WHAT ARE DENTAL IMPRESSIONS?
Dental impressions are defined as the negative imprint of the teeth and other parts of the oral cavity (gums, surrounding tissues, etc.) which is used to create an exact three-dimensional model of these structures.
Where are dental impressions used?
These imprints are often used in oral health care for various purposes including prosthodontics, orthodontics, dental crowns and bridges, maxillofacial prosthetics, oral and maxillofacial surgery, in diagnosis, mouth guards, whitening trays, and for the permanent dental record to document before-and-after phase.
What are the types of dental impressions?
There are three main types of dental impressions — preliminary, final, and bite registration.
- Preliminary impressions are accurate reproductions of a patient’s mouth. This type of impressions is usually used to construct study models for the diagnosis, documentation of dental arches, and as a visual aid for education.
- Final impressions have the exact details of the tooth structures and their surrounding tissues. They are used to make casts and dies with the precise details of the tooth structures and their surrounding tissues. The casts and dies are utilized by dental laboratory technicians for the construction of crowns, bridges, dentures, and other restorations.
- Bite registrations are employed to document the occlusal relationship between arches which is essential when establishing the articulation of maxillary and mandibular casts
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.
Decontamination of impressions
It is necessary to decontaminate impressions to prevent cross-contamination. Once an impression is removed from the patient’s mouth, the following procedure should be undertaken:
Wearing personal protective equipment:
- Inspect the dental impression for blood and/or debris. Wash in a designated sink to remove the same
- The disinfection process will be impeded without adequate removal of blood and/or debris
- Follow the manufacturer’s instructions in relation to the immersion of the dental impression in a disinfectant solution for material compatibility and length of immersion
- After impression decontamination, rinse impression in the designated sink
- It is essential that good cross-infection control practices are followed during impression decontamination; avoid contamination of immersion bath and sink. If contamination occurs, disinfect
- Disinfected impressions should be prepared for the laboratory by placing in a sealable bag, taking care to not cross-contaminate the bag. Seal the bag
- Complete laboratory prescription ensuring that the date, time and solution of disinfectant is recorded
- Staple laboratory prescription to the sealed bag above the seal, taking care to not perforate bag
- Transport to the dental laboratory
What materials are used to create dental impressions?
The most widely used material, especially for preliminary impressions, is irreversible hydrocolloid. Hydrocolloids can exist in a sol or a gel state. The physical transformation from sol to gel called gelation can either be reversible – changed by thermal factors – or irreversible – changed by chemical factors, depending on the type of hydrocolloid used. Irreversible hydrocolloids do not change after gelation.
The most often used irreversible hydrocolloid is alginate. Alginate is a naturally-occurring polymer usually obtained from brown seaweed. This biomaterial has numerous applications in biomedical science as well as engineering due to its biocompatibility.
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 (e.g. sodium alginate) 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.
- Good detail reproduction
- Good dimensional stability (contraction on setting)
- Good patient tolerance
- 2–3 minutes working time
- 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