Flexure strength of methacrylate- and ormocer-based bulk fill resin composites: effect of material thickness and distance to photo-polymerization device

This study evaluated the effect of thickness and photo-polymerization distance on flexure strength of two bulk fill resin composites. In total, 96 rectangular-shaped resin composite bars were divided into eight experimental groups (n = 12/group) according to the three experimental factors; Factor 1: Material (X-tra fil and Admira Fusion X-tra), Factor 2: Thickness (2 mm and 4 mm) and Factor 3: photo-polymerization distance (2 mm and 8 mm). The bars were prepared with fixed 2 mm width and 2 mm length but differed in thickness (2 mm or 4 mm). The bars were light cured either at 2 mm or 8 mm distances. Flexure strength test was run at a crosshead speed of 0.5 mm/min. Data were analyzed using 3-Way ANOVA/Student t-test. Methacrylate-based resin composite exhibited significant higher flexure strength (p < 0.001). At 2 mm distance, there was no significant difference between 2 mm and 4 mm thicknesses for X-tra fil and Admira Fusion X-tra (p = 0.726 and p = 0.486, respectively). At 8 mm distance, both materials showed significantly higher flexure strength with 4 mm thickness compared to 2 mm thickness (p ˂ 0.001). Only X-tra fil with 2 mm thickness had no statistically significant difference on the flexure strength at 2 mm and 8 mm distances (p = 0.437). X-tra fil in 2 mm thickness cured at 8 mm distance and Admira Fusion X-tra in 4 mm thickness cured at 2 mm distance revealed the highest reliability. Methacrylate-based resin composite tested in this study had superior flexure strength compared to ormocer-based resin composite, regardless of the increment thickness and photo-polymerization distance.