1. Influence of the LED curing source and
selective enamel etching on dentin bond
strength of self-etch adhesives in class I
composite restorations
Eduardo José Souza-Junior, Cíntia
Tereza Pimenta Araújo, Lúcia Trazzi
Prieto & Luís Alexandre Maffei Sartini
Paulillo
Lasers in Medical Science
ISSN 0268-8921
Lasers Med Sci
DOI 10.1007/s10103-011-1030-y
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Lasers Med Sci
DOI 10.1007/s10103-011-1030-y
ORIGINAL ARTICLE
Influence of the LED curing source and selective enamel
etching on dentin bond strength of self-etch adhesives
in class I composite restorations
Eduardo José Souza-Junior &
Cíntia Tereza Pimenta Araújo & Lúcia Trazzi Prieto &
Luís Alexandre Maffei Sartini Paulillo
Received: 18 July 2011 / Accepted: 7 November 2011
# Springer-Verlag London Ltd 2011
Abstract The aim of this study was to evaluate the compared to the non-etched specimens (26.7±2.2). UL
influence of the LED curing unit and selective enamel yielded higher μTBS values (24.1±3.2) in comparison to
etching on dentin microtensile bond strength (μTBS) for the photoactivation approach with FL (18.8 ±3.9) and RD
self-etch adhesives in class I composite restorations. On 96 (19.9 ±1.8) for CSE. The two-step CSE was not influenced
human molars, box-shaped class I cavities were made by the enamel etching (p≥0.05). Enamel acid etching in
maintaining enamel margins. Self-etch adhesives (Clearfil class I composite restorations affects the dentin μTBS of
SE – CSE and Clearfil S3 – S3) were used to bond a the one-step self-etch adhesive Clearfil S3, with no
microhybrid composite. Before adhesive application, half alterations for Clearfil SE bond strength. The polywave
of the teeth were enamel acid-etched and the other half was LED promoted better bond strength for the two-step
not. Adhesives and composites were cured with the adhesive compared to the single-peak ones.
following light curing units (LCUs): one polywave (Ultra-
Lume 5 - UL) and two single-peak (FlashLite 1401 - FL Keywords Acid etching . Enamel . Self-etch adhesives .
and Radii Cal - RD) LEDs. The specimens were then Light curing units . Bond strength
submitted to thermomechanical aging and longitudinally
sectioned to obtain bonded sticks (0.9 mm2) to be tested in
tension at 0.5 mm/min. The failure mode was then Introduction
recorded. The μTBS data were submitted to a three-way
ANOVA and Tukey’s (α=0.05). For S3, the selective Resin composites have been widely used in direct aesthetic
enamel-etching provided lower μTBS values (20.7±2.7) restorative procedures. Self-etching bond systems were
introduced in order to reduce the sensitivity of techniques
that affect the bonding ability of adhesive systems; they
E. J. Souza-Junior also decrease the number of steps in the bonding approach
Department of Restorative Dentistry, Dental Materials Area, because they include acidic monomers that simultaneously
Piracicaba Dental School, University of Campinas (UNICAMP),
etch and prime both enamel and dentin [1, 2]. The absence
Areião, SP, Brazil
of the rinsing approach and partial removal of the smear
C. T. P. Araújo : L. T. Prieto layer and smear plugs with these adhesives lead to less
Department of Restorative Dentistry, Operative Dentistry Area, technique-sensitive, time-consuming procedures, and con-
Piracicaba Dental School, University of Campinas (UNICAMP),
sequently, a possible reduction of postoperative sensitivity
Areião, SP, Brazil
results [2–5].
E. J. Souza-Junior (*) : L. A. M. S. Paulillo Moreover, a recent study has shown that enamel smear
Department of Restorative Dentistry, Piracicaba Dental School, layer compromises bonding by mild self-etch adhesives [6].
University of Campinas (UNICAMP),
Separate preliminary etching is known to remove the smear
Avenida Limeira, 901,
Areião 13414-903 52 SP, Brazil layer created by instrumentation and to demineralize the
e-mail: edujcsj@gmail.com dental substrate. The prior enamel phosphoric acid etching

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Lasers Med Sci
as an additional step for self-etch adhesives has been units on dentin microtensile bond strength (μTBS) of self-
proposed to improve the durability of the enamel bond [7– etching bond systems in class I composite restorations. The
10]. Hence, the selective enamel etching increases marginal first hypothesis tested was that the previous enamel etching
adaptation of self-etch adhesives as evidenced by numerous would not affect microtensile bond strength values of the
studies [4, 5, 11–14]. tested adhesive systems. The second hypothesis was that
While it is clear that selective enamel etching may have the third-generation polywave LED would present higher
some beneficial effects on marginal integrity in class V dentin bond strength values for the tested bond systems.
cervical lesions [5, 11, 13], the literature provides no
evidence for the bonding performance of self-etching
adhesives in class I cavities for direct composite restora- Materials and methods
tions in vitro. Since class V cervical lesions restorations
present smaller amounts of dental enamel, it may not be the LED curing units
best model for understanding the real effects of enamel
etching on dentin substrate and the stress development on Three light-emitting diodes (LEDs) were tested: FlashLite
the tooth/adhesive interface, like class I restorations [13, 1401 (FL) (Discus Dental, Culver City, CA, USA), Radii-
15]. In this sense, the literature gives no evidence about the Cal (RD) (SDI Limited, Victoria, Australia) and UltraLume
role of prior enamel acid etching on the bond strength of LED 5 (UL) (Ultradent, South Jordan, Utah, USA).
self-etch adhesives to dentin in class I cavities in vitro. Characteristics of the LCUs used in this work are presented
It is important to consider when restoring class I cavities in Table 1. Irradiance (mW/cm2) was measured dividing the
with self-etch bond systems is the selection of an efficient output power (measured by a calibrated power meter -
light curing unit (LCU). The LCU should guarantee a Ophir Optronics, Har – Hotzvim, Jerusalem, Israel) by the
satisfactory monomer conversion of the adhesive and resin tip end area of the LCUs. The spectra of the LED units was
composite. Consequently, it may improve the material’s measured with a calibrated spectrometer (USB2000, Ocean
physical and mechanical properties, such as ultimate tensile Optics, Dunedin, FL, USA), as shown in Fig. 1. The energy
strength and bond strength to dental substrate [16–20]. dose was standardized to approximately 11 J/cm2 for the
Light emitting diodes (LEDs) have been shown to bonding systems curing approach and 22 J/cm2 for the cure
effectively cure composites and dental adhesives, emitting of each composite increment. Because the RD light source
unfiltered blues light [17, 21]. These LCUs generate a mandatorily operates in ramp mode for 5s, these initial
narrow spectral range that targets the absorption wave- seconds were discarded and further application was realized
length of camphorquinone, yielding low amounts of wasted in order to maintain the standardized irradiance, conse-
energy and minimum heat generation [22, 23]. quently equaling the energy density obtained for each of the
Previous studies have shown that current LED curing LCUs.
units can replace, in most situations, the conventional QTH
light units [24–26]. Some adhesive systems are not well Specimen preparation
cured with single-peak LEDs [21] due to alterative photo-
initiators content; to overcome this problem, polywave Ninety-six freshly extracted, sound human molars were
third-generation LEDs were introduced to the market [24– selected. This study had the approval of the Ethical
26]. These LEDs emit light wavelengths targeting the Committee of the State University of Campinas, Piracicaba
absorption peak of camphorquinone. However, they deliver Dental School. The teeth were cleaned, their roots inserted
additional light output at the UV-Vis region of the in polystyrene resin and the occlusal surfaces wet polished
electromagnetic spectrum (400–415 nm). The second peak with 320-grit SiC paper under running water (Politriz,
in the UV-VIS region may promote a satisfactory cure of AROTEC – São Paulo, SP) in order to expose a flat enamel
these adhesives, containing alternative photoinitiators sys- surface without exposing dentin. Box-shaped class I
tems, such as phenyl propanedione (PPD), bis-alquyl cavities were prepared using #56L diamond burs (KG
phosphinic oxide (BAPO) and trimethylbenzoyl-diphenyl- Sorensen, Barueri, SP, Brazil) at high-speed, under air–
phosphine oxide (TPO) [26]. This point is crucial because water cooling. A custom-made preparation device allowed
some manufacturers do not state all of the photoinitiator the standardization of the cavity dimensions to 5-mm
systems used in their proprietary products, [25, 27] and the mesio-distal length, 5-mm bucco-lingual width, and 3-mm
selection of an adequate LCU is primordial to yielding an depth, all cavity margins on enamel substrate. The used bur
adequate adhesive polymerization and resin/dentin bond was replaced after four-cavity preparation.
strength. After cavity preparation, the teeth were assigned to 12
Thus, this study aimed to investigate the influence of groups (n=8) according to the three studied factors:
selective enamel acid etching and different LED curing selective enamel etching, bond system and curing light (2

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Table 1 Characteristics of the LCUs used in this study
LCU Manufacturer Type Tip diameter Irradiance Radiant exposure
(mm) (mW/cm2) (J/cm2)
FlashLite 1401 Discus Dental, Culver City, CA, USA Single-peak 7 1077 22
Radii Cal SDI Limited, Victoria, Australia Single-peak 7 731 22
UltraLume LED 5 Ultradent products Inc, South Jourdan, Utah, USA Polywave 11 x 7 800 22
conditioning protocols x 2 adhesives x 3 LEDs). Two self- interface. This sectioning was done with a diamond saw
etch adhesive systems (two-step and one-step) were used (Isomet 1000, Buehler Ltd, Lake Bluff, IL, USA) at 300
for the bonding procedure: Clearfil Tri-S Bond (pH=2.7; rpm to obtain sticks with a cross-sectional area of
Kuraray Medical Inc., Tokyo Japan) or Clearfil SE Bond approximately 0.9 mm2. The cross-sectional area of each
(pH = 2.0; Kuraray Medical Inc., Tokyo Japan). The stick thus was measured with the digital caliper to the
composition, application mode, and batch number of the nearest 0.01 mm and recorded for the bond strength
adhesive systems are presented in Table 2. All groups were calculation. The bonded sticks originated from the same
restored with B1-shade Charisma microhybrid composite teeth were assigned to the tested groups. Sticks were
(Heraeus Kulzer, Hanau, Germany) using an incremental individually attached to a jig for microtensile testing with
oblique technique with increments approximately 2 mm cyanoacrylate resin (Super Bond gel, Loctite, Henkel,
thick. Then, the finishing and polishing procedures were Brazil) and subjected to a tensile force in a universal
performed with medium-, fine-, and extra fine-grit alumi- testing machine (EMIC DL500, São José dos Pinhais,
num oxide disks (Sof-Lex – 3M/ESPE), respectively. After Brazil) at a crosshead speed of 0.5 mm/min until failure.
polishing, the specimens were submitted to 200,000 Bond failure modes were evaluated with a scanning
mechanical loading (2 Hz) and 500 thermal cycles (ranging electron microscope (JEOL, JSM-5600LV, scanning elec-
from 5 to 55°C with a dwell time of 60 s in each bath with tron microscope, Japan) and classified as: (1) cohesive
an interval of 5 s) at a thermo-mechanical device ER-11000 (failure exclusive within dentin or resin composite, (2)
(ERIOS, São Paulo, Brazil) in order to simulate similar adhesive (failure at resin/dentin interface or cohesive in
ageing of the composite restorations in oral environment adhesive resin) or (3) mixed (mixed with cohesive failure of
conditions. the neighboring substrates). The obtained data were
subjected to a three-way ANOVA and Tukey’s test at a
Microtensile bond strength pre-set alpha of 0.05.
After storage in distilled water at 37ºC for 24 h, specimens
assigned to the MTBS test were longitudinally sectioned in Results
both “x” and “y” directions across the pulpal wall bonded
Microtensile bond strength
Approximately 16 sticks could be obtained per tooth,
including those with premature debonding. Pre-test failures
were included in the statistical analysis as half of the
minimum bond strength measured for each tooth [27]. The
results of μTBS are shown in Tables 3 and 4. The double
interactions between the factors adhesive x selective
etching (p≤0.05) and adhesive x LED light (p≤0.05) were
statistically significant.
Phosphoric acid-etching pre-treatment of enamel prior to
application of S3 led to lower dentin μTBS values (p≤
0.05). However, for CSE, the pre-etched enamel yielded a
similar dentin μTBS mean compared to the non-etched
specimens. Dentin bond strength of CSE did not statisti-
cally differ from S3 when prior etching was used before the
adhesive application. Moreover, when these adhesives were
Fig. 1 The spectrum range distribution of each LCU used in this study applied following the conventional approach, S3 presented

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Table 2 Composition, application mode, and manufacturer’s information for the adhesive systems tested
Adhesive systems Composition Application mode
Clearfil SE Bond Primer (Batch #00896A): water, MDP, HEMA, Selective H3PO4 conditioning for 15 s. Rinse with water
(Kuraray Medical camphorquinone, hydrophilic dimethacrylate. spray for 10 s, leaving the tooth dry. Apply primer for
Inc., Tokyo, Japan) Adhesive (Batch #01320A): MDP, bis-GMA, HEMA, 20 s. Mild air stream. Apply bond. Gentle air stream.
camphorquinone, hydrophobic dimethacrylate, N,N- Light cure at an energy density of 11 J
diethanol p-toluidine bond, colloidal silica
Clearfil S3 Bond Adhesive (Batch #00116A): MDP, Bis-GMA, HEMA Selective H3PO4 conditioning for 15 s. Rinse with water
(Kuraray Medical hydrophobic dimethacrylate, dl-camphorquinone, spray for 10 s, leaving the tooth dry. Apply the bond
Inc., Tokyo, Japan) silanated colloidal silica, ethyl alcohol, and water system for 20 s. Gentle air stream for 10 s. Light cure
at a energy density of 11 J.
higher bond strength mean in comparison to the two-step step on the bond strength approach to pulpal dentin in
self-etch adhesive. class I composite restoration has not been clarified. In
All LED curing units promoted similar dentin μTBS for this sense, the selective etching increases the free surface
the one-step self-etch bond system S3 (p≥0.05). In contrast, energy of the enamel substrate and favors mechanical
the two-step CSE showed higher dentin μTBS (24.1±3.2) interlocking with the bonding resin [28]. In this study, the
when UL was used in comparison to the single-peak LEDs bonding ability of self-etch adhesives to pulpal dentin with
FL (18.8±3.9) and RD (19.9±1.8). different application approaches was evaluated in class I
The descriptive analysis of the failure mode for each restorations. These class I preparations frequently present a
adhesive is shown in Table 5 and 6. SEM evaluation of the thick enamel layer on the cavosurface angle surrounding all
fractured surfaces showed predominant adhesive failures the restoration margins, compared to a class V or a class III
for CSE when the enamel prior acid etching was used for cavity. In contrast, there are many studies evaluating the
the bonding approach (Fig. 2). A similar failure mode was bonding performance of self-etch adhesives and prior acid
found for S3 in the same condition. However, when the etching on flat enamel surfaces [29–31]. These circum-
self-etch adhesives were conventionally used, mixed fail- stances do not simulate clinical situations for direct compos-
ures were present (Fig. 3), and one cohesive failure was ite application because the bond performance of dental
shown on composite for CSE when photocured with FL. adhesives occurs with the shrinkage stress influenced by the
Figure 4 shows osmosis-induced droplets due to the cavity configuration and the restoration is surrounded by
adhesive hydrophilicity for Clearfil S3. different dental substrates [32, 33].
Based on the results, the first hypothesis tested was
partially validated, since for CSE, the previous acid etching
Discussion did not influence the μTBS results. In contrast, S3 adhesive
showed lower dentin μTBS when this additional etching
The selective enamel etching is related to increased step was performed. The conventional application of the
bond performance of self-etch adhesives to ground self-etch adhesive generally ensure good retention perfor-
enamel [8, 9]. However, the influence of this additional mance for dentin and a poor bonding approach for enamel
Table 3 Means and standard deviation of the microtensile bond Table 4 Means and standard deviation of the microtensile bond
strength for the interaction between adhesive system and selective strength for the interaction between adhesive system and light curing
etching unit
Adhesive system Selective acid etching Adhesive system Light curing unit
Etching No etching FlashLite 1401 Radii Cal UltraLume 5
Clearfil SE bond 20.3 (3.3) Bb 21.6 (3.2) Bb Clearfil SE bond 18.8 (3.9) Bb 19.9 (1.8) Bb 24.1 (3.2) Aa
Clearfil S3 20.7 (2.7) Bb 26.7 (2.2) Aa Clearfil S3 24.1 (2.8) Aa 23.2 (3.4) Aa 23.5 (3.8) Aa
Mean values followed by different small letters in the column and Mean values followed by different small letters in the column and
capital letters in the row differ statistically among themselves for the capital letters in the row differ statistically among themselves for the
Tukey test at the level of 5% Tukey test at the level of 5%

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Table 5 Percentage of specimens (%) from Clearfil SE bond adhesive distributed according to the failure mode
LCU Prior phosphoric acid etching Without prior phosphoric acid etching
A M C A M C
FlashLite 1401 75 25 0 64 24 1
Radii Cal 100 0 0 100 0 0
UltraLume LED 5 60 40 0 70 30 0
A adhesive failure and cohesive failure within the adhesive; M mixed failure; C cohesive failure within dentin or composite resin
[5, 28, 31]. Thus, the slightly inferior enamel bonding therefore an important bonding behavior since gap forma-
performance may yield minor stress at the margins, tion represents the first sign of restoration failure, which
resulting in fewer gaps and paramarginal enamel fractures can be clinically evidenced by marginal staining [5, 8, 11,
[15]. However, when enamel is pre-etched before the self- 34, 35]. However, the S3 can promote a small decrease in
etch adhesive application, higher bond strength between the bonding ability of this single-step adhesive to the
enamel/adhesive and resulting stress may occur and retention on the pulpal dentin. Therefore, for a gold-
consequently affect dentin bond performance. The enamel standard CSE, it is assumed that prior acid etching can
selective etching for the S3 application may have caused promote better marginal sealing without compromising the
higher stress and bond strength for the enamel substrate, dentin μTBS.
especially due to its ultra-mild pH characteristic, while the The impact of the adhesive bonds to the dental substrate
adhesive layer bonded to dentin might have disrupted and of composite restorations is strongly related to LCU
decreased the dentin μTBS. efficacy, which will promote satisfactory materials poly-
In addition, the stress might have been enhanced due to merization [21]. LED technology has been used for
the thermal and mechanical ageing promoted to the class I resinous materials polymerization as a substitute for
restorations, favoring the decrease in the retention values. conventional QTH curing lights [36, 37]. However, the
The failure mode was mainly adhesive when the previous conventional single-peak second-generation LEDs still
acid etching was performed for S3; therefore, mixed harm the cure of dental resins due to the narrower spectra
failures were more present when this adhesive was used emitted to the camphorquinone initiator [38–40]. They do
in the conventional way. For the CSE, the application of the not allow an optimal cure for other photosensitizes, like
hydrophobic resin layer may have supported the stress phenyl propanedione and TPO, present in some dental
promoted by enamel selective etching, but did not affect the materials [26]. Polywave LEDs were introduced in order to
dentin μTBS when compared to the unique layer applied overcome these concerns, promoting the conversion of this
for the S3. It is suggested that previous enamel etching UV–Vis photoinitiator content [24–26].
needs to be considered for S3 in restorations whose In the present work, the second hypothesis was partially
retention primarily depends on a strong bond to the enamel validated, since for CSE, the polywave third-generation
surface, such as large class III and IV or veneer restorations. LED UL promoted higher dentin μTBS values compared to
In recent studies [5, 11, 14, 31], selective enamel etching the single-peak second-generation ones. However, S3
was shown to improve marginal integrity of composite presented similar dentin bond strength regardless of the
restorations using either a two-step or a one-step self- LCU used. For CSE, the results are in accordance with
etching adhesive. The decrease in marginal gaps is some authors [21, 41] who observed that single-peak LEDs
Table 6 Percentage of specimens (%) from Clearfil S3 adhesive distributed according to the failure mode
LCU Prior phosphoric acid etching Without prior phosphoric acid etching
A M C A M C
FlashLite 1401 80 20 0 50 50 0
Radii Cal 70 30 0 66 34 0
UltraLume LED 5 85 15 0 60 40 0
A adhesive failure and cohesive failure within the adhesive; M mixed failure; C cohesive failure within dentin or composite resin

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Fig. 4 SEM observation of Clearfil S3 - osmosis induced droplets due
to the adhesive hydrophilicity, especially because of the presence of
Fig. 2 SEM of the fractured specimen showing a pure adhesive HEMA. It is not a phase separation droplets, because this adhesive do
failure within the bonding interface not present phase separation – Clearfil S3
promoted a lower degree of cure for CSE compared to a
LCU with a large spectrum range distribution, comprising curing light may have excited the material’s photoinitiator
the UV–Vis wavelength. This finding may be attributed to content. Nevertheless, the energy dose for all LEDs was
the photoinitiator content of CSE; the manufacturer may standardized at approximately 11 J/cm2 for the bonding
omit the presence of other photosensitizers that may have system, which may have delivered the same quantity of
larger spectrum ranges that comprise the UV–Vis region. photons to allow the satisfactory cure of S3. Consequently,
Another possible explanation is that the absorption peak of similar dentin bond strength values were achieved, regard-
camphorquinone can be moved to lower wavelengths. The less of the spectrum range and irradiance of the LCUs.
LED unit continues to deliver shining light because the Although the etching approach for self-etch adhesives is
structural part of this initiator, responsible for the short- recommended by many authors [4, 5, 11–14], the inclusion
wavelength absorption, remains unchanged while the of an additional step for these bond systems may be
photoreaction at the initial maximum absorption peak controversial, since these adhesives intend to represent a
(468 nm) ends [42]. It may contribute to better bond clear simplification of the clinical application of bond
strength values with the polywave LED UL, which have systems. In this sense, the adverse points of this selective
broader emission spectra, and it may have promoted a approach include the increase of the bonding steps and the
better monomer conversion compared to the single-peak technique sensitivity [13].
ones. However, it is questionable whether 10–20 s of light Although some in vitro studies exist, final conclusions
activation is sufficient to enable this effect. regarding the role of the enamel selective acid etching for
The dentin bond strength of S3 was not affected by the self-etch adhesives in class I composite restorations will
LCU used. A thin adhesive layer was applied onto the depend on the outcomes of clinical trials. Clinical long-term
enamel and dentin surface of the class I preparation, and the studies and investigations of this approach’s retention
ability for bond systems in the oral environment can best
evaluate the quality and durability of these restorations.
Conclusions
The additional selective enamel acid etching in class I
composite restorations does affect the dentin bond strength
of the ultra-mild one-step self-etch adhesive Clearfil S3.
However, this selective step does not affect the bond
performance of the two-step self-etch bond systems Clearfil
SE. The polywave LED promoted better bond strength for
the two-step adhesive when compared to the single-peak
Fig. 3 SEM of the fractured specimen showing mixed failure within ones. Moreover, all LCUs presented similar cure potential
cohesive in adhesive layer and hybrid layer adhesive failure for the one-step self-etch adhesive.

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