Hardness Development in Resin Composite Core Materials

Purpose To investigate the hardness characteristics of 13 contemporary resin core materials. Materials and methods Specimens (n = 12) were fabricated using stainless steel molds with top surfaces of dual‐cure products photopolymerized while additional groups were allowed to self‐cure. Twelve Knoop hardness indentations 500 microns apart were obtained of photopolymerized top and bottom sample surfaces as well as the self‐cured sample surface with the mean recorded as the representative sample hardness. Testing was completed at 10 minutes, 1 hour, and 24 hours. In addition, hardness values were compared to that obtained from polished coronal dentin samples. Mean data between groups were analyzed with Kruskal‐Wallis/Dunn's, within groups with repeated measures ANOVA/Tukey's. Results Hardness results were material dependent. All but two products demonstrated a 0.8 bottom/top Knoop hardness ratio at 10 minutes. Product's self‐cure cure reaction did not attain hardness similarity with any photopolymerized top surfaces and while some materials were found to have similar dentin hardness to resin top surface ratio similarity, only one product had hardness equal to or greater than that of dentin during any time period. Conclusions Under this study's conditions, hardness development was material dependent and all but two products demonstrated adequate hardness‐derived degree of cure assessment at 10 minutes after preparation. Self‐cured samples demonstrated hardness increase; however, no self‐cured material achieved hardness similarity to photopolymerized top surfaces. None of the materials achieved hardness similarity to dentin and only one product demonstrated hardness greater than that of dentin.