title: Graded structures for damage resistant and aesthetic all-ceramic restorations.
abstract: OBJECTIVE
  Clinical studies revealed several performance deficiencies with alumina- and zirconia-based all-ceramic restorations: fracture; poor aesthetic properties of ceramic cores (particularly zirconia cores); and difficulty in achieving a strong ceramic-resin-based cement bond. We aim to address these issues by developing a functionally graded glass/zirconia/glass (G/Z/G) structure with improved damage resistance, aesthetics, and cementation properties.
  
  
  METHODS
  Using a glass powder composition developed in our laboratory and a commercial fine zirconia powder, we have successfully fabricated functionally graded G/Z/G structures. The microstructures of G/Z/G were examined utilizing a scanning electron microscopy (SEM). The crystalline phases present in G/Z/G were identified by X-ray diffraction (XRD). Young's modulus and hardness of G/Z/G were derived from nanoindentations. Critical loads for cementation radial fracture in G/Z/G plates (20mmx20mm, 1.5 or 0.4mm thick) bonded to polycarbonate substrates were determined by loading with a spherical indenter. Parallel studies were conducted on homogeneous yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) controls.
  
  
  RESULTS
  The G/Z/G structure consists of an outer surface aesthetic glass layer, a graded glass-Y-TZP layer, and a dense Y-TZP interior. The Young's modulus and hardness increase from surface to interior following power-law relations. For G/Z/G plates of 1.5 and 0.4mm thick, critical loads for cementation radial fracture were 1990+/-107N (mean+/-S.D., n=6) and 227+/-20N (mean+/-S.D., n=6), respectively, which were approximately 30 and 50% higher than those for their monolithic Y-TZP counterparts (1388+/-90N for 1.5mm and 113+/-10N for 0.4mm thick; mean+/-S.D., n=6 for each thickness). A 1-sample t-test revealed significant difference (p<0.001) in critical loads for radial fracture of G/Z/G and homogeneous Y-TZP for both specimen thicknesses.
  
  
  CONCLUSIONS
  Our results indicate that functionally graded G/Z/G structures exhibit improved damage resistance, aesthetics, and potentially cementation properties compared to homogeneous Y-TZP.
authors.affiliation: Department of Biomaterials and Biomimetics, New York University College of Dentistry, New York University, New York 10010, USA. yz21@nyu.edu
authors.forename: Yu
authors.initials: Y
authors.lastname: Zhang
authors.forename: Jae-Won
authors.initials: JW
authors.lastname: Kim
mesh_terms: D002516:Ceramics; D003442:Crowns; D018360:Crystallography, X-Ray; D001840:Dental Bonding; D003776:Dental Porcelain / Q000138:chemical synthesis / Q000737:chemistry; D017267:Dental Prosthesis Design; D003799:Dental Stress Analysis; D003830:Denture, Partial, Fixed; D055119:Elastic Modulus; D004955:Esthetics, Dental; D005898:Glass; D006244:Hardness; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D018583:Pliability; D019279:Resin Cements; D015019:Yttrium; D015040:Zirconium
chemical_list: D019279:Resin Cements; C499362:yttria stabilized tetragonal zirconia; D003776:Dental Porcelain; D015019:Yttrium; C023506:Glass ceramics; D015040:Zirconium; C028541:zirconium oxide