Diode laser irradiation increases microtensile bond strength of dentin.
One of the main challenges in adhesive dentistry is obtaining a reliable link between composite restorations and dentin,  especially with the simplified dentin bonding systems (DBSs) that are considered less technique sensitive and less time consuming, [2,3,4,5] but are usually based on complex hydrophilic monomers that determine higher permeability [6,7] and consequently more rapid degradation in the oral environment. 
For better adhesion to dentin, some authors have suggested the use of lasers for dentin conditioning, as laser irradiation can provide an apparently microretentive surface, free of a smear layer and open dentinal tubules. [9,10] However, despite this notable morphology, laser-treated dentin has been reported to provide bond strength values only equal to or lower than those of dentin treated in the traditional manner. [11,12]
Recently, a different approach was suggested to promote better adhesion to dentin via lasers. Gonyalves et al.  proposed the laser irradiation of already infiltrated dentin adhesive systems, prior to their polymerization. This technique promoted increased bond strength values of simplified DBSs irradiated with the Nd:YAG laser in microtensile  and shear bond strength tests.  According to those authors, laser irradiation could increase bond strength by determining a higher penetration of DBSs into dentin  or promoting the development of a substrate where dentin and adhesive are fused by the action of the laser. 
As an alternative, the diode laser also provides a near-infrared irradiation (wavelength = 970 nm) with parameters similar to those provided by Nd:YAG, but with more attractive usage and availability, such as lower size, weight, and cost. Because the influence of diode laser irradiation associated with simplified DBSs has not been well established, the aim of this study was to evaluate, through the microtensile bond strength test, the effect of diode laser irradiation on two simplified DBSs applied to human dentin.
The null hypothesis is that laser irradiation may not determine different bond strength values to dentin with simplified DBSs.
This was a factorial 2 x 2 in vitro study wherein the factors were two adhesive systems [(SB) Adper[TM] Single-Bond 2, 3M ESPE, St. Paul, USA; and (EO) Adper[TM] Easy-One, 3M ESPE, St. Paul, USA] with two levels of laser irradiation [CONTROL, no irradiation; (L) LASER, irradiation with a diode laser, SiroLaser, Sirona, Bensheim, Germany].
The experimental sample consisted of 40 teeth, randomized by crown size and assigned to one of the above four groups (n = 10). The quantitative response variable was bond strength to dentin in megaPascals (MPa), obtained from microtensile specimens tested for each tooth.
Forty sound human third molars (Figure 1A) indicated for extraction were obtained from the dental emergency clinic of Bauru School of Dentistry, Universidade de Sao Paulo--USP, after approval by the Ethics Committee of Research on Human Beings at the same institution (CAAE 05737812.7.0000.5417). Immediately after extraction, the teeth were cleaned with periodontal curettes and stored in a 0.1% thymol solution (ISO TR 11405), renewed weekly. They were kept immersed until the specimens were prepared.
Each selected tooth had its crown sectioned transversely on the occlusal third by means of a sectioning machine (Figure 1B) (Isomet[TM] Low Speed Saw[R], Buehler, Lake Bluff, USA) with a water-cooled diamond disc (Extec Corporation, Enfield, USA) to expose superficial dentin for adhesive procedures, after which it was subjected to wear on a polishing machine (Figure 1C) (Arotec, Cotia, Brazil) with 320-grit silicon carbide paper (Carbimet Paper Discs, Buehler) for the eventual removal of enamel remaining on the surface. The teeth were then polished for 30 s with a 600-grit SiC paper to simulate smear layer formation.
Over the flat dentin (Figure 1D), a 6 mm x 6 mm condensation-cured silicon mold (Zetalabor, Zhermack, Sao Paulo, Brazil) was positioned centrally in each specimen, and nail varnish was applied all around to delimit the testing area (Figure 1E), after which each group received the respective treatment according to the manufacturer's instructions (Table 1, Figures 1F and 1G) followed by gentle air-drying for 10 s for solvent evaporation.
In the laser groups, after application of the dentin bonding system, a diode laser (Figure 1H) was activated in contact mode, and the full testing area was scanned for 30 s with the tip perpendicular to the specimen's surface. Prior to laser application, the operator was trained to standardize the scanning speed, ensuring that the entire testing area was equally irradiated. Other parameters used for laser irradiation are listed in Table 2.
After laser irradiation, the specimens were light-cured for 20 s with light-emitting diodes (LED) (Blue Star 2, Microdont, Sao Paulo, Brazil) (Figure 1I) with 1 W/[cm.sup.2] power density positioned perpendicular to the dentin surface as closely as possible. Composite resin (Filtek[TM] Z250, 3M ESPE) (Figure 1J) was then applied with the incremental technique with three consecutive 1.5-mm thick increments, each also light-cured for 20 s. Specimens were immersed in deionized water at 37[degrees]C for 7 days.
After storage, specimens were sectioned (Figure 1K) perpendicular to the bonding surface to create multiple beam-shaped sticks with a cross-sectional surface area of approximately 1.0 [mm.sup.2] (Figure 1L).
Prior to being tested, the cross-sectional area of each specimen's adhesive interface was measured with a digital caliper (Digimatic Caliper, Mitutoyo Sul Americana, Rio de Janeiro, Brazil), and the values were entered into the BlueHill Lite software (BlueHill[R] Materials Testing Software, Norwood, USA). Specimens were then attached individually with a cyanoacrylate-based adhesive (Loctite Super Bonder Gel Control, Henkel Ltda, Sao Paulo, Brazil) to a microtensile dispositive similar to Bencor Multi T (Danville Engineering, San Ramon, USA) adapted to a universal testing machine (Figure 1M) (Instron 3342, Illinois Tool Works, Norwood, USA). The specimen's adhesive interface was positioned perpendicular to the tensile forces generated by the testing machine. Testing was performed at 0.5 mm/min speed (ISO TR 11405) with a 500 N load cell. Bond strength values (MPa) were calculated on the basis of the maximum force supported by each specimen (in N) divided by the cross-sectional area ratio of each specimen (in [mm.sup.2]).
Failure mode analysis
After being tested, both segments of the fractured specimens were analyzed with a digital portable microscope (Dino Lite Microscope 100*, AnMo Electronics Corp, New Taipei City, Taiwan) and classified by failure modes: Adhesive (A) for failures at the adhesive interface; Cohesive in Dentin (CD) for failures on the dentin substrate; Cohesive in Resin (CR) for failures in the resin composite; and Mixed (M) when none of the types predominated. The percentage of each type of failure was obtained.
Data were subjected to two-way ANOVA (p [less than or equal to] 0.05) followed by Tukey's test (p < 0.05) for individual comparisons. Statistical analysis was performed with the software Statistica 10.0 (StatSoft, Inc., Tulsa, USA).
Both factors (DBSs and laser) were significant by two-way ANOVA. For the adhesive factor, higher bond strength values were obtained when the etch-andrinse adhesive system (SB) was used. For the laser factor, both groups (SB-L and EO-L) yielded higher bond strength values (Table 3). Adhesive failures were predominant in all groups. The distribution of each type of failure mode is described in Table 4.
Results obtained in this in vitro evaluation showed an increase in bond strength in both adhesive systems when the diode laser was irradiated, rejecting the null hypothesis. Similar results have already been reported with the Nd:YAG laser irradiation of simplified DBSs, [14,15] suggesting that near-infrared laser irradiation is a promising technique for achieving a more reliable link to dentin; however, the underlying mechanism is still unknown.
Some authors have speculated that laser irradiation promotes "the development of a new substrate, in which dentin substrate and adhesive would be fused by laser action,"  thus obtaining higher bond strength values. As hydroxyapatite melting occurs only at a very high temperature (approximately 800[degrees]C),  the intra-pulp-chamber temperature is another issue that must be investigated.
Another attempt to explain the immediate increase in bond strength is the heat provided directly by laser irradiation, which could favor adhesive penetration  and solvent evaporation. It was recently found that a warm air stream can also provide immediate increases in bond strength values of simplified DBSs,  which could also explain the favorable results obtained with the laser irradiation technique.
The pronounced but local heat generation caused by laser irradiation may also determine a higher degree of conversion of the adhesives already infiltrating the dentin, especially if the diode laser wavelength is well absorbed by the adhesives. Fourier transform infrared spectrometry and near-infrared spectrometry may be notable instruments for these evaluations.
Despite the favorable results, the immediate increase in bond strength does not guarantee that the resin composite restorations with margins in dentin will last longer, especially in posterior tooth restorations, because the main cause of failures is the degradation of the hybrid layer over time.  Conversely, the immediate increase in bond strength reveals that some favorable interaction is proceeding, which must stimulate more investigations in this field.
Laser irradiation may represent one more step in dentin hybridization; however, the results provide evidence that simplified DBSs can also establish high bond strength values to dentin. Other studies should be performed to investigate the underlying mechanism by which laser irradiation can promote increased bond strength values and also the longevity of the laser-treated hybrid layer after water storage and before clinical usage.
Based on the results of this study, the association of diode laser irradiation with simplified adhesive systems already applied to dentin, but prior to polymerization, is a promising alternative for achieving higher bond strength values.
This manuscript is part of a dissertation submitted by the first author to the Bauru School of Dentistry, Universidade de Sao Paulo-USP, in partial fulfillment of the requirements for the Master's degree in Restorative Dentistry. This study was supported by Conselho National de Desenvolvimento Cientfico e Tecnologico-CNPq (Process #134356/2011-0).
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Rafael Massunari MAENOSONO (a)
Odair BIM JUNIOR (a)
Marco Antonio Hungaro DUARTE (a)
Regina Guenka PALMA-DIBB (b)
Linda WANG (a)
Sergio Kiyoshi ISHIKIRIAMA (a)
(a) Universidade de Sao Paulo--USP, Bauru School of Dentistry, Department of Operative Dentistry, Endodontics and Dental Materials, Bauru, SP, Brazil.
(b) Universidade de Sao Paulo--USP, Ribeirao Preto School of Dentistry, Department of Restorative Dentistry, Ribeirao Preto, SP, Brazil.
Declaration of Interests: The authors certify that they have no commercial or associative interest that represents a conflict of interest in connection with the manuscript.
Sergio Kiyoshi Ishikiriama
Submitted: Mar 28, 2014
Accepted for publication: Jul 15, 2014
Last revision: Oct 13, 2014
Table 1. Application technique for each dentin bonding system according to its manufacturer's instructions. Adhesive system Application technique Adper[TM] Single Bond 2 Etch dentin surface with phosphoric acid (37%) for 15 s 3M ESPE, St. Paul, MN, USA Rinse for 15 s Gently dry with absorbent paper Apply adhesive with a microbrush Gently air-dry for 10 s Adper[TM] EasyOne Apply adhesive over a clean and dry dentin surface and wait for 20 s 3M ESPE, St. Paul, MN, USA Gently air-dry for 10 s Table 2. Diode laser parameters used for irradiation of the testing areas. Parameter Value Energy per pulse 80 mJ Frequency 10 Hz Power 0.8 W Testing area 36 [mm.sup.2] Irradiation time 30 s Total energy 24 J Table 3. M icrotensile bond strength of dentin bonding sys- tems with or without irradiation using a diode laser [Mean (MPa) [+ or -] standard deviations]. Adhesive System Control Diode Laser SB 33.49 [+ or -] 6.77Aa 43.69 [+ or -] 8.15Ba EO 19.67 [+ or -] 5.86Ab 29.87 [+ or -] 6.98Bb N = 10 Uppercase letters represent differences between columns (p = 0.012). Lowercase letters represent differences between rows (p < 0.001). Table 4. Failure mode distribution (%) for each group. A CR CD M SB 94% 3% 3% 0% EO 76% 3% 0% 21% SB-L 65% 25% 5% 5% EO-L 96% 0% 0% 4% (A) Adhesive; (CR) Cohesive in Resin; (CD) Cohesive in Dentin; (M) Mixed.
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|Title Annotation:||Original Research: Operative Dentistry|
|Author:||Maenosono, Rafael Massunari; Bim, Odair, Jr.; Duarte, Marco Antonio Hungaro; Palma-Dibb, Regina Guen|
|Publication:||Brazilian Oral Research|
|Date:||Jan 1, 2015|
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