Evaluation of the microtensile bond strength of highly filled flowable composites to sound and caries-affected dentin surfaces

In this in vitro study, the microtensile bond strength (μTBS) values, interfacial failure types, and SEM findings of two different adhesive systems combined with four different composite materials were evaluated on sound and artificially demineralized (caries-affected) dentin surfaces following thermal cycling. In this thesis, 64 sound human molars were used. Dentin surfaces were prepared using a diamond saw and SiC abrasive papers. To create artificial caries, half of the specimens were subjected to a 14-day pH-cycling protocol consisting of 8 hours in a demineralization solution followed by 16 hours in a remineralization solution. Two universal adhesive systems (GC Premio Bond and G2-Bond Universal) were applied to the dentin surfaces, and restorations were performed using three highly filled flowable composites (Clearfil Majesty Flow, G-ænial Universal Injectable, GrandioSO Heavy Flow) and one universal nanohybrid composite (G-ænial A'Chord). All specimens underwent 10,000 thermal cycles, after which microtensile bond strength testing was performed. Interfacial failure modes (adhesive, cohesive, mixed) were assessed under a light microscope. For each group, one representative specimen was examined by SEM to observe the resin–dentin interface. All statistical analyses were performed in R version 4.1.2. The normality of the data was assessed using the Shapiro–Wilk test and Q–Q plots, while variance homogeneity was evaluated with Levene's test. Differences in microtensile bond strength among various adhesive systems, composite types, and dentin conditions were analyzed using one-way ANOVA followed by Tukey HSD; in cases where variance homogeneity assumptions were not met, Welch's F test and Games–Howell post-hoc tests were used. The significance level was set at p < 0.05. The highest microtensile bond strength values were obtained in sound dentin for the G2-Bond Universal + Clearfil Majesty Flow group (40.75 ± 8.31 MPa). In contrast, the lowest bond strength was recorded in artificially demineralized dentin for the GC Premio Bond + G-ænial A'Chord group (9.19 ± 4.91 MPa). G2-Bond Universal generally demonstrated higher bond strength than GC Premio Bond on both sound and caries-affected dentin. GC Premio Bond showed relatively higher bond strength values only when combined with G-ænial A'Chord on sound dentin (20.25 ± 5.01 MPa); however, no statistically significant difference was detected between the two adhesives. Across all adhesive–composite combinations, microtensile bond strength values were consistently higher on sound dentin compared to artificially caries-affected dentin. Regarding interfacial failure modes, a high percentage of cohesive failures was observed in sound dentin for the G2-Bond Universal + Clearfil Majesty Flow group, whereas adhesive failures predominated in all other groups. The condition of the dentin substrate—whether sound or caries-affected—is one of the most critical factors influencing microtensile bond strength. Adhesive system and composite selection follow as secondary determinants. In this study, G2-Bond Universal showed superior bonding performance compared to GC Premio Bond on both dentin substrates. Notably, the G2-Bond Universal and Clearfil Majesty Flow combination exhibited remarkably high microtensile bond strength values on sound dentin, outperforming all other groups. Keywords: artificial caries; highly filled flowable composite; microtensile; universal composite; universal adhesives