Mechanical performance of a conventional resin composite and its bulk-fill restorative counterpart after long-term accelerated aging

The long-term mechanical properties and stability of various resin composites in areas under stress are questionable. The aim of the study was to determine the effects of long-term hydrothermal aging on the mechanical properties of a microhybrid conventional resin composite and its bulk-fill counterpart. We used a conventional and high-viscosity restorative bulk-fill resin-based composites (RBCs) from one company. Bar-shaped specimens of each type of resin composite were fabricated using steel molds and divided into 2 groups. The specimens were stored at 37°C for 24 h, and half of the specimens in each group were subjected to the 3-point bending flexural test and microhardness measurement. The remaining specimens were aged for 10,000 thermal cycles between 5°C and 55°C, and then subjected to flexural testing and microhardness measurement. A Vickers microhardness tester was used to estimate the surface microhardness of the specimens. Data was analyzed using an independent t test and the Mann-Whitney U test. The statistical significance level was set at p ≤ 0.05. Scanning electron microscopy (SEM) was used to investigate the surface of each material. The bulk-fill RBC showed similar flexural strength and modulus to its conventional counterpart before aging. The flexural strength of both resin composites was significantly decreased after thermocycling (p < 0.001). Hydrothermal aging had no significant effects on the flexural modulus (p = 0.84). There was a significant decrease in the surface microhardness of the bulk-fill RBC. Scanning electron microscopy photomicrographs showed several pits as a result of the exfoliation of the filler particles on the surface of the bulk-fill RBC after aging. The flexural strength of both resin composites decreased significantly after aging. The flexural properties, surface changes and microhardness of the bulk-fill type were additionally affected by the aging process.