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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 (or cast) can be formed. They are made by using a container which is designed to roughly fit over the dental arches ("trays"). Impression materials are designed to be liquid or semi-solid when first mixed and placed in the tray, and then quickly set to a solid (usually a few minutes depending upon the material), leaving an imprint of the structures in the mouth. Impressions, and the study models which are cast from them, are used in several areas of dentistry including prosthodontics (such as making dentures, inlays and plastic casts), orthodontics, restorative dentistry (e.g. to make impressions of teeth which have been prepared to receive indirect extracoronal restorations such as crowns or bridges), maxillofacial prosthetics (prosthetic rehabilitation of intra-oral and extra-oral defects due to trauma, congenital defects, and surgical resection of tumors) restorative, diagnosis and 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.
A correctly made dental impression will capture a part or all of a person's dentition and surrounding structures of oral cavity. The dental impression forms an imprint (i.e. a 'negative' mould) of teeth and soft tissues, which can then be used to make a cast of the dentition. Casts are used for diagnostics, patient record, treatment planning, fabrication of custom trays, fabrication of dentures, crowns or other prostheses and orthodontics.
An impression is made by placing a viscous, thixotropic impression material into the mouth via a custom or stock dental impression tray. The material, then sets to become an elastic solid, and, when removed from the mouth, provides a detailed and stable negative of teeth. Common materials used for dental impressions are sodium alginate, polyether and silicones - both condensation-cured silicones and addition-cured silicones, such as polyvinyl siloxane. Historically plaster of Paris, zinc oxide eugenol and agar have been used.
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.
Impression materials can be considered as follows:
Plaster of Paris (Impression Plaster)
Impression compound Zinc-Oxide Eugenol plaster Impression Waxes
Hydrocolloids are further classed as reversible or irreversible. Reversible - Agar Irreversible - Alginate
Examples of elastomeric impression materials include elastomers, polysulfides, polyethers and silicones.
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 give 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.
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