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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Elastic impression materials
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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Elastic impression materials
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
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