lecture 4 - impression materials

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Impression Materials

Are used to form replicas or copies of teeth and other oral structures. The impression is a negative reproduction while the model or cast is the positive reproduction. Objective is to copy accurately the teeth and other structures in the oral cavity.

Properties of Good Impression Material

Good flow property Good dimensional stability (dont shrink or expand after setting) Reasonable cost Easy to manipulate Biocompatible Should have adhesive property Compatible with the cast

Good storage life Palatable taste Appropriate setting time No toxic agents Produce creamy consistency Enough strength to withstand withdrawal from the patients mouth

Setting Mechanism

Impression materials can set by means of reversible or irreversible reactions. Irreversible Reaction implies that chemical reaction have occurred and that the material cannot revert to its present state, example: alginate, ZOE impression paste, impression plaster and elastomeric impression materials. Reversible materials softened under heat and solidify when they are cooled, with no chemical change taking place, example reversible hydrocolloid and impression compound.

Impression Trays

1. 2.

3.4.1. 2.


5. 6.

Are appliance used to reach the patients mouth Kinds of trays Stock Tray used for modelling compound for preliminary impression Perforated Tray used when impression materials do not adhere to the tray Rim-Lock Tray Individual Tray custom built; self-made, used in final impression for study cast Shellac base plate Type II modelling compound resin Water-Cooled used for agar impression Disposable Tray made of plastic

Impression Trays

Individual TrayStock Tray

Rim-Lock Tray

Perforated Tray

Disposable Tray

Types of Impression Taking

Single Impression Double impression

Preliminary impression Final impression/secondary wash

More accurate Corrects the defect of preliminary impression

Classification of Impression Materials According to Manner of Setting


Set with chemical reaction Irreversible Examples are soluble plaster, ZOE, alginate, irreversible hydrocolloid Set with change in temperature Reversible Examples are modelling compound, wax, agar


Classification On Impression Materials According to Mechanical Properties

Rigid/Inelastic (thermoset) Elastic

Rigid/Inelastic (Thermoset)

Used for taking secondary impression Used for edentulous patients (no undercuts) WP ratio is .50-.75 Examples: compound and wax Manipulation

Same as other gypsum products After mixing, pour on impression trays, tap a little to release air bubbles, then seat the tray inside the patients mouth.

Manner of Withdrawal: teasing movement


For edentulous patients (for undercuts) Hydrocolloid

Reversible agar Irreversible alginate Rubber impression material (elastomers)

Polysulfides, polyethers, silicone:condensation and addition

Manner of Withdrawal: sudden pull, snap jerk

Parallel to the long axes of the tooth

After impression taking, wash it in running water to remove saliva, blood (for patients with periodontal problem) and food debris

Presence of such substances can alter the accuracy of the cast

After washing, shake off excess water Optional; put some separating medium before cast construction: varnish, soapsuds, lacquer

Composition of Soluble/Impression Plaster

80% Plaster of Paris ( -hemihydrate) k2SO4 speed up setting time (3-5 mins.) Potato Starch Makes the plaster soluble) Helps in separating cast and impression material when submerged in hot water (soluble plaster swells when placed in hot water) Coloring Materials Helps in interpreting the impression Provides ease in reading the impression For easy identification of impression from cast Flavoring Material For palatability

Methods of Cast Construction1. 2. 3.

Boxing Method Inversion Method Rubber Base Former

Boxing Method

Place a 1 wax strip around the tray Pour a cast material on tray and level it to the wax Let it set, remove the wax and separate the cast after setting

Inversion Method

Pour cast material on tray Pour the excess on a glass slab or on the tiled working table Invert the impression tray on slab or table Scrape flowing cast material towards the tray to form the base

Rubber Base Former

Instead of inverting on a slab or table, invert on a rubber base former.

Properties of Good Cast

No porosities or no nodules (no bubble formation) No distortion: copy accordingly the given impression Dimensionally stable

Impression Compound

Also called modeling compound, is supplied in the form of sheets and sticks. This compound is softened by heat, inserted in an impression tray, and place against tissue before it cools to a rigid mass. Its primary indication for use has been making an impression of the edentulous ridge.

Types of Impression Compound

Type I

True impression compound Has high flow property For preliminary impression Tray compound More rigid For individual tray

Type II

Components of Modeling Compound

Beeswax and Thermoplastic Resin

Responsible for thermoplastic property Act as plasticizer which can improve workability

Shellac, Gutta Percha, Stearic Acid

French Chalk, Talc, Diatomaceous Earth Fillers

Hardening agents, improve the strength

Coloring Pigments and Flavoring Agent

Types of Modeling Compound

Cake Form Stick Form Cone Form

Uses of Modeling Compound

Cake Form: used for full arch impression during preliminary impression. Stick Form: used for single tooth impression known as impression tube with the se of copper band (matrix band). Cylinder in shape, open on both ends Come in different sizes Should fit the tooth properly Used to make individual tray construction Used to border molds Makes the rim higher Serve as a wedge material to hold the matrix in place: orangewood stick


Moist Heat Method Dry Heat Method

Moist Heat Method

Use of water bath 50-70C Get the modeling compound (cake form) Immerse in the water bath

Because of poor thermal conductivity or poor heat transfer, the outer surface softens while the inner surface remains hard

Knead with fingers to expose the inner layer Repeat until the modeling compound is homogenously soft inside and out.

Dry Heat method

Use of open flame for small amount of modeling compound (stick form) Modeling compound are heated to become soft and not to be melted

Important ingredients are lost during melting Overheating sparks indicate that some components (plasticizers) are leached out. If during heating, the modeling compound does not exhibit shiny surface, discard it (dullness indicates that the plasticizers are lost already)

Properties of Modeling Compound

Poor thermal conductivity Dimensional change: shrinks at 0.3-0.4% With good flow property Advantage enables us to get a more detailed and accurate impression Disadvantage if you fail to construct the cast immediately after withdrawing the impression from the patients mouth, the continuous flow property can be a source of error

Cast Construction and Separation

Wash in running water after withdrawal No need for separating medium Mix plaster of paris to make a study cast Immerse in a hot water bath

MC softens, if MC sticks to the cast, soften a piece of MC and allow it to come into contact with the melted MC.

Zinc Oxide Eugenol (ZOE)

The reaction between zinc oxide and eugenol yields a relatively hard mass that possesses certain medicinal advantages, as well as mechanical property benefits, for some dental operations. This type of material has been involved in a wide range of applications in dentistry, including use as an impression material for edentulous mouths, a surgical dressing, bite registration paste, temporary filling material, root canal filling material, cementing medium, and temporary relining material for dentures.

Uses of ZOE

Secondary impression for edentulous ridge Surgical dressing

After periodontal surgery, open wounds are covered with ZOE to allow healing (medicament covers the wound from debris)

Stabilizes occlusion rims Temporary filling material

When theres pain When time is not enough When there is rampant caries

Uses of ZOE

Temporary relining material for loose dentures

Dentures become loose because the bone resorbs Remedy: relining or rebasing depends on the extent of looseness


Relining resurfacing the tissue side of a denture in order to compensate for changes in the soft tissue occurring during the wearing of the denture and to achieve an accurate fit Use ZOE but temporary only because it is soluble by oral fluids Use Resin permanent reliner


Heating the entire denture base because of too much looseness

Uses of ZOE

Root Canal Sealer During root canal treatment, ZOE is used to cement gutta percha sticks inserted into the canal ZOE is a sealer or obtundant material Cementing Medium To cement crown preparations, onlays, orthobrackets Onlays with missing cusps, restorations that are fabricated outside the mouth, they must be cemented on the prepared cavity Inlays restorations are confined within the walls of the cavity 2 varieties

Weak for temporary cementing medium Strong for permanent cementing medium

Finished Preparation - MOD porcelain onla