ceramicdesignsmayalsoreleasemetal mainissuetheyareconfrontedwithisthefactthatyearsof
ionsreleaseduetometal-on-metalneckimpingementand metalionresearchhaveyettorevealanytoxicsignificance
frettingatmodulartaperjunctions.
associatedwithmetalionsreleasedfrommetal-on-metal
articulations.15-22Therefore,settingamaximumvaluehasnotbeenpossible.Dataontheeffectsof
cobaltandchromiuminthebodyatlevelsmuchhigherthanthatofametal-on-metalimplantcanbe
foundinstudiesperformedoutsideofthefieldoforthopedics,suchasinthemetalindustry.For
generalpurposes,comparisonscanbemadetothemetalionlevelsreportedfrommetal-on-metal
implantstudies.
4
Casereportsandstudiesofcobaltandchromium
inotherindustriessupporttheuseofmetal-onmetalbearingsinorthopedics.
W
orkersinthemetalindustryaretypicallyexposedtohigherthannormallevelsofthemetalsthey
areprocessing.Testshaveshownhigherthannormalbloodionvaluesinfactoryworkersexposed
tochromiumandcobalt.
Thesemetalscanbeintroducedintotheirsystemthroughdermalexposure,inhalationandaccidental
ingestion.27-31Itshouldbenotedthateventhoughtheseworkersdemonstratebloodionvalues15-20
timeshigherthanthatofametal-on-metalimplant,nosystemictoxicityhasbeenestablished.Metal-onmetalionlevelsofchromiumcanalsobecomparedto“studiesonworkercohortswhohavehistorically
beenexposedwellabovethecurrentpermissiblestandards(IARC,1990),andhavenotshowna
consistentpatternofelevatedcancerincidence”(Figure5)(Proctor,2002).27
ChromiumBloodLevels
100
Micrograms/Liter
3
80
75
60
40
20
0
Proctor
(Factory
Workers)
3
3.03
1.28
Schaffer
Metal-onMetal
MacDonald
Metal-on
Metal
Jacobs
Metal-onMetal
Figure5
Metal-on-metalionlevelsofchromiumcanalsobecomparedto“studiesonworkercohortswhohavehistoricallybeen
exposedwellabovethecurrentpermissiblestandards,andhavenotshownaconsistentpatternofelevatedcancerincidence.”27
Forexample,discussionsoftoxicityhavealsoextendedtothefemalepopulationandpregancy.Inthe
1960scobaltwassometimesusedasaremedyforanemiainpregnantwomen.Barcelouxreportedon
onegroupofwomenreceivingover100,000Mcgofcobaltperday,allofwhomdeliveredhealthy
full-terminfants,andexperiencednoadversesideeffectsrelatedtothecobaltmedication30(Figure8).
5
Metal-on-Metalhasbeenusedinwomenofchildbearingage.
Dr.BrodnerfromVienna,Austriarecentlyreported(Montreal,June2003)onthreepregnantwomenwho
wererecipientsofmetal-on-metalhips.Metaliontestingwasperformedatthetimeofdelivery.Trace
amountofionscouldbedetectedinthemothers,withnodetectableamountintheumbilicalcords.
Dr.Brodnerconcludedthatmetal-on-metalwassafeforwomenofchildbearingageas“theplacentaacts
asaneffectivebarrierforcobaltandchromiumdisseminatedfrommetal-on-metalarticulations.”31Not
onlyhascobaltbeenusedasamedicineforrefractoryanemia,butchromiumalsohasbeneficialeffects
inthebody,suchasassistinginpropermetabolicfunction.
Adebateaboutmetalionrelease,similartothatoccurringinorthopedics,iscurrentlybeingwagedin
thefoodindustry.Someinvestigatorshaveexpressedconcernregardingtheleachingofmetals,suchas
chromium,intofoodfrommetalliccookingutensils.Asintheorthopedicfield,somehaveremained
cautious,statingthatmoreresearchinthisareaisneeded.Dr.Aitio,ChiefPhysicianfortheDepartment
ofIndustrialHygieneandToxicologyattheFinnishInstituteofOccupationalHealth,concluded
that:“Notoxicityistobeexpectedfromthechromiumleachedfromkitchenware,itmayinfactbe
beneficialtohealth,sincetheamountsofchromiuminpresentWestern-typedietsaregenerallysmallin
comparisontoamountsconsideredtobeoptimal.”26
Debateaboutmetallicionreleasefromfoodutensils.
6
4
CobaltandChromiummaybebeneficialtothe
bodyasestablishedbyresearchandlistedbytheUS
government.
A
ttimesinthemidstofthedebatesregardingmetalions,itishelpfultorememberthatmetal-
on-metaldevicesareFDAregulatedandclearedfororthopedicapplicationsbytheUSgovernment.
Thegovernmenthasalsoestablishedarecommendeddailyallowanceforchromium.“ChromiumIII
(trivalentchromium)isanessentialmicronutrientwitharecommendeddailyrequirementforhumans
intherangeof50to200microgramsperday(µg/d).”27Asdemonstratedinthechartbelow,ametalon-metalhipisassociatedwithbloodchromiumlevelsof3–5Mcg/l,themajorityofwhichisexcreted
thoughtheurine.18
Aswithmanyelements,chromiumcanexistinseveral
atomicvalencestates.ChromiumVI(hexavalent)and
ChromiumIII(trivalent)arethetwomostcommon.
“ChromiumIIIdisplaysextremelylowornotoxicity
viaallroutesofhumanexposureanddoesnotposea
carcinogenichazard(ATSDR,2000;U.S.EPA,1998a;
IARC,1990;WHO,1988).”27Chromiumpicolinate
isoneformofchromiumIIIthatwassoldinlarge
quantitiesduringthe“chromiumcraze”ofthemid
1990s.Itwaspromotedasawaytoburnfatand
regulatemetabolism,andisstillastandardingredientin
manyhealthfoodsupplementsbeingsoldtoday.34,35
Aswithmanyelementschromiumcanexistinseveralatomic
valencestates.
“ChromiumVI(hexavalentchromium)isimportanttoavarietyofindustriesincludingpigment
manufacturingandpainting,metalplating,woodtreating,andleathertanning…”27Hexavalent
chromiumcanhaveatoxiceffectinphysiologicalenvironmentsespeciallyifintroducedtothebodyin
largeamounts.
Chromiumisastandardingredientinmanyhealthsupplementsbeingsoldtoday.
7
Itiswidelyacceptedthatthevalencestateofchromiumreleasedfromametal-on-metalimplantis
primarilytrivalentwhichisthemorebiocompatibleformofchromium.Also,evenifchromiumions
werereleasedinthehexavalentstate,thelevelsappeartomuchlowerthantheamountsthatmightcause
toxicity,asillustratedinthefindingslistedbelow:
•
•
•
•
“recentkineticsandinvivogenotoxicitydatademonstratethatCr(VI)isreducedtonontoxicCr(III)
insaliva,intheacidicconditionsofthestomach,andintheblood.”27
“AtconcentrationsatleastashighasthecurrentU.S.maximumcontaminantlevelof100
microgramsperliter(µg/L),andprobablyatleastanorderofmagnitudehigher,Cr(VI)isreducedto
Cr(III)priorto,oruponsystemicabsorption.”27
“Redbloodcelllysateshaveasubstantialandregenerativecapacitytoreduce>100McgofCr(VI)
(toCr(III)basedonthehematocritofanaverageadult(DeFloraandWetterhahn,1989;DeFlora,
1996).”29
“EssentiallyalltissuespossessasimilarhighcapacitytoreduceCr(VI)toCr(III),especiallytheliver,
whichisresponsibleforthe“firstpasseffect”biotransformationsofmanychemicals(Sipesand
Gandolfi,1991).”29
Currentmetal-on-metalionreleasestudieshaveshowntrivalentchromiumlevelsinserumtobemuch
lowerthaneventhe100µg/lmaximumlistedforhexavalentchromium.18–22Also,mostresearchers
agreethatchromiumionsreleasedintothebodyfrommetal-on-metalimplantsareinthetrivalentCr(III)
state,andaccordingtothestatementsabove,evenifalloftheionsoccurredinthehexavalentCr(VI)
state–theycouldstillbereducedtothetrivalentstatewithouttoxiceffects(Figure6).
MicrogramsperDay
ChromiumValenceComparison
100
100
80
60
40
20
0
15
(Shafferetal21)
15.15
(MacDonaldetal18)
ChromiumlevelsfromMetal-onMetalImplants
Themaximum
amountofCRVIthe
bodyisabletosafely
converttoCRIII.27
Figure6
Currentmetal-on-metalionreleasestudiesshowtrivalentchromiumlevelsintheserumtobemuchlowerthanthe100µg/d
maximumlistedforhexavalentchromium.18,21,27EvenifalloftheionsoccurredinthehexavalentCr(VI)state–theycouldstill
bereducedtothetrivalentstatewithouttoxiceffects.
Trivalentchromiumisakeyelementofglucoseregulationandmetabolicfunction.Ithasbeenwidely
consumedinpillformasameansofhelpingtoregulatediabetes,controlsugarcravings,andaidin
weightloss.Ithasbeenthesubjectofmorethan35clinicalstudiesinvolvingover2,000participants36.
Therecommendeddailydoseasrecommendedbythesestudiesisaround200Mcgperday(Figure7).
ArecentstudyfundedbytheNationalInstitutesofHealthshowedthat:
“Dailysupplementationwith1,000Mcgofchromiumaschromiumpicolinatesignificantly
enhancedinsulinsensitivity.Theseinitialresultsofferapotentialnewnutritionaltherapyfor
approximately2millionAmericanwomensufferingfromPolycysticOvarianSyndrome(PCOS).
8
PCOSisalittle-understoodhormonalconditionthatisaleadingcauseofinfertility,andis
associatedwithinsulinresistance,gestationaldiabetesandtype2diabetes.”36(Figure7)
ChromiumBloodLevels
1000
MicrogramsperDay
1000
800
650
400
200
200
0
15
Recommended Recommended
Daily
Chromium
Requirement
forPCOS
(USRDA)
15.15
Schaffer
Metal-onMetal
6.4
Jacobs
MacDonald
Metal-on Metal-onMetal
Metal
Figure7
C
Concernsregardingthetotalchromiumvaluesreportedinmetal-on-metalionsstudiesshouldbeweighedagainstthedaily
valuesrecommendinotherstudies.
obaltalsohasaroleinthebody.Cobaltisanessentialelementnecessaryfortheformation
ofvitaminB12,andthemetabolismofproteins.Consequently,consumingextraproteinandvitamin
B12(ie:meatandmultivitamins),maysubstantiallyinfluencetheurinaryconcentrationsofcobalt.The
averagepersonconsumesbetween5and45micrograms(mcg)ofcobaltperday.30Theamountof
cobaltexcretedintheurinefromametal-on-metalimplantappearstobebetween3and350Mcgper
day.19-21Inthe1960ssomeheavybeerdrinkersdevelopedcardiacproblemsacoupleofmonthsafter
cobaltchloridewasaddedtotheirlocalbeersupply.Theestimateddailyintakeofcobaltbythesebeer
drinkerswas6000to8000Mcgperday.Reducedproteinintakeandapoordietwerealsocitedas
contributingtothebeerdrinker’sheartconditions30(Figure8).Assumingacumulativeeffectofcobalt
ionreleasefromametal-on-metalimplant(inotherwordsnoreleasefromthebody)itwouldtake
approximately438yearsofcumulativecobaltionreleasetocomparewithwhatthebeerdrinkerswere
ingestingintwomonths!Furthermore,“cancerinhumansasaresultofexposuretocobaltbyanyroute
hasnotbeendemonstrated.”30TheUSNationalToxicologyProgramdoesnotlistcobaltasarecognized
animalorhumancarcinogen.31
CobaltLevelsintheBody
100000
100000
MicrogramsperDay
90000
80000
70000
60000
50000
40000
30000
20000
10000
0
8000
45
DailyIngestion
(Barceloux)
50.5
MOMUrinary
Excretion
(MacDonald)
BeerDrinkers
(Barceloux)
PregnantWomen
(Barceloux)
Figure8
Studiesofelevatedcobaltlevelsinpregnantwomen,andbeerdrinkershavenotdemonstratedanelevatedriskofcancer
despitelevelsmuchhigherthanthosereportedfrommetal-on-metalimplants.
9
Concernsregardingionreleasemustbe
weighedagainstthewearby-productsofother
bearingsurfaces.
E
Everybearingsurfaceproducesan
environmentalburdenofwearrelated
debris,noneofwhichareinerttothebody.
Occurrenceofosteolysisduetoexcess
polythyelenewearisregularlyreportedinthe
literature.37,38Reportsofosteolysiswithmetalon-metalimplantsarerare.
verybearingsurfaceproducesanenvironmentalburdenofwear
relateddebris,nonebeingtotallyinertinthebody.Occurrence
ofosteolysisduetohighermagnitudesofpolythyelenewearhas
beenreportedintheliterature.37,38Metallicparticlesmayalsobe
associatedwithosteolysis,althoughreportsofthisarelacking,and
occurrenceisrare.Ceramicparticleshavebeenassociatedwith
osteolysis.Dr.Yoon,etal.,reportedon103ceramic-on-ceramic
hipsat7.6yearsfollow-up,finding22%femorallysisand49%
acetabularlysis.39Somehavesuggestedthatthehighlycrosslinked
polyethylenesmaynoteliminateosteolysis,inthatincreased
irradiationenergycanleadtosmaller,rounderparticulate40whichhas
beendirectlyassociatedwithanincreasedosteolyticresponse.41,42A
reducedlocalresponsetometal-on-metalwearproductshasbeen
welldocumentedintwenty-yearstudiesbySchmidt,McKellop,and
Cuckler.1,2,43Dr.Cuckler,amongotherresearchers,hassuggested
thatreasonsforthereducedtissueresponseofmetal-on-metalwear
debrisareduetoultra-lowmetal-on-metalwearrateswhichreduces
theenvironmentalburdenandlocalinflammatoryresponses.43
Themajorityofwearproductsexistatasubmicronlevelenabling
theweardebristobeexcretedfromthebodythroughthebody’s
normalmetabolicprocesses.ThiswasreportedbyDr.Schaffer;“We
concludethattheeliminationofcobaltandchromiumproceeds
overseveralyears,affectingabalancebetweenreleaseand
excretion”(Figure9).Thisreleasefromthebodyinanionic
formmayavoidalocalresponseaswellanytoxicbuild-upona
systemiclevel.
CobaltandChromiumValuesintheUrine
60
MicrogramsperDay
5
50.5
50
50
40
Cobalt
Chromium
30
20
10
0
5.1
11.8
4
2.09
Jacobs
Jacobs
MacDonald Schaffer
Figure9
Theexcretionofionsthroughtheurineisevidencedinclinicaltrialsofmetal-onmetalimplantsaslistedabove.
10
Conclusion
T
hereareavarietyofbearingoptionsavailablefortotalhipreplacement.Metal-on-metalisthemost
clinicallyprovenalternatebearingavailabletoday,withover40yearsofclinicaluse.Concerns
associatedwithmetalionreleasemustbeweighedagainsttherisksandbenefitsofotherbearingoptions.
Metallicdebrisisanenvironmentalby-productofmetal-on-metalwearandispresentinthebodywith
non-metal-on-metalbearingsurfaces.24,33Extensiveresearchandyearsofclinicaltrialshavefailedto
proveanycauseforconcernassociatedwiththeionlevelsexhibitedfrommetal-on-metalimplants.The
lower-wearaspectsandwiderangeofdesignoptionsavailablewithametal-on-metalimplantmakeit
particularlyattractivefortheyounger,moreactivepatientpopulation.
Thelower-wearaspectsandwiderangeofdesignoptionsavailablewitha
metal-on-metalimplantmakeitparticularlyattractivefortheyounger,more
activepatientpopulation.
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