Hardening of Dual-Cure Resin Cements and a Resin Composite Restorative Cured with QTH and LED Curing Units

Objective: The aim of this study was to determine the effects of light intensity and type of light unit (quartztungsten-halogen [QTH] or light-emitting diode [LED]) on the hardening of various resin cements and a resin composite restorative. Methods: Disk specimens were prepared from 4 dual-cured resin cements (Variolink II, Calibra, Nexus 2 and RelyX ARC). Two QTH light-curing units (Visilux 2, at 550 mW/cm2, and Optilux 501, at 1,360 mW/cm2) and a LED unit (Elipar FreeLight, at 320 mW/cm2) were used for curing. Specimens were light-cured or dual-cured for 10, 30 or 40 seconds with 1 of the 3 light units (curing applied to upper surface only) and were tested 24 hours after curing. Additional cement specimens were self-cured and tested at 15, 30 and 60 minutes and at 24 hours. Testing consisted of measurement of Knoop hardness number (KHN) for each specimen. Six KHN values were obtained for the upper surface only of the various cement specimens in each test group. Disk specimens 2.5 mm thick were also prepared from a resin composite restorative (XRV Herculite). These were light-cured as above, and KHN measurements were obtained for both the upper and the lower surfaces. Mean KHNs were determined, and data were analyzed with analysis of variance. Results: The groups were significantly different (p < 0.05). High-intensity light curing resulted in the highest KHN values for all materials with any of the 3 light-curing times. For the cements, LED light curing (with both dual-curing and light-curing modes) resulted in hardness values similar to those achieved with conventional QTH light curing, although there were some exceptions. However, both LED and conventional QTH light curing resulted in inferior hardening of lower surfaces of the XRV Herculite specimens at the 3 curing times. For all cements except Nexus 2, self-curing resulted in significantly lower hardness values than dual curing. The self-curing mechanism of Variolink II cement needed a longer time to activate than those of the other cements. Conclusions: High-intensity light curing and longer curing times resulted in the highest KHNs. The LED curing unit was associated with the lowest hardness values for lower surfaces of the resin composite restorative.