Effect of Light-curing Unit Design and Mouth Opening 
on the Polymerization of Bulk-fill Resin-based Composite Restorations in Molars

Purpose: This study evaluated the effect of the design of the light-curing unit (LCU) and mouth opening on the properties of bulk-fill resin-based composites (RBCs).
Materials and Methods: Eighty molars received a mesio-occlusal-distal preparation and were restored using two different bulk-fill RBCs, Opus Bulk Fill APS (FGM) and Filtek Bulk Fill One (3M Oral Care), using two different designs of LCU, straight (Valo Cordless [Ultradent]) and angled (Radii-Cal [SDI]). Two mouth openings of 25 and 45 mm at the incisors were used. Forty samples were sectioned mesiodistally. One half was immersed in tetrahydrofuran to examine the effect of the solvent on the RBCs (SE), and the other half to measure the degree of conversion (DC) in proximal and occlusal boxes using Fourier Transform Infrared spectroscopy. The other 40 samples were subjected to thermomechanical fatigue and immersed in silver nitrate to examine the gingival margin integrity (GMI) using SEM. The irradiance (mW/cm2) and emission spectrum (mW/cm2/nm) from the LCUs were measured using a MARC Resin Calibrator (BlueLight Analytics). DC data were analyzed by 3-way ANOVA and Tukey’s test; SE and GMI were analyzed by an ANOVA on Ranks and Dunn test (α = 0.05).
Results: The DC values were similar in the occlusal and proximal boxes when using Valo Cordless, irrespective of RBC and the mouth opening used. Lower DC values were measured in proximal boxes when using Radii-Cal. The mouth opening had a greater effect on the irradiance delivered by Radii-Cal. Radii-Cal with 25-mm mouth opening had greater SE and GMI values, whereas the SE and GMI values obtained with Valo Cordless were unaffected by mouth opening.
Conclusion: The choice of LCU did not influence the properties of tested bulk-fill RBCs when the 45-mm mouth opening was used. However, when using the 25-mm mouth opening, the choice of LCU did influence the properties of the RBCs.