title: In vitro fracture strength and thermal shock resistance of metal-ceramic crowns with cast and machined AuTi frameworks.
abstract: BACKGROUND
  AuTi alloys with 1.6% to 1.7% (wt%) Ti provide sufficient bond strength to veneering ceramics, but the strength of entire metal-ceramic restorations fabricated from these alloys is not known. However, this information is important to assess the clinical performance of such materials.
  
  
  OBJECTIVE
  This in vitro study evaluated the fracture strength and thermal shock resistance of metal-ceramic crowns with AuTi frameworks produced by milling or casting.
  
  
  METHODS
  Frameworks of the alloy Au-1.7Ti-0.1Ir (wt%) (Esteticor Vision) were produced by milling or casting (test groups). A high-gold alloy (Esteticor Special) was used as the control. The frameworks were veneered with ceramic (VMK 95). Specimens (n=7) were loaded until fracture. Loads at failure (N) were recorded and the mean values statistically evaluated using 1-way analysis of variance and a post hoc Dunnett test (alpha=.05). To assess the crazing resistance of the veneering ceramic, 6 additional crowns of each group were subjected to a thermal shock test. Fractured surfaces were documented by scanning electron microscopy. Coefficients of thermal expansion of the materials used were measured (n=2) to assess the thermal compatibility between alloys and ceramic.
  
  
  RESULTS
  The mean fracture strength of the crowns with machined AuTi frameworks (1294 +/- 236 N) was significantly lower (P=.012) than that of the cast AuTi frameworks (1680 +/- 150 N), but statistically not different than the high-gold alloy (1449 +/- 159 N). Bonding failure to the AuTi alloy predominantly occurred at the alloy-oxide interface. For the high-gold alloy, more ceramic residues were observed. In the thermal shock test, crowns with milled AuTi frameworks showed significantly higher thermal shock resistance compared to the other groups. The coefficients of thermal expansion (Esteticor Vision cast: 14.5 microm/m.K; Esteticor Vision milled: 14.3 microm/m.K; Esteticor Special cast: 13.7 microm/m.K) did not correlate with the results of the thermal shock test.
  
  
  CONCLUSIONS
  The in vitro fracture strength of crowns with milled AuTi frameworks is lower than that obtained with cast AuTi frameworks, but comparable to those crowns produced with a high-gold alloy.
authors.affiliation: Department of Removable Prosthodontics, School of Dental Medicine, University of Berne, Berne, Switzerland.
authors.forename: Rolf K
authors.initials: RK
authors.lastname: Kellerhoff
authors.forename: Jens
authors.initials: J
authors.lastname: Fischer
mesh_terms: D019245:Compressive Strength; D017076:Computer-Aided Design; D003722:Dental Alloys / Q000737:chemistry*; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis / Q000379:methods*; D003801:Dental Veneers; D006358:Hot Temperature; D008422:Materials Testing / Q000379:methods; D016876:Metal Ceramic Alloys / Q000737:chemistry*; D016474:Weight-Bearing
chemical_list: D003722:Dental Alloys; D016876:Metal Ceramic Alloys; C416206:gold-titanium alloy