Influence of voids in the hybrid layer based on self-etching adhesive systems: a 3-D FE analysis

ABSTRACT The presence of porosities at the dentin/adhesive interface has been observed with the use of new generation dentin bonding systems. These porosities tend to contradict the concept that etching and hybridization processes occur equally and simultaneously. Therefore, the aim of this study was to evaluate the micromechanical behavior of the hybrid layer (HL) with voids based on a self-etching adhesive system using 3-D finite element (FE) analysis. Material and Methods: Three Fe models (Mr) were built: Mr, dentin specimen (41x41x82 μm) with a regular and perfect (i.e. pore-free) HL based on a self-etching adhesive system, restored with composite resin; Mp, similar to M, but containing 25% (v/v) voids in the HL; Mpp, similar to Mr, but containing 50% (v/v) voids in the HL. A tensile load (0.03N) was applied on top of the composite resin. The stress field was obtained by using Ansys Workbench 10.0. The nodes of the base of the specimen were constrained in the x, y and z axes. The maximum principal stress (σmax) was obtained for all structures at the dentin/adhesive interface. Results: The Mpp showed the highest peak of σmax in the HL (32.2 MPa), followed by Mp (30 MPa) and Mr (28.4 MPa). The stress concentration in the peritubular dentin was high in all models (120 MPa). All other structures positioned far from voids showed similar increase of stress. Conclusion: Voids incorporated into the HL raised the σmax in this region by 13.5%. This behavior might be responsible for lower bond strengths of self-etching and single-bottle adhesives, as reported in the literature.


INTRODUCTION
The structural behavior of the adhesive layer plays an important role in maintaining the integrity of the dentin-resin bond over time 1 . Self-etching adhesive systems have been introduced to allow dry bonding based on shallower demineralization with the formation of thinner hybridization of dentin. Consequently, a more homogenous dentin/adhesive interface is expected to be recreated 5 . Self-etching adhesives also reduce the steps necessary for bonding in comparison with etch-and-rinse adhesives.
Because many self-etching adhesives leave the bottom of smear plugs intact, they tend to create resin-dentin bonds that exhibit less dentin sensitivity 5,6 .
Although self-etching adhesives are used in order to form a stable and strong biopolymer 18 , lower bond strength has been reported 4 mainly www.scielo.br/jaos in wet enviroments 14 . Unfortunately, the benefit of saving time by using self-etching adhesives may impair the quality of resin-dentin bonds (e.g. incomplete sealing) 6 .
The complex environment of hybrid layers (HL) created by self-etching adhesives may explain their reduced performance 30 . For instance, nano and micro analysis of one-bottle adhesives show the presence of high concentrations of hydrophilic acid monomers as being responsible for the incorporation of voids in these hybrid layers that, in turn, increase their permeability 24,29 . In these zones, water is incompletely removed, resulting in regions of imperfect polymerization and/or hydrogel formationbetween the remaining water and the HeMA present in the adhesive systems 7,25 .Therefore, partial hybridization of dentin may occur with more aggressive selfetching adhesives (lower pH). The presence of these defects may act as stress raisers 19 in resindentin interfaces, reducing adhesion over time.
A previous study provided data about bond strength and the characteristics of the adhesive interface obtained using scanning electron microscope (SeM) and nanoleakage 9-.
However, little information is available about the mechanical behavior of the dentin/adhesive interface based on hybrid layer quality 2,15,19 .
The aim of this study was to evaluate the mechanic behavior of the hybrid layer containing voids, using 3-D finite element analysis. The voids were incorporated in different proportions (25% and 50% by volume). The null hypothesis is that voids have no effect on stresses within the hybrid layer.

MATERIAL AND METhODS
In order to perform the micromechanical analysis of dentin/adhesive interfaces, a virtual dentin specimen restored with composite resin 18 (41 x 41 x 82 µm) was built using SolidWorks

RESULTS
The peak of σ max was observed in the peritubular dentin ( Figure 3) for all models

DISCUSSION
While a durable seal between several current bonding systems and enamel has been achieved, it is still a challenge to seal the resin-dentin interface, due to the heterogeneous characteristic of the dentin structure, surface morphology 10 , and/or intrinsic shortcomings of the design of these modern adhesives 21 .
Conventional thought is that a perfect seal along the resin-dentin interface can be achieved within the demineralized collagen matrix when Consequently, in order to evaluate the influence of the HL voids on the stress distribution at dentin/adhesive interfaces in this study, the bond established between the adhesive system and dentin was either considered ideal or incorporated with voids in different contents (25% and 50%).
It was observed that the peritubular dentin showed the highest stresses in all models, in accordance with previous studies 2, 3 . The next structures to bear high stress were the resin tags, HL, adhesive layer and intertubular dentin.  indicates that the bottom of the adhesive layer might not be the site to start up failure when the bond between dentin and HL is porous. The peak of σ max in the Mr was in the HL close to the peritubular dentin, very similar to that previously found in a perfect bond scenario 2,3 .
Mollica, et al. 19 found that the presence of voids raised the σ max by 3.7 times. In our study, the influence of voids was able to raise the σ max only 13.5% ( Figure 5). The study of Mollica, et al. 19  tags, in contact with dentinal tubule walls ( Figure   6). Thus, our hypothesis can be accepted, as the σ max was higher in the presence of voids. Further studies on the porous dentin/adhesive interface should be carried out, considering different degrees of bonding between the HL and the adhesive layer, as well as the intertubular dentin.

CONCLUSIONS
Within the limitations of the present study, we can conclude that: -The presence of voids in the hybrid layer raised the maximum principal stress in all structures of the dentin/adhesive interface; -The increase of void content has some influence on stress. The 50% void content was able to raise the stress by 13.5% inside the HL; -In the presence of voids, the maximum stress moved from the peritubular dentin to the HL in contact with the voids.