title: Effect of curing light emission spectrum on the nanohardness and elastic modulus of two bulk-fill resin composites.
abstract: OBJECTIVE
  To determine the nanohardness and elastic moduli of two bulk-fill resin based composites (RBCs) at increasing depths from the surface and increasing distances laterally from the center after light curing.
  
  
  METHODS
  Two bulk-fill dental RBCs: Tetric EvoCeram Bulk Fill (TECBF) and Filtek Bulk Fill Flowable (FBFF) were light cured in a metal mold with a 6mm diameter and a 10mm long semi-circular notch. The RBCs were photo-polymerized for 10s using a light emitting diode (LED) Bluephase Style curing light, with the original light probe that lacked the homogenizer. This light has two blue light and one violet light LED emitters. By changing the probe orientation over the mold, the light output from only two LEDs reached the RBC. Measurements were made using: (i) the light from one violet and one blue LED, and (ii) the light from the two blue LEDs. Five specimens of each RBC were made using each LED orientation (total 20 specimens). Specimens were then stored in the dark at 37°C for 24h. Fifty indents were made using an Agilent G200 nanoindentor down to 4mm from the surface and 2.5mm right and left of the centerline. The results were analyzed (alpha=0.05) using multiple paired-sample t-tests, ANOVA, Bonferroni post-hoc tests, and Pearson correlations.
  
  
  RESULTS
  The elastic modulus and nanohardness varied according to the depth and the distance from the centerline. For TECBF, no significant difference was found between the spatial variations in the elastic modulus or hardness values when violet-blue or blue-blue LEDs were used. For FBFF, the elastic modulus and nanohardness on the side exposed to the violet emitter were significantly less than the side exposed to the blue emitter. A strong correlation between nanohardness and elastic modulus was found in all groups (r(2)=0.9512-0.9712).
  
  
  CONCLUSIONS
  Resin polymerization was not uniform throughout the RBC. The nanohardness and elastic modulus across two RBC materials were found to decline differently according to the orientation of the violet and blue light LED emitters within the curing light.
authors.affiliation: Faculty of Dentistry, Dalhousie University, Halifax, Nova Scotia, Canada.
authors.forename: Yaser
authors.initials: Y
authors.lastname: Issa
authors.affiliation: University of Manchester, School of Dentistry and Photon Science Institute, Manchester, UK.
authors.forename: David C
authors.initials: DC
authors.lastname: Watts
authors.affiliation: Dept of Applied Oral Sciences, Faculty of Dentistry, Dalhousie University, Halifax, Nova Scotia, Canada.
authors.forename: Daniel
authors.initials: D
authors.lastname: Boyd
authors.affiliation: Dept of Dental Clinical Sciences, Faculty of Dentistry, Dalhousie University, Halifax, Nova Scotia, Canada. Electronic address: rbprice@dal.ca.
authors.forename: Richard B
authors.initials: RB
authors.lastname: Price
mesh_terms: D003188:Composite Resins / Q000737:chemistry*; D055117:Curing Lights, Dental; D055119:Elastic Modulus; D006244:Hardness; D055115:Light-Curing of Dental Adhesives / Q000379:methods*; D008422:Materials Testing; D058105:Polymerization; D013314:Stress, Mechanical; D013499:Surface Properties; D013696:Temperature
chemical_list: D003188:Composite Resins; C000614713:Filtek Bulk Fill; C501891:Tetric EvoCeram
keywords: Bulk fill; Elastic modulus; Emission spectrum; Light curing unit; Light emitting diode; Nano-indentation; Nanohardness; Photo polymerization; Resin based composite