Background: Resin composites require optimal mechanical strength for long-lasting restorations. Although preheating improves flowability, adaptation, and degree of conversion, repeated preheating cycles during clinical use may induce thermal stresses that alter material properties. Hence, it is important to evaluate the effect of repeated preheating under simulated oral stresses on the microhardness of nanohybrid composites. Aim: This study aims to assess and compare the microhardness of a nanohybrid composite cured at room temperature and after 1, 15, and 30 repeated preheating cycles at 70°C in Class I restorations, followed by chewing simulation and thermocycling. Materials and Methods: This in vitro study involved Class I cavity preparations on the occlusal surfaces of 60 extracted human premolars. Samples were allocated into four groups ( n = 15): Group 1 – composite placed without preheating; Group 2 – composite placed after 1 preheating cycle; Group 3 – composite placed after 15 preheating cycles; and Group 4 – composite placed after 30 preheating cycles. Following etching and bonding, the composite was preheated according to group allocation, placed in the cavities, and light-cured. Specimens were subjected to cyclic loading for 120,000 chewing cycles and 5000 thermocycles. Microhardness was evaluated using the Vickers microhardness test, and data were analyzed using the Kruskal–Wallis and Mann–Whitney U -tests ( P < 0.05). Results: The results showed no significant difference among the groups; however, a noticeable reduction in microhardness was observed after 30 cycles of repeated preheating. Conclusion: Under simulated oral conditions with chewing simulation and thermocycling, repeated preheating for 30 cycles did not significantly affect the microhardness of the nanohybrid composite, suggesting its durability.
Vemuri et al. (Thu,) studied this question.