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Bond strength to different types of lithium disilicate reinforced ceramic materials
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Content
Bond Strength to Different Types of Lithium Disilicate Reinforced Ceramic Materials
by
Mona Alhomuod, BDS
A Thesis Presented to the
FACULTY OF THE USC HERMAN OSTROW SCHOOL OF DENTISTRY UNIVERSITY OF
SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirement for the Degree
MASTER OF SCIENCE
BIOMATERIALS AND DIGITAL DENTISTRY
December 2021
Copyright 2021 Mona Alhomuod
ii
Dedication
I’d like to dedicate my work to my parents, my husband, and my daughter. Without
whom none of my success would be possible.
iii
Acknowledgements
First, I would like to thank Allah for providing me with health and wellness to complete
my studies and attain this academic degree.
With much appreciation, I acknowledge members of the Committee for all their efforts and
their constant motivation throughout my journey at USC.
A special appreciation is to my supervisor and the committee chairman Dr. Jin-Ho Phark,
who read and revised my manuscript several times. With all the love and gratitude, thank you, Dr.
Phark, for your unlimited time, your fast response, and the constant support you offered to me.
Without you, this project would not have been possible.
Dr. Phark, you have been a great inspiration for me, not only scientifically but also in my
whole life. I learned tremendously from your excellent way of thinking, your dedication to work,
your energy, and your great love for your family, which showed us that family comes first, and
they are a privilege for success. Probably the most essential thing I have been taught was to see
the actual value of research. You kept pushing and supporting me to be a better researcher in
addition to how to look at things accurately, how to be more organized, how to analyze things
critically, and how to deduce and plan and not to accept any excuse or any reason to take failure
at work. I've learned so much from you, thank you.
Dr. Phark, you were one of the great motivators who brought out the best in me. You know
how I started this project and how I ended it!!. You have been a great leader who helped me reach
this stage. Simply, you were behind all this success.
Finally, Dr. Phark, please forgive me if I felt depressed or failed to work correctly one day.
I did not mean to skip the work or to look stubborn, but what we went through was not easy at all.
I hope I made it to become a good student.
iv
Dear Dr. Phark, I had the honor of working with you, and it was a beautiful scientific
experience. Thanking you can never fully express my gratitude to you.
Also, I would like to give Dr. Sillas Duarte, my mentor, co-advisor and the “FATHER of
ENAMEL”, very special thanks with gratitude for teaching me everything I have accomplished so
far, taking me under his wing, and giving me the opportunity to try everything that will help me in
my future career.
Dr. Duarte, thank you so much for giving me the opportunity in the first place, for helping
me to do this dual program.
Dr. Duarte, I encountered some frustrations, and you were always there for me because
you believed in me more than I do! You saw that I have potential, and since then, you have been
great support and inspiration for me to go beyond my ability and always do my best and try to be
the best!
Dr. Duarte, all the words will not express my gratitude toward you. Thank you for your
faith, your constant encouragement, and for helping me become a better clinician.
Dear Dr. Duarte,
I had the honor of being one of your students. I'll always be Dr. Duarte's student. Learning
from you does not only contribute to my academic performance but also influenced my personal
life. Thank you Dr. Duarte from the bottom of my heart.
Additionally, I should thank Dr. Alena Knezevic for her generous heart and support.
Dear Dr. Knezevic, I've learned from you the dedication in work with enjoying the process.
You are an example of the successful, strong, responsible woman that I'm looking forward to. Your
v
achievements inspired me personally. Thank you for all your instructions, for sharing information,
and for your love for spreading science.
I also thank my parents for their support, love, and constant prayers during all my life. They
have never hesitated to offer everything I need in order to succeed and fulfill my ambitions.
My beloved mom "Sharifa Hassen," you are my shield and my strength in this difficult life,
who enlightened my way and flooded me with love. So, I dedicate this work to you.
My lovely dad, "Ali Alhomuod," as always!: "Who is like my father!? I will always be
your spoiled girl, and you'll be my first love. You stood with me in all the moments and gave me
love and safety. You have been very keen to see my success, and I hope I could achieve your
satisfaction. Now, I'm here to fulfill your dream that you saw 11 years ago! Finally, your vision is
a fact today. I love you, Dad.
I should express my gratitude with special appreciation to my amazing man, Dr.
Mohammed Tarrosh, for standing by my side, hand in hand, to reach where I am now and always
believing in me and loving me. Dear husband, I will never forget your great sacrifice for me, your
constant faith in me, and your continued encouragement. You have been the source of pride and
strength. Thank you so much. Dear Dr. Tarrosh, I'm here because you're with me.
To my daughter, Lama Tarrosh, or as I used to call you, "my perfect daughter. You have
been with me from the beginning of this journey, being sweet when life was sour, the source of
healing whenever life was painful, the laughter when life used to disturb my mood. You have been
my best friend when I needed one. My beloved daughter, Lama, how happy and lucky I am to have
you. I wish you would forgive me for being frequently away from you and busy. I love you.
vi
Additionally, I would like to thank all my brothers and sisters despite their far geographical
distance, but their love has always been very near me. They are the hidden magic secret behind
my strength. They have been my best supporters. I love you all.
To all my in-laws, your love and constant support are much appreciated. Thank you for your
patience and dedication
I would like to thank all the doctors at USC, especially Dr. Eddie Sheh and Dr. Jenny Son.
Thank you for teaching me and encouraging me to be a better dentist. I have learned a lot from
both of you. Your humble dedication to work was a great inspiration for me. Thank you for all
your advice and guidance.
I cannot forget all my former doctors and teachers, whom I thank for all their generous
efforts and offered support.
To Dr. Neimar Sartori, I met you during a hard time in my life, but you didn't judge me
through it. Perhaps you were the first to believe in me, the first to see that I have potential. You
constantly challenge me to give my best because you knew my capabilities. You did not fail with
me in teaching everything that benefited me. I insisted on learning 'digital wax-up.' When you saw
my commitment, you went for it and made sure that I did my best. Thanks, from the bottom of my
heart, for all your efforts. Thank you for being an example to follow. Thank you for being kind.
Thank you for being you.
To Dr. Abdullah Meshni, the beginning of my journey as a young clinician was with you. You
believed in me and encouraged me a lot. You were the best teacher and the best doctor. I'll never
forget your great favor to me. Thank you
vii
To Mrs. Karen Guillen, thank you for all your help and kindness. Thank you for being
patient and taking care of everything.
To all my co-residents and fellows that I have encountered through this program, thank
you.
Special thanks to my people! Talal Alshawaf, Erin Anderson, and Reema Gore, it was a
long and tough journey, but being together made things easier and doable. Thank you.
To Hanadi Abu Sharifa and Sarah Alsaleh: you were the good side of being abroad, and
more than a friend, we bonded well together! Thank you for your infinite support and assistance;
thank you for being part of my life. I also thank the program that brought us together.
I am lucky to have this golden chance to meet those exceptionally supportive people beside
me. This program is a remarkable learning experience and a memorable training stage that helped
me become a better dentist and also a better person. I am grateful to all those who helped make it
happen.
viii
Table of Contents
Dedication ........................................................................................................................... ii
Acknowledgements .......................................................................................................... iii
List of Tables ..................................................................................................................... ix
List of Figures .................................................................................................................... x
Abbreviations .................................................................................................................... xi
Abstract ............................................................................................................................ xii
Introduction ....................................................................................................................... 1
CAD CAM Classification ........................................................................................ 2
Glass-Matrix Ceramics ............................................................................................ 3
Two Stage-Process .................................................................................................. 5
Three Stage-Process ................................................................................................ 6
General Concept of Bonding for Indirect Restorative Material .............................. 7
Bonding to Silica-Based Glass Ceramics/Surface Treatment ................................. 9
Objectives and Specific Aims ............................................................................... 11
Materials and Method ................................................................................................... 112
Study Design ....................................................................................................... 112
Sample Preparation ................................................................................................ 13
Surface Treatment ................................................................................................. 17
Composite Cylinders ............................................................................................. 17
Bonding Protocol ................................................................................................... 19
Artificial Aging ..................................................................................................... 21
Failure Mode Analysis .......................................................................................... 22
Statistical Analysis ................................................................................................ 24
Scanning Electron Microscopy (SEM) .................................................................. 24
Results ............................................................................................................................... 25
Notched-Edge Shear Bond Strength (SBS) Analysis ............................................ 25
Failure Analysis ..................................................................................................... 29
SEM Ultra-Structural Analysis ............................................................................ 334
Discussion ......................................................................................................................... 36
Conclusion ........................................................................................................................ 48
Disclaimer ......................................................................................................................... 49
Funding ............................................................................................................................. 50
References ........................................................................................................................ 51
ix
List of Tables
Table 1: Comparison between material composition and physical properties ............................. 14
Table 2: Group assignment ........................................................................................................... 15
Table 3: Firing parameters for IPS e.max CAD crystallization in programat CS3 furnace ......... 16
Table 4: Firing parameters for Amber Mill crystallization in programat CS3 furnace ................ 16
Table 5: Bonding materials ........................................................................................................... 20
Table 6: Overall shear bond strength (SBS) ................................................................................. 25
Table 7: SBS values and standard deviation (SD) in MPa ........................................................... 26
Table 8: Specific failure analysis .................................................................................................. 32
x
List of Figures
Figure 1: Lithium disilicate reinforced ceramics materials ........................................................ 112
Figure 2: Teflon mold for composite cylinder fabrication ........................................................... 18
Figure 3: Bonding side vs. nonbonding side for composite cylinder ........................................... 18
Figure 4: Position of the light curing units during bonding ......................................................... 19
Figure 5: Thermocycler ................................................................................................................ 21
Figure 6: Difference in fluorescence between composite (C) and resin cement (RC) ................. 23
Figure 7: Examples of different failure modes pattern ................................................................. 24
Figure 8: Comparison of mean notched edge SBS in MPa between all the material ................... 27
Figure 9: Comparison between non-aged groups ......................................................................... 28
Figure 10: Comparison between aged groups .............................................................................. 28
Figure 11: General macroscopic analysis ..................................................................................... 29
Figure 12: Amber Mill compared to IPS e.max CAD material .................................................... 29
Figure 13: CEREC Tessera compared to n!ce .............................................................................. 30
Figure 14: Overall specific failure analysis .................................................................................. 31
Figure 15: Specific failure analysis for non-aged groups ............................................................. 32
Figure 16: Specific failure analysis for the aged groups .............................................................. 33
Figure 17: SEM analysis for different types of LDC ................................................................... 35
xi
Abbreviations
A: Aged
NA: Non-aged
SA: Silane + Adhesive
NS: Adhesive only
HEMA: 2-Hydroxyethyl Methacrylate
SBS: Shear bond strength
SEM: Scanning electron microscopy
SBUP: Scotchbond Universal Plus
SBU: Scotchbond Universal
10-MDP: 10-Methacryloyloxydecyl dihydrogen phosphate
HF: Hydrofluoric acid
LDC: Lithium disilicate glass ceramic
ISO: International Organization for Standardization
xii
Abstract
Title: Bond Strength to Different Types of Lithium Disilicate Reinforced Ceramics
Objective: Evaluate the influence of applying universal adhesive only versus universal adhesive
with silane on shear bond strength (SBS) to four types of lithium disilicate reinforced glass ceramic
materials (LDC) using universal resin cement.
Material and methods: Specimens (n=240, 1.5 mm thick) cut from 4 different LDC CAD/CAM
materials (IPS e.max CAD [EX, Ivoclar Vivadent, Schaan, Liechtenstein], Amber Mill [AM,
Hassbio, Kangreung, Korea], n!ce [NC, Straumann, Freiburg, Germany], CEREC Tessera [TS,
Dentsply Sirona, York, PA.USA]) were polished (600 grit). EX and AM were sintered according
to manufacturers’ instructions; NC and TS were not required to be sintered. For bonding,
specimens were etched with HF acid, then either universal adhesive only (NS) or silane + universal
adhesive (SA) was applied, and prefabricated composite cylinders (IPS Empress Direct) were
cemented by applying a dual-cure universal resin cement (RelyX Universal Resin Cement, 3M,
St. Paul, MN, USA). SBS testing was performed after either 24 h non-aged (NA) or after of 20,000
cycles thermocycling + 2 months of water storage (aged). Surfaces were analyzed with ZEISS
stereomicroscope (Carl Zeiss Meditec, Jena, Germany) for failure mode and with the scanning
electron microscope (FEI Nova NanoSEM, Thermo Fisher Scientific) for ultra-structures. Data
were statistically analyzed by applying non-parametric tests with ⍺=0.001.
Result: SBS values for non-aged specimens ranged from 29.08 (TS-SA) to 17.87 (AM-NS) MPa
and for aged specimens from 22.24 (NC-SA) 3.01 (AM-NS) MPa. SBS was significantly reduced
when silane was omitted and after aging (p<0.001). For aged, EX and AM values were
significantly lower when silane was omitted compared to all other materials. Failure mode was
xiii
mostly adhesive at CAD/CAM surface with some cohesive failures in the LDC, especially in
groups that were not required to be sintered (NC, TS).
Conclusion: SBS to LDC differs between different CAD/CAM materials and decreases over time.
Using a silane containing universal adhesive alone is not sufficient. Silane application as a separate
step is advised when using a universal resin cement in combination with a universal adhesive.
Clinical Significance: Using silane separately is advised before bonding to LDC, apart from the
existence of silane within the universal adhesive.
Keywords: Silane, Universal adhesive, Lithium disilicate glass ceramic, SBS, Universal cement,
CAD-CAM.
1
Introduction
During the past 30 years, there have been significant advancements in indirect dental
restorative technology and materials. This is due to the rapid progress in computer technology and
its combination with dental restorative technologies, for instance: 3D printing and computer--aided
design and manufacturing systems (CAD/CAM).
(1)
Moreover, these advancements are also
attributed to the restorative materials themselves, which showed progressive improvement in their
strength, esthetics, and methods of fabrication. Consequently, many different products became
available for clinicians to choose from.
(1)
Nevertheless, due to the rapidly growing number of
products being introduced nowadays, clinicians frequently go through complex decision-making
processes to choose a ceramic material for a certain indication.
(2)
Their choice is rarely based upon
detailed awareness of the chosen material’s characteristics. Usually, their decision becomes based
on certain criteria, e.g., in vitro strength, translucency, the technique of manufacturing, known
laboratory technicians, or advertising claims.
(2)
CAD/CAM technology is being used in the manufacturing process of indirect dental
restorations (e.g., crowns, veneers, fixed partial dentures, inlays, onlays or dental implants).
(3)
This
technology allows scanning of the preparation, designing and milling the restoration, and delivery
with a short turn-around time, even within the course of only one day. There are chairside systems
that don’t require any further processing steps after milling and labside systems that further require
additional lab steps, e.g., crystallization through sintering.
(4)
Currently, clinicians and lab
technicians can select from a broad variety of materials to match the required properties for each
patient (e.g., high aesthetics, biocompatibility, durability and functionality).
(4)
2
CAD CAM Classification
Classification systems for dental ceramic materials are valuable for several reasons, e.g.,
communication or education. Classification systems should provide information that is clinically
relevant regarding the location of use (anterior vs. posterior), the purpose of restoration (partial vs.
full, or short vs. long span), and method of use (adhesively vs. traditionally).
(2)
Several systems of
classification have been suggested, based on clinical indication, composition, the possibility of
etching, processing method, firing temperature, translucency, resistance to fracture, micro-
structure, or antagonist wear.
(5)
Nevertheless, most of these classification systems are generally
vague, inaccurate, or do not permit the addition of new materials.
(2)
The frequently used Kelly and Benetti
(6)
classification system of Kelly and Benetti
(6)
described ceramic materials according to the glass contents: (1) mostly glassy materials, (2)
particle-filled glasses, and (3) polycrystalline ceramics, in which no glass is present. In this glass
content classification system, clinicians may become confused by the low clarity in quantifying
the glass phase content that is necessary for the ceramic to be included in either the largely glassy,
or the particle-filled glasses category.
(2)
Moreover, this classification does not identify basic
improvements in ceramic technology that has been achieved: The manufacturing processes of
these materials have changed from naturally occurring components (e.g., feldspar) to synthetically-
derived ceramics.
(2)
This led to an enhancement in the standardization and quality control of these
materials. Finally, yet importantly, the current ceramic materials classifications do not involve
resin-matrix materials, which are greatly filled with ceramics. These are currently provided by
several manufacturers and are suggested as esthetic substitutes for several clinical indications.
(2)
3
In a more recent classification, Stefano Gracis et al.
(2)
categorized ceramic restorative
materials to 3 families: (1) glass-matrix ceramics, (2) polycrystalline ceramics, and (3) resin-
matrix ceramics (Error! Reference source not found.).
(2)
Subfamilies were illustrated in each
group, along with their composition, permitting recently developed materials to be placed into
existing principal families. The criteria applied to distinguish ceramic materials are based on the
phase of their chemical structure.
(2)
Therefore, an all-ceramic material is classified according to
the presence of a glass-matrix phase (glass-matrix ceramics) or its absence (polycrystalline
ceramics), or whether the material comprises an organic matrix highly filled with ceramic particles
(resin-matrix ceramics).
(2)
Glass-Matrix Ceramics
The glass matrix consists mostly of silicon dioxide (also called silica or quartz) with
different amounts of alumina.
(7)
The purely glass-based ceramic systems are very esthetic due to
their high translucency, however; they have low mechanical properties. For the purpose of
increasing the glass matrix strength, natural or synthetic crystals can be dispersed in it,
(2)
a process
called “dispersion strengthening", since it is more difficult for dispersed crystals to be penetrated
by cracks. Hence, crack propagation into the matrix may be stopped or slowed down.
(8)
Natural leucite crystals derived from potassium feldspar is present in several materials,
e.g., IPS Empress Esthetic, IPS Empress CAD, IPS Classic (Ivocar Vivadent, Schaan,
Liechtenstein), Vitadur, Vita VMK 68, and Vitablocs (VITA Zahnfabrik, Bad Säckingen,
Germany).
(2)
Synthetic crystals consist of other leucite-based crystals, e.g., IPS d.Sing (Ivoclar
Vivadent, Schaan, Liechtenstein), VM7, VM9, VM13 (VITA Zahnfabrik), Noritake EX-3,
Cerabien, Cerabien ZR (Kuraray Noritake Okayama, Japan); lithium disilicate (IPS e.max Press
or CAD, Ivoclar Vivadent, Schaan, Liechtenstein) or lithium-silicate crystals (VITA Suprinity,
4
VITA Zahnfabrik, Schaan, Liechtenstein; Celtra Duo, Dentsply Sirona; York, PA, USA); and
fluorapatite-based crystals (IPS e.max Zir-Press, Ivoclar Vivadent, Schaan, Liechtenstein).
(2)
A new class of materials that contain glass-ceramic with an inter-penetrating resin matrix,
also often described as hybrid ceramic, has been recently introduced to the market.
(2)
Currently,
the only hybrid ceramic material available is VITA Enamic (VITA Zahnfabrik, Bad Säckingen,
Germany). It is comprised of a dual network with two parts, a feldspathic ceramic part (86% by
weight and 75% by volume) and a polymer part (14% by weight and 25% by volume), consisting
of urethane dimethacrylate (UDMA) and triethylene glycol dimethacrylate (TEGDMA).
(9)
Lithium disilicate reinforced glass ceramic (LDC) was first produced by Stookey at
Corning Glass Works in the 1950s.
(10)
These ceramics are obtained from the SiO2–Li2O system.
This system involves up to 70% of fine rod-like Li2Si2O5 crystals, combined with less amount of
lithium orthophosphate (Li3PO4) crystals.
(10)
Together, these crystals disperse in a random but
uniform distribution to make up the unique glassy matrix.
(10)
Compared with leucite glass ceramics, the crystallization mechanism is a volume
crystallization, as crystals nucleate and grow throughout the glass.
(10)
Crystallization of LDC is
heterogeneous and is accomplished via 2 different processing routes, i.e., a 2-stage or 3-stage
process, based on whether the glass ceramic is to be used as a pressable ingot for the lost wax hot-
pressing technique or machinable block for CAD-CAM milling.
(10)
5
Two Stage-Process
When using the technique of "lost wax hot pressing" to manufacture LDC restorations, a
2-stage crystallization process is used.
(10)
In the 1
st
stage the manufacturer fabricates a glass ingot
that contains nuclei, which develop during the steps of cooling or pre-heating. These nuclei
crystallize to Li2O5Si2 in a heating treatment at 750–850°C for about two hrs.
(10)
In the 2
nd
stage,
which is conducted in the dental lab, the crystallized ingot becomes hot-pressed at 920 ° C to flow
viscously to the dental mold, which is made by the lost wax technique, to create the needed
restoration, to be kept at this high temperature for 5 to 15 minutes.
(10)
This last hot-pressing step
gives 3-6 μm-long needle-like crystals of Li2O5Si2 in a volume fraction of about 70%, resulting in
higher biaxial fracture strength and fracture toughness of the pressable ceramic (400 MPa and 2.75
MPa.m1⁄2, respectively) compared with other glass based ceramics.
(11)
Polyvalent ions which are dissolved in the glass, are applied to deliver the preferred color
to the lithium disilicate material. These color-controlling ions are evenly distributed in the single-
phase material, thus removing any microstructure color-pigment defectiveness.
(12)
In 1988, Li2O5Si2 dental ceramics were first used as a heat-pressed core material. It was
marketed as IPS Empress 2 (Ivoclar Vivadent, Schaan, Liechtenstein).
(13)
Empress 2 was
categorized as a glass ceramic, a subgroup of particle-filled glasses, with almost 70% crystalline
Li2O5Si2 filler.
(6, 13, 14)
Applying a pressure casting procedure produced a material that had fewer
defects and further uniform crystal distribution.
(6)
Reformulation and refinement of IPS Empress
2 production process enabled the production of a new ceramic line. In 2005, the new ceramic
formulation (IPS e.max Press) was released .
(15)
Eventually, the introduction of the IPS e.max line
led four years later to the withdrawal of the IPS Empress 2 line.
(16)
6
Three Stage-Process
To manufacture restorations from machinable LDC blocks (e.g., IPS e.max CAD, Ivoclar
Vivadent, Schaan, Liechtenstein), a 3-stage crystallization process is used.
(10)
In the 1
st
stage, a
glass block becomes heated at 450 to 550°C for 5-60 minutes to provide the maximum number of
nuclei to warrant good crystal growth of metasilicate crystals. Also, this treatment can be
conducted during the cooling step as stated before.
(10)
In the 2
nd
stage, the glass block becomes
heated at a high temperature (690 to 710 ° C) for 10 to 30 minutes to form lithium metasilicate
(Li
2
SiO
3
) crystals epitaxially developed from the nano-lithium orthophosphate nuclei and then
becomes cooled down to room temperature. The intermediate metasilicate phase is present in 40%
volume fraction.
(10)
It includes evenly dispersed small platelet-shaped crystals (0.2 to 1 μm) that
enforce the glass to a flexural strength (about 130 MPa).
(10)
This blue block material is weak.
However, due to its microstructure, it can be milled to produce the desired dental part.
(10)
Like the
pressable lithium disilicate, the millable IPS e.max CAD blocks are colored with coloring ions,
but the coloring elements are in a different oxidation state during the intermediate phase than in
the fully crystallized state.
(12)
Consequently, the lithium disilicate shows a blue coloration. The
materials reach their optimal tooth color and opacity when the lithium metasilicate changes to
lithium disilicate during the post-milling firing process, which mostly happens in the 3
rd
stage.
(12)
The material becomes heated at 850 ° C for 20 to 31 minutes in a dental porcelain furnace to
precipitate the final strong lithium disilicate phase and some minor amounts of lithium
orthophosphate crystals. This sintering step is accompanied with a 0.2% shrinkage accounted in
the CAD software.
(10)
During this step, the lithium metasilicate crystals react fully with
surrounding glass silica through a solid-state reaction to provide 1.5 μm- long rod-like, interlocked
crystals of Li2O5Si2 in almost 70% volume fraction.
(10)
This offers high strength for the glass
7
ceramic (360 MPa) in addition to the high fracture toughness (2.25 MPa.m1⁄2).
(17)
These elongated
crystals, compared with the rod-like crystals, formed in the sintered CAD/CAM material, are
responsible for the somewhat greater strength and fracture toughness.
(10)
With the expiration of the LDC patent
(18)
various other companies introduced their own
derivates of the LDC. One of the examples is the product Amber Mill (AM, HassBio Kangreung,
Korea), which is a labside material that is required to be sintered. It was introduced in 2019 as a
Li2O5Si2 glass-ceramic.
(19)
The crystal structure of Amber Mill is denser and to be cross-linked,
which aids in better physical properties. The biaxial flexure strength of Amber Mill is 450MPa
after it is fully crystallized by the end-user.
(19)
Other LDC materials which are fully crystallized by the manufacturer and which are
referred to as “chairside ceramics” such as Nice (Straumann, Freiburg, Germany)
(20)
and Tessera
(Dentsply Sirona), are said to achieve esthetic and mechanical properties, comparable to lithium
disilicate.
(21)
The Nice block contains 64-70% SiO2 and 10.5-12.5% Li2O by weight with a flexural
strength of 350 ± 50 MPa and a fracture toughness >1.5 MPa m
(20)
, while Tessera CAD/CAM
block reaches up to 32% of biaxial strength.
(21)
General Concept of Bonding for Indirect Restorative Material
The main function of luting is to create a reliable retention, a durable seal of the space
between the tooth and the restoration, and to deliver sufficient optical properties. Due to solubility,
using water-based cement, e.g., glass–ionomer cement, depends mainly on a macro-retentive
preparation design and a good marginal fit. Adhesive luting materials show minor solubility and
enhanced esthetic effects. By generating a hybrid layer, resin cement can provide long-term
8
stability for non-retentive restorations.
(22)
The adhesive bonding of ceramic restorations to tooth
structures involves 2 distinctive interfaces which determine and contribute to the outcome of the
restoration, i.e., the dentin/enamel-resin interface and the ceramic-resin interface. These interfaces
should be enhanced to achieve a successful restorative treatment.
(23)
The glass based material silica content allows these materials to be etched with HF acid
before bonding, whereas the polycrystalline ceramics, (e.g., alumina and zirconia), cannot be
etched due to the lack of the glass phase.
(7)
Despite their inherent brittleness and low flexural strength, glass-ceramics gained wide
popularity with the advancement of adhesive bonding strategies. After final adhesive bonding with
resin cement, the fracture resistance of the ceramic restoration and the substrate increases.
(24, 25)
Resin-cements have multiple advantages over conventional luting cement when it comes to
bonding to glass ceramics. With resin cement, the need for a retentive preparation is reduced or
not needed at all, which allows for tooth structure preservation. Hence, the utilization of ultra-thin
glass-ceramics restorations.
(26)
. Resin cement show clinically acceptable marginal fit and
adaptation.
(27)
Also, resin cements are available in various shades for optimizing the esthetics of
the restoration.
(27)
Due to their unique microstructure, lithium disilicate reinforced glass-ceramics
have significantly more flexural strength and fracture toughness compared to those of leucite
reinforced glass- ceramics.
(28)
LDC must have an occlusal restorative thickness of 1.5 mm in order
for self-adhesive or conventional cement to be used.
(29)
9
Bonding to Silica-Based Glass Ceramics/Surface Treatment
The intaglio surfaces of silica-based glass-ceramics should be treated to achieve optimal
adhesion to resin-based cement. This can be achieved by two mechanisms:
1- Micromechanical interlocking after surface modification by mechanical or chemical
means.
(30)
2- Chemical bonding through the application of coupling agents.
(31)
For micromechanical retention, ceramic porosities are created by dissolving the amorphous
silica glass phase selectively.
(32)
The hydrofluoric acid attacks the glassy matrix and exposes the
crystalline structure, which creates an irregular microstructure on the surface
(33)
, leading to a highly
specific surface area which promotes the bonding area at the interface.
(31)
Besides its important
role in adhesion, hydrofluoric acid is a very harmful chemical to the skin and soft tissues. It is also
toxic, corrosive, and highly reactive.
(34)
However, although it is a weak acid, its toxicity is
attributed to the highly reactive fluoride ions. When hydrofluoric acid gets in touch with living
tissue, it rapidly crosses lipid membranes, and the fluoride ions become released to react with
tissues’ calcium and magnesium, leading to rapid tissue necrosis.
(35)
when it directly contacts the
skin, it causes dehydration reactions, that consequently causes 2
nd
or 3
rd
degrees burns.
(36)
Therefore, hydrofluoric acid is not suitable to be used intra-orally.
Regarding chemical adhesion, silanes and hybrid organic-inorganic compounds may act as
mediators that increase adhesion between different inorganic and organic materials via dual
reactivity. These are termed primers, or coupling agents, based on their function and substrates.
Basically, organo-silanes are either "hydrophilic" or "hydrophobic", and can be either "anionic" or
"cationic".
(37, 38)
Silane most frequently used in dental laboratories and chairside is the mono-
functional g-methacryl-oxypropyltrimethoxysilane (or 3-trimethoxysilylpropyl methacrylate).
10
Activated silanol groups are condensed with hydroxylated silica groups. Furthermore, these are
stabilized with the help of silanol intermolecular condensation.
(32)
Silanes are delivered in two
different forms, either pre-hydrolyzed or unhydrolyzed. The pre-hydrolyzed silanes are used as
one-bottle systems in a solvent that contains ethanol and water. However, this form has a limited
shelf life since the hydrolyzed molecules may autocondensate, especially in the presence of
atmospheric moisture. Once the solution changes to a cloudy or milky appearance, the silane
cannot be used.
(7)
On the other hand, two-bottle systems were shown to have increased shelf life
because they consist of a separate unhydrolyzed silane that is mixed with an aqueous acetic acid
solution at the time of the procedure. Upon mixing the components, hydrolysis is initiated.
(39, 40)
Silane is effective in increasing bonding effects by forming siloxane bonds at the interface
between the ceramic and the resins. A silane solution of organo-functional trialkoxysilane esters
can copolymerize with the remaining C=C bonds of the resin cement. Notably, silane’s hydrolyzed
alkoxy groups can react with hydroxyl groups of the lithium disilicate and glass matrix to form
covalent siloxane bonds. Several studies showed that clinical restorations generally depend on the
bonding effect between the ceramic and resin cement, rather than the strength of the ceramic.
(27,
41)
Strong interfacial bonding between the ceramic and resin cement improves fracture resistance
(41)
and marginal adaptation
(42, 43)
and reduces micro-leakage
(43)
, leading to restoration retention.
Although all previously mentioned factors are essential for the long-term and predictable
clinical outcomes of ceramic restorations, there has been a pressing need for materials that work
on both, tooth structure and restoration, which can decrease operation time, diminish errors during
the bonding process, but still accomplish adequate bonding.
(44)
Conventional bonding processes
that usually comprise numerous steps of etching, cleaning, primer, and adhesive on the tooth and
restoration side.
(45)
Universal adhesives try to address both interfaces at the same time by
11
combining silane, which provides a chemical bond to glass-ceramic surfaces, in addition to a
monomer, called 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP provide chemical
bonding to zirconia, alumina, and metals) which helps chemical bonding to various substrates
(tooth structure, direct/indirect restorations) without the need to any additional primers, as claimed
by the manufacturer.
(46)
The current study aims to assess the bonding durability to various types of lithium disilicate
ceramic and to explore the influence of a chemical coupling agent when bonding with a universal
adhesive and a universal resin cement.
Objectives and Specific Aims
Study objectives are to analyze the influence of applying a universal adhesive only versus
the application of silane prior to a universal adhesive on the bond strength to 4 different types of
lithium disilicate reinforced glass ceramic materials before and after artificial aging.
Null Hypothesis
1-There is no difference in shear bond strength to the four different lithium disilicate reinforced
glass ceramic materials.
2- Additional silane application compared to the omission of silane before application of the
universal adhesive does not influence the bond strength to different lithium disilicate reinforced
glass ceramic materials.
3- Artificial aging does not affect the bond strength to different lithium disilicate reinforced glass
ceramic materials.
12
Materials and Method
Study Design
Rectangular slices of IPS e.max CAD (Ivoclar Vivadent, Schaan, Liechtenstein), Amber
Mill (Hassbio, Kangreung, Korea), ), n!ce (Straumann, Freiburg, Germany), and CEREC Tessera
(Dentsply Sirona, York, PA.USA; Figure 1 ) received different bonding protocol; Silane +
Adhesive (SA) vs. Adhesive only (NS), before bonding composite cylinders to them with a dual-
cure universal resin cement. The bonded specimens were assessed regarding shear bond strength
(SBS) with a universal testing machine after storage in distilled water for either 24 hrs (non-aged)
or 2 months and thermocycling (aged).
Figure 1: Lithium disilicate reinforced ceramics materials
A: IPS e.max CAD, B: Amber Mill, C: n!ce, and D: CEREC Tessera
13
Sample Preparation
Four types of lithium disilicate reinforced glass ceramic materials (Table 1), IPS e.max
CAD (EX), Amber Mill (AM), n!ce (NC), and CEREC Tessera (TS) were cut into 240 rectangular
shaped specimens (N=60 per material), with a thickness of 1.5 mm. For this purpose, the
CAD/CAM blocks (12x14x18) were sliced using a precision saw (IsoMet 1000, Buehler, Lake
Buff, IL, USA) equipped with a diamond blade (102 mm diameter, 0.3 mm thickness; IsoMet
Blade 15LCA, Buehler, Lake Buff, IL, USA) at a speed of approximately 800 RPM under
continuous cooling with distilled water. On the non-bonding side of the specimens, all
irregularities were removed, and a notch was carved using a 1.4 mm diameter straight diamond
bur (#856.31.014 Medium Round-End Taper Diamond, Brasseler, Savannah, GA USA) with a
high-speed handpiece. Finally, a 600 grit of silicone paper was used to polished (CarbiMet
Abrasive Sheet, 600, SiC, Buehler, Lake Buff, IL, USA) the bonded side for 30 s to remove any
scratches on the surface and to standardize the surface for all specimens.
The specimens of each material were further divided into 2 subgroups (N=30) according
to the bonding protocol (silane vs. no silane) and were further subdivided (N=15) according to
their artificial aging protocol (non-aged vs. aged; Table 2).
14
Material
(Lot no.)
Compositions in wt% Flexural
Strength
Need for
sintering
Opacity
Manufacturer
IPS e.max
CAD
(47)
LOT#:
W37102
Z018HT
SiO2: 57.0 - 80.0 %
Li2O: 11.0 – 19.0 %
K2O: 0.0 – 13.0 %
P2O5: 0.0 – 11.0 %
ZrO2: 0.0- 8.0 %
ZnO: 0.0- 8.0 %
Other and coloring oxides:
0.0 – 12.0 %
≥ 360 MPa
(biaxial)
Yes HT A1 Ivoclar
Vivadent,
Schaan,
Liechtenst
ein
Amber
Mill
(48)
LOT#:
EBE05MK
1101
SiO2: <78 %
Li2O: <12 %
Other oxides and coloring:
<12 %
≥ 300 MPa Yes HT A1 HassBio,
Kangreung,
Korea
n!ce
(20)
LOT#:
YZ781
CRR61
SiO2: 64-70%
Li2O: 10.5-12.5%
Al2O3: 10.5-11.5%
Na2O: 1-3%
K2O: 0-3%
P2O5: 3-8%
ZrO2: 0-0.5%
CaO: 1-2%
Coloring oxides: 0-9%
350 MPa No HT A1 Straumann
Freiburg,
Germany
CEREC
Tessera
(21)
LOT#:
16007942
Li2O5Si2: 90%
Li3PO4: 5%
LiAlSi2O6 (Virgilite): 5%
700 MPa No HT
A2
Dentsply
Sirona,
York,
PA.USA
Table 1: Comparison between material composition and physical properties
15
Material
name
Number
of
samples
Silane application
Artificial aging
Group
name/number
IPS
e.max
CAD
(EX)
N=60
Silane (SA; N=30)
Non aged (NA; N=15) EX-SA-NA (#1)
Aged (A; N=15) EX-SA-A (#2)
No silane (NS;
N=30)
Non aged (NA; N=15) EX-NS-NA (#3)
Aged (A; N=15) EX-NS-A (#4)
Amber
Mill
(AM)
N=60
Silane (SA; N=30)
Non aged (NA; N=15) AM-SA-NA (#5)
Aged (A; N=15) AM-SA-A (#6)
No silane (NS;
N=30)
Non aged (NA; N=15) AM-NS-NA (#7)
Aged (A; N=15) AM-NS-A (#8)
n!ce
(NC)
N=60
Silane (SA; N=30)
Non aged (NA; N=15) NC-SA-NA (#9)
Aged (A; N=15) NC-SA-A (#10)
No silane (NS;
N=30)
Non aged (NA; N=15) NC-NS-NA (#11)
Aged (A; N=15) NC-NS-A (#12)
CEREC
Tessera
(TS)
N=60
Silane (SA; N=30)
Non aged (NA; N=15) TS-SA-NA (#13)
Aged (A; N=15) TS-SA-A (#14)
No silane (NS;
N=30)
Non aged (NA; N=15) TS-NS-NA (#15)
Aged (A; N=15) TS-NS-A (#16)
Table 2: Group assignment
Prior to surface treatment and bonding, all specimens were thoroughly cleaned by
immersion in distilled water (Arrowhead Mountain Spring Water, BlueTriton, San Bernardino,
CA, USA) in an ultrasonic bath (Ultrasonic Cleaning Systems, Quantrex, Kearny, NJ, USA) for
10 mins. EX and AM specimens required an additional crystallization step. The samples were
crystallized according to the manufacturers’ guidelines using a sintering furnace (Programat CS3
Furnace, Ivoclar Vivadent, Schaan, Liechtenstein; Table 3). Firing parameters differed slightly for
both materials: IPS e.max CAD crystallization was performed at a higher temperature and for a
longer time
(47)
(840°C to 850°C for 20 – 31 mins; Table 4) compared to Amber Mill crystallization
for which the temperature ranged from 550°C to 815°C for 15 mins
(48)
(Table 4).
16
Temperature of stand (B) 403°C
Time of Closing (S) 6:00 [min]
Rate of Heating (t1) 90 [°C/min]
Temperature for Firing
(T1)
820°C
Time for Holding (H1) 0:10 [min]
Rate of Heating (t2) 30 [°C/min]
Firing temperature (T2) 840°C
Holding time (H2) 7:00 [min]
Vacuum (1-11)
Vacuum (1-12)
550°C
820°C
Vacuum (2-21)
Vacuum (2-22)
820°C
840°C
Long – term cooling (L) 7 00°C
Cooling rate (t) 0 [°C/min]
Table 3: Firing parameters for IPS e.max CAD crystallization in programat CS3 furnace
Temperature of stand (B) 400 °C
Time of Closing (S) 3:00 [min]
Rate of Heating (t1) 60 °C
Temperature for Firing
(T1)
HT/815 °C
Time for Holding (H1) 15.00 [min]
Vacuum 1 1°C/ VAC.2°C 550/815 °C
Long – term cooling (L) 690 °C
Cooling rate (tL) 0 °C
Table 4: Firing parameters for Amber Mill crystallization in programat CS3 furnace
17
Surface Treatment
The etching time for EX and NC was 20 seconds with 4.5% HF acid (IPS ceramic etching
gel, Ivoclar Vivadent, Schaan, Liechtenstein). In comparison, AM and TS were etched for 30
seconds according to the manufacture’s instruction. For this purpose, one drop of HF acid etchant
was applied and rubbed on the surface with a micro-brush (Maxmicro Micro Brush Applicators,
Plasdent, Novi, MI, USA). Then, the etching gel was rinsed away thoroughly for 60 seconds with
distilled water. The surface was then dried with oil-free air for 5 seconds at a distance of 30 mm.
After HF etching, the samples were cleaned by etching with 35% phosphoric acid (Ultra Etch,
Ultradent Products, South Jordan, UT, USA) for 60 s, followed by rinsing with distilled water for
60 s. The surface was dried with oil-free air for 5 seconds at a distance of 30 mm. For EX, NC,
and TS, one drop of silane (Silane, Ultradent Products, South Jordan, UT, USA) was used and
spread in scrubbing motion with a micro-brush for 10 s. For AM the silane was applied with
scrubbing motion for 20 seconds according to the manufacturers’ instruction. On all study groups,
the silane was then left for 1 min to evaporate. After that non-evaporated silane was removed with
oil-free air.
Composite Cylinders
Two-hundred and forty composite resin cylinders were fabricated from a nano-hybrid
composite (IPS Empress Direct A1E, Ivoclar Vivadent, Amherst, NY, US) using a preformed
Teflon mold (Error! Reference source not found.) with 2.38 mm internal diameter and of 3 mm
height (bonding mold insert, Ultradent, South Jordan, UT, USA).
18
Figure 2: Teflon mold for composite cylinder fabrication
The mold was put on a glass slab (Surgipath Microslides 1x3x1.0 mm, Leica Biosystems,
Nussloch, Germany), then the composite was inserted into the mold with a composite instrument
(Didier Dietschi CompoSculp #3/4 (Yellow); PFIDD3/48; Hu Friedy, Chicago, USA), and layered
in two increments to fabricate composite cylinders. Each increment was condensed with Tanner
Plugger (#PLG2T, Hu Friedy, Chicago, IL, USA) and light cured for 20 s using a polywave LED
light-curing unit (Valo, Ultradent, South Jordan, UT, USA) at standard power mode (1000
mW/cm2, wavelength 385-515 nm). Before removing the cylinder from the mold, the unbonded
side (Figure 3) of the composite cylinder was marked with a black ultrafine permanent marker
(Sharpie, Newell, Atlanta, GA, USA). Then, the glass slab was removed, and the cylinder was
removed by pushing it out of the mold with the aid of a condenser. All cylinders were fabricated
one day prior bonding and inspected before use. The defective cylinders were discarded. Only
void-free cylinders were stored in dry, clean conditions until bonded.
Figure 3: Bonding side vs. nonbonding side for composite cylinder
A: Flat, shiny, and smooth surface (bonding side)
B: Show the marked area that represent the unbonded side
19
Bonding Protocol
The bonding procedures were performed according to manufacturers’ instructions
(20, 21, 48-
50)
for all specimens in the same way. A universal bonding agent (Scotchbond Universal Plus, 3M,
St. Paul, MN, USA) was used with a micro-brush in a rubbing motion for 20 s to the bonding side
of the specimens and was air-thinned until no ripples were visible. The adhesive was left without
light curing. Then a small amount of a dual-cure universal resin cement (Rely X universal cement.
3M ESPE, St. Paul, MN, USA) was applied to the bonding side of the composite cylinder
(unmarked end), and it was seated on to the middle of the bonding surface of each specimen. Using
a seating device (chewing simulator, CS-3-8; SD Mechatronik), a standardized load of 1 kg was
used to the cylinder. With the specimen mounted in the seating device, extra-cement was cleaned
with a micro-brush. The cement was initially light cured for 20 s with 2 polywave curing lights
(Valo, Ultradent; Figure 4) positioned opposing each other to the left and the right to left of the
specimen as close as possible. Then an air-blocking gel (Sterile Lubricating jelly, Medline,
Northfield, IL, USA) was used around the bonding interface, and the specimens were again light-
cured for 20 s from left and right and for another 20 s from front and back. The samples were
removed from the seating device and rinsed with distilled water to remove the air-blocking gel.
Specimens were then stored in separate containers for each group, filled with distilled water, and
stored at 37°C in an incubator (5510, National Appliance, Portland, OR, USA).
Figure 4: Position of the light curing units during bonding
20
Material
and lot No.
Manufacturer Composition
Lot No. & Expiration
date
Etching gel
IPS ceramic
etching gel
(51)
Ivoclar
Vivadent,
Schaan,
Liechtenstein
Hydrofluoric acid 4.5%
Z015FK
2024-04-25
Etching gel
Phosphoric
acid
Ultra-Etch
(52)
Ultra -Etch
Ultradent
Products,
South Jordan,
UT, USA
35% phosphoric acid, water, cobalt
aluminate blue spinel, glycol,
siloxane
BKJKW
2025-03-31
Coupling
agent
Silane
(53)
Ultradent
Products,
South Jordan,
UT, USA
Methacryloxy propyl trimethoxy
silane; isopropyl alcohol
BKLH2
2024-02-30
Adhesive
Scotchbond
Universal
Plus
(46)
3M, St. Paul,
MN, USA
Monomer: Dimethacrylate, MDP
phosphate monomer
HEMA
Copolymer: Vitrebond
Filler: Silica
Other: Ethanol, Water, Silane,
Photoinitiator, accelerator
7061823, 6840217
2023-03-16
Resin
Cement
RelyX
Universal
cement
(54)
3M, St. Paul,
MN, USA
Monomer: Dimethacrylate
Other: Additives, Initiators, Filler,
Pigments
7602171,6982138
2021-07-27
Composite
IPS
Empress
Direct C1E
(55)
Ivoclar
Vivadent,
Schaan,
Liechtenstein
Monomer: Dimetharylates
Fillers: Mixed oxide along with
Copolymers, trifluoride, Barium
glass, Silicon dioxide and
Ytterbium
Others: Stabilizers, Additives,
Initiators and Pigments
V36125
2021-09-24
Table 5: Bonding materials
21
Artificial Aging
After bonding, all specimens were stored according to their assigned aging protocol:
• Non-aged (NA) groups: stored in a distilled water for 24 hrs
• Aged groups (A): thermocycling + stored in a distilled water for 2 months
Storage temperatures were maintained at 37°C in an incubator. During the two months period,
specimens in the aged groups were further artificially aged by thermo-mechanic cycling (20,000
cycles) in distilled water at temperatures of 5 and 55°C (Thermocycler THE-1100, SD
Mechatronik, Westerham, Germany) for 20,000 cycles for 15 s dwell time and 10 s transfer
time.
(56, 57)
Figure 5: Thermocycler
● Notched-Edge Shear Bond Strength Testing
All specimens were assessed for shear bond strength using a universal testing machine
(Model 6596; Instron, Norwood, MA, USA) with a notched-edge blade (Notched-edge crosshead
blade, Ultradent, South Jordan, UT, USA). The test was performed at a crosshead speed of 1
mm/min.
(58)
Immediately before testing, each specimen was dried using a paper towel (Multi-Fold
Towels, Scott brand, Neenah, WI, USA) by carefully blotting without contacting the adhesive
22
interface or without putting any pressure on the cylinder. The specimen was placed in the middle
of the holder, and the crosshead was aligned as near as possible from the composite cylinder
without touching it. During the test, the maximum load was recorded in the testing software
(Instron, Norwood, MA, V3.04, Bluehill, USA) at the time of failure. SBS was calculated by
dividing the maximum shear load (in N) by the composite cylinder bonding surface (mm
2
). The
study counted the specimens, which spontaneously debonded during thermal cycling or water
storage before SBS testing as 0 MPa (pre-testing failures).
Failure Mode Analysis
After testing SBS, the failure mode was analyzed macroscopically (general analysis) and
microscopically (specific analysis). The primary general analysis (macroscopically) was
performed with naked eye and inspected with an explorer (#EPD1359; Hu Friedy, LLC Chicago,
IL, USA). Failure modes were categorized as adhesive failure (A), cohesive failure (C), and
mixed (M). To further distinguish and specify the type of mixed failure, a microscopic analysis
was performed with a stereomicroscope (Extaro 300, Carl Zeiss Meditec, Jena, Germany) at 20x
magnification with white light and fluorescent light. The white light helps to differentiate
between adhesive failures that occurred at the CAD/CAM or composite material. For adhesive
failures at CAD/CAM (Figure 7), the bonding area on CAD-CAM appeared clean.
The fluorescent light (Figure 7) was used to distinguish between the different types of
cohesive failures when the fracture occurred within the (3) layer of cement, (4) composite cylinder,
or even (5) CAD/CAM material. The composite remnant was more fluorescent than cement
remnants (Figure 6).
23
Figure 6: Difference in fluorescence between composite (C) and resin cement (RC)
In case of CAD/CAM fracture, a defect was noticed under the white light and the remnant
of ceramic that stuck with the composite cylinder appeared black under fluorescent light.
Most of the time, more than one failure occurred, and to determine the predominate type as well
as an actual percentage, a specific failure analysis was run for all the specimens, especially for
mixed failures specimens with an image analysis software (Fiji ImageJ, V1.0, Research Services
Branch, National Institute of Mental Health, Bethesda, Maryland, USA).
(59)
The pictures were imported and arranged based on the assigned groups (Table 2) into
Microsoft PowerPoint (Office 365 for Mac, V16.52, Microsoft, Redmond, WA, USA). The width
and length for both microscopic pictures (white light and fluorescent light) were standardized with
a circle that surrounded the failure pattern (total circle area). The circle outline followed the width
of the composite cylinder (2.38 mm). Then, a screenshot was taken, and the image was imported
into the image analysis software with which the surface areas of the different fracture types were
measured in pixels. All the results were inserted into an Excel sheet under its specific category (1-
Cohesive at the composite cylinder, 2- cohesive at CAD/CAM material, 3-cohesive in cement, 4-
Adhesive in CAD/CAM, 5- Adhesive in the composite cylinder).
All the data were transformed from pixel to percentage by dividing the measured area by
the area of the standard circle and then multiplied by 100.
24
Figure 7: Examples of different failure modes pattern
A: Represent adhesive failure at CAD CAM under white light
B: Shows mixed failure [adhesive (A) + cohesive (C) at CAD CAM] under white light
C: Shows mixed failure (adhesive (A)+ cohesive at composite) under fluorescent light
Statistical Analysis
Collected data were organized using Microsoft Excel (Office 365 for Mac, Microsoft,
Redmond, WA, USA) and were analyzed using the Statistical Package for Social Sciences (SPSS
Inc., Chicago, IL, USA, version 19 for Mac). The statistical analysis was applied using a non-
parametric test for ceramic, silane, and aging factors from the SBS data due to the lacking
heterogeneity of variances (Levene test p<0.05). In addition, the Kruskal-Wallis test was used to
detect overall differences for the factor’s material (EX, AM, NC, and TS), time (24 h vs. 2 months),
and silane application (silane adhesive vs. adhesive only). Group-wise comparisons were
conducted separately for each material with Mann-Whitney test using Bonferroni correction due
to multiple comparisons (⍺=0.001).
Scanning Electron Microscopy (SEM)
Representative specimens from each type of CAD/CAM lithium disilicate glass-ceramic
(EX, AM, NC, and TS) were selected for scanning electron microscopy evaluation. The specimens
were etched with 4.5% hydrofluoric acid according to manufacturer’s instructions, mounted on
aluminum stubs and sputter-coated with gold-palladium. Images of specimens were obtained using
a field-emission scanning electron microscope (FEI Nova NanoSEM, Thermo Fisher Scientific) at
an accelerating voltage of 5 to 10 kV and magnification of 1,000 to 25,000x.
25
Results
Notched-Edge Shear Bond Strength (SBS) Analysis
The Kolmogorov-Smirnov test showed that generally, the data were not normally distributed
(p>0.05). Levene’s test was used to check the homogeneity of variances (p<0.05). Therefore,
applying parametric analysis was not possible. Kruskal-Wallis was performed for all different
factors: ceramic, silane, and aging. (Table 7; Figure 8). Furthermore, multiple Mann-Whitney tests
were performed as post-hoc comparison to compare data between the different groups. The
significance level was adjusted due to multiple comparisons to α=0.001.Table 6 shows the
minimum and maximum values for each group.
Material
Group No and
names
Std.
error
95% Confidence
Interval for Mean
Min
Max
Lower
Bound
Upper
Bound
IPS e.max
CAD
EX-SA-NA (#1) 1.85 23.40 31.35 17.74 44.02
EX-SA-A (#2) 1.46 13.49 19.77 8.86 27.43
EX-NS-NA (#3) 2.51 15.22 26.02 9.52 38.20
EX-NS-A (#4) 1.10 6.39 11.12 1.21 15.26
Amber
Mill
AM-SA-NA (#5) 0.91 23.99 27.90 18.76 32.22
AM-SA-A (#6) 2.17 12.88 22.20 7.69 39.10
AM-NS-NA (#7) 1.80 14.00 17.72 8.64 29.90
AM-NS-A (#8) 0.99 0.88 5.15 0.00 9.57
n!ce
NC-SA-NA (#9) 1.00 24.25 28.55 17.10 33.27
NC-SA-A (#10) 1.94 18.06 26.42 2.94 31.97
NC-NS-NA (#11) 1.93 20.59 28.89 12.35 36.59
NC-NS-A (#12) 1.38 16.79 22.74 8.74 26.88
CEREC
Tessera
TS-SA-NA (#13) 1.21 26.48 31.68 19.93 37.24
TS-SA-A (#14) 1.33 16.04 21.78 10.54 33.46
TS-NS-NA (#15) 1.59 22.46 29.30 18.31 38.18
TS-NS-A (#16) 1.08 14.51 19.17 11.53 25.80
Table 6: Overall shear bond strength (SBS)
26
Mean notched SBS values are displayed in (Table 7 and Figure 8). The results revealed
that the peak mean of SBS was recorded for group #13 TSSANA (29.08 ± 4.68 MPa) with values
ranging from 19.93 to 37.24 MPa. The next was recorded for group #1 EXSANA (27.38 ± 7.17
MPa), ranging from 17.74 to 44.02 MPa. The lowest mean of SBS was recorded for group #8
AMNSA (3.01 ± 6.03 MPa), with values ranging from 0.00 to 9.57 MPa. The second lowest was
group #4 EXNSA 8.75 ± 4.27 MPa, with values ranging from 1.21 to 15.26 MPa.
Pre-test failures were observed in group #8 only (AM-NS-A; 8 of the samples) for which
the silane was omitted. These specimens debonded during thermocycling.
Material Silane No silane
Non-aged Aged Non-aged Aged
IPS e.max
CAD
EX-SA-NA
(#1)
27.38 ±7.17
aA
EX-SA-A (#2)
16.63 ± 5.67
bA
EX-NS-NA (#3)
20.62 ± 9.74
abd A
EX-NS-A (#4)
8.75 ± 4.27
cA
Amber
Mill
AM-SA-NA
(#5)
25.94 ± 3.53
aA
AM-SA-A
(#6)
17.54 ±8.41
aA
AM-NS-NA (#7)
17.87 ± 6.99
aA
AM-NS-A (#8)
3.01 ± 3.84
bA
n!ce NC-SA-NA
(#9)
26.40 ± 3.87
aA
NC-SA-A
(#10)
22.24 ± 7.55
abA
NC-NS-NA (#11)
24.74 ± 7.48
acA
NC-NS-A (#12)
19.77 ± 5.36
bcB
CEREC
Tessera
TS-SA-NA
(#13)
29.08 ± 4.68
aA
TS-SA-A
(#14)
18.91 ± 5.18
abA
TS-NS-NA (#15)
25.88 ± 6.17
aA
TS-NS-A (#16)
16.84 ± 4.20
bB
Within a row: Same lower case letters are not significantly differently from each other
Within a column: Same UPPER CASE letters are not significantly differently from each
other
Table 7: (SBS) values and standard deviation (SD) in MPa
27
Figure 8: Comparison of mean notched edge SBS in MPa between all the material
According to results of the Kruskal-Wallis tests, all three factors significantly influenced
SBS (p=0.000).
For the materials (Figure 8), the overall groupwise comparisons showed that only groups
AM and NC were significantly different from each other (p<0.001). All other materials were not
significantly different from each other.
For IPS e.max CAD groups #1 - #4, and CEREC Tessera groups #13 - #16, the highest bond
strength was achieved for the non-aged groups #1 and #13 when silane was applied as an
additional step along with the adhesive.
0
5
10
15
20
25
30
IPS e.max CAD Amber Mill n!ce CEREC Tessera
Overall SBS
Silane - Non-aged Silane- aged No silane- Non-aged No silane-Aged
28
Figure 9: Comparison between non-aged groups
Silane vs. no silane for non-aged groups (Figure 9) showed no significant difference in SBS
value (Table 7).
Figure 10: Comparison between aged groups
Silane vs. no silane in aged groups (Figure 10, Figure 9) showed a significant drop in SBS
values (Table 7), especially for labside material (EX and AM). For IPS e.max CAD groups #2, #4
SBS values ranged from 16.63 to 8.75 MPa. While Amber Mill glass ceramic groups #6, #8 SBS
values ranged from 17.54 to 3.01 MPa.
For n!ce glass-ceramic groups #10, #12 SBS values ranged from 22.24 to 19.77 MPa.
While CEREC Tessera glass ceramic groups #14, #16 SBS values ranged from 18.91 to 16.84
MPa. The highest value based on the material difference was for group #2 (16.63 MPa), and that
achieved with silane application. While the lowest value was for group #8 (3.01 MPa).
0
5
10
15
20
25
30
IPS e.max CAD Amber Mill n!ce CEREC Tessera
Non-aged Groups
No silane Silane
0
5
10
15
20
25
30
IPS e.max CAD Amber Mill n!ce CEREC Tessera
Aged Groups
No silane Silane
29
Failure Analysis
• General Failure Analysis (Macroscopic)
For failure mode analysis, specimens were analyzed macroscopically immediately after testing.
However, due to the high number of mixed failures (65%, Figure 11) further analysis was needed.
Figure 11: General macroscopic analysis
Figure 12: Amber Mill compared to IPS e.max CAD material
35%
65%
Adhesive at CAD/CAM material Mixed
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
ADHESIVE AT CAD/CAM
MATERIAL
MIXED
Labside Materials
Amber Mill- No silane - Aged
Amber Mill- No silane - Non-aged
Amber Mill- Silane - Aged
Amber Mill- Silane - Non-aged
IPS e.max CAD- No silane - Aged
IPS e.max CAD- No silane - Non-
aged
IPS e.max CAD- Silane - Aged
IPS e.max CAD- Silane - Non-aged
30
Figure 13: CEREC Tessera compared to n!ce
Labside material in comparison to chairside material, shows more adhesive failure at
CAD/CAM material. A higher frequency of “mixed” failures (n=156) was seen generally and
specially in groups that did not require to be sintered (Chairside; NC, TS; Figure 13).
Additional microscopic failure analysis of all failures (specific analysis), including the mixed
failures, was performed. Mixed failure was a combination of the other five failure modes (Table
8): cohesive in composite, cohesive in ceramic, cohesive in cement, along with the adhesive failure
at CAD/CAM material. “Adhesive at composite cylinder” was not in this study.
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
ADHESIVE AT CAD/CAM
MATERIAL
MIXED
Chairside Materials
CEREC Tessera- No silane - Aged
CEREC Tessera- No silane - Non-
aged
CEREC Tessera- Silane - Aged #15
CEREC Tessera- Silane - Non-aged
#13
n!ce- No silane - Aged #12
n!ce- No silane - Non-aged #11
n!ce- Silane - Aged #10
n!ce- Silane - Non-aged #9
31
• Specific Failure Analysis
Figure 14 and Table 8 show the specific analysis for all mixed failures (65%). Upon examination,
Adhesive at CAD/CAM was dominant in comparison to other cohesive failures. Cohesive in
cement was recorded as the least frequent type of failures. Cohesive failures in CAD/CAM
materials were influenced by silane application and the type of ceramic. Hence, it was observed in
all the groups except IPS e.max CAD groups (#1- #4).
Figure 14: Overall specific failure analysis
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
EX-SA-NA (#1)
EX-SA-A (#2)
EX-NS-NA (#3)
EX-NS-A (#4)
AM-SA-NA (#5)
AM-SA-A (#6)
AM-NS-NA (#7)
AM-NS-A (#8)
NC-SA-NA (#9)
NC-SA-A (#10)
NC-NS-NA (#11)
NC-NS-A (#12)
TS-SA-NA (#13)
TS-SA-A (#14)
TS-NS-NA (#15)
TS-NS-A (#16)
Cohesive in composite in % Cohesive in CAD/CAM in % Cohesive in cement in %
Adhesive at CAD/CAM in % Adhesive at composite in %
32
Groups No. and
names
Cohesive in
composite
in %
Cohesive in
CAD/CAM
in %
Cohesive in
cement in
%
Adhesive at
CAD/CAM
in %
Adhesive at
composite
in %
EX-SA-NA (#1) 17.15 0.00 0.00 82.85 0.00
EX-SA-A (#2) 30.94 0.00 1.02 68.04 0.00
EX-NS-NA (#3) 4.20 0.00 0.00 95.80 0.00
EX-NS-A (#4) 3.66 0.00 0.00 96.34 0.00
AM-SA-NA (#5) 21.11 3.54 0.00 75.34 0.00
AM-SA-A (#6) 9.07 2.51 0.00 88.42 0.00
AM-NS-NA (#7) 3.61 4.56 1.16 90.67 0.00
AM-NS-A (#8) 0.00 0.00 0.00 100.00 0.00
NC-SA-NA (#9) 16.01 31.72 0.00 52.26 0.00
NC-SA-A (#10) 24.57 34.22 0.00 41.21 0.00
NC-NS-NA
(#11)
7.72 29.71 0.77 61.81 0.00
NC-NS-A (#12) 19.79 12.97 1.45 65.80 0.00
TS-SA-NA (#13) 7.47 56.40 0.00 36.13 0.00
TS-SA-A (#14) 14.34 2.60 2.72 80.35 0.00
TS-NS-NA (#15) 16.93 36.15 0.00 46.92 0.00
TS-NS-A (#16) 16.51 1.38 1.06 81.05 0.00
Table 8: Specific failure analysis
Figure 15: Specific failure analysis for non-aged groups
CEREC Tessera and n!ce revealed the highest amount of cohesive failure in the ceramic of
all tested CAD/CAM lithium disilicate glass-ceramic blocks, with an average percentage of
46.28% and 30.72% respectively. IPS e.max CAD showed no cohesive failure (0%), and Amber
Mill in an intermediary position (4.05%). Significant increase in number of cohesive in ceramic
was observe for group #13 among all the non-aged group (Figure 15).
0%
20%
40%
60%
80%
100%
EX-SA-NA
(#1)
EX-NS-NA
(#3)
AM-SA-NA
(#5)
AM-NS-NA
(#7)
NC-SA-NA
(#9)
NC-NS-NA
(#11)
TS-SA-NA
(#13)
TS-NS-NA
(#15)
Cohesive in composite in % Cohesive in CAD/CAM in % Cohesive in cement in %
Adhesive at CAD/CAM in % Adhesive at composite in %
33
Figure 16: Specific failure analysis for the aged groups
Aging significantly reduced the number of mixed failures in general and cohesive failure
(Figure 16), which was particularly observed in groups #12, #14, and #16. The number of
cohesive in CAD/CAM significantly reduced particularly for CEREC Tessera. However, n!ce
sustained the high percent of cohesive in CAD/CAM especially when the silane was applied
(group #10; 34.22%).
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
EX-SA-A
(#2)
EX-NS-A
(#4)
AM-SA-A
(#6)
AM-NS-A
(#8)
NC-SA-A
(#10)
NC-NS-A
(#12)
TS-SA-A
(#14)
TS-NS-A
(#16)
Cohesive in composite in % Cohesive in CAD/CAM in %
Cohesive in cement in % Adhesive at CAD/CAM in %
34
SEM Ultra-Structural Analysis
After surface treatment conditioning, several ceramics showed numerous microstructures,
and surface topographies, which ultimately affect the strength of bond between ceramic and
resin-cement.
(31)
IPS e.max CAD: hydrofluoric etching revealed a dense and homogeneous microstructure of
fine-grain needle like lithium disilicate crystals with length of approximately 1.5 μm embedded
into a glassy matrix (Figure 17 A).
Amber Mill: hydrofluoric etching exposed a loose microstructure of numerous fine and delicate
lithium disilicate crystals with length of approximately 0.2 μm in diameter and areas of
apparently unetched glassy matrix (Figure 17 B).
n!ce : hydrofluoric etching revealed loose microstructure of fine-grain needlelike lithium
disilicate crystals with length of approximately 0.75 μm into areas of apparently unattached
etched glassy matrix (Figure 17 C).
Tessera: hydrofluoric etching revealed loose microstructure of numerous lithium disilicate
crystals with length of approximately 0.5 μm as well as lithium aluminum silicate crystals
(virgilite) over an etched glassy matrix (Figure 17 D).
35
Figure 17: SEM analysis for different types of LDC
A: Microstructure of CAD/CAM lithium disilicate glass-ceramic
(IPS e.max CAD, Ivoclar Vivadent) after crystallization followed by etching with 4.5% HF for
20 s (magnification X2,500).
B: Microstructure of CAD/CAM lithium disilicate glass-ceramic (Amber Mill, Haasbio) after
crystallization followed by etching with 4.5% HF for 30 s (magnification X2,500).
C: Microstructure of CAD/CAM lithium disilicate glass-ceramic (n!ce, Straumman) after
etching with 4.5% HF for 20 s (magnification X2,500).
D: Microstructure of CAD/CAM lithium disilicate glass-ceramic (CEREC Tessera, Dentsply
Sirona) after etching with 4.5% HF for 30 s (magnification X2,500).
36
Discussion
This study is an in vitro research that aimed to assess the effect of silane application as an
additional step on the strength of the bond of a universal bonding agent together with universal
resin cement to four different types of LDC.
Our results showed significant differences between all 4 study materials, especially when
comparing AM to NC. Hence, the 1
st
null hypothesis could be rejected. For the 2
nd
null hypothesis,
additional silane application yielded significantly higher bond strength than the application of the
universal adhesive only. Thus, the 2
nd
null hypothesis was also rejected. Moreover, the 3
rd
null
hypothesis was rejected since artificial aging, by means of long-term water storage and thermal
fatigue, was associated with significantly reduced bond strength values for all groups.
The gold standard in bonding to lithium disilicate glass ceramic was achieved by etching
with HF acid, then applying silane coupling agent, ensuring a chemical interaction between the
resin-based agent and the ceramics obtained, forming strong siloxane linkages.
(60)
Etching efficiency of hydrofluoric acid depends on several factors, i.e., the concentration,
etching time, temperature, and dilution of the acid solution.
(61, 62)
According to the manufacturers’
instructions, IPS e.max CAD must be etched with a 5% hydrofluoric acid concentration for 20
s.
(47)
This procedure could be documented as the best accepted surface treatment for glass
ceramics.
(6, 63)
This is less than the time needed for feldspathic and leucite-based ceramics (90
seconds and 60 s, respectively). Higher hydrofluoric acid concentrations (9% to 10%) and longer
etching times were reported to be too aggressive and may provoke relevant damages, not only to
the surface, but also to the internal micro-structure of the material, negatively influencing
mechanical performance (reduction of flexure strength), adhesion potential and long-term success
of ceramic restorations, especially with low thickness.
(64)
37
Several agents were advocated to substitute hydrofluoric acid, such as phosphoric acid
(H
3
PO
4
), acidulated phosphate fluoride, ammonium bifluoride, and titanium tetrafluoride.
(65)
H
3
PO
4
is principally applied to clean ceramic surfaces. When 40% H
3
PO
4
is used for 60 seconds,
it does not change the topography or morphology of the ceramic surface when observed under the
scanning electron microscope (SEM).
(66)
Kussano et al.
(67)
looked at alternatives to hydrofluoric
acid etching and reported that simply roughening porcelain surfaces, followed by treatment with
35% phosphoric acid gel for 60 s and silane priming, was as effective as hydrofluoric acid-treated
samples to bonded composite. Canay et al.
(68)
noted that etching with acidulated phosphate fluoride
gel, even with extended application times, led to very shallow etching patterns compared to
hydrofluoric acid etching for a short time. Ammonium bifluoride (NH
4
HF
2
) reacts with silica
component of glass, acting on the phase boundaries only, creating an etching pattern analogous to
that of hydrofluoric acid etching of lower concentration or shorter duration.
(60)
Nevertheless, in
several aqueous concentrations, surface treatment with titanium tetrafluoride (TiF
4
) did not create
a stable adhesive bond between glass-ceramic and resin-cement.
(69)
In addition to the previously stated chemical agents, other mechanical techniques have been
proposed to alter the ceramic surface (e.g., airborne particle abrasion, tribochemical silica coating,
and laser surface treatment).
(70)
Airborne particle abrasion is carried out using 50 μm alumina
particles (Al2O3) under high pressure. These alumina particles mechanically adjust the ceramic
surface. However, some of these particles can embed in the surface loosely, which might
compromise the intaglio surface bond to resin-cement.
(71)
Tribochemical silica coatings use silica-
coated alumina particles for surface modification by increasing the surface silica content and
surface energy. Next, the immediate silane application forms covalent bonds between the silica-
coated surface and the resin cement.
(70)
38
While some techniques, e.g., airborne particle, might be suitable for oxide-based ceramics,
most of these techniques are not recommended for glass-ceramics
(72, 73)
due to the submicron
cracks and defects that might develop on the ceramic surfaces, which might ultimately lead to slow
crack growth, and eventually catastrophic fracture of the restoration.
(74, 75)
Yet, this procedure, in
addition to laser etching, can cause undue loss of material, with surface modifications that are less
uniformly distributed than hydrofluoric acid etching and that can substantially reduce flexural
strength.
(76)
An effective way to use phosphoric acid was to clean the surface for 30 s due to the
presence of insoluble salts in the surface. Insoluble salts may affect the interaction between the
silane and the silica, leading to lower bond strength values.
(77)
Therefore, it will necessitate
sufficient cleaning of the surface prior to the use of the silane. Results of most studies support the
post-etching protocol used (ultrasonic distilled water bath for two minutes) to provide effective
residue elimination.
(78-80)
However, this study did not display the effectiveness of post-etching
ultrasonic cleaning. According to Swift et al.
(81)
there is no difference or effect on bond strength
in various techniques of post-etching cleaning (i.e., air/water spray, phosphoric acid, or steam).
The success of a ceramic restoration is highly determined by the bonding procedure used.
The longevity of restorative procedures is established by the ability to bond materials to the tissues.
As such, primers play major roles when it comes to bond to glass-based ceramic.
Silane enhances the formation of 3 different layers after application on the ceramic surface,
and only one monolayer is considered the most critical for stability and bond strength to ceramic.
The 3 essentially different structures at the interphase are:
(82, 83)
The outermost layers comprise of small oligomers which are physically absorbed to the glass so
that they can be easily washed away by either organic solvents or water at room temperature. The
39
second region is nearer to the glass surface. It consists of oligomers similar to the outer layers,
apart from a few siloxane bonds connecting the oligomers and is hydrolyzable by hot water. In the
region close to the glass, uniformity and extent of cross linking of the layers increases and a regular
3-D network is formed, which is quite hydrolytically stable. Only this last layer of coupling agent
on the surface is essential for better bonding. Therefore, the major portion of the silane is of no
value in bonding, and may even be unfavorable.
(84)
Thus, improvement can be possible via
removing the outermost layer of the silane film and leaving the most stable and chemically
absorbed layer on the ceramic surface. Moreover, it has been advocated that heat treatment may
remove byproducts, promoting the formation of a covalent bond.
(85, 86)
The latest trend in adhesive dentistry is to simplify bonding procedures and reduce the
application steps by incorporating silane into a universal adhesive.
(87)
Universal adhesives are
simplified adhesives in a one bottle, applicable for different substrates, e.g., dentin, enamel, resins,
alloys and ceramics.
(88)
Universal adhesives contain silane and phosphate monomers, called 10-
methacryloxydecyl dihydrogen phosphate (MDP), which are responsible for the adhesive capacity
between the ceramic, polymeric and dental substrates.
(89)
Moreover, the multipurpose adhesives
are systems with the option of dual-polymerization, or only chemical that are available in
presentations of 2 bottles, reported in different clinical protocols, specially when
photopolymerization is not an option.
Recent studies using a universal adhesive (Scotchbond Universal Plus) showed that the
SBS for IPS e. max CAD when they used the universal adhesive along with the universal cement
was equivalent to a dual-cure and self-adhesive resin cement when used with silane.
(90)
Another study reported that when silane is incorporated in a universal adhesive, it does not
seem to provide the same adhesive strength as when a silane agent is used separately. This is
40
perhaps because the acidic MDP included in universal adhesives neutralizes the silane, rendering
it unstable over time.
(88)
Therefore, silane may be practically ineffective when contained in
universal adhesives.
The difference between the previous (Scotchbond Universal) and new versions of universal
adhesive (Scotchbond Universal Plus) is the presence of amino silane and the monomer removal
(Bis-GMA) that may interfere with the condensation between silane’s silanol groups and the
hydroxyl groups of ceramics.
(46)
According to the manufacturer, Scotchbond Universal adhesive
Plus comprises a pre-hydrolyzed silane (amino-silane), which help to stabilized the silane for at
least 12 months of storage. Yet, the amount of silane in its composition was not reported by the
manufacturer and may be insufficient to improve bond strength to a glass-based ceramic.
The reduced bond strength values that were observed in this study group (EX & AM) in
which the adhesives were used only (Scothbond Universal Plus), may have occurred because of
the degradation that occurred during the two months of water storage plus thermocycling. The
adhesives contain hydrophilic monomers and solvents, being more susceptible to water sorption
and hydrolytic degradation.
(91, 92)
Therefore, the use of silane is an important process following a
mechanical action method and cannot be substituted by a universal or multipurpose adhesive.
(88,
93)
The selection of resin luting material frequently depends on material type, fit of the
restoration, film thickness, and bonding to the tooth requirements
(94)
.
According to the steps of
bonding procedures, resin cement is categorized into etch-and-rinse, self-etch, and self-adhesive
resin cements.
(95)
The etch-and-rinse system utilizes phosphoric acid, which pretreats tissues
before rinsing the applied adhesive. On the other hand, self-etch, has acidic monomers that can
simultaneously etch and prime the tooth. Eventually, self-adhesive resin cements utilize diacrylate
41
resins and can be light-activated and self-cure.
(95)
Moreover, they are categorized according to
their polymerization mode into chemically cured, light-cured, or dual-cured. The resin-cement
type plays an important role in the immediate bond strength and durability of the bond.
Chemically
cured resin-cement has shown to have the highest immediate and long-term bond strength.
(96)
Sattabanasuk et al.
(97)
reported no significant differences between chemically cured and dual-cured
resin cement when bonding to lithium disilicate reinforced glass ceramic.
Mostly, chemically cured materials show higher setting time and a less degree of
conversion (DC) values compared with light-cured or dual-cured materials.
(98)
A high degree of
conversion is important to the material’s physical and mechanical properties.
(99)
The dual-cure
resin cement is usually selected because of the low film thickness of the cement, the high DC, and
the long-term efficacy of the resulting bond.
(100)
The new universal resin cement (RelyX Universal Resin Cement; 3M, St. Paul, MN, USA)
contains amphiphilic adhesion monomers and a novel amphiphilic redox initiator system, which
improve the adhesion monomers and diffuse into the hydrophilic dentin smear layer to form a
strong bond to dentin (which is not the case in this study). Moreover, the manufacturer claims that
the high bond strength of new universal cement is enhanced when combined with the new universal
adhesive for glass based ceramic material.
(101)
Geber et al.
(90)
, confirmed that on his study, which
showed a result that was equivalent to (RelyX Ultimate Adhesive Resin Cement and Variolink)
for immediate SBS value. And that could explain the high result present in this study for CEREC
Tessera. Moreover, he found when the silane was added, the SBS value significantly increases,
which is confirmed in this present study.
42
Ultra-structural analysis of the newer CAD/CAM LDC blocks (Amber Mill, n!ce, and
CEREC Tessera) revealed significant differences in the chemical composition and glass contents
than IPS e.max CAD. Consequently, the etching pattern created by HF on the aforementioned
materials is also significant difference among themselves and to IPS e.max CAD. Interestingly,
n!ce and CEREC Tessera exhibited the highest amount of cohesive failure in the ceramic when
silane was applied. Tessera recorded a significant drop in the percent of CAD CAM cohesive
failures when the silane was omitted in non-aged group (56%, 36%) respectively. That might
explain the presence of loose distribution of the lithium disilicate crystals and virgilite which were
not fully embedded into the glassy matrix. Additionally, areas of etched glass matrix appeared to
be unattached from the main glass matrix. Both occurrences may explain the cohesive failure
within the ceramic.
According to Fasbinder
(102)
, Tessera contains 40-45% of glass with a submicron particle
around 0.5 μm. The amount of lithium disilicate reach to 90% with 5% of lithium phosphate and
5% of Virgilite crystal.
(103)
Virgilite crystal is a lithium aluminum silicate platelet-shaped crystals
(<100 nanometers)
(102)
.
The ultra-structure of n!ce was the same except for the present of virgilite crystal but the
result of cohesive fracture was less and showed no difference between silane (31.72%) vs. no
silane (29.71%) in non-aged group.
IPS e.max CAD lithium disilicated crystals were densely embedded into the glass ceramic
matrix after etching supporting the findings where no cohesive failure was observed for this
material. For Amber Mill, the apparent areas of unetched glass matrix may contribute to the failure
analysis since this group had minimal cohesive failure into the ceramic in non-aged groups (4.56%,
3.54%) silane vs no silane, respectively. And that could be due to the low micromechanical
43
retention that impact the bond strength and lead to more adhesive failure at the CAD CADM.
Which explained the amount of pre-testing failures seen during the thermocycling.
Most of the failure was adhesive at CAD/CAM material speculated that it might be caused by the
loose crystals that were not embedded into the glassy matrix
All experimental groups revealed differences in the size of lithium disilicate crystals. The
size of the lithium disilicate crystals appears not to interfere with the resin permeation into the
etched microstructure of the ceramics. Although all tested materials yielded adequate adhesive
properties after HF etching; their bonding efficacy is significantly affected by type of silane and
hydrolysis and cannot be ignored. All the tested groups revealed significant drop in the amount of
CAD CAM cohesive failure after aging except nice maintain the high percent especially when the
silane add.
The SBS was the chosen test for this study due to its low technique-sensitivity, since it
avoids the specimen sectioning and trimming steps, which can introduce early micro-cracking of
fragile substrates.
Therefore, it has been shown that a shear approach is more applicable compared
with a microtensile approach.
(104-106)
Therefore, the primary purpose for selecting SBS as a test
method was to minimize damage generate by cracks defects that could affect the stress distribution
in the bond interface. This indicates that the substrate itself was assessed rather than the bond. In
this work, the SBS testing method was chosen since the substrate was sufficiently strong for
preventing cohesive failures. The shear bond strength test specimen preparation is less laborious
and time-consuming. In addition, the damage from sectioning and trimming that may occur with
tensile bond testing is prevented. Nevertheless, with shear bond strength, occurrence of cohesive
failure is usually greater, which is unwanted.
(107)
This is due to the complex stresses generated on
44
the specimen’s interface. This explains the number of mixed failures in this study. In addition, it
could be due to the strong bond between the universal resin and LDC .
(108)
An alternative for SBS, some in vitro methods (e.g., micro-shear, and tensile) can be used
to establish the bond strength between resin-luting agents and ceramics. Microtensile bond strength
(MTBS) assesses interfacial adhesives since it produces uniform stress distribution across the
bonded interface
(109)
and limiting cohesive failure in the substrate.
(110)
Nevertheless, these tests
were not known as a universal method, and each method has its specific advantages and
limitations.
(111)
Moreover, numerous aspects should be considered when selecting a test method,
such as the sensitivity of the technique, e.g., MTBS can present a high frequency of premature
failures,
(112)
and it may be affected by cutting speed,
(113)
shape of the sample, brittleness of the
substrate
(114, 115)
and the strength of the bond being investigated (strong bonds tend to generate
more cohesive failures].
(116)
Testing methods can have a great impact on the results as well. The distribution of the force
applied to the shear bond strength test cannot be uniform due to the force that is applied only from
one side or area. This causes less accuracy of the result compared to micro-tensile testing.
(117, 118)
To increase testing accuracy, one of the most important factors of shear bond testing is the
proper application of load at the bonded interface to prevent the composite cylinder from rotating
or bending rather than a shear load.
(119)
Therefore, samples positioning is very important.
(119)
When
comparing between knife-edge blade and notched edge blade, positioning of the notched blade is
much simpler and repeatable since the blade wraps the composite. Moreover, since the area of
contact of notched edge blade is much larger than that of the knife-edge blade, the positioning of
the blade becomes less critical and yields less force concentration. As a result, more distribution
of shear force is anticipated.
(119)
45
Long-term water storage is the most commonly applied artificial aging technique. It
allegedly simulates the wet conditions that a bonded interface is exposed to, in the oral cavity.
Nevertheless, clinically, other factors, e.g., stresses due to masticatory forces, thermal changes,
and chemical degradation through enzymes, bacteria, toxins, might play an additional role in the
degradation of adhesive interfaces.
(91, 120)
Hydrolysis may occur after some time, leading to deterioration of the resin interface by a
plasticizing effect of water infiltration into the resin polymer. This can ultimately lead to breaking
of the resin polymer covalent bonds, leading to loss of the resin mass, monomer leaching, and
bond degradation.
(41, 91)
Molecular bond degradation altered by the hydrophilic-hydrophobic capacity of both
resinous materials and adhesive systems, due to the variation in composition allows greater or
lesser water absorption. Sorption of water encourages siloxane bond weakening which plays a
significant role in weakening the bond.
(121, 122)
In addition, this effect may occur as a result to the
hydrolytic cleavage of siloxane connections in the interfacial layer, which lowers the bond
strength. Several studies
(122-124)
found that high bond strengths, that were obtained after 24 hours,
become less after aging in water with substantial reduction in the bond strength between the resin
and the ceramic.
Moreover, the storage medium influences shear bond strength.
(125)
Therefore, it’s
important to select the right medium when bond strength testing to get accurate results. Mediums
such as hydrogen peroxide, ethanol, and artificial saliva are recommended, while formalin may be
considered extreme.
(126)
However, it’s important to note that the storage media doesn’t have any
effect on the bond-failure site.
46
To judge the bonding performance and stimulate the physiological aging, thermocycling
which involves a repeated alternation of temperatures
(127)
, was performed in combination with
water storge for two months to mimic the intra-oral environment.
(56, 128)
In a study, lithium
disilicate test specimen were bonded using a universal adhesive along with universal cement
revealed a reduction in shear bond strength after being subjected to 500 thermocycles (p <
0.001).
(129)
Therefore, thermocycling affects the shear bond strength of both, universal cements
and universal adhesives, significantly. In the current study, the bond strength was significantly
influenced by aging and thermocycling, especially for IPS e.max CAD and Amber Mill.
Other factors that influence the impact of thermal cycling include water temperature, dwell
time and lack of securing samples when aging.
(127)
These factors can permit movement of the
specimens in the water bath, probably inducing accidental mechanical force on the bonded
samples.
(56)
The regimen for thermocycle in this study was 20,000 cycles which equal two years
of clinical function.
(56)
The ISO- recommended 500 cycles as a protocol.
(130)
On the contrary,
Amaral et al.,
(131)
Stewardson et al.,
(132)
and
Gale et al.,
(133)
found that 500 cycles consider
insufficient, and it represent less than two months of function. Therefore, the amount of aging in
this study was sufficient and valid.
(128)
However, this regime would not adequately simulate the
degradation that occurs in clinical situations.
(56)
Michailesco et al.
(134)
claimed that 30 thermocycles may occur during each meal, which at
3 meals/day, equates to about 33,000 thermocycles/year.
Gale and Darvell
(133)
assumed that approximately 10,000 thermal cycles may correspond
to one year of clinical function. This estimate is based on the hypothesis that these cycles may
occur 20-50 times/day, which is accepted by several studies (Amaral et al.,
(131)
Ulker et al.,
(135)
Wegner et al.)
(136)
Ehrenberg and Weiner
(137)
suggested a different system, using thermal cycling
47
and mechanical loading to simulate clinical condition in the oral cavity. They assumed that 50,000
cycles of occlusal loading and 8,000 cycles of thermal cycling simulate 6–8 weeks of function in
the oral cavity. Recently, Bayne
(138)
suggested that temperature of food and liquids, quantities of
them, and frequency of consumption differ among individuals. If one should drink 100 ounces of
water daily, the potential for cold cycling per year is 50,000 cycles. If we are interested in the 10-
year service life of a restoration, then thermal cycling can be conducted for at least 500,000 cycles.
From the results, it is concluded that all four materials were influenced by the aging and
silane application. EX showed result within the range compared to other studies while AM was
the lowest among all the groups with pre-test failure and that could be due to loose crystal and low
micromechanical retention. For Nice and Tessera the failure pattern were more toward mixed
failure and involved cohesive fracture within the CAD CAM and that could be due to the loose
crystal. The manufacture’s recommended four minutes and half for Tessera to be glazed and
activate the virgilite in order to increase the mechanical and physical properties. And that could be
for feature study to compare the differences in the strength after glazing and after additional
crystallization and see its impact in the mechanical and physical properties.
48
Conclusion
Based on findings and limitations of our in vitro study, the following conclusions could
be drawn:
(1) Bond durability is highly affected by the type of ceramics used and their microstructures.
All tested materials showed lower in SBS value when the silane was omitted, and Amber
Mill obtained the lowest value among all the groups.
(2) Additional application of silane increases bond strength of the universal adhesive to
LDC.
(3) Artificial aging through thermal cycling and water storage reduces bond strength, hence
the lower number of cohesive failures in CAD CAM with aging.
The results showed significant differences between all 4 study materials, especially when
comparing Amber Mill to n!ce. Amber Mill yielded the lowest SBS value over all tested
materials. Thus, the 1
st
null hypothesis was rejected. For the 2
nd
and 3
rd
null hypothesis, the
results show significant drop in SBS value for both IPS e.max CAD and Amber Mill, especially
when the silane was not applied, and the material was aged. Consequently, the 2
nd
and 3
rd
null
hypothesis were rejected. Further studies, preferably in vivo studies, are necessary to validate the
result under different clinical conditions
49
Disclaimer
The authors affirm that there are no conflicts of interest.
50
Funding
The current in vitro study was supported by the program of Advanced Operative and
Adhesive Dentistry (AOAD), Restorative Sciences, at Herman Ostrow School of Dentistry of
University of Southern California (USC).
51
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Abstract (if available)
Abstract
Objective: Evaluate the influence of applying universal adhesive only versus universal adhesive with silane on shear bond strength (SBS) to four types of lithium disilicate reinforced glass ceramic materials (LDC) using universal resin cement. ❧ Material and methods: Specimens (n=240, 1.5 mm thick) cut from 4 different LDC CAD/CAM materials (IPS e.max CAD [EX, Ivoclar Vivadent, Schaan, Liechtenstein], Amber Mill [AM, Hassbio, Kangreung, Korea], n!ce [NC, Straumann, Freiburg, Germany], CEREC Tessera [TS, Dentsply Sirona, York, PA.USA]) were polished (600 grit). EX and AM were sintered according to manufacturers’ instructions; NC and TS were not required to be sintered. For bonding, specimens were etched with HF acid, then either universal adhesive only (NS) or silane + universal adhesive (SA) was applied, and prefabricated composite cylinders (IPS Empress Direct) were cemented by applying a dual-cure universal resin cement (RelyX Universal Resin Cement, 3M, St. Paul, MN, USA). SBS testing was performed after either 24 h non-aged (NA) or after of 20,000 cycles thermocycling + 2 months of water storage (aged). Surfaces were analyzed with ZEISS stereomicroscope (Carl Zeiss Meditec, Jena, Germany) for failure mode and with the scanning electron microscope (FEI Nova NanoSEM, Thermo Fisher Scientific) for ultra-structures. Data were statistically analyzed by applying non-parametric tests with ⍺=0.001. ❧ Result: SBS values for non-aged specimens ranged from 29.08 (TS-SA) to 17.87 (AM-NS) MPa and for aged specimens from 22.24 (NC-SA) 3.01 (AM-NS) MPa. SBS was significantly reduced when silane was omitted and after aging (p<0.001). For aged, EX and AM values were significantly lower when silane was omitted compared to all other materials. Failure mode was mostly adhesive at CAD/CAM surface with some cohesive failures in the LDC, especially in groups that were not required to be sintered (NC, TS). ❧ Conclusion: SBS to LDC differs between different CAD/CAM materials and decreases over time. Using a silane containing universal adhesive alone is not sufficient. Silane application as a separate step is advised when using a universal resin cement in combination with a universal adhesive. Clinical Significance: Using silane separately is advised before bonding to LDC, apart from the existence of silane within the universal adhesive.
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Alhomuod, Mona Ali
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Core Title
Bond strength to different types of lithium disilicate reinforced ceramic materials
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School of Dentistry
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Master of Science
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Biomaterials and Digital Dentistry
Publication Date
11/19/2021
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09/09/2021
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CAD-CAM,lithium disilicate glass ceramic,OAI-PMH Harvest,SBS,shear bond strength,silane,universal adhesive,universal cement
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CAD-CAM
lithium disilicate glass ceramic
SBS
shear bond strength
silane
universal adhesive
universal cement