Download Impression Materials and Techniques

Impression Materials
Dr. Recep Uzgur
Department of Prosthodontics
Terminology
• IMPRESSION : A negative likeliness or copy in reverse of the surface of
an object; an imprint of the teeth and adjacent structures for use in
dentistry.
• IMPRESSION TRAY : A receptacle into which suitable impression
material is placed to make a negative likeliness (or) a device that is
used to carry, confine, and control impression material while making
impression.
• IMPRESSION MATERIAL: Any substance or combination of substances
used for making an impression or negative reproduction.
• IMPRESSION TECHNIQUE : A method and manner used in making a
negative likeness.
Rigid
Material
Use
Material
Use
Edentuolus Ridge
1.Alginate
Teeth And
Soft Tissues
2.Zinc Oxide
Edentuolus Ridge
2.Polysulfide
3.Polyether
4.Condensation
Silicones
5.Addition Silicones
3.Impression
Compound
Preliminary
Impression
6.Agar
Chemical Reaction 1.Plaster Of Paris
(Irreversible)
Thermally
Induced
Physical Reaction
(Reversible)
Elastic
Tracing
Teeth And
Soft Tissues
Edentuolus
Ridge
History
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Philip Ptaff-1756 First described taking impression with softened wax
Christopher Francois Delabarre-1820 Introduced the metal impression tray
Chapin Haris-1853 First used Plaster of Paris for making impressions
Charles Stent-1857 Introduced the first impression compound
Sears-1937- First used agar hydrocolloids for recording crown impressions.
United States-1945 Introduced alginate during World War II.
SL Pearson-1955 Developed synthetic rubber base impression materials
Polysulfide- Late 1950’S First developed as an industrial sealant.
Condensation Silicone- Early 1960’s
Addition Silicone-1970’s
Polyether- Late 1970’s Polyether Urethane Dimethacrylate
Properties of An Ideal Impression
1) Complete plasticity before cure
2) Sufficient fluidity to record fine detail.
3) The ability to wet the oral tissues.
4) Dimensional accuracy.
5) Dimensional stability.
6) Complete elasticity after cure.
7) Optimal stiffness
8) Good taste and smell
1. Plaster
• Plaster of Paris is seldom used as an impression material now that
elastomeric materials are available.
• The main component of impression plaster is calcium sulfate
hemihydrate, which reacts with water to form calcium sulfate
dihydrate.
• The water/powder (W/P) ratio recommended by the manufacturer
should be measured out carefully.
• Impression plaster is rigid and will break easily.
• The plaster must be stored in an airtight container, because it will
absorb water from the air.
2. Impression Compound
• It used to be used as an impression material, but no longer used.
• Thermoplastic property allows it to be used warm (45°C) and then
cooled to oral temperature (37°C), at which it is fairly rigid.
• Impression compound is softened by heating over a flame or in a
water bath.
• Care must be taken to prevent volatilizing ingredients over a direct
flame.
• It is being used for tracing today.
3. Zinc oxide–eugenol
• The main use of zinc oxide–eugenol is for dentures on edentulous
ridges with minor or no undercuts.
• It can also be used as a wash impression over the compound in a tray
or in a custom acrylic tray.
• This material is commercially available in a form that 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.
• The setting time is shortened by increases in temperature and/or
humidity.
• The set material does not adhere to set dental plaster or stone.
• These materials are usually mixed on a mixing pad with a spatula.
Equal lengths of base and catalyst are extruded on the mixing pad.
• Adequate mixing time is 45 to 60 seconds, pastes have an initial set
time of 3 to 5 minutes.
• Zinc oxide–eugenol gives high accuracy of soft tissue impressions.
• The material is stable after setting, has good surface detail
reproduction, and is inexpensive.
• Zinc oxide–eugenol impressions can be disinfected by immersion in
2% glutaraldehyde or 1:213 iodophor solutions at room temperature.
4. Agar Hydrocolloid
• Agar hydrocolloids have been completely replaced by rubber
impression materials, so its no longer used.
• Agar hydrocolloids are available in both tray and syring consistencies.
The material is supplied as a gel in plastic tubes.
• The gel material can be converted to a sol (liquid) by heating; cooling
the sol will return the material to the gel state.
5. Alginate Hydrocolloid
• Alginates are the most widely used impression materials in dentistry.
• They are used for making impressions for removable partial dentures with
clasps, preliminary impressions for complete dentures, and orthodontic
and study casts. They are not accurate enough for fixed partial denture
impressions
• The powder is mixed with water to obtain a paste.
• A lower W/P ratio increases strength, tear resistance, and consistency, and
decreases working and setting times and flexibility.
• One minute of thorough mixing for the regular-set material and 45 seconds
for the fast-set material are generally recommended. Alginates have a
relatively short working time of about 2.5 minutes and set about 3.5
minutes after mixing.
• Alginate impression materials are inexpensive, easy to manipulate,
pleasant tasting, able to displace blood and body fluids, hydrophilic,
and easily poured in stone. They can be used with stock trays.
• Alginates tear easily, must be poured immediately after removal from
the mouth, have limited detail reproduction, are dimensionally
unstable, and can only be used for single casts.
6. Polysulfide
• Due to their high accuracy and relatively low cost, polysulfide rubbers
can be used for fixed partial denture application.
• Polysulfides are supplied in tubes of base paste and catalyst paste,
which are mixed together.
• They are available in low, medium, and high viscosities.
• Equal lengths of base and catalyst are extruded on a disposable
mixing pad, and the components are mixed thoroughly with a stiff
tapered spatula.
• Adequate mixing time is 45 to 60 seconds, and the working time is
about 5 to 7 minutes.
• Both working and setting times are shortened by higher temperatures
and humidity.
• Cast or die should be poured within 1 hour of taking the impression.
• These materials have a long working time, good tear strength, good
flow before setting, good reproduction of surface detail, high
flexibility for easier removal around undercuts, and lower cost
compared with silicones and polyethers.
• Polysulfides have an unpleasant odor and a tendency to run down the
patient’s throat
7a. Condensation Silicone
• They are used mainly for fixed partial denture impressions.
• These materials are supplied either as two-paste and are available in low,
medium, high, and very high (putty) viscosities.
• After condensation reaction a volatile alcohol is formed as a by-product.
• The manipulation of condensation silicones is the same as for polysulfides
and the setting time is less than that of the polysulfides, which saves some
chair time.
• Higher temperatures and humidity shorten the setting time.
• They are highly elastic, and the setting time can be controlled with the
amount of accelerator.
• These materials tend to be inaccurate due to shrinkage on standing
and should be poured within 1 hour.
• They are very hydrophobic, require a very dry field, and are difficult
to pour in stone.
7b. Addition Silicones
• The accuracy of addition silicones is due to a change in
polymerization reactions to an addition type and the elimination of
an alcohol by-product that evaporates, causing shrinkage.
• These materials are available as two-paste systems in four
viscosities—light, medium, heavy, and putty—and a range of colors,
allowing monitoring of the degree of mixing.
• They are expensive and hydrophilic.
• The working and setting times of addition silicones are faster than
polysulfides.
• Addition silicones can be safely poured later or sent to a dental
laboratory. The materials may be poured 1 week after taking the
impression, and multiple pours are possible.
• The material is expensive—twice the cost of polysulfides; it is more
rigid than condensation silicones.
8. Polyether
• Polyethers are available in low-, medium-, and high viscosity materials.
• Polyethers are similar to addition silicones that have a working time of 2.5
minutes and a setting time of 4.5 minutes.
• The manipulation of polyethers is similar to that of polysulfides and
silicones. Equal lengths of base and catalyst paste are mixed rapidly (30 to
45 seconds), because the working time is short.
• A hand held gun-type mixer that provides quick, bubble-free mixes is
available for Permadyne Garant (3M ESPE).
• Also, an automatic mixing device (Pentamix, 3M ESPE) is available for use
with a polyether packaged in polybags (Impregum Penta).
• These materials are more accurate than polysulfide or condensation
silicone impression materials.
• The cost of these materials is high.
• Polyether impressions can be disinfected by immersion in sodium
hypochlorite.