Light cured resin composites are commonly used in daily clinical practice to restore anterior and posterior teeth, because of their many advantages: esthetic, bonding to tooth structure, and mechanical properties. However, these materials undergo significant volumetric shrinkage when polymerized .
Insertion of these contracting composites into bonded preparations induces the development of mechanical stress inside the material . The stress is transmitted via bonded interfaces to tooth structures. In light cured composites, the fast conversion induces fast increase in composite stiffness, causing high shrinkage stress at the interface. Such stress may disrupt the bond between the composite and the cavity walls or may even cause cohesive failure of the restorative material or the surrounding tooth tissue, in addition to postoperative sensitivity .
The rate of monomer conversion depends upon many factors as photo-initiator chemistry, filler morphology, pigment and irradiance (mW/cm2). But the irradiance applied to the composite is fundamental, because it is a factor that can be controlled by the professional through modulated photo-activation methods, differently of the factors mentioned previously. The higher the irradiance the faster the monomer conversion and the higher the stress generation. Photo-activation using low irradiance could reduce the stress, because it would allow flow during the earlier stages of polymerization and enable a certain degree of polymer chain relaxation before reaching the rubbery stage [3–5].
Modulation of the luminous energy has been shown to be efficient in decreasing the shrinkage stress of dental composite polymerization, but its clinical use is difficult, because it increases the clinical time and is dependent on the irradiance of the light curing unit, which the dentist does not usually know. Moreover, these methods can reduce the stress, but they do not reduce the final shrinkage of the material [3–6].
Therefore, with the objective of decreasing polymerization shrinkage, and consequently, the stress generated at the tooth/restoration interface, new monomers have been studied and introduced into the composition of dental composites. The monomers BisGMA, BisEMA, UDMA and TEGDMA, can be substituted by alternative monomers that have low polymerization shrinkage [7–9].
Recently, a silorane-based composite (Filtek P90), a synthesized monomer starting from oxirane and siloxane, was introduced on the market. Silorane-based composites differ from the methacrylate-based composites due to the polymerization process that occurs via a cationic ring-opening reaction, which decreases the volumetric contraction of the composite when compared with other methacrylate-based composites, in which the polymerization reaction is for addition .
When methacrylate monomers are replaced by silorane, not only can the polymerization shrinkage be reduced, but also the stress caused by it. Thus, many problems related to composite restorations, such as micro-leakage and marginal staining, secondary caries and postoperative sensitivity can be overcome [8–10].
Some clinical studies show that different materials for dental restoration can influence the longevity of the restoration increase the marginal integrity, marginal discoloration or surface texture  and thus reduce the need to replace the dental restoration .
Therefore, the aim of this study was to evaluate the Knoop hardness and bond strength between tooth/restoration of conventional methacrylate- and silorane-based composites. The bond strength was evaluated by the push out test, which is very useful for verifying the effect of polymerization shrinkage on composite restorations and its influence on bond strength.
The hypotheses tested were:
The silorane-based composites could produce higher values of bond strength between tooth/restoration that methacrylate-based composites;
The methacrylate-based composites will obtain higher Knoop hardness values that silorane-based composites.