Method: Two resin composites were used; a microhybrid Filtek Z250 (3M ESPE) and a nanohybrid Grandio (Voco) with shade A2. Three different temperatures (22, 39 and 55oC) were used, utilizing a commercially available composite warmer (ENA HEAT, Micerium SpA GE). The heated samples were immediately injected into cylindrical molds measuring 6 mm in diameter and 2 mm in depth and the top surface of the specimens were polymerized for 10, 20 and 40 sec under a matrix strip, using a QTH light curing unit (Elipar 2500, 3M ESPE). Five specimens were prepared for each combination of the parameters resulting in 18 groups and a total of 90 specimens. Vickers microhardness measurements were performed from both the top and bottom surface of the specimens, following dry storage for 24 hours in the dark. Statistical analysis were performed using the multifactor ANOVA at a level of significance of a=0.05.
Result: The results indicated that there was an increase in microhardness as the temperature of the composite resin was increased from 22 to 55oC for both composite resins for either the top or the bottom surface (Filtek Z250: 22oC - 74±1.2 VHN, 55oC - 81±2.0 VHN, 40 sec light-curing time, top surface) (p<0.05). Furthermore, there was a general increase in microhardness for both the microhybrid and nanohybrid composite resins as light-curing time increased (Grandio: 10 sec - 50±0.8 VHN, 20 sec - 68±1.4 VHN, 40 sec - 71±1.2 VHN, 22oC, top surface) (p<0.05).
Conclusion: Preheating composite resins, with a commercially available composite warmer increases the microhardness of the tested composite resins. Light-curing time influence microhardness values of the composite resins.