EDENTULOUS IMPRESSIONS
ZOE impression paste and impression plaster
ADV Examples of mucostatic impression materials, do not compress the tissue during seating of tray. Ideal material for taking impression of Edentulous structures.
Disadv Are inelastic, cannot be removed past undercuts without fracturing or distorting.
Solution Use very fluid, light body elastomeric impression materials
ELASTOMERIC IMPRESSION MATERIALS
a) Polysulfide, byproduct water. Custom trays recommended for impression making to reduce quantity of material and hence dimensional changes. Lowest viscosity and so one of the least stiff allowing it to be removed from undercut areas with minimum stress
b)Condensation silicone, byproduct ethyl alchohol
c)Additional silicone/ Polyvinyl Siloxanes. No reaction byproduct but reaction between moisture and residual hydride polymers can lead to production of hydrogen gas which can result in pinpoint voids in gypsum if casts are poured soon after removal from mouth. So wait an hour.
Sulphur in latex/vinyl gloves inhibit the setting of this material. Touching the tooth with gloved finger can inhibit the setting of impression material in critical area producing major distortion
d) Polyether
PUTTY refers to high viscosity materials which are highly filled so that less polymer is present and there is less polymerization shrinkage.
IMPRESSION MAKING
Longer the impression material remains in the mouth, less the distortion on removal
I) Multiple Mix technique
a)LIGHT body material- used with syringe and placed directly on hard and soft tissues
b)HEAVY body material- used in tray to support light body material
II) Monophase or Single Viscosity technique : MEDIUM Viscosity materials are used. Single mix is made and part of it is placed in syringe and part on tray. When the medium viscosity material is pushed through the syringe the viscosity decreases (pseudoplastic property). Material in tray retains its medium viscosity and when seated forces the syringed material past critical areas.
III) Putty Wash Technique:
a) Two step technique: Make a preliminary impression with thick putty material in a stock tray using thin polyethylene sheet as a spacer for light body material. This makes a custom tray in which light body material can be used to make a final wash impression. Some light body material can be placed directly onto the preparation.
b)Single step technique: Light body material is syringed in place and unset putty is seated in a tray and then in mouth. Disadv is that the higher viscosity material may displace the lighter material and the critical areas may be reproduced in putty rather than lighter material and the required detail may not be captured in the material.
Both these techniques have distortion as their problem. Inadequate space for light body or distortion of set putty can cause problems.
POURING THE CAST
a) Two or three dies can easily be constructed as these materials are dimensionally stable. Each successive die will be less accurate than the first.
b)The time interval between the impression pours should be less than 30 mins.
c)Polyvinysiloxane materials are hydrophobic which make it difficult to wet the surface by gypsum forming slurry. Use the surfactant spray to form a bubble free cast.
WORKING AND SETTING TIME
Store or Mix the material on cool slab to increase the working time and then the setting time is decreased in mouth at high temp. Working and setting time decrease as viscosity increases.
DIMENSIONAL STABILITY
Polysulfides and Condensation silicones lose reaction byproducts, water and alcohol respectively, so for maximum accuracy, pour these impressions within 30 mins.
Additional silicone and Polyether can be stored from 24 hours to 1 week.
DISINFECTION
Condensation silicones, Polysulfides and Additional silicones can be disinfected with any EPA disinfectant.
Polyethers are suseptible to dimensional changes if immersed for longer than 10 mins due to absorption of water and leaching of water soluble plasticizer.
Long immersion time with Polyvinylsiloxanes may cause the surfactant to leach out rendering the material less hydrophilic and hence difficult to pour.
Rinse and dry the impression after 10 mins of immersion in disinfectant.
EDENTULOUS IMPRESSIONS
ZOE impression paste and impression plaster
ADV Examples of mucostatic impression materials, do not compress the tissue during seating of tray. Ideal material for taking impression of Edentulous structures.
Disadv Are inelastic, cannot be removed past undercuts without fracturing or distorting.
Solution Use very fluid, light body elastomeric impression materials
ELASTOMERIC IMPRESSION MATERIALS
a) Polysulfide, byproduct water. Custom trays recommended for impression making to reduce quantity of material and hence dimensional changes. Lowest viscosity and so one of the least stiff allowing it to be removed from undercut areas with minimum stress
b)Condensation silicone, byproduct ethyl alchohol
c)Additional silicone/ Polyvinyl Siloxanes. No reaction byproduct but reaction between moisture and residual hydride polymers can lead to production of hydrogen gas which can result in pinpoint voids in gypsum if casts are poured soon after removal from mouth. So wait an hour.
Sulphur in latex/vinyl gloves inhibit the setting of this material. Touching the tooth with gloved finger can inhibit the setting of impression material in critical area producing major distortion
d) Polyether
PUTTY refers to high viscosity materials which are highly filled so that less polymer is present and there is less polymerization shrinkage.
IMPRESSION MAKING
Longer the impression material remains in the mouth, less the distortion on removal
I) Multiple Mix technique
a)LIGHT body material- used with syringe and placed directly on hard and soft tissues
b)HEAVY body material- used in tray to support light body material
II) Monophase or Single Viscosity technique : MEDIUM Viscosity materials are used. Single mix is made and part of it is placed in syringe and part on tray. When the medium viscosity material is pushed through the syringe the viscosity decreases (pseudoplastic property). Material in tray retains its medium viscosity and when seated forces the syringed material past critical areas.
III) Putty Wash Technique:
a) Two step technique: Make a preliminary impression with thick putty material in a stock tray using thin polyethylene sheet as a spacer for light body material. This makes a custom tray in which light body material can be used to make a final wash impression. Some light body material can be placed directly onto the preparation.
b)Single step technique: Light body material is syringed in place and unset putty is seated in a tray and then in mouth. Disadv is that the higher viscosity material may displace the lighter material and the critical areas may be reproduced in putty rather than lighter material and the required detail may not be captured in the material.
Both these techniques have distortion as their problem. Inadequate space for light body or distortion of set putty can cause problems.
POURING THE CAST
a) Two or three dies can easily be constructed as these materials are dimensionally stable. Each successive die will be less accurate than the first.
b)The time interval between the impression pours should be less than 30 mins.
c)Polyvinysiloxane materials are hydrophobic which make it difficult to wet the surface by gypsum forming slurry. Use the surfactant spray to form a bubble free cast.
WORKING AND SETTING TIME
Store or Mix the material on cool slab to increase the working time and then the setting time is decreased in mouth at high temp. Working and setting time decrease as viscosity increases.
DIMENSIONAL STABILITY
Polysulfides and Condensation silicones lose reaction byproducts, water and alcohol respectively, so for maximum accuracy, pour these impressions within 30 mins.
Additional silicone and Polyether can be stored from 24 hours to 1 week.
DISINFECTION
Condensation silicones, Polysulfides and Additional silicones can be disinfected with any EPA disinfectant.
Polyethers are suseptible to dimensional changes if immersed for longer than 10 mins due to absorption of water and leaching of water soluble plasticizer.
Long immersion time with Polyvinylsiloxanes may cause the surfactant to leach out rendering the material less hydrophilic and hence difficult to pour.
Rinse and dry the impression after 10 mins of immersion in disinfectant.
Polysulfide (Coe-Flex,Permlastic)
Advantages High tear resistant,Modest Cost
Disadvantages Long working time,Requires custom tray,Odor,Pour within 1 hour,Stains clothes
Vinyl Polysiloxane (Aquasil,Express/Imprint/Imprint II,Extrude,Polysil)
Advantages One material,Easily seen margins,Pour repeatedly,Stable delay pour
Disadvantages Hydrophobic,No flow If sulcus moist,Low tear strength,High cost,Difficult to pour cast
Polyether (Impergum,Permadyne)
Advantages Fast setting,Least hydrophobic,Easily seen margins,Good stability,Delay pour
Disadvantages Stiff, high modulus, Bitter taste, Needs to block undercuts,Absorbs water,Leaches components,High Cost.
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