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1. A Seminar on
IMPRESSIONS IN FIXED PARTIAL DENTURES
1

2. 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
2

3. 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)
 Complete plasticity before use
 Sufficient fluidity to record fine detail
 The ability to wet the oral tissues
 Dimensional accuracy
 Dimensional stability
 Complete elasticity after cure
 Optimal stiffness
 Have a good shelf life
 Be non-toxic and non-irritating
3

4.  Have acceptable taste and odor
 Have suitable working and setting times
 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 line
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
 Addition silicone
4

5.  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
Agar
Borates
Percentage by
w eight
13 – 17%
0.2 – 0.5%
Potassium sulfate
1 – 2%
Wax hard diatomaceous earth 0.5 – 1.1%
Silica
Function
Basic constituent
Improves strength of the gel
retards plaster or stone
Gypsum hardener
Filler
5

6. Clay
Alkyl benzoates
Thizotrophic materials
Coloring agents
Water
0.1%
0.3%
Balance
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
(Tube)
----------
Sol
-------------  Gel
(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
6

7. 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 fillers (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 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
 Light body
7

8.  Regular body
 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 silica or micronize metal oxide filler (Depending upon the
viscosity)  35 – 75%
 Coloring agents
Accelerator paste
 Orhtoethyl silicate – cross linking agent
 Stannous octate – catalyst
Available as
 Light body
 Regular body
 Heavy body
 Putty
8

9. 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
 Poly methyl hydrogen siloxane
 Other siloxane polymers
 Fillers
Accelerator paste
 Divinyl siloxane
9

10.  Other siloxane prepolymers
 Platinum salt (chloroplatinic acid) catalyst
 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 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.
10

11. Trays used
 Custom tray / resin tray
 Stock tray
Polyether
Base paste
 Polyether polymer
 Collidal silica
 Glycoether or dibutyl phathlate
Accelerator paste
 Aromatic sulfonate ester – cross linking agent
 Collidal silica
 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
11

12. 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
 Custom / resin trays
 Stock tray
Comparison of advantages and disadvantages
Material
Reversible and
hydr ocolloid
Advantages
Disadvantages
Hydrophili 

c
Pour
i mme diatel y
use onl y with
stone

Multiple
preparations

Problems
with
moisture

Most
impressions
Latex gloves
use in contraindicated for
all the
elastomers.
Pour within 1
hour.

Most
impressions
Dela y pour
some
Long
working ti me

Low
stabilit y

Low cost

Equipmen
t needed

Mess y

Unpleasa
nt odor

Long
setting ti me

Stabilit y
onl y fair

No custom
tra y required

High tear
strength

Addition silicone
Precautions
Low tear
resistance

Pol ysulfide
Recomme nded
uses
Easier to
pour than
other
elastomers

Dimension
al stabilit y
Hydropho

bic
12

13. 

Short
setting ti me


Automix
available
Some
material
release
hydrogen

Pleasant
to use

Short
setting ti me

Poor
wetting

Low
stabilit y

Set
material ver y
stiff

Pol yether
materials,
care to avoid
bubbles when
working
Poor
wetting

Condensation
silicone
Pleasant
to use

Dimension
al stabilit y
Accurac y



Most
impressions
Pour
i mme diatel y,
care to avoid
bubbles when
pouring

Most
impressions
Care not to
break teeth
when
separating
cast
bic
Short
setting ti me

Hydropho
Automix
available
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
13

14. 3. Water cooled rim lock trays
Custom trays
Advantages
 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 line 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
14

15. until the marked pencil line .(Rigid or stiff material like polyether
requires of wax spacer of 4mm thickness).
 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 till 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 it 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)
15

16. Advantages of stock tray
 Eliminating the time and expenses of fabricating a customized tray.
 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
16

17. Because only one accurate cast can be made from a hydrocolloid
impression, two impressions are made
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.
17

18.  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
 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 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
18

19. Advantages
 Eliminates time and expense of fabricating custom tray
 Metal stock trays are rigid and are not susceptible to distortion.
Disadvantages
 More impression material is required.
 Must be sterilized
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 till 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 filling the
syringe.
 Remove the retraction cord gently and syringe inaccessible areas
first (e.g.) disto lingual finish lines.
 Now insert the tray with low viscosity impression material
19

20.  Position the tray over the arch
 Apply
force
in
a
vertical
direction
until
further
seating
is
impossible.
 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 fill 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.
20

21.  Syringe the material in the inaccessible area first then subsequently
removing the retraction cord gently and then syringe the impression
material.
 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
21

22.  Physical deformation of the impression by mandible during opening
is minimized.
 Seating of teeth during maximum intercuspation is captured.
 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 limited 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.
22

23.  Gingival retraction is done.
 Ready the tray material and the syringe material which is placed on
the tooth after cord removal.
 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
23

24. 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
 Several manufacturers often supply impression material in prepackage 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
24

25.  A matrix of occlusal registration elastomeric material is made over
the tooth preparation.
 Matrix is trimmed to prescribed dimension and retraction cord is
removed.
 A definitive 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 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
 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.
 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 it to collapse as the medium viscosity material in the
stock tray a seated for the pick up impression.
 The matrix facilitates the formation of the optimum flange.
25

26.  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
 Select a copper band of correct diameter by trial and error method.
Deform the tubes to semi ellipsoidal cross section and try in.
 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.
 Evaluate the fit of the band such that it 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
26

27. 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
compound is removed from the impressed occlusal surface. This
creates a space for the heavy body polyvinyl siloxane.
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 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
 Cement tube or Lentulo spiral to avoid any bubbles
27

28.  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 crow n 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 till it 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 till contact is felt wait till the acrylic becomes hard.
 An outline is marked and excess is removed. Space is created for
the impression materials except the shoulder margin area.
28

29.  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
 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.
29

30.  Disinfection is an essential step for prevention of cross infection
and exposure of laboratory personnel.
Impression material
1. Reversible hydr ocolloid
2.Irreversible hydr ocolloid
3. Pol ysulfide
4. Addition silicone
5. Condensation silicone
6. Pol yether
Recomme nded disinfection procedures
Diluted bleach, Iodophor spra y
Diluted bleach, Iodophor spra y
I mme rsion in iodophor or 2% glutaraldehyde
I mme rsion in 2% glutaraldehyde or hypochloride solution
I mme rsion in 2% glutaraldehyde or hypochlorite solution
Iodophor spra y or chloride dioxide
 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 Powers11th edition2002, pg 329-378
2. Phillips science of dental materials:11th edition
3. GPT Academy of prosthodontics JPD,july 2005 vol 94
30

31. 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.
31