Shear bond strengths of pressed andlayered veneering ceramics to highnoble alloy and zirconia cores
Motoaki Ishibe, DDS,a Ariel J. Raigrodski, DMD, MS,b
Brian D. Flinn, PhD,c Kwok-Hung Chung, DDS, MS, PhD,dCharles Spiekerman, PhD,e and Robert R. Winter, DDSf
University of Washington, Seattle, Wash
Statement of problem. Heat-pressed ceramics to metal alloys and zirconia have been available for some time. However, information regarding their shear bond strengths is limited.
Purpose. The purpose of this study was to evaluate the shear bond strengths of heat-pressed and layered ceramics
with regard to their corresponding high-noble alloy and zirconia cores.
Material and methods. Forty cylinders (approx. 5 mm in diameter) of high-noble alloy (Olympia) were cast and divided into 4 groups (n=10). Metal cylinders were veneered with ceramics to produce shear test specimens: Group PMIwith IPS InLine POM; Group LMI with IPS InLine; Group PMC with Pulse press-to-metal; and Group LMC with Authentic Pulse Metal ceramic. Forty cylinders (approx. 5 mm in diameter) of zirconia (Lava) were obtained and divided
into 4 groups (n=10). These cylinders were veneered with ceramics to produce shear test specimens: Group PZI with IPS
e.max ZirPress; Group LZI with IPS e.max. Ceram; Group PZV with VITA PM9; and Group LZV with VITA VM9. The veneering ceramics, 3 mm in thickness, were either pressed or layered to their corresponding cylinders. Thermal cycling was
performed at 5°C and 55°C for 20,000 cycles with a 20 second dwell time. Shear bond strength testing was conductedin a universal testing machine, and the failure strengths were recorded. Fracture surfaces were characterized visually, under a stereomicroscope, and with a scanning electron microscope (SEM). Data were analyzed using rank-based KruskalWallis and Mann-Whitney tests with Bonferroni correction to adjust for multiple comparisons (α=.05).
Results. For metal ceramic specimens, the mean (SD) shear bond strengths ranged from 37.8 (20.6) MPa to 66.4
(22.1) MPa. There were significant differences between Groups PMI and PMC and between Groups LMI and PMC, inwhich Groups PMI and LMI had significantly higher strength values than Group PMC (P=.041). For zirconia ceramic
specimens, the mean (SD) shear bond strengths ranged from 30.03 (9.49) MPa to 47.2 (13.0) MPa, with Group LZV
having a significantly higher shear bond strength value than Group LZI (P=.012). Half of the Group PZV specimens
failed during thermal cycling, and Group PZV was, therefore, excluded from statistical analysis. For all shear bondstrength testing specimens, cohesive failures in the veneering ceramics were observed.
Conclusions. For shear bond strength of veneering ceramics to high-noble alloy, there was no significant difference
between pressing and layering with the same manufacturer. For shear bond strength of veneering ceramics to zirconia,
there was no significant difference between the pressed and layered groups. (J Prosthet Dent 2011;105:29-37)

Clinical Implications

The results of this in vitro study suggest that, for shear bondstrength, clinicians and dental laboratory technicians should consider the use of pressed ceramics to high noble alloy and zirconia as
an alternative to traditional layering procedures.
Graduate student, Graduate Prosthodontics, Department of Restorative Dentistry, School of Dentistry.
Professor and Director, Graduate Prosthodontics, Department of Restorative Dentistry, School of Dentistry.
cResearch Associate Professor, Materials Science and Engineering.
dProfessor, Department of Restorative Dentistry, School of Dentistry.
e
Research Associate Professor, Department of Dental Public Health Sciences.
fAffiliate Professor, Graduate Prosthodontics, Department of Restorative Dentistry, School of Dentistry.
ab

Ishibe et al

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Volume 106 Issue 1
For complete fixed dental prostheses, metal ceramic restorations,for which there are long-term clinicaldata, have been a preferred restoration.1-3 Currently, the clinical data4-6
suggest that zirconia-based restorations should be considered as an
emerging restorative option. Unlike
monolithic restorations, such as goldcomplete crowns and unveneered
lithium disilicate-based restorations,these restorations consist of core
(coping, framework, or substructure)and veneering ceramic.7 Traditionally, veneering ceramics are layeredon metal or zirconia core materials toestablish an optimum esthetic outcome. An alternative technique is topress veneering ceramics to the corematerials. Although the pressing technique is not a new technology,8-10 aprocess for pressing ceramics to metaland zirconia cores using the lost-wax
technique and glass-ceramic ingots
has been recently developed.
In pursuit of superior esthetics andbiocompatibility, the use of ceramicmaterials has increased in clinicalpractice and research.11-14 Esthetic ceramics such as feldspathic porcelain
are brittle in nature and weak in tension.6 Therefore, high strength corematerials have been used to reinforce
the esthetic, but brittle, veneering
ceramics. Such high strength cores
not only support the veneering ceramic but also allow clinicians to use
a wide range of conventional or adhesive luting protocols during insertion of the restoration.15,16 It should
also be emphasized that adequate
tooth preparation and carefully managed laboratory procedures, including
maintaining a smooth, uniform thickness of the veneering ceramic on the
cores, are also important.17
Metal alloys have been extensively
used as cores of metal ceramic restorations ranging from single crowns tolong-span fixed dental prostheses.1
Disadvantages of metal alloys are thatthey do not allow any light transmission and hinder esthetic harmony withthe veneering ceramics. Therefore,
the masking of metal with opaqueporcelain following airborne-particle
abrasion and oxidation is required.
In contrast, zirconia allows some lighttransmission,18 and thus, veneering
ceramics can be applied directly to
the zirconia cores without masking. In
addition, zirconia has better mechanical properties such as flexural strengthand fracture toughness compared toother ceramic materials.19
The layering technique has been
the principal method of applying veneering ceramics to the core material. With this technique, porcelain
powder is mixed with modeling liquid, and the mixture is layered on thecore using a brush. The layer is usually
over built to compensate for condensation and firing shrinkage. Overall,this technique requires skill and multiple applications and firings. With
the pressing technique, a complete
contour anatomical waxing is performed on a core, and subsequently
a sprue is attached to the wax, andthe wax-core complex invested. The
wax is eliminated in an oven and ceramics are heat-pressed into the moldand to the core, thereby reproducing
the anatomy created in the wax and allowing for the creation of the desired
tooth anatomy. Moreover, the firing
shrinkage experienced with the layering technique is minimized, resulting
in a better fit of the porcelain margins
to the abutments.20,21 Distortion of themetal may be reduced during veneering because of support from the investment.22 However, the heat-pressed
method makes development of excellent esthetics more difficult, because
the appearance relies on precolored
ingots. However, this esthetic limitation can be minimized by a combined
technique in which a foundation layer
is created on the core with the pressing technique, leaving space for a subsequent and more esthetic ceramic tobe applied as a powder.
Both in vivo prospective and retrospective studies evaluating the clinical performance of metal ceramic andzirconia-based restorations have been
published.1-6,23-25 Since metal ceramic
restorations have a record of docu-

The Journal of Prosthetic Dentistry

mented outcomes, the performanceof ceramic restorations is often compared to that of metal ceramic restorations. Sailer et al25 reported a similar survival rate for both metal andzirconia-based posterior partial fixed
dental prostheses (FDPs) at 3 yearsof function and with a 100% survival
for each. Both types of restorations
exhibited minor chipping of the veneering ceramic with more extensive
fracture occuring with zirconia-based
restorations. When evaluating mechanical complications, chipping anddelamination of the veneering ceramic have been more frequently reported than damage to the core.4,24,25
An understanding of the properties ofveneering ceramics and the bonding
mechanism at the interface between
the core and the veneer material isessential. Interestingly, Beuer et al26
evaluated posterior zirconia-based
3-unit FDPs that were veneered using pressable glass-ceramic and reported no chipping of the veneering
porcelain. However, 2 restorations
needed to be replaced due to zirconia
framework fracture and loss of retention. This may imply that the application technique of veneering ceramics
has the potential to reduce the complication of cohesive fracture. Someauthors have used various methods
to evaluate the core-veneer bondstrengths of veneering ceramic tometal alloy and zirconia cores, and astandard for bond strength has been
established for metal ceramics, but
not for zirconia.27-30
Shear strength is the maximum
stress that a material can withstand
before failure in a shear mode ofloading and is particularly valuable
in the study of interfaces between
materials.31 For both metal and zirconia ceramic restorations, studies
regarding shear bond strengths havebeen published.32-40 According to theInternational Standards Organization(ISO), the bond between the metal alloy and the veneering ceramics should
be a minimum of 25 MPa.41 However,to the authors’ knowledge, a study
evaluating shear bond strengths of

Ishibe et al

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July 2011
both pressed and layered ceramics tothe corresponding metal and zirconia
cores has yet to be published.
The purpose of this in vitro study
was to evaluate the shear bondstrengths of pressed and layered ceramics to their corresponding highnoble alloy and zirconia cores. The
first null hypothesis was that the shear
bond strength of pressed ceramic
would not be different from that oflayered ceramic to high-noble alloy.
The second null hypothesis was thatthe shear bond strength of pressedceramic would not be different fromthat of layered ceramic to zirconia.

MATERIAL AND METHODSIn this study, pressed ceramics tocore materials were defined as theexperimental groups, and layered ceramics to the core materials were control groups. Fabrication of specimens
and shear bond tests were performed

by a single operator.
For the metal ceramic experiment,
40 cylindrical metal specimens were
fabricated and ceramics were pressedor layered on one end of the cylinders.
The materials used, the corresponding lot number, and the manufacturers’ information are presented inTable I. Firing cycles are presented inTable II. To fabricate the metal cylinders, a stainless steel mold, which had
holes approximately 5 mm in diameter and depth, was used, and autopolymerizing acrylic resin (Pattern Resin
LS; GC America Inc, Alsip, Ill) was
injected into the holes and allowed
to polymerize. The patterns were
evaluated under a stereomicroscope(Inoue Attachment, Tokyo, Japan)
using x10 magnification, and the defects were corrected with wax (GEO
wax wire medium hard blue 3 mm;
Renfert GmbH, Hilzingen, Germany)
when needed. Wax sprues (GEO wax
wire medium hard blue 3 mm; Ren-

fert GmbH) were attached to thepatterns, which were then invested
with a carbon-free phosphate investment (Cera-Fina; Whip Mix Corp,
Louisville, Ky). The invested patterns
were placed in an oven (KaVo Dental
GmbH, Biberach, Germany). Highnoble metal alloy (Olympia; Jelenko,
San Diego, Calif ) was melted using
an oxygen-gas torch and cast using a
non-vacuum centrifugal casting machine (Centrifico; Kerrlab, Orange,
Calif ). After divesting and removing
the sprues, the metal cylinders weredivided into 4 groups and coded byveneering process and manufacturer(n=10): Group PMI: IPS InLine POM(Ivoclar Vivadent, Schaan, Liechtenstein), Group LMI: IPS InLine (Ivoclar
Vivadent), Group PMC: Pulse pressto-metal (Ceramay, Neu-Ulm, Germany), Group LMC: Authentic Pulse(Ceramay).
The surfaces to be veneered were
finished using 240 grit silicon carbide

Table I. Study materials evaluatedMaterial

Lot Number(s)Manufacturer

Metal ceramic specimens:
3547839, 3302082, 3342076

Jelenko, San Diego, Calif

L47540 (Pa), L28110 (L)Ivoclar Vivadent, Schaan, Liechtenstein

L34080

Ivoclar Vivadent

IPS InLine Dentin A2

L53664 (Po) L39576 (L)Ivoclar Vivadent

IPS InLine/IPS InLine POM Glaze

Olympia metal alloy
IPS InLine/IPS InLine PoM Opaquer
IPS InLine PoM Ingots S2

L49281 (Pa), L59382 (L)Ivoclar Vivadent

Pulse Paste Opaque 950°C

3213120947

Ceramay, Neu-Ulm, Germany

Pulse Press to Metal Ingot interface 23030130108

Ceramay

3200120306 (Po), 1290010108 (L)Ceramay

1110600308, 1100650907

Ceramay

Pulse Dentin A2
Authentic Glaze Paste flour
Zirconia ceramic specimens:
Lava zirconium dioxide

3M ESPE, Seefeld, Germany
L32974 (Po), H32800 (L)Ivoclar Vivadent

K23214

Ivoclar Vivadent

IPS e.max Ceram A2 Dentin

L11240 (Po), L06423 (L)Ivoclar Vivadent

IPS e.max Ceram Glaze Paste

L60044(Pa) K49036 (L)Ivoclar Vivadent

IPS e.max Ceram ZirLiner
IPS e.max ZipPress Ingots A2 HT

15800

VITA Zahnfabrik, Bad Säckingen, Germany

VM9 Base Dentin 2M228900 (Po), 22080 (L)VITA Zahnfabrik

AKZENT

16140 (Po) 16000 (L)VITA Zahnfabrik

PM9 2M2P-T

Po: Powder, Pa: Paste, L: Liquid

Ishibe et al32Volume 106 Issue 1

Table II. Firing and pressing temperatures for veneering ceramics used in study

Firing Cycle

Final
Temperature(°C)

Rate of
Temperature
Increase(°C/Min)HoldingTime §(Min)

Olympia

Oxidation

1010

56

0

InLine/InLine POM

Opaque

930

100

2

InLine POM

Press

940

60

20

1st Dentin

910

60

1

2nd and 3rd Dentin

900

60

1

InLine/InLine POM

Glaze

800

60

2

Pulse

Opaque

950

55

1N

Pulse press-to-metal

Press

920

60

20

1st Dentin

790

45

1N

2nd and 3rd Dentin

780

45

1N

Glaze

760

45

1N

IPS e.max Ceram/ZirPress

ZirLiner

960

40

1

IPS e.max ZirPress

Press

910

60

15

IPS e.max Ceram

Wash firing

750

40

1

1st and 2nd Dentin

750

40

1

IPS e.max Ceram/ZirPress

Glaze

725

60

1

PM9

Press

1000

50

20

VM9

Wash firing

950

55

1

1st Dentin

910

55

1

2nd Dentin

900

55

1

Glaze

900

80

1

Materials

InLinePulse

PM9/VM9§N for holding time without vacuum

paper (Allied High Tech Products, Inc,
Rancho Dominguez, Calif ) and a rotary machine (RotoForce-3, Struers
Inc, Cleveland, Ohio) to abrade thesurfaces perpendicular to the longaxis of the cylinders. Airborne-particle abrasion was performed on thesurfaces using 50 μm aluminum oxide particles (Kavo Dental GmbH) for
10 seconds at 0.2 MPa. Subsequently, the cylinders were ultrasonicallycleaned for 30 minutes in distilledwater, and the surfaces were steam
cleaned and dried. Finally, oxidation
was performed according to the manufacturers’ instructions.
Before pressing and layering of
the veneering ceramics, first and second opaque firings for each veneering ceramic were performed using
the respective opaque according to
the manufacturers’ instructions. For
the pressed group, cylindrical wax
patterns (approx. 4 mm in height)

The Journal of Prosthetic Dentistry(Prowax; Ivoclar Vivadent) were fabricated on the opaque surfaces; sprues
were attached to the top of the wax
patterns; and then 5 wax-metal specimens were invested within the samepressing ring. The ring was thenplaced in the preheated oven (KavoDental GmbH), as with the lost wax
technique, and ceramic ingots werepressed into the mold according to
the manufacturer’s instructions in the
furnace (EP 5000; Ivoclar Vivadent).

Ishibe et al

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July 2011
For the layering group, a thin layer ofdentin porcelain was applied to cover
the opaque surfaces and fired in thepressing furnace (EP 5000; Ivoclar
Vivadent). Subsequently, the dentin
porcelain was applied and fired twice.
To maintain a uniform cylindricalform for both core and veneer, excess
ceramic was adjusted using a sintereddiamond rotary instrument (Brasseler
USA, Savannah, Ga). After finishing
the specimens, glaze firing was performed for all specimens.
For the zirconia ceramic group,
40 zirconia (Lava; 3M ESPE, St. Paul,
Minn) cylinders were obtained and ceramics were layered or pressed on one
end of the cylinders (Table I, Table II).
The cylinders were examined undera stereomicroscope using x15 magnification for any defects and werethen divided into 4 groups, coded byveneering process and manufacturer(n=10); Group PZI: e.max ZirPress(Ivoclar Vivadent), Group LZI: e.max
Ceram (Ivoclar Vivadent), Group PZV:VITA PM9 (VITA Zahnfabrik; Bad
Säckingen, Germany), Group LZV:VITA VM9 (VITA Zahnfabrik).
For Group PZI and Group LZI, aliner (ZirLiner; Ivoclar Vivadent) wasapplied on one end of each zirconia
cylinder to be veneered and fired, after which waxing and layering procedures were conducted on the surfaces(Fig. 1). For Group PZV and Group
LZV, waxing and layering procedures
were conducted directly on the zirconia surface as per manufacturer’s
instructions. The procedure for pressing and layering was performed in thesame manner in the metal ceramic
group and following manufacturers’
instructions. Oxidation of the metal
cylinders and ceramic application
for all specimens were performed ina press-firing combination furnace(EP5000; Ivoclar Vivadent).
Thermal cycling was performed on
all specimens between water temperatures of 5°C and 55°C with a dwell
time of 20 seconds for 20,000 cycles
using an automated thermal cyclingmachine (Proto-tech: Version 2.1a;
Portland, Ore). Subsequently, shear

Ishibe et al

1 Example of specimen: zirconia ceramic group (pressed).

2 Example of specimen (metal ceramic group) mounted
in shear testing fixture. Load was applied to veneer adjacent to and parallel to interface between veneer and core.
bond strength testing was conductedon the specimens in a universal testing machine (Instron model 5500R;
Instron Corp, Norwood, Mass) at a
crosshead speed of 0.5 mm/min. A
diagram of the testing method is presented in Figure 2. The shear stress
was calculated by dividing force by interface area in the same manner as ina previous study.33
After shear testing, all of the specimens were examined visually andwith a stereomicroscope (×15) todetermine the mode of failure. Representative fractured specimens fromeach group were mounted on alumi-

num blocks via colloidal silver liquid(Electron Microscopy Sciences, Hatfield, Pa). The specimens were sputtered with platinum in an argon gas
environment (SPI Module Sputter
Coater; Structure Probe, Inc, West
Chester, Pa) and examined with ascanning electron microscope (SEM)(JEOL 7000; JEOL Ltd, Tokyo, Japan)
with secondary electron imaging andbackscattered electron imaging. Digital images of these specimens wererecorded at various magnifications toevaluate the fracture surfaces and toverify the mode of failure.
Rank-based Kruskal-Wallis and34

Volume 106 Issue 1
Mann-Whitney tests were used tocompare shear bond strengths within the same cores. Within each coregroup, a Kruskal-Wallis test was used
to test the null hypothesis that allgroups were the same. If the KruskalWallis test proved significant (a<.05)
then Mann-Whitney tests with the
Bonferroni correction for multiple
comparisons were used to assess all
pairwise comparisons between the individual groups. In the metal ceramic
group, adjustments were made for the
6 comparisons. In the zirconia ceramic group, adjustments were made forthe 3 comparisons. Statistical software (SPSS 16.0; SPSS Inc, Chicago,Ill) was used for all of the calculations.

RESULTS
The mean (SD) shear bond strengths
for the metal ceramic groups were obtained. Groups PMI and LMI had values of 66.42 (20.60) MPa and 63.69(22.08) MPa, and Groups PMC and
LMC had values of 37.80 (20.57) MPa
and 43.74 (6.64) MPa, respectively.
The Kruskal-Wallis test demonstrated
that all metal ceramic groups weresignificantly different (P<.001). The
Mann-Whitney tests with the Bonferroni correction for multiple comparisons showed that there were significant differences between Groups PMI
and PMC, and between Groups LMI
and PMC, in which Groups PMI andLMI had higher strength than GroupPMC (P=.041) (Table III). For the zirconia ceramic groups, the mean shear

Table III. P values from Mann-Whitney statistical analysis
after adjustment for multiple comparisons and shear bondstrength test mean and SD values for metal ceramic groups

bond strengths and standard deviations of Group PZI and Group LZI
were 40.41 (10.28) MPa and 30.03(9.49) MPa, and Group PZV andGroup LZV were 21.34 (24.30) MPa
and 47.18 (12.99) MPa, respectively.
The Kruskal-Wallis test demonstrated
that all zirconia ceramic groups weresignificantly different (P=.006). The
Mann-Whitney tests with the Bonferroni correction for multiple comparisons showed that there was a significant difference between Group LZI
and Group LZV (P=.012), in whichGroup LZV had higher mean shear
bond strength. In Group PZV, 5 out of
10 specimens were completely separated during thermal cycling and, asa result, the group was excluded fromthe statistical analysis (Table IV).
For all specimens, cohesive failure

Table IV. P values from Mann-Whitney statisticalanalysis after adjustment for multiple comparisonsand shear bond strength test mean and SD values forzirconia ceramic groups

PMI

LMI

PMC

Mean (SD)in MPa

Group

PZI

LZI

Mean (SD)in MPa

PMI

–

–

–

66.42 (20.60)

PZI

–

–

40.41 (10.28)

LMI

1

–

–

63.69 (22.08)

LZI

.106

–

30.03 (9.49)

PMC

.041

.041

–

37.80 (20.57)

PZV

Excluded

Excluded

21.34 (24.30)

LMC

.088

.174

.210

43.74 (6.64)

LZV

.84

.012

47.18 (12.99)

Group

Results significant for P<.05

3 SEM image of LMI specimen (metal ceramic group)
after shear testing: cohesive failure in veneering ceramic(original magnification ×10).

The Journal of Prosthetic Dentistry

Results significant for P<.05

4 SEM image of LZI specimen (zirconia ceramic group)
after shear testing: cohesive failure in veneering ceramics(original magnification ×10).

Ishibe et al

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July 2011
was observed (Figs. 3, 4). For both
metal ceramic groups and zirconia
ceramic groups, fracture initiated atsmall defects such as porosities (Figs.
5, 6). Cohesive failure within the veneering ceramic was confirmed by athin layer of veneering ceramic covering the zirconia surface (Fig. 7).

DISCUSSION

5 SEM image of PMC (metal ceramic group): fractures initiated at small inherent processing defects such as porosity (core side, original magnification ×100).

6 SEM image of PZI (zirconia ceramic group): Typically fracture initiated atsmall inherent defects such as porosity (core side, original magnification ×10).

7 SEM image of LZV (zirconia ceramic group): Thin layer of veneering
ceramics covered zirconia surface (core side, original magnification ×1000).

Ishibe et al

This study evaluated the shear
bond strengths of pressed and layered veneering ceramics to their corresponding high-noble alloy or zirconia cores. For the metal ceramic
groups, there were significant differences between Groups PMI and
PMC and between Groups LMI and
PMC, in which Groups PMI and LMI
had higher strength than Group PMC(P=.041). Therefore, the first null hypothesis was rejected. For the zirconia
ceramic groups, the results demonstrated that there was no significant
difference between the pressed andlayered groups. Thus, the second null
hypothesis was not rejected.
To reduce the variables in the
study, pairs of pressable and layering
ceramics from the same manufacturer
were selected. Thus, the metal ceramic specimens in Groups PMI and LMIor Groups PMC and LMC had similar
procedures performed in terms of theprocess of opaque application. However, application of veneering ceramics was done either via pressing or layering. In addition, when comparing
group PZI and Group LZI or GroupPZV and Group LZV in the zirconia
ceramic specimens, veneering ceramics were either pressed or layered, withGroup PZI and Group LZI having aliner applied prior to the ceramic application.
For pressed specimens, the desired
shape of the specimens was achieved
with a single pressing procedure.
However, the layering specimens required 3 applications and firings,
and subsequently, adjustments wereneeded to acquire the definitive shapefor shear testing. This was due to thefiring shrinkage of the layering proce-36

Volume 106 Issue 1
dure, but this adequately represents
clinical practice for fabricating therestorations. Finally, glaze firing was
performed on all specimens to simulate the situation in the dental laboratory and to enhance the surface
quality. A variety of factors, such as
material composition and properties,
firing temperatures, cooling rates,
operator’s skill, porosities, and fabrication process, may affect the quality and strength of the bond between
the core and the veneering materials.
In this study, 5 out of 10 specimensof the Group PZV (Vita PM9) experienced complete separation during
thermal cycling. Exact reasons for the
separations are uncertain, althoughporosities were observed at the interface. As a result, the group was excluded from the study.
Various testing methods have beenused to evaluate shear bond strengths
between veneering ceramics and thecores of metal alloy and zirconia.27
Some of the studies compared theeffect of application of veneering
technique, pressed and layered, andfound similar bond strengths between pressed ceramics and layered
ceramics.29,30 In this study, shear bondstrengths were evaluated by a method
used in a study by Ashkanani et al,33
which is similar to a circular interface
test described in a prior study and ina review.36, 38
Limitations of this study include
the fact that the design of the specimens does not represent the clinicalsituation, the sample size was limited,
and the fabrication of the specimens
was performed by a single operator. For
the layered groups, only a single category of porcelain (dentin) was used.
After aging, shear bond strengthfor both the metal ceramic and zirconia ceramic groups analyzed,
pressed and layered, exceeded thevalue required by ISO 9693.41 In the
future, studies evaluating shear bondstrengths with different combinationsof cores and veneering materials, theeffect of surface treatments such as
airborne-particle abrasion on corematerials, and the effect of differ-ent thermal cycling protocols should
be conducted. In addition, different methodologies to evaluate shear
bond strengths might be considered.
Additional studies may evaluate themechanical properties of the different
veneering ceramics, such as flexuralstrengths and fracture toughness, andthe effect of application methods. A
quantitative evaluation of the residual
stresses in zirconia and veneering ceramics for the pressing technique, ascompared to the layering technique,and the effect of veneering application procedures and the cooling rate
might also be conducted.

CONCLUSIONS
Within the limitations of this invitro study, the following conclusions
may be drawn:
1. In terms of the shear bondstrength of veneering ceramics tohigh-noble alloy, there were significant differences between Group PMI(IPS InLine POM) and Group PMC(Pulse press-to-metal) and between
Group LMI (IPS InLine) and Group
PMC, in which Group PMI and GroupLMI had a higher value.
2. In terms of the shear bondstrength of veneering ceramics to zirconia, there were no significant differences between the pressed and thelayered groups. Within the layeredgroups, Group LZV (VITA VM9) had asignificantly higher value than GroupLZI (IPS e.max. Ceram).

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Dr Ariel J. Raigrodski
Graduate Prosthodontics
School of Dentistry, University of Washington
D-780 Health Sciences Center
1959 NE Pacific St.; Box 357456
Seattle, WA 98195-7456
Fax: 206-543-7783
E-mail: araigrod@u.washington.edu
Acknowledgments
The authors thank 3M ESPE, Seefeld, Germany,for providing the zirconia specimens, and Ivoclar Vivadent, Schaan, Liechtenstein for providing a portion of materials used in this study.
Copyright © 2011 by the Editorial Council for
The Journal of Prosthetic Dentistry.

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