Department of:
HERE
U N I V ER S I TY O F M O S U L
C O L L EG E O F D E N TI S TR Y
Dental Impression 2020-2021
Materials
By:
Lec. Abdullah J.
Department of
Mohammed
Prosthodontics
عبدهللا جاسم محمد
Impression Materials
Department of: U N I VER SI T Y O F M O SU L
Introduction HERE
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Impression materials are used to make replicas of oral
structures. All impression materials must be in a plastic or fluid
state while the replica is being made. Physical change, chemical
reaction, or polymerization convert these fluid materials into
either elastic or nonelastic (ie, plastic or brittle) negative
replicas of the soft and/or hard tissues of the mouth.
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A model or cast material (eg, high-strength stone) is poured into the
impression and, upon setting, produces a positive impression of the tissues of
interest
020-2021
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Requirements
The requirements of impression
Department of: materials can be conveniently discussed under four
main headings: HERE
(1) Factors which affect the accuracy of the impression.
(2) Factors which affect the dimensional stability of the impression, that is, the way in
which the accuracy varies with time after recording the impression.
(3) Manipulative variables such as ease of handling, setting characteristics, etc.
(4) Additional factors such as cost, taste, colour etc.
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Department of:
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Department of: U N I VER SI T Y O F M O SU L
HERE
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Nonelastic Materials
Impression plaster
Plaster of Paris is seldom used as impression material now that
elastomeric materials are available, but it can be used as a
"wash" (a thin lining material placed over a stiffer base
material or tray) material for edentulous impressions. The
main component of impression plaster is calcium sulfate
hemihydrate, which reacts with water to form calcium sulfate
dihydrate. Manufacturers incorporate additives to adjust
the setting time and setting expansion.
Department of: U N I VER SI T Y O F M O SU L
The water/powder (W/P) ratio recommended
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manufacturer should be measured out carefully. The powder
should be sprinkled into the water, allowed to sit for 30
seconds to wet the powder, and then mixed for the minimum
time necessary to obtain a homogeneous mix. Impression
plaster is rigid and will break rather than bend. The plaster
must be stored in an airtight container because it will absorb
water from the air and its setting time will be
adversely affected.
Department of: U N I VER SI T Y O F M O SU L
HERE
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Impression compound:
There are two types of dental compound as defined by the
ADA. Type I is used for impression taking, and Type II is used
for tray preparation. Although dental compound has fallen into
disuse, it can be used for full-crown impressions (Type I),
impressions of partially or completely edentulous jaws (Type I),
and impression trays in which a final impression is taken with
another material (Type II). Compound
cannot be used to record undercuts since it is not elastic.
Department of: U N I VER SI T Y O F M O SU L
HERE
C O L LEGE O F D E NT IS T RY
Impression compound is available in either cakes or
sticks in various colors.
Dental compound is thermoplastic; it is used warm
(45C) and then cooled to oral temperature (37C), at
which it is fairly rigid. The setting mechanism is
therefore a reversible physical process rather than a
chemical reaction.
Department of: U N I VER SI T Y O F M O SU L
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Composition
Dental compound contains several ingredients. Natural resins,
which comprise about 40% of the formulation, make the
compound thermoplastic. Shellac is often used. Waxes (about
7%) also produce thermoplastic properties. Stearic acid (about
3%) acts
as a lubricant and plasticizer. Fillers and inorganic pigments
account for the remaining 50% of the formulation.
Diatomaceous earth, soapstone, and talc are examples of
commonly used fillers.
Department of: U N I VER SI T Y O F M O SU L
thermal conductivityHERE
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of dental impression compounds is
very low. These materials do not conduct heat very well
and therefore require heat soaking to attain a uniform
temperature throughout the mass. When heated or
cooled, they soften or harden quickly on the outside, but
time is needed for the temperature to become uniform
throughout the entire mass. If the impression is removed
from the mouth before it has cooled completely, severe
distortion may occur.
Department of: U N I VER SI T Y O F M O SU L
HERE
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Advantages
Dental impression compound is compatible with die and
model materials
Disadvantages
The handling of dental impression compound is very technique
sensitive. If it is not prepared properly, volatiles can be lost on
heating, or low-molecular-weight ingredients can be lost
during immersion in a water bath. Also, excessive wet
kneading can incorporate water into the mix and change the
flow properties of the compound. Due to a high coefficient of
thermal expansion, the dimensions of the impression are not
likely to be the same as the dimensions in the mouth. These
materials are nonelastic and may distort on removal from the
mouth. The casts should be poured within 1 hour.
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Troubleshooting
1. Distortion. If the material is not completely cooled, the inner
portions of the impression will still be soft when the impression is
removed, resulting in distortion. Also, if water has been incorporated 020-2021
as the result of wet kneading, the material could have excessive flow
at mouth temperature, producing distortion during removal from the
mouth. If the tray used to carry the compound to the mouth is too
flexible, distortion can result. It is important to select a tray that is
strong and rigid. A delay in preparing the stone cast also may cause
distortion. The cast should be poured as soon as possible after the
impression has been removed from the mouth.
2. Compound is too brittle or grainy. Prolonged immersion in the
water bath will cause low-molecular- weight components to leach
out.
Department of: U N I VER SI T Y O F M O SU L
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Zinc oxide-eugenol
Zinc oxide-eugenol's main use as an
impression material is for dentures on
edentulous ridges with minor or no
undercuts. It can also be used as a wash
impression over compound in a tray or in a
custom acrylic tray.
Department of: U N I VER SI T Y O F M O SU L
HERE
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Composition This material is commercially available in a
powder and liquid form and as two pastes. One paste, called
the base or catalyst paste, contains zinc oxide (ZnO), oil, and
hydrogenated rosin. The second paste, the accelerator,
contains about 12% to 15% eugenol, oils, rosin, and a filler
such as talc or Kaolin. These two pastes have contrasting colors
so it can be determined when the pastes are thoroughly
mixed.
Department of: U N I VER SI T Y O F M O SU L
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Note:
The shrinkage of these materials during the hardening process
is approximately 0.1%. Subsequently, no additional
dimensional change should occur.
Advantages
The advantages of zinc oxide-eugenol include high accuracy of
soft tissue impressions due to its low viscosity. The material is
stable after setting, has good surface detail reproduction, and
is inexpensive. It also adheres well to dental impression
compound.
Department of: U N I VER SI T Y O F M O SU L
HERE
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Disadvantages
The Disadvantages of this material are messiness and a
variable setting time due to temperature and humidity.
Eugenol is irritating to soft tissues. This material is nonelastic
and may fracture if undercuts are present.
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4- Impression wax C O L LEGE O F D E NT IS T RY
Waxes are thermoplastic materials, they do not set by
chemical reaction . Normally used to correct small
imperfection in other impression especially zinc oxide eugnol.
They consist of a combination of a low melting paraffin wax
and beeswax in a ratio of approximately 3:1 to ensure the wax
flows at room temperature. A cast should be poured up
immediately after taking the impression to avoid distortion
that readily occurs in wax.
Clinical uses:
Preliminary impression for edentulous patients
Bite registration
Baseplate wax used to be used for provisional crown and
bridge work
Melted to correct voids in gypsum casts
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References:
Sakaguchi RL and Powers JM( Craig's restorative dental materials)
Mccabe JF and Walls AWG (applied
Department of:
dental materials)
William J. O'Brien (Dental
HERE Materials and Their Selection)
U N I VER SI T Y O F M O SU L
C O L LEGE O F D E NT IS T RY
2020-2021
THE END
