title: Effect of repeated firings on flexural strength of veneered zirconia.
abstract: OBJECTIVE
  Chipping and/or delamination represent a clinical failure of porcelain fused to zirconia (PFZ) prostheses. Causes and solutions have not been completely clarified. The present study was aimed at evaluating the effects of number of firings on the flexural strength of PFZ specimen.
  
  
  METHODS
  Forty-five zirconia specimens in shape of bars were cut, sintered and divided in 3 groups (n=15). Group 1: veneering ceramic was layered "in bulk" and fired. Group 2: veneering ceramic was layered in three layers, individually fired. Group 3: veneering ceramic was layered in five layers, individually fired. Each layer thickness was controlled by the use of calibrated molds. The total veneering ceramic thickness for all the specimens was 1.2mm, and the total thickness of the specimen of 2.0mm. Three-point bending test was performed. Fracture load was recorded in Newton and MPa value was calculated taking into account the bi-layered nature of the specimen. Data were statistically analyzed.
  
  
  RESULTS
  Specimens obtained with on single firing cycle obtained a statistically significant (p<0.001) lower flexural strength (54.61±8.98MPa) than specimens veneered with 3 or 5 firing cycles. The last two obtained very similar results (77.63±13.17MPa and 73.62±12.38MPa respectively) and the differences was not statistically significant.
  
  
  CONCLUSIONS
  In bi-layered PFZ specimen, three to five layers and firings determine higher flexural resistance when compared to a single firing. Thus, a 3-layers veneering procedure is recommended to increase flexural resistance. If a 5-layer procedure is necessary to improve esthetics, it does not decrease flexural resistance.
authors.affiliation: Department of Medical Biotechnologies, University of Siena, Italy. Electronic address: alessandrovichi1@gmail.com.
authors.forename: A
authors.initials: A
authors.lastname: Vichi
authors.affiliation: Department of Medical Biotechnologies, University of Siena, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Sedda
authors.affiliation: Department of Medical Biotechnologies, University of Siena, Italy.
authors.forename: G
authors.initials: G
authors.lastname: Bonadeo
authors.affiliation: Department of Dentistry, University of Pavia, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Bosco
authors.affiliation: Department of Medical Biotechnologies, University of Siena, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Barbiera
authors.affiliation: Department of Medical Biotechnologies, University of Siena, Italy.
authors.forename: N
authors.initials: N
authors.lastname: Tsintsadze
authors.affiliation: Department of Medical Biotechnologies, University of Siena, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Carrabba
authors.affiliation: Department of Medical Biotechnologies, University of Siena, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Ferrari
mesh_terms: D001840:Dental Bonding; D003764:Dental Materials / Q000737:chemistry; D003776:Dental Porcelain / Q000737:chemistry*; D003799:Dental Stress Analysis; D003801:Dental Veneers; D006358:Hot Temperature; D008422:Materials Testing; D018583:Pliability; D013314:Stress, Mechanical; D013499:Surface Properties; D015040:Zirconium / Q000737:chemistry*
chemical_list: D003764:Dental Materials; D003776:Dental Porcelain; D015040:Zirconium; C028541:zirconium oxide
keywords: CAD/CAM; Ceramic; Ceramic firing; Ceramic veneering; Chipping; Fracture load; Zirconia