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Impressions in FDP

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CLASSIFICATION OF IMPRESSIONMATERIALS

BASED ON SETTING MECHANISM

1. Reversible ( Temperature changes)

2. Irreversible( Chemical changes)

BASED ON MECHANICAL PROPERTIES

1. Rigid ( Edentulous ridge )

2. Elastic ( tooth form )

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ELASTIC

IMPRESSION MATERIALS

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WHAT IS AN IMPRESSION MATERIAL ?

ANY SUBSTANCE OR COMBINATION OF

SUBSTANCES USED FOR MAKING AN

IMPRESSION OR NEGATIVEREPRODUCTION ..

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ELASTIC IMPRESSION MATERIALS

1.Aqueous irreversiblea) Alginate

2. Aqueous reversiblea) Agar

3. Non aqueous irreversible

a) Polysulfidesb) Polyethersc) Condensation siliconed) Addition silicones

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HISTORY

These materials were developedto mimic natural rubber whenthere was shortage duringWorld War II

They are classified as synthetic

rubbers

ADA Specification no. 19

The first synthetic rubberlike

materials were produced by aprocessVulcanization orcuring

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Classification of Elastomers based on Viscosity :

a) light body

b) medium body

c) heavy bodyd) putty

Addition silicones available in ( Extra-low andmonophase too )

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Elastomers

Term rubber and elastomer are scientifically identicaland interchangeable

Rubber generally referred to natural rubber

Elastomer is synthetic

They are a special group of a wider group calledpolymers

Made of long flexible chain or string like molecules

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The inside of rubber can be imagined as pile of cookedspaghetti

Each chain of elastomers will be joined occasionallyalong its length to one or more nearby chains with fewchemical bridges

Called cross links

Whole structure forms a coherent network whichprevents the chain from sliding past one another

The process by which cross links addedvulcanisation

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MIXING SYSTEMS

Three systems available :

1) Hand mixing

2) Static automixing

3) Dynamic mechanical mixing

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Hand Mixing

Impression paste dispensed from collapsible tubes

Equal length of base and catalyst taken

Initial mixing in circular motion

Final mixing with broad strokes of spatula

Mixing accomplished in 45 seconds

In case of two putty system kneading with fingers is

performed

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Kneading with fingers

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STATIC AUTOMIXING

Base and catalyst in separate plastic cartridges

Cartridge placed in mixing gun having 2 plungers

The base and catalyst forced through static mixing tip containinginternal spiral

The two components folded over each other many times as theyare pushed through the spiral

Uniform mix obtained

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DYNAMIC MECHANICAL MIXING

Catalyst and base supplied in large plastic bags housed in acartridge

Inserted on top of mixing machine

Plastic mixing tip ( motor driven ) in front of the machine

Parallel plungers push collapsible bags

Material forced into the mixing tip

Mixing accomplished by rotation and forward motion

Thus higher viscosity material mixed with ease

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Problems in hand mixing :

possibility of contamination

more air incorporation

more material wastedmight not be a uniform mix (homogenous)

Vigorous mixing while hand mixing can lead toincorporation of air

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Mixing tips : available with 11 and 13 spirals

the spirals participate in shear thinning of material thus giving us

more working time ..

material mixed with 13 spirals cant be mixed with 11 spirals as the no.

of spirals might not be enough to provide a homogenous mix

In automix after the first ejection of material the tip should be placed in

the material and further material ejected to avoid air incorporation

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Automixing and dynamic mixing in comparison to

hand mixing ::superior physical properties

uniform mix

reduction of voidsavoidance of contamination

During automixing air can get incorporated whileplacement of cartridge in the gun .. Incorporating lotof air before closing it in the gun

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( accdg to an article by JUNGNAM et al )

dynamic mixing was given overall preference to

automixingon the basis of following criteria

ease of mixing

control of loading

quality of mixing

level of cleanliness

contamination

Duration of mixing for dynamic mixing was said to beslower than automixing

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Ideal Requirements

Dimensionally stable Radioopaque Tear resistance Hydrophillic No reaction by products Easy to use Superior colour No odour

Biocompatible accurate surface detail reproduction Maintain accuracy with multiple pours Optimal working time

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Elastic recoveryaddition > condensation > polyether > polysulfide

A set impression must be sufficiently elastic so that it will

return to its original dimensions without significant distortion

upon removal from mouth

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Flexibility of the material is related to the

Glass Transition Temperature( low temperature behaviour)

GTT of different elastomers is differentEg . Natural rubber it is70 C

Therefore it means below this temperature the material behaves like

glass and on hammering can break like glass.

GTT is generally a range 10 degrees

At a temperature above GTT the material will be rubbery .

( flexible )

Strictly speaking we should we should only use the termelastomer to describe a material only if it is above its GTT

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HIGH TEMPERATURE BEHAVIOUR

The upper temperature to which the elastomer can be used dependson its chemical stability

At high temperature the elastomer gets attacked by oxygen

This attack results in a chemical reaction

2 type of reaction

a) degradativebreaks cross linksmakes rubber soft

b) addition of cross linkshardens the rubber

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Dimensional shrinkagepolysulfide > condensation > polyether > addition

The amount of shrinkage a material undergoes once thepolymerization process is allowed to proceed

Factors which cause dimensional shrinkage :

1) loss of reaction by products2) polymerization shrinkage

3) thermal contraction

Polysulfides and condensation silicones have highest dimensionalchange during setting ( 0.40.6 ) %

shrinkage here is due to evaporation of volatile by products and

rearrangement of bonds with polymerization

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Addition silicone have a dimensional change of

maximum of 0.15 %

Polyether dimensional change0.2%

Less in addition silicone and polyether as there are no reactionby products

Dimensional stability is important if the impression has to besent to the lab and impression poured for that time the

material should be dimensionally stable

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Dimensional accuracy :-

1) Greatest dimensional accuracy occuring

immediately after polymerization complete

2) But declining as the impression is stored for extended

periods of time

3) pvs and polyether dimensionally stable for

1-2 weeks after taking impression

4) polysulfide accurate if poured within 1-2 hours of makingan impression

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Tear strengthpolysulfide > polyether > addition = condensation

Tear strength refers to the property of the material to resisttearing when removed from undercuts

In deep sulcus the impression can tear thus compromising theaccuracy of the impression

While retrieving the cast from the impression the material mustnot tear so that repeated pours can be made

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Hydrophillicity

Also referred to as Wettability relates to the ability of amaterial to flow in small areas

Impression that wet teeth well , displace moisture wellcreate

less voids

If material has high wetting angle it does not flow in smallcrevices ( thus not a good material for FPD )

For FPDmaterial must reproduce detail in area of 20-70 m

For RPD100-150 m of detail reproduction is ok

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Hydrocolloids , Polyether , Polysulfides have relatively lowcontact angle

PVS requires surfactants to lower con