Effects of preheating on degree of conversion, flexural strength and flexural modulus of bulk-fill resin composites

Purpose: The aim of this study was to investigate the effects of preheating on the degree of conversion (DC), flexural strength (FS) and flexural modulus (FM) of bulk-fill resin composites with varying viscosities and filler contents.
Methods: Four bulk-fill resin composites - EverX Posterior (EXP), EverX Flow Bulk (EXF), Filtek One Bulk Fill (ONE), SDR Plus Flow (SDR) - and a conventional composite - Filtek Z250 (Z250) - as a control group were evaluated. Each composite was divided into three subgroups: [1] Room temperature (20±1°C), [2] preheated to 39°C, and [3] preheated to 55°C. DC, FS and FM results of the specimens were evaluated. Statistical analysis included one-way ANOVA and post hoc Tukey tests (p<0.05).
Results: At room temperature, Z250 exhibited the lowest DC (41.32%), significantly lower than EXP (52.11%) and other bulk fill composites. FS values for Z250 (145.68 MPa), EXF (152.19 MPa) and EXP (157.24 MPa) at room temperature were comparable but higher than for ONE (100.70 MPa) and SDR (114.95 MPa). Preheating increased the DC and FS of Z250 and EXP but not of EXF, ONE or SDR.
Conclusion: Preheating is recommended to enhance the properties of viscous, fiber-reinforced (EXP) and conventional hybrid (Z250) composites for stress-bearing restorations. In contrast, it offers no significant mechanical advantage for the flowable bulk-fill composites tested (EXF, SDR, ONE), underscoring that the benefit of preheating is highly material-dependent.

Keywords: Bulk-fill, degree of conversion, flexural modulus, flexural strength; preheating.