INTRODUCTION
Despite improvements in the current formulations of resin composite, polymerization shrinkage remains a problem.1 The polymerization shrinkage of resin com- posite may induce mechanical stresses on tooth struc- ture through the bond to enamel and dentin.2 These stresses can contribute to failure of the weakest por- tions of a composite-dentin interface.2-3 This process can cause micrometer-wide marginal gaps, thus opening a path for the migration of microorganisms and poten- tially inducing secondary caries.4 Several materials and methods have become advocated to minimize develop- ment of this gap. Current dentin adhesive systems that created a hybrid layer between resin and dentin have shown improved marginal seal due to the use of acidic molecules and improved bonding technology.5 Another approach to creating a gap-free bond is the use of an elastic intermediate layer of resin between the compos- ite and adhesive resin, which may absorb contraction stress of the composite during polymerization.6-7
However, in 1995, Sano and others8 described a leakage pathway through the porous zone at the hybrid layer- adhesive interface without gap formation. This leakage is not the classical microleakage that can be seen by conventional 10-30x microscopic magnification. Rather, it represents a leakage that occurs within nanometer- sized spaces in the base of the hybrid layer that have not been filled with adhesive resin or which were left when poorly polymerized resin was extracted by denti- nal fluid. To distinguish this leakage from the typical microleakage, it was called nanoleakage. In order to quantify the degree of nanoleakage, silver nitrate pen- etration has been used for scanning electron microscopy (SEM) investigation.9-10 The amount of sil- ver penetration depends on the type of bonding agent and the different parameters of the application tech- nique, such as etching time and dentin moisture.11
Flowable composites were introduced in late 1996.12
They have a filler size similar to hybrid composites but lower filler content (60%-70% by weight and 60%-75% by volume).13 The reduced filler loading of flowable com- posites compared with their hybrid analogs leads to enhanced flow and reduced elastic modulus.13 The lower elastic modulus indicates that flowable compos- ites have a greater ability to flex with the tooth than stiffer restorative materials.14 Also, this material may act as a stress-breaker and seems to wet the cavity more completely than conventional sticky resin-based restorative materials.14-15 However, Braga and others16
showed that flowable composites produced polymeriza- tion contraction stress similar to hybrid composite