Objectives: The use of dual cure self-etching resin cements to retain fixed restorations has increased significantly over the past 10 years. This study examined the effect time and temperature has on the micro-hardness of four self-etching dual-cure resin cements when they were not light cured.
Methods: Four dual cure cements, Unicem (3M ESPE), G-Cem (G-C), Maxcem Elite (Kerr), SmartCem2 (Dentsply) were mixed according to their manufacturer's instructions and placed in a mold. The specimens were not light cured. To prevent the formation of an air-inhibited layer and simulate the inside of a crown, the top and bottom surfaces were covered with Mylar strips that were left in place during storage. The Knoop microhardness was tested 30 minutes, 60 minutes, 24 hours and at 7 days after mixing and storage in the dark at 22ºC or at 37ºC. Five specimens were made and tested at each time interval. The Knoop microhardness results were analyzed with ANOVA and Fisher's PLSD at α = 0.05.
Results: Temperature and time had a significant effect on the Knoop microhardness of all the cements. At the 30 minute and 60 minute time interval, Unicem was too soft to measure, and significantly softer than the other cements. However, after one week storage in the dark at 22ºC or 37ºC, Unicem was the hardest cement. G-Cem was the hardest cement at 30 min., 60 min., and at 24 hours.
| G-Cem | Maxcem Elite | SmartCem 2 | Unicem | ||||
Time | 22ºC Mean KHN | 37ºC Mean KHN | 22ºC Mean KHN | 37ºC Mean KHN | 22ºC Mean KHN | 37ºC Mean KHN | 22ºC Mean KHN | 37ºC Mean KHN |
30mins | 9.5(a) | 12.2(d) | 8.6 | 16.8 | 5.6 | 11.2(g) | 0.0(b) | 4.6 |
60 mins | 17.4 | 33.8 | 15.0 | 31.9(e) | 9.6(a) | 20.8 | 0.0(b) | 11.8(d,g) |
1 day | 32.7 | 50.8 | 20.6 | 47.9 | 18.8(c) | 31.2(e) | 19.5(c) | 59.8 |
1 week | 37.5 | 52.3(f) | 29.4 | 52.4(f) | 21.5 | 31.7(e) | 46.4 | 70.4 |
Letters in parentheses = Homogeneous subsets (Fisher's α=0.05)
Conclusions: Although Unicem was the hardest cement at 7 days after mixing. At 22ºC this cement takes time to set and was the softest cement at the 30 minute and 60 minute time intervals. Wherever possible these cements should be light cured.
Dalhousie University supported this study.