Influence of Pre-curing warming on micro-leakage, micro-hardness and degree of conversion of high viscosity bulk fill composite resins: An in vitro study

Objective: This study aimed to investigate the Influence of preheating on micro-leakage, micro-hardness and degree of conversion of high viscosity bulk fill and conventional nanohybrid composite resins
Materials and methods: For micro-leakage assessment, thirty extracted human molars were used. Sixty class II cavities were prepared in the mesial and distal surfaces of teeth, and were randomly divided into two groups according to type of composite resin Group I : nanohybrid high viscosity bulk fill composite resins (Tetric Evoceram bulk fill, Ivoclar Vivadent), and Group II: conventional nanohybrid composite resins (Tetric Evoceram,Ivoclar Vivadent). Each group was further divided into three subgroups according to the precure temperature of composite (subgroup A: 24 °C (control subgroups), subgroup B: 37°C, and subgroup C: 68°C). where subgroups (IA, IB and IC received high viscosity bulk fill nanohybrid composite resins), and subgroups (IIA, IIB and IIC received conventional nanohybrid composite). Micro-leakage was determined by dye penetration test with scoring criteria from 0-4. Sixty composite discs were used in Vickers micro-hardness test (VHN), and degree of conversion with the same experimental grouping as used in micro-leakage assessment .VHN was determined on top and bottom surfaces of the specimens using Vickers microhardnss indenter and monomer conversion was determined using Fourier transform infrared spectroscopy. Data were collected, tabulated and statistically analyzed by using ANOVA F-test, Mann-Whitney testand Wilcoxon Signed rank test (p<0.05).
Results: Concerning micro-leakage; There was no significant difference between the three subgroups for each type of the tested material. Concerning micro-hardness; results proved a significant difference in the top and bottom VHN among the two groups with the highest mean top and bottom VHN value observed in preheating both types of composite to 68°C and the lowest mean top and bottom VHN value seen in control subgroups (composite resin stored at room temperature). Both composites attained bottom / top % ranges between 80-90 %. The degree of conversion of Tetric EvoCeram Bulk Fill was significantly increased (p = 0.006)with composite pre-heating, but no effect on monomer conversion of the other investigated material was observed. Tetric EvoCeram Bulk Fill achieved the significantly highest Vickers micro-hardness and monomer conversion, irrespective of the precuring temperature.. Conclusions: Both the composite material and the pre-cure temperature affect Vickers micro-hardness and degree of conversion. Pre-heating of composites prior to photo activation significantly increased the Vickers micro-hardness and degree of conversion but have no effect on micro-leakage.