Effect of diode laser irradiation on the bond strength of polymerized non-simplified adhesive systems after 12 months of water storage

Abstract Objectives: The aim of this in vitro study was to evaluate the bonding strength of non-simplified dentin bonding systems (DBS) to dentin irradiated with a diode laser (970 nm) immediately and after 12 months of water storage following either primer or bond application. Material and methods: The experimental design included three different factors: DBS type [AdperTM Scotchbond Multipurpose (MP) and Clearfil™ SE Bond (CSE)], irradiation [without irradiation - control (C), irradiation after primer application (AP), and irradiation after bond application (AB)], and time [initial (I) and after 12 months of water storage (12 m)]. Sixty sound human third molars (n = 10) were obtained, and their flat occlusal dentin areas were prepared and standardized. Laser irradiation was performed in the contact mode perpendicular to the dental surface over an automatically selected scanning area at a pulse energy of 0.8 W, frequency of 10 Hz, and energy density of 66.67 J/cm2. After 7 days of treatment, the specimens were cut, and half of them were subjected to microtensile testing (500 N/0.05 mm/min), whereas the remaining sticks were examined after 12 months of water storage. The obtained data were analyzed by three-way analysis of variance (ANOVA) followed by a Tukey test (p<0.05). The observed fracture modes were investigated using a portable digital microscope with a magnification of 40x. Results: Among the utilized DBS, MP generally exhibited higher bond strengths, but did not always differ from CSE under similar conditions. The irradiation factor was statistically significant only for the MP/AB groups. After 12 months of storage, all groups demonstrated a significant reduction in the bond strength, whereas the results of fracture analysis showed a predominance of the adhesive type. Conclusions: The laser treatment of non-simplified DBS was not able to stabilize their bonding characteristics after 12 months.


Introduction
Polymeric restorative procedures performed on dental substrates rely essentially on the ability of the currently available dentin bonding systems (DBS) to promote interaction with demineralized dentin via mechanical imbrication or chemical bonding. [1][2][3] Margin restorations on enamel typically exhibit good adhesion characterized by high longevity, which is comparable with that of amalgam restorations. 4 However, the dentin substrate forms a relatively weak bond with the resin due to its complexity and fragility, 5 causing clinical failures related to marginal infiltration and aesthetics. 6 Different strategies have been proposed to improve the long-term dentin bonding strength. [7][8][9][10][11] Some of them focused on the obtention of more hydrophobic polymer resins and/or their ability to chemically interact with dentin, while other works aimed at reinforcing collagen fibers. One of such approaches combined the laser technology with DBS which has been considered promising, since high values of their bonding strengths were observed in laboratory studies. [12][13][14] Maenosono, et al. 14 (2015) found that noticeably higher bond strengths were achieved when simplified DBS were irradiated with a diode laser (970 nm). This phenomenon was attributed to the solvent evaporation process enhanced by laser irradiation due to either the existence of laser-solvent interactions or high temperature. However, no studies on non-simplified DBS were performed to validate this assumption.
Since the use of such systems involves multiple steps including primer application, the same rationale can be applied in their case because the primers of nonsimplified DBS also contain solvents. In the majority of previous studies, DBS were irradiated by various types of lasers after adhesive application as had been initially suggested by Gonçalves, de Araujo, and Damião 13 (1999).
In the present work, it is hypothesized that the irradiation of DBS after primer application can increase their bonding strength due to the presence of solvent in the primer. Therefore, to achieve a better understanding of the mechanism of laser-solvent interactions, a diode laser was used at different stages of the bonding process. The objective of this study was to estimate the bonding strengths of non-simplified DBS to dentin irradiated with a diode laser after the primer or bond application.

Experimental design
The conducted in vitro study involved the following

Specimen preparation
After the approval of the Ethics Committee of Research on Human Beings (CAAE 310887/4.1.0000.5417), sixty sound human third molars were obtained and randomized into six groups according to the sizes of the exposed dentin areas (n=10). To expose dentin, the occlusal face was sectioned perpendicular to the long axis of the tooth using a sectioning machine (Isomet™ Low Speed Saw ® , Buehler; Lake Bluff,

Microtensile test
The specimens were tested using a universal to determine a particular type of failure among the following categories: adhesive, cohesive in dentin, cohesive in resin, and mixed.

Statistical analysis
The obtained data were analyzed statistically using the Statistica software (Statsoft ® ; Tulsa, OK, USA).
The assumptions of normal distribution and equality of variances were checked for all variables using the Kolmogorov-Smirnov and Levene tests, respectively.
After validating these assumptions, the data were subjected to three-way analysis of variance (ANOVA) and Tukey's test (p<0.05).

Scanning electronic microscopy (SEM)
Two additional specimens from each group were prepared for scanning electron microscopy (SEM) observations using the same protocol. Briefly, they were placed in stubs, sputter coated with gold, and then examined using an SEM instrument (JSM T220A, JEOL USA; Peabody, MA, USA) at a magnification of ×1,500 15 .

Bond strength
The mean values of the measured bond strengths and their standard deviations are summarized in Table   1    Therefore, in the present work, a high-power laser was used to isolate the solvent and confirm the mechanism proposed in a previous study. 14 Furthermore, the bonding of two non-simplified DBS to the dentin substrate was investigated. Because the durability of the bonding interface is an important factor that affects the quality and longevity of polymeric adhesive restorations, different laboratory strategies are utilized to simulate interface aging. In this study, it was achieved via the water storage at a temperature of 37°C for 12 months with frequent water exchange. 22 The obtained results showed no differences between the control group and the specimens treated with the diode laser before the application of the primer or   The limitations of this work are related to the standardization of the irradiated areas. In order to achieve the total irradiation of the sticks, the specimens were standardized in the dentin center using the matrix with an area of 36 mm 2 . In this region, the presence of the pulp chamber can significantly influence the material performance; as a result, the data obtained in some tests were negatively affected, leading to higher standard deviation values. To resolve this problem, future studies will be performed at an amplitude of the irradiated area.
In order to develop a proper surface treatment procedure for each clinical situation, it is important to understand the effect of different strategies on the mechanical properties of DBS. This study was mainly focused on the potential use of high-power lasers for the irradiation of non-simplified DBS containing etch-and-rinse adhesives. For such systems, stable chemical bonding can be apparently achieved without any additives. Further studies will be conducted to determine the advantages of using diode and other high-power lasers for the improvement of the longterm durability of the dentin bond interface.

Conclusion
The results of this study indicate that the application of a diode laser did not improve the bond strength of etch-and-rinse DBS after 12 months of water storage.
Effect of diode laser irradiation on the bond strength of polymerized non-simplified adhesive systems after 12 months of water storage