Elastic Impression Materials-Hydrocolloids-Prosthetic Dentistry Lecture note

Introduction to Elastic Impression Materials

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

Introduction to Hydrocolloids

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. A sol can be converted into a gel in one of two ways:
1. Reduction in temperature, reversible because sol is formed again on heating (eg agar).
2. Chemical reaction which is irreversible (eg alginates). A gel can lose (syneresis which results in shrinkage) or take up (imbibition which results in expansion) water or other fluids.
Hydrocolloids are placed in the mouth in the sol state when it can record sufficient detail, then removed when it has reached the gel state. Hydrocolloid materials especially the alginates, may display a lack of incompatibility with some makes of dental stones. The resultant model may show reduced surface hardness and possibly surface irregularities and roughness.

Agar Impression Materials

CHEMISTRY

• Agar (colloid)
• Borax (strengthen gel)
• Potassium Sulphate
• Water (dispersion medium)

In its natural state it a gel, but on heating becomes a sol.
PROPERTIES

• Good surface detail
• Can be used on undercuts, but liable to tear on deep undercuts
• Evaporation or imbibition
• Non toxic and non irritant
• Slow setting time
• Poor tear resistance
• Adequate shelf life
• Can be sterilised by an aqueous solution of hypochlorite.

ADVANTAGES
1. Good surface detail
2. Reusable and easily sterilised
DISADVANTAGES
1. Need special equipment (water bath) and special technique
2. 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.

Alginate Impression Materials

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). The impression should be soaked in hypochlorite for 60 seconds and then cast as soon as possible.

1) An alginate impression of the upper arch in a special tray

2) An alginate impression of the lower arch

Properties of Alginates

CHEMISTRY
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

The above reaction occurs too quickly often during mixing or loading of the impression tray. It can be slowed down by adding trisodium phosphate to the powder. This reacts with the calcium sulphate to produce calcium phosphate, preventing the calcium sulphate reacting with the sodium alginate to form a gel.
This second reaction occurs in preference to the first reaction until the trisodium phosphate is used up, then the alginate will set as a gel.
There is a well-defined working time during which there is no viscosity change.
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

ADVANTAGES
1. Non toxic and non irritant
2. Good surface detail
3. Ease of use and mix
4. Cheap and good shelf life
5. Setting time can be controlled with temperature of water used
DISADVANTAGES
1. Poor dimensional stability
2. Incompatibility with some dental stones
3. Setting time very dependent on operator handling
4. Messy to work with

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