Impression Materials Elastic- Dr. Sumit Goel

DEFINITION

“An

impression

is

essentially

a

negative or reverse replica of any entity. In dentistry, this replica is of the teeth and its supporting and surrounding structures.”

According to the ability of the set material to be withdrawn over undercuts : Elastic impression materials Hydrocolloid  Agar Alginate

Rubbers Polysulfide.  Poly silicones.  Polyether.

Non-elastic or Rigid impression materials  Impression compound  ZOE

Ideal Requirements of Impression Materials

1.

Fluid enough to adapt to the oral tissues

2.

Viscous enough to be contained in the tray

3.

In mouth should transform (set) into a rubbery or rigid solid in a reasonable amount of time: setting time less than 7min

•

Set impression should not distort or tear when removed from mouth

•

Impressions must be dimensionally stable until the cast can be poured



Impression should maintain dimensional stability after removal of cast to enable making of a second or third cast from same impression



Biocompatible



Material associated, processing equipment, and processing time should be cost effective

Elastomers or Rubber base Impression materials fulfills most of these



Soft and rubber-like & are known as elastomers or synthetic rubbers.



As per ADA Sp. No. 19 they are called as - non-aqueous Elastomeric dental impression materials.

Liquid polymers which are converted

into solid rubber at room temperature

Contains large molecules with weak

interaction between them, tied together at certain points and form 3D network

Basically

used for dentulous cases

by virtue of their ability of withdrawal from under cuts and their excellent surface detail reproduction.

Types: III.According to chemistry ( setting rxn) 1) Polysulfides 2) Polysilicones – condensation & addition 3) Polyether

2. According to Viscosity 1) Light body or syringe consistency 2) Medium or regular body 3) Heavy body or tray consistency 4) Very heavy body or putty consistency

3. ADA Classification Based on selected elastic properties & dimensional changes 1) Type I 2) Type II 3) Type III

General Properties:



Excellent reproduction of surface details



Generally hydrophobic (except poly ether)

Good elastic properties

(repeated pouring is possible) Dimensional inaccuracies are

lower but exists due to various reasons

 Excellent tear strength.  Extended shelf-life  Generally higher cost  Requires tray adhesive or

mechanical interlocking with the tray.

CHARACTERSTICS

Rheological

properties

(viscoelastic )- play major role Introduced

as Viscous pastes-

setting reaction- viscoelastic solids.

imp.

Mat. should be free of any

distortion dev due to strain during its removal, thus the mat. Should be visco elastic i.e. intermediate behavior between elastic solid and viscous liquid.

Supplied as: •

All elastomers - two paste systems (base & catalyst)

•

Putty consistency – supplied in jars

Uses: 

Impression material for all applications including 

Fixed partial dentures



Dentures and edentulous impressions



Border molding of special trays (poly ether)



Bite registration



As duplicating material for refractory casts

1.

POLY SULFIDES:

Ist elastomer to be introduced mercaptan/Thiokol

Polysulfide  First

dental elastomers  Indications  complete

denture  removable fixed partial denture tissue

 crown

and bridge

Composition: Base Paste:  Liquid

Polysulfide Polymer 80 to 85 %

 Inert

-

fillers (Titanium dioxide, zinc sulfate ,copper carbonate or silica) 16 to 18 %  Plasticizer-- dibutylpthalate (viscosity)  Accelerator---

0.5% Sulfur

Reactor /catalyst/accelerator Paste Lead dioxide

-

60 to 68 %

Dibutylphthalate (plasticizer) - 30 to 35 % Sulfur (accelerator)

-

3%

Retarder --- oleic acid or stearic acid Other substances like Magnesium & deodorants- 2 %

Tray adhesive: Butyl rubber or styrene/acrylonitrile dissolved in a volatile solvent such as chloroform or a ketone.

Chemistry and Setting Reactions The lead dioxide reacts with the poly sulfide polymer Oxidizing agent like lead dioxide initiates polymerization rxn through 

Chain lengthening by oxidation of the terminal -SH groups and



Cross-linking between pendant -SH groups.

Exothermic reaction - 3 to 40 C rise in temp. leaving water as by product.  It is accelerated by heat and moisture 

PbO2 + S HS - R- SH

Mercaptan + Lead dioxide

HS - R-S-S-R-SH + H2O Poly sulfide +Water

T-butyl hydro peroxide – alternative to PbO2

Polysulfide Reaction --SH

HS---------------------SH =

Pb

S H

O H S

=

=

Pb

-S-S---------------S-S-

=

O

H O S--

O = Pb = O

O

mercaptan + lead dioxide

S + 3PbO + H O 2 S

polysulfide rubber + lead oxide + water

O’Brien Dental Materials & their

Properties: • Unpleasant odor and colour - stains linen & messy to work with • Extremely viscous and sticky - mixing is difficult • Mixing time is 45 seconds • Long setting time of 12.5 (at 370C) - Patient discomfort • Excellent reproduction of surface detail









Dimensional stability: - Curing shrinkage is high 0.45%. It has the highest permanent deformation (3 to 5%) among the elastomers It is hydrophobic - so the mouth should be dried thoroughly before making an impression The shelf life is good (2 years)

Advantages Lower

cost

compared

to silicones and

polyethers Long

working time High tear strength High flexibility Good detail reproduction Phillip’s Science of Dental Materials 1996

Disadvantages  Poor

dimensional stability

 water

by-product  pour within one hour  single pour  Custom

trays

 Messy  paste-paste

mix

 bad

odor  may stain clothing  Long

setting time Phillip’s Science of Dental Materials 1996

Silicone Rubber Impression Material: Developed to over come some of the disadvantages of poly sulfide

Types Based

on

polymerization

the

type

of

reaction

1) Condensation silicones 2) Addition silicones

CONDENSATION SILICONE:

Also known as conventional silicone. •

•

Available in light, medium and putty consistency

Indications

complete crown

dentures

and bridge

COMPOSITION: Base

paste

Accelerator paste

Polydimethyl siloxane Tetraethyl (hydroxyl-terminated) Orthosilicate – (cross linking agent) Colloidal silica or Stannous octoate – microsized metal (catalyst) oxide (filler) 35-75% Color pigments

SETTING REACTION: CH3

OC2H5

OH-Si-OH + C2H5O-Si- OC2H5 Sn octoate CH3

OC2H5

Silicone + CH3CH2OH

Cross-linking between terminal group of the silicon polymers and alkyl silicate to form 3D network

Dimethyl siloxane

+ Tetraethyl orthosilicate

Stannous octate Silicone rubber

+ ethyl

alcohol

Ethyl alcohol – byproduct-evaporate -shrinkage &dimensional instability



Tray adhesive – Polydimethyl siloxane & Ethyl silicate

Properties: • Pleasant odor and color. • Mixing time of 45sec & setting time of 8-9mins. • Excellent reproduction of surface details and highly elastic. • Lesser dimensional stability - high curing shrinkage (0.4 - 0.6%)

permanent

deformation due to shrinkage caused by the evaporation of ethyl alcohol is also high (1-3%).

Hydrophobic

- needs a dry field..

Biologically

inert.

Compatible

with all gypsum

products.

Advantages Better

elastic properties

Clean,

pleasant

Stock

tray

putty-wash

Good

working and setting time

Disadvantages Poor

dimensional stability

high

shrinkage

polymerization evaporation

pour

of ethanol

immediately

within

30 minutes

Hydrophobic poor

wettability

Addition silicones: Also called as polyvinyl siloxanes Better properties than condensation silicones.

Indications crown

and bridge denture bite registration -Improvement over condensation silicones -no by-product

Composition:

Base: Poly (methyl hydrogen siloxane) Other siloxane prepolymers Fillers

Accelerator: Divinyl poly siloxane  Other siloxane pre polymers  Platinum salt: Catalyst (chloroplatinic acid)  Palladium (Hydrogen absorber)  Retarders  Fillers 

SETTING REACTION: CH3

CH3

CH3

CH3

Pt salt

Si-H + CH2=CH-Si

Si-CH2-CH2-Si activator

CH3

Vinyl + siloxane

CH3

Silane siloxane

CH3

CH3

Silicone rubber

No by product, but imbalance  hydrogen gas  air bubbles in the stone models  To avoid this palladium /platinum is added. 

CLINICAL SIGNIFICANCE:

Sulfur compounds retard the setting of silicones One source of sulfur contamination is from latex gloves Vinyl gloves should be used

Properties: • Pleasant odor and color • Excellent reproduction of surface details • Mixing time of 45 secs ,setting time of 5-9 mins. • Best dimensional stability - low curing shrinkage (0.17 %)

-

lowest permanent deformation (0.05 – 0.3 %)  Stone pouring delayed by 1-2 hours  Extremely hydrophobic, some manufacturers add a surfactant (detergent) to make it more hydrophilic  Good shelf life of 1-2 yrs  Good tear strength (3000gm / cm2)

Advantages  Highly

accurate  High dimensional stability  pour

 Stock

up to one week

or custom trays  Multiple casts  Easy to mix  Pleasant odor

Disadvantages Expensive Sulfur

in latex gloves + ferric and Al sulfate in retraction solution inhibits setting

Polishing

teeth with pumice req before impression.

Short

working time

Lower

tear strength

Possible bubbles

hydrogen gas release on die

palladium

or platinum salts added to absorb gases and act as scavenger

Addition Silicones  Surfactants

added

 reduce

contact angle  improved castability gypsum

wettability?? still

need dry field clinically

Pratten J Dent Res 1987 Mandikos Aust Dent J 1998

Poly ether Rubber Impression material Introduced in Germany in late

1960’s Good mechanical properties and

dimensional stability, but short working time, very stiff material and expensive

Composition: Base Paste Poly ether polymer Colloidal silica (filler) Glycol ether or phthalate (plasticizer)

Accelerator paste Aromatic sulfonate ester (crosslinking agent)  Colloidal silica (filler)  Phthalate or glycol ether (plasticizer) 

 Available

as 3 viscosities: light, medium & heavy bodied.

CHEMISTRY & SETTING H

O

REACTION O

H

CH3 – C - CH2 – C – O – R – O – C - CH2 – C - CH +  Crosslinked rubber N N CH2-CH2 Polyether + Sulfonic ester  Exothermic reaction  4-50 C

CH2-CH2 Crosslinked rubber

Properties: 2. Pleasant odor and taste 3. Mixing time is 30 secs, setting time of 8 mins 4. Dimensional stability is very good. Curing shrinkage is low (0.24%) The permanent deformation is also low (1-2%).



  

Very stiff (flexibility of 3%), needs extra space, around 4 mm is given. 5. Hydrophilic (moisture control not critical) 6. Shelf life extends upto 2 years

Advantages  Highly

accurate  Good dimensional stability  Stock or dual-arch trays  Good surface detail  Pour within one week  kept

dry

 Multiple

casts  Good wettability

Disadvantages  Expensive  Short

working time

 Rigid  difficult

to remove from undercuts

 Bitter

taste  Low tear strength  Absorbs water  changes

dimension

AUTOMATIC DISPENSING & MIXING DEVICES

ADVANTAGES: - More uniform mix - Less air bubbles - Reduced working time

Different methods of making impressions 1. Single mix or mono-phase technique: -

Regular viscosity elastomer is used (addition silicone/polyether)

-

Paste is mixed, part of it is loaded on to the tray & the remaining onto the syringe

-

Syringe material is injected onto the prepared area & tray material is seated over it.

Multiple mix technique

DISADV: Higher viscosity material may displace more fluid wash material

RELINE OR 2-STAGE PUTTY WASH TECHNIQUE • Preliminary impression is made with a putty consistency using a thin plastic sheet or spacer over it (acts as a spacer) • Light body is injected around the prepared tooth • The plastic sheet is removed & putty impression is seated back

ELASTOMERIC IMPRESSION MATERIALS: PROPERTIES

WORKING AND SETTING TIMES  Working time- begins at the start of mixing and ends just before the elastic properties have developed

 Setting

time- time elapsing from the beginning of mixing until the curing has advanced sufficiently so that the impression can be removed from the mouth with a minimum of distortion

 Increase

in temperature- accelerates

curing time  Increase

in viscosity- decrease in working

and setting time

Dimensional stability 1. 2. 3. 4. 5.

Polymerization shrinkage Loss of byproduct Thermal contraction from oral to room temperature Imbibition Incomplete recovery of deformation because of viscoelastic behavior

Reproduction of oral detail The rubber impression materials are capable of reproducing detail more accurately than can be transferred to the stone die or cast

ELASTICITY Elastic properties of elastomeric impression materials improve with an increase in curing time in the mouth Permanent deformation following strain in compression increases in following orderaddition silicone, condensation silicone, polyether and polysulfide

Tear strength  Measures

the resistance to fracture of elastomeric material subjected to a tensile force acting perpendicular to a surface flaw Biocompatibility Polysulfide- lowest cell death count Polyether- highest cell toxicity

Elastomeric Impression Materials Rapidly

remove

decreases

permanent deformation chains

recoil

from a recoverable distance increases

strength

tear Phillip’s Science of Dental Materials 1996

Comparison of Properties Working longest

time to shortest

agar

> polysulfide > silicones > alginate = polyether

Setting

time

shortest

to longest

alginate

< polyether < agar < silicones < polysulfide O’Brien Dental Materials & their Selection 1997

Stiffness most

to least

polyether

> addition silicone > condensation silicone > polysulfide = hydrocolloids

Tear

strength

greatest

to least

polysulfide

> addition silicone > polyether > condensation silicone >> hydrocolloids O’Brien Dental Materials & their Selection 1997

Cost lowest

to highest

alginate

< agar = polysulfide
Dimensional best

stability

to worst

addition

silicone > polyether > polysulfide > condensation silicone > hydrocolloid Phillip’s 1996 O’Brien Dental Materials & their Selection 1997

Wettability best

to worst

hydrocolloids >

polyether > hydrophilic addition silicone > polysulfide > hydrophobic addition silicone = condensation silicone

Castability best

to worst

hydrocolloids >

hydrophilic addition silicone > polyether > polysulfide > hydrophobic addition silicone = condensation silicone O’Brien Dental Materials & their Selection 1997

Thank you