Influence of storage time in water on the integrity of adhesive interface in resin composite restorations

When immersed in oral fluids, water absorption by the restorative resin composite material can occur, which is identified by some researchers as one of the causes of loss of aesthetic features and reduction of mechanical properties over time. On the other hand, some authors have suggested that the fluids sorption may contribute to the reduction of shrinkage stress generated at the adhesive interface and reduce the width of gaps. The aim of this study was verifying if the storage time in water of restorations carried out with different filling techniques could influence on the integrity of tooth-restoration adhesive interface. Eighteen cavities were built in bovine incisors and they were divided into 3 groups after the adhesive procedure: group B (“Bulk”) received one single increment of light-cured resin composite; group I (“Increments”) received the same composite in three oblique increments; and the group B+S (“Bulk + Self-cured resin composite”). The last one firstly received a flowable, self-cured resin composite, and then, it was inserted one single increment of light-cured composite. After 48 h of storage, the restorations were sliced, the first measurement was accomplished, and the analysis of the adhesive interface was made each 30 days over nine months of immersion in water. The results were subjected to a split-plot analysis of variance and Tukey’s test. It was not verified significant influence of immersion time in water on the gap width, or regular increase or decrease of percentage interface free of gaps over time for any of three filling techniques. Some hypothesis could explain this occurrence, such as gain of mass without significant increase in the volume; the expansion of restoration in directions that did not contributed to the gaps closure; and the simultaneous occurrence of hygroscopic expansion and hydrolytic degradation of the resin processes.