8. impression materials

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

7

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

8

Mixing techniquesManual:

Low viscosityLess viscous is easierSilicones are easiest to mix followed by polyethers and polysulphides

9

Mixing techniquesAutomatic mixing

Reduces mixing timeReduces bubblesHomogenous mixUse correct mixing proportions

2x2x2x2x2x2x2x2 = 256 folds

10

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

11

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.

12

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

13

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)

14

Flexibility of impression materials

0

1

2

3

4

5

6


Flexible

Moderate

Hard

15

Resistance to tearing

0

1

2

3

4

5

6


High

Medium

Low

16

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)

17

WettabilityGood wetting is the ability of a liquid to

cover the surface of the substrate completely

non-wetting partial wetting perfect wetting

18

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.

19

Wettability of impression materials

0

1

2

3

4

5

6


Excellent

Good

Fair

20

Dimensional accuracy and stability

Choice and use of traySetting shrinkagePermanent setStorage stability

21

Dimensional change of impressions

0 1 2 3 4

HAS

AS

CS

PE

PS

IDEAL

Low

Medium

High

22

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

24

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 polymerFiller (colloidal silica)Organo-tin compound

Activator pasteSilicone polymerFiller (colloidal silica)Tetra-ethyl silicate


•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


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

http://www.dental-tours.ch/beh/kons/keramikinlay/index.html

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.


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


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.


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

8. impression materials

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