Lecture 5 a Impression

11

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

22

IndicationsIndications• Diagnostic casts

• Working casts for indirect reconstruction

• Bite registration

33

Poly sulfides

44

TypesTypes

Non-elastic

*Impression compound

*Impression plaster

*Zinc oxide eugenol

*impression wax

Must not have undercuts

55

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

66

TraysTrays

Individual, special trayIndividual, special tray

Stock traysStock trays

77

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

88

Mixing techniquesMixing techniques

• Manual:• Low viscosity

• Less viscous is easier

• Silicones are easiest to mix

• followed by polyethers and polysulphides

99

Mixing techniquesMixing techniques

• Automatic mixing• Reduces mixing time

• Reduces bubbles

• Homogenous mix

• Use correct mixing proportions2x2x2x2x2x2x2x2 = 256 folds2x2x2x2x2x2x2x2 = 256 folds

1010

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

1111

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

1616

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.

1717

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

1818

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)

1919

Flexibility of impression materialsFlexibility of impression materials

0

1

2

3

4

5

6


Easy

Moderate

Hard

Most stiff to least , or flexiblepolyether > addition silicone > condensation silicone > polysulfide = hydrocolloids

2020

Resistance to tearingResistance to tearing

0

1

2

3

4

5

6


High

Medium

Low

greatest to leastpolysulfide > addition silicone > polyether > condensation silicone >> hydrocolloids O’brien

2121

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)

2222

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

2323

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

2424

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

2525

Wettability of impression materialsWettability of impression materials

0

1

2

3

4

5

6


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

2727

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

2828

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%

2929

Elastic recoil Elastic recoil

Impression

Impression Tray

VERYFAST

(SNAP)

Strain rate sensitive elastomers !

Fast removal

Slow removal

3030

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

3131

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

3333

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

3434

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)

3535

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

3636


• High viscosity– Mucocompressive

• Rigid

3737

Impression plasterImpression plaster

3838

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

4040

Zinc oxide / eugenolZinc oxide / eugenol

4141

Impression washImpression paste (SS White)Luralite (Kerr)

Temporary filling materialKalzinol (De Trey)

Temporary cementationKalzinol (De Trey)

Periodontal pack Endodontic sealer

Tubli-Seal EWT (Kerr)


4242

Zinc oxide eugenolZinc oxide eugenol

Two pastes, different size opening•sticks to dry surfaces•Put vasiline

4343

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

4545


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

4646

Impression waxImpression wax

•See waxes lecture later

4747

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

4848

HydrocolloidsHydrocolloids

Reversible hycrocolloids – e.g. agar

Gel Sol Gel heating cooling

Irreversible hycrocolloids – e.g. alginate

Sol Gel chemical reaction

4949

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

5050

ManipulationManipulation

• Gel in tubes– Syringe and tray material

5151

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

5252


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

5353

AlginateAlginate

5454

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

5555

Alginate impression materialsAlginate impression materials

COMPOSITIONSodium alginate hydrogel former

5656


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

5757


1. Acid-base Reaction

NanAlg + n/2CaSO4 n/2Na2SO4 + Can/2Alg

Setting Process

2. Crosslinking reaction

5858


Sodium phosphate: controls working time

Na3PO4 3Na+ + PO43-

3Ca2+ + 2PO43- Ca3(PO4)2

5959


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,

6161


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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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

6363

To be continued

Lecture 5 a Impression

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