Impression materials
By
Dr. Neven Aref
Assistant Professor of Dental Biomaterials, Faculty of Dentistry,
Qassim University
1.
Define the impression.
2.
Classify impression materials and indication of
each.
Recognize setting reaction of impression
materials.
3.
4.
Recognize types of hydrophilic impression
materials
5. Recognize types of plastic impression materials.
6. Recognize composition, types of alginate impressions and their
properties and manipulation.
7. Describe the composition, types, manipulation and properties
of impression compound.
8. Recognize types of materials used for pouring impression for
study casts.
• Impression material: the substance that is used to
take an impression of the patient’s oral structure.
4
• Impression:
Negative replica of the oral
hard (teeth) or soft (gingiva,
mucous membrane)
structures.
(Types: primary &
secondary)
• Cast (model):
positive replica of the oral
hard (teeth) or soft (gingiva,
mucous membrane)
structures.
5
•
Tray:
An instrument used to carry the impression
material.
*Types:
1. Stock tray >> for 1ry imp. >> metallic or
Metallic stock tray
plastic
2. Special (custom) tray >> for 2ry imp.
>>Shellac, acrylic, compound.
Acrylic special tray
Plastic stock tray
6
Purpose of taking impression:
1. To study alignment of teeth
2. Treatment planning
3. Fabrication of indirect restorations
4. Construction of special (custom) tray
7
Ideal requirements of impression material
1. Accuracy (dimensionally accurate) = ability to record fine details
of oral structures (by high flow of the unset impression material).
2. Dimensional Stability = ability to retain its dimensional size over
time (minimal dimensional changes during and after setting).
3. Should adhere to the tray during removal from the mouth.
8
4. Adequate elasticity on removal from the mouth (after
setting), so that undercuts can be recorded without
distortion of the impression.
(Undercut = area under inclined plane)
9
5. Should be compatible with gypsum products
(cast material):
a. Not need separating medium, which may
occupy space and affect accuracy.
b. Not react with the cast material or release by
products which affect the model surface.
6. Biologically acceptable: non-toxic, non-irritant
and of acceptable taste & odor.
10
7. Easily disinfected without loss of accuracy.
8. Easily manipulated with minimal equipment.
9. Have suitable working and setting time.
10.Have suitable shelf life.
11. Not expensive.
11
Classification of impression materials
I. According to elasticity after setting
12
Plaster
Non-elastic
(rigid)
Compound
Impression
Materials
ZnO - Eugenol
Aqueous
Hydrocolloids
Elastic
Agar-Agar (reversible)
Alginate (irreversible)
Polysulfide
Non-aqueous
Elastomers
(rubber base)
Condensation
Silicones
Polyether
13
Additional
• What is the importance of classifying impression
materials according to their elasticity after setting?
Rigid >>>> edentulous patient without undercut
Elastic >>>>>dentulous or edentulous patient
14
Another classification:
According
to setting
reaction
Physical
(reversible)
Impression
compound
Agar- Agar
Chemical
(irreversible)
Zinc Oxide
Eugenol
Alginate
15
Rubber base
A. Rigid impression materials
1. Impression Compound
❖ Thermoplastic impression
material that is softened by
heating & hardened by
cooling.
❖ Also known as “modeling
compound”.
❖ Forms: sheets & sticks of
different colors
16
* Composition:
1. Thermoplastic materials:
To give thermoplasticity, flow and cohesion.
e.g. natural resins & waxes
2. Fillers:
To give body and suitable working consistency.
e.g. talc powder
3. Plasticizers:
To act as lubricants & control consistency (with fillers) and improve workability.
e.g. stearic acid or stearin.
4. Coloring agents:
to give characteristic color.
e.g. rouge
17
• Types and uses:
Type I: “Lower fusing compound” (<70°C):
a. Sheet form for recording primary
imp. of edentulous pts. without
undercut…. Why?
b. Stick form for border tracing of
special tray.
Type II: “Higher fusing compound”
(>70°C):
Used as tray material for wash impression
technique.
18
• Setting reaction:
Thermoplastic physical reversible reaction
Hard
(Set)
45-55° C
37° C
Soft
(unset)
19
• Manipulation (for 1ry impression):
1. Compound is softened by heating over a
flame or in a water bath.
2.
As
the
material
has
low
thermal
conductivity, it must be immersed and &
kneaded under water for sufficient, but not
Thermostatically-controlled
water bath
too long, time to ensure complete
softening but without leaching out of its
ingredients.
3. Type of tray used…
Plain stock tray
(non perforated).
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1. Impression compound
21
Manipulation (continued)
4. Cooling & removal:
Use water spray to cool the impression and to
accelerate hardening of the material.
Remove impression from the mouth after complete
hardening to avoid its distortion.
5. Sterilization: Can be autoclaved for 10 minutes.
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Properties
1. Accuracy:
Compound is not sufficiently fluid to record fine details.
2. Dimensional stability:
It has high coefficient of thermal expansion and contraction, so it
shows considerable amount of shrinkage on cooling (during
hardening & cooling to room temperature).
3. Elasticity:
the material is non-elastic .… so cannot be used in undercut
areas.
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Properties (continued)
4.
Compatible with cast & die materials & does not need separating medium.
To separate cast from the impression, use warm water.
5.
As the material is reversible, it can be re-used, but after sterilization.
6.
Compound impression can be added or corrected.
7.
Other properties:
* Non-toxic or irritant.
* Suitable setting time.
* Long shelf life.
* Can be copper plated.
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Hydrocolloid impression
materials
Colloids:
• Heterogeneous two phase system like suspension but the
particle size of the dispersed phase is smaller (1-200 nm)
than that of the suspension.
Hydrocolloids
• Colloids are called hydrocolloids when the dispersion
medium is water.
• Used for dental impression.
• It exists in the sol and gel state.
• It is inserted in the mouth in sol state, when it is
sufficiently fluid to record details.
• Then, it is removed after the gel is formed and exhibited
elastic properties.
• Sol
Cooling (physical reaction)
I. (Agar-Agar)
Gel
Heating
Types of hydrocolloids
• Sol
II. (Alginate)
Chemical reaction
Gel
Irreversible hydrocolloid (Alginate)
impression
• It is a viscoelastic impression material which
changed from a sol to a gel state through a chemical
reaction, so it is irreversible
• It is indicated as a primary impression that is used
for both dentulous and edentulous patients
Presentation forms
→Traditional powder form packed either in
A- bulk form in container with suitable measures for
dispensing the correct quantity of powder and water
(scoop and plastic measuring cylinders).
B- Small sealed packets/ individually sealed pouches
→ Modified forms
• Two pastes system
Composition
1. Potassium (or sodium) alginate 15 %
•
Soluble salt of alginic acid.
•
Main Rx ingredient because it forms sol with water.
2. Calcium sulfate dihydrate ( CaSO4 .2H2O ) 16 %
•
It is the reactor of the material, it reacts with the soluble alginate form to produce
insoluble form.
3. Sodium phosphate Na3(Po4)2 2%
▪
It acts as a retarder ,as it allow longer working time where Ca+ react with Po4+
before the cross linking occurs.
4. Potassium sulfate 3 %
• It is accelerator for the setting time of the gypsum products
used in pouring models
5. Fillers
As zinc oxide or diatomaceous earth which strengthens the gel
6.
Flavoring,
coloring
and
disinfecting
agents
(
chlorhexidine)
7. Chemical indicator (present in some products)
• Changes color with PH change, to indicate different stages in
manipulation (chromatic alginate)
Setting reaction
Gelation reaction
• The reaction of alginate is chemical, done through several
steps
• When the powder is mixed with water, calcium sulfate is
dissociated into calcium ions and sulfate ions
CaSo4 + 2H2O →Ca + So4
• Sodium phosphate is also dissociated
Na4P2O7 → 4Na + P2O7
• Then, the Calcium reacts with Phosphate to form
insoluble Calcium phosphate.
• This process continuous until the whole amount of
phosphate is depleted , then the calcium ions start to
react with the soluble alginate (Sodium or
Potassium alginate) to form insoluble alginate
(Calcium alginate)
• Ca + Sodium alginate → Calcium alginate (final
elastic gel)
Manipulation
І- Equipment
• Alginate powder (bulk container or reweighed package)
• Mixing rubber bowel
• Metal spatula
• Perforated or rim locked stock tray
ІІ- Proportioning and mixing
-
Proportioning of powder/water
manufacturer instructions.
ratio
acc.
to
• Water is inserted into a clean rubber bowel, and
then the powder is incorporated into the water by
vigorous mixing with a metal spatula against the
wall of the bowel.
• Mixing time is 45 sec-1 min & setting time 1.5-5
min from start of mix.
• The resulting mix should be creamy and smooth.
• Avoid movement of the tray during gelation.
III. Tray loading and impression taking
• Avoid movement of the tray during gelatin.
• Do not clean the mixing bowl immediately. Use the alginate
remaining in the bowl as an indicator for setting time. If the
alginate in the bowl is set, then the material in the mouth is
also set.
IV- Impression removal
• The impression should be removed with a snap movement as this
increases the tear strength and elastic recovery.
V- Disinfection
• The disinfection procedure should be relatively rapid to prevent
dimensional changes.
Disinfectant agents:
• Iodophors, sodium hypochlorite and glutaraldehyde
Technique:
Spraying technique
• The impression is rinsed in water.
• The disinfectant is sprayed on the surface.
• The impression is wrapped in a disinfectant soaked paper towel and
placed in a sealed plastic bag for 10 minutes.
• The impression is removed from the bag,
unwrapped, rinsed, and shaken to remove excess
water then pouring.
Immersion technique
• An alternative disinfection method is by immersion
for 10 minutes.
VI. Impression pouring without need for
separating medium
Alginate impression
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Properties of alginate
Flexibility
• Flexible → easy to be removed from undercut with minimum stresses.
Tear strength
• Poor tear strength, so should be removed with snap movement.
Elasticity
• Viscoelastic
• Elastic recovery: 97.3%
Clinical importance
• The impression should be removed suddenly to reduce the amount of
permanent deformation.
Compatibility with gypsum
• Don't need separating medium.
• Affect the setting and surface properties of the gypsum
materials.
Causes
1. The water content of the impressions inhibits the setting of
the gypsum at the surface and results in soft surface.
2. The hydrocolloids are hydrophilic and so absorb water from
the gypsum mix which results in chalky appearance.
How to avoid?
1. Immersion of the impression in 2 % K2SO4
before pouring (gypsum accelerator).
2. Incorporation of the gypsum accelerator in the
alginate impression material itself, such as
potassium titanium fluoride.
Dimensional stability
• Dimensionally unstable on storage
Causes
A. Water loss
• Occurs either by phenomenon called syneresis (exuding of
fluids onto surface) or evaporation.
• Results in shrinkage.
B. Water uptake
• Occurs by phenomenon called imbibition
• Results in swelling and expansion.
How to reduce dimensional changes?
• Pouring the impression immediately (within 10
min).
• If the pouring must be delayed, the impression is
then wrapped in a wet towel (saturated with water)
and placed in a sealed plastic bag or a humidor
(stored in 100% humidity)
Advantages
• Inexpensive
• Easy to use
• Hydrophilic. So, good wetting for oral tissues
• Stock trays with no need for special tray
Disadvantages
• Dimensionally unstable
•
•
•
•
- Immediate pour
- Single cast
Lower detail reproduction
Difficult to disinfect
Can not be electroplated
Low tear strength
Materials used for pouring impressions
• Gypsum products are used in dentistry for preparation
of study models.
• The most common gypsum materials used for
pouring study casts are:
• Dental plaster or model plaster (Type II)
• Dental stone (Type III) – It has higher strength than
Dental plaster.
1. Sakaguchi RL, Power JM. Craig's Restorative
Dental Materials. Mosby CO., St. Louis; 12th ed.
(2006) and 13thed. (2012).