lecture 5 a impression
TRANSCRIPT
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Dental impression materialsDental impression materials
Dr Layla Abu-Naba’aDr Layla Abu-Naba’a
BDS, PhD, MFD RCSBDS, PhD, MFD RCS
Assistant professor of prosthodonticsAssistant professor of prosthodontics
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IndicationsIndications• Diagnostic casts
• Working casts for indirect reconstruction
• Bite registration
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Poly sulfides
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TypesTypes
Non-elastic
*Impression compound
*Impression plaster
*Zinc oxide eugenol
*impression wax
Must not have undercuts
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TypesTypesElastomer 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
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TraysTrays
Individual, special trayIndividual, special tray
Stock traysStock trays
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Choice and use of traysChoice and use of trays
• The tray should have enough spacer• Avoid flexible trays• Use perforated trays• Make sure the tray is clean• Apply only a thin layer of adhesive• Wait appropriate time before taking
impression• Don’t mix adhesives and impression
materials
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Mixing techniquesMixing techniques
• Manual:• Low viscosity
• Less viscous is easier
• Silicones are easiest to mix
• followed by polyethers and polysulphides
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Mixing techniquesMixing techniques
• Automatic mixing• Reduces mixing time
• Reduces bubbles
• Homogenous mix
• Use correct mixing proportions2x2x2x2x2x2x2x2 = 256 folds2x2x2x2x2x2x2x2 = 256 folds
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Impression techniquesImpression techniques
Full dentures Plaster of Paris Single stage Stock tray / special trayZnO-Eug Single stage Special trayCompo/ZnO-Eug Two stage Stock tray
Partial Alginate Single stage Stock tray / special traydentures Elastomers Single stage Special tray
Fixed Compo/wax Single stage Copper ringdentures Elastomers Single stage (M) Special tray
Twin mix (H+L) Special trayTwin mix (P+L) Stock trayTwo stage (P+L) Stock tray
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Layering techniques Layering techniques
• Single stage: – One material, one setting
• Two 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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RequirementsRequirements
• 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?What’s important?
Patient• Neutral taste and odour• Short setting time• Small tray• Easily removed impression• No retakes• Non-toxic• Cost
– 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 timesWorking and setting times
0
1
2
3
4
5
6
7
8
Ideal HAS AS CS PE PS
To
tal
tim
e (m
in)
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Flexibility of impression materialsFlexibility of impression materials
0
1
2
3
4
5
6
Ideal HAS AS CS PE PS
Easy
Moderate
Hard
Most stiff to least , or flexiblepolyether > addition silicone > condensation silicone > polysulfide = hydrocolloids
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Resistance to tearingResistance to tearing
0
1
2
3
4
5
6
Ideal HAS AS CS PE PS
High
Medium
Low
greatest to leastpolysulfide > addition silicone > polyether > condensation silicone >> hydrocolloids O’brien
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Reproduction of surface detailReproduction of surface detail• Viscosity
–High - mucocompressive
–Low - 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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Reproduction of surface Reproduction of surface detaildetail
Wettability
–Hydrophobic contact angle is from 40 to
70 degrees.1
–Hydrophilic contact angle is from 80 to
105 degrees
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Why do we want good wetting?Why do we want good wetting?
• Fewer voids• Less 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 retakes• Improve adaptation of gypsum so as to prevent voids
in the casts.• Disinfection procedure should be altered
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WettabilityWettabilityGood wetting is the ability of a liquid Good wetting is the ability of a liquid
to cover the surface of the to cover the surface of the substrate completelysubstrate completely
non-wetting partial wetting perfect wetting
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Wettability of impression materialsWettability of impression materials
0
1
2
3
4
5
6
Ideal HAS AS CS PE PS
Excellent
Good
Fair
•hydrocolloids > polyether > hydrophilic addition silicone > •polysulfide > hydrophobic addition silicone = condensation silicone
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Dimensional accuracy and stability Dimensional accuracy and stability
• Choice and use of tray
• Setting shrinkage
• Permanent set
• Storage stability
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Dimensional change of impressionsDimensional change of impressions
0 1 2 3 4
HAS
AS
CS
PE
PS
IDEAL
Low Medium Highbest to worst
addition silicone = hydrophilic addition silicone > polyether > condensation silicone > polysulfide > hydrocolloid
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Rating of impression materialsRating 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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Elastic recoil Elastic recoil
Impression
Impression Tray
VERYFAST
(SNAP)
Strain rate sensitive elastomers !
Fast removal
Slow removal
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Permanent deformationPermanent deformation
Elastic Recovery
)%(98.8 97.3 94.5 – 96.9 98.2 – 99.6 99 – 99.9
98.3 – 99.0
0
10
20
30
40
50
60
70
80
90
100
100%
70%
95%
Original
Under pressure
Recovered
Agar Alginate PolysulfideCondensation Silicone
Addition Silicone
Polyether
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Permanent deformationPermanent deformation
In 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.
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Impression compositionImpression composition
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FormsForms
• Sheet– Primary impression of
edentulous mouth– Close fitting special tray used
with zinc oxide /eugenol wash
• Stick– Single crown impression
(copper ring technique)– Border molding
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CompositionComposition• Natural or synthetic resins
– Shellac, dammar, colophony, sandarac
• Plasticisers– Stearic acid or gutta
percha (avoid brittleness)
• Fillers– Talc, calcium carbonate or
limestone (avoid tackiness)
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PropertiesProperties
• Softening temperature (Tg) 55-60oc
• Poor thermal conductivity– Subject to stress relief
• High coefficient of thermal expansion– Cooling contraction
flow by temp45-60 flow37 solid
temper with warm water
do not over heat (loose oils& burn pt)
with solid tray
pour immediately
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PropertiesProperties
• High viscosity– Mucocompressive
• Rigid
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Impression plasterImpression plaster
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Impression plaster Impression plaster type 1type 1
•Mucostatic impressions of edentulous ridges •ST 3 min•High W/P•Score, fracture, reassemble, put separator•Can be combined with impression compound
Calsium hemihydrateK sulfate, K chloride
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Properties of impression plasterProperties of impression plaster
Advantages Easy to mix
– Working time 2-3 min
– Setting time 2-3 min
Low viscosity– Mucostatic
Good dimensional stability & accuracy
Cheap
Disadvantages Low strength Rough surface finish Poor abrasion
resistance Rigid once set Dry sensation in the
mouth
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Zinc oxide / eugenolZinc oxide / eugenol
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Impression washImpression paste (SS White)Luralite (Kerr)
Temporary filling materialKalzinol (De Trey)
Temporary cementationKalzinol (De Trey)
Periodontal pack Endodontic sealer
Tubli-Seal EWT (Kerr)
Zinc oxide / eugenolZinc oxide / eugenol
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Zinc oxide eugenolZinc oxide eugenol
Two pastes, different size opening•sticks to dry surfaces•Put vasiline
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Setting time factorsSetting time factors
•Initial ST 3-6 min
•Final ST 10 min
Inside the mouth it sets faster by increased Temperature, and moisture
Or add drops of water , alcohol, zinc acetate crystals to mixture
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Zinc oxide eugenolZinc oxide eugenol•Euginol irritant, causes burning sensation of lips
•Mucostatic impressions edentulous ridges•Not elastic
•Protect unsupported edges, it has low tear resistance
Use non euginol mixes, use rubbers
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Zinc oxide / eugenolZinc oxide / eugenol
HydrophilicAccurate reproduction of surface details
Dimensionally stablePour any time,
Compatible with dental stoneNo separator neededSeparated by softening the set paste (thermoplastic)
in water at 60C
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Impression waxImpression wax
•See waxes lecture later
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HydrocolloidsHydrocolloids
Hydro=water Kola=glue Oid=Like
• A solution is a homogeneous mixture consisting of a single phase
• A suspension is a mixture of two phases
• A colloid is a heterogeneous mixture of two phases, where the two phases are not readily differentiated
colloidal silica in resin
Agar and alginate impression materials
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HydrocolloidsHydrocolloids
Reversible hycrocolloids – e.g. agar
Gel Sol Gel heating cooling
Irreversible hycrocolloids – e.g. alginate
Sol Gel chemical reaction
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AgarAgar
Sea weedUsesCompositionAgar 15%Borax 0.2%Sodium sulphate1%Anti fungal (alkyl benzoate)Water 85%
Similar to gelatine1. Crown & bridge imp2. Cast duplication- lab
Strengthens gelAffect gypsum setting against?Good storageTwo viscosity forms
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ManipulationManipulation
• Gel in tubes– Syringe and tray material
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AgarAgar
Manipulation Water-cooled traysConditionerHysteresis, hydrophilicWetting agent on teeth
5 min cooling - snap?Space for thick impCool to 30-45 > 70 degree, 60-66, 45-47prevent burns and pulp death
Intimate contact
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PropertiesProperties
Dimensional change Syneresis
Imbibition
Tear strength
Elastic limit Cheap
Pour immediatelyAffects detail accuracyAnd prosthesis fittingShort storage < 1H 100% humidity (wet tissue in sealed bag) Spray disinfectantPoured only once
= Alginate,< rubber snapSupport (tray)Rigid box storageExpensive instruments
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AlginateAlginate
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AlginateAlginate
Ca- alginate
Widely used?
Not C&B, inlays, onlays
Diagnostic casts, primary imp for CD, PD, opposing jaw for CB, repairs of prosthesis, provisional restorations, fluoride & bleaching trays, night guards, sport protectors Not fine details, low flow
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Alginate impression materialsAlginate impression materials
COMPOSITIONSodium alginate hydrogel former
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Alginate impression materialsAlginate impression materials
Calcium sulphate dihydrate:provides calcium ions
Don’t use a bowl with previous gypsum mix
CaSO4.2H2O 2Ca2+ + 2SO4- + H2Ocalcium ions displace the sodium ions in the
sodium alginate polymer
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Alginate impression materialsAlginate impression materials
1. Acid-base Reaction
NanAlg + n/2CaSO4 n/2Na2SO4 + Can/2Alg
Setting Process
2. Crosslinking reaction
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Alginate impression materialsAlginate impression materials
Sodium phosphate: controls working time
Na3PO4 3Na+ + PO43-
3Ca2+ + 2PO43- Ca3(PO4)2
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Alginate impression materialsAlginate impression materials
Potassium sulphate enhances setting
Fillers controls consistency
Sodium silicofluoride controls pHtri Na phosphate retarder
glycols for dustless alginatecolour, disinfectants, flavour, filler
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Properties: Setting TimeProperties: Setting Time
WtRegular 2-3 min Or fast 1.25-2 minSetting timeRegular 2-5 min, fast 1-2 min
+ 1-2min in mouth after bowl ST
Spacer 2-4 mm Snap removal
Reduced if long mixing
Cold water st, warm water stW/P severely affects properties so not used
Tear resistance, elastic limit
Tear resistance, permanent deformation,
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PropertiesProperties
Dimensional stability as agarUse wet napkins but not soaked one
Tear strengthThin < thick Compatibility with tray and agar
Pouring > 8-10 minBut not more than 1 hour
Only pour one gypsum cast
Store several hoursWash from saliva Drops of water cause local imbibtion- distort
W/P reduce it
Allows for rebound, disinfect 10 minTo prevent syneresis
If another cast is poured in the same impression, then it is not accurate because of previous imbibition
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Alginate impression materialsAlginate impression materials
PROPERTIES Dust free powder Cheap Limited shelf life Well controlled working and setting times Mucostatic Hydrophilic Poor surface reproduction Poor storage stability
–Imbibition (water sorption)–Syneresis (water loss)
Low tear strength Excessive permanent deformation
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To be continued