Impression Materials: Plaster of Paris, Alginates, Hydrocal, Elastomers and much more. The name originated during the Renaissance, when a good sculpture plaster was obtained from gypsum mined in montmartre, Paris. Uses Impressions,especially of edentulous mouths.
Impression Materials: Plaster of Paris, Alginates, Hydrocal, Elastomers and much more
Plaster of Paris
The name originated during the Renaissance when a good sculpture plaster was obtained from gypsum mined in Montmartre, Paris.
Uses
- Impressions, especially of edentulous mouths.
- Model of teeth for reference purposes, and for construction of dentures, crowns, bridges, and orthodontic appliances.
- In refractory investment materials used during the casting of inlays and crowns.
Technique
Plaster is slowly sifted on the surface of water contained in a bowl.
This is continually tapped against the bench, causing water to be taken up by the powder and air-bubbles to be expelled. Plastic is added until no free water remains. Gentle spatulation would incorporate air-bubbles which cause inaccuracies. Insufficient powder leads to delayed setting and weak plaster. If an impression breaks during removal from the mouth the pieces are rejoined with model cement (sticky wax).
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Alginates (Hydrocolloids)
There are many varieties of alginate that can be purchased but all consist of powders to be mixed with water. It is most important to carry out the manufacturer's instructions carefully.
What are the uses of Alginates?
The main uses are:
Impressions for partial dentures, orthodontic appliances, and splints.
Technique
A measured quantity of powder is spread on to the surface of a measured volume of water and the two are mixed to a homogeneous paste. A long bladed, flexible spatula, allows a firm pressure against the side of the plastic or rubber bowl, in which mixing takes place.
if the powder is left in a damp atmosphere it becomes wet and solidifies. Therefore the packet must always be kept tightly closed.Retention in the tray is by using:
- Perforated trays, some material passing through the holes to rivet the impression into position.
- Molten sticky(yellow) wax.
- Cotton wool fibers stock on the tray.
- Quick drying adhesive.
- Elastoplast.
Elastomers (Rubber polymers)
There are two types of rubber like polymers (elastomers) are used as impression materials.
- Polysulphides (rubber base, Thiokol).
- Silicones.
What are the uses of Elastomers (rubber polymers)?
Impressions for crowns, bridges, inlays, and partial dentures where deep undercuts exist.
What are the general properties of elastomers?
- Very accurate for recording detail.
- Little contraction with a setting.
- Tougher than alginate.
- Non-toxic or poisonous.
- Viscosity varies with the composition of a material.
- Very elastic, thus less chance of breakage when withdrawn from undercuts.
What are the properties of Polysulfides?
- May be obtained with a diluent, to produce a thinner viscosity at the chairside
- An odor of pastes containing lead dioxide is unpleasant.
- Supplied as two pastes.
The accelerator of lead dioxide (brown), plus sulfur and oil.
4. May be obtained with a diluent, to produce a thinner viscosities:
a. Light bodied for injection by syringe.
b. Heavy bodied for using in a tray.
What are the Properties of silicones?
- Sold as two pastes, or a paste and a liquid.
- Viscosity varies with the material:
b. Puttylike for a tray.
Technique for elastomers
Usually mixed with a broad-bladed spatula on a waxed paper pad for about 45 seconds, until no streaks remain, in a similar way to zinc oxide eugenol pastes. Rubber solution is used to hold the material in the tray. Patients lips are coated with vaseline.
Zinc oxide and eugenol impression pastes
These are not elastic and so it does not record undercuts accurately. The setting is due to a change instead of due to heat.
No separating medium is required prior to casting.
What are the uses of zinc oxide and eugenol impression pastes?
- Impression for relining full dentures. Used as a thin wash inside the denture, which is the first cutaway on the fitting surfaces.
- Accurate impressions for full dentures are taken inside a preliminary composition impression, a base plate of the bite block, or old denture.
- Which wisps of cotton wool as a periodontal pack.
- Technique
No separating medium is required prior to casting.
Hydrocal
Chemically it is very similar to plaster of Paris but differs physically. It is much stronger and harder and is thus suitable for models.
Casting models from impressions:
Impression provides a negative picture of the dental structures. To construct a positive model, Hydrocal or plaster of Paris is poured into the impression. It is gently mixed and slowly runs into one side of the impression, gently vibrating the tray to ensure full contact between plaster and impression. Unless great care is taken at this time air bubbles will be trapped with loss of accuracy.
In the case of a plaster impression, a separating medium is applied before adding the model plaster, which would otherwise stick to it. Substances used are dilute water glass, collioidin varnish, and soap solution.
After the model has set the impression material is removed; plaster is chipped away, composition and zinc oxide paste are heated, and the others are cut away. Finally, the model is trimmed to ensure a neat finish.
Impression compounds
Whereas plaster of Paris hardens by chemical means, compositions are thermoplastic. They soften with heat and harden upon cooling to mouth temperature. As this property is retained the material can be re-used.
These are divided into three groups
Group 1
Little or no elasticity, so do not reproduce undercuts.
Uses
Copper ring impressions for inlays and crowns.
Functional or compressive impressions for dentures.
Group 2
Sufficiently elastic to reproduce two-thirds of an undercut.
Uses
Impressions of mouths with small undercuts.
Group 3
Relatively tough
Uses
Support for other materials, as special trays.
The technique of impression compounds
They are softened in hot water. Gauze or cloth around the inside of the bowl prevents the material from sticking to it. The composition should be thoroughly kneaded with the fingers. The softened material is placed in a warm dry tray and roughly shaped to the dental arch.
The surface is then slightly vaselined; passed through a flame to make it sufficiently soft, and an impression is taken. After cooling to mouth temperature it is removed and placed in cold water to harden it finally, thus reducing the possibility of distortion.
For crowns and inlays, the softening composition is used in a copper rind instead of an impression tray
Plaster is poured directly into the composition, no separating medium is needed. The composition is removed from the model by softening water.
Other impression materials:
Gutta-percha
A thermoplastic and rubbery material.
Uses of Gutta-percha
- The lining of tissue borne.
- Root canal filling points
- Temporary fillings
- Temporary denture relining material
- Impression for cleft palate plates.
Inlay wax
This material is softened over a flame. It hardens at mouth temperature. This enables it to be carved accurately with fine margins in the mouth. Although hard enough for the wax pattern to be withdrawn from the cavity, it sufficiently brittle to break rather than bend, thus preventing inaccuracy. The color contrasts with the tooth tissues and is usually blue.
Uses
Accurate direct impressions for gold inlays and crowns.
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