A Seminar on

IMPRESSIONS IN FIXED PARTIAL DENTURES

PRESENTED BY, SIGNATURE OF H.O.D

Dr.U.Varalakshmi,

II Year P.G.Student.

Department Of Prosthodontics

SIBAR Institute Of Dental Sciences

GUNTUR.

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IMPRESSIONS IN FIXED PARTIAL DENTURES

CONTENTS

DEFINITIONS

IDEAL REQUIREMENTS OF IMPRESSION MATERIALS

CLASSIFICATION OF IMPRESSION MATERIALS

IMPRESSION TRAYS

IMPRESSION TECHNIQUES FOR DIFFERENT IMPRESSION

MATERIALS

SPECIAL CONSIDERATIONS

CONCLUSION

REFERENCES

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Definitions

Impression

An imprint or negative l ikeness of the teeth and/or edentulous areas

where the teeth have been removed, made in a plastic material which

becomes relatively hard or set while in contact with these t issues.

Impression material

Any substance or combination of substances used for making a

negative reproduction or impression.

Impression tray

A device which is used to carry, confine and control an impression

material while making an impression.

Ideal requirements of impression material (According to Tylman)

Complete plasticity before use

Sufficient fluidity to record fine detail

The abili ty to wet the oral tissues

Dimensional accuracy

Dimensional stability

Complete elasticity after cure

Optimal stiffness

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Have a good shelf l ife

Be non-toxic and non-irritating

Have acceptable taste and odor

Have suitable working and setting t imes

Have strength to resist tearing

Be compatible with model and die material

Facilitate clinical identification of beginning and end of cure

Facilitate visualization of the finish l ine

An impression for a fixed restoration should meet the following

requirements

It should be an exact duplication of the prepared tooth, including all

of the preparation and enough uncut tooth surface beyond the

preparation for the dentist and technician to be certain of the

location and configuration of the finish line.

Teeth and tissues adjacent to the prepared tooth must be accurately

reproduced to permit proper articulation of the cast and contouring

of the restoration.

It must be free of bubbles, especially in the area of the finish line

and occlusal surfaces of other teeth in the arch.

Impression materials that are used in fixed prosthodontics are

Reversible hydrocolloids

Polysulfide

Condensation silicone

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Addition sil icone

Polyether

urethane dimethacryalte

Classification

According to chemical nature

Hydrocolloid impression material

Elastometric impression material

Each of them are further classified according to the viscosity

• Heavy body

• Regular/ medium body

• Light body

• Very heavy/ putty

Reversible hydrocolloid (Agar)

The credit for its first use in United States for fabricating cast

restorations is given to J.D. Hart (1930). It is a polysaccharide extracted

from certain types of seaweed.

Composition

Ingredients Percentage by weight

Function

AgarBorates

Potassium sulfateWax hard diatomaceous earth

13 – 17%0.2 – 0.5%

1 – 2%0.5 – 1.1%

Basic constituentImproves strength of the gel retards plaster or stone

Gypsum hardenerFiller

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SilicaClayAlkyl benzoatesThizotrophic materialsColoring agentsWater

0.1%0.3%-Balance

PreservativesPlasticizers-Dispersion medium

Gellation process

The setting process of reversible hydrocolloid is called as a

gellation process i t is due to change in temperature.

It is available as tray material, or as syringe material as semi-solid

gel in polyethylene tubes.

The cycle is

Gel ---------- Sol ------------- Gel

(Tube) (Conditioner) (Tray)

Agar requires a conditioning unit which has three compartments one

for liquefying were the tubes are placed at 1000C as it is too hot for intra

oral use i t is cooled in two stages: storage and tempering. Addition to

lowering the temperature it also increases the viscosity. The storage

temperature is 700C and can be kept for 5 days. Tempering is done just

before the impression is made that is between 370C – 500C. After loading

in the tray and placing in the mouth water is circulated at 170C – 210C

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until the material is completely gelled. This is done using water cooled

rimlock tray

Elastomeric impression materials

Polysulfide (Mercaptan, Thiokol)

Composition

Base

Liquid polysulfide polymer 80 – 85%

Inert fi llers (TiO, Zn Sulfate, Copper carbonate) 16 – 18%

Plasticizers (Dibutyl phathalate)

Accelerator paste

Lead di-oxide 60 – 68% - Reactor

Dibutyl phathalate 30 – 35% - Plasticizer

Sulfur 3% - Retarder

Setting reaction

This polymer is usually cross l inked with an oxidizing agent such as

lead dioxide. It is this lead dioxide that gives polysulfide its characterist ic

brown color. This is a condensation reaction where all the polymer chains

grow simultaneously and a reaction by product is formed like water.

They are available as

Light body

Regular body

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Heavy body

Polysulfide impression material is hydrophobic and should be

poured within 1 hour of removal from the mouth and it is a radio opaque

impression material because of the presence of lead dioxide.

Condensation silicone

Composition

Base paste

Poly dimethyl siloxane 25 – 65%

Colloidal sil ica or micronize metal oxide fi ller (Depending upon the

viscosity) 35 – 75%

Coloring agents

Accelerator paste

Orhtoethyl si licate – cross linking agent

Stannous octate – catalyst

Available as

Light body

Regular body

Heavy body

Putty

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Setting reaction

This is condensation polymerization reaction which reacts with

trifunctional and tetrafunctional alkyl si licates l ike orthoethyl sil icate in

the presence of stannous octate. Ethyl alcohol is a by product which is

responsible for shrinkage of the material.

The condensation sil icone impression materials are supplied as a

base paste and a low-viscosity liquid or catalyst paste. Since ethyl alcohol

is the by product they are dimensionally stability is poor and they should

be poured immediately after the removal from the mouth. They have a

very poor shelf l ife because of the instability of alkyl sil icates in the

presence of organo-tin compounds, which may result in the oxidation of

tin.

Addition sil icone

Base paste

Poly methyl hydrogen siloxane

Other siloxane polymers

Fillers

Accelerator paste

Divinyl siloxane

Other siloxane prepolymers

Platinum salt (chloroplatinic acid) catalyst

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Palladium – hydrogen absorber

Fillers

Available as

Light body

Regular / medium body

Heavy body

Putty

Setting reaction

It is addition polymerization reaction terminated with vinyl groups

and is cross linked with hydride groups activated by a platinum salt

catalyst. There is no reaction by products as long as correct proportions of

vinyl silicone and hydride sil icone are manipulated and there are no

impurities, if the proportion is out of balance or impurities are present

then side reactions will produce hydrogen gas. This is avoided by the

manufacturer adding noble metals or palladium or platinum to act as

scavengers another way is to wait for 1 hour before pouring up the

impression.

Trays used

Custom tray / resin tray

Stock tray

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Polyether

Base paste

Polyether polymer

Collidal sil ica

Glycoether or dibutyl phathlate

Accelerator paste

Aromatic sulfonate ester – cross linking agent

Collidal sil ica

Dibutyl phathalae or glycoether

Available

Single viscosity (both syringe and tray material)

Regular

Setting reaction

It is a addition polymerization reaction with no by products the

reaction is between aziridine rings which are at the end of branched

polyether molecules. The main chain is a copolymer of ethyle oxide and

tetrahydrofuran. Cross linking and setting is brought about by an aromatic

sulfonate which Acts as an initiator by releasing a cation R+ an alkyl

group. This produce the cross l inking by cationic polymerization via the

imine end groups.

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Trays used

Custom / resin trays

Stock tray

Comparison of advantages and disadvantages

Material Advantages Disadvantages Recommended uses

Precautions

Reversible and hydrocolloid

Hydrophil ic

Long working t ime

Low cost No custom

tray required

Low tear resistance

Low stabili ty

Equipment needed

Multiple preparations

Problems with moisture

Pour immediately use only with stone

Polysulfide High tear strength

Easier to pour than other elastomers

Messy Unpleasa

nt odor Long

set t ing time Stabil ity

only fair

Most impressions

Latex gloves use in contra-indicated for all the elastomers. Pour within 1 hour.

Addition si licone Dimensional stabili ty

Pleasant to use

Short set t ing time

Automix available

Hydrophobic

Poor wetting

Some material release hydrogen

Most impressions

Delay pour some materials , care to avoid bubbles when working

Condensation sil icone

Pleasant Hydropho Most Pour immediately,

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to use Short

set t ing time

bic Poor

wetting Low

stabili ty

impressions care to avoid bubbles when pouring

Polyether Dimensional stabili ty

Accuracy Short

set t ing time Automix

available

Set material very sti ff

Most impressions

Care not to break teeth when separating cast

Trays

Various type of trays are used for fixed partial dentures impression

procedures.

1. Stock tray

Metallic

Non metallic

2. Custom made trays

Autopolymerizing acrylic resin

Thermoplastic or photo initiated resin

3. Water cooled rim lock trays

Custom trays

Advantages

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Improves the accuracy of an elastomeric impression by limiting the

volume of the material, thereby reducing the 2 sources of error

a. Stress during removal

b. Thermal contraction

No need for sterilization

Uniform thickness of the impression material minimizes distortions

resulting from curing shrinkage

Procuring of the tray material is not required

Disadvantages

Time taken for the fabrication

Aging for 24 hours to minimizes the distortion

Sensitivity to monomer

Steps in fabrication

Soak replicas of diagnostic casts in slurry water for 10 mins.

Tray extensions are marked with the pencil l ine at either the

cervical region of the teeth or 5mm below the cervical line.

Base plate wax is adapted (for 2 sheets thickness – 2-3mm) after

softening on the Bunsen burner and trim the excess with the knife

until the marked pencil l ine .(Rigid or stiff material like polyether

requires of wax spacer of 4mm thickness).

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Cover the wax with tin foil , or aluminium foil for preventing the

wax to melt during the exothermic heat produced during the

polymerization of the resin (autocuring).

Wax is removed either in 4 or 3 tripodal areas 3 sq.mm and located

not on the prepared tooth but on non-functional or non-centric

cusps. If all the teeth are prepared then soft tissues stops either on

the palatal area or the crest of the ridge is placed.

Needed amount of polymers and monomers of autopolymerizing

resin is mixed as per manufacturer instructions t ill the dough stage

is attained.

Then it is flattened to almost 4mm thick putty. It is then adapted

and molded over the tin foil separation and excess is trimmed off.

A handle is formed along with 2 buccal wings or ridges on either

side for the easy removal.

After the set i t is slowly removed form the cast, check if the stops

are proper.

It can then be placed in the water for 9 – 24 hours (Rosenstiel) or 5

minutes in boiling water before use.

A tray adhesive can be applied 15 minutes before placing the

impression material and allow it to dry (inside and outside the bond

of the tray)

Advantages of stock tray

Eliminating the time and expenses of fabricating a customized tray.

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They are rigid metal and susceptible to distortion.

Impression techniques

For reversible hydrocolloid

It requires a hydrocolloid conditioning unit. A conditioning unit has

three units.

Liquefying bath

Storage bath

Tempering bath

1. Liquefying bath

Tubes of impression materials and syringes are boiled at 2120F

(1000C) for 10 – 12 minutes.

2. Storage bath

Stored at 1500F (650C) at least for 10 minutes. The material can be

stored for 5 days.

3. Tempering bath

Loaded impression trays are tempered in this bath at 1100F to

1150F (about 400C) for 5 – 10 minutes immediately before placing in the

mouth.

Procedure

Because only one accurate cast can be made from a hydrocolloid

impression, two impressions are made

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1. A sectional impression for making a die (made first)

2. A full arch impression for the working cast

After selecting a proper tray and placing stops in the proper area.

Tray material is kept on the tray for tempering.

Low viscosity syringe material is placed in the bath which is then

removed and applied after the retraction cord is removed and then

the tray with heavy body is placed into the mouth and cold water is

circulated.

It is then removed after holding without movement with a rapid

motion, washed with cold water, inspected, disinfected and

immediately type 4 stone is poured.

Wet field technique

The areas of the teeth and tissues are flooded with warm water.

Syringe material is taken directly from tempering compartment and

added to prepared cavities, first at base of preparation and then the

tooth is covered.

The material used to fill the tray should be cooler or tempered.

Gelation is accelerated by circulating cool water, through the tray

for 3 – 5 minutes.

It is postulated that the hydraulic pressure of the viscous tray

material forces the fluid syringe hydrocolloid into the area to be

restored.

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Laminate technique

A recent modification to the traditional agar procedure is the

combination of agar and alginate impression materials.

The tray hydrocolloid is replaced with a mix of chilled alginate,

that bonds with the syringe agar.

The alginate gels by chemical reaction, but agar gels by means of

contact with cool alginate rather than water circulating through the

tray

Advantages

Less preparation time and less complicated when compared to the

wet field technique.

Disadvantages

Bond between agar and alginate is not strong

High viscosity alginate displaces agar during seating

Dimensional inaccuracy of alginate l imits the use to single units.

Techniques for electrometric impression materials

1. Using stock tray

Synonyms – Putty wash

Mixing method – double mix and single mix

Advantages

Eliminates time and expense of fabricating custom tray

Metal stock trays are rigid and are not susceptible to distortion.

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Disadvantages

More impression material is required.

Must be steril ized

Technique

Select a stock tray and coat with adhesive

Mix high viscosity l putty and roll it in the shape of the cylinder

and load it on the tray, give a space which is a sheet of polyethylene

and seat with rocking motion and wait t ill the initial set (2 minutes)

and then remove from the mouth with the minimal sideward

movement.

Gingival retraction done

After lubricating the O-ring of the syringe, the needed amount of

the low viscosity material is mixed on a pad and either loaded in the

syringe by scraping or by making a paper cone and then fill ing the

syringe.

Remove the retraction cord gently and syringe inaccessible areas

first (e.g.) disto lingual finish l ines.

Now insert the tray with low viscosity impression material

Position the tray over the arch

Apply force in a vertical direction until further seating is

impossible.

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After material is set, insert two index fingers under each side of the

tray to break the seal.

Remove the tray in the direction parallel to the preparation.

Evaluate the set impressions.

Difference between single mix and double mix technique

In single mix technique one viscosity material (regular body) is

used to fil l the tray and load the syringe.

In double mix technique, the light body is loaded in the syringe and

syringed around the tooth prepared and heavy body is mixed and

loaded in the tray.

2. Using custom tray

Procedure

Do the gingival retraction and Prepare the syringe

In a mixing pad mix the low viscosity impression material as per

manufacturer’s instruction. First use circular motion combining the

two strands then a finger of eight motion, obtaining a streak free

mixture in a less than 1 minute.

Load the syringe as well as the tray.

Syringe the material in the inaccessible area first then subsequently

removing the retraction cord gently and then syringe the impression

material.

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Now insert the tray. Seal from posterior to anterior allowing excess

to extrude in an anterior direction.

Continue seating in a vertical direction until the trays stops prevent

further progress.

After the material is completely set , remove the impression parallel

to the preparation path.

Evaluate the impression.

Closed bite double arch method

Also called as Dual Quad tray, double arch, triple arch and closed

mouth impression.

Minimum conditions

Either natural teeth or an incisal pin and table should be provided in

the articulator as vertical stops.

Sufficient space distal to the last tooth should be present to allow

tray approximation.

Intact dentition, class I occlusion, bounded on either side by intact

teeth, opposing tooth having intact occlusal contact is ideal.

Advantages

Physical deformation of the impression by mandible during opening

is minimized.

Seating of teeth during maximum intercuspation is captured.

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Less material is needed and patient is more comfortable.

Disadvantages

Tray is not rigid and it depends on the impression materials rigidity.

Not a functionally generated technique – so l imited to one casting

per quadrant.

Types of dual arch trays

Metal

Plastic (both with or without side wall)

Depending on the location

Posterior / anterior sexant

Quadrant

¾ of an arch

Full arch

Technique

Fit of the tray is checked such that the tray extend distal to the last

teeth of the arch by and then ask the patient to close, observe for

the bilateral closure and see that the patient is comfortable.

Gingival retraction is done.

Ready the tray material and the syringe material which is placed on

the tooth after cord removal.

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Tray placed inside mouth while observing the distal extent and

slowly asking the patient to close the mouth.

After waiting for the set (2 minutes), the patient is asked to open

the mouth, and then the tray adheres to one arch. After placing

fingers on either side of the tray it is removed with equal pressure

bilaterally to minimize the distortion of the tray.

The handle should not be used for removal of the tray and the

material is removed from the sulcus the impression is then washed

and the retraction cords is removed and checked.

Different techniques for making a dual arch impression

One step technique

Stock trays are used, light body is injected around the prepared

tooth and putty or bite registration paste is inserted on both sides of the

tray and the patient is asked to bite in centric occlusion.

Dual arch – hydraulic pressure technique

Low or medium viscosity impression material is injected on the

prepared and preoperative impression of the unprepared tooth. The tray is

placed and patient is asked to bite in centric occlusion, the hydraulic

pressure created, will force the material into the sulcus and through the

vent holes created in the buccal or the lingual sides of the pre-operative

impression.

Dual arch – laminar impression technique

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A pre-operative impression is made. The post operative impression

of the tooth to be prepared is relieved to a depth of 0.5mm in the cervical

area. Two holes are drilled from the buccal surface and one on the mesial

and one of the distal .

Auto-mix technique

Several manufacturers often supply impression material in pre-

package cartridges to which a disposable mixing tip is attached.

The cartridge is inserted in a caulking-gun like device, and the base

and catalyst are extruded into the mixing tip, where mixing occurs

as they progress to the end of the tube.

The homogenously incorporated material can be directly placed on

the prepared tooth and impression tray.

Matrix impression system

This is a new system that requires a series of three impression

procedures using three types and/or viscosities of impression materials.

Steps

A matrix of occlusal registration elastomeric material is made over

the tooth preparation.

Matrix is trimmed to prescribed dimension and retraction cord is

removed.

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A definit ive impression is made in matrix of the preparation with a

low viscosity elastomeric impression material.

After the matrix impression is seated, a stock tray is fi lled with a

medium viscosity elastomeric impression material is seated over the

matrix and remaining teeth to create an impression of the entire

arch.

Advantages

This system effectively controls the four forces (relapsing,

retraction, displacement and collapsing) that impact on the gingiva

during the crit ical phase of making impression when attempting to

register the subgingival margins.

The design of the matrix also gently forces the high viscosity

impression materials along the preparations and in to the sulcus

where it cleanses the sulcus of unwanted debris and fills the sulcus.

The high viscosity material gently extends into the sulcus and does

not permit i t to collapse as the medium viscosity material in the

stock tray a seated for the pick up impression.

The matrix facil itates the formation of the optimum flange.

Tearing is virtually eliminated because of the improved

configuration of the sulcular flange and by the elimination of voids

or contaminants in the sulcus.

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Copper band impression

Fitting copper band to preparation

Select a copper band of correct diameter by trial and error method.

Deform the tubes to semi ell ipsoidal cross section and try in.

Approximate posit ion of the finish line and mark i t on the band with

an explorer and cur with a scissors. Smoothen the rough edges using

a carborundom stone.

Evaluate the fit of the band such that i t extends 1mm beyond the

finish line.

Orientation holes are cut on the top one fifth of facial surface of the

tube.

Make compound plug

Fingers are covered with petrolactum jelly

A warm red stick compound is inserted in the top one third of the

copper tube. The compound should touch the occlusal surface. It is

then cooled and removed by Bachins towel clamp by grasping top

one fifth of the copper band.

Excess is relieved by cutting with a no. 6 or no. 8 round bur and

under slow speed and by frequently removing debris. 0.2mm of

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compound is removed from the impressed occlusal surface. This

creates a space for the heavy body polyvinyl si loxane.

Making an impression

Coat the inner surface with adhesive

Mix heavy body polyvinyl siloxane

Load it in the copper band

Position the finger and on the top of band, orient and seat

customized copper band

Stabilize the band

Wait ti ll the final set. Using towel clip remove the band evaluate

the impression and followed the orientation impression.

Special considerations

Pin retained restorations

Elastomeric impression materials are strong enough to be used in

these situations but should be introduced with

Cement tube or Lentulo spiral to avoid any bubbles

Special nylon bristles or Prefabricated plastic pinscan be put in the

pin hole and the impression is made

Post and cores

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This procedure involves reinforcing the impression with a plastic

pin or suitable wire (e.g. orthodontic wire) – elastomeric impression

materials can be used.

Impression technique with the use of preformed crown shells

Select preformed crowns (polycarbonate) adjust the gingival margin

slightly apical to finish line.

Adhesive applied to inner side

Regular body is loaded

Then seated properly

Wait ti ll i t sets and remove

Then full arch pick up impression made

Impression procedure for a subgingivally prepared shoulder margin

An individual tray is prepared directly with acrylic, which should

not cover the shoulder margin. Gingival retraction done.

The tray is rebased with fluid resin and some placed on the teeth

and pressed ti ll contact is felt wait ti ll the acrylic becomes hard.

An outline is marked and excess is removed. Space is created for

the impression materials except the shoulder margin area.

An adhesive is then applied and the tray fi lled with elastomer. This

impression is then picked up with elastomer or hydrocolloid in a

stock tray.

Impressions for CAD/CAM procedures

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Prepared tooth

Apply rubber dam

Imaging powder + l iquid is coated on the prepared area for photo

receptivity

Symmetrical optic beam is passed

3D picture is produced on a computer for milling procedure

This technique is mostly advocated for the CEREC II system.

Evaluation of impression

Upon removal the impression must be inspected for accuracy.

Bubbles or voids in the margin necessitate discarding the

impression and starting over.

An intact uninterrupted cuff of impression material should be

present beyond every margin.

Streaks of base and catalyst material indicate improper mixing and

may render an impression useless.

Immediately after removal from the mouth impression is rinsed

under tap water and dried with an air syringe.

Disinfection is an essential step for prevention of cross infection and exposure of

laboratory personnel.

Impression material Recommended disinfection procedures1. Reversible hydrocolloid2.Irreversible hydrocolloid3. Polysulfide4. Addition si licone5. Condensation si licone6. Polyether

Diluted bleach, Iodophor sprayDiluted bleach, Iodophor sprayImmersion in iodophor or 2% glutaraldehydeImmersion in 2% glutaraldehyde or hypochloride solut ionImmersion in 2% glutaraldehyde or hypochlorite solutionIodophor spray or chloride dioxide

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Done properly i t has no clinically significant effects on the

accuracy or surface reproduction of the elastomers.

Conclusion

Devan states that the impression should be in the dentists mind before it is in his

hand. Based on this statement I conclude the use of impression material and technique

should be purely the dentists choice.

REFERENCES:

1. Restorative Dental materials:G Craig & John M Powers-

11th edition2002, pg 329-378

2. Phillips science of dental materials:11th edition

3. GPT Academy of prosthodontics JPD,july 2005 vol 94

4. Tylman’s Theory and Practice Of Fixed Prosthodontics, 8th

edition, pg 237-254.

5. Fundamentals Of Fixed Prosthodontics by Herbert T.

Shillingburg, 3rd edition, pg 281

6. Contemporary Fixed Prosthodontics, by Rosenstiel, 4th

edition, pg452- 462.

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7. Notes on dental materials by E.C. Combe, 6th edition,pg

115- 126.

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