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.
Uses
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
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 relieved such that both the advantages of muco static and muco compressive techniques are achieved.
User Guide of Dental Impression material: https://www.youtube.com/watch?v=-keGMbCHC2A
Special techniques
"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
The preparation border must be accurately captured by the light bodied impression material when taking impressions for crown and bridge work. As a result, the gingival tissues must be pushed away from the preparation margin in order for the impression material to be accessible. Inserting a retraction cord into the gingival crevice is one method of retracting gingival tissues away from the tooth.
Impression materials
Impression materials can be considered as follows: