The accuracy of the model depends on the accuracy of the impression in which it was cast. As we say, first impression should be good,likewise a dental impression recorded should cover all minute details of the mouth. The requirements of impression materials can be conveniently discussed under four main headings: In order to record the fine detail of the hard or soft oral tissues, the impression material should be fluid on insertion into the patient’s mouth. Temporary. However, the latter terms have been used for many years and are therefore likely to be familiar to dentists. – Is it a Burning Mouth Syndrome?. Classification of Impression materials. Figure 16.5 shows diagramatically the way in which a set material is placed under stress during the withdrawal of the impression. 16.5 Diagram illustrating how an impression material is placed under stress during removal from an undercut area. Hence, we have silicone materials, alginates, etc. In addition to the requirements given above, there are two further requirements which apply specifically to materials used for recording undercuts. (a) Impression in place before removal. w 1848 Gutta percha was introduced as an impression material. Individual with deep undercuts, the material of choice for recording impression is alginate with perforated stock trays. Zoe impression paste, Impression plaster. Three Impression Material Classifications: A Comparison. Model ² a study purpose cast which has a positive reproduction of prosthetic bed tissue relief in real sizes received after taking impressions with Impression Materials A. We need to understand which impression material and technique are used in an individual. 5 IMPRESSION MATERIALS. Elastomeric impression materials are in common use. – As mentioned earlier, those that replace lost tooth structure and restore the function of the teeth mus t be strong and hard. According to philips scienceof dental materials impression materials are classified on thebasis of setting and elasticity.Mode of setting rigid ElasticSet by chemicalreaction(irreversible)Impression plaster,zinc oxide eugenol.Alginate,polysulfide,polyether, silicone,Set by temperaturechange(reversible)Compound, waxes Agar hydrocolloid. The behaviour of viscoelastic materials is described on p. 15–16, where the influence of time as an important parameter is discusse/>. Menu and widgets The effects of thermal changes are minimized if the values of coefficient of thermal expansion of the impression material and tray material are small. Impressions are the Key for a good prosthesis. impression waxes) and in other cases are very rigid but show little evidence of plastic deformation (e.g. 2020/2021 Agar; b) Irreversible. a) reversible. It is of great importance, therefore, that inaccuracies are minimized at this stage, otherwise they will be carried through and possibly compounded later on. Uploaded by. E.g. These supporting structures include gingiva, a lveolar bone or residual ridge, hard and soft palate, and frenums, which are muscle attachments. The accuracy of these final restorations is highly dependent on the impression materials and techniques utilized. The accuracy of ‘fit’ and the functional efficiency of the appliance depends upon how well the model replicates the natural oral tissues. For most materials, the dimensional change is a contraction and, providing the impression material is firmly attached to the impression tray, this produces an expansion of the impression ‘space’ and an oversized die, as illustrated in Fig. General Comments – Impression materials are used to make replicas (models or casts) of t eeth and other oral tissues. w 1840 Charles De Loude gave the first references to impression trays. It is difficult to calculate the precise value of the thermal contraction or to predict accurately the direction in which it operates since the contraction of the tray and that of the material act in opposite directions, providing the impression material remains attached to the tray. 1, a). 16.4. Punj et al. (a) Impression in place before removal. This may effect the fine detail which can be recorded in impressions of hard tissues and may influence the degree of tissue compression or displacement achieved with soft-tissue impressions. This factor is primarily responsible for governing the principal applications of the materials. what are preliminary impressions used to make? 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