Abstract: Objective To determine both short-term and long-term effect of preheating on the microhardness of 4 composite resins. Methods 24 samples (5 mm diameter×2 mm height) of each of the 4 composite materials (Z250, Dyract, Tetric and SDR), were divided randomly into 3 groups (n=8 per group). These groups were cured respectively with curing light at 3 different temperatures (room temperature, pre-heated to 37 ℃, pre-heated to 55 ℃). After curing, microhardness of both top and bottom surfaces of each sample was measured by microhardness tester at moments, 24 hours later, 1 week later and 1 month later. Data were analyzed by SPSS. Results There was no significant difference in microhardness between top and bottom of the 3 composite resins at 3 different temperatures except Dyract. There was no significant difference in microhardness between the 4 composite resins at 3 different temperatures. With the time lapsing, the microhardness of the 4 composite resins gradually increased and finally tended to stability. Conclusion Preheating has no effect on microhardness of these 4 composite resins.

Key words: preheating, composite resin, microhardness