title: Effect of the irradiance distribution from light curing units on the local micro-hardness of the surface of dental resins.
abstract: OBJECTIVE
  An inhomogeneous irradiance distribution from a light-curing unit (LCU) can locally cause inhomogeneous curing with locally inadequately cured and/or over-cured areas causing e.g. monomer elution or internal shrinkage stresses, and thus reduce the lifetime of dental resin based composite (RBC) restorations. The aim of the study is to determine both the irradiance distribution of two light curing units (LCUs) and its influence on the local mechanical properties of a RBC.
  
  
  METHODS
  Specimens of Arabesk TOP OA2 were irradiated for 5, 20, and 80s using a Bluephase® 20i LCU in the Low mode (666mW/cm(2)), in the Turbo mode (2222mW/cm(2)) and a Celalux® 2 (1264mW/cm(2)). The degree of conversion (DC) was determined with an ATR-FTIR. The Knoop micro-hardness (average of five specimens) was measured on the specimen surface after 24h of dark and dry storage at room temperature.
  
  
  RESULTS
  The irradiance distribution affected the hardness distribution across the surface of the specimens. The hardness distribution corresponded well to the inhomogeneous irradiance distributions of the LCU. The highest reaction rates occurred after approximately 2s light exposure. A DC of 40% was reached after 3.6 or 5.7s, depending on the LCU. The inhomogeneous hardness distribution was still evident after 80s of light exposure.
  
  
  CONCLUSIONS
  The irradiance distribution from a LCU is reflected in the hardness distribution across the surface. Irradiance level of the LCU and light exposure time do not affect the pattern of the hardness distribution--only the hardness level. In areas of low irradiation this may result in inadequate resin polymerization, poor physical properties, and hence premature failure of the restorations as they are usually much smaller than the investigated specimens. It has to be stressed that inhomogeneous does not necessarily mean poor if in all areas of the restoration enough light intensity is introduced to achieve a high degree of cure.
authors.affiliation: Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, Zlin, Czech Republic; Bonn-Rhein-Sieg, University of Applied Sciences, Department of Natural Sciences, Rheinbach, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Haenel
authors.affiliation: Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, Zlin, Czech Republic; Tomas Bata University in Zlin, Faculty of Technology, Department of Production Engineering, Zlin, Czech Republic.
authors.forename: Berenika
authors.initials: B
authors.lastname: Hausnerová
authors.affiliation: Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, Zlin, Czech Republic; Bonn-Rhein-Sieg, University of Applied Sciences, Department of Natural Sciences, Rheinbach, Germany.
authors.forename: Johannes
authors.initials: J
authors.lastname: Steinhaus
authors.affiliation: Dalhousie University, Department of Dental Clinical Sciences, Faculty of Dentistry, Halifax, Canada.
authors.forename: Richard B T
authors.initials: RB
authors.lastname: Price
authors.affiliation: Dalhousie University, Department of Dental Clinical Sciences, Faculty of Dentistry, Halifax, Canada.
authors.forename: Braden
authors.initials: B
authors.lastname: Sullivan
authors.affiliation: Bonn-Rhein-Sieg, University of Applied Sciences, Department of Natural Sciences, Rheinbach, Germany. Electronic address: bernhard.moeginger@fh-brs.de.
authors.forename: Bernhard
authors.initials: B
authors.lastname: Moeginger
mesh_terms: D055117:Curing Lights, Dental; D006244:Hardness; D008027:Light; D008422:Materials Testing; D012117:Resins, Synthetic / Q000737:chemistry*; D012542:Scattering, Radiation; D017550:Spectroscopy, Fourier Transform Infrared; D013499:Surface Properties; D013997:Time Factors
chemical_list: D012117:Resins, Synthetic
keywords: Degree of conversion; Dental resin; FTIR; Hardness mapping; Irradiance distribution; Knoop micro-hardness; Light curing units