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.
As a result, the gingival tissues must be pushed away from the preparation margin in order for the impression material to be accessible.
Additionally, more water is added to the powder than with the casting material to aid in good flow during impression taking.
If a special tray is to be used, impression plaster requires 1–1.5mm spacing for adequate thickness.
Due to its poor flow characteristics, it is unable to reproduce fine detail and so its use is somewhat limited to the following scenarios: Impression compound is a thermoplastic material; it is presented as a sheet of material, which is warmed in hot water (> 55–60 °C) for one minute, and loaded on a tray prior to impression taking.
Impressions with compound should be poured within an hour as the material exhibits poor dimensional stability.
It is available in Putty and light body consistencies to aid dentists make perfect impressions for fabrication of crowns, bridges, inlays, onlays and veneers.
[1] 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 storage container can maintain agar tubes and cartridges at temperature 63–66 °C for several days for convenient immediate use.
It exists in two phases: either as a viscous liquid, or a solid gel, the transition generated by a chemical reaction.
Improper storage can either result in syneresis (the material contracts upon standing and exudes liquid) or imbibition (water uptake which is uncontrolled in extent and direction).
For example, this includes the creation of study casts to plan dental cases and design prosthesis, and also to create the primary and working impressions for denture construction.
Due to the increased accuracy of elastomers, they are recommended for taking secondary impressions over alginate.
Patients both preferred the overall experience of having an impression taken with an elastomer than with alginate, and also favoured the resultant dentures produced.
[4] Advantages: Disadvantages: As stated above, there are times clinically where the accuracy of an alginate impression is not acceptable, particularly for the construction of fixed prosthodontics.
This property makes it a commonly used material in general practice as it more likely to capture preparation margins when moisture control is not perfect.
Advantages:[2] Disadvantages:[2] Indications: There are two types of silicone resin impression material, addition and condensation (reflecting each of their setting reactions).
Advantages:[2] Disadvantages:[2] Indications: Contraindications Condensation silicones are commonly used as a putty, paste or light bodied material.
The systems are usually presented as a paste or putty and a liquid/paste catalyst; meaning accurate proportioning is difficult to achieve resulting in varied outcomes.
For example, the setting reaction of putty is started by kneading a low viscosity paste accelerator into a bulk of silicone with high filler content.
As stated the material sets by a condensation reaction forming a three-dimensional silicone matrix whilst releasing ethyl alcohol as a by-product.
Stock trays are used to take primary impressions and come in a range of sizes and shapes, and can be plastic or metal.
The stock tray with the closest size and shape to the patient's own arch dimensions is selected for impressions.
The accuracy of the results is however subject to the ability of the patient to close their teeth when the tray is present in the mouth.
Digital impressions using extra-oral or intra-oral scanner systems are being adopted in dentistry.
