Tensile Bond Strength ofGold and Porcelain Inlaysto Extracted Teeth Using

Three Cements

Francesco S. Michelini, Dr Med Dent'

Urs G, Beiser, Prof Dr Med Dent**

Susanne S. Scberrer, Dr Med Dent*

University of GenevaGeneva, Switzerland

IVaidemar G. De Rijh, MS, PhD, DDS"

University uf tllinais at ChicagoChicago, Illinois

This in viiro sfudy compared ihe tensile bond slrengfh of gold and porcelaininiays to exlracfed molars in standardized cavities. Tfiree cements were used:zinc phosphate, glass-ionomer, and a resin composite cement. The goldinlays were cemented using zinc phosphate or glass-ionomer cemenf, andfhe porcelain inlays were luied using resin composite or glass-ionomercement. Surface treatments included, for gold iniays, either no trealment(zinc phosphate cement) or airborne particle abraded and tinplafed (glass-ionomer cement); and i or porcelain inlays, eifher no freafmenf Iglass-ionomer cemenf) or efched and silane-treafed (resin composife cemenf).Sfatistical analysis was performed using the Weibull disfribufion, Resulfsshowed no significant differences between gold inlays cemented using zincphosphate or glass-ionomer cemenfs and porcelain inlays luted using glass-ionomer cements. The bonded porcelain inlays (resin composife cement)showed fensile bond sfrenglhs two lo three times higher fhan those obtainedfor cemented gold inlays, Int i Prosthodont ¡995:8:324-331.

Currently a variefy of cemenfs are available tolute metal and porcelain restorations to tooth

structure. The choice of the adequate cementdepends on both the restorative material and thetooth structure involved. The selection of the typeof prosthetic restoration is mainly dictated byclinical criteria such as longevity and esthetics.

*Assistant Professor. Department of Fixed Prosthodontics andOcciusion.

—Professor and Chairman. Department of FixedProsthodontics and Occlusion.

'**Associate Professor. Department oí Restorative Dentistry.

Condensed from a medical thesis presented in partiai luilill-ment ol the requirements lor the degree of Dr Med Dent Iromthe Faculty of Medicine of the University of Geneva.

Portions of tbis paper have been read belore the 71st GeneralSession of the international Association for Dental Research inChicago, liiinois, March 10-U. 1993.

Reprint requests: Dr Susanne S. Scherrer, Assistant Professor.Department of Fixed Prosthodontics and Occlusion. School ofDentistry. University of Geneva. 19. rue Barthélemy-Menn.1211 Geneva 4. Switzerland,

Although gold alloy restorations have performedvery well over time,' ceramic restorations arebecoming increasingly popular because of fheiresthetic qualities. The introduction of adhesivecements and related surface treatments has signifi-cantly improved the fracture resistance of the brit-tle ceramic restorations.^-^ The resin compositecements and glass-ionomer cements are the mostprominent among the luting materials with adhe-sive properties. Their ability to bond to mineralizedtoofh structures and ceramics has been studiedextensively over the past decade. In particular, ten-sile bond strengths of resin composite cements toetched and s i lane-treated ceramics range between9 and 27 MPa/-^ whereas their adhesion to toothstructure is reported to be in the range of 8 lo 20MPa for etched enamel, and 3 to 6 MPa for pre-treated denfin," Class-ionomer cements, which areless controversial in terms of biocompatibility thanresin composite cements or zinc phosphatecement, adhere significantly to enamel, with bondstrengths (shear and tensile) of 5 to 9 MPa, but littleto dentin (1 to 4 MPa), as reported by Craig,^ Hotzet al,^ and Aboush and Jenkins,'" Class-icinomer

I of PrO!(llodorl]C5 324

Mkhelini el al Teiisilo Liond SlrcnRll) ol Giild ,ind PurceUin Irl.iv

< . 4 mm > < 2,95 mm

i2 mmDentin

Enamel

Dentin

2.65 mm2.65 mm

12 mm

Fig 1 Standardized cavities with and without bevel prepared on the buccal aspect of extracted mandibular rroiars using a rniilingmachine (drawing not to scale).

materials have also demonstrated retention to goldinlays superior to that of zinc phosphate cement^'as well as some bonding capacity to airborne parti-cle abraded and subsequently tinplated gold alloys(5.5 MPa¡.^ The literature seems to favor the use ofadhesive cements when retention is the key param-eter tested. Nevertheless, in studies involving brittlecements such as zinc phosphate, glass-ionomers,and resin composites, as well as tooth substrateswith a huge variation in their ability to bond, themechanical reliability of a luting medium is a criti-cal issue- The use of appropriate statistics that allowdefining failure probability as a function of stress isnecessary when comparing the performance of brit-tle cements. These cements possess a distribution ofcritical flaws that varies from one sample to anotherand is responsible for the variability in strengthamong a population of tested samples. The use ofthe Weibull distribution'- can quantify the strengthvariability with respect to a multiple flaw distribu-tion, and is based on extreme value statistics mostsuitable for brittle materials.

The purpose of this study was to evaluate Ihe over-all performance of cement-to-in lay restorations and

tooth structure in one investigation using standard-ized cavity preparations, well-controlled inlay geom-etry, and a statistical analysis based on the Weibuildistribution. Tensile bond strengths of gold andporcelain inlays luted into standardized cavities ofextracted molars were compared using three mate-rials, a resin composite cement (CP), a glass-ionomer luting agent (Gl), and a zinc phosphatecement ¡ZP],

Materials and Methods

Cavity Design

Forty-five extracted intact maxillary and man-dibular molars were collected and embedded inresin, leaving only the buccal enamel surfaceexposed. The exposed enamel was flattened manu-ally under running water using 220 to 1,000 gritSiC paper on a Lunn minor Struers apparatus(Struers, Copenhagen, Denmark). Standardizedcavities, 2 mm in depth, 2,95 mm wide, and with a6-degree taper, were prepared under constantwater-spray with a manually controlled milling

Mumber4, 1995 325 The imernarional Journal of Pr

rmíilr Hivid SlrmgLii i,l Goici and Purœliiiii liilayí Miüidini el í

ENAMEX

• ENTiN

a

ENAMEL

DENTIN

C

GZP

\zJPCP

l í jAf l ¡^

DENTIN

b

ATENAMEL

DENTIN

d

GGI

PGI

Fig 2 Inlay dislriDulic-'i (a)Group GZP = gold/ZP cornent,ib) GGi = gold/GI cement, (c)PCP = porceiain/RB cement;(d) PGI ^ porcelain/GI cement.

Table 1 Inlay Disfribution, Cavity Designs, Surface Treatments, and Luting Cements

Inlays

GZPGGi

PCP

PGi

Numberol

inlays

1211

11

11

Cavifydesign

BevelBsvel

No bevei

No bevel

Inlaymaterial

GoldGold

Porcelain

Porcelain

Surfacetreatmentot inlays

NoneAiriDorne parficle abraded

+ finplatedEfched

4. si lane-treatedNone

Surfacetreatmentof enamel

NoneNone

Efohing

None

Surfacefreafmenfof denfin

NoneNone

Dentin bondingagentsNone

Cemenl

Zinc phospfiateGlasB-ioncmer

(Aqua Gem)Resin composite

cement (Dicor LAC)G1 ass-ion 0 mer

machine (Metzner SA, Geneva, Switzerland) anddiamond burs of specific dimensions (Fig 1). Anadditional 23 cavities were beveled using a rounddiamond bur with a diameter of 4.4 mm (Fig 1).

Inlay Distribution

A total of 23 gold inlays (Protor 3, Cendres etMétaux, Bienne, Switzerland), and 22 porcelaininlays (Ivoclar IPS Classic, Ivoclar AG, Schaan,Liechtenstein) were arbitrarily distributed into fourgroups as shown in Fig 2 and Table 1. Group GZPconsisted of 12 gold inlays cemented with ZP(DeTrey/Dentsply, Konstanz, Germany) into beveledcavities without any surface treatment. Group GGIcomprised 11 gold inlays thaf were first airborne

particle abraded using 50 |jm AI^O^ at 2 atm pres-sure, held approximately 1 cm away from the toolno22le (Unltool AG, Aarburg, Switzerland), subse-quently tinplated (electrodeposition of 2 to 5 |jm ofSn) following the manufacturer's instructions (OVS:Opaque Coupling System, DeTrey/Dentsply), andfinal ly cemented with CI (Aqua Cem,DeTrey/Dentsply) into beveled cavities. Croup PCPreceived 11 porcelain inlays etched and silanized(Dicor Retention Gel and Dicor Coupling Agent,DeTrey/Dentsply), and subsequently bonded withCP into nonbeveled, pretreated cavities (etching anddentin bonding agents, Prisma Universal Bond,DeTrey/Dentsply). Finally, group PGI had n porce-lain inlays cemented with Gl into nonbeveled cavi-ties without any surface treatment.

The International iourn.ii of Prosthodontii 326

Tensile Bond Strength of Coid und Porcelain Inlays

Fabrication of tbe Inlays

Single impressions front all cavities were madeusing silicone (Reprosil HF, DeTrey/Dentsply) andpoured with either Fujirock stone (CC DentalIndustrial, Tokyo) for the gold inlays, or with arefractory die material (Vita Hi-Ceram, VitaZahnfabrik, Bad Säckingen, Germany) for theporcelain inlays. A T-bar shaped metal frame withlateral extensions to rest upon enamel next to theinlay cavity was prepared and duplicated for eachgold and porceiain inlay (Fig 2) to facilitate thehandling of the iniay, to allow the sintering ofporceiain onto it, to control the cementation pro-cedure, and to provide a grip for tensile testing. Ailgold iniays including their metal frame were madewith Type IV gold (Protor 3, Cendres et Métaux],whereas the metal frames for the porcelain inlayswere made with Esteticor Pius (Cendres et Métaux),a high Au-Pt alloy (96% Au-Pt¡ routinely used inthe metal ceramic technique. Feldspar-hasedporcelain (Ivoclar IPS Classic) was sintered to thecentral axis of each metal trame positioned on itsrespective refractory die. After application of theporcelain, the refractory dies were eliminated byairborne particle abrading with 50 fjm Ai,O, parti-cles under a pressure of 2 atm as descrihed ahove.All inlays were tried into their original cavity usinga si l icone material (Fit Checker, GC DentalIndustrial) to evaluate the overall adaptation priorto cementation.

Cementation Procedure

The surface treatments as described in Table 1were performed according to the manufacturer'sinstructions prior to luting. All inlays, ideally seatedby using their metal frame positioned on the flat-tened enamel surface next to the cavity, werecemented using finger pressure. Croup PCP bondedwith CP was exposed to 50 seconds of light-polymer-ization (Visilux 2, 3M Schweiz AG, Ruschlikon,Switzerland). Ten minutes after cementation, all sam-ples were stored in water for 2 days prior to tensiletesting.

Tensile Testing

Tensile ioading was performed with a UniversalTesting Machine (Instron 1114, Instron, HighWycombe, England) at a crosshead speed of 1mm/min using bundled Kevlar fibers (Kevlar 220deci Tek, Du Pont Nemours Int, Geneva,Switzerland) to connect the inlays to the testingmachine. Failure loads were recorded in kg and

converted to MPa as the total surface for each cay-ity design could be calculated from the knownstandardized cavity dimensions and taper angles.

Mode of Failure Analysis

The mode of failure (adhesive, cohesive, combi-nations) was determined for each sample under aiight microscope (Photomakroskop M400, Wild-iHeerbrugg Ltd, Heerbrugg, Switzerland) usingmagnifications of 10 to 20 X. Typical failures foreach group were documented by means of SEMphotomicrographs (Autoscan, Elmics, Munich,Germany) at magnification x 20.

Statistical Analysis

The Weibuli'^ distribution was used to calculatefailure probabilities as a function of appliedstress.^-' The Weibuil distribution function thatreiates the cumulative prohability of failure F(S) tostress (S) is described by two parameters: the char-acteristic strength SQ, the point at which 63% of thespecimens have failed, and m (Weibull's moduius),a parameter related to the shape of the distributionfunction. When data conform to the Weihull distri-bution, the logarithmic plot of the distribution willproduce a straight line. The linear correlation coef-ficient R for the data provides a measure of appli-cability of the distrihution.

The maximum likelihood estimates for the char-acteristic strength values S and m were determinedfor each cementation group using iterative proce-dures (Newton-Raphson) as described by Mann etal.^'' The 90% confidence intervals were calculatedfor Sy and m using the tables by Thoman et al.^^When these confidence intervals for the two datasets do not overlap, the difference is designated assignificant at 81 % level of confidence.

Results

Statistical Analysis

Table 2 summarizes the results obtained for eachinlay group by listing the Weibuli's modulus m, thecharacteristic strength S , their respective confi-dence intervals (CI), as well as the correlation coef-ficient R determined from the loglog-transformedfailure data. The cumulative probability of failureas a function of the applied stress is plotted in Fig 3for all groups. The corresponding loglog transfor-mations are shown in Fig 4. A bar graph represen-tation of the characteristic strength for each inlaygroup is given in Fig 5, where the error bars repre-

Njniber4, 1995 327 The International tournai of Proîihodontics

iie IJond Slr.-jmth ol Cokl jnd Por, eUm Inlays

Table 2 Tensile Bonid Sfrengfhs for Gold and Porceiain Inlays: Characteristic Strengfh (SJ, Weibull Modulus im).Confidence Infervals (at 90%), R Values for the Logiog Transfórmete Data

IniaygroupsGZPGGIPCPPGI

Inlay maferialand

cement

Gold/ZPGold/GI

Pcrcelain/CPPorcelatn/Gi

Numbercf

inlays

111111

Weibullmodulus

m

2.3-74a.2-

Confidenceintervai

m

1.2-2.9

Characteristicstrength S

(MPa)

8.02.2*

Confidenceinterval

SglMPa)

2.2-3.12.7-4.67.3-8.71.6-2.9

Coirelafion

0.9680.977

' Indicates no slatislically signiticant difference.2P = 2inc phosphate, Gl = glass-ioncmer; CP = resin composite cement.

en

f fai

lure

[F

ty o

3bab

iul

ativ

e pr

Cum

0

0.80

0.60

0.40

0.20

0.00

0.00

PGI a PCP • GZP • GGISg = 2.2MPa Sj = 8.0MPa SQ = 2.6 MPa Sg = 3.6 MPa

if .y2.20 4.40 6.60 8.80 11.00

Stress (MPa)

Fig 3 Cumuiafive probabilifyof failure expressed lor allgroups combined.

sent the upper limit of the asymmetric confidenteinterval for the value of Sg.

The results showetd no significant differencesbetween the groups CZP (gold/ZP), CGI igoM/GI],and PGI ¡porcelain/G/), neither for their Weibull'smodulus nor for their S,.,. The characteristicstrength (S ) of group PCP (porcelain/CP), how-ever, was significantly greater (three to four times]than that of group PGI (porcelain/GI). In addition,the m value of PCP was significantly higher whencompared fo PGI, indicating less data scatteringfor the porcelain inlays bonded wifh resin com-posite cement.

Mode of Failure

Analysis of the failed samples showed exclusivelyan adhesive mode of failure for GZP, GGI, and PGI.The adhesive failures seen with ZP and Gl wereequally distributed at the interface cement/inlay or

cemenl/tooth slruclure. The porcelain inlays bondetdwith resin composite cement failed in a combinedmode involving fractures within enamel, porcelain,and fhe cemenf inferface (Figs 6a and 6b).

Discussion

When comparing performance of materials it isimportant fo standardize as much as possible allvariables involved. The use of a milling deviceproved to be an effective method for obtaining per-fect and identical cavity preparations. This made itpossible to calculate a tolal bonding surface andexpress failure stresses in MPa. Some variabilityinherent to the material, such as dentin, porcelain,and iuting cements, will always be present and inpar! be responsible for scatter in the data. The useof adequate statistical analysis is therefore a keyfactor when comparing the performances of prod-ucts. Thus, the Weibull distribution provides infor-

328

TeníileaondStrenglhof Goidand Por

Fig 4 Bond strength vaiuesshown as Weibuil ioglog-trans-formed dala. The siope isexpressed by the Weibj l l 'sshape parameter m.

Logl

og [

1/(1

-F

2

1

0

-1

-2

-3

m = 2,2 MPa

r

•

o /

-2 -1

PCP ^ GZPm = 7,4 MPa m = 3,2 MPa

/ / . /

0 1

Log stress (MPa)

o

r

2

GGIm = 2,3 MPa

/

3

Fig 5 Characteristic tensilebend strengths of goid andporcelain iniays cemenfed toflentin using three iutingmedia, GZP - goid iniay/zincphosphate cement; GGI = goldiniay/glass-ionomer cemenl:PCP = porcelain inlay/resincomposite cement; PGI =porcelain inlay/glass-ionomercement.

12,00

7,20 -

4,ao -

2,40 -

0,00

3,6

2,6

GZP GGI PCP

Inlay group

PGI

mation about the probability of failure of a givenmaterial at any stress level, as opposed to theGaussian distribution, which is not available inintegral (ie, cumulative) form, A higher Weibullmodulus m means a more uniform behavior of the

material tested, A comparison of the m values (seeTable 2) and respective confidence intervalsbetween the groups showed a high m for theporcelain inlays bonded with resin compositecement, which can be attributed to the uniformity

, Number 4, 1995 329 The International journal of Prosllioclontk

Tensile Bond Slieiigdi ol Cold and Port

Figs 6a and 6b SEM P' i°t '^ '" ' f ^ :graphs (original magnrticatior ^ ¿uj ua nonbeveleä cavity of a porceiam iniayfrom group PCP. Mixed mode oí tailureincluding failure through porceiam,through enamel, and at the cementinterface.

of the bond for these specimens. On the contrary,glass-ionomer cement did not perform well on theporcelain group PG[, showing a very low m, whichindicates a great amount of scatter in the data pro-duced by the inhomogeneity of the adhesion of Glwith porcelain, enamel, and dentin substrate. Alow m was also found for the gold inlays cementedwith Gl. In tbis group, the tinplating of the goldneither improved the characteristic tensile bondstrength (S ) when compared to the zinc phosphatecement (GZP), nor contributed to raising the mvalue. The large dispersion in the data seems to beinherent to the Gl and ZP cement being studied.This is the result of the asymmetry in the probabil-ity distribution function and not necessarily areflection oí ihe quality of the experimental tecb-nique. Tbe cumulative probability of failure plotsgive interesting information about tbe stress levelsneeded for early failures to occur. The stress levelsfor the probability of having 10% of a populationof samples fail were the same for the gold inlayscemented with ZP as with Gl. For the two porce-lain inlay groups, these stress levels were signifi-cantly different.

Thus, failure predictions for inlay restorationscemented with different luting media can be madefrom strength tests and depend on the experimentalparameters (m and S ) that measure the strengthdistribution. The reliability of the luting mediumincreases as m and SQ increase. When testing thestrength of materials it is important to determinethese parameters, as the inherent variability instrength results from the wide range of strength-controlling flaws.

The analysis of fractured samples and thedescription of the failure mode into either single ormixed mode was only based on a low-magnifica-tion analysis of the fractured surface. The failureorigin was not determined, as this implicates theuse of fractography^'''^^ and fracture mechanicsprinciples."'•''* It would be of considerable value tounderstand the mode of failure, and to calculatethe stress necessary to initiate and propagate acrack in multiple-layered restorations (ie, ceramic-composite cement-dentin) by applying these engi-neering techniques to failure analysis.

Conclusions

Within the limitations of this in vitro study, thefollowing conclusions can be drawn based on theWeibull analysis of tbe data:

1. Porcelain inlays bonded with resin compositecement to tooth structure using adhesive tech-niques reveal a Weibull parameter (m) and acharacteristic tensile strength (S„] respectivelytwo and three times greater than conventionalgold inlays cemented with zinc phosphate.

2. The use of glass-ionomer cement on tinplatedgold inlays does not significantly affect the Stensile strength nor the m parameter whencompared to conventional gold inlays lutedwith zinc phosphate cement.

3. The Sy tensile strength and ni parameter ofporcelain inlays cemented with resin compositecement are three to four times greater than forglass-ionomer cement.

ol Pro! [fiodon tics 330

Bond Slrenglh of Goid and Porcelain Inlavs

Acknowledgments

Tfie autfiors wish lo thank Cendres el Métaux (Bienne,Switzerland), DeTrey-Deniiply (Konslanz, Germany), andIvoclar (Sch.ian, Liechtenstein) for providing the material loconduct tfiis study.

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Literature Atustract

Mercury Vapor Suppression by Various Liquid Media

Many reoommendatrons fiawe been proposed for tbe safe bandling and temporary storage ofscrap amalgam and mercury wasfe The purpose of this sfudy was to investigafe tbe effective-ness of fresh and used phofograpbic fixer, Merconvap, and wafer for fheir ability to suppressthe vaporizafion of meroury. Triple-disfilled denial mercury was dispensed into fest fubes con-taining fresb and used photographic fixer. Merconvap, and water. Test specimens were mea-sured for mercury vapor confent above the fluid level at various times between 10 minutes and335 days Resulfs showed that mercury vapor concenfration for used and fresh photographicfixer and fwlerconvap decreased to zero over fhe measuremenf penod. In contrasf, mercuryvapor concentration for the samples sfored in water initially decreased between 10 and 30 min-ufes. but fhen increased continuously beyond 1.75 hours. Visual inspection of fhe fubes al335 days revealed no defectable changes in the surface appearance of fhe mercurydrops in tfie water specimens, wfiereas the mercury in ihe fixer and Merconvap was cov-ered with a black layer. Fresfi and used iixer and Merconovap stjppressed the vaporiza-tion of mercury to below tfie detection limit ol the measuring instrument.

Sutow EJ, Foong WC, Rlzkalla AZ, Jones DW, Power NL. J Oral Flehabil 1994:2^553-558. References:22 Reprints: Eiiiott J. Sutiow, Division of Dental Biomatenais. Department ol Applied Oral Sciences, Facultyof Dentistry. Dalhousie University, Haiifa«. Nova Scotia B3H 3J5, Canada.—Howard Jeon, DDS, andStephen D. Campbell. DDS. MMSc. Department ol Restorative Deniistry, University of Illinois at Chicago,

Chicago

Number 4, 1995 331 ai of Prosthodontii