Assalamu-alaikum
DR. TAJAMMUL AHMED
a.tajammul@gmail.com
IMPRESSIONS & TECHNIQUES IN FIXED
PARTIAL DENTURE
Impression
GPT-8

Impression : a negative likeness or
copy in reverse of the surface of an
object; an imprint of the teeth and
adjacent structures for use in
dentistry.
Considerations for
Precision Impression Taking

Before Impression Making (Extra-oral):
- Easy to mix
- Long working time

During Impression Making (Intra-oral):
- Hydrophilic
- Ability to flow around preparation without slumping
- Detail-capturing
- Snap set
- Good taste & odor

Upon Removal from Mouth:
- Easy to remove
- High tear strength
- Good recovery from deformation

After Impression Making (Storage):
- Dimensional stability
- Easy to read
- Can be disinfected
During Pouring:
- Compatibility with gypsum
- Reproduction of details (ADA Specification 19:
- Ability for multiple pouring

20 microns)
CLASSIFICATION OF IMPRESSION MATERIAL
Nonelastic
Elastic
Hydrocolloids Synthetic elastomers
Waxes, gums, resins
Agar-Agar
Polysulphides
Plaster of paris
Zinc oxide eugenol
Alginate
Condensation-silicone
(Type I silicone)
Impression compound
Polyether
Eugenol-free paste
Addition silicone
(Type II silicone)
ELASTOMERIC IMPRESSION MATERIALS
Depending on viscosity
Light body
Medium body / regular body
Heavy body
Putty
Reversible hydrocolloid (Agar-Agar) :
Introduced by Sears 1937
First elastic impression material
Longest continuous use
Available as :
Tray material
Syringe material
Hydrocolloid conditioning with three baths : liquefying, storage and tempering
Tubes and cartridges of
Tubes and cartridges of
Tray filled with
hydrocolloid placed in hydrocolloid transferred from
impression material
liquefying bath
liquefying bath to storage bath placed in tempering bath
Cartridge of liquefied hydrocolloid
is removed from storage bath
Cartridge of impression material
is loaded into syringe
Blunt needle is attached to
impression syringe
Irreversible hydrocolloid (Alginate) :
During World War II
Salt of alginic acid (anhydro d mannuronic acid)
Available as :
• Dust free alginate
• Chromatic indicator
ELASTOMERIC IMPRESSION MATERIALS
Accuracy / Detail capture :
ADA No. 19 fine detail of 25m
PVS impression material  best
Reversible hydrocolloid  least
Reproduction of fine detail of gypsum die materials  50 m
Lower the viscosity  Increased fine detail
Putty material  reported fine detail of 75m
Tear strength:
Measures the resistance to fracture of an elastomeric material
subjected to tensile force acting perpendicular to a surface flaw.
Point of consideration  Interproximal, subgingival areas
 Slot and groove
Polymerized material
in the gingival crevice
Torn material during
removal
Hydrophilic group
Hydrocolloids
Polyether
• Works in wet environment
(blood and saliva)
Hydrophobic group
Polysuphide
C – silicone
A – silicone (PVS)
• Resistant to wetting (repelled by
hemorrhage or tissue fluids)
• Readily wettable by the gypsum • Not readily wettable by the gypsum
CONTAMINATION
Inhibition of polymerization of PVS impression material.
Direct inhibition
• Direct contact with latex gloves
• Impression are made with rubber dam in place
Mechanism of inhibition  Element sulphur react with
chloroplatinic acid catalyst present in PVS impression material.
Indirect inhibition
Indirect contact of tooth and periodontal tissues during tooth
preparation and gingival displacement procedure.
Limited to small isolated areas
Unpolymerized material remain
adherent to prepared tooth surface
•Contaminated surface will be slippery and sticky to touch
Clinical consideration :
Use of non latex gloves – Vinyl gloves
– Polyethene gloves
Avoid touching the tooth preparation and gingival areas
Avoid handling retraction cord with gloved hands.
TRAY SELECTION AND PREPARATION OF THE TRAY
• Tray should be rigid.
• Dimensionally stable.
• Resist the deformation (loading heavy body material).
• Provides the space for optimal thickness.
Plastic vs metal stock trays
Tray should be 2-3mm thick to ensure sufficient rigidity.
Incorporation of stops
 Guide the clinician for positive seating
 Three widely spaced stops
 Non functional cusp of unprepared teeth, edentulous areas
or the palate.
Tin foil or aluminium foil should be used over the wax spacer to
prevent decrease in adhesive bond
Adhesion of the impression material to the tray
For accurate impression  proper adherence to the tray
Tray adhesive composition
– poly dimethyl siloxane and ethyl silicate.
Adequate mixing and loading :
Earlier materials  base and catalyst tubes
Present materials  cartridges with mixing tips
 Gun like dispenser
Automixing system :
• Lesser inherent voids
• Extend the essential working time
• Reduced waste
• Uniformity in proportioning
ELECTRIC MACHINE DEVICE
(Dynamic Mechanical Mixer)
Provides uniform mix with touch of a button
Far fewer voids in the set elastomers
AGAR IMPRESSION TECHNIQUE
Armamentarium :
Tray and syringe
material
Water cooled trays
Conditioning unit
Procedure :
Tray material
Syringe material
Loaded tray in
tempering bath
Dentate arch is
flooded with water
Water cooling tubes
connected to seated
tray
Completed
impression
Disadvantages –
• Dimensionally unstable / distortion during gelation
• Initial expenditure for instruments
• Multiple pouring is not possible
ALGINATE IMPRESSION TECHNIQUE
Clinical interest :
For the study cast / diagnostic cast – diagnostic wax up
– mock preparations
For interim restoration (temporization)
Clinical considerations
Tray selection
Perforated metal / rigid plastic trays
Tray adhesive
Perforations – insufficient
To prevent detachment of impression,
Extend 2-3 mm outer surface
Loading the tray
Sectional placement
Wet gloved finger
Making the impression
HYDROCOLLOID LAMINATE TECHNIQUE
ALGINATE – AGAR METHOD
Prepared tooth
surfaces
Injection of reversible
hydrocolloid
Combined alginate –agar impression
Stock tray with chilled
alginate
Master cast
Critical to success  bond between agar and alginate
Advantages :
• Satisfactory surface reproduction
• Good wettability (smooth, dense model)
• Simple conditioning (no tempering bath / water cooled trays)
• More economical
Disadvantages :
• The bond between agar and alginate is not sound
• Higher viscosity alginate displaces agar hydrocolloid
PUTTY WASH IMPRESSION TECHNIQUES
(Stock tray impression technique)
One stage or one step putty wash impression
(Simultaneous technique / squash technique)
In this technique putty and wash are recorded simultaneously
Disadvantage :
Difficult to control thickness of wash material
Higher viscosity material displaces the wash material
- Critical areas captured in putty rather than light body
Setting distortion of the putty add to over all distortion of
impression
TWO STAGE OR TWO STEP PUTTY WASH
TECHNIQUE
Two step unspaced impression technique
Preoperative putty impression is recorded
Preset putty impression is relined with thin layer of wash
Two step spaced impression technique
Preoperative putty impression is made
Space for the light body wash material is provided to reline
the putty
DUAL PHASE IMPRESSION TECHNIQUE
(Multiple mix technique, laminate single impression
technique, custom tray impression technique)
Two consistencies
• Light body syringe material
(wash type)
Heavy body – tray material
• Tray material will force the light
body material into the gingival
sulci and preparation detail
Custom tray with 2–3mm space
ADVANTAGES
Uniform thickness of impression material (2-3 mm)
Viscosities of 2 materials reduces drags and improves the
blending of 2 phases.
Good marginal definition.
Less impression material is required.
DISADVANTAGES
Construction of custom tray (time / cost)
Differential shrinkage
Monophase impression technique
(Single viscosity technique)
•Medium viscosity – polyether or addition silicone
•Used as both tray material and syringe material
•Custom tray with 3 mm spacer
Depends on pseudoplastic property
DISADVANTAGES
•Reduced ability to flow into intra coronal features and gingival crevice
•Greater amount of polymerization shrinkage compared to heavy body
•Surface reproduction may not be as good as light body material
Copper band or tube method
The copper band is used to salvage an impression of multiple preparation
where there is only vague margins on one or two preparation that are not
adequately replicated in the impression.
Copper band with orentation holes
Evaluating the fit of the copper band
Copper band contoured
Copper band with the final impression
Close bite double arch method .
synonyms: dual quad tray, double arch ,triple arch,
accu-bite, closed mouth impression.
Close bite double arch method .
synonyms: dual quad tray, double arch ,triple
arch, accu-bite, closed mouth impression.
Technique
Evaluate the fit of the tray in the patients mouth & observe the complete
bilateral closure and the patients comfort.
The high viscosity material is used in the tray & then the low viscosity material
is syringed over the high viscosity in the tray & positioned on the arch.
Instruct the patient to close the mouth and observe for the interdigitation on the
opposite arch. Wait for the material to set .
Instruct the patient to open the mouth & remove it by applying equal pressure
bilaterally. Evaluate the impression.
Close bite double arch method .
synonyms: dual quad tray, double arch ,triple
arch, accu-bite, closed mouth impression.
Closed mouth impression
FLAWS RELATED TO IMPRESSION TECHNIQUE
Visible flaws :
Finish line not visible  Inadequate gingival displacement
• Gingival inflammation and bleeding
• Crevicular fluid and heamorrhage
Poor moisture
control
Dry field
Air bubbles / voids in critical areas
Mixing
Syringing
 Keep syringe tip in expressed
material
 Start syringing from distal
interproximal area (distoligual)
 Fill the base of grooves, boxes first
Tray loading and tray seating
Drags
Reason :
Poor flow of putty
Premature syringe of impression material
Poor bond between the tray and syringe material
Unset / unpolymerized impression material  contamination
INVISIBLE FLAWS
Tray and impression recoil
Permanent deformation
Detachment of impression from the trays
DISINFECTIONS OF IMPRESSIONS
• To prevent exposure and cross infection
• Disinfection before poring the cast.
Disinfection methods :
• Spray-on disinfectants
• Immersion of impression in chemical disinfectant (Submerged /
soaking)
Types of chemical disinfectant used :
• Chlorine compounds
• Synthetic phenolic compounds
• Glutaraldehyde
• Idophores
• Combination of phenols and alcohols
• If an impression has not been disinfected, or if the laboratory has
no assurance that an appropriate disinfection protocol was
followed, it is necessary to disinfect the stone cast.
 ADA recommends the use of
 1: 2.13 dilution of iodophor
 1: 10
dilution of sodium hypochlorite
 Stone cast be disinfected by
 Spraying until wet
 Immersing for the time recommended for tuberculocidal
disinfection.
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