title: Strength and microstructure of IPS Empress 2 glass-ceramic after different treatments.
abstract: OBJECTIVE
  This investigation was designed to determine whether heat pressing and/or simulated heat treatments affect the flexure strength and microstructure of the lithium disilicate glass-ceramic of the IPS Empress 2 system.
  
  
  METHODS
  Four groups of the lithium disilicate glass-ceramic were prepared as follows: group 1 = as-received material; group 2 = heat-pressed material; group 3 = heat-pressed and stimulated initial heat-treated material; and group 4 = heat-pressed and simulated heat-treated material with full firings for a final restoration. Three-point bending tests and scanning electron microscopy (SEM) analysis were conducted.
  
  
  RESULTS
  The flexure strength of group 2 was significantly higher than that of group 1. However, there were no significant differences in strength among groups 2, 3, and 4, or between groups 1 and 4. The SEM micrographs of the lithium disilicate glass-ceramic showed a closely packed, multidirectionally interlocking microstructure of numerous lithium disilicate crystals protruding from the glass matrix. The crystals in the glass matrix of the heat-pressed materials (groups 2, 3, and 4) were a little more homogeneous and about 2 times bigger than those of the as-received material (group 1). These changes of the microstructure were greatest between groups 1 and 2. However, there were no marked differences among groups 2, 3, and 4.
  
  
  CONCLUSIONS
  Although there were significant increases in the strength and some changes of the microstructure after the heat-pressing operation, the combination of heat pressing and simulated subsequent heat treatments did not produce an increase of strength of IPS Empress 2 glass-ceramic.
authors.affiliation: Department of Prosthodontics, School of Dentistry, Wonkwang University, Iksan, South Korea 570-749. scoh@wonnms.wonkwang.ac.kr
authors.forename: S C
authors.initials: SC
authors.lastname: Oh
authors.forename: J K
authors.initials: JK
authors.lastname: Dong
authors.forename: H
authors.initials: H
authors.lastname: Lüthy
authors.forename: P
authors.initials: P
authors.lastname: Schärer
mesh_terms: D003460:Crystallization; D003776:Dental Porcelain / Q000737:chemistry*; D006358:Hot Temperature; D018020:Lithium Compounds / Q000737:chemistry*; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D018583:Pliability; D013674:Technology, Dental / Q000379:methods
chemical_list: C118557:Empress 2; D018020:Lithium Compounds; D003776:Dental Porcelain