Most dental technical procedures arc carried out on a cast (model) which is a positive likeness of the tooth-bearing and associated areas of the oral cavity. This is obtained by taking an impression (a negative likeness) of those areas, and pouring a suitably strong material into it to form the cast.
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It is of prime importance that the cast is an accurate duplicate of the mouth. This normally involves a two-impression technique. The first or primary impression is taken in irreversible hydrocolloid (alginate) or impression compound and supported in a stock tray. The second impression may be taken in irreversible hydrocolloid, rubber base, zinc oxide—eugenol paste or plaster of Paris. These impressions are supported in a tray, termed a special tray, which is constructed to fit the preliminary cast of the patient’s oral tissues.
Impressions taken in a thermoplastic material such as impres-sion compound, generally displace the oral tissues because pressure is required to scat them. Such an impression is termed mucocom-pressive. Materials such as hydrocolloid, rubber base and plaster of Paris exhibit good flow properties and, if handled correctly, do not displace the oral tissues. They are therefore called mucostatic impression materials. if muscles of the oral cavity are flexed during impression-taking, a functional impression is formed, which indi-cates the boundaries of muscular activity. This information is essen-tial when constructing appliances for the mouth. When the natural dentition is complete it is said to be fully dentate; if some teeth are missing the mouth is referred to as being partially dentate; and a mouth devoid of teeth is termed edentulous. The terms dentate, partially dentate and edentulous are used to describe impressions and casts alike.
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