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Effect of erosive challenge and Nd:YAG laser irradiation on bond strength of adhesive systems to dentin
International Journal of Adhesion & Adhesives 64 (2016) 60–64
Contents lists available at ScienceDirect
International Journal of Adhesion & Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Effect of erosive challenge and Nd:YAG laser irradiation on bond strength of adhesive systems to dentin Fernando Akio Maeda a, Karen Akemi Fukushima a, Tamara Kerber Tedesco b, Ana Cecília Corrêa Aranha c, Daniela Prócida Raggio b, Walter Gomes Miranda Junior a, Paulo Francisco Cesar a,n a
Department of Biomaterials and Oral Biology, School of Dentistry, University of São Paulo, Av. Prof. Lineu Prestes, 2227, 05508-000 São Paulo, SP, Brazil Department of Orthodontics and Pediatric Dentistry, University of São Paulo, Av. Prof. Lineu Prestes, 2227, São Paulo, SP, Brazil c Special Laboratory of Lasers in Dentistry (LELO), Department of Restorative Dentistry, University of São Paulo, Av. Prof. Lineu Prestes, 2227, São Paulo, SP, Brazil b
art ic l e i nf o
a b s t r a c t
Article history: Accepted 5 September 2015 Available online 8 October 2015
Purpose: To evaluate the effect of Nd:YAG laser irradiation and erosive challenge on bond strength of two adhesive systems to dentin. Methods: Twenty bovine incisors were cut and grounded to obtain eighty slabs of flat dentin. Specimens were allocated into eight groups, based on: adhesive system—a two-step etch-and-rinse and a two-step self-etch; laser irradiation—Nd:YAG (1 W/10 Hz) or control (no laser irradiation); and erosive challenge after restorative procedure—presence or absence of erosive challenge. Nd:YAG laser groups were submitted to laser irradiation before the restorative procedure. Blocks of composite resin were built up on the bonded surfaces with a Southern Dental Industries device to perform shear bond strength (SBS) test. s After, each specimen of erosive challenge, groups were subjected to immersion in Sprite Zero (20 ml/ 2 h/24 °C/under agitation). The SBS test (0.5 mm/min) was performed after 24 h of water storage at 37 °C. Failure mode was evaluated with a stereomicroscope (X400). Data were analyzed with three-way ANOVA and Tukey’s post hoc tests (α ¼ 0.05). Results: The etch-and-rinse adhesive system presented higher bond strength values than self-etch adhesive. Laser irradiation increased the bond strengths values when erosive challenge was present. The predominant failure mode observed was adhesive. Conclusions: The irradiation of Nd:YAG laser positively influences the bond strength values when erosive challenges are present. Moreover, the etch-and-rinse adhesive system is a better option to be used in dentin in this clinical condition. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Self-etch adhesive Total-etch Dentine Macro-shear Dental erosion
1. Introduction Changes in eating habits have been reported as one of the main reasons for the prevalence of health problems in a large part of the population [1,2]. Nowadays, the increasing ingestion of citric fruits, soft drinks, juices and yogurt has been claimed as the main causing factor of erosion lesions in permanent and primary teeth [3–6]. Dental erosion is the result of surface dissolution of the mineralized dental tissues caused by acids without any microbiological involvement [7]. This phenomenon can be responsible for morphological changes in the dentinal tissue, such as an increase in the number of widened dentinal tubules [8,9]. n Correspondence to: University of São Paulo, Av. Prof. Lineu Prestes, 2227, Cidade Universitária, São Paulo, SP, Brazil. Tel./fax: þ 55 11 3091 7849. E-mail address: [email protected] (P.F. Cesar).
http://dx.doi.org/10.1016/j.ijadhadh.2015.09.008 0143-7496/& 2015 Elsevier Ltd. All rights reserved.
Therefore, these alterations in the dentin tissue will facilitate the fluid displacement and will ultimately stimulate the pulp mechanic-receptors, leading to acute and localized pain [10]. Dental erosion has been associated with the prevalence of dentin hypersensitivity [11,12]. The effective treatment of dental erosion should decrease teeth sensibility and prevent the formation of new lesions by means of the elimination of predisposing factors, like harmful eating habits or gastroesophageal reflux. Changing the patients eating habits is not an easy task. Some desensitizing agents have been recommended in an attempt to reduce tooth sensitivity; however, their effectiveness is transitory as they act only as pain relievers [13]. The lack of an effective treatment to dental erosion has encouraged the development of new approaches to control dentin hypersensitivity. Application of Nd:YAG laser to the exposed dentin surface showed satisfactory results in clinical trials, with
F.A. Maeda et al. / International Journal of Adhesion & Adhesives 64 (2016) 60–64
reductions of 88% in the sensibility rate [14–16]. These results were further confirmed in a recent systematic review [17]. This type of laser treatment occludes the dentinal tubules by dentinal fusion and subsequent tissue solidification [18], reducing the demineralization process and progression of the lesion [19,20]. One important aspect of laser treated erosion lesions is that they usually need to be restored with resin composites in order to reconstruct the correct dental anatomy, improving esthetics and function of the affected teeth [21]. To the authors’ knowledge, there is no study in the literature that investigated the influence of Nd:YAG laser on the bond strength of adhesive systems to dentin when the erosive challenge is maintained after the restorative treatment. This last feature is particularly important because it is has been shown that the erosion challenge usually persists after the restorative procedure and little information is available on the effect of such challenge on the longevity of bonded restorations. Thus, the purpose of this study was to evaluate the effect of Nd: YAG laser irradiation and erosive challenge on the bond strength of two adhesive systems to dentin. The hypothesis tested is that there is no influence of both Nd:YAG laser irradiation and erosive challenge on the bond strength to dentin.
2. Materials and methods Twenty freshly extracted bovine incisors were selected and stored in 0.01% (w/v) thymol solution at 4 °C for 1 month until the beginning of the study. The root portion of each teeth was removed in the transversal plane, and the coronal portion was sectioned in the mesiodistal and cervico-occlusal direction using a cutting machine with low speed water-cooled diamond saw (Isomet 1000, Buhler, Lake Bluff, IL, USA) to obtain eighty dentin slabs with dimensions of 4.5 4.5 mm2. Specimens were then
61 s
embedded in self-curing acrylic resin (JET Clássico , São Paulo, SP, BR) and the surfaces were ground with 180, 400 and 600-grit SiC paper for 30 s under running water to ensure a flat dentin and uniform smear layer. 2.1. Experimental groups Specimens were randomly assigned into eight experimental groups, based on: adhesive system (one two-step etch-and-rinse, Adper Single Bond 2; 3M ESPE, St. Paul, MN, USA, and one twostep self-etch, Adper SE Plus; 3M ESPE, St. Paul, MN, USA); laser irradiation (Nd:YAG laser or control—no laser irradiation); erosive challenge after restorative procedure (presence or absence of erosive challenge) (Fig. 1). The absence of erosive challenge was simulated in distilled water. These groups resulted in a 2 2 2 factorial experimental design with 10 specimens in each subgroup. Compositions and manufacturer’s instructions of the adhesive systems are summarized in Table 1. 2.2. Laser irradiation Specimens from laser irradiation groups (G2, G4, G6, G8) were irradiated with a Nd:YAG laser (Power LaserTM ST6, Lares s Research , Chicago, CA, USA) with a quartz fiber of 300 mm and wavelength of 1064 nm. The irradiation protocol used was: power of 1 W, repetition rate of 10 Hz, 100 mJ of energy and 80 J/cm2 of energy density, in contact mode [15,22], with scanning movements, two times for 10 s each (one in horizontal and the other in vertical direction), with an interval of 20 s between them to allow thermal relaxation of the tissue, without air cooling, in VLP mode (very long pulse). The quartz fiber was always cleaved after 5 irradiated specimens.
Fig. 1. Experimental groups description.
Table 1 Adhesive systems; characteristics, general composition, manufacturers and manufacturers' instructions.
Characteristics Composition
Manufacturer Manufacturers' instruction
Adper Single Bond 2
Adper SE Plus
Two-step etch-and-rinse adhesive system Etchant: 35% phosphoric acid Adhesive system: HEMA, ethanol, water, Bis-GMA, dimethacrylate, amines, methacrylic copolymer of polyacrylic and polyitaconic acids, and photo initiator. 3M ESPE St. Paul, MN, USA Etch tooth surface with acid etchant for 15 s then rinse for 10 s; blot excess water with cotton pellet; apply 2 coats of bond with rubbing motion for 15 s; air-thin for 5 s; and light-curing for 10 s.
Two-step self-etch adhesive system Primer: water, HEMA, surfactant, pink colorant Adhesive: UDMA, TEGDMA, TMPTMA, HEMA, methacrylated phosphates, bonded zirconia nanofiller, initiator system based on camphorquinone 3M ESPE St. Paul, MN, USA Apply primer; apply adhesive with a brush and rubbing motion for 20 s; apply other coat of adhesive and air-thin for 10 s; and light-curing for 10 s.
Abreviations: Bis-GMA: bis-phenol A diglycidylmethacrylate; HEMA: 2-hydroxyethyl methacrylate; UDMA: urethane dimethacrylate; TEGDMA: Triethylene glycol dimethacrylate; TMPTMA: hydrophobic trimethacrylate.
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2.3. Adhesive and restorative procedures Adhesive systems were applied on the specimen’s surface in accordance to the manufacturer’s instructions and experimental groups (Table 1). After light curing of adhesive systems, specimens were attached to a device for standardization of the shear bond strength test by SDI (Southern Dental Industries, Bayswater, VIC, AUS) (Fig. 2). As illustrated in Fig. 2, a cylinder of composite resin (Z250, A2, 3M ESPE, St. Paul, MN, USA) with a cross-sectional area of 9.62 mm2 was built inside the metal ring of the device on the dentin, and the shear test was performed to the set after 24 h to determine the bond strength. 2.4. Erosive challenge Specimens submitted to the subsequent erosive challenge (G3, s G4, G7, G8) were individually immersed in 20 ml of Sprite Zero (The Coca-Cola Company, Porto Real, RJ, BR) for 2 h under agitation after restorative procedure at 24 °C. Specimens were then rinsed with distilled water and stored in 20 ml of artificial saliva for 24 h at 37 °C [23].
contact with the bonded specimen at the composite and dentin interface, so that forces were applied parallel to the bonded surface. Shear load was applied at a crosshead speed of 0.5 mm/min until failure occurred. 2.6. Fracture pattern The fracture pattern was determined at a magnification of X400 under a stereomicroscope (Stereo Discovery V20, Zeiss, Göttingen, NI, GER) and classified as: adhesive, mixed or cohesive failure in dentin or resin composite. 2.7. Statistical analysis All data were presented as mean and standard deviation (MPa) value. Equality of variances was assumed after Levene’s test (p¼ 0.252). Bond strength values were analyzed with three-way ANOVA (adhesive system*laser irradiation*erosive challenge after restorative procedure) and Tukey’s post-hoc test. Statistical analysis was performed using Minitab Statistical software V16 (Quality Analysis Results, State College, PA, USA) and the significance set at 5%.
2.5. Shear bond strength test
3. Results
Specimens were submitted to shear bond strength test in a universal testing machine (Kratos IKCL3-USB, Kratos Industrial Equipment, Cotia, SP, BR) with the loading device placed to make
Mean bond strength values and standard deviations (MPa) are summarized in Table 2. Three-way ANOVA indicated statistically significant differences for two of the main factors, i.e., “adhesive
Fig. 2. (A) Shear device of SDI; (B) metal cylinder and acrylic pin; (C) shear device mounted; and (D) specimen ready to shear bond test.
Table 2 Means of bond strength (MPa) and standard deviations for the experimental groups. No erosive challenge
Adper Single Bond 2 Adper SE Plus
Erosive challenge
Control
Nd:YAG laser irradiation
Control
Nd:YAG laser irradiation
G5 - 19.0 7 0.6a,b G1 - 15.0 7 0.4c,d
G6 - 18.7 71.0a,b G2 - 13.5 70.9d,e
G7 - 17.1 71.0b,c G3 - 11.4 71.5e
G8 - 21.3 7 1.3a G4 - 16.6 7 3.7b,c,d
*Different lowercases indicate statistically significant difference to interaction adhesive system*laser irradiation*treatment of restorative procedure.
F.A. Maeda et al. / International Journal of Adhesion & Adhesives 64 (2016) 60–64
Fig. 3. Failure modes (percent) for experimental groups (SB: Adper Single Bond 2; SE: Adper SE Plus; Nd: Nd:YAG laser irradiation; Ec: Erosive challenge).
system” (p o0.001) and “laser irradiation” (p ¼ 0.001). The following interactions were also statistically significant: “laser irradiation*erosive challenge” (p o0.001) and “adhesive system*laser irradiation*erosive challenge” (p¼ 0.032). The factor “erosive challenge” and the other interactions were not statistically significant (p 40.05). According to Table 2, the etch-and-rinse adhesive system (Adper Single Bond 2) (G5–G8) showed significantly higher bond strength values than those obtained for the self-etch adhesive (G1–G4), regardless of the presence of laser irradiation and erosive challenge. Regarding to the effect of the erosive challenge on bond strength, it was possible to note a significant deleterious effect on bond strength only for the control condition of Adper SE Plus (G1– G3). Laser irradiation resulted in increased mean bond strength in comparison to the control group for both adhesive systems only in the presence of erosive challenge (G1–G4, G5–G8). Fig. 3 shows the prevalence of failure modes observed for each experimental group. Adhesive failures were the most frequently identified failure types for all groups. No cohesive failures in dentin were observed.
4. Discussion Nd:YAG laser irradiation has been suggested as an option to hypersensitive dentin treatment. However, the influence of subsequent erosive challenge on the bond strength to dentin submitted to this type of laser application has not been reported yet. In our study, Nd:YAG laser irradiation improved the resin–dentin bonds for both adhesives when the erosive challenge was present, in comparison to control group. Therefore, the study hypothesis had to be rejected. The better bond strength results found for laser irradiated dentin can be explained by the fact that dentin irradiated with Nd: YAG laser is more resistant to the dissolution promoted by the erosive challenge [24-26]. This acid-resistant dentin probably protects the resin–dentin interface from the continuous erosive challenge that occurs after the restorative procedure. The interaction between dentin and laser leads to surface melting, reducing the tubular diameter or obstructing the dentin tubules and sometimes could cause some protein denaturation. Furthermore, it has been hypothesized that the temperature increase that occurs after laser irradiation of the dentinal substrate results in chemical and structural changes, leading to loss of water and carbonate [27,28]. This reduction in carbonate content leads to fluoride incorporation into apatite and, after repeated dissolution events, the dentin substrate becomes more resistant to new acid
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challenge episodes [29,30]. Thus, it is likely that a similar phenomenon took place in the adhesive interface around the resin cylinders used in the present investigation, protecting the resin– dentin bonds from degradation during the erosive challenge. This finding corroborates with results from previous study that showed a better behavior of adhesive systems in dentin submitted to erosive challenge when laser irradiation was performed [31]. However, this study used Er,Cr:YSGG laser, which can result in a different dentin surface after laser application than that obtained with Nd:YAG irradiation [31]. One important fact that should be taken into account to understand the effect of laser application on the specimens submitted to erosive challenge is the decrease in bond strength observed for the control groups of both adhesives after submitting them to the erosive challenge, though only for Adper SE Plus such decrease was statistically significant. These lower values observed for the control groups after erosive challenge corroborate with a previous study [32] and is a consequence of the deterioration of resin–dentin interface in the erosive environment. The erosive challenge performed in this investigation created an outer zone of demineralized dentin and, as a consequence, exposed collagen fibrils and weakened the adhesive interface. The better performance of the etch-and-rinse adhesive system is in accordance with previous studies that showed higher bond strength values for this system in comparison to self-etch adhesives. However, it is important to consider that these studies were performed in adhesive interface without subsequent erosive challenge [33,34]. It has been demonstrated that the relatively high bond strength values obtained for etch-and-rinse adhesives are related to better micro-mechanical interlocking to dentin surfaces when phosphoric acid is applied, compared to the acidic monomers found in self-etch adhesive compositions [35]. In fact, even the decreased thickness of the weak dentin zone without resin infiltration obtained with the self-etch approach did not prevent the inferior performance observed for this type of adhesive in the current investigation. Such negative behavior can be explained by the combination of the high amount of water ( 80%) with the HEMA molecules found in the composition of self-etch adhesives. The association of these two components (waterþHEMA) results in lower rates of water evaporation with subsequent material phase separation [34]. Furthermore, these satisfactory results obtained with etch-and-rinse adhesive system in dentin corroborate a previous study that showed that this adhesive system approach results in better performance than other restorative materials, such as glass ionomer cements when applied in eroded dentin [36,37]. On the other hand, when another laser was applied, a better performance was reported for a two-step self-etch adhesive system when compared to an etch-and-rinse system [31]. The difference between these findings can be related to the type of self-etch adhesive system used, since in earlier works a MDP-containing adhesive system was used [31], which associates micromechanical interlocking with a possible chemical interaction with the calcium in hydroxyapatite crystals that can lead to the higher bond strength values [38]. The shear bond strength test carried out in this investigation was based in a recently developed and standardized device. Usually, the shear bond strength test is performed in bonding cross-sectional areas of up to approximately 1 mm2. Such test is called microshear bond strength test and has the advantage of making possible the construction of several specimens in the same tooth. Moreover, the greatest advantage compared to microtensile test is producing less stress during specimen production, since there is no need of slicing the specimens [39]. Placido et al. showed, by finite element analysis, that the microshear test can result in more heterogeneous stress concentrations than conventional shear test [40]. Thus, in an attempt to find a test with less
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disadvantages and more reliable results of bond strength, we chose the shear test using this new device. Also, the shear bond strength test seems to better simulate the adhesive interface exposed in cervical lesions, unlike what happens with microtensile specimens which leave the resin–dentin interface exposed to water and erosive challenge action, resulting in faster degradation. Further studies still need to be conducted to evaluate the longterm behavior of restorative procedures in dentin submitted to Nd: YAG laser irradiation and continuous erosive challenge, since both adhesive systems evaluated here can suffer degradation resulting of the hydrolysis of resin matrices by esterase [41] and collagen matrices by host-derived enzymes [42]. In fact, there is no data available about the influence of this type of laser irradiation on the bond stability of adhesive systems.
5. Conclusions Based on the results of the present investigation, it was possible to conclude that, for both adhesives systems evaluated, the irradiation of Nd:YAG laser only had a positive effect on bond strength when the erosive challenge was present. The importance of this finding is related to the fact that laser irradiation can be considered as an alternative treatment before restorative procedures are subjected to erosive challenge. Another conclusion of this work is that the etch-and-rinse adhesive resulted in superior performance than self-etch system in this clinical condition.
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Contents lists available at ScienceDirect
International Journal of Adhesion & Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Effect of erosive challenge and Nd:YAG laser irradiation on bond strength of adhesive systems to dentin Fernando Akio Maeda a, Karen Akemi Fukushima a, Tamara Kerber Tedesco b, Ana Cecília Corrêa Aranha c, Daniela Prócida Raggio b, Walter Gomes Miranda Junior a, Paulo Francisco Cesar a,n a
Department of Biomaterials and Oral Biology, School of Dentistry, University of São Paulo, Av. Prof. Lineu Prestes, 2227, 05508-000 São Paulo, SP, Brazil Department of Orthodontics and Pediatric Dentistry, University of São Paulo, Av. Prof. Lineu Prestes, 2227, São Paulo, SP, Brazil c Special Laboratory of Lasers in Dentistry (LELO), Department of Restorative Dentistry, University of São Paulo, Av. Prof. Lineu Prestes, 2227, São Paulo, SP, Brazil b
art ic l e i nf o
a b s t r a c t
Article history: Accepted 5 September 2015 Available online 8 October 2015
Purpose: To evaluate the effect of Nd:YAG laser irradiation and erosive challenge on bond strength of two adhesive systems to dentin. Methods: Twenty bovine incisors were cut and grounded to obtain eighty slabs of flat dentin. Specimens were allocated into eight groups, based on: adhesive system—a two-step etch-and-rinse and a two-step self-etch; laser irradiation—Nd:YAG (1 W/10 Hz) or control (no laser irradiation); and erosive challenge after restorative procedure—presence or absence of erosive challenge. Nd:YAG laser groups were submitted to laser irradiation before the restorative procedure. Blocks of composite resin were built up on the bonded surfaces with a Southern Dental Industries device to perform shear bond strength (SBS) test. s After, each specimen of erosive challenge, groups were subjected to immersion in Sprite Zero (20 ml/ 2 h/24 °C/under agitation). The SBS test (0.5 mm/min) was performed after 24 h of water storage at 37 °C. Failure mode was evaluated with a stereomicroscope (X400). Data were analyzed with three-way ANOVA and Tukey’s post hoc tests (α ¼ 0.05). Results: The etch-and-rinse adhesive system presented higher bond strength values than self-etch adhesive. Laser irradiation increased the bond strengths values when erosive challenge was present. The predominant failure mode observed was adhesive. Conclusions: The irradiation of Nd:YAG laser positively influences the bond strength values when erosive challenges are present. Moreover, the etch-and-rinse adhesive system is a better option to be used in dentin in this clinical condition. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Self-etch adhesive Total-etch Dentine Macro-shear Dental erosion
1. Introduction Changes in eating habits have been reported as one of the main reasons for the prevalence of health problems in a large part of the population [1,2]. Nowadays, the increasing ingestion of citric fruits, soft drinks, juices and yogurt has been claimed as the main causing factor of erosion lesions in permanent and primary teeth [3–6]. Dental erosion is the result of surface dissolution of the mineralized dental tissues caused by acids without any microbiological involvement [7]. This phenomenon can be responsible for morphological changes in the dentinal tissue, such as an increase in the number of widened dentinal tubules [8,9]. n Correspondence to: University of São Paulo, Av. Prof. Lineu Prestes, 2227, Cidade Universitária, São Paulo, SP, Brazil. Tel./fax: þ 55 11 3091 7849. E-mail address: [email protected] (P.F. Cesar).
http://dx.doi.org/10.1016/j.ijadhadh.2015.09.008 0143-7496/& 2015 Elsevier Ltd. All rights reserved.
Therefore, these alterations in the dentin tissue will facilitate the fluid displacement and will ultimately stimulate the pulp mechanic-receptors, leading to acute and localized pain [10]. Dental erosion has been associated with the prevalence of dentin hypersensitivity [11,12]. The effective treatment of dental erosion should decrease teeth sensibility and prevent the formation of new lesions by means of the elimination of predisposing factors, like harmful eating habits or gastroesophageal reflux. Changing the patients eating habits is not an easy task. Some desensitizing agents have been recommended in an attempt to reduce tooth sensitivity; however, their effectiveness is transitory as they act only as pain relievers [13]. The lack of an effective treatment to dental erosion has encouraged the development of new approaches to control dentin hypersensitivity. Application of Nd:YAG laser to the exposed dentin surface showed satisfactory results in clinical trials, with
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reductions of 88% in the sensibility rate [14–16]. These results were further confirmed in a recent systematic review [17]. This type of laser treatment occludes the dentinal tubules by dentinal fusion and subsequent tissue solidification [18], reducing the demineralization process and progression of the lesion [19,20]. One important aspect of laser treated erosion lesions is that they usually need to be restored with resin composites in order to reconstruct the correct dental anatomy, improving esthetics and function of the affected teeth [21]. To the authors’ knowledge, there is no study in the literature that investigated the influence of Nd:YAG laser on the bond strength of adhesive systems to dentin when the erosive challenge is maintained after the restorative treatment. This last feature is particularly important because it is has been shown that the erosion challenge usually persists after the restorative procedure and little information is available on the effect of such challenge on the longevity of bonded restorations. Thus, the purpose of this study was to evaluate the effect of Nd: YAG laser irradiation and erosive challenge on the bond strength of two adhesive systems to dentin. The hypothesis tested is that there is no influence of both Nd:YAG laser irradiation and erosive challenge on the bond strength to dentin.
2. Materials and methods Twenty freshly extracted bovine incisors were selected and stored in 0.01% (w/v) thymol solution at 4 °C for 1 month until the beginning of the study. The root portion of each teeth was removed in the transversal plane, and the coronal portion was sectioned in the mesiodistal and cervico-occlusal direction using a cutting machine with low speed water-cooled diamond saw (Isomet 1000, Buhler, Lake Bluff, IL, USA) to obtain eighty dentin slabs with dimensions of 4.5 4.5 mm2. Specimens were then
61 s
embedded in self-curing acrylic resin (JET Clássico , São Paulo, SP, BR) and the surfaces were ground with 180, 400 and 600-grit SiC paper for 30 s under running water to ensure a flat dentin and uniform smear layer. 2.1. Experimental groups Specimens were randomly assigned into eight experimental groups, based on: adhesive system (one two-step etch-and-rinse, Adper Single Bond 2; 3M ESPE, St. Paul, MN, USA, and one twostep self-etch, Adper SE Plus; 3M ESPE, St. Paul, MN, USA); laser irradiation (Nd:YAG laser or control—no laser irradiation); erosive challenge after restorative procedure (presence or absence of erosive challenge) (Fig. 1). The absence of erosive challenge was simulated in distilled water. These groups resulted in a 2 2 2 factorial experimental design with 10 specimens in each subgroup. Compositions and manufacturer’s instructions of the adhesive systems are summarized in Table 1. 2.2. Laser irradiation Specimens from laser irradiation groups (G2, G4, G6, G8) were irradiated with a Nd:YAG laser (Power LaserTM ST6, Lares s Research , Chicago, CA, USA) with a quartz fiber of 300 mm and wavelength of 1064 nm. The irradiation protocol used was: power of 1 W, repetition rate of 10 Hz, 100 mJ of energy and 80 J/cm2 of energy density, in contact mode [15,22], with scanning movements, two times for 10 s each (one in horizontal and the other in vertical direction), with an interval of 20 s between them to allow thermal relaxation of the tissue, without air cooling, in VLP mode (very long pulse). The quartz fiber was always cleaved after 5 irradiated specimens.
Fig. 1. Experimental groups description.
Table 1 Adhesive systems; characteristics, general composition, manufacturers and manufacturers' instructions.
Characteristics Composition
Manufacturer Manufacturers' instruction
Adper Single Bond 2
Adper SE Plus
Two-step etch-and-rinse adhesive system Etchant: 35% phosphoric acid Adhesive system: HEMA, ethanol, water, Bis-GMA, dimethacrylate, amines, methacrylic copolymer of polyacrylic and polyitaconic acids, and photo initiator. 3M ESPE St. Paul, MN, USA Etch tooth surface with acid etchant for 15 s then rinse for 10 s; blot excess water with cotton pellet; apply 2 coats of bond with rubbing motion for 15 s; air-thin for 5 s; and light-curing for 10 s.
Two-step self-etch adhesive system Primer: water, HEMA, surfactant, pink colorant Adhesive: UDMA, TEGDMA, TMPTMA, HEMA, methacrylated phosphates, bonded zirconia nanofiller, initiator system based on camphorquinone 3M ESPE St. Paul, MN, USA Apply primer; apply adhesive with a brush and rubbing motion for 20 s; apply other coat of adhesive and air-thin for 10 s; and light-curing for 10 s.
Abreviations: Bis-GMA: bis-phenol A diglycidylmethacrylate; HEMA: 2-hydroxyethyl methacrylate; UDMA: urethane dimethacrylate; TEGDMA: Triethylene glycol dimethacrylate; TMPTMA: hydrophobic trimethacrylate.
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2.3. Adhesive and restorative procedures Adhesive systems were applied on the specimen’s surface in accordance to the manufacturer’s instructions and experimental groups (Table 1). After light curing of adhesive systems, specimens were attached to a device for standardization of the shear bond strength test by SDI (Southern Dental Industries, Bayswater, VIC, AUS) (Fig. 2). As illustrated in Fig. 2, a cylinder of composite resin (Z250, A2, 3M ESPE, St. Paul, MN, USA) with a cross-sectional area of 9.62 mm2 was built inside the metal ring of the device on the dentin, and the shear test was performed to the set after 24 h to determine the bond strength. 2.4. Erosive challenge Specimens submitted to the subsequent erosive challenge (G3, s G4, G7, G8) were individually immersed in 20 ml of Sprite Zero (The Coca-Cola Company, Porto Real, RJ, BR) for 2 h under agitation after restorative procedure at 24 °C. Specimens were then rinsed with distilled water and stored in 20 ml of artificial saliva for 24 h at 37 °C [23].
contact with the bonded specimen at the composite and dentin interface, so that forces were applied parallel to the bonded surface. Shear load was applied at a crosshead speed of 0.5 mm/min until failure occurred. 2.6. Fracture pattern The fracture pattern was determined at a magnification of X400 under a stereomicroscope (Stereo Discovery V20, Zeiss, Göttingen, NI, GER) and classified as: adhesive, mixed or cohesive failure in dentin or resin composite. 2.7. Statistical analysis All data were presented as mean and standard deviation (MPa) value. Equality of variances was assumed after Levene’s test (p¼ 0.252). Bond strength values were analyzed with three-way ANOVA (adhesive system*laser irradiation*erosive challenge after restorative procedure) and Tukey’s post-hoc test. Statistical analysis was performed using Minitab Statistical software V16 (Quality Analysis Results, State College, PA, USA) and the significance set at 5%.
2.5. Shear bond strength test
3. Results
Specimens were submitted to shear bond strength test in a universal testing machine (Kratos IKCL3-USB, Kratos Industrial Equipment, Cotia, SP, BR) with the loading device placed to make
Mean bond strength values and standard deviations (MPa) are summarized in Table 2. Three-way ANOVA indicated statistically significant differences for two of the main factors, i.e., “adhesive
Fig. 2. (A) Shear device of SDI; (B) metal cylinder and acrylic pin; (C) shear device mounted; and (D) specimen ready to shear bond test.
Table 2 Means of bond strength (MPa) and standard deviations for the experimental groups. No erosive challenge
Adper Single Bond 2 Adper SE Plus
Erosive challenge
Control
Nd:YAG laser irradiation
Control
Nd:YAG laser irradiation
G5 - 19.0 7 0.6a,b G1 - 15.0 7 0.4c,d
G6 - 18.7 71.0a,b G2 - 13.5 70.9d,e
G7 - 17.1 71.0b,c G3 - 11.4 71.5e
G8 - 21.3 7 1.3a G4 - 16.6 7 3.7b,c,d
*Different lowercases indicate statistically significant difference to interaction adhesive system*laser irradiation*treatment of restorative procedure.
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Fig. 3. Failure modes (percent) for experimental groups (SB: Adper Single Bond 2; SE: Adper SE Plus; Nd: Nd:YAG laser irradiation; Ec: Erosive challenge).
system” (p o0.001) and “laser irradiation” (p ¼ 0.001). The following interactions were also statistically significant: “laser irradiation*erosive challenge” (p o0.001) and “adhesive system*laser irradiation*erosive challenge” (p¼ 0.032). The factor “erosive challenge” and the other interactions were not statistically significant (p 40.05). According to Table 2, the etch-and-rinse adhesive system (Adper Single Bond 2) (G5–G8) showed significantly higher bond strength values than those obtained for the self-etch adhesive (G1–G4), regardless of the presence of laser irradiation and erosive challenge. Regarding to the effect of the erosive challenge on bond strength, it was possible to note a significant deleterious effect on bond strength only for the control condition of Adper SE Plus (G1– G3). Laser irradiation resulted in increased mean bond strength in comparison to the control group for both adhesive systems only in the presence of erosive challenge (G1–G4, G5–G8). Fig. 3 shows the prevalence of failure modes observed for each experimental group. Adhesive failures were the most frequently identified failure types for all groups. No cohesive failures in dentin were observed.
4. Discussion Nd:YAG laser irradiation has been suggested as an option to hypersensitive dentin treatment. However, the influence of subsequent erosive challenge on the bond strength to dentin submitted to this type of laser application has not been reported yet. In our study, Nd:YAG laser irradiation improved the resin–dentin bonds for both adhesives when the erosive challenge was present, in comparison to control group. Therefore, the study hypothesis had to be rejected. The better bond strength results found for laser irradiated dentin can be explained by the fact that dentin irradiated with Nd: YAG laser is more resistant to the dissolution promoted by the erosive challenge [24-26]. This acid-resistant dentin probably protects the resin–dentin interface from the continuous erosive challenge that occurs after the restorative procedure. The interaction between dentin and laser leads to surface melting, reducing the tubular diameter or obstructing the dentin tubules and sometimes could cause some protein denaturation. Furthermore, it has been hypothesized that the temperature increase that occurs after laser irradiation of the dentinal substrate results in chemical and structural changes, leading to loss of water and carbonate [27,28]. This reduction in carbonate content leads to fluoride incorporation into apatite and, after repeated dissolution events, the dentin substrate becomes more resistant to new acid
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challenge episodes [29,30]. Thus, it is likely that a similar phenomenon took place in the adhesive interface around the resin cylinders used in the present investigation, protecting the resin– dentin bonds from degradation during the erosive challenge. This finding corroborates with results from previous study that showed a better behavior of adhesive systems in dentin submitted to erosive challenge when laser irradiation was performed [31]. However, this study used Er,Cr:YSGG laser, which can result in a different dentin surface after laser application than that obtained with Nd:YAG irradiation [31]. One important fact that should be taken into account to understand the effect of laser application on the specimens submitted to erosive challenge is the decrease in bond strength observed for the control groups of both adhesives after submitting them to the erosive challenge, though only for Adper SE Plus such decrease was statistically significant. These lower values observed for the control groups after erosive challenge corroborate with a previous study [32] and is a consequence of the deterioration of resin–dentin interface in the erosive environment. The erosive challenge performed in this investigation created an outer zone of demineralized dentin and, as a consequence, exposed collagen fibrils and weakened the adhesive interface. The better performance of the etch-and-rinse adhesive system is in accordance with previous studies that showed higher bond strength values for this system in comparison to self-etch adhesives. However, it is important to consider that these studies were performed in adhesive interface without subsequent erosive challenge [33,34]. It has been demonstrated that the relatively high bond strength values obtained for etch-and-rinse adhesives are related to better micro-mechanical interlocking to dentin surfaces when phosphoric acid is applied, compared to the acidic monomers found in self-etch adhesive compositions [35]. In fact, even the decreased thickness of the weak dentin zone without resin infiltration obtained with the self-etch approach did not prevent the inferior performance observed for this type of adhesive in the current investigation. Such negative behavior can be explained by the combination of the high amount of water ( 80%) with the HEMA molecules found in the composition of self-etch adhesives. The association of these two components (waterþHEMA) results in lower rates of water evaporation with subsequent material phase separation [34]. Furthermore, these satisfactory results obtained with etch-and-rinse adhesive system in dentin corroborate a previous study that showed that this adhesive system approach results in better performance than other restorative materials, such as glass ionomer cements when applied in eroded dentin [36,37]. On the other hand, when another laser was applied, a better performance was reported for a two-step self-etch adhesive system when compared to an etch-and-rinse system [31]. The difference between these findings can be related to the type of self-etch adhesive system used, since in earlier works a MDP-containing adhesive system was used [31], which associates micromechanical interlocking with a possible chemical interaction with the calcium in hydroxyapatite crystals that can lead to the higher bond strength values [38]. The shear bond strength test carried out in this investigation was based in a recently developed and standardized device. Usually, the shear bond strength test is performed in bonding cross-sectional areas of up to approximately 1 mm2. Such test is called microshear bond strength test and has the advantage of making possible the construction of several specimens in the same tooth. Moreover, the greatest advantage compared to microtensile test is producing less stress during specimen production, since there is no need of slicing the specimens [39]. Placido et al. showed, by finite element analysis, that the microshear test can result in more heterogeneous stress concentrations than conventional shear test [40]. Thus, in an attempt to find a test with less
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disadvantages and more reliable results of bond strength, we chose the shear test using this new device. Also, the shear bond strength test seems to better simulate the adhesive interface exposed in cervical lesions, unlike what happens with microtensile specimens which leave the resin–dentin interface exposed to water and erosive challenge action, resulting in faster degradation. Further studies still need to be conducted to evaluate the longterm behavior of restorative procedures in dentin submitted to Nd: YAG laser irradiation and continuous erosive challenge, since both adhesive systems evaluated here can suffer degradation resulting of the hydrolysis of resin matrices by esterase [41] and collagen matrices by host-derived enzymes [42]. In fact, there is no data available about the influence of this type of laser irradiation on the bond stability of adhesive systems.
5. Conclusions Based on the results of the present investigation, it was possible to conclude that, for both adhesives systems evaluated, the irradiation of Nd:YAG laser only had a positive effect on bond strength when the erosive challenge was present. The importance of this finding is related to the fact that laser irradiation can be considered as an alternative treatment before restorative procedures are subjected to erosive challenge. Another conclusion of this work is that the etch-and-rinse adhesive resulted in superior performance than self-etch system in this clinical condition.
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