Influence of different exposure times required to stabilize hardness values of composite resin restorations

Aim: The purpose of this study was to evaluate if Knoop hardness values (KHN) for top and bottom surfaces of resin composite materials can reach a plateau within a clinically acceptable photoactivation time. Methods and Materials: Four light-curing units (LCUs) were evaluated in this study (n=5): QTH (Optilux501: 550 mW/cm2) and LEDs (FreeLight2: 1100 mW/cm2; UltraLume5: 900 mW/cm2; and Radii: 750 mW/cm2). Composite resin discs (4 mm x 2 mm) of Heliomolar (Ivoclar/Vivadent) and Herculite XRV (Kerr) were tested using five photoactivation times (20, 40, 60, 80, and 100 seconds). KHN were obtained for each test specimen and comparisons between LCUs, depths, and photoactivation times were analyzed using two-way analysis of variance (ANOVA) and polynomial regression analysis. Results: Data for Heliomolar discs using linear regression found a relationship between the independent variables KHN and time with the Optilux501 at the top and bottom surfaces (r2=0.68/ r2=0.66). Radii presented a linear regression at the top surface (r2=0.75) and a quadratic regression at the bottom (r2=0.94). A quadratic regression was also detected for UltraLume5 and FreeLight2 at both top (r2=0.84/ r2=0.94) and bottom surfaces (r2=0.97/ r2=0.90), respectively, reaching a plateau at 80 seconds in all cases. For Herculite XRV, a quadratic regression was observed for all LCUs at the top and bottom surfaces and 80 seconds irradiation time was needed to reach a plateau in KHN. Conclusion: There is a specific, but not clinically acceptable, photoactivation time that KHN at both top and bottom surfaces can reach a plateau and is dependent on LCUs and the resin-composite tested. Clinical Significance: The LCUs and the resin-composite formulation affected the exposure time required to stabilize hardness values. The overall performance of LED LCUs was better than the QTH LCU regardless of the material evaluated.