2017年12月29日金曜日

20171229 Published by Springer Written by Carlos-Bergmann and Aisha-Stumpf Dental Ceramics: Microstructure, Properties and Degradation pp.15-16より抜粋引用

Published by Springer   Written by Carlos-Bergmann and Aisha-Stumpf
Dental Ceramics: Microstructure, Properties and Degradation
Chapter 3 
『Ceramic Materials for Prosthetic and Restoration Use』
ISBN:3642382231 

pp.15-16

『The first uses of ceramics as dental material date from the early XV century.
 Today, Ceramics are being used on a large scale as restorative materials in Dentistry. 
This includes materials for dental crown, prosthesis cementation and total and partial dentures. 
The increase in the use of ceramic restorations has virtually removed the use of metal restorations, since they do not meet the esthetic demands of patients.
Ceramic materials are best suited to replace metal, especially in erosive and corrosive environments.
Dental ceramics allow regular and diffuse transmission, as well as diffuse and specular reflectance of light, and therefore have the potential to reproduce the depth of translucency, depth of color, and texture of natural teeth. 
In addition, dental ceramics have a relative good resistance to degradation. 
The oral cavity is biologically compatible, and has a coefficient of thermal expansion that is similar to that of tooth structure.
The first dental ceramics had their use limited to the front teeth because of their low flexural strength. 
New technologies and manufacturing techniques have allowed the development of different ceramic systems for dental use with higher resistance, expanding the options available to dentists and allowing the manufacture of prostheses for posterior teeth.
Ceramics have been used to fabricate a wide variety of restorations including inlays, onlays, implant, crowns and fixed partial dentures on account of their biocompatibility, were resistance and better esthetics. Due to their better esthetics, in particular, patients have become more demanding regarding the appearance of their restorations.
The use of all-ceramic crowns has been questioned because of their lack of strength.
 Their counterparts, the metal-ceramic crowns, have been used successfully; the majority of all full-coverage restorations and fixed prostheses are fabricated from metal-ceramic systems that have a failure rate of only 1-3% over 5 years. 
Metal-ceramic systems (MSC) have come under scrutiny, however, because of (1)potential alloy corrosion leading to toxicity and allergy concerns; (2)esthetic problems such as lack of translucency, discoloration of some ceramics from silver in the alloy, and excessive value in the cervical third; (3)the amount of tooth reduction necessary, and associated tendency to overcountour the restoration; and (4)incompatibility between metal and ceramic, and the difficulty in establishing standard test for bond strength and thermal compatibility.
Fixed prosthodontic treatment, whether involving complete or partial coverage and natural tooth or dental implant abutments, commonly relies on indirect fabrication definitive prostheses in the dental laboratory. 
Historically, the necessity for provisional treatment has been primarily derived from this methodological process.
The importance of interim treatment, however, is more far-reaching than is portrayed by this procedural necessity and the requirements for satisfactory provisional restorations differ only slightly from the definitive treatment they precede
In the last few decades, ceramic materials for restorative. 
Dentistry have evolved significantly, and esthetic restorations, such as all-ceramic crowns and veneers, are routinely used in practice.
Different materials and production systems are available for all-ceramic dental restorations; a single layer ceramic that is attached to the tooth structure or a two-layered structure with a high-strength ceramic core material, which supports the more fragile veneering ceramic.
Some high strength materials are designed for computerized milling techniques (CAD/CAM) where the core structure is milled from the presintered block of an oxide ceramic. 
A relative recent dental ceramic is yttria-stabilized zirconium oxide polycrystals. 
These materials are manufactured from fine particles of ZrO2 and 3-5% Y2O3, which from a partly stabilized tetragonal structure at room temperature after heat treatment. 
Due to the optical opacity of these materials, they are covered with veneering ceramic, usually feldspathic types, with esthetic characteristics similar to the natural tooth substance.

The veneering process involves a firing procedure at high temperatures(750-900)and subsequent cooling of the restoration. 
This process is performed at least once, usually two to five times. 
It is not fully known what effect this thermal history has on the properties of the core ceramic. 
Patients have become more and more demanding regarding the esthetic and biocompatibility of their dental restorations, ceramic as material for inlays, onlays, crowns and bridges has become a main goal for scientific interest especially from the material point of view.』

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