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Introduction

Resin-composites have become very popular; however, some problems could restrict the clinical success of bonded restorations. Interfacial failure has been pointed out as the main problem due to bonding sensibility and shrinkage stress . Interfacial quality seems to be particularly related to the failure in the bonding to dentin and/or hybrid layer formation . The etch-and-rinse bonding approach has been demonstrated to present higher incidences of hybridization failures than self-etching approach . Moreover, polymerization shrinkage creates contraction forces (shrinkage stress), leading to stress accumulation at the preparation walls. As a consequence, gap formation and subsequent microleakage can occur . Several approaches have been proposed to reduce shrinkage stress such as controlling the cavity configuration (C-factor) , modulating the light intensity using different polymerization techniques , using different cavity filling methods , and applying stress-absorbing intermediate layers . New resin-composite formulations also have been proposed by increasing the volume of inorganic particles, by increasing the monomer molecular weight, or by modifying the chemical structure of certain monomers and/or by replacing them .

In this way, an attempt to reduce shrinkage stress relies on the alteration of the composite resin matrix. Recently, a silorane-based composite was introduced containing cationic ring-opening monomers: a compensating mechanism for shrinkage stress achieved during polymerization . This monomer system was obtained from the reaction of oxirane and siloxane molecules . This resin-composite claimed to have combined the two key advantages of the individual components: low polymerization shrinkage due to the ring-opening oxirane and increased hydrophobicity and to the presence of the siloxane. The silorane-based composite revealed a decreased water sorption, solubility, and associated diffusion coefficient compared to those observed when methacrylate-based composites were tested . In a previous study , it was found that the cuspal deflection caused by the polymerization shrinkage was significantly lower when extracted teeth were restored with an experimental silorane material in comparison to that observed when a methacrylate-based composite was applied. Besides, in another study , no microleakage was found when Class II MOD preparations were restored with a silorane-based composite.

A wide variety of light sources are currently available, with increased power density and different spectral irradiance, with inherent characteristics and claimed advantages . On the other hand, the best irradiation procedure to polymerize resin-composites has not been determined yet . The kinetics of polymerization is dependent upon the energy dose . Thus, it could be expected that the longer the light exposure time, the higher the degree of conversion. An increase in the energy dose for silorane-based composites might not be recommended, as the polymerization stress can be developed by increases in the degree of conversion and elastic modulus. In this case, the application of longer photoactivation times might lead to the formation of interfacial defects could be increased.

This study evaluated the influence of the energy dose on the Knoop hardness, polymerization depth, and internal adaptation of Class II restorations filled with different posterior restorative systems (silorane-based or methacrylate-based systems). The following research hypotheses were tested: (1) bonding approach can influence the internal gap formation; (2) restorations filled with the silorane-based composite will present reduced gap formation when compared to methacrylate-based composite, irrespective of the applied energy dose and the bonding approach; (3) higher energy dose will produce superior hardness and gap formation, regardless of the type of resin-composite and the bonding approach; (4) the hardness of the resin-composites will not be affected by the depth of evaluation.