Dr M Slabbert
Dept Prosthodontics
Wits
First Impressions count
Classification of Impressions
 Preliminary impressions
 Taken either by the dentist or an expanded-function
dental assistant.
 Used to make a reproduction of the teeth and
surrounding tissues.
 Used to make (1) diagnostic models, (2) custom
trays, (3) provisional coverage, (4) orthodontic
appliances, and
(5) pretreatment and post-treatment records.
Classification of Impressions- cont’d
 Final impressions
 Taken by the dentist.
 Used to make the most accurate reproduction of
the teeth and surrounding tissues.
 Used to make indirect restorations, partial or full
dentures, and implants.
Classification of Impressions- cont’d
 Bite registrations
 Taken by the dentist or dental assistant.
 Make a reproduction of the occlusal relationship
between the maxillary and mandibular teeth.
 Provide an accurate registration of the patient’s centric
relationship between the maxillary and mandibular
arches.
Study model (cast)
Oral Examination
Primary Impression
Treatment Planning
• duplicate model
Final Impression
Mouth and tooth
preparation
Master model
Denture
Laboratory procedure
• investment cast
• refractory cast
Delivery
Definition
An impression is a negative record of the tissues of the
oral cavity which constitutes the basal seat of the denture.
An impression is made in a material which has plasticity
and which hardens or sets while in contact with the tissue.
Prof Owen Fundamentals of Removable partial dentures 2nd Ed pg. 124 -131
Wits Pros Book Vol 2 Pg. 14
Study pages 14- 31 Vol 2
www.health.wits.ac.za/Prosthodontics Primary impressions in alginate…
The Ideal Impression Material
 Easy to mix and handle.
 Suitable working time.
 Suitable setting time.
 Compatible with die and stone
 Not toxic or allergenic to the patient.
 Dimensionally stable on setting.
 Accurate to record the fine details of the prepared tooth
 Has acceptable odor and taste.
 Adequate strength.
 Adequate shelf life.
The Ideal Impression Material
cont.
 Economical
 Ready to disinfected without loss of accuracy.
 Fluid or plastic when inserted into the mouth.
 It must be an exact record of all the aspects of the prepared tooth
and sufficient unprepared tooth structure immediately adjacent to
margins, to allow the dentist and the technician to be certain of the
location and configuration of the finish line.
 Other teeth and tissue surrounding the abutment tooth must be
accurately reproduced to permit proper articulation of the cast and
contouring of the restoration.
 It must be free from air bubbles especially in the finish line area.
• High accuracy
Biocompatibility
(very small contraction <0.5%)
•
•
•
•
High dimensional stability
High elastic recovery
Ease of use
Hydrophilic
and Hydrophobic
• Proper setting time
Compatibility to stone
High tear strength
Long shelf life
Pleasant color &taste
Cost
Elastic recovery
The amount of rebound after a cylinder of material is
strained 10% for 30 seconds.
98%
Hydrophilic VS Hydrophobic
Which one of the impression
materials we choose?
•What’s the job you do?
primary impression
final impression
•How accurate do you want?
removable denture
fixed prostheses
•What technique do you use?
IMPRESSION MATERIALS
Key Properties
a. Accuracy = ability to replicate the intraoral surface details.
b. Dimensional Stability = ability to retain its absolute
dimensional size over time.
c. Tear Resistance = ability to resist tearing in thin sections
(such as through the feather-edged material within the
gingival sulcus.
Impression Tray
Impression
Accuracy
Dimensional
Stability
Tear
Resistance
PROBLEM ANALYSIS
What are tolerable limits for “error” in indirect procedures?
a.
b.
c.
d.
e.
f.
g.
Impressions
Casts, Dies
Waxing
Investing
Casting
Finishing, Polishing
Cementation
Prepped
Tooth
Width
=
=
=
=
=
=
=
+/- 0
+/- 0
+/- 0
+ 1.5%
- 1.5%
+/- 0
+/- 0
8 mm (=8,000 m)
8,000 m x 0.5% = 40 m = 20 m/side
Typical clinical error = >100 m/side
CLASSIFICATION SYSTEM
Based on Properties of Set Materials
Properties:
Reaction:
Set:
Rigid
Rigid
Rigid
Irrev
Rev
Irrev
(Chem)
(Phys)
(Chem)
Water-Based Gel:
4. Alginate (Irreversible Hydrocolloid)
5. Agar-Agar (Reversible Hydrocolloid)
Flexible
Flexible
Irrev
Rev
(Chem)
(Phys)
Elastomers:
6. Polysulfide (Rubber Base, Thiokol)
7. Silicone (Conventional, Condensation)
8. Polyether
9. Polyvinyl Siloxane (Addition Silicone)
Flexible
Flexible
Flexible
Flexible
Irrev
Irrev
Irrev
Irrev
(Chem)
(Chem)
(Chem)
(Chem)
Rigid:
1.
2.
3.
Impression Plaster
Impression Compound
Zinc Oxide/ Eugenol
Impression materials
Plaster
Nonelastic
Impression Compound
Zinc oxide Eugenol
Impression waxes
Impression material
Hydrocolloids
Elastic
Non-aqueous
elastomers
Agar
Reversible
Polysulfides
Alginate
Irreversible
Polyethers
Condensation
silicone
Addition
silicone
Nonelastic
Gypsum (Plaster)
Impression Compound
Zinc oxide eugenol
Impression wax
Dental Plaster
Dental Plaster
• Type I – impression plaster
• Type II – model (laboratory) plaster (used for mounting casts)
plaster is composed of the β form of calcium sulfate hemihydrate Crystals
• plaster is weaker than dental stone due to:
1.) porosity of the particles, requiring more water for a plaster mix
2.) irregular shapes of particles prevent them from fitting together tightly
Dental Stone
• Type III - dental stone
(diagnostic casts)
• Type IV - high strength
dental stone (working
models)
• Type V - high-strength, high-expansion dental stone
Stone is the α hemihydrate form Types of Gypsum Products
Calcination
 Calcination
H or other means
 Mineral gypsum ---------------- > Model plaster + Water
(CaSO4 . 2H2O) Dental stone
High-strength dental stone (CaSO4 . 1/2H2O)
Reverse Reaction
When calcium sulphate hemihydrate (dental plaster, stone, etc.)
is mixed with water, the reverse reaction takes place, and the
hemihydrate is converted back to the dihydrate:
CaSO4 . 1/2H2O + 11/2H2O ---> CaSO4 . 2H2O + 3900 cal/g mol
Elastomeric Impression
Materials
A material that is used when an extremely
accurate impression is essential. The term
elastomeric means having elastic or rubberlike
qualities.
Elastic
Agar
Hydrocolloids
Alginate
Hydrocolloids
Reversible and Irreversible
 Introduced by Sears 1939
 First elastic
 Sears AW. Hydrocolloid impression technique for inlays and fixed
bridges. Dent Digest 1937; 43: 230-234.
 Lin C, Zeiber G J. Accuracy of impression materials for complete
arch fixed partial dentures. J Prosthet Dent 1988; 59: 288-291
 Philips Science of Dental Materials 11th Ed. Part 2 Pg. 231
Irreversible Hydrocolloid
 Material that cannot return to a solution state after
it becomes a gel.
 Alginate is the irreversible hydrocolloid most
widely used for preliminary impressions.
Makeup of Alginate
 Potassium alginate (Alginic Acid) (12-15%)
 Comes from seaweed; is also used in foods such
as ice cream as a thickening agent.
 Calcium sulfate (8-12%)
 Reacts with the potassium alginate to form the gel.
 Trisodium phosphate
 Added to slow down the reaction time for mixing.
Makeup of Alginate- cont’d
 Diatomaceous earth (70%)
 A filler that adds bulk to the material.
 Controls the stiffness of the set gel
 Zinc oxide
 Adds bulk to the material.
 Potassium titanium fluoride (~10%)
 Added so as not to interfere with the setting and surface
strength.
 Sodium Phosphate (retarder) (2%)
 Coloring and flavouring agents (traces)
Physical Phases of Alginate
 The first phase is a sol (as in solution). In the sol
phase, the material is in a liquid or semiliquid form.
(sol: resembles a solution, but is made up of
colloidal particles dispersed in a liquid)
 The second phase is a gel. In the gel phase, the
material is semisolid, similar to a gelatin dessert.
“gel” entangled framework of solid colloidal particles
in which liquid is trapped in the interstices and held
by capillary forces (Jello)
Packaging and Storing of
Alginate
 Containers about the size of a coffee can are the
most commonly used form of packaging.
 Premeasured packages are more expensive, but
save time by eliminating the need for
measurement of the powder.
 Shelf life of alginate is approximately 1 year.
Fig. 46-7 Examples of packaging for alginate.
Causes for Distortion and
Dimensional Change of Alginate
 If an alginate impression is stored in water or in a
very wet paper towel, the alginate will absorb
additional water and expand. This condition is called
imbibition.
 If an alginate impression remains in the open air,
moisture will evaporate from the material, causing it
to shrink and distort. This condition is called
syneresis.
ADA Specifications
 <3% deformation with a 10% strain
Altering the Setting Times of Alginate
 Cooler water can increase the setting time if
additional time is needed for the procedure.
 Warmer water can reduce or shorten the setting
time of the procedure.
Water-to-Powder Ratio
 An adult mandibular impression generally
requires two scoops of powder and two measures
of water.
 An adult maxillary impression generally requires
three scoops of powder and three measures of
water.
Fig. 46-8 Scoop and water measure for alginate.
Taking an Alginate Impression
 Explain the procedure to the patient:
 The material will feel cold, there is no unpleasant taste,
and the material will set quickly.
 Breathe deeply through your nose to help you relax and be
more comfortable.
 Use hand signals to communicate any discomfort.
 Mouth Preparation
 Rinse and dry the patient's teeth
 If teeth are too dry, alginate will stick
An Acceptable Alginate
Impression
 The impression tray is centered over the central and






lateral incisors.
There is a complete "peripheral roll," which includes all of
the vestibular areas.
The tray is not "overseated," which would result in
exposure of areas of the impression tray.
The impression is free from tears or voids.
There is sharp anatomic detail of all teeth and soft
tissues.
The retromolar area, lingual frenum, tongue space, and
mylohyoid ridge are reproduced in the mandibular
impression.
The hard palate and tuberosities are recorded in the
maxillary impression.
Trouble Shooting
 Inadequate working or setting time:
 temperature of the water, incomplete spatulation
 W/P too low
 improper storage of alginate powder
 Distortion:
 Tray movement during gelation or removed from mouth prematurely
 weight of tray compressing or distorting alginate
 impression not poured up immediately
 Tearing:
 removing impression from mouth before adequately set
 thin mixes (high W/P ratio)
 presence of undercuts (blocking out these areas before an impression may help)
 inadequate amount of impression material in tray (avoided by minimum 3 mm of impression
material between tray and oral tissues)
 • Loss of detail:
 removed from mouth prematurely
 Consistency:
 preset mix is too thin or thick
 The W/P ratio is incorrect (avoid by fluffing powder before measuring;
do not overfill powder dispenser)
 inadequate mixing (avoided by vigorous spatulation and mixing for
recommended time)
 using hot water: grainy and prematurely thick mix
 • Dimensional change:
 delay in pouring
 alginate impression stored in air: results in distorted, undersized cast
due to alginate impressions losing water when stored in air
 Porosity:
 whipping air into the mix during spatulation (proper mixing: after initial
 wetting of powder by the water, mix alginate so as to squeeze the
material between the spatula blade and the side of the rubber bowl)
 • Poor stone surface (of cast)
 set gypsum remaining in contact with the alginate for too long a period
of time
Reversible Hydrocolloid
 An impression material that changes its physical
state from a sol to a gel and then back to a sol.
Chemical Makeup of Reversible
Hydrocolloid
 85% water
 13% agar
 Agar is an organic substance derived from seaweed.
 Additional chemical modifiers are added to aid in the
handling characteristics.
Conditioning Bath for Reversible
Hydrocolloid
 Three Compartments



The first bath is for liquefying the semisolid
material. A special water bath called a
“hydrocolloid conditioner” at 212° F liquefies the
material. After liquefying, the preset thermostat
cools the temperature to 150° F automatically.
The second bath becomes a storage bath that
cools the material, readying it for the impression.
At this temperature, the tubes are waiting for use.
A third bath is kept at 110° F/44° C for tempering
the material after it has been placed in the tray.
Types of Reversible Hydrocolloid
 Tray material
 Packaged in plastic tubes. Each tube has enough
material to fill a full arch, water-cooled tray.
 Syringe material
 Packaged in plastic or glass cartridges that fit a
syringe or in preloaded syringe or preformed sticks
that refill special hydrocolloid inlay syringes.
Application of Reversible Hydrocolloid
Impression Material
1.
2.
3.
4.
5.
6.
A stock water-cooled tray is selected, making sure
that the tray does not impinge on any of the teeth
or soft tissue.
Plastic stops are placed in the tray.
Tubing is connected to the tray and to the water
outlet for drainage.
The material is liquefied and moved to the storage
bath.
The light-bodied material is placed in the syringe,
and heavy-bodied material is placed in the tray.
The light-bodied material is expressed around the
prepared tooth, and the dentist seats the tray.
Elastic
Polysulfides
Polyethers
Non-aqueous
elastomers
Condensation
silicone
Addition
silicone
GENERAL FORMULATION
For Elastic Impression Materials
1.
Flexible Matrix (Continuous Phase):
a. Multifunctional Pre-Polymer or Polymer
b. Crosslinking Agent
c. Curing Agent (Catalyst or Initiator)
d. Modifiers (Accelerators, Retarders,
Plasticizers, Flavoring Agents, Colorants)
2.
Filler or Extender (Dispersed Phase):
Polymer (high shrinkage)
Filled Polymer (low shrinkage)
ELASTOMERIC IMPRESSIONS
Management of Shrinkage
Use 2-step techniques:
(a) Light-Heavy Body, OR
(b) Wash-Putty
Impression Tray
Impression
Load tray with
heavy-bodied IM
Cover prep with
light-bodied IM
from syringe
During setting shrinkage, distortion of impression is toward tray.
Impression
Dies tend to be oversize and
so the casting is oversize.
ELASTOMERIC MATERIALS
Mixing and Delivery Systems
Polyether,
Polyvinylsiloxane
2x2x2x2x2x2x2x2 = 256 folds
Polysulfide Rubber,
Silicone Rubber
Mixing Options:
 2 Pastes on Mixing Pad
 2 Pastes in Mixing Gun
 2 Pastes in Mixing Machine
IMPRESSION MATERIALS
Management of Distortion During Tray Removal
Strain rate sensitive elastomers !
Impression Tray
Fast removal
Impression
VERY
FAST
(SNAP)
Slow removal
IMPRESSION MATERIALS
REACTION
CONVERSION (%)
Distortion Time Related to Setting Reaction
100
VPS Pe
PS, Silicone
log TIME (minutes)
Polysulfide Impression
Material
 Chemical makeup
 Base: Mercaptan polysulfide.
 Cross-linking agent: Sulfur and/or lead peroxide.
 Catalysts: Copper hydroxides, zinc peroxide, organic
hydroperoxide.
 Fillers: Zinc sulfate, lithopone, or calcium sulfate
dihydrate.
Manipulation and Technique
Considerations for Polysulfide Material
 Dispense pastes at the top of the mixing pad.
 Mix pastes with the tip of a spatula to incorporate the






material first.
Transfer the material to the fresh surface of the mixing pad.
Water, saliva, and blood affect polysulfide material.
Impression should be removed quickly after setting-do not
rock the tray.
Adhesive must be thin and dry before adding the
impression material.
Wait 20 to 30 minutes before pouring the impression for the
stress relaxation to occur in the material.
Be careful of glove powder contamination of the
impression.
Polyether Impression Material
 Chemical makeup
 Base: Polyether
 Cross-linking agent: Sulfate
 Catalysts: Glycol-based plasticizers
 Filler: Silica
Manipulation and Technique
Considerations for Polyether Material
 Material is very stiff, which makes it difficult to




remove without rocking.
When removing the impression, break the seal and
rock slightly to prevent tearing.
Water, saliva, and blood affect polyether material.
Added moisture will increase the impression’s
marginal discrepancy.
Increased water absorption occurs if a thinning
agent is used.
Silicone Impression Material
 Chemical makeup
 Base: Poly dimethyl siloxane
 Cross-linking agent: Alkyl ortho silicate or organo
hydrogen siloxane
 Catalyst: Organo tin compounds
 Filler: Silica
Manipulation and Technique
Considerations for Silicone Material
 The material has a limited shelf life.
 The tray requires a special tray adhesive.
 No syneresis or imbibition but does respond with
shrinkage over time.
 The material is more flexible, so there is more
chance for distortion during removal.
 Wait 20 to 30 minutes before pouring of models for
stress relaxation to occur.
Polyvinyl Siloxane Impression Material
 Chemical makeup
 Base: Silicone polym
 Catalyst: Chloroplatinic acid
 Filler: Silica
Manipulation and Technique
Considerations for Polyvinyl Siloxane
Material
 For dimensional stability, this is the best impression
material.
 Pouring of the model can be delayed up to 7 to 10
days.
 Stiffness of the material makes removal of the tray
difficult.
 Material dispensed using auto-mixing unit and
mixing tips.
POLYSULFIDE RUBBER
Chemistry and Setting Reaction
CONTINUOUS PHASE:
Polymer
Crosslinking Agent
Catalysts
DISPERSED PHASE:
Fillers
= Mercaptan Functional Polysulfide
= Sulfur and/or Lead Peroxide
= PbO2 or Copper Hydroxides (Type I)
Zinc Peroxide or
Organic Hydroperoxide (Type II)
= TiO2 or Zinc Sulfate or
Lithopone or Calcium Sulfate Dihydrate
SETTING REACTION:
Stepwise (relatively slow), Exothermic, Affected by temperature
POLYSULFIDE RUBBER
Manipulation and Technique Considerations
a.
b.
c.*
d.
e.
f.
g.
Two-step techniques recommended: Reduces air entrapment and
surface tension effects.
Material adversely affected by H2O, saliva, and blood.
Set impression should be removed quickly - do not rock tray.
No syneresis or imbibition, but distortion due to continued reaction.
Ideally need uniform thickness and at least 2 mm thick for accuracy.
(1) Adhesive must be thin
(2) Adhesive must be dry
Paste-Paste Mixing Recommendations:
(1) Dispense pastes at the top of the mixing pad
(2) Mix pastes with tip of spatula only for 5 seconds
(3) Transfer mass to fresh surface at center of mixing pad
(4) Wipe spatula off with paper towel; Strop mass for 15s to constant color
(5) Load syringe or tray
(6) Use pad excess to monitor setting time
Pouring of models:
(1) Wait 20-30 minutes before pour for stress relaxation to occur
(2) RB is non-reactive with model and die materials
(3) Be careful of glove powder contamination of impression
(4) RB can be electroplated
SILICONE RUBBER
Chemistry and Setting Reaction
CONTINUOUS PHASE:
Polymer
Crosslinking Agent
Catalysts
Modifiers
DISPERSED PHASE:
Fillers
= Polydimethyl Siloxane
= Alkyl Orthosilicate or Organo H-Silane
= Organo Tin Compounds (e.g., tin octoate)
(but not dibutyl tin dilaurate)
= Colorants, Flavorants
= Silica
SETTING REACTION:
Stepwise (relatively slow), Exothermic, Affected by temperature
H2O by-products
SILICONE RUBBER
Manipulation and Technique Considerations
a.
b.
c.
d.
e.
Limited shelf-life: Unstable in tubes.
Requires mechanical retention or special tray adhesives
No syneresis or imbibition, but continued polymerization shrinkage.
Better dimensional stability than RHC but more expensive
Pouring of models:
(1) More flexible so more chance for distortion during removal
(2) Wait 20-30 minutes before pour for stress relaxation to occur
POLYETHER RUBBER
Chemistry and Setting Reactions
CONTINUOUS PHASE:
Polymer
Crosslinking Agent
Catalysts
Modifiers
= Amine-terminated Polyether
= Aromatic Sulfonate
=
= Colorants, Glycol Plasticizers, Flavorants
DISPERSED PHASE:
Fillers
= Silica
SETTING REACTION:
Stepwise (relatively slow), Exothermic, Affected by temperature
POLYETHER RUBBER
Manipulation and Technique Considerations
a.
b.
c.
d.
e.
Excellent impression accuracy and dimensional stability.
Stiff and therefore difficult to remove without rocking.
Break seal and rock slightly to prevent tearing: Low tear resistance.
Negatively affected by H2O, saliva, and blood.
(1) Since hydrophobic, moisture increases marginal discrepancy
(2) Increased water absorption occurs if use thinning agents
Can be dispensed from automated extruder and mixer (ESPE PentaMix)
POLYVINYL SILOXANE
Chemistry and Setting Reactions
CONTINUOUS PHASE:
Polymer
Crosslinking Agent
Catalysts
Modifiers
= Double-bond-functional Silicone Polymer
= Chloroplatinic Acid
=
= Colorants, Flavorants, Plasticizers
DISPERSED PHASE:
Fillers
= Silica
SETTING REACTION:
Chain (very fast), Exothermic, Affected by temperature
Hydrogen gas released by decomposition of crosslinking agent.
POLVINYLSILOXANE
Manipulation and Technique Considerations
a.
b.
c.
BEST impression material for dimensional stability:
Pouring should be delayed at least 4 hours for H2 out-gassing.
Pouring can be delayed up to 7-to-10 days (or indefinitely).
Stiffness makes removal difficult.
Most material dispensed using auto-mixing gun and mixing tips
COMMERCIAL PRODUCTS
1980-1995
Predominantly
POLYSULFIDE and
SILICONE elastomers
before 1995.
COMMERCIAL PRODUCTS
1996-2004
Predominantly POLYETHER and
PVS elastomers after 1996.
IMPRESSION MATERIALS
Comparison of Key Properties
Poor >---------------------------------> Good
1.
Accuracy (Reproduction of Detail):
a. Wetting of Tissues:
b. Wetting by Dental Stone:
ALG < SIL, PS < PE <= RHC, PVS
SIL, PVS < PS < PE <= ALG, RHC
2.
Dimensional Stability:
(Resistance to Distortion):
a. Polymerization Shrinkage
b. Thermal Shrinkage
c. Loss of Components
d. Stress Relaxation
ALG <= RHC < PS, SIL < PE, PVS
3.
Tear Resistance (Elasticity):
a. Elastic Deformation
b. Strain Rate Sensitivity
ALG, RHC, PE < PVS , SIL < PS
IMPRESSION IMPRESSIONS
PERCENT
CHANGE (%)
Dimensional Stabilty
PRODUCT EXAMPLES
Stock tray
Rim lock tray
Perforated tray
Stock tray
Special tray
(Custom tray)
Nonelastic
Plaster
Impression Compound
Zinc oxide eugenol
Impression wax
Elastic
Agar
Hydrocolloids
Alginate
Non-aqueous
elastomers
Polysulfides
Polyethers
Condensation
silicone
Addition
silicone
Elastic
Agar
Hydrocolloids
Alginate
Polysulfides
Polyethers
Non-aqueous
elastomers
Condensation
silicone
Addition
silicone
(Ideal Properties)
• High accuracy
Biocompatibility
(very small contraction <0.5%)
•
•
•
•
High dimensional stability
High elastic recovery
Ease of use
Hydrophilic
and Hydrophobic
• Proper setting time
Compatibility to stone
High tear strength
Long shelf life
Pleasant color &taste
Cost
Impression plaster
ADA type I gypsum product
Impression plaster
For
edentulous impression
Contraindication
for undercut
Composition
Calcium sulfate hemihydrate
Potassium sulfate
(CaSO4)2.H20
Reduce expansion
Borax
Reduce the rate of setting time
Starch
Help disintegration of
the impression from plaster model
Reaction
Calcium sulfate hemihydrate + water
calcium sulfate dihydrate
Mechanical properties
Very low viscosity (mucostatic)
Hydrophilic
Good dimension stability 0.06%
Great accuracy
Manipulation
 Easy to mix, trapping air bubbles
 W/P ratio must be measured out carefully
 Use in special tray, thickness of 1.0-1.5 mm.
 Setting time 2-3 minutes
 Must apply separating medium before pouring
the impression
 Powder must be stored in air tight
Advantage
Accuracy to soft tissue impression
Good accuracy
Short setting time
Disadvantage
Can not be used in undercut ridge
Able to flow to pharynges
Heat due to reaction
Sensation of dryness
Impression compound
Type I
Impression compound
Type II
Green stick compound
Impression with compound
Type I
Type II
Composition
Natural resin
Waxes
Stearic acid
Filler&inorganic pigment
40%
7%
3%
50%
RESINS
 Amorphous organic substance which
are insoluble in water
 Make Thermoplastic
 Shellac, dammar, rosin or sandarac
WAXES
 Straight chain hydrocarbon
CH3(CH2)nCH3
 Tasteless, Odorless, Colorless
 Make Thermoplastic
 beeswax and colophany
Stearic acid
•Lubricant and Plasticizer
Fillers
Control
degree of flow
Minimize shrinkage
Improve rigidity
Diatomaceous earth, Soapstone, talc
Mechanical properties
Thermoplastic
Reversible
37-45 C๐
physical process
Very
low thermal conductivity
High
thermal expansion
High
contraction coefficients
High
viscosity
0.3%
Manipulation






Soften by heating over the flame or water bath
Do not heat too much
volatile
Take impression
Room temperature water cooling
Pour impression as soon as possible
Warm the impression before take the cast
Water bath
Alcohol lamp
Alcohol torch
Advantage

Can take impression again
 Compatible with model material
Disadvantage
Very
technique sensitive
High
coefficient of thermal expansion
=> dimensional change

Mucocompressive

Must be poured within one hour

Low detail reproduction
Troubleshooting
 Distortion
-Material is not completely cooled
-Flexible tray
-Delay in pouring
Troubleshooting
 Compound is too brittle or grainy
-Prolong immersion in the water bath
Zinc oxide-eugenol
Impression paste
Zinc oxide-eugenol
Impression paste
Zinc oxide-eugenol
 For full arch edentulous impression without or minor
undercut

Wash impression

Bite registration
Temporary cementation
 Temporary filling

Composition (base)
 Zinc oxide (ZnO)
 Oil
 Hydrogenated rosin
 Zinc acetate
 Trace of water
Accelerator
Initiator
Composition (reactor)
 Eugenol
12~15%
 Oil
 Rosin
 Filler (kaolin)
Non eugenol paste
Carboxylic acids
Reaction
ZnO + eugenol
(powder)
Zn eugenolate + ZnO(unreacted)
(liquid)
(Solid)
Mechanical
properties
@ Very low viscosity
@ Irreversible chemical change
@ Low dimensional change
0.1%
Manipulation
Manipulation
 Equal length of the two paste is extruded
Mixed with stainless steel spatula in paper
pad or glass slab
 Mixing time
45-60 s


Working time
3-5 min
shorten by increase
Temperature&Humidity
Advantage
 Accuracy of soft tissue impression (mucostatic)

Good dimensional stability

Good surface detail reproduction
Inexpensive
 Adhere well to dental compound

Disadvantage
Messiness
Non elastic Time
Unstable setting
 May irritating to soft tissue

Troubleshooting
 Inadequate working or setting time
-excessive humidity and/or temperature
 Distortion
-unstable tray
Impression wax
KORECTA WAX , IOWA WAX
Manipulation
 Functional Impression
Disinfection of Impression
Materials
 Irreversible Hydrocolloids (Alginate)
 Diagnostic Casts:

Soak 10 min in Gluteraldehyde
 Final Impressions:

Dip in Gluteraldehyde ,rinse in sterile water, dip again let stand under damp gauze 10 min

Spray with Sodium Hypochlorite rinse, spray again & let stand under damp gauze 10 min
 Reversible Hydrocolloid



Dip in Gluteraldehyde ,rinse in sterile water, dip again let stand under damp gauze 10 min
Spray with Sodium Hypochlorite rinse, spray again & let stand under damp gauze 10 min
Addition –reaction Silicones



In Gluteraldehyde 1 hr.,
Rinse sterile water
Soak in fresh solution Gluteraldehyde 10 min
 Zinc oxide eugenol

Soak in Gluteraldehyde 10 min
 Silicone Impressions

Soak for 10 min Gluteraldehyde
 Polyether Impressions

Dip in Gluteraldehyde ,rinse in sterile water, dip again let stand under damp gauze 10 min
Fundamentals of Removable Partial Dentures 2nd Ed. CP Owen
REFERENCES
•Skinner’s Science of Dental Materials
•Dental Materials and Their Selection (William J. O’Brien)
•Introduction to dental materials (Richard Van Noort)
•Dr Sukontip Arwatchanakan
•Science of Dental Materials Phillips 11th Ed.
•Stephen C. Bayne Department of Operative Dentistry School of Dentistry
University of North Carolina
• Mr. Robert Seghi Alginate Impression Materials (alginate lecture 2006.ppt)
Tray Selection Criteria
 Feel comfortable to the patient.
 Extends slightly beyond the facial surfaces of the
teeth.
 Extends approximately 2 to 3 mm beyond the third
molar, retromolar, or tuberosity area of the arch.
 Is sufficiently deep to allow 2 to 3 mm of material
between the tray and incisal or occlusal edges of the
teeth.
Characteristics of Impression Trays
 Quadrant tray
 Covers one half of the arch.
 Section tray
 Covers the anterior portion of the arch.
 Full arch tray
 Covers the entire arch.
 Perforated tray
 Holes in the tray create a mechanical lock to hold the
material in place.
 Smooth tray
 Interior of the tray is painted or sprayed with an
adhesive to hold the impression material.
Impression Trays
 Must be sufficiently rigid to:
 Carry the impression material into the oral cavity.
 Hold the material in close proximity to the teeth.
 Avoid breaking during removal.
 Prevent warping of the completed impression.
Table 46-1 Types of Stock Trays
Fig. 46-3 Examples of quadrant, section, and
full-arch impression trays.
Tray Adhesives
 VPS adhesives (blue)
 For polyvinyl siloxane and polyether impression
materials.
 Rubber Base Adhesive (brown)
 Used with rubber base impression materials.
 Silicone Adhesive (orangish-pink)
 Used with silicone impression materials.
Characteristics or Elastomeric
Impression Materials
 Base
 Packaged as a paste in a tube, as a cartridge, or
as putty in a jar.
 Catalyst
 Also known as the accelerator, is packaged as a
paste in a tube, as a cartridge, or as a liquid in a
bottle with a dropper top.
Forms of Elastomeric Materials
 Light-bodied
 Also referred to as syringe type, or wash type. This
material is used because of its ability to flow in and
about the details of the prepared tooth. A special
syringe, or extruder, is used to place the light-bodied
material on and immediately around the prepared
teeth.
Forms of Elastomeric Materials- cont’d
 Regular and heavy-bodied
 Often referred to as tray-type materials, they are
much thicker. As the names imply, they are used to fill
the tray. Their stiffness helps to force the light-bodied
material into close contact with the prepared teeth
and surrounding tissues to ensure a more accurate
impression of the details of a preparation.
Basic Impression Technique
1.
2.
3.
4.
5.
6.
7.
The material selected depends upon the dentist’s preference
and the type of impression required for the procedure.
The dentist prepares the tooth or teeth for the impression.
The light-bodied material is prepared and loaded into the
syringe and transferred to the dentist.
The dentist places the light-bodied material over and around
the prepared teeth and onto the surrounding tissues.
The heavy-bodied material is prepared and loaded into the
tray and transferred to the dentist.
When the impression material has reached final set, the
impression is removed and inspected for accuracy.
The impression is disinfected, placed in a biohazard bag,
labeled, and readied for the laboratory technician.
Curing Stages of Elastomeric Materials
 Initial set
 The first stage results in stiffening of the paste
without the appearance of elastic properties. The
material may be manipulated only during this first
stage.
 Final set
 The second stage begins with the appearance of
elasticity and proceeds through a gradual change to
a solid rubberlike mass. The material must be in
place in the mouth before the elastic properties of
the final set begin to develop.
 Final cure
 The last stage occurs from 1 to 24 hours.
Types of Elastomeric
Materials
 Polysulfide
 Polyether
 Silicone
 Polyvinyl siloxane
Occlusal Registration
 An accurate registration of the normal centric
relationship of the maxillary and mandibular
arches. Also commonly referred to as the bite
registration.
Types of Occlusal Registration
 Wax bite
 Useful when the diagnostic casts are trimmed.
 The most common technique is to use a softened
baseplate wax.
Fig. 46-27 Wax bite registration.
Types of Occlusal Registration- cont’d
 Polysiloxane bite registration paste: Supplied
both as a paste system and also as cartridges.
 The material is fast setting.
 There is no resistance to biting forces.
 There is no odor or taste for the patient.
 It gains dimensional stability over time.
 It is convenient to use.
Types of Occlusal Registration- cont’d
 Zinc oxide-eugenol (ZOE) bite registration paste
 ZOE paste has little to no resistance to bite closure
and is a fast-setting material.
 Material is supplied in a paste system and
dispensed onto a paper pad, mixed, and placed
onto a gauze tray for the patient to bite into.