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A dental impression is a negative record of the tissues of the mouth. It is used to reproduce the form of the teeth and surrounding tissues.

The negative reproduction of the tissues given by the impression material is filled up with dental stone or other model materials to get a positive cast. The positive reproduction of a single tooth is described as a “die”, and when several teeth or a whole arch is reproduced, it is called a “cast” or “model”.

ADVANTAGES OF USING A CAST OR MODEL

1. Models provide a three-dimensional view of the oral structures, thus aiding in diagnosis and treatment planning.

2. Many restorations or appliances are best constructed on casts. It may be inconvenient to both dentist and patient if these have to be made directly in the patient’s mouth.

3. Models can be used to educate the patient. 4. They serve as pre- and post-treatment records.

5. By using casts, technical work can be passed on to technicians, saving valuable clinical time.

DESIRABLE PROPERTIES OF AN IMPRESSION MATERIAL

An impression material should:

1. Have a pleasant taste, odor and esthetic color. 2. Not contain any toxic or irritating ingredients.

3. Have adequate shelf life for storage and distribution. 4. Be economical.

5. Be easy to use with the minimum equipments.

6. Have adequate setting characteristics that meet clinical requirements. 7. Possess a satisfactory consistency and texture.

8. Have adequate strength so that it will not break or tear while removing from the mouth.

9. Possess elastic properties with freedom from permanent deformation after strain. 10. Exhibit dimensional stability.

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11. Be compatible with the die and cast materials. 12. Faithfully reproduce the tissues of mouth. 13. Be able to be electroplated.

CLASSIFICATION OF IMPRESSION MATERIALS

There are several classifications:

According to Mode of Setting and Elasticiy

Mode of Setting Rigid Elastic

Set by chemical reaction Impression plaster Alginate hydrocolloid (irreversible or thermoset). Zinc oxide eugenol Non-aqueous elastomers,

e.g. Polysulfide polymer Polyether, Silicone Set by temperature change Compound Agar hydrocolloid. (reversible/thermoplastic). Waxes

According to their Uses in Dentistry

Impression Materials used for Complete Denture Prosthesis

Impression plaster, impression compound and impression paste set to a hard rigid mass and hence cannot be removed from undercuts without the impression being fractured or distorted. Therefore these materials are best suited for edentulous mouth.

Impression Materials used for Dentulous Mouths

On the other hand alginates and rubber base impressions are sufficiently elastic to be withdrawn from undercut areas. Such elastic impression materials are suitable for impressions in removable partial denture prosthesis and crown and bridge work, where the impressions of the ridge and teeth are required.

The Rigid Impression Materials

As mentioned earlier the rigid impression materials are: 1. Impression plaster

2. Impression compound

3. Zinc oxide eugenol impression paste 4 Impression waxes.

(Impression plaster is described in the chapter on Gypsum Products).

IMPRESSION COMPOUND

Impression compound is described as a rigid, reversible impression material which sets by physical change. On applying heat, it softens and on cooling it hardens. It is mainly used for making impressions of edentulous mouth.

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ADA specification No.3 two types: Type I — Impression Compound Type II — Tray Compound

Type II: Tray compound is used to prepare a tray for making an impression. A second material is then carried in it in order to make an impression of mouth tissues. Since reproduction of the fine details is not essential, it is generally stiffer and has less flow than regular impression compound.

SUPPLIED AS

Sheets, sticks, cakes and cones (Fig. 4.1).

FIGURE 4.1: Impression compound cakes and sticks

APPLICATIONS

1. For making a preliminary impression in an } edentulous (mouth without teeth) mouth }

2. For individual tooth impression } Type I 3. Peripheral tracing or border moulding }

4. To check undercuts in inlay preparation }

5. To make a special tray } Type II

Single Tooth Impression

In operative dentistry, an impression is made of a single tooth in which a cavity is prepared. The compound is softened and carried in a copper band. The filled band is pressed over the tooth and the compound flows into the prepared cavity. It is referred to as a tube impression. Tube impressions were also used to make electroformed dies.

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Complete Denture Impressions

In complete denture fabrication it is common to make two sets of impressions— the preliminary and the final impression. The preliminary impression is made in a stock tray. A study cast is made from this is used to construct a custom tray or special tray. The custom tray is used to make the final impression. The technique of making a preliminary and final impression greatly improves the accuracy of the complete denture.

REQUIREMENTS OF IDEAL IMPRESSION COMPOUND

It should:

• harden at or little above mouth temperature,

• be plastic at a temperature not injurious or harmful to oral tissues, • not contain irritating or toxic ingredients,

• harden uniformly when cooled without distortion,

• have a consistency when softened which will allow it to reproduce fine details, • be cohesive but not adhesive,

• not undergo permanent deformation or fracture while withdrawing the impression from the mouth,

• be dimensionally stable after removal from the mouth and during storage, • exhibit a smooth glossy surface after flaming,

• withstand trimming with sharp knife without flaking or chipping after hardening. • should not boil and lose volatile components on flaming,

• should remain stable without losing soluble plasticizers when immersed in water for long periods.

COMPOSITION

In general impression compound is a mixture of waxes, thermoplastic resins, organic acids, fillers and coloring agents.

Ingredients Parts Resin 30 Copal resin 30 Carnauba wax 10 Stearic acid 5 Talc 75

Coloring agent Appropriate amount

Plasticizers Bees wax is brittle, so compounds such as shellac, stearic acid and gutta-percha are added to improve plasticity and workability. These substances are referred to as plasticizers. Synthetic resins are being used in increasing amounts, usually in combination with natural resins.

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