title: Quantification of the amount of light passing through zirconia: the effect of material shade, thickness, and curing conditions.
abstract: OBJECTIVE
  This study aimed to evaluate the amount of light (360-540 nm) passing through shaded zirconia with respect to material thickness, exposure distance, and different curing modes.
  
  
  METHODS
  The specimens were divided into groups according to thickness as follows: 0.5, 1, 1.5, 2, 2.5, and 3 mm. Thirty-five zirconia and seven glass-ceramic (control group) specimens were fabricated for each group (N=252). Zirconia was divided into five subgroups (n=7) and stained to the following shades: CL1, CL2, CL3, and CL4. One zirconia group remained unstained (CL0). Irradiance passing through the different specimens was measured using a violet-blue LED curing unit in three curing modes (Xtra-power, high-power, and standard-power mode) with a fibre-optic USB4000 spectrometer. Irradiance was measured at varying exposure distances, ranging from direct contact of the curing unit with the surface to a distance of 7 mm from the surface, increasing in 1 mm steps. Data were analyzed using a multivariate analysis and linear mixed models (p<0.05).
  
  
  RESULTS
  The control group, the glass-ceramics, transmitted the highest irradiance values, followed by CL0 (unshaded zirconia), CL1 (~A1/B1), CL2 (~A3/A3.5/A4/B3/B4), and CL3 (~A3.5/B3/B4/C3/D3), respectively. The highest transmitted irradiance was measured at a specimen thickness of 0.5 mm for all materials, decreasing exponentially with increased ceramic thickness. Within one type of ceramic, one thickness, and one polymerization mode, a decrease in transmitted irradiance with increased exposure distance could be observed only at a distance of 3 mm and above.
  
  
  CONCLUSIONS
  Unshaded zirconia was significantly less translucent compared with the glass-ceramic, but the translucency decreased slower with material thickness. The Beer-Lambert law describes well the decrease of transmitted irradiance with an increase of the specimens' thickness for all materials. Except for dark ceramics, this would allow for calculating the transmitted irradiance through any material thickness and any initial irradiance.
  
  
  CONCLUSIONS
  The amount of light passing through ceramics is an important aspect for an adhesive cementation, since many dual-cured luting materials reveal a high sensitivity to additional occurrence of blue light. For restorations thicker than 1.5 mm in light-shaded zirconia and 0.5 mm in darker-shaded zirconia the use of less-light-sensitive dual-cured cements are recommended.
authors.affiliation: Department of Operative/Restorative Dentistry, Periodontology and Pedodontics, Dental School, Ludwig-Maximilians University, Munich, Germany. Electronic address: nilie@dent.med.uni-muenchen.de.
authors.forename: Nicoleta
authors.initials: N
authors.lastname: Ilie
authors.affiliation: Department of Prosthodontics, Dental School, Ludwig-Maximilians University, Munich, Germany.
authors.forename: Bogna
authors.initials: B
authors.lastname: Stawarczyk
mesh_terms: D000538:Aluminum Silicates / Q000737:chemistry / Q000528:radiation effects; D002516:Ceramics / Q000737:chemistry / Q000528:radiation effects; D002712:Chlorides / Q000737:chemistry; D003116:Color; D004396:Coloring Agents / Q000737:chemistry; D055117:Curing Lights, Dental / Q000145:classification; D003764:Dental Materials / Q000737:chemistry / Q000528:radiation effects*; D004871:Erbium / Q000737:chemistry; D005290:Ferric Compounds / Q000737:chemistry; D006801:Humans; D008027:Light; D008422:Materials Testing; D017680:Potassium Compounds / Q000737:chemistry / Q000528:radiation effects; D011897:Random Allocation; D013057:Spectrum Analysis / Q000295:instrumentation; D013499:Surface Properties; D015040:Zirconium / Q000737:chemistry / Q000528:radiation effects*
chemical_list: D000538:Aluminum Silicates; D002712:Chlorides; D004396:Coloring Agents; D003764:Dental Materials; D005290:Ferric Compounds; D017680:Potassium Compounds; C053952:erbium chloride; C016447:feldspar; D004871:Erbium; C023506:Glass ceramics; D015040:Zirconium; C028541:zirconium oxide; C024555:ferric chloride
keywords: Blue light; Irradiance; Translucency; Zirconia