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SD Vasudevan, Shobha Sundareswaran
10.5005/jp-journals-10021-1256
Original article
Bonding Characteristics of Improved
Low Viscosity Adhesives for Orthodontic Use
1
SD Vasudevan, 2Shobha Sundareswaran
Abstract
INTRODUCTION
Aim: To compare the bonding characteristics of low viscosity
adhe­sives, Transbond Supreme LV (TSLV-3M, Unitek, Monrovia,
California) and Enlight LV (ELV, Ormco, Orange, California) with
the traditional orthodontic adhesive, Transbond XT (TXT-3M,
Unitek).
One of the most important landmarks in dentistry is the
introduction of the acid etch technique by Buonocore.1 This
was followed by the contributions of Bowen2 and Newman3
resulting in the formulation of Bis-GMA. Thus was born
the concept of direct bonding in orthodontics. Further work
carried out by Newman and Snyder,4 Retief et al,5 etc.
have greatly refined the system for bonding orthodontic
attachments directly to enamel surface. Basically, a resin
based composite is defined as a three-dimensional mixture
of two or more chemically different materials having
distinct interfaces.6 The three phases incorporated are as
follows:
• Organic phase comprising of the matrix (Bis-GMA/
modified Bis-GMA/ TEG-DMA/ UDMA) in which filler
particles are suspended.
• Interfacial phase consisting of coupling agents like
silanes which serve as bonding agents between the
inorganic filler particles and organic matrix.
• Inorganic phase comprising of filler particles to reduce
shrinkage, provide strength and wear resistance. A
classification was introduced by Lutz and Phillips7 in
1983 based on filler particle size, categorizing them
into macrofillers (10-100 microns), midifillers (1-10
microns), minifillers (0.1-1 micron), and microfillers
(0.01-0.1 micron). More recently, we have polymer
nanocomposites with nanofillers (0.005-0.01 microns).
However, according to Bishara et al,6. the fairly thick
consistency of this material made it unsuitable for
orthodontic use as the paste needed to be forcibly pushed
into the retention pad of the bracket bases. This implies
that a flowable consistency is preferable for bonding as
it has better penetration.
Progress in this field has taken place in rapid strides, so
much, so that, it is not easy for a practicing orthodontist to
stay updated. The continuous search for better adhesives and
simpler procedures has lead to the introduction of flow­able
composites. This material merits great ortho­dontic attention
because of its desirable clinical handling charac­teristics like:
• Fluid injectability
• Better penetration
• Nonstickiness
Materials and methods: Forty-five extracted maxillary human
pre­molars, divided into three groups of 15 each, were bonded
with stainless steel brackets using TSLV in Group 1, ELV in
Group II, and TXT in Group III. After 24 hours of storage in distilled water, shear bond strength was evaluated. The adhesive
remnant index was assessed after debonding. In addition, the
flow characteristics were also examined.
Statistical analysis: Data were analyzed using analysis of
variance (ANOVA), Duncan’s multiple range and Kruskal-Wallis
tests.
Results: The mean SBS for Group I low viscosity resin was
12.59 ± 2.79 MPa (TSLV), Group II was 11.97 ± 2.42 MPa (ELV),
both well above the clinically acceptable range, but signi­­ficantly
lower than Group III (TXT) 14.92 ± 2.52 MPa. The ARI scores
for Groups I and II were significantly higher than Group III.
Evaluation of flow characteristics indicated superior flowability
for Group I whereas Group II exhibited least flow.
Conclusion: The low viscosity resin Transbond Supreme
LV and Enlight LV can be conveniently applied for bonding
orthodontic brackets.
Keywords: Flowable composites, Low viscosity resins, Shear
bond strength.
How to cite this article: Vasudevan SD, Sundareswaran S.
Bonding Characteristics of Improved Low Viscosity Adhesives
for Orthodontic Use. J Ind Orthod Soc 2014;48(4):262-266.
Source of support: Nil
Conflict of interest: None
Received on: 18/9/13
Accepted after Revision: 5/10/13
1
Senior Lecturer, 2Professor and Head
1
Department of Orthodontics, Kamineni Institute of Dental
Sciences, Nalgonda, Andhra Pradesh, India
2
Department of Orthodontics, Government Dental College
Kozhikode, Kerala, India
Corresponding Author: SD Vasudevan, Senior Lecturer
Department of Orthodontics, Kamineni Institute of Dental
Sciences, Nalgonda, Andhra Pradesh, India, Phone: 09849970170
e-mail: [email protected]
262
JIOS
Bonding Characteristics of Improved Low Viscosity Adhesives for Orthodontic Use
These low viscosity composites were created by retaining
the small particle size of traditional hybrid composites by
reducing the filler content and allowing the increased resin
to decrease the viscosity of the mixture. A plethora of new
low-viscosity composite resin materials have been marketed
during the past few years. However, these materials can be
used by the clinician only if they can guarantee clinically
acceptable shear bond strength (SBS) to acid etched enamel.
Studies published in this regard have come up with
conflic­ting results. Use of flowable composites has been
recom­mended by Ryou et al8 as well as Tecco et al.9 But,
the study by Uysal et al10 reported significantly lower SBS
values with flowable composites and they have, therefore,
not recommended their use for direct bonding of orthodontic
brackets.
The enamel surface after removal of orthodontic brackets
bonded with flowable orthodontic composites was studied
by Tecco et al11 in comparison with a traditional orthodontic
com­posite resin. It was concluded that flowable composites
seem to show no relevant differences in terms of enamel
lesions and cracks compared with traditional composites.
Another low viscosity resin claiming improved shear
bond strength over the currently available flowable adhe­
sives is Transbond Supreme LV (3M Unitek, Monrovia,
California). The manufacturers claim that these resins have
on demand flowable characteristics. At the same time, the
combination of silica and zirconia nanofillers (65% by
weight) which are claimed to give the material excellent
strength, flow and wear properties would make it an ideal
orthodontic adhesive but little has been published about
it till date. Yet another low viscosity resin in the market
is Enlight LV marketed by Ormco, Orange, California.
There are relatively few studies available on the bonding
characteristics of these materials.
Hence, the aim of this study was to investigate the
bonding characteristics of the two flowable composites
(Transbond Supreme LV and Enlight LV) using shear
bond strength test and site of bond failure as evidenced by
adhesive remnant index score (ARI) in comparison with
the conventional bonding adhesive Transbond XT. It was
also decided to evaluate the flow characteristics for each
material using a method similar to Bayne et al.12 The null
hypothesis generated was that there would be no difference
in shear bond strength or adhesive remnant index score of
the two low viscosity resins being tested and the traditional
composite resin Transbond XT.
MATERIALS AND METHODS
The samples consisted of 45 human maxillary premolar teeth
that had been extracted as part of orthodontic treatment. Only
intact, noncarious, nonrestored teeth with no developmental
defects on the facial surfaces were used. These samples
were collected and stored at room temperature in distilled
water which was changed periodically to inhibit bacterial
growth. Each sample was embedded in an acrylic block of
polymethyl methacrylate (PMMA) so that only the coronal
portion of the specimen was exposed. The crowns were
oriented along the long-axis of the blocks. The samples were
randomly divided into three groups—Groups I, II and III;
each group having fifteen samples. The teeth were polished
using a rubber cup and a nonfluoridated glycerine-free
polishing paste, then washed and air dried.
All teeth were etched using 3M ScotchbondTM Etchant
(3M ESPE Dental Products, St Paul, Minn) containing
35% phosphoric acid. Etching procedure was carried out
according to the manufacturer’s instructions.
Forty-five metal maxillary premolar brackets were used
(Gemini series, 3M Unitek); the bracket base area was esti­
mated as 12.2 mm2.
The teeth were bonded as follows:
• Group I (n = 15): Acid, Transbond XT primer, Transbond
Supreme LV Paste (3M Unitek) and Light cure.
• Group II (n = 15): Acid, Ortho Solo Primer, Enlight LV
Paste (Ormco, Orange, Cailfornia, USA) and Light cure.
• Group III (n = 15): Acid, Transbond XT Primer,
Transbond XT Paste (3M Unitek) and Light cure.
Intermediate low viscosity resin was used here as directed
by manufacturers. In all cases, the brackets were placed
on the teeth with firm pressure and excess adhesive was
removed from around the base of the bracket. The adhesive
was then light-cured for 40 seconds, positioning the light
source (Ortholux, 3M Unitek) for 10 seconds on each side.
All the procedures were done by a single operator to
avoid interoperator variability. After bonding, the specimens
were stored in distilled water at 37°C for 24 hours.
Method of Shear Bond Strength Evaluation
The SBS of bonded teeth was tested using an Instron
universal testing machine model no. 3365. The sample testing
was carried out using a sensitive load cell of 2 K Newton
capacity, at a crosshead constant speed of 0.5 mm/minute.
The testing external environment recorded 51% humi­dity
and room temperature was 24°C.
The acrylic block was mounted and the bracket held
precisely by hooking a 21 gauge stainless steel wire of
sufficient length under the gingival tie wings of brackets.
The brackets were always loaded from beneath the tie-wings
to reduce the peeling moment for in vitro shear testing. The
other end of the wire was hooked to the upper arm of the
machine.
The Journal of Indian Orthodontic Society, October-December 2014;48(4):262-266
263
SD Vasudevan, Shobha Sundareswaran
The force at which the bond failure occurred was recor­
ded on the computer, calculated in megapascal (MPa) and
tabulated for each subgroup.
Adhesive Remnant Index
Once the brackets were debonded, the enamel surface
of each tooth was examined with a digital micro­scope
(Keyence digital HD microscope, VH 8000 series) under
×35 magnifications to determine the amount of residual
adhesive remaining on each tooth. ARI suggested by Artun
and Bergland13 was used to quantify the amount of remaining
adhesive using the following scale:
• 0—no composite left on enamel surface.
• 1—less than half of composite left on enamel surface.
• 2—more than half of composite left on enamel surface.
• 3—all composite left on enamel surface.
Flow Characteristics
A disposable 1 ml syringe without a needle tip was filled
with the test material. Then a standard volume (0.5 ml) was
extruded onto a glass plate. This was immediately covered by
three stacked glass slides. After 30 seconds, the samples were
transferred to a curing unit and were cured for 60 seconds. This
resulted in a nearly circular disk and its diameter was recorded
twice (along perpendicular lines) and the relative flow
(n = 10) for each composite was recorded by measuring the
diameter. For each material, the average diameter of ten
disks was used as the comparative flow result.
Statistical Analysis
Data were analyzed using computer software, statistical
package for social sciences (SPSS) version 10. Data are
expressed in its mean and standard deviation. Analysis of
variance (one-way ANOVA) was performed as parametric
test to compare different groups. Duncan’s multiple range
(DMR) test was used as post hoc test to elucidate the diffe­
rence between each group. Kruskal-Wallis ANOVA was used
to compare nonparametric index variable. For all statistical
evaluations, a two-tailed probability of value, < 0.05 was
considered significant.
RESULTS
Shear Bond Strength
The mean shear bond strength for brackets bonded with
Transbond Supreme LV in Group I was 12.59 ± 2.79 MPa;
for Enlight LV in Group II, it was 11.97 ± 2.42 MPa; for
Transbond XT in Group III, it was 14.92 ± 2.52 MPa
(Table 1).
Samples bonded with Transbond XT in Group III showed
the highest mean shear bond strength in the study followed
264
by brackets bonded with Transbond supreme LV in Group I,
while the lowest value was seen in brackets bonded with
Enlight LV in Group II.
One-way ANOVA and Duncan’s multiple range test
showed that there was no statistically significant difference
between the mean SBS of brackets bonded with the two low
viscosity resins (Groups I and II). However, the analyses
did show that there was a statistically significant difference
between the mean SBS of the two low viscosity orthodontic
adhesives (Groups I and II) and the conventional orthodontic
adhesive (Group III) which was found to have the highest
mean SBS when compared to the other two materials used
in the study.
Adhesive Remnant Index (Table 2)
• Group I: One bracket (6.6%) showed failure at composite
tooth interface. 26.7% brackets showed less than half of
the adhesive left on the tooth, 40% brackets showed more
than half of the adhesive left on the tooth and 26.7%
brackets had failure at the bracket—adhesive interface.
• Group II: 73.4% brackets showed more than half of the
adhesive left on the tooth, while 26.6% of the brackets
showed failure at the bracket—adhesive interface.
• Group III: 60% brackets showed less than half of the
adhesive left on the tooth, 40% brackets showed more
than half of the adhesive left on the tooth.
The mean ARI scores for Group I was 1.8 ± 0.94, for
Group II, it was 2.27 ± 0.46, and, for the conventional
adhe­sive in Group III, it was 1.40 ± 0.51. However, the
median scores for the low viscosity adhesives in Groups I
and II was 2 whereas for Group III it was 1. Kruskal-Wallis
ANOVA was used as nonparametric test to compare ARI
score between groups. For all statistical evaluations, a twotailed probability value, < 0.01 was considered significant.
Table 1: Analysis of variance (one-way ANOVA) comparing
mean adhesive strength in MPa of different groups
Groups
Mean
±SD
I
12.59
a
2.79
II
11.97a
2.42
III
14.92b
2.52
F-value
p-value
5.442
< 0.01
a, b
Means with same superscripts do not differ each other (Duncan’s
multiple range test)
Table 2: Analysis of variance (Kruskal-Wallis ANOVA)
comparing adhesive remnant index of different groups
Groups
Median
score
Mean
score
±SD
I
2
1.8
0.94
II
2
2.27
0.46
III
1
1.4
0.51
H-value
p-value
11.239
< 0.01
JIOS
Bonding Characteristics of Improved Low Viscosity Adhesives for Orthodontic Use
Fig. 1: Composite disks showing flow characteristics Sample/
resin
1 mm
2 mm
3 mm
Mean
relevant property of an adhesive system as it determines
the clinical longevity of the bonded attachments. The com­
position of the adhesive used is one among the many factors
that affect bond strength.
Recently, flowable composites are being put to ortho­
dontic use by many clinicians. Flowable composites would
provide good surface contact between bracket base and
enamel and better adherence at molecular levels. More
flowable the composite, smaller would be the contact angle,
better the wetting and better the adhesion. However, previous
studies with other low viscosity adhesives are controversial
regarding their clinical use with some reporting acceptable
bond strengths8,9 while others do not recommend their use
for direct bonding due to significantly low SBS values.10
In the current study, the mean SBS values of Group I
(TSLV) at 12.59 ± 2.79 MPa and Group II (Enlight LV)
at 11.97 ± 2.42 MPa, were significantly higher than that
obtained by Ryou et al and Uysal et al.8,10 Their values are
well above 5.9 MPa considered adequate for clinical use
by Reynolds14 and also significantly above 7 MPa recom­
mended by Lopez15 to be the maximum bond strength for
suc­cessful clinical bonding. However, the bond strength for
Group III (Transbond XT) at 14.92 ± 2.52 MPa was signi­
ficantly higher than both the flowable composites tested
in the current study. Group I showed higher values than
Group II. But, a post-hoc Duncan’s analysis revealed no
statis­tically significant difference between the two.
A stated advantage of low viscosity resins is that there is
no necessity to use an intermediate bonding resin thus saving
chairside time. However, in the present study, following
recommendations by the manufacturers, intermediate resins
were used.
Group I
12.5
13.5
14.5
13.5
Adhesive Remnant Index
Group II
6
7.5
9
7.5
Group III
9.5
12.5
10.5
10.8
Of primary concern to the clinician is the maintenance of
a sound, unblemished enamel surface after removal of the
Comparing the ARI scores between groups, the test showed
that there was no significant difference between the ARI
scores of low viscosity adhesives (Groups I and II); however,
there was a significant difference between the ARI scores
of low viscosity adhesives and the conventional orthodontic
adhesive.
Flow Characteristics
The flowable composites in Group I produced a significantly
larger disk diameter than the other two indicating their
superior flowability (Fig. 1). The average disk diameter
dimen­sions exhibited by Group II composites were the
least of the three. This would suggest inferior flowability
as compared to the other two groups. Conventional resin
(Group III) exhibited intermediate flow characteristics
(Table 3).
DISCUSSION
The bond strength of an orthodontic adhesive should be
sufficient to withstand the forces exerted by the archwires,
mechanical impact from mastication, biochemical changes in
oral cavity as well as allow controlled tooth movement in all
three planes, so as to minimize unexpected debonding during
treatment. At the same time, on completion of treatment,
debonding should be hassle free with no damage to enamel.
Precise quantification of the various shear, tensile
and torsional forces acting on the brackets is difficult.
Traditionally, SBS has been accepted as the most clinically
Table 3: Flow characteristics of the different groups
The Journal of Indian Orthodontic Society, October-December 2014;48(4):262-266
265
SD Vasudevan, Shobha Sundareswaran
bracket, yet bracket failure at each of these two interfaces
has its own advantages and disadvantages.
In the present study, the median value for the ARI scores
of low viscosity resins (Groups I and II) was 2, while that
of the conventional orthodontic adhesive (Group I) was 1.
As reflected by the ARI scores with median value
being 2, a larger resin remnant was left on the enamel surface
with the low viscosity resins (Groups I and II) meaning that
the primary failure site for the flowable composites was
within the material or at the bracket composite interface.
This type of failure would seem to be more desirable to
minimize the enamel fractures.
Conversely, the median value for the ARI scores of the
conventional orthodontic adhesive (Group III) being 1,
indicating fracture at the enamel-adhesive interface, implies
that enamel fractures and damage tend to increase in this
score. Thus, although this adhesive can provide more stable
bonding between the bracket and composite, it may not be
optimal in terms of enamel damage. Therefore, great care
is required to avoid damaging the enamel surface during
debonding.
Flow Characteristics
Flow properties are a special feature and an important
consideration that influences penetration of adhesives into the
retentive base of brackets as well as acid etched enamel and
the ability of an adhesive to resist bracket drift during bonding.
Increased ability to infiltrate acid etched enamel and
bracket bases would obviously result in superior bond
strengths. On analyzing the flow results, it was seen that
Group I produced a significantly larger disk diameter than
the other two groups indicating their superior flowability.
This would permit better penetration into the retentive bases
of the brackets as well as acid etched enamel. The flowable
composites of Group II produced significantly inferior
flow properties. Interestingly, the conventional orthodontic
adhesive (Group III) showed flow properties that were
superior to that of the flowable composites of Group II.
SUMMARY AND CONCLUSION
The mean shear bond strength of both low viscosity adhesives
tested in the current study were adequate and acceptable for
clinical use as their values were above the clinically acceptable
range. However their values were significantly lower than
Transbond XT, the conventional orthodontic adhesive. Thus,
the null hypothesis that there would be no difference in shear
bond strength of Transbond SLV and Enlight LV as compared
to Transbond XT was rejected.
The median value of ARI scores of low viscosity resins
was 2, indicating failure within the material or at the bracket
266
adhesive interface, signifying decreased cohesiveness, but
desirable from the point of view of minimizing enamel
fractures.
Evaluation of the flow characteristics of the three ortho­
dontic adhesives indicated superior flowability for Group
I (Transbond Supreme LV). Group II (Enlight LV) though
claimed to be a low viscosity resin exhibited least flow. The
conventional resin in Group III (Transbond XT) also showed
acceptable flow.
Considering the SBS, ARI and flow characteristics,
the improved low viscosity resin Transbond Supreme LV
and Enlight LV can be conveniently used for orthodontic
bonding.
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