INELASTIC IMPRESSION MATERIALS

INTRODUCTION

IMPRESSION: A negative likeness or copy in reverse of the surface of an object; an imprint

of the teeth and adjacent structures for use in dentistry.(GPT8)

INELASTIC IMPRESSION MATERIALS: It is a group of materials that exhibit an

insignificant amount of elastic deformation when subjected to bending or tensile stresses.

CRITERIA OF A MATERIAL FOR ACCURATE IMPRESSION MAKING

1. Fluid enough to flow around area of interest and adapt to oral tissue

2. They should be viscous enough to be contained in the tray that is seated in the mouth.

3. Must “set” in reasonable amount of time. (Ideally the total setting time should be less

than 7 minutes)

4. Can remove from mouth without distortion

5. No harmful effects on tissues

6. Relatively tasteless & odorless

7. Dimensionally stable until a cast is created

8. Give detailed reproduction

9. Cost effective

10. Adequate shelf life for storage

HISTORY

PHILIP PFAFF- 1756- 1st described making impression with softened wax.

CHRISTOPHE FRANCOIS DELABARRE-1820- Introduced the Metal impression tray.

CHAPIN HARIS-1853-First used Plaster of Paris for making impressions.

CHARLES STENT-1857- Introduced the 1st Impression compound.

Inelastic material was used for soft and hard tissue impressions.

Hydrocolloids then were introduced to make impression of hard tissue.

After world war II – Elastomers – to make impression of soft and hard tissue.

CLASSIFICATION OF IMPRESSION MATERIALS

BY SETTING

REACTION

RIGID/INELASTIC ELASTIC

chemical reaction.

(irreversible)

Plaster of Paris ,

ZOE impression

paste.

(Edentulous ridge ,

inter-occlusal

records)

Alginate

hydrocolloid,

Non aqueous-

Elastomers

(Teeth and soft

tissue)

temperature

changes

(reversible)

Impression

compound,

Impression waxes

(Preliminary

impression)

agar hydrocolloid

(Teeth and soft

tissue)

INELASTIC IMPRESSION MATERIALS

1. Impression plaster

2. Impression compound

3. Zinc oxide eugenol paste

4. Impression waxes

IMPRESSION PLASTER

Type I gypsum product (ADA specification 25)

Setting reaction: β-Calcium sulfate hemi hydrate + Water = Calcium sulfate dihydrate.

Chemical reaction:

CaSO₄.2H₂O→ (110-130°C) →CaSO₄.1/2 H₂O (130-200°C) →CaSO₄ (Hexagonal

anhydrite)

Composed of POP+Modifiers(accelerators or retarders)

(Potassium sulfate +Borax+Coloring & Flavouring Agent.)

Muco-static impression material

Differences from dental plaster

Rapid setting

Smaller setting expansion

Lower strength

Potassium sulfate – to reduce expansion but accelerates setting.

Borax – a retarder to prolong setting time.

Diatomaceous earth, quartz, lime - to make the plaster more brittle.

Potato starch-Impression plaster containing potato starch to make it soluble(soluble

plaster). After cast has hardened on putting in hot water starch swell making cast

separation easy.

Rarely used - inability to sustain elastic deformation without fracture.

Restricted to final/wash impression in construction of full dentures.

Setting time: 4+1 minutes

Setting expansion: Minimum- 0.00

Maximum- 0.15

Compressive strength at 1 hour- 40+20 Kg/cm2

W:P Ratio: 0.45-0.75

The setting reaction is associated with expansion of 0.3-0.6%. When this is within the

confines of an impression tray, will lead to significant reduction in accuracy .Mixing

the plaster with anti-expansion solution (4% K₂SO₄ &0.4% Borax) will reduce this.

K₂SO₄ reduces expansion to 0.05% but this also accelerates the setting reaction &

borax is added as a retarder which gives more time for making impression.

Proportioning, mixing, and caring for impression plaster:

The water and powder should be measured by using an accurate graduated cylinder

for the water volume and a weighing balance for the powder. The powder should not

be measured volume (use of a scoop), because powder varies from product to product

and does not pack uniformly. Powder will pack harder as the container remains

unused. If the container is shaken, the volume increases as a result of entrapment of

air. Preweighed envelopes have become popular, because they promote accuracy,

reduce waste, and save time.

Mixing: If mixing is performed by hand, the bowl should be parabolic in shape,

smooth, and resistant to abrasion. The spatula should have a stiff blade and a handle

that is convenient to hold. Entrapment of air in the mix must be avoided to prevent

porosity, leading to weak spots and surface inaccuracies. The use of an automatic

vibrator of high frequency and low amplitude is helpful. Place a measured amount of

water in the bowl, and sift the weighed powder into the water as initial hand mixing is

performed. Then stir the mixture vigorously, periodically wiping the inside of the

bowl with the spatula to ensure the wetting of all of the powder and the breaking up of

any agglomerates, or lumps. Continue mixing until a smooth mix is obtained, usually

within a minute. The guesswork of repeatedly adding water and powder to achieve the

proper consistency must be avoided, because this yields a lower strength and it may

cause distortion and is one of the main causes of inaccuracy in the use of gypsum

products. The preferred method of mixing of impression plaster is to add the

measured water first, followed by gradual addition of the preweighed powder. The

powder is incorporated during approximately 15 sec of mixing with a hand spatula,

followed by 20 to 30 sec of mechanical mixing under vacuum.

Caring for the Cast:

If the surface of the cast is not hard and smooth when it is removed from the

impression, its accuracy is questionable. The cast should be an accurate reproduction

of the oral tissues, & any departure from the expected inaccuracy will probably result

in a poorly fitting prosthesis. Therefore the cast should be handled carefully. Once the

setting reactions in the cast have been completed, its dimensions will be relatively

constant thereafter under ordinary condition of room temperature and humidity. The

gypsum cast is slightly soluble in water. When a dry cast is immersed in water,

negligible expansion may occur, provided that the water is saturated with calcium

sulfate. If it is not so saturated, gypsum may be dissolved. If the stone cast is

immersed in running water; its linear dimension may decrease approximately 0.1%

for every 20 min of immersion. The safest method for soaking the cast is to place it in

a water bath made for the purpose, in which plaster debris is allowed to remain

constantly on the bottom of the container to provide a saturated solution of calcium

sulfate. If the storage temperature is raised to between 30° and 110°, shrinkage occurs

as the water of crystallization is removed and the dihydrate reverts to the hemihydrate.

The contraction of plaster at high temperature is greater than that of the stone, and it

also loses strength. It is not safe to store or heat a stone cast in air at a temperature

higher than 55°C (130°F). Gypsum surfaces made with thinner mixes appear to be

affected more than those with a low W/P ratio. The best means of storage is to seal the

product in a moisture-proof metal container, When gypsum products are stored in

closed containers, the setting time is generally retarded only slightly, approximately 1

or 2 min, per year. This may be counteracted by a slight increase in the mixing time if

necessary.

Advantages:

1. Cheap and long shelf life, easy to prepare

2. Excellent surface detail reproduction

3. Excellent dimensional stability

4. Rate of setting reaction can be controlled by the clinician.

5. Mucostatic.

Disadvantages:

1. Cannot be used for mucodisplacive impression materials.

2. Very rigid – often need to be fractured when removed from the mouth

3. Fractures if undercuts are present

4. Non-toxic but may dry soft tissues - unpleasant to patients

IMPRESSION COMPOUND

(ANSI/ADA SPECIFICATION NO.3)

Also called modeling plaster.

One of the oldest dental impression material and it is supplied as sheets, sticks, cakes

&cones

Softened by heat , inserted in impression tray , pressed against tissue before it hardens

and cools to a rigid mass.

Composition

Waxes

Thermoplastic resins

Filler

Coloring agent

Shellac, stearic acid and gutta percha

Classified : Type I – low fusing

Type II – high fusing

Indications

1. Making impression of edentulous ridge (tray compound).

2. In operative dentistry for impression of single tooth preparation (tube impression).

3. To stabilize matrix bands.

4. To border mold acrylic custom tray.

5. Viscous tray material is used for primary impression and then as tray to support

secondary impression material.

Properties of impression compound

1. Thermal property:

When impression compound is heated in a hot water bath, the material looses its hardness at

approximately 39°C. However at this stage it is still not plastic or soft enough for making an

impression. This temperature at which the material loses its hardness or brittleness on

heating or forms a rigid mass on cooling is called GLASS TRANSITION TEMPERATURE.

FUSION TEMPERATURE-On further heating at approximately 43.5°C ,the material softens

to a plastic mass that can be manipulated .This is called fusion temperature. It is the

temperature at which crystalline fatty acid melts (or solidify).

SIGNIFICANCE OF FUSION TEMPERATURE-

Above this temperature 43.5°C the fatty acids are liquid & lubricate the softened material to

form a smooth plastic mass, while the impression is being obtained. Thus all impressions

with compound should be made above this temperature. Below this temperature an accurate

impression can’t be expected.

SIGNIFICANCE OF GLASS TRANSITION TEMPERATURE

Once the impression tray is seated, it should be held firmly in position until first fusion

temperature & later Glass transition temperature is reached , thus the impression is made

above the fusion temperature where as it is removed from the mouth only after it cools down

to its Glass Transition Temperature.

2. Thermal conductivity

Impression compound has very low thermal conductivity -they are poor conductors of heat.

SIGNIFICANCE-During softening of the material the outside will soften first & the inside

last, so to ensure uniform softening compound be kept for sufficiently long time in a water

bath. Kneading of the material ensures further uniform softening. Due to poor thermal

conductivity, the layer adjacent to the mouth tissues will remain soft ,removal of which will

cause distortion .Thus it is important to cool the compound thoroughly before removing it

from the mouth.

3. Coefficient of linear expansion

Comparatively high due to presence of resins & waxes .The linear contraction from mouth

temperature to room temperature is 0.3%-0.4%.

4. Softening of impression compound

In an oven

Over a flame

Water bath

When direct flame is used, the compound should not be allowed to boil or ignite so that the

constituents are volatilized. Prolonged immersion or overheating in the water bath is not

indicated; the compound may become brittle and grainy if some of the low molecular weight

ingredients leach out of the material.

Softening of the compound in a warm water bath is the method recommended for separating

the cast from a compound impression.

5. Flow

Good flow is desirable in impression making. The softened material should flow in to all the

details of the tissue contour. Once the compound hardens it should have minimum flow lest it

gets distorted.

Type I- flow not more than 3% at 370C

Type II- flow not more than 2% at 370C

6. Dimensional stability

Since the release of strains is unavoidable, the safest way to prevent distortion is to pour the

cast immediately or at least with in the hour. Another cause of warpage is the removal of the

impression too early before it is thoroughly cooled in the mouth.

7. Disinfection: 2% alkaline glutaraldehyde solution.

Advantages

1. Non irritant & nontoxic

2. Reusable.

3. Can be reheated & readapted

4. Can support other materials for wash Impressions.

5. Easy manipulation.

6. Low cost

7. Good dimensional stability but excessive water incorporation during wet kneading

can cause dimensional change

Disadvantages

1. Highly viscous

2. Can displace tissue surface

3. It does not record surface details very accurately due to poor dimensional stability.

4. Will distort if removed from undercuts

ZINC-OXIDE EUGENOL IMPRESSION PASTES

(ANSI / ADA SPECIFICATION No.16)

Zinc oxide & eugenol are widely used in dentistry.

Under proper conditions the reaction between ZnO & Eugenol yields a relatively hard

mass that has certain medicinal advantages as well as mechanical property

benefits ,for some dental operations.

Wash impression for edentulous jaws.

Applications in dentistry:

1. Impression material for edentulous mouth

2. a surgical dressing

3. bite registration paste

4. temporary filling material

5. Root canal filling material

6. cementing medium

7. Temporary relining material for dentures.

Classified as

Type I – hard paste (Final setting time10 min. Initial setting time 3-6 min.)

Type II – soft paste (15 min. 3-6 min.)

INITIAL SETTING TIME- Period from the beginning of mixing until the material ceases to

pull away or string out when its surface is touched with a metal rod.

FINAL SETTING TIME: When a needle of specified dimension fails to penetrate the surface

more than 0.2mm,under a load of 50 gms.

Setting time :

Shortened - adding small amount of accelerator

- drop of water

Prolonged - cool mixing slab

- plasticizer – inert oil or wax

Dimensional stability

- Less than 0.1% shrinkage

Manipulation

The mixing of the two pastes is generally accomplished on an oil-impervious paper or a glass

mixing slab.

The proper proportion of the two pastes is generally obtained by squeezing two strips of

paste of the same length, one from each tube, onto the mixing slab.

A flexible stainless steel spatula is typically used for the mixing procedure.

The two strips of contrasting colors are combined with the first stroke of the spatula, and the

mixing is continued for approximately 1 min, or as directed by the manufacturer, until a

uniform color is achieved.

Setting reaction

The setting reaction is a typical Acid-Base reaction to form a chelate.The reaction is called

chelation & the product is Zinc Eugenolate.

Factors controlling setting time

1. Particle size of ZnO powder- If the particle size is small & if it is acid coated, the

setting time is less.

2. Setting time can be decreased by adding zinc acetate or a drop of water or acetic acid.

(acetic acid increases speed of formation of the zinc hydroxide.)

3. Longer the mixing time ,shorter is the setting time .

4. High atmospheric temperature & humidity accelerates setting.

5. Setting can be delayed by cooling the mixing slab, spatula or adding small amounts of

retarder or plasticizer (wax/inert oil)

Disinfection

The recommended disinfectant solution for ZOE impression paste is 2% alkaline

glutaraldehyde solution. Phenolic spray can be used.

Non-eugenol pastes

Eugenol leaches out causing burning sensation on contact with soft tissues

ZO + CARBOXYLIC ACIDS = ZOE – LIKE MATERIAL

Orthoethoxybenzoic acid (EBA) - substitute for eugenol

Bactericidal agents and other medicaments

Surgical pastes

After gingivectomy aid in the retention of a medicament and promote healing

Softer and slower in their setting reaction

Bite registration pastes

ZOE inter-occlusal record established are more stable than one made in wax

IMPRESSION WAXES

Also called bite wax or corrective wax.

Distort when withdrawn from undercut areas.

Limited for use in edentulous portions of mouth or in occlusal surface areas.

Corrective wax are soft at mouth temperature and register the details of the soft tissue

and are rigid at room temperature.

Corrective Impression Wax

Used to record Posterior palatal seal in dentures.

Functional impression for obturators.

It is used as a wax veneer over an original impression to contact & register the details

of the soft tissues.

To make functional impression of free end saddles(Class 1 &2) R.P.D.

Consists of paraffin, ceresin, beeswax, metal particles.

Flow at 37 0C is 100%.

Should be poured immediately as these waxes are subjected to distortion during

removal from the mouth.

Bite Registration Wax

Composition- Beeswax, paraffin, ceresin, aluminium or copper particles

Supplied as: U-shaped rods or wafers. One side of the wax is usually covered with foil

Manipulation: The wax is softened in warm water. The soft wax is then placed

between the teeth & the patient is asked to bite. The wax is then taken out & placed in

chilled water. The cast of the patient is placed in the indentation formed by the teeth.

It is then mounted with plaster on the articulator.

Uses- It is used to record the relationship between the upper & lower teeth. This is

necessary in order to mount the casts correctly in the articulator.