Polymerization of a resin composite luting agent: effects of temperature, preheating duration, and light-curing duration

Objective This study evaluated how preheating temperature and duration, and light-curing time affect the microhardness (HV) and degree of conversion (DC) of a preheated resin composite used as a luting agent. Materials and methods A microhybrid composite was preheated to 54 °C or 68 °C for 5, 10, or 15 min, and then placed between a 4.0 mm lithium disilicate overlay and a natural tooth as a luting agent. Light-curing was performed for 120, 180, or 270 s. The microhardness and degree of conversion were measured. Three-way ANOVA analyzed the effects and interactions of the variables. Pearson's correlation assessed the relationship between HV and DC. Results DC was not significantly affected by preheating temperature, duration, or light-curing time ( p > 0.05). HV was significantly influenced by all variables ( p ≤ 0.001), with the highest HV at 68 °C for 15 min and longer curing. A significant three-way interaction was noted ( p = 0.039). No consistent correlation was found between DC and HV, except for one condition (54 °C, 5 min, 90 s; ρ = 0.97, p = 0.005). Conclusions Within the preheating conditions evaluated, higher preheating temperatures, longer preheating durations, and extended curing times were associated with increased microhardness, but not the degree of conversion, of the resin composite under ceramic restorations. Clinical significance Among the preheating protocols evaluated, higher preheating temperatures, longer preheating durations, and extended multidirectional light-curing times were associated with higher microhardness of resin composites used as luting agents beneath thick (4 mm) lithium disilicate restorations, without affecting the degree of conversion. Because no non-preheated control group was included, these findings should be interpreted as comparisons among different preheating protocols rather than evidence of the benefits of preheating itself. Furthermore, the extended curing times investigated in this study represent optimal experimental conditions for the evaluated restoration configuration and may not always be practical or feasible in routine clinical settings. Minimizing the delay between preheating and curing may help preserve the effects associated with the investigated preheating protocols.