Effect of Light-Curing Tip Distance on the Top and Bottom Surface Microhardness of Two Single-Shade Resin Composites: An In Vitro Study

Abstract

Aim: The purpose of this in vitro experiment was to measure the difference in surface microhardness and bottom-to-top hardness ratio between two single-shade resin composites as a function of increasing light-curing tip distance. Materials and Methods: We used Palfique OMNICHROMA and ONEshade Universal to create sixty disk-shaped specimens (n = 30 for each). Using the light-curing tip distance as a criterion, we separated each material into three subgroups (n = 10). After being polymerized for 30 seconds at 1000 mW/cm² in an LED curing machine, the specimens were immersed in distilled water and left to cure at 37°C for 24 hours. We determined the bottom-to-top hardness ratio by measuring the Vickers microhardness on both the top and bottom surfaces. We used three-way mixed ANOVA and two-way ANOVA to analyze the data, with a significance level of α = 0.05. Results: Both composites showed a significant decrease in microhardness as the curing distance increased (p < 0.001). At all curing distances, ONEshade showed considerably stronger top and bottom microhardness compared to OMNICHROMA (p < 0.001). Hardness ratios measured from bottom to top fell somewhere between 83.96% and 90.07%, with no group falling below the 80% mark.

Conclusion: Both single-shade composite microhardness decreased as the light-curing tip distance increased, but the relative polymerization remained sufficient at a specimen thickness of 2 mm. To get the most out of single-shade resin composites mechanically, you should keep the curing-tip distance to a minimum.