Rubbing time and bonding performance of one-step adhesives to primary enamel and dentin

Abstract Objectives: This study investigated whether increasing the concentration of acidic monomers in one-step adhesives would allow reducing their application time without interfering with the bonding ability to primary enamel and dentin. Material and methods: Experimental one-step self-etch adhesives were formulated with 5 wt% (AD5), 20 wt% (AD20), or 35 wt% (AD35) acidic monomer. The adhesives were applied using rubbing motion for 5, 10, or 20 s. Bond strengths to primary enamel and dentin were tested under shear stress. A commercial etch-and-rinse adhesive (Single Bond 2; 3M ESPE) served as reference. Scanning electron microscopy was used to observe the morphology of bonded interfaces. Data were analysed at p<0.05. Results: In enamel, AD35 had higher bond strength when rubbed for at least 10 s, while application for 5 s generated lower bond strength. In dentin, increased acidic monomer improved bonding only for 20 s rubbing time. The etch-and-rinse adhesive yielded higher bond strength to enamel and similar bonding to dentin as compared with the self-etch adhesives. The adhesive layer was thicker and more irregular for the etch-and-rinse material, with no appreciable differences among the self-etch systems. Conclusion: Overall, increasing the acidic monomer concentration only led to an increase in bond strength to enamel when the rubbing time was at least 10 s. In dentin, despite the increase in bond strength with longer rubbing times, the results favoured the experimental adhesives compared to the conventional adhesive. Reduced rubbing time of self-etch adhesives should be avoided in the clinical setup.


Introduction
Adhesive materials have been increasingly used to prevent and treat dental caries. However, application time, technical complexity 24 and unpleasant taste 3 sometimes are complicating factors for the use of adhesive systems in paediatric dentistry. Self-etch adhesives have been recommended as an alternative to reduce such problems 9 . To be considered ideal, an adhesive system needs, among other features, to be easy to use and to have minimal technical sensitivity 24 .
The use of single-step self-etch adhesive systems can save clinical time 9 and reduce the discrepancy commonly associated with etch-and-rinse adhesives 24 .
Other advantages of using self-etch adhesives in paediatric dentistry include the fact that the technique does not involve washing and moisture control of the dentin, which are additional steps required in the conventional technique 9 .
Self-etch adhesive systems are composed of hydrophilic monomers, represented mostly by the monomer 2-hydroxyethyl methacrylate (HEMA) and hydrophobic co-monomers, in addition to acidic monomers. The solvent component usually combines water, which is necessary for ionization of the acidic monomers, with ethanol or acetone as co-solvents to increase the vapour pressure of the mixture, which becomes an azeotrope and facilitates evaporation of residual water 17 . The acidic monomer is responsible for etching the dental substrate, creating retention and promoting bonding. It has been shown that the concentrations of acidic monomer and water have of the bond to enamel or dentin in permanent teeth 16 .
Regarding the method of application of selfetch adhesive systems, it is known that their active application (i.e. with rubbing motion) increases the bond strength and interactions with enamel 5 and dentin 2,12 . The bonding process involves the removal of calcium phosphate from both the enamel and the dentin, which creates surface micropores. These micropores allow the formation of an interdiffusion zone between the enamel and the hybrid layer of the dentin 21 Table 1. The adhesives were prepared using two distinct bottles (A and B), which were mixed before application.
The concentration of acidic monomer in the mixed adhesives was 5 wt%, 20 wt%, and 35 wt%, thus the materials were labelled as AD5, AD20, and AD35. The pH of the mixed adhesives (n=3) was measured using Preto, SP, Brazil). The formulations were based on a previous investigation 6  The adhesives were actively applied (with rubbing motion) to enamel surfaces using microbrushes for the corresponding time for each group. The solvent was evaporated for 10 s with compressed air. After testing the enamel surfaces and classifying the failure modes, the same teeth were further wet-polished with 600-grit SiC abrasive papers until medium dentin was exposed. The dentin specimens were randomly divided again into groups, and the adhesives were applied the same way described for enamel. Two additional groups were obtained, testing a conventional etch-and-rinse,  In case of premature failure, the hemisection was eliminated and replaced by a new specimen.

Statistical analysis
Bond strength data were subjected to a two-   Table 2.

Adhesive failures predominated in both
Rubbing time and bonding performance of one-step adhesives to primary enamel and dentin substrates. While only a few failures in enamel were group, failures in dentin were mostly mixed. Failure modes were not influenced by either the acidic monomer concentration or the rubbing time.
SEM images of the bonded interfaces of groups presented in Figure 3 are shown in Figure 4 (enamel) and Figure 5 (dentin). The differences are not appreciable among the experimental materials with distinct acidic monomer concentrations. The adhesive layer was thicker ( Figure 4D and Figure 5D) and more irregular ( Figure 5D) for the commercial etch-andrinse compared with the self-etch adhesives. More resin tags in dentin seemed to be formed for AD35 ( Figure 5C) compared with the other experimental adhesives, but no other clear differences in interfacial morphology were noticed.   Alteration in the water concentration of self-etch adhesives may be enough to increase the etching aggressiveness of primary enamel 13 . In primary teeth, the minimum concentration of water needed to cause study was 20% 13 . Higher water concentration may hinder its elimination by evaporation 7 . Another study for adequate ionization of acidic monomers but the concentration of the monomers cannot be altered to 14 . In this study, the concentrations of water and solvent were standardized, and the concentration of GDMA-P was altered by reducing the HEMA content. Dental adhesive materials are increasingly common in dentistry, whether to prevent caries or to restore carious lesions and fractures. The basic adhesion mechanism to dentin or enamel for either primary or permanent teeth is based on an exchange of substances in which the minerals in the hard tissues are replaced by resinous monomers present in adhesives that bond micromechanically to the porosities created by the acid material. The proportions of minerals are different between primary and permanent teeth, as are the depths of the dentin and enamel. These two substrates have important differences that cannot be overlooked. The use of adhesive materials should consider all of these factors. In paediatric dentistry, the child's age must also be acknowledged. Therefore, the development of an adhesive material that addresses all of these factors needs to be undertaken because, hard tissue trauma is highly prevalent. In both of these cases, adhesive restorations are common.