A Seminar on
IMPRESSIONS IN FIXED PARTIAL DENTURES
PRESENTED BY, SIGNATURE OF H.O.D
Dr.U.Varalakshmi,
II Year P.G.Student.
Department Of Prosthodontics
SIBAR Institute Of Dental Sciences
GUNTUR.
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IMPRESSIONS IN FIXED PARTIAL DENTURES
CONTENTS
DEFINITIONS
IDEAL REQUIREMENTS OF IMPRESSION MATERIALS
CLASSIFICATION OF IMPRESSION MATERIALS
IMPRESSION TRAYS
IMPRESSION TECHNIQUES FOR DIFFERENT IMPRESSION
MATERIALS
SPECIAL CONSIDERATIONS
CONCLUSION
REFERENCES
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Definitions
Impression
An imprint or negative l ikeness of the teeth and/or edentulous areas
where the teeth have been removed, made in a plastic material which
becomes relatively hard or set while in contact with these t issues.
Impression material
Any substance or combination of substances used for making a
negative reproduction or impression.
Impression tray
A device which is used to carry, confine and control an impression
material while making an impression.
Ideal requirements of impression material (According to Tylman)
Complete plasticity before use
Sufficient fluidity to record fine detail
The abili ty to wet the oral tissues
Dimensional accuracy
Dimensional stability
Complete elasticity after cure
Optimal stiffness
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Have a good shelf l ife
Be non-toxic and non-irritating
Have acceptable taste and odor
Have suitable working and setting t imes
Have strength to resist tearing
Be compatible with model and die material
Facilitate clinical identification of beginning and end of cure
Facilitate visualization of the finish l ine
An impression for a fixed restoration should meet the following
requirements
It should be an exact duplication of the prepared tooth, including all
of the preparation and enough uncut tooth surface beyond the
preparation for the dentist and technician to be certain of the
location and configuration of the finish line.
Teeth and tissues adjacent to the prepared tooth must be accurately
reproduced to permit proper articulation of the cast and contouring
of the restoration.
It must be free of bubbles, especially in the area of the finish line
and occlusal surfaces of other teeth in the arch.
Impression materials that are used in fixed prosthodontics are
Reversible hydrocolloids
Polysulfide
Condensation silicone
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Addition sil icone
Polyether
urethane dimethacryalte
Classification
According to chemical nature
Hydrocolloid impression material
Elastometric impression material
Each of them are further classified according to the viscosity
• Heavy body
• Regular/ medium body
• Light body
• Very heavy/ putty
Reversible hydrocolloid (Agar)
The credit for its first use in United States for fabricating cast
restorations is given to J.D. Hart (1930). It is a polysaccharide extracted
from certain types of seaweed.
Composition
Ingredients Percentage by weight
Function
AgarBorates
Potassium sulfateWax hard diatomaceous earth
13 – 17%0.2 – 0.5%
1 – 2%0.5 – 1.1%
Basic constituentImproves strength of the gel retards plaster or stone
Gypsum hardenerFiller
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SilicaClayAlkyl benzoatesThizotrophic materialsColoring agentsWater
0.1%0.3%-Balance
PreservativesPlasticizers-Dispersion medium
Gellation process
The setting process of reversible hydrocolloid is called as a
gellation process i t is due to change in temperature.
It is available as tray material, or as syringe material as semi-solid
gel in polyethylene tubes.
The cycle is
Gel ---------- Sol ------------- Gel
(Tube) (Conditioner) (Tray)
Agar requires a conditioning unit which has three compartments one
for liquefying were the tubes are placed at 1000C as it is too hot for intra
oral use i t is cooled in two stages: storage and tempering. Addition to
lowering the temperature it also increases the viscosity. The storage
temperature is 700C and can be kept for 5 days. Tempering is done just
before the impression is made that is between 370C – 500C. After loading
in the tray and placing in the mouth water is circulated at 170C – 210C
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until the material is completely gelled. This is done using water cooled
rimlock tray
Elastomeric impression materials
Polysulfide (Mercaptan, Thiokol)
Composition
Base
Liquid polysulfide polymer 80 – 85%
Inert fi llers (TiO, Zn Sulfate, Copper carbonate) 16 – 18%
Plasticizers (Dibutyl phathalate)
Accelerator paste
Lead di-oxide 60 – 68% - Reactor
Dibutyl phathalate 30 – 35% - Plasticizer
Sulfur 3% - Retarder
Setting reaction
This polymer is usually cross l inked with an oxidizing agent such as
lead dioxide. It is this lead dioxide that gives polysulfide its characterist ic
brown color. This is a condensation reaction where all the polymer chains
grow simultaneously and a reaction by product is formed like water.
They are available as
Light body
Regular body
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Heavy body
Polysulfide impression material is hydrophobic and should be
poured within 1 hour of removal from the mouth and it is a radio opaque
impression material because of the presence of lead dioxide.
Condensation silicone
Composition
Base paste
Poly dimethyl siloxane 25 – 65%
Colloidal sil ica or micronize metal oxide fi ller (Depending upon the
viscosity) 35 – 75%
Coloring agents
Accelerator paste
Orhtoethyl si licate – cross linking agent
Stannous octate – catalyst
Available as
Light body
Regular body
Heavy body
Putty
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Setting reaction
This is condensation polymerization reaction which reacts with
trifunctional and tetrafunctional alkyl si licates l ike orthoethyl sil icate in
the presence of stannous octate. Ethyl alcohol is a by product which is
responsible for shrinkage of the material.
The condensation sil icone impression materials are supplied as a
base paste and a low-viscosity liquid or catalyst paste. Since ethyl alcohol
is the by product they are dimensionally stability is poor and they should
be poured immediately after the removal from the mouth. They have a
very poor shelf l ife because of the instability of alkyl sil icates in the
presence of organo-tin compounds, which may result in the oxidation of
tin.
Addition sil icone
Base paste
Poly methyl hydrogen siloxane
Other siloxane polymers
Fillers
Accelerator paste
Divinyl siloxane
Other siloxane prepolymers
Platinum salt (chloroplatinic acid) catalyst
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Palladium – hydrogen absorber
Fillers
Available as
Light body
Regular / medium body
Heavy body
Putty
Setting reaction
It is addition polymerization reaction terminated with vinyl groups
and is cross linked with hydride groups activated by a platinum salt
catalyst. There is no reaction by products as long as correct proportions of
vinyl silicone and hydride sil icone are manipulated and there are no
impurities, if the proportion is out of balance or impurities are present
then side reactions will produce hydrogen gas. This is avoided by the
manufacturer adding noble metals or palladium or platinum to act as
scavengers another way is to wait for 1 hour before pouring up the
impression.
Trays used
Custom tray / resin tray
Stock tray
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Polyether
Base paste
Polyether polymer
Collidal sil ica
Glycoether or dibutyl phathlate
Accelerator paste
Aromatic sulfonate ester – cross linking agent
Collidal sil ica
Dibutyl phathalae or glycoether
Available
Single viscosity (both syringe and tray material)
Regular
Setting reaction
It is a addition polymerization reaction with no by products the
reaction is between aziridine rings which are at the end of branched
polyether molecules. The main chain is a copolymer of ethyle oxide and
tetrahydrofuran. Cross linking and setting is brought about by an aromatic
sulfonate which Acts as an initiator by releasing a cation R+ an alkyl
group. This produce the cross l inking by cationic polymerization via the
imine end groups.
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Trays used
Custom / resin trays
Stock tray
Comparison of advantages and disadvantages
Material Advantages Disadvantages Recommended uses
Precautions
Reversible and hydrocolloid
Hydrophil ic
Long working t ime
Low cost No custom
tray required
Low tear resistance
Low stabili ty
Equipment needed
Multiple preparations
Problems with moisture
Pour immediately use only with stone
Polysulfide High tear strength
Easier to pour than other elastomers
Messy Unpleasa
nt odor Long
set t ing time Stabil ity
only fair
Most impressions
Latex gloves use in contra-indicated for all the elastomers. Pour within 1 hour.
Addition si licone Dimensional stabili ty
Pleasant to use
Short set t ing time
Automix available
Hydrophobic
Poor wetting
Some material release hydrogen
Most impressions
Delay pour some materials , care to avoid bubbles when working
Condensation sil icone
Pleasant Hydropho Most Pour immediately,
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to use Short
set t ing time
bic Poor
wetting Low
stabili ty
impressions care to avoid bubbles when pouring
Polyether Dimensional stabili ty
Accuracy Short
set t ing time Automix
available
Set material very sti ff
Most impressions
Care not to break teeth when separating cast
Trays
Various type of trays are used for fixed partial dentures impression
procedures.
1. Stock tray
Metallic
Non metallic
2. Custom made trays
Autopolymerizing acrylic resin
Thermoplastic or photo initiated resin
3. Water cooled rim lock trays
Custom trays
Advantages
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Improves the accuracy of an elastomeric impression by limiting the
volume of the material, thereby reducing the 2 sources of error
a. Stress during removal
b. Thermal contraction
No need for sterilization
Uniform thickness of the impression material minimizes distortions
resulting from curing shrinkage
Procuring of the tray material is not required
Disadvantages
Time taken for the fabrication
Aging for 24 hours to minimizes the distortion
Sensitivity to monomer
Steps in fabrication
Soak replicas of diagnostic casts in slurry water for 10 mins.
Tray extensions are marked with the pencil l ine at either the
cervical region of the teeth or 5mm below the cervical line.
Base plate wax is adapted (for 2 sheets thickness – 2-3mm) after
softening on the Bunsen burner and trim the excess with the knife
until the marked pencil l ine .(Rigid or stiff material like polyether
requires of wax spacer of 4mm thickness).
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Cover the wax with tin foil , or aluminium foil for preventing the
wax to melt during the exothermic heat produced during the
polymerization of the resin (autocuring).
Wax is removed either in 4 or 3 tripodal areas 3 sq.mm and located
not on the prepared tooth but on non-functional or non-centric
cusps. If all the teeth are prepared then soft tissues stops either on
the palatal area or the crest of the ridge is placed.
Needed amount of polymers and monomers of autopolymerizing
resin is mixed as per manufacturer instructions t ill the dough stage
is attained.
Then it is flattened to almost 4mm thick putty. It is then adapted
and molded over the tin foil separation and excess is trimmed off.
A handle is formed along with 2 buccal wings or ridges on either
side for the easy removal.
After the set i t is slowly removed form the cast, check if the stops
are proper.
It can then be placed in the water for 9 – 24 hours (Rosenstiel) or 5
minutes in boiling water before use.
A tray adhesive can be applied 15 minutes before placing the
impression material and allow it to dry (inside and outside the bond
of the tray)
Advantages of stock tray
Eliminating the time and expenses of fabricating a customized tray.
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They are rigid metal and susceptible to distortion.
Impression techniques
For reversible hydrocolloid
It requires a hydrocolloid conditioning unit. A conditioning unit has
three units.
Liquefying bath
Storage bath
Tempering bath
1. Liquefying bath
Tubes of impression materials and syringes are boiled at 2120F
(1000C) for 10 – 12 minutes.
2. Storage bath
Stored at 1500F (650C) at least for 10 minutes. The material can be
stored for 5 days.
3. Tempering bath
Loaded impression trays are tempered in this bath at 1100F to
1150F (about 400C) for 5 – 10 minutes immediately before placing in the
mouth.
Procedure
Because only one accurate cast can be made from a hydrocolloid
impression, two impressions are made
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1. A sectional impression for making a die (made first)
2. A full arch impression for the working cast
After selecting a proper tray and placing stops in the proper area.
Tray material is kept on the tray for tempering.
Low viscosity syringe material is placed in the bath which is then
removed and applied after the retraction cord is removed and then
the tray with heavy body is placed into the mouth and cold water is
circulated.
It is then removed after holding without movement with a rapid
motion, washed with cold water, inspected, disinfected and
immediately type 4 stone is poured.
Wet field technique
The areas of the teeth and tissues are flooded with warm water.
Syringe material is taken directly from tempering compartment and
added to prepared cavities, first at base of preparation and then the
tooth is covered.
The material used to fill the tray should be cooler or tempered.
Gelation is accelerated by circulating cool water, through the tray
for 3 – 5 minutes.
It is postulated that the hydraulic pressure of the viscous tray
material forces the fluid syringe hydrocolloid into the area to be
restored.
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Laminate technique
A recent modification to the traditional agar procedure is the
combination of agar and alginate impression materials.
The tray hydrocolloid is replaced with a mix of chilled alginate,
that bonds with the syringe agar.
The alginate gels by chemical reaction, but agar gels by means of
contact with cool alginate rather than water circulating through the
tray
Advantages
Less preparation time and less complicated when compared to the
wet field technique.
Disadvantages
Bond between agar and alginate is not strong
High viscosity alginate displaces agar during seating
Dimensional inaccuracy of alginate l imits the use to single units.
Techniques for electrometric impression materials
1. Using stock tray
Synonyms – Putty wash
Mixing method – double mix and single mix
Advantages
Eliminates time and expense of fabricating custom tray
Metal stock trays are rigid and are not susceptible to distortion.
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Disadvantages
More impression material is required.
Must be steril ized
Technique
Select a stock tray and coat with adhesive
Mix high viscosity l putty and roll it in the shape of the cylinder
and load it on the tray, give a space which is a sheet of polyethylene
and seat with rocking motion and wait t ill the initial set (2 minutes)
and then remove from the mouth with the minimal sideward
movement.
Gingival retraction done
After lubricating the O-ring of the syringe, the needed amount of
the low viscosity material is mixed on a pad and either loaded in the
syringe by scraping or by making a paper cone and then fill ing the
syringe.
Remove the retraction cord gently and syringe inaccessible areas
first (e.g.) disto lingual finish l ines.
Now insert the tray with low viscosity impression material
Position the tray over the arch
Apply force in a vertical direction until further seating is
impossible.
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After material is set, insert two index fingers under each side of the
tray to break the seal.
Remove the tray in the direction parallel to the preparation.
Evaluate the set impressions.
Difference between single mix and double mix technique
In single mix technique one viscosity material (regular body) is
used to fil l the tray and load the syringe.
In double mix technique, the light body is loaded in the syringe and
syringed around the tooth prepared and heavy body is mixed and
loaded in the tray.
2. Using custom tray
Procedure
Do the gingival retraction and Prepare the syringe
In a mixing pad mix the low viscosity impression material as per
manufacturer’s instruction. First use circular motion combining the
two strands then a finger of eight motion, obtaining a streak free
mixture in a less than 1 minute.
Load the syringe as well as the tray.
Syringe the material in the inaccessible area first then subsequently
removing the retraction cord gently and then syringe the impression
material.
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Now insert the tray. Seal from posterior to anterior allowing excess
to extrude in an anterior direction.
Continue seating in a vertical direction until the trays stops prevent
further progress.
After the material is completely set , remove the impression parallel
to the preparation path.
Evaluate the impression.
Closed bite double arch method
Also called as Dual Quad tray, double arch, triple arch and closed
mouth impression.
Minimum conditions
Either natural teeth or an incisal pin and table should be provided in
the articulator as vertical stops.
Sufficient space distal to the last tooth should be present to allow
tray approximation.
Intact dentition, class I occlusion, bounded on either side by intact
teeth, opposing tooth having intact occlusal contact is ideal.
Advantages
Physical deformation of the impression by mandible during opening
is minimized.
Seating of teeth during maximum intercuspation is captured.
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Less material is needed and patient is more comfortable.
Disadvantages
Tray is not rigid and it depends on the impression materials rigidity.
Not a functionally generated technique – so l imited to one casting
per quadrant.
Types of dual arch trays
Metal
Plastic (both with or without side wall)
Depending on the location
Posterior / anterior sexant
Quadrant
¾ of an arch
Full arch
Technique
Fit of the tray is checked such that the tray extend distal to the last
teeth of the arch by and then ask the patient to close, observe for
the bilateral closure and see that the patient is comfortable.
Gingival retraction is done.
Ready the tray material and the syringe material which is placed on
the tooth after cord removal.
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Tray placed inside mouth while observing the distal extent and
slowly asking the patient to close the mouth.
After waiting for the set (2 minutes), the patient is asked to open
the mouth, and then the tray adheres to one arch. After placing
fingers on either side of the tray it is removed with equal pressure
bilaterally to minimize the distortion of the tray.
The handle should not be used for removal of the tray and the
material is removed from the sulcus the impression is then washed
and the retraction cords is removed and checked.
Different techniques for making a dual arch impression
One step technique
Stock trays are used, light body is injected around the prepared
tooth and putty or bite registration paste is inserted on both sides of the
tray and the patient is asked to bite in centric occlusion.
Dual arch – hydraulic pressure technique
Low or medium viscosity impression material is injected on the
prepared and preoperative impression of the unprepared tooth. The tray is
placed and patient is asked to bite in centric occlusion, the hydraulic
pressure created, will force the material into the sulcus and through the
vent holes created in the buccal or the lingual sides of the pre-operative
impression.
Dual arch – laminar impression technique
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A pre-operative impression is made. The post operative impression
of the tooth to be prepared is relieved to a depth of 0.5mm in the cervical
area. Two holes are drilled from the buccal surface and one on the mesial
and one of the distal .
Auto-mix technique
Several manufacturers often supply impression material in pre-
package cartridges to which a disposable mixing tip is attached.
The cartridge is inserted in a caulking-gun like device, and the base
and catalyst are extruded into the mixing tip, where mixing occurs
as they progress to the end of the tube.
The homogenously incorporated material can be directly placed on
the prepared tooth and impression tray.
Matrix impression system
This is a new system that requires a series of three impression
procedures using three types and/or viscosities of impression materials.
Steps
A matrix of occlusal registration elastomeric material is made over
the tooth preparation.
Matrix is trimmed to prescribed dimension and retraction cord is
removed.
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A definit ive impression is made in matrix of the preparation with a
low viscosity elastomeric impression material.
After the matrix impression is seated, a stock tray is fi lled with a
medium viscosity elastomeric impression material is seated over the
matrix and remaining teeth to create an impression of the entire
arch.
Advantages
This system effectively controls the four forces (relapsing,
retraction, displacement and collapsing) that impact on the gingiva
during the crit ical phase of making impression when attempting to
register the subgingival margins.
The design of the matrix also gently forces the high viscosity
impression materials along the preparations and in to the sulcus
where it cleanses the sulcus of unwanted debris and fills the sulcus.
The high viscosity material gently extends into the sulcus and does
not permit i t to collapse as the medium viscosity material in the
stock tray a seated for the pick up impression.
The matrix facil itates the formation of the optimum flange.
Tearing is virtually eliminated because of the improved
configuration of the sulcular flange and by the elimination of voids
or contaminants in the sulcus.
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Copper band impression
Fitting copper band to preparation
Select a copper band of correct diameter by trial and error method.
Deform the tubes to semi ell ipsoidal cross section and try in.
Approximate posit ion of the finish line and mark i t on the band with
an explorer and cur with a scissors. Smoothen the rough edges using
a carborundom stone.
Evaluate the fit of the band such that i t extends 1mm beyond the
finish line.
Orientation holes are cut on the top one fifth of facial surface of the
tube.
Make compound plug
Fingers are covered with petrolactum jelly
A warm red stick compound is inserted in the top one third of the
copper tube. The compound should touch the occlusal surface. It is
then cooled and removed by Bachins towel clamp by grasping top
one fifth of the copper band.
Excess is relieved by cutting with a no. 6 or no. 8 round bur and
under slow speed and by frequently removing debris. 0.2mm of
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compound is removed from the impressed occlusal surface. This
creates a space for the heavy body polyvinyl si loxane.
Making an impression
Coat the inner surface with adhesive
Mix heavy body polyvinyl siloxane
Load it in the copper band
Position the finger and on the top of band, orient and seat
customized copper band
Stabilize the band
Wait ti ll the final set. Using towel clip remove the band evaluate
the impression and followed the orientation impression.
Special considerations
Pin retained restorations
Elastomeric impression materials are strong enough to be used in
these situations but should be introduced with
Cement tube or Lentulo spiral to avoid any bubbles
Special nylon bristles or Prefabricated plastic pinscan be put in the
pin hole and the impression is made
Post and cores
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This procedure involves reinforcing the impression with a plastic
pin or suitable wire (e.g. orthodontic wire) – elastomeric impression
materials can be used.
Impression technique with the use of preformed crown shells
Select preformed crowns (polycarbonate) adjust the gingival margin
slightly apical to finish line.
Adhesive applied to inner side
Regular body is loaded
Then seated properly
Wait ti ll i t sets and remove
Then full arch pick up impression made
Impression procedure for a subgingivally prepared shoulder margin
An individual tray is prepared directly with acrylic, which should
not cover the shoulder margin. Gingival retraction done.
The tray is rebased with fluid resin and some placed on the teeth
and pressed ti ll contact is felt wait ti ll the acrylic becomes hard.
An outline is marked and excess is removed. Space is created for
the impression materials except the shoulder margin area.
An adhesive is then applied and the tray fi lled with elastomer. This
impression is then picked up with elastomer or hydrocolloid in a
stock tray.
Impressions for CAD/CAM procedures
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Prepared tooth
Apply rubber dam
Imaging powder + l iquid is coated on the prepared area for photo
receptivity
Symmetrical optic beam is passed
3D picture is produced on a computer for milling procedure
This technique is mostly advocated for the CEREC II system.
Evaluation of impression
Upon removal the impression must be inspected for accuracy.
Bubbles or voids in the margin necessitate discarding the
impression and starting over.
An intact uninterrupted cuff of impression material should be
present beyond every margin.
Streaks of base and catalyst material indicate improper mixing and
may render an impression useless.
Immediately after removal from the mouth impression is rinsed
under tap water and dried with an air syringe.
Disinfection is an essential step for prevention of cross infection and exposure of
laboratory personnel.
Impression material Recommended disinfection procedures1. Reversible hydrocolloid2.Irreversible hydrocolloid3. Polysulfide4. Addition si licone5. Condensation si licone6. Polyether
Diluted bleach, Iodophor sprayDiluted bleach, Iodophor sprayImmersion in iodophor or 2% glutaraldehydeImmersion in 2% glutaraldehyde or hypochloride solut ionImmersion in 2% glutaraldehyde or hypochlorite solutionIodophor spray or chloride dioxide
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Done properly i t has no clinically significant effects on the
accuracy or surface reproduction of the elastomers.
Conclusion
Devan states that the impression should be in the dentists mind before it is in his
hand. Based on this statement I conclude the use of impression material and technique
should be purely the dentists choice.
REFERENCES:
1. Restorative Dental materials:G Craig & John M Powers-
11th edition2002, pg 329-378
2. Phillips science of dental materials:11th edition
3. GPT Academy of prosthodontics JPD,july 2005 vol 94
4. Tylman’s Theory and Practice Of Fixed Prosthodontics, 8th
edition, pg 237-254.
5. Fundamentals Of Fixed Prosthodontics by Herbert T.
Shillingburg, 3rd edition, pg 281
6. Contemporary Fixed Prosthodontics, by Rosenstiel, 4th
edition, pg452- 462.
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7. Notes on dental materials by E.C. Combe, 6th edition,pg
115- 126.
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