22: Dental ceramics

Chapter 22 Dental ceramics

Introduction

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.

Table 22.1 Comparison of the composition of decorative and dental ceramics

Composition Decorative ceramic (%) Dental ceramic (%)
Kaolin 50–70 3–5*
Quartz (silica) 15–25 12–25
Feldspar 15–25 70–85
Metallic colourants <1 1
Glass 0 Up to 15 depending on fusing temperature

* Note the small amount of kaolin in dental ceramic with a consequent increase in the percentage of feldspar.

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 feldspathicporcelains’. 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.

Table 22.3 Metallic oxides convey various colours to the ceramic

Metallic oxide of Colour
Chromium Green
Cobalt Blue
Copper Green
Iron Brown
Manganese Lavender
Nickel Brown
Titanium Yellow/brown

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 shoulderporcelains’ (see p. 389), or to correct minor defects and to add surface colouring and shading.

Low fusing ceramics should not be subjected to multiple firings as this is likely to lead to distortion. They must also be supported by a substructure otherwise they are likely to sag (see p. 390).

Dental laboratory procedure

The technical process to construct a ceramic restoration in the dental laboratory is time-consuming and requires considerable care to achieve a satisfactory result. This process is described below.

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.

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).

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).

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.

Some considerations in firing

• This process must be carefully controlled as the temperature of the furnace and time that the ceramic is in it is critical.

• 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.

Properties of fired dental ceramics

Aesthetics

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.

Jan 31, 2015 | Posted by in Dental Materials | Comments Off on 22: Dental ceramics
Premium Wordpress Themes by UFO Themes