| Abstract|| |
Aim: The aim of this in vitro study was to evaluate the influence of dentin surfaces with different tubule orientation on bond strength of a multimode adhesive system.
Materials and Methods: Twenty human molars were selected and prepared in different ways in order to produce different dentin regions. The teeth were randomized (n = 5) according to the application modes of a multimode adhesive system (etch-and-rinse and self-etching) and to the dentin region (occlusal and proximal - 1.5 mm depth from dentinoenamel junction). Cylindrical restorations were performed on dentin with a starch tube. The samples remained immersed in distilled water for 24 h and the microshear bond strength (μSBS) test was performed. The μSBS values were expressed in MPa and analyzed with ANOVA and Tukey's test (P < 0.05).
Results: There was no significant difference in the bond strength values between the application modes of the adhesive system (P = 0.19); however, the dentin regions (P < 0.05) significantly affected the μSBS. The proximal dentin presented higher bond strength values than occlusal dentin.
Conclusion: The dentin surfaces with different tubule orientation influenced the bonding effectiveness of the adhesive system tested.
Keywords: Adhesive system, bond strength, composite resin, dentin, microshear
|How to cite this article:|
Montagner AF, Carvalho MP, Susin AH. Microshear bonding effectiveness of different dentin regions. Indian J Dent Res 2015;26:131-5
The concept of hybrid layer formation is widely accepted by researchers all over the world. It is basically obtained by the replacement of minerals removed from dental hard tissues by hydrophilic and hydrophobic monomers, in such a way that a polymer becomes micro-mechanically retained into the tooth substrates. 
|How to cite this URL:|
Montagner AF, Carvalho MP, Susin AH. Microshear bonding effectiveness of different dentin regions. Indian J Dent Res [serial online] 2015 [cited 2019 Oct 14];26:131-5. Available from: http://www.ijdr.in/text.asp?2015/26/2/131/159137
Adhesive systems have been launched to offer bonding procedures faster, safer, and friendly handling to the practitioners, although histologic differences presented by the different hard tissues in the tooth may require extra care by the operator during the bonding technique. Enamel and dentin have the chemical composition and histological differences, mainly concerning the characteristics and disposition of each structure perceived according to the region of the tooth. 
Multi-mode or universal one-bottle adhesives can be used as self-etch or etch-and-rinse. The manufacturers suggest that those adhesive may also be used with selective enamel etching. Its composition is an important factor to be taken account, since most of these adhesive contain 10-methacryloyloxydecyl dihydrogen phosphate (MDP), a monomer that can chemically adhere with hydroxyapatite (HAp), and could influence the bonding effectiveness. 
Regional variations in enamel characteristics (aprismatic enamel structure) or its absence on cervical margins may be responsible for lower bond strengths values , and consequently precipitate failure of adhesive procedures long time. Dentin tubules disposed perpendicularly to the long axis of the tooth are mainly presented in cervical margins in class V cavities  as well as on the axial wall of class II cavities. It was reported that the orientation of the dentin tubules influences the presence of resin tags and hybrid layer formation, which presents thicker in regions with perpendicular tubule orientation than in regions with parallel tubule orientation, where resin tag formation is not possible. ,,
Class V lesions are common injuries that require frequent adhesive restorations irrespective of their different etiology or diagnosis. , Defects in marginal adaptation have been shown in the cervical margins, and they are much more expressive considering that dentinal tubule orientation on this area is distinctive from that on the pulpal wall. ,
Based on those aspects, the adhesive performance of the adhesives systems could be not uniform inside the cavity walls. Thus, the purpose of this study was to evaluate the influence of dentin surfaces with different tubule orientation on bond strength of a multimode adhesive system.
| Materials and Methods|| |
This in vitro study was approved by the Local Ethics Committee in Research (protocol 23081.010339/2008-22). Twenty extracted caries-free human third molars were used in the current study. The teeth were cleaned and stored in 0.5% chloramine-T solution, at 4°C for up to 30 days. Then the teeth were stored in distilled water at the same temperature until use in this study.
The root portions of the teeth were removed, and the crowns were sectioned in two different ways in order to obtain proximal and occlusal dentin substrates. The proximal substrate was obtained from a longitudinal section of the distal or mesial third of the crown, exposing the axial dentin at 1.5 mm from dentinoenamel junction. The occlusal substrate was prepared with a complete cross-section at the middle third of the crown, exposing the occlusal dentin at the same distance of the longitudinal section. The cuts were carried out using a low-speed water-cooled diamond saw in a cutting machine (Labcut 1010, Extec Co, Enfield, CT, USA). The sections were embedded using acrylic resin (JET Clássico ® , São Paulo, SP, Brazil) in PVC cylinders of 20.0 mm in height and 25.0 mm in diameter.
After the acrylic resin had completed cured, the samples were taken to a circular mechanical polishing machine with 600-grit SiC paper, to level the acrylic resin and prepare a standardized smear layer on the dentin surface. The samples were evaluated in a stereomicroscope to confirm the nonexposure of the pulp chamber and the absence of "islands" of enamel; if necessary, the remaining enamel was removed with 180-grit SiC paper for 60 s in a polishing machine (Arotec PL 4, São Paulo, SP, Brazil) under cooling.
The samples were randomly divided into four groups (n = 5) according to the mode of application of the adhesive system (etch-and-rinse and self-etching) and regarding the dentin regions (proximal and occlusal). The materials, composition, batch number, and manufacturers' instructions for the materials are summarized in [Table 1]. The multimode adhesive system single bond universal (SBU) (3M ESPE, St Paul, MN, USA) was applied on dentin following both application modes and the manufacturer's instructions.
Starch tubing (Renata, Pastifício Selmi, Londrina, PR, Brazil) was used as a tube to build up the cylindrical composite resin restorations for microshear specimens. The technique is described elsewhere (Tedesco et al., 2013). After the adhesive system application following the manufacturer's instructions, the starch tubes were positioned on the dentin surfaces prior to light curing. Three tubes were placed over in each bonded surface. The starch tubes have a 0.96 mm internal diameter, respectively, and were prepared in 1.0 mm high. After light-curing with a light emitting diode (Shuster Light Emitting - Shuster Ind Com, Santa Maria, Brazil) with ≈ 800 mW/cm 2 light output, the tubes were carefully filled with a composite resin (Shade A2, Filtek Z250, 3M/ESPE, St. Paul, MN, USA), and light-activated for 20 s. All adhesive procedures and restorations were carried out by a previously trained operator at a room temperature of 24°C. [Figure 1] presents a schematic design of the study.
The specimens were stored in distilled water at 37°C for 24 h. After this period, the tubes were carefully removed with a gentle water stream. The specimens were examined using a stereomicroscope at ×10 (Discovery V20, Zeiss, Berlin, Germany) for bonding defects. Specimens with interfacial gaps, bubble inclusions, or other defects were eliminated from the test and replaced.
The specimens were placed in a jig attached to a universal testing machine (EMIC DL 1000, Equipamentos e Sistemas de Ensaio Ltda., São José dos Pinhais, PR, Brazil). A thin stainless steel wire loop (0.20 mm in diameter) was placed flush against enamel surface in contact with the lower half-circle of the cylinder. A shear load was applied at a crosshead speed of 1.0 mm/min until failure occurred. Care was taken to keep the composite cylinder in line with the center of the load cell and to keep the wire loop parallel to the load cell movement direction and to the bonding interface.
The microshear bond strength (μSBS) was expressed in MPa as derived from the division of the imposed force (N) at the time of fracture by the bonded area (mm 2 ). The software embedded in the machine performed this calculation automatically.
The μSBS values (MPa) data were subjected to a Levene test to evaluate homogeneity of variances, and then analyzed using two-way ANOVA (tested variables: Dentin region and application mode of the adhesive system) followed by a post-hoc Tukey test. For premature failures, the value fixed was "zero." The tests were conducted using a statistical software package Statistical Package for Social Sciences (SPSS, version 20.0, Chicago, IL, USA) and the statistical significance was set at P < 0.05.
| Results|| |
The μSBS values are summarized in [Table 2]. There was no statistically significant difference between μSBS values for the application's mode of the adhesive system (ANOVA, P = 0.19); however, there was statistically difference for dentin regions (ANOVA, P < 0.05). There was no interaction between application mode and dentin region (ANOVA, P = 0.21). The axial dentin presented higher bond strength values than occlusal dentin for both application modes of the multimode adhesive system [Table 2].
| Discussion|| |
The findings of the present study showed no difference between the two different approaches for the multi-mode adhesive used in this methodology. Both etch-and-rinse and self-etching techniques did not present any statistical difference between each other. These results corroborate previous studies , indicating that the technique does not influence the dentin bond strength of multipurpose adhesive system, as the addition of an extra step (etching step) did not significantly affected the dentin bond strength when compared to the self-etch application mode.
The dentin region proved to be an important effect factor on bond strength values indicating that the adhesive performance may not be similar inside the cavity walls. Some discrepancies on bond strength may be explained by the influence of tubule orientation and depth of dentin, positively affecting monomeric infiltration when the tubules are wider, as on the depth dentin and perpendicularly configured, as on the pulpal wall. The proximal surface showed bond strength values higher than the occlusal surface. Dentinal tubules direction is an important variable in determining the dentin bond strength. , Dentin is a hydrated biological complex with microstructural design, which can represent one of the reasons why the bond strength is not uniform along all the dentin surfaces.
The present study confirmed that the different sites of substrate localization are important factors in the adhesive performance. Further differences regarding the substrate regions (occlusal × proximal) there are other factors that are able to influence the mechanical properties of the adhesive systems, as for instance on occlusal dentin the bond strength decreases 50% in deep dentin when compared to superficial. 
Several structural components and properties of dentin can directly affect the adhesion to dentin. The difference in the bond strength values of dentin surface tends to decrease when the smear layer is left intact since this layer is able to decrease the permeability.  However, in the present study, this factor did not affect the dentin-resin adhesion.
The adhesive system used in the present study showed similar performance irrespective of the mode of application or dentin region. It can be used either with an etch-and-rinse or with a self-etching technique, which has been considering a simple and easy technique with only one-coat/step application. This fact could be explained by the composition, which contains the MDP monomer that has been attributed to be the reason for the enhanced adhesive performance when compared to adhesives without the possibility to promote potential chemical interaction with HAp-based substrates. These adhesives are called as multimode adhesives, and there are presenting adequate performance in both laboratory , and clinical  studies using both bonding strategy.
Other factor that could explain the suitable performance of the adhesive tested is the presence of the polyalkenoic acid copolymer (Vitrebond Copolymer), which in combination with MDP has shown some interesting results in the literature. Perdigão et al. reported higher dentin bond strength of SBU when compared to Clearfil SE bond, which has only the MDP monomer.  On the other hand, Muñoz et al. reported a lower dentin bond strength of SBU when compared to the Clearfil SE bond. It could be explained because the complete of polyalkenoic acid copolymer with the MDP monomer for Ca-bonding sites in HAp and due to its high molecular weight, which could prevent monomer approximation during polymerization. 
In the present study, the microshear evaluates the dentin bond strength comparing different regions (occlusal dentin and proximal dentin) and different application modes (with acid application and without acid application) of a multipurpose adhesive system. The results show that the adhesive performance is independent of the technique and reported good dentin bond strength independent of the region tested, as there was no interaction between application mode and dentin region. Based on that, it is important to consider the self-etch technique more appropriate to treat this substrate, since this technique is less sensitive than the etch-and-rinse, where the dentist could cause an aggression on the substrate by phosphoric acid application directly on the dentin substrate. Other advantage of the self-etch technique is the less time for application since it is necessary fewer steps when compared with the etch-and-rinse technique.
Results of in vitro studies evaluating the bond strength of the SBU have shown promising results, which has been clinically confirmed.  This has led researchers to continue studying about MDP and other monomers that promote a chemical bond with HAp without requiring the use of acid etching of dentin surface. Yoshihara et al.  2014 tested a similar monomer to MDP but with better results regarding the bond strength. The monomer 6-methacryloxy-2, 2, 3, 3, 4, 4, 5, 5-octafluorohexyl dihydrogen phosphate also presented chemical adhesion to HAp, maintaining hydrophobic properties, such as the MDP. Thus, further studies should be focused on this aspect.
Based on the finding of this study, it can be concluded that the bond strength is region dependent on microshear tests; however, application mode did not affect it. It seems that the self-etch application mode is a tendency on adhesive dentistry, mainly in dentin substrate, where you can use an easier, safer, and quicker technique.
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Prof. Alexandre H Susin
Department of Restorative Dentistry, Federal University of Santa Maria, Santa Maria, RS
Source of Support: None, Conflict of Interest: None
[Table 1], [Table 2]