Impression materials and techniques

PRESENTED BY Dr EKTA GARGMDS 1st YEAR

DEPARTMENT OF CONSERVATIVE DENTISTRY & ENDODONTICS

• Introduction• Definition• History• Ideal Requirements• Classification• Impression materials - Elastomeric materials - Hydrocolloids • Latest advances • Impression Techniques

Introduction

Impression materials are used to produce the accurate replicas of intraoral tissues.

There are a wide variety of impression materials available each with their own properties, advantages and disadvantages.

Materials in common use can be classified as elastic or non-elastic according to the ability of the set material to be withdrawn over undercuts.

Definition

“Any substance or combination of substances used for making an impression or negative reproduction.”

- GPT

“An impression is an “imprint” or negative likeness of the teeth and/or edentulous areas, made in plastic material which becomes hardened or set while in contact with the tissue.”

-Hartwell 1992

“An impression is the perpetual preservation of what already exists and not the meticulous replacement of what is missing.”

-M.M.Devan 1995

• 1782 : William Rae used wax with Plaster of Paris.

• 1940s: American dentists used Plaster OF Paris for impression & the technique

was presented to the profession at large by Chaplin Harris in 1953.

• 1925 : Alphous Poller of Vienna was granted a British Patent for a totally

different type of impression material which was later described by Skinner as

Colloidal Sols of Emulsoid type. The possibility of using colloidal substance for

dental impression became apparent when Poller’s Negacoll was modified &

introduced into the dental profession as Dentacol in 1928. Agar hydrocolloid

was introduced to the dental impression.

• 1930 : JD Hart of Oklahoma began to use Agar for fabrication of cast

restoration.

1930 : AW Ward and EB Kelly introduced ZnO Eugenol.

1890s : A chemist from Scotland noticed that Brown Seaweed yielded peculiar

mucous extraction. He named it ‘Algin’.

1936-40 : S William Wilding used Algin as a dental impression material

1950s : Development of Rubber base impression materials (Polysulphides and

Condensation Silicones)

1960s : Polyether impression material developed in Germany

1970s : Addition silicone was introduced as a dental impression material

1988 : Latest addition and light cure elastomers

1990-2000 : New auto devices and delivery systems

Ideal Requirements

• Fluid enough to adapt to the oral tissues

• Viscous enough to be contained in the tray

• Setting time - less than 7 minutes

• Adequate tear resistance

• Dimensionally stable

• Biocompatible

• Cost effective

Classification

I. Based on the degree of tissue compression/ amount of pressure applied

• Mucostatic

Ex: Impression plaster

• Mucocompressive

Ex: Impression Compound

II. Based on Mode Of Set t i ng & Elas t i c i ty

A. Chemical reaction (irreversible)

By elasticity and use

Inelastic or rigid Elastic

Materials Use Material Use

1.Plaster of Paris 2.Zinc-oxide eugenol

Edentulous ridge Interocclusal records

1. Alginate 2. Non-aqueous elastomers -Polysulfide - Polyether- Condensation silicone - Addition silicone

Teeth and soft tissue

B. Physical reaction (reversible)

3.ImpressionCompound

Preliminary impression

3.Agar hydrocolloid Teeth and soft tissue

III. Based on the chemical composition

IV. Based on type of impression & area of use

V. According to the use of the materials in dentistry

A) Materials used for obtaining impression of dentulous mouth

Alginate Agar Non-aqueous Elastomers

B) Materials used for obtaining Impression of edentulous mouth:

• Impression Compound• Impression Plaster• Zinc Oxide eugenol• Wax

VI. Based on Manipulation

VII According to their method of use

VIII. Based on the tray used for impression

Hydrocolloids

Colloids

often classed as fourth state of matter as colloidal state. can be considered as a compromise between the very small molecules in solution & very large particles in suspension. Two phases

• Dispersed phase or dispersed particles• Dispersion phase or dispersion medium

All colloidal dispersions are termed as sols.

Types Of ColloidsTypes Of Colloids

The only exception is the gaseous state, i.e. two gases.

Hydrocolloids If the dispersion medium is water, the material is known as hydrocolloid. Particles larger than those in solution. Particle size ranges from 1-200 nm. There is no clear line of demarcation among the solutions, colloids & suspensions.

Gels Colloids with a liquid as a dispersion medium can exist in two different forms known as Sol & Gel.

Sol- has a appearance & many characteristics of a viscous liquid. Gel- is a semisolid & produced from a sol by the process of gelation.

GelationIt is a process of conversion of sol to gel, to form fibrils, micelles of the dispersed phase which become interlocked to give characteristic jelly like consistency.

Within the gel, the fibrils branch & intermesh to give a brush heap structure.

micellesmicelles brush-heap brush-heap structurestructure

Gelation Temperature The temperature at which gelation occurs is known as

gelation temperature.

• Agar - secondary bonds (weak) hold the fibrils together-

break at slightly elevated temperatures and become re-

established as it cools to room temperature - reversible

• Alginate - the fibrils are formed by chemical action -

irreversible

The Gel may lose water by evaporation from its surface or by exuding fluid onto the surface by a process known as SYNERESIS. The gel shrinks as a result of evaporation & syneresis.

If a gel is placed in water, it absorbs water by a process known as IMBIBITION. The gel swells during imbibition, thereby altering the original dimensions.

The effects of syneresis, evaporation & imbibition on the dimensional changes are of considerable importance in dentistry, since any change in dimension that occurs after the impressions are removed from the mouth will lead to inaccurate casts & models.

AGAR(REVERSIBLE HYDROCOLLOID)

Reversible hydrocolloids undergo transition from gel to sol on heating.

On cooling they return back to the original state.

The hydrocolloid supplied as the gel is heated above its liquefaction temperature, cooled to a temperature compatible with the oral tissues & placed in the mouth. Gelation occurs as the tray continues to cool, after which the impression is removed & poured.

Agar It is a sulphuric ester of a linear polymer of galactose.• Gelation Temperature of agar is approx- 37˚-50˚c.• The temperature at which the gel changes to sol i.e. liquefaction temperature is 70˚-100˚c.

Although it is an excellent impression material & yields accurate impressions, it has been largely replaced by alginates & rubber impressions because-• Of the minimum equipment required• Possibility of obtaining metal dies from rubber impression.

Component Function Composition Agar Brush – heap structure 13 – 17%

Borates Strength 0.2 – 0.5%

Sulfates Gypsum hardener 1.0 – 2.0%

Wax Filler 0.5 – 1.0%

Thixotropic materials

Thickener 0.3 – 0.5%

Water Reaction medium Balance 84%

Alkyl Benzoates Preservatives 0.1%

Color and Flavors Taste & appearance Trace

Mode of supply :Gel in collapsible tubes (for impressions)

A number of cylinders in a glass jar (syringe material)

In bulk containers (for duplication)

• Full mouth impressions without deep undercuts

• Quadrant impressions without deep undercuts

• Single impressions

• Can be used for crown and bridge impressions because of their

accuracy

• Cast duplication

• Tissue conditioner

ManipulationPreparation, conditioning and tempering of the agar material:

Before use the material is subjected to a controlled regimen in three water baths.

Liquefying bath -100°C for 10 min.Storage bath – 63-66°C Tempering bath- 44-46°C for 3-10 min.

Conditioning unit

Making the Agar impression

• Syringe material, directly taken from storage compartment - applied on to the prepared tooth.

• Water-soaked outer layer of tray hydrocolloid is removed from the

tempering basin, outer layer removed and impression made.

• Gelation is accelerated by circulating cool water (18-21oC) through

the tray for 3-5 min.

According to ADA specification no.11 :-

• Compressive strength : 8000 gm/cm2

• Tear strength : 700 gm/cm2

• Viscosity : Sufficient fluid to allow detailed reproduction of hard and soft tissues

• Flexibility: 11% when a stress of 14.2 psi applied

• Elastic recovery : 98.8%

• Accuracy : 25µm

• Working time : 7-15 min.

• Setting time : 5 min.

• Gelation temperature : 37-50°C

• Liquefaction temperature : 70-100°C

• Hydrophilic

• Good elastic properties

• Can be re-used as a duplicating material

• Long working time and low material cost

• No mixing technique, the potential for errors are eliminated

• Palatable and well tolerated by patients

• High accuracy and fine detail recording

• Compatible with die and cast materials

• Low tear resistance

• Low dimensional stability

• Only one model can be used

• Extensive and expensive equipment required

• It can not be elecroplated

• Impossible to sterilize for re-use

ALGINATE (IRREVERSIBLE HYDROCOLLOID)

• Developed as a substitute for the agar impression material when its supply became scarce during World War II.

• Based on a natural substance extracted from certain

brown seaweed. This substance is called as-

anhydro-ß-d-mannuronic acid or

alginic acid (sodium or potassium or

triethanolamine acid) (insoluble in water)

Currently, alginate is more popular than agar because-• easy to manipulate• comfortable for the patient.• relatively inexpensive• does not require elaborate equipment

Component Function Weight percentage

Potassium alginate Soluble alginate 15

Calcium sulfate Reactor 16

Potassium titanium Fluoride

Accelerator 3

Zinc oxide Filler particles 4

Diatomaceous earth

Filler particles 60

Sodium phosphate Retarder 2

Composition

SETTING REACTION :-

Two main reactions occurs during setting :

i) 2Na3PO4 + 3 Ca SO4 Ca3(PO4)2 + 3 Na2SO4

(Sodium phosphate) (Calcium sulphate) (Retarder) (Reactor)

i) Sodium Alginate + CaSO4 + H2O Ca Alginate + Na2SO4

(Powder) (Reactor) (Gel)

• I. According to concentration of sodium

phosphate

• Fast set

• Regular set

• II. According to concentration of filler

• Soft set

• Hard set

Classification of alginate Classification of alginate (( Robert Robert G. CraigG. Craig ))

Types Mixing time Working time Setting time

I- Fast set 45 sec 1.25 mins 1-2 mins

II- Normal set 60 sec 2 mins 2-4.5 mins

MANIPULATION :- Prepare a proper mixture of water & powder. The measured powder is sifted into the premeasured water. Water added first to ensure complete wetting of powder particles. Clean equipment is important. Avoid incorporating air into the mix. Figure –of-eight motion is best, with the mix being swiped or stropped against

the sides of the bowl with intermittent rotations (180˚) of the spatula to press out air bubbles.

All of the powder must be dissolved.

PROPERTIES :-

According to ADA specification No 18: Flexibility : 14% at a stress of 1000gm/cm2 Elastic recovery : 97.3% Tissue detail reproduction : 0.075mm Compressive strength : 5000-8000/cm2 Tear strength : 350-700gm/cm2 Poor dimensional stability

USES :- Impression making in complete denture prosthesis and orthodontics In undercuts and in excessive salivary flow For impressions for mouth protectors For impression in study models and working cast.

ADVANTAGES :- Easy to mix and manipulate Minimum requirement of equipment Flexibility of set impression Accuracy if properly handled Low cost Hygienic, fresh material for each impression Good surface details even in saliva Comfortable to the patient

DISADVANTAGES :- Cannot be electroplated Distortion occurs easily Poor dimensional stability Poor tear strength

RECENT ADVANCES IN ALGINATES

Sol containing water but no source of calcium ions .A reactor of Plaster of Paris can be added to the sol later.

The two component system may be in the form of two pastes- One contains the alginate sol and the other contains the calcium reactor

Siliconised alginates Chromatic alginates eg. Integra & KromaFaze (Dux dental) ,Take one

alginate(Kerr) Dustless alginates( glycerin is added on the alginate powder to

agglomerate the particles) eg.Identic dust free alginate (Dux dental),Jeltrate Dustless(Dentsply Caulk) , Integra dustfree alginate(Kerr)

Alginot Alginator 2

Laminate technique (Alginate – Agar Method)

• The hydrocolloid in the tray is replaced with a mix of chilled alginate that bonds to the syringe agar.

• Alginate gels by a chemical reaction whereas agar gels by means of contact with the cool alginate rather than with the water circulating in the tray.

• Since the agar, not the alginate is in contact with the prepared

tooth, maximum detail is produced.

ADVANTAGES :-Most cost effective way of producing an impression with adequate detail. less preparation time required.

DISADVANTAGES :-The bond between the agar & alginate is not always sound.The higher viscosity of the alginate material displaces the agar hydrocolloid during seating.Dimensional inaccuracy of the alginate hydrocolloid limits its use to single units.

Elastomeric Impression Materials

• Soft and rubber-like & known as elastomers or synthetic rubbers• As per ADA Sp. No.19- non-aqueous elastomeric dental impression

materials• Liquid polymers when mixed with a suitable catalyst are converted into

solid rubber at room temperature.

TYPES :-

I. According to chemistry – Polysulphides Condensation polymerizing silicone Addition polymerizing silicone Polyether Visible light curable polyether urethane dimethacrylate ( a new class

added recently)

II.ADA Classification

Based on selected elastic properties & dimensional changes(sp no.19)

Maximum permanent Maximum flow Maximum dimensional

deformation in compression change in 24 hrs

Type I 2.5 0.5 -0.5

Type II 2.5 0.5 -1

Type III 5.5 2.0 -0.5

Each type is further divided into four viscosity classes;

III. According to Viscosity(ISO 4823)

1) Light body or syringe consistency

2) Medium or regular body

3) Heavy body or tray consistency

4) Very heavy body or putty consistency

Excellent reproduction of details Generally hydrophobic Good elastic properties Excellent tear strength High coefficient of thermal expansion Electroplatable :-silver or copper plated Extended shelf-life : 2 years Generally higher cost Requires tray adhesive and mechanical interlocking.

Dimensional changes occur due to :-

- Curing shrinkage - Loss of by products of reaction - Condensation silicone lose alcohol - Polysulphide lose volatile accelerators(Hydroperoxide type) - Polyethers absorb water and loses soluble plasticizers - Thermal contraction when transferred from mouth to room temperature. - Incomplete elastic recovery after removal - Increased filler content reduces the shrinkage - Uniform thickness of the material : shrinkage more uniform - Good adhesion of tray : Shrinkage is directed towards outer surface of the tray - Time of pouring : after elastic recovery

SUPPLIED AS :-

All elastomers –two paste systems(base and catalyst)

Putty consistency – supplied in jars

USES :-

1) Impression material for all applications including

- Fixed partial dentures

- Dentulous and edentulous impressions

2) Border moulding of special trays(polyether)

3) Bite registration

4) As duplicating material for refractory casts

Polysulfide (Synonyms : Rubber base, Mercaptan,

Thiokol rubber)

• First elastomeric impression to be introduced

• Composition : Base paste : Liquid Polysulphide Polymer - 80-85% Inert fillers( Titanium dioxide, Zinc sulfate , - 16-18% copper carbonate or silica) Plasticizers Reactor Paste Lead dioxide - 60-68% Dibutyl phthalate - 30-35% Sulfur - 3% Other substances like Magnesium Stearate(retarder) & deodorants - 2%

Tray adhesive :- Butyl rubber or styrene/acrylonitrile dissolved in a volatile solvent such as chloroform or a ketone is used with polysulphides

Mode of supply: - Available as 2 systems : Base and accelerator - 3 viscosities: light, medium and heavy bodies

Chemistry and Setting Reactions :-

-On mixing base and accelerator pastes the liquid polymer sets to form a solid, but flexible and elastic rubber like material.

-Lead dioxide reacts with the polysulfide polymer-Chain lengthening by oxidation of the terminal –SH groups-Cross-linking by oxidation of the pendant –SH groups

PbO2 + S

HS - R - SH HS - R-S-S-R-SH + H2O

OR

Mercaptan+ Lead dioxide Polysulphide + Water

Commercial products :

Permlastic (Lead dioxide system), Coeflex, Omniflex (Copper hydroxide system)

Advantages Disadvantages

- High tear strength - Dimensionally unstable

- Long working time - Unacceptable odor

- Established precision - Untidy and stains clothing

- Economic - Long setting time

- Extensive shelf life - Least elastic recovery

- Less hydrophobic - Subsequent pours are less accurate.

Silicones

These materials were developed to overcome some of the disadvantages of

polysulphide materials :-

• Objectionable odor

• Staining of linen by lead dioxide

• Amount of effort required to mix the base with the accelerator

• Long setting times

• Moderately high shrinkage on setting

• Fairly high permanent deformation

• Pouring should be within one hour

TYPES :-

- Condensation silicone

- Addition silicones

• Polymer chains grow simultaneously and a reaction byproduct is formed with associated shrinkage

• Products available:-

- Accoe(GC)

- Cuttersil(Heraeus Kulzer)

- Silene(Bosworth)

- Speedex(Coltene Whaledent)

- Xantopren(Heraeus Kulzer)

• Advantages :-

-It is clean and pleasant

- Good working time

- Easily seen margins

CONDENSATION SILICONE

• DISADVANTAGES :-

- High polymerization shrinkage

- Volatile alcohol byproduct

- Low tear strength

- Hydrophobic

- Pour immediately

• COMPOSITION :-

Base Paste

- Polydimethylsiloxane (Hydroxy-terminated)

- Colloidal silica or microsized metal oxide(filler) 35-75%

- Colour pigments

Accelerator Paste

- Orthoethyl silicate - (cross linking agent)

- Stannous octoate - catalyst

• SETTING REACTION :-

CH3 OC2H5

OH-Si-OH + C2H5O-Si-OC2H5

Sn

Silicone +

CH3CH2OH

OctoateCH3 OC2H5

Dimethyl siloxane

Orthoethyl

silicate

Silicone rubber

Ethyl alcohol

+ +

• These reactions are effected at ambient temperatures and the materials are therefore called RTV( Room Temperature Vulcanization) silicones.

• The ethyl alcohol formed evaporates gradually leading to shrinkage & instability. So a condensation silicone should be poured as soon as possible after removal from the mouth.

• Also called as polyvinyl siloxanes

• Has better properties than condensation silicones

• MODE OF SUPPLY :-

Available in various viscosities and paste and jars with contrasting colors

Eg: - Impressiv( Cosmedent)

- Imprint 3 Penta (Putty impression material with automix)

- Aquasil/Reprosil/Hydrosil(Dentsply Caulk)

- Exaflex/ Examix/Exafast/Hydroflex(GC America)

- Express, Imprint/Imprint II(3M ESPE)

- Flexitime(Heraeus Kulzer)

ADDITION SILICONE

• Composition:-

Base :-

- Poly(methyl hydrogen siloxane)

- Other siloxane prepolymers

- Fillers

Accelerator :

- Divinyl polysiloxane

- Other siloxane prepolymers

- Platinum salt : Catalyst (chloroplatinic acid)

-Palladium (Hydrogen absorber)

- Retarders

- Fillers

• CHEMISTRY AND SETTING REACTION :-

CH3

Si-H +

CH3

CH3

CH2= CH-Si

CH3

CH3 CH3

Si-CH2-CH2-Si

CH3 CH3

Pt salt

activator

Vinyl siloxane + Silane siloxane Silicone rubber

Complications from latex gloves :-

• Contact of polyvinyl siloxanes with latex gloves should be avoided• Especially true when handling and placing gingival cords• Mechanism :- - Migration of particulate sulfur and sulphur chloride compounds to the

surface and are transferred to prepared teeth and adjacent soft tissues - When mixing by hand, mechanism of inhibition is thought to be

contamination of chlorplatinic acid catalyst with free sulfur left in glove

- Residues from acrylics,methacrylates and petroleum jelly lubricants may interfere with setting reaction of material

• Contact of internal surface of impression with gloved hands :- - Failure of the material adjacent to the tray to polymerize - Separation of the tray from the impression material

• Prevention :- - Use vinyl or nitrile gloves to eliminate the problem

- Preparations should be cleaned after temporary prosthesis before making final impression

- Avoid handling retraction cords with latex gloves

TRAY ADHESIVE :-Polydimethyl siloxane & Ethyl silicate

• Available in 4 consistencies • Light body• Medium body • Heavy body • Putty

Advantages Disadvantages - Excellent dimensional accuracy - Hydrophobic

- Long term dimensional stability - Expensive

- Hydrogen gas evaluation in some materials - Pleasant to use

- Short setting time - Hydrophilic formulations imbibe moisture- Auto mix available

- If hydrophilic, good compatibility

with gypsum - Sulfur contamination by

latex glove

Polyether• Introduced in Germany in late 1960s

• Good mechanical properties and dimensional stability, but short

working time, very stiff material and expensive

• Commercial names:

- Impregnum(ESPE,Germany)

- Permadyne(ESPE,Germany)

• Available as 3 viscosities: light,medium & heavy bodied

• COMPOSITION :-

Base Paste

- Polyether polymer

- Colloidal silica(filler)

- Glcol ether or phthalate(plasticizer)

Accelerator paste

- Aromatic sulfonate ester (cross-linking agent)

- Colloidal silica (filler)

- Phthalate or glycol ether (plasticizer)

• CHEMISTRY & SETTING REACTION

Polyether + Sulfonic ester Crosslinked rubber

• PROPERTIES :-

- fast setting time of less than five minutes

-Undergo addition based polymerization reaction

- Have no reaction by product so are dimensionally stable

- Must be stored dry as they absorb water and swell under conditions of

high humidity

- Are relatively hydrophilic and hence more forgiving of inadequate

moisture control

- Have good tear resistance, but are rigid, hence considerable force may

be required when removing from mouth or cast

Advantages Disadvantages

Dimensional stability - Set material very stiff

Accuracy - Imbibition

Shorter setting time - Short working time.

Automix available - Allergic hypersensitivity in some cases.

PROPERTIES OF ELASTOMERIC IMPRESSION MATERIALS

• Accuracy :-

- According to ADA sp no 19, elastomeric impression materials must be able to reproduce fine detail of 25µm.

• Differences in ability to record fine detail of various viscosities :-

-Lower the viscosity of impression materials , better it will record fine details

• Elastic recovery :-

-No impression material has 100% elastic recovery

- Eliminate or block out any undercuts in the tooth preparation before making impression

• Dimensional stability

- PVS materials possess ideal dimensional stability

- Hydrocolloids are subject to imbibation & syneresis

• Flow & flexibility

- Addition silicone and polyether are pseudoplastic impression materials

• Rigidity

- Polyether impression materials tend to be more rigid than the other

materials

- PVS materials are reasonably stiff, but fracture of the dies are

uncommon

- Reversible hydrocolloid is the least rigid of all impression materials

-With dual arch impressions, use a very rigid impression material.

• Workability

- Automix devices, electronic mixing devices, and simpler mixing guns

provide a standardized mix

• Hydrophilicity :-

- Reversible hydrocolloid impression materials are truly hydrophilic

- Polyether impression materials are also hydrophilic but require a dry field for making impressions

- The original PVS were hydrophobic but wettability is improved in the later formulations

• Shelf life :-

-No more than 6 months’ supply of impression material should be kept on hand at any time

• Patient comfort :-

- Polysulphide & Agar is less patient friendly

• Economic factors :-

- Reversible hydrocolloid is less expensive than elastomeric impression materials

RECENT ADVANCESVisible light cured polyether urethane dimethacrylate

• In early 1988, a visible light cured impression was introduced(Genesis

L.D. caulk).

• Two viscosities :-Light and heavy bodied

• Composition :-

- Polyether urethane dimethacrylate

- Photoinitiators (camphoroquinone)

- Photoaccelerators (Diethyl amino ethyl methacrylate)

- Silicone dioxide (Filler)

• Chemistry :-

Similar to light cured composites

Properties :- - Long working time and short setting time

- Blue light is used for curing with transparent

impression trays

- Tear strength-6000-7500 gm/cm2 (Highest among

elastomers)

- Dimensional stability, flow, detail reproduction,

permanent deformation, wettability, compatibility with cast

and die materials and electroforming is similar to addition

silicone

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

preparation

-Heavy body is loaded onto a 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 mins

• Advantages :-

- Controlled working time

- Excellent properties

• Disadvantages :-

- Need special transparent trays

- Difficult to cure in remote area

Automatic dispensing and mixing devices-This latest technique consists of a double barrel caulking gun with mixing

tip. The tip contains spirals on the inside. Forcing of the base & accelerator results in its mixing.

e.g. Volume mixer (Kerr), Pentamix(3M ESPE)

-Advantages :-More uniform mixLess air bubblesReduced working time

Senn (GC America) :-

Hybrid polyether/polysiloxane material that has both hydrophilicity and

dimensional accuracy, before, during and after set.

MATERIALMATERIAL ADVANTAGESADVANTAGES DISADVANTAGESDISADVANTAGESALGINATEALGINATE Rapid set, HydrophilicRapid set, Hydrophilic

Low cost, No custom tray Low cost, No custom tray requiredrequired

Poor accuracy & surface Poor accuracy & surface details, Low tear résistancedetails, Low tear résistanceLow stability.Low stability.

AGAR-AGARAGAR-AGAR Long working timeLong working timeHydrophilicHydrophilic

Low tear résistanceLow tear résistanceLow stabilityLow stabilityEquipment requiredEquipment required

POLYSULFIDEPOLYSULFIDE High tear resistanceHigh tear resistance Long setting timeLong setting timeUnpleasant odor, StickyUnpleasant odor, Sticky

ADDITION ADDITION SILICONESILICONE

Dimensional stability, Dimensional stability, excellent accuracy, Auto excellent accuracy, Auto mix availablemix available

Hydrophobic, release of Hydrophobic, release of hydrogen gas, hydrogen gas, Most expensiveMost expensive

CONDENSATIOCONDENSATION SILICONEN SILICONE

Fair accuracy, short setting Fair accuracy, short setting timetime

Release of Alcohol Release of Alcohol byproduct, Hydrophobic.byproduct, Hydrophobic.

POLYETHERPOLYETHER Dimensionally stable, Dimensionally stable, accurate, Auto mix accurate, Auto mix availableavailable

Rigid, imbibitionRigid, imbibitionshort working time, short working time,

METHODS OF DISINFECTION OF IMPRESSIONS

Material Method Recommended dis infectant

Comments

Irreversible hydrocolloid (alginate)

Less than ten min. immersion with caution

Chlorine compounds or iodophors

Short term glutaraldehyde also has to be acceptable.

Reversible hydrocolloid

Do not immerse in alkaline glutaraldehyde

Polysulfide Immersion Glutaraldehydes, chlorine compounds, iodophors, phenolics

Disinfectants requiring more than 30 min exposure times are not recommended

Material Method Recommended dis infectant

Comments

Polyether Immerse with caution. Use only disinfectant with a short exposure time (<10 min)

Chlorine compounds or iodophors

ADA recommends any of the disinfectant classes, however short term exposures are essential to avoid distortion.

ZOE impression paste

Immersion preferred, spraying can be use for bite registration.

Glutaraldehydes or iodophors

Not compatible with chlorine compounds! Phenolic spray can be used.

Impression compound

Iodophors or chlorine compounds

Phenolic spray can be used

Classification• Based on the position of the mouth

Open mouth techniqueClosed mouth technique (functional impression )

• Based on the pressure applied Positive pressureNegative pressureSelective pressure technique

• Based on the mixing techniqueDouble mix

Heavy-light body technique Putty wash technique

• One stage – simultaneous

• Two stage – relief channel

- 2mm spacer techniqueSingle mix or monophase technique

• According to the tray used Custom tray impressions Stock tray impressions

Three most common methods for making impressions for fixed restorations are-

Simultaneous dual-viscosity technique,

Single-viscosity or monophase technique, Putty-wash technique

Simultaneous dual-viscosity technique

• low-consistency material is injected with a syringe into critical areas & the high-consistency material is mixed & placed in an impression tray.

• After injecting the low viscosity material, the tray containing the higher viscosity material is placed in the mouth.

• Since both the materials are mixed together, they join, bond & set together.

•After the materials have set, the tray & the impression are removed.

• Advantages

Over comes the polymerization shrinkage of the light body material

Margins duplicated in light body

• Disadvantages

Use of custom tray

An assistant required for mixing the material - tray/syringe simultaneously

Margins duplicated in heavy body in case of excess pressure

Single viscosity or monophase technique

• Impressions often made with a medium viscosity impression material.

• Addition silicone & polyether are well suited for this technique because both have a capacity for shear thinning.

• When the medium viscosity material is forced through an impression syringe, viscosity is reduced, whereas the viscosity of the same material residing in the tray is unaffected. This behavior of the materials is referred to as PSEUDOPLASTIC.

• Advantages

Reduced wastage of the material

Less time consumption

Avoids the time involved in fabrication of custom tray

• Disadvantages

Relatively high viscosity and reduced flow of the

monophase materials, makes their injection onto the

preparation more difficult to control -increased incidence

of surface voids

- Stephen M. Dunne et al 1998

PUTTY -WASH TECHNIQUE

Two steps impression procedure whereby a preliminary impression is taken in high or putty consistency material before the tooth preparation is made.

Space is provided for a low consistency material by a variety of techniques, & after tooth preparation, a low consistency material is syringed in to the area & the preliminary impression reinserted.

The low & high consistency materials bond, & after the low consistency material sets, the impression is removed. This procedure is sometimes called a WASH TECHNIQUE.

The Putty consistency materials & this technique were developed for condensation silicones to minimize the effects of dimensional change during polymerization. This technique was extended to addition silicones after their introduction even though their polymerization shrinkage is significantly lower.

Putty wash simultaneously

technique (One stage)

Light body material syringed on to the

preparation while the putty material loaded in a

stock tray is simultaneously inserted into the

mouth.

• Advantages Reduced chair side time

• Disadvantages Absolute lack of control in the bulk of wash material

By mixing putty, syringe material simultaneously, setting distortion of putty included in over all distortion of impression

- Chee and Donovan 1992

Possibility of margins duplicated in putty medium

Tendency of bubbles to be formed and occluded in the set impression

Putty wash relief channel technique

Pre-operative putty impression is made intra orally

In the area where the tooth is to be prepared,

impression material is removed or channels prepared

using putty cutter instrument

The impression is then relined with low viscosity

material

• Advantages

Impression can be captured with the wash materials

• Disadvantages

To confine the wash material to area of relieved impression

If entire area is washed - creates hydraulic displacement of

putty impression resulting in smaller dies

- Donovan TE 2004

Putty wash 2mm spacer technique

2 mm thick wax spacer is prepared on a diagnostic cast, occlusal stops are provided on non-functional cusps.

A putty impression is made with a stock tray resulting in a putty custom tray with 2mm space for the wash material.

Putty custom tray is then washed with light body material.

• Advantages

Wash stage carried out after the putty has set and

contracted

Controlled wash bulk compensates for this contraction

with minimal dimensional change

• Disadvantages

Extra chair side time

Extra material

Clinicians have an excellent array of impression materials & techniques for fabrication of tooth restorations.

Based on the quality of impressions sent to the commercial laboratories, it seems that many impressions fall short of the quality made possible by current impression materials.

Thus it is important to review the contemporary principles of impression materials & to take impressions consistent with those principles. It is thus important to familiarize with the various specialty impression techniques available & use them when indicated.