Resistance to Fractureof Metal Ceramic and

All-Ceramic Crowns

Dario Castellani, MD, DDS'

Tiziano Baccetti, DDS"

Alberto Giovannonl, DDS"

Ubaldo Dino Bernardini, MD, DDS'

Department af ProsthodonticsSchool of Dental MedicineUniversity of FlorenceFlorence, Italy

This study evaluated the fracture resistance of three types of all-ceramiccrowns and compared these to the fracture values of metal ceramics.Uniform metal ceramic specimens; veneered, cast glass-ceramic; andporcelain fused to two different dispersion-strengthened ceramic cores(Hi-Ceram and In-Ceram) were investigated. The metai ceramic specimensdemonstrated a significantly higher resistance to fracture than did theHi-Ceram or veneered glass-ceramic units but did not significantly differ fromthe In-Ceram specimens. The metal ceramic crowns showed cracks only inthe ceramic layer, whereas the all-ceramic specimens underwent globalfracture. Int I Prosthodont I994;7:149-154.

R ecent progress in the technology and researchof new materials has broadened the choices for

esthetic single-crown restorations. Traditionalmetal ceramic restorations have been augmentedby alternative solutions that eliminate the metalsubstrate and allow better translucency and simu-lation of natural tooth appearance. The metal sub-structure may be replaced by an aluminous core orfused vitreous ceramic that improves translucencyand increases the strength.

In the authors' opinion, the use of such optionsis limited to single-unit restorations, for even rein-forced all-ceramic units do not now offer sufficientguarantees for use with fixed prostheses. Strength,marginal precision, and esthetic results are mattersof concern even for single-unit restorations.

Studies testing the fracture resistance of metalceramic'-'' and all-ceramic crowns''" have beendescribed, and different elements have been con-sidered responsible for the limited durability of all-ceramic restorations. The rapid development ofnew materials makes clinical verification of these

'Clinicai Professor."Research Fellow.

"-Professor and Head.

Reprint Requests: Dr Dario Casteilani, Viale Don Minzoni 39,

1-50129 Florence, Italy.

innovative technologies difficult, and marketinginterests are often overly optimistic.

The purpose of this study was to evaluate thefracture resistance oí three types of all-ceramiccrowns and to compare these values with those ofmetal ceramic units. For lack of other standards ofreference,' the values of the metal ceramic restora-tions were used as parameters.

Materials and Methods

The study was designed to determine the frac-ture resistance of metal ceramic specimens,veneered glass-ceramic specimens (Dicor,Dentsply, York, PA), aluminous-core porcelainjackets (Hi-Ceram, Vita Zahnfabrik, BadSackingen, Germany [Hl-C]), and glass-infiltratedaluminous-core porcelain jackets {In-Ceram, VitaZahnfabrik |IN-CI). Seven specimens were fabricat-ed for each of the materials. To minimize varia-tions in crown form, dimension, or thickness, thespecimens were fabricated using an assemblydevice designed at the University of Michigan thatcontrolled the predetermined thickness of symmet-ric crown-shaped specimens (Fig 1). The stainlesssteel master die was machined to approximate thedimensions of a premolar abutment (Fig 2a). Theassembly device was constructed in such a way as

i/oljme 7, Number 2, 1994 149 Tlie Irlernalional loumal of Pn

istance ol Motal Ce

to provide chambers with a constant thickness of0.5 mm for the substructure and a total crownthickness of 1.5 mm following ceramic veneering(Fig 2b).

The alloy and cast glass substructures of themetal ceramic and veneered glass-ceramic speci-mens were cast using the lost wax method.Following preheating of the assembly in a 6O''Cwater bath to prevent possible distortions, meltedwax was poured into the assembly chamber.Placement of the upper portion of the assemblyshaped the sample and controlled the thickness ofthe occlusal portion while pressing the wax intothe chamber. After bench cooling, the assemblywas opened, the wax patterns were removed andinspected, and incomplete samples were dis-carded. Thickness was randomly checked using awax caliper.

Fig 1 Stainless steel master die andttie crown construction assembly.

The metal ceramic specimens were cast in agold-palladium alloy (V Delta, Metaux-Precieux SAMetalor, Neuchatel, Switzerland) using a gas-airtorch and centrifugal casting technique. The inter-nal cores for tbe veneered glass-ceramic crownswere cast using materials and equipment specificfor the Dicor technique (Dentsply).

The Dicor core was divested and then embeddedin the ceramming investment for thermal treatment.All the castings obtained were inspected andrefined, and their thickness was controlled by usingtbe brass templates designed for this purpose (Figs3a to 3c). Glass-ceramic castings that had grossdefects or were incomplete were eliminated. Thecerammed layer was maintained to strictly followthe indications of the instruction manual."

The fabrication procedures for the Hl-C andIN-C crown cores necessarily entailed tbe use of arefractory die onto which the porcelain was con-densed. For this reason, the specimens could notbe fabricated using the methods previously out-lined. The stainless steel master die was duplicatedin the appropriate refractory material, the ceramiccore material was condensed and fired in threesuccessive stages, and the thickness was controlledat every successive stage using the brass template.The IN-C specimens were subjected to vitreousinfiltration according to the requirements of thatspecific technique.

All the cores thus obtained were refined andcarefully inspected by an optical stereomicroscope(Model PM-10M, Olympus Optical, Tokyo, Japan)to relieve any possible irregularities of the internalor exiernal surfaces. Complete seating of the speci-mens on the master die was also ensured andchecked by using a silicon fit checker (CC Fit-Checker, GC Dental, Tokyo, Japan).

In a preliminary phase of tbe study, the meansand the standard deviations for the measurementsof the thickness of the four different groups ofmaterials (seven specimens for each group) were

Fig 2a (Left) Stainless steei masterdie mactiined to approximate thedimensions of a premolar abutment witha round shoulder finishing iine.

Fig 2b (Right) Assembly device fabri-cated to provide "chambers" with a con-stant thickness of 0.5 mm for the sub-structure and 1.5 mm for the total crownwith the ceramic coating.

of Prosthodortics 150

Figs 3a to 3c Brass templates used to control the uniform sample thickness, check the core thickness, and check the completespecimen thickness.

Fig 4a Inlernal and external aspect of a metal ceramicspecimen.

Fig 4bmen.

Internal and externai aspect of an all-ceramic speci-

tested by analysis of variance (ANOVA] and no sig-nificant differences were found.

Once the cores were fabricated, they were seat-ed on the stainless steel master die in the assemblydevice that initially allowed a 1,5-mm thicknessof the occlusal and axial walls, A 1.0-mm spaceremained for the porcelain veneering after thecores were placed. The porcelain veneer materialswere then condensed using a mechanical vibrator(Dental Farm, Torino, Italy) to ensure uniform over-all thickness and complete distribution on thewalls and peripheral margins. For the metal ceram-ic crowns. Vita VMK 68 (Vita Zahnfabrik) porce-lain was used, and all-ceramic specimens wereveneered using Vitadur N (Vita Zahnfabrik] porce-lain. The firing schedules were those recommend-ed by the ceramic manufacturers (Table 1),

After the first firing, a second addition and firingwere required to compensate for porcelain shrink-age and to fill in potential voids, A third firing wasperformed to mimic the glazing cycle. The metalceramic crowns were constructed by veneering the

Table 1 Specimen Processing Temperatures (°C)

PFM

Hl-CIN-C

VGC(Dicorcore)

Core

1450 (Casting temperaturefcr V Delta metal copings]1170 (Alumina)1120 x2h (Alumina]Then 1100x4h(Glass infiltration]1350 (Casting temperaturefor Dicor core)1070 X 6h (Dicor ce ram m ing)

Veneeringporcelain

920 (Vita VMK 68]

960 (Vitadur N)960 (Vitadur N)

960 (Vitadur N)

metal core with just body porcelain and withoutapplying any opaque layers. The rationale for thisprocedure was to maintain consistency with thenumber of firing cycles used for the other samples(Figs 4a and 4b],

Specimens were tested for resistance to fractureusing an Instron machine (Instron, Canton, MA) ata crosshead speed of 1 mm/min. Two coppersheets were placetd above and below the speci-

7, Number 2, 1994 151 The international Journal ol Prosthodont i i

Fr.icturc R™,lance oí Metal Cpramic and All-Ceramic Cron

Fig 5a Upper membei of Instron apparatus (1)\ lower (fixed)member of Instron apparatus (2): copper foiis (3); core (4):veneenng porcelain (5): and points ot maximum concentra-tion ot stress dunng loading (S', S").

Fig 5b Crown specimen loading in theInstron testing mactime.

mens to better distribute the contact and attempt toavoid stress concentration (Figs 5a and 5b), Theapplied stress was measured at the moment of frac-ture following the experimental model proposedby Hondrum," Pressure (ratio of load to surface)was not calculated, because it was impossible toprecisely and uniformly measure the area of theapplied load. As a result, the values obtained canbe compared only within the sphere of this experi-mental design.

The results were subjected to ANOVA andStudent's ; test for paired comparison to identifysignificant differences (P < ,05¡ between the fourmaterials groups tested.

Results

Metal Ceramics Hl-Ceram Veneered in-Ceramg i ass-ceramic

Fig 6 Fracture load versus crown type.

The mean fracture loads ranged from a maxi-mum of 22,37 Kg (SD 5,62) for the IN-C specimensto a minimum of 5,55 Kg (SD 0.518) for tbeveneered glass-ceramic specimens (Table 2, Fig 6),The statistical evaluation was completed using anANOVA and a paired comparison.

The metal ceramic specimens demonstrated asignificantly higher fracture resistance than did theHl-C (P < ,001) and veneered glass-ceramic (P <,001) specimens. There was no significant differ-ence between the metal ceramic and IN-C speci-mens (P> .05) (Table 3),

There were substantial differences between themanner in which the all-ceramic and metal ceram-ics units fractured. The all-ceramic specimens frac-tured gfobaliy ("catastrophic fracture"). Crackswere visible through the entire thickness of these

Table 2 Mean Fracture Loads (kg)

Specimen n

Metal ceramicHigh-CeramVeneered g i ass-ceramicin-Ceram

Table 3 Statistical Analysis

ANOVAF = 37.08

Student's t testHigh-Ceram • Metal-ceramics

Mean

7 17,0487 8,4457 5.1557 22,370

SD

±3.801+ 0.746±0,518± 5,620

Veneered glass-ceramic • M eta I-cera m tes P.Meta I-ce ram i es • In-Ceram

P < ,001 S.001,05

S = significant: NS = ror signifitart.

SNS

The Intómütional lournal of Proslliodonti. 152

units, and Ihey shattered macroscopically into atleast 2 lo 3 pieces. The metal ceramic specimensshowed superficial cracking that affected only theceramic layer, sometimes with a few chips break-ing away.

Discussion

This study gives evidence of the different behav-ior between the materials tested. Significant differ-ences were found between the four types of speci-mens tested, with the exception of ihe comparisonbetween the metal ceramic and IN-C crowns.

The hypothesis could be made that crack propa-gation of the all-ceramic crowns follows a differentpath than that of the metal ceramic materials. Inthe all-ceramic specimens, the crack starled in themore porous layer;" from there, the crack easilyextends for the entire thickness because of theintrinsic fragility of the ceramic material. Theporcelain of the metal ceramic specimens tends toadhere to the metal substrate even when complete-ly fractured.

This diversity in behavior essentially seems to bedependent upon the properties of the substructure(core) material, including the Young's modulus,'thickness,' porosity,'"" and geometry.' Accordingto Campbell,' the veneer material's resistance tosiress is closely related to the rigidity of the core.This means that an indirectly conducted testexpresses the coefficient of elasticity of the corematerial. It can be stated thai the metallic substruc-ture's high coefficient of elasticity gives increasedresistance to stress, which limits the extension ofthe crack to the superficial layers. The ceramicmaterials generally present lower values for thecoefficient of elasticity;" therefore, they offer lessresistance to stress and favor an extension of thecrack to all layers.

An even Ihickness of the internal core is particu-larly important, as this has an influence on defor-mation in a relationship to the third power. Smallvariations in thickness can have a considerableeffect on the overall resistance of the restoration.'The specimens in this experimental model werefabricated using the Michigan assembly device toensure even thickness, thus minimizing ihis sourceof variability.

Porosity is also important in determining aceramic material's resistance to stress.""' In-Ceramcrowns are infiltrated with moiten glass, providinga homogenous, bubble-free core consisting of fineparticles in a vitreous matrix. This probablyexplains the greater stress resistance of this materi-al. Moreover, the cylindrical form of the samples

icLure Resistance o( Metai Ceramic and Ail-Ceramic Crowns

causes a maximum stress concentration on theintemal surface, related lo the application point ofthe load (Fig 5al,'- This corresponds to the internalcore of Ihe specimens, and thus it may emphasizethe greater stress resistance of IN-C crowns ascompared to the other all-ceramic samples.

From a clinical viewpoint, the resistance of all-ceramic crowns to fracture is improved by etchingand bonding techniques,""'" However, the reliabili-ty of resin cements over the course of time has notyet been clearly established. Furthermore, occlu-sion plays a primary role in determining the phe-nomenon of "static" fatigue, which causes the slowpropagation of cracks in all-ceramic crowns. Invitro evaluation is undotjbtedly a valid method ofsingling out variables that affect the strength ofceramic restorations, but such tests do not trulyrepresen! the clinical environment. To demonstratethe actual clinical strength of prosthetic crownsand the materials used in their fabrication, it willbe nL'cessary to subject appropriate experimentalmodels to a more complete verification of the pre-viously obtained results.

Conclusions

This study evaluated resistance to load in all-ceramic and metal ceramic specimens. Within thelimitations of the study design the following state-ments can be made:

1, All-ceramic crowns constantly exhibited anoverall (catastrophic) fracture, independent ofthe values of load resistance. Metal ceramicspecimens showed multiple cracks with porce-lain chipping, and single ceramic particlesremained bonded to the metal,

2, The In-Ceram specimens showed a significantlyhigher average resistance to fracture than theother all-ceramic specimens but were not statis-tically different from Ihe values obtained fromthe metal ceramic samples.

Acknowledgments

The authors wish to thank Arianna Bracconi Dental Lab andMr Giovanni Furno for constructing the samples.

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