IMPRESSION MATERIALS (DENTAL MATERIALS)

IMPRESSION MATERIALS
Dental impression: It is a negative record of tissue of the mouth. It is used to reproduce the form of the teeth and surrounding tissues. A positive reproduction is obtained by pouring dental stone or other suitable material into the impression and allowing it to harden.
The positive reproduction of a single tooth is described as die, and when several teeth or a whole arch is reproduced, it is called cast or model. The impression material is carried to the mouth in a tray, which either stock tray or special tray.
Requirements (desirable properties) of impression materials:
1- Accurate reproduction of surface details.
2- A pleasant odor, taste, and esthetic color.
3- Absence of toxic or irritant constituents.
4- Adequate shelf life for requirements of storage and distribution.
5- Reasonable cost.
6- Easy to use with the minimum of equipment.
7- Setting characteristics that meet clinical requirements.
8- Satisfactory consistency and texture.
9- Readily wets oral tissues.
10- Elastic properties with freedom from permanent deformation after strain.
11- Adequate strength so it will not break or tear on removal from the mouth.
12- Dimensional stability over temperature and humidity ranges normally found in clinical and laboratory procedures for a period long enough to permit the production of a cast or die.
13- Compatibility with cast and die materials.
14- Readily disinfected without loss of accuracy.
15- No release of gas during the setting of the impression or cast and die materials.

THERE ARE SEVERAL CLASSIFICATIONS OF IMPRESSION MATERIALS
I- According to the properties of set material (elasticity)

1- RIGID IMPRESSION MATERIALS
They cannot engage undercuts, so their use is restricted to edentulous patient without undercut.
a- Impression plaster.
b- Impression compound.
c- Zinc oxide eugenol.
d- Impression wax.

2- ELASTIC IMPRESSION MATERIALS
They can engage undercuts, and they may be used in edentulous, partially dentate, and fully dentate patients.
• Hydrocolloid:
a- Reversible hydrocolloid (agar-agar).
b- Irreversible hydrocolloid (alginate).
• Elastomers
a- Polysulfide.
b- Silicone:
- Condensation polymerizing silicone.
- Addition polymerizing silicone.
c- Polyether.



II- According to mode of setting

1- SET BY CHEMICAL REACTION (IRREVERSIBLE)
? Impression plaster.
? Zinc oxide eugenol.
? Alginate.
? Polysulfide.
? Polyether.
? Silicones.

2- SET BY TEMPERATURE CHANGE (REVERSIBLE) (THERMOPLASTIC)
? Impression compound.
? Impression wax.
? Agar-agar.

III- According to viscosity of material before set

1- MUCOSTATIC
It is not compress tissue during seating of the impression.
? Impression plaster.
? Zinc oxide eugenol.
? Alginate.
? Agar-agar.

2- MUCOCOMPRESSIVE
It compresses tissue during seating of impression, the material more viscous.
? Impression compound.

3- PSEUDOPLASTIC
Material fairly viscous whilst under low stress conditions may become fluid during recording of impression.
? Polysulfide.
IV- According to the way in which the material interact with saliva

1- HYDROPHILIC
The material is compatible with moisture and saliva.
? Impression plaster.
? Alginate.
? Addition polymerizing silicone.
? Polyether.

2- HYDROPHOBIC
Ability of material to repel saliva, a dry field is essential for such materials.
? Polysulfide.
? Condensation polymerizing silicone.