IMPRESSIONS IN FIXED PARTIAL DENTURES
CONTENTS
Ø DEFINITIONS
Ø IDEAL REQUIREMENTS OF IMPRESSION MATERIALS
Ø CLASSIFICATION OF IMPRESSION MATERIALS
Ø IMPRESSION TRAYS
Ø IMPRESSION TECHNIQUES FOR DIFFERENT IMPRESSION MATERIALS
Ø SPECIAL CONSIDERATIONS
Ø CONCLUSION
Ø REFERENCES
Definitions
Impression
An imprint or negative likeness 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 tissues.
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)
Ø Complete plasticity before use
Ø Sufficient fluidity to record fine
detail
Ø The ability to wet the oral tissues
Ø Dimensional accuracy
Ø Dimensional stability
Ø Complete elasticity after cure
Ø Optimal stiffness
Ø Have a good shelf life
Ø Be non-toxic and non-irritating
Ø Have acceptable taste and odor
Ø Have suitable working and setting times
Ø Have strength to resist tearing
Ø Be compatible with model and die
material
Ø Facilitate clinical identification of
beginning and end of cure
Ø Facilitate visualization of the finish
line
An impression for a fixed restoration
should meet the following requirements
Ø 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.
Ø Teeth and tissues adjacent to the
prepared tooth must be accurately reproduced to permit proper articulation of
the cast and contouring of the restoration.
Ø 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
Ø Reversible hydrocolloids
Ø Polysulfide
Ø Condensation silicone
Ø Addition silicone
Ø Polyether
Ø urethane dimethacryalte
Classification
According to chemical nature
Ø Hydrocolloid impression material
Ø 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
|
|
Agar
Borates
Potassium sulfate
Wax hard diatomaceous earth
Silica
Clay
Alkyl benzoates
Thizotrophic materials
Coloring agents
Water
|
13 – 17%
0.2 – 0.5%
1 – 2%
0.5 – 1.1%
0.1%
0.3%
-
Balance
|
Basic constituent
Improves strength of the gel retards plaster or stone
Gypsum hardener
Filler
Preservatives
Plasticizers
-
Dispersion medium
|
Gellation process
The setting process of reversible
hydrocolloid is called as a gellation process it 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 ----------à
Sol -------------à 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 it 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 until the
material is completely gelled. This is done using water cooled rimlock tray
Elastomeric impression materials
Polysulfide (Mercaptan, Thiokol)
Composition
Base
Ø Liquid polysulfide polymer à 80 – 85%
Ø Inert fillers (TiO, Zn Sulfate, Copper
carbonate) à 16 – 18%
Ø Plasticizers (Dibutyl phathalate)
Accelerator paste
Ø Lead di-oxide à 60 – 68% - Reactor
Ø Dibutyl phathalate à 30 – 35% - Plasticizer
Ø Sulfur à 3% - Retarder
Setting reaction
This polymer is usually cross linked
with an oxidizing agent such as lead dioxide. It is this lead dioxide that
gives polysulfide its characteristic 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
Ø Light body
Ø Regular body
Ø 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
Ø Poly dimethyl siloxane à 25 – 65%
Ø Colloidal silica or micronize metal
oxide filler (Depending upon the viscosity) à 35 – 75%
Ø Coloring agents
Accelerator paste
Ø Orhtoethyl silicate – cross linking agent
Ø Stannous octate – catalyst
Available as
Ø Light body
Ø Regular body
Ø Heavy body
Ø Putty
Setting reaction
This is condensation polymerization
reaction which reacts with trifunctional and tetrafunctional alkyl silicates
like orthoethyl silicate in the presence of stannous octate. Ethyl alcohol is a
by product which is responsible for shrinkage of the material.
The
condensation silicone 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 life because of
the instability of alkyl silicates in the presence of organo-tin compounds,
which may result in the oxidation of tin.
Addition silicone
Base paste
Ø Poly methyl hydrogen siloxane
Ø Other siloxane polymers
Ø Fillers
Accelerator paste
Ø Divinyl siloxane
Ø Other siloxane prepolymers
Ø Platinum salt (chloroplatinic acid)
catalyst
Ø Palladium – hydrogen absorber
Ø Fillers
Available as
Ø Light body
Ø Regular / medium body
Ø Heavy body
Ø 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 silicone 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
Ø Custom tray / resin tray
Ø Stock tray
Polyether
Base paste
Ø Polyether polymer
Ø Collidal silica
Ø Glycoether or dibutyl phathlate
Accelerator paste
Ø Aromatic sulfonate ester – cross
linking agent
Ø Collidal silica
Ø Dibutyl phathalae or glycoether
Available
Ø Single viscosity (both syringe and tray
material)
Ø 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 linking by cationic
polymerization via the imine end groups.
Trays used
Ø Custom / resin trays
Ø Stock tray
Comparison of advantages and disadvantages
|
Material
|
Advantages
|
Disadvantages
|
Recommended uses
|
Precautions
|
|
Reversible and hydrocolloid
|
Ø Hydrophilic
Ø Long working time
Ø Low cost
Ø No custom tray
required
|
Ø Low tear resistance
Ø Low stability
Ø Equipment needed
|
Ø Multiple
preparations
Ø Problems with
moisture
|
Pour immediately use only with
stone
|
|
Polysulfide
|
Ø High tear strength
Ø Easier to pour than
other elastomers
|
Ø Messy
Ø Unpleasant odor
Ø Long setting time
Ø Stability only fair
|
Ø Most impressions
|
Latex gloves use in
contra-indicated for all the elastomers. Pour within 1 hour.
|
|
Addition silicone
|
Ø Dimensional
stability
Ø Pleasant to use
Ø Short setting time
Ø Automix available
|
Ø Hydrophobic
Ø Poor wetting
Ø Some material
release hydrogen
|
Ø Most impressions
|
Delay pour some materials, care
to avoid bubbles when working
|
|
Condensation silicone
|
Ø Pleasant to use
Ø Short setting time
|
Ø Hydrophobic
Ø Poor wetting
Ø Low stability
|
Ø Most impressions
|
Pour immediately, care to avoid
bubbles when pouring
|
|
Polyether
|
Ø Dimensional
stability
Ø Accuracy
Ø Short setting time
Ø Automix available
|
Ø Set material very
stiff
|
Ø 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
Ø Metallic
Ø Non metallic
2. Custom made trays
Ø Autopolymerizing acrylic resin
Ø Thermoplastic or photo initiated resin
3. Water cooled rim lock trays
Custom trays
Advantages
Ø 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
Ø No need for sterilization
Ø Uniform thickness of the impression
material minimizes distortions resulting from curing shrinkage
Ø Procuring of the tray material is not
required
Disadvantages
Ø Time taken for the fabrication
Ø Aging for 24 hours to minimizes the
distortion
Ø Sensitivity to monomer
Steps in fabrication
Ø Soak replicas of diagnostic casts in
slurry water for 10 mins.
Ø Tray extensions are marked with the
pencil line at either the cervical region of the teeth or 5mm below the
cervical line.
Ø 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 line .(Rigid or stiff material like
polyether requires of wax spacer of 4mm thickness).
Ø 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).
Ø 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.
Ø Needed amount of polymers and monomers
of autopolymerizing resin is mixed as per manufacturer instructions till the
dough stage is attained.
Ø 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.
Ø A handle is formed along with 2 buccal
wings or ridges on either side for the easy removal.
Ø After the set it is slowly removed form
the cast, check if the stops are proper.
Ø It can then be placed in the water for
9 – 24 hours (Rosenstiel) or 5 minutes in boiling water before use.
Ø 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
Ø Eliminating the time and expenses of
fabricating a customized tray.
Ø 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.
Ø Liquefying bath
Ø Storage bath
Ø 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
- A sectional impression for making a die (made first)
- A full arch impression for the working cast
Ø After selecting a proper tray and
placing stops in the proper area.
Ø Tray material is kept on the tray for
tempering.
Ø 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.
Ø 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
Ø The areas of the teeth and tissues are
flooded with warm water.
Ø Syringe material is taken directly from
tempering compartment and added to prepared cavities, first at base of
preparation and then the tooth is covered.
Ø The material used to fill the tray
should be cooler or tempered.
Ø Gelation is accelerated by circulating
cool water, through the tray for 3 – 5 minutes.
Ø It is postulated that the hydraulic
pressure of the viscous tray material forces the fluid syringe hydrocolloid
into the area to be restored.
Laminate technique
Ø A recent modification to the
traditional agar procedure is the combination of agar and alginate impression
materials.
Ø The tray hydrocolloid is replaced with
a mix of chilled alginate, that bonds with the syringe agar.
Ø The alginate gels by chemical reaction,
but agar gels by means of contact with cool alginate rather than water
circulating through the tray
Advantages
Ø Less preparation time and less
complicated when compared to the wet field technique.
Disadvantages
Ø Bond between agar and alginate is not
strong
Ø High viscosity alginate displaces agar
during seating
Ø Dimensional inaccuracy of alginate
limits 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
Ø Eliminates time and expense of
fabricating custom tray
Ø Metal stock trays are rigid and are not
susceptible to distortion.
Disadvantages
Ø More impression material is required.
Ø Must be sterilized
Technique
Ø Select a stock tray and coat with
adhesive
Ø 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 till the initial
set (2 minutes) and then remove from the mouth with the minimal sideward
movement.
Ø Gingival retraction done
Ø 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
filling the syringe.
Ø Remove the retraction cord gently and
syringe inaccessible areas first (e.g.) disto lingual finish lines.
Ø Now insert the tray with low viscosity
impression material
Ø Position the tray over the arch
Ø Apply force in a vertical direction
until further seating is impossible.
Ø After material is set, insert two index
fingers under each side of the tray to break the seal.
Ø Remove the tray in the direction
parallel to the preparation.
Ø Evaluate the set impressions.
Difference between single mix and double mix technique
Ø In single mix technique one viscosity
material (regular body) is used to fill the tray and load the syringe.
Ø 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
Ø Do the gingival retraction and Prepare
the syringe
Ø 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.
Ø Load the syringe as well as the tray.
Ø Syringe the material in the
inaccessible area first then subsequently removing the retraction cord gently
and then syringe the impression material.
Ø Now insert the tray. Seal from
posterior to anterior allowing excess to extrude in an anterior direction.
Ø Continue seating in a vertical
direction until the trays stops prevent further progress.
Ø After the material is completely set,
remove the impression parallel to the preparation path.
Ø Evaluate the impression.
Closed bite double arch method
Also called as Dual Quad tray, double
arch, triple arch and closed mouth impression.
Minimum conditions
Ø Either natural teeth or an incisal pin
and table should be provided in the articulator as vertical stops.
Ø Sufficient space distal to the last
tooth should be present to allow tray approximation.
Ø Intact dentition, class I occlusion,
bounded on either side by intact teeth, opposing tooth having intact occlusal
contact is ideal.
Advantages
Ø Physical deformation of the impression
by mandible during opening is minimized.
Ø Seating of teeth during maximum
intercuspation is captured.
Ø Less material is needed and patient is
more comfortable.
Disadvantages
Ø Tray is not rigid and it depends on the
impression materials rigidity.
Ø Not a functionally generated technique
– so limited to one casting per quadrant.
Types of dual arch trays
Ø Metal
Ø Plastic (both with or without side
wall)
Depending on the location
Ø Posterior / anterior sexant
Ø Quadrant
Ø ¾ of an arch
Ø Full arch
Technique
Ø 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.
Ø Gingival retraction is done.
Ø Ready the tray material and the syringe
material which is placed on the tooth after cord removal.
Ø Tray placed inside mouth while
observing the distal extent and slowly asking the patient to close the mouth.
Ø 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.
Ø 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
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
Ø Several manufacturers often supply
impression material in pre-package cartridges to which a disposable mixing tip
is attached.
Ø 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.
Ø 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
Ø A matrix of occlusal registration
elastomeric material is made over the tooth preparation.
Ø Matrix is trimmed to prescribed
dimension and retraction cord is removed.
Ø A definitive impression is made in
matrix of the preparation with a low viscosity elastomeric impression material.
Ø After the matrix impression is seated,
a stock tray is filled with a medium viscosity elastomeric impression material
is seated over the matrix and remaining teeth to create an impression of the
entire arch.
Advantages
Ø This system effectively controls the
four forces (relapsing, retraction, displacement and collapsing) that impact on
the gingiva during the critical phase of making impression when attempting to
register the subgingival margins.
Ø 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.
Ø The high viscosity material gently
extends into the sulcus and does not permit it to collapse as the medium
viscosity material in the stock tray a seated for the pick up impression.
Ø The matrix facilitates the formation of
the optimum flange.
Ø Tearing is virtually eliminated because
of the improved configuration of the sulcular flange and by the elimination of
voids or contaminants in the sulcus.
Copper band impression
Fitting copper band to preparation
Ø Select a copper band of correct
diameter by trial and error method. Deform the tubes to semi ellipsoidal cross
section and try in.
Ø Approximate position of the finish line
and mark it on the band with an explorer and cur with a scissors. Smoothen the
rough edges using a carborundom stone.
Ø Evaluate the fit of the band such that
it extends 1mm beyond the finish line.
Ø Orientation holes are cut on the top
one fifth of facial surface of the tube.
Make compound plug
Ø Fingers are covered with petrolactum
jelly
Ø 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.
Ø 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 compound is removed from the impressed occlusal surface. This
creates a space for the heavy body polyvinyl siloxane.
Making an impression
Ø Coat the inner surface with adhesive
Ø Mix heavy body polyvinyl siloxane
Ø Load it in the copper band
Ø Position the finger and on the top of
band, orient and seat customized copper band
Ø Stabilize the band
Ø Wait till 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
Ø Cement tube or Lentulo spiral to avoid
any bubbles
Ø Special nylon bristles or Prefabricated
plastic pinscan be put in the pin hole and the impression is made
Post and cores
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
Ø Select preformed crowns (polycarbonate)
adjust the gingival margin slightly apical to finish line.
Ø Adhesive applied to inner side
Ø Regular body is loaded
Ø Then seated properly
Ø Wait till it sets and remove
Ø Then full arch pick up impression made
Impression procedure for a subgingivally prepared shoulder margin
Ø An individual tray is prepared directly
with acrylic, which should not cover the shoulder margin. Gingival retraction
done.
Ø The tray is rebased with fluid resin
and some placed on the teeth and pressed till contact is felt wait till the
acrylic becomes hard.
Ø An outline is marked and excess is
removed. Space is created for the impression materials except the shoulder
margin area.
Ø An adhesive is then applied and the
tray filled with elastomer. This impression is then picked up with elastomer or
hydrocolloid in a stock tray.
Impressions for CAD/CAM procedures
Prepared tooth
¯
Apply rubber dam
¯
Imaging powder + liquid 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
Ø Upon removal the impression must be
inspected for accuracy.
Ø Bubbles or voids in the margin
necessitate discarding the impression and starting over.
Ø An intact uninterrupted cuff of
impression material should be present beyond every margin.
Ø Streaks of base and catalyst material
indicate improper mixing and may render an impression useless.
Ø Immediately after removal from the
mouth impression is rinsed under tap water and dried with an air syringe.
Ø Disinfection is an essential step for
prevention of cross infection and exposure of laboratory personnel.
|
Impression
material
|
Recommended
disinfection procedures
|
|
1. Reversible hydrocolloid
2.Irreversible hydrocolloid
3. Polysulfide
4. Addition silicone
5. Condensation silicone
6. Polyether
|
Diluted bleach, Iodophor spray
Diluted bleach, Iodophor spray
Immersion in iodophor or 2% glutaraldehyde
Immersion in 2% glutaraldehyde or hypochloride
solution
Immersion in 2% glutaraldehyde or hypochlorite
solution
Iodophor spray or chloride dioxide
|
Ø Done properly it 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.
7. Notes on dental materials
by E.C. Combe, 6th edition,pg 115- 126.
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