Compatibility ofCeramic-Ceramic Systems

for Fixed Prosthodontics

Patrick I. Steiner, ODS, MS"Mutional Naval Dental CenterBethesda, Maryland

/. Robert Kelly, DDS, MS, DMedSc"

Anthony A. Ciuseppetti"'

National Institute of Standards and TechnologyGaithersburg, Maryland

This Study evaluated dilatometric data for predicting ceramic-ceramiccompatibility for porcelains fired on central incisor copings (n = 72) of highexpansion colored IPS-Emprcss porcelain. Nine body porcelains (leucite 0 wt%to 51 wt7o) were each fired onto eight copings. Cracks were detected at 10 Xrragnification using transilluminating light. Failure was defined as the presenceof at leastonecrackandprobability of failure (P,)a5 the ratio of failed to totalcrowns. Thermal contraction coefficients (o;} were determined using four hars ofeach porcelain following the protocol of ISO 9693. Absolute differences inthermal contraction, |Act|, between core and test porcelains were plotted againstPi and curve fit. Significant differences in o: were found among the porcelainstested (ANOVA, 95% Tukey): the a values ranged from 7.92 to 1 7.83 x10-^/°C; Pf ranged from 0 to 1. Compatible porcelains (no cracks during anyfiring of all eight crowns) had | i a | values less (han 1 x 10"'/"C. Absolutevalues (|Aa|) were surprisingly predictive of P¡ given the very different coolingrates Idilatometry versus dentai lab) and the relatively complex crown shape.Standard dilatometry may be useful for predicting the compatibility of ceramic-ceramic systems. Three porcelains, IPS-Empress dentin, Duceram, andWill-Ceram were successfully used for veneering IPS Empress cores,!r}tj Prosthodont 1997; ¡0:375-380.

A -ceramic systems that provide increasinglyhigher strengths and esthetic potential for

'Lieutenant Commander, Dental Corps, United States Navy,Prosthodontics [Department, Naval Dental School.

"Commander, Dental Corps, United States Navy.'"American Dental Association Pafienbarger Research Center.

Reprint requests: Dr I. Robert Kelly, National institute of Standardsand Technology, Building 2Z4-A143. Gaithersburg, Maryland20899.

Presented as a finalist in the 1996 lADR Arthur R. FrechetteProsthodontic Research Competition, San Francisco, CA.

This work is a contribution from the Nationai Institute of Standardsand Technology. Not subject to copyright.

Certain commercial materials are identified in this paper. In no in-stance does such identification imply recommendation or endorse-ment by the Nationai Institute of Standards and Technology or theUnited States Navy.

replicating natural teeth continue to be developed.Many of these systems are unusual in that theyhave high coefficients of thermal expansion, oftenmore than twice that of conventional all-ceramicmaterials.' Crystalline leucite is the strengtheningphase in these systems and is the same mineral giv-ing metal ceramic porcelains their high expansionproperties. Leucite is often chosen as a strengthen-ing filler because its close index of refractionmatch with feldspathic porcelains minimizes theopacification seen with other fillers (eg, alumina).Esthetic porcelains are now available that range inleucite content from 0% to 51% by weight (wt%),bracketing both sides of traditional metal-ceramicsystems that contain approximately 12 wt% to 14wt%.^ With such a complete range becoming in-creasingly available the possibility of cross-systemcompatibility {ie, between manufacturers) is raised.

10, Number 4, 1597 3 7 5 The Inlernational Journal of Prosthodontics

: Systems (or Fixed Piosdiodomii

Figs 1a and 1b Cross-sectioned crown showing the inner metal coping (gray) veneered wilh porcelain (white), (Left) When thecoefticient o¡ thermal expansion ct the porcelain (n) is much lower than that ot the metai, tangentiai cracks wiil form upon cooling asa result ot tensiie stresses oriented perpendicuiar to Ihe e>cternal surtace (Right) When the thermal expansion of the porcelain ishigher than the metai, craciting wiii occur in a radiai fashion upon cooling.

For example, a traditional metal ceramic porcelainmay be completely compatible with a newly de-veloped all-ceramic core material. Ceramic-ceramic compatibility becomes of practical interestfor both economic and esthetic reasons.Economically, establishing compatibility betweensystems might help dental laboratories moderatesome expense by combining systems rather thanhaving to stock special veneer porcelains,Esthetlcally, the range of color, translucency, andopalescence represented in nature is usually notcompletely encompassed by any one system.

There are instances when advantage was takenof cross-manufaclurer, ceramic-ceramic compati-bility. One highly esthetic, all-ceramic crown wasbased on copings cast from a low expansion glass-ceramic IDicor, Dentsply/York Division, York,PA), These glass-ceramic copings were veneeredwith a compatible low expansion porcelain avail-able for use with an alumina-containing core ce-ramic (Vitadur-N, Vita Zahnfabrik, Bad Säckingen,Germany), This combination of materials alloweda great latitude in developing final translucency,color, surface texture, and form."

IPS-Empress (Ivoclar AG, Schaan, Liechtenstein) isanother easily formed glass-ceramic that may alsobe veneered for improved esthetics. The IPS-Empress material offers advantages over Dicor inbeing intrinsically colored in a variety of dentinshades. Until recently only translucent incisai porce-lains and external colorants were provided to aid in

matching IPS-Empress to natural teeth. The broadrange of tones that natural teeth exhibit were not en-compassed by the modifiers provided, limiting theesthetic results attainable. For the more basic scien-tific purpose of this study, IPS-Empres5 serves as amodel material for investigating ceramic-ceramiccompatibility among high expansion porcelains.

In evaluating potential veneering porcelains forIPS-Empress, the same concerns regarding thermalexpansion mismatches apply as for traditionalmetal ceramic crowns. If the coefficient of thermalexpansion of the porcelain is much lower than thatof the metal, tangential cracks will form upon cool-ing as a result of tensile stresses oriented perpen-dicular to the external surface (Fig la]. Similarly, ifthe thermal expansion coefficient {a) of the porce-lain is higher than that of the metal, cracking willoccur in a radial fashion upon cooling (Fig 1b),The stresses that cause such cracking are directlyproportional to the difference in thermal expansionbetween the two materials, as well as the magni-tude of the temperature drop during cooling,^

Successful material combinations should havewell-matched coefficients of thermal expansion toavoid the incompatibility stresses that lead to crack-ing. This premise is conceptually simple, but in ap-plication it is often complicated by subtle yet highlyinfluential factors, Eor example, the temperaturebL'low which stresses develop (the glass transitiontemperature) is sensitive to the rate at which porce-lains are heated and cooled. Cooling conditions

The inlemälJoriäl fournai of Prostliodorti< 376 VoJiimdO, NLiiiber4, 1997

lic-Cerarric Sysiems for Fixed Prostliodonlies

specified for laboratory research (usually 5"G perminute] can be quite different from the rapid cool-ing rates employed in dental laboratories (500 to1800"C per minute].'''^ Furthermore, crown shapesare complex, and metals and porcelains transmitheat very differently. An oxide layer also exists be-tween the metal and porcelain that can influencethe crack formation. All these factors affect the finalstress state and explain in part why standardizedthermal expansion coefficient measurements aloneare not considered sufficient to predict compatibil-ity in metal ceramic systems."

For ceramic-ceramic systems, many of these com-plications may not exist, and simple standardizedexpansion data may be sufficient to predict compati-bility. To the authors' knowledge, no research hasyet addressed this issue in the dental literature. Thepurposes of this study were both practical and scien-tific. They were (1) to evaluate a variety of porce-lains for compatibility with an IPS-Empress core in aclinically relevant fashion, and 12) to test the hypoth-esis that standardized expansion data may be suffi-cient for predicting ceramic-ceramic compatibility.

Materials and Methods

Nine commercially available body porcelains wereused in this study. The leucite content of these se-lected porcelains varied from 0% to over 50% byweight. The porcelains included in this study were:Vita VMK-68, Vita Omega, Vita Omega 800,Vitadur Alpha (Vident, Brea, CA]; Ceramco II(Ceramco, Burlington, Nj]; Duceram (Degussa,South Plainsfield, NJ); Optec-HSP (Jeneric/Pentron,Wallingford, CT]; Will-Geram, and IPS-Empressdentin (Ivoclar, Buffalo, NY). Six of these porcelainswere originally developed for the veneering of metalcopings. Two porcelains are marketed for firing onceramic copings: (1) Vitadur Alpha for veneering thelow expansion In-Geram core ceramic (Vita); and (2)IPS-Empress dentin for firing on IPS-Empress cop-ings. Optec-HSP was designed to be used for fabri-cation of single unit crowns.

A poly(vinyl siloxane] (PVS] template (ExtrudeType 1 medium viscosity, Kerr Manufacturing,Romulus, Ml) was used to fabricate four bars ofeach porcelain to the nominal dimensions of 32 x8 x 8 mm. The template rested on flat plates madeof ADA type V high-expansion die stone (DieKeen, Miles Dental Products, South Bend, IN].Porcelains were mixed with deionized water to aworkable consistency. Powder-water mixtureswere added to the template under gentle vibration,and excess water was blotted off the surface usingabsorbent paper or was absorbed into the stone.

Manufacturer's recommended first body bake firingschedules were followed for the sintering of barsunder vacuum. The ends of the fired bars weremade parallel to each other using 800-grit car-borundum paper. Parallelism was visually verifiedusing the measuring arms of a digital micrometer(Mitutoyo, Kawasaki, japan]. Each bar length wasmeasured to the nearest 0.001 mm and recorded.

Standard single rod dilatometry (Thermo-dilatometric Analyzer model TD-720C, FHarropIndustries, Golumbus, OFH) was used to calculate theheating and cooling curve of each porcelain in ac-cordance with the protocol of ISO 9693 for dentalmetal ceramic systems.* Determination of the coeffi-cient of thermal expansion was achieved by heatingeach specimen from 25°C to 500°C at a rate of 5°Cper minute. Cooling curves were measured from500°C to 2 5 ^ at 5°C per minute regulated by liquidnitrogen. A platinum standard was used to calibratethe dilatometer after every fourth cycle.

An ivorine tooth (Columbia Dentoform, NewYork, NY] representing a maxillary central incisorwas prepared to receive an all-ceramic crown.Tooth preparation included a circumferential 1-mmshoulder having a rounded axiogingival line angle,1-mm cervical-third reduction, and a 1.5-mm re-duction of the middle to incisai third. The incisaiedge was reduced 2 mm. A complete contour wax-up was fabricated, and the labial aspect was cutback to a thickness of 0.6 mm. A PVS mold wasmade of the modified wax pattern from which nu-merous DuraLay (Reliance Dental, Worth, IL] pat-tern replicas having identical dimensions could beproduced. Seventy-two copings were fabricated,and the 0.6-mm facial dimension was verified usinga thickness caliper (Miltex, Lake Success, NY] capa-ble of measuring to approximately 0.05 pm.Crowns were randomly assigned to the treatmentgroups to mitigate any systematic bias.

After investing and pressing the copings usingshade A-2 colored IPS-Empress ingots, the copingswere devested using 50 jm glass beads at 0.28MPa. Sprues were removed using high-speed dia-mond burs and copious irrigation. The nine bodyporcelains were fired onto eight copings each.Individual copings received 0.18 g of porcelainthat had been mixed with deionized water to aworkable consistency. All copings were veneeredby the same operator, and efforts were made tofabricate crowns having a uniform veneer of porce-lain. Weighed portions of powder were used toachieve control. The manufacturers' recommendedfiring cycles for each porcelain were followed for afirst and second body bake under vacuum. Thesefirings were followed by a glaze bake in air.

VolumdO, Number 3 7 7 The Inlernational Joiirnsl of Prosthodontics

lor Fiífd Proslhodonm

8

7

6O^ 5o

i. "^ 3

2

1

— 1

1 11 1, , 1 1[ ll 1

1 1

1 2 3 4 5 6 7 8 9 10Porceiain

Fig 2 Absoiute differences in thermai conlraction cooling co-efficients between the lest porceiains and the IPS-Empresscore ceramic. Ceramics are numbered as per the Tabie.

2 4iAc<i(x

Fig 3 Probability of faiiure piotted as a function of lAai, Ttieseabsolute ditferences in cooling coefficients (reiative to that ofIPS-Empress core) were weli fit by a steep, step-like cumulativeprobability function (degrees of freedom adjusted r - 0.986).

Table Test Porcelains Ranked by CoolingCoefficients (a)

D era míe Type

Vitadur AlphaVita OmegaCeramco liDuceramPS-Empress corePS-Em press denim

Wili-CeramVila VMK 68Vita Omega 800Optec-HSP

« (X 1O^/"C) i i o l -

7,92 6.0812,8012.9513.7014,00

1.201.050.30

14,5514,58

—

15,15

0.550.581.15

16.70 2.7017.83 3.83

P,

1.00.375.125.00—

.00

.00

.1251.001.00

'Absolute uaiue ut tlie diffeEmpress core. Verticai bai(ANO VA. 95% Tu key Test ¡

3nce in o as compared with the IPS-( indicate statisticaiiy simiiar groups

Between each firing cycle, the specimens wereobserved at 10 x magnification under a stereomi-croscope using transilluminated light to detectcracking in both veneering and core ceramics.Serial drawings of crack systems for each crownwere recorded on a standardized form. Failure wasdefined as the presence of at least one crack fol-lowing any firing. Probability of failure was definedas the ratio of failed crowns to total crowns foreach specimen set. Because this study was basedon clinically relevant conditions and phenomena,only macrocracking was evaluated.

Results

The ceramics are ranked, lowest to highest, bycooling coefficients (a) in the Table. The verticallines group cooling coefficients that did not differsignificantly as calculated using analysis of variance(ANOVA, P < .05) and a 95% Tukey test. The ab-

solute differences between the cooling coefficientsrelative to the IPS-Empress core material (|Aa|) andthe probability of failure for each porcelain (ratio offailed to total crowns) are listed in the Table.

Figure 2 illustrates graphically the absolute dif-ferences in thermal contraction as between testporcelains and the IPS-Fmpress core ceramic,keeping the same order of porcelains as in theTable, Porcelains below the horizontal line werecompletely successful as veneering porcelainsbased on the failure criterion previously definedand under the conditions of this study. Successfulporcelains (no cracks during any firing of all eightcrowns! were characterized as those having |Aa|valuesof less than 1 x 10-V°C.

Figure 3 illustrates that the probability of failure,plotted as a function of |Ac(], was well modeled by asteep step-like cumulative probability function, to adegrees of freedom adjusted r of 0.986 (TableCurve2-D, )andel Scientific, San Rafael, CA). Probability offailure appears to be a very sensitive, but well-fitfunction of absolute differences in cooling coeffi-cients. A thermal expansion coefficient difference ofonly 1.5 to 1.6 X l t r V C resulted in complete fail-ure (at least one crack in every crown fired).Examples of crowns having large cooling coefficientdifferences are shown in Figs 4 and 5. Copings inFig 4 were veneered with the nonleucite-containingporcelain (Vitadur Alpha) and had the largest ab-solute cooling coefficient difference. Figure 5 showscrowns veneered with porceiain (Vita Omega 800],the second highest contraction porcelain. The threeporcelains that did not exhibit cracking wereIPS-Empress dentin, Duceram, and Will-Ceram.

The Interrational loumal of Prosthodoniics 378 • 1Û, Number 4, 1997

c-Cer,imic Syslems for Fixed Proilliocluntil

Fig 4 Copings veneered with the nonleucite-containingVitadur Alpha (lowest relative a) had the largest absolutecooling coefficient difference. Fracture clearly involved thecore ceramic as well as the veneer porcelain.

Fig 5 Crowns veneered with the second highest relativecontraction a (Vita Omega 800) demonstrated extensivecracking, mainly limited to the veneer.

Discussion

This study illustrated that a range of contractionbehavior is demonstrated by currently marketedporcelains. It can be 5een from Fig 2 that testporcelains ranged fairly uniformly to either side ofthe IPS-Empress core, with the porcelains on theleft having cooling coefficients that were lower andthoie on the right having higher coefficients. Thisranking seems to generally, but not entirely, followavailable literature values for leucite content:Vitadur Alpha (0 wt%] ' ; Ceramco II ¡21,5 wt%)^(25 wt%]\- IPS-Empress core (23,6 wtX)^; VitaVMK 68 (19.3 wt%]2; and Optec-HSP (50,6 wt%)^(41 wt%¡'. Part of this scatter between cooling co-efficients and leucite content may be related to ex-perimental and/or product manufacturing varia-tions. It is also possible that glassy phases in theseporcelains also contribute to their contraction be-havior. Eor example, one very high expansionporcelain, not included in this present study, hasbeen reported to contain no leucite,^

A "beneficial" range of mismatch probably existsfor metal-ceramic systems. Increasingly sophisti-cated viscoeiastic finite element analysi ^ is con-firming previous thermoelastic analysis regardingthe development of compressive principal stresseswhen metal contracts slightly faster than the porce-lain. It must be remembered, however, that thereare "protective" compressive stresses in the porce-lain only because there exist equal tensile "sys-tems" in the metal. The metal can withstand thissituation, which is a clever "systems" approach toimproving the bimaterial structure. The importantfactor for ceramic-ceramic systems is whether thecore porcelain can withstand constant tensile load-ing. In the present study, specimens mismatched inthis direction appeared to have failed from cracksinvolving the core material.

Based upon the probability of failure curve-fit inFig 3, a "safe" tolerance in cooling coefficient mis-match appears to be less than 0,6 X 10"^/°C, Formetal ceramic systems, a safe tolerance of 0,225x lO-VC has been proposed based upon thermalstress calculations,^ It may be that ceramic-ceramicsystems are more tolerant because of their relativesimplicity (ie, no oxide layer, no opaque porcelain,similar thermal diffusivities). It may also be thatthese two safe tolerances cannot be considered assignificantly different, given the experimental vari-ation in their measure and probable variations incooling coefficients as a function of firing history.The steepness of the curve in Fig 3 demonstratesthat once the small cooling coefficient differencefor success is exceeded, failure probabilities riseextremely rapidly over a very narrow range.

Most experts, however, would disagree with theconcept that standard dilatometry data (eg, the safetolerance discussed above) is sufficient to predictmetal-ceramic compatibility. In the InternationalOrganization for Standardization (ISO) specifica-tion for dental metal ceramics, it is stated, "Themeasured values for coefficients of linear thermalexpansion are compared with the manufacturer'svalues as a means of quality control, but the valuesdo not themselves provide an assurance that thealloy and ceramic are compatible,"^ In light of this,it is rather remarkable that such a strong empiricalfit was achieved in this study between researchlaboratory values of lAcd and the presence or ab-sence of cracking on clinical crowns.

The physical simplicity of ceramic-ceramic sys-tems compared to metal ceramics has already beenbriefly discussed. Another faclor related to glasschemistry might also have played a role in allowinglaboratory data to be extrapolated to cooiing ratesover one thousand times higher to predict dentallaboratory performance. All of the ceramics used in

VolumelO, Number 4, :'>'I7 379 The Internationjl lour nal of Prosthodonlii

: Systems for Fixed Pro!(liotiontics

this Study are based upon feidspathic glass composi-tions, including iPS-Empress.'" Residual stress levelsin porcelains coupled to other materials are quitesensitive to shifts in the glass transition temperature(T ) of the porcelain and changes in thermal behav-ior near the T . It may be that the glassy phases wereso similar in tbe porcelains studied that all wereshifting simultaneously. This is obviously one verylarge difference compared to metal ceramic systems.

Benefits that the clinician might derive from thisstudy are that valid, predictive information may be-come readily available on the compatibility ofproducts between manufacturers. Ceramic-ceramiccompatibility has both economic and esthetic im-portance. As new ceramics are developed, dentallaboratories could benefit by being able to combinesystems. Combined systems may allow for abroader range of esthetic capabilities. Further workon ceramic-ceramic compatibility seems to be war-ranted to determine whether the results of this studycan be expanded for practical application.

Conclusions

Nine feldspathic porcelains having varying coeffi-cients of thermal expansion were fired on con-trolled IPS-Fmpress cores and evaluated for crack-ing. Standardized dilatometry was used to obtainthermal contraction coefficients, and these datawere evaluated for their value in predicting crack-ing. Within the parameters of the study design, thefollowing conclusions may be made:

1. Porcelains that did not exhibit any crackingduring firing of all eight crowns had l i a i val-ues of less than 1 X 10-V'C.

2. Thermal expansion coefficient differences of only1,5 to 1.6 X lO-^/X resulted in complete failuredefined as at least one crack in every crown fired.

3. IPS-Empress dentin, Duceram, and Will-Ceramporcelains were successfully veneered on IPS-Empress copings without cracking in any of theeight units.

4. Based on the probability of failure curve fit, asafe tolerance in cooling coefficient mismatchappears to be less than 0.6 x 10-V°C for theceramic-ceramic systems studied.

5. Standard dilatometry may be useful for predict-ing tbe compatibility of ceramic-ceramic sys-tems.

References

1. Piché PW, O'Brien W|, Groh CL, Boenke KM Leucite conteníof selected dental porcelains. J Biomed Maler Res1994;2a:603-609.

2. Denry IL, Rosenstiel SF, Holloway ]A. Characterization ofcrystalline leucite in feldspalhic dental porcelains |IADR ab-stract 21 39), I Denl Res 1994;73:369.

3. Cdler W, Kwiatkowskj S). The Willi's glass crown: A new so-lution in the dark and shadowed zones of esthetic porcelainrestorations. Quintessence DentTechnol 1987;11:233-242.

4. Campbell SD. Aesllietic modification of cast dental-ce ramierestorations. In! | Prosthodoni ]990;3:123-129.

5. Kingery WD, Bowen HK, Uhlmann DR. Introduction toCeramics, ed 1. New york: John Wiley & Sons, 1976:816-840.

6. Fairliurst CW, Hashinger DT, Twiggs SW. The effect of ther-mai history on porcelain expansion behavior. | Dent Res19S9;68:1313-1315.

7. Asaoka <, Kuwayama N, Tesk JA. Influence of temperingmethod on residual stress in dental porcelain. | Dent Res1992;7I:1623-1627,

8. Internationa] Standards Organization, ISO/TC 106 Dentistry.Dental Porcelain Fused to Metal Restorations [SpecificationiSO 9693:19911, Geneva, Switierland: 1991.

9. Nielsen IP, Tuccillo JJ. Calculation of interfacial stress in den-ial porcelain bonded lo gold alloy substrate, J Denl Res1972;51:1043-1047.

10. Dong |K, Luthy H, Wuhlwend A, Scharer P. Heal-pressed ce-ramics: Technology and strength. Int J Proslhodont1992;5:9-16.

The International Journal of Pcosthodonlii 380 •10, Nijmber4, 1997