title: Polymerization of resin composite restorative materials: exposure reciprocity.
abstract: OBJECTIVE
  To examine whether there is reciprocity between irradiation time and irradiance with regard to the mechanical properties of filled, resin composite restorative materials (RCs).
  
  
  METHODS
  Four visible light-cured RCs, all of shade A3, were used: Heliomolar Radiopaque (HR) and Tetric Ceram (TC) (Ivoclar, Schaan, Liechtenstein), Filtek Z250 (FZ) (3M, St Paul, MN, USA) and Prodigy condensable (PR) (Kerr, Orange, CA, USA). Bar specimens (1.0 x 1.5 x 16.0 mm(3)) were cured at irradiances (I) ranging from 25-1500 mW/cm(2) and irradiation times (t) of 1-3000 s. Six specimens at 250 combinations of t and I were prepared and stored in artificial saliva of pH 6, at 37 degrees C for 7d before performing three-point bend tests for flexural strength (F), flexural modulus (E) and total energy to failure (W). Contour plots of property value vs. t and I on log scales were prepared.
  
  
  RESULTS
  The contour plots showed three regions: unset at low I.t, a plateau corresponding to more or less full property development, and connecting ramp. The boundary between the plateau and the ramp suggests the minimum acceptable exposure. No practical lower limit to irradiance was detected, but there may be no benefit from increasing I beyond about 1,000 mW/cm(2). The slopes of the contours in the log-log plots provided a test of the hypothesis of reciprocity. These slopes were approximately -1.5 for HR, TC and PR; and approximately -1 for FZ, compared with the expected value of -1. The general hypothesis therefore fails. The existence of localized maxima in property values is further evidence of that failure, even for FZ.
  
  
  CONCLUSIONS
  Dentists may use any lamp, including LED sources, and attain satisfactory results providing irradiation time is long enough. Manufacturers ought to supply a graph indicating the minimum acceptable exposure for each product for specified curing lamps. Calculations based on total energy delivered to guide irradiation protocols are invalid and do not recognize product behavior.
authors.affiliation: Dental Materials Science, Faculty of Dentistry, The University of Hong Kong, Prince Philip Dental Hospital, 34 Hospital Road, Hong Kong, People's Republic of China.
authors.forename: L
authors.initials: L
authors.lastname: Musanje
authors.forename: B W
authors.initials: BW
authors.lastname: Darvell
mesh_terms: D000180:Acrylic Resins / Q000528:radiation effects; D003188:Composite Resins / Q000528:radiation effects*; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis; D004548:Elasticity / Q000528:radiation effects; D008027:Light; D008422:Materials Testing; D018583:Pliability / Q000528:radiation effects; D011108:Polymers / Q000737:chemistry; D011140:Polyurethanes / Q000528:radiation effects; D011829:Radiation Dosage; D013674:Technology, Dental / Q000379:methods; D013997:Time Factors
chemical_list: D000180:Acrylic Resins; C029012:Composite Dental Resin; D003188:Composite Resins; C444701:Filtek Z250; C513708:Heliomolar; D011108:Polymers; D011140:Polyurethanes; C477351:Prodigy Condensable; C121818:Tetric ceram