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An evaluation of bond strength using sham lingual brackets with differences in base morphology and preparation

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An evaluation of bond strength using sham lingual brackets with differences in base morphology and preparation

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Content An evaluation of bond strength using sham lingual brackets
with differences in base morphology and preparation.
By
Christopher Oviedo, DDS
A Thesis Presented to the Faculty of the USC Graduate School
University of Southern California In Partial Fulfillment of the
Requirements for the Degree Master of Science (Craniofacial Biology)
May 2015
Copyright 2015 Christopher Oviedo
Table of Contents
List of Figures ……………….…………………………………………………….……. iii
List of Tables…….……………………………………………………………..…………iii
List of Graphs…………………………………..…………………………….…………. iv
Acknowledgements ………….………….……………………………………………… v
Abstract ……………………………………….………………………………………… vi
Chapter 1: Introduction …………………….….……………………………………….. 7
Chapter 2: Literature Review ……………….…………………………………………10
I. Sandblasting …………………………………………………………… 10
II. Tests for Bond Strength …….………………………………………… 11
III. Bracket Design…………………………………………………………12
IV. Adhesive Thickness……………………………………………………15
V. Lingual Appliances…………………………………………………….. 16
VI. Experimental Design………………………………………………… 18
VII. Bracket-Adhesive Interface…………………………………………..22
Chapter 3: Objective …………………………………………………………………. 24
Chapter 4: Hypothesis ……………………………………………………………….. 25
Chapter 5: Materials and Methods ………………………………………………….. 26
Details of Protocol ……………………………………………………….. 29
Chapter 6: Statistics …………………………….…………………………………….. 32
Chapter 7: Results ……….…………………….……………………………………… 33
Chapter 8: Discussion ………………………….………………………………………39
Chapter 9: Conclusions ………………………….……………………………………..46
References ………………………………………….………………………………….. 47
ii
List of Figures
Figure 1: Examples of two types of sham brackets .………..………………………..26
Figure 2: The resin block ………………………………………………………………..27
Figure 3: Virtual resin block ……………………………………………………………..28
Figure 4: Resin block design ……………………………………………………………28
Figure 5: Pre-fabrication …………………………………………………………………29
Figure 6: The bond angle ………………………………………………………………..30
Figure 7: The bonding jig ………………………………………………………………..30
Figure 8: The laboratory setup ………………………………………………………….31
Figure 9: Orientation of the tooth substrate …………………………………………..39
Figure 10: Load delivery …………………………………………………………………40
Figure 11: Scanning electron microscopy ……………………………………………..44
List of Tables
Table 1: Instron test results ………………………………………..……………………35
Table 2: 2x2 Between Subjects Factorial Design …………..……….………………. 35
Table 3: Test of Homogeneity of Variance ………..………………..….………………36
Table 4: One-way ANOVA …………….……………..……….….……………………..36
Table 5: Tukey Test for multiple comparisons of means …………………………….36
Table 6: Summary of location of failure …………….…….……….…..…………….. 37
iii
List of Graphs
Graphs 1-4: Raw data generated from Instron testing —
Graph 1: Loading curve for flat base with sandblasting………..……….…33
Graph 2: Loading curve for flat base with laser-etching..……………….…33
Graph 3: Loading curve for custom base with sandblasting………………34
Graph 4: Loading curve for custom base with laser-etching………………34
Graph 5: Violin Plot ………………………………………………..…………………….35
iv
Acknowledgements
I would like to thank Dr. Hongsheng Tong for allowing me to work with him on this
project. Dr. Tong provided a great deal of support throughout the entire project, which
was integral to its completion. I would also like to thank Dr. Jin-Ho Phark for giving me
his time and allowing me to work in his lab. The collection of data would not have been
possible without his accommodation. Also, thanks to Robert Lee for helping to design
the resin block, and Coco Chengliang Dong for her assistance with the statistics.
v
Abstract
Title
An evaluation of bond strength using sham lingual brackets with differences in base
morphology and preparation.
Background
In vitro lab investigations are useful to gain insights into how different bracket bases
perform under controlled conditions. It is possible to evaluate the characteristics of
these bases to design one that is both, clinically effective and affordable to manufacture.
Purpose
To test the effects of base morphology and preparation on bond strength, using sham
lingual brackets and a customized resin substrate for bonding.
Methods
Eighty sham lingual brackets were divided into four equal groups (n=20): flat base with
sandblasting, flat base with laser-etching, custom base with sandblasting, and custom
base with laser-etching. The brackets were bonded to a resin substrate using a custom
jig and tested for bond strength.
Results
The brackets with a custom base and laser-etching showed a significantly lower bond
strength when compared to both sandblasted groups. All groups, however, had
acceptable bond strengths.
Conclusion
When comparing different sham lingual brackets, one that has a custom base or
preparation with laser etching may not have a greater bond strength than a base that is
flat and sandblasted. All bracket types demonstrated acceptable performance,
however, and may be considered for clinical use. Further clinical study of custom
brackets may be hindered by issues associated with feasibility and increased costs.
vi
Chapter 1: Introduction
The use of a lingual appliance is a desirable treatment option in modern orthodontic
practice. Many aspects of bonding brackets to the facial and lingual surfaces can be
the same, such as the materials used and procedures followed. A major difference in
bonding between the two surfaces, however, is the surface itself. While the labial
surfaces of teeth are convex, the lingual surfaces vary considerably. In the anterior
region, teeth are concave on their lingual surfaces. While in the posterior region, they
are convex. Due to this variability in morphology, many lingual bracket systems are
designed with a customized base to improve adaptation of the bracket to the crown.
Lingual appliances with customized alloy bases have several perceived benefits. The
brackets tend to adapt better to the tooth, making placement more reproducible and
accurate (Weichmann, 2003). As a result of better adaptation to the tooth, there is a
thinner and more uniform amount of adhesive applied. This aspect has been
associated with greater bond strength (Weichmann, 2003; Chumak, 1989). Custom
lingual bases also can make speech effects more tolerable through better adaptation
and a lower profile (Creekmore, 1989). Because the bracket may be closer to the tooth,
and, therefore, closer to the center of resistance of the tooth, movements in the
presence of a customized base can be more controlled (Ludwig, 2000).
In spite of the benefits, however, customized lingual appliances can be more costly to
produce and deliver to patients. Custom lingual setups can dramatically increase
overhead as a result of the amount of reliance and communication that the practitioner
must place upon the manufacturer. Also, these appliances can require more chair time
during bonding and routine adjustment appointments, which add to the overall expense.
Lingual appliances may also require the increased use of expensive wires. If the
custom appliance breaks, it may be necessary to contact the manufacturer to create
additional custom wires or replace brackets, which can increase treatment time.
Prolonged treatment times can result from lack of operator experience and unfamiliar
mechanics (Ludwig, 2000). 7
A lingual system which does not rely on the manufacturer to replace custom brackets is
advantageous for the practitioner. Understanding the feasibility of creating such an
appliance is a different story. Currently, there are different lingual bonding systems,
which allow the clinician to create customization through the use of a resin base
(Komori, 2013; Sung, 2013; Ludwig, 2000). These systems tend to be bulky and
uncomfortable for the patient. They also use less efficient mechanics because the wire
is an increased distance from the tooth. Non-customized lingual systems require
varying amounts of bonded material to make up the difference between the bracket
base and tooth. The additional thickness of material may create weaker bonds
(Bordeaux, 1994; Bradburn, 1992; Pender, 1988; Knoll, 1986), which are compensated
by increasing the surface area of bonded attachment to the tooth (Komori, 2013; Sung,
2013). This circumstance can be related to the maxillary anterior, where the lingual
anatomy is more pronounced and variable.
The object of this study was to pursue the role of varying bracket design when bonding
to the lingual. In a previous study conducted at this University (Vrontikis, 2015), it was
determined that sandblasting, as a means of preparation of the bracket base,
contributed to increased bond strength and was superior to laser-etched and double
mesh preparations. As a follow-up to that study, the current investigation was conducted
to better understand how differences in base morphology and preparation of brackets
affected bond strength.
An in vitro investigation was designed to find out whether it will make a difference to
have a flat base on the lingual surface of the teeth, rather than a close-fitting custom
base, and to understand how different surface preparations, namely sandblasting and
laser-etching, affect the bond strength when applied to the aforementioned base
morphologies. The lingual surface of a lateral incisor was chosen due to this tooth’s
variable nature and relative concavity. A resin substrate was designed and fabricated
with the anatomical configuration of this tooth. Specifically, the micro-mechanical bond
between the adhesive and base of the bracket was being investigated. This interface is
8
a function of the surface area of the bracket’s base, the design of the base used,
adhesives, and other experimental factors. By varying the surface morphology and
preparation of sham metallic brackets, it may be possible to establish a feasible base
design for clinical practice — one which relates to the fabrication of a standardized
lingual appliance system.

9
Chapter 2: Literature Review
In order to fulfill this investigation, it was necessary to examine the existing literature in
several areas. The background information provided in this section served as a
foundation to design the experiment and contributed to the formation of a knowledge
base that was integral to the interpretation of the results and formation of conclusions.

I. Sandblasting
Sandblasting is a technique commonly employed in clinical orthodontic practice. The
technique involves the propulsion of small particles, usually made of aluminum oxide,
under high pressure to impact the surface of a substrate. For research purposes,
typical substrates include bovine or human enamel, the bracket base, a custom resin
base (for indirect bonding), or, as in this experiment, a resin block. Clinically, the most
common use for sandblasting in orthodontics involves the removal of residual cement
from the surface of a failed bracket’s base, thus, allowing the bracket to be reused.
Several studies show that sandblasting increases bond strength when applied to a
bracket, tooth, or other substrate (Al Jabbari, 2014; Ziebura, 2014; Chacko, 2013;
Lombardo, 2011; Faltermeier, 2009; Thompson, 2008; SharmaSayal, 2003; Johnston,
1999; MacColl, 1998; Millet, 1993). This increase is attributed to added surface
roughening or the formation of microporosities through the impact of the pressurized
particles on the surface. These effects are thought to have a role in increasing the
capacity for micro-mechanical retention between an adhesive and substrate.
Sandblasting can be integral to create an effective bond to metal and other surfaces
prior to initiating the bonding of an orthodontic attachment and is shown to be more
effective than merely “roughening” or cleaning the surface with a carbide or diamond
bur (Gange, 2015; Aksu, 2013).
Espinar Escalona looked at the relationship between particle size and bond strength.
As a whole, sandblasted brackets significantly increased bond strength when compared
10
to controls. Although 600 ℳm aluminum oxide (Al2O3) particles were associated with an
increase in surface topography over 80 and 200 ℳm sizes, there was no significant
increase in bond strength except with one bracket type. Silicone carbide (SiC) particles
of different sizes also showed no significant effect on bond strength. Furthermore,
preparation with SiC particles did not significantly increase the bond strength of
brackets relative to controls (Espinar-Escalona, 2012). These findings were opposite to
those of Bouschlicher, who found that SiC particles increased SBS more than Al2O3
when bonding to resin (Bouschlicher, 1999).
II. Tests for Bond Strength
In vitro testing for bond strength is the most common way to evaluate performance of
orthodontic brackets. Tests for bond strength can be performed in vitro in a variety of
ways. Some of these methods include shear, shear-peel, tensile, and fatigue bond
strength tests. Shear bond strength (SBS) testing is the most common method applied
to test orthodontic brackets. This method is relatively simple to perform and involves the
delivery of a parallel force to the bracket’s base. Unfortunately, this method only
presumes to be a reliable simulation of actual occlusal forces (Eliades, 2000).
The use of an Instron machine is the most common instrument used to measure bond
strength. The shear application of this machine is used in bond strength studies within
the orthodontic literature and is characterized by a cross-head blade that approaches a
bonded complex at an angle perpendicular to the floor. Upon striking the bonded
complex at a controlled rate, a force is generated within the adhesive layer of the
bonded complex until failure of the bonded layer occurs. This entire loading sequence
is digitally recorded and displayed on a graph using software designed for the machine.
Debonding forces are recorded in Newtons (N) and divided by the surface area of the
bracket’s base (in mm
2
), thus, giving the bond strength in megapascals (MPa). It is
possible to compare the adhesion of brackets of different shapes and sizes using this
common unit of measurement.
11
Shear-peel, tensile, and fatigue bond strength tests are worth mentioning. These tests
are unique, and when compared with shear bond strength testing, may lack correlation.
Shear-peel bond strength tests can be relevant because they are based on the creation
of a moment that “peels” the bracket away from the tooth when the force is not exactly
shear in nature. This may be a more realistic approach to how an orthodontic bond may
experience failure.
Tensile bond strength (TBS) tests involve the application of a force perpendicular to the
interface of the bracket. This force “stretches” the adhesive bond until failure. Based
on the orientation of this force delivery, tensile bond strength methods have been
questioned for their clinical relevance to bracket debonding forces (Eliades, 2000).
Fatigue bond strength (FBS) tests replicate the way brackets fail — with incremental
and repeated force application over time, which can send cracks propagating
throughout the adhesive layer (Eliades, 2000; Zachrisson, 1996). FBS testing is
realistic, but the interactions occurring within an adhesive layer are complex and
unlikely to be understood using any laboratory test that relies on catastrophic failure to
measure results (Eliades, 2000). Finite element model (FEM) analysis is a technique,
therefore, that has shed light on these complex relationships and gives information
regarding the actual stresses involved when adhesive bonds are brought to failure
(Katona, 1997).
III. Bracket base design
Controlled in vitro studies highlight the variables which affect bracket bases. These
factors — such as base material (i.e. metal or ceramic), nominal size and surface area,
mesh size, the presence of spot welds, and surface preparation — are all attributed to
having an effect on bond strength values. Several authors have investigated these
characteristics and conclude that bond strength can be affected by varying the design of
the base of a bracket (Lombardo, 2011; Samruajbenjakul, 2009; Gibb, 2006; Wang,
12
2004; Sharma-Sayal, 2003; Knox, 2000; Willems, 1997; Bordeaux, 1994; Buzzitta,
1982; Maijer, 1981; Dickinson, 1980; Lopez, 1980).
There are significant differences between metal and ceramic brackets. Metallic
brackets adhere to their bonded substrate via a mechanical interlocking that is
influenced by the design of the bracket base and the specific adhesive used (Knox,
2000). While ceramic brackets can also be bonded through mechanical retention, many
have been designed to form chemical bonds via silane-coupling interactions between
the adhesive and bracket (Britton, 1990). Reynolds reported that minimal bond
strengths need to be in the range of 6-8 MPa, and Reteif reported enamel fracture at
bond strengths as low as 14 MPa. Many ceramic bond strengths were reported with
ranges that are much greater than these values (i.e. 25-29 Mpa), however, and have the
potential to cause significant damage to the enamel surface (Bishara, 1997).
Increased surface area has been associated with greater bond strength, greater
debonding forces and less bond failure (Ziebura, 2014; Sung, 2013; Weichmann, 2003;
Hanson, 1983; Buzzitta,1982; Maijer, 1981). In a study by Hanson, a more porous
network with increased surface area was created by adding a powdered metal coating
to brackets through a special sintering process. The presence of this layer increased
tensile bond strength by 88% over foil mesh brackets (Hanson, 1983). Buzzitta and
Maijer found that weld spots within the bracket mesh decreased nominal size and
available bonding surface area and led to decreases in bond strength (Buzzitta, 1982;
Maijer, 1981). Sung compared three types of custom bases for bonding of lingual
appliances — limited resin base (conventional), gold alloy base (Incognito), and
extended resin base (Kommonbase). All bases formed adhesive bonds that were
suitable to withstand occlusal and masticatory forces, and increased surface area of the
bracket’s base was associated with greater debonding force (Sung, 2013).
Not every study showed a clear and positive association between bracket base design
and shear bond strength (SBS). In 1998, MacColl studied the effect of bases of
different sizes on SBS. He found that when the surface area of the base was between
13
6.82 to 12.35mm
2
there was no significant change in bond strength. However, from
6.82 to 2.38mm
2
there was a relative decrease in bond strength (MacColl, 1998). Cucu
evaluated bracket bases with surface areas between 8-14mm
2
and found no significant
difference in SBS between the brackets (Cucu, 2002). Also, Lopez found that pairs of
brackets, which were identical except for their size, did not have significantly different
bond strengths. He, thus, concluded that the smaller brackets could be used without
sacrificing SBS (Lopez, 1980).
Several articles have evaluated the role of mesh size in bond strength. Reynolds
evaluated the effects of varying mesh size on tensile bond strength (TBS) and found
that those brackets in the finer mesh group (100-150 ℳm) yielded lower bond strengths
than those in the group with more coarse meshes (50-70 ℳm). Smaller aperture size
was also associated with lower TBS (Reynolds, 1976). Buzzitta’s findings agreed with
Reynolds’, when he found that 60 ℳm mesh had greater TBS than 100 ℳm (Buzzitta,
1982). Wang’s results showed 60 ℳm mesh increased bond strength more than 100 ℳm
mesh, also. This result led to his belief that bond strengths are a function of mesh size
and adhesive type. Filled adhesives required larger mesh sizes to better penetrate the
mesh space (Wang, 2004).
Others reported limited correlation between brackets with different mesh sizes and bond
strength (Cucu, 2002; DIckinson, 1980). Dickinson felt that bond strength was
independent of mesh size and cited the presence of spot welds and reduced nominal
area as greater contributing factors (Dickinson, 1980). Cucu found no significant
difference in bond strength after comparing brackets with 80 and 100 ℳm mesh sizes
(Cucu, 2002).
In general, surface properties are shown to have important roles in bond strength
testing. After finding significant differences between bracket types, Sharma-Sayal
concluded that superior designs may have allowed more efficient and complete
penetration of the cement, thus, increasing SBS (Sharma-Sayal, 2003). Sorel, et al.
14
compared laser-structured and foil mesh bracket bases. The results showed that the
laser-structured brackets had twice the bond strength as the foil mesh and were equally
safe despite 80% of failure at the adhesive-enamel interface. This, however, translated
into less cement to remove when debonding (Sorel, 2002). Surface preparation with
silanized silicone particles or aluminum oxide was shown to increase bond strength
when bonding to resin (Bouschlicher, 1999). There may be a level of hierarchy of bond
strengths because the silanized silica coated particles were associated with greater
SBS than aluminum oxide.
IV. Adhesive thickness
The role of adhesive thickness in orthodontic bonding is not clear. While a variety of
studies have been conducted testing for changes in bond strength with different
amounts of adhesive, there is a lack of standardization among these studies. Several
studies cite a decrease in bond strength with increased adhesive thickness (Bordeaux,
1994; Bradburn, 1992; Pender, 1988; Knoll, 1986). Other studies found an increase in
bond strength by increasing the thickness (Jain, 2013; Arici, 2005). And one study
showed no significant effect (Schiffer, 1992). A study by Mackey also showed minimal
change in SBS with increasing adhesive thickness, however, there was a trend to
decrease (Mackay, 1992).
Several authors addressed the concern regarding the role of adhesive thickness in
bracket adaptation (Jain, 2013; Machado, 2012; Mackey, 1992; Chumak, 1989). Each
of these investigators encouraged a uniform thickness of adhesive with well-adapted
brackets. Additionally, Mackey and Jain elaborated a link between adhesive thickness,
bracket placement, and control of tooth position (Jain, 2013; Mackay, 1992). With these
factors in mind, Lombardo added that indirect bonding methods, utilizing a custom resin
base, are standard in lingual orthodontics (Lombardo, 2011).
Type of adhesive also appears to have a role in bond strength. Jost-Brinkmann and
Schiffer showed that the type of adhesive used may be the most important factor when
15
determining the effect of adhesive thickness on bond strength. When adhesive
thickness was greater than 0.2mm, their recommendation was to use a macrofilled,
chemically cured, paste-paste composite (Jost-Brinkmann, 1992; Schiffer, 1992).
Buyuk found that Filtek Silorane, a low-shrinking composite, allowed less microleakage
than Transbond XT, a conventional bonding adhesive. Despite less microleakage,
however, this low-shrinking composite produced insufficient SBS, and the ARI
(Adhesive Remnant Index) scores indicated a less effective bond between the adhesive
and bracket. This type of adhesive was, therefore, not recommended for routine clinical
use (Buyuk, 2013).
Although the role of adhesive thickness in orthodontic bonding may not be clarified by
the literature, the thickness of adhesive must be purposeful and take into consideration
its effect on the teeth. Further investigation of adhesive thickness as it pertains to
customized and non-customized appliances and the lingual surface, in particular, is
warranted. Information gained from this type of investigation can better our under-
standing of the properties of adhesives and their role in the development of orthodontic
systems.
V. Lingual appliances
With the revival of lingual appliances, customized bracket systems are being advocated
over conventional types. Weichmann explained that the customized base of the CAD/
CAM lingual appliance allows for a much lower profile and, thus, contributes to less
tongue irritation and speech problems, reduced trauma from occlusion (and bracket
failures), and better torque control. The CAD/CAM system, he explained, requires
individualization of all appliance components and fuses the processes of bracket
fabrication and optimum positioning into one unit (Weichmann, 2003). In another
article, he relates that customized lingual appliances can be as efficient as conventional
labial types once the system becomes a part of the clinical routine. Much of the
previous burden of the lingual appliance is being relieved from the practitioner, and even
the inexperienced clinician can use the appliances with a little practice (Weichmann
16
2000). In conjunction with Weichmann’s approach, Lombardo cites the use of indirect
bonding techniques as standard in lingual orthodontics to achieve more precise bracket
and tooth positions (Lombardo, 2011).
George stated that customized lingual appliances have significant benefits over
prefabricated ones. The base of the bracket is very close to the tooth and eliminates
the need for a composite filler to make up for poor adaptation. This also allows the wire
to run in close proximity to the surface of the teeth and can provide more accurate
torque control and better finishing (George, 2013). Grauer reported that tooth
movement using customized bracket systems is very predictable and can achieve
results within 0.1mm or 5 degrees (Grauer, 2011). In a separate article, Grauer
advocates customizable systems as a way to treat patients as individuals while using
clinical techniques that can be faster and more precise than conventional methods
(Grauer, 2012). Other considerations when using lingual orthodontics include the use of
smaller brackets to facilitate better cleaning with less plaque retention (Cucu, 2002).
This is despite the fact that less plaque accumulation and minimal iatrogenic
decalcification occur when bonding to the lingual surface (Machado, 2012; van der
Veen, 2010).
Auluck expressed that there is a lack of research discussing the disadvantages of
customized appliances. Known disadvantages, however, may include treatment delays
to re-order brackets or wires, increased laboratory fees, technique sensitivity,
technological limitations, and a learning curve (Grauer, 2012; Ludwig 2000).
In a clinical study, Ziebura found that lingual brackets with a larger surface area
experienced less bond failure. Also, bond failure was significantly higher in the posterior
when compared with the anterior of both lingual and labial groups (Ziebura, 2014). In
contrast to this evidence, a 1989 investigation by Chumak evaluated the role of lingual
surfaces in determining shear bond strengths of stainless steel brackets. This
investigation showed that the variable nature of the lingual surfaces of the dentition
affected bond strength. These differences were most prominent when comparing
17
mandibular premolars and incisors, which have convex and concave lingual surfaces,
respectively. The convex surfaces were associated with greater bond strengths due to
greater bracket adaptation and more uniform adhesive thickness. The author, therefore,
recommended a more custom approach to lingual appliance design with identical
bonding protocols for all teeth (Chumak, 1989).
VI. Experimental Design
It is well documented that in vitro studies cannot emulate the dynamic nature of the oral
cavity (Samruajbenjakul, 2009; Eliades, 2000; Zachrisson, 1996). The presence of
water, proteins, minerals, and differences in pH levels and temperature can have an
impact on the way bonds are created with the teeth (Samruajbenjakul, 2009; Bishara,
2007). In spite of this obstacle, however, in vitro studies are useful to determine
bonding protocol and validity should be given to their results (Ziebura, 2014).
Fox and Eliades described a need for standardization in orthodontic bond strength
testing (Eliades, 2000; Fox, 1994). After reviewing the experimental designs for a large
sample of orthodontic literature on bond strength, Fox recognized several variables
within the studies. These variables included time elapsed before debonding, storage
medium, statistical analysis, force delivery, units of measurement, and failure location.
Based on these variables, Fox developed a rationale for a standard experimental
protocol for bond strength studies (Fox, 1994).
In terms of time elapsed before debonding, Fox stated that the amount of time is not
critical as long as 24 hours has elapsed (Fox, 1994). A study by Lopez supports this
recommendation because there was no significant difference in the bond strengths
between brackets that were placed in distilled water for 24 hours versus 30 days
(Lopez, 1980). Furthermore, Sharma-Sayal found that, prior to the first 24 hours, bond
strength increased (Sharma-Sayal, 2003).
18
In spite of the recommendation by Fox, there is research that supports the use of early
force applications. Ching showed that brackets bonded via a chemical cure were able
to withstand static shear and tensile forces within 15 minutes of bonding. Upon
debonding the brackets two weeks later, there was no significant difference in the bond
strengths between the brackets which were loaded and the control (Ching, 2000).
When testing shear-peel bond strength, Chamda showed that testing bond strengths
after 10 minutes, and within the initial 24 hours after bonding, can be successful,
regardless of whether a chemical- or light-cured adhesive is used (Chamda, 1996).
The immersion of natural tooth specimens in 37°C distilled water prior to debonding
may also be prudent when designing an in vitro bonding experiment. There was limited
data to support this practice, but it was the storage medium of choice in 24 of 66 papers
reviewed by Fox (Fox, 1994). Alternatively, Eliades contended that storage media may
have little, if any, influence on the adhesive bond strength to enamel, due to the largely
inorganic content of the hardened tooth surface (Eliades, 2000).
Some studies used plastic and hydroxyapatite as a bonding substrate instead of natural
teeth. The studies which compared plastic and natural teeth did not find any significant
difference in the tensile bond strengths between the the two bonding media (Buzzitta,
1982; Dickinson, 1980). When compared to enamel (16.62 MPa), the hydroxyapatite
(20.83 MPa) had a significantly greater shear bond strength. The SBS value for the
hydroxyapatite was in a clinically acceptable range, however (Imthiaz, 2008). Thus,
plastic cylinders and hydroxyapatite have been deemed as appropriate substitutes for
teeth when conducting in vitro studies of bond strength.
Although not widespread in its use, Fox recommended employing a Weibull analysis for
statistical testing with inclusion of a survival analysis, which gives clinical relevance to
the study (Fox, 1994). Eliades stated that the Weibull analysis does not require a
normal distribution and gives information about the tail of the sample, which can be
integral to the safety of the bonding system (Eliades, 2000). Bishara corroborated this
point of view after reviewing the increased bond strengths of ceramic brackets (Bishara,
19
1997). Fox also warned against assuming a normal distribution of specimens and
discouraged the use of t-tests and ANOVAs. Eliades added that a sample size greater
than thirty is appropriate to establish a normal distribution (Eliades, 2000; Fox, 1994).
In terms of bond strength, Fox stated that the average bond strength should be used
and measured in MPa or its equivalent, N/mm
2
. (Fox, 1994) Katona countered this
suggestion and used finite element model (FEM) analysis to show that the average
value used to describe bond failure (i.e. MPa) was not, in fact, an accurate predictor of
bond strength. Stresses varied throughout a bond and depend on the locus of the force
load. Distributing the force at failure by finding the average bond strength, in turn, was
not representative of the actual bond strength (Katona, 1997).
Fox further discussed how direction of force has variable effects on the specimen being
tested. A true shear force will take place at the exact interface of the bracket and
adhesive. A force applied distant to this interface, however, may create a “peel” effect
and, thus, affect the force magnitude needed to dislodge the bracket (Fox, 1994).
Katona explained how the load angulation affects the shear forces generated within a
cement layer. The load may contribute compressive or tensile loads to the specimen,
which ultimately can affect bond strength. Additionally, the distance that the force is
delivered from the cement layer creates a moment and further contributes to the forces
generated within the system (Katona, 1994). Other studies added that laboratory
techniques should be representative of true clinical debonding conditions (BenGassem,
2013; Littlewood, 1997) and would, thus, discourage the use of the uniform delivery of
force loads (Zachrisson, 1996).
Fox also addressed the location of bracket failure. Most bonding studies utilize the
Adhesive Remnant Index (ARI) to describe bond failure and its location. Studies show
that an ARI = 0 indicates that there is no adhesive remaining on the tooth. An ARI = 3,
on the other hand, implies that all of the adhesive is remaining on the tooth and there
will be an imprint of the bracket (Buyuk, 2013; Klocke, 2005; Bradburn, 1992). Fox
commented that ideally failure will occur at the adhesive-enamel interface between the
20
bracket and tooth. This will allow easier debonding and assumes no damage to the
enamel surface (Fox, 1994). Several other studies are in agreement with this idea
(Sorel, 2002; McAlarney, 1993; Hocevar, 1988; Pender, 1988). Ceramic brackets may
be the exception, however. The increased bond strengths established with these
brackets make it preferable to have failure occur away from the cement-enamel junction
where damage to enamel is less likely to occur (Bishara, 1997).
McAlarney investigated bond strengths and failure locations between conventional and
modified direct bonding systems. The modified system consisted of a cured resin layer
on the bracket pad as a part of bracket fabrication. Several factors associated with the
experimental conditions were identified that could affect the results in vitro. These
factors included adhesive properties and thickness, adhesive porosity, properties of the
bonding agent and bracket, direction and magnitude of applied force, rate of applied
force, and chemical and mechanical degradation. By keeping the experimental design
constant, it was possible to minimize the effects of variables and test for differences
between the methods. His results indicated a reduced, although acceptable, bond
strength and an increased percentage of failure at the interface of the bracket and
adhesive for the modified direct bonding technique (McAlarney, 1993).
Like McAlarney, Sinha showed that bond strength and the ARI are a function of many
factors. The applicable factors to his study included bracket type, bonding materials
used, and bonding technique (direct v. indirect). The results showed that all
combinations yielded adequate clinical bond strengths and consistent ARI scores.
Significant differences in SBS and ARI scores were seen based on bonding technique
and type of adhesive used. Further research was suggested to evaluate the indirect
method which yielded the lowest ARI score (Sinha, 1995).
Further evidence reveals that the type of adhesive plays a role in the shear bond
strength (SBS). In an earlier study, Pender demonstrated that a light-activated
orthodontic bonding agent showed reduced bond strength when compared with one that
was chemically-cured (Pender, 1988). Machado found that Transbond XT, a
21
conventional orthodontic bracket adhesive, had a greater SBS than Filtek-Z350, a
nanofilled flowable composite. ARI scores were not significant, however (Machado,
2012). In a study by Bishara, it is explained that orthodontic bonding systems may
contain two types of diacrylic resin adhesives - one filled and one unfilled. Generally,
the unfilled resin is placed on the tooth, while the filled resin engages the bracket pad
and is associated with increasing the bond strength (Bishara, 1997). Lastly,
BenGassem, et al. found a significantly greater SBS with a nanofilled adhesive when
comparing it to a conventional light-cured adhesive. The mean fatigue bond strengths
(FBS) between these adhesives were not significant, however (BenGassem, 2013).
The translation of laboratory findings to clinical practice should always be done with
caution (Aksu, 2013) and was addressed by several authors. Despite the statistical
significance of the reported bond strength values in vitro, these authors believed that
the clinical outcomes would not be significant in vivo. This comes as a result of bond
strength values for each material that were above a minimum threshold to achieve
clinical acceptability (BenGassem, 2013; Samruajbenjakul, 2009; Hildebrand, 2007;
McAlarney, 1993).
VII. Bracket-Adhesive interface
The interface between the bracket and tooth is a principle area of interest within bond
strength investigations. Studies controlling for the bracket-adhesive interface are more
favorable in vitro because of highly controlled experimental conditions in the laboratory.
In the human oral cavity, moisture contamination can affect the strength of the adhesive
bonds and confound results. Different authors cited contamination, voids, and issues
with access as reasons for reduced bond strength in vivo (Shiau, 1993; Maijer, 1981;
Dickinson, 1980). Furthermore, several studies identify the bracket-adhesive interface
as the site of failure in their experimental results (Sung, 2013; Lombardo, 2011; Arici,
2005; Cucu, 2002; Bordeaux, 1994; Shiau, 1993; Hocevar, 1988; Knoll, 1985; Buzzitta,
1982), and Knox isolated this interface when evaluating the influence of base design on
bond strength (Knox, 2000).
22
In a 1988 article by Oldegaard, it is highlighted that the bond strength is weaker
between the bracket and adhesive versus the adhesive and tooth. Due to the fact that
failure occurs at the bracket, as seen by the adhesive remnant on the tooth, one can be
certain the bond between the adhesive and tooth is stronger. Furthermore, the opposite
is true when ceramic brackets are bonded to the tooth. A chemical bond may be
formed, and thus, contributes to greater bond strength between the adhesive and
bracket, thus, leading to bond failure closer to the tooth (Oldegaard, 1988). Other
studies also showed that an increase in bond strength can lead a shift in bond failure
closer to the enamel (Samruajbenjakul, 2009; Wang, 2004; Sorel, 2002; Bradburn,
1992). Katona used FEM analysis to oppose this viewpoint regarding the association of
bond strength and failure location. He states that the location of failure does not
indicate a weaker bond in that area, but indicates the location where enough force was
built-up to to cause separation. Thus, a different area of the bond may be weaker, but a
lack of force in this area left the bond intact (Katona, 1997).
With regards to the enamel bonding surface, Chumak advocated identical bonding
protocols for all teeth, whether labial or lingual, and cited concerns regarding enamel
fracture, especially for the lingual surface, where the enamel is thinner. Mandibular
incisors were especially at risk due to microscopic differences in surface enamel rod
concentrations. It is believed damage to enamel may be overlooked on the lingual
surface and should be monitored, in particular when brackets fail due to occlusal force
(Chumak, 1989). Arici showed that when bracket failures occur at the bracket-adhesive
interface or completely within the adhesive, one can assume that there is no damage to
the enamel (Arici, 2000). Retief confirmed, however, that bond failure as low as 14MPa
could cause damage to enamel (Retief, 1974).
23
Chapter 3: Objective
This investigation was carried out for two reasons. First, it was believed that a study of
orthodontic bonding involving custom and non-custom brackets and different base
preparations would add to the existing body of evidence for this topic. Additionally, it
was valuable to study how different brackets can be bonded to the lingual surface.
Secondly, this project was undertaken to aid in the research and development of a novel
bracket system. This study was designed with the hope that the data generated would
give insight regarding the necessary character of a bracket base preparation technique
and morphology that will function well in the clinic, while also being cost-effective to
manufacture.
24
Chapter 4: Hypothesis
I. Research Hypothesis, Ha: There may be a difference in the mean bond strength
between four types of sham lingual brackets — sandblasted with a flat base,
sandblasted with a custom base, laser-etched with a flat base, and laser-etched with
a custom base.

II. Null Hypothesis, H0: There may not be a difference in the mean bond strength
between four types of sham lingual brackets — sandblasted with a flat base,
sandblasted with a custom base, laser-etched with a flat base, and laser-etched with
a custom base.
.
25
Chapter 5: Materials and Methods
This in vitro investigation tested sham brackets with flat and custom bases, while
varying surface preparation (Figure 1), to determine bond strength when bonded to the
lingual surface of a tooth-shaped resin substrate (Figure 2). One half of each group of
brackets were sandblasted and the other half were laser-etched, thus, creating four
distinct groups. The resin substrate was derived from an image of a real tooth and
manipulated using Solidworks, a software program for engineers (Figure 3). This
manipulation involved adding a block base to the digitized lateral incisor to create a
custom fit in the Instron machine (Figure 4). Once a good fit was established between
the block and the Instron machine, the resin block was fabricated using a 3D printer
(EnvisionTEC Ultra 3SP, Dearborn, MI) at the Viterbi School of Engineering at the
University of Southern California (Figure 5).
In order to ensure reproducible bracket placement when bonding to the resin substrate,
a putty material was used to fabricate a bonding “jig”. To create the jig, a bracket with a
custom base was bonded by hand to the resin substrate several times. This was done
to ensure that the custom bracket placement was repeatable and precise. Horizontal
and vertical orientation was evaluated prior to bonding to ensure perpendicularity. After
bonding, the “bond angle” (Figure 6) was evaluated between specimens to verify that
the bracket orientation was consistent. Once the repeatability of the procedure was
confirmed, the putty matrix (jig) was fabricated by using one of the bonded samples.
The matrix was formed to the resin substrate-bonded complex and the setting reaction
26
b) Flat, laser-etched bracket (with sprue)
Figure 1: Examples of two types of sham
brackets — a) Custom, laser-etched bracket
(with sprue)
was allowed to occur. The matrix was then trimmed so that it accommodated the
bracket (with sprue) and resin substrate in a way that was amenable to easy and
repeated use (Figure 7).
Once the resin block was seated into the putty matrix with the applied primer coating, a
bracket was coated with the adhesive and placed into its respective groove within the
putty matrix. Care was taken to ensure both a precise and accurate bracket position
once any flash was removed, and error was minimized by verifying that each aspect of
the bracket, resin substrate, and bonding “jig” were oriented properly. Once the bracket
position was confirmed, the bracket-resin block complex was cured and stored. The
entire population of eighty brackets were bonded in this way, and there were no
deviations in the protocol.
27
Figure 2: The resin block — Coronal (a) and sagittal (b) views of tooth-
shaped resin substrate. Note the anatomical features of the crown.
28
Figure 3: Virtual resin block — Occlusal and sagittal views of the resin substrate with
accompanying flat- (a) and custom-base (b) brackets. Note the characteristic anatomy of
the lateral incisor crown. Also, the casting sprue is not present on the bracket.
Figure 4: Resin block design — Diagrams demonstrating the unique fit of the resin
substrate within the Instron machine.
29
Figure 5: Pre-fabrication — Digital image
of resin substrate as depicted prior to
being printed.
Bonding brackets
1. Apply layer of adhesive (Assure Universal Bonding Resin, Reliance Orthodontic
Products, Itasca, IL) to lingual surface of resin block and bracket pad using a
microbrush.
2. Thin adhesive on both surfaces for <3 seconds with moderated application of
compressed air.
3. Place resin block in jig without completely seating it.
4. Apply Transbond PLUS Color Change Adhesive (3M Unitek, Monrovia, CA) to base
of bracket.
5. Place bracket into jig using cotton forceps and apply light pressure to create
adhesion with resin block.
6. Seat bracket and resin block together into jig.
7. Remove flash with sickle scaler and microbrush.
8. Confirm orientation of bracket on resin block and within jig.
9. Cure adhesive using Valo Ortho Cordless curing light (Ultradent, South Jordan, UT)
on Standard Mode (1200 mW/cm²) for 10 seconds.
Preparation of sandblasted brackets
1. Color unprepared bracket bases with black Sharpie.
2. Sandblast brackets at a distance of 5mm and for
5 seconds using 50 micron white aluminum oxide
(Danville Engineering, San Ramon, CA) under a
pressure of 65 psi with Basic Classic sandblaster
(Renfert, Hilzingen, Germany).
3. Verify that black ink from Sharpie is no longer
present.
4. Rinse brackets in distilled water.
5. Place all brackets (laser-etched and sandblasted)
in ultrasonic cleaner (Healthsonics, Algonquin, IL)
in 70% alcohol solution for 20 min.
6. Air dry.
Details of Protocol
30
10. Remove bracket-resin block complex from jig.
11. Remove any uncured composite and flash
from the obscured side of resin block.
12. Cure adhesive for an additional 5 seconds
from previously obscured side using Valo
Ortho Cordless curing light on Standard Mode.
13. Bracket-resin block complex is now ready to be
stored.
Figure 6. The bond angle — Photos of bonded
brackets identifying similar “bond angles” made
possible through use of the bonding “jig”.
Figure 7: The bonding jig — (a) Deliberate cuts
made to incorporate bracket and resin substrate
into bonding “jig”. The jig was trimmed in this way
to eliminate any interferences, which could
interfere with seating of the bracket or resin
substrate prior to bonding. It was possible to use
a single jig to bond all specimens. (b) Precise fit
of bracket-resin substrate within bonding jig.
Storing brackets
1. Store in a temperature-controlled oven at 37 °C
for 48 hours.
Testing for Bond Strength
1. Calculate the surface area of the brackets in mm²
2. Place bonded brackets in pre-determined
orientation in a universal testing machine
(System 5960, Instron, Norwood, MA), so that the
shear blade strikes the bracket-adhesive
interface of all specimens in the same location
and direction.
a
b
31
Figure 8: The laboratory setup — Photo of the resin substrate-bonded
complex placed within the Instron machine
3. Brackets are subjected to shear bond strength testing by loading specimens
under compression until failure at a crosshead speed of 0.5 mm/minute.
4. Debonding force is measured in N and converted to bond strength in MPa (N/mm²).
Failure analysis
1. Upon bond failure using the Instron machine, document the nature and degree of failure.
2. Label the failure as follows: adhesive at the bracket, adhesive at the substrate, or mixed.
3. Note the location of the fracture (i.e. at the substrate), if any occurs.
4. Note any other findings that may be significant to the experiment’s outcome.
5. Assign an ARI score (0-3).
Scanning Electron Microscopy
1. One before/after sample from each group was rinsed in 70% alcohol solution.
2. Samples were air-dried overnight.
3. Samples were placed within scanning electron microscope (JEOL JSM-7001F,
Nanofabrication Cleanroom Facility, Denton, TX)
4. Micrograph images were taken at 25x, 50x, 100x, and 200x magnification
Chapter 6: Statistics
In order to find the average bond strength of each tested specimen, it was necessary to
establish two things: (1) the bond strength obtained from each trial of the raw
experimental data, and (2) the surface area of both, the flat- and custom-base brackets.
The debonding force for each specimen was digitally obtained using Blue Hill 3 software
(Instron, Norwood, MA) after each successive test with the Instron machine and
recorded in Newtons (Graph 1-4). The calculation for surface area was made possible
using the Solidworks program by multiplying the length and width of each bracket base,
while taking into account the added morphology of the custom base. Both values were
reported in mm
2
(Table 1). It was not possible to quantify the change in surface area
that resulted from the different surface preparations (i.e. from the sandblasting and
laser-etching). These alterations of the surface were examined and qualified, however,
through scanning electron microscopy (SEM).
The type of bond failure was documented for each specimen. These failures were
categorized as follows: adhesive at the bracket, mixed, or adhesive at the substrate. In
addition, the nature of the failure was noted because there were varying degrees of
fracture at the substrate.
Statistical tests
1. Debonding force values (in N) were divided by individual bracket surface area (in
mm
2
) for each group, thus, producing the bond strength for all specimens in N/mm
2
,
or MPa.
2. A near-normal distribution of specimens was identified for each group.
3. Statistical analysis in R was performed using one-way ANOVA to evaluate for a
difference between groups.
4. A post-hoc Tukey test was performed to identify which groups were significantly
different from each other.
5. Quantification of the bracket failures was applied.
32
Chapter 7: Results
Graphs 1-4: Raw data generated from Instron testing
Graph 1-2: Sample of loading curves for bracket types with flat bases. (1) specimens with flat
base and sandblasting, (2) specimens with flat base and laser-etching. Note: linear character of
the loading curves.
33
(1)
(2)
34
Graphs 3-4: Samples of loading curves for bracket types with custom bases. (3) specimens with custom
base and sandblasting, (4) specimens with custom base and laser-etching. Note: There is a characteristic
“dip” in the loading curves for both groups.
(3)
(4)
Table 1: Instron test results

Group 1: C/SB - custom bracket with sandblasted base
Group 2: C/LE - custom bracket with laser-etched base
Group 3: F/SB - flat bracket with sandblasted base
Group 4: F/LE - flat bracket with laser-etched base
Table 2: 2x2 Between Subjects Factorial Design
Bond strength in MPa

MPa

Group No. Bracket/
Surface
type
n Surface
Area
(mm
2
)
Newtons
(avg.)
sd (N) MPa (N/
mm
2
)
sd (MPa)
1 C/SB 20 9.66 438.01 65.55 45.34 6.78
2 C/LE 20 9.66 350.41 58.29 36.27 6.03
3 F/SB 20 7.65 363.63 58.36 47.53 7.62
4 F/LE 20 7.65 319.70 62.82 41.79 8.21
Sandblasted Laser-etched
Custom bracket
45.34±6.78 36.27±6.03
Flat bracket
47.53±7.62 41.79±8.21
35
Graph 5: Violin Plot
Table 3: Test of Homogeneity of Variances
Table 4: One-way ANOVA
*p <0.05 = statistically significant
Table 5: Tukey test for multiple comparisons of means
Means for groups in homogenous subsets are displayed
Using harmonic mean sample size (n=20)
95% family-wise confidence interval
*p <0.05 = statistically significant
Levene statistic df1 df2 p value
0.555 3 76 0.646
Df Sum of
squares
Mean square F-value p value
Within groups 3 1449 483 9.286 *2.64 x10^-5
Between
groups
76 3954 52
Total 79 5403 535
Goups compared difference in
mean
lower limit upper limit p value (adj.)
C/SB - F/LE 3.552223 -2.4389361 9.5433821 0.4089397
F/SB - F/LE 5.743029 -0.2481302 11.7341881 0.0651153
C/LE - F/LE -5.515929 -11.5070883 0.4752299 0.0821618
F/SB - C/SB 2.190806 -3.8003531 8.1819651 0.7721231
C/LE - C/SB -9.068152 -15.0593113 -3.0769931 *0.0008961
C/LE - F/SB -11.258958 -17.2501173 -5.2677990 *0.0000269
36
Table 6: Summary of location of failure
^ARI <2 and identified “two-part” failure
*at bracket; ARI > 2

Bond strength testing
Table I provides a summary of the raw data generated from the experiment, a sample of
which is presented in Graphs 1-4. All debonding force values (N) were converted to
mean bond strength (MPa) for purpose of comparison as a function of surface area.
The 2x2 Between Subjects Factorial Design that was used for this study is illustrated in
Table II and shows mean bond strength values with standard deviation for each group.
Analysis of a histogram (not pictured) and violin plot (Graph 2) for each group revealed
a normal distribution pattern and homoscedasticity of variability of the bond strengths for
the four groups is shown in Table III. Based on the satisfaction of these assumptions for
normality, it was elected to proceed with a one-way ANOVA test to determine whether
there was a significant difference in mean bond strength between any of the groups
(Table IV). Based on the results from the ANOVA, it was evident that the mean bond
strength from at least one group was significantly different from the others (p < 0.0001,
F-test). Tukey test was then applied for the pair-wise comparisons of these four groups
(Table V). The results indicated that the custom, sandblasted group (Group 1) had an
estimated mean bond strength that was 9.07 MPa greater than the custom, laser-etched
group (Group 2). This difference was statistically significant (P=0.0009 by Tukey test,
95% CI: -15.06, -3.08). The flat, sandblasted group (Group 3) had an estimated mean
bond strength that was 11.26 MPa greater than the custom, laser-etched group (Group
2). This difference was also statistically significant (P=0.00003 by Tukey test, 95% CI:
-17.25, -5.27). As a result, the research hypothesis was validated — there was a
significant difference in mean bond strength present within the four groups.
Group
No.
Bracket
type
n Mixed
failure^
%
Mixed
failure
Adhesive
failure*
%
Adhesive
failure
1 C/SB 20 20 100 0 0
2 C/LE 20 20 100 0 0
3 F/SB 20 1 5 19 95
4 F/LE 20 2 10 18 90
37
Analysis of failure location
Table VI reports a summary of the location of failure for each group. Groups 1 and 2
can be characterized by mixed bond failure for each sample, which coincided with a
characteristic “dip” for these groups (Graphs 3 and 4). The ARI < 2 for all mixed failure
specimens. Bracket failure for Groups 3 and 4 occurred at the bracket-adhesive
interface 95% and 90% of the time, respectively. The loading curves were more linear
for these groups (Graphs 1 and 2), and ARI > 2 for all specimens. Thus, the mixed
failures for these two groups were more mild than those for Groups 1 and 2.
38
Chapter 8: Discussion
When fabricating the materials for this experiment, it was necessary to create a system
where all parts fit. The use of a software program for engineers, Solidworks, was
integral to this process (Figure 3). This software allowed us to design a resin substrate
with ideal dimensions to fit within the Instron machine. The 3D printer allowed us to
manifest the resin substrate design in a precise and reproducible way, thus, allowing the
anatomical lingual surface of the lateral incisor to act as a reliable control and
reproducible custom bonding surface. With the aid of a “jig”, it was possible to place the
bracket in a consistent way. We found that a putty material, identical to that used in
restorative dentistry to fabricate a provisional crown, was suitable to fabricate the jig.
Care was required to form and cut the putty matrix so that the bracket and tooth
substrate could be placed in an ideal way. The putty matrix conformed to all the printed
resin blocks in the same way, which was a testament to the accuracy of the 3D printer
(Figure 7b). Moreover, the process was aided by the excellent dimensional stability of
the putty. This technique proved to be cost-effective and easy relative to the other
methods that were attempted.
As a part of the procedure to cast the brackets used in this
study, a sprue was attached to the bracket wax pattern
(Figure 1). When designing the experiment, it was
important to leave the sprue intact on the bracket, for it
made handling and placing the bracket more manageable
during the bonding process. The sprue was also useful to
evaluate the “bond angle”, the angle formed by the sprue
and the long axis of the resin substrate (Figure 6). No
significant deviation of bond angle was noted in a cross-
sectional analysis of bonded substrate-bracket complexes
selected from each group. This finding further validated
the reproducibility of the jig, which was invaluable when
bonding the flat brackets.
39
Figure 9: Orientation of the tooth
substrate.
The tooth-shaped element of the resin substrate was digitally oriented so that the
Instron blade would contact the bracket at an off-angle (Figure 9). It was believed that
the off-angle orientation of the Instron force load added a more realistic aspect to the
experiment and mimicked the natural occlusion. In a true shear bond strength test,
however, the blade of the Instron would load the bracket parallel to the vertical plane of
its base. Regardless of whether the base was flat or custom, all brackets were oriented
in the same way, and the Instron blade loaded each sample in the same location next to
the “marginal ridges” (Figure 10). Due to this deviation in load delivery, there was an
effect on the stresses created within the adhesive interface.
For the groups with flat-bases, the blade of the Instron struck each bracket at its
interface with the adhesive and away from the “marginal ridges” of the resin substrate.
The force, however, was not directed down the vertical face of the bracket (Figure 10a).
Despite this deviation, most bracket failure occurred at the bracket-adhesive interface,
and there was a shear nature of the force. The design of the experiment was to allow
40
Figure 10: Load delivery. Diagrams depicting the angle and location of the bonding interface
relative to the Instron blade for (a) the bracket with flat base and (b) the bracket with custom
base. Note that the blade does not impede the marginal ridge (not labeled).
a
b
the Instron blade to load the flat brackets at their incisal aspect without interference by
the resin substrate design, and this was achieved.
For the custom brackets, the “marginal ridges” limited the ability of the Instron blade to
access the bracket-adhesive interface (Figure 10b). Nonetheless, the blade of the
Instron machine struck each bracket at the same angle, and there were not any
apparent deviations. The force created when the bonded custom-bracket complex was
loaded away from its interface, therefore, produced different stresses within the
adhesive (compared to the flat brackets) and influenced the results.
The effect of loading the custom base samples is shown in Graphs 3 and 4. The
loading curves for these graphs are characteristic of the custom base groups, which
experienced different internal stresses when compared to the brackets with flat bases
(Graphs 1 and 2). Within the initial 0.4 - 0.8mm of loaded extension, there is a sign of
failure for both custom-base groups, which is characterized by a “dip” in the debonding
force load (N). In most cases, the specimen was able to withstand this fatiguing and
persist for an extended period. During the subsequent period, the samples continued to
show compressive response, and, in many cases, were associated with fractures within
the resin substrate (ARI = 0). This is what would be expected with the generation of
elevated debonding forces (N). However, once the surface area of the custom base
samples was factored in, the mean bond strength (MPa) values were lower for the
groups with custom bases when compared to the groups with flat bases (Table 1). The
conversion to mean bond strength to assess relative bond strengths per unit surface
area is conventional in the literature, but may be misleading when determining whether
or not a bond is adequate to sustain clinical debonding forces. This is perceived as a
limitation when considering mean bond strength values for clinical interpretation.
Graph 1 and Graph 2 illustrate the more linear loading curves for the groups with flat
bases. Based on these curves, it is apparent that a more straightforward internal force
system was present for these groups. Furthermore, the groups with flat bases required
41
lower debonding forces (N), but maintained mean bond strengths (MPa) that were
superior to those of the custom base groups.
Based on these graphs, it is apparent that the custom base groups were subject to a
different set of stresses along the adhesive interface when compared to the flat base
groups. The distinction between these two disparate force systems may be further
evaluated using a finite element model (FEM) analysis and was beyond the scope of
this investigation. Like any other in vitro investigation, these results must be viewed
with caution, and further investigation may be necessary prior to application to the
clinical setting.
The bracket-adhesive interface is the more pertinent interface to study when evaluating
the effectiveness of bonded brackets. Many studies use natural teeth to study this
interface. After being validated in both, pilot studies for this investigation and in
previous research (Buzzitta, 1982; Dickinson, 1980), resin was chosen as the bonding
substrate for this investigation. The use of resin for a substrate was advantageous for
two reasons: (1) Like enamel, the strength of the bond between the resin block and
adhesive is greater than that between the adhesive and bracket base. This created a
situation suitable to evaluate the bond between the bracket and adhesive because this
aspect of the adhesive bond failed first. (2) It was possible to create a reproducible
custom lingual surface in resin, which made it possible to test custom brackets. It would
not have been practical to make individual custom brackets if natural teeth were used.
Like many other bond strength studies, the present study may not be subject to direct
comparison with other investigations. Concessions were made, however, to design this
experiment in a manner consistent with existing evidence. For example, it was decided
to wait >24 hours after bonding to administer the experiment. Contrary to other in vitro
bonding experiments, however, all brackets were placed in a temperature-controlled
oven at 37°C for 48 hours. Using the existing body of evidence, Fox recommended that
all bonded tooth substrates be placed in 37°C water bath for at least 24 hours. Because
42
this experiment did not use natural teeth and there was no trial conducted to assess
how the bonded specimens would behave in the 37°C water bath, it was preferable to
use dry heat for storage. This type of treatment may have affected the results, but all
groups were treated in the same way. Therefore, the effects from the temperature-
controlled oven would be universal and yield no net effect within any single group.
The sample size of all groups in this study is somewhat small (n=20), so the results
should be interpreted with this consideration. Despite the sample size, the data
generated from this study are substantiated, by a near-normal distribution for all groups
and homoscedasticity. Any outlying values obtained could be attributed to an uneven
adhesive layer, presence of flash, imperfections in the bracket or resin substrate,
variation in surface preparation, or other contamination. There were no noted
deviations in protocol throughout the experiment.
The design of this experiment may be viewed as lacking a true control group. Due to
the practical nature of the study and objective to find a clinically useful bracket, it was
deemed unnecessary to test the sham bracket base without any surface preparation.
Based on previous trials by the author, an unadulterated sham bracket produced inferior
results. As such, the use of such a base did not make sense. It was, therefore, decided
not to expend the time or resources to test this aspect. If one were to study a sham
bracket base without preparation, it may be beneficial to try different adhesives to
promote a greater bond strength for that specific type of base. Furthermore, the use of
a 2x2 Between Subjects Factorial Design made it possible to establish an idea of
relative superiority of bond strength by varying bracket base preparation and
morphology between groups. Meaningful data was generated by keeping one aspect
constant within each group, testing each group, and then comparing the results.
Isolating the bracket-adhesive interface was a goal in the design of this experiment.
The groups with flat bases were associated with ARI > 2, indicating that the bond failed
at or near the bracket-adhesive interface. For the groups with custom bases, bond
failure occurred at, or within, the tooth substrate and, thus, were labeled as “mixed”
43
failures and assigned ARI scores < 2. This mode of failure was attributed to the “off-
axis” loading in the custom-base groups, which incited greater compressive forces.
Nonetheless, one would have expected groups with lower mean bond strengths to have
greater ARI scores, especially in a controlled environment. Based on the fact that the
groups with lower ARIs failed within the substrate interface, there is an indication that
other factors are involved, and mean bond strength alone cannot predict location of
failure for bonded brackets.
This experiment was designed to identify whether adequate bond strength could be
achieved by varying the surface character of a sham lingual bracket. The results
indicate significant differences in mean bond strength when varying the bracket base
and provide information that may be useful in the design of a lingual appliance system.
Surface preparation had a significant effect on the mean bond strength when a custom
44
Figure 11: Scanning electron microscopy. Flat, sandblasted bracket base (a) before and (b)
after debonding. Custom, laser-etched base (c) before and (d) after debonding. — 50x image
Note: (b) was reported as an “adhesive failure”, and (d) was reported as “mixed”.
bracket base was used. In the the presence of a flat base, however, only the
sandblasted bracket had a significantly greater mean bond strength than the laser-
etched custom base. Despite these findings, the bond strengths of all groups were well-
above the necessary strength requirement to be effective clinically, so the statistically
significant results of this study may not, in fact, be clinically significant. Furthermore,
these results lead to a speculation that the base of a bracket may be more amenable to
different characteristics than was previously thought.
Several factors were considered to understand why the bond strengths were so high in
this study. These factors included the bonding agent and adhesive used, the “off-angle”
and compressive loading of the bonded specimens, the use of a resin substrate, and
the application of dry heat during storage. Despite the consideration of these factors,
the principal cause for the increase in mean bond strength across all groups remains to
be investigated, and clinical interpretation of these results should be made with caution.
Several statements can be made based on the results of this experiment. First,
sandblasting, alone, may be a sufficient surface preparation method to bond a bracket
to the lingual surface, making it unnecessary to laser-etch metallic brackets. Next, it
may be feasible to use a bracket with a flat base on the lingual surface, regardless of its
surface preparation. This finding may lead to a significant reduction in the production
costs to fabricate an appliance. Finally, despite great demand for customization in
modern lingual appliances, there may be a cost-effective way to fabricate an appliance
without the use of complex mesh designs and customized features. These results,
therefore, suggest further investigation and clinical translation.
45
Chapter 9: Conclusions
The following conclusions can be made based on this study:
• When sandblasted or laser-etched, the base of a sham bracket can produce
acceptable bond strengths regardless of whether the bracket base is custom or flat.
• Sham brackets with custom, laser-etched bases were associated with significantly
lower bond strengths when compared to both types of sandblasted bracket bases.
This difference may not be significant clinically, however.
• There was no significant difference in bond strength between either, sandblasted or
laser-etched, flat brackets.
• The ARI is a function of more than mean bond strength and can be influenced by
compressive forces, which act on the adhesive layer.
• 3D printing technology can be useful in the design of customized in vitro experiments.
• Further investigation can give more insight into the results of this study.
46
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52

Abstract (if available)

Abstract Background: In vitro lab investigations are useful to gain insights into how different bracket bases perform under controlled conditions. It is possible to evaluate the characteristics of these bases to design one that is both, clinically effective and affordable to manufacture. ❧ Purpose: To test the effects of base morphology and preparation on bond strength, using sham lingual brackets and a customized resin substrate for bonding. ❧ Methods: Eighty sham lingual brackets were divided into four equal groups (n=20): flat base with sandblasting, flat base with laser-etching, custom base with sandblasting, and custom base with laser-etching. The brackets were bonded to a resin substrate using a custom jig and tested for bond strength. ❧ Results: The brackets with a custom base and laser-etching showed a significantly lower bond strength when compared to both sandblasted groups. All groups, however, had acceptable bond strengths. ❧ Conclusion: When comparing different sham lingual brackets, one that has a custom base or preparation with laser etching may not have a greater bond strength than a base that is flat and sandblasted. All bracket types demonstrated acceptable performance, however, and may be considered for clinical use. Further clinical study of custom brackets may be hindered by issues associated with feasibility and increased costs.

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Creator Oviedo, Christopher J. (author)

Core Title An evaluation of bond strength using sham lingual brackets with differences in base morphology and preparation

School School of Dentistry

Degree Master of Science

Degree Program Craniofacial Biology

Publication Date 04/15/2015

Defense Date 03/09/2015

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

Tag base,bond strength,lingual brackets,OAI-PMH Harvest

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Advisor Sameshima, Glenn T. (committee chair), Grauer, Dan (committee member), Paine, Michael L. (committee member)

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Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-547843

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