PURPOSE: 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. MATERIALS AND 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. CONCLUSION: 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.

Crystallization↗

PubMedDate not supplied

Effects of preparation and luting system on all-ceramic computer-generated crowns.

PURPOSE: Computer-aided design/computer-integrated machining (CAD/CIM) allows defect-oriented custom-shaping of the inside surfaces of all-ceramic crowns. The purpose of this study was to examine the effect of inside crown form on fracture strength of cemented and bonded crowns. MATERIALS AND METHODS: Four preparation types were used: (1) "classic" with a butt shoulder of 1.2 mm, abutment height of 4 mm, and 6-degree convergence, (2) like type 1 with mesio-occlusodistal cavity, (3) like type 1 with height reduced by 50%, and (4) like type 1 with abutment reduced by 100% plus a pulp chamber cavity. Crowns were CAD designed on preparations 1 to 4 using identical outside morphology. Machined crowns were placed on abutments (a) without any media as controls (n = 15), (b) cemented (n = 15), and (c) bonded (n = 15), and were loaded until fracture. RESULTS: Zinc phosphate-cemented crowns (1b, 2b, 3b, and 4b) showed significant (P < 0.001) increase of fracture load values compared to uncemented control crowns (1a, 2a, 3a, 4a). Fracture load values of bonded crowns (1c, 2c, 3c) were significantly (P < 0.001) higher than those for cemented crowns. Bonded crowns with thick occlusal dimensions (3c and 4c) showed the highest fracture load values. CONCLUSION: Bonded all-ceramic CAD/CIM crowns with defect-oriented inside morphology and increased occlusal dimensions showed high fracture load values.

Analysis of Variance↗