title: Influence of different exposure time required to stabilize hardness values of composite resin restorations.
abstract: OBJECTIVE
  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
  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.
  
  
  CONCLUSIONS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Restorative Dentistry, Dental School at The University of Passo Fundo, Brazil. lara_cavalcante@yahoo.com.br
authors.forename: Larissa Maria Assad
authors.initials: LM
authors.lastname: Cavalcante
authors.forename: Thiago Assunção
authors.initials: TA
authors.lastname: Valentino
authors.forename: Bruno
authors.initials: B
authors.lastname: Carlini
authors.forename: Nick
authors.initials: N
authors.lastname: Silikas
authors.forename: Luiz André Freire
authors.initials: LA
authors.lastname: Pimenta
mesh_terms: D000180:Acrylic Resins / Q000737:chemistry / Q000528:radiation effects; D003188:Composite Resins / Q000737:chemistry* / Q000528:radiation effects; D055117:Curing Lights, Dental / Q000145:classification; D003764:Dental Materials / Q000737:chemistry* / Q000528:radiation effects; D003793:Dental Restoration, Permanent; D006244:Hardness; D006801:Humans; D008422:Materials Testing; D011140:Polyurethanes / Q000737:chemistry / Q000528:radiation effects; D011829:Radiation Dosage; D019279:Resin Cements / Q000737:chemistry / Q000528:radiation effects; D013499:Surface Properties; D013997:Time Factors
chemical_list: D000180:Acrylic Resins; D003188:Composite Resins; D003764:Dental Materials; C513708:Heliomolar; D011140:Polyurethanes; D019279:Resin Cements; C065770:Herculite XR