title: Influence of thickness and cooling rate on development of spontaneous cracks in porcelain/zirconia structures.
abstract: BACKGROUND
  Clinical studies reporting the outcome of zirconia-based restorations indicate that the strength of the zirconia frameworks is sufficient to withstand masticatory forces. However, a significant incidence of cohesive fracture of the veneering porcelain has been reported. The aim of this study was to investigate spontaneous crack development (chipping, rupture) in a range of porcelains veneered to a zirconia core as a result of thermal stresses induced by changes in thickness and cooling rate. The hypothesis tested was that crack incidence would increase with increased veneer thickness and faster cooling rates.
  
  
  METHODS
  Zirconia spheres (diameter 7.8 mm) were veneered with 1.5 gm (thickness ratio 1:2) and 2.5 gm (thickness ratio 1:1) of five nominally compatible commercially available porcelains. The manufacturers' firing cycles and a rapid cooling firing cycle were followed.
  
  
  RESULTS
  Multiple regression analysis showed positive associations between the occurrence of cracks and the three covariates (materials, thickness and cooling rate). The incidence of cracks and rupture of the veneering porcelain increased with a faster cooling rate and increased thickness of the specimens in three porcelain-zirconia combinations.
  
  
  CONCLUSIONS
  Crack incidence increased with increased porcelain veneer thickness and faster cooling rates in nominally compatible porcelain/zirconia systems in the geometrically configured specimens tested.
authors.affiliation: Discipline of Prosthodontics, Faculty of Dentistry, Westmead Centre for Oral Health, Westmead Hospital, The University of Sydney, Sydney, New South Wales. mguazzato@yahoo.com.au
authors.forename: M
authors.initials: M
authors.lastname: Guazzato
authors.forename: T R
authors.initials: TR
authors.lastname: Walton
authors.forename: W
authors.initials: W
authors.lastname: Franklin
authors.forename: G
authors.initials: G
authors.lastname: Davis
authors.forename: C
authors.initials: C
authors.lastname: Bohl
authors.forename: I
authors.initials: I
authors.lastname: Klineberg
mesh_terms: D055598:Chemical Phenomena; D003764:Dental Materials / Q000737:chemistry*; D003776:Dental Porcelain / Q000737:chemistry*; D003801:Dental Veneers; D006801:Humans; D008422:Materials Testing; D013314:Stress, Mechanical; D013499:Surface Properties; D013696:Temperature; D013816:Thermodynamics; D015019:Yttrium / Q000737:chemistry; D015040:Zirconium / Q000737:chemistry*
chemical_list: D003764:Dental Materials; C499362:yttria stabilized tetragonal zirconia; D003776:Dental Porcelain; D015019:Yttrium; D015040:Zirconium; C028541:zirconium oxide