Adhesive systems have been widely used in dentistry, especially because they are used for a large number of direct and indirect restorations. The clinical success of restorations depends on the quality and durability of the bonding interface (restoration tooth structure) and requires knowledge of the dental substrate to which the adhesive system is applied.1 Bonding strength is influenced by the time, the dental substrate treated, and the type of adhesive system utilized.2,3
Advancements in the understanding of adhesive systems have prompted the development of a well-known simplified etch-and-rinse system (conventional), comprising two clinical steps: Total etching and washing/drying, followed by the application of a primer/bond. A previous study reported that the development of the simplified adhesive system is attractive for clinical use because it seems to reduce the number of clinical steps.4
To reduce the number of clinical steps, more versatile adhesive systems have been developed including the etch-and-rinse system (two steps) and the self-etching system (one or two steps). These novel adhesive systems can be classified as “multipurpose”, “multimode”, or “universal”.5 The applicability and effectiveness of these materials on dental substrates have been recently assessed, whereas the universal systems appear to be material dependent.5-7
Previous studies evaluated the effect of enamel pretreatment before the application of self-etching adhesive systems in order to determine optimal adhesive conditions.8,9 Enamel pretreatment with phosphoric acid increases the bonding strength and results in the formation of longer resin tags and greater adhesive penetration depth in the intact enamel.6,8 However, the adhesive penetration depth does not necessarily lead to an increase in bonding strength.10
At present, lasers are utilized in dentistry for tooth cavity preparations, cavity tissue removal, oral cavity decontamination, and enamel etching.11 The Er:YAG laser is one of the most widely utilized techniques for hard dental tissue etching because it produces surfaces with a greater roughness, without a smear layer.12
Furthermore, irradiation with an Er:YAG laser can promote structural and morphological changes in hard dental tissues.13 However, further studies are required to evaluate the effects of Er:YAG laser application on dental substrates with respect to the bonding technique utilized.
Adhesive systems should be tested in vitro and compared with previously established parameters and outcomes before clinical application.10 Novel materials are constantly introduced into the market and their effectiveness requires both in vitro and in vivo evaluation.
The aim of this in vitro study was to evaluate the effect of Er:YAG laser application on tooth enamel before the utilization of etch-and-rinse and universal adhesive systems.
MATERIALS AND METHODS
A total of 32 human permanent molars were extracted from dental clinic patients involved in the undergraduate dentistry course at Ceuma University. The Human Research Ethics Committee under protocol n. 543.409 approved the research.
Healthy teeth free from caries, cervical lesions, cracks, enamel defects, and restorations were selected, cleaned, immersed in 0.1% thymol solution, and stored in distilled water at 37 ± 1°C. These samples were sectioned in the mesiodistal direction, producing 64 vestibular and lingual halves. Following this, the samples were embedded in polyvinyl chloride tubes using chemically activated acrylic resin (JET-CLÁSSICO, São Paulo, Brazil) so that the vestibular and lingual surfaces of each sample were exposed on a flat level, facing up.
The vestibular and lingual halves of each sample were randomized into two groups (n = 32) (Flow Chart 1) to receive application of either an etch-and-rinse bonding (SB2; total-etching Single Bond 2 adhesive system (SB2) (3M ESPE, Saint Paul, Minnesota, USA) adhesive system, or a universal adhesive system (SBU) (3M ESPE, Saint Paul, Minnesota, USA) (Table 1). Before the application of a universal or etch-and-rinse adhesive system, phosphoric acid (37%) was applied to the tooth enamel. Samples in the SB2 and SBU groups were subdivided into either subgroup L (n = 16), in which the enamel surfaces were irradiated with an Er:YAG laser (SBU-L and SBU-L) or subgroup C (control group, n = 16) in which the enamel surfaces were not irradiated (SB2-C and SB2-C).
The irradiated area corresponded to the central area of the molar surface, delimited by the mesial and distal faces of the vestibular or lingual surfaces, while the remaining tooth surface was used as a control. Irradiation was administered using an Er:YAG laser (KaVo Key Laser 2; Kavo Co, Biberach, Germany) for enamel etching at an energy of 60 mJ per pulse and at a frequency of 10 Hz. The laser emitted a wavelength of 2.94 μm through a handpiece (#2051) with a spot diameter of 0.63 mm. The handpiece was positioned perpendicularly to the tooth surface and the distance was calibrated at 12 mm (focused mode). Irradiation was performed manually in the scan mode under constant water cooling; the entire tooth surface was irradiated. Care was taken to avoid area overlapping in order to obtain a homogeneously irradiated surface.
Three transparent cylindrical matrices (Tygon Tubing, TYG-030, Saint-Gobain Performance Plastic, Maime Lakes, Florida, USA) were placed on the vestibular surface of each sample. The TPH spectrum composite resin (Dentsply Caulk, Milford, Delaware, USA) was applied to each matrix, filling the internal volume. All photoactivation procedures were performed using a fast-curing cordless LED photopolymerizer (3M ESPE dental, Landsberg am Lech, Germany) under a light intensity of approximately 800 mW/cm2, measured with a radiometer (GNATUS, Ribeirão Preto, São Paulo, Brazil). After enamel bonding, the matrices were removed to expose the small cylinders of composite resin to the areas intended to be treated.
To perform the microshear test, a 0.2-mm diameter stainless steel wire was positioned around the cylinder and aligned with the bonding interface of the adhesive system. The test was conducted using a universal testing machine EMIC (DL 2000-EMIC, São José dos Pinhais, Paraná, Brazil) at a speed of 0.5 mm/min until fracture was achieved. The mean fracture values of the three composite resin cylinders were then calculated.
SB2: Single bond; SBU: Single bond universal; TPH: Bis-GMA: Bisphenol A glycidyl methacrylate; HEMA: Hydroxyethyl methacrylate; MDP: Methacryloyloxydecyl dihydrogen phosphate; EDAB: Ethyl-4-dimethylamino benzoate
After the bonding strength tests, the surface of each sample was examined under a microscope to determine the type of fracture that occurred in the region of rupture between the enamel and the composite resin. Fractures were classified as follows: (1) Adhesive fractures: fractures in the resin–adhesive interface; (2) cohesive enamel fractures: fractures in the tooth enamel; (3) cohesive fractures in the composite resin: fractures in the body of the composite resin cylinder; and (4) mixed fractures: fractures involving resin, adhesive, and tooth structure (combination of the fracture types).
Bonding strength data were not normally distributed in the same groups (Shapiro Wilk; p < 0.05). Therefore, bonding strength values were compared with the amount of adhesive material remaining using the nonparametric Kruskal–Wallis and Mann–Whitney tests. For non-normally distributed data or for groups without homogeneity of variance, the median and interquartile ranges of the values obtained were calculated (Table 2). A value of α = 0.05 using the Bonferroni correction was used for multiple comparisons, resulting in an α = 0.012 for each of the four comparisons. Statistical analyses were performed using both PAWS Statistics software (version 17; SPSS Inc., Chicago, USA) and GraphPad Prism software (version 5-2007, release 18.104.22.1689; San Diego, USA).
|Variables||Mean (SD)*||Median||95% CI|
|SB2-L||22.83 (4.0) A||24.00||20.25–25.42|
|SB2-C||19.67 (2.4) A||19.00||18.10–21.23|
|SBU-L||21.50 (3.2) A||21.00||19.43–23.57|
|SBU-C||20.58 (4.5) A||19.00||17.72–23.44|
CI: Confidence interval; SD: Standard deviation; *Similar uppercase letters denote no statistically significant difference within each group of teeth
Adhesive Strength (MPa)
Descriptive and inferential statistics of bonding strength for the study groups are shown in Table 2. Kruskal–Wallis and Mann–Whitney tests showed that there was no statistically significant difference in bonding strength between the groups evaluated (Table 2).
The absolute and relative frequencies of the variable type of fracture are presented in Table 3. Adhesive fractures were the predominant fracture type in groups SB2-C, SBU-L, and SBU-C.
In this study, there was no significant difference in tooth enamel bonding strength between the utilization of a universal adhesive system and a etch-and-rinse bonding system following Er:YAG laser irradiation.
Enamel treatment with phosphoric acid before adhesive system application results in greater bonding strength compared with unetched enamel.6,9,14 Phosphoric acid creates microporosities and more pronounced bonding patterns in the enamel.4 With regard to novel universal adhesive materials, McLean et al6 found that etching significantly increased the bonding strength of these materials. In the present study, before the application of a universal or etch-and-rinse adhesive system, phosphoric acid (37%) was applied to the tooth enamel, independent of the use of Er:YAG laser. This is because acid etching increases the bonding strength of different adhesive systems.15
The analysis of the composition of the adhesive materials used in this study indicated that, in addition to the components present in the etch-and-rinse adhesive system, the universal adhesive material also contains methacryloyl oxide decamethylene phosphoric acid, methacryloyloxydecyl dihydrogen phosphate (MDP), and silane (Table 1). Self-etching adhesive systems are known to cause a partial demineralization of the tooth substrate and studies investigating the Clearfil SE Bond have demonstrated that 10-MDP forms a stable chemical bond with the tooth substrate. This knowledge led to the development of the “universal,” “multipurpose,” and “multimode” adhesive systems that contain 10-MDP, in addition to other components. Accordingly, the bonding mechanism occurs through micromechanical and chemical nanointeractions. Nanolayers of 10-MDP molecules result in the formation of a stable salt, MDP-calcium, which increases the resistance of the adhesive interface to biodegradation. This may explain the improved interfaces obtained with the use of 10-MDP-based adhesive systems.16
Some studies utilizing universal adhesive systems containing 10-MDP have been recently conducted because these systems have previously been shown to yield greater and more stable bonding strengths, with reduced nanoleakage.6,7,17
Although previous studies have shown that acid etching improves the penetration of universal adhesives in the dentin, it should be noted that total etching could remove the inorganic content, leaving the organic matrix exposed.18 Therefore, the formation of the MDP-calcium salt, which facilitates the chemical interaction with nanomaterials, becomes restricted.
The results obtained in this study indicate that enamel etching using Er:YAG laser prior to the use of the phosphoric acid did not significantly change the bonding strength in the groups treated with a conventional or a universal adhesive system. Ciucchi et al19 by employing the Er:YAG in dental pits before etching, for applying sealants, also found no significant changes in the microscopic images obtained. A previous study has demonstrated that enamel etching with Er:YAG laser produces less stress traction and demineralization compared with phosphoric acid.20 In the present study, a low-energy Er:YAG laser (60 mJ) was utilized for enamel etching, as previously described because low-energy levels (60–80 mJ) can promote demineralization without causing major changes in the enamel.21
However, it is difficult to compare the findings of previously published studies due to the variation in the experimental techniques employed: some studies combined the use of laser and acid etching, while others used laser alone to promote etching.13,20,22 The previous study has proposed the application of enamel etching using laser parameters similar to those used in cavity preparations. In these cases, despite an adequate bonding strength, severe and permanent damage to the tooth enamel has been recorded.22
In contrast to acid etching, the application of a tooth enamel laser has been shown to produce a region comprising fissures and debris in the subsurface layer of the enamel in 80% of the irradiated samples.23 Accordingly, Ceballos et al24 highlighted that the enamel laser irradiation is a nonviable alternative to acid etching pretreatment for the composite resin. These results provided the rationale for the current study; to perform acid etching after irradiating the tooth enamel with Er:YAG laser.
Yung et al25 showed that the lower bonding strength values obtained after laser etching may be due to thermal damage, unfavorable changes in the enamel surface by excessive laser energy, composition of the adhesive system utilized, or due to differences in the laser parameters.
The most frequent fractures in the SB2-C, SBU-L, and SBU-C groups were observed at the adhesive–enamel interface. The SB2-L group presented homogeneous adhesive, cohesion, and mixed fractures. In contrast, McLean et al6 performed a microtensile assessment and recorded a higher frequency of mixed fractures (adhesive and cohesive fractures in the enamel) with acid etching followed by the application of the universal adhesive system.
Therefore, there was no statistically significant difference in the tooth enamel bonding strength between a universal adhesive system and a two-step etch-and-rinse adhesive system following application of Er:YAG laser irradiation.