The word ceramic is derived from the Greek word keramos which literally means ‘burnt stuff’ but which has come to mean more specifically a material produced by burning or firing. The first ceramics fabricated by man were earthenware pots used for domestic purposes. This material is opaque, relatively weak and porous and would be unsuitable for dental applications. It consisted mainly of kaolin. The blending of this with other minerals such as silica and feldspar produced the translucency and extra strength required for dental restorations. Material containing these additional important ingredients was given the name porcelain.
Fused porcelain has long been used in the construction of works of art. It can be produced in almost every shade or tint and its translucency imparts a depth of colour unobtainable by other materials. Although the technique for porcelain fusing is exacting it can be initially moulded by hand as a paste and alterations can be made at various stages of the work. It is not surprising; therefore, that dentistry has turned to porcelain for the production of artificial teeth, crowns, bridges and veneers.
Injection moulded and pressed ceramics:
The technique for fabricating ceramic copings from this type of material involves the formation of a wax pattern on an epoxy resin die. A variation of the lost wax technique is used to maintain the correct shape and size in the ceramic material. The mixed material, containing magnesium and aluminium oxides, glass, kaolin, calcium stearate and wax is injected under pressure at 180ºC into the prepared mould. Firing is then carried out in a special furnace using controlled temperatures up to 1300ºC during which formation of the spinel occurs. Veneer porcelains are baked onto the surface of the coping to produce the finished crown.
Cast glass and polycrystalline ceramics:
Crowns are formed from wax patterns which are invested in a phosphate-bonded investment. The wax burn-out and heat soak of the investment is carried out at about 950ºC. The molten ceramic is cast centrifugally into the mould at around 1350ºC. This result in a transparent glass crown which is then heat treated in an oven at 1075ºC for 10 hours. This heat treatment or ceramming causes partial crystallization to form mica-like which has a dual effect. They lightly reduce the translucency of the previously clear material and significantly increase the strength. That the strength of these cast ceramics can match that of aluminous porcelain.
Colour matching is achieved by applying a series of the appropriate tinted porcelains to the surface and re-firing. This procedure may be circumvented with the development of new candidate materials in which the shading can be incorporated within the body of the crown. Further improvements in appearance are achieved by selecting the correct shade of luting cement. The surface glazes used to provide colour to cast glasses can produce a very realistic result. Unfortunately, if there is a need to undertake any adjustment of the shape of the crown they are rapidly removed, resulting in poor aesthetics unless the crown is re-glazed.
Porcelain veneers:
Porcelain veneers offer a means of improving the appearance of stained or discolored teeth. The veneer consists of a thin shell-like structure which is ideally fabricated in such a way that it can be closely adapted to the prepared tooth. There is some controversy as to whether the veneers can be attached to unprepared teeth – a technique which would obviously conserve sound tooth substance, or whether some reduction in the tooth contour is necessary. Most authorities do advise the removal of about 0.5 mm of labial enamel.
An accurate assessment of the shade of a porcelain veneer cannot be made at trial without ‘coupling’ the veneer to the underlying tooth. This process involves optically linking the veneer to the underlying tooth to see what effect the colour of the tooth has on the finished restoration. In its simplest form this can be achieved using water, but a better alternative is a water-soluble trial paste. This commercial product has similar colour
Characteristics to the luting resin but can be washed off the veneer surface with water prior to luting the veneer in place.
Porcelain fused to metal:
The requirements of the alloy used to form the substructure are similar to those for non-porcelain bonding work with additional requirements as follows
- The alloy, having been previously cast into the desired shape, should be capable of withstanding porcelain firing without melting or suffering creep. Hence the alloy must have a high fusion temperature.
- The alloy should be sufficiently rigid to support a very brittle porcelain veneer otherwise fracture of the veneer is inevitable.
- The alloy should be capable of forming a bond with the porcelain veneer in order that the latter does not become detached
- The alloy should have a value of coefficient of thermal expansion similar to that for the porcelain to which it is bonded.
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