Non-elastic Impression Materials
Four main types of products form the group of impression materials classified as non-elastic materials:
They are classified together for convenience rather than for reasons of similarity in composition or properties. A factor which links the materials is their inability to accurately record undercuts. One of the materials (plaster) is brittle when set and fractures when withdrawn over undercuts. The other products are likely to undergo gross distortions due to plastic flow if used in undercut situations.
Impression plaster is similar in composition to the dental plaster used to construct models and dies (Chapter 3). It consists of calcined, β-calcium sulphate hemihydrate which when mixed with water reacts to form calcium sulphate dihydrate.
The material is used at a higher water/powder ratio (approximately 0.60) than is normally used for modelling plasters. The fluid mix is required to enable fine detail to be recorded in the impression and to give the material mucostatic properties. The setting expansion of dental plaster is reduced to minimal proportions by using anti-expansion agents. Potassium sulphate is the most common of these and has the secondary effect of accelerating the setting reaction, details of which are discussed on p. 37. A retarder, such as borax, is normally incorporated, in order to give a material in which the setting characteristics are controlled. A pigment such as alazarin red is also commonly used, in order to make a clear distinction between the impression and the model after casting of the model. The anti-expansion agent, retarder and pigment are incorporated into the impression plaster powder by some manufacturers. As an alternative an anti-expansion solution, containing potassium sulphate, borax and pigment, may be prepared and used with a standard white plaster.
Freshly mixed plaster is too fluid to be used in a stock impression tray and is normally used in a special tray, constructed using a 1–1.5 mm spacer. The tray may be constructed from acrylic resin or shellac. Another technique is to record the plaster impression as a wash in a preliminary compound impression. The compound is deliberately moved during setting to create space for the plaster wash. The technique for insertion of the impression into the mouth involves ‘puddling’ the impression into place. With other materials the tray is simply seated home in a single movement. With plaster the tray is gently moved from side to side and antero-posteriorly to best take advantage of the handling characteristics of the material, particularly its fluidity.
Before casting a plaster model in a plaster impression, the impression must be coated with a separating agent, otherwise separation is impossible.
The mixed impression material is initially very fluid and is capable of recording soft tissues in the uncompressed state. In addition, the hemihydrate particles are capable of absorbing moisture from the surface of the oral soft tissues, allowing very intimate contact between the impression material and the tissues. The fluidity of the material, combined with the ability to remove moisture from tissues and a minimal dimensional change on setting, results in a very accurate impression which may be difficult to remove.
The water-absorbing nature of these materials often causes patients to complain about a very dry sensation after having impressions recorded. Disinfection of a plaster impression can be achieved with a 10 minute soak in sodium hypochlorite solution as described previously.
Following setting, the plaster impression material is very brittle. It can undergo virtually no compressive or tensile strain without fracturing. The material is, therefore, not suitable for use in any undercut situations.
One technique for recording impressions of undercut areas, commonly used before the advent of elastic materials, was to allow the impression plaster to ‘set’ and then to fracture it in order to facilitate removal from the mouth. The material is weak and easily fractured due to its high water/ powder ratio. The fragments are then reconstructed in order to form the completed impression.
The properties of impression plasters can be compared with those of model plasters and stones by reference to Table 3.2. The main differences between impression plaster and model plaster are: more rapid setting in order to avoid inconvenience/discomfort to both the patient and dentist; smaller setting expansion for greater accuracy – the expansion is actually equivalent to that observed for a low expansion die stone (type 4) and much lower strength so that fracture can occur easily if the material engages an undercut.
Dental impression plaster remains a useful material, particularly when recording impressions of patients with excessively mobile soft tissues overlying the residual alveolar bone (a ‘flabby’ ridge). It is important to capture such tissue at rest rather than risk an abnormal pattern of displacement with a more viscous impression material. A two-stage technique is commonly used in which a special tray is made with appropriate spacing for zinc oxide/eugenol paste where the mucosa is well supported and having a window overlying the ‘flabby’ area. An impression of the bulk of the ridge is recorded in zinc oxide/eugenol paste. Any excess material is removed from the window and the impression re-seated in the mouth. The shape of the flabby ridge at rest is then recorded by painting plaster into its surface with a brush and keying this impression into the impression tray.
Impression compound (see Fig. 17.1) is a thermoplastic material, having properties which in many ways are similar to those of the dental waxes discussed in Chapter 4. The composition varies from one product to another but an indication of typical composition is given in Table 17.1. Two types of impression compound are available. These are usually classified as type I (lower fusing) and type II (higher fusing). The type I materials are impression materials whereas the type II materials are used for constructing impression trays. The difference in fusing temperature between type I and type II materials naturally reflects a difference in the composition of the thermoplastic components of each.
The lower fusing, type I impression materials may be supplied in either sheet or stick form. The sheet material is used for recording impressions of edentulous ridges, normally using stock trays. The stick material is used for border extensions on impression trays or for recording impressions of single crowns using the copper ring technique.
The sheet material is normally softened using a water bath. Both the temperature and time of conditioning in the water bath affect the performance of the material. If the conditioning temperature is too low the material does not soften properly, and if too high, it becomes sticky and unmanageable. A temperature in the range 55–60°C is normally found to be ideal.
|Thermoplastic material (47%)||Natural or synthetic resins and waxes||Characterises the softening temperature|
|Filler (50%)||Talc||Gives ‘body’ by increasing viscosity of the softened material; reduces thermal contraction|
|Lubricant (3%)||Stearic acid||Improves flow properties|
The conditioning time must also be carefully monitored. It should not be so long that important constituents, such as stearic acid, can be leached out, nor should it be so short that the material is not thoroughly softened. The materials are poor conductors of heat and it may take several minutes for the centre of the material to become softened. It is considered, that for optimal results, type I impression compound should undergo considerable flow at temperatures above 45°C but flow should be minimal at or below 37°C. The stick material is generally softened using a flame. A measure of skill and experience is required in order to soften the material sufficiently witho/>