Impression Materials PDF

Impression Materials  Introduction A variety of impressions are made in the dental clinic. Each variety requires a material of slightly different properties. In complete denture work, a material is needed that accurately registe...

Impression Materials  Introduction A variety of impressions are made in the dental clinic. Each variety requires a material of slightly different properties. In complete denture work, a material is needed that accurately registers all the denture-bearing areas. In partial denture work, there is an additional requirement. The material must be capable of registering both tooth and soft tissue undercuts. In many dental impression procedures, two materials and sometimes even three are used in sequence to take advantage of the most favorable qualities of each.  History of Impression Materials Wax was the only impression material used in dentistry until the mid-19th century when gutta-percha first appeared. Then in 1857, Charles Stent created a thermoplastic modeling compound similar to today's impression compound. Still, the problem with this material was that it was rigid and could not reproduce undercut areas. All the impression materials used until that date became rigid after setting and could not copy the oral tissues accurately. Thus, there was always a need for an impression material that could remain elastic even after setting. That is when agar, a reversible hydrocolloid manufactured from algae, was introduced in dentistry. Although this jelly-like material was elastic, it required a complicated procedure to be used as an impression material. When the algae used to manufacture agar was unavailable during the second world war, Americans used local algae to manufacture another elastic impression material known as alginate, which has gained popularity since then. Alginate and agar have disadvantages, like dimensional instability and low tear strength, which led to the manufacture of elastomeric (also known as rubber-based) impression materials. First came polysulfide, then condensation silicone followed by polyether, and then addition silicones. With the advancement in technology, digital dentistry is also making its way into the field. However, there is always a scope for new impression materials, as no impression material is 100% accurate until the date. What is an impression? An impression is a negative replica of tissues. It is used to copy the teeth and surrounding oral structures by creating a dental impression which will be poured with dental plaster to fabricate a dental cast. What is a dental cast? A cast is a positive reproduction of tissues. This procedure provides a tridimensional and accurate mouth replica, allowing dental work even in the absence of the patient. A dental cast is used to analyze and diagnose a clinical case, manufacture removable and fixed prostheses, and build occlusal splints and mouthguards Ideal requirements of Impression Materials Sufficiently fluid (should capture fine details) Suitable setting time (to produce suitable working time) Sufficient mechanical strength (no to tear or deform during insertion/removal) Sufficient dimensional stability Compatible with die and cast materials Safe (non toxic or irritable) Acceptable to patients (good taste-colour-odour) Good keeping qualities Classification of Dental Impression Based on Use Based on Setting Reaction For primary impressions, e.g., alginate, Reversible (physical): impression impression compound, elastomeric putty. compound, impression waxes, agar. For secondary impressions, e.g., light Irreversible (chemical): alginate, body elastomers and zinc oxide eugenol. impression plaster, elastomeric impression materials.  Classification of Dental Impression Based on the State of the Based on Compression of Based on Consistencies Impression Material After Setting Underlying Tissues Light body in the Oral Cavity Mucostatic: impression plaster, light Medium body Rigid: impression compound, zinc body elastomers. Heavy body oxide eugenol, impression plaster, Mucocompressive: impression impression waxes. Putty compound, putty elastomers. Elastic: alginate, agar, polysulfides, addition and condensation silicones, polyether. Non Elastic Impression Materials 1. Impression Plaster 2. Impression compound 3. Impression Wax 4. Impression Paste Impression Plaster and Soluble Impression Plaster: Impression plaster is a plaster that has been specially compounded by the manufacturer for use in the mouth. It must set rapidly to reduce the time it is held in the mouth. Because plaster will not flex over an undercut as it is withdrawn from the mouth, it must be broken into pieces and reassembled outside the mouth. For this reason, it should be weak and brittle so it will fracture cleanly, and it must have a very low setting expansion to make an accurate impression. Various accelerators and retarders are added to give the plaster the required properties. In addition, coloring and flavoring agents are often added. Because of the difficulty sometimes experienced in removing a plaster impression from the cast, some impression plasters are made water soluble by adding cornstarch. If a separator is used, the plaster can be dissolved off the cast with hot water. This decreases the possibility of breaking the cast when it is separated from the impression.  Impression Compound/Modeling Compound Impression compound is a thermoplastic, rigid and reversible impression material (meaning it can be reused in the same patient if an impression needs to be repeated). It is softened by heat into a soft plastic mass and then hardened by cooling it with either a stream of cold water or a blast of air. It is used in the clinic for preliminary impressions, to make custom impression trays, and to modify stock trays. Because it does not accurately spring around an undercut and return to its former shape, impression compound has very limited use in partial denture work. Although impression compound is a basic material in the clinical phases of prosthetic dentistry, it is not often used in the laboratory. The laboratory technician uses it periodically to attach a cast to its mounting in an articulator.  Types Type 1: Low fusing impression compound. Available as sheet, rope, and stick. The sheet material impresses edentulous arches, and the stick form is used for border molding. Type 2: High fusing impression compound, also known as tray compound. It is used for making impression trays. However, there is no uniformity among manufacturers as to exactly what constitutes high or low fusing. High Fusing Compound One brand of “high-fusing” impression compound may have about the same fusing range as another brand labeled “medium-fusing,” and the two may be the same or different colors. In general, high-fusing impression compounds flow at approximately 135 to 140 o F while low-fusing types m ay flow at 115 o F. Several of the manufacturers make a “tray” compound which is high fusing (about 140 o F). It is almost always black in color and is the type of impression compound most suitable for a custom impression tray. The type 2 impression compound is used to make an impression tray to make a wash impression. Low fusion Impression Wax Low-fusing impression wax is specially formulated to flow under controlled pressure in the mouth. It is melted in a water bath and painted in an individual impression tray as a corrective liner for final impressions. It is also used for reline impressions for complete dentures and RPDs. Low-fusing wax impressions must be handled with extreme care in the laboratory because the wax is so easily distorted. Fingers must never touch the tissue side of the impression, including the periphery. If an impression is rinsed, it should be done very carefully with room -temperature water. A separator is not necessary when a cast is poured. However, it is not used due to its inaccuracy of details in the impression. The type 1 stick impression compound is softened over a flame and placed on the borders of the custom impression tray. Impression Paste/ Zinc oxide Eugenol Paste Impression paste is usually supplied as two separate components, a base and a hardener/catalyst. The base and hardener are mixed together in specific proportions to form a paste. Impression paste is rigid when it sets, and it does not spring over undercuts. Its principal ingredients are zinc oxide and eugenol or lauric acid. Impression paste is used primarily as a corrective material inside an individual impression tray. One use of impression paste is to reline impressions for both complete and RPDs. Occasionally, it is used in immediate denture work as a lining for a sectional impression. It can also be used to provide a lining for a complete denture record base to make it fit the cast and the mouth more accurately. A separator is not required when the cast is poured into an impression made with this material. Elastomeric Impression Materials Elastomers are required when precision is of utmost importance, like crown and bridge preparation and implants. These materials are supplied as two-part systems; a base paste and an accelerator paste. Some manufacturers supply the accelerator as a liquid. When the two are mixed in the correct proportions, the resulting mixture polymerizes into a rubbery state. Elastomeric materials are not reversible and can be used only once. They are used primarily for fixed prosthodontic units (crowns or onlays), although they can also be used as corrective liners in complete denture impressions and RPD bases. Pastes and accelerators from different brands of elastomeric materials must never be cross-mixed. Materials must be carefully handled because the stains that some of them produce on contact with clothing and towels are impossible to remove. Types of Elastomeric Impression Materials There are four types of elastomeric impression materials available: 1. Polysulfides 2. Silicones ( Condensation reaction silicones) 3. Polyvinylsiloxanes (Addition reaction silicones) 4. Polyethers  Polysulfides The basic ingredient of a polysulfide impression material is polysulfide rubber with various fillers, pigments, and modifiers. The polysulfides are easily recognized because one of the two pastes is usually dark and the other paste is white in color. This material has a very characteristic odor. The polysulfides are more commonly known as rubber base or polysulfide rubber base impression material. The low viscosity of this material allows accurate registration of the soft and hard tissues. It is most often used in removable prosthodontics. Consistencies: Light body Heavy body A DVA N TAG E S D I SA DVA N TAG E S Least rigid (most flexible of all the Requires a custom tray elastomers) Long setting time Good tear strength Must be poured within half an hour Most biocompatible Bad odor Excellent flow so good reproduction of Unpleasant taste details Stains the clothes Hydrophilic Messy to work  Silicones (Condensation Reaction Silicones) Silicone impression material consists of silicone and ethyl silicate. This material exhibits significant setting shrinkage and should be used in thin consistent layers. Silicone manufacturers were the first to offer a two-phase impression method. The dentist first makes an impression, either in the mouth or on the diagnostic cast, of the patient’s arch, using a stock impression tray and a putty form of the silicone material. The resulting custom-fitted tray is used to carry a wash of the lower viscosity silicone material to the mouth for the final impression. Silicone materials are generally lighter in color and translucent when set, and they have a much more subdued odor than the polysulfides. A disadvantage is that shrinkage occurs if the material is allowed to sit for more than 30 minutes before pouring the impression.  Consistencies: extra-low, Low, medium, and putty A DVA N TAG E S D I SA DVA N TAG E S Good elastic recovery Dimensional deformation over the time Pleasant odor and taste Hydrophobic Accurate reproduction Dimensional stability is affected due to reaction byproducts  Polyvinylsiloxanes (Addition Reaction Silicones) Polyvinylsiloxanes are like the conventional silicones in their elastic nature, but they differ in chemical structure and reactions. Because of this, the polymerization shrinkage of polyvinylsiloxanes is well controlled, and a thin uniform thickness of the material is not so significant a requirement for accurate impressions. Polyvinylsiloxanes are also used with a two-stage impression technique. A disadvantage is the rigidity and cost of the material. It is available in all consistencies( extra-low, low, medium, heavy, and putty Setting reaction: The base paste containing hydrosilane-terminated molecules reacts with an accelerator paste containing siloxane oligomers with vinyl end groups and a platinum catalyst. Although no by-product is formed, a secondary reaction leads to hydrogen gas production. That is why it is recommended to wait at least 30 minutes to pour the cast rather than have a pitted cast. A DVA N TAG E S D I SA DVA N TAG E S Excellent dimensional stability - impressions can be Hydrophobic - the impression area must stored or posted before casting. be dry to prevent inaccuracies in the Elastic recovery impression Great accuracy Short setting time The sulfur present in the latex gloves Good tear resistance and rubber dam may interfere with the Automix available polymerization of the base and catalyst. Hydrophilized addition silicone has good Hydrogen gas may lead to pitting in the compatibility with gypsum cast. The impression can be cast multiple times without jeopardizing the details  Polyethers The base of this impression material is a polyether compound, and the accelerator is a sulfonic acid. Laboratory studies have shown that polyethers, along with polysiloxanes, are the most accurate of the elastomeric impression materials. However, when set, polyether impression material is very stiff, making it difficult to remove the impression tray from the mouth if large tooth undercuts are present. This is also a problem when trying to separate casts from the impression without breaking off stone teeth. Mode of supply: Available as light, medium, and heavy body consistencies. A DVA N TAG E S D I SA DVA N TAG E S Hydrophilic Most rigid of all the elastomers makes it Accurate and high dimensional stability - hard to remove (newer polyethers are makes it possible to delay casting and allow slightly more flexible) multiple pouring. May cause allergy due to sulfonate acid Good elastic recovery esters Good compatibility with gypsum Good shelf life It can be used as a single-phase material or with a syringe tray technique Special Characteristics of Elastomeric Impressions Polysulfide rubber base and condensation silicone impressions should be poured within 25 minutes of their removal from the mouth. These materials do not have long term dimensional stability. Second pours may be made from these impressions, but the resulting cast does not have the same accuracy as the cast from the first pour. Addition reaction silicone and polyether impressions show great dimensional stability. Pours of casts may be delayed up to 7 days with no significant loss of accuracy. Second pours maintain accuracy of detail and dimension comparable to first pours. Polyethers are highly hydrophilic (they absorb water) and exposure to liquids must be minimized. They may not be used in die-plating procedures because water sorption leads to unwanted dimensional change. Addition reaction silicones are more difficult to pour or achieve a bubble-free cast due to their hydrophobic nature. Hydrocolloids A hydrocolloid material elastically deforms and then returns to its original shape. The undercuts are thus accurately reproduced in the impression. There are two basic types of hydrocolloids: 1. Agar 2. Alginate They are chemically and physically different and require different handling, but the purposes for which they are used are very similar. They are often referred to as reversible and irreversible, respectively. Hydrocolloid, Agar Type (Reversible). There are two different reversible hydrocolloids. One is designed to be used in the mouth for impressions; the other is compounded for duplication use in the laboratory as follows: 1. Impression Type Impression hydrocolloid is a gelatin-like material that is composed mainly of agar-agar and water. The material is heated in a double boiler or in a special heating syringe to soften it to a thick consistency. It is then tempered, carried to the mouth in a tray, and cooled with 70 o F water to make it set. When it has set, it is removed, and the cast is poured. Impression-type agar can be used for duplicating in the laboratory if laboratory duplicating agar is not available. Its principle use, however, is for making RPD impressions and fixed prosthodontic final impressions. A main disadvantage of this material is that an impression can only be poured one time due to the dimensional change caused by the evaporation of water. 2. Laboratory Type Laboratory duplicating hydrocolloid is specially manufactured for laboratory use. It is stronger and, therefore, more satisfactory for duplicating than the impression type. It can be used repeatedly if it is properly handled and stored. In order to maintain a precise water balance, heat the material in a stainless steel double boiler. The double boiler has a dome-shaped lid that condenses the water and returns it to the mixture. The water balance of the hydrocolloid is critical and is maintained by the double boiler. Store any unmixed hydrocolloid in a sealed container.  Hydrocolloid, Alginate Type (Irreversible) The alginate-type hydrocolloid is supplied in the form of a fine powder. The powder is mixed with a prescribed amount of water to form a mixture that, like the agar-type hydrocolloid, is capable of accurately reproducing an undercut of either a tooth or soft tissue. In general, the ingredients used to make an alginate impression material are sodium or potassium alginate, plaster, magnesium oxide, trisodium phosphate, sodium phosphate, and ditomaceous earth as a filler.  General Uses. Alginate is used as an impression material for partial dentures. It can be used alone or in conjunction with another material for immediate denture impression and is sometimes used for cast duplication in the laboratory. When used for duplicating, alginate is usually mixed with more than the usual amount of water (2 or 3 times more, depending on how fluid a mix is needed). Handling Requirements Alginates are mucostatic impression materials. When freshly spatulated, alginates have low viscosity and record the soft tissues without compressing them Making an accurate cast from a hydrocolloid impression requires following. certain rules. The water balance in a gelled hydrocolloid material is critical to its accuracy. When gelled hydrocolloid is exposed to water or air, it changes its dimensions quickly. This is why hydrocolloid impressions must be poured as soon as possible after they are made ( within 10 minutes after the material is set.) Reversible and irreversible hydrocolloids tend to exude a fluid that causes gypsum surfaces to be soft and chalky. Not all gypsum products are affected in the same manner. Some brands of reversible hydrocolloid material require immersing the impression in a 2 percent solution of potassium sulfate before the cast is poured. This procedure is called fixing. It improves the surface qualities of the cast. Of course, manufacturer’s directions must be followed. Setting Reaction It is a sol to gel transformation divided into two steps: 1) retardation reaction 2) gelation reaction. First, calcium sulfate reacts with trisodium phosphate to form calcium sulfate and sodium sulfate; the second stage starts after all the trisodium phosphate gets consumed, which provides optimum working time in the manipulation of alginate. In the gelation reaction, potassium alginate reacts with calcium sulfate to form calcium alginate gel. Types of Alginate based on the setting time The retarder can slow the setting reaction, giving rise to dental alginates with different working times. Types Type I is a fast set that hardens within 1 to 2 minutes, Type II is a standard set that hardens within 2 to 5 minutes. The setting reaction time of dental alginates also depends on the water temperature used for the mixture. Warmer water speeds up the setting reaction, and colder water slows the reaction down. The water-to-powder ratio also affects the setting reaction; the hardening reaction is faster with more powder. Modified alginates: Dustless alginates: manufactured to prevent inhalation of alginate particles, which may cause respiratory problems. The alginate particles are coated with glycol or glycerine to make them denser. Chromatic alginates: change colors during manipulation to guide mixing time and setting time. In the form of sol or paste (containing water) and reactor (plaster of pairs) separately.  Properties An ideal impression material would have sufficient dimensional stability over time to allow for pouring whenever convenient. However, the dimensional stability of alginate impressions decreases with longer storage time. Alginate impressions can undergo imbibition, evaporation, and syneresis, affecting their dimensional stability. Imbibition is fluid absorption by a colloid resulting in swelling, and syneresis is the expulsion of liquid from a gel. Imbibition and evaporation processes decrease when the impression is poured as soon as possible. If it is not feasible to pour the impression immediately, it must be stored in a humid environment. This environment can be created by wrapping the impression in a moistened paper towel before being shipped to the dental laboratory or stored in a container with some water, which will evaporate. Some studies advocate that a damp paper towel should not be placed in the impression if it can be poured within ten minutes to avoid water absorption by the material. Disinfection: The disinfection of alginate impressions must be carried out with regard for the dimensional instability of the impression material. Excessive immersion in aqueous solutions may cause swelling of the material and should be avoided. Impressions should be rinsed first before disinfection to remove saliva and/or blood.  Alginate dental impressions can be disinfected using 0.5% sodium hypochlorite spray or 2% glutaraldehyde solutions. Some recommend spraying the impression with sodium hypochlorite for 10 minutes, while others recommend dipping the impression quickly or keeping it closed in a plastic bag for 10 minutes after spraying. After disinfecting, you can rinse the impression with room temperature tap water to remove any remaining disinfectant. Disinfecting impressions is important because dental materials can transmit microorganisms that can cause infection. A DVA N TAG E S D I SA DVA N TAG E S Affordable Low tear strength Short manipulation time Dimensional instability Simple technique Few armamentarium required Good impression (even in the presence of undercuts) all in a single step