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Enamel Bond Strength of Self-Etching Adhesives to Orthodontic Brackets: An In Vitro Study
by
Dr. Melissa Cerone
A thesis submitted in conformity with the requirements for the degree of Master of Science (Orthodontics)
Faculty of Dentistry
University of Toronto
© Copyright by Dr. Melissa Cerone 2017
ii
Enamel Bond Strength of Self-Etching Adhesives to Orthodontic
Brackets: An In Vitro Study
Dr. Melissa Cerone
Master of Science (Orthodontics)
Faculty of Dentistry
University of Toronto
2017
Abstract
Background: Self-etch adhesives, which have combined the acidic monomer, primer and
adhesive resin into one bottle, have been extensively tested on dentin and cut-enamel; however,
very limited studies test their performances on uncut enamel. Objective: To assess their shear
bond strength(SBS) on uncut enamel bonded to orthodontic brackets. Methods:160 premolars
were divided into four groups: SU(Scotchbond™ Universal Adhesive), BU(All-Bond Universal),
CU(Clearfil Universal Bond), the control total-etch adhesive, C(Adper™ Scotchbond™ Multi-
Purpose Adhesive). Following storage in distilled water(24 h/6 months), they were tested for
SBS, adhesive remnant index(ARI) and percentage of remaining resin(%RR). Data was analyzed
by one-way ANOVA and Student-t test(α =0.05). Results: C group had the highest SBS, ARI and
mean %RR. BU group had the lowest SBS. SBS and ARI decreased over time for SU and BU,
but remained stable for CU and there was no difference in %RR. Conclusion: None of the self-
etch adhesives achieved the recommended SBS for orthodontic treatment.
iii
Acknowledgments
I would like to thank my supervisor, Dr. Anuradha Prakki, for her constant encouragement,
guidance and support throughout my thesis. You have been a tremendous mentor for me and it
has been a great honor to work with you.
I would also like to extend my sincere gratitude to the members of my committee, Dr. Siew-Ging
Gong and Dr. Wafa El-Badrawy, for their scientific guidance and advice throughout my thesis.
A special thank you to research technicians, Jian Wang, Nancy Valiquette and Douglas
Holmyard for their technical expertise in the use of the Instron machine, microscopes and digital
cameras.
I would like to thank the University of Toronto’s Dental Research Institute for funding this
project. I would also like to thank the dental companies, Kuraray, 3M ESPE, 3M Unitek and
Bisco, for donating the materials for the study.
I am deeply grateful to my family for a lifetime of unconditional love and support. Your constant
support has strengthened me on this wonderful journey.
Lastly, I wish to dedicate my thesis to my husband, who has believed in me and supported me
every step of the way.
iv
Table of Contents
Abstract ................................................................................................................................................. ii
Acknowledgments ...............................................................................................................................iii
Table of Contents ................................................................................................................................. iv
List of Tables ....................................................................................................................................... vi
List of Figures ..................................................................................................................................... vii
Introduction ...................................................................................................................................... 1 1
1.1 Purpose of the Study ................................................................................................................ 3
1.2 Null Hypotheses ....................................................................................................................... 3
Literature Review ............................................................................................................................ 4 2
2.1 Adhesives .................................................................................................................................. 4
2.1.1 Total-Etch System ....................................................................................................... 5
2.1.2 Self-Etch System.......................................................................................................... 6
2.2 Adhesives in Orthodontics ..................................................................................................... 10
2.3 Adhesive Testing .................................................................................................................... 11
2.4 Bond Strength Stability .......................................................................................................... 13
2.5 Factors Affecting the Bond Strength .................................................................................... 14
2.5.1 Cut vs. Intact Enamel ................................................................................................ 14
2.5.2 Mechanical Properties of Enamel ............................................................................. 15
2.5.3 Light Curing Unit ...................................................................................................... 16
2.5.4 Bracket Type .............................................................................................................. 17
2.5.5 Storage Medium ......................................................................................................... 17
2.5.6 Loading Rate .............................................................................................................. 17
2.6 Quantitative Adhesive Remnant Evaluation......................................................................... 28
2.7 Significance of the problem ................................................................................................... 29
v
Materials and Methods .................................................................................................................. 31 3
3.1 Study Design .......................................................................................................................... 31
3.2 Materials Used ........................................................................................................................ 31
3.3 Sample Size Calculation ........................................................................................................ 33
3.4 Definition of Groups and Samples Preparation .................................................................... 33
3.5 Shear Bond Strength Test ...................................................................................................... 37
3.6 ARI and Evaluation of Remaining Resin ............................................................................. 37
3.7 Scanning Electron Microscopy (SEM) ................................................................................. 38
3.8 Statistical Analysis ................................................................................................................. 38
Results ............................................................................................................................................ 39 4
4.1 Shear Bond Strength .............................................................................................................. 39
4.2 Adhesive Remnant Index ....................................................................................................... 40
4.3 Percentage of Remaining Resin ............................................................................................ 42
4.4 Scanning Electron Microscopy (SEM) ................................................................................. 44
Discussion ...................................................................................................................................... 47 5
Conclusion...................................................................................................................................... 53 6
References ...................................................................................................................................... 54 7
vi
List of Tables
Table 1: Summary of the studies assessing the bond strength and ARI of various adhesives....... 19
Table 2: ARI scoring index78
............................................................................................................. 28
Table 3: Modified ARI scoring index59
............................................................................................. 29
Table 4: Composition and pH of the adhesives ................................................................................ 31
Table 5: Mean SBS and standard deviation values .......................................................................... 39
Table 6: ARI scores for each adhesive group at baseline ................................................................ 41
Table 7: ARI scores for each adhesive group at 6 months ............................................................... 41
Table 8: Mean percentage of remaining resin and standard deviation values ................................ 43
vii
List of Figures
Figure 1: Classification of adhesive bonding agents - Adapted from McLean et al.8 ...................... 9
Figure 2: Study Design ....................................................................................................................... 36
Figure 3: Set-up for SBS testing with the Instron Machine ............................................................. 37
Figure 4: Mean SBS and SD values .................................................................................................. 40
Figure 5: Stereomicroscopic images (10x) of representative enamel surfaces. A was taken from
BU-6m group and represents an ARI score of 0. B was taken from the CU-B group and
represents an ARI score of 1. C was taken from the C-6m group and represents an ARI score of
2. D was taken from C-6m group and represents an ARI score of 3 ............................................... 42
Figure 6: Mean Percentage of Remaining Resin .............................................................................. 44
Figure 7: Scanning electron microscope images (1000x) of enamel surfaces after debonding. A
was taken from SU-B group and B was taken from the BU-B group. C was taken from the CU-B
group and D was taken from C-B group. E represents the enamel surface and R represents the
resin. .................................................................................................................................................... 45
Figure 8: Scanning electron microscope images (500x) of enamel surfaces after debonding. A
was taken from SU-6m group and B was taken from the BU-6m group. C was taken from the
CU-6m group and D was taken from C-6m group. E represents the enamel surface, R represents
the resin and AD represents the adhesive. ........................................................................................ 45
Figure 9: Enamel damage seen under SEM (1000x) of a sample from SU-6m (A) and BU-6m (B)
group. SE represents the enamel surface and DE represents the damaged enamel. ....................... 46
1
Introduction 1
The benefits of successful orthodontic treatment to improve a patient’s smile, facial aesthetics
and masticatory function are well known today. The efficiency and efficacy of achieving these
goals relies on the orthodontist’s ability to properly control tooth movement during treatment.
Orthodontic tooth movement relies on the interface between the wire and the bracket to
effectively move teeth. Therefore, the adhesive system that bonds the bracket to the enamel
surface of the tooth must be strong enough to resist all masticatory forces and remain adherent to
the tooth and bracket throughout the entire course of treatment. Reynolds (1975) stated that 5.9-
7.8 MPa of shear bond strength (SBS) was sufficient to withstand masticatory forces and this is
still considered to be the critical SBS for orthodontic adhesives.1 Current orthodontic articles still
cite that this SBS range provides clinically acceptable values.2-4
When a bracket loses its
attachment to the tooth, the orthodontist no longer has control over tooth movement, treatment is
interrupted and an additional appointment is required to rebond the tooth, thus increasing patient
inconvenience with prolongation in treatment time. While it is critical that the adhesive is strong
enough to withstand masticatory forces during the orthodontic treatment, once the treatment is
completed the orthodontist must remove the brackets with least discomfort to the patient and
without damage to the enamel surface.
Currently in orthodontics, the etch and rinse system of adhesion, which involves three steps:
etch, prime and adhesive resin, is used to bond the brackets to the enamel surface of the tooth.
Although this system provides adequate bond strength2 to withstand masticatory forces and
causes minimal damage to the enamel surface when the brackets are removed, the bonding
appointments are technique sensitive and time-consuming for both the orthodontist and the
patient.
The demand by dental professionals for adhesives with reduced technique sensitivity, shorter
clinical application time and less incidence of post-operative sensitivity has led manufacturers to
develop the self-etching system of adhesion. Within this system, there are the self-etching
adhesives also known as universal adhesives that have combined the three steps required for
2
adhesion into a one-step application. These adhesives can be used in either a self-etch mode,
selective enamel-etch mode or a total-etch mode for both direct and indirect dental restorative
procedures.5 When used in a total-etch mode both the enamel and dentin surfaces are directly
etched with the phosphoric acid, while in a selective etch mode, the etchant is isolated to the
enamel, leaving the dentin intact.5 When used in a self-etch mode, the newly developed universal
adhesives are capable of providing a one-step approach, which can significantly simplify the
bonding process by reducing the number of bonding steps and eliminating the need for etching.6
This would, therefore, lessen the risk of contamination and reduce the bonding procedure time.6
These self-etching universal adhesives have been extensively tested on dentin and cut-enamel
surfaces. Although the shear bond strength reported in many of these studies are within the
recommended 5.9-7.8 MPa1 for orthodontics, they are all measured on cut enamel, whereas in
orthodontics, the enamel surface is uncut. Very limited studies, however, exist on the
performance of these universal adhesives on uncut enamel or even within an orthodontic setting.
The objective of this study was, therefore, to investigate the shear bond strength and
characteristics of debonded surfaces of the universal adhesives within an orthodontic setting.
3
1.1 Purpose of the Study
The purpose of this in vitro study was to assess the bond strength of three universal adhesives
currently on the market, Scotchbond™ Universal Adhesive (3M ESPE), All-Bond Universal
(Bisco) and Clearfil Universal Bond (Kuraray), in comparison to Adper™ Scotchbond™ Multi-
Purpose Adhesive (3M ESPE), over time to orthodontic brackets by evaluating:
1- Shear Bond Strength (SBS) at baseline and after 6 month aging
2- Adhesive Remnant Index (ARI) score and Percentage of Remaining Resin
3- Scanning Electron Microscopy (SEM)
1.2 Null Hypotheses
1- There will be no significant difference in the shear bond strength of the orthodontic
brackets to the following adhesive systems: Scotchbond™ Universal Adhesive (3M
ESPE), All-Bond Universal (Bisco), Clearfil Universal Bond (Kuraray) and Adper™
Scotchbond™ Multi-Purpose Adhesive (3M ESPE).
2- There will be no significant differences in shear bond strength values after 6 month
aging.
3- There will be no significant difference in the amount of resin remaining on the teeth
after debonding the brackets among the evaluated adhesive systems.
4
Literature Review 2
In dentistry, the past six decades have witnessed significant progress in material science,
especially in the area of adhesives. The demand by dentists for adhesives with reduced technique
sensitivity, shorter clinical application time and less incidence of post-operative sensitivity has
led to the development of the self-etch system. Within this system, there are the universal
adhesives that have combined the three steps required for adhesion into a one-step application. In
orthodontics, adhesives are important because they allow the brackets to adhere to the tooth.
Orthodontic brackets are constantly loaded by masticatory and orthodontic forces, thus to
efficiently and effectively complete treatment, the adhesive must be able to withstand these
forces. The universal adhesives have been extensively studied for restorative procedures, but
there is very little literature on the efficacy of these universal adhesives in orthodontics.
2.1 Adhesives
Adhesive dentistry was first introduced by Buonocore in 1955.7 Since this time, various
adhesives have been developed with the objective of adequately bonding to both enamel and
dentin, despite their differences in structure, composition and natural variability. Enamel is a
homogenous structure composed of 86 vol% inorganic hydroxyapatite, 2 vol% organic content
and 12 vol% water. On the other hand, dentin is structurally more variable and composed of 50
vol% inorganic mineral, 30 vol% organic collagen and 20 vol% water.8 Adhesives are made up
of three main components: acid, primer and adhesive resin. The acid will remove the smear
layer, expose the enamel prisms, open the dentinal tubules, demineralize the collagen matrix and
roughen the surface of the tooth to increase the mechanical retention of the adhesive to the
tooth.5, 9
A primer acts to wet the surface of the dentin to prevent collapse of the dentinal
collagen fibrils, thereby keeping the collagen fibrils expanded and allowing the adhesive resin to
infiltrate and create a hybrid layer.9 In addition, the primer is also an amphiphilic compound,
therefore it can infiltrate the hydrophilic dentinal structure as well as the hydrophobic adhesive
resin.9 The hydrophobic adhesive resin infiltrates into the demineralized dentin, creating a hybrid
layer.9 The hybridization provides a micro-mechanical retention and increases the bond strength
of an adhesive.9 Research in the past few years had been directed at reducing technique-
5
sensitivity, decreasing clinical application time and reducing the post-operative sensitivity of
bonding procedures.10
The basic mechanism of adhesion between tooth structure and an adhesive bonding agent is
based on an exchange process such that the minerals of hard tissue are replaced with resin
monomers that creates a micromechanical bond.8 Adhesives can be classified into two main
systems based on their mechanism of adhesion, which are the total-etch system and the self-etch
system. The main difference between these 2 systems is the acid component of the adhesive. In
addition to being classified based on their mechanism of adhesion, adhesives can also be
classified in generations, which is based on their type of bonding system. Overall, there are seven
generations of adhesives; however, only adhesives from the fourth to seventh generation are
currently available on the dental market.8 Forth generation adhesives, also known as three-step
etch and rinse adhesives are considered the conventional multi-step adhesives. Fifth generation
or two-step etch and rinse adhesives involves acid etching followed by the combined application
of a primer and adhesive resin. The sixth generation or two-step self-etch adhesives involve the
application of an acidified primer followed by the adhesive resin. The seventh generation or one-
step self-etching adhesive involved the application of a combined acidified primer and adhesive
resin in a single step.8 (Figure 1) Although in the past, the main way to classify adhesives was
based on their generation, today they are mainly classified according to their mechanism of
adhesion.
2.1.1 Total-Etch System
This system, which is also known as etch and rinse adhesives, requires an initial etching step,
followed by a compulsory rinsing procedure, application of a primer and finally the placement of
an adhesive resin.11
The most commonly used acid is 30-40% phosphoric acid.12
Phosphoric acid
is a strong acid with a pH of less than 0.5, thus it is effective in dissolving the hydroxyapatite
minerals of the enamel and dentin, resulting in exposure of the enamel’s crystal structure and the
microporous network of dentinal collagen.5 The phosphoric acid etching step modifies the dentin
and enamel surfaces via demineralization of hydroxyapatite to allow penetration of the adhesive
resin into the tooth surface. Once the phosphoric acid has been rinsed away, the following step
consists of applying a primer, which contains specific monomers that are hydrophilic, like 2-
hydroxyethyl methacrylate (HEMA), dissolved in organic solvents like acetone, ethanol or
6
water.11
HEMA will improve the wettability and promote the re-expansion of the collagen
network while the solvents displace water from the tooth surface, thus preparing the collagen
network for the adhesive resin infiltration.11
Finally, an adhesive resin, which contains
hydrophobic monomers, is applied onto the tooth surface and it will infiltrate into the
interfibrillar spaces of collagen networks, dentin tubules and enamel structure. The presence of
resin tags inside the dentin tubules and enamel structure creates a hybrid layer, which provides
the micromechanical retention.11
In enamel, there are two types of resin tags within the etch pits.
Macro-tags fill the space around the enamel prisms and micro-tags result from resin infiltration
and polymerization within the etch-pits at the cores of the etched enamel prisms. The latter
contributes the most to enamel retention.13
Aside from the traditional three-step etch and rinse adhesives, simplified two-step adhesives also
exist within this system. The only difference between these adhesives is that the two-step
adhesives have combined the primer and adhesive resin into one single solution. These
simplified adhesives perform inferiorly when compared to the traditional three-step adhesives
due to their reduced ability to infiltrate the demineralized tooth structure and since they are more
hydrophilic, they are more prone to water sorption and long term hydrolytic degradation.11
The etching of enamel surfaces continues to be the standard enamel surface preparation
technique.14
The three-step etch and rinse commonly researched in the orthodontic literature is
Adper™ Scotchbond™ Multi-Purpose Adhesive (3M ESPE).14, 15
It is a bonding system that is
able to produce a bond to both enamel and dentin.15
A study by Olsen et al., (1997) which
assessed the shear bond strength of orthodontic brackets on uncut enamel, proved that this
adhesive can be used for orthodontic treatment, because it had a SBS of 13 ± 4.8 MPa.15
These
results are in agreement with studies by Mulcahey et al.(1999)14
and Sreedhara et al.(2015).16
2.1.2 Self-Etch System
With this system, there is no isolated etching step because the primer contains acidic monomers
that can simultaneously etch and prime the dental substrates.11
Within the self-etching system,
there are two subdivisions, which are known as the self-etch primers and self-etch adhesives. For
the self-etching primers, following the application of the acidic primer, an adhesive resin must be
placed and this is known as a 2-step system. On the other hand, the self-etch adhesives have
7
combined the acidic monomer, primer and adhesive resin all in one bottle, thus no mixing is
required. Therefore, these adhesives are also known as a 1-step system.8
Although the required components of an adhesive are included within the self-etch adhesives,
they differ from their etch-and-rinse counterparts in composition and performance. When these
self-etching adhesives are applied to a tooth, the acidic monomers will demineralize and the
adhesive resin will penetrate into the surface simultaneously, thus ensuring that there is no gap
within the hybrid layer.5 The hybrid layer produced by the self-etching system depends on the
ability of the acidic monomers to demineralize the dental substrate.11
The acidity of the
adhesives within the self-etch systems vary considerable depending on their composition, thus
they can be classified as strong (pH ≤1), moderately strong (pH 1-2), mild (pH 2-2.5) and ultra-
mild (pH >2.5) adhesives.17
Strong self-etch adhesives like the Adper Prompt-L Pop (3M ESPE, St Paul, MN, USA) are
capable of deep demineralization, thus creating a micromechanical tooth-adhesive interface that
is similar to the etch-and-rinse system.11, 12
On the other hand, mild self-etching adhesives, that
only partially demineralize tooth substrate, rely on a twofold bonding mechanism, which is a
combination of a micromechanical bond and a chemical bond.11, 12
This twofold bonding
mechanism is believed to be advantageous for bonding effectiveness and durability.11, 12
The
desirable chemical bond, that is achieved by only some self-etching adhesives, is related to the
presence of specific functional monomers in their composition, like 10-MDP (10-
methacryloyloxydecyl dihydrogen phosphate) or 4-META (4-methacryloxydecyl trimellitic
acid).11
These monomers usually contain a carboxylic or phosphoric acid group, which etches the
tooth surface, creating sufficient surface porosity to obtain the mechanical retention through
hybridization.11, 12
Their hybrid layer is usually no deeper than 1 µm and resin tags are hardly
seen but neither the thickness of the hybrid layer, nor the length of the resin tags are important
for achievement of bonding effectiveness and stability.11, 12
In addition, since they only partially
demineralize the enamel and dentin, a large amount of hydroxyapatite crystals remains around
the collagen fibrils.11, 12
These functional monomers (i.e., 10-MDP or 4-META) are able to
ionically bond with calcium in hydroxyapatite, forming complexes with the calcium ions, thus
creating a chemical bond.11
The self-etch adhesives with these functional monomers are able to
8
achieve both the mechanical and chemical retention. The challenge is to keep the calcium-
carboxylate or calcium-phosphate bonds stable within a hydrophilic environment.12
Furthermore, in order for the acid, primer and adhesive resin to be stable within a single bottle,
manufacturers needed to make the self-etch adhesives more hydrophilic than their two-step and
etch-and-rinse counterparts.11
This property may weaken the effectiveness and stability of
adhesion to the dental substrate.11
The hydrophilicity of the self-etch adhesives causes the
adhesive to act like as semi-permeable membrane that attracts water, which can contribute to the
hydrolysis of the resin polymers and degradation of the tooth-adhesion bond over time.11, 13
HEMA is a water soluble methacrylate monomer that is used to increase the wettability and
hydrophilicity of an adhesive. When a relatively high concentration of HEMA is incorporated, it
also improves the compatibility of the hydrophobic and hydrophilic components within the
adhesive solution.11
The differences in the amount of HEMA and its concentration among the
various self-etching adhesives can have a significant effect on the tooth-adhesive interface.
The weakest property of mild self-etch adhesives is their bonding potential to enamel.12
It has
been reported that some self-etch adhesives bond reasonably well to ground enamel, but there are
general concerns about the adhesion to unground, aprismatic enamel where micromechanical
retention is hardly achieved.11
9
Figure 1: Classification of adhesive bonding agents - Adapted from McLean et al.8
The mild self-etch adhesives are the newest development in adhesive dentistry and are known as
universal adhesives or all-in-one adhesives. There are multiple universal adhesives on the market
like All-Bond Universal (Bisco, Schaumburg, IL, USA), ScotchbondTM
Universal Adhesive (3M
ESPE), Xeno IV (Dentsply Caulk, Milford, DE, USA), G-ænial Bond (GC America, Alsip, IL,
USA), Clearfil Universal Bond (Kurraray, New York, NY, USA) and OptiBond XTR (Kerr
Dental, Orange, CA, USA). These self-etching adhesives claim to have the following
advantages:
Reduce the risk of saliva contamination
Less post-operative sensitivity
Bonding procedure is less technique sensitive due to the use of non-rinse acidic primers
Reduce the risk of making errors during application and manipulation
Less chair time due to reduced bonding time and elimination of the rinsing phase
Two mechanisms of bonding to tooth structure – micromechanical and chemical 4, 18
Adhesives
Etch and Rinse
Three-Step
Etch - Prime -Adhesive Resin
4th Generation
Two-Step
Etch - Prime + Adhesive Resin
5th Generation
Self Etch
Two-Step
Etch + Prime - Adhesive Resin
6th Generation
One-Step
Etch + Prime + Adhesive Resin
2 Components
Require mixing
6th Generation
1 Component
All-in-one
7th Generation
10
Three common self-etching adhesives reported in the literature are All-Bond Universal (Bisco),
ScotchbondTM
Universal Adhesive (3M ESPE) and Clearfil Universal Bond (Kurraray). Rosa et
al. (2015) systemic review on the bond strength of universal adhesives demonstrated that All-
Bond Universal (Bisco) and ScotchbondTM
Universal Adhesive (3M ESPE) are the two most
commonly researched adhesives in the literature.19
Clearfil Universal Bond (Kurraray) is one of
the newest universal adhesives on the market as it was only released in 2014 by the company.20
2.2 Adhesives in Orthodontics
In 1965, Newman introduced the concept of bonding orthodontic brackets with adhesives. This
revolutionized the orthodontic profession because it provided the ability to bond brackets directly
to the tooth structure, instead of placing bands with soldered brackets around each individual
tooth.4
Today, bonding orthodontic brackets with adhesives to the tooth’s surface is standard
practice for all orthodontists. Orthodontic brackets are constantly loaded by various forces, thus
to efficiently and effectively complete treatment, the adhesive must be able to withstand these
forces. Reynolds (1975) stated that a resistance of 5.9-7.8 MPa is sufficient to withstand the
masticatory and orthodontic forces encountered during orthodontic treatment.1 In Reynolds
review paper, he stated that occlusal loading is the major force to be withstood by brackets since
the range of occlusal forces during mastication was between 10-100 kg, whereas the maximum
orthodontic force varied between 1-5 kg.1 By comparing various studies’ in vitro bond strengths
to their clinical failure rates, he determined that the maximum value of 60-80 kg/cm2 (5.9-7.8
MPa) would be a reasonable bond strength for orthodontic brackets.1 This SBS range is still
accepted in current orthodontic literature as the ideal bond strength orthodontic brackets should
have in order to withstand the forces applied during orthodontic treatment.2-4, 21
This is due to the
fact that controlled orthodontic tooth movement relies on the interaction between the archwire
and the bracket.22
The success of fixed appliance therapy vastly depends on the capability of
adhesive systems to resist failure to a large number of forces directed to bracket - adhesive -
enamel junction as well as various factors in the mouth. The overall failure rate for orthodontic
brackets is 4.7-6%.23
Bonding failure can lead to an increased number of appointments,
emergency appointments, longer treatment time for the patient, patient inconvenience and
financial cost for the orthodontist. Therefore, orthodontic adhesives should have sufficient bond
11
strength to ensure that the bracket will remain bonded to the unground enamel surface for the
whole duration of treatment in order to effectively and efficiently move the teeth.
While it is critical that the adhesive is strong enough to withstand the various forces applied
during the orthodontic treatment, once the treatment is completed the orthodontist must remove
the brackets with least discomfort to the patient and without damage to the enamel surface.24
Many studies have shown that debonding brackets can cause enamel loss, especially when the
fracture occurs at the enamel-adhesive interface.25
Enamel cracks may occur or propagate during
debonding, which can affect the integrity of the enamel and cause esthetic problems for the
patients. 25-27
Bond failure at the bracket-adhesive interface or within the adhesive is considered
safer than a fracture at the enamel-adhesive interface.15, 16, 25, 28
Studies have shown that an
increased bond strength is associated with bracket failure occurring closer to the enamel-
adhesive interface, thus causing more stress and cracks in the enamel.28
Furthermore, the bracket
material and the method of debond also affect the amount of enamel loss.26
In addition to the
above mentioned properties, the adhesive should be non-irritating to the oral mucosa, allow
adequate working time for proper positioning of the brackets while setting quickly enough for
patient comfort, provide a simple way of application, and a convenient way of curing.24
Given
these particularities in orthodontics, the ideal adhesive must contain these properties:
− Adequate bond to uncut-enamel surface
− Least amount of enamel damage when debonding
− Brackets must remain on the teeth throughout the entire treatment time, thus the
adhesive must withstand the various forces that are encountered during orthodontic
treatment 4
2.3 Adhesive Testing
Laboratory testing on near ideal substrates and under optimal in vitro conditions is valuable as a
screening test of adhesive materials.12
Bond strength testing can reveal valuable clinical
information and determine the effectiveness of adhesive materials within the specific test set-
up.12, 18
A good correlation exists between laboratory and clinical effectiveness, thus it has been
concluded that laboratory testing can predict clinical effectiveness.12
In order for orthodontic
treatment to be successful, the orthodontic brackets must remain on the enamel surface
throughout treatment, thus all new dental adhesives are developed in an attempt to achieve a high
12
bond strength.4 The rational for this test is based on the concept that an adhesive with a stronger
bond strength has a stronger interaction with the tooth and thus is better able to resist stresses
imposed by oral function.12, 18
Bond strengths have been measured in the literature by multiple
testing types; most commonly shear, push-out, tensile and torsion.29
Static tests are classified into
macro-tests when the bond area is > 3mm2 and micro-tests when the area is less than 3mm
2.29
The gold standard test to assess orthodontic adhesives is an in vitro shear bond strength test with
an universal testing machine.3, 30
Shear bond strength will assess the degree to which an adhesive
bond can resist shear forces, which is the nature of occlusal and masticatory forces on the
brackets.29
Therefore, this test best represents clinical situations. Given that these new universal
adhesives have rarely been tested on uncut enamel, it is critical to assess their shear bond
strengths with orthodontic brackets prior to implementation into clinical practice.
In the literature, shear bond strength tests are usually followed by a mode of fracture analysis and
a microscopic evaluation of the bonded surface. In addition to bond strength, orthodontists also
want to ensure that the adhesive will not damage the enamel surface when the brackets are
debonded, thus they are interested in quantifying the amount of enamel damage after
debonding.24
This can be done by performing the Adhesive Remnant Index Score (ARI), which
grades the amount of residual adhesive on each tooth after debonding31
or by quantitatively
assessing the amount of residual resin. Both these methods can be used to assess the quality of
adhesion between the adhesive and tooth and between the adhesive and bracket base. Bond
failure at the bracket-adhesive interface or within the adhesive is considered safer than a fracture
at the enamel-adhesive interface.15, 16, 25, 28
Studies have shown that increased bond strength is
associated with bracket failure occurring closer to the enamel-adhesive interface, thus causing
more stress and cracks in the enamel.28
Scanning electron microscopy (SEM) provides a more
detailed examination of the enamel surface after debonding such that micro-enamel fractures can
be detected. The rational for these tests is that if the majority of the adhesive remains on the
enamel surface, the likelihood of enamel fracture decreases but the orthodontist must spend time
removing the adhesive after debonding, which results, overall, in the least amount of enamel
lost.26
13
2.4 Bond Strength Stability
Given that orthodontic treatment usually takes about 2 years, it is critical that the adhesive’s
bond strength is stable long term. The most validated method to assess the durability of adhesion
is to age the adhesive bonded to either enamel or dentin for a certain period of time.18
In the
orthodontic literature, the most commonly used artificial aging technique involves storing the
bracketed teeth in water.32
After about three months, all adhesive systems exhibit mechanical and
morphological evidence of degradation that resembles in vivo aging.32
In the long term, however,
the bonding effectiveness of some adhesives drops dramatically, whereas the bond strengths of
other adhesives are more stable.18
Studies have shown that self-etching adhesives, due to their
hydrophilic nature, degrade faster than hydrophobic total-etch adhesives.18
While the bonded tooth is stored in water, the adhesive-tooth interface is subjected to chemical
degradation. A decrease in bonding effectiveness can be caused by degradation of the interface
by hydrolysis of the resin and collagen.18
Hydrolysis involves breaking the covalent bond
between the resin polymers by the addition of water, resulting in a loss of resin mass.33
This is
one of the main reasons for resin degradation within the hybrid layer, which contributes to a
reduction in bond strength.33
Furthermore, water can infiltrate and decrease the mechanical
properties (i.e.: modulus of elasticity) of the polymer matrix by swelling and reducing the
frictional forces between the polymer chains, a process known as plasticization.18
In addition,
any uncured monomer and break-down products can be washed away, thus weakening the
bond.18
The hydrophilicity of the self-etch adhesives causes them to act as semi-permeable
membranes, attracting water to the tooth-adhesive interface, which can potentially lead to faster
degradation and reduced bond strength over time.18
On the other hand, other researchers have stated that the two-fold bonding mechanism of the
self-etching adhesives is advantageous for bonding durability. The micromechanical bonding
component provides resistance to abrupt debonding stresses, whereas the chemical interaction is
more resistant to hydrolytic break-down.18
According to studies, the Ca-salt of 10-MDP are
hardly soluble, creating a very stable molecular adhesion to hydroxyapatite.34
Furthermore,
keeping hydroxyapatite around collagen via the partial demineralization may better protect it
against hydrolysis and degradation of the bond.34
14
The bonding effectiveness of the self-etch adhesives has been attributed to their ability to
simultaneously demineralize and infiltrate enamel, therefore theoretically decreasing incomplete
penetration of the adhesion.11, 33
While some studies have shown a discrepancy between the
depth of demineralization and resin infiltration with the self-etch adhesives, it is significantly
smaller than the etch-and-rinse counterparts.34
In addition to the above mentioned factors, excess
solvent, a very thin adhesive layer, incomplete polymerization and insufficient enamel etching
have all been attributed to affecting the bonding performance of self-etching adhesives.13, 33
2.5 Factors Affecting the Bond Strength
2.5.1 Cut vs. Intact Enamel
Adhesives, especially the self-etch system of adhesion, have lower bond strengths to intact
enamel compared to cut enamel surfaces.12, 35-37
The lower bond strength may be due to the fact
that the morphological structure of the intact, peripheral enamel surface is different than that of
the middle enamel layer. The intact enamel surface is prismless, hypermineralized and contains
more inorganic material than the middle enamel layer.37
In addition, changes occur in the
outermost enamel layer after eruption and it can contain more fluoride than instrumented enamel,
which all result in a less pronounced enamel etching pattern for intact enamel compared to cut
enamel.35
This causes the adhesive resin to inadequately penetrate into the microporosities of
intact enamel surface, resulting in lower bond strengths.37
Most of the adhesive materials are
developed through bonding tests to ground enamel surfaces, however due to these inherent
differences, the bonding performance of the current adhesive systems should be evaluated also
on intact enamel surfaces. 37
The different morphologies within the enamel also leads to differences in surface energy, which
can affect an adhesive’s bond strength. Adhesion relies on the fact that the adhesive must come
in close contact with the substrate to perform a chemical adhesion and/or micromechanical
bond.38
The three factors that determine the adhesive’s ability to come into contact with the
substrate are the wettability of the substrate by the adhesive, the viscosity of the adhesive and the
morphology of the substrate.39
The capacity for adhesion between enamel and an adhesive
directly correlates to the surface energy of the enamel and the surface tension of the adhesive.39
Any adhesive with a lower surface tension than the critical surface energy of the enamel will wet
15
the surface and create a proper adhesion.39
Typically, the surface energy of an adhesive resin is
in the range of 34-38 mJ·m2.39
The intact enamel surface has a lower surface energy than the
adhesive resin, resulting in a decreased capacity for adhesion.39
The outer enamel is a low energy
surface, because it has reacted with various elements within the oral environment and is covered
with a strongly adherent organic pellicle.40
Cut enamel has a high surface energy and is highly
reactive, which means that the resin will adapt well to the surface, resulting in a better wettability
and higher bond strength.39
2.5.2 Mechanical Properties of Enamel
It has been well documented in the literature that enamel hardness and other mechanical
properties vary among individuals. These variations can have significant effect on an adhesive’s
bond strength. Cardoso et al. (2009) assessed the cross-sectional hardness of enamel from human
teeth at different posteruptive ages. They found that recently erupted dental enamel has
characteristics that make them more susceptible to demineralization. These characteristics are
higher porosity, higher carbonated apatite content and higher concentration of impurities in
apatite.41
However as the posteruptive age increases, the cross sectional hardness of the enamel
also increases, which suggests that the exposure in the oral cavity results in deep enamel
maturation.41
Park et al. (2008) also found that near the tooth’s surface, both the hardness and the
elastic modulus of enamel was greater in older teeth than younger teeth.42
These structural and
mechanical differences are significant because studies have shown that the shear bond strength
of immature permanent teeth is significantly lower than mature permanent teeth.43
In addition to the biological differences, environmental factors can also affect the enamel’s
mechanical properties and the shear bond strength of dental adhesives. Wongkhantee et al.
(2006) assessed the effect of acidic foods and drinks on the surface hardness of enamel. The
study simulated the washing effect of saliva after an individual drinks one can of soft drink. The
results showed that cola soft drinks, orange juice and sport drinks all significantly reduce the
surface hardness of enamel. The reduction in enamel hardness occurs via dental erosion which is
a loss of minerals from the tooth surface due to a chemical process of acidic dissolution.44
Similarly to above, the loss of mineral content and decrease in enamel hardness will result in a
decreased bond strength.
16
Another environmental factor that affects the composition of enamel and the shear bond strength
of adhesives is fluoride exposure. The application of fluoride promotes the formation of
fluorapatite in enamel, which is less soluble than hydroxyapatite. Several studies report that the
application of different fluoride solutions on sound enamel before orthodontic bonding results in
decreased shear bond strengths.45
In addition, fluorosed teeth have a negative effect on the shear
bond strength of orthodontic brackets.2
2.5.3 Light Curing Unit
Irrespective of the light curing unit used, it should be capable of adequately polymerizing the
material, which is directly related to the light power and irradiation time. An adequately
polymerized resin has a higher bond strength than a material with a lower degree of conversion.46
The light-emitting diode (LED) is commonly used for light polymerization in orthodontics. The
advantages of LED are coincidence of peak irradiance of the light with camphorquinone,
duration of about 10 000 hours, resistance to impacts, little power consumption and can run on
rechargeable batteries.46
Many studies have assessed the influence of LED on the shear bond strength of brackets to
determine the optimal polymerization time. A meta-analysis by Finnema et al. (2010) showed
that the total polymerization time, reported in the literature, varies from 2 to 60 seconds, with a
mean of 25.3 seconds. They found that each additional second of polymerization increased bond
strength by 0.077 MPa.3 However, the meta-analysis included studies that used different light
polymerization devices and different resins which all require different ideal polymerization
times.
In addition, a study by Dall’Igna et al. (2011) looked at the effect of light curing units on SBS of
brackets bonded to bovine enamel. They used the Ortholux LED curing light (3M-Unitek,
Monrovia, CA, USA) to bond metal brackets with Transbond XT (3M-Unitek). They were light
cured for 5, 10 and 15 seconds and after 24 hours, their shear bond strengths were assessed. The
results showed that there was no significant difference among the three groups. The highest
mean SBS was obtained with the LED at 15 seconds (16.68 MPa), which did not significantly
differ from the LED 10 second (14.76 MPa) or 5 second (13.92 MPa) groups. The LED at 5
seconds provided sufficient mean SBS to resist both orthodontic and masticatory forces.46
These
17
results are also supported by a study by Rêgo et al. (2007), which assessed the LED curing times
at 40, 10 and 5 seconds to bond metallic brackets with Transbond XT (3M-Unitek) to bovine
enamel. Group I was light cured for 40 seconds using a halogen light, while Groups II, III, and
IV were light-cured with a LED light unit for 40, 10, and 5 seconds, respectively. The mean
shear bond strengths were 4.87 MPa for Group I, 5.89 MPa for Group II, 4.83 MPa for Group III
and 4.39 MPa for Group IV. There were no statistically significant differences among the groups
regarding the shear bond strength. Neither of the types of light-curing sources or exposure times
influenced the shear bond strength of metallic brackets.47
2.5.4 Bracket Type
There are currently two types of brackets used by orthodontists: stainless steel and ceramic
brackets.2 Stainless steel brackets are more commonly used than ceramic brackets. A review by
Bakhadher et al.(2015) showed that ceramic brackets produce a significantly higher SBS than
stainless steel brackets.2 In addition to the type of bracket, the bracket base design and size can
also influence the SBS.2 Studies have shown that brackets with laser structured base have higher
mean SBS than brackets with foil mesh base. Of the brackets with foil mesh base, the 60-gauge
microetched foil mesh base brackets perform better than the 80-gauge and 100-gauge brackets.2
2.5.5 Storage Medium
The most commonly used artificial aging technique is long-term water storage.18
A compilation
of orthodontic literature, as shown in Table 1, further validates this statement as the majority of
the studies in the literature used distilled water as their storage medium. Artificial saliva solution
can also be used but the bond strength reductions are similar to those obtained with pure water
degradation.18
A study by Jaffer et al. (2009) proved that water, isotonic saline solution, and
chloramine T storage produced comparable bond strengths.48
In addition, a meta-analysis by
Finnema et al. (2009) which assessed the in vitro orthodontic bond strength testing, showed that
57 of the 65 experimental groups they analyzed used distilled water as a storage medium.3
2.5.6 Loading Rate
The crosshead speed for the shear bond test may affect the results of the test since the adhesives
have viscoelastic properties. Therefore, faster rates may produce higher bond strengths.49
The
18
crosshead speed varies greatly in the literature from 0.5 mm/min to 5 mm/min. In a study by
Oshida and Miyazaki (1996), they found that the crosshead speed did not affect the bond strength
as long as it was below 1 mm/min.50
There was no difference between 0.5 mm/min and 1
mm/min, therefore they concluded that these speeds should be preferred for shear bond strength
testing.50
More recently, a study by Shooter et al. (2012) assessed the effect of changing the
crosshead speed on the shear bond strength of orthodontic bonding adhesives. The results proved
that there was no significant difference in the mean SBS between the crosshead speeds of 0.5
mm/min, 1 mm/min, 2 mm/min and 5 mm/min. They concluded that studies using different
crosshead speeds may be used to compare the SBS of other orthodontic adhesives.51
A compilation of the orthodontic literature, which assessed the bond strength of brackets,
including their study design and ARI scoring index, is summarized in Table 1. Table 1 shows the
heterogeneity of the various study designs. The majority of the studies were conducted on either
human incisors, premolars and molars or bovine teeth and the sample size varied between 7-35
teeth per group. Shear bond strength tests were either conducted immediately, after 24h, 48h,
72h, 5-7 days, 6 months or 1 year with a crosshead speed of 0.5-1 mm/min. Most of the studies
stored the teeth in distilled water. In these studies, the mode of failure was either assessed by an
ARI scoring index of 0-3, 0-4 or 1-5, as adhesive, cohesive or mixed fractures or as a percentage
of surface area of the bracket base with remnant adhesive. Lastly, very few studies assessed the
enamel surface under a scanning electron microscope.
19
Table 1: Summary of the studies assessing the bond strength and ARI of various adhesives
Author Article Title Tests Sample
Size
Storage Speed Wire/
Instron
Microscope
/Mag
Mode of
Failure
SEM
Elsaka et al.52
Evaluation of
stresses
developed in
different
bracket-cement-
enamel systems
using finite
element analysis
with in vitro
bond strength
tests
Tensile
bond
strength
Shear bond
strength
Human
incisors
15
Phosphate-
buffered
saline
0.5mm/
min
0.020" SS
wire bent
in U form
Stereomicroscope
20x
ARI 0-3
−
Algera et al.
53
A comparison of
finite element
analysis with in
vitro bond
strength tests of
the bracket-
cement-enamel
system
Tensile
bond
strength –
72 hr
Shear bond
strength –
72 hr
Bovine
teeth
15
Tap water
0.5mm/
min
0.020" SS
wire bent
in U form
Stereomicroscope
25x
ARI 0-3
25x
Algera et al. 54
The influence of
different bracket
base surfaces on
tensile and shear
bond strength
Tensile
bond
strength –
24 hr
Shear bond
strength –
24 hr
Bovine
teeth
10
Tap water
0.5mm/
min
Round SS
1mm
diameter
Stereomicroscope
25x
ARI 0-3
−
20
Sunilkumar
et al. 55
A comparison
study of the
shear and tensile
bond strength
using 3 types of
direct bonding
adhesives on
stainless steel
brackets: an in
vitro study
Tensile
bond
strength –
24 hr
Shear bond
strength –
24 hr
Human
premolars
20
− − − − − −
Li
56
Effect of
flexural strength
of orthodontic
resin cement on
bond strength of
metal brackets
to enamel
surfaces
Tensile
bond
strength –
24 hr
Shear bond
strength –
24 hr
Human
centrals
7
Water
1mm/
min
0.457 x
0.558 mm
Chisel -
edge
plunger
Light microscope
10x
ARI 0-3
−
McLean et
al.8
Enamel bond
strength of new
universal
adhesive
bonding agents
Shear bond
Strength –
24 h and 6
months
Human
molars
10
0.5%
Chloramine
T solution
(6 months)
Distilled
water
1mm/
min
Chisel
Light microscope
10x
Adhesive
Cohesive
Mixed
−
Sharma et al.
4 A comparison of
shear bond
strength of
orthodontic
brackets bonded
with 4 different
orthodontic
Shear bond
strength –
24 hr
Human
premolars
20
0.1%
thymol
Distilled
water
1mm/
min
Flat end of
a steel rod
Fiber optic light
10x
ARI 0-3
3000x
21
adhesives
Zielinski et
al.57
Comparison of
shear bond
strength of
plastic and
ceramic brackets
Shear bond
strength
Bovine
incisors
12
0.5%
Chloramine
T solution
Distilled
water
1mm/
min
−
−
ARI 0-4
−
Vilchis et al. 58
Shear Bond
strength of
orthodontic
brackets bonded
with difference
self- etching
adhesives
Shear bond
strength –
24 hr
Human
premolars
35
0.1%
thymol
Distilled
water
0.5mm/
min
0.017 x
0.025 SS
Flat end of
rod
−
ARI 0-3 −
Harari et al.
31 A new
multipurpose
dental adhesive
for orthodontic
use: an in-vitro
bond strength
study
Shear bond
strength –
72 hr
Human
premolars
20
Saline 0.5mm/
min
Shearing
instrument
− − −
Mulcahey et
al.14
In vitro bracket
bond strength to
acid-etched or
air-abraded
enamel
Shear bond
strength –
5-7 days
Human
molar &
premolars
35
Saline 0.05in/
min
− −
− −
Olsen et al.
15 Evaluation of
Scotchbond
multipurpose
Shear bond
strength –
72 hr
Human
premolars
24
0.1%
thymol
5mm/
min
Steel rod
with
flattened
−
ARI 1-5 −
22
and maleic acid
as alternative
methods of
bonding
orthodontic
brackets
Deionized
water
end
Mirzakouch-
aki et al.59
Effect of self-
etching
primer/adhesive
and
conventional
bonding on the
shear bond
strength in
metallic and
ceramic brackets
Shear bond
strength -
thermocycle
- 1 week
Human
premolars
25
0.2%
thymol
Distilled
water
0.5mm/
min
Steel rod
with flat
end
Stereomicroscope
10x
ARI 1-5 −
Boruziniat et
al.21
Evaluation of
bond strength of
orthodontic
brackets without
enamel etching
Shear bond
strength
Human
premolars
15
Distilled
water
0.5mm/
min
− Stereomicroscope
10x
ARI 0-3 −
Takamizawa
et al.60
Influence of
water storage on
fatigue strength
of self-etch
adhesives
Shear bond
strength –
24 hr, 6
months, 1
year
Fatigue
strength
Human
molars
15
Distilled
water
1mm/
min
Chisel-
shaped
metal rod
Optical
microscope
20x
Adhesive
Cohesive
Mixed
10kv
40x
2500x
Barkmeier et
al.61
Shear bond
strength of
Shear bond
strength –
Human
molars
Distilled
water
5mm/
min
Chisel-
shaped rod
Stereobinocular
microscope
Adhesive
Cohesive
−
23
composite to
enamel and
dentin using
Scotchbond
Multi-Purpose
24 hr 10 20x Mixed
Swift et al.
62 Bond strength of
Scotchbond
Multi-Purpose
to moist dentin
and enamel
Shear bond
strength –
24 hr
Human
molars
10
Distilled
water with
thymol
disinfectant
Distilled
water
0.5cm/
min
− Dissecting
microscope
Adhesive
Cohesive
Mixed
−
Yadala et
al.63
Comparison of
shear bond
strength of 3
self-etching
adhesives: an in-
vitro study
Shear bond
strength -
24h
Human
premolars
15
Formalin
Distilled
water
1mm/
min
− Optical
microscope
50x
ARI 0-3 −
Sreedhara et
al. 16
Effect of self-
etch primer-
adhesive and
conventional
adhesive
systems on the
shear bond
strength and
bond failure of
orthodontic
brackets: a
comparative
study
Shear bond
strength –
48 hr
Human
premolars
20
Deionized
water
5mm/
min
− Stereomicroscope
10x
ARI 1-5 −
24
Takamizawa
et al.64
Influence of
different etching
modes on bond
strength and
fatigue strength
to dentin using
universal
adhesive
systems
Shear bond
strength –
24 hr
Shear
fatigue
strength –
24 hr
Human
molars
15
Distilled
water
1mm/
min
Chisel-
shaped
end
Optical
microscope
20x
Adhesive
Cohesive
Mixed
10kV
40x
1000x
5000x
20000
x
Mirzakouch-
aki et al.28
Shear bond
strength and
debonding
characteristics
of metal and
ceramic brackets
bonded with
conventional
acid-etch and
self-etch primer
systems: an in
vitro study
Shear bond
strength
Human
premolars
30
0.1%
thymol
solution
0.5mm/
min
Steel rod
with
cutting
edge
Stereomicroscope
40x
ARI 1-5 −
Isolan et al.
65 Bond strength of
a universal
bonding agent
and other
contemporary
dental adhesives
applied on
enamel, dentin,
composite and
porcelain
Shear bond
strength –
24 hr
Micro-
tensile bond
strength
Bovine
incisors
20
0.5%
Chloramine
-T solution
Distilled
water
1mm/
min
− Light
stereomicroscope
40x
Adhesive
Cohesive
Mixed
−
Buyukyilaz et Effect of self- Shear bond Human Distilled 0.5mm/ Chisel- − ARI 1-5 20-25
25
al.66
etching primers
on bond strength
- are they
reliable?
strength –
24 hr
premolars
20
water min edge
plunger
kV
1500x
2000x
Kim et al.
67 Phosphoric acid
incorporated
with acidulated
phosphate
fluoride gel
etchant effects
on bracket
bonding
Shear bond
strength –
1 hr and 24
hr
Human
premolars
10
0.12%
thymol
Deionized
water
1mm/
min
Chisel-
edge
plunger
Stereomicroscope
5x
ARI 1-5 15 kV
1000x
Klocke et
al.68
Plasma arc
curing lights for
orthodontic
bonding
Shear bond
strength –
48 hrs
Human
incisors &
premolars
30
1%
chloramine-
T solution
Distilled
water
1mm/
min
Chisel-
shaped rod
Stereomicroscope
10x
ARI 0-3 −
McCourt et
al.69
Bond strength of
light-cure
fluoride-
releasing base-
liners as
orthodontic
bracket
adhesives
Shear bond
strength –
24 hr, 4
weeks
Human
premolars
10
Distilled
water
0.5mm/
min
− − − −
Northrup et
al.70
Shear bond
strength
comparison
between two
orthodontic
Shear bond
strength –
40 hrs
Human
premolars
20
Distilled
water
0.1mm/
min
− Stereomicroscope ARI 0-3 −
26
adhesives and
self-ligating and
conventional
brackets Romano et
al.71
Shear bond
strength of
metallic
orthodontic
brackets bonded
to enamel
prepared with
self-etching
primer
Shear bond
strength –
24 hr
Human
premolars
10
0.1%
thymol
Distilled
water
0.5mm/
min
− Stereomicroscope
8x
ARI 0-3 −
Sayinsu et
al.72
New protective
polish effects on
shear bond
strength of
brackets
Shear bond
strength –
72 hrs
Human
premolars
20
70% ethyl
alcohol
Distilled
water
3mm/
min
0.016 ×
0.022" SS
− − −
Sayinsu et
al.73
Light curing the
primer-
beneficial when
working in
problem areas?
Shear bond
strength –
72 hrs
Human
premolars
15
70% ethyl
alcohol
Distilled
water
3mm/
min
0.016 ×
0.022" SS
− − −
Tecco et al.
74 A new one-step
dental flowable
composite for
orthodontic use:
An in vitro bond
strength study
Shear bond
strength –
72 hrs
Human
premolars
20
0.1%
thymol
Deionized
water
1mm/
min
0.021 x
0.025" SS
Fiber-optic
transillumination
16x
ARI 0-3 −
Usumez et Effect of light- Shear bond Human Distilled 0.5mm/ Chisel- − ARI 1-5 −
27
al.75
emitting diode
on bond strength
of orthodontic
brackets
strength –
24 hr
premolar
20
water min edge
plunger
Vicente et
al.76
Influence of a
nonrinse
conditioner on
the bond
strength of
brackets bonded
with a resin
adhesive system
Shear bond
strength –
24 hr
Human
premolars
25/15
0.1%
thymol
Distilled
water
1mm/
min
− Microscope
connected to
camera
Image analysis
equipment
%
bracket
base
surface
with
remnant
adhesive
ARI 0-3
−
Vincente et
al.77
Shear bond
strength of
precoated and
uncoated
brackets using a
self-etching
primer
Shear bond
strength –
24 hr
Human
premolars
25/15
0.1%
thymol
Distilled
water
1mm/
min
− Microscope
connected to
camera
Image analysis
equipment
%
bracket
base
surface
with
remnant
adhesive
−
28
2.6 Quantitative Adhesive Remnant Evaluation
In addition to bond strength, orthodontists also want to ensure that the enamel surfaces are not
damaged when the brackets are removed at the completion of orthodontic treatment.4, 28, 58
When
the bonded brackets are removed, failure can occur between the enamel and adhesive as well as
between the adhesive and the bracket. These failures are known as adhesive failures. Cohesive
failures can also occur within the adhesive or within the tooth. If an adhesive has a strong bond
to the enamel, the bonding material may tear the enamel surface as it pulls away from it, thus
failure at the enamel surface is undesirable.22
Increased bond strength results in bracket failure
occurring closer to the enamel-adhesive interface, which causes more stress and cracks in the
enamel surface.28
The interface between the bonding material and the bracket is the failure site
preferred by most orthodontists when brackets are removed and it is considered ideal if the
adhesive remains on the tooth surface after debonding.22, 28
Often, bond failures are a
combination of adhesive and cohesive failures. One way of assessing the adhesive remaining on
the tooth surface is with the Adhesive Remnant Index (ARI) that was outlined by Artun and
Bergland (1984).78
The index was used to evaluate the amount of resin remaining and to identify
the location of bond failure (Table 2).
Table 2: ARI scoring index78
ARI Score Description Bond failure location
0 No adhesive left on the tooth Tooth-adhesive interface
1 Less than half of the adhesive left on the tooth Within the adhesive
2 More than half of the adhesive left on the tooth Within the adhesive
3 All of the adhesive left on the tooth Bracket-adhesive interface
Although this index is easy to use, a large drawback is its inability to differentiate between
samples with very little adhesive remaining on the surface. Therefore, the modified ARI was
created with a 5-point scale, as described in Table 3.
29
Table 3: Modified ARI scoring index59
ARI Score Description
1 All of the adhesive remained on the tooth
2 More than 90% of the adhesive remained on the tooth
3 More than 10% but less than 90% of the adhesive remained on the tooth
4 Less than 10% of the adhesive remained on the tooth
5 No adhesive remained on the tooth
While both these scoring indices are widely used, it should be noted that they both tend to mask
minor differences.79
Two teeth with the same ARI score can have significantly different amounts
of adhesive remaining on the tooth surface. These qualitative methods, which are currently used
to assess the amount of remnant adhesive left on the enamel surface, do not closely reflect the
quantitated area measurement of the remnant adhesive.80
On the other hand, quantitatively
assessing the amount of resin remaining on the enamel surface after debond gives a better
indication of the location of the bond failure. In addition, this assessment can also be used to
evaluate the quality of the adhesive-tooth adhesion and the adhesive-bracket adhesion. The
rational for this test is that if the majority of the adhesive remains on the enamel surface, the
likelihood of enamel fracture when debonding decreases.16, 25, 28
2.7 Significance of the problem
Currently in orthodontics, either a three-step or a two-step adhesive is used, based on practitioner
preference. Although these systems provide adequate bond strengths, the bonding appointments
are time-consuming for both the orthodontist and the patient. The newly developed universal
adhesives provide a one-step approach, which can significantly simplify the bonding process.6
These adhesives reduce the number of bonding steps and eliminate the need for acid etch, which
lessens the risk of contamination and reduces the bonding time.6 In the literature, the three self-
etching universal adhesives have been tested on dentin and cut-enamel surfaces. A previous in
vitro study by McLean et al. (2015) aimed to evaluate the shear bond strength of composite to
30
enamel using three universal adhesives, All-Bond Universal (Bisco), Scotchbond Universal
Adhesive (3M ESPE), and Clearfil SE (Kuraray). The results showed that the SBS on cut enamel
was 11 ± 2 MPa, 14 ± 3 MPa and 19.5 ± 6 MPa respectively after 24 hrs and 9 ± 4 MPa, 12 ± 7
MPa and 24 ± 5 MPa after storage in distilled water for 6 months, respectively.8 Manufacturers
of the three commercial adhesives have advertised the SBS to cut enamel to be 24 ± 4 MPa for
Scotchbond Universal Adhesive (3M ESPE)5, 29 MPa for All Bond Universal (Bisco)
81 and 23
MPa for Clearfil Universal Bond (Kuraray)20
. Although these measurements are within the
recommended 5.9-7.8 MPa1, they are all measured on cut enamel whereas in orthodontics, the
enamel surface is uncut. Very limited studies, however, exist on the performance of these
universal adhesives on uncut enamel or even within an orthodontic setting. Prior to implementing
the use of these adhesives to orthodontic practice, they should be investigated further to assess
their properties and bond strengths to ensure that they can withstand the masticatory and
orthodontic forces encountered during treatment. The bond strength of orthodontic brackets must
be able to withstand the forces applied during the orthodontic treatment. Reynolds (1975) stated
that 5.9-7.8 MPa of bond strength was sufficient to withstand these forces and this is the critical
SBS that all orthodontic adhesives should have.1
Bond strength is critical in orthodontics because orthodontic tooth movement relies on the
interface between the wire and the bracket to effectively move teeth, therefore the bracket must
remain bonded to the enamel surface of the tooth throughout the course of treatment.4, 21, 63
Not
only should the adhesive have adequate bond strength, it must also maintain the enamel
unblemished after debonding, thus the ARI and SEM will allow assessment of the mode of
fracture and visualization of the enamel surface to assess for risk of enamel fracture.47
Should the
findings of this in vitro project prove that the self-etching adhesives have SBS within the range
of 5.9 to 7.8 MPa, these adhesives can impact future research direction and be tested in an in vivo
setting. If the results of the study show that the universal adhesives have sufficient bond strength
to be used as orthodontic adhesives, orthodontists may consider using them in their clinical
practices, which may have numerous benefits for the patients and the orthodontist. These
adhesives may result in decreased chair time because their bonding procedures are less technique
sensitive and there is a reduction in bonding steps.
31
Materials and Methods 3
3.1 Study Design
This in vitro study evaluated the bond strength of orthodontic brackets to uncut enamel. The
factors under study are 1) different bonding agents at 4 levels (Scotchbond™ Universal Adhesive
(3M ESPE), All-Bond Universal (Bisco), Clearfil Universal Bond (Kuraray) and Adper™
Scotchbond™ Multi-Purpose Adhesive (3M ESPE), and 2) aging of dental bonding at 2 levels
(baseline and 6 month evaluation). The association of factors (4 × 2) resulted in eight groups (n =
20). The quantitative response variables are enamel shear bond strength, ARI score and
evaluation of remaining resin. Scanning electron microscopy was also used for qualitative
analysis of debonded surfaces.
3.2 Materials Used
The materials used are shown in Table 4.
Table 4: Composition and pH of the adhesives
Adhesive Composition pH
Scotchbond™ Universal
Adhesive5
MDP Phosphate Monomer
(10-Methacryloyloxydecyl dihydrogen phosphate)
Dimethacrylate Resins
HEMA (2-hydroxyethyl methacrylate)
Vitrebond Copolymer
Filler
Ethanol
Water
Initiators
Silane
2.7
All-Bond Universal64
MDP phosphate monomer
Bis-GMA (Bisphenol A-glycidyl methacrylate)
2.3
32
HEMA
Ethanol
Water
Initiators
Clearfil Universal Bond82
Bis-GMA (Bisphenol A diglycidylmethacrylate)
HEMA
MDP Phosphate Monomer
(10-Methacryloyloxydecyl dihydrogen phosphate)
Hydrophilic aliphatic dimethacrylate
Colloidal silica
dl-Camphorquinone
Silane coupling agent
Zirconium oxide
Accelerators
Initiators
Water
Ethanol
2.3
Adper™ Scotchbond™
Multi-Purpose Adhesive 83
Etchant: 35% H3PO4
Primer:
HEMA
Polyalkenoic acid polymer
Water
Adhesive Resin:
Bis-GMA
HEMA
Tertiary amines
Photo-initiator
Primer:
3.3
Adhesive
resin: 8.2
Transbond XT Light Cure
Adhesive84
Silane treated quartz
Bisphenol A diglycidyl ether dimethacrylate
(BISGMA)
Bisphenol A bis (2-hydroxyethyl ether)
33
dimethacrylate
Silane treated silica
Diphenyliodonium hexafluorophosphate
3.3 Sample Size Calculation
Sample Size Calculation was performed according to McLean et al. (2015)8, using the following
equations:
Comparison of 2 means: Equation: [(U+V)^2(σ1^2 + σ2^2)] / (u1-u0)^2
U (for 80% power) = 0.84 V( for 5% significance) = 1.96
Group 1 vs 2
u1=14 MPa u2 = 11MPa σ1=3 σ2=2
> [1.96+0.84)^2(3^2+3^2) / (14-11)^2 = 11. 32 ~ 12 teeth
Group 1 vs 3
u1=14 MPa u3 = 19 MPa σ1=3 σ3=7
> [1.96+0.84)^2(3^2+7^2) / (19-14)^2 = 18. 18 ~ 19 teeth
Group 2 vs 3
u2 = 11MPa u3=19 MPa σ2=2 σ3=7
> [1.96+0.84)^2(2^2+7^2) / (19-11)^2 = 6.49 ~7 teeth
Therefore, a sample size of 20 teeth per group was shown be adequate to achieve the required
level of power and significance for meaningful results.
3.4 Definition of Groups and Samples Preparation
This study was an in vitro experiment on extracted human premolar teeth. Extracted human
premolars were collected from the Oral Surgery Department at the University of Toronto,
34
Faculty of Dentistry and from two oral surgeons’ private offices. This project was approved by
the University of Toronto’s Health Sciences Research Ethics Board (Protocol reference #
31823). All teeth were visually inspected and only caries free human permanent maxillary or
mandibular premolars without buccal restorations were chosen for the study. Any tooth that
presented with endodontic treatment or carious lesions, buccal restorations, enamel defects such
as enamel hypoplasia, enamel hypomineralization or visible cracks were excluded from the
study. The selected teeth were disinfected in 0.5% chloramine T solution for 1 week, stored in
distilled water at 37 ˚C and used within six months following the extraction. Twice a week, the
distilled water was replaced and the teeth were washed and brushed.
A total of one hundred and sixty extracted, caries free human premolars were used in the study.
The teeth were randomly divided equally into four groups (n=40) according to the type of
adhesive used. Prior to bonding, all the teeth were cleaned and pumiced by using a rubber cup
with fluoride-free paste for 10s, thoroughly washed with water and air dried. Stainless steel
orthodontic brackets (American Orthodontic’s Mini Master Series, Sheboygan, WI, USA) with
0.022 slot were bonded to the extracted teeth using one of the following bonding procedures:
Experimental group 1 (SU) – Scotchbond™ Universal Adhesive (3M ESPE) was applied as a
one-step self-etch adhesive following the manufacturer’s recommendations. The adhesive was
applied to the tooth with a microbrush and rubbed in for 20 seconds. The adhesive was then
gently air died for about 5 seconds to evaporate the solvent and light cured with the Ortholux™
Luminous Curing Light (3M Unitek, Monrovia, CA, USA) for 10 seconds.5 The curing light was
a high intensity 1600 mW/cm2 blue LED.
Experimental group 2 (BU) – All-Bond Universal (Bisco) was applied as a one-step self-etch
adhesive following the manufacturer’s recommendations. The adhesive was applied as two
separate coats using a microbrush to scrub the surface for 10-15 seconds per coat. There was no
light curing between the coats. The adhesive was then thoroughly air-dried for at least 10
seconds in order to evaporate excess solvent. There was no visible movement of the adhesive and
the surface had a uniform, glossy appearance. The adhesive was light cured with the Ortholux™
Luminous Curing Light (3M Unitek) for 10 seconds.85
35
Experimental group 3 (CU) – Clearfil Universal Bond (Kuraray) was applied as a one-step self-
etch adhesive following the manufacturer’s recommendations. Using a disposable microbrush
applicator tip, the adhesive was rubbed on the tooth surface for 10 seconds. The adhesive was
dried by blowing mild air for more than 5 seconds until the bond did not move. The adhesive
was light cured with the Ortholux™ Luminous Curing Light (3M Unitek) for 10 seconds.20
Control group 4 (C) – Adper™ Scotchbond™ Multi-Purpose Adhesive (3M ESPE) was applied
as a total-etch system following the manufacturer’s recommendations and used as the gold
standard. The teeth were etched with Scotchbond Etchant for 15 seconds. Then rinsed for 15
seconds and dried for 5 seconds. The teeth had a chalky white appearance. Then the Adper™
Scotchbond™ Multi-Purpose primer was applied on the etched enamel with a microbrush and
dried gently for 5 seconds. Finally, the Adper Scotchbond Multi-Purpose Adhesive was applied
on the tooth with a microbrush and light cured with the Ortholux™ Luminous Curing Light (3M
Unitek) for 20 seconds.86
Stainless steel premolar brackets (American Orthodontic’s Mini Master Series) with 0.022 slot
were used to bracket the teeth. The Maximum Retention™ pads provided dual mechanical
retention by layering 80-gauge mesh over an etched foil base. The pad’s photo-chemically etched
pockets increased its surface area, creating a greater mechanical lock.87
The dimensions of the
bracket were:
Mesial-Distal dimension .120"
Gingival-Occlusal dimension .124"
Surface area of bracket 0.0159510 " squared
The bracket that had the best surface contact and fit on the buccal surface of the tooth was
chosen. The bracket was placed on the tooth and bonded with Transbond XT Light Cure
Adhesive (3M Unitek).88
The brackets were manipulated only with a bracket holding tweezer.
The adhesive was compressed into the mesh of the brackets with a composite plastic instrument.
The brackets were then placed onto the buccal surface of the tooth with their slot parallel to the
incisal edge. The brackets were placed in their ideal position for clinical orthodontics. Once the
brackets were positioned in the correct location, they were compressed onto the buccal surface of
the tooth with a hollenback dental instrument and the excess Transbond XT Light Cure Adhesive
36
Teeth Recruitment
SU
(40 teeth)
SU-B
(20 teeth)
SU-6m
(20 teeth)
BU
(40 teeth)
BU-B
(20 teeth)
BU-6m
(20 teeth)
CU
(40 teeth)
CU-B
(20 teeth)
CU-6m
(20 teeth)
C
(40 teeth)
C-B
(20 teeth)
C-6m
(20 teeth)
was gently removed. The brackets were light cured with the Ortholux™ Luminous Curing Light
(3M Unitek). The curing light was a high intensity 1600 mW/cm2 blue LED, which was held
stationary at a distance of 1-2 mm from the bracket for a total of 12 seconds, with the light beam
directed for 6 seconds at each mesial and distal aspect of the bracket, as determined by the pilot
projects.59, 88
The 40 teeth, from each type of adhesive group, were then divided into two (n=20), according to
the storage period. The baseline groups (B) were stored for 24 hours in distilled water at 37˚C
and the 6 month groups (6m) were stored for six months in 37˚C distilled water (Figure 2).
Figure 2: Study Design
37
3.5 Shear Bond Strength Test
At the defined time points, the shear bond strength of the adhesive was tested. To test the shear
bond strength, each bracket was debonded using the Universal Testing Machine (Instron, model
4301, Norwood, MA, USA). An occluso-gingival load
was applied by a chisel to produce a shear force at the
bracket-tooth interface (Figure 3). Prior to testing the
shear bond strength, each tooth was placed in a circular
mounting jig, made of SR Ivolen’s polymethyl
methacrylate base (Ivoclar Vivadent, Schaan,
Liechtenstein). A template was used when fabricating the
jig in order to standardize the size. The mounting jig was
used to align the facial surface of the tooth parallel to the
chisel such that the chisel blade contacted each bracket
from the incisal aspect on the bracket stand-off, as close
to the bonding interface as possible. The universal
testing machine had a load-cell capacity of 200 N. The shear bond strength was measured at a
constant crosshead speed of 1 mm/minute and the maximum force required to debond a bracket
was recorded in Newtons (N). All values were converted to megapascals (MPa) by dividing the
force in Newtons by the mean base surface area of orthodontic bracket. Mean shear bond
strength was calculated for each group.
3.6 ARI and Evaluation of Remaining Resin
Once the bracket was debonded, the enamel surface of each tooth was examined under a
stereomicroscope (Wild M3Z, Wild Heerbrugg, Gais, Switzerland) at 10x magnification. A
photograph of the buccal surface of each tooth alongside a ruler was taken with a Spot Insight
Color 3.2.0 Camera (Diagnostic Instruments, Sterling Heights, MI, USA) to qualitatively and
quantitatively assess the amount of resin remaining on the tooth. Each photograph was assessed
and each enamel surface was checked with an explorer to determine the amount of resin
remaining on the tooth surface. An ARI score as described by Artun and Bergland (1984) was
Figure 3: Set-up for SBS testing with
the Instron Machine
38
given to each enamel surface.78
The results were expressed as the percentage of each score per
adhesive.
ImageJ Software (National Institutes of Health) was used to quantitatively determine the surface
area of the bracket and remaining resin for each tooth from each image. A straight line from one-
millimeter measurement line on the ruler to the adjacent millimeter line was drawn to calibrate
each image. Following the calibration of each photograph, a freehand tool was used to outline
the area of the bracket base and the software quantitatively computed the area. For each image,
this procedure was then repeated around the various areas of residual resin to obtain a total
surface area of the resin. The residual resin for each tooth was then expressed as a percentage of
the total bracket area.
3.7 Scanning Electron Microscopy (SEM)
Representative samples within each group were qualitatively analyzed with a scanning electron
microscope to assess for enamel damage by the various adhesives. Samples were sonicated for
10 minutes and placed in a desiccator for 24 h. Afterwards, samples were coated with gold
sputter (Leica EM ACE200, Leica Microsystems, Vienna, Austria) and subjected to SEM
analysis (XL30, FEI, Hillsboro, Oregon) at 50x, 500x and 1000x magnification.
3.8 Statistical Analysis
At each defined time point, one-way analysis of variance (ANOVA) and Tukey tests were used
to compare the mean shear bond strengths and the mean percentages of remaining resin among
the different adhesives. For each adhesive, a Student’s t-test was used to compare baseline and 6-
month SBS means as well as the mean percentages of remaining resin. The statistical
significance was set at the two-tailed 5% level for all tests.
39
Results 4
4.1 Shear Bond Strength
For all tests, the assumptions of normal distribution of errors were checked and satisfied by
Shapiro-Wilk test. Table 5 and Figure 4 show the mean SBS value and standard deviation (SD)
for each adhesive group at baseline and 6 months. At each time point, one-way analysis of
variance showed that BU group had the lowest SBS values, followed by SU and CU. At baseline,
SU and CU did not statistically differ from each other (p < 0.05). At the 6 month time point, the
SBS of CU was significantly greater than SU (p < 0.05). The C group had the significantly
highest SBS values at both time-points (p < 0.05).
Table 5: Mean SBS and standard deviation values
Different Adhesive Systems
(Mean ± SD)
Group 1 (SU) Group 2 (BU) Group 3 (CU) Group 4 (C)
Baseline (B) 3.8 ± 2.0Aa
1.9 ± 1.0Bc
4.1 ± 1.4Ae
8.4 ± 3.4Cf
6 months (6m) 2.9 ± 1.0Wb
0.55 ± 0.35Xd
4.6 ± 2.3Ye
6.0 ± 2.0Zg
ANOVA and Tukey; α=0.05
Different upper case letters within each line indicate significant differences among means
Different lower case letters within each column indicate significant differences among means
40
Figure 4: Mean SBS and SD values
When comparing each adhesive at the different time points (Table 5), Student t-test showed
statistically lower mean values for SBS at 6 months than at baseline for SU, BU and C (p <0.05,
p <0.0001 and p <0.005, respectively). There was no significant difference in the SBS values
obtained at baseline and 6 months for the CU group (p =0.1823).
4.2 Adhesive Remnant Index
Table 6 and Table 7 show the ARI scores for the evaluated adhesives at baseline (Table 6) and 6
month (Table 7) time points. An explorer was used to verify the enamel surfaces and identify the
remaining resin. At baseline, the SU, BU and CU groups presented 100% of ARI score 1. The C
group had 84% of ARI score 1 and 16% of ARI score 2. At 6 months, SU group had 6% of ARI
score 0 and 94% of ARI score 1. BU-6m group presented 17% of ARI score 0 and 83% of ARI
score 1. CU group presented 100% of ARI score 1. The C group had 6% of ARI score 1, 76% of
ARI score 2 and 18% of ARI score 3. Figure 5 is a pictorial representation of each ARI score
obtained from the samples. The shiny enamel surface, as seen in Figure 5a, is the adhesive’s
penetration into the enamel microporosities.
41
Table 6: ARI scores for each adhesive group at baseline
ARI Score 0 1 2 3
SU-B 0% 100% 0% 0%
BU-B 0% 100% 0% 0%
CU-B 0% 100% 0% 0%
C-B 0% 84% 16% 0%
ARI score 1: Less than half of the adhesive left on the tooth
ARI score 2: More than half of the adhesive left on the tooth
Table 7: ARI scores for each adhesive group at 6 months
ARI Score 0 1 2 3
SU-6m 6% 94% 0% 0%
BU-6m 17% 83% 0% 0%
CU-6m 0% 100% 0% 0%
C-6m 0% 6% 76% 18%
ARI score 0: No adhesive left on the tooth
ARI score 1: Less than half of the adhesive left on the tooth
ARI score 2: More than half of the adhesive left on the tooth
ARI score 3: All of the adhesive left on the tooth
42
Figure 5: Stereomicroscopic images (10x) of representative enamel surfaces. A was taken
from BU-6m group and represents an ARI score of 0. B was taken from the CU-B group
and represents an ARI score of 1. C was taken from the C-6m group and represents an
ARI score of 2. D was taken from C-6m group and represents an ARI score of 3
Table 6 and Table 7 prove that the most common mode of failure for all the adhesive groups at
baseline and at 6 months was a combination of adhesive and cohesive failures. At both time
points, the C adhesive had more resin remaining on the tooth than the universal adhesives, which
is in agreement with its greater shear bond strength. Due to the inability of this scoring system to
identify minor differences among the adhesive groups, quantitative assessment of the amount of
resin remaining on the tooth was conducted.
4.3 Percentage of Remaining Resin
For all tests, the assumptions of normal distribution of errors were checked and satisfied by
Shapiro-Wilk test. Table 8 and Figure 6 show the mean percentage of remaining resin and
standard deviation (SD) for each adhesive group at baseline and 6 months. At baseline, C group
had a statistically higher mean percentage of remaining resin than the SU, BU and CU (p < 0.05).
A B
C D
43
There were no significant differences among the self-etching adhesives, with CU having more
remaining resin, followed by BU and SU. At 6 months, C group also had a statistically higher
mean percentage of remaining resin than the SU, BU and CU (p < 0.05). There were no
significant differences among the self-etching adhesives, with CU having more remaining resin,
followed by SU and finally BU.
Table 8: Mean percentage of remaining resin and standard deviation values
Different Adhesive Systems
(Mean ± SD)
Group 1 (SU) Group 2 (BU) Group 3 (CU) Group 4 (C)
Baseline (B) 7.0 ± 4.9Aa
7.2 ± 4.6Ab
11.4 ± 6.7Ad
30.4 ± 20.7Be
6 months (6m) 6.4 ± 5.3Ya
1.7 ± 1.6Yc
9.4 ± 6.6Yd
77.4 ± 18.1Zf
ANOVA and Tukey; α=0.05
Different upper case letters within each line indicate significant differences among means
Different lower case letters within each column indicate significant differences among means
44
Figure 6: Mean Percentage of Remaining Resin
When comparing the mean percentages of remaining resin at the different time points, Student t-
test showed statistically significant differences for BU and C groups. BU-6m had less remaining
resin than BU-B, while C-6m had more remaining resin than C-B (p<0.001). There were no
significant differences in the mean percentages of remaining resin for SU and CU groups.
4.4 Scanning Electron Microscopy (SEM)
The following Figure 7 and Figure 8 are representative images of debonded enamel surfaces
respectively at baseline and 6 month time points. As a general finding, the enamel surface of the
universal adhesives after debonding appeared smooth and less porous than the total-etch
adhesive (Figure 7, Figure 8). Superficial microporosities and open tubules can be seen in Figure
7D and Figure 8D. In addition, less adhesive remaining on the enamel surfaces of the universal
adhesives in comparison to the total-etch adhesive was commonly seen (Figure 7, Figure 8).
45
Figure 7: Scanning electron microscope images (1000x) of enamel surfaces after debonding.
A was taken from SU-B group and B was taken from the BU-B group. C was taken from
the CU-B group and D was taken from C-B group. E represents the enamel surface and R
represents the resin.
Figure 8: Scanning electron microscope images (500x) of enamel surfaces after debonding.
A was taken from SU-6m group and B was taken from the BU-6m group. C was taken from
46
the CU-6m group and D was taken from C-6m group. E represents the enamel surface, R
represents the resin and AD represents the adhesive.
Figure 9 illustrates two samples that underwent enamel damage during the shear bond strength
test. Areas of enamel fracture are clearly distinguishable from the smooth surface enamel by
their roughness and exposed enamel pattern.
Figure 9: Enamel damage seen under SEM (1000x) of a sample from SU-6m (A) and BU-
6m (B) group. SE represents the enamel surface and DE represents the damaged enamel.
47
Discussion 5
To summarize the results of this study, at each time point, BU group had the lowest SBS values,
while C group had the highest SBS values. At baseline, there was no statistical difference in the
SBS values of SU and CU, but at the 6 month time point, the SBS of CU was significantly
greater than SU. SU, BU and C groups had lower SBS values at 6 months when compared to
baseline. There was no significant difference in the SBS values obtained at baseline and 6
months for the CU group. At both time points, C group had a higher mean ARI score mean
percentage of remaining resin than the SU, BU and CU. The most common mode of failure for
all the adhesives groups at baseline and at 6 months was a combination of adhesive and cohesive
failures. At both time points, C group had a statistically higher mean percentage of remaining
resin than the SU, BU and CU and there were no significant differences among the self-etching
adhesives. When comparing the mean percentage of remaining resin at the different time points,
BU-6m had significantly less remaining resin than BU-B and C-6m had significantly more
remaining resin than C-B. There were no significant differences in the mean percentages of
remaining resin for SU and CU. On SEM, the enamel surface of the universal adhesives after
debonding appeared smooth and less porous than the total-etch adhesives.
This in vitro study demonstrated that the total-etch system provided a significantly higher shear
bond strength than the three universal adhesives, thus the first null hypothesis was rejected. This
is in agreement with the results from Mousavinasab et al. (2009), which showed higher SBS with
Scotchbond™ Multi-Purpose Adhesive compared to self-etch adhesives.89
While the C group, in
this study, was able to achieve the critical shear bond strength required for orthodontic practice,
the three universal adhesives were unsuccessful at both time points. At baseline, BU group had
the lowest SBS values, followed by SU and CU, which did not statistically differ from each
other. These results are in agreement with Vermelho et al.(2016) study, which showed that the
24 hour SBS of the total-etch system was greater than three self-etching adhesives.90
In addition,
among the self-etching adhesives, the SBS of Clearfil SE Bond (Kuraray) and Scotchbond
Universal (3M ESPE), which did not differ significantly, were greater than the SBS of All-Bond
Universal (Bisco).90
48
In this study, the SBS of CU, at the 6 month time point, was statistically greater than SU, which
was greater than BU. For the self-etching adhesives, the SBS obtained at baseline and at 6
months are in agreement with McLean et al. (2015) study, which showed that the SBS on cut
enamel for All-Bond Universal (Bisco), Scotchbond Universal Adhesive (3M ESPE), and
Clearfil SE (Kuraray) was 11 ± 2 MPa, 14 ± 3 MPa and 19.5 ± 6 MPa respectively after 24 hrs
and 9 ± 4 MPa, 12 ± 7 MPa and 24 ± 5 MPa after storage in distilled water for 6 months,
respectively.8 This study’s SBS results are about five times less than those reported in Mc Lean
et al.(2015), for each adhesive at each time point, thus demonstrating a similar trend.8 The five-
fold decrease in SBS may be due to differences in methodology since McLean et al. (2015) used
cut enamel without orthodontic brackets.8
When comparing the SBS at the two different time points for each adhesive, there was a
significant decrease in SBS for the SU, BU and C group, thus the second null hypothesis was
rejected. Water storage had no significant effect on the SBS of CU, which is in agreement with
McLean et al. (2015)8 and Atash Biz Yeganeh et al. (2015).
91 Atash Biz Yeganeh et al. (2015)
study also showed a great reduction in bond strength for Scotchbond™ Multi-Purpose Adhesive
group after 6 months, which compliments the results of the C group.91
The differences in SBS between the self-etch adhesives and within each adhesive group at
baseline and 6 months may be explained by the fact that functional monomer impurities may
affect an adhesive’s performance and enamel bond durability.92, 93
Yoshihara et al. (2015)
claimed that the functional monomer 10-MDP was originally synthesized by Kuraray.92
In their
study, they compared three different 10-MDP versions made by three different companies. One
was made by Kuraray Noritake, while the other two companies remained anonymous.92
In
comparison to the 10-MDP from Kuraray Noritake, the other two 10-MDP versions contained
more impurities and 10-MDP dimer, which resulted in a lower microtensile bond strength
(µTBS) to dentin immediately after bonding and a significant decrease after thermocycling.92
The µTBS of the 10-MDP by Kuraray was unaffected by thermocycling.92
The results confirm
that both the purity and presence of 10-MDP dimers in adhesives influence the etching efficacy
of hydroxyapatite and bond strength.92
Based on these results, it is possible that the lower SBS
for SU and BU at baseline and at 6 months, in comparison to CU, might be due to impurities in
the 10-MDP functional monomer. These impurities and dimers may undergo hydrolytic
49
degradation more rapidly, thus accounting for the decrease in SBS after 6 months in storage.
Another possible explanation may be based on differences in ratios and components of different
adhesives that are proprietary. For examples, if a certain adhesive has higher ratios of
camphorquinone, it can lead to a higher degree of polymerization conversion, which may
generate higher shear bond strengths.91, 94
In terms of failure mode, Al-Salehi and Burke (1997) claimed that there was a relationship
between bond strength and failure mode, as higher bond strengths correlate with greater mixed
fractures.95
This relationship is clearly seen, in this study, when comparing the ARI score of C
group to SU, BU and CU at both time points, thus we can reject our third null hypothesis. The C
group, which had the greatest SBS at both time points, also had a greater ARI score, which
means that more resin remained on the teeth following the removal of the brackets. These results
are in agreement with Sharma et al.(2015) study, which showed that the total-etch system had a
greater ARI score than the self-etch system.4 The total-etch system had a higher distribution of
ARI scores 2 and 3, while the self-etch adhesives had a higher frequency of ARI score 1 and 2.4
In addition, the differences in mode of fracture between the self-etch and total-etch adhesives are
in agreement with Schnebel et al. (2012) study, which showed that the total-etch adhesives failed
mostly at the bracket/adhesive interface, thus leaving the enamel surface intact but required
increased chair time to remove the residual adhesive.96
On the other hand, the self-etch adhesives
resulted in more enamel-adhesive interface failures, which left less residual adhesive on the tooth
surface.96
However, since bracket failure occurs at the weakest link, it also indicated a weak
bond to the enamel surface, resulting in lower SBS values.96
There were no differences in the ARI scores at baseline between SU, BU and CU, even though
there were significant differences in the SBS of CU and SU compared to BU. This may be due to
the fact that the ARI grading system is not able to detect minor differences in remaining
adhesive. For this reason, a quantitative method to assess the percentage of remaining resin on
the tooth surface was also performed. There was a higher ARI score for the CU group at 6
months compared to SU and BU, which is in agreement with the SBS results at this time point
and the McLean et al.(2015) study.8
50
When comparing the ARI scores per adhesive, at each time point, the overall ARI score for SU
and BU decreased over time, which correlated to its decrease in SBS over 6 months. A lower
ARI score after 6 months signified less resin remaining on the tooth and a weaker bond between
the resin and the enamel. The ARI score of 1 remained constant for CU, which reflected the
stability of the SBS over the time points. The only discordance between SBS and ARI scores was
seen with the C group. While the overall ARI score increased from 1 to 2 over the time period,
there was actually a decrease in the SBS. A study by Burrow et al. (2005), which assessed the
seven-year dentin bond strength of a total-etch and self-etch system, demonstrated similar
results.97
While the SBS of both systems decreased over time, the mode of failure for the self-
etch system showed no difference, whereas the total-etch system had an increase in cohesive
failures in dentin.97
This signifies that the bond between the resin and tooth became stronger over
time and that the weakest point was within the dentin.97
While this study did not use orthodontic
brackets, a comparison can be made. Since the amount of remaining resin increased drastically
from baseline to 6 months for the C group, it may indicate a stronger adhesion between the
enamel and adhesive, therefore causing the mode of fracture to be predominately at the bracket-
adhesive interface. It can be speculated that the polymer expansion due to water sorption may
explain the resin’s increased bond strength to the enamel in 6 month evaluation.98
Sharma et
al.(2014) stated that 70% of failures for light cured total-etch adhesives occurred at the adhesive-
bracket interface due to incomplete polymerization of the resin below the metal base of the
bracket.4
As previously mentioned, due to the inability of the ARI scoring system to identify minor
differences among the adhesive groups, quantitative assessment of the amount of resin remaining
on the tooth was conducted. Lee and Lim (2008) stated that the ARI score was a general and
rough estimation of remaining adhesive on the enamel surface.99
In a study by O’Brien et al.
(1988), the amount of adhesive remnant was expressed as a percentage of the mean bracket
area100
and this method was used, in this study, to assess quantitively the amount of resin
remaining on the teeth for each adhesive group at each time point. At baseline and 6 months, C
group had a higher mean percentage of remaining resin than the SU, BU and CU, which
correlated with its higher SBS and ARI scores. At baseline, there were no significant difference
among the self-etching adhesives, with CU having more remaining resin, followed by BU and
SU. Although this corresponded to the ARI score of 1 at baseline, there was no relationship to
51
the SBS results. The SBS of CU and SU were significantly greater than BU, nonetheless there
was no significant difference in the ARI score nor the percentage of remaining resin. An ARI
score of 1 for these three adhesives signified that less than 50% of the resin remained on the
tooth’s surface, however, the actually percentages of remaining resin were between 11.4 ± 6.7%
and 7.0 ± 4.9%, thus demonstrating a large disproportion between the two methods. At 6 months,
there were no significant differences in percentage of remaining resin among the self-etching
adhesives, but CU had more remaining resin, followed by SU and finally BU. At this time point,
the trend is in agreement with the SBS and ARI scores for each group.
In this study, the percentage of remaining resin decreased significantly for BU, increased for C
group and there was no difference for CU and SU. For all the groups, except for SU, the results
are in agreement with the ARI scores. Overall, the results of the percentage of remaining resin
are in agreement with the ARI scores, thus the ARI scoring index was validated. Cehreli et al.
(2012) did a comparative study of qualitative and quantitative methods for the assessment of
adhesive remnant after bracket debonding and they concluded that qualitative visual scoring
using the ARI is capable of generating similar results with those assessed by quantitative image
analysis techniques.79
The SEM findings can also be related to the values of the SBS and ARI, because when the
enamel surface was more affected by the acid conditioner, a greater bond strength and more
adhesive remnant was found, as seen in the C group (Figure 7, Figure 8). These findings are in
agreement with Sharma et al. (2015) study, which stated that the enamel surface of the universal
adhesives after debonding appeared smooth and less porous than the total-etch adhesives.4 This
signifies that the acid conditioning by the universal adhesives was weak, resulting in a decreased
micromechanical retention, lower SBS results and ARI scores. The SEM findings validate the
results obtained by the SBS and ARI scores at baseline and at the 6 month time point.
In addition, under the scanning electron microscopy, more residual resin was identified on the
selected enamel surfaces than the calculated percentage of remaining resin. However, even with
the additional resin, there was no change in the ARI score for the selected teeth. Enamel damage
was also seen under the SEM for two samples, one from SU group and another from BU group,
which was indistinguishable under the stereomicroscope (Figure 9). The enamel damage may be
52
explained by the fact that the mode of failure for both groups was mostly at the enamel-adhesive
interface, thus placing a significant amount of stress on the enamel surface. Given these findings,
a better representation of the enamel surface after removal of the brackets was obtained with the
scanning electron microscope, rather than the stereomicroscope.
Overall, the main disadvantage of the self-etching adhesives is their hydrophilicity because it
leads to a permeable adhesive layer, which contributes to the hydrolysis of resin polymers,
degradation of the tooth-resin bond over time and a decreased bond strength.8 Bonding to enamel
remains a weak property of mild self-etching adhesives. Therefore, developing monomers with
stronger chemical bonding potential to uncut hydroxyapatite may help improve their bonding
performance and their use in orthodontic pratice.13
In addition, further studies investigating the
purity of monomers in commercial available dental adhesives are needed.92
The limitations of this study include the in vitro study design as it can be difficult to apply the
results to an in vivo situation. In addition, the teeth used in the study were extracted by the use of
forceps, thus craze lines may have been created, which were undetectable and could have
affected the bond strengths. The clinical implications of this study suggest that the universal
adhesives have significantly lower shear bond strengths to uncut enamel when used in a self-
etching mode compared to total-etch adhesives. On careful evaluation, the highest SBS for CU
including its standard deviation creates a reasonable orthodontic bond strength without causing
any enamel damage, in the study, thus it is possible that in the future this adhesive may be used
in an orthodontic setting.
53
Conclusion 6
Within the limits of this study, it can be concluded that the orthodontic shear bond strength of
Adper™ Scotchbond™ Multi-Purpose Adhesive (3M ESPE) on uncut enamel was significantly
greater than the three self-etch adhesives, Scotchbond™ Universal Adhesive (3M ESPE), All-
Bond Universal (Bisco) and Clearfil Universal Bond (Kuraray) at baseline and at 6 months, thus
the first null hypothesis was rejected. In comparison to the two time points, there was a
significant decrease in the shear bond strength for the Scotchbond™ Universal Adhesive (3M
ESPE), All-Bond Universal (Bisco) and Adper™ Scotchbond™ Multi-Purpose Adhesive (3M
ESPE), thus the second null hypothesis was rejected. With respect to the percentage of remaining
resin, the Adper™ Scotchbond™ Multi-Purpose Adhesive (3M ESPE) had a statistically higher
mean than the self-etch adhesives and there were no differences in the mean percentages of
remaining resin between the self-etching adhesives, thus the third null hypothesis was also
rejected. At this time, the shear bond strength of the universal adhesives, when used in a self-
etching mode, do not meet the gold standard required by Reynolds1 for orthodontic treatment and
therefore they should not be used in orthodontic practice.
54
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