title: Slow cooling protocol improves fatigue life of zirconia crowns.
abstract: OBJECTIVE
  To compare the fatigue life and damage modes of zirconia crowns fabricated with and without framework design modification when porcelain veneered using a fast or slow cooling protocol.
  
  
  METHODS
  Composite resin replicas of a first molar full crown preparation were fabricated. Zirconia copings were milled as conventional (0.5mm even thickness, Zr-C, n=20,) or modified (lingual margin of 1.0mm thickness, 2.0mm height connected to two proximal struts of 3.5mm height, Zr-M, n=20). These groups were subdivided (n=10 each) according to the veneer cooling protocol employed: fast cooling (Zr-CFast and Zr-MFast) and slow cooling (Zr-CSlow and Zr-MSlow). Crowns were cemented and fatigued for 10(6) cycles in water. The number of cycles to failure was recorded and used to determine the interval databased 2-parameter probability Weibull distribution parameter Beta (β) and characteristic life value Eta (η).
  
  
  RESULTS
  2-parameter Weibull calculation presented β=5.53 and β=4.38 for Zr-MFast and Zr-CFast, respectively. Slow cooled crowns did not fail by completion of 10(6) cycles, thereby Weibayes calculation was applied. Increased fatigue life was observed for slow cooled crowns compared to fast cooled ones. Groups Zr-MFast and Zr-MSlow presented no statistical difference. Porcelain cohesive fractures were mainly observed in fast cooled groups. Slow cooled crowns presented in some instances inner cone cracks not reaching the zirconia/veneer interface.
  
  
  CONCLUSIONS
  Improved fatigue life in tandem with the absence of porcelain fractures were observed in slow cooled crowns, regardless of framework design. Crowns fast cooled chiefly failed by porcelain cohesive fractures.
authors.affiliation: Department of Prosthodontics, University of São Paulo - Bauru College of Dentistry, Bauru, SP, Brazil.
authors.forename: Vitor G
authors.initials: VG
authors.lastname: Paula
authors.affiliation: Department of Prosthodontics, UNIGRAN University, Dourados, MS, Brazil.
authors.forename: Fabio C
authors.initials: FC
authors.lastname: Lorenzoni
authors.affiliation: Department of Prosthodontics, University of São Paulo - Bauru College of Dentistry, Bauru, SP, Brazil. Electronic address: estevamab@gmail.com.
authors.forename: Estevam A
authors.initials: EA
authors.lastname: Bonfante
authors.affiliation: Department of Restorative Dentistry, Federal University of Minas Gerais, UFMG, Belo Horizonte, MG, Brazil.
authors.forename: Nelson R F A
authors.initials: NR
authors.lastname: Silva
authors.affiliation: Department of Biomaterials, Biomimetics and Biophotonics, King's College London, Guy's Hospital, London, United Kingdom.
authors.forename: Van P
authors.initials: VP
authors.lastname: Thompson
authors.affiliation: Department of Prosthodontics, University of São Paulo - Bauru College of Dentistry, Bauru, SP, Brazil.
authors.forename: Gerson
authors.initials: G
authors.lastname: Bonfante
mesh_terms: D003442:Crowns; D003776:Dental Porcelain; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis / Q000379:methods*; D003801:Dental Veneers; D008422:Materials Testing; D013499:Surface Properties; D013696:Temperature; D015040:Zirconium / Q000737:chemistry*
chemical_list: D003776:Dental Porcelain; D015040:Zirconium; C028541:zirconium oxide
keywords: Ceramics; Cooling rate; Fatigue; Fractography; Weibayes; Zirconia