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The effect of surface treatment and translucency on the shear bond strength between resin cement and zirconia

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The effect of surface treatment and translucency on the shear bond strength between resin cement and zirconia

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“The Effect of Surface Treatment and Translucency on the Shear
Bond Strength between Resin Cement and Zirconia”

By: Dr. Sarah M. Hazime
Advisor: Dr. Neimar Sartori
Co-Advisor: Dr. Sillas Duarte

Thesis presented to the Herman Ostrow School of Dentistry University of Southern California, in
partial fulfillment of the requirements for the degree: Master of Science in Craniofacial Biology.

Advanced Operative and Adhesive Dentistry
Division of Restorative Sciences

December 2017

2
ACKNOWLEDGEMENT
And pray, “O my Lord, let my entry be with honor, and let my exit be with honor, and grant me
power from You which would help (sustain) me”
(17:80) The Holy Qur’an
Every journey has its moments of good and bad, moments of weakness and others of
strength. I started this particular journey with one foot out the door. Yet here I am, two and a half
years later, writing this section and being grateful that I persevered. I would like to take a moment
of gratitude for everyone who made this journey worthwhile.
A special thank you goes to all committee members; my mentor and advisor, Dr. Neimar
Sartori, thank you for your guidance, expertise, dedication and support. My co-advisor and
program director, Dr. Sillas Duarte, thank you for your constant guidance and generous teachings.
Dr. Jin-Ho Phark, thank you for your valued insights and encouragement. Dr. Michael Paine, thank
you for your constant help and guidance.
A special thank you goes to my lovely family whom without I wouldn’t have made it this
far, and to whom I dedicate this work. My loving parents, Mohammed and Etidal, thank you for
always believing in me, loving me unconditionally and accepting me for who I am. My amazing
siblings, Hasan, Hajar and Areej, thank you for always being there and supporting me all the way
through. My 2-year old niece, Lana, thank you for making me laugh every time you speak.
I would also like to thank all my teachers and colleagues as well as the staff of the Herman
Ostrow School of Dentistry and King Abdulaziz University for all their love, support and guidance.
I’m a better person because of your teachings.
Last and definitely not least, a special thank you to my best friends and loved ones for
being everything I could ask for. You complete me!

3
Table of Contents
LIST OF FIGURES AND TABLES ...........................................................................................................5
ABBREVIATIONS ......................................................................................................................................6
ABSTRACT ..................................................................................................................................................8
1. INTRODUCTION ...............................................................................................................................9
2. MATERIALS AND METHODS .....................................................................................................11
2.1. ZIRCONIA BLOCK PREPARATION .................................................................................................11
2.2. ZIRCONIA SURFACE TREATMENT ................................................................................................11
2.2.1. Melted fluoride etchant ......................................................................................................12
2.2.2. Sandblasting ......................................................................................................................13
2.3. SINTERING ..................................................................................................................................13
2.4. COMPOSITE CYLINDERS .............................................................................................................13
2.5. ADHESIVE BONDING PROCEDURES .............................................................................................13
2.6. STORAGE .....................................................................................................................................15
2.7. SHEAR BOND STRENGTH .............................................................................................................15
2.8. STATISTICAL ANALYSIS ..............................................................................................................15
2.9. FAILURE MODE ...........................................................................................................................16
2.10. SCANNING ELECTRON MICROSCOPY (SEM) ...............................................................................16
3. RESULTS ...........................................................................................................................................16
3.1. SHEAR BOND STRENGTH .............................................................................................................16
3.2. FAILURE MODE ...........................................................................................................................18
3.3. SCANNING ELECTRON MICROSCOPY (SEM) ...............................................................................20
4. DISCUSSION ....................................................................................................................................23

4
5. CONCLUSION ..................................................................................................................................26
6. CONFLICT OF INTEREST ............................................................................................................27
7. FUNDING ..........................................................................................................................................27
REFERENCES ...........................................................................................................................................28

5

LIST OF FIGURES AND TABLES
Figure 1: Group distribution according to surface treatment. Groups indicated by dotted lines
(n=10) represent non-aged groups. Dashed lines indicate groups that will be aged (n=10)
Figure 2: Control groups (original magnification x3,000). A. Katana ML zirconia. B. Katana
UTML zirconia.
Figure 3: Katana ML zirconia surface treatments (original magnification x3,000). A. Post-etch
group; B. Pre-etch group; C. SB + Post-etch group; D. SB + Pre-etch group.
Figure 4: Katana UTML zirconia surface treatments (original magnification x3,000). A. Pre-etch
group. B. Post-etch group. C. SB + Post-etch group.
Table 1: Chemical composition of substrate and adhesive.
Table 2: Shear bond strength means, standard deviation and statistical results.
Table 3: Distribution of failure modes in each tested group by percentage.

6
ABBREVIATIONS
ZrO
2
: Zirconium Oxide.
m: Monoclinic.
t: Tetragonal.
c: Cubic.
MgO: Magnesium Oxide.
Y
2
O
3
: Yttrium Oxide.
CaO: Calcium Oxide.
Ce
2
O
3
: Cerium Oxide.
Y-TZP: Yttrium stabilized tetragonal zirconia polycrystal.
ML: Multi Layered.
UTML: Ultra Translucent Multi Layered.
SB: Sandblast.
KHF
2
: Potassium Bifluoride.
Bis-GMA: Bisphenol A Glycidyl Methacrylate.

Bis-EMA: Bisphenol A Polyethethylene Glycol Diether Dimethacrylate.
TEGDM: Triethylene glycol dimethacrylate
UDMA: Urethane dimethacrylate
10-MDP: 10-Methacryloyloxydecyl Dihydrogen Phosphate
TC: Thermocycling.
MPa: Mega Pascal.
N: Newton.
mm
2
: Millimeter square.

7
SEM: Scanning Electron Microscopy.
SBS: Shear Bond Strength.
C-zr: Cohesive-zirconia.
C-cr: Cohesive-composite resin.
C-rc: Cohesive-resin cement.
A-zr: Adhesive-zirconia.
A-cr: Adhesive-resin cement.
M: Mixed.

8
ABSTRACT
Objectives: The verdict on achieving a long-term durable bond to zirconia restorations has not
been reached yet. The present study investigated the effect of different surface treatments on the
long-term bond to two types of zirconia materials.
Methods: Two-hundred and forty monolithic zirconia blocks were randomly assigned into six
groups according to the surface treatment performed (Control, Post-etch, Pre-etch, Sandblast (SB),
SB + Post-etch, SB + Pre-etch). Etching was done using a potassium bifluoride salt preparation
that was allowed to melt over the surface of zirconia in a furnace at 300°C. The blocks were bonded
to resin composite cylinders using a 10-MDP containing resin cement, and were subjected to shear
bond strength (SBS) testing after 24 hours and 20K cycles of thermal aging. The treated surfaces
were examined under SEM and failure modes were reported.
Results: The bond strength was significantly affected by the surface treatment, the aging process,
and the type of zirconia (p < .001). Post-etching of multi layered zirconia had the highest bond
strength values especially when preceded by pre-sintered sandblasting. The bond strength value of
post-etched multi layered zirconia was not significantly affected by Thermocycling (p > 0.05).
SEM analysis revealed that the surface morphology of the zirconia was changed after surface
treatment.
Conclusions: Potassium bifluoride etching of post-sintered multi layered zirconia significantly
improves the zirconia-resin cement bond.
Clinical Significance: Achieving a long-term durable bond to zirconia might be possible when
using a potassium bifluoride preparation for the surface treatment for multi layered zirconia.
Keywords: bonding; fluoride etchant; potassium bifluoride; surface treatment; yttrium-stabilized
tetragonal zirconia; sandblasting

9
1. INTRODUCTION
The evolution in dental materials, CAD/CAM technologies and adhesive techniques
elevate the treatment possibilities and applicability of metal-free dentistry. Since zirconia’s first
reported use in dentistry in the 1970s
(1, 2)
, its dental applications have expanded to include implant
abutments, post and cores, coppings for all-ceramic restorations, partial and full coverage
monolithic restorations, as well as anterior veneers
(3)
.
Dental Zirconia is chemically known as Zirconium Oxide (ZrO
2
), a white crystalline oxide
of Zirconium
(3)
. It was first discovered by chemist Martin Heinrich Klaproth in 1789, and later
isolated in 1824 by Jöns Jakob Berzelius
(3)
.

According to crystalline arrangements, the
microstructure of zirconia can be categorized into three known phases: 1) Monoclinic (m) phase,
stable at room temperature up to 1170 °C; 2) Tetragonal (t) phase, stable between 1170 °C – 2370
°C and; 3) Cubic (c) phase, stable above 2370 °C. The Tetragonal phase represents the strongest
phase in terms of mechanical properties, followed by the Cubic phase, whilst the Monoclinic phase
represents a poor mechanical performance of the zirconia material
(3)
.
Since tetragonal and cubic phases are unstable at room temperature, several attempts to
stabilize the zirconia through the addition of oxides; such as Magnesium Oxide (MgO), Yttrium
Oxide, (Y
2
O
3
), Calcium Oxide (CaO), and Cerium Oxide (Ce
2
O
3
) has been done over the years.
Yttrium stabilized tetragonal zirconia polycrystal (Y-TZP) is the most common stabilized zirconia
used in dentistry nowadays
(3, 4)
.

The percentage of the stabilizing agent in Y-TZP ranges from 2 -
3%. The fraction of t-phase retention within the Y-TZP structure depends on multiple factors
including; the processing temperature, the yttrium content, and the grain size
(3)
.
In addition to mechanical properties, other important factors that determine the clinical
indication of restorative materials are biocompatibility, translucency and color
(4)
. Conventional Y-

10
TZP restorations are opaque and their esthetic outcome as monolithic restorations, is incomparable
to that of glass-ceramic monolithic restorations
(5, 6)
. To overcome this esthetic drawback, newer
zirconia materials made with higher percentage of cubic phase within their microstructure have
been recently developed
(7)
. The cubic crystal configuration enhances the optical properties of
zirconia, making it more translucent
(7)
. However, the downside of this process is the possible
reduction in mechanical properties when compared to conventional Y-TZP
(8)
.
Another clinical disadvantage of zirconia, when compared to silica based ceramics, is its
inability to achieve a durable bond to resin cement with conventional bonding techniques, due to
its inertness and lack of hydroxyl group
(7, 9-14)
. Several studies have investigated the bond
effectiveness of zirconia using different surface treatments such as rotary instrument abrasion
(15)
,
air born particle abrasion with alumina oxide (Al
2
O
3
)
(16-18)
, tribochemical silica coating
(5, 7, 17, 19,
20)
,

grinding
(21)
, laser
(16, 18, 22-31)
, selective infiltration etching
(19, 32)
, modified fusion sputtering
(32,
33)
, as well as etching with fluoride salts
(34, 35)
. Although the aforementioned techniques may
promote adequate short-term bond strength, the long-term bond of zirconia to resin cements is still
unpredictable.
Motivated by this ongoing issue, the present study aims to evaluate the effect of zirconia
pre- and post-sintered surface treatment on long-term bond strength to resin cement. The research
null hypotheses were: 1) Surface treatment does not affect the bond strength of zirconia to resin
cement; 2) Artificial aging does not affect the zirconia-resin bond strength, and; 3) There is no
significant difference in the bond strength of different types of zirconia to resin cement.

11
2. MATERIALS AND METHODS
2.1. Zirconia block preparation
Zirconia CAD/CAM discs with two different translucencies, multi layered (ML), and ultra-
translucent multi layered (UTML) were used in the study (Katana Zirconia, Kuraray Noritake
Dental, Aichi, Japan). The discs were sectioned into 14 x 14 x 2 millimeter (mm) blocks using a
precision low-speed diamond saw (IsoMet 1000, Buehler, Lake Bluff, IL, USA) under constant
cooling with distilled water (n=120/material).
To obtain a scratch free standardized bonding surface, all blocks were polished with
abrasive silicon carbide paper (Carbimet Paper Strips, Buehler, Lake Bluff, IL, USA) in ascending
order (600, 800, and 1200 grit) under running water for 30 seconds each.
2.2. Zirconia surface treatment
Blocks from each zirconia type were randomly assigned into six groups depending on the
surface treatment performed. The group distribution used in this study is described in Figure 1.
• Control group: no surface treatment.
• Post-etch group: the surface was treated with a melted fluoride etchant after sintering.
• Pre-etch group: the surface was treated with a melted fluoride etchant before sintering.
• Sandblast (SB) group: the surface was sandblasted with Al
2
O
3
before sintering.
• SB + Post-etch: the surface was sandblasted with Al
2
O
3
before sintering and etched with a
melted fluoride etchant after sintering.
• SB + Pre-etch: the surface was sandblasted with Al
2
O
3
and then etched with a melted fluoride
etchant before sintering.

12

2.2.1. Melted fluoride etchant
A slurry consisting of distilled water and 4.0g/ml potassium bifluoride (KHF
2
) crystals
(Sigma-Aldrich, St. Louise, MO, USA) was mixed and applied to the bonding surface of zirconia
before or after sintering. The treated blocks were then subjected to heat treatment of 300°C for 10
minutes (Whip Mix PRO 100, Whip Mix Cooperation, Louisville, KY, USA). After that, the
blocks were ultrasonically cleaned in distilled water for 10 minutes, followed by a 10-minute
ultrasonic bath in 70% ethanol.
Figure 1 Group distribution according to surface treatment. Groups indicated by dotted lines
(n=10) represent non-aged groups. Dashed lines represent aged groups (n=10)

13
2.2.2. Sandblasting
Half of the blocks from each zirconia were air abraded with 50µm Al
2
O
3
particles from a
10mm distance using a pressure of 2 bar for 15 seconds in a blasting chamber (Renfert, Hilzingen,
Germany) before sintering.
2.3. Sintering
All specimens were sintered in a zirconia furnace (Zirkonzahn 700, Zirkonzahn, Gais,
Italy) according to manufacturer’s recommendations: temperature increase and decrease of
10°C/min, holding for 2 hours at 1500°C for the ML zirconia and at 1550°C for UTML zirconia.
2.4. Composite Cylinders
Two-hundred and forty composite resin cylinders were fabricated using a micro-filled
composite resin (Filtek Z250, 3M, St. Paul, MN, USA). The resin cylinders were fabricated with
the aid of a preformed Teflon mold with an internal diameter of 3mm and 4mm height. The mold
was placed on a glass slab, filled with composite resin, covered with a second glass slab, and gently
pressed to allow excess material to escape. The material was cured for 30 seconds on each side
using an LED light curing unit (VALO, Ultradent Products, South Jordan, UT, USA) utilizing a
wavelength of 395–480 nm at 800mWcm. The glass slabs were removed and the cylinders were
pushed out of the mold using a small condenser. The cylinders were stored in dry, clean conditions
until bonded.
2.5. Adhesive bonding procedures
All zirconia blocks were ultrasonically cleaned with 70% ethanol for 20 minutes, allowed
to dry for 10 minutes, and were primed with a single coat of primer for 10 seconds (Clearfill
Ceramic Primer, Kuraray, Aichi, Japan). Oil free compressed air was used for 10 seconds at a

14
distance of 3 centimeters to evaporate the solvent. Dual-cure resin cement (Panavia V5, Kuraray,
Aichi, Japan) was applied to the bonding surface of the composite resin cylinder, then the cylinder
was placed on the prepared zirconia surface using a seating device that employed a weight of 1kg
for 20 seconds. Excess cement was cleaned out using a micro-brush and the composite-zirconia
assembly was light cured for 10 seconds with an LED light curing unit (VALO, Ultradent Product,
South Jordan, UT, USA) from all four sides of the block at a 3mm distance. Light intensity was
measured with a radiometer (Bluephase meter, Ivoclar Vivadent, Schaan, Liechtenstein) after
every 10 specimens to ensure that adequate light intensity was maintained throughout the bonding
procedures.
Table 1 – Chemical composition of substrate and adhesive.
Material (batch number) Composition Manufacturer
Zirconia
Katana Multi Layered
(DRECK)
Zirconium oxide (ZrO
2
), Yttrium
oxide (Y
2
O
3
)
Kuraray Noritake;
Aichi, Japan

Katana Ultra Translucent Multi
Layered (DRYTA)
Zirconium oxide (ZrO
2
), Yttrium
oxide (Y
2
O
3
)
Kuraray Noritake;
Aichi, Japan

Composite resin
Filtek Z250 (N685938)
Resin matrix: Bis-GMA,
Bis-EMA, UDMA, TEGDMA.
Filler loading: 60 vol% silanized
zirconia/silica particles (0.01-
3.5microns)
3M ESPE; St. Paul,
USA
Resin cement
Panavia V5 (4T0011)
Paste A: Bis-GMA, TEGDMA,
hydrophobic aromatic
dimethacrylate, hydrophilic aliphatic
dimethacrylate, initiators,
accelerators, silanated barium glass
filler, silinated fluoroaluminosilicate
filler, colloidal silica.
Paste B: Bis-GMA, hydrophobic
aromatic dimethacrylate,
hydrophilic aliphatic dimethacrylate,
Kuraray Noritake;
Aichi, Japan

15
silanated barium glass filler,
silinated alminium oxide filler,
accelerators, dl-Camphorquinone,
pigments.
Primer
Clearfil ceramic primer
(3L0025)
3-methacryloxypropyl
trimethoxysilane,
10-methacryloyloxydecyl dihydrogen
phosphate
(MDP), ethanol

Kuraray Noritake;
Aichi, Japan

2.6. Storage
After bonding, all specimens were stored in distilled water at 37°C for 24 hours. The first
half of the specimens from each group (n=10) was directly tested while the second half was
subjected to 20,000 cycles of artificial aging (Figure 1) in a thermocycler at 5°C and 55°C, 20
seconds dwell time and 5 seconds transfer time (THE-1100, SD Mechatronik, Germany).
2.7. Shear bond strength
A universal testing machine (Model 5965 Instron, Norwood, MA, USA) with a knife-edge
shearing rod at a crosshead speed of 0.5 mm/min was used to perform the shear bond strength
(SBS) testing. Maximum shear load at the point of failure was recorded using a computer software
(Bluehill 3, V3.04, Instron, Norwood, MA, USA). Shear bond strength was then calculated in MPa
by dividing the maximum shear load (in N) by the surface area of the bonding surface of the
composite cylinder (in mm
2
).
2.8. Statistical analysis
The data were organized using Microsoft Excel (Office 365 for Mac, Microsoft, Redmond,
WA, USA) and analyzed using SPSS 20 for Mac (SPSS Inc., Chicago, IL, USA) at a preset level
of significance of 5% (α = 0.05) for all tests. The normality (Kolmogorov-Smirnov test) and

16
homoscedasticity (Levene test) assumptions of the data were assessed before statistical evaluation.
The data were then analyzed using a three-factor analysis of variance to examine the effects of the
surface treatment (i.e. Control vs. Post-etch vs. Pre-etch vs. SB vs. SB + Post-etch vs. SB + Pre-
etch), aging (i.e. 24h vs. after 20K of thermocycling), and type of zirconia (i.e. ML vs. UTML) as
well as the interaction of those factors on shear bond strength. To detect the statistical difference
among the surface treatments, Tukey’s multiple comparison post-hoc test was applied.
2.9. Failure mode
The failure modes were evaluated using a stereo microscope (Wild Heerebrugg,
Heerebrugg, Switzerland) at 20x magnification and were classified as cohesive (failure entirely
within zirconia substrate, composite resin, or resin cement), adhesive (failure at the zirconia/resin
cement or composite resin/resin cement interface), and mixed (failure at one of the adhesive
interfaces along with cohesive failure of either zirconia, resin cement or composite resin).
2.10. Scanning electron microscopy (SEM)
A representative sample from each group was randomly selected for SEM examination.
The specimens were cleaned in 100% ethanol for 30 minutes, sputter coated with gold-palladium,
and examined under Field Emission SEM (JEOL JSM-7001F, CA, USA).
3. RESULTS
3.1. Shear bond strength
Three-way ANOVA reveals a statistically significant difference between the types of
zirconia tested (p < .001), moments (p < .001), surface treatments (p < .001), as well as a positive
interaction among all factors evaluated (p = .002). The results of the bond strength testing are
summarized in Table 2.

17
Table 2 – Shear bond strength means, standard deviation and statistical results.

Surface treatment
Moment
24h 20K TC
ML UTML ML UTML
Control 15.74±2.0
bcAρ
19.88±2.7
aAλ
5.66±2.2
bAτ
3.84±2.2
aAθ

Post-etch 23.28±3.4
abAρ
21.82±4.1
aAλ
20.11±3.2
aAρ
10.69±2.5
aBθ

Pre-etch 11.58±3.1
cBρ
25.89±4.6
aAλ
4.26±2.1
bAρ
7.28±3.9
aAθ

SB 18.33±2.6
bcAρ
21.21±1.9
aAλ
7.44±2.0
bAτ
9.89±1.8
aAθ

SB + Post-etch 30.99±1.5
aAρ
20.86±5.6
aBλ
22.32±3.0
aAτ
4.29±1.3
aBθ

SB + Pre-etch 18.18±4.3
bcAρ
16.19±6.1
aAλ
8.25±3.1
bAτ
4.39±2.5
aAθ

Within the same vertical column, means with same superscript lower-case letters, comparing different surface
treatment for each Zirconia tested (ML or UTML) in each moment (24h or 20K TC) are not statistically different
(p > 0.05). Within the same horizontal row, means with the same superscript upper-case letters, comparing different
zirconia tested (ML vs. UTML) for each surface treatment in each moment (24h or 24K TC) are not statistically
different (p > 0.05). Within the same horizontal row, means with the same superscript Greek letters, comparing
different moments (24h vs. 20K TC) for each surface treatment zirconia tested [ML (ρ and τ) or UTML (λ and θ)]
for each zirconia tested (ML or UTML) are not statistically different (p > 0.05).

The highest SBS mean (30.99 MPa) was obtained with ML zirconia when the composite
resin cylinders were bonded to a zirconia surface that was sandblasted before sintering and then
post-etched with potassium bifluoride. This mean was significantly different from all other surface
treatments for ML zirconia assessed at 24h, except the Post-etch group (23.28 MPa). The lowest
bond strength value for ML zirconia at 24h was seen in the Pre-etch group (11.58 MPa), which
was significant different from all groups tested at the same moment, except for the Control group
(15.74 MPa) and sandblast group (18.33 MPa). After aging, the bond strength means of all groups
decrease significantly, except for Post-etch and Pre-etch groups. Other than a significantly
decrease in bond strength over time, the ML zirconia group that was sandblasted before sintering
and then post-etched had the highest SBS mean (22.32 MPa).
For UTML Zirconia there were no significant differences between the surface treatments
at 24h, with means ranging from 16.19 MPa for the SB + Pre-etch group, to 25.89 MPa for the

18
Pre-etch group. After artificial aging the bond strength values decreased significantly for all tested
groups, raging form 3.84 MPa for the Control group to 10.69 MPa for the Post-etch group.
There were no statistical significant differences in the SBS between both zirconia tested at
24h, irrespective of surface treatment, except for the SB + Post-etch group (ML = 30.99 MPa vs.
UTML = 20.86 MPa) and Pre-etch group (ML = 11.58 MPa vs. UTML = 25.89 MPa), where the
differences remained significant even after aging (ML = 22.32 MPa vs. UTML = 4.29 MPa).
Moreover, the SBS for the aged ML Post-etched group (20.11 MPa) was significantly higher than
that for the aged UTML Post-etched group (10.69 MPa).
3.2. Failure Mode
The percentage distributions of failure patterns are shown in Table 3
Table 3 – Distribution of failure modes in each tested group by percentage.
Zirconia
Failure mode
C-zr C-cr C-rc A-zr A-cr M
ML (24h)
Control 0 0 0 100 0 0
Post-etch 0 10 0 90 0 0
Pre-etch 10 0 0 60 0 30
SB 0 0 10 90 0 0
SB + Post-etch 10 0 30 10 0 40
SB + Pre-etch 30 10 10 40 0 10
ML (20K TC)
Control 0 0 0 100 0 0
Post-etch 0 0 30 60 0 10
Pre-etch 0 0 10 70 0 20
SB 0 0 0 100 0 0
SB + Post-etch 0 10 30 40 0 20
SB + Pre-etch 10 0 60 30 0 0
UTML (24h)
Control 0 0 10 90 0 0
Post-etch 0 0 30 60 0 10
Pre-etch 20 10 10 20 0 40
SB 0 0 0 50 0 50
SB + Post-etch 0 10 0 60 0 30

19
SB + Pre-etch 10 0 50 40 0 0
UTML (20K TC)
Control 0 0 10 90 0 0
Post-etch 10 0 40 50 0 0
Pre-etch 20 0 30 40 0 10
SB 0 0 0 100 0 0
SB + Post-etch 0 0 0 100 0 0
SB + Pre-etch 0 0 0 100 0 0
Classification of failure modes: (C-zr) Cohesive failure within zirconia. (C-cr) Cohesive failure
within composite resin. (C-rc) Cohesive failure within resin cement. (A-zr) Adhesive failure
along zirconia bonded interface. (A-cr) Adhesive failure along composite resin bonded
interface. (M) Mixed failure: adhesive failure along bonded interface associated with a
cohesive failure within zirconia, composite resin or resin cement.

The majority of the failures were adhesive in nature, involving the zirconia-resin cement
interface. Groups with higher bond strength values (i.e. Post-etch and SB + Post-etch for ML and
UTML Zirconia, and Pre-etch for UTML Zirconia) showed higher percentage of mixed and
cohesive failures in the resin ceramic. Cohesive composite resin failures were reported in non-
aged specimens of Post-etch, SB + Post-etch, SB + Pre-etch ML zirconia groups as well as Pre-
etch and SB+ Post-etch UTML zirconia groups. Zirconia fractures were overserved when etching
was done before sintering for all zirconia types.

20
3.3. Scanning electron microscopy (SEM)
The ultrastructural morphology of both zirconia types and the etched groups is presented
in Figures 2-4. It’s clear that the structure of ML zirconia is inherently different than that of UTML
zirconia. Furthermore, surface treatment resulted in different morphological changes in all of the
tested groups.

A B
Figure 2 Control groups (original magnification x3,000). A. Katana ML zirconia. B. Katana UTML zirconia.

21

D C
B A
Figure 3 Katana ML zirconia surface treatments (original magnification x3,000). A. Post-etch group; B. Pre-etch
group; C. SB + Post-etch group; D. SB + Pre-etch group.

22

A B
C
Figure 4 Katana UTML zirconia surface treatments (original magnification x3,000). A. Pre-etch
group. B. Post-etch group. C. SB + Post-etch group.

23
4. DISCUSSION
Zirconia is a highly inert material formed by a high crystalline structure resistant to
conventional acid etching, due to the absence of hydroxyl groups,

which makes bonded zirconia
restorations a clinical challenge
(7-13)
. Researchers have been investigating alternative methods to
bond to zirconia through zirconia surface alterations, and the introduction of new bonding
systems
(19, 36, 37)
.

However, achieving a long-term durable bond to zirconia is still questionable.
This study evaluated the effect of different surface treatments of two types of zirconia on the shear
bond strength values to resin cement. The results show that surface treatments can alter zirconia
surface morphology and significantly affect the SBS of zirconia to resin cement in ML zirconia,
but not in UTML zirconia (Table 2). Therefore, the first null hypothesis stating that, the surface
treatment does not affect the bond strength of zirconia to resin cement nor does it affects the surface
morphology of zirconia, was rejected.
Post-sintered etching with KHF
2
slurry improved the bond strength in ML zirconia and
resulted in a stable zirconia-resin interface even after aging (23.28 vs. 20.11 MPa). The ultra-
morphological changes on the zirconia surface topography, responsible for creating micro-
mechanical interlocking can be observed in the SEM images (Figure 3 A&C).
The use of KHF
2
for zirconium is well known in industrial fields and is referred to as “A
process of blanching zirconium”
(34)
. This fluoride salt acts by removing surface oxides when
allowed to melt at a temperature of 300°C for one minute
(34)
. We hypothesized that extending the
melting time would produce an etching effect rather than a cleaning effect on the zirconia surface.
Therefore, the melting time in this study was extended. The 10 minute melting margin was selected
based on the recommendations mentioned in the literature
(35)
. KHF
2

crystals are formed by
dissolving potassium carbonate (K
2
CO
3
) in excess hydrofluoric acid (HF). On heating, the

24
bifluoride yields potassium fluoride (KH) and HF. It is therefore very important to maintain good
ventilation when working with this product due to the toxic fumes generated during the melting
process
(35, 38)
. It is believed that the etchant primarily works by providing a surface topography that
promotes micromechanical retention and penetration of the resin cement into the zirconia structure.
Furthermore, the blanching effect of the etchant, through the removal of surface oxides, followed
by ultrasonic cleaning seems to improve the surface cleanness of the zirconia block and therefore
improves bonding. This is known as “surface hydroxylation” of zirconia; where the fluoridated
surface promotes the emergence of active hydroxyl sites that improve bonding
(35, 39)
.
The long-term stability of zirconia-resin bond have been investigated in this study. As
shown by the results, the SBS significantly decreases after aging in all tested groups except for the
Post-etch and Pre-etch groups of ML zirconia. Therefore, the second null hypothesis which states,
artificial aging does not affect the zirconia-resin bond strength, was partially rejected.
The use of resin cements containing 10-MDP has been linked to increased bonding
effectiveness between Y-TZP surfaces and resin cements. This is because the monomer contains
a functional group that facilitates chemical adhesion to zirconia
(9, 20, 40-46)
. It is believed that the
durability of the 10-MDP bond to zirconia is influenced by hydrolytic degradation more than the
degradation induced by temperature changes
(9, 47)
. Furthermore, the durability of the bond is also
related to the hydrolytic stability of the bond between MDP-m-ZrO
2
and MDP-t-ZrO
2
. It has been
found that the bond between MDP-m-ZrO
2
is more hydraulically stable than the bond between
MDP-t-ZrO
2
(9)
. Examining the ultrastructure of Post-etch and SB + Post-etch ML groups (Figure
3 A&C) suggests that the latter may have a higher quantity of tetragonal crystals and therefore a
higher quantity of MDP-t-ZrO2 bonds than found in the Post-etch ML group. This could explain
why the SBS of Post-etch ML group was stable after aging as opposed to SB + Post-etch ML

25
group. A quantitative analysis of the KHF
2
etched surfaces by means of x-ray diffraction, is
necessary in order to draw such conclusions.
Pre-sintered etching with KHF2 slurry in ML zirconia, had a detrimental effect on the bond
strength at 24h (11.58 MPa) when compared to the Control (15.74 MPa) and the Post-etch groups
(23.28 MPa). At the ultrastructural level, pre-etched blocks had an estranged structure in both
zirconia types. Although pre-sintered etching seemed to improve the bond strength in UTML
zirconia (25.89 MPa), this improvement was not significant and was unstable overtime (7.28 MPa).
The authors are unaware of other studies that investigated the effect of using a fluoride etchant for
the surface treatment of zirconia at the pre-sintered stage.
The effect of pre-sintered sandblasting has been investigated in several studies. It was
shown that pre-sintering air born abrasion with Al
2
O
3
particles of different sizes, increase surface
roughness, surface energy and wettability, and therefore enhances the bond strength
(48-51)
.
However, the high-energy impact of the Al
2
O
3
particles leads to increased heat generation within
the zirconia and ultimately leading to phase transformation (t®m) and weakening of zirconia
structure
(15-18, 21)
. When sandblasting is done at the pre-sintered stage, the undesirable phase
transformation that had occurred is reversed by the subsequent sintering, resulting in a stronger
restoration that is more resistant to fracture
(48, 49)
. In contrast to previous studies, sandblasting the
pre-sintered zirconia surface did not significantly improve the bond strength in any zirconia type
in the current study. However, sandblasting may have improved the immediate bond strength
values in ML zirconia, especially when followed by post-sintered etching (30.99 MPa), by creating
more surface roughness and therefore permitting a uniform effect of the etchant.
Another factor investigated in this study is the effect of zirconia translucency on the shear
bond strength to resin cements over time and with different surface treatments. Both materials

26
were impacted differently by the surface treatments performed, and thus yielded significantly
different bond strength values. For this reason, the third null hypothesis stating that, there is no
significant difference in the bond strength of different types of zirconia to resin cement, was
rejected. When post-etching was done in ML zirconia, the morphological etching pattern
resembled that of glass ceramics after HF acid etching. The structure of the zirconia was intact
with well-defined crystalline phases (Figure 3 A&C). UTML zirconia showed a completely melted
and undefined crystalline structure (Figure 4 B&C) which is believed to have impacted the bond
strength drastically. It is assumed that the 10-minute melting margin was suitable for ML zirconia
but unsuitable for UTML zirconia due to their inherently different structures and amount of each
crystalline phase. Further studies are needed to explore the effect of reducing the melting time on
ultra-translucent zirconia.
Another important finding is related to the pre-sintered etching groups in both types of
zirconia. With the current protocol, clear morphological changes on the zirconia block were
observed on visual examination after the blocks were sintered. Evidently, pre-sintered etching
resulted in a bluish-green discoloration as well as bending/bowing of the zirconia blocks. Some
blocks completely fractured during sintering, while others were cracked. The effect of KHF
2

etching at both stages (i.e. pre-sintered and post-sintered) on the flexural strength of zirconia is
still unclear. Further studies are needed in order to assess these concerns.
5. CONCLUSION
Potassium bifluoride surface treatment of post-sintered multilayered zirconia is successful
in achieving a strong, reliable zirconia-resin cement bonded interface.

27
6. CONFLICT OF INTEREST
The authors do not report any conflict of interest.
7. FUNDING
This study was funded by the Advanced Operative and Adhesive Dentistry Program at the
Herman Ostrow School of Dentistry of USC.

28
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Abstract (if available)

Abstract Objectives: The verdict on achieving a long-term durable bond to zirconia restorations has not been reached yet. The present study investigated the effect of different surface treatments on the long-term bond to two types of zirconia materials. ❧ Methods: Two-hundred and forty monolithic zirconia blocks were randomly assigned into six groups according to the surface treatment performed (Control, Post-etch, Pre-etch, Sandblast (SB), SB + Post-etch, SB + Pre-etch). Etching was done using a potassium bifluoride salt preparation that was allowed to melt over the surface of zirconia in a furnace at 300℃. The blocks were bonded to resin composite cylinders using a 10-MDP containing resin cement, and were subjected to shear bond strength (SBS) testing after 24 hours and 20K cycles of thermal aging. The treated surfaces were examined under SEM and failure modes were reported. ❧ Results: The bond strength was significantly affected by the surface treatment, the aging process, and the type of zirconia (p < .001). Post-etching of multi layered zirconia had the highest bond strength values especially when preceded by pre-sintered sandblasting. The bond strength value of post-etched multi layered zirconia was not significantly affected by Thermocycling (p > 0.05). SEM analysis revealed that the surface morphology of the zirconia was changed after surface treatment. ❧ Conclusions: Potassium bifluoride etching of post-sintered multi layered zirconia significantly improves the zirconia-resin cement bond. ❧ Clinical Significance: Achieving a long-term durable bond to zirconia might be possible when using a potassium bifluoride preparation for the surface treatment for multi layered zirconia.

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Asset Metadata

Creator Hazime, Sarah Mohammed (author)

Core Title The effect of surface treatment and translucency on the shear bond strength between resin cement and zirconia

School School of Dentistry

Degree Master of Science

Degree Program Craniofacial Biology

Publication Date 11/14/2017

Defense Date 10/24/2017

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

Tag bonding,fluoride etchant,OAI-PMH Harvest,potassium bifluoride,sandblasting,surface treatment,yttrium-stabilized tetragonal zirconia

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Contributor Electronically uploaded by the author (provenance)

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

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