Lecture 5 A Impression

Dental impression materials Dr Layla Abu-Naba’a BDS, PhD, MFD RCS Assistant professor of prosthodontics

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Indications • Diagnostic casts • Working casts for indirect reconstruction • Bite registration

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Poly sulfides

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Types Non-elastic *Impression compound *Impression plaster

Must not have undercuts ✂

*Zinc oxide eugenol *impression wax 4

Types

Elastomer impression materials Can be used in Hydrocolloids aqueous impression (contain water) undercuts *reversible (agar) *Irreversible (alginate) ✂ (Sol ⇔ gel) temperature ✂

Non-aqueous elastomers *Polysulfides (PS) *Silicon rubber Additional (AS, HAS) Condensation (CS)

*Polyethers (PE)

(Sol

✂

⇒ gel)

Set by polymerisation ✂

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Trays

Stock trays

Individual, special tray

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Choice 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 techniques • Manual: • Low viscosity • Less viscous is easier • Silicones are easiest to mix • followed by polyethers and polysulphides

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Mixing techniques • Automatic mixing • Reduces mixing time • Reduces bubbles • Homogenous mix • Use correct mixing proportions 2x2x2x2x2x2x2x2 = 256 folds

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Impression techniques Full dentures Plaster of Paris Single stage Stock tray / special tray ZnO-Eug Single stage Special tray Compo/ZnO-Eug Two stage Stock tray Partial dentures

Alginate Elastomers

Single stage Single stage

Stock tray / special tray Special tray

Fixed dentures

Compo/wax Elastomers

Single stage Copper ring Single stage (M) Special tray Twin mix (H+L) Special tray Twin mix (P+L) Stock tray Two stage (P+L) Stock tray

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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 11

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Requirements • 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 featheredged material within the gingival sulcus. 16

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
DENTIST

• easily mixed • short working times • good quality impressions • low cost • easily disinfected • simple procedure 17

Working and setting times

Total time (min)

8 7 6 5 4 3 2 1 0

Ideal

HAS

AS

CS

PE

PS

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Flexibility of impression materials 6

Easy

5 4

Moderate

3 2

Hard

1 0

Ideal

HAS

AS

CS

PE

PS

Most stiff to least , or flexible polyether > addition silicone > condensation silicone > polysulfide = hydrocolloids 19

Resistance to tearing 6

High

5 4

Medium 3 2

Low

1 0

Ideal

HAS

AS

CS

PE

PS

greatest to least polysulfide > addition silicone > polyether > condensation silicone >> hydrocolloids O’brien 20

Reproduction 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) 21

Reproduction of surface detail

Wettability –Hydrophobic contact angle is from 40 to 70 degrees.1

–Hydrophilic contact angle is from 80 to 105 degrees 22

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 23

Wettability Good 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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Wettability of impression materials Excellent 6 5 4

Good

3 2

Fair

1 0

Ideal

HAS

AS

CS

PE

PS

•hydrocolloids > polyether > hydrophilic addition silicone > •polysulfide > hydrophobic addition silicone = condensation silicone 25

Dimensional accuracy and stability • • • •

Choice and use of tray Setting shrinkage Permanent set Storage stability

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Dimensional change of impressions IDEAL PS PE CS AS HAS 0

1

2

3

Low

Medium

High

best to worst addition silicone = hydrophilic addition silicone > polyether > condensation silicone > polysulfide > hydrocolloid

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Rating of impression materials IDEAL

100%

Hydrophilic addition silicone

HAS

93%

Hydrophobic addition silicone AS

84%

Condensation silicone

68%

Polyether

PE

Polysulphide PS

CS

87% 60% Dental Advisor vol9, no4, 1992 28

Elastic recoil Strain rate sensitive elastomers !

Impression Tray

Fast removal

Impression

VERY FAST (SNAP)

Slow removal

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Permanent deformation 100 90

Under pressure

80 70 60

100%

95%

50 40

70%

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Recovered

20 10 0

Elastic Recovery )%(

Original Agar

Alginate

Polysulfide

Condensation Silicone

98.8

97.3

96.9 – 94.5

99.6 – 98.2

Addition Silicone 99.9 – 99

Polyether

99.0 – 98.3

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Permanent 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. 31

Impression composition

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Forms • 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 33

Composition • Natural or synthetic resins – Shellac, dammar, colophony, sandarac

• Plasticisers – Stearic acid or gutta percha (avoid brittleness)

• Fillers – Talc, calcium carbonate or limestone (avoid tackiness) 34

Properties • Softening temperature (Tg) 55-60oc • Poor thermal conductivity – Subject to stress relief

• High coefficient of thermal expansion – Cooling contraction

↑ flow by temp ✂45-60 ° flow ✂37 ° solid temper with warm water do not over heat (loose oils& burn pt) ↑ with solid tray pour immediately

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Properties • High viscosity – Mucocompressive

• Rigid

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Impression plaster

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Impression plaster type 1 •Mucostatic impressions of edentulous ridges •ST 3 min

↑ Calsium hemihydrate K sulfate, K chloride

•High W/P •Score, fracture, reassemble, put separator •Can be combined with impression compound 38

Properties of impression plaster Advantages  Easy to mix – Working time 2-3 min – Setting time 2-3 min

 Low viscosity – Mucostatic

Disadvantages  Low strength  Rough surface finish  Poor abrasion

resistance  Good dimensional stability &  Rigid once set accuracy  Dry sensation in the  Cheap mouth 39

Zinc oxide / eugenol

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Zinc oxide / eugenol  Impression wash Impression paste )SS White( Luralite )Kerr(

 Temporary filling material Kalzinol )De Trey(

 Temporary cementation Kalzinol )De Trey(

 Periodontal pack  Endodontic sealer Tubli-Seal EWT )Kerr( 41

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

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Setting 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 43

Zinc oxide eugenol •Euginol irritant, causes burning sensation of lips

Use non euginol mixes, use rubbers

•Mucostatic impressions edentulous ridges •Not elastic •Protect unsupported edges, it has low tear resistance 44

Zinc oxide / eugenol Hydrophilic Accurate reproduction of surface details

Dimensionally stable Pour any time, Compatible with dental stone No separator needed Separated by softening the set paste (thermoplastic) in water at 60C 45

Impression wax •See waxes lecture later

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Hydrocolloids 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 47

Hydrocolloids Reversible hycrocolloids – e.g. agar Gel

⇒ Sol ⇒ Gel heating cooling

Irreversible hycrocolloids – e.g. alginate Sol

⇒ Gel chemical reaction 48

Agar Sea weed Uses Composition ✂Agar 15% ✂Borax 0.2% ✂Sodium sulphate1% ✂Anti fungal (alkyl benzoate) Water 85% ✂

Similar to gelatine 1. Crown & bridge imp 2. Cast duplication- lab ✂

Strengthens gel ✂Affect gypsum setting against? ✂Good storage ✂Two viscosity forms ✂

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Manipulation • Gel in tubes – Syringe and tray material

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Agar Manipulation ✂

Water-cooled trays

Conditioner

✂

Hysteresis, hydrophilic ✂

Wetting agent on teeth ✂

5 min cooling - snap? ✂Space for thick imp ✂Cool to 30-45 ✂ > 70 degree, 60-66, 45-47prevent burns and pulp death ✂

✂

Intimate contact

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Properties Dimensional change ✂Syneresis ✂

Imbibition

✂

Tear strength

✂

Elastic limit ✂Cheap ✂

Pour immediately ✂Affects detail accuracy ✂And prosthesis fitting ✂Short storage < 1H 100% humidity (wet tissue in sealed bag) ✂ Spray disinfectant ✂Poured only once ✂

= Alginate,< rubber snap ✂Support (tray) ✂Rigid box storage ✂Expensive instruments ✂

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Alginate

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Alginate 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 ✂

✂

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Alginate impression materials COMPOSITION Sodium alginate

hydrogel former

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Alginate impression materials Calcium sulphate dihydrate: provides calcium ions CaSO4.2H2O ⇒ 2Ca2+ + 2SO4- + H2O calcium ions displace the sodium ions in the sodium alginate polymer

Don’t use a bowl with previous gypsum mix

✂

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Alginate impression materials Setting Process 1. Acid-base Reaction NanAlg + n/2CaSO4 ⇒ n/2Na2SO4 + Can/2Alg 2. Crosslinking reaction

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Alginate impression materials Sodium phosphate: controls working time

Na3PO4 ⇒ 3Na+ + PO433Ca2+ + 2PO43- ⇒ Ca3(PO4)2

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Alginate impression materials Potassium sulphate setting

enhances

Fillers consistency

controls

tri Na phosphate retarder

✂

Sodium silicofluoride ✂glycols for dustless controls pH alginate

colour, disinfectants, flavour, filler ✂

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Properties: Setting Time Wt Regular 2-3 min Or fast 1.25-2 min Setting time Regular 2-5 min, fast 1-2 min ✂

+ 1-2min in mouth after bowl ST

Reduced if long mixing

✂

Cold water ↑ st, warm water ↓ st ✂W/P severely affects properties so not used ✂

↑ Tear resistance, ↑ elastic limit

↑Tear resistance,↓ permanent deformation, ✂

Spacer 2-4 mm Snap removal

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Properties Dimensional stability as agar ✂Use wet napkins but not soaked one

Store several hours ✂Wash from saliva ✂Drops of water cause local imbibtion- distort

✂

✂

Tear strength ✂Thin < thick ✂Compatibility with tray and agar

✂

✂

Pouring > 8-10 min ✂But not more than 1 hour ✂

✂

Only pour one gypsum cast

↑ W/P reduce it

Allows for rebound, disinfect 10 min ✂To prevent syneresis ✂

If another cast is poured in the same impression, then it is not accurate 61 because of previous imbibition ✂

Alginate 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 62

To be continued

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