title: The effect of preparation design on the fracture resistance of zirconia crown copings (computer associated design/computer associated machine, CAD/CAM system).
abstract: OBJECTIVE
  One of the major problems of all ceramic restorations is their probable fracture against the occlusal force. The aim of the present in-vitro study is was to compare the effect of two marginal designs (chamfer & shoulder) on the fracture resistance of zirconia copings, CERCON (CAD/CAM).
  
  
  METHODS
  This in vitro study was done with single blind experimental technique. One stainless steel dye with 50' chamfer finish line design (0.8 mm depth) was prepared using milling machine. Ten epoxy resin dyes were made, The same dye was retrieved and 50' chamfer was converted into shoulder (1 mm).again ten epoxy resin dyes were made from shoulder dyes. Zirconia cores with 0.4 mm thickness and 35 μm cement Space fabricated on the 20 epoxy resin dyes (10 samples chamfer and 10 samples shoulder) in a dental laboratory. Then the zirconia cores were cemented on the epoxy resin dyes and underwent a fracture test with a universal testing machine (GOTECH AI-700LAC, Arson, USA) and samples were investigated from the point of view of the origin of the failure.
  
  
  RESULTS
  The mean value of fracture resistance for shoulder margins were 788.90±99.56 N and for the chamfer margins were 991.75±112.00 N. The student's T-test revealed a statistically significant difference between groups (P=0.001).
  
  
  CONCLUSIONS
  The result of this study indicates that marginal design of the zirconia cores effects on their fracture resistance. A chamfer margin could improve the biomechanical performance of posterior single zirconia crown restorations. This may be because of strong unity and round internal angle in chamfer margin.
authors.affiliation: Associate Professor, Department of Fix Prostodontics, Dental Branch, Azad University, Tehran, Iran.
authors.forename: E
authors.initials: E
authors.lastname: Jalalian
authors.forename: B
authors.initials: B
authors.lastname: Atashkar
authors.forename: R
authors.initials: R
authors.lastname: Rostami