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:

violet phase: mixing time

pink phase: placing the material inside the impression tray

yellow phase: placing the tray inside the mouth

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

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

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
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
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
after setting

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