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.

Uses

Impressions, and the study models, are used in several areas of dentistry including:
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 : condensation-cured silicones, addition silicones, vinyl polyether silicones
  • ** 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. 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 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
  • 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:

Rigid

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 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.
Advantages:
  • Hydrophilic
  • Good detail reproduction
  • Good dimensional stability
  • Good patient tolerance
  • 2–3 minutes working time
Disadvantages:
  • Brittle
  • 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

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. Due to its poor flow characteristics, it is unable to reproduce fine detail and so its use is somewhat limited to the following scenarios:
  • 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 for one minute, 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. The impression can further be hardened by placing it in cold water after use. Impressions with compound should be poured within an hour as the material exhibits poor dimensional stability. There are two main presentations of impression compound: red compound and greenstick. The latter is mainly used for border moulding and recording the post-dam area.

Vinyl polysiloxane impression material (impression material)

vinyl polysiloxane dental impression materials used for making accurate dental impressions with excellent reproducibility. It is available in Putty and light body consistencies to aid dentists make perfect impressions for fabrication of crowns, bridges, inlays, onlays and veneers.
Example
Advantages:
  • Better reproduction detail with two viscosities
  • Exhibits pseudo-plastic properties for precision which is not found in alginates
  • Superior tear strength than any other VPS material
  • Better dimensional stability – multiple models can be poured up to two weeks
  • Good hydrophilicity
  • Compatible with gypsum products
  • Superior wetting characteristics ensuring gypsum working cast is hard with smooth surface
  • Can be subjected to cold sterilization without compromising the details and dimensional stability of the impression

    Zinc-oxide eugenol plaster (impression paste)

Impression paste is traditionally used to take the working impressions for a complete denture. When used with a special tray it requires 1 mm 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 on a paper pad. Zinc-oxide Eugenol plaster will produce a mucostatic impression.
Advantages:
  • Thermoplastic – can be heated to aid removal from the casting material
  • Good detail reproduction
  • Good dimensional stability
Disadvantages:
  • Rigid – presence of undercuts can distort the final material or cause the section engaged to separate from resultant impression

    Impression waxes

Non rigid

Hydrocolloid

Agar
is a material which provides high accuracy. Therefore, it is used in fixed prosthodontics 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 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 which provide high accuracy impressions and are easier to use hence agar is used less frequently.
Advantages:
  • high accuracy
  • hydrophilic
  • reusable
Disadvantages:
  • complex procedural steps
  • significant start-up cost of the hardware