IMPRESSION MATERIALS

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

(don’t 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 patient’s 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 Are appliance used to reach the patient’s mouth Kinds of trays1. Stock Tray – used for modelling compound for preliminary

impression2. Perforated Tray – used when impression materials do not

adhere to the tray3. Rim-Lock Tray 4. Individual Tray – custom built; self-made, used in final

impression for study cast1. Shellac base plate2. Type II modelling compound3. resin

5. Water-Cooled – used for agar impression6. Disposable Tray – made of plastic

Impression Trays

Stock Tray

Perforated Tray

Rim-Lock Tray

Disposable Tray

Individual 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

Thermoset Set with chemical reaction Irreversible Examples are soluble plaster, ZOE, alginate,

irreversible hydrocolloid Thermoplastic

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 patient’s mouth.

Manner of Withdrawal: teasing movement

Elastic

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 ( ß-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 Construction

1. Boxing Method

2. Inversion Method

3. 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

Type II Tray compound More rigid For individual tray

Components of Modeling Compound

Beeswax and Thermoplastic Resin Responsible for thermoplastic property

Shellac, Gutta Percha, Stearic Acid Act as plasticizer which can improve workability

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

Manipulation

Moist Heat Method Dry Heat Method

Moist Heat Method

Use of water bath 50-70°C 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 patient’s 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 there’s 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

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

Rebasing

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 onlay preparation

Uses Of ZOE Pulp Capping Agent

When there is excessive cavitation Depth of Cavitation Class A – depth is 0.2-0.5 mm beyond DEJ; ideal depth of cavity Class B – a little beyond 0.5mm Class C – no pulp involvement yet deeper than class B Class D – with pinpoint pulp involvement

Pulp capping is done on Class C and D For class C – use ZOE, indirect P.C. procedure For class D – not ZOE but a cement-based capping agent

(Calcium hydroxide) direct pulp capping

Uses of ZOE

Bite Registration Paste To stimulate the occlusal relationship between the upper

and lower Use of bite wax (yellow wax) and ZOE paste Seat the horse-shoe shaped wax on ZOE paste on the

mouth Allow ZOE paste to set Once set, transfer the bite to the cast

Heat Insulating Base Used to absorb heat coming from outside stimulus before

amalgam estoration

Composition of ZOE

Dispensed as two separate pastes. Tube 1 Universal/ Base Tube 2 Reactor/Catalyst/Accelerator

Tube 1 Universal/Base

87% Zinc Oxide 13% Fixed Vegetable or Mineral Oil

Acts as a plasticizer and aids in offsetting the action of the eugenol as an irritant.

Tube 2 Reactor/Catalyst/Accelerator Oil of Cloves

Contains 70 to 85% eugenol, produces less burning sensation for patients

Gum or Polymerized Rosin Facilitates the speed of the reaction and yields a smoother,

homogenous product Resinous Balsam

Used to increase flow and improve mixing properties Accelerators

CaCl2 – speed p setting Silica & Lanolin (fillers or hardening agent)

Increase the strength of ZOE Olive Oil

Dilute the content

Manipulation

Accomplished on an oil impervious paper or a glass mixing slab.

Squeeze two strips of paste of the same length, one from each tube, onto the mixing slab

Use stainless steel spatula for mixing procedure Combined the two strips of contrasting colors, with

the first stroke of the spatula, and mixing is continued for approximately 1 min, or as directed by the manufacturer, until uniform color is achieved

Put it on a tray and insert it to the patients mouth.