Effect of preheating on Fracture-toughness of Bulk-fill and Conventional Composite-resins

Objectives: The objective of this study was to evaluate the effect of preheating on the fracture toughness of bulk-fill restorative material in comparison to conventional composite resin. The null-hypothesis tested was that preheating has no effect on the fracture toughness of composite resins.
Methods: Four different groups of composite resin materials were prepared (n=12): Group 1: Bulkfill (Filtek One, 3M) preheated to 68 ± 1°C, Group 2: Bulkfill (Filtek One, 3M) at room temperature, Group 3: Conventional composite resin (Filtek Supreme Ultra, 3M), preheated to 68 ± 1°C, Group 4: Conventional composite resin (Filtek Supreme Ultra, 3M), at room temperature. Each composite was carefully injected into the appropriate mold (at room temperature and preheated) to achieve desired sample dimensions of 3mm*4mm*21mm. Photo-polymerization was performed according to manufacturers recommended time. Samples were removed from the mold and polished using 400-grit silicon-carbide abrasive paper. Samples were then stored in deionized water for 24h at 37°C. A 0.5mm notch was cut into the center of the sample and a razor blade was used to form the final V-shape geometry for testing. A 3-point test was performed using an Instron Universal machine to determine the fracture toughness. Statistical analysis was performed using ANOVA and post-hoc Tukey’s tests (p<0.05).
Results: The fracture toughness of Filtek One was significantly higher than Filtek Supreme Ultra when prepared at room temperature and after preheating (p<0.05). Preheating of both Filtek One and Filtek Supreme Ultra showed no significant difference in fracture toughness when compared to room temperature groups.
Conclusions: Preheating has no effect on the fracture-toughness of both bulk-fill and conventional composite resins. The clinical significance of preheating dental composites, as shown in the literature, need to be interpreted with caution.