Comparative In Vitro Analysis of Mechanical Properties in Three High-Viscosity Bulk-Fill Composite Resins

Bulk-fill composite resins (BFCRs) have emerged as efficient alternatives to conventional restorative systems, enabling placement in thicker increments without compromising polymerization; however, their comparative mechanical performance under clinically demanding conditions remains uncertain. This study aimed to evaluate and compare the mechanical properties—flexural strength (FS), elastic modulus (EM), strain (ε), and displacement (δ)—of three high-viscosity bulk-fill resins: Filtek One™ Bulk Fill (3M ESPE), Tetric^® N-Ceram Bulk Fill (Ivoclar Vivadent), and Opus™ Bulk Fill (FGM). Thirty specimens (n = 10 per group) were fabricated according to ISO 4049:2019 and subjected to three-point bending tests. Statistical analysis included Shapiro–Wilk testing for normality, one-way analysis of variance (ANOVA) with Tukey’s post hoc comparisons, multivariate analysis of variance (MANOVA), and Spearman’s correlation. Filtek One™ Bulk Fill exhibited the highest FS 142.5 megapascals (MPa) and EM 4.2 gigapascals (GPa), with significant differences compared to Tetric^® N-Ceram Bulk Fill and Opus™ Bulk Fill (p < 0.001). Opus™ Bulk Fill demonstrated greater deformation capacity before fracture (p = 0.015). MANOVA revealed a significant effect of resin type on overall mechanical behavior (Wilks’ λ = 0.132; p < 0.001). Strong correlations were observed between strength and stiffness (ρ = 0.82), and between stiffness and deformation (ρ = –0.68). These findings confirm that BFCRs differ significantly in mechanical behavior, with Filtek One™ Bulk Fill exhibiting superior stiffness and resistance, while Opus™ Bulk Fill showed greater deformation capacity. Such differences support material selection based on the functional and anatomical demands of restorations, contributing to improved clinical performance and longevity.