Influence of opacity and curing distance on the microhardness and bottom-to-top hardness ratio of an APS-based nanohybrid resin composite

Abstract

Background: To evaluate the effects of manufacturer-designated opacity category (enamel, body and dentin) and curing distance on the top and bottom surface microhardness and the bottom-to-top hardness ratio of an APS-based nanohybrid resin composite. Materials and Methods: Forty-two disc-shaped specimens (2 mm thick, 8 mm in diameter) were prepared from an APS-based nanohybrid resin composite in three manufacturer-designated opacity categories (enamel WE, body A1, dentin DA1), each divided according to curing distance (0 or 2 mm from the light guide tip). Polymerization used an LED light-curing unit at 1200 mW/cm² for 20 s. After 24 h in distilled water at 37°C, Vickers microhardness was measured on both surfaces and the bottom-to-top ratio was calculated as an indirect indicator of curing adequacy. Data were analyzed by two-way ANOVA with appropriate post-hoc comparisons (p<0.05). Results: Opacity category significantly affected top surface microhardness, bottom surface microhardness, and the bottom-to-top microhardness ratio (p<0.001). Bottom surface microhardness was also significantly influenced by curing distance (p=0.003). Enamel WE was the only category that exceeded the commonly used 80% hardness ratio threshold at both curing distances, whereas body A1 and dentin DA1 remained below this threshold. No significant opacity category × curing distance interaction was detected for any of the evaluated parameters. Conclusions: Within the limitations of this in vitro study, manufacturer-designated opacity category significantly in fluenced the hardness-based curing outcomes of the tested composite. Enamel WE showed the highest bottom-to-top hardness ratio and was the only category to exceed the 80% criterion at both curing distances. Increasing the curing distance significantly reduced bottom surface microhardness but did not significantly affect the hardness ratio. The manufacturer-designated opacity categories of this composite should not be assumed to exhibit equivalent hardness based curing outcomes under an identical light-curing protocol.