7+ +impressions

7/30/2019 7+ +Impressions

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Impressions in FixedProsthodontics


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Impressions

Imprint or negative reproduction, made byplacing soft, semi-fluid material in the mouth tillthe complete setting.

positive reproduction or cast


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ImpressionsRequirements:

Accurate reproduction of the prepared tooth(prepared and uncut surface).

Accurate reproduction of other teeth and tissueadjacent to the prepared tooth.

Absence of bubbles.


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Successful Impression

Choice of Impression Material

Technique of Impression


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Impression materials

Impression materials are commonly classified byconsidering their elastic properties once set.

non-elastic materials elastic materials


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Properties of Impression materials

Wettability

Viscosity Dimensional stability

Cost


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Wettability

Ease of pouring with gypsum products

Defect free impression no air bubbles

hydrophilic, ex.: polyether, hydrocolloids,

hydrophobic, ex.: polysulfide, silicones


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Viscosity

Viscosity fluidity

consistency: light, medium, heavy, putty


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Viscosity


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Viscosity

Thixotropy: property that allows a material to stay in atray without dripping and have low viscosity to beused in a syringe.

viscosity of the material is reduced by increasing the

shear rate


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Elastic impression materials

Hydrocolloids Elastomeres

Reversible Irreversible PolysulfidePolyether

Silicone


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Hydrocolloids Elastomers

Reversible Irreversible

(Agar agar) (alginate)


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Reversible hydrocolloid (Agar)

Passes repeatedly between highly viscous gel and lowviscosity sol simply through heating and cooling


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Reversible hydrocolloid

need expensive conditioning baths and water cooled

trays

poor dimensional stability

low tear resistance


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Irreversible hydrocolloid


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Irreversible hydrocolloid

alginatevery poor dimensional stability

Temp. affects the setting time

low tear resistance


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Hydrocolloids Elastomers

Reversible Irreversible Polysulfide

Polyether

Silicone


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Polysulfides light body (low viscosity), medium or regular body

and heavy body (high viscosity).


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PolysulfidesAdvantages:

long working time (decreased by raising temperature or

moisture) excellent tear resistance

cheap


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Polysulfides

Disadvantages:

setting reaction long

messy to handle

objectionable odor

Shrinkage as a result of a continued settingreaction + evaporation of water produced as aby-product of the setting reaction


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Polyethers single regular viscosity

heavy light bodied system


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PolyethersAdvantages:

no reaction by-product dimensional stability

adequate tear resistance very good elastic properties

hydrophilic

Disadvantages:

high elastic modulus ,rigid when set

absorption of water on storage in conditions of highhumidity


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Silicones Classified according to their method of

polymerisation on setting

condensation curing (or Type I) silicones and addition curing (or Type II) silicones.

Available in: light, medium, heavy and puttyviscosities


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Silicones (condensation-cured) good elastic properties

adequate tear resistance

non-toxic and absolutely neutral in both colorand taste.


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Silicones (condensation-cured) setting reaction produces a volatile by-product (ethyl

alcohol)

shrinkage

Highly hydrophobic


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Silicones (condensation-cured)


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Silicones (addition-cured) Highly dimensionally stable (pouring can be delayed

7-10 days)

Adequate tear resistance

Excellent elastic recovery


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Silicones (addition-cured) release of H2 that produces bubbles in models

prepared within the 1st hour

hydrophobic


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Silicones (addition-cured) Setting inhibition caused by some brands of latex

gloves.


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Silicones (addition-cured)


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Trays

Tray selection depends on impression material used

and viscosity Irreversible hydrocholloid and most elastomeric

impressions can be made using prefabricatedimpression trays

A custom tray improves the accuracy of elastomericimpression by limiting the volume of material

The opposite is true regarding hydrocholloids


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Mixing


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Impression Technique

Usually we use two viscosities

1- high (heavy body or putty): make the bulk of theimpression

2- low (light body or wash): gives details


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Two step putty/wash

One step double mix

Monophase ,single mix

Impression Techniques


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Two-step technique

(high pressure) putty is recorded first and after setting relined with a

thin layer of wash

Used with condensation silicon and additional silicone


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Two-step technique

(high pressure)


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Two-step technique

(high pressure)


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Two-step technique

(high pressure)


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Two-step technique

(high pressure)


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Two-step technique

(high pressure) Suitable for subgingival margins


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One step, double mix technique

(low pressure) putty and wash are recorded simultaneously (also

called twin mix or laminate technique)

Tray: putty or heavy viscosity

Syringe: low viscosity


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Monophase , single mix technique

One viscosity:

In the custom tray and syringe


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Evaluation Proper mix: no visible streaks of base or catalyst

Accurate seating of the tray: custom tray should notshow through

Absence of voids, folds or creases especially in criticalarea around finish line

Even un-interrupted extension of impression material

beyond the margins of the preparation No seperation between impression material and the

tray


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Disinfection


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ThanQ

7+ +impressions

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