Clinically, it should produce an accurate impression secondary to its adaptability to oral structures, have a consistency that is dimensionally stable to resist tearing but results in an atraumatic removal, set within a reasonable amount of time, demonstrate biocompatibility to include a hypoallergenic nature, and have a reasonable cost per use. 2. Disadvantages of polysulfide impression materials can stain clothing, skin, and other surfaces objectionable odor (onion, skunk, natural gas scents result from reaction) After reviewing the types and characteristics of the most common impression materials, it becomes apparent that hydrocolloids have a high hydrophilic nature that allows this material to capture accurate impressions in the presence of some saliva or blood . Making a cast in gypsum materials from an impression of dental anatomy aids dentists in designing and constructing removable and fixed prostheses. Impression materials are used to record intraoral structures for the fabrication of definitive restorations. Often, the choice of impression materials depends on the subjective choice of the operator based on personal preferences and past experience with particular materials. When mixed with water, the alginate material first forms a sol. Polyvinyl siloxanes are generally hydrophobic. Rigid impression materials require less support from trays. (b) Disadvantages – The disadvantage of the material being predominantly water is that water evaporates from the surface of the impression if it is left exposed to air. Because of their low wetting angle, they easily capture full arch impressions. Polyethers are considered to have the highest tear strengths, whereas hydrocolloids have relatively low tear strengths . Compress soft tissue while making impression. These hydrophilic polyvinyl siloxanes have improved wettability , and they are only clinically acceptable under dry conditions . Materials used without adequate knowledge of their characteristics can impair a successful outcome. Materials with high dimensional stability are the polyethers and polyvinyl siloxanes, in contrast to alginate, which has a low dimensional stability. – A polysulfide impression should be poured within several hours after mixing. These surfactants act through a diffusion transfer of surfactant molecules from the polyvinyl siloxane into the aqueous phase. Explain the most advantageous clinical feature that polyether impression materials possess. Difficult to record details because of high viscosity. stiffness Polyeter most stiff makes difficult to remove stone from impression,fracture oof die Flexibility / stiffness : The stiffness of the impression material Lowest Highest Polysulphide, Condensation silicones, addition silicones, polyether Reversible hydrocolloid least rigid The conventional polyether 27 times as stiff as the light body polysulphide impression material. This result has been reported even with the new “hydrophilic” polyvinyl siloxane impression materials. The newer materials are supposedly able to be poured in 5 minutes after the impression material is removed from the mouth. Polysulfides and polyethers are more hydrophilic. It is not accurate enough for fixed partial dentures but is used for partial framework impressions . Either placing a small amount of silicone lubricant over the teeth in a prophylactic paste or rehydrating through a rinse is necessary to produce a new film over the teeth for accurate impressions. Because these materials are moderately hydrophilic, strict attention to disinfection guidelines is necessary to prevent swelling of the material. Impression materials can be classified as readily wettable by gypsum (hydrophilic) or resistant to wetting (hydrophobic) ... hydrocolloids > polyether > hydrophilic addition silicone > polysulfide > … If cooled, the material sets slower. Impressions made in hydrocolloid are easier to remove than other materials and require rigid trays to prevent distortion in impression making and pouring of dental casts. They contain hydrophobic aliphatic hydrocarbon groups around the siloxane bond . Shelf life – 2 yrs. The setting reaction of hydrocolloids is not affected by latex proteins from gloves. Material can be reused several times. This is probably true with respect to any impression material when comparing hand spatulation to automixing. This thin layer causes impressions to remain tacky around new composite placed restorations . messy & stains clothes. Finally, if repetitive impressions are made, the film over the teeth is lost and prevents satisfactory impression. Accurate impressions depend on identifying the applications that do or do not fit each material’s characteristics. The base consists of a polysulfide polymer (terminal/side chain −SH groups), titanium dioxide, zinc sulfate, copper carbonate, or silica. 19. The impression taken should be highly precise, thus, requiring specific care when manipulatingthese materials. After the phosphate ions are depleted, the calcium ions react with the soluble alginate to form insoluble calcium alginate, which with water forms the irreversible calcium alginate gel. It is recommended that one wait at least 30 minutes for the setting reaction to be completed before the gypsum casts and dies are poured . There are several reasons for this the first and foremost is the hazardous formulation of polysulphide sealant. A potential problem when using irreversible hydrocolloid is the tendency for this material to stick to teeth, which occurs when alginate radicals in the impression material form chemical bonds with hydroxylapatite crystals of the enamel. Working times can be varied with respect to standard-set versus quick-set impression materials as prepared by various manufacturers . Claims are made with respect to polyvinyl siloxane materials being hydrophilic, but in reality they are somewhat hydrophobic . The following chemical reaction forms a gel to create the set impression material. Their ability to reproduce detail is excellent and they are dimensionally stable and allow multiple pours of accurate casts for 1 to 2 weeks after impressions are made, provided there is no tearing of the impression. It is not affected by latex gloves. Three 0.1 gram samples were aseptically obtained from each packet. Accurate impressions are necessary for construction of any dental prosthesis. Automixing cartridges tend to create fewer bubbles than hand spatulation. They have low rigidity and require rigid trays to support borders; otherwise they tend to distort. If this material is overheated it may not recover to its normal setting time even after cooling, and it is recommended that this material be stored in a cool place and not in the sun (refrigerator or cool space). Wettability of an impression material relates to the ability of the material to flow into small areas . Another source of contamination is the oxygen-inhibited layer on the surface of resin materials that appears immediately after curing. 18. Polysulfide impression materials are supplied as two paste systems. Master casts are used for producing complete dentures, removable partial dentures, crowns, fixed partial dentures, and implant prostheses. Difficult to remove if there are undercuts. Within a few days, tissue conditioners become stiffer as a result of the loss of alcohol. There may be less waste of material associated with automixing and providing a more bubble-free mix resulting in more accurate casts. Polysulfide impression material is dimensionally accurate if poured within 1 to 2 hours of making the impression . Practitioners should take this characteristic into consideration when selecting impression materials given the time available to the practitioner to pour casts during office hours. Viscosity is altered by changing the amount of silica filler, which produces either a putty or less viscous wash material. Polysulfide material is more sensitive to temperature change than the polyether material. These materials should have low shrinkage upon polymerizing and remain stable, which allows them to be poured days after making the impression. Polyvinyl siloxanes are fairly stiff, and depending on the viscosity of the material, they flow readily to capture areas of detail . Impression Materials - Lecture note 1. Adequate tear strength; Better elastic properties on removal. What do you want from an impression material? – Hydrocolloid impression materials are quite hydrophilic, but the same cannot be said for all impression materials. Aquasil is an exception because it does adhere to itself after setting. unset impression material over wet oral structures, Why agar hydrocolloids have greater accuracy, due to their greater degree of recover after deformation around undercuts (elastic recovery/plastic deformation), combination of elastic and viscous (plastic or permanent) behaviors, seen in plastics and impression materials, Types of hydrophobic impression materials, How addition silicone can be made hydrophilic, Light body elastomers/impression materials, Regular or medium body elastomers/impression materials, Heavy body elastomers/impression materials, Registering oral structures: general considerations, Transferring detail to model: general considerations, can cause voids upon addition of stone slurry, "poly (vinyl) siloxane" or "vinyl polysiloxane", What initiates cross-linking in addition silicones, Contamination of addition silicone impression material. It has moderate ability to reproduce detail and costs relatively little compared with other impression materials. The newer materials are said to contain a proprietary component that eliminates hydrogen bubbles, but it is best to read the guidelines for pouring specific brands of polyvinyl siloxanes before pouring stone. Advantages: Adequate working & setting timer. They adhere to themselves and are excellent for border molding and correctable impression technique. Some water supplies contain large amounts of minerals that can adversely affect the accuracy and the setting time of alginate impression materials, however. These characteristics make it highly desirable for clinical and laboratory use. They contain functional groups that chemically attract and interact with water molecules via hydrogen bonding . The hydrophilization of polyvinyl siloxanes is enhanced with the incorporation of nonionic surfactants. Polyvinyl siloxane materials are also thermally sensitive . They are fairly easy to remove. These properties include improved dimensional accuracy, stability, wettability, excellent elastic recovery, flexibility, ease of handling, tear strength, ability to produce multiple casts from one impression, and superior ability to reproduce detail. Polyethers and some polyvinyl siloxanes fall into this category. Wettability results in fewer voids and less entrapment of oral fluids, providing more accurate impressions . Important properties of impression materials, Classification by physical condition after setting, viscosity (register details but not flow down patient's throat). They are a superior material to hydrocolloids and somewhat better than polyvinyl siloxanes . Polyethers tend to be the most rigid impression materials . Impression materials with low contact angle enable dental stone to flow easily, and relatively bubble-free casts are produced. Water is the ideal example of a material with a low wetting angle. Before they set, however, they are susceptible to contamination. It may allow for more than one pour if it is not too thin in areas. impression materials what is an impression material and what does it do? The basic ingredient of a polysulfide impression material is a polyfunctional mercaptan of the average formula HS-(R-S-S)23-R-SH, where R is C2H4-OCH2-O-C2H4. It has a moderately high wetting angle, which makes it a little more difficult to create an accurate full arch impression than with hydrocolloid, polyether, or polysulfide. Polyvinyl siloxanes are hydrophobic because of their chemical structure. Tissue conditioners are soft elastomers. Polyether Impression Materials. The addition silicones have the smallest change, approximately −0.15%, followed by the polyethers, approximately −0.2%. Polyethers consist of a base paste that is composed of a long-chain polyether copolymer with alternating oxygen atoms and methylene groups (O-[CH 2 ] n ) and reactive terminal groups. 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