![]() ![]() The material is provided in tube-like sachets for loading the tray, or in a syringe for easy adaptation to the teeth. Ideally, the impression material to be used should be sufficiently fluid on placement to prevent displacement of the soft tissues, with such materials being considered to be mucostatic. Such impression materials are classed as being muco-compressive. When this is reproduced in the prosthesis, the soft tissues will need to adapt to the prosthesis rather than the other way round this will cause discomfort to the patient. If the impression material is very stiff, it may displace such tissues and produce a distorted impression. This occurs particularly with edentulous patients, who can present with a flabby ridge. There are instances in which the patient will have mobile soft tissues. If this is not feasible, an alternative material must be used that is compatible with moisture and saliva. A dry field is essential for such materials. If this should happen in a critical area, then important surface detail may be lost as a blow hole is formed on the impression surface due to trapped air. Some materials are hydrophobic (water-repellent) and will be repelled by moisture on the surface. A low viscosity is therefore desirable, but it should not be so low that the material is not easily contained within the impression tray. ![]() ![]() The accuracy of reproduction of the surface detail depends on the viscosity of the mix and the ability of the impression material to adapt closely to both the soft and the hard tissues. Similarly, alginates, when used in a stock tray, do not always give the required degree of accuracy and are then better used in a special tray. Although compo can be used in a stock tray, the impression obtained does not reproduce surface details adequately unless a zinc oxide–eugenol wash is used with it. Others, such as impression compound (compo), plaster of Paris, alginate and the silicones are available in formulations that can be used with a stock tray. Some impression materials are not available in a sufficiently high-viscosity version for use in a stock tray, and these include zinc oxide–eugenol, polyether and polysulphide elastomers. This can be done either by constructing an acrylic special tray from a preliminary model, or by using a high-viscosity material, which is placed in a stock tray once this has set, a special tray is produced. The choice of impression tray is determined, to some extent, by the viscosity of the impression material.Īn impression material that is very fluid when it is first mixed cannot be used with a stock tray, and a close-fitting special tray needs to be produced. The variety of applications of and techniques used with the impression materials are presented in Table 2.7.2. The trays also provide support when the model is poured from the impression. These trays are needed to support the impression material (especially when it is still fluid), so that it can be carried to the patient, inserted in the mouth, and removed once it is set. The choice of impression material may also be affected by the technique to be adopted, with a major consideration being the selection of a stock tray or special tray. The choice will depend on the particular requirements of each individual case. Both are able to engage undercuts and may be used in edentulous, partially dentate and fully dentate patients. The elastic impression materials are subdivided into hydrocolloid and elastomeric impression materials. Consequently, their use is restricted to edentulous patients without bony undercuts. The rigid impression materials cannot engage undercuts that may be present on the teeth or the bone. ![]()
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