Surface microhardness and depth of cure in bulk fill resin composites with and without preheating: An in vitro study.

Background: Preheating bulk-fill resins reduces their viscosity, facilitates their adaptation, and improves polymerization by accelerating the activity of photoinitiators. This would allow for more efficient application and deeper curing. The study's objective was to compare the surface microhardness and depth of cure of three bulk-fill composite resins with and without preheating.

Material and methods: Sixty samples were prepared from three different materials, Aura Bulk Fill, Filtek One Bulk Fill, and Opus Bulk Fill, in standardized molds. A Vickers electronic hardness tester was used to measure the surface microhardness. The depth of cure was calculated using the upper-to-lower microhardness ratio for each sample. Robust Student's T was used to compare two independent measures, and Welch's robust ANOVA was used to compare more than two independent measures with Games Howell's post hoc. Significance was set at p<0.05.

Results: It was observed that the preheated Aura Bulk Fill and Filtek One Bulk Fill resins had significantly higher surface microhardness than the same resins without preheating (p<0.001 and p=0.038, respectively). On the other hand, the non-preheated Opus Bulk Fill resin had significantly higher surface microhardness than the same resin preheated (p = 0.046). In addition, it was evident that the Filtek One Bulk Fill resin had a greater depth of cure than the Aura Bulk Fill resin (p = 0.011). This, in turn, had a greater depth of cure than Opus Bulk Fill resin (p = 0.003). Finally, with and without preheating, no significant differences existed in the depth of cure of the Bulk Fill resins evaluated (p>0.05).

Conclusions: Preheating significantly increased the surface microhardness in Aura Bulk Fill and Filtek One Bulk Fill resin, while it caused the opposite effect in Opus Bulk Fill resin. In addition, preheating did not increase the depth of cure of each Bulk Fill resin evaluated. Finally, with and without preheating, Filtek One Bulk Fill resin presented higher surface microhardness and depth of cure than Aura Bulk Fill and Opus Bulk Fill resins. Key words:Preheating, bulk-fill composite resin, depth of cure, microhardness.