Fracture Toughness, Microstructure and Toughening Mechanism of Leucite and Lithium Disilicate Glass Ceramics

In recent years the use of all-ceramic materials for dental prosthesis has increased rapidly. All ceramic materials offer better optical properties and biocompatibility as compared with the traditional porcelain-fused-to metallic-substrates. The main drawback of these materials has been their brittle nature and the extreme sensitivity to flaws and defects. One class among these systems requires hot pressing by means of a special furnace to produce the required shape (pressable materials). Empress 1 (E1) (leucite reinforced glass-ceramic) and Empress 2 (E2) (lithium disilicate glass-ceramic) (Ivocalr-Vivadent, Schaan, Liechtenstein) are well known pressable materials, which have generated considerable interest because of their ease of fabrication (lost waxing technique) and excellent esthetic features. The first material, El, is recommended for restoring single units, and preferably in low stress bearing areas. E2 was developed in 1998s for the purpose of restoring 3-unit fixed partial dentures up to the second premolar. Another pressable material (experimental) (E3) (also a lithium disilicate glass-ceramic) is about to be introduced into the market by the same manufacturer. This material is claimed to offer better mechanical and optical properties than Empress 2 and is also suitable for the fabrication of 3-unit fixed partial dentures. The aim of the present study was to evaluate the fracture toughness of the 3 above-mentioned all-ceramic materials, using the indentation fracture (IF) and the indentation strength (IS) techniques, and to elucidate the toughening mechanism involved in each system.