Bond strength

COMPARATIVE EVALUATION OF SHEAR BOND
STRENGTH OF CONVENTIONAL HALOGEN LIGHT
WITH LIGHT EMITTING DIODE CURING UNIT AND
STUDY OF DEBONDING CHARACTERISTICS -AN IN-
VITRO STUDY
www.indiandentalacademy.com

CONTENTS
 INTRODUCTION
 AIMS AND OBJECTIVES
 REVIEW OF LITERATURE
 MATERIALS AND METHODS
 STATISTICAL ANALYSIS
 RESULTS
 DISCUSSION
 SUMMARY AND CONCLUSION

INTRODUCTION
History reveals that one of the prominent milestones in the
orthodontic field is the development of firm attachment of brackets
and bands to the teeth, which enables efficient movement of the
teeth. Firm attachment was initially achieved by cementing bands
on all the teeth.
Since the introduction of the acid - etch technique by
Buonocore in 1955 and its subsequent adaptation to orthodontics
by Newman, rapid strides in the phenomenon of bonding has made
the use of bands on anterior teeth nearly obsolete.

INTRODUCTION
The bonded orthodontic brackets are more advantageous than
bands in that they have no inter-proximal contact, easier to
place and remove, separation of teeth not required, more
aesthetic, hygienic and less irritating to the gingiva. However
the frequency of bond failure during treatment has prompted
manufacturer to improve on bonding material and curing
techniques.
Advances in material science have tried to improve the quality
of bonding by refining the composition of bonding materials,
dispensing systems and mode of cure, enhancing bond strength
and handling characteristics simplifying the procedure.

INTRODUCTION
Thus two-paste systems have given way to single paste system
and recent self-etching primers which combine etching and priming
in a single step are yet other strides towards simplifying the
bonding procedure.
Since the introduction of light cure systems in the 1970’s light
curing units have become an indispensable part of the dentist’s
armamentarium. Visible light curing units have become an
important part of modern adhesive dentistry. They are used to cure
resin based composite restorative materials, resin modified GIC,
preventive pit and fissure sealants, certain bases and liners, core
build up materials and provisional restorative materials and most
important to the orthodontist to bond orthodontic bracket to the
teeth with resin.

INTRODUCTION
In orthodontics bonding with light activated system is
popular because the extended working time allows for
precise bracket placement and ease of manipulation. Once
bracket is situated at desired location rapid command-set is
accomplished through photoactivation.
Requirement of an efficient light cure system would be
increased depth of polymerization producing adequate bond
strength. Another consideration would be minimum curing
time required for achieving polymerization, as this would
save the chair side time

INTRODUCTION
Currently four different technologies are available
for curing of dental composites by light:
 Halogen lamps
 Plasma arc lamps
 Lasers
 L.E.D
Of all the techniques mentioned above, halogen
lamps are most commonly used, but they have
certain drawbacks:
 Short service life (40-100 hrs)
 High temperature
 Continuous spectrum must be narrowed by filter system

Nasser Barghi in 1994 studied and evaluated the intensity
output of 209 halogen LCU in private dental offices and reported
that 30% of the units had an output less than 200 mW/cm2,
while 300 mW/cm2 is the minimum required intensity. 10% had
cracked or blistered filters.
In 1995, Mills et al proposed solid-state light emitting diode
for the polymerization of light activated dental materials.
Research has shown that LED’s have an increased lifetime and
undergoes little degradation of output over time. Several studies
have demonstrated the potential of LED technology for light
activated dental materials. No difference in composite hardness
and depth of cure were detected between LED and halogen light
source.
INTRODUCTION

The purpose of this study was to investigate the efficiency
of commercially available LED unit as compared to the
conventional halogen light unit, to determine the curing time
required for adequate bond strength and study of debonding
characteristics.
INTRODUCTION

AIMS AND OBJECTIVES
 To evaluate shear bond strength of
stainless steel bracket cured with two
different lights, i.e. halogen and
commercially available LED curing units.
 To establish the optimum curing time while
using LED curing light.
 To assess the debonding characteristics of
each group by using modified adhesive
remnant index (ARI) and also through
scanning electron microscopy (SEM).

William J. Dunn (2002) compared the shear bond
strength of orthodontic brackets bonded to teeth with conventional
halogen based light- curing units and commercially available LED
curing units and reported no differences in bond strength of both
the lights.
Vittorio Cacciafesta et al (2002) performed an
in-vitro study to evaluate the shear bond strength of a
composite resin cured with two different light sources, a
halogen unit and on LED unit on freshly extracted teeth and
found that the LED light cure bonded more rapidly with no
significant reduction in bond strength.
REVIEW OF LITERATURE

Mills R.W. (2002) studied the cure depth and light output
characterization of light emitting diode (LED) and halogen light curing
unit (LCU) and reported LED achieved greater or equal depth of cure
when compared with the halogen LCU.
Samir E.Bishara et al (2003) studied the effect of light emitting
diode (LED) on the shear bond strength of orthodontic adhesive, which
had 8mm footprints and can simultaneously cure two orthodontic
brackets.
Robin W. Mills (1995) has proposed the blue light emitting
diodes as another method of light curing. LEDs are inexpensive, low
voltage devices, long service life, portable & lighter device, shock
resistant, no need for filter, spectrum ranges from 450-500 nm.

Mills R.W. (1999) has compared the composite depth of
cure with halogen having an irradiance of A 455 mW/cm2 and
blue light emitting diode having an irradiance of 290mW/cm2.
Reported that LED produced a significantly greater depth of
cure as compared to halogen light.

MATERIALS AND METHODS
Teeth used in the study
 120 freshly extracted human
permanent maxillary
premolars were divided into
six groups & stored in distilled
water .
 Each group consisted of 20
specimens. The teeth were
cleansed of soft tissue and the
root portions were embedded
in cold curing fast setting
acrylic with each tooth
oriented so that its labial
surface of the crown would be
parallel to the chisel during
debonding when testing for
shear bond strength.

 Maxillary premolar
brackets of 0.022 slot
Roth prescription 3M
Unitek Gemini series
made of P.H 17-4 type
stainless steel with a
80 gauge braze welded
foil mesh base were
used. The surface area
of the bracket base
was 10.61 mm2 as
described by the
manufacturer.
Brackets

 The adhesive used
for bonding was
Transbond XT
(3M/Unitek,
Monrovta, la)
Adhesive

All the teeth were cleaned with rubber cup and pumice,
etched using 37% phosphoric acid gel for 15 seconds, followed by
thorough rinsing and drying. After application of the primer on the
tooth the brackets were bonded near the center of the facial
surface of the tooth with sufficient pressure to express excess
adhesive, which was removed from the brackets base with a
scaler before polymerization. Radiometer (DEMETRON KERR
Model 100) was used to detect the irradiance of both the lights
Adhesive

Three groups were exposed to a conventional
halogen light-curing unit (light intensity 300mW/cm2).
The remaining three groups were cured with LED (light
intensity 130mW/cm2)

Group A - Stainless steel brackets were cured for 10
seconds with halogen light, 5 seconds each from
the occlusal & gingival side respectively.
A
B
C
D
E
F

Group B - Stainless steel brackets were cured for 10
seconds with light emitting diode, 5 seconds each from
A
B
C
D
E
F

Group C - Stainless steel bracket were cured for 20 seconds
with halogen light, 10 seconds each from the occlusal &
gingival side respectively
A
B
C
D
E
F

Group D - Stainless steel bracket were cured for 20
A
B
C
D
E
F

Group E - Stainless steel bracket were cured for 40 seconds
with halogen light, 20 seconds each from the occlusal &
gingival side respectively.
A
B
C
D
E
F

Group F - Stainless steel bracket were cured for 40
A
B
C
D
E
F

Testing procedure
 All the bonded test
samples were stored in
distilled water at room
temperature for 24
hours before testing.
 Shear bond
strength testing was
done using an Instron
testing machine,
which was connected
to a digital meter, and
the debonding force
was recorded
automatically in
Newtons

 The acrylic block was clamped in
the lower vice and a chisel shaped
rod was placed parallel to the
bracket in the upper vice.
 The test samples were stressed
for debonding at a crosshead speed
of 1mm/min until the bracket
debonded.
 The force required for
debonding was recorded in
Newtons and converted to MPA
using the formula.

Force in Newtons
MPA = --------------------------------
Surface area of the bracket
base
Testing procedure

ARI Index
 To assess the debonding
characteristics, modified
ARI scoring was done using
STEREOMICROSCOPE “MC
80-Zessis, Germany” at
20x and 40x magnification.

SCORE 1
 All composite remained on the tooth surface along with
distinct impression of bracket base.
SCORE 2
 More than 90% of composite remained on the tooth
surface.
SCORE 3
 More than 10% but less than 90% of composite
remained on the tooth surface.
SCORE 4
 Less than 10% of composite remained on the tooth
surface.
SCORE 5
 Indicating that no composite remained on the tooth
surface.
ARI Index

20X 40X
ARI SCORE - 1

20X 40X
ARI SCORE - 2

40X
20X
ARI SCORE – 3

20X 40X
ARI SCORE – 4

40X
20X
ARI SCORE - 5

S.E.M study
Randomly tooth surfaces were studied using a “JOEL”
scanning electronic microscopy machine and then observed in
a JSM-840A scanning microscope (JOEL-JAPAN) at an
operative voltage of 15-20 kV, scanning electron micrographs
were taken at 25x and 50x magnification to confirm the
adhesive remnant index scores.

SCORE –1

SCORE - 2

SCORE - 3

SCORE -4

SCORE - 5

STATISTICAL ANALYSIS
The following analysis was employed to statistically
evaluate the results:
 MEAN AND STANDARD DEVIATION
 ONE-WAY ANOVA.
 MULTIPLE RANGE TEST BY TUKEY – HSD
PROCEDURE
 KAPLAN MEIER SURVIVAL ANALYSIS.
 CHI – SQUARE TEST
 KRUSKAL – WALLIS ONE-WAY ANOVA TEST
All the results were analyzed using statistical software SPSSPC (statistical package for social
science version 4.0).

 The results of shear bond strength were
statistically analyzed by :
 one-way ANOVA test
 multiple range test by Tukey – HSD procedure
 Kaplan Meier survival analysis.
 The modified ARI scoring was statistically
analyzed by:
 Chi - square test
 Kruskal – Wallis one-way ANOVA test.

Mean and standard deviation were estimated from
the sample for each study group. Mean values were
compared by the student’s t – test /one-way ANOVA
appropriately. Multiple range tests by Tukey – HSD
(Honestly significantly difference) procedure was
employed to identify the significant group at 5% level.
Kaplan Meier survival analysis was done to compare the
median survival time between different groups after
correcting for multiple comparisons by Bonferroni
correction method. For this study P < 0.05 was
considered as the level of significance.

Chi – square test was used to assess the association between
ARI scores and different study groups. Mean and standard
deviation of ARI scores were estimated from the sample for
each study group. The mean values were compared by
Kruskal – Wallis one-way ANOVA.

SHEAR BOND STRENGTH
The mean and standard deviation calculated for
each group was:
 Group I (halogen-10 sec): 1.172 ±0.869 MPA.
 Group II (LED- 10 sec): 4.719 ±1.766 MPA.
 Group III (halogen- 20 sec): 7.824 ±2.617
MPA.
 Group IV (LED- 20 sec): 7.534 ± 2.682 MPA.
 Group V (halogen- 40 sec): 8.261 ± 2.390
MPA.
 Group VI (LED- 40 sec): 8.489 ± 2.373 MPA.

One - way ANOVA analysis showed a P value of < 0.001,
which indicated that mean shear bond strength in group I
was significantly lower than the mean shear bond strength
in group II, group III, group IV, group V, group VI (P <
0.05). Also the mean shear bond strength in-group II was
significantly lower than the mean shear bond strength in
group III, group IV, group V and group VI (P < 0.05). This
was further investigated using multiple range test by Tukey
HSD procedure. And it was observed to show the same
inference as one - way ANOVA analysis showed.

Kaplan Meier survival analysis in table-3 showed that median
survival time in-group I (1.538 ± 0.195) was significantly lower
than median survival time in group II (4.363 ± 0.632), group
III (7.351 ± 0.498), group IV (7.297 ±0.419), group V (7.595 ±
0.951) and group VI (8.104 ± 0.171) (P < 0.05). Also the
medial survival time in-group II was significantly lower than
the median survival time in group III, group IV, group V and
group VI (P < 0.05).

Modified ARI scoring
Mean ARI score in:
 group I (1.75 ± 1.07)
 group II (1.80 ±1.06)
 group III (1.95 ± 1.28)
 group IV (1.95 ± 1.15)
 group V (1.70 ± 0.92)
 group VI (1.75 ±1.07)
 No significant difference in mean scores of
ARI index between different study groups
(P < 0.98).

Mean, Standard deviation and test of
significance of mean values between different
study groups
GROUPS
Mean + SD in
MPa
LIGHT
CURING
UNIT
P-Value*
Significant#
groups
at 5% level
GROUP I 1.712+0.869 Halogen
VI, V, IV, III, II
Vs. I
VI, V, IV, III
Vs. IIGROUP II 4.719+1.766 LED
GROUP III 7.824+2.617 Halogen
GROUP IV 7.534+2.682 LED
GROUP V 8.261+2.390 Halogen
GROUP VI 8.489+2.373 LED
<0.0001
(Sig.)
*One-Way ANOVA was used to calculate the P-value.
#
Multiple range test by Tukey-HSD procedure was employed to identify significant groups at 5% level.

Kaplan Meier Survival Analysis
GROUPS
Median
Survival
S.E. 95% CI LIGHT CURING UNIT P-Value*
Significant #
groups at
5% level
I 1.538 0.195 1.157-1.919 Halogen (10 sec) I Vs. II, III, IV, V, VI
II Vs. III, IV, V, VI
II 4.363 0.632 3.125-5.601 LED (10 sec)
III 7.351 0.498 6.376-8.326 Halogen (20 sec)
IV 7.297 0.419 6.475-8.119 LED (20 sec)
V 7.595 0.951 5.730-9.460 Halogen (40 sec)
VI 8.104 0.171 7.769-8.439 LED (40 sec) < 0.0001
(Sig.)
* Log Rank test was used to calculate the P-value.
#
Log Rank test with Bonferroni correction method was used to identify the significant groups at 5% level.

Cross tabulation of ARI score by groups
GROUPS SCORE I SCORE II SCORE III SCORE IV SCORE V
CHI-SQUARE
VALUE
P- VALUE
GROUP I 12(60%) 3 (15%) 3 (15%) 2 (10%) 0 (0%)
GROUP II 11(55%) 4 (20%) 3 (15%) 2 (10%) 0 (0%)
GROUP III 11(55%) 3 (15%) 3 (15%) 2 (10%) 1 (5%)
GROUP
IV
10(50%) 4 (20%) 3 (15%) 3 (15%) 0 (0%)
GROUP V 11(55%) 5 (25%) 3 (15%) 1 (5%) 0 (0%)
GROUP
VI
12(60%) 3 (15%) 3 (15%) 2 (10%) 0 (0%)
d.f=20
X2
=7.16
0.966 (N.S)
d.f – degree of freedom
Chi-square test
Kruskal Walli’s One-way ANOVA
There is no significant difference between different groups.

Mean, Standard deviation and Test of significance of
mean values between different study groups
GROUPS Mean S.D. P – Value*
GROUP – I 1.75 1.07
GROUP – II 1.80 1.06
GROUP – III 1.95 1.28
GROUP – IV 1.95 1.15
GROUP – V 1.70 0.92
GROUP – VI 1.75 1.07 0.98 (N.S)
* Kruskal Walli’s One-way ANOVA was used to calculate the P-value.

0
1
2
3
4
5
6
7
8
9
SHEARBONDSTRENGTHINMPA
GROUPS
GRAPH 1. SHEAR BOND STRENGTH
1
2
3
4
5
6
Mean 1.71 4.71 7.82 7.53 8.26 8.48
1 2 3 4 5 6

Graph 2. Survival Analysis
H - Halogen, L-LED
SHEAR BOND STRENGTH (MPA)
1614121086420
CumSurvival 1.1
1.0
.9
.8
.7
.6
.5
.4
.3
.2
.1
0.0
GROUPS
L - 40 Sec
H - 40 Sec
L - 20 Sec
H - 20 Sec
L - 10 Sec
H - 10 Sec

0
2
4
6
8
10
12
SCORES
GROUPS
GRAPH 3. MODIFIED ARI INDEX
SCORE I
SCORE II
SCORE III
SCORE IV
SCORE V
SCOREI 12 11 11 10 11 12
SCOREII 3 4 3 4 5 3
SCOREIII 3 3 3 3 3 3
SCOREIV 2 2 2 3 1 2
SCOREV 0 0 1 0 0 0
H-10 LED-10 H-20 LED-20 H-40 LED-40
H- HALOGEN

GRAPH 4: SPECTRAL RANGE OF HALOGEN AND LED LIGHT

RESULTS
Hence the test result shows that shear bond
strength in group I was significantly lower than the shear
bond strength in group II, group III, group IV, group V, and
group VI. Also the shear bond strength in group II was
significantly lower than the mean shear bond strength in
group III, group IV, group V and group VI. However there
was no significant difference between any other contrasts

Results of SEM observation for debonding
characteristics show that there were no enamel cracks and
loss of enamel surface from the samples.
S.E.M. OBSERVATION
RESULTS

Statistical analysis of the scoring is illustrated in Table 4,
Table - 5 and graph-3, photographs of each scoring is
illustrated from (Fig. 8-12).
Mean score in group I (1.75 ±1.07), group II (1.90 ± 1.06),
group III (1.95 ±1.28), group IV (1.95 ± 1.15), group V
(1.70±0.92) and group VI (1.75 ±1.07). There was no
significant difference in mean scores of ARI index between any
of the groups (P =0.98).
MODIFIED ADHESIVE REMNANT INDEX
RESULTS

DISCUSSION
In Orthodontics, bond strength is influenced by many
factors such as tooth conditioning, adhesive system used,
design of the bracket base, and the mode of cure.
The most important of these factors is whether the
adhesive composite has reached a level of polymerization
that will adequately retain the bracket to the tooth when
orthodontic forces are applied. With light cure systems
being in vogue, their ability to deliver adequate light at
appropriate levels to optimize polymerization is crucial.

The first light cure unit used ultraviolet light which
had the disadvantage that one minute was required per
millimeter of thickness. Because of the safety concerns
of the long-term use of U-V light, visible light curing
(VLC) was introduced around 1980.
The visible light activated resin systems use a
diketone absorber to create free radicals that initiates
the polymerization process. Most dental photo initiator
systems use camphoroquinone as the diketone
absorber with, the absorption maximum in the blue
region of the visible light spectrum at a wavelength of
470nm.
DISCUSSION

DISCUSSION
Halogen bulbs generate light when electric energy heats a
small tungsten filament to high temperatures. But only a small
portion is given off as light since most of the energy is converted
into heat. Selective filters screen the wavelengths so that only
blue light is emitted. Photoinitiator molecules sensitive to blue
light are activated, creating free radicals that initiate the
polymerization process. The light power output of the halogen-
based lights is less than 1% of the consumed electrical power.
Despite their popularity, halogen bulbs have several
shortcomings like shorter lifetime of approximately 100hrs, need
selective filters to produce effective blue light, high heat
generation necessitating cooling time if entire dentition needs to
be cured, thus the procedure becomes time consuming

DISCUSSION
The argon lasers introduced in the late 1980’s & early
1990’s are capable of curing filled resins in 10sec & unfilled
resins in 5 sec at a wavelength of 488 nm.
More recently xenon arc light curing units have been
introduced for rapid light curing in restorative dentistry. The
plasma arc curing system has filters that narrow the spectrum
of visible light to a band centered on the 470nm wavelength
for activation of the camphoroquinone. This allows curing time
to be as short as those with the argon lasers. Even though
these curing units proved to be efficient they are not
commonly used, as they are very expensive

Mills et al in 1995 proposed solid-state light emitting diode
technology for the polymerization of light activated dental
materials.
This type of curing light was developed specially to overcome
the shortcomings of halogen VLC units. Instead of hot filaments
used in halogen bulbs, light emitting diode use junctions of
doped semiconductors to generate light. Thus heat production is
less and they have a lifetime of over 10,000 hrs & undergo little
degradation of output over this time.
They require no filters to produce blue light, are resistant to
shock & vibration and take little power to operate.
DISCUSSION

The newer gallium nitrite LED’s produce narrow spectrum of
light (400-500nm) that falls closely within the absorption range
of camphoroquinones that initiates the polymerization of the
resin monomers. The longer life span & consistent light output of
the LED is promising for dental application.
Thus, this study was undertaken to compare the shear bond
strength of orthodontic attachments cured by both halogen (Q-
LUX) & LED (APOZA) light curing units. (Fig 3 a & b) Before the
study was conducted, a radiometer (DEMETRON KERR Model
100) was used to detect the irradiance of both the lights.
DISCUSSION

Barghi et al while evaluating LCU’s of 122 private dental
offices stated that many dentists were unaware that the output
of curing light was inadequate for dental office use. Intensity of
light was inversely proportional to the age of the unit
In our study LED showed comparable performance with the
halogen light, even though the irradiance of LED was
130mW/cm2 as compared with halogen light which was
300mW/cm2.
This was similar to the results of Mills et al who compared
LED light having an irradiance of 290mW/cm2 with halogen
light having an irradiance of 455mW/cm2 & found out that
they displayed comparable results. They demonstrated that the
spectral flux of LED is concentrated over a much narrower
wavelength band than for halogen light.
DISCUSSION

The peak of the light absorption of camphoroquinone
photoinitiator is at 468nm. As the graph (No.-4) shows that LED
has a higher irradiance in the region of peak absorption for
camphoroquinone, it may account for the greater depth of cure
even though it has an irradiance of 130mW/cm2. Blue light in
different part of absorption spectrum of camphoroquinone has
different effectiveness & that light closer to the absorption peak is
more effective at curing.
William J. Dunn & Louis J.Taloumis and Vittorio Cacciafesta et
al found that there is no significant difference in the bond
strength with LED or halogen LC units and it goes in accordance
with our study.
DISCUSSION

Fujibayashi et al found an LED source producing the same
irradiance as a halogen source i.e.100 mW/cm2, producing a
significantly greater depth of cure than the halogen source.
Cristina Kurachi found that the samples cured by L6 LED
based device having an irradiance of 79mW/cm2 compared
with the halogen lamp having an irradiance of 475mW/cm2
showed reasonable hardness values considering the low
relative irradiance.
In this study, results of modified adhesive remnant index
revealed that more than 50% of the samples in all groups
showed debonding at bracket adhesive interface, this was true
even in the groups cured for 10 sec even though the bond
strength was deemed insufficient
DISCUSSION

Our study showed that LED is a viable alternative to
halogen light cure unit inspite of its reduced irradiance.
Although the LED does not cure faster than the halogen
LCUs, it is a much less expensive alternative to xenon light
and argon laser. While the cost is comparable to conventional
halogen light, they have additional advantages of low
maintenance cost, half the heat production of halogen lights
and are lighter & portable
DISCUSSION

SUMMARY AND CONCLUSION
 The result of this study showed that LEDs,
which are comparable to halogen LCUs in
terms of bond strength, may be a suitable
alternative to the latter because of the
numerous advantages it offers.
 10 seconds of curing time is not adequate for
both Halogen and LED light.
 20 seconds of curing time is adequate for both
LED and Halogen light, since increasing the
curing time to 40 seconds, showed no
significant difference.

 There was no significant difference in the
debonding characteristics with most of the
debonding taking place at the bracket-adhesive
interface.
 Now days, market is flooded with varieties
of LEDs and each one of them have tall claims
about their products. So it is a challenge for an
orthodontist to choose a better one. Further
clinical investigations are required to assess
other available LED light curing units in
orthodontic field.
SUMMARY AND CONCLUSION

Bond strength

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