Elastic Impression Materials

• DR.HINA ADNAN

• These materials can be stretched and bent to a fairly large degree without suffering any deformation. These are used for recording the patient's mouth where undercuts are present. Usually used for partial dentures, over dentures, implants and crown and bridge work .The elastic impression materials are:

• Hydrocolloids• Elastomers

• A colloid is a state of matter in which individual particles of one substance, are uniformly distributed in a dispersion medium of another substance. When the dispersion medium is water it is termed a hydrocolloid. The colloid is relatively fluid when the solute particles present are dispersed throughout the liquid. This is called a sol. Alternatively, the particles can become attached to each other, forming a loose network which restricts movement of the solute molecules. The colloid becomes viscous and jelly like, and is called a gel. Some colloids have the ability to change reversibly from the sol state to the gel state.

Hydrocolloids

• The following hydrocolloids are discussed in this section : 

• Agars

• Alginates

• CHEMISTRY• Agar (colloid)• Borax (strengthen gel)• Potassium Sulphate• Water (dispersion medium)• In its natural state it a gel, but on heating

becomes a sol.

Agar Impression Materials

• Good surface detail• Can be used on undercuts, but liable to tear on deep

undercuts• Evaporation or imbibition • Non toxic and non irritants• low setting time • Poor tear resistance • Adequate shelf life • Can be sterilized by an aqueous solution of hypochlorite.

PROPERTIES

• ADVANTAGES• 1. Good surface detail

2. Reusable and easily sterilized

• DISADVANTAGES• 1. Need special equipment (water bath) and

special technique2. Dimensional instability

• CLINICAL• Supplied in sealed tubes to prevent evaporation of water. The

tubes are heated in boiling water (in a water bath) for 10-45 minutes. Once the impression is taken the tray can be cooled with water to aid gel formation. A higher temperature is needed to convert the gel into a sol. The first material to set is that which is in contact with the tray since it is cooler than the tissues. Thus it is the material in contact with the tissue which stays in the sol state for the longest time. Agars have been largely superseded by alginates and elastomers, although are still used for complex impressions for advanced restorative work. They are often used in labs to duplicate model because they can be reused many times.

• Container of powder should be shaken before use to get an even distribution of constituents. Powder and water should be measured to manufactures instructions. Water at room temperature should be used, this gives a reasonable working time of a couple of minutes. Faster or slower setting times can be achieved by using warm or cold water respectively. The material nearer the tissues sets first (cf. agar). Retention is needed to the impression tray and is provided by perforations in the tray and/or adhesives. Once removed from the mouth the impression should be rinsed with cold water to remove any saliva or blood. It should then be covered in a damp gauze/napkin to prevent syneresis (not placed in water which would cause imbibition-expansion).

Alginate Impression Materials

• On mixing the powder with water a sol is formed, a chemical reaction takes place and a gel is formed.The powder contains

• 1. Alginate salt (e.g. sodium alginate)2. Calcium salt (e.g. calcium sulphate)3. Trisodium phosphate

• The setting reaction is as follows:• On mixing the powder with the water

• SODIUM ALGINATE SODIUM SULPHATE

• + +CALCIUM SULPHATE CALCIUM

ALGINATE

PROPERTIES>Good surface detail>Reaction is faster at higher temperatures>Elastic enough to be drawn over the undercuts, but tears over the deep undercuts>Not dimensionally stable on storing due to evaporation>Non toxic and non irritant>Setting time can depend on technique>Alginate powder is unstable on storage in presence of moisture or in warm temperatures

ADVANTAGES1. Non toxic and non irritant2. Good surface detail3. Ease of use and mix4. Cheap and good shelf life5. Setting time can be controlled with temperature of water used

DISADVANTAGES

1. Poor dimensional stability2. Incompatibility with some dental stones3. Setting time very dependent on operator handling4. Messy to work with

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