From Feldspar Glass to Zirconia: State of the Art of Ceramics for Dental Applications
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From Feldspar Glass to Zirconia: State of the Art of Ceramics for Dental Applications Isabelle Denry Dentistry, Ohio State University, 305 W 12th Avenue, Columbus, OH, 43210 ABSTRACT The evolution of dental ceramics over the past four decades has been remarkable in light of the increase in the diversity of materials and fabrication techniques that have become available. As a consequence, the applications and indications for all-ceramic materials continue to expand. The purpose of the present article is to review the evolution of dental ceramics from the various aspects of crystalline phases, glassy matrix, flaws and in vivo performance. INTRODUCTION Dental ceramics can be classified according to their fabrication technique and according to their type and amount of crystalline phase. Metal-ceramic dental restorations consist of a leucite-based veneering porcelain sintered on a metal framework. All-ceramic restorations are either monolithic or composed of a high strength ceramic core, on which veneering porcelain is applied. All-ceramics can be sintered, heat-pressed, slip-cast or machined. Cast glass-ceramic restorations relying on the lost wax technique to cast a glass, followed by a crystallization heat treatment are no longer available, due to clinical failures related to low strength and processing defects. Recently introduced processing techniques now combine machining of a core and heatpressing of a veneering ceramic onto the core. Compared to veneering porcelains, all-ceramic core materials encompass a much greater variety of crystalline phases. These include leucite, lithium disilicate, alumina, spinel, zirconia, fluorapatite. Each of these crystalline phases has unique characteristics and properties, in combination with the surrounding glassy matrix.1 The applications and indications of all-ceramic materials in dentistry continue to expand as new products are being introduced. Initially, ceramics and porcelains were only used in the anterior region to satisfy esthetic requirements. High strength all-ceramic materials have recently become available to fabricate anterior as well as posterior fixed partial dental prostheses, implants and implant abutments. The requirements for all-ceramic materials focus on excellent esthetics, high strength, high toughness and long-term performance.2,3 These requirements have not yet been fulfilled at the same time in any one dental ceramic.4 Currently available alumina or zirconia-based ceramics exhibit high strength and toughness ceramics but they are, unfortunately, extremely opaque.5-7 The purpose of the present article is to review the evolution of dental ceramics from the various aspects of crystalline phases, glassy matrix, flaws and in vivo performance. A look at dental ceramics from the crystalline phase aspect • Chemistry and crystalline transformations The chemistry of the crystalline phase is critical to the thermal stability of the ceramic and its interaction with the glassy matrix, since most manufacturing steps include multiple heat treatments. Leu
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