To produce accurate replicas of intra- and extraoral tissues, the impression materials should be (1) sufficiently fluid to adapt to the oral tissues, (2) viscous enough to be contained in a tray, (3) able to transform (set) into a rubbery or rigid solid in the mouth in a reasonable time (less than 7 min), (4) resistant to distortion or tearing when removed from the mouth, (5) dimensionally stable long enough to allow one or more casts to be poured, (6) biocompatible, and (7) cost-effective in terms of time as well as the expense of the associated processing equipment. One or two gingival retraction cords are placed under the margin around the tooth for at least 5 min before making impressions. One method for determining the time of removal is to inject some of the syringe material onto a space that is not in the field of operation before inserting the impression tray. Why is rapid seating of an impression tray not advisable for a pseudoplastic impression material? The catalyst (or accelerator) paste contains lead dioxide, filler, and plasticizer as in the base paste, and oleic or stearic acid as a retarder to control the rate of the setting reaction. Elastomers comprise a group of synthetic polymer-based impression materials that are chemically cross-linked when set and that can be stretched and yet rapidly recover to their original dimensions, like vulcanized natural rubber when the applied stress is released. putty. After being removed from the mouth, the pressure in the impression is released and the putty recovers its “elastic deformation.” The distortion produced by the stiff, compressible putty results in a short, narrow die (, Under no circumstances should the impression be removed until the curing has progressed sufficiently to provide adequate elasticity, so that distortion will not occur. Impression materials that are used today can be classified according to their composition, mechanism of setting, mechanical properties, and applications. The phenomenon of viscoelastic behavior is discussed in subsequent sections. Classification of Elastomeric Impression materials: Till now the elastomeric impression materials are divided into 3 types based on the material used SH groups interact with oxygen released from lead dioxide (, The base paste consists of α-ω-hydroxyl-terminated polydimethyl siloxane (. Prior to making an impression, a uniform thickness of tray adhesive is applied on the tray surface, extending over its edge, and it is allowed to dry (through evaporation of solvent). Chemically, there are three elastomers based on the backbone of polymer chains: polysulfide, silicone (condensation and addition), and polyether. Copyright © 2021 Elsevier B.V. or its licensors or contributors. Hydrocolloid materials include agar, which is reversible, and alginate, which is irreversible. The purpose of this article is to review and understand currently used elastomeric impression materials in the fabrication of complete dentures. The base paste contains polymethylhydrosiloxane, as well as divinylpolysiloxane. The dentist designs and constructs both removable and fixed prostheses on a gypsum cast. What are the steps needed to produce casts and dies for prosthetic procedures? This material is often called a polyvinyl siloxane (PVS) or vinyl polysiloxane (VPS) impression material. Problems can be avoided if one expresses a small amount of material from the cartridge before attaching the mixing tip. A set rigid material is highly resistant to flexure, and it fractures suddenly when stressed, in a manner similar to that of chalk. Remember, however, that polymerization may continue for a considerable time after setting. Aust Dent J 23:178, 1978. This material is often called a polyvinyl siloxane (PVS) or vinyl polysiloxane (VPS) impression material. The mixing tips vary in their diameter, length (number of helical mixer elements), and the size of openings for a specific consistency. Technically, hydrogen gas is a reaction by-product that does not affect the dimensional stability of the impression. They are often formulated in several consistencies, including extra low, low, medium, heavy, and putty, in increasing order of filler content. Retraction cords may be impregnated with a hemostatic agent by dipping the cord in a hemostatic solution prior to placement. Pseudoplasticity—The tendency of a material to become less viscous as the shear rate increases and to recover viscosity immediately upon the elimination of shear stress. In addition, all elastomeric impression materials are viscoelastic, and it is necessary to use a quick snap to minimize plastic deformation of the impression during the final step of the removal process. In using this device, thorough mixing of higher-viscosity materials can be achieved with little effort. Submitted as partial fulfillment for a Post-doctoral Masters Thesis, Loyola University Medical Center, Maywood, Ill. The mixer elements are a series of alternating right- and left-turn 180° helixes positioned so that the leading edge of one element is perpendicular to the trailing edge of the next (Figure 8-9, B). A weak stone cast may fracture during removal. Impression compound is often used to make trays for the construction of full dentures. Addition reaction—A polymerization reaction in which each polymer chain grows to a maximal length in sequence. The reason for this situation is that the elastomeric impression materials are capable of reproducing detail more accurately than can be transferred from the stone die or cast, which may not be capable of such accuracy. Click to share on Twitter (Opens in new window), Click to share on Facebook (Opens in new window), Click to share on Google+ (Opens in new window), Which of the seven criteria that ensure accurate impression making are related to the time the impression material is in the mouth? The article focuses on polyether (PE), polysulphide, … This is usually performed by cutting away the interproximal and gingival areas of the impression. The filled tray is then inserted in the mouth and seated over the syringe material, which has been extruded on hard and/or soft tissue. The latter, outermost cord is removed, leaving the fine cord within the crevice while the impression is made. Among the most popular methods of gingival displacement is the use of gingival retraction cord. The device used to accomplish this mixing is a gun for compressing materials in a two-cylinder cartridge, which contains the base and catalyst separately, as well as a mixing tip (, The base and catalyst are pressed from the cartridge into the mixing tip as one stream of a two-layer material. Cast—A dimensionally accurate reproduction of a part of the oral cavity or extraoral facial structures that is produced in a durable hard material. Currently, elastomeric impression materials are supplied for three modes of mixing: hand mixing, static mixing, and dynamic mechanical mixing (Figure 8-8). The clinical significance of the surface reproduction tests is not entirely evident. With the mechanical devices described earlier, the materials now can be mixed as needed by one individual. From Kim K-N, Craig RG, Koran A: Viscosity of monophase addition silicones as a function of shear rate. Hydrogen atoms along the backbone structure of the vinyl silicone chain move to the vinyl groups during addition polymerization (, One of the disadvantages of the silicone impression materials (including condensation silicones) is their inherent hydrophobic nature. This surfactant migrates toward the surface of the impression material and has its hydrophilic segment oriented toward the surface—a phenomenon that makes the surface more wettable by water. After flowing through “n” elements, the number of layers in the stream of material increases to 2n. Effect of seating tray material that has passed its working time and developed some elasticity. Since the ether-linkages constitute the main component of the polymer chains, these materials behave very much like the first type of polyether impression material. The second step stretches the impression enough to pass under the height of contour of the hard tissue to remove the impression; therefore, using a material of higher rigidity will require a greater force to stretch the impression to facilitate removal. Preliminary, final, and occlusal, or bite registration ... Elastomeric materials use an auto mix system. Surface Detail Reproduction and Effect of Disinfectant and Long-Term Storage on the Dimensional Stability of a Novel Vinyl Polyether Silicone Impression Material. Impression materials are introduced into the mouth as viscous pastes with precisely adjusted flow properties. The curing of this material involves a reaction of tri- and tetra-functional alkyl silicates in the presence of stannous octoate as a catalyst. Even with proper relief of the initial impression, it will be difficult to reseat the tray precisely. – Often, polysulfide materials are called “rubber” or … How are dental impression materials classified? The mass is then scraped up with the spatula blade and spread uniformly back and forth on the mixing pad. The filled tray is then inserted in the mouth and seated over the syringe material, which has been extruded on hard and/or soft tissue. This surfactant migrates toward the surface of the impression material and has its hydrophilic segment oriented toward the surface—a phenomenon that makes the surface more wettable by water. Medium-body polyether and addition silicone are often used for the monophase or single-viscosity technique. The contamination is so pervasive that touching the tooth with latex gloves before seating the impression can inhibit the setting of the critical surface next to the tooth. The term elastic means that the material is flexible and can be deformed and still return to its original form when unstressed. *Numbers within parentheses represent the standard deviation of the mean. This material can be probed with a blunt instrument from time to time; when it is firm and returns completely to its original contour, the impression is sufficiently elastic to be removed. In addition, as the materials make turns along the helix, the rotational circulation causes a radial mixing of the materials. In this chapter, they are called elastomeric impression materials. In particular, hot and humid conditions will accelerate the setting of polysulfide impression material. Sulfur contamination from natural latex gloves inhibits the setting of addition silicone. AIM: The aim of the present study was to assess the accuracy of multi-unit dental implant casts obtained from two elastomeric impression materials… The clinical significance of hydrophilic additives is discussed in subsequent sections. The function of the impeller is only to mix the materials as they are passing through; it does not propel the material. For example, epinephrine, which is used widely, is of particular concern in patients with cardiovascular disease. These impression materials still require a dry field for impression making. Which material should be used if a more durable impression material … When a multiple mix technique is used, it is advisable to test both the syringe and the tray materials in this manner. Elastomer—Any of various polymers having the elastic properties of natural rubber. Loss of this small molecule from the set material has a significant effect on the dimensional stability of the impression. Revised American Dental Association Specification No. Viscosity (×104 cp) of Single-Phase Vinyl Polysiloxanes at 37 °C. 3. The leading edge of the first element splits the material entering the mixer into two streams. This type of device has also been adapted to mix and dispense temporary crown and bridge acrylic materials and cements that are used for luting and for producing restorations (, A syringe material (light body) and a tray material (heavy body) are used in this technique. Imbibition—The displacement of one fluid by another immiscible fluid in a hydrocolloid. An electrosurgical unit or a soft tissue laser can also be used. If one of the components is in liquid form, such as the catalyst for condensation silicones, a length of the base is dispensed from the tube onto a graduated mixing pad and drops of the liquid catalyst corresponding to the length of the base are added. One precaution that should be taken in using these automixing devices is to make sure that the openings of the tubes that dispense the pastes remain unclogged. Both pseudoplasticity and thixotropy are shear-thinning processes; the difference is that changes in pseudoplastic viscosity do not exhibit the time dependency characteristic of thixotropy. In addition, there are fewer possibilities for contamination of the material. – It is interesting to note that dental impression materials are also used to make impressions for medical prostheses (such as artificial eyes) and forensic investigations (bite marks). Advances … Set—The state of being sufficiently rigid or elastic to permit removal from the mouth without plastic deformation. Vinyl polysiloxane impression material (impression material) vinyl polysiloxane dental impression materials used for making accurate dental impressions with excellent reproducibility. The use of a custom tray (Figure 8-7, A) is recommended to reduce the quantity of material required to make impressions; thus, any dimensional changes attributed to the materials are minimized. J Am Dent Assoc. Rheology—Pertaining to the science that describes the fluid or flow characteristic of materials. There are two basic setting mechanisms: reversible and irreversible. What are the three classifications of impressions used in dentistry? Space for the light-body “wash” material is provided either by cutting away some of the “tray” putty or by using a thin polyethylene sheet as a spacer between the putty and the prepared teeth during preliminary impression making. What is the main difference between static mixing and dynamic mixing? This technique transforms two fluid (or paste-like) materials into a homogeneous mixture without mechanical mixing. The use of custom trays for polyether and addition silicone impressions is not critical, since these materials are stiffer and have less polymerization shrinkage than the polysulfide material. 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