The last section of this book deals with the materials used in the process of fabrication of indirect restorations and dentures. In all cases the underlying method of production involves a technique that can be traced back many hundreds of years and which was used extensively in the manufacture of jewellery: the lost wax technique. In this technique, a model of the substructure is first prepared. On this the prototype prosthesis is made using materials such as waxes that can be shaped to the required anatomical shape but which can also be destroyed by heating. Once this prototype is prepared it is invested or surrounded in a material which on setting will form the negative of the prototype pattern. The material for the prototype is then removed by heating. This leaves a space in the investing materials, which is filled either by casting or by applying a dough of the material and closing the mould under pressure. The details of this process will be described for each application.

In clinical practice, it is often necessary to make models of the patient’s teeth. The models are used by the dentist to (in conjunction with other information) to plan a course of treatment or to preoperatively design a prosthesis such as a bridge. Once the treatment plan has been decided, the teeth are prepared and an impression is taken for a new model, and the restoration is then constructed in the dental laboratory by the dental technician on the second model.

Both the preoperative model and the working cast are constructed out of a material based on gypsum. This is commonly referred to as (dental) plaster. As such, plaster is one of those ubiquitous materials which is used in many types of clinical dentistry and in the dental laboratory. Dental plaster is also traditionally used to make an impression of the edentulous mouth prior to the construction of a complete denture (see Chapter 15). A special form of plaster may be used when metal restorations are to be cast using the lost wax technique. This chapter discusses all the dental materials used in the construction of dental models and those used as investment materials.

A model may be defined as a replica of the structures in the oral cavity. It is made by pouring a material such as dental plaster into an impression of the area.

There are a range of materials that the technician can use, the choice of which depends on the purpose and use of the cast. In certain cases, for example when the framework for a metal denture is waxed up, the whole model is required. This is called a refractory model and is made out of a special material – a refractory material – so that it may be invested and subjected to high temperature so that the metal framework can be cast on to it. A refractory material retains its shape and strength, that is, it is physically and chemically stable, at high temperatures. This material should also be resistant to thermal shock and have appropriate thermal properties for the intended purpose.

Models may be made out of dental plaster, dental stone or investment material. All these material are based on gypsum but have different properties, which will determine when and how they are used.

Gypsum is calcium sulphate dihydrate and occurs naturally at many sites around the world. It is crystalline in form (Figure 20.1). To be used as a casting material, the crystalline gypsum is heated at 130°C to remove some of the water contained in it. The product is called plaster of Paris, named after the site where this process was first carried out. The plaster is called calcined and the chemical produced is calcium sulphate hemihydrate. Further heating (up to 200°C) will drive off all of the residual water, leaving behind anhydrous calcium sulphate.

Fig. 20.1 (A) Gypsum stone as it occurs naturally. (B) A micrograph of the microscopic structure of the stone.

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Dental plaster is provided in the hemihydrate form. Once water is added to this, the hemihydrates reverts to the dihydrate with the liberation of heat. It is this reaction which occurs with all dental plasters. The form of the crystalline hemihydrate determines the precise type of plaster which is produced although all types are chemically identical and are dissimilar only in structure and form.

Dental modelling plaster

Conventional dental modelling plaster such as plaster of Paris is produced by heating the gypsum to between 110 and 130°C in an open vessel. The hemihydrate so formed is known as the β-hemihydrate. The powder produced is made up of irregular particles which are porous. These particles are not packed closely together (Figure 20.2). Figure 20.3 shows dental models made out of plaster of Paris.

Fig. 20.2 Illustration of the crystal structure of plaster of Paris. Note the large, irregular particles that are loosely arranged and porous.

Fig. 20.3 A dental model made out of plaster of Paris. Note its white colour.

Dental stone

If the dihydrate is heated under pressure and in the presence of water vapour at 125°C, it produces much more uniformly shaped particles. This material has much reduced porosity (Figure 20.4) and is known as α-calcium sulphate hemihydrate. The variant used in dentistry is known as dental stone. Figure 20.5 shows dental models made out of this material.

Fig. 20.4 Illustration of the crystal structure of dental stone. Note the more uniformly shaped particles and much reduced porosity.

Fig. 20.5 A dental model made out of dental stone (Kaffir D). Note its yellower colour.

However, most dental casts are constructed of a mixture of plaster of Paris and Kaffir D, usually 50/50 by weight. This resultant mixture is called orthodontic plaster.

High-strength dental stone (die stone)

Further treatment of the dihydrate improves the properties of the stone, such as increasing its strength and abrasion resistance. The material produced is called densite, high-strength dental stone or die stone. This material is produced by dehydrating the gypsum in the presence of calcium or magnesium chloride. The combination of chemicals is boiled together, and then the chlorides are washed away with boiling water. The chlorides aid in separating the gypsum particles and the end result is a powder which is even less porous and much less irregular in shape (Figure 20.6). The powder is also the densest of the three types of hemihydrate. Figure 20.7 shows a dental model made out of this material.

Fig. 20.6 Illustration of the crystal structure of die stone. Note the particles are much more uniformly shaped, smaller and denser than dental stone or plaster of Paris.

Fig. 20.7 A working cast made out of die stone, in this case to construct an inlay/onlay for tooth 36. The die stone is only used to make the teeth part of the cast, with the base being constructed out of dental stone. This is done for commercial reasons, as die stone is much more expensive than dental stone and accuracy and hardness are not critical in this region.

Die stones are available in a range of colours (Figure 20.8).

Fig. 20.8 Different types of die stone.