Purpose: the aim of this investigation was to determine efficacy of QTH light curing units used in a sample of dental offices in Toronto through determination of effect of curing time on hardness of a resin composite. Methods: A total of 416 offices were selected following specific criteria for inclusion, including that the dentist routinely placed posterior composite restorations. Only 100 of those offices agreed to take part. Two investigators visited each office to prepare specimens for hardness testing. A resin composite (Point 4 – SDS/Kerr) was used to prepare disc specimens 2.5 mm thick. Two specimens were light cured for 20 s from top surface and 2 were cured for 40 s. Using a Knoop hardness tester, 4 Knoop Hardness Numbers (KHNs) were obtained from top surface and 4 from bottom surface of each specimen. Mean KHNs were calculated and relative hardness (hardness of bottom/top surface) was determined. Light intensity of each curing unit was also determined and correlations were made between light intensity and relative hardness (RH). A total of 214 curing units were included in the study. Results: A wide range of RH values for the different curing units was found. With 20 s curing 124 units with intensity less than 600 mW/cm2 had RH values less than 40%. With 40 s curing a significant increase in RH values occurred with fewer units resulting in RH values less than 40%. Conclusions: Relative hardness values varied considerably among the curing units with some producing values close to zero. A direct correlation was observed between light intensity and RH. Increasing curing time resulted in a significant increase in RH values with a considerable number of curing units. Approved by Faculty of Dentistry Research Committee and supported in part by SDS/Kerr.

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