Chapter 22 Dental ceramics
For many years the term dental porcelain described the material which was used to construct aesthetic restorations such as anterior crowns (Figure 22.1). Dental porcelain or ceramic is related to other ceramics which are used to make objects such as Chinese porcelain vases, engine mouldings, ballistic protection, roof tiles and the heatproof tiles on NASA’s space shuttle (Figure 22.2).
It is now acknowledged that the term ‘dental porcelain’ was incorrect. This is because little or no kaolin is present in the dental version, unlike the other (decorative) ceramics mentioned above (Table 22.1). Kaolin is a clay (chemically hydrated aluminium silicate). The reason for the absence of kaolin from dental ceramics is that it is opaque and this influences the optical properties and therefore the aesthetics of the final restoration.
|Composition||Decorative ceramic (%)||Dental ceramic (%)|
|Glass||0||Up to 15 depending on fusing temperature|
The ceramics now used in dentistry have been specifically produced for dental applications. A ceramic may be defined as a material which is an inorganic non-metal solid produced by the application of heat which is then cooled. It may be amorphous and partly or wholly crystalline. Dental ceramics need to be translucent and so feldspar and silica are incorporated into the material to achieve this. Dental ceramics are therefore really glasses called feldspathic ‘porcelains’. Pigments are also included to improve and optimize the aesthetics.
Conventional Dental Ceramics
Conventional dental ceramics are vitreous ceramics made up of a silica network with either potash feldspar (potassium alumino silicate) and/or soda feldspar (sodium alumino silicate) (Figure 22.3 and Table 22.2). This latter material is also called albite. Feldspars are a mixture of both of these materials with the proportions differing to yield different properties. Feldspar is the lowest fusing component and melts and flows during firing, forming a solid mass uniting the other constituents. Borax is also frequently included to further lower the fusing temperature.
The flux, in the case of a ceramic material, is a material which increases the viscosity of the molten glass and lowers the fusion and softening temperature of the glass. Binders act by holding the ceramic particles together prior to firing. As well as conveying opacity to the final product, cerium also produces fluorescence. Feldspathic porcelains are also referred to as opalescent porcelains as various metallic oxides are added to convey opalescence and provide colour. The amounts and constituents vary as to the requirements of the final product. Table 22.3 lists the metallic oxides used together with the colour which their inclusion imparts.
|Metallic oxide of||Colour|
Manufacture of the ceramic powder
The ceramic is supplied to the dental laboratory as a powder. The manufacturers make this powder by taking the raw materials and grinding them to form fine powders. These are blended together and then fired at a high temperature in a furnace. The molten mass thus produced is then rapidly cooled in cold water, which leads to large internal stresses, cracking and crazing of the mass. The resulting fragments of ceramic are known as frit, with the process called fritting (which is a pyrochemical reaction). The frit is then milled to a very fine powder. This powder may now be mixed with distilled water by the dental technician to form a creamy paste and the restoration built up.
Types of dental ceramic
The feldspathic ceramics form leucite and a glass phase when heated to a temperature of between 1150 and 1500 °C. The leucite material is potassium aluminium silicate, which has almost twice the coefficient of thermal expansion of feldspar. The manufacturer carries out this process to provide the dental technician with a powder with defined amounts of the appropriate components to permit the mass to be fired successfully.
The composition of the ceramic powder is such that a further chemical reaction is not required. Instead the particles of the ceramic powder fuse when it is heated to just above its glass transition temperature. This is called sintering (see Chapter 9). It is very important that the powder particles are very closely packed so that a dense compact structure without air inclusions is produced. During sintering, the glass phase will soften and start to coalesce. This is termed liquid phase sintering. This process takes time and may be halted at any stage by removing the ceramic from the heating oven. During the heating process, the glass phase will initially soften and a friable matrix is established. As the temperature rises the other components tend to fill the voids within the glass matrix. There is controlled diffusion between the particles, and as this continues, a dense solid is formed.
There is a range of dental ceramics, and these may be defined by the firing temperature: the ultra low (fired below 850 °C); low fusing ‘porcelains’ (fired between 850 and 1100 °C); and higher fusing ceramic powders, which are used primarily for denture teeth. All these are manufactured under controlled conditions within a factory environment. The ultra low fusing ceramics are used primarily as shoulder ‘porcelains’ (see p. 389), or to correct minor defects and to add surface colouring and shading.
Dental laboratory procedure
The traditional method involved an impression being cast to produce a working model. The preparation die was then removed from the model and the dental technician laid down a platinum foil onto the die and closely adapted the foil to the surface of the die. The purpose of the foil was threefold:
However, more recently, use of platinum foils has fallen out of favour as the ceramic crowns produced were not strong and tended to fracture. This is now overcome by the use of a substructure which supports the overlying ceramic. Until recently, the primary means to provide this support was to fire the ceramic onto an underlying metal coping, usually a gold alloy. This coping also prevents crack propagation. However, more recently, use of alumina, leucite and zirconia core structures has proved fruitful. Like the metal coping, these materials provide strength and prevent crack propagation. They are relatively opaque but modern techniques using glass infiltration of a friable, part-sintered framework has produced core materials which are very much stronger than conventional dental ceramics. The construction of these cores may be carried out by hand in the dental laboratory or may be produced by the computer-aided design– computer-aided manufacture (CAD-CAM) technique from factory prepared blocks of the sintered materials. This is discussed in detail later in the chapter.
Building up the restoration: dentine portion
The ceramic is either built up on a refractory die which itself is placed in the furnace or onto a core or coping by the technician applying an opaque shade to mask the colour of underlying substructure. This is either fired in the furnace or left to dry. The dentine portion of the restoration is then laid down using the appropriate shade of the ‘dentine’ ceramic powder.
The ceramic powder is mixed with distilled water to form a creamy paste, which is then laid down onto the coping. It is important that the minimum amount of air is incorporated into the powder slurry during this process to avoid porosity and stress concentrations in the final product. To produce minimum shrinkage during the firing process the powder must be condensed to remove water and pull the ceramic particles closer together. This is called compacting and is achieved by either vibration, spatulation or smoothing/burnishing with a brush. Once condensation has been achieved, the excess water is blotted away using absorbent tissue (Figure 22.4).
Building up the restoration: enamel portion
Once the dentine portion of the restoration has been applied, the appropriate shade of enamel is selected and this is built up as previously described. The final built-up mass is substantially oversized than the restoration it will finally become. A combination of the condensation process and firing will reduce the size markedly. At this point, the mass is referred to being in the green state, i.e. the prefiring state. It is very fragile and must be handled very carefully (Figure 22.5).
Firing: first bake
The mass is fired to fuse the particles together and form the final restoration, by a series of ‘bakes’ in the furnace. It is important that the mass is slowly heated initially to eliminate the water from the slurry and allow shrinkage to occur. To achieve this, it is usually held near the entrance to the furnace for sometime before being introduced inside (Figure 22.6).
Fig. 22.6 A furnace used to bake dental ceramic. Note that the ceramic mass in the green state is sitting a short distance to the furnace entrance to allow water to be slowly driven off before it turns into steam. If steam was allowed to form, the powder core would break.
During the first ‘bake’ the water is driven off and the powder particles sinter together. The majority of the shrinkage occurs during this firing and is in the range of 10–20%. The temperature of the furnace is set at about 50 °C below the fusing temperature of the ceramic powder being used. During this time, the binders are burned off and the ceramic particles start to fuse at the points of contact, forming a porous mass. The voids in the porous mass start to disappear as the molten glass flows between the particles, drawing them closer. This is called pyroplastic flow. Shrinkage continues to occur until an almost void-free material results. This is often referred to as the biscuit bake or biscuit firing (Figure 22.7).
Image courtesy of Vita Fabric.
It is important that once this firing cycle has been completed, the ceramic is allowed to cool slowly and uniformly. This will prevent stresses forming, as different portions of the material shrink to different extents. Stresses can lead to cracking and a loss in strength due to thermal shock.
Firing: subsequent bakes
If the technician deems that further ceramic is required to complete the restoration, this is added and the restoration fired again. Figure 22.8 shows a crown just prior to firing with the various ceramic powders built up. The different colours help in differentiating the dentine and enamel powders. These colours are lost in the firing.
Fig. 22.8A–D (A) An all-ceramic crown being built up to restore tooth 11 using dentine ceramic (pink) and enamel (white). (B, C) The crown has been fired and (D) further ‘enamel’ ceramic has been added.
Images courtesy of Vita Fabric.
The firing process is then repeated but in this case the temperature of the furnace is increased. There is a further slight contraction, and the voids between the particles are filled by the molten glass, which is drawn into the spaces between the sintered particles by capillary action to form a solid mass.
Some considerations in firing
• Overfiring can result in molten glass flowing too much and the restoration losing its shape. Modern furnaces are usually computer controlled and the changes required during the firing process can be programmed into the memory. This may involve firing in air or a partial vacuum, producing an atmosphere about 10% of normal. Some ceramic products come supplied with a bar code which is scanned to input the firing cycles required by that particular ceramic. This is of particular importance as each manufacturer’s ceramics have different firing parameters that should be adhered to precisely.
• The size of the particles of the ceramic powder also has an influence on the finished crown. Finer grained powders produce more uniform surfaces than coarser grains. Although the firing process and the densification which occurs will leave a structure which is solid, there is still a risk of small air voids being present. This is the case even when the firing process is carried out under vacuum. Much of the air within the ceramic structure is removed as the vacuum develops. The little that remains will be below atmospheric pressure. Once the furnace temperature has reached to within about 50 °C of the final firing temperature, the vacuum is released and this results in the voids collapsing as the pressure external to the crown is increased by a factor of 10 above the internal pressure in the crown.
Stains may now be applied using a paint brush to characterize the final restoration, such as the staining of occlusal fissures or hypoplastic spots. The staining kit resembles an artist’s palate (Figure 22.9). The stains may be applied to the surface of the restoration or become incorporated within the ceramic. If the stain is applied on surface, it may be lost if any adjustment is made or during function. Generally speaking, stains are better incorporated within the structure of the ceramic (Figure 22.10).
The final stage of the firing process is the glazing of the restoration. This produces a glassy smooth surface on the restoration, sealing it. It will also fill in any small areas of porosity at the surface. Glazing is achieved by either very carefully re-firing the restoration to fuse the outer layer of ceramic completely or by using glazes with lower fusing (transparent glass) temperatures which are applied as a thin layer to the outer surface of the restoration. The restoration may then be adjusted with fine diamonds and polishing rubbers (Figure 22.11).
Properties of fired dental ceramics
It is widely recognized that ceramic is the best dental restorative material with respect to aesthetics. When the shade and any characterization has been carefully prescribed and then replicated into the restoration, the aesthetics can be excellent creating an almost imperceptible result. It can be quite a challenge to notice the difference even for dental care professionals, particularly with all-ceramic restorations. Ceramics are colour stable and achieve a very smooth surface finish and have the ability to retain the finish better than other materials. However, they will be affected by acids. In particular, topical fluoride gels can etch the surface of the ceramic. This etching leads to the surface glaze being disrupted quite rapidly, possibly resulting in surface staining. Bulk colour changes can occur if the tints incorporated during the crown construction are involved.
Dental ceramics provide a very high level of translucency, which is important when matching the restoration to natural tooth tissue. As well as matching the shade of the adjacent natural teeth, dental ceramics must also be able to fluoresce and be opalescent. Natural tooth tissue has this ability, the best example of this is viewing a ceramic crown under ultraviolet light in a nightclub, where it will often appear ‘dead’ and dark compared to the adjacent teeth.
Dental ceramics formed during the firing process are chemically stable (inert) and resistant to chemical attack. They are also biocompatible and have a good soft tissue compatibility. However, strong acids such as hydrofluoric acid can be used to etch the surface of the ceramic. This is used when the ceramic restoration is to be bonded to the tooth surface, for example a veneer or to repair fracturered ceramic.
The thermal properties of dental ceramic and tooth tissue shows a great similarity, i.e. the coefficients of thermal expansion and thermal diffusivity are close to one another. This means that the ceramic restoration will behave in the same way as the underlying dentine with respect to thermal expansion and contraction and will exhibit a slower rate of heat transfer. The restoration will therefore not be stressed during oral thermal cycling. Thermal diffusivity is poor and may present a problem if the dentist carried out a sensitivity (vitality) test on the tooth by applying a very hot or cold material to the ceramic crown. The material will not transmit these extremes of temperature well, making the results of the test difficult to interpret.
Themal shock may be avoided by firing ceramic materials as few times as practicable and allowing them to cool slowly on their removal from the furnace. Soldering of metal components should be avoided after the ceramic has been added for the same reason.
While fully fired ceramic is dimensionally stable, this not the case prior to firing, when a large volumetric shrinkage is seen from the early sintered state to the fully fired product. This property makes ceramic a challenging material for the dental technician and restorative dentist to work with. The construction and maintenance of accurate occlusal contacts is difficult if not impossible unlike the lost wax technique, where the wax and metal may be much more accurately worked with. The inclusion of a try-in appointment is invaluable where the restoration is returned to the clinic in the biscuit stage. The occlusion can then be adjusted to create stable contacts prior to the restoration being glazed and fitted.
The large shrinkage seen has prompted the development of shoulder ceramics. Shrinkage at the margins of the preparation leads to an open margin and potential for leakage with the attendant sequelae. Shoulder ceramics shrink much less and so a more accurately fitting restoration is produced. To overcome the problem of ceramic shrinkage, many restorative dentists prefer metal margins but in reality this solution is only helpful in the non-aesthetic zone.
Dental ceramics are very strong in compression but are also very brittle and have low flexural strengths. However, in tension and flexure the ceramic behaves as a glass. The best analogy for this is the impact of a cricket ball against a pane of glass. The ceramic must always be supported by an underlying structure or it will fracture under load, particularly if the ceramic is unsupported. Ceramic also has a low fracture toughness, which means crack propagation between defects will readily occur.
Dental ceramics show static fatigue, which is the decrease in strength over time even without the application of load. They can also develop slow crack growth during cyclic loading in a moist environment, which may, over time, lead to fracture of the ceramic.
Effects of tooth preparation
Ceramics may also fracture during function if the initial tooth preparation was inadequate. If the height of an anterior crown preparation is reduced excessively then a large area of tooth must be replaced by ceramic. If the unsupported ceramic is thicker than 1 mm it will have no support from the underlying tooth structure and therefore is at risk of flexure during chewing and biting, and thus fracture (Figure 22.12).
Vacuum versus air firing
During the firing cycle, if residual air is retained because air voids were present in the unfired ceramic mass, incomplete fusion of the glass particles will occur. These air voids may be quite extensive and f/>