title: Effects of cement thickness and bonding on the failure loads of CAD/CAM ceramic crowns: multi-physics FEA modeling and monotonic testing.
abstract: OBJECTIVE
  To determine the influence of cement thickness and ceramic/cement bonding on stresses and failure of CAD/CAM crowns, using both multi-physics finite element analysis and monotonic testing.
  
  
  METHODS
  Axially symmetric FEA models were created for stress analysis of a stylized monolithic crown having resin cement thicknesses from 50 to 500 μm under occlusal loading. Ceramic-cement interface was modeled as bonded or not-bonded (cement-dentin as bonded). Cement polymerization shrinkage was simulated as a thermal contraction. Loads necessary to reach stresses for radial cracking from the intaglio surface were calculated by FEA. Experimentally, feldspathic CAD/CAM crowns based on the FEA model were machined having different occlusal cementation spaces, etched and cemented to dentin analogs. Non-bonding of etched ceramic was achieved using a thin layer of poly(dimethylsiloxane). Crowns were loaded to failure at 5 N/s, with radial cracks detected acoustically.
  
  
  RESULTS
  Failure loads depended on the bonding condition and the cement thickness for both FEA and physical testing. Average fracture loads for bonded crowns were: 673.5 N at 50 μm cement and 300.6N at 500 μm. FEA stresses due to polymerization shrinkage increased with the cement thickness overwhelming the protective effect of bonding, as was also seen experimentally. At 50 μm cement thickness, bonded crowns withstood at least twice the load before failure than non-bonded crowns.
  
  
  CONCLUSIONS
  Occlusal "fit" can have structural implications for CAD/CAM crowns; pre-cementation spaces around 50-100 μm being recommended from this study. Bonding benefits were lost at thickness approaching 450-500 μm due to polymerization shrinkage stresses.
authors.affiliation: Department of Dental Materials and Prosthodontics, São Paulo State University, São José dos Campos, SP, Brazil. liligressler@bol.com.br
authors.forename: Liliana G
authors.initials: LG
authors.lastname: May
authors.forename: J Robert
authors.initials: JR
authors.lastname: Kelly
authors.forename: Marco A
authors.initials: MA
authors.lastname: Bottino
authors.forename: Thomas
authors.initials: T
authors.lastname: Hill
mesh_terms: D019245:Compressive Strength; D003198:Computer Simulation; D017076:Computer-Aided Design; D003442:Crowns; D001840:Dental Bonding; D003776:Dental Porcelain; D017267:Dental Prosthesis Design; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis / Q000379:methods; D020342:Finite Element Analysis; D006801:Humans; D008422:Materials Testing; D008963:Molar; D058105:Polymerization; D019279:Resin Cements; D012986:Software Validation; D016019:Survival Analysis; D013718:Tensile Strength
chemical_list: D019279:Resin Cements; D003776:Dental Porcelain