Flexural strength and fracture load of high-flow bulk-fill and conventional composites in ISO 4049 bars and MOD cavity models: an in vitro study

BACKGROUND: High-flow bulk-fill (HFB) resin composites have been introduced to simplify restorative procedures by enabling bulk placement and improved cavity adaptation. However, evidence is limited regarding whether their mechanical performance as stand-alone materials is consistent across different testing geometries. This study aimed to compare the flexural strength and fracture load of HFB composites with a conventional paste-like posterior composite using ISO 4049 bar specimens and MOD shaped restorations. METHODS: Eighty specimens were fabricated and divided into two specimen configurations: ISO 4049 bar specimens (25 × 2 × 2 mm) and MOD shaped restorations (12 × 7 × 5 mm) (N = 40 each). Three HFB composites-Clearfil Majesty ES High Flow (CM), Tetric N Flow Bulk Fill (TF), and Estelite Universal High Flow (EU)-and one conventional paste-like posterior composite (GC G-aenial Posterior; GP) were tested (n = 10 per subgroup). ISO specimens were evaluated for flexural strength (MPa) and MOD specimens evaluated for fracture load (N) using a three-point bending and fracture load test at 1 mm/min. Data were analyzed using one-way ANOVA and Tukey post hoc tests (α = 0.05). RESULTS: Remarkably, the mechanical outcomes varied depending on the tested composite material and the specimen configuration (p < 0.05). In MOD shaped restorations, TF demonstrated the highest fracture load (1198.33 ± 188.83 N), followed by EU (1007.76 ± 182.37 N), GP (871.92 ± 227.67 N), and CM (776.10 ± 93.06 N). In ISO 4049 bar specimens, EU exhibited significantly higher flexural strength (85.84 ± 23.43 MPa) compared with CM (61.76 ± 2.99 MPa), TF (55.27 ± 12.04 MPa), and GP (54.19 ± 11.57 MPa) (p < 0.05). Material ranking differed between ISO bar and MOD specimen configurations. CONCLUSIONS: HFB resin composites showed flexural strength and fracture load values comparable to or higher than those of the conventional paste-like posterior composite. However, performance was material-dependent and influenced by specimen geometry. These findings highlight the importance of combining standardized and geometry-dependent testing approaches for a more comprehensive evaluation of restorative materials.