Effect of preheating and curing lamp distance on the degree of conversion of four nanohybrid resins: An in vitro study.

Background: Inadequate polymerization of resins is a major cause of failure in dental restorations. This study aimed to evaluate the hypothesis that both polymerization distance and preheating of four nanohybrid resins significantly affect their degree of conversion (DC).

Material and methods: Four A2-colored nanohybrid resins were selected: Filtek Z250 XT (3M), Tetric N-Ceram (Ivoclar), Zafira (New Stetic), and Spectra Smart (Dentsply). These resins were chosen due to their varied compositions. Forty-eight discs (6 mm diameter, 2 mm thickness) were manufactured, with 24 discs preheated to 39°C. All discs were polymerized for 40 seconds at distances of 1 mm and 6 mm using the Bluephase N lamp (Ivoclar Vivadent), operating at 385-515 nm and 1200 mW/cm². The polymerized discs were stored in distilled water at 37°C for 24 hours, and the DC was measured using Fourier-transform infrared spectroscopy (FTIR). Statistical analysis was performed using one-way ANOVA and independent samples t-tests.

Results: No statistically significant differences in DC were observed between samples preheated to 39°C and those at room temperature (p> 0.05). Zafira exhibited the highest DC, significantly higher than Z250 XT in all groups (p< 0.005) and higher than Tetric N-Ceram on the surface (p< 0.05). Significant differences were also found between Zafira and Spectra Smart in specific conditions (p< 0.05). No significant differences in DC were found between polymerization distances of 1 mm and 6 mm. Uniform polymerization was achieved throughout the resin discs.

Conclusions: Preheating nanohybrid resins to 39°C had no statistically significant impact on their degree of conversion. Acceptable DC values were achieved using a high-intensity lamp for 40 seconds, even at a curing distance of 6 mm. Among the tested resins, Zafira demonstrated the highest DC under various conditions, significantly outperforming Z250 XT, Tetric N-Ceram, and Spectra Smart in specific comparisons. Key words:Nanohybrid composite, polymerization, degree of conversion.