Auxiliary Materials Flashcards
Six major steps to use elastomeric impression materials to fabricate gypsum models, casts etc
- Preparing a tray
- Managing tissues
3.Preparing impression materials - Making an impression
5.Removing of impression material - Preparing stone casts and dies
Use of custom tray
To reduce the quantity of material required to make an impression
Why is tissue management necessary
To ensure access for tooth preparation and for making an impression
How is tissue managed
Use of gingival retraction cord.
Objective of retraction cord
To displace the gingival tissue laterally away from the margin of the preparation.
One or two gingival retraction cords are placed under the margin around the tooth for atleast 5 min before making impressions.
When is double cord technique used
When the gingival margin is close to gingival attachment.
Manipulation of impression materials are done by?
- Hand mixing
- Static mixing
- Dynamic mixing
What is static mixing
A technique to turn two fluids into homogeneous mixture without mechanical mixing.
In static mixing the mixing top is made of?
Helical mixer elements in cylindrical housing.
What is flow division
The leading edge of the first element split the material entering the mixer into two streams.
The stream flow in either side of the helix will make 180° turn.when they reach the second element.
Both streams are split by the leading edge of the second element and two substream combine to new streams entering the second element.
Common mixing tips have how many elements and striations
11-16 elements and 2048-4069 striations
Dynamic mechanical mixing
Uses a motor to drive parallel plungers, forcing Materials into mixing top and out into an impression tray or syringe.
The use of the motor driven impeller in dynamic mixing
To nic the materials as they are passing through, not propel the material.
Which materials are used to mix in dynamic mixing
Higher viscosity materials
What are the techniques used to make an impression
1.multiple mix technique
2. Monophase technique
3. Putty wash technique
Multiple mix technique
Syringe material and tray material are used.
The syringe material is put around the tooth preparation. The filled tray is then inserted in the mouth seated over the syringe material.
The tray material will force the lighter material to adapt to the prepared tissues.
How to repair an impression in multiple mix technique
By cutting away the interproximal and gingival areas of the impression.
Impression materials surface must be roughened enough to ensure the new material bond to the set impression
Monophase technique is also
Single viscosity technique
Which impression materials are used in monophase technique
Addition silicone and medium bodied polyether
Success of monophase depends on
Pseudoplastic property of material
Putty wash technique was originally developed for which impression material
Condensation silicone
Putty wash technique
Thick putty is placed on a stock tray and made a preliminary impression is made.
Then The light bodied wash is provided in the tray during Preliminary impression making.
A thin consistency wash material is placed into the putty impression and repeated into the mouth to make a final impression
Six main causes of impression inaccuracies
- Polymerization shrinkage
2 loss of condensation reaction byproduct - Thermal contraction from oral temperature to room temperature
- Absorption of water or disinfectant over a period of time
- Incomplete recovery from deformation because of viscoelastic behaviour
- Incomplete recovery because of plastic deformation
How to know when to remove the impression material from the mouth
Inject some of the syringe material onto a space that is not field of operation before inserting the impression tray
Thie material can be probed with a blunt instrument from time to time.
When it’s firm and returns completely to its original contour the material is sufficiently elastic to be removed.
What are the mechanics of removing the impression materials
- Breaking the physical adhesion between material and tissue.
- Stretching of the material enough to pass under the height of contour of hard tissue
Which has the lowest viscosity and the least stiff elastomeric impression material
Polysulfide
Advantage and disadvantage of Hydrophobic characteristic of silicone impression
Advantage:- makes them suitable for pouring of epoxy resin to produce dies.
Disadvantage::- makes pouring with gypsum products challenging. Because it increases risks of voids in gypsum and casts
How is Hydrophobicity overcome to make gypsum products
Use of debubblizers.
Use of surfactant sprays.
Use of dilute solution of soap
An alternative to the use of surfactant is to select hydrophilized addition silicone
Which material do not need the aid of surfactant
Polyether and polysulfide
Who is it not advisable to alter the base/catalyst ratio as means of controlling working or setting time?
Working and setting time decreases as the filler content in material increases.
Altering the base or catalyst will change the curing rate of the material.
Since accelerator paste contains retarder as well as reactor, increasing it may not produce a predictable change
What are two categories of shear-thinning phenomenon
- Thixotropy
- Pseudoplasticity
What is pseudoplastic material
It displays decreasing viscosity with increasing shear stress and recovers it’s viscosity immediately upon decrease in shear stress
What is thixotropic material
It does not flower until sufficient energy in the form of impact force or vibration force is applied to overcome the yield stress of the material.
At rest , it takes specific time to regain previous viscous state
Permanent deformation in compression following strain induced in impression materials increases as:-
1.Addition Silicone
2. Condensation silicone
3. Polyether
4. Polysulfide
The stiffness (elastic modulus) of impression materials of similar viscosity increased in order of:-
Polysulfide
condensation silicone
Addition
Polyether
What is tear strength
The amount of force needed to tear a specified test specimen divided by thickness of specimen
Trea strength from higher to lower:
Silicones (Addition and condensation)
Polyether
Polysulfide
Tear energy
Dividing force needed to crack propagating divided by t/(1+lambda). Where t is thickness of specimen and lambda is extension ratio.
Extension ratio is extended portion of length ∆l divided by orginal length l
Which material has highest resistance to tearing
Polysulfide
Why are hydrophilic materials potentially more susceptible to distortion prior to being poured with a gypsum forming products
Because long immersion time may cause the surfactant in hydrophilic polyvinyl siloxane to leach out and render impression less hydrophilic
How does surfactant added to hydrophilized PVS affect contact angle of water droplets during setting and pouring of casts with gypsum
The water droplets may absorb a sufficient quantity of surfactant sprays and become a liquid of lower surface tension.
Lowering of surface tension makes water droplets spread wider on surface of PVS yielding lower contact angles.
Which impression material has highest and lowest cytotoxicity
Highest :- Polyether
lowest :- Polysulfide
What is colloid
A substance that is microscopically dispersed uniformly throughout another substance
What is hydrocolloid
Colloids have two phase:- dispersed and dispersion phase
When the colloid’s dispersion phase is water, it is hydrocolloid
Physical change of Agar
Reversible.
The sol turns to gel in cooler temperature. This is also called gelation temperature (37-50°C).
The gel turns to sol in warm temperatures. Also called liquefaction temperature (70-100°C)
Composition of Agar
Main component:- polysaccharide extracted from certain seaweed
And water.
Filler:- Borax
Accelerator:- potassium sulphate to counteract borax’s Retarder effect.
Thymol and glycerine added as a bactericidal and plasticizer
The three compartment conditioning unit of Agar making
- Liquefaction (100°C) for 10 mins
- Storage bath (65°C)
- Tempering (45°C)
The Agar syringe material is
Not tempered, as the material should be in a fluid state to enhance tissue adaptation
Gelation of the Agar impression in mouth is accelerated by
Circulation of cool water (18°-21°C) through tray for 3-5 mins
If the Agar material is held rigidly in tray then?
The impression material will shrink towards the centre of its mass thereby creating larger dies
How Alginate sets
By crosslinking of algic acid with calcium ions
Describe making of Agar impression
Requires three compartment conditioning.
1. To allow liquifaction of hydrocolloid in boiling water of 100°C for minimum 10mins
2. Then placed into storage bath of 65°C to retain Sol condition until needed
3. For immediate preparation,the impression tray is filled with the storage bath material and a guaze is place over it and the tray is placed in water-filled tempering compartment at 45°C .
Composition of Alginate
Brown seaweed extraction which is a linear copolymer of beta-D-mannuronic acid and alpha-L-glucouronic acid.
For impression making alginate is richer is mannuronic acid.
Main component -sodium/potassium or triethanolamine alginate.
Filler-Diatomaceous earth, zinc oxide
Reactor- Calcium sulfate dihydrate
Accelerator - potassium titanium fluoride
Types of Alginate
- Dustless
- Disinfectant alginate
- Chromatic Alginate
4.siliconed Alginate
How does the retarder optimise the working time of alginate
The process of production of calcium alginate is so rapid that it does not allow sufficient working time.
A water soluble phosphate salt is added to composition as a retarder.
Calciums ions will react with the phosphate and thus, the rapid reaction between calcium and soluble alginate is deferred until phosphate ions from trisodium phosphate are exhausted.
Setting time for fast setting and normal setting alginate
Fast setting - 1.5-3 mins
Normal setting - 3-4.5 mins
How can clinicians control the setting time of Alginate
By altering the temperature of Water .
Higher the temperature, shorter is setting time
Why is it important to add water to a rubber mixing bowl before adding powder when mixing fast setting alginate
If the powder is placed first in the bowl, penetration of the water to the bottom of the bowl is inhibited and a longer mixing time may be required to ensure homogeneous mix
How much should be the thickness between impression and Tissue in Alginate
3mm
Results of insufficient Spatulation of alginate
Failure of the ingredients to dissolve sufficiently so that chemical reactions can proceed uniformly throughout the mass
Result of Overmixing of Alginate
Breaks up the formation of calcium alginate network and reduces it’s strength
Two major factors that affect shelf life of Alginate
1.Storage Temperature
2.Moisture contamination from ambient air
How can distortion of hydrocolloid impression material be minimised during storage
2% potassium sulphate solution or 100% relative humidity in the storage chamber to reduce dimensional change of Agar impression
Why do agar and alginate include a gypsum hardener and how does it function
Agar includes borax which is Retarder.
In Alginates, it produces not only insoluble calcium alginate but also sodium sulfate which is Accelerater at low concentrations but in High it acts as a retarder.
To overcome the gypsum setting it is done by:-
1. Immersing of Agar in a solution of 2% potassium sulphate prior to pouring impression with gypsum
2. By incorporating gypsum Hardener in material such as sulfate in Agar and Potassium titanium fluoride in Alginate
Optimal method for disinfecting hydrocolloid impression material without causing distortion or degradation
- To use household bleach, iodophors, synthetic phenols.
- Immersion of material But should not exceed 10 mins
What is the principal that makes Laminate technique (agar-alginate Technique) effective?
Agar in the tray is replaced with a mix of chilled alginate that bonds to Agar expressed from syringe.
Alginate gels by chemical reaction whereas,
agar gels by contact with cool alginate rather than water circulation through tray.
Since Agar is the one that contacts the prepared teeth, maximal detail is reproduced. Because only syringe material needs to be heated, equipment cost is lower and less time preparation
Main disadvantage of Laminate technique
- Bond between Agar and Alginate is not always sound
- Viscosity of Alginate displaces the Agar during seating
- Dimensional inaccuracies of alginat limits it’s use to single units
How is The Bonding Issue in the Laminate technique adressed
It requires that the bond between the two materials under Tensile strength should be in excess of 50kPa
Impression plaster is
Beta-calcium sulfate hemihydrate.
Why is Impression plaster suitable for taking impressions of soft tissues
Because of its Fluidity, it has Mucostatic characteristic.
Composition of Impression plaster
Potassium sulfate -antisetting expansion agent
Borax- retarder
Which pigment is added to make a clear distinction between impression and the model after the casting of nodel
Alizarin Red
What is beading. What does it provide?
Long, narrow strips of wax can be adapted around the periphery of impression.
It provides a clear indication of where the impression ends which prevents from over trimming and Overextension
Disinfection of impression plaster
10 min soak in sodium hypochlorite solution
Impression compound is
Also called as modeling plastic.
It’s a thermoplastic material
Types of impression compound
Type 1- low fusing Material for making impressions in oral cavity
Type 2- higher fusing also called tray compound. Used for forming trays to be used in oral cavity
What is the other common application of Impression compound
Border molding of Acrylic custom tray during fitting of the tray
Composition of Impression Compound
Mixture of waxes, thermoplastic resins, filler and a coloring agent.
Shellac, stearic acid, Gutta percha are added to improve plasticity
Disinfectant of impression compound
2% alkaline glutaraldehyde solution
Composition of ZOE
Base - zinc Oxide
Fixed vegetable or mineral oil
Accelerator:- oil of cloves or Eugenol
Gum or polymerized rosin
Filler
Lanolin
Resinous balsam
Accelerator solution (CaCl2) and color
Chief Disadvantage of ZOE
Possible stinging or burning sensation caused by Eugenol as it leaches out and contacts soft tissues
How can the clinician shorten time of setting of ZOE
By adding a small amount of zinc acetate or a drop of water in paste before mixing.
The use of cool spatula and mixing slab is also effective in prolonging the setting time
Disinfectant of ZOE
2% alkaline glutaraldehyde solution
Other applications of ZOE
Bite registration paste.
Surgical dressing
Temporary filling material
Root canal filling material
Cementing medium
Temporary reclining material
