title: Correlation between the beam profile from a curing light and the microhardness of four resins.
abstract: OBJECTIVE
  To demonstrate the effect of localized irradiance and spectral distribution inhomogeneities of one LED-based dental light-curing unit (LCU) on the corresponding microhardness values at the top, and bottom surfaces of four dental resin-based composites (RBCs), which contained either camphorquinone (CQ) alone or a combination of CQ and monoacylphosphine oxide (TPO) as photoinitiators.
  
  
  METHODS
  Localized irradiance beam profiles from a polywave LED-based LCU were recorded five times using a laser beam analyzer, without and with either a 400 nm or 460 nm narrow bandpass filter placed in front of the camera lens. Five specimens of each of the four RBCs (two containing CQ/TPO and two containing CQ-only) were exposed for 5-, 10-, or 30-s with the light guide directly on the top surface of the RBC. After 24 h, Knoop microhardness values were measured at 45 locations across the top and bottom surfaces of each specimen. Microhardness readings for each RBC surface and exposure time were correlated with localized patterns of the LCU beam profile, measured using the 400 nm and 460 nm bandpass filters. Spearman rank correlation was used to avoid relying on an assumption of a bivariate normal distribution for the KHN and irradiance.
  
  
  RESULTS
  The local irradiance and spectral emission values were not uniformly distributed across the light tip. There was a strong significant positive correlation with the irradiance beam profile values from the LCU taken through bandpass filters and the microhardness maps of the RBC surfaces exposed for 5 and 10 s. The strength of this correlation decreased with increasing exposure time for the RBCs containing CQ only, and increased for the RBCs containing both CQ and TPO.
  
  
  CONCLUSIONS
  Localized beam and spectral distributions across the tip end of the light guide strongly correlated with corresponding areas of microhardness in both the top and bottom surfaces among four RBCs with different photoinitiator contents. Significance: A light-curing unit with a highly inhomogeneous light output can adversely affect localized microhardness of resin-based composites and this may be a contributing factor for premature failure of a restoration.
authors.affiliation: Dental Clinical Sciences, Dalhousie University, Halifax, NS, Canada. rbprice@dal.ca
authors.forename: Richard B T
authors.initials: RB
authors.lastname: Price
authors.forename: Daniel
authors.initials: D
authors.lastname: Labrie
authors.forename: Frederick A
authors.initials: FA
authors.lastname: Rueggeberg
authors.forename: Braden
authors.initials: B
authors.lastname: Sullivan
authors.forename: Ivan
authors.initials: I
authors.lastname: Kostylev
authors.forename: John
authors.initials: J
authors.lastname: Fahey
mesh_terms: D002164:Camphor / Q000031:analogs & derivatives / 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*; D006244:Hardness; D006801:Humans; D008422:Materials Testing; D010720:Phosphines / Q000737:chemistry / Q000528:radiation effects; D058567:Photoinitiators, Dental / Q000737:chemistry / Q000528:radiation effects; D058105:Polymerization; D011829:Radiation Dosage; D013499:Surface Properties; D013997:Time Factors
chemical_list: C520678:(2,4,6-trimethylbenzoyl) diphenylphosphine oxide; D003188:Composite Resins; D003764:Dental Materials; C515951:Filtek Supreme Plus; C000590992:Filtek Supreme Ultra; D010720:Phosphines; D058567:Photoinitiators, Dental; C501891:Tetric EvoCeram; D002164:Camphor; C553149:camphorquinone