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The influence of thickness and different resin cements on the flexural strength of high strength CAD/CAM glass ceramics

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The influence of thickness and different resin cements on the flexural strength of high strength CAD/CAM glass ceramics

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Herman Ostrow School of Dentistry of USC
Advanced Operative and Adhesive Dentistry
Division of Restorative Sciences

The Influence of Thickness and Different Resin Cements on the Flexural
Strength of High Strength CAD/CAM Glass Ceramics

Dr. Sara Casado
Advisor: Dr. Sillas Duarte Jr
Co-Advisor: Dr. Jin-Ho Phark

In Partial Fulfillment of the Requirements for the
Degree of Master of Science in Craniofacial Biology
University of Southern California

December 2017

Influence of thickness and different resin cements on the flexural strength of high strength CAD/CAM glass ceramics

2
TABLE OF CONTENTS
TABLE OF CONTENTS ........................................................................................................... 2
ACKNOWLEDGEMENTS ....................................................................................................... 3
LIST OF FIGURES AND TABLES ......................................................................................... 4
Figures.................................................................................................................................... 4
Tables ..................................................................................................................................... 5
ABBREVIATIONS ................................................................................................................... 6
ABSTRACT ............................................................................................................................... 7
INTRODUCTION ..................................................................................................................... 9
OBJECTIVES .......................................................................................................................... 11
MATERIALS AND METHODS ............................................................................................. 12
Specimen Preparation .......................................................................................................... 13
Ceramic Surface Treatment ................................................................................................. 14
Luting Procedures ................................................................................................................ 15
Storage and Artificial Aging ................................................................................................ 16
Flexural Strength Test .......................................................................................................... 16
Statistical analysis ................................................................................................................ 17
RESULTS ................................................................................................................................ 19
DISCUSSION .......................................................................................................................... 23
CONCLUSION ........................................................................................................................ 27
CONFLICT OF INTEREST .................................................................................................... 28
FUNDING................................................................................................................................ 28
REFERENCES ........................................................................................................................ 29
Sara Casado, MS Thesis Advanced Operative and Adhesive Dentistry, CBY | December 2017
3
ACKNOWLEDGEMENTS
It is a great honor to acknowledge the committee members for their generosity with their time and
support.
A very special thanks to Dr. Sillas Duarte Jr., my mentor, advisor, and committee chairman, for
the countless hours of guidance, reassurance, and support, not only in this process, which proved
difficult at times, but in the program as a whole. Thank you for always pushing me to strive for
greatness even when I doubt myself.
I would also like to thank Dr. Jin-Ho Phark, my co-advisor, for impressing on me the importance
of high standards, not only in research, but in all my work.
To Dr. Neimar Sartori, thank you for teaching me the importance of being a well-rounded
clinician and researcher. And for sharing with me your vast knowledge.
To Dr. Michael Paine, for lending your time and expertise in this process, and for always treating
me with great kindness.
To Mrs. Karen Guillen, for her constant support, understanding, and for providing me any
assistance when requested.
I would like to thank my co-residents for their constant support and words of encouragement.
Finally, I would like to thank my family and friends for the constant support and for believing in
me, always.

Influence of thickness and different resin cements on the flexural strength of high strength CAD/CAM glass ceramics

4
LIST OF FIGURES AND TABLES
Figures

Figure 1: Preparation of the specimens from the glass-ceramic CAD/CAM blocks.
Figure 2: Beams originated from the glass-ceramic CAD/CAM blocks, with different
thicknesses.
Figure 3: Experimental groups for LD and ZLS.
Figure 4: Fixtures used for three-point flexural strength testing.
Figure 5: Mean flexural strength of CAD/CAM glass-ceramic of different thicknesses before
and after thermal fatigue.
Figure 6: Scanning Electron Microscopy a) Electron micrograph depicting the microstructure of
zirconia-reinforced lithium silicate glass-ceramic (CELTRA Duo) etched with 5% hydrofluoric
acid for 30 seconds (magnification x10,000); b) Electron micrograph depicting the
microstructure of lithium disilicate glass-ceramic (IPS e.max CAD) etched with 5% hydrofluoric
acid for 20 seconds (magnification x10,000).

Sara Casado, MS Thesis Advanced Operative and Adhesive Dentistry, CBY | December 2017
5
Tables

Table 1: Materials, manufacturer, and composition of materials used in the present study.
Table 2: Crystallization parameters for IPS e.max CAD and CELTRA Duo.
Table 3: Mean flexural strength (± standard deviation) for different surface treatments, before
and after thermal fatigue (20K TF) for IPS e.max CAD.
Table 4: Mean flexural strength (± standard deviation) for different surface treatments, before
and after thermal fatigue (20K TF) for CELTRA Duo.
Table 5: Mean flexural strength (± standard deviation) for IPS e.max CAD and CELTRA Duo in
different thicknesses at 24 hours and after thermal fatigue (20K TF).

Influence of thickness and different resin cements on the flexural strength of high strength CAD/CAM glass ceramics

6
ABBREVIATIONS
ETC: Etched
HF: Hydrofluoric acid 5%
LD: Lithium disilicate-reinforced glass-ceramic (IPS e.max CAD)
NET: Not etched
RLP: RelyX Luting Plus
SEM: Scanning electron microscopy
UN2: RelyX Unicem 2
VRL: Variolink Esthetic LC
ZLS: Zirconia-reinforced lithium silicate glass-ceramic (CELTRA Duo)

Sara Casado, MS Thesis Advanced Operative and Adhesive Dentistry, CBY | December 2017
7
ABSTRACT
The Influence of Thickness and Different Resin Cements on the Flexural Strength of Two
CAD/CAM Ceramic Blocks

Objective: The aim of this study was to evaluate the influence of the thickness, etching, and
resin cement type on the flexural strength of lithium disilicate-reinforced glass-ceramic (IPS
e.max CAD, LD), and zirconia-reinforced lithium silicate glass-ceramic (CELTRA Duo, ZLS).
Materials and Methods: IPD e.max CAD and CELTRA Duo CAD/CAM blocks were
sectioned with a diamond-wafering blade, under constant water, to yield beams of varying
thicknesses of 0.5 mm, 1.0 mm, and 2.0 mm. Specimens were polished, crystallized in a ceramic
furnace, and sorted into different experimental groups (n=15) according to: ceramic material,
thickness, etching, and luting cements (RelyX Unicem 2, Variolink Esthetic LC, and RelyX
Luting Plus). The specimens were also tested at two different moments: 24 h and after 20,000
cycles of thermal fatigue. Flexural strength was assessed with three-point bending test, the values
recorded in MPa. Statistical analysis was performed with t-test, two-way ANOVA, and Tukey
post-hoc.
Results: Statistically significant differences were found for LD, which yielded higher flexural
strengths than ZLS, both before and after thermal fatigue. LD yielded higher flexural strength at
2.0 mm; all other thicknesses (0.5 mm, and 1.0 mm) were not significantly different (p>0.05).
For ZLS, no significant difference (p>0.05) was identified in the flexural strength for any
thickness tested. Different surface treatments did not produce statistical differences among the
experimental groups (p>0.05).

Influence of thickness and different resin cements on the flexural strength of high strength CAD/CAM glass ceramics

8
Conclusion: LD showed superior flexural strength results when compared to that of ZLS.
Thickness did not influence the flexural strength for ZLS, whereas a 2.0mm thick specimens
increased the flexural strength for LD. Etching improved the flexural strengths for ZLS.
Different luting cements had no impact on the flexural strength of the two tested ceramics.
Significance: Different CAD/CAM glass-ceramic yielded different flexural strengths. Thickness
and infiltration of luting cements did not have an influence on the strength of the tested ceramics
for ZLS. Higher flexural strength was observed for LD. Both CAD/CAM ceramic materials are
suitable for minimally invasive restorations.

Keywords: Glass ceramic, Flexural strength, Thickness, Resin cement, Hydrofluoric acid

Sara Casado, MS Thesis Advanced Operative and Adhesive Dentistry, CBY | December 2017
9
INTRODUCTION
Esthetics and mechanical strength are of paramount importance for dental ceramic restorations.
However, in areas of high occlusal stress glass-ceramics may pose a concern due to its inhered
brittleness.
(1)
These concerns unleashed the development glass-ceramics reinforced with oxides,
which not only improved mechanical properties, but also provided superior optical properties.
(2,
3)
More recently, the development of computer-assisted design and computer-assisted
manufacturing (CAD-CAM) technology allowed the development of more homogeneous
ceramic materials
(4)
.
Among those new materials, partially crystallized lithium disilicate glass-ceramic CAD/CAM
blocks were introduced. These CAD/CAM blocks contain 40% lithium metasilicate (Li
2
SiO
3
)
platelet-shaped crystals embedded in a glassy phase and a nanocrystalline lithium disilicate
(Li
2
Si
2
O
5
). These partially crystallized blocks (blue stage) can be easily milled by chairside
CAD/CAM technology
(5)
and

exhibits a flexural strength of 130 to 150 MPa
(6)
. However, once
the restoration has been milled, it must be fully crystallized on a ceramic furnace. During this
process, lithium metasilicate glass–ceramic is transformed into lithium disilicate glass–ceramic
and now it exhibits tooth-color appearance. After the crystallization, it has been reported a
flexural strength of 360 MPa
(7)
.
More recently a zirconia-reinforced lithium silicate glass-ceramic CAD/CAM block has been
introduced. The material consists mainly of lithium silicate glass with 10% dissolved zirconium
oxide (ZrO
2
) that uses diphosphorus pentoxide (P
2
O
5
) as a nucleation agent for crystallization of
lithium metasilicate
(8, 9)
. This reinforced CAD/CAM glass-ceramic block is available in fully
crystallized form and additional heat-treatment is not necessary. After milling the zirconia-
reinforced lithium silicate glass ceramic CAD/CAM yields flexural strength of 210 MPa
(10)
.
Influence of thickness and different resin cements on the flexural strength of high strength CAD/CAM glass ceramics

10
However, the manufacturer claimed that additional heat-treatment increases the flexural strength
by 76% (up to 370 MPa)
(10)
.
Both CAD/CAM ceramic blocks lithium disilicate glass-ceramic and zirconia-reinforced lithium
silicate glass ceramic can be acid etched. Hydrofluoric acid (HF), applied to glass ceramic,
dissolves the glassy phase of the ceramic by reacting with silicon dioxide, and thereby creating a
uniformly porous surface
(11, 12)
. It is this increase in roughness that creates the possibility of a
micromechanical interlock between the ceramic and a resin cement
(11)
. Moreover, the fracture
resistance of an etched glass-ceramic increases significantly with resin infiltration
(13)
, possibly
because the elastic modulus of different resin cements, may play a role in strengthening ceramics
(14)
. However, it is not clear if different resin-based cements or resin-modified glass-ionomer
cements affect the flexural strengths of novel CAD/CAM glass-ceramics after etching and
silanization.
Flexural strength testing is one of the principal techniques available for measuring the uniaxial
tensile strength of a ceramic material. The data generated from this technique can be used to
evaluate the material’s composition and mechanical behavior. Given that most ceramic materials
are brittle in nature, their differences in flexural strength arise from variations in their
microstructures, flaws within this microstructure, and the dimension of the material tested
(15, 16)
.
As preservation of dental hard tissues is of vital importance for the current concepts of minimally
invasive restorative and adhesive dentistry, these new CAD/CAM materials have been indicated
for fabrication of dental restorations with less than 1.0 mm in thickness. Therefore, the
mechanical properties of these CAD/CAM glass-ceramic materials at different thicknesses
should be investigated before clinical use, including the effect of thermal fatigue on the behavior
of these new ceramics
(17)
.
Sara Casado, MS Thesis Advanced Operative and Adhesive Dentistry, CBY | December 2017
11
OBJECTIVES

The aim of this study was to evaluate the influence of thickness, etching, and resin cement type
on the flexural strength of lithium disilicate-reinforced glass-ceramic (IPS e.max CAD – Ivoclar
Vivadent, Schaan, Liechtenstein), and zirconia-reinforced lithium silicate glass-ceramic
(CELTRA Duo – Sirona Dentsply, Milford, DE, USA).

This study was designed to test the following null hypotheses:
(1) different reinforced CAD/CAM glass-ceramic blocks possess similar flexural strengths, (2)
material thickness has no significant effect on the flexural strength of the tested CAD/CAM
glass-ceramics;
(3) ceramic etching does not have an influence on the flexural strength of ceramic materials and;
(4) infiltration of different cements into the etched ceramic surface do not produce differences in
flexural strength for the tested materials.

Influence of thickness and different resin cements on the flexural strength of high strength CAD/CAM glass ceramics

12
MATERIALS AND METHODS
All materials tested in this study are presented in the Table 1.
Table 1: Materials, manufacturer, and composition of materials used in the present study

Material (Lot number) Manufacturer Composition
IPS e.max CAD
(W37102)

Ivoclar Vivadent,
Schaan, Liechtenstein
Silicon dioxide
,
lithium oxide, potassium oxide,
phosphorus pentoxide, zirconium dioxide, zinc oxide,
aluminum oxide, magnesium oxide, pigments.
CELTRA Duo
(18030101)
Dentsply Sirona,
Milford, DE, USA

Silicon dioxide, lithium oxide, potassium
oxide, phosphorus pentoxide, aluminum
oxide, zirconium dioxide, cerium oxide, and pigments.

IPS ceramic etching gel
(W30687)

Ivoclar Vivadent,
Schaan, Liechtenstein

Hydrofluoric acid (5%).

Monobond Plus
(W05075)

Ivoclar Vivadent,
Schaan, Liechtenstein

Adhesive monomers (4%), Ethanol (96%).

RelyX Unicem 2
(666940)
3M ESPE, St. Paul,
MN, USA

Base paste: Methacrylate monomers containing
phosphoric acid groups, methacrylate monomers,
silanated fillers, initiator components, stabilizers
rheological additives.
Catalyst paste: Methacrylate monomers, alkaline
(basic) fillers, silanated fillers, initiator components,
stabilizers, pigments, rheological additives.

Variolink Esthetic LC
(W34586)
Ivoclar Vivadent,
Schaan, Liechtenstein

Monomer matrix: Urethane dimethacrylate,
methacrylate monomers.

Inorganic fillers: Ytterbium trifluoride, and spheroid
mixed oxide, initiators, stabilizers.

RelyX Luting Plus
(N880579)
3M ESPE, St. Paul,
MN, USA

Paste A: Fluoroaluminosilicate (FAS) glass, proprietary
reducing agent HEMA, water, opacifying agent.

Paste B: Methacrylated polycarboxylic acid, BisGMA,
HEMA, water, potassium persulfate, zirconia silica
filler.

Sara Casado, MS Thesis Advanced Operative and Adhesive Dentistry, CBY | December 2017
13
Specimen Preparation

Reinforced glass-ceramic CAD/CAM blocks were sectioned with a diamond-wafering blade in a
precision cutting machine (IsoMet 1000, Buehler, Lake Bluff, IL, USA) under constant water
cooling yielding nine-hundred (n=900) beams of different thickness with the following
dimensions: 0.5 x 4.0 x 18.0 mm, 1.0 x 4.0 x 18.0 mm, and 2.0 x 4.0 x 18.0 mm (Figure 1, and
2). Each beam was further polished with abrasive silicon carbide paper in ascending order (800-
and 1200-grit) under running distilled water. Specimens were then ultrasonicated in 100%
ethanol for 5 minutes to remove debris from sectioning and polishing procedures. Lithium
disilicate glass-ceramic and zirconia-reinforced lithium silicate glass-ceramic were crystallized
on a ceramic furnace (Programat CS3 Furnace, Ivoclar Vivadent, Schaan, Liechtenstein)
according to the manufacturer’s instructions (Table 2).

Figure 1: Preparation of the specimens from the glass-ceramic CAD/CAM blocks.

Figure 2: Beams originated from the glass-ceramic CAD/CAM blocks, with different
thickness.
Influence of thickness and different resin cements on the flexural strength of high strength CAD/CAM glass ceramics

14

Table 2: Crystallization parameters for IPS e.max CAD and CELTRA Duo.

IPS e.max CAD CELTRA Duo
Stand-by temperature (B) 403°C 500°C
Closing time (S) 6:00 min. 3:30 min.
Heating rate (t1) 90°C/ min. 60°C/ min.
Firing temperature (T1) 850°C 820°C
Holding time (H1) 0:10 min. 1:00 min.
Heating rate (t2) 30°C/ min. -
Firing temperature (T2) 840°C -
Holding time (H2) 7:00 min. -
Vacuum (1-11)
Vacuum (1- 12)
550°C
1022°C
Off
Vacuum (2-21)
Vacuum (2 -22)
820°C
1508°C
Off
Long-term cooling (L) 700°C 750°C
Cooling rate (t) 0 min. 3 min.

Ceramic Surface Treatment

Thirty beams for each glass-ceramic material (IPS e.max CAD and CELTRA Duo) of each
different thickness were left untreated (not etched) for flexural strengths testing purposes (NET)
(n=180). The remaining lithium disilicate glass-ceramic (IPS e.max CAD) beams were etched
with 5% hydrofluoric acid for 20 seconds (as per manufacturer’s instructions)
(18)
, rinsed with
distilled water for 60 seconds, and ultrasonic cleansed in distilled water for 5 minutes. Similarly,
the remaining zirconia-reinforced lithium silicate glass-ceramic (CELTRA Duo) beams were
etched with 5% hydrofluoric acid for 30 seconds (as per manufacturer’s instructions)
(18)
, rinsed
with distilled water for 30 seconds, and ultrasonically cleansed in distilled water for 5 minutes.
Sara Casado, MS Thesis Advanced Operative and Adhesive Dentistry, CBY | December 2017
15
Following HF etching, silanization was performed by applying a silane-coupling agent
(Monobond Plus, Ivoclar Vivadent, Schaan, Liechtenstein) on the etched surfaces for 60 seconds.

Luting Procedures

Three different luting materials were tested: a dual-cured self-adhesive resin cement (RelyX
Unicem 2, 3M, St. Paul, MN, USA [UN2]), a light-cured adhesive resin cement (Variolink
Esthetic LC, Ivoclar Vivadent, Schaan, Liechtenstein [VRL]), and a resin-modified glass
ionomer cement (RelyX Luting Plus, 3M, St. Paul, MN, USA [RLP]).
To standardize the cement thickness, a 100 µm-thick adhesive tape (Avery
Ò
) was attached to the
edges of each beam. The cements were mixed in 1:1 ratio, which is according to manufacturer’s
instructions, applied on the silanated surfaces, and covered with a microscope glass slide. A load
of 2 kg was applied to the ceramic for 3 minutes to create a uniform resin cement of
approximately 100 µm, simulating the range of resin luting film thickness observed on all-
ceramic restorations
(12)
. Excess resin cement was removed with a sable brush before the setting
of the material. Each specimen was then light-cured for 20 seconds, using an LED light curing
unit (VALO, Ultradent Products Inc., South Jordan, UT, USA) utilizing a wavelength of 395–
480 nm, with a radiant emittance of >800 mW/cm
2
.
The distribution of different experimental groups is described below (Figure 3).

Influence of thickness and different resin cements on the flexural strength of high strength CAD/CAM glass ceramics

16

Figure 3: Experimental groups for LD and ZLS.

Storage and Artificial Aging

All specimens were stored in distilled water at 37°C. The flexural strength of all experimental
groups was tested in two moments: 24 hours and after artificial aging. Artificial aging was
performed by 20,000 cycles of thermal fatigue at 5°C and 55°C with 20 seconds dwell time and
5 seconds transfer time (20K TC) (THE-1100, SD Mechatronik, Germany).

Flexural Strength Test

Flexural strength was assessed using a three-point bending test (span 14.0 mm) in a universal
testing machine (Model 5965 Instron, Norwood, MA, USA) according to ISO 6872: 2015
(Figure 4). A constant load was applied on the ceramic surface, with the etched surface or luted
cement side facing the opposite direction of the load at a crosshead speed of 0.5 mm/min, until
failure. Testing was performed under standard laboratory conditions. Maximum load at time of
failure was recorded using computer software (Bluehill 3, V3.04, Instron, Norwood, MA, USA).

Sara Casado, MS Thesis Advanced Operative and Adhesive Dentistry, CBY | December 2017
17

The following equation was used for flexural strength (F) calculation:
F = 3PL/2bh2
where P is load at fracture; L is the test span (14.0 mm); b is the thickness of the sample; h is
height of the sample. Each beam thickness b and height h was recorded using a digital caliper
before the failure.

Figure 4: Fixtures used for three-point flexural strength testing.

Statistical analysis

Statistical analysis was performed based on the flexural strength (MPa), using statistical software
(SPSS 20, Chicago, IL USA). The Kolmogorov-Smirnov statistical test was used to assess the
normal distribution of the data, and the Levene’s statistical test to assess the equality of
variances.
The data were then analyzed using a two-factor analysis of variance (two-way ANOVA) to
examine the effects of thickness (i.e. 0.5 mm vs. 1.0 mm vs. 2.0 mm), and type of surface
treatment (NET, ETC, UN2, VRL, and RLP). A post-hoc all paired Tukey test was employed (α
= 0.05) to identify statistically significant differences between paired group with the corrected α -
Influence of thickness and different resin cements on the flexural strength of high strength CAD/CAM glass ceramics

18
value stated. T-test was used to analyze differences in the testing moment (i.e., 24h vs. 20K TC),
on the flexural strength of the two different ceramics.

Scanning Electron Microscopy
An additional beam was prepared for each tested ceramic for evaluation of the etched surface
under scanning electron microscopy (SEM). The specimens mounted in universal stubs with
double tape, sputter coated with gold-palladium, and examined under SEM (Jeol 7001F, Jeol
Inc., Peabody, MA, USA).

Sara Casado, MS Thesis Advanced Operative and Adhesive Dentistry, CBY | December 2017
19
RESULTS
The mean flexural strength values (MPa) and standard deviation (SD) for all experimental
groups are presented in tables 1 and 2, respectively.

Table 3: Mean flexural strength (± standard deviation) for different surface treatments, before
and after thermal fatigue (20K TF) for IPS e.max CAD.

Flexural Strength ± SD (MPa)
Surface
Treatment
24 h 20K TC
Thickness Thickness
0.5 mm 1.0 mm 2.0 mm 0.5 mm 1.0 mm 2.0 mm
NET 300.35±120.80
aA
348.24±42.71
aA
382.42±43.14
aA
316.78±78.21
aA
295.94±71.85
aB
325.94±70.77
aB

ETC 301.52±82.97
aA
277.55±99.99
aA
322.64±53.37
aA
333.82±71.32
aA
347.92±75.30
aB
388.29±74.15
aB

UN2 336.19±82.68
aA
326.24±55.92
aA
367.24±73.01
aA
402.20±126.12
aA
322.98±86.81
aA
370.90±82.54
aA

VRL 358.18±130.65
aA
372.82±65.8
aA
358.18±61.74
aA
275.60±72.82
aB
323.67±46.51
aB
380.15±63.44
aA

RLP 276.54±59.48
aA
332.33±98.51
aA
357.22±52.94
aA
357.97±85.23
aA
324.90±80.28
aA
393.20±90.54
aA

Within the same vertical column, means with same superscript lower-case letters, comparing different surface
treatment are not statistically different (p > 0.05). Within the same horizontal row, means with the same superscript
upper-case letters, tested in different moments (24 h vs. 20K TC) for each surface treatment are not statistically
different (p > 0.05). NET: no surface treatment, ETC: etched, UN2: RelyX Unicem 2, VRL: Variolink Esthetic LC,
RLP: RelyX Luting Plus.

Table 4: Mean flexural strength (± standard deviation) for different surface treatments, before
and after thermal fatigue (20K TF) for CELTRA Duo.

Flexural Strength ± SD (MPa)
Surface
Treatment
24 h 20K TC
Thickness Thickness
0.5 mm 1.0 mm 2.0 mm 0.5 mm 1.0 mm 2.0 mm
NET 223.91±53.39
bA
272.47±56.09
bA
304.04±41.11
bA
284.55±74.97
bB
256.04±58.14
bA
311.15±77.88
bA

ETC 335.58±40.08
aA
314.04±60.33
aA
290.17±76.02
aA
345.94±79.02
aA
372.95±87.57
aB
325.62±72.69
aA

UN2 293.08±88.69
bA
218.04±49.59
bA
201.70±54.79
bA
311.77±80.67
bA
257.24±71.23
bA
229.57±86.48
bA

VRL 243.02±53.23
bA
241.30±56.64
bA
275.77±57.41
bA
234.63±78.63
bA
270.55±87.93
bA
237.16±44.94
bA

RLP 242.63±90.41
bA
266.87±105.12
bA
226.59±86.91
bA
276.59±94.31
bA
252.25±107.71
bA
310.54±58.68
bB

Within the same vertical column, means with same superscript lower-case letters, comparing different surface treatment
are not statistically different (p > 0.05). Within the same horizontal row, means with the same superscript upper-case
letters, tested in different moments (24 h vs. 20K TC) for each surface treatment are not statistically different (p > 0.05).
NET: no surface treatment, ETC: etched, UN2: RelyX Unicem 2, VRL: Variolink Esthetic LC, RLP: RelyX Luting Plus.
Influence of thickness and different resin cements on the flexural strength of high strength CAD/CAM glass ceramics

20

Different surface treatments did not produce statistical differences among the experimental
groups before and after of thermal fatigue (Table 3 and 5), except for CELTRA Duo NET (Table
4) (p>0.05).
Lithium disilicate-reinforced glass-ceramic (IPS e.max CAD) yielded statistically significant
higher flexural strength values than that of the zirconia-reinforced lithium silicate glass-ceramic
(p<0.05). Figure 5 illustrates the behavior of both CAD/CAM glass-ceramic materials before
(24h) and after thermal fatigue (20K TC).
Figure 5: Mean flexural strength of CAD/CAM glass-ceramic of different thicknesses before
and after thermal fatigue.

0
50
100
150
200
250
300
350
400
0.5 mm 1.0 mm 2.0 mm
Mean Flexural Strength (MPa)
Thickness
IPS e.max CAD vs CELTRA Duo
IPS e.max CAD -24h IPS e.max CAD -20K TC CELTRA Duo -24h CELTRA Duo -20K TC
Sara Casado, MS Thesis Advanced Operative and Adhesive Dentistry, CBY | December 2017
21

IPS e.max CAD yielded statistically significant higher flexural strength at 2.0 mm than that of all
other experimental groups. For the remaining thicknesses (1.0 mm and 0.5 mm), no statistically
significant differences were found among the other experimental groups (Table 5).

Table 5: Mean flexural strength (± standard deviation) for IPS e.max CAD and CELTRA Duo in
different thicknesses at 24 hours and after thermal fatigue (20K TF).

Thickness
Flexural Strength ± SD (MPa)
IPS e.max CAD CELTRA Duo
24 h 20K TF 24 h 20K TF
0.5 mm 300.35±120.80
bAλ
316.78±78.21
bAλ
223.91±53.39
aBρ
284.55±74.97
aBτ
1.0 mm 348.24±42.71
bAλ
295.94±71.85
bAθ
272.47±56.09
aBρ
256.04±58.14
aBρ

2.0 mm 382.42±43.14
aAλ
325.94±70.77
aAθ
304.04±41.11
aBρ
311.15±77.88
aBρ

Within the same vertical column, means with same superscript lower-case letters, comparing different thicknesses are
not statistically different (p > 0.05). Within the same horizontal row, means with the same superscript upper-case letters,
comparing different materials tested (IPS e.max CAD vs. CELTRA Duo) for each thickness are not statistically different
(p > 0.05). Within the same horizontal row, means with the same superscript Greek letters, comparing different moments
tested (24h vs. 20K TC) for each thickness [IPS e.max CAD (λ and θ) or CELTRA Duo (ρ and τ)] are not statistically
different (p > 0.05).

Thermal fatigue negatively affected the flexural strength of IPS e.max CAD at 1.0 mm thickness,
whereas a statistically significant increase in the flexural strength was observed for CELTRA
Duo at 0.5 mm thick compared to the other two thicknesses (Table 5).

Influence of thickness and different resin cements on the flexural strength of high strength CAD/CAM glass ceramics

22
Scanning Electron Microscope Micrographs

Figure 6: Scanning Electron Microscopy a) Electron micrograph depicting the microstructure of
zirconia-reinforced lithium silicate glass-ceramic (CELTRA Duo) etched with 5% hydrofluoric
acid for 30 seconds (magnification x10,000); b) Electron micrograph depicting the
microstructure of lithium disilicate glass-ceramic (IPS e.max CAD) etched with 5% hydrofluoric
acid for 20 seconds (magnification x10,000).

Sara Casado, MS Thesis Advanced Operative and Adhesive Dentistry, CBY | December 2017
23
DISCUSSION
The present study revealed that lithium disilicate-reinforced glass-ceramic (L) yielded significant
higher flexural strengths than that of zirconia-reinforced lithium silicate glass-ceramic (ZLS).
The manufacturer stated an increment of 76% in flexural strength (from 210 MPa to 370 MPa)
for ZLS when additional heat treatment is performed
(18)
. However, our findings disagree that
post-crystallization of ZLS increases the flexural strength comparable to that of LD
(9, 19)
. No
microstructural differences have been found between the sectioned CAD/CAM block and post-
crystallized ZLS
(9)
. It has been speculated that an increase on flexural strengths after post-
crystallization is due to healing of possible intrinsic defects at elevated temperatures. However,
our findings dispute this claim. Comparatively to the LD, that shows an interlocked
microstructure composed of Li
2
Si
2
O
5
crystals, ZLS is constituted of two crystal phases: a
submicrometic lithium metasilicate crystallites with lithium orthophosphates of nanometric size.
The lithium disilicate phase present on the fired ZLS is of relatively low intensity, when
compared to the intensity present in the LD
(8)
. The ZLS is advertised as a zirconia-reinforced
glass-ceramic, however, no crystalline zirconia was detected in this ceramic
(9)
. Therefore, the
first null hypothesis was rejected.
The flexural strength evaluation was done following the guidelines of the ISO 6872:2015.
Current data on the flexural strength for lithium disilicate reinforced glass-ceramic (IPS e.max
CAD) and zirconia reinforced lithium silicate glass ceramic (CELTRA Duo) has been reported in
several in vitro studies that tested both materials at thickness of 3.0 mm or more
(9, 12, 19)
.

Unfortunately, thicknesses of 3.0 mm or more have little to no clinical relevance since LD and
ZLS were designed to be bonded and consequently ideal for minimally invasive restorative
techniques. Contemporary restorative dentistry requires maximum preservation of dental tissues
Influence of thickness and different resin cements on the flexural strength of high strength CAD/CAM glass ceramics

24
and consequently bonded restorations of thinner dimensions. Following this rationale, one of the
aims of the present investigation was to determine the influence of different thickness (0.5, 1.0,
and 2.0 mm) on the flexural strength of LD and ZLS. This would provide more realistic data for
clinicians aiming to preserve dental structures and to take advantage of the bonding properties of
these reinforced CAD/CAM glass-ceramic.
Currently, the present study is the first in the literature to evaluate the influence of the thickness
on the flexural strength of CAD/CAM reinforced glass-ceramic. ZLS produced similar flexural
strengths irrespective of the thickness tested (0.5, 1.0, and 2.0 mm); however, thickness of 2.0
mm yielded superior flexural strengths for LD. These findings might be associated with the
toughness/ rigidity of the LD that increases with increased thickness leading to a higher flexural
strength. This might partially justify the results found. However, more studies assessing the
influence of thickness on the flexural strength, Young’s modulus, and fracture toughness of the
glass- based ceramics are required to attain further conclusions. Thus, the second null hypotheses
tested which stated that material thickness has no significant effect on the flexural strength of the
two different ceramics is, therefore, partially rejected.
Standard adhesive protocol requirements for glass-ceramic restorations include etching with
hydrofluoric acid, followed by the application of silane
(20)
. It has been stated that etching with
hydrofluoric acid (HF) increases the bond strength by selective removal of the glassy matrix and
roughening of the ceramic surface
(21-23)
. However, increasing etching times may negatively
influence the mechanical properties of ceramics
(24-26)
. No statistically significant differences
were found for the LD before and after etching with hydrofluoric acid. This result corroborates
that when HF etching is performed according to the manufacturer’s recommendations (5%
hydrofluoric acid for 20 seconds) no detrimental effect on the flexural strength is observed for
Sara Casado, MS Thesis Advanced Operative and Adhesive Dentistry, CBY | December 2017
25
CAD/CAM LD. Conversely, contradictory results were found for the ZLS, which the flexural
strengths increased after etching procedures (5% hydrofluoric acid for 30 seconds). A possible
explanation for this outcome is that etching may diminish the surface cracks by altering the
nature and distribution of flaws and cracks
(12)
. Therefore, the third null hypothesis that ceramic
etching does not have an influence on the flexural strength of the ceramic materials was partially
rejected.
HF etching creates microporosities in the glass-ceramic that can be easily infiltrated with resin
cements. The penetration of resin cements into the ceramic structure can heal eventual cracks by
crack bridging
(12, 13)
and form a barrier against water sorption/corrosion of the ceramics
(12)
.
Although it has been reported that the infiltration of the etched glass-ceramics with resin cements
increases the flexural strength of these ceramics
(12)
, our results showed no improvement in the
flexural strength of both CAD/CAM glass-ceramic infiltrated with different cements. In addition,
there was no statically significant difference between the two resin-based cements (RelyX
Unicem 2, and Variolink esthetic LC) and the resin-modified glass-ionomer (RelyX Luting Plus)
was observed. Thus, the fourth null hypothesis that different cements do not produce differences
in flexural strength was, therefore, accepted. These findings are partially in agreement with a
study
(27)
that reported that the type of cement has no influence on the survival rate of glass-base
ceramic crowns. Interestingly, it was observed that approximately 83% of the etched ceramic
tested infiltrated with the resin-modified glass-ionomer cement (RelyX Luting Plus) lost the
cement coating, independently of the testing moment (24 hours vs. thermal fatigue). Despite of
the recommendations of the manufacturer of the LD includes glass-ionomer cements as a
cementation option, after the results of this study, the use of a resin-modified glass-ionomer on
etched CAD/CAM reinforced glass-ceramics is strongly discouraged.
Influence of thickness and different resin cements on the flexural strength of high strength CAD/CAM glass ceramics

26
Thermal fatigue is commonly performed to mimic the oral environment. Previous studies
evaluating the effect of thermocycling on the flexural strength of ceramic materials have found
that the flexural strength decreases due to an increase in the flaws on the ceramic surface. The
alternation between hot and cold temperatures produces tensile stresses on the ceramic surface,
which increases the growth of the crack and leads to a decrease in the mechanical strength of the
material
(12, 28)
. The results of the present study were varied and inconclusive, regarding the
influence of thermal fatigue on the flexural strength of the tested CAD/CAM glass-ceramic
materials. Further investigation is recommended to determine the influence of thermal fatigue
and hydrolysis on CAD/CAM glass-ceramics.
In conclusion, our results support that different CAD/CAM reinforced glass-ceramic produce
different flexural strengths according to its composition, thickness, and etching. Resin cement
infiltration was not able to increase the flexural strength of CAD/CAM reinforced glass-ceramic.

Sara Casado, MS Thesis Advanced Operative and Adhesive Dentistry, CBY | December 2017
27
CONCLUSION
Within the present in vitro study, it is possible to conclude that:

1. Lithium disilicate reinforced glass-ceramic has superior flexural strength results when
compared to that of the zirconia-reinforced lithium silicate glass-ceramic;
2. Thickness did not influence the flexural strengths for zirconia-reinforced lithium silicate
glass-ceramic, whereas at 2.0 mm the flexural strength of the lithium disilicate reinforced
glass-ceramic was superior;
3. Hydrofluoric acid etching treatment does not interfere with the flexural strength of
lithium disilicate reinforced glass-ceramic; however, HF etching increase the flexural
strength for zirconia-reinforced lithium silicate glass-ceramic;
4. Resin cement infiltration of the etched ceramic surfaces does not influence the flexural
strength for both tested ceramics.

Influence of thickness and different resin cements on the flexural strength of high strength CAD/CAM glass ceramics

28
CONFLICT OF INTEREST
The authors declare no conflict of interest.

FUNDING
The present research study was supported by the program of Advanced Operative and Adhesive
Dentistry, Restorative Sciences, at Herman Ostrow School of Dentistry of University of Southern
California.

Sara Casado, MS Thesis Advanced Operative and Adhesive Dentistry, CBY | December 2017
29
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Abstract (if available)

Abstract Objective: The aim of this study was to evaluate the influence of the thickness, etching, and resin cement type on the flexural strength of lithium disilicate-reinforced glass-ceramic (IPS e.max CAD, LD), and zirconia-reinforced lithium silicate glass-ceramic (CELTRA Duo, ZLS). ❧ Materials and Methods: IPD e.max CAD and CELTRA Duo CAD/CAM blocks were sectioned with a diamond-wafering blade, under constant water, to yield beams of varying thicknesses of 0.5 mm, 1.0 mm, and 2.0 mm. Specimens were polished, crystallized in a ceramic furnace, and sorted into different experimental groups (n=15) according to: ceramic material, thickness, etching, and luting cements (RelyX Unicem 2, Variolink Esthetic LC, and RelyX Luting Plus). The specimens were also tested at two different moments: 24 h and after 20,000 cycles of thermal fatigue. Flexural strength was assessed with three-point bending test, the values recorded in MPa. Statistical analysis was performed with t-test, two-way ANOVA, and Tukey post-hoc. ❧ Results: Statistically significant differences were found for LD, which yielded higher flexural strengths than ZLS, both before and after thermal fatigue. LD yielded higher flexural strength at 2.0 mm

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Asset Metadata

Creator Casado, Sara Raquel Andre (author)

Core Title The influence of thickness and different resin cements on the flexural strength of high strength CAD/CAM glass ceramics

School School of Dentistry

Degree Master of Science

Degree Program Craniofacial Biology

Publication Date 11/17/2017

Defense Date 10/24/2017

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag flexural strength,glass ceramic,hydrofluoric acid,OAI-PMH Harvest,resin cement,thickness

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Duarte, Sillas Jr. (committee chair), Paine, Michael (committee member), Phark, Jin-Ho (committee member), Sartori, Neimar (committee member)

Creator Email saraandre.casado@gmail.com,scasado@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c40-458312

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