Dentin pretreatment and adhesive temperature as affecting factors on bond strength of a universal adhesive system

Abstract Objectives: To evaluate the effects of dentin pretreatment and temperature on the bond strength of a universal adhesive system to dentin. Material and Methods: Ninety-six extracted non-carious human third molars were randomly divided into 12 groups (n=8) according to Scotchbond Universal Adhesive (SbU) applied in self-etch (SE) and etch-and-rinse (ER) mode, adhesive temperature (20°C or 37°C) and sodium bicarbonate or aluminum oxide air abrasion. After composite build up, bonded sticks with cross-sectional area of 1 mm2 were obtained to evaluate the microtensile bond strength (μTBS). The specimens were tested at a crosshead speed of 0.5 mm/min on a testing machine until failure. Fractured specimens were analyzed under stereomicroscope to determine the failure patterns in adhesive, cohesive (dentin or resin) and mixed fractures. The microtensile bond strength data was analyzed using two-way ANOVA and Tukey's test (α=5%). Results: Interaction between treatment and temperature was statistically significant for SbU applied in self-etch technique. Both dentin treatments showed higher bond strength for ER mode, regardless of adhesive temperature. When compared to control group, sodium bicarbonate increased bond strength of SbU in SE technique. Adhesive temperature did not significantly affect the μTBS of tested groups. Predominantly, adhesive failure was observed for all groups. Conclusions: Dentin surface treatment with sodium bicarbonate air abrasion improves bond strength of SbU, irrespective of adhesive application mode, which makes this approach an alternative to increase adhesive performance of Scotchbond Universal Adhesive to dentin.


Introduction
With the development and improvement of aesthetic restorative materials, adhesive systems have become essential elements in various clinical applications. Adhesive systems are responsible for the bonding of restorative material to dental structures.
Thus, the longevity of adhesive restoration is directly associated with the effectiveness of adhesive systems 5 .
The adhesion of composite resins to the dentin substrate is based on smear layer treatment 5 . While some adhesive systems require the conditioning of dentin with phosphoric acid, as etch-and-rinse systems (ER), others preserve the smear layer by incorporating it into the adhesive layer, they are the self-etch systems (SE). Self-etch systems use an acidic primer, which can be separated from the adhesive or can be all-in-one, in which the primer and adhesive are present in a single bottle 5 . New universal or multi-mode adhesives are single-step self-etch adhesives that can be applied both to the ER and SE techniques 12,21 .
The ER technique is more sensitive, as it is susceptible to operator error and can affect the condition of the substrate, since this technique can lead to the collapse of the collagen network due to the rinsing-drying steps. In addition, incomplete resin 17 . Thus, self-etching primer systems have been developed to minimize the sensitivity of the technique and reduce the time of clinical application 8,17 . However, some failures of restoration using this type of adhesive can be associated with the inability of the adhesive to penetrate correctly into the smear layer and reach the underlying dentin 27,30 .
Dentin adhesion depends not only on the successful permeation of the adhesive into the dental substrate, but also on the mechanical properties and quality of the polymer formed. Thus, the solvents and the water present in the adhesive should ideally be removed inhibit the polymerization and negatively affect the mechanical properties of the adhesive 16,26 . Hence, a clinical approach to improve the adhesive properties indicates the use of warm air 9, 13,16,19,20,26 to facilitate the evaporation of the solvent as well as the use of preheated adhesive systems 11,18,25,28 in order to increase the speed of permeation of the monomers and the evaporation of the residual monomeric content. The quality and the stability of the hybrid layer can be stimulation of their chemical reactions and, thus, some properties of the monomeric solutions, such as degree of monomer conversion and viscosity, can be changed by temperature 1, 11,18,23,28 . Temperature the polymerization of the adhesive by increasing the degree of conversion 1 , besides decreasing the viscosity and improving the speed of spreading of the adhesive and deeper penetration into dentin 11,18,25,28 .
Although new adhesive systems should be developed in order to improve performance, we can use different dentin clinical manipulation techniques to increase the bonding and, consequently, long-term clinical success. Alternatively, the use of some dentin pretreatments such air abrasion techniques may improve the bonding strength, through mechanical 8 .
Abrasion with aluminum oxide particles has been used to increase the bonding of metallic surfaces to resinous materials, to prepare and clean the cavity, and to remove decayed tissue and faulty restorations 6,24 .
Moreover, abrasion with sodium bicarbonate has been used for prophylaxis of the dentinal surface and removal of plaque and debris formed during cavity preparation. Sodium bicarbonate abrasion is superior to other cleaning methods, such as the rubber cups, during deeper cavity preparations 3,22 . However, air abrasion has also been used in dentin pretreatment, because it increases the surface roughness and the area available for adhesion and thus improves the interfacial contact between the dentin and the

Abrasive and restorative procedures
In the groups that received surface treatments, the dentin was air-abraded with Laxis Sonic BP (Schuster; São Paulo, SP, Brazil) sodium bicarbonate jet (15-300 μm) for 15 seconds, at a distance of 5 mm, a pressure of 60 psi, and an angle of 90° between the jet and dentin. The dentinal surface received an air/water spray jet for 30 seconds and was dried with absorbent paper. In another treatment, the dentinal surface was     Analysis of the fracture pattern presented predominantly adhesive failures for all groups ( Figure   4a). However, the four types of failures were observed in all groups (Figure 4). Descriptive results are presented in Table 2.

Discussion
Universal dental adhesives were developed mainly with regard to dentin; with the aim of simplifying the clinical steps and reducing the sensitivity of clinical techniques. Due to their versatility, they may be used with the etch-and-rinse and self-etch techniques 10,29 .
As they are a class of recent and little studied adhesives, alternative approaches, such as dentinal pre-treatment 5,8 and pre-heating of the adhesive 11,18 can be carried out to improve adhesive properties.
Cleaning of the dentinal surface with sodium bicarbonate air-powder polishing before adhesive procedures is a very common technique and aims to remove plaque and debris present in the cavity, 3,15 . In addition, acid conditioning removes the smear layer and can remove aluminum oxide particles left on the dentinal surface, thus exposing the dentinal into the dentin and enhancing resin tag formation 5,6,8 .
Dentin pretreatment and adhesive temperature as affecting factors on bond strength of a universal adhesive system Temperature influences some properties of monomers by decreasing the viscosity and by increasing the spreading speed of the adhesive and its deeper penetration into dentin, besides increasing the degree of conversion of monomers, which 11,18,25,28 . In this study, however, pre-heating of the adhesive did not associated with aluminum oxide groups; thus, the second hypothesis tested was partially accepted. This on an adhesive based on ethanol/water, similar to the SbU tested in this study 11,13 . Other studies showed that higher immediate bond strength was associated with the highest temperatures for enamel (40°C) 1 and dentin (50°C) 18 ; however, the adhesive systems that rinse adhesives, thus being different from the SbU.
Another study showed that the heated adhesive (40°C) increased the bond strength for the etch-and-rinse 25 .
The stereomicroscopy analysis revealed a predominance of adhesive failures for all experimental showed an adhesive fracture pattern 11,12,29 . However, other studies showed a pattern of different fractures, predominantly cohesive 6 and mixed failures 22 .
While adhesive failures microscopically represent a rupture in the interface between the resin and dentin characterized by an opening of the dentinal tubules, cohesive failures indicate that the hybrid layer is intact 6 .
Within the limitations of an in vitro study, our alternative approaches prior to adhesive procedures, for example, pretreatment with sodium bicarbonate air abrasion, in order to improve the performance of the Scotchbond Universal adhesive system. More approaches on the adhesive effectiveness of universal adhesive systems in the short and long term.

Conclusion
Pretreatment of dentin with sodium bicarbonate air abrasion increases the bond strength of universal adhesive, regardless of whether the etch-and-rinse or the self-etch technique is used. Treatment of dentin using abrasion with aluminum oxide particles rinse application mode. Pre-heating of the adhesive