Impression materials

IMPRESSION MATERIALS 01. Introduction.

02. Classification.

03. Ideal requirements.

04. Agar.

05. Alginate.

06. Development in Alginate.

07. Non-aqueous elastomeric impression materials.

•

Poly sulphide.

•

Condensational silicone.

•

Additional silicone.

•

Polyether.

•

Visible light cured polyether.

08. In elastic impression materials.

• Impression compound.

• Zn-Oxide eugenol impression paste.

• Impression plaster.

09. Techniques of impression making.

1

INTRODUCTION: A dental impression is a negative record of the tissues of the mouth. It is used to register or reproduce the form and relationship of teeth and surrounding tissues.

Various materials have been used in the past for obtaining impression of these some like clay, waxes, gutta-percha have become outdated.

A constant research for better materials has led to the

development of new materials from time to time.

Classification of impression materials: By setting mechanism: Rigid or inelastic By chemical reaction * Plaster of paris. (irreversible) * Zn Oxide eugenol.

Elastic * Alginate. * Non elastomers.

aqueous

- Polysulpide - Polyether - Poly silicons * Condensation * Additional By temp (Resersible)

change * Imp. Compound.

* Agar hydrocolloid

* Wax

Mucostatic impressive material: ZnoE, PoP. Mucocompressive impressive material: Impressive compound.

Resersible: Agar Agar, Impressive compound.

Irreversible: Alginate, PoP, ZnoE. 2

Ideal requirements of impressive materials: 01. A pleasant odour and taste.

02. Should not be toxic or irritant to the tissues.

03. Adequate shelf life on storage.

04. Accuracy should be high.

05. Should be compatible with stone and plaster and should not interfere with the setting mechanism.

06. Should not distort on storage after setting.

07. Easy to use and economical.

08. Satisfactory consistency and texture.

09. Should wet the oral tissues adequately.

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

11. Readily disinfected without loss of accuracy.

12. Should have adequate elastic properties with freedom from permanent deformation after strain.

Hydrocolloid impression materials: Colloid: Suspension of two phases.

Hydrocolloid: Colloid that contains water as the dispersion phase.

Agar: (Reversible hydrocolloid) Agar is an organic hydrophillic colloid extracted from certain type of sea weed.

3

Composition: Component

Function

Composition %

Agar

Heap structure

13-17%

Borates

Strength

0.2-0.5%

Sulfates

Gypsum hardener

1.0-2.0%

Wax

Filler

0.5-1.0%

Thixotopic materials

Thickness

0.3-0.5%

Alkyl benzoate

Preservative

Water

Reaction medium

Balance

Agar: Main ingredient.

Dispersed phase of the solution.

Forms a colloid with water.

Liquefies between (70o C – 100o C) and gels between (300 C-50o C).

Borax: Increases the strength of the gel by increasing the density of fibril network.

Dis-advantage: Retards the setting of gypsum products.

Potassium sulphate: Accelerates the setting of gypsum product.

Water: Dispersion phase.

Alkyl benzoate: Prevents the growth of mould during storage (preservative).

Fillers: Diatamaceous earth, clay, silica, wax rubber, Thymol and glycerin: bactericidal and plasticizer.

4

Gelation process: Gelation: It is the setting of a reversible hydrocolloid.

It is a solidification process.

Sol ď „ gel Gel is heated to a temperature (70-100o C) to bring it to sol.

Solution when cooled transforms to gel (37-50o C).

Manipulation of Agar: 1. Advanced preparation of the material.

2. Preparation immediately before making the impression.

3. Making of the impression.

Step-by-step procedure: 1. Material is supplied in tubues and exists as gel at room temperature.

2. Liquify it and store it as a solution.

3. Immediately before impression, solution is placed in impression tray and tempered so that temperature is tolerable to oral tissues.

4. Impression is seated against the oral tissues and chilled to form a gel with cooling water circulation 5. Tray is then removed (Rim lock trays).

Preparation of material: Hydrocolloid is supplied in two forms i. Syringe, ii. Tray material.

Reverse the gel to solution by using boiling water for 10 minutes.

5

After liquification the material is stored in solution condition until required for injection into the cavity preparation or the tray.

Conditioning or tempering: As the storage temperature of 65o C is too hot for the oral tissues the material is cooled or “tempered” in a conditioning unit at 45o C for 3-10 minutes.

This tempering also increases the viscosity of the

material.

Making of the impression: Procedure:  Syringe material is taken directly from storage compartment and applied to the prepared cavity.  The tray material is then properly tempered and placed in the mouth to form the bulk of impressive material.  Gelation is accomplished by cold water at 18-21o C for 3-5 minutes.

AGAR ADVANTAGES

DIS-ADVANTAGES

MOST ACCURATE.

DISTORTION DURING GELATION.

HYDROPHILIC BECAUSE GOOD

IMPRESSION CANNOT BE POURED

RESULTS IN WET FIELD.

AGAIN.

LONG SHELF LIFE.

REQUIRES EQUIPMENTS.

LOW COST.

THERMAL DISCOMFORT.

REVERSIBLE BECAUSE CAN BE USED SEVERAL TIMES.

POURING SHOULD BE IMMEDIATE.

FAILURES AND CAUSES 6

EFFECT GRAINY MAT.

CAUSE INADEQUATE BOILING. WATER SOAKED LAYER OF TRAY

SEPARATION OF TRAY AND

MATERIAL NOT REMOVED.

SYRINGE MATERIAL.

PREMATURE GELATION OF EITHER TRAY OR SYRINGE MATERIAL. INADEQUATE BULK.

TEARING. PREMATURE REMOVAL OF TRAY. INADEQUATE CLEANSING OF IMPRESSION. ROUGH OR CHALKY STONE MODEL.

PREMATURE REMOVAL OF DIE. EXCESS WATER OR K2SO4 LEFT IN IMPRESSION. IMPRESSION NOT POURED IMMEDIATELY. MOVEMENT OF TRAY DURING

DISTORTION.

GELATION. PREMATURE REMOVAL OF TRAY. USE OF ICE WATER ∴ IMM GELATION ∴ DISTORTION.

7

Other techniques: 1. Wet field technique: Agar is hydrophilic in nature, in this technique the tooth surface and the tissue surface is kept moist or wet purposefully with warm water.

Syringe material is then introduced.

It is postulated that

hydraulic pressure of viscous tray material forces the fluid syringe hydrocolloid into areas to be restored.

2. Triple tray technique: Alginate: (Irreversible hydrocolloid) or elastic mucostatic material At the end of last century chemist from Scotland noticed mucous extraction from brown sea weed (algae) also called Algin.

These had linear chains of carboxylic acid and was therefore named as “Anhydrous β manuronic acid” or “Algenic Acid”.

Alginate was found during the II world war due to scarcity of Agar.

Composition: Ingredient Ester salts of algenic acid Na or K or Triethyanol amine alginate Calcium sulfate (Reaction)

Percentage

Function

15%

Dissolves the water and reacts with calcium ions.

16%

Reacts with K alginate to form insoluble calcium alginate.

ZnO

4%

Acts as filler.

Potassium titanium fluoride

3%

Acts as a gypsum hardener

Diatamaccous earth

60%

Na phosphate

2%

Acts as filler Reacts preferentially with calcium sulphate 8

Coloring agents

Tracer

Gelation process: Soluble alginate + CaSO4 – Ca Alginate gel.

Dis-advantage: (of this reaction) CaSO4, which is formed, is too rapid and does not allow sufficient working time. Therefore a water-soluble salt such as Trisodium phosphate is added to prolong the working time.

Diatomaceous earth: also added as filler to control;  Flexibility of the set impression.  Increase strength and stiffness.  Imparils smoothness.

Gel structure: In Na or K alginate the cat ion is attached at the carboxyl group to form an ester or a salt.

When the insoluble Ca alginate is formed the Ca ions replace the Na ions in two adjacent molecules to produce a cross linking between the two molecules.

As the reaction progresses a cross-linked

molecular complex or polymer network forms.

Such network

constitutes a “brush heap” structure of the gel.

In Na / K alginate Cat ion attached to carboxyl group Forms Ester of insoluble Ca alginate Ca++ replaces Na++ Forms cross-linking chains Forms polymer network 9

BRUSH HEAP STRUCTURE OF GEL Controlling the gelation time: 1. Gelation time: Is the time from start of mixing until the material is no longer tacky or sticky.

Fast setting alginates – 1-2 minutes.

Normal setting alginates – 2-5 minutes.

Higher the temperature – shorter the gelation time, to overcome this retards are added.

Manipulation: 1. Water dispensing cup – 3 marks on the cup.

Ist mark – 1 cup of powder.

IInd mark – 2 cups.

IIIrd mark – 3 cups.

2. Powder and liquid are mixed and spatulated vigorously with the figure of 8 motion, the spatula being rubbed against the sides of the rubber mixing bowl.

3. Perforated tray is used.

4. The thickness of alginate between the tray and tissues should be 3 mm.

5. Alginate impression should be removed from mouth for atleast 2-3 minutes after gelation has occurred.

Factors affecting the strength: 1. Composition: a. Type of reaction. b. Filler.

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c. Amount of soluble alginate.

2. Manipulation: a. Water : Powder ratio.

3. Mixing: Grainy mix decreses the strength.

Dimensional stability:  Gels

are

invariably

subjected

to

changes

in

dimension

by

synergesis, evaporation and imbibitions.  Gels are subjected to stresses in localized areas, due to exertion of pressure on the tray.  The relaxation of the internal stresses causes syneresis and corresponding dimensional changes.  Thermal changes also contribute the dimensional change.

Precautions:  If pouring is delayed, it should be rinsed in tap water and wrapped in a paper towel, saturated with water, and placed in a closed container to create a 100% humid environment.  2% potassium sulfate suggested to reduce the dimensional change of agar impression.

Compatibility with gypsum: The surface of the gypsum cast prepared from a hydrocolloid impression may be too soft for waxing procedures.

This dis-advantage can be overcome by two ways.

1. Immerse the impression in a solution containing an accelerator for the setting of gypsum products.

11

2. Incorporate a plaster hardener or accelerator in the material by manufacturer.

Various chemicals used as hardening agents are: 1. Potassium sulfate (2%).

2. Zinc sulfate.

3. Manganese sulfate.

4. Potash Alum.

5. Titanium fluoride.

Development in alginate: a. Modified alginates: 1. Alginate is also available in the form of a solution containing water but no calcium ions. A reaction of P.O.P. is added to the solution to complete the reaction.

2.

Also available in two pastes: 1St paste – alginate solution, 2nd paste – Ca+ reaction.

b. Dust less alginate: Alginate is coated with glycerin / glycol. c. Chromatic alginate: During the setting reaction the pH value of the fluid mass changes because of this some manufacturers have included acid / base indicators which change colors indicating the time of insertion of tray into mouth. d. Siliconised alginate:  In this silicon polymers are added.  Two pastes from.

12

ďƒ˜ Material is considered hybrid of alginates, which have better property than alginate. e. Laminate technique: Here the tray agar hydrocolloid is replaced by chilled alginate that bonds to the syringe agar. Alginate gels by chemical reaction and agar gels by means of contact with chilled alginate.

Advantages: 1. Maximum detail is reproduced (because agar is in contact with preparation).

2. Cost of equipment is lower (only syringe materials needs to be heated).

3. Less preparation time.

Dis-advantages: 1. Bond between alginate and agar is not always strong.

2. Higher viscosity alginate displaces agar during reacting.

3. Dimensional accuracy of alginate limits the use to single units.

ALGINATE ADVANTAGES

DIS-ADVANTAGES

HYDROPHILIC GOOD RESULTS IN MOIST FIELD.

LESS ACCURATE, ROUGH.

PLEASANT ODOUR AND TASTE.

TEARS EASILY.

EASY MANIPULATION.

HAS TO BE POURED IMMEDIATELY.

LONG SHELF LIFE.

RETARDS THE SETTING OF STONE SOMETIMES.

LOW COST

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EFFECT

CAUSE IMPROPER MIXING.

GRAINY MATERIAL.

PROLONGED MIXING. W/P RATIO TOO LOW. INADEQUATE BULK.

TEARING.

PREMATURE REMOVAL. PROLONGED MIXING.

BUBBLES / VOIDS.

AIR INCORPORATION. DEBRIS. INADEQUATE CLEANING OF IMPRESSION.

ROUGH / CHALKY STONE MODEL.

EXCESS WATER IN IMPRESSION. PREMATURE REMOVAL OF STONE. IMPROPER MANIPULATION. IMPRESSION NOT POURED IMMEDIATELY.

DISTORTION.

MOVEMENT OF TRAY. PREMATURE REMOVAL FROM MOUTH.

Non-aqueous elastomeric impression materials: Development:  These materials are classified as synthetic rubbers.  They are developed to mimic natural rubber during World War II.  Initially called as rubber impression material but now ADA No.19 identifies this material as “non aqueous elastomeric impression material”.

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Classification: Based on elastic properties.

1. Poly sulfides.

2. Condensational poly silicone.

3. Additional poly silicone.

4. Poly ether.

5. Visible light curable polyether urethane dimetha acrylate.

Each is further divided into four viscosity.

1. Light body.

2. Medium body.

3. Heavy body.

4. Putty.

Poly sulphide impressive material: Chemistry: Basic ingredient of the polymer paste is a poly functional mercapton or poly sulphide powder.

This polymer is usually cross-linked with an oxidizing agent such as lead dioxide.

It is the lead dioxide that gives polysulphide its characteristic brown colour.

During the condensation reaction of the lead dioxide with the SH groups of the polysulfide polymer, two phenomena occurs.

1. Chain lengthening polymerization from the reaction with the termind SH groups (causes increase in viscosity).

2. Cross linking from the reaction with the pendant SH groups (gives the elastic property).

15

Composition: Base Plate

Reaction paste

Poly sulphide polymer

Lead dioxide

Filler (Titanium dioxide, Zn sulfate)

Oleic acid or stearic acid [retarder] to control setting time

Plasticizer (Dibuthyl phthalate) (increases viscosity) Sulfur (0.5%) (to enhance the reaction) Working and setting time: Mean W.T. (min)

Mean S.T (min)

23o C

37o C

23o C

37o C

Poly sulphide

6.0

4.3

16.0

12.5

Cond. Silicone

3.3

2.5

11.0

8.9

Add. Silicone

3.1

1.8

8.9

5.9

Poly ether

3.3

2.3

9.0

8.3

Impression materials

 An increase in temperature accelerates the curing rates.  Exothermic reaction.  Cooling is a practical method of increasing the working time.  A drop of water accelerates the curing rates.

Elasticity: 

Elastic property of these materials improves with curing time.



Longer the impression can remain in the mouth before removal, greater is the accuracy.



Available as, Low (syringe/wash), medium (regular), Heavy (tray set).

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

Poly sulphide exhibit the most permanent deformation following strain in compression, compared with other materials.

Rheology:  Highest resistance to tearing.  Because of its susceptibility to distortion, it may distort rather than tear.  Strain

rate

influences

the

tear

resistance

and

permanent

deformation.

Dimensional stability:  For maximum accuracy cast should be constructed within first 30 minutes.

Sources of dimensional change:  During setting most polymers contract slightly during cross-linking.  After setting the byproduct (water) is lost causing shrinkage.  These material can absorb water.  Incomplete recovery of deformation after setting.

Biocompatibility:  Compared to other impression materials, poly sulphide shows best cell toxicity.

Handling of the tray:  Most accurate poly sulphide impressions are made by using custom acrylic tray.  Adhesion to the tray is by tray adhesive such as butyl rubber.

Handling technique: Two types of material.

1. Syringe material.

2. Tray material.

Multiple mix technique when 1 and 2 are mixed.

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Disinfection of impression:  Polysulphide disinfected by various antimicrobial solutions without adverse dimensional change, provided disinfection time is short.  10 minutes immersion in 10% solution of hypochlorite.

Handling of stone cast:  Possible to construct successive stone cast from polysulphide impressions.  Time interval between successive pours should not be more than 30 minutes.

Shelf Life: To maximize the shelf life, always keep the tubes tightly closed and store in cool environment.

Condensational silicone impression materials:  The condensation polymerization of this material involved a reaction with tri functional and tetra functional alkyl silicates in the presence of stannous octoate.  The properties of impression materials are influenced by property of fillers – known as “Law of mixtures” more filler – more viscous.

Composition:  This material is supplied as a base paste and a low viscosity liquid or catalyst paste.  Silicone polymer is a liquid.  Colloidal silica (or) microsized metal oxide added as a filler to form a paste (5-10 mm).  In putty there are larger filler and highly filled.

18

Manipulation:  A length of base paste is dispensed from the tube to mixing pad.  One drop of liquid catalyst is added for each unit length of base.  A uniform colour shows a homogenous mix.  Putty material also supplied as very thick paste and a liquid accelerator or two paste putty system.  Latex gloves should not be used while mixing the putty because of sulfur component that inhibits the setting of the putty.

Working and setting time:  Chilling the material or mixing on a glass slab slows the reaction.  Altering the base / catalyst ratio changes the curing time.

Elasticity:  More elastic than poly sulphides.  Minimal permanent deformation.  These materials are not very stiff.

Rheology:  They can respond elastically (or) sustain permanent deformation.  Strained rapidly – elastic.  Prolonging the strain – permanent deformation.

Tear energy:  Tear resistance is low.  Applying a force rapidly ensures the highest tear resistance.

Dimensional stability:  Poor.  Linear contraction 2-4 times greater than other impression materials.  Dimensional instability is due to: a. Large setting shrinkage. b. Loss of byproduct (Ethyl alcohol).  Impression should be poured within first 30 minutes.

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Biocompatibility:  One of the most biologically inert materials.  Possibility of leaving torn impression materials in the gingival sulcus.  Not radio opaque.

Shelf life:  Limited because of oxidation of the tin components within the catalyst.  Degradation of the base.  Cross-linking of base during storage.

Handling of the tray:  Stock trays are required to support the putty.  Tray adhesive used contains polydimethyl siloxane and ethyl silicate.

Handling technique:  Two stage putty wash technique.  Combining wash and putty steps together.

Dis-advantages  Putty material may displace wash material  Putty is too thick to replicate critical areas.

Addition silicone impression material: Chemistry: Also called poly vinyl siloxane impression material.

In contrast with cand. silicone, the additional reaction polymer is terminated with vinyl groups and is cross-linked with hydride groups activated by a platinum salt catalyst.

20

Composition: Base

Catalyst

Polymethyl hydrogen silixane.

Divinyl polydimethyl siloxane.

Other siloxane preplolymers.

Other siloxane prepolymers.

Hybrid silicone.

Platinum catalyst.

Fillers.

Retarders.

Surfactant

(to

increase Fillers.

hydrophobic).

Manipulation: Available as: Light body 2 paste – Base + Catalyst Medium body Putty – 2 jars - Base + Catalyst Automatic mixing device: Advantages: 1. Greater uniformity in proportioning and mixing.

2. Less air is incorporated.

3. Mixing time is reduced.

4. Less contamination of material.

Working and setting time: -

Working time and setting time can be extended by addition of a retarder or cooling the slob.

Elasticity: -

More elastic.

-

Lowest strain in compression values.

Rheology: -

Pseudo plastic impression material.

21

Tear Energy: -

Adequate, similar to cond. silicone.

-

These materials are highly visco-elastic, so a rapid strain rate produces an elastic response.

Dimensional stability: -

Mostly dimensionally stable of all existing materials.

-

A cast poured between 1-2 week was accurate as a cast poured the first hour.

-

Multiple casts can be made from the same impression.

Bio-compatibility: -

Highly bio-compatible.

Handling of the tray: -

Stock trays are used along with putty material.

-

Tray adhesive for poly sulfide, poly ether, condensation silicone are quite satisfactory (Polydimethyl siloxane and ethyl silicate).

Handling technique: -

Putty – Wash technique has been popular for this materials.

Advantages: -

More convenient.

-

Low polymerization shrinkage.

-

Avoid applying pressure to the setting putty.

-

Single phase or mono phase viscosity also available.

Disinfection: -

10-15

minutes

of

immersion

in

10%

hypochlorite,

2%

gluteraldehyde.

22

Polyether impression material: Chemistry: It was the 1st elastomer introduced in 1960’s in Germany.

It is a polyether based polymer that is cured by the reaction between aziridine rings, which are at the end of branched polyether molecules.

Composition: Supplied as two pastes.

Base

Accelerator

Polyether polymer.

Alkyl aromatic sulfonate.

Colloidal silica (Filler).

Filler.

Glycolether (Plasticizer).

Plasticizer.

Manipulation: -

Originally, they were supplied in one viscosity.

-

Now, polyether is available in several viscosities.

Working time and setting time: -

Curing rate is less sensitive to temperature change.

-

Modification in base/accelerator ratio used to extend working time.

Elasticity: -

They are considered the stiffest of the impression materials.

Rheology: -

The pseudo elastic characteristics allowed the original single viscosity material to be used as both tray – syringe material.

Tear Energy: -

Tear resistance better than that of cond. and add. silicone.

-

Most radiolucent material.

23

Dimensional stability: -

Dimensional change for this material is small.

-

Like add silicone, no byproducts for polyether.

-

They exhibit the least amount of distortion from the loads on set material.

-

Negative effects of material is absorption of water or fluids and the leaching of water.

-

Soluble plasticizer.

Bio-compatibility: -

Contact dermatitis from polyether has been reported.

-

The set polyether impression material did produce the highest cell toxicity scores after multiple exposures.

Handling of the tray: -

Two arguments for the need of custom tray.

-

Those in favor of custom trays suggest that a thin uniform layer (2 mm) makes it easy to remove.

-

Other argue that the extremely stiff material does not need the support of a custom tray.

Handling technique: -

Polyether is supplied in several viscosities with an automatic mixing device.

-

The stiffness of good elastic properties prevent distortion of the impressions made from single viscosity.

Disinfection: -

2% glutaraldehyde for 10 minutes.

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PROPERTY

POLYSULFIDE

COND.SILIONE > THAN POLYSULFIDE

ADDTN.SILICONE

ELASTICITY

VERY LESS

RHEOLOGY

VERY FLEXIBLE

VISCOELASTIC

PSEUDO ELASTIC

TEAR RESISTANCE DIMENSIONAL STABILITY BIOCOMPATIBLE SHELF LIFE

HIGHLY RESISTANCE

LOW

ADEQUATE

POOR

POOR

MOST STABLE

MOST

GOOD

HIGHLY

ADEQUATE

LIMITED

2 YEARS

TRAY ADHESIVE

BUTYL RUBBER IN CHLOROFORM

POLYDIMETHYL SILOXANE

SAME AS C.S.

BYE PRODUCS

WATER

ETHYL ALCOHOL

-

MIXING TECH

MULTIPLE MIX

PUTTY WASH MULTIPLE MIX

WITHIN 1 HR.

MUST POUR IMMEDIATELY

SINGLE/DOUBLE MIX PUTTY WASH REPEATED POUR POSSIBLE

LONG (4.3 MIN)

GOOD (2.5 MIN)

LESS (1.8MIN)

ODOUR

OBNOXIOUS

PLEASANT

PLEASANT

MARGINS

EASILY READ

EASILY READ

WATER PHOBIA

HYDROPHOBIC

HYDROPHOBIC

TRADE NAMES

COE-FLEX NEO-FLEX OMI-FLEX PERMLASTIC

ACCOE CUTTERSIL ELASTICON XANTOPREN

EASILY READ HYDROPHOBIC SURFACTANT IS ADDED PALLADIUM IS ALS ADDED TO REDUCE H2 TO MAKE IT LESS HYROPHOBIC. HYDROSIL IMPRINT REPROSIL EXPRESS EXTRUDE

POURING MODEL WORKING TIME

MOST ELASTIC

POLYETHER STIFFEST PSEUDO ELASTIC BETTER THAN C.S. AND A.S. SMALL CHANGE HIGHEST CELL TOXICITY GOOD SAME AS POLYSULFIDE AND C.S. AND A.S. SINGLE MIX DOUBLE MIX DELAY POUR IS POSSIBLE LESS (2.3 MIN) SOME TASTE PR. EASILY READ

LEAST HYDROPHOBIC

IMPREGUM PERMADYNE POLYJEL-NF

25

NON-AQUEOUS ELASTOMERIC IMPRESSION MATERIAL FAILURES ROUGH/UNEVEN SURFACE ON IMPRESSION.

CAUSES INCOMPLETE POLYMERIZATION. TOO RAPID POLYMERIZATION. ACCELERATOR BASE RATIO TOO RAPID POLYMERIZATION PREVENTING PROPER FLOW.

BUBBLES / VOIDS.

AIR INCORPORATED DURING MIXING. MOISTURE / DEBRIS ON SURFACE OF TEETH. INADEQUATE CLEANING OF IMPRESSION. EXCESS WETTING AGENT.

ROUGH /CHALKY STONE MODEL.

EXCESS WATER. PREMATURE REMOVAL OF CAST. IMPROPER MANIPULATION OF STONE. POLYMERIZATION SHRINKAGE. SETTING BEFORE SEATING. EXCESSIVE BULK OF MATERIAL. MOVEMENT OF TRAY.

DISTORTION.

INSUFFICIENT RELIEF OF RELINE MATERIAL. PREMATURE / IMPROPER REMOVAL OF TRAY .

Recent advances in impression materials: -

Visible light cured polyether urethane dimetha acrylate.

26

-

It is based on the polyether urethane dimetha acrylate resin with visible light cured photo initiators and photo accelerators.

Advantage: -

Working time is controlled by the operator.

Dis-advantages: -

Inability to set against a wet field.

-

Direct the light to all areas of the mouth.

Consistency: -

Light body,

-

Heavy body

.

Properties: -

Long working time.

-

Short setting time.

-

Visible light is used.

-

Transparent impression tray is used for making impression.

-

Highest resistance to tearing among the elastomers (6000-7500 gm/cm).

-

Dimensional stability, flow, detail reproduction, permanent deformation, wettability, compatibility with cast and die materials is similar to addition silicone.

Automatic dispensing and mixing: It consists of a double barrel caulking gun with mixing tip. The tip contains spirals on the inside, forcing of the base and accelerator through these spirals in the tip result in its mixing.

Advantages: Improved properties due to: 27

1. More uniform mix.

2. Less air bubbles incorporated in mix.

3. Reduced working time.

In elastic impression materials: They represent a group of materials that exhibit an in significant amount of elastic deformation when subjected to bending or tensile stresses.

They include: -

Plaster of paris.

-

Impression compound.

-

Zn-oxide eugenol impression paste.

Impression compound: Softened by heat, inserted in impression tray and pressed against tissue before it hardens.

Two types: (1) Tube impression (2) Tray compound.

Indication: -

Used for making impression of edentulous ridge.

-

Used for making impression of single tooth preparation.

28

-

To stabilize matrix bands.

-

For border moulding of an acrylic impression tray.

Tube impression: -

For single tooth preparation, a cylindrical copper band is filled with the softened compound.

-

The band is pressed over the tooth.

Tray compound: -

It is used to form a tray for the construction of dentures.

-

Primary impression.

-

Secondary impression.

Two forms of impression compounds: (1) Cake (2) Stick.

Composition: It is a mixture of – Waxes (carnoba – wax) - Thermoplastic resins (Copal resin, rosin) - Filler (French chalk, diatomaceous earth, soap stone, talc) - Colouring agent - Plasticizer (Shellac or gutta-percha added to improve plasticity and workability) Fillers: -

Increases strength.

-

Increases viscosity.

-

Increases rigidity.

29

Fusion temperature: -

Above this temperature, the softened material remains plastic while the impression is being obtained.

Softening: -

Oven, flame, water batch.

Zn-oxide eugenol impression paste (ADA-16):

Application: -

Impression material for edentulous arches.

-

Surgical dressing.

-

Bite registration.

-

Temp filling material.

-

Temp relining material.

Chemistry: Setting mechanism consists of zinc oxide hydrolysis and a subsequent reaction between zinc hydroxide and eugenol to form a chelate.

ZnO + H20 → Zn (OH)2 Zn (OH)2 + 2HE → ZnE2 + 2H20 Base

Acid (Eugenol)

Salt (Zn Eugenalate)

Composition: dispensed in two pastes: Base Plate

Accelerator Paste

Zn Oxide (87%)

Oil of clove or eugenol (12%)

Fixed vegetable or (Plasticizer) Mineral oil (13%)

Gum or polymerized resin (50%) Filler (20%) Lanolin (3%) Resinous balsam (10%) Colour (5%) Cacl2 (accelerator) Znacetde (accelerator) Glacial acetic acid 30

Manipulation: -

Oil impervious paper or glass mixing slab is used.

-

Flexible stainless steel spatula.

-

Two strips are mixed for 1 minute until a uniform colour is observed.

Setting time: Working time → 3-6 minutes.

Final setting time → Type 1 paste → 10 minutes.

Type 2 paste → 15 minutes.

Dis-advantages: -

Possible stringing or burning sensation.

-

ZnOE reaction is never completed (free eugenol leaches out).

-

Taste disagreeable.

Impression plaster (Type 1): -

Composed of POP to which modifies has been added to regulate the setting time and the setting expansion.

-

Rarely used now a days.

-

Replaced by hydrocolloids and elastomers.

Techniques for impression making:

1. Double impression technique, putty wash technique, reline technique: First the thick putty material is placed in a stock tray and a preliminary impression is made.

Space for the light bodied “wash

material” is provided either by cutting away some of the tray putty or using a thin polyethylene sheet as a spacer between the putty and prepared teeth. After the spacer is removed, a mix of thin consistency wash material is placed onto the putty impression and then the putty wash is seated in the mouth to make the final impression.

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Advantage: Accurate.

Dis-advantages: -

Deformation due to release of compressive forces.

-

i.e. if too much force is applied while taking impression, it can be compressed.

2. Modified double impression: Procedure is similar to the above, except for that the primary impression is taken before the abutment tooth is prepared. The space provided by the removed tooth structure will provide space for the secondary impression. During the secondary impression the pressure should not be applied.

3. Multiple mix: It is a procedure of recording impressions simultaneously with two different viscosities of silicon elastomers.

The two viscosities are mixed separately.

The light body is loaded in a syringe.

Heavy body is loaded in a tray.

Syringe material is injected around the gingival margin on inlay preparation.

Then a heavy body in tray is seated over this.

Both

materials set together to produce single impression.

4. Single phase or Mono-phase: Single paste is mixed and loaded in syringe and tray and impression is taken together i.e. syringe material 1 st and then tray material.

5. Laminate technique: Here the tray agar hydrocolloid is replaced by chilled alginate that bonds to the syringe agar. Alginate gels by chemical reaction and agar gels by means of contact with chilled alginate.

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Advantages: 1. Max. detail is reproduced (because agar is in contact with **).

2. Cost of equipment is lower (only syringe materials needs to be heated).

3. Less pulp time.

Dis-advantages: 1. Bond between alginate and agar is not always strong.

2. Higher viscosity alginate displaces agar during reacting.

3. Dimensional accuracy of alginate limits the use to single units.

REFERENCES:  Anusavice – Science of Dental Materials.  Subba Rao – Notes on Dental Materials.

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