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.