Impression materials
‫أحمد عبد العزيز الحسين‬
‫أخصائي تركيبات سنيه في وزارة الصحة األردنية‬
‫محاضر غير متفرغ في جامعة العلوم والتكنولوجيا األردنية‬
Ahmad Abdul Aziz Alhussien
Prosthodontist at M.O.H
Part time lecturer at J.U.S.T
BSc , MSc, Jor. Board/ prosthodontics
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• The function of an impression material is to make
a negative copy of the oral tissue … that is poured
to produce a positive replica for these tissues
2
Requirement of an impression
material
• Must be a semi-liquid material that will flow and adapt
itself around the structure of interest.
• It must set and harden into a solid ?? that is rigid enough
“not too rigid” to be removed from the mouth without
becoming deformed.
• Copy details accurately
• Dimensional stability after removal from the mouth
• Appropriate working time (from the start of mix)
• Appropriate time to harden in the mouth (setting time)
• Bio-compatibility/Aesthetic, odor/taste
• Chemically compatible with material used to pour cast or
dies
3
Impression trays
• Used to carry the impression material into the oral cavity
• Can be
–
–
–
–
metal or plastic
Perforated or non perforated
for edentulous or dentate
pre-manufactured (stock trays) or custom-made (made in the lab.
From resin).
• Trays need to be rigid enough to support impression
material.
• For impression material to be retained in tray, tray maybe
perforated, and adhesive spray can be used to enhance
retention.
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Variety of trays
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• Cast
replica of dental arch
• Die
replica of a single tooth (cut from a cast)
6
Classification of dental impression
materials
• Inelastic / Elastic
• Mechanism of hardening
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Impression material (rigid)
Inelastic materials
Setting mechanism
Impression
Cooling to mouth
temperature
compounds
‘Dental compounds’
Zinc-oxide
eugenol (ZOE) Chemical reaction
Impression
Chemical reaction
Cooling to mouth
temperature
plaster
Impression wax
8
Impression materials (aqueous)
Elastic material
(hydro-colloid)
Setting mechanism
Reversible
Cooling
(Agar)
Irreversible
(Alginate)
Chemical reaction
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Impression materials (rubber)
Elastic material (non- Setting mechanism
aqueous)
Polysulfides
Polymerization reaction
Condensation
silicon Polymerization reaction
Polyether
Polymerization reaction
Addition
Polymerization reaction
silicon
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Inelastic impression materials (rigid)
• Plaster of Paris (Type I gypsum)
• Dental compound/impression compounds
– Softened by heat and pressed against tissue before it hardens.
Thermoplastic softens at 60°C, remains plastic at 45°C and becomes
firm at mouth temperature. Used for primary impression
• ZOE (Zinc oxide eugenol)
– Other uses ???
• Impression wax
Elastic impression materials (non-rigid)
• Hydrocolloids
• Elastomers (Rubber)
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Hydrocolloids
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Hydrocolloids
• The colloid state represents a highly dispersed phase of
fine particles within another phase, some where between
solution and a suspension.
• The colloid can exist as a viscous liquid known as a sol or a
solid known as gel.
• If the particles are suspended in water, the suspension is
called a hydrocolloid (hydrosol, hydrogel
• Hydrocolloids come in two forms:
– Reversible agar
– Irreversible alginate
13
Agar
Composition of agar:
Material
Agar
(seaweed
extract)
Percentage purpose
12-15%
Colloidal particles as basis of
the gel
Potassium
sulfate
Borax
1%
0.2%
Ensures set of gypsum
materials
Strengthens gel
Alkyl
benzoate
water
0.1%
Antifungal agent
85%
Dispersing medium for the
colloidal suspension
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Properties
• Flows well and adapts readily to hard and soft
tissue contours
• Accurate reproduction (hydrophilic nature),
however, the impression need to be poured
immediately.
• Need to be stored in 100% humidity for short
period of time if not poured immediately
• Distortion is more likely to occur if impression
not poured within an hour.
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• Tear strength is not high.
• If compressed it might rebound if compression is not too
sever otherwise deformation occurs.
• Material suffers from loss or gain of water:
– Syneresis: a process by which the gel contracts and
some of the liquid is squeezed out, forming an
exudate on the surface.
– Evaporation: loss of water which causes the material
to shrink, and impression is distorted.
– Imbibition: uptake of water, this will swell and distort
the impression.
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Clinical application
• The use of agar requires special equipment
consisting of hydrocolloid conditioner (has 3
water bath chambers) and water -cooled tray
connected to a rubber hose that delivers water.
• Usually used in lab for cast duplication
• Agar is supplied in two viscosities, thick and thin
(depending on the amount of agar).
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• Preparing the material for
impression:
– Gel heated to 71-100°C, becomes liquid (sol)
– Tray and agar syringe are liquefied in 8-12
minutes in boiling water in 1st chamber
– 2nd chamber is used to store the agar at 6066°C.
– 3rd camber kept at 45-47°C to temper agar
before use to prevent burns.
• Agar exhibits hysteresis, melting
at 85 °C and solidifying from 3240 °C
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Alginate
•
•
•
•
Inexpensive
Easy to manipulate
Requires no special equipment
Reasonably accurate
Common uses:
•
•
•
•
Diagnostic cast (study model)
Preliminary impression for complete denture
Partial denture framework
Custom trays for fluoride or bleaching
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Composition/setting rxn/
working time
Material
Percentage purpose
Na, K alginate
15-20%
Basis of the gel
Ca sulfate
dihydrate
14-20%
Creates irreversible gel with
alginate
Potassium
sulfate
10%
ensures set of gypsum
Trisodium
sulfate
2%
Retarder to control setting
Diatomaceous
earth
55-60%
Filler to increase thickness and
strength
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Setting rxn/working time
• Setting rxn occurs when the powder is mixed with water:
Ca sulfate dihydrate + NaAlginate
Ca alginate
• Working time: total time from start of mixing to the final time at
which an impression tray can be fully seated without distortion
– Regular set: 2-3 minutes
– Fast-set: 1.25-2 minutes
• Setting time: elapsed time from the start of mixing until impression
material becomes firm enough to resist permanent deformation.
Regular set: 2-5 minutes, Fast set: 1-2 minutes
21
Setting rxn/working time
• Controlling water temperature shortens or
lengthens setting time
• Changing P:L has adverse effect on
materials property and strength
• Allow extra 1-2 minutes, tear strength
increases, and rebound from undercuts
w/out deformation improves
22
Important considerations to ensure
accurate impression
• 2-4 mm bulk material in tray
• Snap action removal from mouth
• Allow extra 1-2 minutes after setting to improve tear
strength
• Stored in a moist environment to avoid loss of water and
deformation
• Disinfect in less than 10 minutes to avoid dimensional
instability
23
Impression making
• Fluff powder, wait 30 seconds for the dust to
settle
• Measure powder and water according to
manufacturer instruction
• Use spatula to stir water into powder to mix
• Stir vigorously to create homogenous mix (45
seconds for regular, 30 seconds for fast setting
alginate)
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• Tray should cover all anatomical features
• Utility wax can be added on tray borders to
extend its coverage
• Adhesive spray is used to prevent separation of
alginate from tray.
• Tray is loaded from posterior towards anterior
• Moisten and smooth alginate
• Posterior part of tray is seated first then anterior.
• *tips to control gag reflex*
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Alginate impression
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Elastomers
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Elastomers
• Often called rubber materials since they
have properties similar to rubber.
• Clinical uses:
– Bridges
– Implants
– Partial dentures complete dentures
– Indirect esthetic restorations
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Polymerization reaction
• Involves formation of long-chain polymers and
cross-linking of chains
• General properties:
– Not as sensitive to water as hydrocolloids
– Have a certain amount of rebound, the highest is for
addition silicon and polyethers, medium for
condensation silicon and poorest for Polysulfides
– Not wet well by water (hydrophobic) possible solution:
surfactants
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Polysulphides
Dispensing & Composition
• Supplied in two tubes as base and catalyst, equal lengths
are mixed. Light, regular, heavy viscosities
• Chemical composition of base:
– 80% low-molecular-weight organic polymer containing
mercaptan reactive groups (-SH), and 20% reinforcing
agents: titanium dioxide, silica, zinc sulfide
• Chemical composition of catalyst:
– Lead dioxide or copper hydroxide
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Setting reaction
Mercaptan + lead dioxide
polysulfide+H2O
• The reaction is sensitive to temperature and
moisture so increase in any will accelerate the
setting.
• Also sensitive to correct mixing ratio
31
Uses
• Crown and bridge impressions
• Partial and complete denture impressions
Clinical considerations when used
• Used with custom trays
• Allow 2mm thickness
• Use tray adhesive
• Dry field
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Properties of clinical interest
• Setting time: 8-14 minutes
• Higher tear strength than hydrocolloids
• Accuracy improves if impression is poured within 30
minutes
• very unpleasant taste and odor
• Messy (orange solvent to remove stains)
• Can be irritant to oral mucosa
For all these reasons, Polysulfides have been largely
replaced by other rubber materials
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Silicon rubber
Addition (A) silicon
Condensation (C) silicon
34
Condensation silicon
• Developed as alternative to Polysulfides
• Has more desirable qualities in
comparison:
– Easy mix
– Better taste and odorless
– Shorter setting time (5-7 minutes)
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Dispensing & Composition
•
•
•
•
Two pastes, base and catalyst.
Comes as light, medium, or heavy viscosity
Base: dimethylsiloxane + filler (silica)
Catalyst: suspension of stannous octoate +
alkyl silicate
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Setting rxn
• Condensation reaction that produces ethyl
alcohol as by product. The alcohol
evaporates which causes dimensional
instability.
• The material continues to contract with
time, so needs to be poured within
minutes.
37
Addition silicon (PVS)
• Desirable clinical qualities:
–
–
–
–
–
Dimensional stability
Accuracy
Clean
Easy to mix
No foul odor or taste
the most expensive
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Dispensing & composition
• Light, regular and heavy viscosities and also putty
• Dispensed as cartridge with 2 chambers (pastes),
or two putty- system
– Base: low-molecular-weight silicon with vinyl
groups (paste system), or low-molecularweight silicon with silane hydrogens (putty) +
silica filler
– Catalyst: chloroplatinic acid
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Setting rxn
• Polymerization rxn of chain lengthening and
cross-linking with reactive vinyl groups,
producing a stable silicon rubber.
• No ethyl-alcohol by product
• Some addition silicons produce hydrogen as by
product, manufacturers incorporated palladium
powder that absorbs hydrogen
• setting time: 3-7 minutes
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Impression making techniques
• Putty/wash technique
– One step
– Two step
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Polyethers
• Also used for crown and
bridge work since they
are very accurate and also
more hydrophilic than
other silicons
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Dispensing & composition
• Dispensing same as other rubber materials (2 tubes)
• In addition it’s supplied in pouches of base and catalyst
placed in mechanical mixer.
• Composition:
– Base: low- molecular-weight polyether with cation
reactive group
– Catalyst: aromatic sulfonic acid
*clinical tip: mix well to avoid irritation from unmixed
catalyst.
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Properties
•
•
•
•
•
Stiff, difficult to remove from undercuts
Short working and setting times
Setting time 3-5 minutes
Sensitive to moisture and temperature
More hydrophilic (must not be stored in water or
disinfectant)
• accurate
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Inelastic impression materials
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Impression compound:
• Impression compound:
softens with heat, hardens
in the mouth.
• 2 main forms:
– Cakes (sheets) /low fusing
– Sticks /high fusing
• Clinical uses:
– Sheets:
• Primary impression in
metal trays
• To make custom trays
– Sticks are used for
border molding
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Composition & properties
• Composition
–
–
–
–
–
Thermoplastic resin and waxes
Fillers to reduce flow
Plasticizers
Organic acids or oils
Pigments
• Properties
–
–
–
–
Softened at 60°C, remains plastic at 45°C, firm at 37°C
Heated in water not by flame
Should be poured ASAP to avoid distortion
Flame used to heat sticks
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Impression plaster
• Impression plaster: seldom used
– Composed of plaster gypsum (Ca hemihydrae)
– Used for primary impression (high P:L)
– Scored with a knife in the mouth then removed and
reassembled in the lab (distortion?)
– Wash impression
– Easy to use
– Inexpensive
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ZNO eugenol
• Secondary impression for complete dentures, or wash
impression (mucostatic impression)
• 2 tube paste system, different colors
Zinc oxide (80%) and fillers, eugenol (15%) with oils, resin,
fillers. In addition to chemical accelerator (zinc acetate)
• Dispensed in 2 equal lengths and mixed to homogenous
color forming zinc eugenolate
• Initial set:3-6 minutes, final set:10 minutes
• To accelerate the setting, a drop of water or zinc acetate
is added.
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Properties
• Brittle, so not suitable for areas with
undercut
• Flows readily
• Accurate
• Eugenol can be irritant (burning sensation)
• Once set, dimensionally stable
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Impression 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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Disinfection of impressions
• The disinfectant should be compatible with the
impression materials.
• After taking the impression, it should be rinsed
with water, excess water shaken off, and
disinfectant sprayed or impression immersed in
disinfectant
• Protective gloves should be worn
• Rinse after disinfection is complete
52
Disinfecting casts
• Maybe necessary if impression was not
properly disinfected, or if immersion of
impression adversely affects the
impression
• Casts should be set and stored for 24 hours
before disinfection.
• Solution used: Na hypochlorite, iodophors.
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Sterilizing trays
• Trays should be properly sterilized before
use
• Disposable trays are recommended if
appropriate
• Sterilization can be achieved by heated
steam, dry heat, chemical vapors
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