title: In vitro evaluation of fracture strength of zirconia restoration veneered with various ceramic materials.
abstract: OBJECTIVE
  Fracture of the veneering material of zirconia restorations frequently occurs in clinical situations. The purpose of this in vitro study was to compare the fracture strengths of zirconia crowns veneered with various ceramic materials by various techniques.
  
  
  METHODS
  A 1.2 mm, 360° chamfer preparation and occlusal reduction of 2 mm were performed on a first mandibular molar, and 45 model dies were fabricated in a titanium alloy by CAD/CAM system. Forty-five zirconia copings were fabricated and divided into three groups. In the first group (LT) zirconia copings were veneered with feldspathic porcelain by the layering technique. In the second group (HT) the glass ceramic was heat-pressed on the zirconia coping, and for the third group (ST) a CAD/CAM-fabricated high-strength anatomically shaped veneering cap was sintered onto the zirconia coping. All crowns were cemented onto their titanium dies with Rely X™ Unicem (3M ESPE) and loaded with a universal testing machine (Instron 5583) until failure. The mean fracture values were compared by an one-way ANOVA and a multiple comparison post-hoc test (α=0.05). Scanning electron microscope was used to investigate the fractured interface.
  
  
  RESULTS
  Mean fracture load and standard deviation was 4263.8±1110.8 N for Group LT, 5070.8±1016.4 for Group HT and 6242.0±1759.5 N for Group ST. The values of Group ST were significantly higher than those of the other groups.
  
  
  CONCLUSIONS
  Zirconia crowns veneered with CAD/CAM generated glass ceramics by the sintering technique are superior to those veneered with feldspathic porcelain by the layering technique or veneered with glass ceramics by the heat-pressing technique in terms of fracture strength.
authors.affiliation: Department of Prosthodontics and Dental Research Institute, School of Dentistry, Seoul National University, Seoul, Korea.
authors.forename: Yu-Sung
authors.initials: YS
authors.lastname: Choi
authors.forename: Sung-Hun
authors.initials: SH
authors.lastname: Kim
authors.forename: Jai-Bong
authors.initials: JB
authors.lastname: Lee
authors.forename: Jung-Suk
authors.initials: JS
authors.lastname: Han
authors.forename: In-Sung
authors.initials: IS
authors.lastname: Yeo