Chapter 20 Model and investment materials
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.
Types of Model
• In the case of a cast restoration such as a crown or bridge, individual teeth may be removed from the rest of the cast so that the restoration can be waxed up and worked on more easily. An individual tooth structure or preparation on a model is known as a die.
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.
Dental Plaster and Dental Stones
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.
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.
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.
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. 20.7 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.
Commercially available products
As indicated above, the setting reaction for all these hemihydrate materials is initiated by mixing with water. The amount of water required to achieve a suitable mix varies with the plaster type. It is possible to calculate the exact amount of water required to mix with a specific weight of water. Due to the porous nature of the powder and its particle irregularities, the amount of water to achieve a suitable mix of plaster of Paris must be increased so that the powder is wetted. The mass of water required for the other two types of stone is reduced in proportion to the porosity of the powder and the shape and density of the particles. The consequence of using more water in the rehydration of the hemihydrates is that the plaster so formed will be weaker and more friable. With all types of dental plaster the amount of water used should be the minimum required to produce a creamy mix that can be effectively manipulated into an impression to produce an air-blow-free model. The manufacturer will provide this information.
• Potassium sulphate is added to accelerate the setting time. A 2% solution is used as an alternative to water and will reduce the setting time of model plaster from 8–10 to 4–5 minutes. The compound crystallizes very quickly and encourages further crystal growth. A 4% solution decreases the setting expansion.
• Borax is used to retard the set. A 2% solution will prolong the setting time of some gypsum materials by up to a few hours. The addition leads to the formation of a calcium salt of the borate. This is deposited on the dihydrate crystals, preventing further crystal growth.
• The addition of sodium chloride has the effect of reducing the setting expansion by providing extra sites for crystal growth. This in turn reduces the degree of growth at individual sites so preventing the crystals from being pushed apart.
• Calcium sulphate dihydrate provides nuclei of crystallization and therefore it acts as an accelerator. These set particles have a marked effect when used at very low concentrations between 0.5 and 1%. However, its effects above this value are less apparent.
The setting process was originally described by Le Chatelier and confirmed by van’t Hoff in 1907. The process has been described as being the result of differences in the solubilities of the dihydrate and hemihydrates of calcium sulphate.
As the hemihydrate powder is added to the water, some of the powder dissolves. A reaction occurs and this hemihydrate is converted to the dihydrate. The solubility of the dihydrate is very low and a supersaturated solution is rapidly formed. Since the stability of the supersaturated solution is very low, the dihydrate crystals start to precipitate out. This process continues as more hemihydrate dissolves in the water. This is a quite an aggressive exothermic reaction and has potential for tissue damage due to burning if handled incorrectly.
Due to the high exothermic reaction of gypsum products with water, care should be exercised when it is being used. This was recently demonstrated by an English schoolgirl who placed her fingers in a bowl of unset plaster. The material was allowed to set with her fingers in it and as a consequence of thermal damage, she suffered serious injuries resulting in the loss of several fingers.
The compressive strength of plaster-based materials ranges from 12 to 45 MPa 1 hour after setting depending on the type of hemihydrate used. β-hemihydrate has the lowest compressive strength, that is, greater porosity leads to lower compressive strengths. After 1–2 hours the model appears dry but over a period of time further water is lost to the atmosphere. As this water is lost from the model, the compressive strength rises significantly. Once about 7% water has been lost, the compressive strength reaches approximately 60 MPa.
Surface hardness and abrasion
The hardness and compressive strength are linked in that higher compressive strengths are associated with higher hardness values. After the loss of water to the atmosphere the hardness also increases significantly. However, there is still a risk that dies and models will be damaged during any construction process. Attempts have therefore been made to make the model more abrasion resistant. For example, impregnation of the die with a variety of materials such as epoxy resin, methylmethacrylate, glycerin or h/>