Effect of fiber reinforcement on the fracture resistance of bulk-fill composite restorations in endodontically treated premolars: an in vitro study

This in vitro study aimed to evaluate the fracture resistance of endodontically treated premolars restored with different bulk-fill composites with and without polyethylene fiber reinforcement. A total of 144 extracted premolars were prepared with standardized mesococclusodistal cavities and allocated to 12 groups (n = 12). Root canal treatment was performed in all but the intact control group. Restorations were carried out via a nanohybrid composite (Filtek Z250) or one of four bulk-fill composites (SDR, SonicFill 2, Tetric EvoCeram Bulk Fill, Filtek Bulk Fill Posterior), each with or without polyethylene fiber reinforcement. After 24 h of water storage at 37 °C, the samples were loaded axially with a steel sphere until fracture. Fracture resistance values (mean ± standard deviation, N) were recorded, and the data were analyzed via one-way ANOVA and Tukey’s test (α = 0.05). Intact teeth presented the highest fracture resistance (1278.82 ± 124.19 N), whereas unrestored teeth presented the lowest fracture resistance (255.88 ± 59.17 N). Among the restored groups, Filtek Bulk Fill Posterior (996.77 ± 55.90 N) provided the greatest resistance, approaching intact values. Fiber reinforcement did not consistently increase fracture resistance and significantly reduced it in the Filtek Bulk Fill Posterior group (p < 0.05). Within the limitations of this in vitro study, high-viscosity bulk-fill composites, particularly Filtek Bulk Fill Posterior, effectively restored the fracture resistance of premolars to levels approaching intact teeth. The routine use of polyethylene fiber reinforcement does not provide consistent benefits and may compromise performance.