title: Influence of veneering porcelain thickness and cooling rate on residual stresses in zirconia molar crowns.
abstract: OBJECTIVE
  The aim of this study was to investigate the influence of increasing veneering porcelain thickness in clinically representative zirconia molar crowns on the residual stresses under fast and slow cooling protocols.
  
  
  METHODS
  Six veneered zirconia copings (Procera, Nobel Biocare AB, Gothenburg, Sweden) based on a mandibular molar form, were divided into 3 groups with flattened cusp heights that were 1mm, 2mm, or 3mm. Half the samples were fast cooled during final glazing; the other half were slow cooled. Vickers indentation technique was used to determine surface residual stresses. Normality distribution within each sample was done using Kolmogorov-Smirnov & Shapiro-Wilk tests, and one-way ANOVA tests used to test for significance between various cusp heights within each group. Independent t-tests used to evaluate significance between each cusp height group with regards to cooling.
  
  
  RESULTS
  Compressive stresses were recorded with fast cooling, while tensile stresses with slow cooling. The highest residual compressive stresses were recorded on the fast cooled 1mm cusps which was significantly higher than the 2 and 3mm fast cooled crowns (P<0.05). There was a significant linear trend for residual stress to decrease as veneering porcelain thickness increased in the fast cooled group (P<0.05). No significant differences were found between the various cusp heights during slow cooling (P≥0.05).
  
  
  CONCLUSIONS
  Cooling rate and geometric influences in a crown anatomy have substantially different effects on residual stress profiles with increasing veneering porcelain thickness compared to the basic flat plate model.
authors.affiliation: Sir John Walsh Research Institute, University of Otago, Dunedin, New Zealand. Electronic address: basil.al-amleh@otago.ac.nz.
authors.forename: Basil
authors.initials: B
authors.lastname: Al-Amleh
authors.affiliation: Sir John Walsh Research Institute, University of Otago, Dunedin, New Zealand.
authors.forename: J
authors.initials: J
authors.lastname: Neil Waddell
authors.affiliation: Sir John Walsh Research Institute, University of Otago, Dunedin, New Zealand.
authors.forename: Karl
authors.initials: K
authors.lastname: Lyons
authors.affiliation: Sir John Walsh Research Institute, University of Otago, Dunedin, New Zealand.
authors.forename: Michael V
authors.initials: MV
authors.lastname: Swain
mesh_terms: D019245:Compressive Strength; D003442:Crowns; D003776:Dental Porcelain / Q000737:chemistry*; D003799:Dental Stress Analysis / Q000379:methods*; D003801:Dental Veneers; D008422:Materials Testing; D008963:Molar; D013499:Surface Properties; D013696:Temperature; D013718:Tensile Strength; D015040:Zirconium / Q000737:chemistry*
chemical_list: D003776:Dental Porcelain; D015040:Zirconium; C028541:zirconium oxide
keywords: Chipping; Cooling protocol; Crowns; Porcelain; Residual stresses; Thickness of veneer; Vickers indentation; Zirconia