FDP Impressions

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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