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A dental impression is a negative imprint of hard (teeth) and soft tissues in the mouth from which a positive reproduction (cast or model) can be formed. It is made by placing an appropriate material in a stock or custom dental impression tray which is designed to roughly fit over the dental arches. Impression material is of liquid or semi-solid nature when first mixed and placed in the mouth. It then sets to become an elastic solid (usually takes a few minutes depending upon the material), leaving an imprint of person's dentition and surrounding structures of oral cavity.
Impressions, and the study models, are used in several areas of dentistry including:
- diagnosis and treatment planning,
- prosthodontics (such as making dentures),
- restorative dentistry (e.g. to make impressions of teeth which have been prepared to receive indirect extracoronal restorations such as crowns, bridges, inlays and onlays),
- maxillofacial prosthetics (prosthetic rehabilitation of intra-oral and extra-oral defects due to trauma, congenital defects, and surgical resection of tumors),
- oral and maxillofacial surgery for both intra-oral and or extra-oral aims (e.g. dental implants).
The required type of material for taking an impression and the area that it covers will depend on the clinical indication. Common materials used for dental impressions are:
- non rigid materials:
- rigid materials:
- plaster of Paris,
- impression compound,
- zinc oxide and eugenol-based impression paste.
- 1 Techniques for Taking Impression
- 2 Special techniques
- 3 Impression materials
- 4 Impression trays
- 5 References
Techniques for Taking Impression
Impressions can also be described as mucostatic or mucocompressive, being defined both by the impression material used and the type of impression tray used (i.e. spaced or closely adapted). Mucostatic means that the impression is taken with the mucosa in its normal resting position. These impressions will generally lead to a denture which has a good fit during rest, but during chewing, the denture will tend to pivot around incompressible areas (e.g. torus palatinus) and dig into compressible areas. Mucocompressive means that the impression is taken when the mucosa is subject to compression. These impressions will generally lead to a denture that is most stable during function but not at rest. Dentures are at rest most of the time, so it could be argued that mucostatic impressions make better dentures, however in reality it is likely that tissue adaption to the presence of either a denture made with a mucostatic or a mucocompressive technique make little difference between the two in the long term. Another type of impression technique is selective pressure technique in which stress bearing areas are compressed and stress relief areas are relived such that both the advantages of muco static and muco compressive techniques are achieved.
- "Wash impression"- this is a very thin layer of low viscosity impression material which is used to record fine details. Usually it is the second stage, where the runny impression material is used after an initial impression taken with a more viscous material.
- two phase one stage: the putty and low body weight impression materials are inserted to the mouth at once .
- two phase two stage: first the putty is set in the mouth then low body weight material is added on the top of ready impression and inserted to the mouth to get the final accurate impression
- Functional impression (also known as secondary impression)
- Neutral zone impression
- Window technique
- Altered cast technique
- Applegate technique
Impression for Provision of Fixed Prosthesis
When taking impression for provision of crown and bridge work, the preparation border needs to be accurately captured by the light bodied impression material. For this reason, the gingival tissues need to be pushed away from the preparation margin in order to make it accessible to the impression material. One way to retract gingival tissues away from the tooth is by insertion of a retraction cord into the gingival crevice.
Impression materials can be considered as follows:
Plaster of Paris (Impression Plaster)
Plaster of Paris is traditionally used as a casting material once the impression has been taken, however its use as an impression material is occasionally useful in edentate patients. The tissues are not displaced during impression taking, hence the material is termed mucostatic. Mainly composed of β-calcium sulphate hemihydrate, impression plaster has a similar composition and setting reaction to the casting material with an increase in certain components to control the initial expansion that is observed with Plaster of Paris. Additionally, more water is added to the powder than with the casting material to aid in good flow during impression taking. As the impression material is very similar to the casting material to be used, it requires the incorporation of a separating medium (e.g. sodium alginate) to aid in separating the cast from the impression. If a special tray is to be used, impression plaster requires 1-1.5mm spacing for adequate thickness.
- Good detail reproduction
- Good dimensional stability (contraction on setting)
- Good patient tolerance
- 2-3 minutes working time
- No recovery from deformation. Therefore, if an undercut is present the material will have to be broken off the impression and then glued back together prior to casting
- Excess salivation by the patient could have adverse effect on detail reproduction
Impression compound has been used for many years as an impression material for removable prostheses. Although its use has recently declined with the advent of better materials it is still used for:
- Primary impressions of complete dentures
- Border moulding of trays
- Extension of trays
- Achieving mucocompression in the post-dam area when working impressions are taken for complete dentures
Impression compound is a thermoplastic material; it is presented as a sheet of material, which is warmed in hot water (> 55-60 °C) and loaded on a tray prior to impression taking. Once in the mouth, the material will harden and record the detail of the soft tissues.
Zinc-Oxide Eugenol Plaster (Impression Paste)
Impression paste is traditionally used to take the working (secondary) impressions for a complete denture. When used with a special tray it requires 1mm of spacing to allow for enough thickness of the material; this is also termed a close fitting special tray. It is available as a two-paste system:
- Base paste: zinc oxide
- Catalyst paste: eugenol
The two pastes should be used in equal amounts and blended together with a stainless steel spatula (Clarident spatula) on a paper pad. Zinc-oxide Eugenol plaster will produce a mucostatic impression.
- Thermoplastic - can be heated to aid removal from the casting material
- Good detail reproduction
- Good dimensional stability (0.15% shrinkage on setting)
- Rigid - presence of undercuts can distort the final material or cause the section engaged to separate from resultant impression
Agar is a material which provides high accuracy. Therefore, it is used in fixed prosthodontics (crowns, bridges) or when a dental model has to be duplicated by a dental technician. Agar is a true hydrophilic material, hence the teeth do not need to be dried before placing it into the mouth. It is a reversible hydrocolloid which means that its physical state can be changed by altering its temperature which allows to reuse the material multiple times. The material comes in form of tubes or cartridges. A special hardware is required in the process of taking agar impressions, namely a water bath and rim lock trays with coiled edges allowing passage of cold water for cooling the material to set while in the mouth. The bath consists of three containers filled with water at different temperatures: the first is set at 100°C to liquefy the agar, the second is used to lower down the temperature of the material for safe intra-oral use (usually set at 43-46°C) and the third one is used for storage and is set at 63-66°C. The storage container can maintain agar tubes and cartridges at temperature 63-66°C for several days for convenient immediate use. The tray is connected to a hose, material is loaded onto the tray and placed in the mouth over the preparation - an adequate thickness of the material is required, otherwise distortion may occur upon removal from the mouth. The other end of the hose is connected to a cold water source. The hydrocolloid is then cooled down through the tray wall which results in setting of the material. The models should be poured as soon as possible to avoid changes in dimensional stability.
Modern dentistry offers other materials (e.g. elastomerics) which provide high accuracy impressions and are easier to use hence agar is used less frequently.
- high accuracy
- complex procedural steps
- significant start-up cost of the hardware
Alginate on the other hand is an irreversible hydrocolloid. It exits in two phases: either as a viscous liquid, or a solid gel, the transition generated by a chemical reaction . The impression material is created through adding water to the powdered alginate which contains a mixture of sodium and potassium salts of alginic acid. The overall setting double composition reaction is as follows:
Potassium (sodium) alginate + calcium sulphate dihydrate + water → calcium alginate + potassium (sodium) sulphate
Sodium phosphate is added as a retarder which preferentially reacts with calcium ions to delay the set of the material.
Alginate has a mixing time of 45 – 60 secs, a working time of 45 secs (fast set) and 75 secs (regular set). The setting time can be between 1 – 4.5 mins which can be varied by the temperature of water used: the cooler the water, the slower the set and vice versa. You want to ensure that the material is fully set before removal from the mouth.
The water content that the completed impression is exposed to must be controlled. Improper storage can either result in syneresis (the material contracts upon standing and exudes liquid) or imbibition (water uptake which is uncontrolled in extent and direction). Therefore the impression must be stored correctly, which involves wrapping the set material in a damp tissue and storing it in a sealed polythene bag until the impression can be cast. Alginate is used in dental circumstances when less accuracy is required. For example this includes the creation of study casts to plan dental cases and design prosthesis, and also to create the primary and working impressions for denture construction.
Several faults can be encountered when using an alginate impression material, but these can generally be avoided through adequate mixing, correct spatulation, correct storage of the set material, and timely pouring of the impression.
Due to the increased accuracy of elastomers, they are recommended for taking secondary impressions over alginate. Patients both preferred the overall experience of having an impression taken with an elastomer than with alginate, and also favoured the resultant dentures produced. 
- Easy flow
- Reproduction of adequate detail
- Fast setting time
- Minimal tissue displacement in the mouth
- 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
Examples of elastomeric impression materials include elastomers, polysulfides, polyethers, additional silicone, and condensation silicone
An impression tray is a container which holds the impression material as it sets and supports the set impression. There are 2 main types of trays. Stock trays are manufactured en masse in a range of sizes and shapes, from which the closest size and shape tray is selected that matches the dimensions of the dental arches of the person who is to receive the impression. Special trays (or custom trays) are made to fit a specific individual's mouth by a denturist (dental technician). Special trays are constructed on a cast from a preliminary impression which utilizes stock trays.
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 they can be a partial coverage tray, designed to fit over about 3 teeth (used when making crowns). Stock trays can be made out of a range of materials from plastics to metals, and they can be perforated (to allow the impression material to run through the holes and increase the bond of the impression material to the tray when set). Adhesives are commonly used to bond the impression material to the tray, and vary in composition depending upon what material is used.
Special trays are commonly made from acrylic or shellac. They are classed as spaced (leaving about 3 mm space between the tray and the mucosa for the impression material to occupy) or closely adapted, where less space is left for the impression material. Special trays can be given perforations if required by drilling many holes in tray.
All trays are designed to be rigid and have a handle to remove them from the mouth. Due to increasing legislation about infection control in medicine and dentistry, single use disposable trays are used more commonly than trays which are capable of being disinfected and reused.
- J., Bonsor, Stephen (2013). A clinical guide to applied dental materials. Pearson, Gavin J. Amsterdam: Elsevier/Churchill Livingstone. pp. 243–251. ISBN 9780702031588. OCLC 824491168.
- John F. McCabe, Angus W. G. Walls (1990). Applied Dental Materials. Wiley. p. 143. ISBN 978-1-118-69712-2.
- Stephen J. Bonsor, Gavin J. Pearson (2013). A Clinical Guide to Applied Dental Materials. United Kingdom: Elsevier. pp. 237–273. ISBN 978-0-7020-3158-8.
- Bonsor, Stephen J.; Pearson, Gavin J. (2013). A clinical guide to applied dental materials. Elsevier/Churchill Livingstone. pp. 245–248. ISBN 9780702031588.
- Hyde, T.P; Craddock, H.L; Gray, J.C; Pavitt, S.H; Hulme, C; Godfrey, M; Fernandez, C; Navarro-Coy, N; Dillon, S; Wright, J; Brown, S; Dukanovic, G; Brunton, P.A (2013). "A Randomised Controlled Trial of complete denture impression materials". Journal of Dentistry. 42 (8): 895-901.
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