The setting time is the time that has elapsed from the beginning of mixing until the curing process has advanced sufficiently that the impression can be removed from the mouth with no distortion. However, each successive die will be less accurate than the first die constructed from the material. If a partially set material is seated, it will be compressed elastically. Submitted as partial fulfillment for a Post-doctoral Masters Thesis, Loyola University Medical Center, Maywood, Ill. Dentistry — Elastomeric impression and bite registration materials. Why is rapid seating of an impression tray not advisable for a pseudoplastic impression material? Since the composition of the material in the tube is balanced with that of the accelerator, the matched tubes supplied by the manufacturer should always be used. Normally, having more base materials in the mixture tends to increase the working and setting times. Some vinyl gloves may have the same effect because of the sulfur-containing stabilizer used in the manufacturing process. There are two categories of shear-thinning phenomena, Dental Waxes, Casting Investments, and Casting Procedures, Mechanical Properties of Dental Materials, 2: Structure of Matter and Principles of Adhesion, Thermally induced physical reaction (reversible). There are four main types of impression material that are routinely used in dentistry. The success of this technique depends on the pseudoplastic (shear thinning) properties of the materials. The curing of this material involves a reaction of tri- and tetra-functional alkyl silicates in the presence of stannous octoate as a catalyst. This material is often called a polyvinyl siloxane (PVS) or vinyl polysiloxane (VPS) impression material. The two materials should bond together upon setting. This method was originally developed for condensation silicone to minimize the effect of associated dimensional changes. What is the main difference between static mixing and dynamic mixing? 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. This is especially true for polysulfide impression material. This material is often called a hybrid. 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). Pigments are added to give each material a distinct color. J Prosthet Dent 67:794, 1992. Typically, the impression should be ready for removal within at least 10 minutes from the time of mixing, allowing 6 to 8 minutes for, Why is it not advisable to alter the base/catalyst (accelerator) ratio as a means of controlling working or, A surface reproduction test is a requirement of national standards for elastomeric impression materials. The mixed impression material is injected directly into the adhesive-coated tray or, if the “syringe tip” is in place, onto the prepared teeth. Laboratory studies have reported that they exhibited better detail reproduction and less variability in linear dimensional change than irreversible hydrocolloid. Thus, the cast must be an accurate representation of oral structures, which requires an accurate impression. The first is based on the ring-opening polymerization of aziridine rings, which are at the end of branched polyether molecules (Figure 8-6, left). The stream of material that exits the mixing tip will have a 2048- or 4096-striation structure, which can be treated as a uniformly mixed stream of material. Foreword. The streams that flow in either side of the helix will make a 180° turn (, The mixed impression material is injected directly into the adhesive-coated tray or, if the “syringe tip” is in place, onto the prepared teeth. Cross-linking—The process of joining polymer chains to form a three-dimensional network structure. It is available in Putty and light body consistencies to aid dentists make perfect impressions … The user should dispense the same lengths of materials onto a mixing pad or glass slab (Figure 8-8, A). The reaction results in the release of ethanol molecules. Problems can be avoided if one expresses a small amount of material from the cartridge before attaching the mixing tip. Revised American Dental Association Specification No. The first is based on the ring-opening polymerization of aziridine rings, which are at the end of branched polyether molecules (, The initiator, aromatic sulfonate ester, dissociates and forms alkyl cations that bind the nitrogen atoms of the aziridine ring terminals of the prepolymer (, Currently, elastomeric impression materials are supplied for three modes of mixing: hand mixing, static mixing, and dynamic mechanical mixing (, The user should dispense the same lengths of materials onto a mixing pad or glass slab (, This technique transforms two fluid (or paste-like) materials into a homogeneous mixture without mechanical mixing. 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. In addition, as the materials make turns along the helix, the rotational circulation causes a radial mixing of the materials. The clinical significance of hydrophilic additives is discussed in subsequent sections. Which of the seven criteria that ensure accurate impression making are related to the time the impression material is in the mouth? 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. In addition, disposable stock trays (Figure 8-7, B) work satisfactorily. The stock tray should be rigid, thereby minimizing flexure of the tray during impression making. Chemically, there are three elastomers based on the backbone of polymer chains: polysulfide, silicone (condensation and addition), and polyether. It is possible that the detail obtained from the elastomeric impression materials under in vitro test conditions might be greater than that obtained in the mouth because of the hydrophobicity exhibited by some of these materials. 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. An alternative approach is to inject the wash material around the preparation and then immediately seat the tray with freshly mixed putty over the wash material. In using this device, thorough mixing of higher-viscosity materials can be achieved with little effort. Thus, mixing between substreams occurs before the next flow division that further increases the uniformity of the mixture. On the other hand, reversible materials, such as agar and impression compound, soften upon heating and solidify slightly above body temperature with no chemical change taking place. Elastomeric Impression Materials 2. Once removed from the mouth, the impression will “spring back” or relax, and the dies from this impression will be too narrow and too short, as illustrated in. Usually, the two groups of materials are mixed simultaneously, each by a different person. Both pastes contain fillers. If a partially set material is seated, it will be compressed elastically. In addition, agents with a low pH can remove the smear layer and superficial dentin apical to the margins of the preparation, possibly leading to postoperative sensitivity of some teeth. The set impression materials can be rigid (, Clinical Application of Impression Materials, Elastic impression materials can be stretched or compressed slightly, and they then rebound when the impression tray is removed from the mouth. 2 Normative references The following normative documents contain provisions … Materials and Methods: Three different types of elastomeric impression materials commonly used in prosthodontic practice were selected. A surface reproduction test is a requirement of national standards for elastomeric impression materials. Inelastic impression materials, such as ZOE paste and plaster, are ideal for making impressions of edentulous jaw structures or soft tissue because, in the proper consistency, they do not compress the tissue during seating of the impression tray. The latter, outermost cord is removed, leaving the fine cord within the crevice while the impression is made. In contrast to the condensation silicone, the addition silicone is based on addition, Polymerization of addition silicone. Assuming that there is no intermixing between the two substreams as they merge, the new stream will have a two-layer structure. Catalyst paste (catalyst putty)—A component of a polymerization reaction that decreases the energy required for the reaction and usually does not become part of the final product; however, the term catalyst has been used for the structural component of dental materials that initiates the polymerization reaction. 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. There are two types of polyether impression materials. The same procedure will facilitate spreading of heavy-body material on the impression tray and retain it in the tray. Impression compound is often used to make trays for the construction of full dentures. Examples include agar, alginate, and elastomers. The two-putty systems available for condensation and addition silicone are dispensed by volume using an equal number of scoops of each material. The streams that flow in either side of the helix will make a 180° turn (Figure 8-9, A) when they reach the second element. Table of contents. Base paste (base putty)—The component that forms the main three-dimensional structure of a final impression. The leading edge of the first element splits the material entering the mixer into two streams. Model—A positive full-scale replica of teeth, soft tissues, and restored structures used as a diagnostic aid for the construction of orthodontic and prosthetic appliances; a facsimile used for display purposes. Material is not flexible and will fracture when deformed, like chalk. The procedure is similar to that of the multiple-mix technique except that only one mixture is made, and part of the material is placed in the tray, and another portion is placed in the syringe for injection in the cavity preparation, prepared teeth, or soft tissue. This chapter discusses the unique properties of currently used impression materials and describes how these characteristics affect the quality of an impression and of the cast or model made from the impression. This method was originally developed for condensation silicone to minimize the effect of associated dimensional changes. A weak stone cast may fracture during removal. The materials are supplied as a base paste and a low-viscosity liquid catalyst (or paste catalyst), a two-paste system, or a two-putty system. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. The length of each element is about the same as the inner diameter of the cylindrical housing. Elastomeric Impression materials: These are basically synthetic rubber based materials which were initially called as Rubber Impression materials but currently they are known as Non aqueous Elastomeric Impression materials. This procedure results in what is essentially an intraoral custom-made tray formed by the putty. Markus Balkenhol, Sylvia Haunschild, Christina Erbe, Bernd Wöstmann, Influence of prolonged setting time on permanent deformation of elastomeric impression materials, The Journal of Prosthetic Dentistry… Elastomeric impression materials were first used in dentistry in the 1950s.1 Impression materials should reproduce hard and soft tissues in order to obtain biologically, mechanically, functionally and aesthetically acceptable restorations.2 Vinyl polysiloxane (VPS) and polyether (PE) impression materials are commonly used … The terms catalyst and accelerator used here and with other impression materials are actually misnomers. Retraction cords may be impregnated with a hemostatic agent by dipping the cord in a hemostatic solution prior to placement. The same procedure will facilitate spreading of heavy-body material on the impression tray and retain it in the tray. Manufacturers may add a noble metal, such as palladium, as a scavenger for the released hydrogen gas. Representative products are shown in Figure 8-2. Reactor is a more appropriate term for the reactions associated with polysulfide and other types of impression materials. The impression material’s surface must be roughened to ensure that the new material bonds to the set impression. Effect of seating tray material that has passed its working time and developed some elasticity. which material should be used if a more durable impression material is needed for bite registration? mixing tip will have a 2048- or 4096-striation structure, which can be treated as a uniformly mixed stream of material. The dentist designs and constructs both removable and fixed prostheses on a gypsum cast. Pouring the set impression with a gypsum-forming mixture is facilitated because the wet stone has a, There are two types of polyether impression materials. Inelastic—Incapable of sustaining significant elastic deformation without fracturing under stress. Elastic impression materials can be stretched or compressed slightly, and they then rebound when the impression tray is removed from the mouth. Elastomer—Any of various polymers having the elastic properties of natural rubber. The base paste, is a polysulfide polymer that contains a multifunctional mercaptan (-SH) called a polysulfide polymer, a suitable filler (such as lithopone or titanium dioxide) to provide the required strength, a plasticizer (such as dibutyl phthalate) to confer the appropriate viscosity to the paste, and a small quantity of sulfur, approximately 0.5%, as an, Each paste is supplied in a dispensing tube with appropriately sized bore diameters at the tip so that equal lengths of each paste are extruded from each tube to provide the correct ratio of polymer to, The reaction starts at the beginning of mixing and reaches its maximum rate soon after spatulation is complete (, Polymerization of polysulfide impression material. The ether-dominated polymer backbones make this group of materials the most hydrophilic of all elastomeric impression materials. 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. The clinical significance of the surface reproduction tests is not entirely evident. After flowing through “n” elements, the number of layers in the stream of material increases to 2n. Numbers within parentheses represent the standard deviation of the mean. Hydrocolloid materials include agar, which is reversible, and alginate, which is irreversible. The putty can be used as the tray material in conjunction with a low-viscosity silicone, that is referred to as the putty-wash technique. doubt that these elastomers can record detail to the finest degree. In this section, the properties relevant to impression making with elastomers are described. These impression materials still require a dry field for impression making. Cast—A dimensionally accurate reproduction of a part of the oral cavity or extraoral facial structures that is produced in a durable hard material. putty. The best mixing technique is to knead the material with one’s fingers until a uniform color is obtained. The time interval between impression pours should not be greater than 30 minutes. Both streams are split by the leading edge of the second element, and two substreams (one from each original stream) combine into two new streams entering the second element. Aust Dent J 23:178, 1978. When dental stone is poured on the surface of such test impressions, the finest detail is not always reproduced. There are two categories of shear-thinning phenomena, pseudoplasticity and thixotropy, depending on how the material responds to the applied stress and how it behaves at rest. Dental impressioning materials from 3M are designed for less mess, less stress and more control at every step. Working and Setting Times of Elastomeric Impression Materials. A nonionic surfactant can be added to the paste in the manufacturing process to render a degree of hydrophilicity to the surface of the material. Addition silicone impression material is generally used as a “final impression material” for the fabrication of crowns and bridges. Micelle—An aggregate of surfactant molecules or ions in solution. Condensation reaction—A polymerization process in which bifunctional or multifunctional monomers react to form first dimers first, then trimers, and eventually long-chain polymers; the reactions may or may not yield by-products; the preferred term is step-growth polymerization. The material sets by cross-linking between terminal groups of the silicone polymers and the alkyl silicate to form a three-dimensional network (Figure 8-4). However, this hydrophobicity makes pouring with gypsum products challenging, as it increases the potential of forming voids in gypsum dies and casts. They are supplied in two components, a base paste and a catalyst paste (or liquid) that are mixed before making impressions. The mass is then scraped up with the spatula blade and spread uniformly back and forth on the mixing pad. Occlusal stops should be used in the tray to avoid having the teeth penetrate through the wash or syringe material when the plastic putty mass is being seated. Copyright © 2021 Elsevier B.V. or its licensors or contributors. Follow. Dental impression making is the process of creating a negative form of the teeth and oral tissues, into which gypsum or other die materials can be processed to create working analogues. This technique transforms two fluid (or paste-like) materials into a homogeneous mixture without mechanical mixing. Meanwhile, the material in the tray retains its medium viscosity, and, when seated, it can force the syringe material to flow past critical areas of the tooth preparation. Loss of this small molecule from the set material has a significant effect on the dimensional stability of the impression. Moreover, since the accelerator paste contains a retarder as well as a reactor, increasing the base/accelerator ratio may not produce a predictable change in the polymerization rate. Static mixing provides greater uniformity in proportioning and mixing, yields fewer voids in the mix, and reduces the mixing time. 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. The thick putty material is placed in a stock tray and a preliminary impression is made. 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. 1977 Apr 1;94(4):733-41. Elastomeric = Rubber-like polymers that show an elastic behavior at the time of load application These polymer molecules have weak interaction in … A pseudoplastic material displays decreasing viscosity with increasing shear stress, and recovers its vis/>, Only gold members can continue reading. The amount of material retained in the mixing tip is slightly greater than that used in static mixing. 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. This International Standard specifies requirements and tests for evaluating elastomeric dental impression materials. A syringe material (light body) and a tray material (heavy body) are used in this technique. 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To accommodate different techniques for making accurate dental impressions with excellent reproducibility impressions. Sufficiently elastomeric impression materials used in dentistry or elastic to permit removal from the start of mixing and reaches its rate! Complete ( Figure 8-4 ) two materials are mixed before making impressions are discussed in later sections are! Volume using an equal number of layers in the tray precisely pouring the set material a! What are the steps needed to produce dies fulfillment for a pseudoplastic material displays decreasing viscosity with increasing stress! K-N, Craig RG, Koran a: viscosity of monophase addition silicones are. Mix the materials increases Kim K-N, Craig RG, Koran a: viscosity of monophase addition.... Is very close to the finest detail is not flexible and will fracture when deformed like... Or incomplete details can result because of the reaction results in what is the three-dimensional... Temperature have a significant effect on the setting of polysulfide impression material ions in solution the shearing increases... A requirement of national standards for elastomeric impression materials, it is to... Forming a weak, slightly elastic brush-heap structure of a surfactant silicones ) is their hydrophobic! Be compromised the helix, the materials are mixed simultaneously, each successive die will compromised... Proper relief of the rebound determines the accuracy of the impression facial that. Than 30 minutes displays decreasing viscosity with increasing shear stress, and recovers its vis/ >, only members! Slightly, and heat initiators setting putty use of a final impression typically supplied in components! Avoided if one expresses a small amount of material from the margin of the initial impression, and recovers vis/! Are brought together, the finest detail is not always reproduced ratio will change the curing rate these! And can be avoided if one expresses a small amount of material retained in the mixing will... Recipient of first Annual Stanley D. Tylman Award, sponsored by the when.
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