the mouth disdavantages: it has poor dimensional stability poor tear strength if it is unsupported, it distorts easy to include air during mixing a minimum thickness of 3 mm is required which is hard to achieve in thin areas in between the teeth non-aqueous elastomeric impression materials as stated above, there are times clinically where the accuracy of an alginate impression is not acceptable, particularly for the construction of fixed prosthodontics. agar may be used but as discussed has a number of technical difficulties in its use. as such elastomers were developed to capture the fine detail and accuracy required. polysulphides polysulphides have become increasingly unpopular due to their unpleasant taste/smell. the material is presented as a paste to paste system mixed by a dental nurse prior to use. the material sets by a condensation polymerisation reaction. initially the polymer chains increase in length causing a slight increase in temperature, of 3–4 °c. this is then followed by cross linking of the polymer chains and finally the release of water as a by product. this later reaction slightly contracts the material making it stiffer and more resistant to permanent deformation. when poured and cast this slight contraction means the resulting model is slightly larger and as such creates space for the luting cement. advantages: good tear resistance dimensionally stable – some shrinkage on set with release of by-product good accuracy most flexible elastomer disadvantages: reduced patient satisfaction – distinct unpleasant taste and smell long setting time requires excellent moisture control difficult to mix polyethers polyethers are the most hydrophilic impression material of the hydrophobic elastomers. this property makes it a commonly used material in general practice as it more likely to capture preparation margins when moisture control is not perfect. presented as a paste to paste system the material is often used with a monophase impression technique, meaning both the material syringed round the preparation and the bulk within the tray are the same material. note when mixing polyether the base to accelerator ratio is not 1:1 like with most elastomers, but 1:4. advantages: most hydrophilic elastomeric impression material dimensionally stable – minimal shrinkage on set with release of by-product good accuracy monophase impression good tear resistance disadvantages: can be too stiff – deep undercuts and space under a bridge pontic should be blocked out with soft (modelling) wax to prevent inadvertently removing bridge with impression indications: indirect cast restorations, especially in cases where moisture control cannot be guaranteed locating and the pick up of implant analogues in preparation for placement of superstructure functional impression taking in removable prosthodontics silicones impression taken with silicon there are two types of silicone resin impression material, addition and condensation (reflecting each of their setting reactions). silicones are inherently hydrophobic and as such require excellent moisture control for optimal use. addition silicone addition silicones have become the most used impression material in advanced restorative dentistry. there are many forms available, based on their differing amounts of filler content. this dictates the flow properties of each type with more filler resulting in a thicker, less flowable material. the most common forms are: extra light-bodied (low filler content), light-bodied, universal or medium-bodied, heavy-bodied and putty (high filler content). however each type follows the same addition polymerisation reaction and is presented as a paste to paste system. the reaction does not produce any by-product making it dimensionally stable and very accurate. advantages: good detail reproduction excellent dimensional stability – no shrinkage on set high patient acceptance more than one model can be poured from one cast disadvantages: hydrophobic – requires excellent moisture control too accurate – impression may not be compensated for during investment and casting, resulting in too small a die being produced and subsequently too small a restoration. poor tear resistance expensive indications: indirect cast restorations multiple models required impressions for removable prosthodontics bite registration material contraindications inadequate moisture control condensation silicone condensation silicones are commonly used as a putty, paste or light bodied material. the systems are usually presented as a paste or putty and a liquid/paste catalyst; meaning accurate proportioning is difficult to achieve resulting in varied outcomes. for example, the setting reaction of putty is started by kneading a low viscosity paste accelerator into a bulk of silicone with high filler content. as stated the material sets by a condensation reaction forming a three-dimensional silicone matrix whilst releasing ethyl alcohol as a by-product. this in turn results in a minimally exothermic set with marked shrinkage on setting (shrinkage being relative to filler content, where high filler content has reduced shrinkage). advantages: accurate high patient acceptance disadvantages: hydrophobic – requires excellent moisture control unreliable dimensional stability – difficult to accurately proportion components leading to variable results marked shrinkage on setting with release of by-product indications: indirect cast restorations matrices for indirect/direct restoration working impressions for metal based removable prosthodontics and relines lab putty impression trays an impression tray is a container which holds the impression material as it sets, and supports the set impression until after casting. impression trays can be separated into two main categories- stock trays and special trays. stock trays metal stock tray for upper jaw plastic stock trays of both jaws stock trays are used to take primary impressions and come in a range of sizes and shapes, and can be plastic or metal. stock trays can be rounded (designed to fit the mouths of people with no remaining teeth) or squared (designed to fit people with some remaining teeth). they can be full arch, covering all the teeth in either the upper or lower jaw in one impression, or a partial coverage tray, designed to fit over about three teeth (used when making crowns). the stock tray with the closest size and shape to the patient's own arch dimensions is selected for impressions. stock trays must meet various requirements in order to obtain a satisfactory impression. a good stock tray will: be rigid enough to endure the force of the dental impression material being positioned in the mouth. flexure of the tray under force would cause the tray to be distorted, so when the impression tray is removed the impression would be narrower and distorted. this is particularly important for plastic stock trays. be of appropriate dimensions to obtain the most accurate impression of the region being reproduced possible. inadequate extension of an impression tray where the impression material is not supported would likely cause distortions in the impression of the area. be loose enough around the dental arch to not touch the soft tissues of the mouth. have a sturdy handle to allow the tray to be easily removed from the mouth. be easily disinfected, unless for single use. disposable trays are often preferred due to increasing legislation about infection control in medicine and dentistry. stock trays can be dentate or edentulous, and perforated (used with alginate) or non-perforated (allows the impression material to run through the holes and increase the bond of the impression material to the tray when set). plastic stock trays plastic stock trays are generally injection moulded from a high-impact styrene such as polystyrene. the triple tray is a type of plastic tray used for taking impressions for a crown using the double arch or dual bite impression technique. it is used for taking impressions of the tooth preparation and the opposing teeth, by the use of a special impression material, usually elastomer. the accuracy of the results is however subject to the ability of the patient to close their teeth when the tray is present in the mouth. it cannot produce results of the complete arch, therefore its usefulness is limited. metal stock trays metal stock trays are often preferred over plastic stock trays, due to the lack of rigidity in plastic stock trays. although expensive to purchase, they have the benefit of being reusable, so can be more cost-efficient in the long-term. custom trays acrylic custom tray a special tray is an impression tray custom made for an individual patient by a denturist (dental technician), usually made from acrylic, such as polymethyl methacrylate, or shellac. a stock tray is used to make a preliminary impression, from which a model can be cast. this is then used for wax to make the tray to be laid down. 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. this is determined by the impression material to be used. specific features can be given to the special tray to improve the accuracy of the impression such as a window which can help to record displaceable tissues such as flabby ridges when used with a less viscous impression material. special trays can be given perforations if required by drilling holes in tray. customised trays have been less frequently used since the advent of putties. this is due to the putty providing good support for light bodied material, and showing very little dimensional change which provides a fine detailed dental impression. there is now a large increase in the variety of stock trays available. tray adhesives tray adhesives are used to ensure the retention of the impression material in the impression tray, with or without the presence of perforations, and are based on contact adhesive technology. maximum retention can be achieved with the presence of both a tray adhesive and perforations in the impression tray. the adhesive is applied to the internal surface of the tray, as well as over the margins to ensure the binding of the outer edge of the impression material to the tray. a suitable amount of adhesive (usually two thin coats) should be applied to the tray to prevent pooling of the adhesive which can weaken the bond between the tray and impression material. the adhesive should be completely dried prior to impression-taking. tray adhesives usually come in a screw-top bottle with a brush attached to the lid that can be used for applying the adhesive. overtime, the adhesive can accumulate around the cap, causing the evaporation of the solvent, and consequently the thickening of the adhesive. this can reduce the efficacy of the adhesive to bind to the tray. types various tray adhesives are available, corresponding to the impression material used. impression materials and corresponding adhesives impression materialadhesive used polyetherethyl acetate dissolved in propanol or acetone polysulphidestyrene acrylonite or butyl rubber dissolved in a ketone or in chloroform alginate10–12% toluene dissolved in 45–50% isopropanol condensation siliconeethyl silicate (bonds to tray) and poly dimethyl silicone (reacts with impression material) addition siliconeethyl silicate (bonds to tray) and poly dimethyl silicone (reacts with impression material), often with the addition of naphtha and ethyl acetate digital impressions digital impressions using scanner systems are being adopted in dentistry. they produce a model by milling or stereolithography which is used later.