IMPRESSION MATERIALS

First put your patient in a hot baking oven or a searing furnace, then

turn them round and round in a centrifugal machine and pour hot

metal on them. Or better still, heat them above 1000°C in a ceramic

furnace and fine till well done. Don’t look shocked, - not a very

pleasant idea right. Well, that what impression materials same us

from.

Let’s confabulate on this topic under these myriad subheadings:

Introduction

History

Definition and Classification

Ideal requisities

Impression materials and Respective techniques

- Hydrocolloids.

- Elastomeric materials.

- Inelastic or rigid.

Latest Advances

Summary and conclusion

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Bibliography

INTRODUCTION

One of the cornerstones of modern dental practice is the accurate

recording and reproduction of tooth tissue details. Material science

plays a pivotal role in dentistry and impression materials form a vital

core in that foundation. Mimicking the intricate details required in

dental practice in the demanding and challenging oral conditions is an

acid test for any material. Right from the advent of initial crude

impression materials to the modern sophisticated elastomeric

chemistries the quest for the perfect replicating material has spanned

a diverse range of materials, techniques and devices. From dentures to

implants, inlays to crowns, orthodontic to pedodontic to

prosthodontist to restorative and endodontists, there is no branch of

high quality dentistry untouched by the magic of these materials and

no dentist who has not marveled at their ingenuity. Let in delve

deeper into this fascinating and vitally important class of material

science and understand them for their successful and ideal use.

An impression, in general terms is a mark produced on a surface by

pressure. The word impression is divided from latin word

“impression”.

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From impression, it is possible to produce an exact replica of

the dental structures of interest using a cast or die material such as

dental stone or some type of plastic.

There are various categories of impression materials. Each type

processes characteristics which influence the purpose to which it is

best suited, i.e. different types of impression materials have been

developed few different application. These materials have different

physical properties and each has certain advantage or disadvantages.

Thus, an understanding of physical characteristics and

limitation of each material is necessary for its successful use in

clinical dentistry. It would be virtually impossible to perform high

quality restorative dentistry without impression materials.

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History:

The history of dentistry has been influenced a great deal by

development of impression materials. From the cumbersome and

highly unpredictable materials of yore replicating materials now

match highly exacting standards.

In fact the earliest dental impression materials was waxes that

was used in 18 th and 19th century. Beeswax was apparently the

materials first used in making impression in the mouth.

In 1782 William Rae said that he get the measurement of jaws

in a piece of wax pushed into the gums, afterward making a cast of it

with plaster of paris.

In 1842, Montgemery discovered gutta-percha. In 1848 Colburn

or Blake said that it should thoroughly soaked in boiling water, then

kneaded and moulded in the same way as wax and immediately by

placed in the mouth and firmly pressed to its place.

In 1930 according to Applegate, a series of true physiologic

waxes was developed by cooperative effort of Drs. G.C. Bawles, S.G.

Applegate.

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The first real impetus in the use of the zinc oxide eugenol for

impression materials came from two pioneer dentists, A.W. Ward and

E.B. Kelly, during early 1930’s.

Alginate type materials were experimented with over many

years, however, the first patient was awarded in 1936. This in early

1940’s the first irreversible hydrocolloidal (alginate) impression

material was developed.

In middle 1950’s the elastomeric impression materials were

introduced. Polyethers were introduced to dental profession in

Germany in late 1960’s.

Recently, a new polyether urethane diamethacrylate photo-

initiated elastomeric impression material has entered the market.

Definition and Classification:

Impression material is a dental materials whose function is to

accurately record the dimensions of the oral tissues and their spatial

relationship.

An impression essentially is a negative replica of some

structure. In dentistry, this replica usually is made of teeth or gingival

tissue of maxillary such as mandibular arch.

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Classification:

Impression Materials:

Elastic Non-elastic

Hydrocolloid Rubbers ZOE Impression compound

Agar Alginate

Mercaptam Silicones Polyether

Lead peroxide Clean Condensation addition

Catalyst catalyst

Many criteria may be used to classify impression materials.

They are listed as follows:

1) By their generic chemical same:

For example, one may refer to silicone materials or zinc oxide

eugenol materials or even particular commercial brands of these

materials.

2) According to the manner in which they harden:

A) Set by chemical reaction (Irreversible)

Plaster of paris

Zinc oxide eugenol.

Alginate

Non-aqueous elastomers.

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B) Set by temperature change (reversible)

These materials are again subclassified as:

1) Thermoplastic materials.

- Impression compound.

- Wax.

2) Non-thermoplastic material and agar

3) According to the ability of set material to be withdrawn over

undercuts.

A) Elastic impression materials:

- Alginate.

- Agar.

- Non-acqueous elastomers.

B) Non-elastic or rigid impression materials:

- Impression compound.

- Impression plaster.

- Zinc oxide eugenol.

- Wax.

4) According to the use of material in dentistry:

A) Materials used for obtaining impression of dentulous

mouth.

- Alginate.

- Agar.

- Non-acqueous elastomers.

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These materials are used in crown and bridge, partial denture

and inoperative dentistry.

B) Materials used for obtaining impression of edentulous

mouth:

- Impression compound.

- Impression plaster.

- Zinc-oxide eugenol.

- Wax.

5) According to the viscosity or the tissue displacement:

Materials which are initially vary fluid are often

classified as mucostatic impression materials, because they are

less likely to compress soft tissues, while materials which are

initially more viscous are classified as mucocompressive.

A) Mucostatic materials:

- Impression plaster.

- Agar.

- Alginate.

B) Muco-compressive materials:

- Impression compound.

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Ideal Requirements of Dental impression materials:

To produce an accurate impression, the materials used to

produce replicas of intraoral and some extraoral tissues should fulfill

the following criteria.

1. Pleasant taste, odor and esthetic color.

2. Not contain any toxic or initiating ingredient.

3. Have adequate shelf life for storage and

distribution.

4. Be economical.

5. Be easy to use with minimum equipment.

6. Have adequate setting characteristics that meet

clinical requirements.

7. Possess satisfactory consistency and texture.

8. Adequate strength so that it will not break or tear

while removing from the mouth.

9. Possess elastic properties with freedom from

prominent deformation after strain.

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10. Exhibit dimensional stability over temperature

and humidity ranges.

11. Readily wets oral tissues.

12. Compatibility with cast and die materials

13. Accuracy and faithful reproduction of details.

14. Ability to be electroplated.

15. Readily disinfected.

16. No release of gas during setting of impression or

cast and die materials.

- They should be fluid enough to adapt to the

oral tissue and viscous enough to remain content in the tray that

delivers impression to the mouth.

- While in the mouth they should transform

(set) into a rubbery solid in a reasonable amount of time

(should be less than 7 minutes).

- The set impression should not distort or tear

when removed from the mouth. Material should dimensionally

stable so the cast can be poured.

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- In this seminar I will be discussing about

the following impression material:

Hydrocolloids

Reversible Irreversible

1) Hydrocolloids – It is a suspension of time part less the 1µm).

Colloids are often classified as a faster state of matter, the

colloidal state, because of their difference in structure, constitution

and reaction. If the particles are large and can be seen by the naked

eye as through a microscope, the system is turned a suspension or

emulsion. There suspended particles do not readily diffuse and tend to

fall out of the suspending medium unless some type of bonding is

employed to maintain the suspension or emulsion.

The molecules of the colloid remain dispersed nature of fact

that they carry small electrical changes and repel one another within

the dispersion medium.

Types of colloids:

With the exception of the gaseais state (two gases), colloidal

sol may be composed of combination of any other states of matter.

For example:

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- Liquid / solid in air (aerosol).

- Liquid / solid in liquid (Lysol).

- Gas /liquid / solid in solid.

All solids are termed as sols, and not just more in which a

liquid is the dispersion medium. Since hydrocolloid impression

materials are solids (polysaccharides) suspended in liquid (water)

they are lyophilic (liquid leaving) solutions. In general, organic

colloids are lyophilic, whereas the metallic dispersion tend to

lyophobic (liquid heating).

Dental hydrocolloid impression materials exist in two form: sol

or gel form. In the sol form they are fluid with low viscosity and there

is random arrangement of polysaccharide chains. In gel form the

materials are more viscous and may develop elastic properties if the

long polysaccharide chain become aligned.

Gels:

If the concentration of the dispersed phase in the hydrocolloid

is proper amount, the sol may be changed to a semisolid material

known as a gel or jelly when the temperature is decreased. The

temperature at which this change occurs is known as gelation

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temperature is about 37°C as slightly higher few agar. The process is

reversible. Thus agar is known as reversible hydrocolloids.

Reversible hydrocolloids:

These impression materials are compounded from reversible

agar gels, when heated, they liquefy argo into the solution state and

on cooling they return to gel state. Since this process can be repeated,

a gel of this type is described as reversible.

Agar impression are dimensionally unstable on standing and so

models should be made so soon as possible after the impression is

taken. If the agar type of impression material is used carefully with an

understanding of its physical properties it is an excellent elastic

impression material of high accuracy in registering fine detail.

Chemical Ingredients:

The main active constituent of a reversible hydrocolloid

impression product is agar-known commercially as agar-agar, which

is a sulfuric ester of a galactan complex. This material forms a colloid

with water which will liquefy between 71°C and 100°C and set a gel

again between 30°C and 50°C.

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Composition and function:

1) Agar (12.5%) – To prevent the dispersed phase of the solution

and the continuous fibril structure of the gel.

2) Potassium sulfate (1.7%) – To counteract the inhibiting effect

of beewax and agar on the setting of gypsum model materials.

3) Borax (0.02%) – To produce intermolecular attraction in order

to improve the strength of the gel.

4) Alkyl Benzoate (0.1%) – To prevent the growth of the mold in

the impression material during storage.

5) Water (85.5%) – To provide the continuous phase in the

solution and second continuous phase is the gel, the amount

centrals the flaws properties of the solution and the physical

properties of the gel.

6) Colour and flavour (trace) : To improve the appearance and

taste.

The agar content is reduced in the syringe type of materials so

that it is much more fluid at the time of injection than the tray

material at the time of insertion.

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Properties:

1) Viscosity of the sol:

In sol form agar is sufficiently fluid to allow detailed

reproduction of hard and soft tissues. Its low viscosity classifies it as

a mucostatic materials as it does not compress or displace soft tissues.

Agar is a visco-elastic material.

2) Strength:

The strength values of importance for agar impression are tear

strength and the compressive strength. Since the agar impression are

viscoelastic the strength preparation are time dependent and higher

compression and tear strong occur at higher rates of loading.

The compressive strength of a typical agar impression material

is 8000gm/cm2 (0.245Mpa or 35.6Psi). The tear strength is about

700gm/cm though there is no ANSI/ADA specification requirement.

3) Gelation temperature:

The temperature at which the hydrocolloid impression material

sets to a gel is important. The gel must be heated to a higher

temperature. Known as liquefaction temperature (70°C to 100°C) to

return to its sol condition.

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It transforms into a gel between 37° and 50°C. The exact

gelation temperature depends on several factors, including the

molecular weights, the purity of agar and the ratio of agar to other

ingredients.

4) Dimensional stability:

Storage condition

Dimensional change

Causes

1) Air Shrinkage Evaporation of water foam gel

2) Water Expansion Inhibition and absorption of water

3) 100% relative humidity

Shrinkage Syneresis

4) Inorganic salt solution

Expansion / Shrinkage

Depends on relationship of electrolyte in gel and solution.

When stored in air, agar gels losses water and contract, when

they started in water it results in absorption and swetting. So agar

impression are best stored in 100% relative humidity not more than 1

hour.

5) Flexibility:

The ADA specification requirement for flexibility allows a

range of 4% to 15% when a stresses of 14.2 psi (1000gm/cm 2) and

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most agar impression materials meet this requirement. A few hard

setting materials have a flexibility of 1% to 2% agar gel has very poor

mechanical properties and tears at very low levels of stress. Inter

proximal and subgingival areas are very difficult to record with this

type of impression materials.

Advantages:

17. Hydrophilic impression materials since it

respondless critically to moisture, fluids in the sulcus are less

tolerated.

18. Long working time.

19. It does not require any custom tray.

20. Because there is no mixing of separated

components, the potential for errors of measurement are

eliminated.

21. It is clean and pleasant.

22. It is compatible with die stone, enabling bubble

fice casts.

23. It is a material of high accuracy and registers

fine detail.

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24. Casts are easily removed.

25. Reliability.

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Disadvantages:

1. Low tear resistance.

2. Low dimensional stability and the special

handling to prevent dimensional change.

3. Potential hazard of a bown to the patient.

4. Initial purchase of the conditioning unit.

5. Easily distorted as a result of movement

during gelation.

6. Rapid cooling can cause concentration of

stresses near the tray during gelation.

Clinical Presentation:

The agar impression material is supplied as a gel in a metal,

plastic or other types of collapsible disposable or as a number of

cylinders in a glass jar. The first form is used with a water cooled

impression tray and the second for injector with a syringe. The

syringe material may be used in combination with a tray material or a

copper-band technique as with impression compound.

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Types of failure:

Type Cause

1. Grainy material. A. Inadequate boiling.

B. Storage temperature too low.

C. Storage time too low.

2. Seperation of tray and syringe material

A. Water-soaked layer of tray material not removed.

A. Gelation of either syringe or tray material.

3. Tearing A. Inadequate bulk.

B. Premature removal from mouth.

C. Syringe material partially gelated when tray seated.

4. External bubbles A. Gelation of syringe material preventing flaw.

5. Irregularly shaped voids A. Material too cool or grainy.

Laminate technique (Agar alginate combination impression):

In a recent modified procedure, the tray hydrocolloid is

replaced with a mix of chilled alginate that will bond with the syringe

agar. The alginate gels by a chemical reaction, where the agar gels by

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means of contact with the cool alginate rather than the water

circulating through the tray.

The equipment needed for taking an agar impression can be

minimized by use of agar alginate syringe tray combination

impression. In this procedure a syringe type of agar in a cartridge is

heated in boiling water for 6 minutes and stored in 65°C water bath

10 minutes before use. The tray alginate of the regular set type is

mixed with 10% more water than normally recommended and it is

placed in a tray. The agar is injected around the preparation, and the

mixed alginate is promptly seated on the top of agar. The alginate sets

in about 3 minutes and agar sets with this time as a result of being

cooled by alginate. During the setting of alginate and gelling of the

agar a bond forms between them. The impression may be removed in

about 4 minutes.

The accuracy of the agar-alginate impressions was determined

with a laboratory model. Impressions were taken and paired in high

strength stone. The accuracy of 1) The interpreparation distance, 2)

buccolingual diameter and 3) the preparation height of the models

were measured and compared to values obtained with polysulfide

condensation silicone, polyether and addition silicone impression

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materials. Except for the interpreparation distance the agar-alginate

system had same order of accuracy as rubber impression materials.

The advantage of agar-alginate combination impression

compared to agar system alone is the simplification of the heating

equipment, the elimination of water cooled impression trays and the

overall simplification of the procedure. In addition, the agar is more

compatible with gypsum model materials than alginate, the accuracy

is acceptable and the cost of the material is low.

Wet field technique:

Another recent technique has become popular for making

impression in a wet field. It differs in than the tooth surface and tissue

are purposely left wet. The areas are actually flooded with warm

water. Then the syringe material is introduced quickly, liberally and

in bulk to cover the occlusal and / or incisal areas only. While the

syringe material is still liquid, the tray material is seated. The

hydrolic pressure of the viscous tray material forces the fluid syringe

hydrocolloide down into the areas to be restored. This motion

displaces the syringe material, blood and debris with the stronger tray

material throughout the sulcus.

Irreversible hydrocolloide:

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At the end of the last century, a chemist from Scotland noticed

that certain brown seated (algal). He named it algin. This natural

substance was later identified as a linear polymer with numerous

carboxyl acid groups.

When agar impression material become scarce because of

World war II (Japan was a pumice scarce of agar), research was done

to find a suitable substitute. The result was of cause the present

irreversible hydrocolloid, or alginate.

Alginate is an elastic mucostatic impression material. It is more

widely cured than any other elastic impression material. The principle

factors responsible for the success of this type of impression material

are:

1. Early to manipulate.

2. It is comfortable to the patient.

3. It is inexpensive.

4. Does not require elaborate equipment.

Uses:

1. Used widely in complete and partial denture prosthesis and

orthodontics.

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2. To prepare study models of either the entire dental arch or a

segment of it.

3. For master impression in rigid or in divided trays.

4. Whenever there is undercut and not suitable for rigid materials.

5. Useful to a limited extent in inlay, crown and bridge

procedures.

6. In mouth where there is excessive flow of saliva.

7. As a duplicating mateial.

8. To prepare gypsum models of patients for the preparation of

alternate in mouth protectors.

Composition:

Ingredients Function

1. Potassium alginate 18% To dissolve in water and react

with calcium ions.

2. Calcium sulfate dehydrate 14% To react with potassium alginate

to form an insoluble calcium

alginate gel.

3. Potassium sulfate, potassium

zinc fluoride silicate or borates

10%`

To counteract the inhibiting effect

of the hydrocolloids on the setting

of gypsum, giving a high quality

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surface to the die.

4. Sodium phosphate 2% To react preferentially with

calcium ions to provide working

time before gelation.

5. Diatomaceous earth or silicate

powder 56%

To control the consistency of the

mixed alginate and the flexibility

of set impression.

6. Glycols-small amount To make powder distlys.

7. Winter gum, peppermint trace To produce a pleasant taste.

8. Pigments – Trace To provide colour.

Change in the water / powder ration will alter the consistency

and setting times of the mixed material. Mixing time for regular set is

1 minute. Fast set alginate should be mixed with water for 45 seconds.

According to ADA specification No 18 for alginate impression

material requires that it no less than 1.25 minutes.

Flexibility:

The ANSI/ADA specification permits a range of 10% to 20% at

a stress of 1000gm/cm2. The compressive strength ranges from 5000-

8000gm/cm2.

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ADA specification requires a compressive strength of atleast

3500gm/cm2. The tear strength vary from 350 to 600gm/cm.

Types of failure:

Type Cause

1. Grainy material a. Improper mixing.

b. Prolonged mixing.

c. Undue gelation.

d. Water / powder ratio too low.

2. Tearing a. Inadequate bulk.

b. Moisture contamination.

c. Premature removal from mouth.

d. Prolonged mixing.

c. Bubbles a. Undue gelation, preventing flow.

4.Irregularly shaped voids a. Air incorporated during mixing.

b. Moisture or debris on tissue.

5. Rough or chalky stone cast. a. Inadequate cleaning of impression.

b. Excess water left in impression.

c. Premature removal of cast.

d. Learning cast in the

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impression too long.

6. Distortion a. Impression not paired immediately.

b. Movement of tray during gelation.

c. Premature removal from mouth.

d. Improper removal from mouth.

e. Tray held in mouth too long.

Development in alginate:

1) Dustless alginate:

Many materials have been formulated which give off

little or no dust particles, so avoiding dust inhalation. This can

be achieved by coating the material with a glycol / glycerine.

2) Chromatic alginates:

During the setting reaction, the pH value of the fluid

mass changes. Because of this, some manufacturers include

acid/base indications in their formulation so that a colour

change of the setting mass indicates that a certain point has

been reached, usually the point at which the tray should be

loaded or inserted into the mouth.

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3) Modified alginate:

The traditional alginate is supplied as powder and water.

There is no reaction until the water is added to the powder to

initiate the reaction.

There is yet another modification of this concept. The

two component system may be in the form of two pastes. One

certain alginate sol, while the second contain the calcium react

or impression materials of this type are said to contain (9/50)

silicone and humeetants to stabilize. It is said to be better than

water / powder alginate.

4) Silicone alginates:

Alginate modified by the incorporation of silicone

polymers have been developed. There are supplied as two parts

which are mixed together. The materials are considered as

hybrids of alginates and silicone elastomers but their properties

are closely related to more of alginates.

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Next we come to the most important categories of impression

material:

Elastomers or rubber dam impression materials:

Non-aqueous elastomeric dental impression materials as per ADA Sp.

No. 19 are liquid polymers that cross link or polymerize with various

reagents to become solid elastic rubber at room temperature. They are

essential in today’s high teeth dental age of metal free ceramics and

high precision castings.

Rubber impression materials:

Three major types of rubber impression materials are used to

record dental impressions. They are polysulfides, silicones (poly

siloxanes) and polyethers. The silicone type is subdivided into two

classes, condensation and addition or vinyl polysiloxane.

Polysulfide:

These materials are supplied in three consistencies:

1. Low (syringe or wash).

2. Medium (regular).

3. High (tray).

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These are supplied as two pastes in collapsible when, one

labeled base and the other labeled accelerator or catalyst.

Base:

- Polysulphide polymer – 80-85% weight.

- Titanium dioxide, zinc sulfate, copper

carbonate or silica – 16-18% weight.

Accelerator:

- Lead dioxide – 60-68%.

- Dibutyl or dioctyl – 30-35%.

- Sulfur – 3%

Other substances such as Magnesium stearate and deodorants

– 2%

Properties:

1. The elastic properties of these rubber impression materials

improve with curing time (i.e. the longer the impression can

remain in the mouth before removal, the greater the

accuracy).

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2. Polysulfide ranks as one of the best stiff of elastomeric

impression materials.

3. They have the highest resistance to tearing (thin sections of

polysulfide impression material are less likely to tear than

similar thickness of polyether or silicone impression

material).

4. The stone should be poured immediately because, the

impression in the most accurate immediately after removing

it from mouth.

5. The ADA specification for testing biocompatibility includes

dental impression materials, despite the fact that the

probability of allergic or toxic reactions from impression

materials or their components is small.

6. Perhaps the most likely elastomer induced biocompatibility

problem occurs when a piece of the impression material is

left in the gingival sulcus. The irritation can range from

minor to severe. The radioopacity of the load containing

polysulfide materials is an advantage in these situations, as

in the materials resistance to tearing.

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7. One way to minimize the effects of polymerization

shrinkage, less reaction by-product and deformation

associated with distortion is to minimize the amount of

material that is used to make the impression. The most

accurate polysulfide impression are made by using a custom

acrylic tray.

8. The polysulphide polymer has a molecular weight of 2000

to 7000 with terminal and pendant mercaplan groups. The

terminal and pendant groups of adjacent molecules are

oxidized by the accelerator to produce chain extension and

crosslinking. The reaction results in a rapid increase in

molecular weight, and the mixed part is converted to a

rubber. The weight percent of the filler in the base increases

from low medium to high consistencies. The particle size of

the filler is about 0.3µm. Although the most common active

ingredient in the accelerator is lead dioxide, some

magnesium oxide may also be present.

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Other oxidizing agents such as hydrated copper oxide or

organic peroxides such as amine hydroperoxide have been used

as a substitute for lead dioxide.

Properties:

1. Condensation silicone impression materials are more ideally

elastic than polysulfides. They exhibit minimal permanent

deformation and recover rapidly when strained. Like

polysulfides, there materials are not very stiff, which means it

is not difficult to remove them from undercuts without

distortion.

2. The viscoelastic characteristics of these materials suggest that

they can respond elastically as an viscous liquids that easily

sustain permanent deformation.

3. Tear resistance is low for condensation silicone impression

materials. Although they do not tear as easily as alginates or

agar hydrocolloids, they must be handled carefully to avoid

relining a margin of a crown preparation when it is run.

4. The excessive polymerization shrinkage of the condensation

silicones requires a modification of the impression making

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technique to produce accurate impressions. A putty-wash

technique is used for condensation silicones.

5. Silicone is one of the most biologically inert materials.

6. The alkyl silicates are slightly unstable, particularly if they are

mixed with a tin compound to form a single catalyst liquid.

Thus a limited shelf life may result because of oxidation of the

tin component within the catalyst.

7. The condensation silicones are compatible with all gypsum

products.

II) Silicone Rubber Impression materials:

Developed to overcome disadvantages of polysulfide material, these

are based on silicone technology and are of 2 types:

- Condensation silicones.

- Addition silicones.

Available in various color and viscosities.

Silicones (Polysiloxane):

Condensation silicones are usually supplied in low and putty

like consistency. These are supplied as base and an accelerator. The

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base contains a moderately low molecular weight silicone called a

dimethyl siloxane which have reactive terminal hydroxyl groups.

Fillers may be copper carbonate or silica having particle size from 2

to 8µm in concentrations from 35% to 75% for low to putty

consistencies. The accelerator may be a liquid that consists of

stannous octoate suspension and alkyl silicate or it may be supplied as

a paste. The reaction proceeds a three dimensional network rubber

with the liberation of ethyl alcohol and an exothermic temperature

vice of about 1°C. Besides viscosities common to the polysulfides the

condensation silicones are also supplied in an extremely high

viscosity or putty material. These materials are used in the putty wash

techniques.

Polyethers:

Polyether impression materials are supplied as a medium

consistency type in a base and an accelerator tube. The base is a

moderately low molecular weight polyether with ethylene imine rings.

Properties:

1. The polyethers have always been considered the stiffest of the

impression materials, excluding the high viscosity putties.

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Some of the new formulations of regular or medium-bodied

material are actually less stiff.

2. The pseudoplastic characteristics allows the original single-

viscosity materials to be used as both syringe and tray

materials.

3. Tear resistance is better than that of the condensation silicone

impression materials. However, polyether is more prone to

tearing than polysulfide.

4. The dimensional change of the polyether impression material is

small. Like the addition silicones, polyether have no by-

product.

5. The most likely elastomeric induced problem for the patient

arises from pieces of the impression materials being left in the

sulcus. The irritation can range from mild to severe.

6. Storing in a cool, dry environment prolongs the shelf life.

The terminal groups. The catalyst paste contains 2.5-dichloro

benzine sulfonate as a cross-linking agent, along with a thickening

agent. A separate tube contains a thicker that includes actylpindate

and about 5% methyl cellulose as a thickening agent. The rubber is

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formed by cationic polymerization and opening of imine rings. The

setting reaction is slightly more exothermic than that of the other

rubber impression materials with a temperature rice of about 4°C.

Addition silicone (poly vinylsiloxanes):

The addition type is available in low, medium, heavy and very

heavy putty consistencies and is also polysiloxane. The base contains

a moderately low molecular weight polymer with siloxane (-Si-H)

groups and filler. The accelerator (or catalyst) contains a moderately

low molecular weight polymer with vinyl terminal groups, pulls filler

and chloroplatine acid catalyst. Several products contain finely

divided palladium or platinum, which absorbs hydrogen.

A retarder, a liquid low molecular weight polymer of the same

type as the base polymer is available to extend the working and

setting time. Silicone rubber impression are hydrophobic when mixes

of gypsum products are poured into them.

Properties:

1. As one of the most pseudoplastic impression materials, the

effect of increased strain rate on the unset material is quite

pronounced for vinyl polysiloxane.

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2. The resistance to tearing is adequate, similar to that of

condensation silicone. If not handled correctly, these materials

will fear rather than stretch like polysulfides.

3. Vinyl polysiloxane impression materials are most

dimensionally stable of all existing materials. No volatile by-

product is released to cause the material to shrink.

The base and catalyst putty of addition silicones are mixed by

hand. If they are mixed by the operator while later gloves are being

warm, the setting time is lengthened or the material will not set.

Sulphur compounds used in the vulcanization of latex rubber gloves

can migrate to the surface on storage during mixing of the two putties,

there compounds are incorporated into the mix and poison the

platinum containing catalyst resulting in retarded or no

polymerization.

In general polysulfides have the longest working time, followed

by silicones and polyethers.

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Comparison of certain characteristics of elastomeric dental impression:

Polysulfide Condensation silicone Polyether Addition

silicone

Mixing Fair to easy Fair to easy Easy Easy

Flow Variable Good Good Good

Stock Fair to good Fair Good Fair to Good

Elastic recovery Fair Very good Very good Excellent

Advanced Inpleaus Acceptable Acceptable Acceptable

Clean up Difficulties Easy Easy Easy

Types of failures:

Type Cause

1. Rough or uneven

surface on impression

a. Inc

omplete polymerization caused by

premature removal from mouth.

b. To

o rapid polymerization from high

humidity or temperature

2 Bubbles a. Too rapid polymerization preventing

flow.

b. Air incorporated during mixing.

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3. Irregularly shaped

voids

a. Inadequate cleaning of impression.

b. Excess water left on the surface of the

impression.

c. Premature removal of cast.

d. Improper manipulation of stone.

4. Distortion a. Lack of adhesion of rubber to the tray

caused by not enough coats of

adhesive, filling tray with material too

soon after applying adhesive, or using

wrong adhesive.

b. Lack of mechanical retention for more

materials where adhesive is

ineffective.

c. Excessive bulk of material.

d. Movement of tray during

polymerization.

6. Improper removal

from mouth

a. Premature removal from mouth

Advantages of the elastomeric impression material:

1. They are vary elastic in nature.

2. They have good dimensional stability.

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3. The impression material do not have to be paired within

half hour unlike hydrocollides.

4. More than one cast can be made successfully within or

half hour.

5. Exceptionally smooth dies can be made.

6. Gum retraction or gingival retraction is not always

necessary.

7. Accuracy is comparable to that of the hydrocollides.

Visible light cured impression material:

In early 1988, a visible light cured impression material was

introduced (Genesis, L.D. Caulk). This material is available in two

viscosities. The light body material is packed in disposable syringe

and the heavy body material is packed in tubes. This material has

excellent elasticity and very low dimensional shrinkage upon storage.

It may be poured immediately or upto 2 weeks later. The

material is rigid and it is recommended that severe undercuts should

be blocked out to case removal of the impression.

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Manipulation:

No mixing as syringe loading is necessary. The light body

material is syringed into the sulcus, around and over the preparation

and portion of the adjacent teeth. A clear tray is loaded to the full

time with heavy body material. After the tray is seated in the mouth,

both viscosities are cured simultaneously using a visible light curing

unit having an 8mm or larger diameter probe. The curing time is

approximately 3 minutes.

Advantages:

1. Control over the working time.

2. Curing time is relatively short (3 minutes).

3. Excellent clinical, physical and mechanical properties.

Disadvantages:

1. Need for special tray which should be transparent to the

visible light received to case the material.

2. If delay occurs before placement, the material should be

stored in a dark place away from light.

3. Difficult to light cure the remote area.

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III) Polyethers:

Introduced in Germany in late 1960’s it has good mechanical

properties and dimensional stability but is expensive.

Again available in different viscosities and a base and

accelerator.

Composition:

Base:

Polyether polymer.

Colloidal silica – filler.

Glycolether or phthalate – plasticizer.

Accelerator:

Aromatic sulfonate ester – cross linking agents.

Colloidal silica-filler.

Phthalate or glycolether – plasticizer

Chemistry and setting reaction:

It is cured by the reaction between azinidine rings which are at the

end of branched polyether molecule. The main chain is a copolymer

of ethylene oxide and tetrahydrofuran. Cross linking is via the

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aromatic sulfonate ester via the imina end gps reaction is exothermic

(4-5°C).

Properties:

1. Pleasant odor and taste.

2. Mixing time of 30 seconds, setting time of 8.3 minutes.

3. Curing shrinkage is low (0.24%) permanent deformation is

also low (1-2%) can absorb water and change dimension.

4. Very stiff (flexibility of 3%) requires extraspacing of upto

4mm.

5. Tear strength is good (3000gm/cn2).

6. It is hydrophilic, so moisture control is not a critical. Has

best compatibility with stone.

7. Can be electroplated with silver and copper.

8. Excellent shelf tip above 2 years.

The material should not be used with patients with a known always or

sensitivity to urethanes, acrylic or methacrylates.

Impression techniques:

There are two techniques:

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- Multiple mix technique.

- Reline technique.

a) Multiple mix technique:

Two consistencies of material are provided one for use with the

tray and other for use with the syringe type has longer working and

setting time, and a greater polymerization shrinkage and thermal

contraction.

In the multiple mix technique, both the syringe and tray

material are used for same impression. The tray material is usually

mixed first and filled into the tray to a uniform thickness and set

aside. The syringe material is mixed on a separate mixing pad, loaded

into a syringe and injected into the prepared teeth. The filled tray is

then carried to place.

b) Reline technique (putty-wash technique):

Here a preliminary impression is taken with a putty silicone is a stock

impression tray. This forms a custom made tray in which by cutting

away some of the tray silicone or by using thin resin, rubber or wax

sheet as spaces between the teeth and the silicone. This area is then

filled with a thinner consistency silicone and the tray is repeated into

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the mouth for reproduction of sharp angles accurately, a light bodied

silicone is injected.

The latest technique is mixing is the use of automatic spanars and

mixes. These consist of a double barrel caulking gun with mixing tip.

The tip contains spirals on the inside. Foving of the base and

accelerator through these spirals results in mixing. Advantages

include improved properties, more uniform mix, lesser air bubbles

and reduced working time.

Finally, the impression is removed after chocking is set by providing

with a blunt instrument. It become firm and returns to its original

contour. Removal is done quickly and is one motion for best result

disinfection is done by 10 minutes in 2% glutaraldehyde or 3 minutes

in chlorine dioxide solution, phobe iodophor can also be used.

Recent Advances in Elastomers:

Visible light cured polyether urethane dimethacrylate:

In early 1988, a visible light cured impression material was

introduced (Genesis, L.D. Caulk).

Available in 2 viscosities – Light and heavy bodied.

Composition:

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1. Polyether urethane dimethacrylate.

2. Photoinitiates.

3. Photo-accelerators.

4. Silicone dioxide filler.

Properties:

They have long working but short setting times. Blue light is used for

curing along with transparent impression trays. Highest resistance to

tearing among the elastomers (tear strength of 6000-7500gm/cm2).

Dimensional stability, flow, detail reproduction, permanent

deformation, wettability, compatibility with cast materials and

electroforming is similar to addition silicones. The material is rigid

and severe undercuts should be blocked to ease impression removal.

Manipulation: Light body is syringed into the sulcus and over the

preparation while heavy body is loaded onto the clear tray and seated

over the light body. Both are simultaneously cured with a visible light

curing unit having an 8mm or larger diameter probe. Curing time is

approximately 3 minutes.

Advantages include – Controlled working time.

Excellent properties.

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Disadvantages include;

1. Need for special transparent trays.

2. Difficult to cure in remote areas.

It is contraindicated in patients with a known allergy or sensitivity to

wethers, acrylics or methacrylates.

Lastly, we come to the inelastic impression materials – due to this

limited use in operative dentistry, we shall have a brief overview of

those:

1. Impression plaster – Type I gypsum i.e. calcium sulfate with

modifier was used earlier but is really used now. It is brittle

and rigid. It may be used as a final or wash impression in

complete denture prosthesis.

2. Impression compound or modeling plastic – A thermoplastic

material, it is used primarily for edentulous complete denture

primary impression and for single tooth tube impression with a

copper band, greenstick compound, a type of impression

compound is used for border moulding.

It is composed of thermoplastic resins, copal resins, carnauba wax,

steam acid, talu, coloring agents and fillers along with plasticizers.

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The compound softens at 39°C(glass transition) and is manipulate at

43.5°C (fusion temperature). It can be softened over a flame or in

warm water. It is then baded on to tray and binding seated till rigid.

Dimensional stability is less with distortions occurring and surface

details reproduction is comparatively less. Casts should be poured

immediately.

Advantages include repeated cure and reparability. Disadvantages

include distortions and difficult manipulation as well as rigidity.

3) Zinc oxide eugenol pastes: Available as 2 pastes, composition is as

follows:

Base paste Accelerator

Zinc oxide – 87%

Fixed vegetable or mixed oil – 13%

Oil of cloves or eugenol – 12%

Gum or polymerized resin – 50%

Filter (silica type) – 20%

Lasolin – 3%

Resinous bulsam – 10%

Accelerator solution (CaCl2) – 5%

Coloring agents.

Setting reaction is an acid base reaction forming zinc eugenolate.

Final setting range from 10-15 minutes.

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Manipulate – equal length are dispersed, mixed quickly (1 minute

mixing time) and loaded onto the tray, seated till set and removed.

Advantages include good accuracy, dimensional stability and

compatability with casts. Disadvantages include requirement of

special tray, burning reaction of eugenol and inability register

undercuts.

Non eugenol pastes have been developed to overcome eugenol

initiation by adding carboxylic acids like orthoetching benzoic acid.

They can also be used as bite registration pastes.

4) Lastly, a material not brief an impression material but used as such

inlay wax used in direct or indirect techniques to record single tooth

impressions. It has type I and II and is composed of paraffin wax,

gum damer, canaculi were and coloring agents. Candidia wax, natural

resins and other waxed are also added.

The wax softens at about 40-45°C and flows at 56°C or higher and

vaporizes at 500°C. it is heat flamed, softened and manipulated as

desired, and invested immediately to avoid distortion.

CONCLUSION:

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The famous saying goes “The first impression is the best impression”.

That should also be the endeavor of every dental surgeon. Realizing

that a restoration or prosthesis can only be as good as the preparation

and the impression will encourage dentists to master the art and

science of impression making and recording. This can only be

fulfilled by having an indepth information of material science and

unraveling their intricacies as well as being update on the driving

technologies and techniques governing those materials. Only this

holistic knowledge will enable clinicians deliver ideal dental care and

“impress” the patient.

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Bibliography:

1. Philipps Science of Dental Materials.

2. Restorative Dental Materials – Craig.

3. Materials in Dentistry – Jack L. Ferracane.

4. Basic dental material – John J. Manappallil.

5. Notes on dental materials – C. Combe.

6. Dental materials. – Richard Van Noort.

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