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IMPRESSION MATERIALS
HYDROCOLLOIDS
INTRODUCTION
IDEAL REQUIREMENTS TO
OBTAIN AN ACCURATE
IMPRESSION
COLLOIDS IN DENTISTRY
DIMENSIONAL EFFECTS
Syneresis
Imbibition
AGAR- AGAR- (REVERSIBLE
HYDROCOLLOID)
Composition
Gelation temperature
Liquefaction temperature
Gelation process
Viscosity of material
Manipulation
Making the Agar
impression
Laminate technique
(alginate – agar method)
ALGINATE (IRREVERSIBLE
HYDROCOLLOID)
Composition
Mode of supply
Classification of alginate
Gelation process
Manipulation
Removal of impression
Compatibility with gypsum
Biocompatibility
Applications
CONCLUSION
INTRODUCTION
An accurate impression plays a pivotal role in success of
a prosthesis. This accuracy develops through thorough
knowledge of impression materials and in the ability of the
operator to understand the material.
An impression is an “imprint” or negative likeness of the
teeth and/or edentulous areas, made in plastic material which
becomes hardened or set while in contact with the tissue
-Heartwell
An impression is the perpetual preservation of what already
exists and not the meticulous replacement of what is missing
- M.M.Devan
IDEAL REQUIREMENTS TO OBTAIN AN ACCURATE
IMPRESSION
Fluid enough to adapt to the oral tissues
Viscous enough to be contained in the tray
Adequate Setting time
Adequate tear resistance
Dimensionally stable
Biocompatible
Cost effective
COLLOIDS
A colloid is a substance that is microscopically dispersed
uniformly throughout another substance. This description
appears to be similar to that of a solution.
PHASES OF COLLOIDS:
A colloidal system consists of two separate phases:
THE DISPERSED PHASE- particles in a solution. molecules held
by primary or secondary forces
THE DISPERSION PHASE- solutions suspending the particles.
The sizes of the colloid particles ranges from 1 to 200 nm.
COLLOIDS IN DENTISTRY
Colloidal silica in resins
Fillers in impression materials
4th state of matter
Size of particle is 1 to 200nm.
TYPES OF COLLOIDS
Aerosol-liquids or solids in air.
Lyosol-liquids or solids in a liquid.
Foams-Gas, liquid and solids in solid.
HYDROCOLLOIDS
If the dispersion phase of a colloidal system is water, it is
called a hydrocolloid.
CHARACTERISTICS OF HYDROCOLLOIDS
During impression making, the material sets from a flowable
state to a solid state. This change of state associated with
hydrocolloids is called THE SOL-GEL TRANSFORMATION.
A SOL: is a colloidal dispersion of very small particles in a
continuous liquid medium
A GEL: is a suspension that behaves like an elastic solid.
If a hydrocolloid contains an adequate concentration of the
dispersed phase, the sol, under certain conditions, may change to a
gel.
In the gel state, the dispersed phase forms agglomerates in the
form of chains or fibrils, also called micelles.
The fibrils may branch and intermesh to form a brush-heap
structure, which can be envisioned as resembling the intermeshing of
tree branches or twigs in a brush pile. The dispersion medium is held
in the interstices between the fibrils by capillary attraction or
adhesion.
TYPES OF HYDROCOLLOIDS
Reversible Irreversible
(AGAR) (ALGINATE)
AGAR (REVERSIBLE HYDROCOLLOID)
Organic hydrophillic colloid which is a galactose sulphate
Polysaccharide extracted from seaweeds
Sulphuric ester of a linear polymer of galactose.
AGAR : Secondary bonds (weak) hold the fibrils together- break
at slightly elevated temperatures and become re-established as it
cools to room temperature
Thus, it is called a reversible hydrocolloid.
MODE OF SUPPLY:
As gel in collapsible tube
Number of cylinders in a glass jar
GELATION TEMPERATURE
Temp at which hydrocolloid material sets to gel.
If temp higher than oral temperature of 37oc,material may
injure soft tissue.
LIQUIFACTION TEMPERATURE
Gel heated to a temperature to return to sol
Temperature is(70-100oc)
Higher than gelation temperature
Temperature lag is hysteresis
COMPOSITION
GELATION PROCESS
Gel Sol (70 – 100oC) liquefaction temperature
Sol Gel (37 – 50oC) gelation temperature
Temperature lag makes it possible to use agar as
dental impression material.
DISTORTION DURING GELATION
Some contraction occurs because of the physical change in
the hydrocolloid transformation from a sol to a gel. If the material is
held rigidly in the tray, the impression material will shrink toward the
center of its mass, thereby creating larger dies.
Since the sol is a poor thermal conductor, rapid cooling may
cause a concentration of stress near the tray where the gelation first
takes place.
Consequently, water at approximately 20 °C is more
suitable for cooling the impression than is ice water.
VISCOSITY OF MATERIAL
Plays an important role in impression material
Fillers and other modifiers like borate can decrease viscosity.
Triple tray technique
MANIPULATION
Preparation of material
Making of impression
Removal of impression
CONDITIONING UNIT
LIQUIFYING : Placed in boiling water for 10 min
STORAGE: Stored in sol condition at 65oC
TEMPERING : at 45oC for 3-10 minutes
MAKING THE AGAR IMPRESSION
It’s a syringe material, directly taken from storage
compartment - applied on to the prepared tooth.
Tray hydrocolloid is removed from the tempering basin,
outer layer is removed and impression made.
Gelation is accelerated by circulating cool water (18-
21oC) through the tray for 3-5 min.
Perforated water cooled tray to circulate water so that the
material gels fast.
Wet Field Technique
LAMINATE TECHNIQUE (ALGINATE – AGAR METHOD)
The tray hydrocolloid is replaced with a mix of chilled alginate
that bonds to the syringe agar.
Alginate gels by a chemical reaction.
Agar gels by means of contact with the cool hydrocolloid.
ADVANTAGES
Maximum detail is produced
As hydrocolloid and not alginate is in contact with prepared
tooth
SHORT COMINGS
Bond between agar and alginate is not always strong.
Higher viscosity alginate displaces the agar during seating.
Dimensional accuracy of alginate limits the use to single units.
PROPERTIES
FLOW sufficiently fluid to record details if correctly
manipulated.
GELATION TEMPERATURE agar sol should revert to
a gel at temp of 37.c(98.6 f).
FLEXIBILITY when removed from mouth flexibility
should be4-15%,when a stress of 14.2 psi is applied. on
an average flexibility of 11% is desired
ELASTICITY AND ELASTIC RECOVERY occurs to
extent of 98.8%.
ACCURACY AND DIMENSIONAL CHANGES least
dimensional changes occur when stored in 100%
humidity, prompt pouring is recommended.
Reproduction limit a reproduction limit up to 25um is
achievable.
WORKING AND SETTING TIME ranges between 7-15
min and setting time is 15 min.
TEAR AND COMPRESSIVE STRENGTH tear strength
tear strength of 4 psi and compressive strength of 116psi.
ADVANTAGES
Accurate dies can be prepared, if material is handled properly.
Produces most undercut areas correctly
Well tolerated by patients
Cheap as compared to elastic materials.
DISADVANTAGES
Cannot be electroplated
Painful during insertion/gelation.
Tears easily
Expensive armantarium required
Impossible to sterilize material
DIMENSIONAL STABILITY
After 1 hour shrinks in air
.15% in another one hour shrinks
Syneresis and imbibition make it dimensionally unstable
Storage in 100%humidity makes it dimensionally stable
DISINFECTION
Even though unsuitable with disinfectant solution, Powell
showed that IODOPHOR disinfectant prevents microbial
growth.
Other agents may be bleach or gluteraldehyde.
APPLICATIONS
Full mouth impressions without deep undercuts.
Quadrant impressions without deep undercuts and single
impressions.
Duplicating dental casts and models.
Crown and bridge impressions because of accuracy.
ALGINATE
(IRREVERSIBLE HYDROCOLLOID)
Developed as a substitute for the agar impression material
when its supply become scarce during World War II.
A chemist from Scotland perceived that certain brown seaweed
(algae) succumbs an atypical mucous extraction
anhydro-B-d-mannuronic acid or alginic acid (insoluble in water)
England chemist received patent for using alginate as dental
impression material.
United States-1945
Introduced alginate during World War II.
STRUCTURAL FORMULA OF ALGINATE
COMPOSITION:
DIATOMACEOUS EARTH?????
Diatoms are tiny, single-celled algae found in plankton.
Diatomaceous earth is a fine, white, crystalline powder made
up of the fossilized shells of diatoms.
Lightweight, gritty, and porous.
The structure of the fossilized diatoms in diatomaceous earth
resemble small sponges with many openings. With so many
places for particles to become stuck, diatomaceous earth makes
excellent filler material
Diatomaceous earth is the remains of microscopic
phytoplankton, similar to coral, found in prehistoric freshwater
sea beds.
DE is approximately
3% magnesium,
86% silicon, 5% sodium,
2% iron
and trace minerals such as titanium, boron, manganese,
copper and zirconium
MODE OF SUPPLY
• As powder
In bulk in tin or sachets
In preweighed individual containers.
CLASSIFICATION OF ALGINATE (Robert G. Craig)
• I. According to concentration of
sodium phosphate
• Fast set 1.25 – 2min
• Regular set 3 – 4.5 min
• II. According to concentration of filler
• Soft set
• Hard set
GELATION PROCESS
Soluble alginate + calcium sulfate Insoluble calcium alginate gel
trisodium phosphate (retarder)
• Na3Po4+3CaSo4-----Ca3Po4+3Na2So4
• Sodium alginate+CaSo4+H2o----Calcium alginate+Na2So4
Final set structure differs from agar in that these are held
together by primary bonds rather than intermolecular forces so no
effect of temperature.
The fibrils are formed by chemical action - irreversible
GELATION TIME
Amount of retarder added (manufacturer)
By altering the temperature of water.
Colder the water - longer is gelation time
Warmer the water - Shorter is gelation time
Cooling mixing bowl and spatula
Prolong the gelation time.
MANIPULATION
Preparing the mix
Loading the tray
Seating tray
Removal of impression
LOADING TRAY
Sufficient bulk of material to be inserted for.
Impression material should adhere to tray so that it can
be withdrawn from around teeth.
Thickness of alginate impression between tray and mouth
3mm atleast.
Adhesives used in plastic or rim lock trays.
SEATING THE TRAY
Saliva to be wiped off from palate
To be held in place firmly without any movement.
Strength increases after gelation
2-3 min after gelation tray to be kept in mouth
Tackiness lost during this time.
REMOVAL OF IMPRESSION
Impression to be removed with a jerk.
Gentle ,long continued pull causes tearing of impression.
Causes deformation.
Pouring as soon as possible.
PROPERTIES
• Permanent deformation : 1-1.5% for 10% strain
• Flexibility : 14% at a stress of 1000 g /cm2
• Strength : Compressive strength= 5000-9000 g/cm2
Tear strength= 380-700 g / cm
COMPATIBILITY WITH GYPSUM
To overcome surface softness of gypsum cast for waxing
procedures from a hydrocolloid impression material
Immersing impression in solution containing accelerator for
setting of gypsum
Incorporating plaster hardener in material
HARDENING AGENT PREVENTS
Syneresis
Retarding action of gel
Zinc sulphate,magnese sulfate, potash alum, titanium fluoride
DIMENSIONAL STABILITY AND SHELF LFE
AIR– looses water by evaporation and shrinks
WATER—Absorbs water and swells
If held in mouth for 6 to 7 min,distortion occurs
Short shelf lie
STORAGE
Should be poured at once, however storage if becomes
necessary then in
In wet paper towel
In plastic bag
Humidor
If storage is unavoidable, keeping in environment 100%
relative humidity results in least dimensional change.
BIOCOMPATIBILITY
Fine silica particles become airborne from the can when lid is
removed silicosis and pulmonary hypersensitivity
Incorporation of glycerin to agglomerate alginate particles to
reduce dusting
ADVANTAGES
Easy to mix
Minimum equipment
Flexibility of set impression
Low cost
Comfortable to patient
DISADVANTAGES
Not accurate for crown and bridge impressions
Cannot be corrected.
ACCURACY
Agar is among the most accurate of impression materials.
Most alginate impressions are not capable of reproducing the finer
details observed in impressions with other elastomeric impression
materials.
Nevertheless, alginate materials are sufficiently accurate that
they can be used to make impressions for removable partial dentures.
RECENT ADVANCES IN ALGINATE
HIGH TEAR STRENGTH ALGINATES
SILICON REINFORCED 2 PASTE SYSTEM
CHROMATIC ALGINATE
Sodium Silico Fluoride --PH Controller
DUST FREE ALGINATE
organic glycol---- dedusting agent
ALGINATE WITH DISINFECTANT SOLUTION
SOFT SET AND HARD SET-according to filler content.
ALGINATE WITH DISINFECTANT SOLUTION
Alginate compatible with disinfectant solutions such
as sodium hypochlorite,
gluteraldehyde chlorhexidine and organic peracetic acid
Musil reported peracetic acid disinfestation for 1 min did not
disturb the qualities of impression
A quaternary ammonium salt has been added to alginate powder,
BLUE PRINT ASEPT. And it provides high degree of
disinfection when compared with plain material.
BDJ.(1990,169:83)
F GHANI,J.A KIRK,M,.WILSON
APPLICATIONS
Impression making
For duplicating models.
Making impressions for orthodontic and study models
Preparation of athletic mouth protectors for sports people.
when undercuts are present
in mouth with excessive flow of saliva
For partial dentures with clasps
Preliminary impressions for complete dentures.
For duplicating models.
Making impressions for orthodontic and study models
Preparation of athletic mouth protectors for sports people.
CAUSES TO REMAKE HYDROCOLLOID IMPRESSIONS
EFFECT AGAR ALGINATE
Grainy
material
1. Inadequate
boiling
2. Storage time too
long
3. Improper mixing
4. Undue gelation
Separation of
tray and
syringe
material
Water soaked
layer of tray
material not
removed
Not applicable
tearing 1. Inadequate bulk 3. Inadequate bulk
2. Premature
removal from
mouth
4. Moisture
contamination
External
bubbles
Gelation of syringe
material preventing
flow
Air incorporated
during mixing
Irregularly
shaped voids
Material too cool
or grainy
1. moisture or
debris on tissue
Rough or
chalky stone
model
1. Inadequate
cleansing of
impression
2. Excess water in
impression
3. Improper
manipulation of
stone
4. Premature
removal of
model
distortion 1. Impression not
poured
immediately
2. Moment of tray
during gelation
3. Impression not
poured
immediately
4. Premature
removal from
mouth
PROPERTIES OF HYDROCOLLOID IMPRESSION
MATERIALS
The quality of a stone cast depends on the properties of the
hydrocolloid impressions before pouring and the potential
interaction between gypsum and alginate during setting of the
gypsum.
DUPLICATING MATERIALS
Both types of hydrocolloids are used in the dental laboratory
to duplicate dental casts or models for the construction of prosthetic
appliances and orthodontic models.
Agar hydrocolloid is more popular in the lab because it can
be used many times. In addition, with intermittent stirring, agar
hydrocolloid can be kept in a liquid form for 1 or 2 weeks at a
constant pouring temperature. These factors make the cost of
reversible impression materials quite reasonable.
The hydrocolloid-type duplicating materials have the same
composition as the impression materials but their water
content is higher.
Consequently, the agar or alginate content is lower,
which influences their compressive strength and percent
permanent set.
These property requirements are identified in ANSI/ADA
Specification No. 20 (ISO 14356; Dentistry—Duplicating material).
CONCLUSION
The need to understand and evaluate the composition,
properties and handling characteristics of impression materials is
vital as it helps us use materials in a better and more rational manner
thereby eliminating causes of failures.
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