8. impression materials
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WRSuarez, DMD, MPH
IMPRESSION MATERIALS
2
Diagnostic castsWorking casts for indirect
reconstructionBite registration
Indications
3
Poly sulfides
4
TypesNon-elastic
*Impression compound
*Impression plaster
*Zinc oxide eugenol
*impression wax
Must not have undercuts
5
TypesElastomer impression materials Hydrocolloids aqueous impression (contain water)
*reversible (agar)*Irreversible (alginate)
Non-aqueous elastomers*Polysulfides (PS)*Silicon rubber Additional (AS, HAS)Condensation (CS)*Polyethers (PE)
Can be used in undercuts
(Sol gel) temperature(Sol gel)
Set by polymerisation
6
Trays
Individual, special tray
Stock trays
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Choice and use of traysThe tray should have enough spacerAvoid flexible traysUse perforated traysMake sure the tray is cleanApply only a thin layer of adhesiveWait appropriate time before taking impressionDon’t mix adhesives and impression materials
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Mixing techniquesManual:
Low viscosityLess viscous is easierSilicones are easiest to mix followed by polyethers and polysulphides
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Mixing techniquesAutomatic mixing
Reduces mixing timeReduces bubblesHomogenous mixUse correct mixing proportions
2x2x2x2x2x2x2x2 = 256 folds
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Layering techniques Single stage:
One material, one settingTwo stage (double impression):
First material set then another material added as a second layer
Twin mix (two phase impression): Two material layers are put together and set
together
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Requirements of impression materials
Accuracy = ability to replicate the intraoral surface details.Dimensional stability = ability to retain its
absolute dimensional size over time.Tear resistance = ability to resist tearing in
thin sections (such as through the feather-edged material within the gingival sulcus.
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What’s important?PatientNeutral taste and odourShort setting timeSmall trayEasily removed
impressionNo retakesNon-toxicCost
Lowest to highest Alginate < agar =
polysulfide <condensation silicone < addition silicone < polyether
DENTIST
• easily mixed
• short working times
• good quality impressions
• low cost
• easily disinfected
• simple procedure
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Working and setting times
0
1
2
3
4
5
6
7
8
Ideal HAS AS CS PE PS
Tota
l tim
e (m
in)
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Flexibility of impression materials
0
1
2
3
4
5
6
Ideal HAS AS CS PE PS
Flexible
Moderate
Hard
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Resistance to tearing
0
1
2
3
4
5
6
Ideal HAS AS CS PE PS
High
Medium
Low
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Reproduction of surface detailViscosity
High - mucocompressiveLow - mucostatic
Type 0: very high viscosity: (i.E., Putty)
Type 1: high viscosity: (i.E., Heavy body)
Type 2: medium viscosity: (i.E., Medium body)
Type 3: low viscosity: (i.E., Light body)
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WettabilityGood wetting is the ability of a liquid to
cover the surface of the substrate completely
non-wetting partial wetting perfect wetting
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Why do we want good wetting?
Fewer voidsLess entrapment of oral fluids by
displacing water and other oral fluids from the surfaces in the mouth
Bubble-free in the impression then in dies and models
Fewer retakesImprove adaptation of gypsum so as to
prevent voids in the casts.
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Wettability of impression materials
0
1
2
3
4
5
6
Ideal HAS AS CS PE PS
Excellent
Good
Fair
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Dimensional accuracy and stability
Choice and use of traySetting shrinkagePermanent setStorage stability
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Dimensional change of impressions
0 1 2 3 4
HAS
AS
CS
PE
PS
IDEAL
Low
Medium
High
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Rating of impression materials
Dental Advisor vol9, no4, 1992
IDEAL 100%
Hydrophilic addition silicone HAS 93%
Hydrophobic addition silicone AS 84%
Condensation silicone CS 68%
Polyether PE 87%
Polysulphide PS 60%
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Permanent deformation
Elastic Recover
y)%( 98.8 97.3 94.5 – 96.9 98.2 – 99.6 99 – 99.9 98.3 –
99.0
010
20
30
4050
60
70
80
90100
100%
70%
95%
Original
Under pressure
Recovered
Agar Alginate PolysulfideCondensatio
n Silicone
Addition Silicone Polyether
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Permanent deformationIn dentate patients removal of an impression will result in some deformation (strain) due to the presence of undercuts.The elastic impression materials have visco-elastic characteristics.This means that one may not see 100% recovery of the original dimensions in the mouth.This lack of recovery is described as permanent set.
Elastomeric Impression Materials
Polysulphides (rubber base, mercaptan, thiokol)
PolyethersSilicones
Condensation cured (conventional silicone rubber base)
Addition cured (vinyl polysiloxane)
Polysulphide impression material
PRESENTATIONBase paste
PolysulphideFiller (TiO2)
Activator pasteLead dioxidesulphurPlasticiser (dibutyl
phthalate)VISCOSITIES Light bodied (wash) heavy bodied Must be used with a special tray
Polysulphide impression material•Exothermic polymerization
•Affected by moisture & temperature
•For maximum accuracy, should be poured w/in 1 hour of removal from mouth
•(Hydrophobic) No moisture, blood or any fluid on preparation when taking impression
Polyether impression materialPRESENTATION (8:1 ratio)Base paste
PolyetherFiller (colloidal silica)Plasticiser
Activator pasteAromatic sulphonate
esterFillerPlasticiser (dibutyl
phthalate)VISCOSITIES Medium bodiedThinner
Must be used with a special tray
Polyether impression material
•Good accuracy•Excellent dimensional stability even when pouring is delayedAffinity to water (do not store in moist)
Silicone impression material- condensation cured
VISCOSITIES Light bodied (wash) Medium bodied heavy bodied Putty
May be used with a stock tray or special tray
Silicone impression material- condensation cured
PRESENTATIONBase paste
Silicone polymerFiller (colloidal silica)Organo-tin compound
Activator pasteSilicone polymerFiller (colloidal silica)Tetra-ethyl silicate
Silicone impression material- condensation cured
•Poor dimensional stability due to evaporation of alcohol: (must be poured immediately after removal from mouth)
Silicone impression material- addition cured (polysiloxane)
VISCOSITIES Light bodied (wash) Medium bodied heavy bodied Putty
May be used with a stock tray or special tray
Silicone impression material- addition cured
PRESENTATIONBase paste
Silicone polymerFiller (colloidal
silica)Silanol
Activator pasteSilicone polymerFiller (colloidal
silica)Platinum catalyst
PLATINUM SALT CATALYSTChloroplatinic acid (Dihydrogen hexachloroplatinate (IV) hexahydrate)
It is thought to be the source of Hydrogen gas release, if contaminated with water
Solved by the addition of palladium compounds to absorb H2
It is retarded if in contact with latex glovessulfur rubber stabilizers present in rubber
glovesWashed hands Vinyl gloves are used to mix the materialDentist should not touch the tooth with
latex gloves
ApplicationsImpression of teeth for precise laboratory
procedures
Impression bite registrations
Die fabrication for indirect composite techniques
Silicone impression material- addition cured
Vinyl terminated polydimethyl siloxane
Crosslinking reaction (no byproduct)
Relative merits of elastomeric impression materials
HANDLING• Ease of mixing
– polysulphides and condensation-cured silicones somewhat difficult to mix due to different amounts of base and catalyst paste required.
– Gun delivery system for addition-cured silicones makes them easiest to handle
• Working and setting times– polysulphides are susceptible to environmental
variations with long working (6min) and setting times (12min).
– Condensation cured silicones can show erratic setting due to moisture contamination .
– Polyethers have very consistent working and setting times
– Addition-cured silicones have good working and setting times but setting may be inhibited by latex gloves.
Relative merits of elastomeric impression materials
MECHANICAL PROPERTIES• Stiffness (relates to ease of removal from
undercuts)– PS<CCS<ACS<PE
• Permanent set– PS>PE>CCS>ACS
• Tear strength– PS>>PE>CCS=ACS
Relative merits of elastomeric impression materials
SURFACE DETAIL• All show excellent reproduction of surface
detail on dry surfaces.• Polyethers are generally best as these are
more hydrophilic than the other impression materials.
• Surfactants have been added to the addition-cured silicones to improve their wettability.
Relative merits of elastomeric impression materials
DIMENSIONAL ACCURACY AND STABILITY• Setting shrinkage
– PE=ACS<PS<CCS• Thermal contraction
– CCS=ACS<PS<PE• Storage stability
– polysuphides contract on storage– condensation-cured silicones contract on
storage– polyethers are stable but will expand on
storage if exposed to moisture– addition-cured silicones are the most
stable
Disinfection of impressions• Chlorine solutions
– avoid contact with skin, eyes etc. – Bleaches clothes– Unpleasant odour– Highly corrosive
• Aldehyde solutions– avoid contact with skin, eyes etc. – unpleasant odour– potential for sensitisation
• Others– Iodine solutions– Phenols
Disinfection of impressions• Addition reaction silicones
– place in glutaraldehyde for 1 hour, rinse in sterile water, and soak in fresh solution of glutaraldehyde for at least 10 hours
• Condensation reaction silicones– soak for 10 minutes in glutaraldehyde
• Polyethers– dip in glutaraldehyde, rinse in sterile
water, dip again, and stand under damp gauze for 10 minutes(Owen and Goolam. Int J Prosthodont 1993)
Bad impressions• Extra chair-side time• Repeat visits for the patient• Patient distress• waste of expensive materials• Remake bill from the laboratory• Spoil relationship with laboratory
…it was considered impossible to produce satisfactory restorations in up to 13% of the
impressionsWinstanley et al. Br Dent J 1997; 183: 209
Failures with impressionsPoor reproduction of surface detail
rough or uneven surface¨ incomplete set (premature removal, improper
mixing, contamination¨ rapid set (humidity, temperature, wrong mix)
air bubbles¨ rapid set¨ improper mixing¨ surface contamination (moisture)
irregular shaped voids¨ surface contamination (moisture)¨ premature movement
Failures with impressionsPoor fit
distortion¨ adhesive failure between tray and impression material¨ tray not sufficiently rigid¨ excessive seating pressure resulting in too much
permanent set¨ movement of tray during setting
casting too big¨ inappropriate impression technique¨ model poured too late (excessive shrinkage on storage)¨ impression stored under wrong conditions
casting too small¨ inappropriate impression technique¨ model poured too early (insufficient time for elastic
recovery)¨ impression stored under wrong conditions
“... tests confirm that dithiocarbamates are the chemical components which cause the inhibition of the setting of vinyl polysiloxane putty........the use of dithiocarbamate-free gloves is recommended.”
Causton BE, Burke FJ, Wilson NH. Dent Mater 1993, 9, 209-213
Summary An impression or negative likeness of
teeth & surrounding structures is used to obtain cast, on w/c a planned restoration is fabricated
A good impression is an EXACT negative replica of each prepared tooth & must include all prepared surfaces as well as an adequate amount of unprepared tooth structure adjacent to the margins
Summary Health of soft tissues & control of
salivary flow are critical to successful impression
Caution: avoid gingival injury
Thank You
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