Influence of a budget single-peak light-curing unit on the microhardness and bond strength of bulk-fill resin composites to the gingival floor of proximal class II cavity preparations

Abstract

ObjectiveTo evaluate the Vickers microhardness (VH) and long-term microtensile bond strength (μTBS) of bulk-fill resin-based composites (RBCs) at the bottom of the proximal boxes of Class II restorations when photo-cured using two different light-curing units (LCUs). MethodsMesio-occlusal-distal (MOD) cavities were prepared in 99 third molars (n = 7 per group) and restored with Tetric N-Ceram Bulk-Fill (TNC, Ivoclar), Filtek Bulk-Fill Flow (FBF, 3 M ESPE), or SDR flow+ (SDR, Dentsply Sirona). The RBCs were photo-cured with a dual-peak (Bluephase 20i, Ivoclar) or a single-peak budget LCU (Rainbow Curing Light, SML). The energy delivered by the lights were similar. The amount of light transmitted (% of maximum transmission) through the RBCs and the light beam profile through the restored teeth were recorded (n = 5). VH values were measured at the periphery of the RBC in the proximal box. After 24-h storage in water or after additional thermocycling, the teeth were sectioned to make 1 mm2 sticks. The μTBS of bulk-fill RBCs to the dentin on the gingival floor of the proximal boxes was then measured. Two-way repeated-measures ANOVA and 2-way ANOVA were used to analyze the VH and μTBS data, respectively, and Bonferroni's post-hoc test (α = 5%). No comparisons were made among RBCs. ResultsWhen the bonding agent was photo-cured with the budget light, all the specimens failed. Therefore, the Bluephase 20i photo-cured all the bonding resins and the budget light was only used to photo-cure the RBCs. The Bluephase 20i delivered a more homogeneous and wider light distribution through the restored teeth, higher VH values for TNC and SDR groups (p < 0.05), and higher μTBS in the TNC group (p < 0.05) than those produced by the single-peak budget LCU. The μTBS values to the dentin at the gingival floor of the proximal boxes decreased after thermocycling in the SDR group (p < 0.05). ConclusionWhen the bonding agent was photo-cured with the budget light, all the specimens failed. Photo-curing the RBC with a single-peak, budget LCU compared to the Bluephase 20i reduced the VH and μTBS values of bulk-fill RBCs to the gingival floor.