Effects of repeated preheating on polymerization shrinkage kinetics and Vickers microhardness of resin composites: an in vitro study

BACKGROUND: Preheating resin composites can reduce viscosity and improve handling; however, resin syringes or capsules used in multiple clinical applications may undergo repeated heating and cooling. The purpose of this study was to examine the effect of repeated preheating on the polymerization shrinkage kinetics and microhardness of resin composites. METHODS: Three capsule-type nanohybrid resin composites, Filtek Z350 XT (FZ), Estelite Sigma Quick (ESQ), and EsCom250 (EC), were tested under three conditions: non-heated, one-cycle preheating, and five-cycle preheating. Under the one-cycle preheating condition, the specimens were preheated once before curing. Under the five-cycle preheating condition, the specimens underwent five preheating cycles before curing, with cooling to room temperature between cycles. Preheating was performed using a composite warmer set to 55 °C. The polymerization shrinkage strain, maximum shrinkage rate, and time to maximum shrinkage rate were measured using a real-time displacement-based system. The Vickers microhardness was measured on the upper and lower specimen surfaces after 24 h of water storage. Data were analyzed using two-way analysis of variance (ANOVA) with material type and preheating conditions as fixed factors, including their interactions. Post hoc comparisons were performed for each material when appropriate. RESULTS: Two-way ANOVA showed significant material × preheating-condition interactions for polymerization shrinkage strain and maximum shrinkage rate, but not for time to maximum shrinkage rate or upper- or lower-surface Vickers microhardness. Within-material comparisons showed that the preheating conditions significantly affected the shrinkage strain in the FZ and EC, the maximum shrinkage rate in all three materials, and the time to maximum shrinkage rate in the ESQ and EC. Five-cycle preheating reduced the maximum shrinkage rate in all materials and delayed the time to reach the maximum shrinkage rate in ESQ and EC. Preheating did not reduce the Vickers microhardness of the upper or lower surfaces of any of the materials. CONCLUSIONS: Within the limitations of this in vitro study, one-cycle preheating produced limited, material-dependent changes in polymerization shrinkage kinetics and did not reduce Vickers microhardness. Five repeated preheating cycles altered the shrinkage kinetics in a material-dependent manner but did not reduce the Vickers microhardness.