Abstract

The study assessed microhardness, microleakage of Class I, and the degree of conversion of composite resins preheated at 55 °C. This in vitro study evaluated two nanofilled composite resins (NCR), Filtek™ P60 and Filtek™ Z350 XT, used in the restorations. The degree of conversion (DC) was determined by Raman spectroscopy; the study compared the vibrational bands of the residual unpolymerized methacrylate C=C stretching band at 1640 cm−1 to the aromatic C–C stretching band at 1610 cm−1, used as an internal standard. Microleakage tests considered standard Class 1 cavities prepared on human premolars randomly divided. Preheating Filtek™ P60 and Filtek™ Z350 XT increased significantly VMH values (p = 0.0001). The highest microleakage occurred in preheated Filtek™ Z350 XT, while the lowest was in unheated Filtek™ P60. Unheated Filtek™ Z350 XT showed a significant increase of microleakage than unheated Filtek™ P60. (p = 0.024). Preheating of composite resins improved mechanical properties increasing DC and VMH. However, it also increases microleakage promoting bacterial penetration, secondary caries, marginal discoloration and hypersensitivity.