Dental impression

A dental impression is a negative imprint of hard and soft tissues in the mouth from which a positive reproduction, such as a cast or model, can be formed. It is made by placing an appropriate material in a dental impression tray which is designed to roughly fit over the dental arches. The impression material is liquid or semi-solid when first mixed and placed in the mouth. It then sets to become an elastic solid, which usually takes a few minutes depending upon the material. This leaves an imprint of a person's dentition and surrounding structures of the oral cavity. Digital impressions using computerized scanning are now available. Impressions, and the study models, are used in several areas of dentistry including: diagnosis and treatment planning prosthodontics (such as making dentures) orthodontics 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: reversible hydrocolloids: agar irreversible hydrocolloids: sodium alginate elastomeric materials: silicones (polyvinyl siloxane): condensation-cured silicones, addition silicones, vinyl polyether silicones (VPES) polyethers polysulphides rigid materials: plaster of Paris impression compound zinc oxide and eugenol-based impression paste 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.