Resistance of endodontically treated roots restored with different fibre post systems with or without post space preparation: in vitro analysis and SEM investigation.

ORIGINAL

ARTICLE/ARTICOLO

ORIGINALE

Resistance

of

endodontically

treated

roots

restored

with

different

fibre

post

systems

with

or

without

post

space

preparation:

in

vitro

analysis

and

SEM

investigation

Resistenza

di

radici

trattate

endodonticamente

e

restaurate

con

sistematiche

diverse

di

perni

in

fibra

con

o

senza

preparazione

del

post

space:

analisi

in

vitro

e

al

SEM

Daniele

Angerame

*

,

Matteo

De

Biasi

,

Mauro

Cattaruzza

,

Vittorio

Franco

,

Gianluca

Turco

,

Julia

Filingeri

,

Fernando

Zarone

,

Roberto

Sorrentino

a

UniversityClinicalDepartmentofMedical,SurgicalandHealthSciences,UniversityofTrieste,Trieste,Italy

b

PrivatePractice,Pordenone,Italy

c_{PrivatePractice,Rome,Italy} d

DepartmentofNeurosciences,ReproductiveandOdontostomatologicalSciences,University‘‘FedericoII’’of Naples,Naples,Italy

Received8June2016;accepted13September2016 Availableonline18October2016

KEYWORDS Fibrepost;

Fractureresistance; Postspacepreparation;

Abstract

Aim: Tocomparethemechanicalresistancetofractureoftwoconicalpostsystemsplacedwith nopreparationoftherootcanalwiththatofdoubletaperfibrepostsseatedinendodontically treatedsinglerootsafterstandardpostspacepreparationusingdedicateddrills.

PeerreviewunderresponsibilityofSocieta` ItalianadiEndodonzia.

* Correspondingauthorat:UniversityClinicalDepartmentofMedical,SurgicalandHealthSciences,PiazzaOspedale1,I-34125Trieste,Italy. Fax:+390403992665.

E-mail:d.angerame@fmc.units.it(D.Angerame).

Availableonlineatwww.sciencedirect.com

ScienceDirect

jo u rn al ho m e p ag e: ww w. el s ev i e r.c o m /l o ca t e/ gi e

http://dx.doi.org/10.1016/j.gien.2016.09.006

Introduction

Overthelastdecade, thefailureofseverely compromised teethhasbeenreportedtobedramaticallyreduced1,2thanks toprogressinmaterialengineeringandrestorative techni-ques.Themostfrequentcauseoftoothweakeningistheloss ofdentalstructureduetodecay,traumaandcavity prepara-tion,inparticularincaseswheretheintegrityoftheroofof the pulp chamber is lost.3 Furthermore, it is generally acceptedthat thelossof enamel anddentineis themost criticalfactoraffectingtheretentionoftherestoration.4,5 Themechanicalresistanceoftherestoredtoothis influ-encedbytheamountofresidualtoothstructure,6the proper-tiesoftherestorativematerials,thepresenceofaferrule,7 theforcepatternanddistribution,aswellastheocclusionof thepatient.4,8Theliterature hasnotclarifiedyet whether theendodontictreatmentbyitselfcould weakenatooth,9

butitcertainlydoesnotstrengthentheroot.10Theremoval ofintra-canaldentinecouldaffectthedeformabilityofthe root; hence, it is conceivable that the more invasive the endodontictreatmentis,as inthecase ofpostspace pre-paration, the less stable and resistant the root will be.11 Consequently, minimally invasive treatments are recom-mended in both the preparation of the access cavity and intheshapingoftherootcanalsystem,11inordertoavoidthe detrimentalremovalofsoundtoothstructure.

To restore a tooth affected by a critical loss of dental structure,theuseofoneormorepostsisusefultoimprove the retentionof the restorative material. The mechanical propertiesofthepostsareparamountforthedetermination of possible fractures of the restored tooth, and the post systemwasprovedtohaveasignificantinfluenceonfracture resistance.2,12_{Someauthorshavereportedthatthepresence}

ofafibrepostenhancesthemechanicalresistancetofracture Preservationofroot dentine; Self-adhesivecement; Taper. PAROLECHIAVE Pernoinfibra; Resistenzaafrattura; Preparazionedelpost space;

Risparmiodidentina canalare;

Cementoautoadesivo; Conicita`.

Methodology: Thirtyfibrepostswithdouble(G1,n=10,DTLightPost)andsingletaper(G2, n=10,SurgiPost Multiconical;G3,n=10, TechESEndoshape) wereluted withself-adhesive cementinendodonticallytreatedsinglerootsusingdifferentpostspacepreparationtechniques. Thebondedpostswereexperimentallyloadeduntilfailureandthemaximumloadtofracturewas registered.FracturepatternswerequalitativelyevaluatedandSEManalysiswasperformedto assessthequalityofendodontictreatmentsandcementation.Datawerestatisticallyanalysedby meansofone-wayANOVA.

Results: Themeanmaximumloadtofracturewas165.0523.46NinG1,151.5216.23Nin G2and129.0915.25NinG3.StatisticallysignificantdifferenceswerepointedoutbetweenG1 andG3(p<0.01)andG2andG3(p<0.05).Norootfractureswereevidenced.SEManalyses showedslightlythickercementlayersattheapicalandmiddlethirdsofsingletaperposts(G2and G3).

Conclusions: DTLightPostandSurgiPostMulticonicalfibrepostsshowedsimilarpropertiesin terms of mechanical resistance to fracture and higher than those of Tech ES Endoshape. Unrestorablerootfracturesdidnotoccurwithanyofthetestedposts.

ß2016Societa` ItalianadiEndodonzia.ProductionandhostingbyElsevierB.V.Thisisanopen accessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Riassunto

Obiettivi: Confrontarela resistenzameccanicaafratturadiduesistemidiperniconici posi-zionatisenzapreparazionedelcanaleconquelladiperniinfibraaconicita` doppiaposizionatiin radici trattate endodonticamente dopo preparazione di un post space standard con frese dedicate.

Materialiemetodi: Trentaperniaconicita` doppia(G1,n=10,DTLightPost)esingola(G2, n=10, SurgiPost Multiconical; G3, n=10, TechES Endoshape) sono stati cementati con un cementoautoadesivoin radicisingoletrattateendodonticamenteusandodiverse tecnichedi preparazionedelpostspace.Ipernicementatisonostatisottopostiatestdicaricoafratturaede` statoregistratoilcaricomassimo.Ipatterndifratturasonostativalutatiqualitativamenteede` stata svolta un’analisi SEMper la verifica dellaqualita` dei trattamenti endodontici e della cementazione.IdatisonostatianalizzatistatisticamenteconANOVAaunavia.

Risultati: Ivalorimedidicaricomassimoafratturaeranoiseguenti:165,0523,46NinG1, 151,5216,23N in G2and 129,0915,25N in G3.Sono emersedifferenze statisticamente significativetraG1eG3(p<0,01)etraG2eG3(p<0,05).Nonsonostateosservatefratture radicolari.L’analisialSEMhamostratostratidicementoleggermentepiu` spessineiterzimedioe apicaledeiperniaconicita` singola(G2eG3).

Conclusioni: I perni in fibra DT Light Post e SurgiPost Multiconical hanno fatto riscontrare proprieta` similiinterminidiresistenzameccanicaafrattura,lequalisonorisultatesuperiori aquelledeiTechESEndoshape.Nonsisonoverificatefrattureradicolarinonrestaurabilicon alcunodeisistemidipernitestati.

of atooth andreduces therisk ofunrestorablefailures in comparisonwithteethrestoredwithoutfibreposts.6,13The mainadvantageoffibrepostsisrepresentedbytheirelastic modulus thatisveryclose to thatofdentineandallows a stressdistributionsimilartothatofanaturalsoundtooth;on thecontrary,metalpostsexerthighstresslevelsatthe post-dentineinterface.6,14—16 _{Further advantages of fibreposts}

includebiocompatibility,resistancetocorrosionandfatigue, aestheticpropertiesandretrievability,sincetheycaneasily beremovedfromtherootcanalifnecessary.1Fibrepostsare usually luted inside the root canal by means of dentine adhesives and resin cements, allowing for a conservative preparation of the post space.17 Moreover, the adhesive cementationprovidestherestorativesystemwithanelastic modulus similar to that of dentine, resulting in a stress distributionpatterncomparabletothose arisingduringthe occlusalloadofanaturalsoundtooth.15,18,19

Ni—Tirotaryinstrumentsareroutinelyusedin endodon-ticstogive apredefinedtaperto rootcanalsandpreserve radiculardentine,thusperformingaminimallyinvasivecanal shaping.11_{Consequently,itwouldbedesirabletouseapost}

withthesametaperofthelastendodonticinstrumentused toshape thecanal,withoutanyfurtherpreparationofthe coronalandmiddlecanalthirds.

Theaimofthepresentinvitrostudywastocomparethe mechanicalresistance tofractureofstandarddoubletaper fibrepostslutedwithself-adhesivecementafterdedicated postspacepreparationwiththatoftwoconicalpostsystems placedin endodontically-treated single roots with no pre-parationofthepostspace.Thenullhypothesiswasthatthere was nodifference inthe resistanceto fractureamong the threepostsystems.

Materials

and

methods

Collectionofthespecimens

Thirtystraightsingle-rootedpermanentteethwithcomplete apex, no decays or previous restorations, extracted for periodontal reasons, were selected from a poolof freshly extractedteeth.Dentalplaque,calculusandexternaldebris wereremovedusingmanualandultrasonicscalers.Theteeth werestoredin1%thymolsolutionat378C.

For eachtooth,two siliconeimpressions (AquasilPutty, Dentsply DeTrey, Konstanz, Germany) were taken, one in buccolingualand oneinmesiodistal directionto make the positionofeachsamplerepeatableduringtheradiographic phases.Radiographsweretakenwithadigitalsensor(Kodak RVG6100,KodakDentalSystems,Rochester,NY,USA)anda radiographic digital system (2200 Intraoral X Ray System, KodakDentalSystems)setat70kV, 7mAand0.12s.Teeth withmorethanonecanal,withasingleovaland/orirregular canal wereexcluded from thestudy in order to limit the

anatomicalvariablesaffectingthebiomechanicalbehaviour oftherestorations.

The length and diameters of the selected teeth were measuredwithadigitalcaliper(AbsoluteDigimataticCaliper, MitutoyoDigimatic,Sakado,Japan);specifically,the bucco-lingualandmesiodistaldiametersweremeasuredatthelevel ofthecemento-enameljunction(CEJ).

Thespecimenswererandomlydividedinto3groupsof10 teetheach.Thesimilarityamonggroupsintermsoflength anddiameterwasassessedbymeansofone-wayanalysisof variance(p<0.05). The meanmeasurements( standard deviation) ofthespecimens included inthe studyandthe outcomeofthestatisticalanalysisareshowninTable1. Preparationofthespecimens

Thecrownofeachtoothwasremovedbycuttingthetooth 1mmabove theCEJ with amicrotome(Micromet,Remet, CasalecchiodiReno,Italy)underconstantwaterirrigation. During all the preparation stages, the specimens were manipulated with wet gauzes in order to avoid dentine dehydration.

Asize#10Kfile(Dentsply-Maillefer,Ballaigues, Switzer-land)wasinsertedintothecanalorificetoscoutthecanal andcheckthe apicalpatency,byleading thefileinapical directionuntilitstipwasvisibleattheapicalforamenunder 4magnificationsandmarkingthislengthwitharubberstop. Theendodonticworkinglengthwasestablished0.5mmshort of this length.6 _{The root canals were shaped using Ni—Ti}

rotaryinstruments(Mtwo,Sweden&Martina,DueCarrare, Italy)toISO size40,0.06taper(300rpmat maximum tor-que). Once activated, each instrument was progressively taken to working length using a single-length technique without any pressure in apical direction; the instrument wasremovedonceitrotatedfreelyattheworkinglength.20 Duringcanalinstrumentation,constantirrigationwascarried outwith2.5mL5.25%sodiumhypochlorite(Niclor5,Ogna, Muggio`, Italy).Allcanalswerefinally rinsedwith5mL17% EDTA(Pulpdent,Watertown,MA,USA)for120s,followedby 5mL5.25%sodiumhypochloriteandabundantrinsing with salinesolution,and dried withpaper points (Roeko Paper, ColteneWhaledent,Langenau,Germany).EachsetofNi—Ti instruments was discarded after preparing 10 canals. The canalswerefilledwiththecontinuouswaveofcondensation technique (System B, Elements Obturation Unit, Sybron Endo, Orange, CA, USA). An XF or F Buchanan plugger (SybronEndo)was chosento fit 3mm shortof theworking length,avoidingcontactwiththecanalwalls.Thisdepthwas marked using a rubber stop at the reference point. The SystemBunitwassetatfullpowerat2008Cwiththeswitch intouchmode.Foreachspecimen,thetug-backofa40/.06 gutta-perchamasterpoint(MtwoGutta,Sweden&Martina) was checked 0.5mm short of the working length and

improved,ifneeded,bytrimmingthepointwithascalpel. The point was then inserted into the canal with its tip covered with resin sealer (AH26, Dentsply-Maillefer). The gutta-perchamasterpointwasslowlydown-packeddriving theactivatedpluggerthroughthepoint1mmshorterofthe depthmarkedwiththerubberstopper;atthatlevel,heat activationwasinterruptedandthepluggerpushedapically for10s. Afurtherheatactivationof1swas performedto detachthecompactedapicalgutta-perchafromtherestof thepoint and then the plugger was extracted. The back-fillingwasperformedusinganObturasyringe(ObturaSpartan Endodontics,Algonquin,IL,USA)andhandpluggers,leavinga coronalemptyspaceof9mm.Thequalityoftheendodontic obturationwasverified radiographically.The orificeofthe rootcanalwassealedwithglassionomercement(FujiII,GC Corporation,Tokyo,Japan).21

The specimens werestored in sterile salinesolution at 378C.After24h,theglassionomercementwasabradedwith #240siliconcarbidediscsunderconstantwaterirrigation.21 The characteristics ofthetested postsare describedin

Table2.Doubletaper(.02—.10)quartzfibreposts(DTLight

Post#3,RTD,St-Egreve,France)wereusedinGroup1(G1) (Fig.1).Thepostspacewaspreparedusinga#3calibrated burmountedonalowspeedhandpieceunderconstantwater irrigation.Thepostdiameterat9mmfromtheapicaltipwas measured,resultingin1.70mm.

InGroup2(G2)andGroup3(G3),singletaper(.10)glass (SurgiPostMulticonical,MC Italia,Lainate,Italy) andsilica fibreposts(TechESEndoshape,ISASAN,RovelloPorro,Italy) wereused (Fig. 1).Inordertocomparethefracture resis-tancevaluesamonggroups,thediameterofallthepostsat thecanalorificewasstandardisedbymarkingallsingletaper

posts at the point where the diameter was 1.70mm and cutting their tips 9mm from this reference point with a 0.2mmthicksiliconcarbideseparatingdisc (Dedeco Inter-nationalInc.,LongEddy,NY,USA)(Fig.1).Thefitofeachpost in the root canal was verified by means of standardised radiographsinboththebuccolingualandmesiodistal direc-tions using the positioning silicon templates as described above.Beforecementation,allthepostswereremovedfrom therootcanalsandsectionedat14mmfromthetipusingthe separatingdiscs,sothateachpostprotruded5mmfromthe canalorifice.

The post space was cleanedwith an endodonticrotary brush(VersaBrush,VistaDentalProducts,Racine,WI,USA) andthendriedwithpaperpoints.Aself-adhesivelutingagent (RelyXUnicemAplicap,3MESPE,St.Paul,MN,USA)wasused tocementallthepostsusingspecificintra-canaltips.After extrudingthelutingcementintothepostspace,thepostwas seatedwithagentlerotation.Theresincementexcesswas removedwithamanualinstrumentandthecementwas light-curedfor 40s using ahalogenunit at600mW/cm2(Elipar 2500,3MESPE).Aftercementation,thespecimenswerekept at378Cand100%relativehumidity for24h.

Simulationofthe supportingperiodontaltissues Theexternal surfacesoftherootswereisolatedusing gly-ceringel.Each specimenwasembeddedinamassof self-curing acrylic resin (Jet Kit, Lang Dental Manufacturing, Wheeling,IL,USA)andpouredintoasteelhollow cylinder having a height and diameter of 30mm and a lumen of 15mm.Thecoronalportionoftherootprotruded2mmfrom

Table2 Postcharacteristicsdeclaredbythemanufacturers.

Group n Post Taper Composition Tensile strength(MPa)

Elasticity modulus(GPa) G1 10 DTLightPost .02—.10 Pretensionedquartz

fibres(64%vol.) Epoxyresin Silane 2050 15 G2 10 SurgiPost Multiconical

.10 Glassfibre(80%vol.) Epoxyresin

3100 80 G3 10 TechES

Endoshape

.10 Silicafibres(60%vol.) Diphenylpropane+ methyloxirane Bariumsulphate

1426 14

thesurfaceoftheacrylicresin.Assoonastheresinstartedto set,each specimen wasremoved from theblock,so as to avoid dehydration caused by the exothermic reaction of polymerisation.Inorder to simulatetheviscoelastic beha-viour of the periodontal ligament, a polyvinyl siloxane impressionmaterial(Flexitime,HeraeusKulzer,Hanau, Ger-many)wasinjectedinthespacecreatedbytherootsinthe resin;then eachspecimen was insertedagainin thesame spacebeforethepolymerisationoftheimpressionmaterial, allowingthepolyvinylsiloxanetosetinathicknessofabout 200—400mm. Excess impression material was carefully trimmedafteritscompletepolymerisation.

Loadto failure

Astaticcontrolledloadwasappliedonthelingualsurfaceof theheadofeachpostat458tothelongitudinalaxisofthe toothbymeansofauniversaltestingmachine(GaldabiniSun 500,CardanoalCampo,VA,Italy).Aflatheadstyluswitha diameterof3mmandacrossheadspeedof0.75mm/minwas used(Fig.2).Allsampleswereloadeduntilfractureandthe maximum failure loads were recorded in Newtons. After mechanical failure, the fracture mode was evaluated at 4magnifications.

Scanningelectronmicroscopeanalysis

Afurtherevaluationofthequalityofendodontictreatment andcementationofthepostswasperformedbymeansofa ScanningElectronMicroscope(SEM)analysis(Quanta250,Fei Company,Hillsboro,NE,USA).

Tworepresentative specimensper groupwereprepared for SEM observation after the mechanical testing. Two grooves werecreated on themesial anddistalsurfaces of therootswithacuttingdisc,avoidingcontactbetweenthe cuttingdiscandcanalwalls.Therootswerethensplitinto halveswithachisel.Sampleswerefixedina0.2Mbuffered solutionof4%glutaraldehyde,dehydratedthroughmultiple stepsinalcoholic solutionswith increasingconcentrations, driedandsputter-coatedwithgold(Sputter CoaterK550X, FeiCompany).Imagesat11and250wereacquiredatthe coronal,middleandapicalthirdofeachpost.

Statisticalanalysis

Thedataobtainedfromthemechanicaltestswere statisti-callyanalysedby meansof dedicated software(Statistical PackageforSocialSciencesv.15,SPSSInc.,Chicago,IL,USA). Allthedatasetswereanalysedfordistributionnormalityand varianceequalitywithShapiro—Wilktest andLevene test, respectively,to verifytheassumptionsfor theuseof para-metrictests.Aone-wayanalysisofvarianceandaSheffe` post hoctest forpairwise comparisonswere usedto assessthe significanceofthedifferenceamonggroupsintermsofmean maximum breaking load. In all the analyses,the level of significancewassetatp<0.05.

Results

The mean values ( standard deviations) of the fracture loadsrecorded inNineachexperimental groupareshown

inTable 3.The highest values offracture resistance were

recorded in G1, followed by G2 and G3 respectively. No statisticallysignificant differenceswere reportedbetween G1 and G2 (p>0.05), while significant differences were evidenced between G1 and G3 (p<0.01) and G2 and G3 (p<0.05). The qualitative analysis of the failure pattern onlyshowedcoronalfracturesofthepostsbendingtofailure atthecanalorifice,whilenorootfractureswereobserved. ComparingtheSEMimagesofthevariousgroups(Fig.3), differences were noticed in terms of preservation of the original root canal anatomy and cement thickness at the post/dentine interface. Slight discontinuities in the root canal taper in the area of the post tip were observed in G1andwereascribabletotheactionofthecalibrateddrill usedtopreparethepostspace.Furthermore,thickerlayers

Figure2 (A)Theuniversaltestingsystemusedfortheloadingtest.(B)Adetailoftheflatheadstylusoftheuniversaltestingmachine loadingthespecimensat458tothelongitudinalaxisofeachtooth.

Table3 Meanvalues(standarddeviations)ofthefracture loads(N)recordedintheexperimentalgroups.

Group Post Fractureload G1 DTLightPost 165.0523.46 G2 SurgiPostMulticonical 151.5216.23 G3 TechESEndoshape 129.0915.25a

ofcementwereevidentbetweenpostsandcanalwallsboth intheapicalareaandatthemiddlethirdofthepostsinG2 andG3ratherthaninG1.

Discussion

Accordingtothe resultsoftheload-to-failure testing,the null hypothesiswas rejected, since statistically significant differences were evidenced among groups in terms of mechanicalresistancetofracture.DTLightPostand Surgi-PostMulticonicalfibrepostsshowedsimilarmechanical resis-tanceto fracture andgreaterthan TechESEndoshape. As widely demonstrated in literature, the post system has a relevantinfluence on fracture resistance.2 With regard to fibreposts,thereisahostoffactorsthatactsimultaneously andcanfurtheraffecttheirmechanicalproperties,namely thediameterofthepostatthecanalorifice,thefibre/matrix ratio,dimensions,orientation, densityandchemo-physical treatmentofthefibres.22Ourfindingssuggestthatthefibre contentaloneisnotindicativeofthefractureresistanceof thebondedpost,sinceSurgiPostMulticonicalpostshavethe highestfibre/matrix ratio (80%),followed bythe DTLight Postsystem(64%)andTechESEndoshape (60%).The great mechanicalresistancefoundinG1,despitetherelativelylow fibrecontent, could bejustified by the number of quartz fibrespersurfaceunitandtheir mechanicaltreatment;in fact,DTLightPostshaveaveryhighdensityofpre-tensioned fibres(diameter12mm,32fibrespermm2).23Furthermore,

thispostsystemistheonlyonethatmakesuseofasilaneto improvethebondstrengthbetweenthefibresandtheresin matrix, and this could also contribute to improve the mechanical performance. According to the knowledge of theauthors,todatenodataareavailableinliteratureabout the mechanicalperformance of SurgiPost Multiconical and TechESEndoshapeposts,soacomparisonoftheresultsisnot feasible. It is nonetheless noteworthy that the resistance values we obtainedin all groups are considerably greater thanthoseregisteredinpreviousstudieswithsimilar experi-mentalset-up,testingthefractureresistanceofotherfibre postsystemswithcomparableorslightlysmallerdiameters (1.38—1.70mm).21,24 In addition, during the mechanical testsperformedinthepresent investigation, noroot frac-tureswerenoticedinanyoftheexperimentalgroups;suchan occurrence couldbeexplainedconsideringthattheelastic modulusoffibrepostsislowerthanmetallicorzirconiaposts andcanbeclosetothatofdentine.Fibrepostsareknownto causesignificantlylesscatastrophicfailuresthanotherkinds ofposts.25Inlightoftheseconsiderations,allthetestedpost systemsexhibitedaperformancethatsupports theirusein theclinicalsetting.

Changes in the experimental set-up can considerably affectthefindingsoftheinvitrofractureresistancetests. Aninsertiondepthof9mmwaschoseninordertoachievethe bestpostretentionavoidingtoweakentheroot,asreported inrecentinvestigationsthatshowednodifferencesin frac-ture resistance seating fibre posts at depths of 5, 7 and 9mm.26,27 Posts with main taper of.10 were selected

becausethesinglecanalofsingle-rootedteethisgenerally wide enough toreceive such posts. Also, accordingto the instructionsofthemanufacturer,Mtwofileswereusedwitha brushing motion against canal walls, thus causing a slight wideningofthecanalthatincreasedthefinaltaperofthe coronalandmiddlecanalthirds,exceedingthatofthelast rotaryfile.

Theworstscenariowasobtainedbyapplyingthe experi-mentalloaddirectlyonthepost,inordertoeliminateany interference of other variables involving the restorative materialsand/ortheresidualtoothstructure.21,24The simu-lationoftheperiodontalligamentwasperformedasithad already been demonstrated that it could influence both fractureresistanceandthefailurepatternofendodontically treatedteeth.28_{Moreover,severalthree-dimensional}

aniso-tropic Finite Element Analysis (FEA) investigations have proved theparamount roleofthe periodontal ligament in distributingstrainsanddissipatingstressesinpostandcore restoredteeth.15,18InanFEAstudy29itwasevidencedthat theoccurrenceofrootfractureswasreducedinthepresence oftheperiodontalligament,duetothefactthatitselastic modulus ismuchlower thanthatofthealveolar boneand ensuresamoreuniformstressdistributiononboththetooth andthesurroundingtissues.

The mean cement thickness around the post mainly depends on the congruence achieved between the shape ofthepost andtherootcanalwalls.Thepreparationof a circularpostspaceintight,ovalorirregularrootcanalswith theaimofachievingthebestfitforapostnecessitatesthe removalofasignificantamountofdentine,weakeningthe rootandincreasingtheriskofperforations.30_{Forthisreason,}

the post space preparation should be minimally invasive, with minimum or no widening of the shaped root canal. Thedimensionsofthepostsusedintheexperimentalgroups with single taper posts were standardised in length and diameter in order to compare the fracture values among different post systems: mechanical comparability was achievedbystandardisingthediameteratthecanalorifice andthe taperof themainportion of thepost. Therefore, whilethecoronalfitofthesingletaperpostswasoptimal,the SEMobservation revealedthatthelayerofcementaround the postwas moderately thicker at themiddle andapical thirds of the postsin G2and G3.Considering the optimal mechanicalandadhesiveproperties ofcontemporary resin cements,thepartiallyunevendistributionofthelutingagent atthemiddleandapicalthirdsofthepostsdoesnotinterfere with the mechanical performance of the restorative sys-tem.1,31,32Ithasalready been shownthat afitting post is notanessentialrequirementfortheimprovementoffracture resistance.33Inaddition,thepresenceofathickerlayerof cementatthe tipofthepost wouldresultinan improved passive fit,preventing the negative influenceof the well-known‘‘wedgeeffect’’.34,35Clinically,thepostsizeshould bechosenaccordingtothebestpassivefitandtheminimal taper,ideallycorrespondingtothatoftherootcanalshaping with no further post space preparation. Despite all the operative effortsto preparethe rootcanalto receive the postand,viceversa,adapttheposttothecanal, standar-disedshapesofprefabricatedpostsdonotfitexactlywiththe complexanatomyofrootcanals.Consequently,thethickness of the resin cement around the post could differ signifi-cantly.36 Astothepossiblelossof retention,someauthors

haveperformedinvitroanalysesontheinfluenceofdifferent cementthicknessesontheretentiveforcesoffibrepoststo intra-canal dentine; however, no consensus has been reportedabouttheidealcementthicknesstoimprovepost retention.37

Asaresultofthedehydrationcausedbythepreparationof thesamplesfortheSEManalysis,cracksandostensiblegaps appearedattheinterfacebetweenthedentineandtheresin cement,asalreadyobservedinpreviousstudies.38,39Owing tosuchaphenomenon,theinterfacebetweendentineand cement usually presents microcracks, since the dentine desiccatesbecause of its hydrophilic characteristics while thecementandthepost,bothhydrophobic,keepthe adhe-siveinterfaceintact.

With regard to the cementation of fibre posts, resin cementshave to bepreferredto zinc oxide lutingagents, sincetheyprovidebetteradhesionandarenotaffectedby significant resorption. Moreover, resin cements present mechanical properties similar to those of fibre posts and dentine.3,40_{Inthepresentstudy,RelyXUnicemself-adhesive}

cementwas used:nosignificantdifferenceswerereported betweenitsbondingstrengthandthatprovidedbyPanaviaF (Kuraray), a dual cure cement with self-etching primer adhesive system.41 _{Furthermore, RelyX Unicem showed}

valuesofadhesioncomparabletothoseachievedwithresin cementsusingetch-and-rinseadhesivesystems.42The push-outforcerecordedforRelyXUnicemresultedtobe compar-abletothat ofPanavia F2.043 andVariolinkIIdualcuring cement.41_{Accordingtoarecentreviewthattookinto}

con-sideration the in vitro data available in literature, it is possible to state that the bonding strength of the most validated self-adhesive cements is comparable with the adhesionofdifferentmulti-stepresincementsand satisfac-toryforclinicaluse.44Moreover,easyhandling,repeatability andreducedchair-side time, as wellasless operator-and technique-sensitivity, could explain how widespread self-adhesivecementshavebecomeinclinicalpractice.1,45

Conclusion

Withinthelimitationsofthepresentinvitroinvestigation,DT Light Post and SurgiPost Multiconical fibre posts showed similarmechanicalresistancetofracture,whichwashigher thanTechESEndoshape.Nevertheless,allthetestedposts hadhigh fractureresistance and none ofthe tested posts causedunrestorablerootfractures.

Inthesameanatomicalconditions,theuseofsingletaper conicalpostswouldbepreferablethandoubletaperposts, sincethey allowedforamoreconservativepostspace pre-parationornopreparationatall.Theslightlyunevencement distributionaroundthemiddleandapicalthirdofthesingle taperconicalpostscanbeminimisedintheclinicalsettingby choosingapostwithlowertaper.

Thefindingsofthepresentstudylaythegroundforfurther investigationsto evaluatethe clinical performance of the testedpostsystems.

Conflict

of

interest

References

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