Original article

Evaluation of shear bond strength of veneeringceramics and zirconia fabricated by the digitalveneering method

Ji-Young Sim MSa, Wan-Sun Lee MPHa, Ji-Hwan Kim MPH, PhDa,*,Hae-Young Kim DDS, PhDb, Woong-Chul Kim MPH, PhDa

aDepartment of Dental Laboratory Science and Engineering, Graduate School, Korea University, Anam-dong 5-ga,

Seongbuk-gu, Seoul 136-713, Republic of KoreabDepartment of Public Health Science, Graduate School & BK21+ Program in Public Health Science, Korea University,

Anam-dong 5-ga, Seongbuk-gu, Seoul 136-713, Republic of Korea

j o u r n a l o f p r o s t h o d o n t i c r e s e a r c h 6 0 ( 2 0 1 6 ) 1 0 6 – 1 1 3

a r t i c l e i n f o

Article history:

Received 6 June 2015

Received in revised form

14 October 2015

Accepted 4 November 2015

Available online 8 December 2015

Keywords:

Zirconia core

Shear bond strength

Layering method

Heat pressing method

Digital veneering method

a b s t r a c t

Purpose: The purpose of this study was to evaluate the shear bond strength (SBS) of

veneering ceramic and zirconia fabricated by the digital veneering method.

Methods: A total of 50 specimens were fabricated, i.e., 10 specimens each for the metal-

ceramic (control) group and the four zirconia groups. The zirconia groups comprised speci-

mens fabricated by the digital veneering method, the heat pressing method, and hand

layering method for two groups, respectively. Furthermore, the shear bond strength was

measured with a universal testing machine (Model 3345, Instron, Canton, MA, USA) and

statistically analyzed using one-way ANOVA set at a significance level of P < 0.05. The

corresponding mode of failure was determined from Scanning Electron Microscope (FESEM

JSM 6701F, Jeol Ltd., Japan) observations.

Results: One-way analysis of variance (ANOVA) revealed that the metal-ceramic

group had the highest SBS (43.62 MPa), followed by the digital veneering method

(28.29 MPa), the heat pressing method (18.89 MPa), and the layering method

(18.65, 17.21 MPa). The samples fabricated by digital veneering had a significantly

higher SBS than the other zirconia samples (P < 0.05). All of the samples exhibited mixed

failure.

Conclusions: Veneering ceramic with a zirconia core that was fabricated via the digital

veneering method is believed to be effective in clinical use since, its shear bond strength

is significantly higher than that resulting from the conventional method.

# 2015 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.

* Corresponding author at: Department of Dental Laboratory Science and Engineering, College of Health Science, Korea University, Anam-dong 5-ga, Seongbuk-gu, Seoul 136-713, Republic of Korea. Tel.: +82 2 3290 5666; fax: +82 2 940 2879.

E-mail address: kjh2804@korea.ac.kr (J.-H. Kim).

Available online at www.sciencedirect.com

ScienceDirect

journal homepage: www.elsevier.com/locate/jpor

http://dx.doi.org/10.1016/j.jpor.2015.11.0011883-1958/# 2015 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.

j o u r n a l o f p r o s t h o d o n t i c r e s e a r c h 6 0 ( 2 0 1 6 ) 1 0 6 – 1 1 3 107

1. Introduction

Porcelain fused to metal (PFM) has been used extensively in

the last 40 years for fixed partial dentures (FPDs) and is still

widely used [1–3]. However, owing to the internal metal core,

the esthetics of natural teeth cannot be completely re-created

with metal-ceramic restorations [4].

Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP),

which was introduced in the early 1990s, has been widely used

in the fabrication of all-ceramic crowns with CAD/CAM

zirconia cores owing to its color, which is similar to that of

natural teeth, its excellent biocompatibility, and its physical

and mechanical properties [5–8]. However, in the last 3 years,

delamination and chip-off in these crowns have resulted in a

25% clinical failure rate of the corresponding restorations [9].

This failure rate is significantly higher than that reported

[10,11] for metal-ceramic restorations (2.7–5.5%).

Failure of all-ceramic crowns with zirconia cores has been

attributed to low bonding strength between the zirconia core

and the upper porcelain [12], design of the zirconia core [13],

stress concentration resulting from the difference between

the coefficient of thermal expansion of the zirconia core and

that of the veneering ceramic [14], structural defect of zirconia

from the zirconia coloring pigment [15], and flaws resulting

from the production processes [16].

Previous studies revealed that the CAD/CAM technique

applied to the production of ceramic prostheses reduced the

porosity and flaws that accelerate failure of the upper

porcelain. This technique also resulted in a uniform zirconia

core; the all-ceramic crown with a zirconia core fabricated by

the CAD/CAM veneering method exhibited significantly less

chipping and failure than that fabricated by other methods

[7,17]. In addition, the bond strength varied with the

production method of the upper porcelain [12].

Most all-ceramic crowns with zirconia cores are fabricated

by forming the upper porcelain on top of the zirconia core via

layering [18,19] and a heat pressing method [18]. The Digital

Veneering System (DVS) (LavaTM; 3M ESPE, Seefeld, Germany),

which was introduced as a CAD/CAM veneering method, can

reduce both the time required for fabrication and the flaws

resulting from the production process. DVS produces all-

ceramic crowns by combining the upper porcelain and the

zirconia core through firing; this firing is performed after

veneering the fusion powder between the upper porcelain and

the zirconia core, which were fabricated by milling the glass

ceramic block and by using the CAD/CAM technique,

respectively. By conducting this procedure, a zirconia all-

ceramic restoration that closely replicated the shape and color

of the natural teeth could be fabricated with minimum firing.

There are several available test methods to evaluate the

bond strength between dental materials. The shear, tensile,

three-point flexure, and four-point flexure tests are commonly

used. The bond strength between a metal core and veneering

porcelain has been investigated using flexure tests. Although

the flexure test may be appropriate for metal-ceramic

materials, it is not suitable for all ceramic system because

of the brittle character of the ceramic core [20]. Some

researchers prefer the tensile test. However, it has been

reported that the tensile test results are greatly influenced by

the geometry of the specimen, and non-uniform stress

distributions arise during load application [21].

The shear bond strength test was selected for this study for

certain important reasons, i.e., the result is not influenced by

the Young’s modulus of the substrate, and it allows simple

specimen fabrication procedure and easy to perform the test

with rapid acquisition of the results [20,22,23].

Numerous studies have been conducted on the bonding

strength of the veneering ceramic and zirconia; however,

studies on the shear bond strengths produced by different

veneering methods, including the digital veneering method,

are rare.

The goal of the present study was to evaluate the effective

clinical use of zirconia-core all-ceramic crowns that were

fabricated by the digital veneering method. This evaluation

was made by comparing the shear bond strength (SBS) of all-

ceramic crowns with zirconia framework fabricated by digital

veneering and also by conventional methods, with the SBS of

metal-ceramic restorations.

The null hypotheses were that the shear bond strength of

an all-ceramic crown with zirconia would not be significantly

different from that of a metal-ceramic crown and the digital

veneering method would not affect the shear bond strength

between the zirconia and ceramic.

2. Materials and methods

A metal-ceramic (MC) was used as the control group. Speci-

mens with their upper porcelain fabricated by the layering

method and digital veneering method on top of the Lava

zirconia (3M ESPE, Seefeld, Germany) core were defined as LZL

(Lava zirconia layering method) and LZD (Lava zirconia digital

veneering method), respectively. In addition, specimens with

their upper porcelain fabricated by layering and by heat

pressing method on top of the IPS e.max zirconia (Ivoclar-

Vivadent, Schaan, Leichtenstein) core were referred to as IZL

(IPS zirconia layering method) and IZP (IPS zirconia heat

pressing method), respectively. A total of 50 specimens (i.e., 10

specimens each for the metal-ceramic group and the four

zirconia groups) were fabricated (Table 1).

2.1. Preparation of the metal-ceramic specimens

A silicone mold (blue eco, Detax GmbH, Ettlingen, Germany)

was used to fabricate wax in the shape of a rectangular

parallelepiped; the hardened wax pattern was removed from

the mold and replicated as a metal casting by a traditional

investment and burn out process. The metal specimens

(Bellabond plus, BEGO, Bremen, Germany) were polished to

sizes of 5 mm (length) � 5 mm (width) � 10 mm (height) using

a stone point (carborundum point #11, SC, Korea), wheel

(carborundum wheel #301, SC, Korea), and technical carbide

bur (TC-cutter, Edenta, Switzerland). A 10 mm distance of the

surface of the cast was blasted for 20 s with 110 mm aluminum

oxide at a pressure of 2.5 bar and then cleaned ultrasonically

with distilled water for 10 min. Porcelain (Vita VM13; Vita

Zahnfabrik, Bad Sackingen, Germany) was veneered in a

separable silicone mold located on top of the metal casting in

which degassing and opaque (Vita VM13; Vita Zahnfabrik, Bad

Table 2 – Firing schedules for the veneering ceramics used in the study.

Materials Starttemperature (8C)

Dryingtime (min)

Temperatureincrease (8C)

Finaltemperature (8C)

Holdingtime (min)

VM13

Degassing 500 4 75 950 2

Opaque 500 4 75 890 2

Dentin 500 6 55 880 1

Lava

MO 450 6 45 810 1

Lava

Dentin 450 6 45 810 1

IPS e.max

ZirLiner 403 4 60 960 1

IPS e.max

Dentin 403 4 50 750 1

Lava DVS

Fusion 500 4 10 770 1

IPS e.max

Zirpress 700 0 60 910 15

Table 1 – Names and manufacturers of the materials used in the experiments.

Group Materials Lot. number Manufacturer

MC Core Ni-Cr Bellabond plus LOT #12217 BEGO, Bremen, Germany

Opaque Vita VM13 OP2 LOT #32280 Vita Zahnfabrik, Bad

Veneer Vita VM13 2M2 LOT #659964 Sackingen, Germany

LZL Core Lava Zirconia LOT #68584 3MESPE, Seefeld, Germany

Veneer Lava Ceram A2 LOT #296014 3M ESPE

LZD Core Lava Zirconia LOT #68584 3M ESPE, Seefeld, Germany

Fusion Lava fusion D3 LOT #365026 3M ESPE

Veneer Lava DVS Ceramic Block E2 LOT #433457 3M ESPE

IZL Core IPS e.max ZirCad MO1 LOT #M50816 Ivoclar-Vivadent, Schaan, Leichtenstein

Zirliner IPS e.max Ceram ZL2 LOT #R37480 Ivoclar-Vivadent

Veneer IPS e.max Ceram A2 LOT #T45644

IZP Core IPS e.max ZirCad MO1 LOT #M50816 Ivoclar-Vivadent, Schaan, Leichtenstein

Zirline IPS e.max Ceram ZL2 LOT #R37480 Ivoclar-Vivadent

Veneer IPS e.max Zirpress HT A2 LOT #T10948

MC: Metal Ceramic, LZL: Lava Zirconia hand layering, LZD: Lava Zirconia Digital veneering, IZL: IPS e.max Zirconia hand layering, IZP: IPS

e.max zirconia heat Press.

j o u r n a l o f p r o s t h o d o n t i c r e s e a r c h 6 0 ( 2 0 1 6 ) 1 0 6 – 1 1 3108

Sackingen, Germany) firing were performed. The firing was

performed according to the manufacturer-specified recom-

mendations (Table 2) using a furnace (Austromat 3001,

Dekema, Freilassing, Germany) (Fig. 1). The firing was

conducted twice to compensate for the shrinkage of the

veneering porcelain. Finally, a 5 mm (width) � 4 mm

(length) � 4 mm (height) upper porcelain was produced for a

metal-ceramic specimen (Fig. 2).

2.2. Preparation of the all-ceramic specimens

The two types of zirconia blocks (Lava, IPS e.max ZirCad) were

fully sintered in a manufacturer-specified furnace (Lava

Furnace 200; 3M ESPE, Seefeld, Germany) and in a compatible

furnace (in-fire1; Sirona, Germany), respectively. Each of the

fully sintered zirconia blocks was cut into 20 rectangular sticks

of equal size (5 mm in width, 5 mm in length, and 10 mm in

height) using a precision diamond saw (IsoMet 2000, Buehler,

Dusseldorf, Germany). The specimens were cleaned using an

ultrasonic cleaner (SD-80H, Sung dong, Korea) for 10 min and

then dried. The surface treatment of all the zirconia speci-

mens was performed according to the manufacturer’s

instructions. The upper porcelain was produced by the

following three methods.

2.2.1. Hand layering method (LZL and IZL groups)Zirconia specimens, which compose the LZL and IZL groups,

were fired with furnace (Austromat 3001, Dekema, Freilassing,

Germany), (Ivoclar Vivadent Programat EP 3000) after being

coated with a 0.1-mm-thick by modifier powder (Lava Ceram

MO; 3M ESPE, Seefeld, Germany), and ZirLiner (Ivoclar-

Vivadent, Schaan, Leichtenstein), respectively. As in the case

of the MC sample, Dentin powder (Lava Ceram, IPS e.max

Ceram) was applied via the silicone mold in order to produce

upper porcelain with identical contact surface and size in all

of the specimens. Final upper porcelain sizes of 4 mm

(length) � 5 mm (width) � 4 mm (height) were obtained after

approximately two rounds of firing.

Fig. 1 – Separated mold for hand layering method.

Fig. 2 – Design of the Schmitz–Schulmeyer specimens [24].

j o u r n a l o f p r o s t h o d o n t i c r e s e a r c h 6 0 ( 2 0 1 6 ) 1 0 6 – 1 1 3 109

2.2.2. Heat pressing method (IZP group)Investment and burn out processes were performed after

5 mm (width) � 4 mm (length) � 4 mm (height) wax patterns

were formed on top of the 0.1-mm-thick ZirLiner-coated, just

as was done for the IZL group of zirconia specimens. The space

created by the burn-out of the wax pattern was then filled by

the veneering ceramic (IPS e.max ZirPress, Ivoclar Vivadent,

Liechtenstein), which was produced via the heat pressing

method using a furnace (Ivoclar Vivadent Programat EP 3000).

2.2.3. Digital veneering method (LZD group)A 20-second air abrasion (RocatecTM Soft; 3M ESPE, Seefeld,

Germany) at a pressure of 2.8 bar, and over a distance of

10 mm was performed on the Lava zirconia specimen

according to the manufacturer’s recommendation. The Lava

DVS Glass ceramic block was cut to 15% larger than the desired

final size (5-mm-width, 4-mm-length, 4-mm-height) to allow

for shrinkage after firing. This cut ceramic was soaked in water

for 1 minute to allow for water absorption, and subsequently

dried for 30 s. Fusion powder (3M ESPE, Seefeld, Germany) was

then layered between the surface-treated Lava zirconia

specimen and the DVS ceramic, which were then fired using

the furnace (Austromat 3001, Dekema, Freilassing, Germany)

according to the manufacturer-recommended schedule.

2.3. Measurement of shear bond strength

The SBS was measured using a universal testing machine

(Model 4465, Instron, Canton, MA, USA). Specimens were fixed

in the testing jig in order for the load to be exerted along the

direction of contact between the zirconia and the upper

porcelain. The maximum load (in N), at a cross head speed of

0.5 mm/min, was taken as the applied shear force at which

failure occurs. In addition, the SBS was calculated by dividing

the maximum load (N) by the surface area (mm2) of the

contacting upper porcelain [24].

2.4. Observation of the failure surface

After measuring the shear bond strength, the fractured

surfaces were visually analyzed using a scanning electron

microscope (FESEM JSM 6701F Jeol Ltd., Japan) to determine the

failure mode and observe the fractured surfaces. Before the

SEM evaluation, platinum was also sputter-coated (JSM670-1F,

Jeol Ltd., Japan) on the fractured surfaces for 90 s.

The failure mode was classified as cohesive if the fracture

occurred within the veneer or core material, adhesive if the

fracture occurred between the core and veneer, and mixed

failure if it consisted of both adhesive and cohesive failures [25].

In addition, the fractured surfaces were observed using a

microscope at an original magnification of �40 (HK-7700,

HIROX, Tokyo, Japan). Then, the surface that remained, with

the veneering ceramic on the zirconia, was measured (HK-

7700 Image measure software HIROX, Tokyo, Japan), and it was

divided by the total interface area to calculate the percentages

Fig. 3 – SEM images at T35 showing the fracture surfaces of the (a) MC, (b) LZL, (c) LZD, (d) IZL, and (e) IZP groups. All groups

exhibited a mixed failure mode which combined cohesive fracture with adhesive failure at the interface.

Table 3 – Mean values and standard deviations of theshear bond strength (MPa). N = 10.

Group Mean S.D. P-value

MC 43.62a 2.13

LZL 17.21b 1.11

LZD 28.29c 2.25 0.001

IZL 18.65b 1.76

IZP 18.89b 1.54

Values with the same letter are not statistically different based on

Tukey’s test at P < 0.05.

j o u r n a l o f p r o s t h o d o n t i c r e s e a r c h 6 0 ( 2 0 1 6 ) 1 0 6 – 1 1 3110

of the cohesive fractured area and adhesive fractured area in

each specimen.

2.5. Statistical analysis

The Shapiro–Wilk test (P > 0.05) was conducted to verify that

the data were consistent with a normal distribution. One-way

analysis of variance (ANOVA) using the Statistics Program

SPSS 12.0 (SPSS Inc. USA), was performed in order to determine

the difference in the mean SBS of the different groups. Tukey’s

post hoc analysis was performed to precisely evaluate the

differences among these groups at a statistical level of

significance of 0.05.

3. Results

3.1. Shear bond strength

The result of the Shapiro–Wilk test (P > 0.05) showed that the

data were normally distributed. Based on the one-way ANOVA

analysis, the groups can be listed in descending order of mean

SBS as MC (43.62 � 2.13 MPa), LZD (28.29 � 2.25), IZP (18.89 �

1.54), IZL (18.65 � 1.76), and LZL (17.21 � 1.11). Statistically

significant intergroup differences were also observed (P <

0.05); i.e., Tukey’s post hoc analysis revealed significant

differences between the MC group and the zirconia groups

as well as between the LZD and the other groups; there was,

however, no significant difference among the LZL, IZL, and IZP

groups (Table 3).

3.2. Observation of the failure surface

All of the specimens exhibited a mixed failure mode, which

combined a cohesive failure in the veneer and adhesive failure

at the interface (Fig. 3). None of the specimens showed only an

adhesive or a cohesive failure.

The SEM images showed that the MC group primarily had

an adhesive fractured area at the interface of each specimen

(Fig. 4), and every zirconia group except LZD had several pores

in the veneering ceramic (Fig. 5).

In the LZD group, the mean percentage of the cohesive

fractured area within veneer was higher than that of the

adhesive fractured area at the bonded area (Table 4).

4. Discussion

The success of the all-ceramic crowns, which are composed of

a zirconia core and veneering ceramic, is influenced primarily

by the bonding between the core and the upper porcelain

as well as the strength of the material itself. In previous

studies, most of the failures in all-ceramic restorations with a

zirconia core resulted from chipping and delamination of

the ceramic [18,26]. Hence, the bonding between the zirconia

and the upper porcelain must be sufficiently strong in order to

yield a long-term and stable outcome in ceramic restorations

that use a zirconia core.

Fig. 5 – SEM images of the (a) LZD, (b) LZL, (c) IZL, (d) IZP, (e) LZL, and (f) LZD groups. Images a, b, c, and d were recorded at

T100; e and f were recorded at T5000. The LZD had the fewest pores of all four groups. In the case of the LZL, IZL, IZP

groups, structural defects and pores were observed in the veneering ceramic.

Fig. 4 – SEM images showing primarily the adhesive failure mode of MC. (a) Metal-original magnification T40, (b) metal-

ceramic-original magnification T5000.

j o u r n a l o f p r o s t h o d o n t i c r e s e a r c h 6 0 ( 2 0 1 6 ) 1 0 6 – 1 1 3 111

SBS experiments are the most commonly used methods

for measuring bonding strength [27,28]; however, there

are some important factors that must be considered, i.e.,

there are large variations in results and the results cannot be

estimated directly to clinical situations [29]. Therefore,

experimental design that takes into account the shape of

the specimen, the area of the contacting surface, and a

crosshead speed, is essential to achieving measured values

with small deviation.

The shape of the specimens chosen for this experiment

was designed to facilitate the Schmitz–Schulmeyer test; i.e.,

the planar interface shear bond test [30]. Hammad et al.

reported that, compared to other methods, this test is the most

appropriate for measuring the bond strength as it requires

minimal experimental variables; in addition, this testing

method results in a uniform interfacial stress by directly

applying the force to the boundary surface [31,32].

Although the MC sample has the highest SBS (43 MPa) of

the materials measured in the current study, this value is still

lower than those (61.40–96.80 MPa) obtained in previous

studies [33,34]. This difference may result from the metals

used in the current experiments and various experimental

factors. Beryllium-free Ni-Cr non-precious alloys (Bellabond

plus, BEGO, Bremen, Germany) were used as the metals in this

study. Wu et al. [35] obtained lower bonding strengths in

beryllium-free Ni-Cr alloys compared to their Be-containing

counterparts. In addition, De Melo et al. [36] reported that the

metal-ceramic bonding strength depends on the composition

of the alloy, coefficient of thermal expansion, and thickness of

the oxidation film.

Table 4 – Means of adhesive and cohesive fractured areasin percentages and failure mode. A: adhesive fracturedarea at interface, C: cohesive fractured area withinveneer.

Group Mean of A and C inpercentage

Failure mode (%)

A (%) C (%)

MC 76.2 23.8 Mixed 100

LZL 63.6 36.4 Mixed 100

LZD 37.5 62.5 Mixed 100

IZL 62.2 37.8 Mixed 100

IZP 64.7 35.3 Mixed 100

j o u r n a l o f p r o s t h o d o n t i c r e s e a r c h 6 0 ( 2 0 1 6 ) 1 0 6 – 1 1 3112

Ishibe et al. [18] reported that the sample with upper

porcelain fabricated by hot pressing had a higher SBS

(40.41 MPa) than that of the hand-layered sample (30.03 MPa).

Furthermore, Henriques et al. [37] showed that, compared to the

hand layer method, heat pressing increased the bonding

strength between the zirconia and the ceramic by reducing

the porosity of the ceramic. Although the heat-pressed (IZP)

sample in the present study had a slightly higher mean SBS than

that of the hand-layered (IZL, LZL) samples, the statistical

difference in the strengths was insignificant. The similarity

in the IZP and IZL interfacial bond strength is believed to

result from the ZirLiner coating on the zirconia surface of these

two samples.

Kanat et al. [12] measured a SBS of 49 MPa for a zirconia

sample whose upper porcelain was fabricated via CAD-on

veneering; this samples was stronger than its counterpart,

which was fabricated by the conventional veneering method.

In addition, Guess et al. [38] reported that a CAD/CAM-

veneered zirconia crown, which had fewer inherent flaws, had

a higher resistance to failure than a crown fabricated by the

hand-layering method.

Ceramic tends to fail as a result of the propagation of flaws

and cracks [39]. In a ceramic restoration, the number and size

of the flaws are associated with the materials and fabrication

method [17]. Hence, a digital veneering system, in which the

veneering process is simple and the number of firings is

minimized, was predicted to have the least number of flaws.

The LZD group presented a significantly higher bond strength

compared to the other zirconia groups. This was thought to be

due to the use of a ceramic block and the fabrication of a

specimen by conducting only one firing process for bonding

between the upper ceramic block and the zirconia.

Previous studies reported that the firing process causes

shrinkage of the porcelain and a reduction in the bond

strength of the interface as a result of the rapid change in

temperature [40].

In addition, Al-Dohan et al. [20] reported SBS’s of 22–

31 MPa for all-ceramic restorations with zirconia cores.

Although the bonding strength (28.29 MPa) of the LZD

samples examined in this study fell within the suggested

range, the strengths of the hand-layered and heat-pressed

samples were not within this range. However, direct

comparison may be inappropriate since the factors influenc-

ing the bonding strength (i.e., the shape of the specimens and

area of the contacting surface) in the previous studies differ

from those of the present study.

Cohesive failure, which occurs in the intra-ceramic layer,

was the dominant mode of failure in previous studies of failed

metal-ceramic specimens [18,41]. However, adhesive failure,

in which the oxidation film becomes delaminated from the

metal while remaining attached to the ceramic, occurred

predominantly in this study. This failure results from the

separation of the metal from the ceramic owing to deforma-

tion that stems from the generation of dislocations or cracks

owing to temperature changes that occur at the boundary

surface with increasing thickness of the oxidation film.

All the zirconia samples exhibited mixed failure, which is

consistent with the failure mode of a previous study [41].

Observations of the failed veneer surfaces revealed the

least number of defects, including pores, in the LZD group. As a

result, a higher cohesive strength for the veneering ceramic

compared to the other zirconia groups was predicted.

However, the percentage of the surface that remained with

the veneering ceramic was the highest. This indicated that the

LZD group had the best interface bond strength. Therefore, the

ceramic in the LZD sample should provide better long-term

stability in the intra-oral environment than those fabricated

by either the hand-layering or heat-pressing methods.

In this study, samples fabricated by digital veneering

exhibited the highest SBS of all the zirconia samples. However,

the fact that even this group of samples was still significantly

weaker than those of the MC group, reveals the need for

continuous research in order to improve the bonding strength

of zirconia prostheses. This study is also limited in some

respects. For example, the specimens used did not reflect the

shapes of clinical dental restoration materials and the

experimental conditions did not replicate the intra-oral

environment. Therefore, research that replicates the intra-

oral environment and uses specimens with similar shapes to

those of the dental restoration materials, is required in order

to avoid these limitations.

5. Conclusions

The results of the present study revealed that the method used

to produce veneering ceramics influences the SBS of the

zirconia prosthesis. The digital veneering method is consid-

ered to be effective in clinical use since the consequent SBS is

significantly higher than that resulting from the heat pressing

or layering method.

Conflict of interest

The authors report no conflict of interest directly relevant to

the content of this work.

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