Porcelain teeth

The glass polyalkenoate cement uniquely combines translucency with the ability to bond to untreated tooth material and bone. Indeed, the only other cement to possess translucency is the dental silicate cement, while the zinc polycarboxylate cement is the only other adhesive cement. It is also an agent for the sustained release of fluoride. For these reasons the glass polyalkenoate cement has many applications in dentistry as well as being a candidate bone cement. Its translucency makes it a favoured material both for the restoration of front teeth and to cement translucent porcelain teeth and veneers. Its adhesive quality reduces and sometimes eliminates the need for the use of the dental drill. The release of fluoride from this cement protects neighbouring tooth material from the ravages of dental decay. New clinical techniques have been devised to exploit the unique characteristics of the material (McLean Wilson, 1977a,b,c Wilson McLean, 1988 Mount, 1990). 

Plastic acrylic denture teeth are made by injection or transfer molding. Acrylic teeth have a higher strength than porcelain teeth and break less readily. However, they resist cold flow, have greater water absorption, and have higher wear rate than porcelain teeth. 

In 1931 Brill published the article Uber die Implantation von Porzellan Wurzeln (On the transplanting of procelain dental roots) and which was followed in 1934 by Abel s Die Befestigung implantierten Porzellanzahnersatzes mittel Wiplakieferschrauben (Attaching porcelain teeth by means of Wipla jaw screws). 

Sairenji E, Moriwaki K, Shimizu M, et al. 1980. Estimation of radiation dose from porcelain teeth containing uranium compound. 59 1136-1140. 

In 1851 vulcanite, a hard rubber, was discovered and used as a denture base for replacing swaged gold in full dentures. Porcelain teeth were set and aligned within the rubber. The rubber was less expensive and more comfortable than the swaged gold. 

Cast gold alloys were used to attach the denture to the remaining teeth for partial dentures. These alloys were replaced with the introduction of the cobalt-chromium alloy Vitallium in 1930. A methyl methacrylate resin was also introduced in the 1930s replacing the vulcanite rubber for denture bases. This resin was easier to use and softer in the mouth, and could be made pink in color for esthetics. The partial denture casting has metal webbing (Figure 14.3) to support the resin base which in turn supports the porcelain teeth. 

This does not mean that there are no shortcomings to acrylic teeth. In a recent study (5), the wear of acrylic against acrylic teeth was found to be about seven times more than the wear of porcelain teeth. Improvement in the wear resistance of acrylic teeth without loss of the other desirable aesthetic properties would be highly desirable. 

Veneering Investments. These are phosphate bonded and contain finely ground quart2, 2irconium oxide, and/or titanium oxide to produce highly refractory, low expansion dies of fine detail. The dies are formed within impressions taken of teeth that the dentist has prepared in anticipation of covering the front surface with an aesthetic ceramic veneer. Porcelain or ceramic powders are shaped to detail on the dies and these are fired at high (- 1000° C) temperatures to produce the veneers. The veneers are then cemented to the front surface of the previously prepared teeth. 

The most common oral condition and dental emergency is dental caries, which is a destructive disease of the hard tissues of the teeth due to bacterial infection with Streptococcus mutans and other bacteria. It is characterized by destruction of enamel and dentine. Dental decay presents as opaque white areas of enamel with grey undertones and in more advanced cases, brownish discoloured cavitations. Dental caries is initially asymptomatic and pain does not occur until the decay impinges on the pulp, and an inflammation develops. Treatment of caries involves removal of the softened and infected hard tissues, sealing of exposed dentines and restoration of the lost tooth structure with porcelain, silver, amalgam, composite plastic, gold etc. 

There are ways of obtaining an instant gleaming smile, such as having your teeth covered with resin or porcelain veneers. The cost of these can be prohibitive, with the porcelain ones being twice as expensive as the plastic ones. Over time, these too can become stained. 

First reported use of porcelain false teeth in France... 

Today, a large segment of utilisation ofzirconia as colour-adapted tooth veneers in dental restoration exists (Cales, 1998). At this point, it is appropriate to mention the ancient French dental doctor Pierre Fauchard (1678-1761) who may be considered the vanguard of modern tooth restoration. He has been credited with recognising the potential of porcelain enamels and initiating research with porcelain to imitate the natural colour of teeth and gingival tissue (Fauchard, 1728). 

Dental crowns and bridges consist of a metal substrate covered by multiple layers of porcelain and glass. The first layer of porcelain must be opaque to hide the metal, while subsequent layers provide color matching to the surrounding teeth, and the correct luster and translu-cency to match the appearance of natural teeth. Good thermal compatibility between the various layers is essential to prevent thermal shock cracking, and to minimize residual stresses which could lead to stress-corrosion failures, or failure due to the combined residual and masticatory stresses. 

Before leaving the topic we should mention that this is not the only way to prepare glass-ceramics. Another group of these materials used for dentistry go by the label In-Ceram . The material is prepared as a porous crystalline ceramic (alumina, spinel, and zirconia have all been used) and then infiltrated with an La-rich glass. The resulting glass-infiltrated ceramic can be quite translucent, can be colored to match other teeth, and is mechanically tough (for a ceramic). It can be coated with porcelains for improved appearance. 

Dental Porcelain. Feldspathic porcelain, shaped, tinted and fired for use as false teeth the firing is sometimes carried out in a partial vacuum to remove small air bubbles and thus ensure maximum density and strength. X-ray diffraction studies have shown fired dental porcelain to be some 80% glassy phase. Dental porcelain jacket crowns are specified by BS 5612, denture teeth by BS 6817. 

Vacuum Firing. A process for the firing of special types of ceramic either to prevent oxidation of the ware or to reduce its porosity. Vacuum firing is used, for example, in the firing of dental porcelain to produce teeth of almost zero porosity. 

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