title: Contact damage as a failure mode during in vitro testing.
abstract: OBJECTIVE
  The purpose of this study was to identify the failure mode(s) for all-ceramic crowns tested in vitro and to determine whether measured failure loads in this type of testing would be influenced by indenter radii (a classic Hertzian cone-crack variable) and specimen thickness.
  
  
  METHODS
  Fracture surfaces and failure probability data from glass-ceramic cuspids (n = 50) tested in a previous in vitro study were examined to determine their mode of failure. To further evaluate the failure mode identified for the crowns, 100 ceramic platelets (50 glass-ceramic, 50 feldspathic porcelain) were loaded to failure beneath spherical indenters (radii, 0.75-94 mm).
  
  
  RESULTS
  Glass-ceramic cuspids failed from blunt contact damage at the point of loading (with Hertzian stress-state damage evident). Ceramic platelets exhibited failure from either the indentation surface (Hertzian cone-cracking present) or from the supported surface (ie, mimicking bending failure). Failure-loads increased with the indenter radius for both failure modes. Failure from blunt contact damage occurred at markedly higher loads than did failure from support-surface sites.
  
  
  CONCLUSIONS
  Blunt indentation damage was identified as being the failure source for the glass-ceramic cuspid crowns and a major failure mode for both feldspathic porcelain and glass-ceramic platelets loaded beneath spherical indenters. This failure mode is not similar to that reported for clinically failed glass-ceramic crowns. Influential testing variables were contact radius, ceramic thickness, and the surface finish of both the ceramic specimen and test platen.
authors.affiliation: US Navy, Prosthodontics Department, Naval Dental School, Bethesda, MD, USA.
authors.forename: C K
authors.initials: CK
authors.lastname: Harvey
authors.forename: J R
authors.initials: JR
authors.lastname: Kelly
mesh_terms: D000704:Analysis of Variance; D002516:Ceramics; D003442:Crowns; D003776:Dental Porcelain; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis; D004867:Equipment Design; D019544:Equipment Failure Analysis; D006244:Hardness; D016014:Linear Models; D008422:Materials Testing / Q000295:instrumentation / Q000379:methods; D015203:Reproducibility of Results
chemical_list: C047475:Dicor ceramic; C082340:Vita Mark II; D003776:Dental Porcelain; C023506:Glass ceramics