INFLUENCE OF SURFACE TREATMENT OF Y-TZP AND LUTING CEMENTS ON
RETENTION OF Y-TZP CROWN
by
MEHDI KARIMIPOUR-SARYAZDI
PERNG-RA LIU, COMMITTEE CHAIR
JON BURGESS
DANIEL GIVAN
AMJAD JAVED
KEITH KINDERKNETCH
A THESIS
Submitted to the graduate faculty of The University of Alabama at Birmingham,
in partial fulfillment of the requirements for the degree of
Master of Science
BIRMINGHAM, ALABAMA
2012
i
Copyright by
MEHDI KARIMIPOUR-SARYAZDI
ii
INFLUENCE OF SURFACE TREATMENT OF Y-TZP AND LUTING CEMENTS ON
RETENTION OF Y-TZP CROWN
MEHDI KARIMIPOUR-SARYAZDI
MASTER OF SCIENCE
ABSTRACT
Clinicians for a single tooth restoration, due to improved esthetics, often select all
ceramic crowns. Zirconium Oxide has been utilized for many years to full fill the
requirement of esthetics, high mechanical properties and biocompatibility.
The purpose of this study is to evaluate the effect of different surface treatments
and luting agents on retention of a zirconium oxide crown.
Material and Methods: Ninety extracted teeth were obtained. Teeth were mounted
in Ortho JetTM resin (Lang Dental Manufacturing Co., Inc. IL, USA). Occlusal surface
was cut flat using Isomet (Buhler Ltd). Diamond burs were used to cut axial walls under
water spray at an angle of 20 degrees using Milling Machine (Shop-Task Modle 12-22).
Impressions were made with Aquasil monophase and XLV (Destply International, York,
PA) and poured with ADA Type IV gypsum. CAD/CAM design was used to fabricate
Zirconia copings with 1 mm of marginal width, and a hole with 2mm in diameter for
crown pull test. Fifty microns of space were incorporated into the crowns for cement
thickness. The intaglio surfaces of crowns were treated with Silica-modified Al 2 0 3 and
Al 2 0 3 . Crowns were cemented with different luting cements such as Panavia F2.0,
RelyXTM Unicem 2 and Duo-linkTM under 2500 gram pressure. Specimens were restored
in distilled water for 3 days. All groups were thermocycled (5000 cycle) and fatigue
loaded for 100,000 cycles while emerged in distilled water.
iii
Two-Way ANOVA and One-way ANOVA was used to analyze effect of different
surface treatments and cements on retention of Y-TZP crowns and followed by Tukey’s
post hoc test. Also, Two-way ANOVA was used to evaluate interaction between internal
fit, axial wall height, total surface area of prepared tooth and crown retention in all the
tested groups.
Surface treatment increased crown retention, but it was not statistically significant
(p>0.05), except Duo-Link cement group (p< 0.05).
The use of surface treatment does not increase crown retention in every cement
type. Crown retention is dependent to cement type.
Keywords: Y-TZP, CAD/CAM design, Diamond Bur, Ortho JetTM resin, Aquasil
monophase and XLV, Type IV gypsum
iv
DEDICATION
This thesis is dedicated to my parents and my oldest brother Dr. Mahmoud
Karimipour, who made it possible for me to immigrate here in order to achieve higher
education.
Also, this thesis is dedicated to my wife, Dr. Yichen (Jenn) Wei, who has helped
me to get through this process and the duration of my residency program. Finally, this
thesis is dedicated to the member of my committee, especially Perng-Ra Liu.
v
ACKNOWLEDGEMENT
I am indebted to the many individuals who contributed to my learning and in
achieving this Master of Science degree. I would like to thank my supervisor, Dr. PerngRa Liu, who has shown extraordinary amounts of patience along with confidence in my
work. Drs. Jon Burgess, Daniel Givan, Amjad Javed and Keith Kinderknetch who gave
me assistance and advice and kept me motivated to complete this degree. I also
acknowledge of great help, Drs. Ramtin Sadid-Zadeh, Deniz Cakir, Mr. Preston Beck and
graduate prosthodontics program.
I reserve special thanks for my parent, family and my wife who have never failed
to help me in achieving my goal.
vi
TABLE OF CONTENTS
Page
ABSTRACT ….………………………………………………………………………… iii
DEDICATION …………………………………………………………………………. v
ACKNOWLEDGMENTS ……………………………………………………………… vi
LIST OF TABLES ………………………………………………………………............ ix
LIST OF FIGUERS …………………………………………………………………..… xi
LIST OF ABBREVIATIONS …………………………………………………………. xiv
INTRODUCTION ...............................................................................................................1
Zirconia: Definition and history...............................................................................2
Transformation toughening.......................................................................................4
Characteristics of zirconia-based ceramics ..............................................................6
High strength and toughness ........................................................................6
Fatigue failure ..............................................................................................7
Fracture resistance after fatigue ...................................................................9
Subcritical crack growth resistance .......................................................................10
Thermal and environmental aging .........................................................................11
Longevity ..............................................................................................................13
Factors influenced mechanical properties of zirconia ceramics ............................17
Critical composition ...............................................................................................17
Manufacturing and sintering process .....................................................................18
Experimental designs .............................................................................................20
vii
Surface treatment ...................................................................................................22
Roughening abrasion .................................................................................22
Silicatization and silane coupling agent .....................................................23
Other Coupling agent .................................................................................27
Resin and resin composites ....................................................................................29
Bonding capacity related to surface treatment .......................................................34
Previous studies on retention and bond strength to zirconia ceramic ....................39
Evaluation methods ...................................................................................39
Taper mode ................................................................................................40
Cements and surface treatments tests ........................................................40
HYPOTHESIS AND AIMS ..............................................................................................41
METHODS AND MATERIALS .......................................................................................42
RESULT ……... ................................................................................................................54
DISCUSSION .................................................................................................................65
CONCLUSION .................................................................................................................78
REFERENCES ..................................................................................................................79
viii
LIST OF TABLES
Tables
Page
1
Comparison of flexural strength and fracture toughness of ceramics from different
system ……............................................................................................................ 7
2
Materials used in this study for cementation and Intaglio surface treatment of
crowns .................................................................................................................. 42
3
Study design groups with designated surface treatment, primer and cement ...... 48
4
Cements specification .......................................................................................... 49
5
Cement Specifications with crown and tooth surface treatment materials ........... 49
6
Crown cementation steps with RelyXTM Unicem 2 ............................................. 50
7
Crown cementation steps with Panavia F2.0 ....................................................... 51
8
Crown cementation steps with Duo-LinkTM ........................................................ 51
9
Characterization of Cement Failure Mode ........................................................... 53
10
ANOVA for evaluating fit for each group ........................................................... 57
11
Specimens and Maximum load values of Panavia F2.0 control groups ...............58
12
ANOVA table for MPa vs. 2 Independents ......................................................... 58
13
Regression Coefficients, MPa vs. 2 Independents ............................................... 59
14
ANOVA table, MPa vs. 3 Independents .............................................................. 59
15
Regression Coefficients, MPa vs. 3 Independents ................................................59
16
ANOVA table, MPa vs. 4 Independents ...............................................................59
17
Regression Coefficients, MPa vs 4 Independent ..................................................60
ix
18
Means and Std. Dev. of total load, SA and stress for all groups ..........................60
19
ANOVA table for Stress (MPa) of all groups .......................................................61
20
Mean and Std. Dev. of Stress (MPa) for all groups ..............................................62
21
Tukey’s/Kramer for Stress (MPa) and different Cements ....................................62
22
Tukey’s/Kramer for Stress (MPa) and different surface treatments .....................62
23
Recorded Failure mode for all groups ..................................................................64
24
Published previous cement retention studies ........................................................69
x
LIST OF FIGURES
Figure
Page
1
Crystallographic phases of zirconia dioxide .......................................................... 4
2
Micro-blasting aluminum oxide: 1. Aluminum oxide is blasted onto the surface to
clean it. 2. On the surface a micro-retentive roughness is achieved. 3. The
aluminum oxide leaves the cleaned activated surface. ........................................ 23
3
Sandblasting with silica-coated aluminum oxide: 1. Silica coated aluminum oxide
is blasted onto the surface. 2. On the surface a triboplasma is created in
microscopic ranges. 3. The aluminum oxide, which is after the ceramization only
partially coated, leaves the surface, which is itself now partially coated
with SiO 2 . ............................................................................................................ 25
4
The silane molecules (on the right) approach the inorganic surface, which is
covered with hydroxide groups and water molecules ............................................27
5
The silane molecules have made a chemical bond with the SiO 2 component of the
coated surface ....................................................................................................... 27
6
Chemistry of MDP monomer bonded to metal oxide .......................................... 34
7
Mounted Tooth ..................................................................................................... 42
8a
Occlusal cut .......................................................................................................... 43
8b
Axial cut ............................................................................................................... 43
9
Inverse Sin = a/c ............................................................................................................. 44
10a
Final impression ................................................................................................... 45
10b
Stone model ......................................................................................................... 45
11
Milled crown ........................................................................................................ 46
12
Retention pull hole (2mm in diameter) ................................................................ 46
xi
13
XLV material under 2.5 kg ...................................................................................47
14a
Replica Technique ................................................................................................47
14b
XLV duplicated crown cut to two halves ............................................................ 47
15a
30X Magnification ............................................................................................... 47
15b
XLV thickness measurements section A ............................................................. 47
16a
Impression lined with temporary cement ............................................................. 48
16b
Cemented prepared tooth ..................................................................................... 48
17a
Panavia F2.0 cement ............................................................................................ 49
17b
RelyXTM Unicem 2 .............................................................................................. 49
17c
Duo-LinkTM .......................................................................................................... 49
18a
Final cementation with pressure of 2.5 kg ........................................................... 52
18b
Cleaning cement with micro-brush ...................................................................... 52
19a
UAB Thermocycling machine .............................................................................. 52
19b
Compressive fatigue testing machine .................................................................. 52
20a
Mounted crown for pull test ................................................................................. 53
20b
Dislodged crown after pull test ........................................................................... 53
21
Comparisons of Total Occlusal Convergence for the sample groups (Key: RURelyX Unicem 2, DL- Duo-Link, PF- Panavia F2.0, A- Al2O3 abrasion surface.
S- Silica modified surface, and C- control surface.) ............................................ 54
22
Comparisons of axial wall height (h) for the sample groups (Key: RU-RelyX
Unicem 2, DL- Duo-Link, PF- Panavia F2.0, A- Al2O3 abrasion surface. S- Silica
modified surface, and C- control surface.) ........................................................... 54
23
Comparisons of preparation surface area for the sample groups (Key: RU-RelyX
Unicem 2, DL- Duo-Link, PF- Panavia F2.0, A- Al2O3 abrasion surface. S- Silica
modified surface, and C- control surface.) ........................................................... 55
24
Impression material thickness used in evaluation of the fit for each group ......... 56
xii
25
Maximum Load plot Panavia F2.0 control group ................................................ 57
26
Interaction Bar plot for effect of cements and surface treatment on stress (MPa) 62
27
Interaction Bar Plot for Stress (MPa) of all groups (Key: RU-RelyX Unicem 2,
DL- Duo-Link, PF- Panavia F2.0, A- Al2O3 abrasion surface. S- Silica modified
surface, and C- control surface.) ........................................................................... 62
28a
AC failure ............................................................................................................. 64
28b
AP failure ............................................................................................................. 64
28c
CC failure ............................................................................................................. 64
xiii
LIST OF ABBREVIATIONS
3-MPS
3-methacryloxypropyltrimethoxysilane
6-MHPA
6-methacryloxyhexylphosphonoacetate
10-MDP
10-methacryloyloxydecyl dihydrogen phosphate
AC
ADA
AP
Adhesive to crown
American Dental Association
Adhesive to preparation
Bis-GMA
bisphenol A glycidyl-methacrylate
C phase
Cubic phase
CAD
Computer-Aided Design
CAM
Computer-Aided Analysis
CaO
Calcium oxide
CC
Cohesive within cement
CeO
Cesium oxide
Co
Cobalt
Cu
Cupper
DL
Duo-LinkTM
DLA
Duo-LinkTM Al 2 O 3
DLC
Duo-LinkTM control
DLS
Duo-LinkTM Silica-Modified Al 2 O 3
xiv
FDA
Food and drug administration
Fe
Iron
FPD
Fixed partial denture
H
Axial wall height
HEMA
hydroxethyl methacrylate
HF
Hydrofluoric acid
HfO 2
Hafnium oxide
LTD
Low temperature degradation
M phase
Mono phase
MDP
Methacryloyloxydecyl dihydrogen phosphate
MgO
Magnesium oxide
Mg
Magnesium
MPa
Megapasscal
Nb 2 O 5
Niobium pentoxide
OH
Hydroxyl
PF
Panavia F.20
PFA
Panavia F2.0 Al 2 O 3
PFC
Panavia F2.0 control
PFS
Panavia F2.0 Silica-Modified Al 2 O 3
PMMA
Polymethyl methacrylate
PVC
polyvinyl chloride
rpm
Revolutions per Minute
RU
RelyXTM Unicem 2
xv
RUA
RelyXTM Unicem 2 Al 2 O 3
RUC
RelyXTM Unicem 2 control
RUS
RelyXTM Unicem 2 Silica-Modified Al 2 O 3
SA
Surface area
SiO 2
Silica oxide
STD. Coeff
Standard Coefficient
Std. dev.
Standard deviation
Std. Err
Standard Error
Srf. trt
Surface treatment
Ta 2 O 5
Tantalum pentoxide
T phase
Tetragonal phase
TDS
Dental Laboratory, Taiwan
TEGDMA
Triethylene glycol dimethacrylate
TOC
Total occlusal convergence
VLC
Visible light-cured
VPS
Vinyl polysiloxane
XLV
Extra light viscosity
Y2O3
Yittrium oxide
Y-TZP
Yttria-stabilized tetragonal zirconia polycrystal
Zr
Zirconia
ZrO2
Zirconium dioxide
ZrSiO 4
Zirconium silicate
xvi
INTRODUCTION
The increased popularity of all-ceramic materials as an alternative to metalceramic restorations is attributed to their excellent aesthetics, chemical stability, and
biocompatibility (1). However, the brittleness and low tensile strength of conventional
glass-ceramics has limited their clinical applications (1). Several developed glassceramics have been introduced such as high alumina-content glass-infiltrated ceramic
core material (In-Ceram® Alumina) and disilicate glass-ceramic (Empress 2®), which
have been used successfully for crowns (2), anterior fixed partial dentures (FPDs) and
three-unit FPDs replacing the first premolar (3. 4). However, these materials do not have
sufficient strength to withstand high loads in posterior sites especially in the molar region
(3, 4). Recently, the development of advanced dental ceramics has led to the application
of partially stabilized zirconia in Restorative Dentistry, which can be produced from a
computer-assisted design/computer-aided manufacture (CAD/CAM) system. The use of
zirconia-based ceramics for dental restorations has risen in popularity due to their
superior fracture strength (5-8) and toughness compared with other dental ceramic
systems (9).
Zirconia is a polymorphic material that exists in three allotropes: monoclinic,
tetragonal and cubic. It can be transformed from one crystalline phase to another during
firing. Pure zirconia is monoclinic (m) at room temperature and this phase is stable up to
1170°C (10). Above this temperature, it transforms into the tetragonal phase (t) and then
into the cubic phase (c) at 2370°C, which exists up to its melting point of 2680°C (10).
1
However, this transformation produces crumbling of the material on cooling (11).
In 1951, researchers found that zirconia alloying with lower valance oxides such as CaO,
MgO, Y 2 O 3 , or CeO can retain tetragonal or cubic phases in the room temperature
depending on the amount of dopant (12-14). However, the tetragonal form is in fact
‘metastable’ at the room temperature. External stresses such as sandblasting (15),
grinding (16-18), and thermal aging (19) can trigger the transformation of partially
stabilized tetragonal zirconia polycrystalline ceramics from a tetragonal into a monoclinic
state (16). This transformation is associated with 3-5% volume expansion (20), which
induces compressive stress; thereby closing the crack tip and preventing further crack
propagation (20). This unique characteristic confers zirconia with superior mechanical
properties compared to other dental ceramics.
Zirconia crowns are frequently selected by clinicians for single tooth restorations
due to enhanced esthetics (21), high mechanical properties (22, 23) and biocompatibility
(24) of zirconium oxide ceramics.
Zirconia: Definition and History
Zirconium is represented by the chemical symbol Zr and has the atomic number
40. It is one of the transition metals (elements whose atom has an incomplete d sub-shell)
of the D.I. Mendeleev’s periodic chart. Zirconium exits in two forms: the crystalline
form, a soft, grayish-white and lustrous metal; and the amorphous form, a bluish-black
powder. From ancient times, zirconium has been known as zircon, which probably
originated from the Persian word zargun (golden in color) (25, 26). Zircon or zirconium
silicate, ZrSiO 4 (67.2% of ZrO 2 and 32.8% of SiO 2 ), is the most important zirconium
mineral (25). A German chemist, Martin Henrich Klaproth discovered the mineral and
2
analyzed a zircon from Ceylon (Sri Langa) in 1789 (25-27). Jöns Jakob Berzelius, a
Swedish chemist, first separated the metallic zirconium in 1824 by heating a mixture of
potassium and potassium zirconium fluoride in a small iron tube. However, it was
impossible to obtain pure zirconium at that time until the beginning of the 19th century
(28). The pure zirconium oxide was first prepared in 1914 by Herzfield (28). He invented
the process of crystallizing zirconium oxychloride octahydrate from a concentrated
solution of hydrochloric acid to remove large amounts of silica and the oxychloride
octahydrate then crystallized out upon cooling (28). In 1925, Akel and de Boer produced
pure zirconium by an iodide decomposition process (29). However, Hafnium is always
found in zirconium cores because separation is very difficult (30), so commercial grade
zirconium contains from 1 to 3% Hafnium (29).
At the beginning of 19th century, a solid solution of Yttria-stabilized tetragonal
zirconia polycrystal ceramic (Y-TZP) was widely used in the refractory as Nernst lighting
element rods and later used as a solid electrolyte (14). It was considered for biomedical
implants from as early 1969 (31), and from 1985, the majority of zirconia balls were
made for total hip arthroplasty. Zirconia ceramic was extended into dentistry in the early
1990s as endodontic posts (32) and more recently as implant abutments (33, 34) and hard
framework cores for crowns and fixed partial dentures (35-37). Zirconia has unique a
characteristic called transformation toughening, which it can give higher strength and
toughness compared with other ceramics.
3
Transformation toughening
The transformation is believed to be martensitic. The martensitic transformation
occurs by the systematic coordinated shearing of the lattice of the old phase in such a way
that the distance moved by any atom is less than one atomic space. This means that the
atom retains the same neighbors. Martensitic transformation can only lead to changes in
crystal structure, and not in composition of the phases (38).
Monolithic
Teteragonal
Cubic
Figure 1. Crystallographic phases of zirconia dioxide (Anderson et al. 1990)
Zirconium dioxide can exist in one of three crystallographic phases (Figure 1);
monoclinic phase (a deformed prism with parallelepiped sides); tetragonal phase
(a straight prism with rectangular sides) and cubic phase (square sides). The monoclinic
phase appears from room temperature up to 1170°C. Above this temperature, it
transforms into the tetragonal phase and then at 2370°C into the cubic phase, which exists
up to its melting point of 2680°C (10). Volume changes on cooling associated with
transformation are substantial enough to make the pure material unsuitable for
applications requiring an intact solid structure: cubic to tetragonal approximately 2.31%
4
(39); tetragonal to monoclinic approximately 3-5% (20). Researchers have studied the
phase relationships between zirconia and metal oxides (12-14). They discovered that
zirconia alloying with lower valance oxides such as CaO, MgO, Y 2 O 3 , or CeO can retain
tetragonal or cubic phases at room temperature depending on the amount of dopant (1214). In the case of 8 mol% Y 2 O 3 dopant, the cubic is stabilized at room temperature while
tetragonal phase is stabilized when using 2-5 mol% Y 2 O 3 (10). The stabilized tetragonal
phase has satisfactory properties when compared to other phases and it also provides an
advantage because of its martensitic transformation to a monoclinic phase (40).
The transformation from tetragonal into monoclinic form can occur by absorption
of mechanical or thermal stress such as grinding (5, 16, 18), sandblasting (18), and high
temperature (thermal ageing) (41, 42), and is associated with a volume increase (20). This
transformation leads to the development of localized compressive stresses being
generated around and at the crack tip preventing further crack propagation (20). This
mechanism is known as transformation toughening and makes zirconia-based ceramics
exhibit high strength (16, 40) and toughness compared to other ceramics (9). However,
severe transformation to the monoclinic phase can induce a deterioration of materials
such as a reduction in the strength and the fracture of the materials. Gupta et al (1977)
reported that a sintered specimens containing high amounts of the monoclinic phase
(~90%) exhibited low strength in a range of 50-100 MPa with the evidence of
micro-cracks in the materials. In contrast, high amounts of the tetragonal phase (~90%)
showed high strength, which was approximately 700 MPa (40). Therefore, transformation
toughening can provide either advantage or disadvantage depending on the degree of
transformation.
5
Characteristics of zirconia-based ceramic
Zirconia ceramics have superior properties compared to other ceramics and (9)
show their biocompatibility (43, 44). However, the properties of zirconia ceramic may be
reduced when it contacts thermal and humid environments (41, 45).
High strength and toughness
Ceramics are weak in tension, so, this aspect should be tested. However, the direct
tensile test is difficult to perform. This is due to the difficulty in preparing specimens to
have the required geometry and it is also difficult to hold the brittle specimens without
pre-stressing and fracturing them (46). The flexure test is an alternative test to investigate
the stress at fracture of brittle materials, which is known as flexural strength (46).
Fracture toughness identifies the resistance of the brittle materials to the catastrophic
propagation of flaws under an applied stress.
Yttria partially stabilized tetragonal zirconia polycrystal ceramics exhibit flexural
strength ranging from 800-1300 MPa (Table 1) (5-8) with a toughness of approximately
5-10 MPa.m1/2 (47) depending on processing methods, composition and microstructures
(48-52). Guazzato et al (2004) found DC-Zirkon (5% Y-TZP) had a flexural strength of
1150 (±150) MPa. (53) Similarly, Curtis et al (2005) reported that as-received Lava™
specimens tested in a wet environment had a flexural strength of 1308 (±188) MPa,
which was not significantly different from the specimens tested in a dry condition
(1267±161 MPa). (5) However, Denry and Holloway (2005) found the biaxial flexural
strength of the as sintered Cercon® was 944 (±156) MPa. (6)
Other dental ceramics have lower flexural strength and toughness compared with
Yttria stabilized tetragonal zirconia ceramics. Empress 2 has flexural strength in a range
6
of 250-350 MPa (54, 55) and fracture toughness approximately 2.8 MPa.m1/2 (56). InCeram Alumina ceramic has been reported to have flexural strength ranging from 300600 MPa (9, 58, 59, 60, 61) and a fracture toughness approximately 3.1-4.8 MPa.m1/2
(62, 56, 59, 60). In-Ceram Zirconia has been reported to have flexural strength and
fracture toughness in a range of 475-630 MPa (63, 9, 58) and 4.8-4.9 MPa.m1/2 (63, 9)
respectively. High alumina content ceramics or Procera has a mean flexural strength in a
range of 469-699 MPa (64, 59, 60, 61) and fracture toughness 3.84-4.48 MPa.m1/2 (59,
60).
Table 1: Comparison of flexural strength and fracture toughness of ceramics from
different systems
Systems
Core materials
Empress II
In-Ceram Alumina
In-Ceram Zirconia
Procera AllCeram
Cercon,
Invizion
Flexural
strength (MPa)
Lithium disilicate
250-350
Glass-infiltrated alumina
300-600
Glass-infiltrated alumina with 475-630
35%
partially
stabilized
zirconia
Densely sintered high-purity 469-699
alumina
Lava, Yttria partially stabilized 800-1300
tetragonal zirconia
Fracture toughness
2.8
3.1-4.8
4.8-4.9
3.84-4.48
5-10
Fatigue failure
Fatigue is the mode of failure, in which a structure eventually fails after being
repeatedly subjected to loads that are so small that one application does not cause failure.
All-ceramic materials are susceptible to fatigue mechanisms that can considerably reduce
their strength over time. The reduction of mechanical strength due to fatigue is caused by
the propagation of natural cracks initially present in the component’s microstructure. (65)
7
The influence of moisture contamination has also previously been identified to affect the
fracture strength of ceramic-based dental ceramics, resulting in a 20% decrease in the
mean fracture strength (65). However, Curtis et al. (2006) highlighted that the biaxial
flexural strength of Y-TZP ceramic was not detrimentally influenced by water immersion
during simulated masticatory forces of 500, 700 and 800 N at 2000 cycles. (66) However,
the duration of the fatigue test that they performed was rather short to allow water to
affect the properties of zirconia. Similarly, it was mentioned that a loading of 100,000
cycles (force of 50 N) did not affect the strength of YTZP framework (67). The
maximum stress, which was recommended to apply during cycling tests due to higher
mechanical strength of zirconia was 500 MPa and for In-Ceram and Empress II, 300 MPa
and 160 MPa respectively (68).
Kim et al (69) studied the fracture patterns during fatigue on flat porcelain
veneers on gold-infiltrated alloy (P/Au), porcelain on palladium silver alloys (P/Pd) and
porcelain on Y-TZP (P/Zr) bilayers. They used a spherical tungsten carbide indenter with
mouth-motion simulator using 200 N forces and a chewing frequency of 1.5 Hz. At
10,000-15,000 cycles for the P/Au system, cracks reached at the porcelain/metal interface
and a radial crack (cracks that emanate outward from the contact axis) initiated at the
lower surface of the porcelain veneer at 20,000-25,000 cycles. For the P/Pd system, inner
cone cracks (cracks that initiated inside the contact circle and form only in cyclic loading
in water) reached the interface at 25,000-50,000 cycles and no radial cracks were found
even after 400,000 cycles and increased force of 600 N. Inner cracks in P/Zr system
reached the veneer/core interface after 550,000 cycles. These cracks did not penetrated
8
either the Y-TZP core or along the porcelain/Y-TZP interface. There was also no
evidence of the radial crack at the lower surface even when a 600 N force was used.
Fracture resistance after fatigue
All ceramic crown and bridge restorations are subjected to masticatory loading on
a daily bases, which places the restoration under repeated loading throughout its servicelife. Repetitive stresses during the chewing cycle may lead to fatigue of the material and
eventually fractures when they are exposed to the oral environment (70).
Previous studies showed that ceramic containing glass such as ProCAD, In-Ceram, and
IPS Empress had a decrease in fracture strength after cyclic loading (70, 71, 72). Chen et
al (1999) reported a significant decrease in fracture strength and considerable increase in
failure probability of ceramic crowns (Vita MarkII, ProCAD and IPS Empress I, II) when
subjected to the cyclic loading at 50,000 cycles using a force of 200 N (70). Similarly, the
fracture strength of In-Ceram and IPS Empress was significantly reduced after 10,000
cyclic using 300N force (73). One study reported that fatigue (20,000 cycles) slightly
reduced the flexural strength of glass-infiltrated zirconia (In-Ceram Zirconia) and high
purity alumina (Procera AllCeram) but it was not statistically significant (64).
On the other hand, the reduction of the fracture strength of high purity zirconia
ceramic after cyclic loading has not been reported. A recent study showed that when
using high forces of 500, 700 and 800 N at low numbers of cycles (2,000 cycles) and also
low force of 80 N at 10,000 and 100,000 cycles, there was no significant difference in the
mean biaxial flexural strength between unloaded and zirconia discs that had undergone
cyclic loading in both dry and wet conditions (66). However, weibull modulus was lower
in zirconia discs when loaded for 100,000 cycles due to accumulation of micro-crack
9
damage (66). Sundh et al (2005) found that cyclic loading (100,000 cycles) of zirconia
three-unit bridges using 50 N force in water did not significantly affect fracture resistance
whereas heat-treatment or veneering zirconia bridges resulted in significantly lower
fracture resistance (67).
Subcritical crack growth resistance
Brittle materials are susceptible to time-dependent failure under static loads,
caused by the subcritical growth of cracks to critical lengths (74). The subcritical crack
growth refers to environmentally enhanced crack propagation at subcritical stress levels.
The propagation of the pre-existing natural defects occurs at low rates (slow crack
growth), and causes delayed failure of ceramics when the flaw size reaches a critical
value (75). The subcritical parameter A and n are usually examined for estimating crack
growth resistance and lifetime of materials. Where A is a constant in meter per second
and n is an exponent, which in most ceramics has a value >10 (74). The n value can vary
from 20 to100 (68, 76, 77) dependent on the test methods. Previous studies reported that
two common methods, double torsion and flexural tests, show a difference in subcritical
n values of zirconia ceramics (76, 78). Li and Pabst (1980) reported that the n value of
received ZrO 2 specimens tested using the double torsion test (n=80) was higher than the
flexural test (n=51). (78) In contrast, Chevalier et al found the n value produced from the
double torsion method (n=19) was lower than the n value obtained from dynamic fatigue
test (n=100). (76)
It is difficult to give a definite conclusion whether zirconia has a higher
subcritical parameter than other ceramics since the results from studies differ. One study
calculated the n and A values of 3Y-TZP, In-Ceram Zirconia and Empress 2 ceramic and
10
found that 3Y-TZP, In-Ceram Zirconia and Empress 2 ceramics had a values of 3.15x1024, 2.61x10-25 and 1.69x10-21 m.s-1 and n values of 28.9, 26.3 and 38.0, respectively.
(68) Conversely, Morena et al (1986) found fine-grain ceramic (Cerestore) had a high nvalue of 80.8 compared with feldspathic (14.6) and aluminous porcelain (28.9). (79) One
study found that the subcritical crack growth parameter n tested in air and water was not
significantly different. (65) Studart et al (2007) studied the fatigue properties of three unit
zirconia bridges using prismatic beams to simulate the bridges. They found no significant
difference in an exponent n between the specimens tested in the air (29) and water
(28.95). (65) They indicated that the operative mechanism of cyclic degradation and
water-assisted crack growth are the same for both environmental conditions (65).
However, the faster crack growth observed in the presence of water since the n value of
water value is lower than n value of air. Studart et al (2007a) also reported that zirconia
has a failure probability of only 5% after 20 years if the maximum stress does not exceed
346 MPa. (65) Therefore, they suggested that dental prostheses in patients with Parafunctional behavior such as clenching and grinding, which can reach forces as high as
800 N, should have increased connector dimensions for extending life time. (65)
Thermal and environmental ageing
The major issue concerning zirconia ceramics is their sensitivity to low
temperature degradation (LTD). (80) Aging occurs by a slow surface transformation from
metastable tetragonal phase to a more stable monoclinic phase in a humid environment
such as humid air, water vapor and aqueous fluids at a relatively low-temperature ranging
from 65-500ºC (45), (81). The tetragonal to monoclinic transformation can be of benefit
due to the compressive layer on the surface of the ceramic, which improves its properties.
However, further aging can result in the reduction of material properties. (82) The
11
transformation of one grain is accompanied by a volume increase causing stresses on the
neighboring grains and micro-crack. This offers a path for the water to penetrate and
exacerbate the process of surface degradation and the transformation process. (39, 41)
The growth of the transformation zone results in severe micro-crack and grain pull out
and finally surface roughening which leads to strength degradation. (39) There are few
factors influence aging rate of zirconia such as temperature, grain size and stabilizing
agent. If the temperature rises up to 200-300ºC, the transformation proceeds most rapidly.
(41, 83) Chen and Lu (1988) found the highest amount of monoclinic presented at the
aging temperature of 250ºC, which led to the lowest flexural strength 340 MPa. (84)
Chevalier studied aging of zirconia in distilled water at different temperatures from 70100ºC. He found the amount of monoclinic phase increased with aging time and
temperature. However, some studies reported zirconia showed no loss of strength both in
vitro and in vivo. (85, 86) Cales et al examined aging of zirconia in Ringer’s solution at
37ºC at pH7 for 1 year and also implanted zirconia ceramic in animals (rats, rabbits and
sheep) for 2 years. They found zirconia ceramics were stable after in vitro and in vivo
long-term aging. There was also no mechanical degradation caused by the different
conditions of in vitro aging and implantation in animals. (85) Similarly, Shimizu et al
found no significant decrease of flexural strength when specimens were placed in saline
solution at 50 and 95ºC for 3 years and also in distilled water at 121ºC for 2000 hr. (86)
Papanagiotou et al found that the transformation tetragonal to monoclinic was higher on
the In-Ceram YZ ceramics boiled for 7 days than as-sintered and boiled for 24 hours. (87)
The transformation presented the highest rate on the specimens after storage in
12
humidified air at 250ºC for 7 days. However, the low-temperature aging treatments in
their study did not affect the flexural strength. (87)
In addition, the critical tetragonal grain size, which is Y 2 O 3 dependent, affects the
aging degradation behavior. (45) Lange (1982) reported that the critical grain size
increased from 0.2 to 1 μm for compositions ranging between 2 and 3mol%, respectively.
(88) However, Watanabe et al (82) identified that critical grain size increased from 0.2 to
0.6 um as the Y 2 O 3 content increased from 2 to 5 mol%. The monoclinic phase increased
with aging when the grain size was larger than the critical value. Conversely, the
transformation to monoclinic phase was either induced or resisted when the grain size
was lower than critical value. The level of stabilization also affected the aging resistance.
A higher stabilization reduced the phase transformation during aging. Therefore, this
review concluded that there was transformation toughening of zirconia due to thermal
and environmental ageing, which lead to the degradation process of zirconia. However,
this process was found to have either reduced or no affect on its flexural strength both in
vitro and in vivo within the limited time of the studies.
Longevity
A restoration’s longevity is an important factor when discussing treatment options
with a patient. Survival and success rates are commonly used to assess longevity. (89)
Survival indicates that a restoration is functioning without losing all the initial desired
parameters. (89) Success implies that the restoration remains unchanged without any
treatment over the observation period. (89) The successful outcome is presence of all
three aspects of the prostheses: health, function and aesthetics. (89)
13
In order to be successful clinically, all-ceramic reconstructions need to achieve
longevity and ideally a high survival rate similar to metal ceramic restorations, which are
currently used for high strength aesthetic posterior restorations. From meta-analysis, the
5-year survival rate of metal ceramic crowns was 95.6% (95% confident interval) and
93.3% (95% confident interval) for all-ceramic crown. (90) The most common
complications found in all-ceramic and metal ceramic were loss of pulp vitality and
caries. (90) Single unit zirconia based crowns have been evaluated; however, these
studies do not provide details of the success or survival rates of single unit zirconia
crowns. Most of the clinical publications relate to zirconia fixed partial dentures. (39, 91)
Zirconia ceramic fixed partial dentures have been reported to have survival rates ranging
from 73.9-100% at 2-5 years follow up periods. (92, 93, 94, 95) However, 5 years is
considered relatively short. A systematic review reported that metal-ceramic fixed partial
dentures had a mean survival rate of 94.4% estimated after 5 years (ranging from 9197.9%). (90) Survival rate of IPS Empress 2 fixed partial dentures was 70% at 5 years,
which was slightly lower than other restorations. (2) In-Ceram fixed partial dentures had
a survival rate of 90% after 5 years. (96) Zirconia restorations offer sufficient stability
and good clinical performance in terms of fracture resistance, marginal integrity,
marginal discoloration, and secondary caries. (92, 93, 94, 95)
Two studies found that all zirconia ceramic fixed partial dentures were still in use
with no caries detected after 2-3 years with a 100% survival rate. (92, 94) Only minor
complications were found such as chipped veneer, marginal crevice (can catch by an
explorer) and/or endodontic problem, but no restorations were needed replacement at that
time. (92, 94) However, some studies reported major complications including framework
14
fracture, root fracture, endodontic complications, extensive fracture of veneering ceramic
(93), and secondary caries due to marginal discrepancies and inappropriate cementation
(93, 95). One case report of a 5-unit bridge described how it fractured through the
connector area because of biting on piece of stone. They assumed that the fracture was
attributed to the trauma or fatigue of the ceramic rather than the connector dimensions,
which were already adequate for the span of the restoration (18.49-19.28 mm2). (93) The
most frequent technical problem found in all studies of zirconia restorations was minor
chipping or fracture of the veneering ceramic. (92-95) The high incidence of chipping of
veneer ceramic may be due to the use of new low fusing ceramics that have a thermal
expansion coefficient compatible with zirconia (>11x10-6K-1). These possess poor
mechanical properties, in particular low fracture toughness and flexural strength. (92, 93)
Also, the produced framework from the CAD/CAM may result in an insufficient occlusal
shape to support the veneer ceramic. An unsuitable thickness ratio between veneer and
the framework is also possible. (96)
One study indicated that a weak bond exists between veneer and zirconia
ceramics when compared to metal ceramics, which may cause the veneer to chip. (97) In
addition, a systematic review reported that the chipped veneer of zirconia restorations
occurred more frequent than glass ceramic and metal ceramic restoration whereas the
fractured framework of glass ceramic restoration occurred more frequently than zirconia
restoration. (90) However, Raigroski et al (2006) reported that none of their restorations
demonstrated an adhesive failure between veneering porcelain and the zirconia
framework after a 3 years observation. (92) Also, in vitro study found no difference in the
bond strength of veneer and framework between all-ceramic and metal restorations. (98)
15
The difference in bond strength outcomes of veneer and zirconia framework in various
studies may be due to types of zirconia frameworks, types of veneering materials and
surface finishing on zirconia framework before veneering. (99) However, it is difficult to
compare the success rate or survival rate between different studies that set different
criteria for failure of a restoration. The failure set can be varied from decementation of a
restoration to fracture. If minor fracture of a restoration were considered a failure, many
restorations would be deemed unsuccessful. (92, 94) Vult von Steyern et al (94)
suggested that if patients were satisfied and unaware of a minor fracture to a veneer
restoration, the chipped veneer was not a failure. One study focused on the condition of
zirconia frameworks rather than the restorations. (93) They reported that zirconia
framework had a success rate of 97.8%; however, there were several complications such
as secondary caries and major fractures of veneer ceramic that they did not include in the
failure criteria. Additionally, several studies presented the survival rates in different ways
and rendered comparison difficult. Use of percentages at certain time intervals is less
informative than median survival as many studies include restorations of differing ages.
There are only a few studies evaluating the clinical results of zirconia framework based
ceramics for fixed prostheses and may have short observation times.
When examining the medical literature, a high fracture rate of zirconia hip
prostheses due to aging sensitivity of zirconia has been reported (100, 101) and early
zirconia products (Prozyr) were recalled and suspended by the United States food and
drug administration (FDA) in 2001. (101) Due to the short time, which zirconia has been
used as a substructure in dental restorations, it is difficult to make a definite conclusion
regarding longevity. Short-term complications have been reported and discussed in this
16
section. Long-term evaluation will be important to assess the aging susceptibility of
zirconia.
Factors influenced mechanical properties of zirconia ceramics
Mechanical strength is an important factor that controls the clinical success of
dental restorations. (102) Strength values in ceramics are affected by several factors such
as chemical composition, manufacturing and sintering process, test methodology, clinical
procedures.
Chemical composition
The strength of zirconia can be varied depending on its chemical composition
such as coloring agents and stabilizing agent. Because zirconia is nit white in color, the
zirconia frameworks can be shaded before sintering in order to achieve an aesthetic and
natural appearance for the veneering layers of FPD. Coloring oxide such as Fe, Cu, Co,
and Mg can be added to dental ceramics (50); however, whether these oxides affect the
strength of zirconia ceramics depends on the manufacturing process (48), amount of
coloring oxide (103) and duration of color shading (104). Ardlin (48) found colored
zirconia (P17) had higher strength than white shaded zirconia (P0). He suggested that the
difference in strength might be related to components such as CeO 2 , Fe 2 O 3 and Bi 2 O 3
that were added to obtain different shades. However, he found higher porosities in white
shaded specimens, which indicated that colored and uncolored specimens had different
manufacturing processes. However, two studies reported that there was no significant
difference in flexural strength between colored and uncolored zirconia specimens (105,
106). Shah et al reported that an increase in amounts of cerium acetate, cerium chloride,
17
and bismuth chloride to 5 wt% and 10 wt% reduced the biaxial flexural strength of
zirconia specimens. (103) In contrast, a small amount (1 wt%) of these coloring agents
did not affect the biaxial flexural strength. They also found that coloring agents did not
induce phase transformation; however, there was a slight increase in lattice parameters
compared to the control group (from a= 3.60, b= 5.17 to a= 3.61, b= 5.18 Å), which
agrees with one study. Longer shading time also reduced the flexural strength of zirconia.
(104)
Another factor that has been reported to influence properties is an alloy added in
zirconia. One study examined various alloying oxides mixed with 2-3% Y-TZP. They
found Tantalum pentoxide (Ta 2 O 5 ), niobium pentoxide (Nb 2 O 5 ) and hafnium oxide
(HfO 2 ) enhanced transformability of the Y-TZP. (107) HfO 2 had the lowest effect on
transformability compared to Ta 2 O 5 and Nb 2 O 5 and conversely the addition of Y 2 O 3
decreased transformation. In addition, the amount of stabilizer affected the phase content
in zirconia. High tetragonal phase-content could not be achieved for the composition
containing 1.5 mol% Y 2 O 3 (88). Gross and Swain also reported that Y 2 O 3 content below
2.3 mol% produced micro-cracks in all samples. Additional components have been added
to Y-TZP in order to improve the strength. Some studies tried to improve the strength of
zirconia by adding some other components to zirconia powder. One study (Nakayama
and Sakamoto 1998) reported that there was an increase in flexural strength (from 950
MPa to >1000 MPa) of 2.5 mol% Y-TZP by adding B 2 O 3 –Al 2 O 3 –SiO 2 .
Manufacturing and sintering process
Manufacturing and sintering processes have a major effect on the properties of
zirconia ceramic. One of the important steps for producing materials is powder pressing.
18
There are three basic powder compaction procedures: uni-axial, isostatic (or hydrostatic),
and hot pressing. For uni-axial pressing, the powder is compacted in a metal die by
pressure in a single direction (46). The formed piece takes on the configuration of die and
platens (metal cylinder) through which the pressure is applied. This method and the
process are confined to shapes, which are relatively simple and non-complicated (46). In
addition, production rates are high and the process is inexpensive. (46) For isostatic
pressing, the powdered material is contained in a rubber envelope and the pressure is
applied by fluid, isostatically with the same magnitude in all direction (46). Isostatic
pressing yields denser compacts compared with uni-axial pressing, and results in better
quality specimens (108). More complicated shapes are possible compare to uni-axial
pressing; however, the isostatic technique is more time consuming and expensive (46).
For both uni-axial and isostatic procedures, a firing cycle is required after the pressing.
With hot pressing, the powder pressing and heat treatment are performed simultaneously.
(46) Powder is compacted at the elevated temperature. The procedure is used for
materials that do not form the liquid phase except at very high and impractical
temperatures (46). This process takes much longer, since in both methods the mold and
die must be heated and cooled down during each cycle. Additionally, the mold is
expensive to fabricate and has a short lifetime (46). A powder mass contain a small
amount of water or binder, which is compacted into the desired shape by pressure (46).
One function of the binder is to lubricate the powder particles as they move over one
another in compaction process (46). The temperature and duration of the sintering
process have been reported to affect grain size and phase content, which they influence
the strength of zirconia (49, 52). Ruiz and Ready proposed that the grain size increased
19
with increasing sintering temperature, which led to an increase in fracture toughness,
owing to larger transformation zones. (52) However, no significant difference in biaxial
flexural strength of various grain size zirconia ceramics was reported in this study. This
contrasts with the results of Casellas et al (2001) who found that a decrease in grain size
gave a slight increase in flexural strength. This is attributed to greater phase
transformation around the crack in coarser microstructures than smaller grain materials.
(49) The factors caused by the manufacturing and sintering processes affecting the
property of zirconia are flaws or defects and crack initiation, which can lead to early
restoration failures (109). The strength of ceramic specimens and prostheses depend on
the size of microscopic cracks and pores. It has been reported that the presence of
numerous surface flaws, including submicroscopic Griffith can lead to failure. This flaw
can act as stress concentrators when the object is under load, with microscopic stress
occurring at the flaw tip and causing fracture as soon as a critical breaking stress is
reached. (110) Crack initiation may be influenced by quality of the ceramic surface and
internal structure of ceramic materials, which they can provide resistance to crack
growth. (109) The strength of material is dependent on the size of the pre-existing
initiating cracks present in a particular sample or component. (111) In addition, a large
number of cracks together with a low fracture toughness of materials will limit the
strength of ceramics and cause a large variability in strength. (111)
Experimental design
Experimental design can affect properties of zirconia such as specimen
preparation, equipment and geometry, or environmental condition. Specimen preparation
will be required for most of tests, which it may affect the results; for example pre-loading
20
on the surface of a specimen or containing a notch, groove, and hole. Some test methods
such as the crack growth measurements require an indentation notch before testing,
which it may affect the results when using higher indentation loads. Zhang and Lawn
found Y-TZP ceramic had high sensitivity to sharp-indenter damage in cyclic flexural
stressing. (112) An indented specimen under a force of 10 N caused significant
degradation in strength after 10 cycles compared to those with natural flaws, 0.1 and 1 N
force. The strength was also reduced after 100 cycles when using 1 and 0.1 N forces
compared with natural flaws. In addition, strength variation can be caused by different
equipment’s design. Albakry et al studied the biaxial flexural strength of ceramic using a
piston on three balls. (113) They suggested that the small piston tip (0.75 mm) might
improve the strength values because only a smaller area of the specimen was subjected to
the maximum tensile stresses. Consequently, there was less chance of the specimen
having a critical flaw in that area. (113) In addition, a difference in geometry for the
strength tests such as uni-axial (3 or 4 point bending test) and biaxial test leads to a
variation in strength values. Ban and Anusavice studied failure stress of four brittle
materials (zinc phosphate cement, opaque porcelain, body porcelain and VLC resin
composite) using biaxial flexural test (piston on three balls) compared with a four point
flexural test. (114) They found that the mean strength obtained from biaxial flexural test
was higher than a four-point flexural test. Shetty et al reported that there was no
significant difference between the strength value of glass ceramic tested by three and four
point bending; however, biaxial flexural test (ballon-ring) provided a higher strength
value in comparison. (115) One of the reasons that different geometries provide different
results is the specimen preparation. The specimens for uni-axial flexural test are prepared
21
in bar shape whilst those for biaxial tests are prepared in disc shaped. Undesirable edge
fracture could occur when preparing bar specimens and this may ultimately affect
strength.
Surface treatment
Roughening abrasion
Airborne particle abrasion, i.e. sandblasting, of dental materials is often used to
clean the substrate surfaces and to achieve both a micro-retentive topography and
increased surface area for bonding. It has been suggested that when using particles of
Al 2 O 3 for airborne particle abrasion, complex reactions on the substrate surface take
place, which consist of the separation and accumulation of certain elements at the
substrate surface (116, 117, 118, 119). The result is an activated and chemically reactive
surface, which can be demonstrated by the increased wettability of the material (116).
Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from
inter-molecular interactions when the substrates are brought together. A force balance
between adhesive and cohesive forces determines the degree of wetting, i.e. wettability.
According to the manufacturers’ instructions, many chemical bonding systems either
recommend or require airborne particle abrasion of the prosthetic structure in order to
achieve a high bond strength (118, 120, 121). Airborne particle abrasion with aluminum
oxide particles is the preferred surface treatment method for high strength ceramic
material, such as aluminum and zirconia ceramics, which created high surface energy and
promotes micro-retention. (21, 122, 123, 124, 125, 126)
Roughening the surface promotes the adhesion, since it allows the polymer to
flow into the surface and forms irregularities on the substrate surface. (127) Sandblasting
22
with pure aluminum oxide activates the surface and creates a uniform pattern of surface
roughness, which it is ideal for the ensuring micro-retentive anchorage of the resin.
(Figure 2)
Figure 2. Micro-blasting aluminum oxide: 1. Aluminum oxide is blasted onto the surface
to clean it. 2. On the surface a micro-retentive roughness is achieved. 3. The aluminum
oxide leaves the cleaned activated surface. (127)
Silicatization and Silane Coupling Agent
There are several alternative ways to silicatize (silica-coat) various prosthodontics
materials surfaces, e.g. restorations, crowns and bridges, in order to clean the surfaces,
create a highly retentive surface and foremost, enhance their silanizability and thus
promote the proper bonding to the substrate surface. Few techniques can be used at the
dentist’s office and some are restricted for laboratory use only. A thermal silica-coating
system (Silicoater MD system, Heraeus Kulzer, Wehrheim, Germany) from early 1980’s
required sandblasting of the metal prior to the silica-coating process. Next, the surface
was coated with a liquid of silane, which it formed at increased temperature silica coating
for the substrate in Silicoater MD apparatus. This system had the advantage of avoiding
flame adjustment problems. (128) Tribochemical silicatization utilizes the same principle
with a specifically surface-modified alumina with SiO 2 coating on the surface of the
23
particles. The silica particle diverges and attaches with the surface and intrudes into the
surface of ceramic, thus providing the surface with a active silica-rich outer surface prone
to silanization and the following resin adhesion for cementing with suitable resin
composites. This method can be considered as a ceramic coating forming a ceramic
interface. Tribochemistry involves creating chemical bonds by applying kinetic energy,
e.g. in the form of sandblasting, without any application of additional heat or light. (129)
The first tribochemical silica-coating system for dental use (Rocatec™ system,
3M ESPE, Seefeld, Germany), introduced in 1989, applied two sandblasting steps to the
metal surface prior to the application of silane and resin. (130) Tribochemical silicacoating using CoJet™ at the dental office is a widely used conditioning method in
ceramic and metal alloy structure repairing and cementing. The system has been
criticized for possibility of subcritical crack propagation within zirconia in when the
restoration are thin. (131, 132, 133)
Airborne particle abrasion of prosthodontics materials has the potential to remove
significant amount of material, which could affect their clinical adaptation. Therefore, the
material loss is an important factor that is affecting the clinical fit of restorations. (125)
Both morphological and compositional changes in the substrate surface occur through
airborne particle abrasion and tribochemical coating procedures. (116, 134) However, the
knowledge of the qualitative and quantitative changes in the surface through these
procedures is limited. The reported initial bond strengths for the silica-coating systems
are high and appear durable on certain metals surviving severe fastened fatigue. (135,
136) In addition, in studies reporting failure mode for silica-coating systems, failures
were partly or mostly adhesive, i.e. the failure was observed in the interface between
24
resin composite and zirconia surface. (135, 136) Therefore, the long-term studies to
investigate the effects of various surface treatments are needed to extend our
understanding of the bonding mechanisms and failure modes involving (137).
High-content Alumina and zirconia core-based ceramics are highly resistant to
chemical attack from hydrofluoric acid (138, 139,140), therefore different approach will
be required for enhance the bonding of these restorations using resin-based adhesives and
luting cements.
A method that has been shown to be quite effective in increasing bond strength to
zirconia-based ceramics is treating the intaglio surface with silica-modified aluminum
oxide followed by silane application. (141, 142) According to the manufacturer,
sandblasting with this material uses impact energy to apply a silica coating to the target
surfaces. (143)
It is unclear whether this transfer of silica is caused by particles actually become
embedded in the target surface, an actual mechanical/chemical transference
(Tribochemistry) or both. It is possible that the silica-coated particles actually “bounceoff” of these surfaces, but before bouncing off the surface there is an actual transference
of silica from the particles to the target substrate (Tribochemistry). (Figure 3)
Figure 3. Sandblasting with silica coated aluminum oxide: 1. Silica coated aluminum
oxide is blasted onto the surface. 2. On the surface a triboplasma is created in
microscopic ranges. 3. The aluminum oxide, which is after the ceramization only
partially coated, leaves the surface, which is itself now partially coated with SiO 2 . (143)
25
Silane coupling agents, i.e. silanes, are hybrid inorganic-organic synthetic
compounds. They are being used to enhance the bonding of resin composite to glassbased fillers and HF-etchable prosthodontics structures or silicatized metals and oxide
ceramics, e.g. silica- coated surfaces. (144) Silanes can provide chemical, and promote
surface wettability. Silane coupling agents have an organo-functional group that can copolymerize with the non-reacted carbon–carbon double bonds in monomers of a resin
composite. Silane compound is hydrophobic, which it first must be chemically activated
prior to silanizing. Their hydrolysable alkoxy groups (-Si-OR) need to react in aqueous
alcohol solution, at a moderate acidic (pH of 4–5) as a catalyst, to form labile reactive
silanols (≡Si–OH). Hydrophilic silanols condense and deposit a hydrophobic siloxane
film with the siloxane bonds, – Si–O–Si–O. The siloxane film thickness depends mainly
on the silane concentration and reaction conditions and it is usually more than a simple
monolayer. (145, 146) At the moment, 3-methacryloyloxypropyltrimethoxysilane (also in
literature known as 3-methacryloxypropyltrimethoxysilane, 3-MPS, Figure 4) is the most
commonly used silane in commercial dental materials. (147, 148)
In any case, this technique is very effective (more so than conventional
sandblasting) not just with high strength alumina and zirconia based ceramics, but also
when bonding to composite (149, 150) and metal surfaces (151, 152). Silica coating of
metal or composite substrates not only mechanically roughens the substrates, but also
increases the number of available hydroxyl groups for surface silane coupling. (Figures 4
and 5)
26
Figure 4. The silane molecules (on the right) approach the inorganic surface, which is
covered with hydroxide groups and water molecules. (152)
Figure 5. The silane molecules have made a chemical bond with the SiO 2 component of
the coated surface. (152)
Other coupling agents
It is noteworthy that silanes are not the only active materials in primers and
silanization is not only method that is used to promote the long-term durability of resin
composite bond to oxide ceramics. There are, indeed, several primers with reactive
monomers that have been evaluated in vitro to investigate the bond longevity and the
capability of the primer to form a chemical bond or, at least, a long-term micromechanical bond to alumina and Y-TZP. There are several products in market to promote
resin oxide ceramic bonding but not all have been found suitable to be used for zirconia
oxide. As a result, a numerous amount of literatures have been published and innovative
adhesive strategies combining new surface roughening procedures (153, 154),
27
precipitated and sintered alumina coating on zirconia (155) and chemical bonding have
been developed. (121, 124, 156-160, 161) Blatz et al compared the bond strengths of
different combinations of bonding/silane coupling agents and resin cements to Y-TZP.
They concluded that conditioning with a bonding/silane agent that also contains a
phosphate ester monomer, 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP,),
could exhibit superior resin bonding to Y-TZP which was airborne particle abraded with
Al2O3 particles. (156) 10-MDP containing materials have been suggested for use with
oxide ceramics in order to achieve sufficient long-term reliability. (156, 162) It has been
shown that even if Y-TZP is bonded using a 10-MDP-containing organo-phosphate resin
without airborne particle abrasion of the surface, specimens demonstrate low bond
strength values.(154) This suggests that without airborne particle abrasion as a
pretreatment no beneficial effect can be attributed by the phosphate monomer alone. This
conclusion is supported by the observation that when the same bonding resin composite
is used on zirconia specimens prepared by using selective infiltration etching (154), a
significant increase in bond strength can be observed.
A new primer containing a phosphoric acid monomer, 6-MHPA (6methacryloxyhexylphosphonoacetate), has been marketed for promoting bonding resin
composite cements to alumina and zirconia ceramics. In principle, in vitro investigations
are of utmost importance to assist in choosing materials for comparative studies of
primers, silanes and resin composite luting cements. Their clinical evaluation can be
commenced after preliminary careful comparative studies. Kitayama et al concluded that
primers containing a silane-coupling agent were effective in improving the bonding of
resin cements to silica-based ceramic. The primers containing a phosphoric acid
28
monomer or a phosphate ester monomer, including 6-MHPA and MDP, were the most
effective ones in improving the bonding of resin cements to zirconia ceramic. Without
any primer, the resin cement containing MDP was found to be effective in bonding to
zirconia ceramic. (163)
Resins and resin composites
Unfilled resins were first introduced in 1937. They were polymers based on
methyl methacrylate and they were called acrylic resins. Polymerization process occurs
as
covalent
bonds
between
molecules
for
formation
of
larger
molecules.
Polymethyl methacrylate (PMMA) is consists of two basic components, powder and
liquid. The powder is composed of small particles of PMMA with benzoyl peroxide as an
initiator. The liquid is methyl methacrylate with an amine as an activator. When these
two components mix together the polymerization process starts and longer molecule
chains are formed to provide the acrylic resin’s final shape. (164) Methyl methacrylate
polymerization is accompanied by a 21 percent reduction in volume. However, continued
development in the dental industry has been able to drastically reduce this reduction in
volume.
Reduction in volume is considered a disadvantage, which it is associated with
many clinical problems seen in composite restorative materials and resin luting agents
that may eventually lead to failure.(164) Filler content and resin matrix composition
dictate the amount of volumetric shrinkage and elastic modulus values of the material.
(165)
In 1962, a major advancement in esthetic dentistry was made by Bowen (166) as
he developed a new type of composite with a resin based on bisphenol A glycidyl
29
methacrylate (Bis-GMA) and inorganic filler particles, which were chemically bonded to
resin matrix through an organic silane coupling agent. Bis-GMA is extremely viscous and
has a high molecular weight, which makes it difficult to mix with large filler loadings.
Triethylene glycol dimethacrylate (TEGDMA) with lower molecular weight has been
used to dilute the highly viscous Bis-GMA and enhances its ability to flow. (167)
The resin-based luting agents are commonly preferred in the application of
various types of indirect restorations because of their superior properties in comparison
with other types of luting agents. (168, 169) They are characterized by a wide range of
clinical use including cementation of ceramic veneers, posts, nonmetallic inlays, onlays,
crowns, and fixed partial prostheses. (170-172) Resin luting agents have excellent
potential esthetic shade matching, better physical and mechanical properties compared to
other dental cements. (173)
In the early 1980s, conventional Bis-GMA resin cement was modified by adding
a phosphate ester to the monomer component, introducing to dentistry a unique group of
resin luting agents that have a degree of chemical bonding as well as a micromechanical
bonding to tooth structure and metal-based alloys. The first product was marketed,
Panavia,
it
was
contained
bifunctional
adhesive
monomer
MDP
(10
methacryloyloxydecyl dihydrogen phosphate) and it was a powder-liquid system. (174)
Bond strength to etched base metal greatly exceeded over the bonding to tooth
structure. Panavia quickly became the luting agent of choice for resin retained fixed
partial dentures. (174) In 1994, Panavia was modified to include a dentin/enamel primer
containing hydroxethyl methacrylate (HEMA), N-methacryloyl 5-aminosalicylic acid and
MDP, intended to improve bond strength to dentin. Under a new name, Panavia 21, it
30
was marketed as a two-paste system in three shades: tooth colored (TC, translucent),
white (EX, semitranslucent), and opaque (OP). Panavia 21’s polymerization required
exclusion of oxygen, and a covering gel was provided. The current product, Panavia F is
a two pastes system that is dual-cured, self-etching and self-adhesive, plus fluoride
releasing. (174)
Before the introduction of Panavia, Bis-GMA composite was modified by
decreasing filler and adding %3 2-hydroxy-3b-napthoxypropyl methacrylate in methyl
methacrylate with 4- methacryloyloxyethyl trimellitate anhydride (4- META) and
tri-n-butyl borane and marketed as C&B Metabond. (174) C&B Metabond has physical
characteristics similar to other resin cements, but also has an extremely high tensile
strength, which is useful for providing retention in restorative situations where less than
optimal conditions exist. It is a powder/liquid auto-curing system and may be used for
resin-bonded prostheses. (175) The strong cohesion forces in the specific net structure of
adhesive resin allow a better stress distribution on the surface of restored tooth (176).
It has been showed that glass ionomer cement may produce a superior bonding to
Y-TZP compared with a phosphate monomer (10-MDP) containing adhesive resin
composite. (177) However, some previous studies showed opposite results and thus 10MDP containing cement produces a more durable bond after airborne particle abrasion of
the Y-TZP surface. (124) 4-META containing adhesive resin has been pointed out to
have a bond strength superior to 10-MDP-containing resin composite. (178)
An early study suggested the use of either conventional luting cements, e.g. zinc
phosphate or glass ionomer cements, or resin cements for clinical application as luting
cements for bonding ceramics such as alumina or Y-TZP. (179, 180) There is some data
31
suggesting that any type of resin composite cement bond to Y-TZP might not capable of
sufficient adhesion for Maryland type FPDs. (181) The conclusion is that it is of
importance to evaluate carefully the substrates, pretreatments, all the materials used and
the presence and the method of fastened fatigue.
Adhesive resins have developed significantly during recent years with new
product generations. (182, 183) It is widely accepted that the key factors in successful
bonding to teeth are micro-mechanical entanglements of monomer resins to etched
enamel and dentin by hybridization, and also marginal seal can be improved
considerably. (184, 185, 186) As well, there is a notable problem with chemical bonding
with resin to Y-TZP, as it is an inert, nonreactive and complex surface with Zr atoms on
the outer surface. Bond strength tests have always drawn a lot of scientific attention, and
that is also the case with evaluating bond strength of different luting materials to YTZP.
It is also well established that the data obtained from different bond strength tests
depend on the actual test setup used and that may differ between individual studies.
Therefore, the bond strength data substantially vary among different studies. All these
interacting variables, i.e. surface pretreatment, silicatizing, silanes, primers and different
resins, make direct comparison between different studies very difficult and ultimately
irrelevant. (187, 188)
In contemporary dental research literature, there can be found several studies
suggesting the use of resin luting containing phosphate monomer will provide
significantly higher retention of zirconia ceramic crowns than conventional luting
cements. (158, 159, 161, 180, 189) Widely used and suggested use of phosphate
monomer composite resin has led to the development of competing products. Due to
32
intellectual property rights (IPR) concerning the structure of MDP-monomer, other
manufacturers have produced new phosphate monomers designed not only to bond to
zirconia but have also cross-linking branches for bonding to resin matrix as well. One of
the recently developed phosphate monomers (RelyX Unicem 2) has a characteristic of
self-etching phosphorylated methacrylates that is designed to bond directly to both
enamel and dentin. With two phosphate groups and at least two double bonded carbon
atoms, good bond strength to zirconia plus adequate crosslinking to the resin matrix is
possible to achieve. Another new self-etch phosphate monomer characterized by
hydrolytic stability has one phosphate terminal and at least two sites capable of bonding
to resin matrix through oxygen bond. This molecule has a terminal hydroxyl group as a
substituent that gives the monomer stability under water and in acidic conditions. (188) A
recent in vitro study on the differences of the above mentioned phosphate monomer
agents suggested that both Multilink and Panavia demonstrated predominantly cohesive
failure within resin cement due to covered zirconia surface with resin cement. In contrast,
RelyX Unicem revealed predominantly adhesive failure where the surface of zirconia
was exposed. Thus, 10-MDP and the reactive molecule used in Multilink automix could
exhibit superior adhesion to RelyX Unicem. (188)
There is some evidence that a good bond to Y-TZP ceramics is obtained using
resin cements with phosphate ester monomers, (MDP monomer). (158, 190) The
phosphate ester group chemically bonds to metal oxides (Figure 6) such as zirconium
dioxide. Lehmann et al evaluated the durability of the bond to zirconia with two resins
cements (Bis-GMA based & MDP based). The MDP based cement presented with higher
bond strength to zirconia surfaces on 150 days of water storage. (158)
33
The anhydride group present in 4-META monomer and the phosphoric
methacrylate ester also chemically bonds to zirconia ceramics. (158, 190) However, the
bond strength of a polymethylmethacrylate (PMMA) resin cement containing META is
not strong enough to resist thermal aging. (21, 121)
Figure 6. Chemistry of MDP monomer bonded to metal oxide (Mehta 2007)
Bonding capacity related to surface treatment
Long-term clinical success of ceramic restorations depends on successful
adhesion between ceramic and tooth substance. To achieve high bond strength of luting
cement to the ceramic, a micro-mechanical interlocking and chemical bonding to the
ceramic surface are required. (21) To enhance the bond strength of luting cements to
ceramics, a number of techniques have been reported; however, different types of
ceramics need different surface treatment techniques in order to gain maximum bond
strength. Glass-based ceramics or silica-based ceramics such as feldspathic, leucite and
lithium disilicate ceramics are acid-sensitive ceramics; therefore, the common treatment
options are acid etching, silanisation, and air borne particle abrasion with aluminum
oxide or combinations of any of these methods. Acid etching with a solution of
34
hydrofluoric acid (HF) and acidulated phosphate fluoride (APF) are commonly used in
dental literatures because they can induce the proper surface texture and roughness of
glassy ceramics (191-194). The acid dissolves the glassy phase and produces a porous
irregular surface. This leads to an increase in surface area and wettability of ceramics
enhancing the micromechanical retention (191). Examples of acid etching used in the
clinic are 5-10% HF and 1.23% APF. HF acid gel has been reported to produce higher
bond strength than APF (191, 195). In addition, the application of HF acid gel generates
more adequate micro-morphological surface topography for micromechanical retention of
the resin cement (191).
However, HF is more hazardous than APF with extreme caustic effects to soft
tissues (191). The danger for its clinical use is also well known due to its rapid
vaporization and the danger of inhalation (191). Therefore, HF can only be used in
clinical procedures if sufficient care during handling is taken in order to avoid its side
effects (192). For this reason, some studies questioned whether 1.23% APF gel might
serve as a safe and effective substitute for etching ceramic surfaces before bonding resin
composite. Tylka and Stewart (1994) supported the use of APF as they found no
significant difference of shear bond strength between 9.5% HF (5 minutes) and 1.23%
APF acids (10 minutes) (194) and these results were similar to another study (193).
Although, they found that 1.23% APF etching produced lower bond strength than 9.6%
HF overall, etching with 1.23% APF for 7-10 minutes did not produce a significant
difference in shear bond strength compared to etching with 9.6% HF for 4 minutes.
Another study (196) also reported that etching with 9.6% HF for 2 minutes had
comparable tensile bond strength to etching with 4% APF for 2 minutes.
35
In addition, the etching period and addition of a silane-coupling agent affects the
bond strength of glass ceramics. Etching time depends on the type of acid and
manufacturer recommendation. For example, etching time of HF acid ranging from 5-30
minutes has been used for bond strength tests (121, 191, 192, 193, 194, 195, 197).
However, it is often applied between 1-2 minutes in several studies (123, 191, 195, 197,
198). Additionally, the etching time of APF acid is longer than that of HF etch (191, 193,
194). Over-etching did not improve the bond strength and it may decrease the bond
strength because the ceramic surface becomes over-etched and the resin cannot penetrate
into the deep micro-porous surface (192).
Apart from acid etching, an application of a silane-coupling agent is commonly
used after the etching process. Silane contains hybrid organic-inorganic compounds,
which act as a mediator to promote adhesion between inorganic and organic substrates
(199). It can bond silicone dioxide with the OH groups on the ceramic surface and also
have a degradable functional group that copolymerizes with the organic matrix of the
resin (199). Therefore, it has chemical bonding ability to both ceramic and composite
resin. Use of a silane-coupling agent in combination with acid etching has been proved to
increase the bond strength (200) and bonding durability (191) of composite to the silicabased ceramics. Brentel et al (191) found that micro-tensile bond strength was increased
by silanization of the feldspathic ceramic surface after APF and HF acid etching. After
thermocycling for 12,000 times and water storage for 150 days, the bond strength
reduced dramatically when the ceramics were not silanized. Although the etching has
been successfully used to increase the bond strength of silica-based ceramics, this
technique does not improve the bond strength of high purity alumina and zirconia
36
ceramics. Özcan and Vallittu (198) reported that acid etched glass ceramics exhibited
significantly higher shear bond strength (26.4-29.4 MPa) than glass infiltrated alumina
ceramics (5.3-18.1 MPa) or zirconia dioxide (8.1 MPa). Bona et al reported that 9.5% HF
etching on alumina and zirconia-based reinforced ceramic surfaces for 1 minute achieved
low tensile and shear bond strength (3.5 and 10.4 MPa) compared with sandblasting (7.6
and 13.9 MPa) and silica coating (10.4 and 21.6 MPa). Sandblasting using aluminum
oxide particles is a surface treatment option that produces irregularities in acid-resistant
ceramics. It results in initially relatively low bond strength when cemented with
conventional Bis-GMA resin composite (124); however, the bond strength reduced after
long-term thermocycling in distilled water (124). The reason for this can be assumed that
sandblasting of zirconia ceramic produces a certain roughness but only limited or
minimal undercuts (124). Therefore, the bonding between Bis-GMA resin and zirconia
were not water resistant and the sample deboned. Addition of a silane did not enhance the
bond strength of the Bis-GMA resin composite to zirconia. In addition, the silane did not
improve the durability of the bond in water (124). This indicates that silane does not bond
to zirconia. In conventional dental ceramics, it is thought that silane bonding is mediated
through the silica at the ceramic surface. As zirconia contains no silica, the silane cannot
promote a resin bond to zirconia. Using a tribochemical silica coating system has been
reported to produce a significant increase in the initial bond strength of the conventional
Bis-GMA resin to YTZP ceramic (124,180); however, the resin bond was decreased after
thermocycling (124, 180, 198). Della Bona et al (2007) reported that silica coating
produces the highest tensile (10.4 ±1.8 MPa) and shear bond strength (21.6 ± 1.7 MPa) of
37
In-Ceram Zirconia ceramic compared with sandblasting (7.6±1.2 and 13.9± 3.1 MPa) and
HF etching (3.5± 1 and 10.4± 3.1 MPa) (197).
The bond strengths of the phosphate monomer containing resin composites (such
as Panavia F and Panavia 21), which is 10-methacryloxy-decry dihydrogenphosphate
(MDP), to sandblasted zirconia ceramic surface were significantly higher when compared
with other techniques and other resin (124, 180) reported that tensile bond strength
decreased about 17% over storage time but was not statistically significant. Kern and
Wegner (124) used conventional resin composite for control and silanization groups.
However, Lüthy et al found shear bond strengths of Panavia F and Panavia 21 were
higher after thermocycling (10,000x) for 14 days. Nevertheless, Lüthy et al (180)
suggested that their study was too short (14 days) to provide the information on long-term
bonding stability compared with Kern and Wegner (124) who used a longer aging time
(150 days). Therefore, the results from several researchers indicate MDP promotes a
water-resistant chemical bond to zirconia ceramics. The phosphate ester group of the
monomer is reported to bond directly to metal oxides (201). However, Atsu et al (2006)
(161) found that MDP containing composite resin luting agent alone or with silane
coupling agent had significantly lower shear bond strength compared with the
combination of silica coating and MDP-containing composite resin groups. Therefore,
they recommended the application of silica coating, MDP-bonding, and silanization
combination to increase the bond strength. However, thermocyling test was not
performed in this study.
Although chemical-cured polyacid-modified resin composite initially provided
relatively high bond strength to sandblasted Y-TZP, it decreased statistically significantly
38
over long-term storage with thermal cycling (124). The anhydride group of 4-META
such as Superbond C&B was reported to have chemical affinity to metal oxides. One
study found that 4-META provided initially high bond strength (44.5 MPa); however, the
bond strength decreased statistically after thermocycling (180). This reduction could be
explained by water absorption of Polymethylmethacrylate (PMMA) during the
thermocycling test, which seemed to weaken the chemical bond (180). Dérand and
Dérand found 4-META provided the highest bond strength (~17-20 MPa) compared with
Panavia 21 (~5-8.9) and Twinlook (autocuring cement) (~2-3 MPa). Superbond also
presented the same bond strength regardless of whether the ceramic surface was etched,
sandblasted or ground while Panavia and Twinlook had a stronger bond if the ceramic
surface was coated with Rocatec (178).
Previous studies on retention and bond strength to zirconia ceramic
Evaluation methods
Material selection and clinical recommendations on luting to ceramics are based
on mechanical laboratory tests, which show great variability in materials and methods
(196, 202). Chemical (203, 204), thermal (126, 180, 205) and mechanical (206)
influences under intraoral conditions were generally used.
The simulation of such influences in the laboratory is compulsory to draw
conclusions on the long-term durability of a specific luting procedure and to identify
superior materials and techniques. Long-term water storage (207) and thermal cycling of
bonded specimens are accepted methods to simulate aging and to stress the luting
interface. Most studies that apply these methods reveal significant differences between
39
early and late bond strength values (208, 209, 210, 211). Also application of mechanical
cyclic loading (fatigue load) might cause significant reduction of bond strengths (212,
213).
Taper mode
Luting to zirconium oxide ceramic was the subject of a number of studies. The
details of these studies have been contradictory with regard to cementation mode,
preparation geometry of the margin, angle of convergence, and extent of tooth removal.
Some studies showed that comparing to metal-based restorations, all-ceramic restorations
should not involve any primary retention, as this would produce crack-inducing tensile
stresses from the inner surface of the restoration. Certain preparation guidelines that
differ from recommendations for metal-supported systems have to be taken into
consideration for all-ceramic FPDs. Nevertheless all-ceramic restorations exhibit an
inferior overall fit compared with cast metal or metal-ceramic restorations (214, 215,
216).
Cements and surface treatment tests
Comparing the different types of cements, Tinschert et al. found that fullcoverage zirconium-oxide ceramic restorations and FPDs may not require adhesive
cementation (216). However, a sufficient resin bond has the aforementioned advantages
and may become necessary in some clinical situations, such as compromised retention
and short abutment teeth (217). Osman et al. compared the film thickness and rheological
properties of zinc phosphate cement with different polymerizing cements, including
Panavia 21, Superbond, All Bond C&B Cement, and Variolink. An initial film thickness
of 25 μm was observed and was not significantly different between the cements (218).
40
Zirconia ceramic surface treatment was also subject to many studies. Derand and
Derand evaluated different surface treatments and resin cements and found that an autopolymerizing resin cement (Superbond C&B) exhibited the significantly highest
retentions regardless of surface treatment (silica coating, airborne particle abrasion, HF
etching, or grinding with a diamond bur). Water storage for 60 days had mixed effects on
crown retention. (178)
Kern and Wegner evaluated different adhesion methods and their durability after
long-term storage (150 days) and repeated thermal cycling. As surface treatment they
used air-abrasion alone and the additional use of a silane or acrylizing. Only the
phosphate-modified resin cement after airborne particle abrasion provided a long-term
durable resin bond to zirconia ceramic. These findings were confirmed by a long-term
study in which specimens were subject to 2 years water storage and repeated thermal
cycling (124). Wegner and Kern found also that restorations made of yttrium-oxidepartially-stabilized zirconia ceramic (Y-TZP) can be cemented non-adhesively or
adhesively with self-curing composite Panavia 21 or the dual curing composite Panavia F
(Kuraray). They found that durable bonding could be achieved with the resin composite
products with the air-abraded surface of the zirconia substructures without the need for
silication and silanization of the surfaces (124).
NULL HYPOTHESIS AND AIMS
The retention of Y-TZP crowns is unaffected by surface treatment or luting agent
after artificial aging (thermocycling) and compressive fatigue loading.
41
MATERIALS AND METHODS
99 (including nine teeth for backup) extracted non-caries molars were
obtained and stored in 0.5% sodium hypochlorite (1:10 dilution of household bleach).
The roots of the teeth were notched for retention. Teeth were centered on PVC mold with
using rope wax and they were mounted along their vertical alignment in a PVC mold
(3cm Height, 2 cm Diameter) with the cemento-enamel junction positioned 1 mm above
the top of the mold. The cold cure auto-polymerizing acrylic resin (Ortho-Jet Resin
Acrylic, Lang Dental Manufacturing Co., Inc.IL, USA, Figure 7) was used to retained the
teeth. An indentation was created on the top surface of mounting resin to orient the final
milled coping to the correct position.
Figure 7. Mounted Tooth
Table 2: Materials used in this study for cementation and Intaglio surface treatment of
crowns
Materials
Panavia F2.0
RelyX Unicem 2
Duo-Link
Alloy Primer
ESPE™ Sil
Z-Prime plus
Silica-Modified Al 2 O 3
Al 2 O 3
Composition
Paste A: BPEDMA/10-methacryloxydecyl
dihydrogen-phosphate (MDP)/DMA
Paste B: Al-Ba-B-Si glass/silica
Methacrylate monomers containing
phosphoric acid groups
Methacrylate monomers, Silanated fillers, Alkaline
Initiator components, Stabilizers, Rheological
Bis-GMA, Triethyleneglycol Dimethacrylate,
Urethane dimethacrylate
Glass Filler
10-methacryloyloxyidecyl- dihyidrogenphosphatea
(MDP Primer)
6-(4-Vinylbenzyl-N-propyl)amino-1,3,5-triazine2,4-dithione / Aceton
Silane [3-trimethoxysilylpropyl methacrylate (MPS
primer)]
Biphenyl dimethacrylate, Hydroxyethyl
methacrylate, Ethanol
30 μ Al 2 O 3 particles coated with silica
50 μ Al 2 O 3 particles
42
Manufacturer
Kuraray Dental, New York,
NY
Lot #
061112
3M ESPE, St.Paul, MN
458395
Bisco, Inc. Schaumburg, IL
11000109
07
Kuraray Dental, New York,
NY
0398BA
3M ESPE, St.Paul, MN
446248
Bisco, Inc. Schaumburg, IL
11000102
71
346850
0960511
3M ESPE, St.Paul, MN
Henry Schein, Melville,
Specimens were stored in distilled water at 37o C in incubator during the study.
Materials used in this study for cementation and Intaglio surface treatment of crowns are
mentioned in Table 2.
Specimens were mounted on the milling machine; occlusal surface was cut flat (4
mm above the CEJ) by using straight hand-piece with diamond bur (Brasseler, Model
920, medium, Grit Size 100 micron, USA) under water spray Figure 8a. In a dental
laboratory, a straight hand-piece diamond bur (Brasseler, Model 840, medium, Grit Size
100 micron, USA) was set on 20 degrees to cut axial walls under water spray Figure 8b.
Preparations were stored in distilled until impressions were made.
Figure 8a. Occlusal Cut
Figure 8b. Axial Cut
Surface area on each preparation was calculated using the following formula:
Área = π/4 d 1 2 + πh/2 (d 1 + d 2 )+ π/4(d32-d22)
d1
h
d3
d2
43
“d1” and “d2” are diameters of the bottom and top of the crown preparation, respectively.
Collected natural teeth for this study have different anatomy such as root shape, occlusal
morphology and crown shape. Some teeth were more trapezoidal and some square. Based
on this observation, a decision was made to use the diameter of the bottom circle d3,
regardless of the width of margin. Thus, “d3” is the bottom diameter plus two 1mm
margins (d2+2). “H” is axial height. Dimensions of each preparation were measured
using a digital caliper (Ultratech, Penn Tool Co. Maplewod, NJ) with a ± 0.02 mm of
accuracy. Microsoft Office Excel spreadsheet was used to automatically calculate the
surface area for each preparation. Also, TOCs were calculated for all the specimens by
using the formula of inverse sine of a/c, which a is equal b 1 -b 2 /2. So, TOC is equal to A 1
+A 2 .
Figure 9. Inverse Sin = a/c (219)
Impressions were made using perforated PVC rings custom-made tray, which was
purchased from a local warehouse (h=3cm and d=2cm). The VPS tray adhesive (GC
44
America Inc. Alsip, IL) was previously applied in the internal surface of the custom made
tray and dried at room temperature for 5 min. Impressions were made by using an
addition silicon impression material (Aquasil Monophase and XLV Fast set,
DENTSPLY, Figure 10a). Impressions were poured with ADA Type IV gypsum (Whip
Mix Corp. Louisville, KY). The master die was recovered from the impression and its
excess was trimmed (Figure 10b).
Figure 10a. Final Impression
Figure 10b. Stone Mode
The master dies were scanned in a laboratory (TDS Products, Taiwan), and 50
microns space was applied digitally on the scanned dies to simulate two coats of die
spacer (220). Thickness of the copings on axial wall and margins were requested to be
1mm even. Occlusal surface of the coping was designed for a 2mm diameter hole, which
is located 1mm above the occlusal surface and 3mm coping above the hole, (Figure 11).
This allowed the crowns to be held by a wire while crown pull retention test was
performed (Figure 12). In the TDS Laboratory, pre-sintered Zirconia blocks (TZP BIOHIP®, METOXIT AG. Thayngen, Switzerland) were milled and sintered at 1530ºC.
Crowns were fabricated in shade A2 (Classic VitaPan shade guide). Path of insertion of
each crown was marked using an indentation marker.
45
Figure 11. Milled Crown
Figure 12. Retention pull hole (2mm in diameter)
To evaluate internal fit of crowns, a replica technique was used. Teeth were
gently dried, Aquasil XLV Fast Set (DENTSPLY Caulk, Milford, DE, USA) were
applied to the intaglio surface of crowns and seated on paired preparation and allowed to
set under constant pressure 2.5kg (221) weight for 3min, Figure 13. The crowns were
removed and Aquasil Heavy Body Fast Set (DENTSPLY Caulk, Milford, DE, USA) was
injected into the crown to reinforce Aquasil XLV Fast Set (Replica Technique, Figure
14a) The specimens obtained from the replica technique were sectioned in two pieces by
using number 15 blade, Figure 14b. Thickness of the XLV impression material was
measured in 6 areas using a digital microscope (Keyence VHX 6000 Series, KEYENCE
America, USA, Figure 15a) at 30x magnification. The six areas of measurement consist
46
of junction axial wall and margin (A), 1 mm below the occlusal surface (C) and middle of
points A and C, called B, Figures 15b. All measurements were recorded and transferred
to Xcel software for statistical analysis. The mean and standard deviation were calculated
for each specimen and it groups.
Figure 13. XLV material under 2.5 kg
Figure 14a. Replica Technique
Figure 15a. 30X Magnification
Figure 14b. XLV duplicated crown cut to two halves
Figure 15b. XLV thickness measurements section A
Specimens were randomly divided to 9 groups of 10 (Table 3). In order to
stimulate the clinical situation, natural teeth needed to be temporized at this time, and due
47
to lack of temporary crowns, original impressions were used for this purpose.
Impressions were lined with temporary cement (Tempbond®, Kerr Corp. CA, USA) and
seated on the preparation to simulate temporary crown cementation. Tempbond cement
did not consist of eugenol, Figures 16a and 216b.
Table 3: Study design groups with designated surface treatment, primer and cement
Study Groups
Surface Treatment
Primer
Cements
A (PFC)
None
None
Panavia F 2.0
B (PFS)
Silica-modified Al 2 O 3
Alloy Primer
Panavia F 2.0
C (PFA)
Al 2 O 3
Alloy Primer
Panavia F 2.0
D (RUC)
None
None
RelyXTM Unicem 2
E (RUS)
Silica-modified Al 2 O 3
ESPE™ Sil-Silane
RelyXTM Unicem 2
F (RUA)
Al 2 O 3
ESPE™ Sil-Silane
RelyXTM Unicem 2
G (DLC)
None
None
Duo-Link™
H (DLS)
Silica-modified Al 2 O 3
Z-Prime™ plus
Duo-Link™
I (DLA)
Al 2 O 3
Z-Prime™ plus
Duo-Link™
Figure 16a. Impression lined with temporary cement
Figure 16b. Cemented Prepared Tooth
Specimens were stored in 37ºC-distilled water for 3 days, after which the
impressions were removed and the cement was cleaned with a prophylaxis cup and flour
of pumice.
48
The different surface treatments and cements used in this study include Roctac
soft (Silica-modified Al 2 0 3 ), Al 2 0 3 and control group (no surface treatment). Three
different cements used were Duo-Link (Bis GMA, Total Etch), RelyXTM Unicem 2
(Phosphate ester, Self Adhesive) and Panavia F2.0 (Phosphate ester, Self etch), Figures
17a-17c.
Figure 17a. Panavia F2.0 cement
Figure 17b. RelyXTM Unicem 2
Figure 17c. Duo-LinkTM
All three cements are Dual-Cured resin with different surface treatment
requirements on teeth, Table 4.
Table 4: Cement Specifications
Cements
Curing Mechanism
Tooth treatment
Composition
Panavia F2.0
Dual-Cured
Self-Etch Primer
Phosphate ester
RelyXTM Unicem 2
Dual-Cured
Self -Adhesive
Phosphate ester
Duo-LinkTM
Dual-Cured
Total-Etch
Bis-GMA
Intaglio surface of crowns and surface of the teeth preparation were treated
with recommended primer and conditioner, which are both listed in Table 5.
Table 5: Cement Specifications with crown and tooth surface treatment materials
Cements
Tooth surface Primer
Panavia F2.0
RelyX
TM
Unicem 2
TM
Duo-Link
Tooth treatment
Crown primers
ED PRIMER II
Self-Etch Primer
Alloy Primer
None
Self–Etch Adhesive
ESPE™ Sil-Silane
ALL-BOND 2
Total-Etch
Z-PrimeTM plus
49
Groups A, D and G were control groups and no treatment was done to intaglio
surface of crowns. The intaglio surface of crowns in groups C, F and I were sandblast
with Al 2 O 3 (50 μ) under 45 psi pressure for 15s at a distance of 10mm, then crowns were
cleaned ultrasonically for 5min in distilled water. The intaglio surface of groups B, E and
H crowns were received Silica-Modified Al 2 O 3 (30 μ) under 45 psi pressure for 15s at a
distance of 10mm. Residues were cleaned by gentle oil free air spray. Primers were
applied for Groups E, I, C, F, H and B following manufacturer’s instructions (Table 6-8).
Crowns for groups A, D and G were not surface treated as a control group and they were
cemented following manufacture’s direction. Cementation procedures were performed
under constant 2.5 kg weight for 5 minutes, Figure 18. All cemented crowns were cured
at the margin with G Light (GC America Inc.) with 400±20 nm range (light intensity was
measured every two cementation procedures) for 160s, 40s each side and excess was
cleaned with micro-brush Figure 27. All specimens were placed in incubator at 37 C0
degrees in distilled water for 24 hours.
Table 6: Crown cementation steps with RelyXTM Unicem 2
RelyXTM Unicem 2: Groups E and F (RUS and RUA)
Crown Intaglio
Surface
•
Blast the intaglio of crown with
Al 2 O 3
•
Blasted surfaces were cleaned
with alcohol and dry it oil free
air.
•
Blast the intaglio with SilicaModified- Al 2 O 3
•
Remove the residue with water
free and oil free air.
•
Wet the surface with EPSE Sil
•
Allow to volatile for 5 minuets
Tooth Surface
•
Clean with pumice
slurry, rinse with water
spray, gently air dry (do
not desiccate the dentin)
50
Cementation
•
Apply to intaglio surface of crown by
using appropriate tip
•
Seat the crown
•
Apply 2.5kg weight for 60 s
•
Clean the excess with micro-brush
•
Cure for 160s, 40 each side
•
Store in 37 C0 degrees distill
water
for
24
hours
before
thermocycling procedure
Table 7: Crown cementation steps with Panavia F2.0
Panavia F 2.0 (Shade A2): Groups B and C (PFS and PFA)
The paste brought to room temperature 15 minutes after it removed from refrigerator.
Crown Intaglio
Surface
Tooth Surface
•
Wet the surface with
Alloy Primer
•
Allow to dry for 60s
Cementation
•
Mix ED PRIMERS II
A and B and apply on
the tooth
•
Mix paste A and B for 20s and
apply it intaglio surface of crown
•
•
Seat the crown
Allow to dry for 30s
•
•
Apply 2.5kg weight for 60 s
Air dry the excess
•
Cure for 160s, 40 each side
•
Store in 37 C0 degrees distill
water for 24 hours before
thermocycling procedure
Table 8: Crown cementation steps with Duo-LinkTM
Duo-LinkTM: Groups H and I (DLS and DLA)
Crown Intaglio
Tooth Surface
Cementation
Surface
•
•
Wet the surface with
•
Etch the tooth for 15s
1-2 coats of Z-Prime
with UNI-ETCH (32%
Dry it with air for 3-
Phosphoric Acid)
5s
•
•
•
•
Apply a layer of PRE-BOND
RESIN on the tooth, and air thin
•
Rinse and dry with cotton
Apply Dual Syringe cement to
intaglio surface of crown
palette
•
Seat the crown
All-Bond 2 (Mix
•
Apply 2.5kg weight for 60 s
PRIMERS A and B) and
•
Clean the excess with micro-brush
apply five coats
•
Cure for 160s, 40 each side
Dry 5s (do not dry
•
Store in 37 C0 degrees distill
between the each coat)
water
for
24
hours
thermocycling procedure
51
before
Figure 18a. Final cementation with pressure of 2.5 kg
Figure 18b. Cleaning cement with micro-brush
The specimens were thermal cycled between water temperatures of 5°C and 55°C
for 5000 cycles with a 15-second dwell time at each temperature (total of 30 second
dwell time and 15 second transfer time) Figure 19a. Then they were fatigue loaded under
compressive fatigue load (70 N) using Wear Machine (Fabricated by UAB Research
Machine Shop, Birmingham USA, Figure 19b), for 100,000 cycles, while specimens
were emerged in distilled water.
Figure 19. UAB Thermocycling (a) and compressive fatigue testing machine (b)
Zirconia coping were pulled with a 5 kN load cell using a Universal Testing
Machine (Instron, Model 5565, MA, USA), which they were loaded in tension at a crosshead speed of 0.5mm/min until debonding occurred, Figures 21 and 22.
52
Figure 20. Mounted crown for pull test (a) and dislodged crown after pull test (b)
The Universal Testing machine automatically plotted the numbers (load N) into a
graph. The force (N) at dislodgment was recorded and the stress removal calculated using
the surface area. In addition, following the coping dislodgement, a calibrated observer
using the criteria set in Table 9 (at 30X Magnification) recorded the nature of debonding.
Table 9: Characterization of Cement Failure Mode
Classification
Description
Nature
1
Cement principally on tooth > 50%
Adhesive to preparation (AP)
2
Cement equally on both casting and tooth
Cohesive within cement (CC)
3
Cement principally on crown > 50%
Adhesive to crown (AC)
A two-way ANOVA was used to analyze interaction of cement and surface
treatment on crown retention; followed by Tukey’s Post hoc test. One-way ANOVA and
Dunnett Post hoc test were used to evaluate influence of surface treatment on crown
retention within each cement group. Two-way ANOVA was used to analyze the fit (or
cement layer thickness) of milled crowns between all groups and one-way ANOVA to
analyze the fit measurements in each group. The interaction between total surface area,
axial height and total occlusal convergence (TOC) of prepared teeth with total load of
crown dislodgement was analyzed by one-way ANOVA followed by Tukey’s Post hoc
test. The mode of failure statistic analysis was performed with Pearson’s chi-square test.
All statistical analyses were performed using a significance level of 5% (p = 0.05).
53
Result
The prepared tooth preparations were compared to assess consistency among the
specimens. The total occlusal convergence, axial wall height, and the total preparation
surface area varied within the sample groups despite controlled milling on a surveyor.
Means and standard deviations for each parameter are shown in Figures 21, 22, and 23.
Comparisons using one-way ANOVA with Tukey’s HSD post hoc tests did not
demonstrate significant differences with the groups for each parameter (p>0.05).
29.5
TOC (Degree)
29
28.5
28
27.5
27
26.5
26
25.5
25
24.5
RUC
RUA
RUF
DLC
DLA
DLS
PFC
RUC RUA RUF DLC DLA DLS
PFC
PFA
PFS
h (mm)
Figure 21. Comparisons of Total Occlusal Convergence for the sample groups (key: RU – RelyX
Unicem 2, DL – Duo-Link, PF – Panavia F2.0, A – Al 2 O 3 abrasion surface, S – silica modified
surface, and C – control surface.)
4.8
4.7
4.6
4.5
4.4
4.3
4.2
4.1
4
3.9
3.8
PFA
PFS
Figure 22. Comparisons of axial wall height (h) for the sample groups (key: RU – Rely X Unicem
2, DL – Duo-Link, PF – Panavia F2.0, A – Al 2 O 3 abrasion surface, S – silica modified surface,
and C – control surface.)
54
180
160
SA (mm2)
140
120
100
80
60
40
20
0
RUC
RUA
RUF
DLC
DLA
DLS
PFC
PFA
PFS
Figure 23. Comparisons of preparation surface area for the sample groups (key: RU –
Rely X Unicem 2, DL – Duo-Link, PF – Panavia F2.0, A – Al 2 O 3 abrasion surface, S –
silica modified surface, and C – control surface.)
The overall fit of the CAD/CAM restorations was assessed by evaluation of the of
the cement space through measurement of the thickness of the low-viscosity impression
material as described previously. A two-way ANOVA showed a significant difference
between the groups without a significant interaction of the treatment effects, Table 10.
Further analysis by a one-way ANOVA with a Tukey’s HSD post hoc test did not reveal
significant differences of the thicknesses of the impression material for in each surface
treatment for each cement group (p>0.05).
However, when comparing cements
significant differences existed between the RelyX Unicem 2 and other two cement
groups, Panavia F2.0 and Duo-Link. There was no significant difference between Panavia
F2.0 and Duo-link specimens groups. Mean and standard deviation for the cement and
surface treatment groups are shown in Figure 24.
55
Table 10: ANOVA for evaluating fit for each group
DF
Sum of Square
Mean Square
F-Value
P-Value
Sur. Trt
2
448.383
224.192
2.008
0.1409
Material
2
1730.098
865.049
7.748
.0008
Sur.Trt& Material
4
555.312
138.828
1.244
0.2992
Residuals
81
9043.069
111.643
Figure 24. Impression material thickness used in evaluation of the fit for each group
One example of the maximum pull load data and graph for Panavia F2.0 control
group was shown in the figure 25 and table 11.
Panavia F2 Control
Specimen #
Load (N)
500
400
300
200
100
0
-1
0
1
2
3
4
5
6
Extension (mm)
Figure 25. Maximum load plot Panavia F2.0 control group
56
7
1
2
3
4
5
6
7
8
9
10
Table 11: Specimens and Maximum load values of Panavia F2.0 control group
Specimens number
Graph number
Maximum Load (N)
61
1
288.94249
62
2
290.21762
67
3
367.71122
66
4
366.10465
65
5
281.05947
70
6
388.16524
64
7
325.75887
63
8
407.58725
69
9
421.99712
68
10
408.42038
After all the crown pulling with Instron machine, the two-way ANOVA ran which
it did not show a significant difference between each groups by comparing the mean of
maximum dislodging forces with different variables, such as surface area, axial height
and TOC (P>0.05). Also, Regression analysis was also used to understand which
independent variables are related to the dependent variable. The results are shown in
tables 12-17:
Table 12: ANOVA table for MPa vs. 2 Independents
DF
Sum of Squares
Mean Square
F-Value
P-Value
Regression
2
114.524
57.262
379.478
<.0001
Residual
87
13.128
0.151
Total
89
127.652
57
Table 13: Regression Coefficients, MPa vs. 2 Independents
Coefficient
Std. Err
STD. Coeff
t-Value
P-Value
Intercept
0.134
0.275
0.134
0.488
0.6269
Load
0.008
3.119E-4
0.951
26.392
<.0001
SA
-0.001
0.002
-0.014
0.382
0.737
Table 14: ANOVA table, MPa vs. 3 Independents
DF
Sum of Squares
Mean Square
F-Value
P-Value
Regression
3
114.524
38.189
250.992
<.0001
Residual
86
13.085
0.151
Total
89
127.652
Table 15: Regression Coefficients, MPa vs. 3 Independents
Coefficient
Std. Err
STD. Coeff
t-Value
P-Value
Intercept
-0.014
0.543
-0.014
-0.210
0.8342
Load
0.008
3.213E-4
0.956
25.735
<.0001
SA
-0.002
0.003
-0.030
-0.632
0.5293
H
0.083
0.156
0.024
0.531
0.5967
Table 16: ANOVA table, MPa vs. 4 Independents
DF
Sum of Squares
Mean Square
F-Value
P-Value
Regression
4
114.737
28.684
188.784
<.0001
Residual
85
12.915
0.152
Total
89
127.652
58
Table 17: Regression Coefficients, MPa vs. 4 Independent
Coefficient
Std. Err
STD. Coeff
t-Value
P-Value
Intercept
0.273
0.654
0.273
0.418
0.6772
Load
0.008
3.221E-4
0.953
25.596
<.0001
SA
-0.002
0.003
-0.035
-0.750
0.4552
h
0.105
0.157
0.030
0.670
0.5047
TOC
-0.016
0.015
-0.037
-1.058
0.2932
Mean and standard deviation of force at dislodgement (N), stresses of crown
removal (MPa) and crown preparation area (mm2) for each group have been showed in
Table 18. Data reflects a uniform distribution of preparations by surface area between
each group. One-way ANOVA did not reveal any significant difference between the
surface areas of tested groups (p>0.05).
Table 18: Means and Std. Dev. of total load, SA and stress for all groups
Type of failure
Force Load (N)
Surface Area of
Stress (MPa)
Preparation (mm2)
Study Groups
PFC
354.60±54.1
134.25±23.2
3.33±0.5
PFS
376.93±104
119.16±16.2
3.96±0.8
PFA
353.99±138
130.69±28
3.52±1.6
RUC
341.13±130.5
122.25±22.1
3.34±0.6
RUS
314.97±108.8
112.47±9.8
3.52±1
RUA
366.67±130.6
122.84±14.8
3.70±1
DLC
71.87±32.4
125.62±16
0.72±0.3
DLS
358.80±122.7
122.13±14.7
3.7±1.1
DLA
229.27±86.6
120.48±15.3
2.40±0.8
59
The mean retention in MPa and the standard deviation are presented in tables
19-22 and figure 28. The highest mean retention was PFS group (3.96 MPa) and the
lowest was recorded for DLC (0.7 MPa). Two-way ANOVA showed a significant
influence of the two tested factors on crown retention (p<0.05). Tukey’s post hoc test
revealed effect of cement and surface treatment on crown retention. Total-etch adhesive
composite resin cement produced significantly lower retention than self-etch composite
resin and self-etch prime composite resin cements (p<0.05), but there was no significant
difference between the latter cements (p>0.05). Also, Tukey’s post hoc test was used to
exercise effect of surface treatment on crown retention. The results showed that Al 2 0 3
and silica-modified Al 2 0 3 treated groups had significantly higher retention than the
control group (p<0.05), but no significant difference was observed between Al 2 0 3 and
silica-modified Al 2 0 3 treated groups (p>0.05). Multiple comparisons of 9 groups were
made with Tukey’s post hoc test to compare each group together. The results revealed
that DL-Control group had significantly lower retention when compared to the other
groups (p<0.05). Except for crown retention of DLA and PFS groups, no significant
difference was observed between other groups (p>0.05).
Table 19: ANOVA table for stress (MPa) of all groups
DF
Sum of Square
Mean Square
F-Value
P-Value
Lambda
Power
Substrate
2
21.788
10.894
19.337
<.0001
38.678
1.000
Sur. Trt
2
13.579
6.789
12.051
<.0001
24.103
0.997
Sur. Trt &
4
15.789
3.947
7.006
<.0001
28.026
0.996
81
45.633
0.563
Material
Residuals
60
Table 20: Mean and Std. Dev. of stress (MPa) for all groups
Study Groups
Count
Mean
Std. Dev.
Std. Err
RUC
10
2.763
0.651
0.206
PFC
10
2.663
0.379
0.120
DLC
10
0.577
0.252
0.080
RUA
10
2.948
0.815
0.258
PFA
10
2.804
1.222
0.386
DLA
10
1.899
0.644
0.204
RUS
10
2.774
0.841
0.266
PFS
10
3.134
0.660
0.209
DLS
10
2.937
0.852
0.269
Table 21: Tukey’s/Kramer for stress (MPa) and different cements
Groups
Mean Diff.
Crit. Diff
Sig.
DL and PF
-1.063
0.464
Yes
DL and RU
-1.024
0.464
Yes
PF and RU
-0.039
0.464
No
Table 22: Tukey’s/Kramer for Stress (MPa) and different surface treatments
Groups
Mean Diff.
Crit. Diff
Sig.
C and Control
0.550
0.464
Yes
Al 2 O 3 and Silica
-0.398
0.464
No
Control and Silica
-0.947
0.464
Yes
61
Figure 26. Interaction Bar plot for effect of cements and surface treatment on stress
(MPa)
One-way ANOVA showed a significant effect of surface treatment on crown
retention of Duo-Link group (p < 0.05). Dunnett post hoc test showed that Al 2 O 3 and
silica-modified Al 2 0 3 treated groups had significantly higher crown retention than the
control group when Duo-Link was used as cement (p < 0.05). One-way ANOVA did not
show any significant difference between control and surface treated groups for self-etch
composite resin (RelyXTM Unicem 2) and self-etch prime composite resin (Panavia F2.0)
Stress Mean (Mpa)
cements p>0.05), Figure 27.
3.5
3
2.5
2
1.5
1
0.5
0
RUC
PFC
DLC
RUA
PFA
DLA
RUS
PFS
DLS
Figure 27. Interaction Bar Plot for Stress (MPa) of all groups, (key: RU – Rely X Unicem
2, DL – Duo-Link, PF – Panavia F2.0, A – Al 2 O 3 abrasion surface, S – silica modified
surface, and C – control surface.)
62
The results for characterization of the failure mode are presented in Table 23.
Pearson’s chi-square test did not show significant difference in failure modes between the
groups (p>0.05). The failure mode was observed between the luting agent and tooth
structure, since more 50% of cement was attached to the intaglio surface of crown
(84.44%), followed by cement remained mainly on the preparation (8.89%). When
RelyXTM Unicem 2 and Panavia F2.0 were used as cement, surface treatment created
100% of the cement to stay in the crown (except for PF- Al 2 O 3 which was 90%).
Overall, the predominant mode of failure was adhesion to crowns where the cement was
found principally on the copings, then adhesion to preparation and finally failure mode of
cohesive within cement, figures 28a-c. Duo-linkTM cement with surface treatment showed
more CC failure. In RelyXTM Unicem 2, a majority of failure mode was AC, table 23.
Table 23: Recorded Failure mode for all groups
Type of failure
Adhesive to Preparation
Cohesive within Cement
Adhesive to Crown
(AP)
(CC)
(AC)
Study Groups
PFC
1
2
7
PFS
0
0
10
PFA
1
0
9
RUC
1
1
8
RUS
0
0
10
RUA
0
0
10
DLC
1
0
9
DLS
0
2
8
DLA
4
1
5
63
Figure 28a: AC failure
Figure 28b: AP failure
64
Figure 28c: CC failure
Discussion
As a result of the good properties of dental ceramics, such as esthetics, chemical
resistance, hardness, compression resistance and biocompatibility, a significant effort has
been made over the years to improve their weak points, which include brittleness and low
tensile strength. (221,222) The zirconia systems, which are currently available for use in
dentistry has a 90% or higher content zirconium dioxide, such as yttrium, stabilized
tetragonal Zirconia (Y-TZP) and glass infiltrated ceramics. Zirconia has been used in
dentistry (Implant, Post and Coping and Full contour restoration) since 1990s.
The retention and bond strength between the substrate and any type of dental
restoration (i.e. full or partial coverage crowns, resin bonded restoration) can be divided
to categories based on bonding such as micromechanical, chemical, or both. The
adhesion and bonding can occur between the tooth structure and luting agent, and also
between luting agent and dental restoration.
There are many factors in regard to tooth structure and dental materials that may
effect the retention of crown restoration including different tooth structure (enamel,
dentin and cementum), surface roughness, TOC, axial wall height of tooth preparation,
luting agent (different retention mechanism), total surface area of prepared tooth, surface
treatment and conditioning of prepared tooth. In the study by Kaufman, there are six
factors involved with tooth preparation that can influence retention. These variables
include: a) surface area, b) height of prepared surfaces, c) convergence angle, d) surface
texture of the tooth, e) the presence of intra-coronal retentive devices in the preparation,
and f) the degree of retention provided by various components of the prepared area. (224)
In addition, the crown restoration can be altered to increase their retention to luting
65
agents. These factors are crown fabrication material (i.e. metal, porcelain or zirconia),
different surface treatments (i.e. etching, sandblasting) and different luting agents. A
commonly thought conventional method of adhesive cementation included prior acid
etching of the ceramic surface with hydrofluoric acid and further silanation, which are not
efficient for Y-TZP ceramics because of their lack of silica and glass phase. (124,156,180)
In present study, the influence of different surface treatments and luting agents
were studied. The mean values of crown retention ranged from 0.7 (Bis-GMA, DueLinkTM Control group) to 3.96 (Panavia F2.0, SiO 2 ) MPa after 5000 thermal cycles (5°C
and 55°C) and 100,000 fatigue loading cycles (70N). By comparing all the control
groups, Bis-GMA had the lowest retention value (mean 0.72 and std 0.3) and phosphate
ester monomer base cement with the highest (mean 3.34, std 0.6). The luting material
agent had a significant influence on crown retention, as we see in this study Panavia F2.0
and Rely X Unicem2 provided a significantly higher retention than Bis-GMA based resin
cement (Duo-Link). In a previous study, phosphate ester monomers, such as MDP (10methacryloyloxyi- decyl- dihyidrogenphosphate), chemically react with zirconium
dioxide, promoting a water-resistant bond to densely sintered zirconia ceramic, and
resulting in strong adhesion to zirconia crowns. (124) Another study stated that metal
primers containing MDP and other monomers, including VBA TDT (6-[4- vinylbenzyl-npropyl] amino-1,3,5-tri- azine-2,4-dithione), MEPS (thiophosphoric methacrylate) and
MTU-6 (6- methacryloyloxyhexyl-2-thiouracil-5-carboxylate). (225, 226)
Also, Lüthy et al (2006) examined the shear bond strength between sandblasted
zirconia surface with resin cements and found that Panavia possessed the highest mean
shear bond strength (63.4 MPa) compared with other cements, and another similar study
66
was also found by Kern and Wegner (1998). The phosphate ester group of the monomer
within the Panavia resin cement is reported to bond directly to metal oxides, which
promotes a water-resistant chemical bond to zirconia ceramic (Lüthy et al. 2006). Also,
the application of a silica coating, silanisation and a resin cement suggested to create a
good bond strength between resin cement and ceramic (Atsu et al. 2006).
In other studies, shear and tensile bond strength tests were used to compare resin
cement adhesion to Y-TZP crowns, and they all resulted in height shear and tensile bond
strength in phosphate monomer base resin cements. (161, 227, 228, 229)
Also, air-abrasion (Al 2 O 3 and silica-modified Al 2 O 3 ) significantly increased crown
retention when Bis-GMA based resin cement was used. The mean value of retention
increased from 0.72 MPa (control) to 2.40 MPa (Al 2 O 3 ) and to 3.7 MPa (silica-modified
Al 2 O 3 ). The reason for higher bond strength in a study group treated with silicamodified Al 2 O 3 could be the formation of tribochemically. Tribochemistry means the
creation of a chemical bond by the use of mechanical energy. This energy can be supplied
by rubbing, grinding or blasting. The silica particles will penetrate up to 15 micron into
the surface of the ceramics and thus, create a very high surface energy, which creates
bond with the primer. Currently, there is no study showing the permanent penetration of
silica particles into the surface of porcelain. The occlusal forces and repetitive loading of
the crown in the oral cavity might cause de-bonding if the particles dislodge from the
surface. There was no significant difference between adhesive phosphate monomer based
resin cements (Panavia F2.0 and RelyXTM Unicem 2).
Crown retention in current studies had a lower range of retention, when compared
to adhesive resin cement data published in previous studies (Table 24). Different zirconia
67
brand, treatment of zirconia crown intaglio surface with different size of Al 2 O 3 grain,
aging conditions, and degrees of convergence, surface area measurement, and cement
type are different from previous studies. In the current study, fatigue loading was
performed under 70N load, higher than Cai et al. (230) and Shahin and Kern (189)
studies. Although, the conditions were different from our study, but still Ernst et al. (231)
crown retention range (1.3 to 4.1MPa) was comparable to this investigation. In addition,
the current data for Rely X Unicem 2 and Panavia F2.0 in control groups (3.34 and 3.33
MPa) were comparable to Johnson et al data for Rely X Unicem and Panavia F2.0 (3.6
MPa and 2.6 MPa). (232)
Table 24: Published previous cement retention studies
Ernst et al 2005
5 degree convergence
7 days in water
T 5000
Compolute
Non-treated
110 μ Al 2 0 3
Silica-Modified
Al 2 0 3 110 μ
Silane
Palacios et al
20degree convergence
T 5000
Ernst et al 2009
10degree convergence
Al2O3 50 μ
Immediately
1year in water
Al2O3 110 μ
Al2O3 110 μ
Shahin and Kern
12degree convergence
3 days in water
150 days in water
T/ 3750
L 50N/300,000
Johnson et al
20degree convergence
Thermo 5000 cycle
RelyX
Unicem
6.7
Panavia F 2.0
Panavia F
2.0
4.0
-
Rely X
Unicem
4.9
-
Multilink/
Monobond
S
RelyX
Unicem
Aplicap
RelyX Unicem
Aplicap/Rocatec Plus
5.4
7.5
7.2
Panavia F 2.0
6.9
Multilink/Metall
primer
2.1
5.3
Panavia 21
Non-treated
5.6
Al2O3 50 μ
Non-treated
7.2
4.8
Al2O3 50 μ
5.2
Non-treated
Cai et al
20degree convergence
20N Load 100,000
cycle
Thermo 10,000
1.7
3.0
Superbond C/B
monomer
4.8
8.1
50 μ Al 2 0 3
50 μ Al 2 0 3
Silica-Modified
Al 2 0 3 30 μ
Panavia F 2.0
RelyX Unicem
2.6
3.6
Exp.Cement
Exp.Cement/Sila
ne
Exp.Cemen
t/Silane
Multilink
Automix
Panavia F2.0
5.4
-
6.5
-
5.1
3.4
-
3.8
-
68
In studies by Palacios et al (235) and Ernst et al (231, 233, 234) a lower 5 and 10
degree of total occlusal convergence were compared to 20 degrees in the present study.
Larger Al 2 O 3 (110μ) particles in sandblasting will create larger surface area for bonding
of the primer to the crown. The lower angle of draw may increase the retention resistance
to crown removal regardless of the type of cement. In the past study, Wilson attempted to
investigate the relationship between preparation TOC and crown retention using brass
dies. He found an inverse relation between the TOC and crown retention; and also
noticed a significant difference in the retention of crowns with different convergence
angles, but not a complete inverse relationship. The highest retention did not occur at 0o
convergence. The peak retention was between 6o and 12o. Thus, where TOC was greater
than 12o, the rate of retention is lost at approximately 0.25MPa per degree. (237) In our
study TOC was measured and the mean value ranged from 26.61 o to 28.97 o with large
standard deviation ranges, which may be due to using the natural teeth.
In the current study, although there was no difference in surface area, axial wall
height and TOC of the preparations between tested groups, standard deviations were
rather high for crown retention and by looking at previous studies, most data show rather
high standard deviation. The greater preparation total occlusal convergence may better
assess the contribution of the cement and surface treatment to retention of the crowns.
(238, 239)
In our study, the occlusal and axial walls were measured as surface area since they
were prepared with medium grit diamond bur and had similar roughness. There were no
significant differences noticed between all the groups. All the preparations were done
69
using a laboratory diamond bur (medium grit, 100 micron), which were changed every
two preparations. A study by Luque showed that there was no significant difference
between the bond strength of self etch-adhesive luting agent in relation to sclerotic
dentin. (240).
Physical properties of the cement may influence the retentive quality of adhesive
cementation. Our lower range data are in agreement with many other studies on luting to
zirconia ceramic. (153, 241-243) The decrease in the retention can be explained by
material fatigue as a result of micro-leakage, changes in the elastic modulus, and plastic
deformation over the time under thermal cycling and mechanical loading. (244, 245) The
new resin luting agents with self-adhesive capability are prepared to dispense in an acidic
state, with a very low pH well below the neutral level of 7 (approximately pH 2). This
low pH allows demineralization and penetration into the tooth. Reactions in the oral
environment cause the pH levels to increase as these materials polymerize. In most cases,
however, these cements do not reach neutral pH. Neutralization allows the luting agent to
become more hydrophobic, a prerequisite to remaining intact in a moist environment.
(246) Many other studies compared the conventional luting agent with zinc phosphate
and glass ionomer cements, and the results showed that conventional resin luting agents
have provided the greatest bonding ability for indirect restorations, mostly when required
on over-tapered preparations when compared to. (247) The self-etching/self-adhesive
luting agents simplify the cementation technique and have the potential to decrease postoperative sensitivity and technique sensitivity. (185,248)
The over simplification of new luting agent is very attractive to practitioner
because of a decreased number of steps and a wide range of applications of use. With the
70
simplified application procedure, there is also a decrease in the technique-sensitivity of
the procedure.1 Other traditional cements such Bis-GMA require an additional etch and
rinse step, compared to self-etching/self-adhesive luting agents, which contain the
necessary chemical components all in one product. According to a study by Van
Meerbeek, the newer resin luting cement do not remove the smear layer, but incorporate
it into the cement, as compared to having the smear layer completely removed and rinsed
away. This mechanism decreases patient discomfort because the smear layer is
incorporated in the dentin/resin hybrid layer, thus not exposing dentinal tubules. (185,
248)
Controversial to some of the studies and our study, total etch rinse and bonding
system which mostly remove the smear layer and open the dental tubules to create more
surface area for bonding show less crown retention values. Removal of the smear layer
results in increased dentin permeability and wetness (249, 250)
Studies based on tooth structure show that dentinal bonding may be influenced by
dentinal depth preparation, tubules orientation, and age of tooth and proximity of dentin
to pulp tissue. It is very important to control dentin depth because the adhesive strength is
1.6 to 10.7 times greater in the superficial layer than in the deep layer. (251) Another
study, defined deep dentin as 0.2-0.9 mm of residual dentin and superficial dentin as 1.42.1 mm of residual dentin. A more recent study, confirmed that bond strengths are
generally lower in deeper bovine dentin than superficial bovine dentin. Bond strength is
also higher in superficial human dentin than in deep human dentin. (252, 253,254,255)
Another important dentin bonding factor is the age of the teeth. The ISO 66
recommended use of third permanent molars from 16- to 40-year-old humans if possible
71
and bovine teeth should be mandibular permanent incisors of cattle not more than 5 years
old. Since the age of collected extracted human teeth is usually not known, it is
reassuring that one study showed that the bond strengths in proximal dentin of young
(age 9-21 years) and old (age 42-64 years) patients were similar. (257)
Also, other factors, such as bonding location (i.e. surface, orientation of dentinal
tubules), and mineralized versus demineralized dentin, have been investigated with
respect to others. These factors displayed a high variation observed in bond strength
measurements. Total or partial destruction of the tubules and inter-tubular dentin in
hypermineralized dentin resulted in lower bond strength than to normal dentin. (258)
Controlling factors in which the bonding of resin to tooth structure is effected
cannot be completely controlled, due to using natural non-caries third molars. In order to
control these variables, an in vitro study with prefabricated prepared teeth can be used
and long-term clinical studies will be needed to investigate these factors.
Chewing cycles generate continuous loads in the presence of water and can lead
materials to fatigue and degrade. The reduction of the mechanical strength due to fatigue
is caused by the propagation of natural cracks initially present in the component’s
microstructure (Studart et al. 2007a). In addition, cements like Pavania F and Pavania 21
were found to have higher bond strength value after thermocycling (Lüthy et al. 2006)
Aging of the specimen has been shown to possibly have a significant effect on bond
strength to dentin. (262, 263)
Shahin and Kern (189) are the only investigation where zirconia crown retention
was evaluated for treated and non-treated zirconia intaglio surfaces cemented with
adhesive resin cement. Their data for adhesive phosphate monomer based resin cement
72
(Panavia 21) was comparable with our data for adhesive phosphate monomer based resin
cement (Panavia F2.0 and RelyX Unicem 2). In either study, there was no significant
difference between crown retention of treated and non-treated groups after artificial
aging. In the current study, although surface treatment significantly increased retention of
zirconia coping with Duo-linkTM cement, overall Duo-linkTM cement showed
significantly lower crown retention than the other cements. This might be explained by
cement composition. Duo-link is Bis-GMA based cement. Shear bond strength of BisGMA based resin cement to zirconia has been reported lower than adhesive phosphate
monomer based resin cements. (124, 180, 241, 246) That might be the reason that 27%
failure mode of Duo-Link group is category 1 (>50% cement on preparation) while
category 1 failure mode is 7% and 13% for Rely X Unicem 2 and Panavia F2.0,
respectively.
Crown retention in Duo-Link group was significantly improved when
combination of surface treatment and MDP primer (Z-prime plus) were used. MDP
primer has the ability to bond to zirconia and silica. Studies support that Bis-GMA resin
cement bond strength increases when zirconia surface is treated with Al 2 O 3 or silicamodified Al 2 O 3 and MDP or MPS primers. (124, 180, 241, 246)
The other factor, which could influence the retention, is the excess cement around
the margin of the crowns. As mentioned in methods and materials, the excess cement was
removed using a micro-brush. The study showed excess cement pass the margins of the
gelatin capsule or "flash" could increase bond strength than if the material was contained
within the specified area. (265, 266) in our study, natural third molar teeth were used and
they all are different anatomically, therefore, the thickness of margin was requested to be
milled 1mm regardless of actual preparation margin width, due to control the surface area
73
variation. In order to control this factor, either prefabricated model should used or the
margin should be taken of the equation, which it would be very difficult to ignore.
Another factor that may also influence the retention is the relative adaptation to
the prepared tooth surface. (224) Following our request, the manufacturer created a 50μ
cement space, making the coping fit passively on the preparation. Replica technique
results showed a good internal fit (84.02-105.05 μ) but higher than 50μ. The mean values
of replica technique in the literature, values of 49 to 136 μ have been reported for the
internal fit of all ceramic restorations. (220, 267) In this study, the mean value for the
thickness of XLV VPS impression material was recorded from 84.02 μ (Bis-GMA
control) to 105.05 μ (Phosphate monomer treated with silica-modified Al 2 O 3 ). There was
a significant difference in thickness of XLV impression material in RelyXTM Unicem 2
group and other study groups. In crowns treated with silica-modified Al 2 O 3 , the highest
mean value was found in RelyXTM Unicem 2 group and lowest in RelyXTM Unicem 2
control group. There have been many studies regarding crown fitness and thickness of the
cement layer between the substrate and dental prosthesis. In the latest version of
CAD/CAM systems such as CEREC® 3, smaller marginal gaps in the range of 53-67 μm
for crowns have been improved. (272) However, one study reported that the marginal gap
of CEREC® 3 was between 75 and 102 μm when resin composite was used. (273) All
these different cement thickness are far from what is theoretically said to be ideal, which
should be between 25 and 40 μm (Christensen 1971) As Beuer et al. reported, zirconia
crown framework marginal opening ranged between 36.6 to 45.5 μm; but, the internal
gaps were still large (50-75 μm for axial walls and 74-100 μm for occlusal gaps). (275)
Tooth and all materials used in restorations have certain elasticity; their strain
74
deformation will reduce stress or at least its perpendicular component. So, as the space
between the substrate and crown walls decreases, the stress decreases as well. As a result
of a thinner luting layer the less stress will be created. As mentioned above, the luting
space should be kept to a minimum to improve the fit of the restoration, expose a
minimum of luting material to oral fluids, and minimize any polymerization contraction
stress. There is no agreement on this minimum, but a 50–100-μm range seems
convenient. (279,280) Other reports show the film thickness of a number of luting
materials that they can range from 152 μm to 10 μm, depending on the material
tested.(281-284) This can be acceptable (<50 μm) at the chamfer, the occlusal reduction,
or the vertical walls, but it can be as high as 200 μm for some dentin-bonding agents at
the dentin line angles between the chamfer and vertical walls.(285) In our study,
however, the thickness of XLV layer (space for cement in Due-Link control) has the
lowest mean value and shows the lowest bonding retention value. Also, phosphate
monomer based resin cement (RelyX unicem2 treated silica modified-Al 2 O 3 ) has the
largest mean value of XLV thickness and shows high bonding retention. The result of our
study is somewhat controversial to previous studies, possibly due to different cement’s
flow rate, sizes of the filler, and also force in which the replica technique was performed.
In previous zirconia crown retention studies, the fitness of zirconia crowns were
not measured, thus the current study data cannot be discussed in this regard. Data from
this current study shows the internal fit of crowns was not significantly different between
the tested groups. It is assumed that the internal fit of copings did not influence the
coping retention.
Failure mode is categorized into three different groups, which are adhesive to
75
preparation (AP), adhesive to crown (AC) and cohesion (CC, 50%-50% AP and AC). In
this study, data shows failure mode of AC (84.44%), followed by cement remaining
mainly on the preparation (8.89%). When RelyXTM Unicem 2 and Panavia F2.0 were
used as cement, surface treatment created 100% of the cement to stay in the crown
(except for PAN- Al 2 O 3 which was 90%). Overall, the predominant mode of failure was
adhesion to crowns, where cement was found principally adhere to intaglio of crowns,
but on the Due-link cement showing more CC failure. Panavia F2.0 CC and AC failure
mode were almost the same; and in RelyXTM Unicem 2, a majority of failure mode was
CC. In studies by Blatz and others, the failure modes were observed to be completely
adhesive at the ceramic surface after water storage and thermal cycling which it agrees
with our observation in this study. As discussed, many variables affect bonding to dentin.
Many studies have discussed different steps in which errors can occur. A study conducted
by Ferrari et al (2005) stated that the use of 2 and 3 steps adhesives with prior etching,
and, self-etching adhesives (2 steps) resulted in an interface in dentin with the formation
of blisters possibly due to residues from solvents retained the interface. However, the
study by Van Landuyt showed that single-step self-etching adhesives are more favorable
to the formation of these droplets due to phase separation and insufficient evaporation of
the solvent (Van Landuyt et al., 2005). Another factor, which could affect the weaker
adhesion to tooth structure, is using air jet to improve the evaporation of solvent and
water, thereby reducing the thickness of adhesive layer and becoming more uniform, but
in our study using the air-jet in Bis-GMA group to gently dry the tooth might cause
collapsing of dental tubules. The time of air-jet application observed according to the
manufacture’s instructions varies between 5 and 10 seconds. The solvent remainder in the
76
adhesive can compromise its polymerization due to dilution of monomers, resulting in
permeability of the adhesive interface (Hashimoto et al., 2004; Cho & Dickens). Also,
the effect of time of application of the adhesive in bond strength was also evaluated in a
study by (Reis et al., 2003). A study by Toledano et al. evidenced that by increasing the
time of application, the bond strength was improved when simplified adhesive systems
were used. In the same way, the application of more than one adhesive layer contributes
for a more effective adhesion, especially in dentin (Ito et al., 2005b; Albuquerque et al.,
2008). As the solvent is evaporated to each adhesive application, the co-monomer
concentration increases improving the quality of the hybrid layer and the correlation of
adhesive layer cured versus non-cured due the oxygen inhibition (Kim et al., 2006). In
review of the results of our study, in all phosphate monomer based resin cement groups,
the failure mode was almost all AC in surface treated groups as oppose to control groups,
which there were some AP and CC failure mode. The possible rational for the
observation could be that stronger bonding occurs between the adhesive and crown after
surface treatments compared to the control groups, which did not have surface treatments.
77
CONCLUSION
Within the limitations of this in vitro study, the following conclusions can be
drawn:
1. Y-TZP crown retention is dependent to adhesive cement type. Bis-GMA (Duo-linkTM)
cement in this study had significantly lower crown retention compared to adhesive
phosphate monomer based resin cement (Panavia F2.0 and RelyXTM Unicem 2).
2. Surface treatment of crown improved the Y-TZP crown retention of Bis-GMA cement
(Duo-Link), but adhesive phosphate monomer based resin cements (Panavia F2.0 and
Rely X Unicem2) retention was not influenced by surface treatment.
3. Surface treatment of the Y-TZP crown (primer and or sandblasting) did not improve
bonding strength for crowns cemented with either adhesive phosphate monomer based
resin cement.
4. Comparing Al 2 O 3 and silica-modified Al 2 O 3 air abrasions, the type of air abrasion did
influence crown retention in Bis-GMA group with the higher retention in silica-modified
Al 2 O 3 group.
5. CAD/CAM design restoration will provide an adequate internal fit.
78
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