A dental impression is a negative imprint of hard (teeth) and soft tissues in the
mouth from which a positive reproduction (or cast) can be formed. They are made
by using a container which is designed to roughly fit over the dental arches
("trays"). Impression materials are designed to be liquid or semi-solid when first
mixed and placed in the tray, and then quickly set to a solid (usually a few minutes
depending upon the material), leaving an imprint of the structures in the mouth.
Impressions, and the study models which are cast from them, are used in several
areas of dentistry including prosthodontics (such as making dentures, inlays and
plastic casts), orthodontics, restorative dentistry (e.g. to make impressions of teeth
which have been prepared to receive indirect extracoronal restorations such as
crowns or bridges), maxillofacial prosthetics (prosthetic rehabilitation of intra-oral
and extra-oral defects due to trauma, congenital defects, and surgical resection of
tumors) restorative, diagnosis and oral and maxillofacial surgery for both intra oral
and or extra-oral aims (e.g. dental implants).
The required type of material for taking an impression and the area that it covers
will depend on the clinical indication.
A correctly made dental impression will capture a part or all of a person's dentition
and surrounding structures of oral cavity. The dental impression forms an imprint
(i.e. a 'negative' mould) of teeth and soft tissues, which can then be used to make a
cast of the dentition. Casts are used for diagnostics, patient record, treatment
planning, fabrication of custom trays, fabrication of dentures, crowns or other
prostheses and orthodontics.
An impression is made by placing a viscous, thixotropic impression material into
the mouth via a custom or stock dental impression tray. The material, then sets to
become an elastic solid, and, when removed from the mouth, provides a detailed
and stable negative of teeth. Common materials used for dental impressions are
sodium alginate, polyether and silicones - both condensation-cured silicones and
addition-cured silicones, such as polyvinyl siloxane. Historically plaster of Paris,
zinc oxide eugenol and agar have been used. Impression materials can ceas Rigid,
Elastic ( Hydrocolloid and Elastomeric)
DEFINITION: A dental impression is defined as the negative record of the
tissues of the mouth. It is used to reproduce the form of the teeth and
DESIRABLE PROPERTIES OF AN IMPRESSION
Have pleasant taste, odor and esthetic color.
Not contain any toxic or irritating ingredients.
Have adequate shelf life for storage and distribution.
Be easy to use with minimum equipments.
Exhibit dimensional stability.
Have adequate strength so that it will not break or tear while removing from
Posses’ elastic properties with freedom from permanent deformation after
Be able to be electroplated
Be compatible with die and cast materials
There is several classifications. According to philips science of dental
materials impression materials are classified on the basis of setting and
Mode of setting rigid Elastic
Set by chemical
Impression plaster, zinc
Alginate, polysulfide, polyether, silicone,
Set by temperature Compound, waxes Agar hydrocolloid.
Impression compound is rigid, reversible impression material sets by
It softens on heating and hardens on cooling.
ADA specification no 3
Supplied as sheets, sticks, cakes and cones.
Making preliminary impression of edentulous mouth.
For individual tooth impression.
To make a special tray.
To check undercuts in inlay preparations.
Copal resin 30
Carnauba wax 10
Stearic acid 5
Glass transitional temperature =39 degree centigrade.
Fusin temperature=43.5 degree centigrade.
They are poor conductors of heat.
Coefficient of Linear Expansion is high.
Good dimensional stability.
Fine reproduction of detail.
Material can be reused several times.
Inaccurate portions can be remade without having to remake the entire
Accuracy can be improved by flaming surface material.
Distortion due to poor dimensional stability.
Compress soft tissue while making impression.
Difficult to remove if there are undercuts.
Difficult to record details because of high viscosity.
ZINC OXIDE EUGENOL IMPRESSION PASTE:
Cementing and insulating medium.
Root canal filling material.
Bite registration paste.
Impression material for edentulous patients.
Temporary relining material for dentures.
base paste(white in color)
Accelerator or reactor or catalyst paste(red in color)
Setting reaction is an acid base reaction to form a chelate.
This reaction is also known as chelation and the product is called zinc
Zno + H2O Zn(OH)2
Zn(OH)2 +2HE ZnE2 +2H2O
(Base) (Acid) ( Salt)
Initial setting time Final setting time
Type 1 3-6 min 10 min
Type 11 3-6 min 15 min
Factors controlling setting time:
Particle size of zinc oxide powder.
Longer mixing time shortens setting time.
High atmospheric temperature and humidity accelerate setting.
Setting can be delayed by cooling mixing slab, spatula or adding small
amount of retarder or oils or waxes.
Good consistency and flow.
Rigid and having good strength.
Good dimensional stability.
Mixing is done on an oil-impervious paper or glass slab.
Two ropes of paste of same length and width, are squeezed on mixing slab.
A flexible stainless steel spatula is used for mixing.
Two ropes are combined until a uniform colour is obtained.
Mechanical rotary mixing are also used.
Mixing time is one minute
It has enough working time to complete border moulding.
It can be checked in mouth repeatedly without deforming.
It registers accurate surface details.
It is dimensionally stable.
Does not require separating media since it does not stick to cast material.
Minor defects can be corrected.
Requires special tray for impression making.
Sticky in nature and adheres to tissues.
Burning sensation of eugenol causes tissue irritation.
Highly inelastic in nature.
ELASTIC IMPRESSION MATERIALS;
Agar hydrocolloid was the first successful impression material to be used in
It is an organic hydrocolloid extracted from certain types of seaweeds.
Agar is a reversible hydrocolloid.
Since it is reversible can be reused.
As tissue conditioner.
For full mouth impression without deep undercuts.
Used extensively for crown and bridge impression before elastomers came
to the market.
Widely used at present fore cast duplication.
Gel in collapsible tube(for impression)
A number of cylinders in a glass jar(syringe material).
In bulk containers(for duplication)
INGREDIENTS PERCENTAGE BY WEIGHT
Potassium sulphate 1-2%
wax, hard 0.5-1%
Thixotropic material 0.3-0.5%
water Around 84%
Coloring and flavoring agents Traces
Function of each ingredient;
Potassium sulphate -It counters retarding effects of borates.
Coloring agents-for patients comfort
Agar hydrocolloid requires special equipments;
Water cooled rim lock tray.
Main disinfectants used include iodophor, bleach or glutaraldehyde.
Accurate die can be prepared, if material is properly handled
Has good elastic properties.
Good recovery from distortion
Palatable and well tolerated by patient
not flow well compared to newly available material
It cannot be electroplated
Tears relatively easily
Only one model can be poured
Its an irreversible elastic hydrocolloid
It is a mucous extract yielded from certain brown sea weeds
type 1 – fast setting
type 2- normal setting
Used for impression making;
when there are undercuts
in mouth with excessive flow of saliva
For impression to make study models and working casts
For making preliminary impression
Sodium or pottasium alginate-15% React with calcium ion
Calcium sulphate(reactor) - 16% React with potassium alginate
Zinc oxide - 4% Acts as filler
Potasium titanium flouride -3% Gypsum hardener
Diatomacous earth -60% Acts as filler
Sodium phosphate(retarder) -2% React with calcium sulphate
Colouring and flavouring agents Wintergreen, orange etc.
Two main reaction occur
2Na3Po4 + 3CaSo4 Ca3 (PO4)2 +3Na2So4
Here firstly the retarder is completely getting reacted to delay the reaction
Sodium alginate + CaSo4+H2O calcium alginate +Na2So4
Properties of Alginate
Alginate has pleasant taste and smell
It is highly flexible
Good elasticity and elastic recovery
Fine reproduction of tissue detail
Compressive strength – ranges from 0343-70 mpa
Tear strength- 350-700gm/cm2
Poorly adheres to the tray
Biological properties- inhalation of silica particles cause health hazards.
steps in manipulation
Fluff or aerate the powder by investing can several times.
For mixing a clean flexible plastic bowl and clean wide bladed stiff metal
spatula is being used.
Proper water powder ratio specified by the manufacturer should be used(1
measure water with two level scoops of powder)
Mixing is started with a stirring motion. vigorous figure 8 motion can also
Fast setting - 45 sec
Normal setting- 60sec
Working time –
Fast setting – 1min
Normal setting – 2 min
Fast setting - 1 to 2min
Normal - 2 to 4.5 min
Easy to mix and manipulate
Comfortable to patient
Good surface detail even in presence of saliva
Accuracy if properly handled
Minimum requirement of equipment
Cannot be electroplated
It cannot be corrected
Poor dimension stability
Poor tear strength
Distortion may occur if not properly handled
ELASTOMERIC IMPRESSION MATERIAL
A) According to chemistry
2) Condensation polymerizing silicones
3) Addition polymerizing silicon
B) According to viscosity
1) Light bodied (syringe consistency)
2) Medium (regular)
3) Heavy body (tray consistency)
4) Very heavy (putty consistency)
A) Base paste
liquid polysulfide polymer – 80 to 85%
Inert fillers – 16 to 18%
B) Reactor paste
lead dioxide -60 to 68%
Dibutyl phthalate -30 to 35%
retarders like magnesium steerages
Base and acceleration paste are mixed together which undergoes chemical
reaction to form polymer
The reaction is exothermic – 3-4 degree Celsius rise in temperature
Mercaptan + lead oxide polysulfide +water
Unpleasant order and colour
Material is extremely viscous and sticky and exhibits pseudo plasticity
Long setting time of 12.5min
High tear strength
Excellent reproduction of surface detail
SILICON RUBBER IMPRESSION MATERIAL
Also known as conventional silicones.
Available in three viscosities namely light, medium bodied and heavy.
Dimethyl siloxane+orthoethyl silicate silicone in presence of
Base paste and catalyst paste;
Vinyl siloxane+silane siloxane silicone rubber
Polyether impression materials;
◦ Good mechanical and compressive strength comparing other
◦ Available in three viscosities.
◦ Consist of base paste and catalyst.