Nomura et al.

Int Chin J Dent 2010; 10: 35-40.

Effect of high-power LED light curing unit on bond strength ofcomposite resin to dentin
Tomomasa Nomura, DMD,a Ichiro Ikeda, DMD, PhD,a Masayuki Otsuki, DDS, PhD,aand Junji Tagami, DDS, PhDa,ba

Cariology and Operative Dentistry, Department of Restorative Sciences, Graduate School of Medicalband Dental Sciences, and Global Center of Excellence Program, International Research Center for
Molecular Science in Tooth and Bone Diseases, Tokyo Medical and Dental University, Tokyo, Japan
Purpose: This study evaluated the effect of a high-power solid-state light emitting diode (LED) curing unit on
the dentin bond strength of a composite resin with self-etching primer adhesives by means of microtensile bondtest.
Materials and Methods: The flat dentin surfaces of the extracted human molars were prepared. Each surface
was treated with either two self-etching adhesive systems (Clearfil SE Bond, Clearfil Tri-S Bond) and aphotocure composite was placed. The adhesive and composite were photocured with a high-power LED, aconventional LED, or a halogen light curing unit. In high-power LED group, the exposure time was reduced 3
or 5 s for the adhesive and 5 or 10 s for the composite. The specimens were trimmed to obtain the stick shapespecimens and the micro-tensile bond strengths were measured. The spectrum of the wavelength of each lightcuring unit was also measured.
Results: In Clearfil SE Bond group, the microtensile bond strengths of high-power LED light curing unit groups
were statistically lower than that of a halogen light curing unit group. In Clearfil Tri-S Bond group, there wereno statistical differences in the microtensile bond strengths between halogen and high-power LED light curing
unit groups, when the composite was photocured for 10 s. The spectrum of both high-power and conventional
LED light curing units had a single sharp peak around 465 nm. The conventional halogen light curing unit
showed broad spectra.
Conclusion: A high-power LED curing unit affected the dentin bond strength of a composite resin with
self-etching primer adhesives. (Int Chin J Dent 2010; 10: 35-40.)
Key Words: composite resin, LED light curing unit, microtensile bond test

Introduction
The photo-curable adhesive and composite resin are widely used for the tooth restorative treatment. This
technology had been based on the combination with camphorquinone (CQ) as a photo-initiator and the quartz
tungsten halogen (QTH) light with a bandpass filter as a light source for the polymerization.
Since the solid-state light emitting diode (LED) technology was introduced for polymerizing dental materials,1
there have been many researches for the LED light curing units (LCUs) concerning various properties2-17
including the bond strength.1,18-20 The output power of the LED LCU has been increased expecting the reductionof the exposure time for both adhesive and composite resin. The highest light intensity of the LED LCU iscomparable to the plasma arc LCU. In a busy practice, the exposure duration can have an impact on the total
time of a treatment procedure.17
However, it is unknown how long the high-power LED LCU can reduce the exposure time. The purpose ofthis study was to evaluate the effect of a high-intensity LED LCU on the dentin bond strength of a composite
resin with self-etching primer adhesives by means of microtensile bond test.

Materials and Methods
Specimen preparation
Forty-two caries-free extracted human molars were used for this study. This research design was subjected to
the guideline of the ethical committee, Faculty of Dentistry, Tokyo Medical and Dental University. After

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Int Chin J Dent 2010; 10: 35-40.

cleaning with a scalar, the roots were removed. Occlusal surfaces of the teeth were ground using a model
trimmer to obtain flat mid-coronal dentin surfaces perpendicular to the long axis of the tooth, and then polishedwith a 600 grit silicon carbide paper (Sankyorikagaku, Saitama, Japan). A black plastic ring of 8 mm indiameter and 2 mm in height was placed on the center of the prepared dentin surfaces and fixed using a wax.
The resinous materials and LCUs used in this study were listed in Table 1 and 2 respectively. A two-step
self-etch adhesive (Clearfil SE Bond, SE, Kuraray Medical, Tokyo, Japan), a one-step self-etching adhesive(Clearfil Tri-S Bond, TS, Kuraray Medical) and a universal photo-curable composite resin (Clearfil AP-X,
Kuraray Medical) were employed for this study. A QTH LCU (CoBee, GC, Tokyo, Japan), a conventional LED
LCU (G-light, GC) and a high-power LED LCU (Mini LED III, Satelec, Boudreaux, France) were used forcuring the adhesives and the composite resin. The original G-light contained one violet and seven blue LEDs as
light source. It was modified disconnecting a violet LED and only blue LED was used in this study.
Table 1.

Materials used in this study.

Materials, Code
Clearfil SE Bond, SE
PrimerBond
Clearfil TriS Bond, TS
Clearfil AP-X (A3 Shade)

Ingredients
MDP, HEMA, Hydrophilic dimethacrylate, dl-Camphorquinone,
N,N-Diethanol-p-toluidine, Water
MDP, Bis-GMA, HEMA, Hydrophobic dimethacrylate, dl-Camphorquinone,
N,N-Diethanol-p-toluidine, Silanated colloidal silica
MDP, Bis-GMA, HEMA, Hydrophobic dimethacrylate, dl-Camphorquinone, Ethyl
alcohol, Water, Silanated colloidal silica
Silanated barium glass, Silanated colloidal silica, Silanated silica, Bis-GMA,
TEGDMA, dl-Camphorquinone

Manufacturer, Kuraray Medical; MDP, 10-Methacryloyloxydecyl dihydrogen phosphate; HEMA, 2-Hydroxyethyl
methacrylate; Bis-GMA, Bisphenol A-glycidyl methacrylate; TEGDMA, Triethyleneglycol dimethacrylate

Table 2.

Light curing units used in this study.

Light curing units, Code (Manufacturer)
Co-Bee, CB (GC)G-Light*, GL (GC)Mini LED III, ML (Satelec)

Light sources
HalogenBlue LED
Blue LED

Light intensity
770 mW/cm2
650 mW/cm2
1,900 mW/cm2*Modified from an original model

The dentin surface in the plastic ring was treated with either SE or TS and irradiated with each LCU. Then
Clearfil AP-X was injected into the ring as a bulk and photocured with same LCU as irradiation to the adhesive.
Experimental groups, the irradiation time and energy in each group were shown in Table 3. The total energy
provided is calculated by multiplying the power density of the light by its exposure duration.
Microtensile bond test
After storage in water at 37ºC for 24 hours, each specimen was vertically crosscut and re-crosscut toapproximately 0.7 mm x 0.7 mm using a low speed diamond saw (Isomet, Buehler, Lake Bluff, IL, USA) underwater cooling to obtain the stick shape specimens. The final width and thickness of the bonded areas were
measured using a digital caliper (Mitsutoyo, Kawasaki, Japan). The specimen were attached to a testing device(Bencor-Multi-T, Danville Engineering, San Ramon, CA, USA) with a cyanoacrylate adhesive (Model Repair II
Blue, Dentsply-Sankin, Otawara, Japan) and subjected to the microtensile bond test at a crosshead speed of 1
mm/minute using a table-top material tester (EZ-Test, Shimadzu, Kyoto, Japan).

The number of theexperimental specimen was 30 in each group (n=30). After the microtensile bond strengths (µTBS) were
measured, all of the specimens were inspected visually and microscopically (x20, Dentcraft Dent-Optics DX,

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Int Chin J Dent 2010; 10: 35-40.

Yoshida, Tokyo, Japan) to determine the modes of failure.
Spectrum of wavelength of LCU
The spectrum of the wavelengths of each LCU was measured using a spectroradiometer (USR-40, Ushio,
Tokyo, Japan).
Statistical analysis
The µTBS data were analyzed by using a two-way and a one-way ANOVA followed by Tukey’s honestly
significant difference test at the 95% level of confidence.
Table 3.

Experimental groups.

Experimental groups
SE-CB-10-40
SE-GL-05-10
SE-GL-10-40
SE-ML-03-05
SE-ML-05-05
SE-ML-03-10
SE-ML-05-10
TS-CB-10-40
TS-GL-05-10
TS-GL-10-40
TS-ML-03-05
TS-ML-05-05
TS-ML-03-10
TS-ML-05-10

Adhesives

LCU

SE
SE
SE
SE
SE
SE
SE
TS
TS
TS
TS
TS
TS
TS

CB
GL
GL
ML
ML
ML
ML
CB
GL
GL
ML
ML
ML
ML

Irradiation time (s)Adhesive
Composite
10
40
5
10
10
40
3
5
5
5
3
10
5
10
10
40
5
10
10
40
3
5
5
5
3
10
5
10

Irradiation energy (mJ)
Adhesive
Composite
32.5
154.0
16.3
32.5
32.5
130.0
28.5
47.5
47.5
47.5
28.5
95.0
47.5
95.0
32.5
154.0
16.3
32.5
32.5
130.0
28.5
47.5
47.5
47.5
28.5
95.0
47.5
95.0

Results
The µTBS of each group was shown in Table 4 and the failure mode was summarized in Fig. 1. For SE
groups, µTBSs of ML groups were statistically lower than that of SE-CB-10-40 and SE-ML-05-10 showed nodifference comparing with that of SE-GL-10-40. There were no differences among SE-ML groups. For TS
groups, µTBS of TS-ML-03-05 and TS-ML-05-05 were statistically lower that of TS-CB-10-40. The µTBS of
TS-ML-03-10 and TS-ML-05-10 showed no differences comparing with that of TS-CB-10-40. The µTBS of
TS-ML-05-10 was statistically higher than those of TS-ML-03-05 and TS-ML-05-05. There were no interaction
between the adhesive systems and the irradiation condition on the µTBS. There was not specific tendency in the
failure mode.
Table 4.

Microtensile bond strength (µTBS).

Experimental groups
SE-CB-10-40
SE-GL-05-10
SE-GL-10-40
SE-ML-03-05
SE-ML-05-05
SE-ML-03-10
SE-ML-05-10µTBS (SD)Experimental groups

84.8 (11.4) a
42.6 (14.6)
77.4 (10.9) a, b
61.2 ( 9.6)
c59.2 ( 7.5)
c63.4 ( 9.1)
c69.3 (14.9)b, cTS-CB-10-40
TS-GL-05-10
TS-GL-10-40
TS-ML-03-05
TS-ML-05-05
TS-ML-03-10
TS-ML-05-10µTBS (SD)63.2 (10.4) d, e
59.2 (13.6) d, e
62.0 ( 9.8) d, f
31.5 ( 9.4)
g44.9 ( 7.5)g, h52.0 ( 9.2)
e,h57.8 ( 8.5)e, f

Identical letters showed no statistical differences (p<0.05).

The spectrum of the wavelengths of each LCU was shown in Fig. 2. CB had relatively broad spectra (390-530nm) with peak wavelength of 490nm. GL showed sharp peak wavelength of 467 nm and ML showed higher
sharp peak wavelength of 463 nm.

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Fig. 1.

Int Chin J Dent 2010; 10: 35-40.

Mode of failure

Fig. 2.

Spectrum of wavelength of LCUs

Discussion
Since “first generation LED LCU”9 was marketed, the intensity has been dramatically increased expecting the
reduction of the exposure time to polymerize the resin materials. However, under higher irradiance, the shorter
lifetime of free radicals would cause insufficient polymerization.20 This study showed that the microtensile bondstrengths of self-etching primer adhesives to the dentin were affected by the exposure time of the high-power
LED LCU.
In this study, the microtensile bond test using the stick shaped specimens was employed. The microtensile
bond test was widely used and was established as the evaluation of adhesive property.18,19,21,22 The different
specimen designs, such as stick, dumbbell or hourglass shape, had no influence on the bond strength to dentin.21
The stick shape specimen seemed to be the favorable design, as it is easy preparation, and is not subject to theeffects of specimen geometry.21
Although, all ML groups showed less µTBS than CB group among SE groups, ML-03-10 and ML-05-10
showed no significant differences compared with CB group among TS groups. It was reported that the bondstrengths of resin composites cured with LED LCUs were significantly lower than values produced with a QTH
LCU.8 The dentin bond strength of SE cured with the LED LCU had a lower value than that cured with the
QTH LCU.10 The other hand, there were no statistically significant differences between bond strengths of SE toenamel with QTH or with LED LCU.18 The microhardness of SE was also affected by light source.5 There aremany factors of the LCU affected bond strength; wavelength spectra, output power, exposure time for adhesiveand composite, distance between light tip and irradiated surface, focus of light, temperature, exposure methods.
One possible explanation might involve the narrow emission spectrum of the LED LCU.10
The high intensity LCU is expected to penetrate through composite resin and to contribute the polymerizationof the underlying adhesives. It was reported that the second photoirradiation to the adhesive through the resin
composite might increase dentin bond strength.23 However, another research suggested that the required light ofshorter wavelength could not penetrate through the overlying composite and that the bonding system should bepolymerized before it was covered by the resin composite.24 Many factors affect light penetration through acomposite.17 Although the emission spectra of LCUs varies greatly, as does the power delivered at different
wavelengths,17 it was suggested that the adhesive and restorative materials themselves might be a more criticalfactor in adhesion than the light source or curing method.1,19
Some dental resins and bonding systems employ other photoinitiators that are not as chromogenic as CQ.
These alternative photoinitiators are more sensitive to the shorter wavelengths (<420 nm) of blue light.24 Two

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adhesives used in this study contain not only CQ as a photoinitiator, but also another one (personalcommunication with the manufacturer) which reacts with low wavelength of visual light as well as
phenylpropanedione (PPD). Generally the manufacturers of the adhesive materials have not disclosed theirphotocatalyst systems in detail. The manufacturers should inform the absorption profile of the dental resins in
the product data sheet, as well as the spectral band emitted by the LCU.7 Although the LED is not inferior to the
halogen light for CQ activation, its narrow emission spectrum has a theoretical weakness for activating
co-initiator with a different absorption spectrum from CQ.10,24 Curing of materials containing co-initiators
should be more challenging for LED LCUs.4
The light exposure time and intensity affect not only bond strength but also various mechanical properties of
the adhesives and composite resins. The low intensity LED LCU required the extension of the exposure duration
for achieving the adequate mechanical property of the composite resin.2-4,6,17

The high-power LED LCU

produced an acceptable level of polymerization of composites as well as QTH LCU.14,15 The effect of lightsources on microhardness was material-dependent12 and the different composites required different exposure
times.16
The LED LCU might reduce the risk of pulp injury, because of the lesser temperature rise compared tohalogen units.11 However, higher power LED LCU would produce more heat. The intensity of LCU, the curing
time and the enthalpy of polymerization of the resin composite were the most important factors in causing
potentially dangerous temperature increases in the pulp. The composite is a good insulator and the greatest risk
occurs when using the light to cure the thin layer of bonding resin or in deep restorations.25 Such hightemperatures are detrimental to the dental pulp.13
If µTBS of TS-CB-10-40 is clinically acceptable, those of TS-ML-03-10, TS-ML-05-10 and all SE-ML
groups might be acceptable and the exposure time could be reduced using high power LED LCU, because there
were no significant differences among them. In this study, it was concluded that the high-power LED LCU
affectedthedentinbondstrengthofacomposite

resin

with

self-etchingprimeradhesives.

Manufacturer-recommended exposure duration values are not reliable indicators of optimal composite
performance.17 Care must be taken when interpreting the conclusions of any study that demonstrates thesuperior performance of LCU, because the results are only valid for the particular resin composite and shade
used in the study.24
Acknowledgment
This research was partially supported by Global Center of Excellence Program, International Research Center for Molecular Science inTooth and Bone Diseases, Tokyo Medical and Dental University.

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Correspondence to:Dr. Tomomasa Nomura
Cariology and Operative Dentistry, Department of Restorative Sciences,
Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University,
1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan.
Fax: +81-3-5803-0195 E-mail: t-nom501@wj9.so-net.ne.jp
Accepted March 26, 2010.

Copyright ©2010 by the International Chinese Journal of Dentistry.

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