title: An in vitro study of the compressive load at fracture of Procera AllCeram crowns with varying thickness of veneer porcelain.
abstract: BACKGROUND
  When designing and cutting a crown preparation, a balance must be achieved between the amount of tooth structure conserved and the space created for the restoration. All-ceramic crowns, which contain a core, are usually veneered with porcelain to produce the desired aesthetics and contour. The effect of veneer porcelain on the strength of the restoration has been a cause of debate.
  
  
  OBJECTIVE
  The purpose of this study was to investigate the effect of different thickness of veneer porcelain on the compressive load at fracture of Procera AllCeram crowns.
  
  
  METHODS
  Sixty brass dies were fabricated with a crown-like preparation and a chamfer margin. Sixty crowns were fabricated with a 0.6-mm-thick core: Procera crowns with either a 0.4-mm- or 0.9-mm-thick veneer of AllCeram (Groups 1 and 2 respectively) and In-Ceram crowns with a 0.9-mm-thick veneer of Vitadur Alpha porcelain (Group 3). Each group consisted of 20 crowns. In-Ceram crowns were used as the control group. All crowns were measured at 4 axial and 1 occlusal random locations before autoglazing. They were air abraded and adhesively bonded onto the appropriate brass die using Clearfil Newbond Bonding Agent, Clearfil Porcelain Bond Activator, and Panavia 21 TC Dental Adhesive as the luting agent. After storage in distilled water at 37 degrees C for 24 hours, the specimens were placed in a compressive test rig within an Instron universal testing machine and loaded in the center of the occlusal surface with a 4-mm diameter stainless steel ball. An axial preload of 20 N was applied before compressive testing at a crosshead speed of 0.1 mm/minute until fracture occurred. An analysis of variance revealed no significant difference in the load at fracture between the 3 groups (P=.3). Using a Bonferroni correction, independent t tests revealed no significant difference in load at fracture for any of the pair-wise comparisons between the 3 test groups (P < .05).
  
  
  RESULTS
  The mean load at fracture for Group 1 was 2197.6 N (SD = 776.4); Group 2, 2401.4 N (SD = 699.1); and Group 3, 2581.0 N (SD = 715.6).
  
  
  CONCLUSIONS
  Within the parameters of this study, the axial thickness of veneer porcelain did not have a significant effect on the compressive load at fracture of Procera AllCeram crowns.
authors.affiliation: Department of Conservation, Eastman Dental Institute for Oral Health Care Sciences, University College London, London, United Kingdom.
authors.forename: Briony
authors.initials: B
authors.lastname: Webber
authors.forename: Ailbhe
authors.initials: A
authors.lastname: McDonald
authors.forename: Jonathon
authors.initials: J
authors.lastname: Knowles
mesh_terms: D000537:Aluminum Oxide; D000704:Analysis of Variance; D019245:Compressive Strength; D003442:Crowns; D003776:Dental Porcelain; D017267:Dental Prosthesis Design; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis; D003801:Dental Veneers; D008422:Materials Testing; D019279:Resin Cements
chemical_list: C105314:AllCeram; D019279:Resin Cements; D003776:Dental Porcelain; C070870:In-Ceram; D000537:Aluminum Oxide