elastomeric impression materials

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Elastomeric Impression Materials

Seminar by Dr. Anuradha .G.Mohite

Guided by Dr. Smita Athavale

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Contents

• Introduction.• Definition & objectives Of Impression Making.• Classification of Dental Impression Materials.• Ideal requirements of Impression materials.• Types of Elastomeric Impression Materials.• Various Consistencies Of Elastomeric

Impression Materials.• Making Impressions with Elastomeric materials.

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Contents

• Compositions and Reactions of Elastomeric materials.

• Properties of Elastomeric materials.• Disinfection Of impressions. • Failures In impressions Making.• Conclusion • References

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Introduction

Elastomers refer to a group of rubbery polymers, which are chemically or physically cross-linked.

They can be easily stretched and rapidly recover their original dimensions when applied stress is released.

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Impression

• Definition : Impressions are negative reproduction of oral structures.

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Objectives of impression making

• To provide a. support,

b. retention,

c. stability,

d. foundation of improved appearance of lips,

e. maintain the health of tissues.• The impression should record all the potential

denture-bearing surfaces available

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Classification of dental impression materials.

By elasticity and use

Inelastic or rigid Elastic

Material use Material Use

By setting Mechanism

Chemical reaction (irreversible)

Plaster of parisZinc oxide eugenol

Edentulous ridgeInteroccusal records.

Alginate,Nonaqueous elastomers.

Teeth and soft tissues.

Thermally induced physical reaction (reversible)

Compound wax Preliminary impression

Agar Teeth and soft tissues.

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Ideal Requirements of impression materials

1. Adequate flow properties,

2. Ease of manipulation,

3. Appropriate working and setting times,

4. Sufficient mechanical strength,

5. Good dimensional accuracy,

6. Acceptability by the patient,

7. Biocompatible,

8. Cost-effective.

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Types of Elastomeric Materials

• Polysulfides (1950) : 1st material introduced• Condensation Silicones (1955)• Polyether (1965)• Addition Silicones (1975)

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Consistencies

• Light body : used with a syringe and placed directly on hard and soft tissues,

• Medium body : preparation of custom tray required to support this material,

• Heavy body : placed in the tray to support the light-body material,

• Putty : useful for materials that exhibit significant polymerization shrinkage.

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Consistencies

Elastomeric material are supplied in several consistencies.

• Polysulfides : a. low/light ( syringe or wash )

b. medium ( regular )

c. high/heavy ( tray )

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Consistencies

• Condensation Silicones : a. low/light

b. putty/heavy

• Addition Silicones : a. plus extra low

b. monophase

c. putty (extra high)

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Consistencies

• Polyether : Previously available as medium but now available as,

a. low/light

b. medium

c. high/heavy

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Making an impression with elastomeric material

• 5 steps are included in fabricating models, casts and dies from elastomeric materials.

1. Preparing a tray,

2. Preparing the material,

3. Making an impression,

4. Removing the impression,

5. Preparing stone, casts and dies.

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Preparing a tray

The use of a custom tray

reduce the quantity of the materials used to make impressions,

any dimensional changes of the materials can be minimized.

• Particularly true for Polysulfides.• Adhesion of impression material to the tray is

essential.

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Preparing a tray

• The use of custom trays not so critical for

addition silicone & polyether as these materials are stiffer and have less polymerization shrinkage.

• Disposable stock trays works satisfactory.• Tray adhesives is also needed for stock trays.

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Preparing the material

Mixing systems:• 3 systems available for mixing Catalyst and

Base paste:1. Hand mixing,

2. Static automixing,

3. Dynamic mechanical mixing.

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Preparing the material

• Hand mixing:– Equal lengths of catalyst and base,– Initial mixing– circular motion,– Final mix – broad strokes,– Mixing completed in 45 seconds.– Hand mixing is available for all 4 types of

elastomeric materials.

Hand mixing

• Manipulation

Base & Accelerator on mixing pad

Mixing of the base & Accelerator with a tapered stiff bladed spatula

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Preparing the material

• Two putty system

– variation in hand mixing.– Available with condensation and addition silicone.– Scoops available for dispensing,– Putties kneaded with fingers until free from

streaks.

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Preparing the material

• Static automixing:– Base and Catalyst supplied in separate cylinders of

the plastic cartridge.– Cartridge placed in mixing gun containing 2

plungers– advanced by ratchet mechanism to extrude equal quantities of base and catalyst.

– Uniform mix is obtained

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Preparing the material

• Static automixing:• Advantage : 1. Fewer voids than hand mixing,

2. 3 to 4 times less material wasted than hand mixing,

3. Less air incorporated in mix,

4. Mixing time is reduced,

5. less contamination of the material

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Preparing the material

• Automixing guns and mixing tips available for all consistencies except putty.

• Addition silicone, condensation silicone and polyethers available with this type of mixing.

Automatic mixing gun

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Preparing the material

• Dynamic mechanical mixing:– Catalyst and base supplied in large plastic bags

housed in a cartridge which is inserted into the top of the mixing machine.

–When button is depressed, parallel plungers push against collapsible bags thereby opening the bags and forcing material into the dynamic mixing tip. mixing is accomplanished by rotation plus forward motion of the material since internal spiral is motor driven.

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Preparing the material

• Dynamic mechanical mixing:• Advantages: ease of use, speed, thoroughness of

mixing and higher viscosity materials can be mixed with ease.

• Disadvantage : costly and more material wasted than automixing but less than hand mixing.

• Polyether and Addition silicone available with this system.

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Impression making

Impression techniques:

• 3 common methods for impression making:1. Simultaneous, dual-viscosity technique,

2. Single-viscosity or monophase technique,

3. Putty-wash technique.

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Impression making

• Simultaneous, dual viscosity technique:– Low consistency material is injected with a syringe

into critical areas.– High consistency material is mixed, placed in an

impression tray and inserted in the mouth– The more viscous tray material forces the lower

viscosity material to flow into fine aspects– The materials join, bond and set together

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Impression making

• Single viscosity or monophase technique:– Impression taken with medium viscosity material.– Addition silicone and polyether can be used with

this technique because they have a capacity for shear-thinning.

–When medium viscosity material is forced through impression syringe, the viscosity is reduced, whereas viscosity of material in tray is unaffected.

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Impression making

• Putty wash technique:– Two approaches used-- two stage and one stage

procedures.

Two stage procedure: – Preliminary impression taken in putty consistency

material before the preparing the teeth.– Space is provided for low consistency material in the

impression.– Low consistency material is syringed into the impression

and the preliminary impression is re-inserted.

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Impression making

• Putty wash technique:Single stage procedure:–Wash material is syringed into place– Unset putty is seated over the light body material– Occlusal stops are provided in the tray to avoid

pushing through the wash material when seating putty mass.

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Impression making

• Low and high consistency material bond and after the low consistency material sets, impression is removed.

• Putty consistency and this technique was developed for condensation silicone to minimize effects of dimensional change during polymerization.

• It can also be used for addition silicone.

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Removing the impression

• Impression should not be removed until curing has progressed to the extent to provide adequate elasticity so that distortion will not occur.

• Typically, impression should be ready for within atleast 10 mins from time of mixing, allowing 6 to 8 mins for the impression to remain in the mouth.

• All elastomeric materials are viscoelastic and it is necessary to use a quick snap to minimize plastic deformation of the impression during removal.

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Preparing stone casts and dies

• All elastomeric materials are compatible with all types of gypsum products.

• The excellent dimensional stability of addition silicone and polyether materials makes it possible to construct 2 or 3 casts or dies from these materials but time interval should not be greater than 30 mins.

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Polysulfide

• Polysulfides: First rubber impression materials.• Available in 3 consistencies:1.low(syringe or

wash),

2. medium

(regular),

3.high(tray)

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Composition

Base paste: 1.Polysulfide polymer,

2. Filler (e.g,lithopone or titanium dioxide)- for strength,

3. Plasticizer- appropriate viscosity to paste,

4. Sulfur (0.5)- Accelerator

• Catalyst paste:

1. Lead oxide,

2. Same filler and plasticizer

as base paste,

3. Alongwith oleic acid or

stearic acid

-- retarder.

Types of polysulphide materials

light,regular& heavy- bodied

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Polysulfide

• Setting reaction:–Main component: Multifunctional Mercaptan(--

SH) or polysulfide polymer.

+

Lead dioxide ( oxidizing agent)

Polysulfide + Water( byproduct )

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Polysulfide

• The reaction results in a rapid increase in the molecular weight , converting the mixed paste to a polysulfide rubber.

• The reaction is slightly exothermic- increase in temperature of 3 to 4 degree.

• Lead dioxide- gives characteristic Brown color.

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Mechanical properties

• Elastic recovery– 96% , • Flow– 0.4 to 1.9% , indicating tendency to

distort• Least stiff • High flexibility, advantage over silicones and

polyether is that removal from undercuts is easier

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Polysulfide

• Advantages:

1. Long working time,

2. High tear strength,

3. High flexibility for easier removal from undercuts,

4. Good flow before setting,

5. Good reproduction of surface detail

• Disadvantages:

1. Requires custom tray due to greater chance of distortion,

2. Stretching leads to distortion,

3. Obnoxious odor,

4. Pour within 1 hour,

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Condensation Silicone

• Condensation Silicones are supplied as

1. Low

2. Putty

The catalyst of the Condensation Silicone can be supplied as putty or a liquid

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Condensation Silicone

• Base paste: 1.Polydimethyl siloxane(which has reactive terminal -

OH groups),

2.Orthoalkylsilicate(for cross-linking)

3.Fillers- calcium carbonate or silica

(from 35% for low consistencies to 75% for puttylike

consistencies).

• Catalyst paste:

(low viscosity liquid or paste catalyst)

1. liquid consisting of stannous

octoate suspension and

alkyl silicate.

Condensation Silicone having low consistencies & puttylike consistency

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Setting reaction

• Reaction produces a 3 dimensional network with the liberation of Ethyl alcohol as a byproduct and exothermic rise in temperature of about 1 degree.

• Polydimethyl siloxane + Tetraethyl orthosilicate + Stannous octoate

Silicone rubber+ Ethyl alcohol ( byproduct)

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Condensation Silicone

• Evaporation of Ethyl Alcohol (byproduct) causes polymerization shrinkage of the silicone rubber impression.

• This shrinkage was more in low consistency silicone , therefore, to overcome the large shrinkage a high viscosity material referred as “Putty” was developed.

• These putties have more fillers and less polymer so they have less shrinkage.

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Mechanical properties

• Excellent elastic recovery of 99%• Low flow, less than 0.1%, thus, less distortion is

likely to be caused by light pressure on standing.

• Stiffer than polysulfides because of lower flexibility values

• Greater shrinkage than polysulfide and polyether – 0.2 to 1% because of release of alcohol byproduct.

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Condensation Silicone

Advantages :

1. Clean and pleasant,

2. Putty for tray,

3. Good working time,

4. Highly elastic and the setting time can be controlled with the amount of accelerator.

Disadvantages:

1. High polymerization shrinkage,

2. Volatile by-product,

3. Low tear strength,

4. Hydrophobic,

5. Pour immediately.

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Polyether

• Polyether was introduced in Germany in the late 1960’s

• Available in: 1. low

2. medium

3. heavy

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Polyether

• Base paste:

1. Polyether polymer with ethylene-imine terminal groups,

2. Filler- colloidal silica,

3. Plasticizer - glycolether or phthalate.

• Catalyst paste:

1. Alkyl-aromatic sulfonate,

2. Filler,

3. Plasticizer.

Polyether Materials

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Setting reaction

• Polyether + Aromatic sulfonate ester(initiator)

Cross-linked polyether rubber.

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Polyether

• The main chain is a copolymer Ethylene oxide and Tetrahydrofuran.

• The cross linking is produced by polymerization via the imine end groups.

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Mechanical properties

• 0.3% shrinkage in 24 hrs, thus good accuracy but inferior to addition silicones,

• Since polyether absorbs water and changes its dimensions, storage in water is not recommended.

• Elastic recovery is between polysulfides and addition silicones– 98.5%

• Flow is very low and thus, contributes to accuracy• Low flexibility i.e high stiffness; it may cause

problems in removal of impression from mouth or the die from impression.

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Polyether

• Advantages:

1. Clean,

2. Ease of mixing,

3. Good surface detail reproduction,

4. Least hydrophobic,

5. Good stability,

6. Delay pour.

• Disadvantages:

1. Short working and setting times,

2. High stiffness after setting,

3. Bitter taste,

4. Needs to block undercuts,

5. Leaches components,

6. High cost.

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Addition silicone

• Frequently called as Polyvinyl siloxane or

vinyl polysiloxane impression materials• Available in : a. extra low

b. low/light

c. medium

d. heavy

e. extra heavy.

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Composition

• Base paste:

1.Polymethyl hydrogen siloxane (low molecular weight polymer) ,

2.Hybrid silicone and

3.Fillers.

• Catalyst paste:

1.Divinyl polydimethyl siloxane ,

2.Platinium salt activator (chloroplatinic acid)and

3.Fillers (Retarders may also be present.)

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Setting reaction

• Polymethyl hydrogen siloxane + divinyl polydimethyl siloxane + platinium salt activator Silicone rubber.

• No reaction by-products.

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Addition silicone

• However, a secondary reaction between moisture and residual hydrides of the base polymer can lead to the development of hydrogen gas that can result in pinpoint voids in the gypsum casts that are poured immediately

• Therefore, Platinium or Palladium is added to act as a scavenger for the released hydrogen gas or to wait 30 mins or more for pouring the impression.

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Addition silicone

• Sulfur contamination from latex gloves have been shown to inhibit the setting of addition silicone. These compounds can contaminate platinium containing catalyst which results in retarded or no polymerization.

• Instead Vinyl or Nitrile gloves can be used.

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Addition Silicone

• Disadvantage of this material is their inherent hydrophobic nature.

• Distortion at the impression margins is caused by undetected moisture in the area to be replicated.

• A Nonionic surfactant is added to make surface of impression hydrophilic.

• This will allow impression material to readily wet soft tissues and capture maximum details.

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Mechanical properties

• Working and setting time faster than polysulfides. Retarder is often supplied to extend working time.

• Excellent elasticity and very low dimensional shrinkage upon storage, thus can be poured up later.

• Greater rigidity, hence, difficult to remove from undercuts as they have lower flexibility values

• Tear strength less than polysulfides

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Addition silicone

• Advantages:

1. Clean and pleasant,

2. Highly accurate high dimensional stability after setting,

3. Ideally elastic,

4. Pour repeatedly.

• Disadvantages:

1. Hydrophobic,

2. No flow if sulcus is moist,

3. Low tear strength,

4. Putty too stiff,

5. High cost.

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Properties of Elastomers

1. Working and setting times,

2. Dimensional stability,

3. Rheological properties,

4. Elasticity,

5. Strain in compression,

6. Tear strength,

7. Hardness,

8. Anatomical detail reproduction,

9. Wettability,

10. Biocompatibility and shelf Life.

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1.Working and Setting times.

Mean working time (mins)

Mean setting time(mins)

Impression material

23 deg.Cel. 37 deg.Cel. 23 deg.Cel. 37 deg.Cel.

Polysulfide 6 4.3 16 12.5

Condensation silicone

3.3 2.5 11 8.9

Addition silicone

3.1 1.8 8.9 5.9

Polyether 3.3 2.3 9 8.3

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Working and Setting times

• Polysulfides have the longest working & setting times followed by Silicones and Polyethers.

• An increase in temperature accelerates curing rates of all elastomeric materials and thus decreases both setting and working times.

• Working times can be increased either by storing materials at room temperature or using chilled, dry glass slab.

• Working and setting times decreases as the viscosity increases from low to high.

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2.Dimensional stability

5 major sources of dimensional changes are:

1. Polymerization shrinkage,

2. Loss of by-product(water or alcohol) during the condensation reaction,

3.Thermal contraction from oral temperature to room temperature,

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Dimensional stability

(cont..)

4. Imbibition when exposed to water, disinfectant or high humidity environment over a period of time,

5. Incomplete recovery of deformation because of viscoelastic behavior.

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Dimensional stability

• Polysulfides >condensation silicone >addition silicone >polyether

• Polysulfides and condensation silicone have larger dimensional change due to loss of reaction by-products.

• Therefore, to obtain maximal accuracy with polysulfides and condensation silicone they have to be poured immediately (within 1st 30 mins).

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Dimensional stability

• The shrinkage of addition silicone and polyether is less since there is no loss of

by-products.• Therefore, they do not have to be poured

immediately.• Polyether absorbs water or fluids and

simultaneous leaching of water soluble plasticizer. Thus, must be kept in dry, cool environment.

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3.Rheological properties

• The rheological properties of these materials play major role in their application as high accuracy impression materials.

• Introduced in mouth as viscous pastes.• Setting reaction – Viscoelastic solids.• For accurate impressions– appropriate flow

behavior of solid form,• Viscosity and flow behavior of unmixed

components is also important.

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Rheological properties

- Polysulfides has the lowest viscosity and is least stiff. Therefore, can be easily removed from undercut areas and with minimum stress.

- Addition silicone and polyether are pseudoplastic materials. Because of this property ,these materials are more stable and resistant to distortion.

-- Viscosity increases from low to high consistencies.

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4. Elasticity

– Elastic properties improve with increase in curing time in the mouth.

– The relative amount of permanent deformation increases in the following order,

addition silicone > condensation silicone >

polyether > polysulfides.

-- Elastic recovery following strain is less rapid for polysulfides than other 3 materials.

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Elasticity

• Addition silicone is ideally most elastic of all materials.

• Excluding the very high viscosity putty class of elastomers , the stiffness ( elastic modulus) increases in following order. Polysulfide > Condensation silicone > Addition silicone > polyether.

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5. Strain in compression

Strain in compression under a stress of 0.1MPa is a measure of flexibility of the material. The low consistency materials are more flexible than high consistency materials.

Polyethers (stiffer) > addition silicone > condensation silicone > polysulfides.

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6. Tear strength

--Tear strength indicates the ability of a material to withstand tearing in thin interproximal areas and subgingival areas.

--Tear strength:

Addition silicone < condensation silicone < polyether < polysulfide.

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Tear strength

• Tear strength is influenced by chemical composition of the material, consistency and manner of removal from mouth.

• Increased consistency = increased strength• Rapid rate of force during removal =>

increases strength.

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7. Hardness

Low, medium and high viscosity addition silicones and polysulfides --- hardness does not change with time.

Condensation silicone, addition silicone putties and polyethers --- hardness increases with time.

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9. Anatomical details reproduction

The rubber impression materials are capable of reproducing details accurately

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10. Wettability

–Wettability may be assessed by measuring the advancing contact angle of water on the surface of the set impression material. –All the elastomeric materials possess

advancing and receding contact angles greater than 45 degrees.

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Wettability

• The hydrophilic addition silicones and the polyethers were wetted the best, and the condensation silicones and hydrophobic addition silicones the least.

• The wettability was directly correlated to the ease of pouring high strength stone models.

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11.biocompatibility

– Comparing the cell cytoxicity, polysulfides results in lowest cell death count whereas, polyether produces highest cell cytoxicity.

– Biocompatibility problem also occur when a segment of impression material is lodged in gingival sulcus that can cause severe gingival inflammation.

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Shelf Life

-- Material will not deteriorate when stored in dry, cool environment.

– Tubes should be tightly closed when not in use.

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Disinfection

• Condensation silicones, addition silicones and polysulfides can be disinfected with all EPA- registered disinfectants without adverse dimensional changes, provided , disinfection time is short.

• Long immersion may cause surfactant in addition silicone to leach out and render impression less hydrophilic.

• Polyethers are also susceptible to dimensional change if immersed for long time (> 10 mins ) because of their hydrophillic nature.

• 2% Glutaraldehyde is satisfactory for most elastomers.

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Property Poly sulfide Condensation silicone

Addition silicone

Polyether

Working time 4-7 2.5-4 2-4 3

Setting time 7-10 6-8 4-6.5 6

Tear strength-N/m

2500-7000 2300-2600 1500-4300 1800-4800

Percent contraction (24 hrs)

0.40-0.45 0.38-0.60 0.14-0.17 0.19-0.24

Custom tray yes No No No

Unpleasant odor

yes no no No

Multiple casts no no yes yes

stiffness 3 2 2 1

Distortion on removal

1 2 4 3

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Failures

Type of failure Causes

A. Rough or uneven surface on impression

1.Incomplete polymerization caused by premature removal from mouth, improper ratio or mixing of components, presence of oil or other organic material on teeth.2.Too rapid polymerization from high humidity or temperature.3.Excessively high accelerator/base ratio with condensation silicone.

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Failures

Type of failure Causes B. Bubbles. 1.Too rapid

polymerization, preventing flow,2.Air incorporated during mixing.

C. Irregularly shaped voids.

1.Moisture or debris on surface of teeth.

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Failures

Type of failure Causes D. Rough or chalky stone cast.

1.Inadequate cleaning of impression.2.Excess water left on surface of the impression.3.Excess wetting agent left on impression.4.Premature removal of cast. Improper manipulation of stone.5.Failure to delay pour of addition silicone at least 20 mins.

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Failures

Type of failure Causes E. Distortion 1.Resin tray not cured

sufficiently, still undergoing polymerization.2.Lack of adhesion of rubber to the tray due to insufficient adhesive.3.Development of elastic properties in the material before tray is seated.4.Excessive bulk of material.5.Movement of tray during polymerization.

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Accuracy of elastomeric impression materials on repeated pours– JPD 2006

• The accuracy of elastomeric impression materials on repeated pours is reviewed in this article.

• It is evident that all materials change dimensionally over time. The present review on the accuracy of elastomeric impression materials on repeated pours would suggest that addition silicones and polyethers to a certain extent were least affected with delay in pouring the impression.

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Accuracy of elastomeric impression materials on repeated pours– JPD 2006

• However, since condensation silicones and polysulphide materials are not dimensionally stable, the time interval between pours should not be greater than 30 min and re-pouring would result in significant loss of accuracy.

• Addition silicones are the most versatile, widely used, dimensionally accurate and stable of all materials followed by polyether.

• This stability exhibited by both these materials suggest that, these impressions do not have to be poured with gypsum products immediately

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Procedure to prevent cast breakage during separation from elastomericimpression- JPD 1999

• This article describes a quick and simple procedure for removing dental stone casts from elastomeric impression. The materials used in this technique i.e Slurry water and dishwashing soap are inexpensive and easy to use.

• The soapy water flows into the space between the cast and the impression, lubricating the 2 surfaces. After soaking for at least 10 minutes, the cast is removed from the impression with the use of minimal force.

• Proper use of this procedure will help improve the quality of poured casts, preventing the occurrence of tooth fractures and saving time and materials.

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Conclusion

• It should be remembered that a good impression results from good handling of a good material.

• The “ perfect impression” can only be approached by a knowledgeable and competent operator, using an impression material that meets the required specifications, with skill and concern for the biologic, psychologic and preventive considerations.

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References

• Restorative dental materials– Craig• Science of dental materials– Anusavice • Dental materials and their selection– William

O’ Brien

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Article References

• J. P.D. 2000 volume 34• JIPS 2006 volume 6 issue 2• J. P.D. 1999 volume 81• DCNA July 1981- occlusion and functions

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