The whole dissertation^

1
Abstract.
Introduction: From the discovery of zirconia in 1789 to the biomedical uses as implants or
fixed restorative devices, zirconia has been utilised for many reasons, but most of all in
dentistry because of its high impact and flexural strength owed to an internal toughening
mechanism. Zirconia is now being found indicated for crowns, bridges and more in the
dental field, plus with the increasing use of CAD/CAM systems. Veneer porcelains are
weaker, and were found to be more likely to crack and de-bond from zirconia substructures
than those of the conventional metals.
Methods: Through searching on MMU library which gave results from Science Direct,
PUBMED, Quintessence (and any dental journal that MMU had paid the subscription for).
Journals were found from specific search categories using the BOOLEAN technique. Then
exclusions were made and included data was studied.
Results: A flowchart to show the process of searching methods was created displaying the
fields of excluded and included data, with amount of included results.
Discussion: The survival rates that were found, showed that crowns of zirconia-ceramic
substructures didn’t last as long in the oral environment than those of metal-ceramics.
Generally cracking of the porcelain and de-bonding was found to occur more in zirconia
crowns. However zirconia was shown to be a longer surviving substructure material in the
use of posterior bridges compared to metals.
Authors Conclusion: Zirconia doesn’t ensure the longevity as other factors have a role in the
survival rates of crowns and bridges, and often it was shown that the porcelain would be
zirconia’s limiting factor. If high strength porcelains veneered to zirconia substructures or
zirconia full contour crowns were utilised, the results might be different and zirconia could
therefore be the dominating factor in the survival, without the porcelain fracturing,
rendering the restoration a failure.

2
Acknowledgements.
I wouldlike toexpresssincere gratitudetoDr,S.A.Horne, Dr, R Taylor, HannahBates andmy
parentsforthe advice andforesightonthisdissertation.

3
Contents
Abstract………………………………………………………………………………………………………………………………………….. 1
Acknowledgments…………………………………………………………………………………………………………………………… 2
Contents…………………………………………………………………………………………………………………………………………. 3
1 Introduction.......................................................................................................................... 5
1.1 Zirconia Background.......................................................................................................5
1.1.1 Discoveryleading to use in biomedical science......................................................... 5
1.1.2 Chemistry and Structure. ........................................................................................ 6
1.1.3 Dental metal alloys:................................................................................................ 7
1.2 Zirconia in Dentistry.......................................................................................................7
1.2.1 Zirconia as a dental ceramic.................................................................................... 7
1.2.2 Processing Zirconia into a substructure for use in fixed prosthodontics.................... 10
1.2.2.1 Slip cast method............................................................................................... 10
1.2.2.2 CAD/CAM......................................................................................................... 11
1.2.3 Clinical survival rates............................................................................................. 12
1.2.4 Technical Indicationsfor use. ................................................................................ 15
1.2.4.1 Strength........................................................................................................... 15
1.2.4.2 Aesthetics......................................................................................................... 16
1.2.4.3 Sandblasting and surface treatments. ................................................................ 17
1.2.5 Clinical Indications for use..................................................................................... 17
1.2.5.1 Properties to aid indications.............................................................................. 18
1.2.5.2 Biocompatibility................................................................................................ 19
1.2.6 Technical Limitations of use. ................................................................................. 20
1.2.6.1 Ageing.............................................................................................................. 20
1.2.6.2 Veneer cracking................................................................................................ 21
1.2.7 Clinical Contra-Indications of use........................................................................... 22
1.2.7.1 Veneer separation. ........................................................................................... 23
1.2.7.2 Veneer cracking................................................................................................ 23
1.2.7.3 Difficultyin adherence to the porcelain/ceramic veneer. .................................... 24
1.2.7.4 Workability....................................................................................................... 24
1.3 Aim of Investigation..................................................................................................... 24
2 Methods............................................................................................................................. 26
2.1 Type of study............................................................................................................... 26
2.2 Selection criteria.......................................................................................................... 26
2.2.1 Keywords:............................................................................................................ 26

4
2.2.2 Language.............................................................................................................. 27
2.2.3 Criteria for success rates evaluations..................................................................... 27
2.2.4 Date or time scale of publication. .......................................................................... 27
2.2.5 Mechanical properties selection criteria:................................................................ 28
2.3 Databases.................................................................................................................... 28
2.3.1 Search terms........................................................................................................ 28
2.3.2 Online Databases:................................................................................................. 28
2.4 Data collection and analysis.......................................................................................... 28
2.4.1 Study selection:.................................................................................................... 28
2.4.2 Data extraction:.................................................................................................... 29
3 Results................................................................................................................................ 30
4 Discussion........................................................................................................................... 32
4.1 Advantages.................................................................................................................. 32
4.2 Limitations. ................................................................................................................. 35
4.3 Future......................................................................................................................... 38
5 Authors conclusions. ........................................................................................................... 39
6 References:......................................................................................................................... 40

5
Hypothesis:The selection of Zirconiaasasubstructure material assureslongevityof the final
restoration.
1 Introduction
1.1 Zirconia Background
1.1.1 Discovery leading to use in biomedical science.
In 1789 the Germanchemist,MartinHeinrichKlaproth,foundthatheatinggemstones
produceszirconiaasa metal dioxide(AdvamegInc,2014). Inthe early1960’s the use of zirconiawas
introducedintobiomedical applications,firstlyusedasa replacementhipjointinorthopaedic
solutionswhere titaniumandaluminahadbeenpredominantlyused(Madfaetal,2014). Piconi
(1999) figuresthroughthe ResearchandDevelopmentdone byHelmurandDriskell inthe late
1960’s, furtheredthe expandingindicationsforzirconiainbiomedical applications.
Increasedinterestinmetal freedentistry,meaningthe replacementof metal usage with
materialswhichhave more biocompatible propertiesi.e.the use of non-toxiczirconiaceramic.This
has become apopulartopicin the dental communityoverthe late 20th
centuryand intothe present,
and has helpedleadtothe increaseduse of zirconiaasabiomaterial withspecificrelationto
dentistry(Manicone,IommettiandRaffaelli,2007). In-ceramzirconiabecame commerciallyavailable
indentistry in1989, by altering‘In-ceramalumina’withthe additionof 35% zirconia,to the slip,
producing‘In-ceramzirconia’(Vitadental technicians,nodate).Thenforuse ina CAD/CAM
manufacturingtechniquein2002, the In-ceramYZwas available (Vitadental technicians,nodate).

6
1.1.2 Chemistry and Structure.
Zirconiaderivations(Knovel,2008):
Zirconiaischemicallythe dioxideof the metal Zirconium, whichinturnderivesfromZircon.Zircon,
whichnaturallyoccurs, isoftenassociatedwithsilica(ZrSiO4) whichisagemstone,hence the
original findbyKlaproth.The lesspure depositsfromzirconare usedforstabilizedzirconiain
ceramicsand the higherpuritydepositsformzirconium.Zirconiumisproducedby:extractionfrom
zircon(mainly) andbaddeleyite byuse of chlorination;subsequentlyrefinedtoZrCl4;followedby
solventextractiontopurifythe material;finallyareductionprocesswithmagnesium, producingthe
Zr element.
Zirconiaisa polymorphicmaterial whichmeansthatatdifferenttemperaturesitsstructure will
change.There are 3 differentstructuresandwhentheyare understoodtheycanbe utilisedfor
desiredmechanical properties.
The forms are as follows(PiconiandMaccauro 1999):
1. At Tr-1170⁰C = monoclinicform
2. At 1170⁰C-2370⁰C = tetragonal form
3. At ≥2370⁰C = cubic form.
The abilityof zirconiatotransformintodifferent
grainstructuresis whatallowsforstrengtheningin
itssolidstate. Piconi andMaccauro (1999) mention
that the abilityof the tetragonal grainsto
transformcan be utilisedinapositive manner the
tetragonal grainsmentionedinFig1.1as
‘untransformedparticles’ come undertensile
stresseswhichresultinstress-induced
transformationtoughening.Whichiswhere the
Fig1.1: Particleinteraction around a crack
propagation (Piconi and Maccauro 1999).

7
tetragonal grainstransform,martensiticinnature, intomonoclinicgrains(‘transformedparticles’)
infrontof the crack, stoppingitfromfurtherpropagation.
Garvie (1972) goesintofurtherdetail tostate that the compressive stresseswhichare associated
withclosingthe crack are sufficientenoughdue toavolume expansion of ~3-5% whenthe grains
transform(tetragonal tomonoclinic).
1.1.3 Dental metal alloys:
The majorityof substructuresinfixedrestorationshave alwaysbeenmade usingametal of some
alloyingmixture.Here inFig1.2 showsa range of relevantdental alloysthatwouldbe usedas
substructurestofixedrestorationssuchascrownsand bridges.
1.2 Zirconia in Dentistry
1.2.1 Zirconia as a dental ceramic
In dentistrythere ismanyformsinwhichzirconiaisusedforfixedrestorations:
 Glass-infiltrated Zirconiumtoughened Alumina (Vita,nodate).
Whichis foundas ‘In-CeramZirconia’.
Thiscan be an alternative techniqueusedtoachieve densificationof ZirconiaToughened
Aluminaproductswithoutthe intensive graingrowth.Intensive grain growthleadstolarger
grainsdeemedundesirable,due tolowerstrengths. Indental ceramicsthe glassyphasescan
be significantlyimprovedbyabout25 – 50%, by a melt-glassinfiltrationprocesswhere the
ceramicobtained hasa homogeneousdefect-free microstructure andhasfavourable
compressive stressesforincreasedstrength.However,the glass-phase contentmust
Dental CastingAlloy
Types:
III IV Noble,goldbased
alloys
Highnoble,gold
alloys
Use: Crowns,short
bridges.
Long-spanbridges.
Yieldstrength: 201-340MPa >340MPa 300-520MPa 240-600MPa
Fig1.2 ‘Dental castingalloy types and their strengths’ (Sakaguchi and Powers, 2012).

8
controlled,astoomuch wouldweakenthe material.Thisprocessresultsflexural strengthsof
495.2 – 633.5±41 MPa (Zhanget al,2012). The glassinfiltrationprocesscanbe appliedto
zirconiawithotheradditivesdependingonthe mechanical propertiesrequiredof the
product. In Tinschertetal’s(2007) workin the lifetimesof zirconia-ceramicsforbridge work.
It was foundthatzirconiawithan aluminaoxide hadthe betterlong-termstrength.
 Yttrium cation-doped tetragonal Zirconia polycrystals.
3Y-TZP, found as ‘Cercon Zirconia’ (DENTSPLY International, no date) plus many other
producers due to its popularity.
It’s most often soft machined, and has a flexural strength of 1087±173 MPa. It’s highly
crystalline and is also known for having the highest fracture toughness of an all-ceramic
material (Sakaguchi and Powers, 2012).
It is produced using tetragonal zirconia polycrystals, with 3 mol% yttrium oxide as the
stabilizer.The stabilizerisneededtoensurethatzirconia remainsinitstetragonal grainswhen
cooled,anddoesn’ttransformto monoclinicgrainsuntil tensile stresses are appliedallowing
the transformation toughening process to occur.
Twokeypropertiesof 3Y-TZPare:Low porosityandhighstrength.These properties,andmore,
make it the most popular choice for application in dental restorations according to Zarone,
Russo and Sorrentino (2011).
 Magnesium partially stabilized Zirconia, Mg-PSZ.
Produced by clusters of tetragonal structured crystals in cubic stabilized matrix with 8-10
mol% stabilizer, magnesium oxide. However the maintained stability isn’t guaranteed long-
term,hence whymagnesium oxideisn’tthechoice overyttrium.Due toitslarge grainsizethe
followingpropertiesoccur: higherporositylevels;lowerdensityandlessstrengthagainstslip.
Furthermore with low dimensional stability and high framework wear. Generally Mg-PSZ is
not seen being indicated for frequent use (Zarone, Russo and Sorrentino2011).

9
 Shrinkage free ZrSiO4 ceramic.
Seen in the Kavo Everest system ‘Shrinkage free ZrSiO4 ceramic’ (Kavo, no date).
Flexural strength of 328.3MPa.
The ZrSiO4 ceramictestedin Binderetal’s(2005) study shows mechanical propertiesthatare
comparable tootherall-ceramicdentalmaterials.Full crownsmade fromthe zirconiaceramic
withstand masticatory forces found in the posterior area. While the material appears to be
reliable for clinical use for posterior full crowns, clinical assessment of these all-ceramic
restorations is required.
In conventional CAD/CAM machining of pre-sintered blocks there is significant shrinkage
occurring,soanenlargedamountofmaterialisneededforthe process.Whichcanbe resolved
by using ‘shrinkage free zirconia’ (Heydecke et al, 2007).
Dense, shrinkage-free ZrSiO4-ceramics.
Produced by a reaction-bonding process using; ZrSi2, ZrO2, and a polysiloxane, as starting
materials. Sinter shrinkage is compensated by the volume increase during oxidation of ZrSi2.
In addition, the use of a Si-containing so called low-loss-binder (PMSS) reduces shrinkage
further(Hennigeetal,1999). A denserceramicislesssusceptible tomicrobial attack Askeland
(2011), but harder to machine (Helvey, 2008).

10
1.2.2 Processing Zirconia into a substructure for use in fixed prosthodontics.
1.2.2.1 Slip castmethod.
Traditional wayof shapingceramicsaccordingto Bauew,Ritzhaupt-Kleissel,andHausselt
(1998) whoalsostateswhy thismethodisbyfar one of the mostpopular,due to the allowance for
controllingthe resultingmechanical properties.
The designedshape isformedinthe followingstagesaccordingtoSakaguchi andPowers,(2012):
 Throughcondensationandcapillaryreactionsthe liquidisremovedfromthe zirconiaslurry
(slip),whichcontainsthe fine ceramicparticlesinanaqueousstate.
 It isincrementallybuiltup,shapedandfinallysintered,whichiswhere amaterial isheated
up to a highenoughtemperature tocoherentlybindthe masswithoutbeingmelted.
 The producedsinteredporouscore isthenglass-infiltrated,bywayof capillaryaction
drawingmoltenglassintothe pores.
 Finallygeneratingtwointerpenetratingnetworks,acrystallineinfrastructure andaglassy
phase.
 The combinationof twostrengtheningmechanismsexplainswhyalumina-zirconiaslip-cast
ceramicsofferthe highestflexural strength andfracture toughnessof all slip-castceramics
strengtheningmechanisms(DenryandHolloway,2010).
1. The stress-inducedtransformationinzirconiagrainsproducescompressive stresseswithin
the transformedgrainsandsurroundingglassymatrix,aswell ascircumferentialtensile
stressesaroundthe grains,accompaniedbymicro-cracknucleation.
2. Crack deflectionisexpectedfromthe presenceof large aluminagrains.
In the processingstagesitisvital to control the phase transformationsforstrengtheningandso
there isno crackingwhen uponcooling.
VITA (2005) have issuedtheirlatestzirconiaslipproductcalled‘VITA In-Ceram’.Theyare usingan
aluminamatrix with~21% tetragonal zirconiaoxide,forstrengthening. Advantagesof thissystem:

11
‘noshrinkage duringglassinfiltration,the material iseasytoprocessinthe intermediate stage and
showsa highdegree of marginal accuracy afterstrengtheningbyinfiltration’.
1.2.2.2 CAD/CAM.
The use of CAD/CAMfor zirconiasubstructureshasbecome amore populartechnique asthe
Research& DevelopmentinCAD/CAMtechnologyhasincreasedinthe presenttime,and
demonstrates‘limitlessimprovements functionallyregardingform’ accordingtoKurtzman(2014).
Usingthis methodnegatesimpressionmaterialsand insteadadigital impressioniscreatedwith
intra-oral scanning.Thismoderntechnique canbe seentobe more accurate due to the removal of
shrinkage encounteredwhentakingthe impressionorusingthe Plasterof Parisforthe casted
models.ResearchconductedbyPelusoetal (2004) explainsintheirarticle that,Plasterof Paris
modelscanbe damagedand have ratesof shrinkage andexpansionof whichaCAD model would
negate.
Or the designonthe computercan be waxedupbya technicianandthen scannedintothe computer
or usinga pantographicdevice,likeinkeycopying,whichiswhere aresinpattern ismade andthen
copiedontothe computerformachiningand/orfurtherdesigns (SakaguchiandPowers,2012).
CAD/CAMindicatestwomainformsof processingceramics.
1. Soft-machiningwithpre-sinteredblocks:
Zirconiaisin the fashionof a ‘block’whichisonlypartiallysinteredbyamanufacturerand later
fullysinteredbythe dental laboratory,which producesarecorded900-1500 MPa flexural
strengthfracture toughnesswhichisgreaterthanall otherall-ceramicsystems (Sakaguchiand
Powers,2012). Thismethodhelpszirconiatoachieve ahigherflexural strengthaccordingtothe
theorybyTinschertet al, (2007) that sinteringaftermillinggivesthe resultof bettermechanical
propertiescomparedtothe denselysinteredcore (hardmachining).‘Pre-sintered’3Y-TZPblocks
are usedto make eitherasingle ormultiunitrestoration.The zirconiasubstructure isthen
veneered,withaporcelainof asimilarcoefficientof thermal expansion.Renishaw (2008) have a

12
systemcalled‘Incise’,the mainbenefitof the soft-machiningtheysayisthat it‘takesa fraction
of the time tomill’,comparedtohard machining.
2. Hard-machiningwithfullysinteredblocks:
Renishaw(2006) have a systemwhichisusedforhard machiningof dental substructures,called
‘Incise’ (differentbranchof productto softmachinedIncise),whichutilisesthe Y-TZPstructure for
transformationtougheningeffects.The use of fullysintered‘blocks’isproducedviathe ‘hotisostatic
pressing’methodwhichaccordingto3M(2008) producessurface defectsandresultsinlower
strength. Plushasbeenshowntocontaina significantamountof monocliniczirconia,whichis
usuallyassociatedwithsurface micro-cracking,highersusceptibilitytolow temperature degradation
and lowerreliability (Denryand Gandhewar, 2007). Additionally theyalsoconcludedthatdue tothe
highhardness,thereforelowmachinabilityof fullysintered3Y-TZP, sothe millingsystemhastobe
particularlyrobustwithspecialiseddiamondburredsystemsneeded.Howeveritwasfoundby(Kou,
Molinand Sjogren,2006) that followingthisprocessthe productiongave averysmoothfinishtothe
surface.
1.2.3 Clinical survival rates.
Throughreviewsof clinical papersonthe in vitro studyof zirconia-ceramicandmetal-
ceramiccrowns andbridges forcomparison. Fig1.5showsthe collationof successrates for such
restorations.Resultswouldindicate forthe crowndatathat zirconiaoverall isalesssuccessful
substructure overa longerperiodof time comparedtometal (alsoshowninfig1.6).Whereaswhen
zirconiaisthe substructure fora posteriorbridge theyoutperformedthe metal comparisonsshown
alsoin Fig1.7.

13
Device Material Success rate % Length of study Reference
Crown Zirconia-ceramic 94 4 (Roedigeretal,
2010)
Crowns Zirconia-ceramic 95 3 (Rinke,2014)
Crowns Zirconia-ceramic 94 3 (Raigrodski,
2004)
Crowns Metal-ceramic 94 12 (Zarone,Russo
and
Sorrentino2011)
and
Sorrentino2011)
and
Sorrentino2011)
and
Sorrentino2011)
Crowns Metal – ceramic 98 3 (Rinke,2014)
Posterior
Bridge.3 unit
(EndAbutment
Design)
Zirconia-ceramic 96 4 (Wolfartetal,
2009)
Posterior
Bridge.3 unit
(Cantilever
Design)
Zirconia-ceramic 92 4 (Wolfartetal,
2009)
Posterior
Bridge.3 unit
Zirconia-ceramic 91 3 (Cercon,nodate)
Posterior
Bridge.3 (and5
unit).
Zirconia-ceramic 100 3 (Saileretal,
2009)
Posterior
Bridge.3 Unit.
Metal-ceramic 83 5 (Sorrensenetal,
1998 )
Posterior
Bridge.3 Unit.
Metal-ceramic 74 5 (Kern,2005)
Posterior
Bridge.3 Unit.
Metal-ceramic 65 3 (Sorensonetal,
1998)
AnteriorBridge.
3 Unit.
Metal-ceramic 100 3 (Sorensonetal,
1998)
(Fig1.5: collatedsuccessratesof zirconiaandranging metal-ceramics).

14
(Fig1.6: comparisonof single crownrestoration,survivalrates).
(Fig1.7:Posteriorbridge comparisonbetweenzirconiaand metal-ceramicforsurvival rates)
75
80
85
90
95
100
105
Zirconia -
ceramic
Zirconia -
ceramic
Zirconia -
ceramic
Metal -
Ceramic
Metal -
Ceramic
Metal -
Ceramic
Metal -
Ceramic
Metal -
Ceramic
Survivalrates%
Crown types
Comparisonof Zirconia-ceramic toranging Metal-ceramic
crowns
0
20
40
60
80
100
120
Zirconia -
ceramic
Zirconia -
ceramic
Zirconia -
ceramic
Zirconia -
ceramic
Metal -
Ceramic
Metal -
Ceramic
Metal -
Ceramic
Survivalrates%
Bridge Types
Posterior Bridge comparison

15
1.2.4 Technical Indications for use.
1.2.4.1 Strength.
Fig1.3: Mechanical properties of zirconia and alumina based ceramics (Piconi and Maccauro,1999).
The flexural strength definedbyMeckolsky(1995) as the ‘final force requiredtocause
fracture and isstronglyaffectedbythe size of flawsanddefectsonthe surface of the material
tested.’ The fixedrestorationundergoesacomplex range of stresses,due tothe mastication,tongue
and cheek muscles(Wangetal,2013), therefore the indicationof how muchstressa material can
withstandiscrucial. The flexural strengthof zirconiawasfoundtobe ~ 900-1200MPa (Piconi and
Maccauro 1999) comparedto ~350MPa for lithiumdisilicate andonly~100-120MPa forglass+
aluminabaseddental ceramics(Yoshida,TsuoandAtsuta2014). Therefore zirconiacanundergofar
more ‘bending’stressesthanotherceramicmaterialsoutthere,howeveronlyzirconiaandlithium
disilicate wouldbe indicatedforuse asa substructure orfull-contouredrestoration. The flexural
strengthmustbe ≥100MPa as a requirementaccordingtothe ISOstandard 6872:2008 whenusing
zirconiainthe CAD/CAMprocessingroute (ISO,2008).
Anotherkeystrengthmeasuredisthatof the abilitytocontaina crack, therefore resisting
fracture, whichisa vital qualityof anydental material,ascracks can be causedin multiple waysbut
if keptcontainedthenthe material canremainclinicallyacceptable. Zirconiatestedagainstlithium
disilicate showeditwasupto ‘5x stronger’inthisfield(Wangetal,2013). Howeverlithium
disilicatescanbe indicatedforfull-coveragerestorationsinthe posterior,likethatof zirconia,and
veneersof whichzirconia cannotbe indicatedfor(Ferencz,2015).

16
The fracture load i.e.‘indicationof the material'sabilitytoresistrapidcrackpropagationand
catastrophicfracture’(Scherreretal,1998) whichfora zirconia-ceramiccrownis~804-1067MPa
(Stawarczyketal,2011) comparedto metal-ceramiccrownswhichcanonlybe shownto withstand
~228-349MPa (Kangetal, 2003). Owedto zirconiatougheningmechanismclosingthe opening
cracks.
Due to the strengthof zirconiaitwas foundthatthe coping/substructure canbe made
thinner,thanwithsubstructure metal materials,withthicknessessuchas0.4mm. Thisleavesmore
space for the aesthetical porcelainwhichneedsagreaterthicknessdue toitbeingaweakermaterial
(Sakaguchi andPowers,2012). Lithiumdisilicate canbe reducedto0.3mm but onlyforveneers,
therefore zirconiaat0.4mm isstill the thinnestsubstructure forfixedrestorationsincluding
posteriorbridges.
Ozcan and Vallittu(2003) has alsoreportedthatzirconiahadsuperiorstrengthcomparedto
otherall-ceramicmaterials.Howeveritwasalsoseen,thatzirconiaisdifficulttoetch,due to its
increasedhardness,whichcancause problemsinthe bondingprocess,hardmachiningorwhen
grinding.
1.2.4.2 Aesthetics.
Zirconia,foruse indental technology,isthe natural colourof teeth. Due topropertiessuch
as: ‘the grainsize isgreaterthan the lengthof light,highrefractive index,low absorptioncoefficient
and highopacityinthe visible andinfraredspectrum’accordingtoHeffernanetal,(2002). The
colourhas sufficientopacityasto mask out dis-colouredoral structuresbelowthe restoration
(Sakaguchi andPowers,2012). Whereasina metal-ceramicrestoration the opaque ceramic
preparationstagesare utilised tohide the metal underthe veneerporcelain.The veneerporcelain
that isadded have to be carefullyselectedsothattheydon’tshow throughto the underlying
preparation.Withzirconiawhichisaceramic shade already,nomaskingoutof underlyingcolouris

17
needed,leavingthe choicesof porcelainveneer,tocovera widerspectrumof shades,includingthe
more commonlydesiredhightranslucencyshades. Whilethe translucencyof zirconiaisnotas high
as that for dental porcelains,theiraestheticsare improvedrelativetothose of conventional core
materials(Zangetal,2012). This positive attitude isnotedthroughoutthe dental communityin
reviewpaperssuchas (Dangra and Gandhewar,2014) and companyliterature (ZirconiumCrowns,
no date) and(IvoclarVivodent,nodate).
1.2.4.3 Sandblasting and surfacetreatments.
RegardingFig1.4the sandblastedmaterialswere more resistanttowards
hydrothermal degradationthanpristineceramicsof the same chemical
compositionandgrainsize,indicatingthattetragonal andmonocliniczirconiagrainsinthe surface of
the sandblastedY-TZPhinderthe propagationof the diffusion-controlledtransformationduring
subsequentexposure toanaqueousenvironment,suchasisthe oral environment.Hence whythe
strengthinair and artificial salivawashigherthananyothertreatmentprocessedzirconia.
1.2.5 Clinical Indications for use.
Zirconia’srange of use or ‘indication’ has become widerthe researchingandtestingzirconia
has beencontinuedsincethe 1960’s to findproperties(suchasthose mentionedin1.2.3) that can
be utilisedindentistryandbeyond.
Indicationsforspecificrelationtofixedrestorationswhere zirconiaisthe substructure (all canbe
anterioror posteriorlysubscribed):
Fig1.4: Aging in zirconia
after differingprocessing
treatments (Kosmac et al,
2007).

18
1. Single Crowns.
2. Implantabutments.
3. Bridge 3-6 units.
4. Bridge – curvedand long-spanupto 48mm in length.
5. CantileverBridge.
6. Inlay/OnlayBridge.
1.2.5.1 Propertiesto aid indications.
Christensen(2007) reportsthat:
 Betteraestheticsthantypical metal-ceramicrestorations.
 Won’tdiscolouranynatural tissues,suchas silvercanhave a greeneffectonlocal tissues.
 The marginsof the restorationshave amore acceptable appearance thanthose of metal-
ceramicrestorationswhengingivarecedes,orincasesof thingingivathereforenodark
shadowingisseen,suchaswouldbe foundinmetal substructures.
 Gingival sensitivitytometal willbe eliminatedwithuse of zirconia-basedrestorations.
Winter(2012) foundthatzirconiaisbetterat maskingoutmoderate tosevere discolourationof
underlyingtoothstructure andbecause of zirconia’shighstrengththe substructure canbe more
conservative aszirconiacrownsneedlessspace,especiallycomparedto metal-ceramics,zirconia
crownscompare closerto a single goldcrowninthis laterrespect.
Zirkonzahn’s(nodate) use of zirconiahasfoundthattheyprescribe nocontra-indications
aboutpreparationtypes,especiallynotrulingoutthe knife-edge preparations.Whichallowsthe
dentistmore freedomof choice fordesignaspects.
Zarone,Russoand Sorrentino(2011) statesthat ‘Zirconiahasa favourable radio-opacity’,
whichisuseful in case of swallowingand when viewingthe clinicalplacementsof restorationsinthe
mouthviaX-ray.

19
1.2.5.2 Biocompatibility.
Accordingto Warashinaetal (2012) zirconiahasa lessphlogisticreactioninthe oral cavity
comparedto titaniumand othermetals.Scarano(2004) foundthatthe bacterial adhesion
comparisonbetweentitaniumandzirconiaresultedwiththe lattershowingabetterresultdue to7%
lessmicrobial adhesion,thiswasbackedupbyZarone,RussoandSorrentino(2011), whostatedthat
‘zirconiadoesn’tenhance bacterial adhesion’.AlsonotedbyAskeland(2011) whomconcludedthat
it isharderfor bacteriato adhere toa densermaterial.Throughtestingitwasevidentthatzirconia
had the highestdensityvalue (5.8g/cm3or99.8%) comparedto aluminaoxide porcelains
(3.98g/cm3) and alumina– titaniumrestorations(96%) accordingtoBeresnev,etal (2014).
Piconi andMaccauro (1999) alsofoundthat zirconiahasno cytotoxicaffects.The scientificreport
was conductedbyuse of fibroblastswhichwere culturedwithzirconiaandbyway of monitoring
themit wasnotedthat nosignificanteffectswere causedby the zirconiadeemingittobe classedas
‘biocompatible’.
Regardingthe gingival cellsresponseZarone,RussoandSorrentino (2011) foundthatzirconiaalso
has a lowcorrosionpotential,certainly comparedtothatof goldalloysthatmightrelease ionsinto
the oral environmentcausinginflammatoryreactions.
Furthermore zirconiahasthe smallestof,all restorative materials,for thermal aconductivity rating
whentestswere conductedincomparisontoa goldalloywhichhasa thermal conductivityvalue of
200 W/ (mK),zirconiais100x lessconductive (2W/(mK)) accordingtoCeramtec(nodate).This
meanswhenputin-situthe material will notabsorbthe heatfromfoodor drink shockingthe nerves,
as much as gold does,certainlyapositive inrespectto the patient.
HoweveritwasseeninVagkopouloetal’s(2009) researchthat significantamountsof alpha
radiationinzirconiabasedceramics(surgical implants) because of highlevelsof ionization,which
coulddamage cellsof oral tissues.Forgamma radiationconversely,the radiationlevelisnot
worrisome.

20
1.2.6 Technical Limitations of use.
1.2.6.1 Ageing
Agingor more specificallyzirconia’sdegradationat‘low temperatureisaprogressive and
spontaneousphenomenonthatisexacerbatedinthe presence of water,steamorfluids’according
to Volpato(2011), whichcan cause consequences suchassurface deterioration,microcracksand
decreasedresistanceinshortandlongterm. Hence whyit isof mostimportantinfixed
prosthodontics,due tothe lowtemperature andwetnessof the oral environment. Devilleetal
(2003) explainsthatif the zirconiacontentinthe composite,iskeptlow enoughthenthe micro-
cracks aren’tformedandthe percolationdoesn’toccur,whichmeansthatno ageingensues.When
zirconiaisat 6.7Vol%,ageingcan be avoidedinmedical appliancessuchasfixedprosthodonticsand
will still be slowerif below16vol%. Hence whyconcentrationsof zirconiamustbe consideredand
testedtoensure thatthe material doesn’tdegrade quicklyleadingtodevice failure. Additional
factors to be consideredare:microstructure patterns,porosity,residual stresses,andparticle size,
can affectthe ageingability(Chevalier,2006).
In an in-vivoacceleratedstudyover24monthswitha chemicallyaggressive wet
environment(suchisthe oral cavity),researchbyKosmac,JevnikarandKocjan(2011) foundthat the
‘naked’surfacesof the as-sintereddental Y-TZPare vulnerabletoT-Mtransformation.
There wasseento be an acceleratedageingprocessinzirconiacomparedto metal-ceramics.
(Zarone,RussoandSorrentino,2011). Howeverthiscanbe argued byKosmac andKocjan (2012)
whomundera 24 studynoticednostrengthdegradationof acceleratedin-vitroageing.
Chevalier(2006) foundthatadditives,suchasalumina,helpedbyslowingthe ageingof
zirconia,leavingthe desiredpropertiestobe perceived.Theyalsofoundthatthe ageingprocessis
acceleratedwhenmechanical stressandwetnessexposure isincreased.
In relationthentozirconiain companyliterature suchas‘ICE zirconia’ whichwasrecordedat
1400 MPa, showed anaverage strengthdegradationof ~30% (980 MPa) whichisa higherflexural
strengththanconventional metal evenpostdegradation(Zirkonzahn,nodate).

21
Usingthe colloidal processingroute i.e.powdermixing,itispossible toincrease the zirconia
concentrationwhichgivesgoodmechanical properties.Butnottoo high,sothat it isstill possibleto
avoidthe ageingphenomenaof grainsbunchingintoaggregates. Whichisadvantageous, twofold,
by keepingthe nanometre grainsize,the importantresidual strainsaftercoolingwillimprovethe
resistance tocrack propagation.Alsobyavoidingzirconiaaggregates,it’spossible toincrease the
zirconiavolume fractionsothatthe transformationtougheningbecomeseffectiveinthe material,
and therefore bothfurtherincreasingthe toughness. ItwasseeninthispaperthatZTA ceramicshad
a betterresistance toageingthanthe monolithic3Y-TZP(Devilleetal,2003).
Kosmac, Dakskobler,OblakandJevnikar(2007) investigatedthe agingof zirconia(Y-TZP)
withspecificfocusoncommonlyusedlutingcements.Itwasfoundthattheyabsorbedwatervia
dentine tubules,therebyexposingthe zirconiacore tomoisture which mayleadto agingproblems
overa shorterperiodof time thananticipated.
1.2.6.2 Veneer cracking.
Whenzirconiaisusedwitha veneeredporcelainthena strongbondisneededbetweenthe
twothat won’tundermine the strengthsof the twocomponentsi.e.nomatterhow greatthe
strengthof zirconiaor the veneeringporcelain, neitherwill matterif the bondstrengthbetweenthe
twois weak,asit will cause themtoseparate whichwill see the device fail.
Researchanddevelopmenthascreatedmodificationsystems tomake more successfulbonding
betweenzirconiaandporcelain,suchas:
 Primerswhichaidschemical retention(Zandparsaetal,2013) and (Griffinetal,2002) and
(YoshidaTsuoand Atsuta2014).
 Tribochemical coatingsusingbondingof silicaparticlestothe surface of zirconia,creating
heightenedroughnessforincreasedmechanical retention(Matinlinna,LassilaandVallittu
2007) and(Kernand Wegnera,1998) and (Ozcanand Vallittu2003).

22
 Silane couplingagentshelpbyincreasingthe wettabilitywill improve the bondingtopolar
surfacessuchas conventional porcelainceramics(Yoshida,TsuoandAtsuta2014) and
(Matinlinnaetal,2006).
 Acidetching,sandblastingandothersurface rougheninghelpstoaidthe mechanical
retention:(Zandparsaetal,2013) and (Cassuci etal, 2009) and (Kosmacetal, 1999) and
(Papanagiotouetal,2006) and(Pharket al,2009) and(Kosmacet al,1999) and(Curtis,
Wrightand Fleming,2006) and (Blatz,SadanandKern,2003).
1.2.7 Clinical Contra-Indications of use.
Zirconiais foundnotto be suitable all the time forevery fixedprosthodonticrestoration,regardless
of itsattributes, here are some examplesof that:
 Prosthesesrequiringprecisionattachmentsorstressbreakersare bestmade with metal-
ceramicrestorations (Christensen,2007).
 Veneers,onlaysandinlays(IvoclarVivodent,nodate) – zirconiaasa monolithicstructure
couldbe indicatedforthese restorations(Glidewell Labs,2015).
 The cost of zirconia-basedrestorationsis oftenhigherthanthatof metal-ceramic
restorationswhenseenindentaljournalssuchasChristensen’s(2007) howevercompany
literature suchasZirkonzhan(nodate) almostagreesstatingthat‘Zirconiarestorationsare
equal toor marginallymore expensive thanmetal-ceramicrestorations.’
 HeavyBruxismwasresearchedbyPerry etal (2012) and foundthatdue to the overlay
porcelainsweakness, agoldalloycrown wouldbe more clinicallysuccessful–howevera
metal-ceramicrestoration cansustainlessstressthanthe zirconia,so they wouldn’tbe
recommendedforanybruxistpatientseither.

23
1.2.7.1 Veneer separation.
Sakaguchi andPowers (2012) referstoissues post-production, causedbyunrecognisedCTE
differencesinearlierproductionstagesof azirconiacrown.It was shownthatthe twolayers,
zirconiaandveneeredporcelain,canseparate leavingmicro-gapsforpenetrationfrombacteria,
where theycan multiply incoloniesandhide awayfromthe mechanical removal suchas brushing.
The separationisalsoof majorconcernfor the strengthwhichcouldcause loosening,complete
separationof the twoparts leadingtofailure andpatientdissatisfaction.The separationcouldeven
be dangerousif leadtomaterialsbeingswallowed. Thiscanalsobe causedby othermechanical and
processingtechniques,suchassandblastingandgrinding done bytechniciansordentists.Causing
surface phase changes to the grainstructure because stresses onthe surface particles,whichare
opento attack unlike the protectedunderlayerof tetragonal grains, are then‘free’to transform
(Sakaguchi andPowers,2012). These structural changescan leadto the internal stressesseveral
micronsbelowthe surface waitingtoformcracks andlater cause failure of the material (Piconi and
Maccauro 1999).
1.2.7.2 Veneer cracking.
Veneercracking/chippingwasinvestigatedbyAugustin-Panadero(2014) whofoundthat
zirconia-ceramicrestorationsfailedinthisway‘between 6% - 15% overa 3 – 5 year period,whilefor
metal-ceramicsthe fracture rate rangesbetween4% - 10% overtenyears’. Sailer(2009) foundthat
minorchippingof the ceramicveneerof zirconia-ceramicrestorationswas25% comparedto 19.4%
of the metal-ceramiccrowns. Bothcrownsand bridgesinRekow etal’s(2011) study,showeda
degree of chipping,butthe size wasmuchgreaterwithzirconiacoresshowing 25% comparedto
metal-ceramicbridgeswith19.4%chippingrate. Zarone,RussoandSorrentino(2011) andTinschert
et al’s(2007) workmentionsthatoverloadingandfatigue inthe clinical settingcouldbe another
factor forveneercracking. Ingreaterdetail explainedbyLee etal (2000), the accumulating
microcracks,fromloadinginan aqueousenvironment(suchasoral cavity),causessurface defects
that may enhance tensioninareasof localisedconcentration,initiatingfracture underlowlyapplied
stresses.

24
1.2.7.3 Difficulty in adherenceto theporcelain/ceramicveneer.
It has beenseenthatusingconventional lutingagentstobondthe zirconiasubstructure to
the ceramic veneer,isshortlived.Thisissue wasfoundinMagnea,ParanhosaandBurnett’s(2010)
work. Whichhas helpedtofurthershowwhy substantial researchanddevelopmentintoluting
agentsfor a sufficientbondtobe made iscrucial indevelopingthe workinglife of arestoration.
1.2.7.4 Workability.
Due to zirconia’shighlevel of hardness,there are laterissuesinthe processingsuchasat the
chair side inthe dental practice,whichcanmeanthat dentistsmaybe lesskeentowantto work
withzirconiadue toextratime or the needforextrastrengthenedtools(likethatof the more
expensive diamondburs) tobe able tosuccessfullygrind(Cehreli,KokatandAkca2009). Helvey
(2008) foundthisto be true in retrospectof the time ittakesto grinda conventional metal
substructure.
1.3 Aim of Investigation.
In answeringthe hypothesis throughthoroughcollationof informativedata, finally
concludingonwhether‘The selectionof Zirconiaasasubstructure material assureslongevityof the
final restoration’inspecificregardtofixedrestorations.
Longevityof the device canbe seenasperformingeithertoa desiredlengthof time or, inthiscase
specifically,surpassingthe average workinglife of otherrestorative materials.Thiscouldbe seenas
a bettermarkerfor whetherzirconiaisinfactthe correct selectionornot.
The longevityof adevice inthe oral environment,isdeterminedbythe mechanical andchemical
wearit undergoesona constantto frequentbasis.The device failure canoccurin manywaysfor
instance:ina mechanical sense i.e.breaksorisweakened;the device couldbecome less
aestheticallypleasingtothe patient,i.e.discolouredbywatersorptionorcouldalterthe colourof
the oral tissues.

25
To gauge a ‘benchmark’for Zirconia,itmustfind whatdental devices are indicatedwithzirconia
substructures,sothatthe comparisontothe conventional/othermaterialsused canbe assessed,
plushowlongthose materialssurvive before theyare deemedinadequate fortheirjob.
To fullyanalyse whyzirconiaisoutlivingorantagonisticallynot,zirconiaasamaterial needstobe
understood,sothatthe propertiesandstructure of the material can be relatedandusedto answer
for the foundlongevityof zirconiaina fixedrestoration.

26
2 Methods.
2.1 Type of study.
Thisis a systematicreview.Informationwassearched,collatedandreviewedincomparison
to otherfindingsonsimilarorcontrastingworkforanalysis.
2.2 Selection criteria.
The methodsearchingforspecificinformationwasthatof ‘BOOLEAN’,thissimpleaddition
of a ‘code word’i.e.‘AND’,‘NOT’,‘OR’,meantthatthe searchcouldbe specifiedandtherefore
wouldreduce the resultsfound.
2.2.1 Keywords:
 Zirconia
 Substructure
 Limitations
 Indications
 Contra-indications
 All-ceramic
 Fixedprosthodontics/restorations
 Fixedpartial denture
 Crowns
 Bridges
 Mechanical properties
 Aesthetical properties
Whensearchingforrestorationsthatzirconiacouldbe indicatedfor,the term‘fixedpartial
denture’(inAmericanpapers) keptbeingused,whichwasfoundtomeana ‘bridge’inthe British
terminology.Therefore itisnecessarytoinclude thisterminmysearch,as theirfindingsare justas
validas British work,butmighthave beenexcludedfromresultsif notincluded.
 For thismethodIusedthe ‘thisOR that’ searchmethodi.e.‘bridge’OR‘fixedpartial
denture’.

27
2.2.2 Language.
OnlypapersinEnglish,were used,nomethodof translationwasusedbyme asit may
translate wrongandthat wouldgive me invalidresults.
 I didcome across a paperthat waspublishedabroadinanon-Englishspeakingcountry,but
it hadbeenprofessionallytranslatedforEnglishreading.Therefore thiswasacceptedinmy
results.
2.2.3 Criteria for success rates evaluations.
There were nobiasestowardsage,genderorrace in myreview of clinical successrates,and
these papersfindingswere myonlyneededscientificpublicstudy.The restwere all mechanical
propertytestingstudiesof zirconia.
Two specificexclusioncriteriaImade was:
 Animal studies,onthe basisof differingpropertiesneededforanimal e.g.dogoral
healthcare overhumanoral healthcare.
 Anystudiesthatwere a mix of crownsand bridgesetc.weren’tused,astheirfindings
weren’tspecificenoughtoeachcomponentseparatelye.g.‘9yearstudy,Crowns+ bridges,
52.66% - mainissue foundwaschipping’,here the wordmainissue istoovague for a
scientificstudy.
2.2.4 Date or time scale of publication.
Anyinvestigationlessthan3yearswas excludeddue toshorttime span,deemedinsufficient
for a longevitystudyandtherefore fortrue validresultstobe extrapolated.
Whensearchingfordefinitionsof ageingforzirconia,Ifoundsome oldsourcese.g.1949. Thiswas
deemedtoooldforzirconiaspecifically,muchmore relevanttoasimple metal definition,butas
zirconiawasn’tunderstoodnorutilisedinthatperiodIexcludedthatfrommyresults.

28
2.2.5 Mechanical properties selection criteria:
I beganto findstudiesrelatedtozirconiaasa claddingmaterial infuel cellsforradioactive
uses.Althoughthe material isthe same Iwantedtoexclude thisfrommysearch,as theywill alter
zirconia’spropertiesforhighheatneedsoverwhatisexpectedinthe oral environment.
2.3 Databases.
2.3.1 Search terms.
Whensearchingfora specifictopicinmind,the use of double quotationmarks(“…..”) tells
the search engine thatyouwantto findresultswiththose specifictermsorphrases.Thishelpsto
make the searchingmuchmore efficient.
2.3.2 Online Databases:
MMU librarydatabase searchesthroughthe Dental journalswe have accessto,andall books
that are available foruse tous.
PUBMEDwas usedonoccasionto go directto a source if the MMU librarysearchturnedupa paper
but couldn'tgetthroughto the full text– passwordswere suppliedformembershipforfull accessto
journals.
Google scholarwasalsoa useful resource forthe same reasonasPUBMED, butalso widensthe
searchrange comparedto MMU library,as longas the full textwasviewableeitherforfree orwitha
givenpassword.
Google searchengine wasusedtofindcompanyliterature.
Journalsreferencelist/ booksreferencelist. If more informationthanthatgivenbythe paperbeing
readwas required,thenfindingthe paperinquestionandmakingareview wasquite agoodsystem.
Howeverthismethodonlyworksforextrainvestigationof similardata,butfor a deeperanalysisof a
subjectitwouldbe recommendedtouse a broadersearch,i.e.the online databases.
2.4 Data collection and analysis.
2.4.1 Study selection:
Once paperswere foundfroma database search,the papersthenare openedonline.If
there can’tbe accessedinfull thentheyare disregardedwithoutview,asinformationwasnever
takenmerelyfromanabstract.

29
The paper abstractsare reviewedtoensure thatsufficientqualifiedinformativedatawasincludedin
the paperand that it wasrelevanttomy inclusionof search,andif sothe paper wasthensavedas a
bookmarkforthoroughreading.Once all papersof that searchresults/enoughpaperswere found,
thenthe reviewandnotingprocesstookplace.
2.4.2 Data extraction:
All publishingpaperdetailsi.e.author/s,date,journal title etc.were notedforthemtobe
writtenuplaterinthe formof ‘MMU Harvard referencing’.
Thena readoverof the journal itself,beforegoingbackoverforfurtherdeepernotingof theirand
othersreferencedwork.Thisiseasiestdone whenafirstreadis done,tofindareasof interesttothe
topicin hand.

32
4 Discussion
To criticallyappraise the selectionof zirconia asa substructure infixedprosthodontics, the
hypothesis mustbe eitherprovedordisproved.Sofarthe history,background,use andlimitationsof
zirconiaindentistryhave been analysed,buttowhatextentdotheyimpactthe longevityof the final
fixedrestorationi.e.crownorbridge?Throughoutthe literaturereview ithasbeenexpressedthat
zirconiahashighstrength,associatedwithitsowntougheningmechanism, thereforesuggestingthat
it will succeedandlastlongerthanweakerconventionalsubstructuresinfixedprosthodontics.
However,the strengthof zirconiamaynotbe able to outweighthe veneercrackingandseparation,
difficultyof bonding,andworkabilityforthe technicianandthe dentist.
4.1 Advantages
One of the greatadvantagesfor the use of zirconiain dentistryisbecause of itsnatural
tooth- like aesthetics.Plusitcanhave a sufficientopacitytomask-outunderlyingstructuresandwith
companiessuchas IvoclarVivodentreleasingnew opacityandtranslucencyranges,the aesthetics
are improvingconstantly(IvoclarVivodent,nodate).
In additiontoaesthetics,the mainmechanical propertythatzirconiapossessesisthe
‘transformationtoughening’effectwhichliesdormantuntil needed.If zirconiawaspure andcooled
slowlythenitwouldtransformfromcubictotetragonal andfinallytomonoclinicatroom
temperature (PiconiandMaccauro, 1999). However;thisisnotpracticallyseen,asitcoolstoo
quicklyandthe mostcommonlyusedzirconiasare dopedwithastabilizere.g.3mol% yttrium(3Y-
TZP) to ensure thatthe tetragonal grainsremain.Withthe tetragonal grainsremainingatroom
temperature then,whentensile stressesare applied,suchasthat froma crack, it isclosedwiththe
counteringcompressive stress(3-5%expansionaccordingtoPiconi andMaccauro, 1999) as the
tetragonal grainstransformintomonoclinicgrains.Thistougheningmechanismcanbe realised
whensandblastingmodificationsare done,whichremovesufficientlayerstoexpose the underlying
TZP (Kosmacetal 2007), thismachininggave the outcome of 100% survival rate inair and artificial
salivasolutions,comparedto50% survival rate ina control sample of as-sinteredzirconiastructure.

33
The strengtheningeffectcanbeenshownasthe fracture toughnesswhichforTZPincomparisonto
aluminais3-6KIC greater(Piconi andMaccauro, 1999), showingthatimpactfrom the opposing
dentitionislesslikelytoaffectthatof the TZP restorations,hence theirindicationforposterioruse
where the masticationisstrongest.ThiscanbeenshowndiagrammaticallyasinFigs1.4 and 1.6 that
the use of zirconia-ceramicbridgesinthe posteriorconstantlyout-performedthe metal-ceramic
bridges.
Due to thisincreasedstrengththe substructure itself canbe thinner(0.4mminsteadof >0.5mm for
conventional metal-ceramics) allowingforthe porcelain,whichis weaker,tobe veneeredinlarger
quantities(SakaguchiandPowers,2012) or kepttothe same ratioallowingforthe zirconia
restorationtobe indicatedforsmall occlusal cases.Plusif the porcelainisallowedmore space,this
givesextraroomfor the aesthetical appearancetobe perfectedi.e.viathe porcelainlayering
technique.
Wang et al (2013) alsoconfirmedthishighstrengthwiththe quantifiable dataof fracture resistance
whichforzirconiawas foundtobe ‘5 timesgreaterthanthat of lithiumdisilicate’.Inaddition,the
flexural strengthvaluescanbe seentobe up to andabove 1GPa forTZP (Piconi andMaccauro, 1999)
comparedto that of lithiumdisilicate,alumina’sandglassaluminas whichall can’texceed 500MPa.
These comparisonsare crucial as the flexural strengthiswhatindicatesthe level towhichthe
material inloadingconditionscanwithstandcrackswithoutfracture.
Anotherpropertyof zirconiawhichindicatesthe density,comesfromthe sintering
processes’,thispropertycanhave greatbiocompatibilityeffects,relatingtozirconia’sincreaseduse
inmetal-free-dentistrywhichisbecomingmore popularasmetalsare seen asold fashioneddue to
corrosive elementsandlessaesthetical appeal.Literature thatsuggeststhis, suchasScarano(2004)
whoprovedthat zirconiahad7% lessmicrobial adhesioncomparedtotitaniumsubstructures,which
can correlate withBeresnevetal (2014) who showedthatthe greatestdensitywaszirconia(99.8%)
comparedto alumina-titanium(96%).Zirconiaalsoproducedthe leastphlogisticreaction,showing

34
lesstissue cell interaction,comparedtotitaniumandotherconventional metalsforsubstructure use
(Warashinaetal,2012).
Otherreasonsforzirconia’sincreaseduse couldbe due tothe move intomodernprocessing
techniques,withcomputerisation,we seethe increaseddevelopmentof milledmachinedzirconia
substructures.Kurtzman(2014) foundthatwiththe use of CAD/CAMthe formsand functionalityof
fixedprosthodonticsislimitless.Suchasthe possibilityfornegationof likelyshrinkageinimpression
and model castingstages(Pelusoetal,2004) isleadingtowardsthe indicationsof use.However;this
can be outweighedbythe factthat the final productionof asubstructure,shrinksby20-25%
(Komine,BlatzandMatsumura,2010) viasoft-machining,therefore needinganover-sizeddesign,
and if thishas beenproducedwithwax/hardresin,thiscanbe seenaswasteful.Thenagainwiththe
increased R&Dintothe CAM area, companiessuchas Kavo(nodate) have createda product to
reduce saidissue e.g. shrinkage free(KavoEverestsystemof ZrSiO4 ceramics) therefore resolving
that issue,andtheirresultscanbe backedup byresearchfrom Heydecke etal (2007) and the use of
‘dense shrinkage freeZrSiO4
ceramicsby(Hennige etal,1999). At the same time as the development
of the CADarea, databasesof anatomical morphologybecomemore accurate andare stored
reducingthe demandforanypreparationworkto be done manuallysuggestingthatinfact this
methodismore conservative asnoproductsare lostcomparedto the conventional methodof wax-
ups,burnoutsand castingof metalswithsprues.

35
4.2 Limitations.
The hardnessof zirconiaisaccentuatedwhenmilledinthe hard-machiningprocess,due tothe fully
sintered state of the zirconia‘blocks’.Whichmakesforatoughersurface leadingtohigherratesof
tool wear,and forthe technicianthiscanmake foran expensiveprocessingtechnique,in
comparisontosoft-machiningwhichusespre-sintered‘blocks’,andlatersintering(Kou,Molinand
Sjorgen,2006). Regardingthe type of millingused,the hardmachiningapproachwhichhasahigher
tool wearrate (Denry,2007) leadingtomore cost ontoolshas beenshowntobe a popularmethod
whichcouldbe due to the use of the systembutalsothe manufacturerssalestechnique i.e.
Renishaw (whomakeshardandsoftmillingmachines) who have ‘spun’the negative,of highertool
rate lossintoa positive bysayingthattheirmachinerywill measurethe lossof the tool during
processingandcompensate forthisinthe amountthat isremovedfromthe machinedproductso
that itremainsaccurate to itsdesign(Renishaw,2006).Yet the consumer,be itthe technician,
dentistorpatient,will be payingmore thanincomparisontosoftmachining.Howeverusingthis
methodthere isnoshrinkage thatthe technicianwill have tocompensate for,asthe blockwasfully
sinteredbeforehand(3M,2008).
Thislevel of hardnesscanalsohave laterimplications,inwhichevermethod the zirconia
substructure ismachined,asthe dentistoftenhastodo minorbutnecessaryadjustmentswhichmay
nowonlybe possible withthe use of highstrengthexpensivediamondburs.Plusthisgrindingwill
take more time than the conventional metal-ceramics(Helvey,2008).
In the same methodmentionedbeforebyKosmacetal (2007) whengrindingthe surface,theyfound
that thismachiningprocesscouldintroduce andincrease the surface flaws. Itissuggestedthatwhen
surface modificationsare beingmade tothe zirconiasubstructure,the manufacturer’sinstructions
are followedtocontrol the effectsthe technicianhasonthe materialsstructure

36
The T-M transformationhowevercanbe detrimental tothe substructure if certainfactors
are notcontrolled. Grindingof the substructuressurface canincrease the rate of ageingof zirconia,
broughtabout bywaterdiffusionin the created cracks.Kosmac(2007) foundthatY-TZP with
commonlyusedlutingagentsshowedageingproblemsoverashorterperiodof time thaninmetal-
ceramicsbondedsimilarly.The waterwasfoundtobe absorbedbythe dentine tubulesandthen
contact withthe zirconiasubstructure.InresearchbyKosmac,JevnikarandKocjan(2011) and
Zarone,Russoand Sorrentino(2011) whomall foundthatinaccelerated in vivo studies(with
chemicallyaggressive wetenvironments,suchasthe oral cavity) there wassignificantageing
degradationcomparedto metal-ceramics.Howeverthese studiesdonotallow forthe fact that the
zirconiasubstructure istobe veneeredwithporcelainwhichactsas a barrierbetweenthe wet
environmentandthe zirconia,therefore suggestingthatthese findingsare lesslikelytooccur in
practice. The companyliterature of Zirkonzahn(nodate) whoproduce ‘ICEzirconia’state thateven
withan average degradationof 30% (420MPa) to flexural strengthfromageing,theirzirconiawill
still be at a higherlevel (980MPa) thanthat of conventionalmetalsfor metal-ceramic’s.Itwasstated
by bothChevalier(2006) and Devilleetal (2003) that withthe additionof aluminatothe zirconia
substructure there will be sufficientlylessageingi.e.aZTA ceramic.
Ageingisn’tthe onlyfactorforlimitingthe longevityof zirconia.Inathoroughreview of
clinical studiesonzirconia-ceramicandmetal-ceramiccrownsandbridges,itwaseasyto see that
zirconiaisoftenout-performedbythe longevityof metal-ceramiccrownrestorations(asseeninFig
1.4). In fact the meanresultsof those zirconiarestorationswas94.5% survival rate overa year range
of 3-4 comparedto the metal-ceramicstudieswhichrangedupto11 yearsand demonstratedand
average of 95% (howeverthere were more metal-ceramicstudiestobe used,soa more valid
average wasfound,thatdid includedone resultof muchlowervalue thatbroughtthe average down
by 2.5%).The newerzirconiasystemswill needtobe testedusinglonger studiestobe as validasthe
metal-ceramicstudies,butforthattime isneeded.

37
The limitedlongevityof zirconiarestorationshasseentobe linkedtothe failure of the restoration as
a whole unitnotnecessarilythe failure of the zirconiasubstructure.Therefore the failure reasonsfor
the veneeringceramicsmustbe reviewed.One of the mostcommoncausesisveneer
cracking/chippingwhichcan have multiple reasonssuchas:incorrectCTE matchingof porcelainto
zirconia,porosities,improperframeworksupport,unnecessarilyhighfurnace temperatureswhich
will increase the tensilestressesresiduallyleftinthe porcelainveneerafterfiring.Lastlydue tothe
complex nature of zirconia’schemical compositiononitssurfaces,bondingbetweenthe porcelain
and zirconiaor at the laterstage of cementingthe all-ceramiccrowntothe abutmentinthe mouth
may nothave beensufficient(Zarone,RussoandSorrentino,2011) and (Tinschertetal,2007).
Austin–Panadero(2014) foundthatthisresultedina ‘6%-15% failure rate overa 3-5 yearperiod
comparedto that of metal-ceramicswithafailure rate of 4%-10% over10 years’.
Veneerbondinghasreceivedaplethoraof attention,asitmay be a solutiontoincreasing
zirconia’slongevityasafixedrestoration.Due tothislimitationof zirconia,therehasbeenmuch
recentResearchandDevelopment whichhasbeenlinkedwiththe R&Dfromthe 1980’s into
improvingthe adherence of zirconiaasa biomaterial,butnow focusingthatattentionondental
appliancestosee if theycanimprove the bonding,whichshouldincrease the lifespanof the
restorationasa result.If the veneerbondisstronger,thenthe strengthof zirconiacanbe realised
otherwise the separationof the twocomponentswill certainlyleadtofracture of the porcelain,
whichleftunnoticedwill become abacteriatrapaccording to Sakaguchi andPowers(2012).
Airborne particle abrasion(APA) isone of ahighlyconsideredmethodof raisingthe wettabilityand
shearbondstrength(SBS) of the zirconia’ssurface forcementing(Zandparasa,2013) and (Cauca,
2009). Kosmac (1999) suggeststhatitalso inducesthe T-Mtransformation,increasingflexural
strengthandSBS, butif overworkedthe processwill introduce micro-cracks(Curtis,2006).Research
by Phark(2009) provedto showthatAPA in fact couldendupsmoothingthe surface of zirconiaas it

38
was suggestedthatzirconiabeganwithmoderate roughness,whichwouldseemtocounteractthe
needforimprovedSBSinthe firstplace.
Therefore alternative researchof chemical surface alterationsshouldbe considered.The use of silica
whichattacks anddisplacesthe hydroxyl groupsoverthe surface,the tribochemical coatingmethod
whichincreasesthe roughnessformechanicalattachment(Kern,2006). Backedup byMatinlinna
(2006) whodescribesthe surface tensionaslowered,increasingthe surface energy,makingthe
zirconia(oftenY-TZPorY-PSZ) hydrophilicforattachmenttothe hydrophobicresin.
4.3 Future.
Withzirconiabeingrealisedforitsstrengthpotentialsandaestheticstoo,the usesof
systemssuchas ‘ZenostarFull ContourZirconia’(IvoclarVivodent,2013) may become more widely
indicatedbecause itremovesthe needforbondingbetweentwostructuresi.e.substructure and
veneer.Due to itbeingshownthat veneeringporcelainsare one of the mainissuescausingfailure of
zirconiaall-ceramicrestoration.Additionallythe strengthof zirconiacanbe utilisedforcaseswhere
occlusal space islimited.
As mentioned beforeanyongoingclinical studiesreporting survivalratesforzirconiawill be
more validforcomparisonto othersubstructure materialsused,astheywill have beenin-situfor
longerperiodsof time thanthose usedforcomparisonandanalysisnow.
High-strengthporcelainssuchaslithiumdisilicate (IPSe.max CAD),seenreportingaround
400MPa flexural strength(Kang,Chang,Song,2013) demonstratingahighstrengthevencompared
to conventional metalsubstructure valuese.g.dental castingalloystypesIIandIV (yieldstrengthsof
>201MPa). Therefore there isdefinitereasonforindicationforthe same use as the metal
substructuresandveneeringe.g.crownsandlongspanbridges,additionallythe aestheticsof lithium
disilicate are betterthanthatof zirconiadue to hightransparency. Sothe use in veneeringtothe
zirconiasubstructure maythenreduce the veneercrackingproblemswhichhave causedsuch

39
detrimental resultsforlongevity.Howeverlithiumdisilicatemaybecome more widelyusedonits
own,as it can be indicatedforposteriorfull-coveragerestorations,with asmall thickness of 0.3mm
whichisgreat fortight occlusal spaces,whilstmaintainingthathighstrength(Ferencz,2015) plus
lithiumdisilicate canbe indicatedforveneers,somethingthatzirconiacannot.
5 Authors conclusions.
Throughthe literature searchit isevidentthatzirconiaisbeingusedmore infixed
prosthodontics,especiallyasitcan be seentobe a more successful indicationforposteriorbridges,
due to the transformationtougheningmechanismpossessed,opposedtothe use of the
conventional metal-ceramicmethod.Pluswiththe increasingutilisationof CAD/CAMsystemswhich
helppromote the use of zirconiafurther.Alongwiththe aesthetical toothsimilarityfoundinthese
all-ceramicrestorationswhichare beingpushedforwardasthe metal-free dentistryalternative.
Howeverthe crackingand difficultyforbondingof the veneeredporcelainstothe zirconia
substructure,especiallyinacrown situation,iscausingissuesforlongevityresults.Yetwiththe
researchand developmentintothese fields,increasinglyfindingnew methodsfor achievingbetter
‘shearbondstrength’coupledwithusingahigherstrengthporcelainsuchaslithiumdisilicate,could
helpthe zirconia-ceramicrestorationstoimprove theirsurvival ratesandsurpassthose of the metal-
ceramics.
At the presenttime the literatureshowsthat zirconiacannotassure the longevityof fixed
restorationsasa whole entity. Until the veneeredporcelainsandbondingstrengthsare improvedor
negatedfromuse i.e.the indicationof fullcontouredzirconiacrowns,instead.

40
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