Highly repeated preheating cycles seem to negatively influence the flexural strength of resin composites. Assuming dental clinicians are aware they are using the same composite syringe for more than 20 cavities and a preheating procedure is steadily adopted, then the use of single-use composite compules instead of syringes should be adopted.

The aim of this study was to assess the flexural strengths of three resin composites prepared at room temperature or cured after 20 or 40 cycles of preheating to a temperature of 45°C. Three resin composites were evaluated: Enamel Plus HFO (Micerium) (HFO), Enamel Plus HRi (Micerium) (HRi), Opallis + (FGM) (OPA). One group of specimens for each composite was fabricated under ambient laboratory conditions, whereas in the other groups, the composites were cured after 20 or 40 preheating cycles to a temperature of 45°C in a preheating device. Ten specimens were prepared for each group. A three-point bending test was performed using a universal testing machine at a crosshead speed of 0.5 mm/min. Data were analyzed with a two-way analysis of variance (ANOVA) test and a Games-Howell test (α = 0.05). The two-way ANOVA showed that both the material and the number of heating cycles were significant factors, able to influence the flexural strength values (p<0.05). However, there was not a statistically significant interaction (p>0.05). For all three composites flexural strengths were not affected after 20 preheating cycles in comparison with the control groups (0 preheating cycles) but were, however, significantly decreased when 40 prewarming cycles were conducted. The HRi and OPA groups had the highest flexural strengths, with no statistically significant differences among them. HFO presented significantly lower flexural strengths in comparison with HRi.