IMPRESSION MATERIAL Dental Impression It is a negative

IMPRESSION MATERIAL

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.

CLASSIFICATIONS OF IMPRESSION MATERIALS 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.

Impression Plaster • The chemical formula is (Ca SO 4. 1/2 H 2 O), • this impression material is known to be very accurate with minimum setting expansion (<0. 06%). • The water/powder ratio is higher than that for dental plaster. • This impression material contains additives that control and regulate both setting time and expansion (K 2 SO 4 ).

Impression Compound Impression compound is described as a rigid, reversible impression material which sets by physical changes. On applying heat, it softens and on cooling it hardens. They supplied as sheet, stick, and cake.

Types: 1 - Type I Impression Compound (Lower Fusing) *Sheet Form Material: it is softened using water bath, a temperature in range (55 -60 °c), knead the material after it has been heated in water to ensure its being at a uniform temperature. Storage in hot water should not be long that important constituents such as stearic acid may be leached out. Overheating make the compound sticky and difficult to handle. *Stick Form Material: it is softened over a flame. The compound should not be allowed to boil otherwise, the plasticizers are volatilized.

2 - Type II Tray Compound (Higher Fusing) It is used to prepare a tray for making an impression. It is generally stiffer and has less flow than regular impression compound.

Zinc Oxide Eugenol Impression Material Classification 1 - Type I (Hard). 2 - Type II (Soft). Available as 1 - base paste (white in color). 2 - Accelerator or reactor or catalyst paste (red in color).

Factors Controlling the Setting Time a- particles size of zinc oxide powder: if the particle size is small, the setting time is less. b- By varying the lengths of the two pastes. c- By adding a dropping of water, the setting time can be decreased. d- Longer the mixing time, shorter is the setting time. e- High atmospheric temperature and humidity decrease the setting time. f- Cooling the mixing slab, spatula increase the setting time. g- By adding a dropping of oil or wax, the setting time can be increased.

Reversible hydrocolloid (agar-agar). • This impression material is liquid state at elevated temperature that changes into viscoelastic state upon cooling. • The material preparation requires special equipment as well as special water-cooled trays for impression making. • Although it is an excellent impression material and yields accurate impressions, presently it has been largely replaced by alginate hydrocolloid and rubber impression materials.

Reversible hydrocolloid (Alginate impression material) • Alginate was developed as a substitute for agar. • Currently, alginate is more popular than agar for primary impression. • Its good elastic property, its long term stability is quite poor. • Hand mixing is required and the powder/water ratio can affect the setting reaction, strength and consistency.

Polysulfide Impression Material • Polysulfide rubber impression material has a polysulfide polymer base that reacts with sulfur with help of a catalyst. • This impression material has good elastic property • and excellent tear strength. • However, this impression material has unpleasant smell. • It is available in different consistencies.

Silicone Impression Material a- Condensation - cured silicone: They consist of a base material and a catalyst. The base material is a dimethyl siloxane with 35 - 70% silica filler, with the help of alkyl silicate catalyst the polymerization is carried out by liberation alcohol. The material has rapid setting reaction but high setting shrinkage.

b- Addition –cured silicone: They are known as polysiloxanes, this impression material is also provided as a base and a catalyst tubes with no byproduct resulted upon reaction. This material has long term stability as well as being hydrophobic with excellent elastic properties.

Polyether Impression Material The base is a polyether with terminal imine rings that undergo cross-linkage by 2, 5 -dichlorobezene sulfonate. It has a rapid setting time, with moderate dimensional change on setting, good elastic recovery. It can induce allergic reaction.