Depth of cure of bulk-fill light cured composite resins with different initiators

Abstract

Introduction: Bulk-fill composites’ manufacturers claim that these materials can be completely cured on a single layer up to 5mm [1], reducing the risk of bacterial and salivary contamination or air bubbles incorporation between layers [2]. Therefore, bulk-fill resin composites are also less time-consuming [2]. The aim of this study was to evaluate the depth of cure of bulk-fill composites with different times of curing, according to the photoinitiator and the resin shade. Materials and methods: Specimens were prepared incorporating an increment of composite (Tetric EvoCeram Bulk-Fill [TECBF] IVA and IVB - Ivoclar Vivadent or Filtek One Bulk-Fill [FOBF] A3 and B422 - 3M ESPE) in standardized metal molds with 4 mm of diameter and height, using the buccal surface of a superior central incisor as a reflection substrate. Twelve groups (n = 10) were defined according to curing time (10, 20 or 40 seconds), shade (lowest/highest value) and photoinitiator (lucirin/camphoroquinone). Vickers microhardness (VMH) was measured at the top and bottom of each specimen by 5 indentations with a force of 29,42 N for 5 s. The depth of cure (DC) was obtained by the ratio of the bottom/top VHM [3]. Results above 80% were used to consider the sample well cured [4]. Comparative inferential statistical analysis was performed through IBM SPSS Statistics v.24 software using a three-way ANOVA at a significance level of 5%. Results: Regardless the shade, DC of the composite resins studied showed an improvement tendency with an increased polymerization time. In both of resins, time was the main factor in obtaining a suitable depth of cure (p < 0.001 and partial eta squared =0.771), being 10 s of polymerization insufficient to reach the minimum of 80% of polymerization (p < 0.001). So, 20 s is the time needed to ensure an adequate polymerization, since there is no statistically significant differences between 20 and 40 s of polymerization. Color was not found to be a significant factor in the DC of bulk-fill composites (p = 0.916). FOBF showed a similar depth of cure but higher superficial hardness when compared to TECBF. Discussion and conclusions: Curing time is the crucial factor in obtaining a proper depth of cure. Color, on the other hand, does not appear to be a significant factor in the depth of cure of the bulk-fill resin composites studied. The presence of different photoinitiators may justify the differences found between the two resin composites.