An impression is imprint of the mouth. Materials which expand during setting result in undersized dies or casts. Hence, we have silicone materials, alginates, etc. Receiving a qualitative impression is one of the main goals which the doctor faces at the prosthodontics treatment of patients with various diseases of a dento- Since no pressure is applied, Tissue health is preserved.Hence, it is also known as Minimal pressure technique. Individual with deep undercuts, the material of choice for recording impression is alginate with perforated stock trays. 1. The impression stage is the first of many stages involved in the production of dentures, crowns, bridges, orthodontic appliances etc. A dental impression is an imprint of teeth and/or soft tissues, formed with specific types of impression materials that is used in different area of dentistry including Prosthodontics.. A correctly made dental impression will capture a part or all of a person's dentition and surrounding structures of oral cavity. (a) If the impression material is bonded to the tray, contraction occurs towards the tray. Once positioned in the patient’s mouth, the materials undergo ‘setting’ by either a chemical or physical process. 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. Dental impression.Materials. Fig. – NUTRITION AND ORAL HEALTH. 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. It is of great importance, therefore, that inaccuracies are minimized at this stage, otherwise they will be carried through and possibly compounded later on. watch our you tube video on dental pathshala to understand the classification of impression materials. When a substance behaves in this way, it is said to be pseudoplastic. 16.2 Impression materials. Fig. – Some restorative materials simulate the appearance of the tissues that are being replaced. Figure 16.6 gives a series of diagrams to illustrate what happens when an impression of an undercut tooth is recorded with (a) an elastic material, (b) a plastic material, and (c) a viscoelastic material. On being withdrawn from the patient’s mouth, which is typically at a temperature of 32–37°C, into the dental surgery, at a temperature of around 23°C, the impression undergoes approximately 10°C cooling. Additional retention is achieved by using perforated trays. The newest class of impression material is the vinyl-polyether hybrids that include SENN (GC America, Alsip, IL; soon to be introduced in the U.S.). The ability of impression materials and gypsum products to reproduce detail in the cast is normally determined by measuring the contact angle which a drop of aqueous calcium sulphate solution makes with the surface of the impression material. Fig. It may be less confusing if the terms rubbery and non-rubbery were used instead of elastic and non-elastic. CLASSIFICATION OF IMPRESSION MATERIALS. For such products, a dry field of operation is essential. This factor is primarily responsible for governing the principal applications of the materials. CRAIG Department of Biomaterials, School of Dentistry, University of Michigan, Ann Arbor, Michigan 48109 Adv Dent Res 2(l):51-64, August, 1988 ABSTRACT Major advances in impression materials and their application have occurred during the … After ‘setting’, the impression is removed from the patient’s mouth and the model cast using dental plaster or stone. Another complicating factor is the spacing of the impression tray. (a) The tray contracts and reduces the impression space. Manufacturers of impression materials often supply tray adhesives which are used to enhance bonding. The accuracy of ‘fit’ and the functional efficiency of the appliance depends upon how well the model replicates the natural oral tissues. Based on material use A. Inelastic or rigid. Thus, materials which are initially very fluid are often classified as mucostatic impression materials because they are less likely to compress soft tissues, whilst materials which are initially more viscous are classified as mucocompressive. This can result in blow holes and loss of fine detail. The effects of thermal changes are minimized if the values of coefficient of thermal expansion of the impression material and tray material are small. The thickness corresponds to specific spacing, and can be classed as spaced, where about 3mm of space is left between the tray and the mucosa for the impression material to occupy, or closely adapted, where less space is left for the impression material. Title: Impression materials (requirement and classification) 1 Impression materials (requirement and classification) Dr. Waseem Bahjat Mushtaha ; Specialized in prosthodontics; 2 Definition . (a) Impression in place before removal. 16.4. In addition to the requirements given above, there are two further requirements which apply specifically to materials used for recording undercuts. A low contact angle is favourable as it indicates good wetting. But retention is not good for mucostatic impression technique. Fig. E.g. When standing teeth are to be recorded, or when the patient has deep soft-tissue under-cuts, the set impression material must be flexible enough to be withdrawn past the undercuts and elastic enough to give recovery and an accurate impression. Classification of materials according to viscosity is not, therefore as simple as it may seem. a) reversible. Impression materials: basic properties Gen Dent. The figures show the different test apparatus, instruments and components. 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