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10.5005/jp-journals-10021-1147
ORIGINAL ARTICLE
Piush Kumar et al
Bracket Bond Strength with Transillumination
of a Light Activated Orthodontic Adhesive and
the Effect of Curing Time and
Tooth Thickness on it: An in vitro Study
1
Piush Kumar, 2Rabindra S Nayak, 3Kenneth Tan, 4KA Mohan, 5Azam Pasha
ABSTRACT
Objective: This study was conducted to compare the shear bond strength of brackets bonded with the conventional technique, transillumination
technique and the combination of both the techniques; the effect of curing time on the bond strength of the brackets bonded with the transillumination
method; and the amount of light passing through different thickness of teeth.
Materials and methods: The study was conducted in vitro on 175 extracted human teeth (50 incisors and 125 premolars). In the first part of the
study, the amount of light passing through different thickness of teeth was studied and, for second part, brackets were bonded with transillumination
method and conventional method followed by shear bond strength testing using an instron machine.
Results: The amount of light passing through the tooth depends on the thickness of the tooth. Only a small fraction of the original light intensity
passes through the tooth but this is adequate to achieve clinically acceptable bond strengths.
The bond strength achieved with transillumination method is comparable to the conventional light cure technique. Ten seconds of transillumination
followed by conventional curing leads to an increase in bond strength and this increase is clinically significant.
Conclusion: Transillumination technique is a viable method for bonding orthodontic attachments.
Keywords: Transillumination, Orthodontic bracket, Bond strength.
How to cite this article: Kumar P, Nayak RS, Tan K, Mohan KA, Pasha A. Bracket Bond Strength with Transillumination of a Light Activated
Orthodontic Adhesive and the Effect of Curing Time and Tooth Thickness on it: An in vitro Study. J Ind Orthod Soc 2013;47(3):148-153.
INTRODUCTION
Buonocore in 1955 demonstrated the increased adhesion of
attachments to tooth surface by conditioning enamel surface
with 85% phosphoric acid for 30 seconds.1 This finding of
Buonocore was brought into use in orthodontics by Newman
in 1965 when he used epoxy resins to bond orthodontic
attachments to teeth.2
Since then various advances in bonding systems in the form
of better bonding materials, increase in bond strength, different
types of curing systems, decrease in curing times as well as
combining the various steps of bonding have led to ease of
bonding and thus their increased popularity.
1
Reader, 2Professor and Head, 3,5Professor, 4Former Professor and Head
Department of Orthodontics, ITS Dental College, Ghaziabad, Uttar
Pradesh, India
2,4,5
Department of Orthodontics, MR Ambedkar Dental College
Bengaluru, Karnataka, India
3
Department of Orthodontics, Oxford Dental College, Bengaluru
Karnataka, India
1
Corresponding Author: Rabindra S Nayak, Professor and Head
Department of Orthodontics, MR Ambedkar Dental College, 1/36 Cline
Road, Cooke Town, Bengaluru-560005, Karnataka, India
e-mail: [email protected]
Received on: 7/2/12
Accepted after Revision: 4/9/12
148
The normal or conventional method of bonding by light
cure technique is through labial curing of the adhesive. It has
been noted that light can be passed through teeth, that is, teeth
show the property of transillumination. Transillumination
directs the light through the tooth to the composite material
placed on the other side of the tooth. This can be used when
metal covers the majority of the composite material on the
tooth, such as, in the case of metal orthodontic brackets. Thus,
transillumination from the lingual side to light cure the
composite resin has been advocated by some authors.3,4
Oesterle et al5 studied the amount of light passing through
teeth but the effect of the buccolingual tooth dimension on
the light transmittance was not dealt with. Also there is no
data to check the bond strength when both labial curing and
transillumination are used simultaneously. Besides, the
appropriate time required for curing and the effect of the
buccolingual dimension of the tooth on the bond strength has
not been dealt with adequately.
AIMS AND OBJECTIVES
The objectives of this study were as follows:
1. To compare the shear bond strength of brackets bonded
with conventional technique, transillumination technique
and a combination of the two.
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Bracket Bond Strength with Transillumination of a Light Activated Orthodontic Adhesive and the Effect of Curing Time
2. To study the effect of curing time on the bond strength of
the brackets bonded with the transillumination method.
3. To study the amount of light passing through different
thickness of teeth and to evaluate the effect of tooth
thickness on the bond strength of brackets bonded with
the transillumination technique.
MATERIALS AND METHODS
Ethical clearance for this study was obtained from the college
and there was no conflict of interest for any author. The study
was self-financed and self-funded.
The present study was divided into two parts. In the first
part of the study, the effect of tooth thickness on the amount
of light passing through the teeth was studied.
In the second part of the study, comparison of the bracket
bond strength between conventional light cure technique and
the transillumination technique was done.
This study was conducted on 175 extracted human teeth—
125 maxillary first premolars (orthodontic extractions) and
50 maxillary central incisors (from patients who had
periodontally compromised dentition and had to undergo
extraction of maxillary central incisors). The teeth were stored
in a solution of 0.1% (wt /vol) thymol to prevent dehydration
and bacterial growth.
A total of 125 upper premolar and 50 maxillary incisor
brackets were used for the study (Gemini series, 3M Unitek,
Monrovia, California). The bonding material used was
Transbond XT with Transbond XT primer (3M Unitek,
Table 1: Groups to study the amount of light passing through
different thickness of teeth
Groups
No. of teeth
Group A
25
Group B
25
Type of teeth
Central incisor which were used in
intact condition
Maxillary first premolars which
were used in intact condition
Monrovia, California). A conventional tungsten-quartz halogen
curing light (3M Unitek) was used for the study.
To study the amount of light passing through the teeth, a
Dual Channel Optometer (Model S 380, United Detector
Technology) was used with a sensitivity of 1µW and a range
of 450 mW. For the first part of the study, one group of
premolars and incisors of 25 teeth each were taken as
designated in Table 1. Following this the average thickness of
teeth in each of the two groups was calculated using a Vernier
Caliper.
Now the amount of light passing through the teeth for both
the groups was calculated. For this the curing unit and the power
meter were kept at standardized distance of 14 mm (fixed to
accommodate the buccolingual thickness of the premolars and
the bracket) and the light was made to pass through the tooth.
A clay mould was made individually for each tooth to prevent
the scattering of light. Then the readings from the power meter
were recorded. Also the original intensity of light was
calculated by placing the power meter and light guide at the
standardized position with no tooth structure in between to
facilitate comparison. The results obtained were subjected to
Students t-test to know the difference in light passing through
different thickness of teeth.
For the second part of the study five groups of maxillary
premolars with 25 teeth each and two groups of maxillary
incisors with 25 teeth each were taken. The teeth used in the
first part of the study were also included for the second part
and they were distributed randomly. The buccal surfaces of
the teeth were polished with pumice slurry, then washed with
distilled water and dried using oil free air from a three way
syringe. This was followed by bonding of brackets on to the
tooth surface as described in Table 2.
The bond strength in shear mode was recorded with Instron
universal testing machine. A load side density of 0 to 50 kg
was set in the Instron universal testing machine and the
crosshead speed was adjusted for 1 mm per minute. Load was
progressively applied till the bracket was debonded from the
tooth surface.
Table 2: Bonding protocol for various groups of incisors and premolars
Groups
Type of tooth
Curing protocol
Group 1
Maxillary central incisors
Group 2
Maxillary central incisors
Group 3
Maxillary first premolars
Group 4
Maxillary first premolars
Group 5
Maxillary first premolars
Group 6
Maxillary first premolars
Group 7
Maxillary first premolars
Brackets bonded with conventional technique, i.e. curing time of 20 seconds
mesially and 20 seconds distally.
Brackets bonded with transillumination technique from lingual fossa with a curing
time of 40 seconds.
Brackets bonded with conventional technique, i.e. curing time of 20 seconds
mesially and 20 seconds distally.
Brackets bonded with transillumination technique from occlusal surface with a
curing time of 40 seconds.
Brackets bonded with transillumination method from occlusal surface with a
curing time of 30 seconds.
Brackets bonded with transillumination method from occlusal surface with a
curing time of 50 seconds.
Brackets bonded with transillumination method from occlusal surface with a
curing time of 10 seconds followed by the conventional method.
The Journal of Indian Orthodontic Society, July-September 2013;47(3):148-153
149
Piush Kumar et al
The values obtained in Newton were converted into MPa.
For this the surface area of the bracket was calculated with a
stereomicroscope.
Modified ARI was used to check for the site of bond failure
in all the groups.
RESULTS
The results show that when no clay blocker was used and with
the light guide placed just against the sensor the power
recorded was 427 mW. When no clay blocker was used and
with the light guide in the standardized position the power
recorded was 318 mW. When clay blocker was used and with
the light guide in the standardized position the power recorded
was 346 mW. With the incisor of mean thickness of 6.1 mm
(Group A) placed in standardized position with clay blocker
the power meter reading was 2.02 ± 0.23 mW. With a premolar
of mean thickness of 13.1 mm placed in standardized position
with clay blocker the power meter reading was 1.08 ± 0.31
mW (Tables 3 and 4).
The shear bond strength was recorded for each of the 25
teeth in all the seven groups (Table 5).
The ‘p’values showed that there was no significant
difference between the shear bond strengths of various groups
except for groups 5 and 7 (Table 6).
The mean ARI values (Table 7) for incisor teeth were 3.1
for Group 1 and 1.9 for group 2. For the premolars the ARI
values were 3.3 for Group 3, 2.5 for Group 4, 2.6 for Group 5,
2.2 for Group 6 and 3.0 for Group 7.
DISCUSSION
The introduction of light cure adhesives to orthodontics was
done in the year 1979 by Tavas and Watts.3 They had used the
transillumination technique for bonding orthodontic
attachments. Since then, light curing from the same side as
that of the attachment placed has been the conventional method
of bonding attachments to the tooth surface and the
transillumination technique has been long forgotten.
Table 3: Average tooth thickness in each group
Groups
Group A
Group B
Mean tooth thickness
in mm along with
standard deviation
6.1 ± 0.40
13.1 ± 0.71
Maximum
(mm)
6.9
14.5
Minimum
(mm)
5.2
11.9
Table 5: Shear bond strength of incisor and premolar brackets
bonded with conventional technique and transillumination technique
Groups
Mean shear
bond strength (MPa)
Maximum
(MPa)
Minimum
(MPa)
Group 1
Group 2
Group 3
Group 4
Group 5
Group 6
Group 7
19.75 ± 3.29
20.66 ± 3.99
19.84 ± 3.57
19.57 ± 4.41
19.05 ± 4.07
20.08 ± 4.31
21.30 ± 3.45
25.08
30.01
26.46
27.71
26.10
27.99
29.49
11.49
12.89
12.89
11.68
10.71
12.37
16.19
One of the major drawbacks of composite materials is its
high shrinkage toward the light source. 6 When curing
composite material from the same side (conventional
technique) there is a tendency for the bulk of the material to
move away from the tooth structure (etched enamel rods)
toward the light source and thus it may cause not only a
weakening of the bond strength but also increased
microleakage at the tooth composite interface.7 Various
methods have been advocated to overcome this drawback
including the use of dual cure composites, pulsed- light
sources and ramped light 8,9 where the intensity of light
increases gradually. Although these methods can reduce this
shrinkage to some extent, yet they cannot overcome this
drawback totally.10 Using transillumination technique for
polymerization of composite material, this shrinkage may be
more of a benefit than a drawback. When light source is
directed at the tooth surface from the opposite side, the
composite tends to move toward the light source, that is, it
will move toward the tooth or into the etched enamel rods,
thereby increasing the bond strength.
Another drawback of the conventional method is that most
of the composite material is covered by the bracket. Little or
no light passes through the bracket and the central part of the
composite below the bracket may remain uncured. The curing
of the composite is dependent on the total light energy
reaching it. The total light energy in turn is dependent on the
intensity of the light source, the duration of cure and the
distance between the light guide tip and the composite
material.11 Rueggeberg et al have shown that a minimum of
400 mW/cm2 of light intensity is required for complete
polymerization of composite material. 12 The general
assumption has been that the use of transillumination technique
which involves directing the light through the opposite enamel,
the dentin, the pulp chamber, and through the adjacent dentin
Table 4: Results of light intensity measurements in different experimental conditions and both the study groups
Experimental condition
Light source placed against sensor without tooth in between
Light source at standard position
Light source at standard position with clay blocker
Light source at standard position with incisor and clay blocker
Light source at standard position with premolar and clay blocker
150
Light intensity (mW)
Maximum (mW)
Minimum (mW)
427
318
346
2.02 ± 0.23
1.08 ± 0.31
–
–
–
2.49
1.74
–
–
–
1.57
0.69
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Bracket Bond Strength with Transillumination of a Light Activated Orthodontic Adhesive and the Effect of Curing Time
Table 6: Comparison of test of significance in premolars and incisors for brackets bonded with different techniques
Groups
Group 1
Group 2
Group 3
Group 4
Group 5
Group 6
Group 7
Group 1
Group 2
Group 3
Group 4
Group 5
Group 6
Group 7
–
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
–
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
–
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
–
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
–
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
–
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
—
Table 7: Modified ARI scores of brackets bonded with conventional technique and transillumination technique
Group 1
Group 2
Group 3
Group 4
Group 5
Group 6
Group 7
Score 1
All adhesive on tooth
24%
48%
20%
44%
40%
56%
32%
Score 2
More than 90% adhesive on tooth
20%
28%
12%
4%
16%
8%
8%
Score 3
10-90% adhesive on tooth
8%
8%
24%
16%
4%
8%
8%
Score 4
Less than 10% adhesive on tooth
16%
16%
12%
28%
12%
16%
28%
Score 5
No adhesive remaining on tooth
32%
0%
32%
8%
28%
12%
24%
and enamel to reach the composite material would require
considerably more light energy and thus more curing time.13,14
This is explained on the basis that most of the light is scattered
within the pulp chamber and absorbed by the tooth surface, so
only a very small fraction of the original intensity of light is
utilized for the purpose of polymerization of the composite
material present below the bracket. Behrents et al13 suggested
the increase in curing time to 2 minutes to ensure complete
polymerization when using transillumination technique for
bonding. Cheng et al15 in their study found that even by
increasing the curing time up to three times, complete
polymerization of the composite material was not seen with
transillumination technique. They also suggested that complete
curing may not in fact be necessary for adequate bond strength.
Cacciafesta et al16 suggested the use of high powered lights
for curing by transillumination.
In the first part of the current study the buccolingual
thickness of maxillary central incisors and maxillary first
premolars was calculated and the amount of light passing
through various thicknesses of teeth was calculated.
According to the Beer-Lambert Law the intensity of light
(or any other form of electromagnetic radiation) passing
through a sample diminishes exponentially with the
concentration and the thickness of the sample. In case of our
study this would depend only on the thickness of the sample,
the rest of the parameters being constant.
The results suggested that approximately 90% of the light
intensity was lost when transillumination technique is used
for bonding. Further it also shows that as the buccolingual
dimension of the tooth increases there is a corresponding
decrease in the amount of light that passes through the tooth.
In spite of only a small amount of light reaching the composite
material it was found that the bond strength of brackets cured
by conventional method and transillumination technique were
almost the same.
The shear bond strength values obtained by transillumination method in this study were higher than what would be
expected from the light transmittance data. This suggests that
even with the less than the recommended light energy, achieved
polymerization was sufficient to give adequate bond strength
values. This can be explained due to two phenomena. In the
conventional technique due to the presence of bracket and due
to the convexity of the teeth almost none of the composite
material gets cured directly by the light source. In order that
the light reaches the composite adhesive, the light must be
refracted within the enamel, the dentin and the pulp chamber
back to the composite beneath the metal bracket base.
Additionally the physical presence of the bracket prevents the
light guide tip from being placed directly on the tooth surface,
further decreasing the total light energy as a function of
distance. This is similar to the light interference and the
increased distance that occurs in transillumination, where light
must be transmitted and refracted through the increased
thickness of enamel, dentin and pulpal tissues. This means that
probably the degree of polymerization of the composite
material is almost the same whether we use conventional
technique or transillumination technique.
The second phenomenon that may explain this finding is
that with the transillumination technique, the composite
material moves toward the tooth tissue into the enamel rods
rather than moving away from the rods as in case of bonding
with conventional light cure technique. This in turn may lead
to the formation of deeper resin tags with a better retention
on to the enamel surface and thereby increase the bond
The Journal of Indian Orthodontic Society, July-September 2013;47(3):148-153
151
Piush Kumar et al
strength. In a study by King et al, no clear relationship was
found between the tooth thickness and the bond strength.17
The present study found that an increase in tooth thickness
is related to a decrease in the intensity of light passing through
it. In case of incisor teeth which were cured by 40 seconds of
transillumination the bond strength was found to be slightly
higher than 40 seconds of conventional cure whereas in
premolars which have a greater buccolingual diameter the bond
strength of teeth cured by transillumination was slightly less
than that cured with 40 seconds of conventional light cure
method. This finding suggests that on increasing the tooth
thickness there is a corresponding decrease in the shear bond
strength.
Brackets cured by 10 seconds of transillumination
followed by 40 seconds of conventional curing the bond
strength was higher than that of any other group. This can again
be explained due to the fact that with initial lingual curing the
resin tags would have moved into the enamel surface and then
a better cure might have been established with the conventional
curing method. The other finding was that in this group the
standard deviation was minimum suggesting more uniform
bond strength with this method. One of the methods to evaluate
whether there is any increase in the resin tag formation by the
transillumination method as compared to the conventional
light cure technique is by evaluating the site of failure
following the debonding procedure. Hence, the modified ARI
index was used to evaluate the site of failure for all the groups.
The results indicated that in brackets bonded with the
transillumination technique the mean ARI index scores were
higher than those bonded with the conventional light cure
technique. This also suggests that there may be a lower
microleakage at bracket composite interface by transillumination technique.18
One of the problems of using transillumination technique
for bonding orthodontic attachments is the increased
transmission of heat as well as light energy to the tooth and
the dental pulp tissues. The exothermic reaction of the
polymerizing composite also generates additional heat.
Increases in pulpal temperature above 42.5° are reported to
produce irreversible damage to the pulpal tissues. Hannig and
Bott19 in a study on Class II restorative preparation with l mm
of dentin between pulp and composite resin found the
temperature increase in pulpal chamber up to 6°C with
40-second exposure with conventional tungsten-quartz curing
light. With a xenon plasma arc curing unit they found the
increase in temperature to be up to 7.8°C with 10 seconds of
exposure. During orthodontic attachment bonding procedure
onto intact tooth structure the healthy enamel and dentin
should provide much greater insulation than in the above study
where cavity preparation was done and transillumination
technique was used. However, it is advisable not to use
transillumination for extended periods of time especially when
using transillumination technique with high intensity xenon
plasma arc curing lights. Another study by Gritsch et al reported
152
no difference in the buccal cell morphology when the mucosa
was exposed to blue light. They did find an increase in
mitochondrial activity of these exposed cells.20
The bond strength of brackets bonded with 10 seconds of
transillumination along with conventional technique was much
higher than in any other group. This suggests its use in cases
of repeated bracket failure as well as in cases when bonding
posterior teeth including the second molars where the
accessibility for curing with conventional technique is low. It
may also be a viable method when using bonding materials
such as moisture insensitive primers in contaminated
conditions where the bond strength is generally lower.
Further research needs to be done to check the
microleakage and to establish transillumination technique for
bonding orthodontic attachments as a routine in clinical
practice.
SUMMARY AND CONCLUSION
Transillumination technique is a viable method for bonding
orthodontic attachments. The bond strength achieved with this
method is comparable to the conventional light cure technique.
Ten seconds of transillumination followed by conventional
curing leads to an increase in bond strength, which may be
clinically significant. The amount of light passing through the
tooth depends on the thickness of the tooth. Only a small
fraction of the original light intensity passes through the tooth
but this is adequate to achieve clinically acceptable bond
strength.
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