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Dental impression. Materials

author icon By Dr. George Ghidrai

A dental impression is an imprint of teeth and/or soft tissues, formed with specific types of impression materials that is used in different area of dentistry including Prosthodontics.

A correctly made dental impression will capture a part or all of a person's dentition and surrounding structures of oral cavity. The dental impression forms an imprint (i.e. a 'negative' mould) of teeth and soft tissues, which can then be used to make a cast of the dentition.

dental impression

dental impression


Impression materials

Dental impressions must be highly accurate. To achieve this goal, specific impression materials are used. These materials have the ability to reproduce very precisely the clinical situation.

Qualities

A proper impression material must have several qualities :

  • Fidelity : the ability of the impression material to record the finest details.

  • Elasticity : the property of the material to keep the fidelity of the impression when removed from the mouth.

  • Stability : the quality of the material to maintain its exact shape for a certain period of time. That means the fidelity of the impression will be preserved until reaching the dental lab.

  • Setting time : has to be short, but enough.

  • Pleasant smell, taste and color.

  • Not irritating or toxic to the oral tissues.


Preparation

Usually, impression materials are prepared by mixing 2 substances: the base substance and the catalyst. The 2 substances can be 2 paste substances, a paste and a liquid or a powder and a liquid (which may be water).

Setting time may vary between 1 and 3 minutes, depending on several factors :

  • The type of material
  • The outdoor temperature
  • The catalyst temperature
  • Material consistency

Some materials can change their color depending on the stage of the process (e.g. alginate). This way the practitioner knows exactly when the right time for each step of the process is. Below an example of such a material :


impression materials : alginate violet phase

violet phase : mixing time

impression materials : alginate pink phase

pink phase : placing the material inside the impression tray

impression materials : alginate yellow phase

yellow phase : placing the tray inside the mouth

impression materials : alginate white phase

white phase : after setting




Preparation is done by mixing the base with the catalyst according to manufacturer's instructions. Preparation time should not be too long ; otherwise, there is the risk that the setting of the material starts before placing it inside the mouth.

There is also the possibility of using a self-mixing impression material syringe.


self mixing impression material syringe

self mixing impression material syringe

Most often, after preparation, the material is placed in an impression tray.


Impression trays

An impression tray is a container, which holds the impression material as it sets and supports the set impression. All trays are designed to be rigid and have a handle to remove them from the mouth. There are 2 main types of trays :

  • Stock trays

    Stock trays are manufactured "en masse" in a range of sizes and shapes, for the upper or lower arch, from which the closest size and shape tray is selected that matches the dimensions of the dental arch to receive the impression.

    stock impression trays

    set of stock trays

    Stock trays can be made out of a range of materials from plastics to metals, and they can be perforated to allow the impression material to run through the holes and increase the bond of the impression material to the tray when set.

  • Custom trays

    Custom trays (or special trays) are made to fit a specific individual's mouth by a dental technician. Special trays are constructed on a cast from a preliminary impression which utilizes stock trays.

    custom tray

    custom tray on a dental cast

    Special trays are commonly made from acrylic or shellac. They are very closely adapted and are mainly utilized for the custom tray impression technique.


Classes of impression materials

Over the years, many types of impression materials have been developed and the qualities have constantly improved. There are several classes of materials, each serving different purposes.

Many types of classifications are available. The following is based on materials rigidity and elasticity.


Rigid impression materials

The base has a putty consistency and the catalyst is a paste or a liquid. After setting, the material becomes hard and rigid. The fidelity is not very high, but the impressions are very resistant after setting and the stability is excellent.

Rigid materials are used for preliminary impressions or for the first stage of the two-step technique (or wash technique) impression.

Materials in this category : plaster, hard silicones, waxes, Stents blocks


rigid impression material before preparation

rigid impression material
before preparation

rigid impression material after setting

rigid impression material
after setting


Elastic impression materials

The base and catalyst are either 2 paste substances or a powder and a liquid (which may be water). After the preparation, they are placed inside impression trays.

Elastic materials have a soft consistency but after setting, they become elastic. They are easy to handle and their accuracy is pretty good.

Materials in this category : alginate, medium consistency silicones


elastic impression material before preparation

elastic impression material
before preparation

elastic impression material after setting

elastic impression material
after setting


Fluid impression materials

These materials have a very soft ("runny") consistency when placed inside impression trays. After setting, they become hard and elastic.

Their fluidity combined with specific features makes them the most accurate impression materials. The fluid materials are used in the second stage of the wash technique impression after an initial impression taken with a more viscous material.

Materials in this category : polyether, polysulfide, fluid silicones


fluid impression material before preparation

fluid impression material
before preparation

fluid impression material after setting

fluid impression material
after setting

There is also the possibility of using a self-mixing impression material syringe.

Last review and update: November 2020



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