Influence of irradiation time on subsurface degree of conversion and microhardness of high-viscosity bulk-fill resin composites

Abstract

Objectives: To evaluate the influence of irradiation time on degree of conversion (DC) and microhardness of high-viscosity bulk-fill resin composites in depths up to 6 mm.

Materials and methods: Four bulk-fill materials (Tetric EvoCeram Bulk Fill--TECBF; x-tra fil--XF; QuixFil--QF; SonicFill-SF) and one conventional nano-hybrid resin composite (Tetric EvoCeram--TEC) were irradiated for 10, 20, or 30 s at 1,170 mW/cm(2). DC and Knoop microhardness (KHN) were recorded after 24-h dark storage at five depths: 0.1, 2, 4, 5, and 6 mm. Data were statistically analyzed using ANOVA and Bonferroni's post-hoc test (α = 0.05).

Results: With increasing bulk thickness, DC and KHN significantly decreased for TEC. TECBF and SF showed a significant decrease in DC and KHN at 4-mm depth after 10-s irradiation, but no decrease in DC after 30-s irradiation (p > 0.05). XF and QF demonstrated no significant DC decrease at depths up to 6 mm after irradiation of at least 20 s. At 4-mm depth, all materials tested achieved at least 80 % of their maximum DC value, irrespective of irradiation time. However, at the same depth (4 mm), only XF and QF irradiated for 30 s achieved at least 80 % of their maximum KHN value.

Conclusions: Regarding DC, the tested bulk-fill resin composites can be safely used up to at least 4-mm incremental thickness. However, with respect to hardness, only XF and QF achieved acceptable results at 4-mm depth with 30 s of irradiation.

Clinical relevance: Minimum irradiation times stated by the manufacturers cannot be recommended for placement of high-viscosity bulk-fill materials in 4-mm increments.