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.