LD

IMPRESSIONS

IN

FIXED PARTIAL

DENTURE.

Dr.Sandeep George.

CONTENTS:

1. INTRODUCTION

2. IMPRESSION TRAYS

-TYPES

-REQUIREMENTS

3. TRAY ADHESIVES-PURPOSE-CONTENTS

4. IMPRESSION MATERIALS-TYPES-PROPERTIES

5. GINGIVAL RETRACTION-ROLE-TYPES

6. IMPRESSION TECHNIQUES

7. DISINFECTION OF IMPRESSION

8. CAST POURING

9. CONCLUSION

10. REFERENCES

INTRODUCTION:

Many Impression materials are suitable for use in dentistry. Impression materials are used to

record intraoral structures for the fabrication of definitive restorations. Accurate impressions are

necessary for construction of any dental prosthesis. The relationship between static and mobile

oral structures must be reproduced accurately for an optimum cast. Making a cast in gypsum

materials from an impression of dental anatomy aids dentists in designing and constructing fixed

prosthesis. The accuracy of these final restorations depends greatly on the impression materials

and techniques. The more common types of impressions are used for fabricating diagnostic and

master casts. Diagnostic casts are used to aid in treatment planning while Master casts are used

for producing fixed partial dentures. Accurate impressions depend on identifying the applications

that do or do not fit each material’s characteristics. Materials used without adequate knowledge

of their characteristics can impair a successful outcome. Often, the choice of impression

materials depends on the subjective choice of the operator based on personal preferences and

past experience with particular materials.

The dentist has a host of excellent impression materials available for making impressions in

fixed prosthodontics. With proper material selection and manipulation, accurate impressions can

be obtained for fabrication of tooth supported restorations. An acceptable impression must be an

exact record of all aspects of the prepared tooth. This means it must include sufficient

unprepared tooth structure immediately adjacent to the margins for the dentist and laboratory

technician to identify the contour of the tooth and all prepared surfaces. The contour of the

unprepared tooth structure cervical to the preparation margin is critical information that must be

available when the restoration is fabricated in the dental laboratory. Dent Clin N Am 51(2007)

629-642

All teeth in the arch and the soft tissues immediately surrounding the tooth preparation must be

reproduced in the impression. They will allow the cast to be accurately articulated and will

contribute to proper contouring of the planned restoration. Particular attention is given to

reproducing the lingual surfaces of anterior teeth because they influence anterior guidance,

which determines the occlusal morphology of the posterior teeth. The impression must be free of

air bubbles, tears, thin spots, and other imperfections that might produce inaccuracies.

The patient's mouth is a challenging environment to make an accurate impression. Moisture

control is probably one of the most important aspects of successful impression making. Except

for the polyethers, all elastomeric impression materials are hydrophobic i.e. they do not tolerate

or displace moisture. Any moisture will result in voids. Consequently, saliva flow into the area

must be reduced and diverted to obtain the necessary dry field of operation. (Dent Clin N Am

51(2007) 629-642)

SALIVA CONTROL

Depending on the location of the preparations in the dental arch, several techniques can be used

to create the necessary dry field of operation. In areas where only supra gingival margins are

present, moisture control with a rubber dam is probably the most appropriate method. However,

in most instances a rubber dam cannot be used, and absorbent cotton rolls must be placed at the

source of the saliva. In the maxillary arch, placing a single cotton roll in the vestibule

immediately buccal to the preparation and a saliva evacuator in the opposing lingual sulcus is

usually sufficient. When working on a maxillary second or third molar, multiple cotton rolls

must sometimes be placed immediately buccal to the preparation and slightly anterior to block

off the parotid duct, which opens just anterior to the maxillary first molar. If the cotton roll does

not stay in position, it can be retained with a finger or the mouth mirror.

When making a mandibular impression, placing additional cotton rolls to block off the

sublingual and submandibular salivary ducts is usually necessary. An alternative to multiple

cotton rolls is placement of one long roll "horseshoe fashion" in the maxillary and mandibular

mucobuccal folds. However, when part of the cotton is saturated, the entire roll must be replaced.

The use of moisture-absorbing cards is another method for controlling saliva flow. These cards

are pressed paper wafers covered with a reflective foil on one side. The paper side is placed

against the dried buccal tissue and adheres to it. The tongue can be a problem when working in

the mandibular arch for that a disposable saliva ejector designed to displace the tongue may also

be effective. One more method is by using svedopter, which is a metal saliva ejector attached

with tongue deflector that is used for isolating mandibular teeth when the patients in a near

upright position.

When saliva control is especially difficult, a medication with antisialagogic action may be

considered. Dry mouth is a side effect of certain anticholinergics. This group of drugs includes

atropine, dicyclomine, and methantheline. Anticholinergics should be prescribed with caution in

older adults and should not be used in any patient with heart disease. They are also

contraindicated in individuals with glaucoma, because they can cause permanent blindness. (JPD

1984; 52: 663)

Clonidine, an antihypertensive drug, has successfully reduced salivary output. It is considered

safer than anticholinergics and has no specified contraindications. However, it should be used

cautiously in patients who take hypertension medication.

IMPRESSION TRAYS:

Impression tray is a device which is used to carry, confine and control an impression material

while making an impression. Commonly used trays are Stock metal trays and Custom trays. The

other types of trays available are Rimlock trays, metal water coolant trays for reversible

hydrocolloid impression and Dual arch impression trays.

Classification of Impression Trays:

A. Based on whether they are prefabricated or individualized:

1. Stock trays

2. Custom trays

B. Depending on presence or absence of perforations:

1. Perforated

2. Non perforated

C. Depending on whether they are meant for dentate or edentate individuals:

1. Dentulous trays

2. Edentulous trays

3. Combination trays are also available for various partially edentulous situations based

on kennedy’s classification.

Stock trays are readily available and are rigid. They are of two types, Perforated and non

perforated trays. They are also available in Metal and Plastic materials. Stock trays are used for

taking putty-wash impression. For alginate, a perforated tray is preferred in order to retain the

impression. Non perforated trays may also be used but only in conjunction with an adhesive.

Non perforated trays are preferred for impression compound.

Advantages -

• Eliminates the time & cost of fabricating custom tray

• Metal stock trays are rigid & less susceptible to distortion

Disadvantages-

• Metal trays must be sterilized

• More impression material required so thickness of the impression will be uneven

Custom trays (Individualized tray/special tray) are needed for atypical arch forms where an

appropriate stock tray is not available. Custom tray improves the accuracy of an elastomeric

impression by limiting the volume of the material, thus reducing stresses during removal and

thermal contraction. Although reducing the bulk of an elastomeric impression material increases

its accuracy, the opposite is true for reversible hydrocolloid impressions. Generally a custom tray

is made from autopolymerizing acrylic resin, although thermoplastic or photo polymerized resins

are sometimes used. Thermoplastic materials can be softened in a water bath and adapted either

manually or with a vacuum former with a heating element. The accuracy of impressions made

with a thermoplastic tray material or light-polymerized materials is comparable to that made with

an autopolymerized resin. Light-polymerized materials are convenient because a storage period

is not needed for the completion of polymerization. (JPD 1999; 82:714-5)

Requirements:

1. Should be rigid

2. Should be dimensionally stable

3. Should be easy to fabricate.

Advantages:

• Less impression material is required than for stock tray.

• Sterilization is not a big issue(single use)

• Less distortion of the impression.

Disadvantages:

• Time consuming

• The tray should age for 24 hours to minimize distortion

Procedure of custom tray fabrication:

1. Using a pencil, mark the border of the tray on the diagnostic cast approximately 5 mm apically

to the crest of the free gingiva. Maxillary trays do not always necessitate covering the entire

palate, although this may be desirable if a removable appliance is planned after completion of the

fixed prostheses. Under no circumstances should the posterior border extend farther than the

demarcation between hard and soft palates.

2. Adapt a wax or other suitable spacer onto the diagnostic cast. Two layers of baseplate wax

will result in a combined thickness of approximately 2.5 mm.

3. Soften the wax by carefully heating it over a Bunsen burner or in hot water. Overheating may

melt it and produce an undesirable thin spot. Only light pressure should be applied.

4. After the second sheet of wax has been applied, trim it back until the pencil line is just visible.

An alternative technique involves repeated dipping of the cast in molten wax. The cast is

thoroughly wetted and then dipped three or four times to obtain a sufficient and uniform wax

thickness (about 2 or 3 mm). This creates the space needed for the impression material. Three

stops are needed in the tray to maintain even space for the impression material in the oral cavity.

These are placed on noncentric cusps of teeth that are not to be prepared (buccal cusps of the

maxillary teeth and lingual cusps of the mandibular teeth). If all teeth are involved, a larger soft

tissue stop can be placed on the crest of the alveolar ridge or in the center of the hard palate.

Stops are made by removing wax at an angle of 45 degrees to the occlusal surfaces of three teeth

that have a tripod arrangement in the arch. This will lend stability to the tray, and the 45-degree

slope will help center the tray during insertion.

5. Apply a layer of tinfoil over the wax (which may melt from the polymerization heat of the

material) to prevent it from contaminating the inside of the tray.

6. Mix autopolymerizing acrylic resin according to the manufacturer's recommendations. The use

of vinyl gloves is recommended to prevent the development of sensitivity to the monomer.

7. After the resin is mixed, set it aside until it is doughy (with the consistency of putty). A

template or a wooden slab and roller may help obtain a consistent thickness, although with

practice the resin can be thinned out accurately by hand. Care must be taken not to stretch the

material when manipulating it; thin areas in the resin may lead to a flexible tray and produce

distortions.

8. Gently adapt the resin to the cast. A handle made from the excess resin can be attached at this

time. If working time is unavailable, it can also be attached later with a separate second mix of

acrylic resin. Buccal ridges, which are helpful with impression removal, can also be added.

9. After the material has polymerized, remove it from the cast and trim it with an acrylic-

trimming bur where the indentation made by the wax ledge is visible. All rough edges should be

rounded to prevent soft tissue trauma.

10. If necessary, fill defects in the stops with additional resin, wetting the set tray material with

monomer to ensure a good bond. To prevent the material from lifting up, some pressure should

be maintained during this phase.

Custom tray fabrication: A, Armamentarium

B. Outlining the border of the tray on the diagnostic cast. C. The tray should extend 3 to 5 mm from the gingival margin and about 3 mm beyond the most distal tooth. D. Softened baseplate wax is adapted to form a spacer. Typically two thicknesses will provide the recommended 2 to 3 mm of space. E. Spacer is trimmed to the pencil line.F. Wax is removed to form the tray stops. G, Covered with tinfoil. H, The foil is adapted to the stops.

I. Custom tray resin.J. While it is still doughy, the resin is molded to a horseshoe shape (semicircle for maxillary trays).K. Wooden slab and roller used in an alternative method.L. The resin is gently adapted to the cast, and the excess is trimmed. M and N, Resin is moistened with monomer to attach the handle. O. When the resin has cured, the periphery is shaped with an acrylic-trimming bur.

P. Maxillary and mandibular custom trays.

'Rimlock' Impression Trays offer dentists the chance to produce ideal impressions in a solid,

non-flexible tray, without the need to use impression adhesives. Rimlock trays incorporate a

unique 'beaded' edge that retains the impression material securely. They are of high quality

stainless steel that can be autoclaved. These are recommended for Putty wash and alginate

impressions. Its unique locking assures secure retention of material. Rim-Lock trays are available

in different sizes and arch configurations to suit practitioner’s needs and preferences.

Dual arch trays: The types of dual arch impression trays can be categorized as metal or plastic

with side walls or sideless and by the amount and location of the arch enclosed by the tray.

Metal trays are rigid and will not be deformed during the impression procedure. Some metal

trays are curved to approximate arch form while others are straight. Some trays can be bent to

more closely adapt to the particular arch form while others are too rigid. One type of metal tray

is disposable and has a fixed insert and very short side walls.

Plastic trays are flexible to varying degrees, depending on the shape and dimensions of the side

wall. So called sideless trays actually have very short side walls. They are the most flexible tray

design. One objective of this design feature was to produce a tray that would have the least

chance of impinging on anatomic structures thus avoid being deformed or displaced during the

impression procedure. Second objective of this tray design was to avoid problems with elastic

rebound from putty impression materials that are placed under loading by being forced between

the teeth, arch and walls of the tray during the impression procedure. Plastic trays come in many

sizes and shapes. Side walls can be long and made of thick, heavy plastic and are relatively

inflexible. Others have walls that are shorter or made of more flexible plastic. However none of

the plastic trays is as rigid as metal trays.

Dual arch trays can also be categorized by the amount and location of the arch enclosed by the

tray. These categories include posterior sextant, three quarters of an arch, anterior sextant and

full arch. (JADA 1996; Vol 127:234-240)

With any system, tray rigidity is important, because even slight flexing of the tray will lead to a

distorted impression. For this reason, thin, disposable plastic trays are unacceptable. Resin

thicknesses of 2 to 3 mm are needed for adequate rigidity. It should extend about 3 to 5 mm

cervical to the gingival margins and should be shaped to allow muscle attachments. It should be

stable on the cast with stops that can maintain an impression. Clearance between the tray and the

teeth should also be 2 to 3 mm; however, greater clearance is necessary for the more rigid

polyether materials. To avoid distortion from continued polymerization of the resin the tray

should be made at least 9 hours before its use. When a tray is needed more urgently, it can be

placed in boiling water for 5 minutes and allowed to cool to room temperature. Direct contact of

custom or stock tray with gingival tissue in the critical portion of the impression can result in a

failed impression. (JPD 1990; 63:12-5)

TRAY ADHESIVES:

Although stock trays often provide mechanical retention for elastomeric impression materials,

manufacturers typically recommend the use of an adhesive, whether a stock or custom tray is

used. The adhesive is applied to the impression tray and allowed to dry. Although minimal

information about adhesive composition for elastomeric impression materials is available, more

accurate and consistent impressions are obtained when adhesives are used. The minimally

required strength for adhesion between the elastomeric impression material and tray to prevent

debonding is not known. However, clinical experience has demonstrated that a strong bond is

desired to prevent undetected inaccuracies in impressions that contribute to non-fitting

restorations. (JPD 2005; 94: 209-13)

However an impression made with a highly accurate material can be rendered absolutely useless

if it detaches from the tray, as the material is withdrawn from contact with the oral tissues.

During this withdrawal the bond between the impression material and the tray is highly stressed

in both tension and shear and often the adhesive is the main factor resisting these stresses. So it is

extremely important that the adhesive bond between the impression material and tray should be

of sufficient strength to withstand the forces generated during removal of the set material and

this bond is a critical factor in the production of a satisfactory final restoration. (Journal of oral

rehab 1997; 24: 63-69)

The adhesives furnished with the various types of rubber impression materials are not

interchangeable. Adhesives employed with the poly-sulfide rubber impression materials include

butyl rubber or styrene/acrylonitrile dissolved in a suitable volatile solvent such as chloroform or

a ketone. The base for the adhesive employed with the silicone rubber materials may contain

poly dimethyl siloxane or a similar reactive silicone, and ethyl silicate. The poly dimethyl

siloxane acts as an adhesive for the rubber, and hydrated silica forms from the ethyl silicate to

create a physical bond with the impression tray.

Available in two types:

1. Paint on adhesives Eg Coltene, Kerr Universal VPS.

2. Spray adhesives Eg Sili spray.

Applied for 10-15 minutes before making impression

Blockout technique for impressions of teeth with increased open gingival embrasures:

The gingival embrassure that is normally filled with gingival papilla can become patent and

enlarged. The cause of these enlarged spaces is varied and can result from increased crown

length after periodontal surgical treatment, increased gingival recession, supra-eruption, or

improperly aligned and/or angulated teeth. Accurate impressions of teeth with these undercut

gingival embrasures can be a challenge. An impression with these clinical conditions can tear

when removed or result in a distorted impression and cast.

A common method to minimize the effect of undercuts has been to fill these areas with wax or an

alternative material, such as Oraseal caulking material or Cavit temporary stopping material This

method prevents tears of the impression and the potential distortion. Waxes and other commonly

selected materials for blockout can be swiftly placed intraorally, but they are easily displaced and

could result in distortion. (JPD 1999: 82: 100-2 )

IMPRESSION MATERIALS:

There is an extensive variety of materials for making a precision negative mold of soft and hard

tissues (JPD 1986; 56: 4). In order of their historical development, the impression materials used

in fixed partial denture construction include:

1) Aqueous Hydrocolloids

a) Agar (reversible)

b) Alginate (irreversible)

2) Non-aqueous Elastomers

a) Polysulfide

b) Silicones

- Condensation

- Addition

c) Polyether

d) Hybrid Elastomers

Each material has advantages and disadvantages, and none is entirely free of shortcomings.

However, they all share one important characteristic: when handled correctly, they can produce

casts of sufficient accuracy and surface detail for the fabrication of clinically acceptable fixed

prostheses.

Nevertheless, there are reasons for selecting one material over another: If it becomes necessary

to store the impression before a cast will be made, the polyethers and addition silicones are

preferable because they exhibit sufficient long-term dimensional stability, the other materials,

particularly the reversible hydrocolloids, must be poured immediately. If the impression will be

poured in epoxy or will be electroplated, reversible hydrocolloid should not be selected because

it is compatible only with die stone.

Depending on the manufacturer, many of the materials are available in cartridges for automixing

and tubes or containers for hand spatulation. The automixing products require no mixing pads or

spatulation, and training in their use is less time consuming. There may be less waste of material

associated with automixing and provide more bubble-free mix resulting in more accurate casts.

Ideal characteristics of impression materials:

An ideal impression material should exhibit certain characteristics in the clinical and laboratory

environment. Clinically it should produce an accurate impression secondary to its adaptability to

oral structures, have a consistency that is dimensionally stable to resist tearing but results in an

atraumatic removal, set within a reasonable amount of time, demonstrate biocompatibility to

include a hypoallergenic nature, and have a reasonable cost per use. In a laboratory setting, it

should be dimensionally stable for accurate pouring of multiple casts and should not affect

dimensional accuracy upon disinfection.

Reversible Hydrocolloid (Agar):

Reversible hydrocolloid (also called agar hydrocolloid or simply hydrocolloid) was originally

derived as a natural product of kelp. However, the material currently available is considerably

different.

History:

• In 1925, Alphous Poller of Vienna was granted a British patent for a totally different type

of impression material.

• Later Poller's 'Negacoll’ was modified and introduced to the dental profession as

'Dentacol’ in 1928.

• In 1937 introduced by Sears, Agar was first into dentistry for recording crown

impressions and was the first elastic impression material available.

Example

• Slate Hydrocolloid (Van R)

• Superbody

• Super syringe

Composition:

• Fillers; diatomaceous earth, clay, silica, wax, rubber, and similar inert powders.

• Thymol and glycerine act as plasticizer and bactericide.

If poured immediately, reversible hydrocolloid produces casts of excellent dimensional accuracy

and acceptable surface detail. At elevated temperatures, it changes from a gel to a sol. This

change is reversible-i.e., as the material cools; the viscous fluid sol is converted to an elastic gel.

Agar changes from gel to sol at 99° C (210° F) but remains a sol as low as 50° C (122° F),

forming a gel only slightly above body temperature. These unique characteristics (hysteresis) are

very favorable for its use as an impression material.

Reversible hydrocolloid is supplied in a range of viscosities. Generally a heavy-bodied tray

material is used with a less viscous syringe material. The required temperature changes are

effected with a special conditioning unit and water-cooled impression trays.

Advantages:

• Accurate reproduction of surface detail

• Hydrophilic

– displace moisture, blood, fluids

• Inexpensive

– after initial equipment

• No custom tray or adhesives

• Pleasant

• No mixing required

Disadvantages:

• Initial expense where special equipment is needed

• Material prepared in advance

• Thermal shock to the patient.

• Tears easily

• Dimensionally unstable

– immediate pour

– single cast

• Difficult to disinfect

This impression technique requires a special conditioning unit which consists of 3

thermostatically controlled water baths.

1. A liquefaction bath.

2. A storage bath.

3. A tempering bath.

Procedure:

Select the correct size of water cooled impression trays. Place small modeling compound or

prefabricated stops in the tray to prevent over seating. Fill the impression tray with heavy

bodied material from the storage bath and place it in the tempering bath. Load the syringe

material in the syringe and replace it in the storage bath. Now carefully remove the retraction

cord and flood the tooth with water. Inject the syringe material on the tooth and cover the

entire tooth. Remove the impression tray from the bath wipe with the gauze and place it in

the mouth. After seating cold water is circulated through the tray until the impression

material is set. Remove the impression with a rapid motion, wash with cold water and

evaluate for accuracy.

Irreversible Hydrocolloid (Alginate):

At the end of the 20th century, a chemist from Scotland noticed that certain brown seaweed

(algae) yielded a peculiar mucous extraction. He named it algin. This was later identified as a

linear polymer with numerous carboxyl acid groups and named anhydro-β-d-mannuronic acid

(also called alginic acid).

When the agar impression material became scarce because of World War II (Japan was a prime

source of agar), research was accelerated to find a suitable substitute. This result was present

alginate impression material. The general use of irreversible hydrocolloid far exceeds that of

other impression materials available because it is easy to manipulate, it is comfortable for the

patient and relatively inexpensive also.

Examples:

– Jeltrate (Dentsply/Caulk)

– Coe Alginate (GC America)

– Integra

– Superjel

– Tropicalgin

_ Xantalgin

Classification of alginate:

I. According to setting time.

• Fast set 1.25 – 2min

• Regular set 3 – 4.5 min

II. According to concentration of filler

• Soft set

• Hard set

Composition:

Component Function Weight percentage

Na / Potassium alginate Soluble alginate 15

Calcium sulfate dihydrate Reactor 16

Potassium titanium

Fluoride

Accelerator 3

Zinc oxide Filler particles 4

Diatomaceous earth Filler particles 60

Sodium phosphate Retarder 2

Modified alginates:

1. Alginate in the form of sol containing the water but no source of calcium ions

Two component system (paste form)

- alginate sol

- calcium reactor

2. Alginates modified by the incorporation of silicone polymers (paste form) that results in fine

detail reproduction, good tear resistance and good dimension stability

3. Dustless alginate: Glycerin is incorporated to agglomerate the particles making the powder

denser so that no dust is formed when the lid is opened after tumbling.

4. Newer alginates: MILLENIUM ALGIN is considered an alginate of new generation with

maximum preciseness of 15 µ. The formulation was optimized to reduce natural contraction with

water loss. The impression can be stored for many hours in a Long Life Bag before casting the

model without undergoing any change for over 100 hours. Here no hand mixing is required so

that it saves time and material. Delivery options is by Cartridge or automix dispensing and

impressions remain stable where pouring can be done immediately or when it is convenient.

Impressions retain dimensional stability for months.

Manipulation:

Fluff or aerate the powder by inverting the can several times. This ensures uniform distribution

of the filler before mixing. Then measured amounts of powder and water are taken as specified

by the manufacturer. Mixing is done in a clean flexible plastic bowl with a clean wide bladed,

reasonably stiff metal spatula and it is better to use separate bowls for plaster and alginate as

plaster contamination can accelerate setting. The mixing is started with a stirring motion to wet

the powder with water. Once the powder has been moistened, rapid spatulation by swiping or

stropping against the side of the bowl is done or vigorous figure-eight motion can also be used.

• Can also be mixed with MIGMA™ Alginate Mixing Machine

Properties:

Reproduction of Tissue Detail

• ADA specification requires the material to reproduce a line that is 0.075 mm in width.

• Detail reproduction is lower when compared to agar hydro-colloid.

Dimensional Stability:

• Set alginates have poor dimensional stability due to evaporation, syneresis and

imbibition. Therefore, cast should be poured immediately.

• If storage is unavoidable, keeping in a humid atmosphere of 100% relative humidity

(humidor) results in the least dimensional change.

Elasticity and Elastic Recovery

• Alginate hydrocolloids are highly elastic but less when compared to agar and about

97.3% elastic recovery occurs.

• Thus permanent deformation is more for Alginate (about 1.2%).

Tear strength:

• Varies from 350-700 gm/cm2

• Increased when the impression is removed with a snap

Compatibility:

• Pouring of stone mixture to fill the impression should start from one end of the arch.

After the impression has been filled with stone, it may be placed in either a humidor or a

2% potassium sulfate solution while the stone hardens in an atmosphere of approximately

100% relative humidity.

• Cast should be kept in contact with the impression, preferably for 60 min or minimum 30

min.

• If the cast is allowed to remain in contact with the hydrocolloid impression overnight, a

chalky stone surface may be produced.

Advantages:

• Inexpensive

• Easy to use

• Hydrophilic

– displace moisture, blood, fluids

• Can be used in stock trays

Disadvantages:

• Poor tear resistance

• Dimensionally unstable

– Should be poured immediately within 10-12 minutes.

– Only single pouring possible.

• Lower detail reproduction

- Unacceptable for working cast of fixed prosthodontics but is used for partial frame

work impressions.

• High permanent deformation

• Difficulty in sterilization and disinfection

Non aqueous elastomeric impression materials:

These are synthetic rubbers that mimic natural rubber and initially called as rubber impression

materials. Currently it is referred as elastomers or elastomeric impression materials. ADA

Specification no. 19 identifies as “nonaqueous elastomeric dental impression materials”

Based on viscosity it is classified as:

• low, medium, high, putty

• Single phase or monophase – single component material with sufficient shearing

potential that it can be used as the syringe material and the tray material.

Polysulfide

• The is the first elastomer used for dental impressions

• Sometime it is referred as :

– By the type of material, such as rubber base impression material

– By the processing terminology, such as vulcanizing impression material

– By chemistry, such as mercaptan impression material or

– By the name of one of the first manufacturers, such as the Thiokol Corporation.

Polysulfide is supplied as Paste in collapsible tubes as base and accelerator where Base is white

and accelerator is brown or grey in color.

Examples:

– Permlastic (Kerr)

– Omni-Flex (GC America) – copper hydroxide system.

– Coe-flex (GC-Amer)

– Neo-plex (Miles)

Composition:

1. Base

Polysulfide polymer

Lithopone and titanium dioxide filler

Dibutyl phthalate plasticizer

Small quantity of sulfur

2. Catalyst or accelerator paste or reactor-

Lead dioxide,

Plasticizer,

Oleic and stearic acid as Retarders

Available in 3 viscosities,

• Light body, medium body and heavy body.

Tray adhesive:

• Butyl rubber or styrene/ acrylonitrile dissolved in a volatile solvent such as chloroform or

a ketone

Manipulation:

With the proper lengths of the two pastes squeezed onto a mixing pad or glass slab, the

catalyst paste is first collected on a stainless steel spatula and then distributed over the

base, and the mixture is spread out over the mixing pad. The mass is then scraped up with

the spatula blade and again smoothed out. If sufficient force is applied and spatulation is

performed rapidly, the material will seem thinner and easier to handle and this

phenomenon is known as pseudoplasticity. Most accurate impressions are made using

custom acrylic trays because of uniform thickness of the material.

Mean working time

• 6.0 min at 23 degree


Mean setting time

• 16 min at 23 degree


Elasticity improves with time, so longer the impression can remain in the mouth before removal, greater

will be the accuracy. Polysulfide ranks as one of the least stiff of the elastomeric impression materials.

Unset material has high level of viscosity. This thick consistency of the uncured material helps displace

an unwanted fluid present while seating the impression. Also, the excess material extruded from the

tray does not flow easily because of the high viscosity, reducing the potential discomfort to the patient

during seating of a tray.

Advantages Disadvantages

Long working time Requires a custom tray

Proven accuracy Must be poured in stone immediately

High tear resistance Potential for significant distortion

Less hydrophobic Odor offends patients

inexpensive to use Messy and stains clothes

Long shelf life Second pour is less accurate

Condensation silicone:

It is also known as conventional silicone. In condensation reaction, polymerization reaction in

which the polymer chains all grow simultaneously and a reaction byproduct is formed.

Available in 3 viscosities

– Light body

– Medium body

– Putty body

Paste – two pastes in collapsible tubes

Putty – jars

Examples

– Speedex (Coltene/Whaledent)

– Primasil (TISS Dental)

– Accoe (GC-Amer)

– Xantopren (Unitek)

– Elasticon (Kerr)

– Cuttersil (Miles)

Composition:

The base is a paste form, which is mainly Polydimethyl Siloxane.

The catalyst is of low-viscosity liquid to which colloidal silica filler is added to form a paste.

The polymerization occurs as a result of cross linkage between the orthoethyl silicate and the

terminal hydroxy group of the dimethyl siloxane, to form a three dimensional network. The

reaction is exothermic (1deg C. rise).

Properties

• Setting time is 8-9 minutes

• Mixing time is 45 sec

• Tear strength 3000gm/cm lower than polysulfide

• Hydrophobic – area should be dried. Avoid air bubbles while pouring the cast

• It is stiffer and harder than polysulfide. The hardness increases with time.

• The spacing in the tray is increased to 3mm to compensate for the stiffness.

• Can be plated with silver/ copper. Silver plating is preferred.

• Shelf life is slightly less than polysulfides due to the unstable nature of the orthoethyl

silicates.

Advantages:

• Better elastic properties

• Clean, pleasant

• Stock tray

- putty-reline

• Good working and setting time

Disadvantages:

• Permanent deformation – high – 1-3%

• Poor dimensional stability

– high shrinkage due to polymerization and evaporation of ethanol

– pour immediately within 30 minutes

• Hydrophobic that cause poor wettability

Addition Silicones:

They are frequently known as polyvinylsiloxane or viny polysiloxane impression materials.

Supplied in 4 viscosities:

– Light body

– Medium body

– Heavy body

– Putty

• Examples

– Extrude (Kerr)

– Express (3M/ESPE)

– Aquasil (Dentsply Caulk)

– Genie (Sultan Chemists)

Virtual (Ivoclar Vivadent

Composition:

Base contains polymethyl hydrogen Siloxane and filler.

Catalyst paste contains divinyl Polydimethyl Siloxane, platinum catalyst and filler.

Improper balance between the vinyl siloxane and silane siloxane contribute to the

liberation of hydrogen gas and to overcome this manufacturer adds palladium (scavenger)

or wait for an hour before pouring the impression.

Properties:

• Setting time – 5-9 min

• Mixing time – 45 sec

• Best dimensional stability among elastomers.

• Curing shrinkage 0.17%

• Permanent deformation 0.05-0.3%

• Tear strength -3000gm/cm

• Extremely hydrophobic

• Electroplated with silver or copper.

• Low flexibility

• Harder than polysulfide – extra spacing 3 mm should be provided in the impression tray.

• Care should be taken while removing the cast from the impression to avoid any breakage.

• Shelf life ranges from 1-2 years

Advantages:

• Excellent reproduction of surface details

• Highly accurate

• High dimensional stability

– pour up to one week

• Stock or custom trays

• Multiple casts

• Easy to mix

• Pleasant odor

Disadvantages:

• Expensive

• Polyvinyl siloxane contamination is usually a result of sulfur or sulfur compounds usually

seen in dental office in the form of latex gloves or rubber dams. Sulfur as a reactive

element may interfere with the silicon polymerization with the chlorplatinic acid catalyst

in VPS impression material. The sulfur containing chemical zinc diethyl dithiocarbamate

used during latex glove fabrication is a preservative and vulcanizing accelerator. (J

Prosthet Dent 2005; 93: 433-8)

• Short working time

• Lower tear strength

• Possible hydrogen gas release that can lead to

– bubbles on die

– palladium added to absorb

– Moisture control – impression making

Modifications in PVS:

• Adding palladium scavenger to tie up any hydrogen gas

• Less hydrophobic, due to surfactant that has been added to overcome this drawback.

Most commonly, nonionic surfactants are used and various surfactant sprays are also

available. The mode of action is believed to be a diffusion controlled transfer of

surfactant molecules.

• HYDROFLEX having early wetting property and superior elasticity and tear strength.

Polyether:

First material introduced for dental impressions and introduced in Germany in late 1960s. It is

available as base and accelerator in collapsible tubes. The accelerator tube is usually smaller and

earlier it was supplied in single viscosity. A third tube containing a thinner was also provided.

• Available in 3 viscosities

– Light body

– Medium body

– Heavy body

• Examples

– Impregum F (3M/ESPE)

– Permadyne (3M/ESPE)

– Pentamix (3M/ESPE)

– P2 (Heraeus Kulzer)

– Polygel (Dentsply Caulk)

Composition:

Base contains Polyether polymer, colloidal silica as filler and a plasticizer such as phthalate.

Catalyst paste contains the alkyl aromatic sulfonate, a colloidal silica filler and phthalate

plasticizer.

Polyether based polymer that is cured by the reaction between aziridine rings and the main chain

is a copolymer of ethylene oxide and tetrahydrofuran. Cross linking is brought about by an

aromatic sulfonate ester and it is an exothermic reaction of 4-5deg C.

Properties:

• Elasticity – stiffest of impression materials

– New formulations of medium or regular body are less stiff.

– When used in a thinner section decreases the stiffness.

• Impressions must be kept in cool, dry environment.

• Sulfonic ester may cause skin reaction. To avoid this, mix thoroughly before making an

impression and direct skin contact should be avoided.

• Setting time is around 8.3 min.

• Mixing time - 30 sec.

• Heat increases the setting time.

• Is extremely stiff – flexibility 3%

• Its hardness is higher than polysulfides and increases with time. Removing it from

undercuts is difficult, so extra spacing 4mm should be given.

• Tear strength is good 3000gm/cm

• Hydrophilic – so moisture in the impression field is not so critical. It has the best

compatibility with stone.

• Can be electroplated with silver or copper.

• Shelf life > 2 years.

Advantages:

• Highly accurate

• Good dimensional stability

• Least hydrophobic

• Good surface detail

• Pour within one week

– kept dry

• Multiple casts

• Good wettability

• Good shelf life

Disadvantages:

• Expensive

• Short working time

• Rigid so that it is difficult to remove from undercuts

• Bitter taste

• Low tear strength

• Absorbs water that changes dimension and leaches its components.

Modifications:

• Reducing the stiffness and producing polyether in low and heavy viscosities have been

the major changes.

Mixing systems:

Three types of mixing systems can be used for elastomeric impression materials:

1. Hand Mixing

2. Auto Mixing

3. Mechanical Mixing.

Hand mixing:

Equal length of catalyst and paste are dispensed on paper pad/glass slab and the mixture

is spread up with the spatula blade until a uniform color is obtained.

All the 4 types of elastomers are available for mixing in this fashion.

In case of putties, the best mixing technique is to knead the material with base fingers.

Automixing:

Here base and catalyst are in separate cylinders of the plastic cartridge that is placed in a

mixing gun.

The gun has 2 plungers advanced by ratchet mechanism to extrude equal quantities of

base and catalyst.

The 2 components are forced through mixing tip that consists of internal spiral resulting

in a uniform mix.

Mechanical mixing:

The catalyst and base supplied in plastic bags housed in a cartridge is inserted into the top

of mixing machine.

When the button is depressed, uniform mix is available from the plastic mixing tip that is

placed on front of the machine.

Comparison of Properties of impression materials:

Working time:

- longest to shortest

Agar > polysulfide > silicones > alginate = polyether

Setting time:

- shortest to longest

Alginate < polyether < agar < silicones < polysulfide

Stiffness:

- most to least

Polyether > addition silicone > condensation silicone > polysulfide = hydrocolloids

Tear strength:

- greatest to least

Polysulfide > addition silicone > polyether > condensation silicone >> hydrocolloids

Cost:

- lowest to highest

Alginate < agar = polysulfide <condensation silicone < addition silicone < polyether

Dimensional stability:

- best to worst

Addition silicone > polyether > polysulfide > condensation silicone > hydrocolloid

Wettability:

- best to worst

Hydrocolloids > polyether > hydrophilic addition silicone > polysulfide > hydrophobic

addition silicone = condensation silicone

Compatibility with gypsum products:

- best to worst

Hydrocolloids > hydrophilic addition silicone > polyether > polysulfide > hydrophobic

addition silicone = condensation silicone

DISPLACEMENT OF GINGIVAL TISSUES:

Tissue displacement is commonly needed to obtain adequate access to the prepared tooth to

expose all necessary surfaces, both prepared and not prepared. This is most effectively achieved

by placement of a displacement cord (generally impregnated with a chemical agent). Sometimes

gingival tissue is excised with a scalpel or with electrosurgery and must be done without

jeopardizing periodontal health but gingival retraction can cause recession and damage to free

gingiva.

Displacement Cord: Some enlargement of the gingival sulcus can be obtained by placing a non-

impregnated cord (mechanical) and leaving it in place for a sufficient length of time. The cord is

pushed into the sulcus and mechanically stretches the circumferential periodontal fibers where

single cord technique is used for 1-3 prepared teeth with healthy gingival health and double cord

technique is used for multiple prepared teeth with compromised gingival health. Better sulcus

enlargement can be achieved with a chemically impregnated cord (chemo mechanical) or by

dipping the cord in an astringent (e.g., Hemodent) where these materials contain aluminum or

iron salts and cause a transient ischemia, shrinking the gingival tissue. Even so, the sulcus closes

quickly (less than 30 seconds) after the cord is removed; therefore, the impression must be taken

immediately. A series of knitted and twined gingival retraction cords was introduced that was

impregnated with epinephrine or aluminium sulphate. Placement is often easier if a braided (e.g.,

Gingibraid)* or a knitted (e.g., Ultrapak) cord is used. Epinephrine should be used with caution,

because it may cause a tachycardia," particularly if it is placed on lacerated tissue. Unfortunately

retraction with chemically impregnated cord is not always sufficient for the reversible

hydrocolloid impression technique, therefore electro surgery is recommended. (JPD 1999; 81:

258-61)

Electro surgery: An electro surgery unit is used for minor tissue removal before impression

making. In this technique, the inner epithelial lining of the gingival sulcus is removed, thus

improving access for a sub gingival crown margin and effectively controlling postsurgical

hemorrhage. Unfortunately, there is the potential for gingival tissue recession after treatment.

An electro surgery unit works by passage of a high-frequency current (1 to 4 million Hzt)

through the tissue from a large electrode to a small one. At the small electrode, the current

induces rapid localized polarity changes that cause cell breakdown. It is contraindicated on

patients with any electronic medical device (e.g., a cardiac pacemaker, TENS unit, insulin

pump)" or patients with delayed healing as a result of debilitating disease or radiation therapy

and it is not suitable on thin attached gingiva (e.g., the labial tissue of maxillary canines).

(JPD1978; 40: 304)

Another method of tissue removal which is comparable to electrosurgery, is use of a 12-fluted

flame shaped carbide bur. This method can provide adequate retraction for reversible

hydrocolloid impression techniques with less damage to the gingival tissues. Introducing copper-

tube or resin-coping procedure also keep the tissue retracted until complete set of impression

materials. (fig.7) (JPD Feb. 1985: vol 53:num2)

Retraction cord procedure seems to be less traumatic when compared to electrosurgery or

rotatory gingival curettage and lowers the risk of gingival recessions caused by the impression.

Various materials used are:

1. Heavy weight Rubber dams.

2. Aluminum shell.

3. Mechanical Pack of Zinc oxide eugenol.

4. Rolled cotton or synthetic cord.

Recent Advances:

1. Gingifoam.

2. Expasyl TM.

3. Affinis/Magic foam cord.

4. Meroce1

5. Gel-cord.

6. Stay-put retraction cord.

7. Comprecap.

8. Z-twist weave

9. Lasers.

IMPRESSION TECHNIQUES:

1. Copper tube and resin coping method.

2. Monophase / Single viscosity technique

3. Dual viscosity technique/Multiple mix single step technique

4. Putty wash technique

a. Single stage procedure/Simultaneous/Squash technique

b. Two stage procedure

c. Injection moulded technique

5. Dual arch impression technique

a. Monophase impression

b. Multiple mix technique

c. Laminar impression technique

d. Hydraulic pressure technique

6. Segmental impression technique

7. Matrix impression system

8. Wet Technique/ Reversible hydrocolloid technique

9. Hydrocolloid Laminate technique

10. Impression using preformed crown shells

11. Functional check-bite impression

COPPER TUBE / RESIN COPING SYSTEM

It is a modified method of making impression with impression compound. Here rigid carrier of

impression material becomes the part of impression. It is a retraction less procedure where

retraction cords are not used.

A copper band is selected and annealed by heating on a flame and quenching in alcohol and

mark the finish line with the explorer and round off the edges. Then evaluate the fit and cut

orientation hole in top one fifth of the facial surface. Now red stick compound is heated over the

Bunsen flame and the warm compound mass is inserted to fill approximately the top third of the

copper band then seat and orient on to preparation, and compress excess in to the band. The

compound should just touch the occlusal surface. Remove and evaluate the impression; only the

occlusal surface should be impressed and remove 0.2 mm of the compound to create space for

the heavy body poly vinyl silicone. Drill a hole through the centre of the compound plug.

For making the impression, Make 4 to 5 holes evenly distributed above the bottom of the copper

tube for the retention of the impression material and coat the internal surface sparingly with

adhesive .Clean and isolate the preparation and mix heavy viscosity material and inject in to the

band and position it on the tooth and wait the material to set remove the band and inspect the

impression. (JPD 1971; 26(2): 154-58)

MONOPHASE / SINGLE VISCOSITY TECNIQUE

In single viscosity, impressions are often taken with medium-viscosity impression materials of

polyether and addition silicone with resin custom tray of 2-4 mm spacing. Only one mix with

part of the material loaded in tray and another portion loaded in the syringe. When medium-

viscosity material is forced through the syringe the viscosity is reduced (shear thinning effect)

whereas viscosity of same material in the tray is unaffected. Success depends on the

PSEUDOPLASTIC properties of these two materials.

Advantages:

Easy to use

Simple technique

Excellent handling properties,

Accuracy

Medium viscosity material in a custom tray with 3mm spacer

Useful in cases where utmost accuracy is not needed

Disadvantages:

More polymerization shrinkage than heavy body

Surface reproduction is not very good

Selection of tray

In single viscosity technique resin custom trays are used.

Preparation of custom tray:

Adapt a wax or other suitable spacer to the diagnostic cast. Two layers of baseplate wax will

result in a combined thickness of approximately 2.0 to 4.0mm. Soften the wax by carefully

heating it over a Bunsen burner or in hot water. Three stops are needed in the tray to maintain

even space for the impression material in the oral cavity and these are placed on noncentric cusps

of teeth that are not to be prepared. Mix autopolymerizing acrylic resin according to the

manufacturer's recommendations and gently adapt the resin to the cast. A handle made from the

excess resin can be attached at this time. After the material has polymerized, remove it from the

cast and trim it with an acrylic-trimming bur.

Making final impression:

Impressions are often taken with medium-viscosity impression materials of polyether and

addition silicone with resin custom tray. One mix with part of the material loaded in tray and

another portion loaded in the syringe.

Insertion of the tray:

Seat the tray with the impression material in patients mouth and hold and wait for the final set.

Removal of the tray:

Remove the tray parallel to the preparation path of withdrawal and the impression is evaluated

for the finish lines and for any distortion and tear.

DUAL VISCOSITY TECHNIQUE:

Here low-viscosity material is injected with a syringe on the tooth surface and high-viscosity

material is mixed and placed in impression tray. After injecting the low-viscosity material, tray

containing high-viscosity material is placed in the mouth.

Selection of tray:

In dual viscosity technique too resin custom trays are used.

Preparing of custom tray:

Custom trays are fabricated the same way as for single viscosity technique.


Impressions are often taken with low-viscosity and high-viscosity impression materials of

addition silicone with resin custom tray. Here low-viscosity material is injected with a syringe

on the tooth surface and high-viscosity material is mixed and placed in impression tray. After

injecting the low-viscosity material, tray containing high-viscosity material is placed in the

mouth.


Seat the tray with the high-viscosity impression material in patient’s mouth and hold with gentle

pressure until the final set.




PUTTY-WASH TECHNIQUE/ RELINE TECHNIQUE

The putty wash technique is commonly used in making impressions with silicone elastomers and

it is recommended to overcome the problems associated with polymerization shrinkage of the

condensation silicone impression materials. The instability of condensation type is attributed to

the production of an alcohol byproduct during polymerization. For this two-step putty/wash

technique was developed in which the use of a thin layer of wash material minimizes the amount

of alcohol byproduct and thereby retains the dimensional stability within acceptable limits. It is

also recommended for additition silicone impression materials even though these materials

appear to be dimensionally stable.

1. TWO STAGE PROCEDURE:

This is a 2 step procedure where a preliminary impression is made with stock tray where high or

putty consistency material is used before cavity preparation is made. These results in intraoral

custom made tray formed by the putty. Now space is provided for a low viscosity material by

cutting away some of the “tray” material or by using a thin polyethylene sheet, rubber or wax

sheet as a space. After cavity preparation, low-viscosity material is syringed into the area and

preliminary impression reinserted.

Advantages:

1) Eliminates time and expense of fabricating custom trays.

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

3) Impression of teeth can be captured with the wash material.

Disadvantages:

1) More impression material is required

2) Extra chair side time

3) Distortion

Technique:

A Stock tray is selected based on shape and size of the patients arch. Now coat the tray evenly

with adhesive on the inside. Then mix the high viscosity putty impression material according to

manufactures instruction and roll putty into elongated cylinder and insert into the stock

impression tray. Cover putty with the spacer (a sheet of polyethylene) then insert and seat the

tray with a rocking type of motion and hold and wait until initial set (approximately 2min). Now

remove from the mouth with minimal side ward movement and wait and test for final set till the

impression rebound completely. Then Peel of the spacer and remove excess impression material

with a sharp knife.

For making final impression, mix the low viscosity impression material according to the

manufactures instructions and load the low viscosity impression material on to a syringe .The

syringe material is loaded to the inaccessible area first eg :disto lingual line angle, and insert the

low viscosity impression material into the tray without overfilling it. Seat the tray and wait for

the final set. Now remove the tray parallel to the preparation path of withdrawal and the

impression is evaluated for the finish lines and for any distortion and tear. (DCNA 2004; 48:445-

70)

Selection of tray:

Here Stock tray is selected based on shape and size of the patients arch.


Mix the high viscosity putty impression material according to manufactures instruction and roll

putty into elongated cylinder and insert into the stock impression tray. Cover putty with the

spacer and seat the tray and hold and wait until it set . Then Peel of the spacer and remove excess

impression material with a sharp knife.

For making final impression, mix the low viscosity impression material according to the

manufactures instructions and load the low viscosity impression material on to a syringe .The

syringe material is loaded to the inaccessible area of prepared tooth first and insert the low

viscosity impression material into the tray without overfilling it. Seat the tray and wait for the

final set.


Seat the tray with the high-viscosity putty impression material in patient’s mouth with a rocking

type of motion and hold and wait until initial set. Then insert the low viscosity impression

material into the tray without overfilling it. Seat the tray and wait for the final set.




2. SINGLE STAGE PROCEDURE/ Squash technique

For single stage procedure, stock tray with the unset putty material should already be loaded in

the tray and the preparation syringed with low viscosity impression material. Now seat the tray

with unset putty over the unset light body material and wait for the final set. Now remove the

tray parallel to the preparation path of withdrawal and the impression is evaluated for the finish

lines and for any distortion and tear. This method is unacceptable because it is impossible to

control the thickness of impression material and excess bulk is used. It is impossible to control

what material records the margin details of the preparations. Usually portions of the prepared

margin are captured in the putty, and putty materials are essentially deficient in their ability to

record marginal detail due to hydraulic pressure of putty pushes the syringe material in critical

areas.

Advantages:

a. Reduce chair side time

b. Savings of impression material.

Disadvantage:

a.Putty may displace wash material so that critical areas of the preparation will be reproduced in

putty.

b.Occasional ledges can be seen at the junction of the putty and wash material. (DCNA 2004;

48:445-70)

Selection of tray:

Stock tray is selected based on shape and size of the patient’s arch..


Here stock tray with the unset putty material should already be loaded in the tray and the

preparation syringed with low viscosity impression material


Seat the tray with unset putty over the unset light body material and wait for the final set.




Another Approach:

In this method putty material is used to fabricate a custom tray. It is fabricated in the same

manner as with PMM materials or light cure. One layer of base plate wax is placed over the

diagnostic cast as a spacer and wax is removed from nonfunctional cusps to provide occlusal

stops. A putty impression is made in a stock tray and a custom tray results. For making final

impression, mix the low viscosity material and load it into the syringe. Now syringe the material

onto the prepared teeth and Seat the tray and wait for the final set. Now remove the tray parallel

to the preparation path of withdrawal and the impression is evaluated for the finish lines and for

any distortion and tear. (DCNA 2004; 48:445-70)

Preparation of tray:

Here putty material is used to fabricate a custom tray. It is fabricated with a putty impression is

made in a stock tray and a custom tray results.


For making final impression, mix the low viscosity material and load it into the syringe . Now

syringe the material onto the prepared teeth and Seat the fabricated putty material custom tray

and wait for the final set.


Seat the fabricated custom tray with low viscosity material in the mouth and wait for the final

set.




Injection moulded putty wash technique:

Preoperative full arch putty impression is recorded in a rigid plastic tray and the putty is prepare

to make the temporary restoration. Now the putty is modified by drilling a hole through the putty

from the outer surface of the tray to allow greater flow of light body material and buccal escape

channel is created. After tooth preparation, tooth is cleaned & retraction cord is removed. Now

putty is reinserted & checked for position on the tooth and mixed light body is injected thru the

hole into the preparation. Tray must be held while injecting light body to prevent it from lifting

off the tooth and wait for the final set.

Selection of tray:

Here rigid plastic trays are used to make preoperative putty impression to make the temporary

restoration.


For making final impression mixed light body is injected thru the hole prepared in preoperative

putty impression into the preparation.


Seat the fabricated custom tray with low viscosity material in the mouth and wait for the final

set.




Most wash/reline procedures are used because of their simplicity. The theory supporting these

procedures is more subject to question, and success is more random than predictable. The

wash/reline methods generate unfavorable collapsing forces on the gingival tissue that affects the

degree of tissue displacement and registration of sub gingival tissues.

The wash/reline method provides even less control in the delivery of sulcular material. The low

viscosity reline material is expected to be driven into the sulcus by the heavier viscosity tray

material or the closely adapted lined tray. Contradictions occur in this concept. The same factors

that are expected to drive the impression material into the sulcus have an impact on the gingival

tissue collapsing the tissue against the prepared teeth and, in effect, inhibiting the entry of

impression material into the sulcus. The heavier the consistency of tray material, the greater will

be the impact on gingival tissue. The closer the adaptation of the lined tray the greater the

collapsing forces on the gingival tissue. All impression procedures should make detailed

Injection moulded putty wash impression technique

reproductions of the prepared teeth and allow the flow of material into the sulcus, while

simultaneously facilitating displacement of gingival tissue.

In putty-wash technique, errors in manipulation may lead to inaccurate impressions which are the

following:

1. An excess bulk of wash material can result in dimensional changes proportional to the

thickness of the material during setting

2. Undercuts or projections into the putty affect the accuracy of the impression.

3. Inaccurately seating the putty tray containing the wash material may lead to an excess bulk of

wash material around the abutment teeth, resulting in wash material being forced away from the

abutment tooth because of over seating and a non-uniform layer of wash left around the

abutment.

4. Some manufacturers provide a polyethylene spacer to be placed over the putty for making the

putty impression. This procedure has the major drawbacks of (1) no landmarks, guiding planes,

or posterior stops left in the putty impression, making accurate replacement of the tray during the

wash stage difficult, and (2) an inaccurate or arbitrarily repositioned tray that cannot achieve a

uniform wash space. For this a technique is presented ensuring exact reseating of the putty

impression tray and creation of a uniform wash space, which are essential for accurate results.

TECHNIQUE:

The following technique is suggested to overcome the problems of errors in manipulation:

1. The putty impression is made with resin provisional restorations in place on the prepared

teeth and allowed to set. The provisional’s act as occlusal stops for seating of the

impression tray. If the restoration is a fixed partial denture, individual resin restorations

made on a stone cast are used so that a uniform impression is made of the edentulous

space.

2. When the impression is removed, the provisional restorations are detached from the set

putty. A wash space, equivalent to the dimensions of the provisional restoration, will be

created in the putty surrounding the prepared tooth. When design of the finish line is such

that an adequate bulk of wash material cannot be achieved because of the thin margins of

the provisional restoration, a strip of putty corresponding to the finish line should be cut

away.

3. To enable accurate reseating of the putty impression and venting away excess wash

material, all undercuts, projections into embrasures or tooth material should be cut away

from the putty before loading of the wash material.

4. The exact amount of wash needed to fill the wash space cannot be clinically calculated.

An excess of wash is usually present. To ensure venting out this excess, escape channels

should be cut into the putty from the gingival margin to the outer border of the tray. A

No. 8 round bur or No. 15 scalpel blade can be used for this purpose. The previous

removal of the embrasure projections will also assist in venting away excess wash

material.

5. Large embrasure spaces in the dental arch should be blocked out before impression

making. Utility wax can be used for this purpose. This block out facilitates removal of the

tray once the wash material has set, avoiding distortion on withdrawal, possible

separation of the putty wash complex from the stock tray, and damage to teeth with

diminished periodontal support. (JPD 1990;64:635-6)

DUAL ARCH IMPRESSION TECNIQUE

Synonyms:

Dual quad tray, triple arch, accu -bite, closed mouth impression, close bite double arch method.

The dual-arch technique can be used successfully as long as the operator understands the

indications and contraindications of the procedure. This technique captures the prepared teeth,

the opposing arch, and the occlusal articulation in maximum intercuspation simultaneously. This

technique should be used with a maximum of two prepared teeth. This should be used only with

patients that have existing anterior guidance. For this technique to be successful, the patient must

be able to close completely in MIP with the impression tray in place. In this technique rigid

metal trays are ideal. Cox et al supported the use of dual arch technique for the fabrication of 3

unit FPD also.

Requirements

The articulator should provide for a positive stop or there should be sufficient teeth to maintain

the vertical dimension.

There should be sufficient space distal to the terminal tooth in the arch to allow tray

approximation.

Advantages

• Clinical simplicity and accurate recording of MIP position.

• Here closed-mouth technique is used that eliminates any mandibular flexure that might be

associated with opening.

• Less impression material is needed.

• Less gagging may occur.

Disadvantages

• The tray is not rigid.

• Limited to one casting per quadrant.

• The distribution of impression material is not uniform

Technique –a. Dual arch-Multiple mix technique

Evaluate the fit of the tray in the patient’s mouth and observe the complete bilateral closure and

the patient’s comfort. Now practice till the patient is familiar with the task. Make the final

impression by mix the low viscosity material and load the syringe. Then the high viscosity

material is loaded in the tray. After the low viscosity material is syringed the tray is positioned

on the arch. Instruct the patient to close the mouth and observe for the interdigitation on the

opposite arch and wait for the material to set. Instruct the patient to open the mouth remove the

tray by applying equal pressure bilaterally and evaluate the impression.

Selection of tray:

Here custom dual arch trays are used


For making final impression, mix the low viscosity material and load the syringe. Then the high

viscosity material is loaded in the tray. After the low viscosity material is syringed the tray is

positioned on the arch.


The tray is positioned with high viscosity material on the arch and observed for the

interdigitation on the opposite arch and waited for the material to set.


Remove the tray by applying equal pressure bilaterally and evaluate the impression.

b.Dual arch-Monophase Impression

In single viscosity/monophase, impressions are often taken with medium-viscosity impression

materials. Only one mix with part of the material loaded in tray and another portion loaded in the

syringe. When medium-viscosity material is forced through the syringe the viscosity is reduced

whereas viscosity of same material in the tray is unaffected. After the low viscosity material is

syringed the tray is positioned on the arch. Instruct the patient to close the mouth and observe for

the interdigitation on the opposite arch and wait for the material to set. Instruct the patient to

open the mouth remove the tray by applying equal pressure bilaterally and evaluate the

impression.

Selection of tray:

Here custom dual arch trays are used.


For making final impression, mix the low viscosity material and load the syringe. Then the high

viscosity material is loaded in the tray. After the low viscosity material is syringed the tray is

positioned on the arch.


The tray is positioned with high viscosity material on the arch and observed for the

interdigitation on the opposite arch and waited for the material to set.


Remove the tray by applying equal pressure bilaterally and evaluate the impression.

c. Laminar impression technique

The laminar impression technique is a precise, rapid, and predictable alternative to traditional

methods of impression-making in fixed prosthodontics. A preliminary impression is made using

plastic “triple-arch” type trays and high-stiffness vinyl polysiloxane jaw relation registration

material. After tooth preparation, tissue management, and retraction this registration can be used

for provisional fabrication.

Two holes are then drilled through the facial wall of the tray into the region of the preparation.

The tray is replaced in the mouth and light-bodied vinyl polysiloxane impression material is

injected into the holes by using an “automixing” gun system.

Advantages:

When the laminar impression technique is used, the occlusal registration, opposing arch

impression, and final impression are combined in one modular unit. By achieving multiple

objectives, a significant time-saving is achieved over more traditional methods. Precision

injection of the wash material avoids waste of excessive impression material. In addition,

injection into an open system does not generate compressive forces that can force impression

material intramucosally and cause a foreign body reaction until removed. The flushing action of

injecting a continuous flow of impression material aids in the removal of sulcular contaminants

and produces clear, detailed impressions of critical regions. (JPD 1989; 62: 391-6)

D.Hydraulic pressure technique:

Low or medium viscosity impression material is injected around the prepared tooth and into the

preoperative impression of the unprepared tooth. The dentists reinsert the preoperative

impression and the patient closes into maximum intercuspation. The generated hydraulic

pressure forces the wash material into the sulcus and around the preparation. Vent holes can be

drilled on the buccal or lingual aspect of the impression material and tray to allow the escape of

excess wash material. (JADA 1996; Vol.127:234-240)

SEGMENTAL IMPRESSION TECNIQUE

The segmental impression technique offers a procedure that predictably permits making a

successful impression with multiple prepared teeth. With this technique the arch to be impressed

is broken down into easily managed segments. This usually is arbitrarily determined as two

prepared teeth per segment. Individual custom trays for each segment with 1 mm of wax spacer

are used on the diagnostic cast. The tray should extend 3mm past gingival margin of the prepared

teeth because there are no occlusal stops and the gingival tissue must prevent over-seating of the

trays and the PMMA custom tray made 24hrs in advance.

Procedure:

Here Light body is loaded into a syringe and one of the segmental trays. Now the impression

material is injected around the preparations and the tray with the material is seated into the place

and excess extruded material is removed from around the tray, and the impression material is

allowed to set. The tray is not removed. This procedure is repeated with each of the segments

until each segmental impression is in place. At this point an over-impression is made with a

compatible impression material in a stock tray. This impression is then handled in a conventional

manner. This technique has proven useful in extensive cases and moisture control is difficult in

specific patients.

Selection of tray:

Here both segmented custom trays with 1mm spacer are used to take multiple prepared teeth and

stock trays are used to take over impression.


Here Light body is loaded into a syringe and one of the segmental trays.

Now the impression material is injected around the preparations and the tray with the material is

seated into the place and the impression material is allowed to set. The tray is not removed. This

procedure is repeated with each of the segments until each segmental impression is in place. At

this point an over-impression is made with a compatible impression material in a stock tray. This

impression is then handled in a conventional manner.


The segmented tray is positioned with light body material and allowed to set and an over

impression is made with compatible impression material.


Remove the tray parallel to the preparation path of withdrawal and the impression is evaluated.

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. It attempts to overcome the deficiencies of the older

systems and at the same time incorporate their best features

Series of impressions using three different viscosities of impression materials are made.

Step I: Matrix is made in occlusal registration elastomeric impression material over the

prepared teeth.

Step II: Definitive impression is made with high viscosity impression material in matrix.

Step III: Matrix impression(s) are seated in position and impression of entire arch is

made with medium viscosity impression material in stock tray (seated over matrix and

remaining teeth).

Procedure:

A matrix of occlusal registration elastomeric material is made over the tooth preparations. The

matrix may be made in one piece or in two or more sections, depending on the distribution and

complexity of the preparations. The matrix is trimmed to prescribed dimensions and, after the

retraction cord is removed, a definitive impression is made in the matrix of the preparations with

a high viscosity elastomeric impression material. After the matrix impression(s) is seated, a stock

tray filled with a medium viscosity elastomeric impression material is seated over the matrix and

remaining teeth to create an impression of the entire arch.

This system effectively controls the four forces (relapsing, retraction, displacement, and

collapsing) that impact on the gingiva during the critical phase of making the impression when

attempting to register the sub gingival margins. The design of the matrix also gently forces the

high viscosity impression material along the preparations and into the sulcus where it cleanses

the sulcus of unwanted debris and fills the sulcus. The high viscosity material gently extends the

sulcus and does not permit it to collapse as the medium viscosity material in the stock tray is

seated for the pick-up impression. The matrix facilitates 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. (J Prosthet Dent 1998; 79:208-16)

Reversible hydrocolloid technique / wet technique

This impression technique requires a special conditioning unit which consists of 3

thermostatically controlled water baths.

Inject the syringe material on the tooth cover the entire tooth and then remove the impression

tray from the bath wipe with the gauze and place it in the mouth. After seating cold water is

circulated through the tray until the impression material is set. Now, remove the impression with

a rapid motion, wash with cold water, evaluate for accuracy.

Procedure

Select the correct size of water cooled impression trays and place small modeling compound or

prefabricated stops in the tray to prevent over seating .Fill the impression tray with heavy bodied

material from the storage bath and place it in the tempering bath. Now Load the syringe material

in the syringe and carefully remove the retraction cord and flood the tooth with water and inject

the syringe material on the tooth and cover the entire tooth. Remove the impression tray from the

bath and wipe it with the gauze and place it in the mouth. After seating, cold water is circulated

through the tray until the impression material is set. Remove the impression with a rapid motion,

wash with cold water, evaluate for accuracy.

Selection of tray:

Select the correct size of water cooled impression trays and place small modeling compound or

prefabricated stops in the tray to prevent over seating.


Fill the impression tray with heavy bodied material from the storage bath and place it in the

tempering bath. Now Load the syringe material in the syringe and flood the tooth with water and

inject the syringe material on the tooth and cover the entire tooth. Remove the impression tray

from the bath and wipe it with the gauze and place it in the mouth. After seating, cold water is

circulated through the tray until the impression material is set.


Remove the impression with a rapid motion, wash with cold water, evaluate for accuracy.

Hydrocolloid Laminate technique:

Syringe agar : Injected around preparation.

Chilled Alginate mix: In tray, promptly seated on the top of agar.

Advantages:

- More accurate surface reproduction by agar.

- Agar more compatible with gypsum.

- Elimination of water-cooled impression trays.

- Economical than newer elastomeric impression materials

Disadvantages:

- Stiff and therefore difficult to remove without rocking.

- Break seal and rock slightly to prevent tearing: Low tear resistance.

Impression using preformed crown shells/ Polycarbonate crown:

Prefabricated temporary crown shells are used for each tooth preparation & a final over

impression using a stock tray.

Procedure: Adjust gingival margin and proximal contacts and coat internal & external surfaces

of provisional crown with an adhesive. Then mix regular body & fill crown shell. While filling

entrapment of air should be avoided and seat crown shell until it covers the finish line. Allow

material to set and finally make a pick up impression using regular body in a complete arch stock

tray. (J Prosthet Dent. 1995; 73: 95–96)

Functional check-bite impressions:

The functional “checkbite-impression” is a combination record derived from adding functional

records to the static simple “checkbite-impression.” This static “checkbite-impression” is a

combined record used for making inlays, crowns, and fixed partial dentures. Because of its

simultaneous recording of prepared teeth and their precise relationship to the opposing teeth, the

cast restorations made from such a record need less adjusting in the mouth than castings made

from separate impressions and interocclusal records.

The basic “checkbite-impression” procedure is a prerequisite to the functionional “checkbite-

impression.” Therefore, the making of a simple checkbite-impression for a fixed partial denture

will be described starting at the point where the abutments have been prepared and the gingival

crevices packed. A unilateral checkbite-impression tray without the occlusal insert is tried in the

mouth. The patient is told about the procedure and what is meant by the request to “bite,” and is

cautioned to maintain contact between the upper and lower teeth until instructed to “open,” at

which time he is to separate the teeth forcefully but carefully. In the meantime, the dental

assistant mixes sufficient regular body rubber-base impression material to fill both the tray and

the syringe and the dentist injects impression material from the syringe over and around the

preparations while the assistant is filling the “checkbite-impression” tray on both sides of the

occlusal insert. The dentist places the tray in the mouth and instructs the patient to bite. The tray

is allowed to come to rest against the lips and is not held while the material sets. Then the patient

is told to open the mouth. The “checkbite-impression” obtained is an accurate static record of the

teeth and their occlusal relationship.

Making the functional check-bite impression:

After the gingival packing has been removed and it is certain that the abutments are dry, the

impression material from the syringe is injected over and around the preparations, the filled tray

positioned in the mouth, and the patient is instructed to bite. The impression material is allowed

to cure until prodding with an instrument shows that it is set, at which time the patient is told to

“open.” (JPD 1971; VOL 26:NUM2, 146-153)

DISINFECTION:

After being removed from the patient's mouth, the impression is immediately rinsed with tap

water and dried with an air syringe. Suitable chemicals should be used, such as glutaraldehyde

solutions or iodophor sprays (JPD 1989; 62: 468) are most commonly recommended techniques

for the impression materials and chemical agents such as sodium hypochlorite can also be used.

Some are perfectly acceptable for one material but unsuitable for others. Because of its tendency

to distort and absorb moisture, polyether or "hydrophilic" addition silicone impression materials

should be sprayed and stored in a plastic bag rather than submerged and soaked in a

gluteraldehyde solution. Disinfection is an essential step for preventing cross-infection and

exposure of laboratory personnel. If it is performed properly, disinfection will not affect the

accuracy or surface reproduction of the elastomer. Special care must be taken with water-based

materials and polyether’s to insure that adequate immersion times are used to eliminate

microorganisms but that extended immersion times are avoided to prevent excess imbibition of

the disinfecting solution and distortion of impression. (JPD 1990; 63: 233)

POURING OF IMPRESSION MATERIALS:

One of the most important manipulative variables with impression materials is the time limit

after removal from the mouth to when the impression is poured. Water-based materials should be

poured within 10 minutes of removal from the mouth. The major component of these

impressions is water, which evaporates at room temperature. This water loss is accompanied by

distortion and is minimized by rapid pouring.

Condensation silicones produce ethyl alcohol as the by-product of the setting reaction, and

evaporation of the alcohol results in distortion. The identical phenomenon occurs with

polysulfide rubber, where the by-product is water. These materials should be poured within 30

minutes for maximum accuracy.

Polyether materials can absorb water from the atmosphere and thus should be poured within 1

hour for maximum accuracy. PVS impression material is stable because there is no volatile by-

product to the reaction and because they do not give off or absorb water. This dimensional

stability permits pouring of the impression at the convenience of the operator. (DCNA2004; 48:

445-70)

REVIEW OF LITERATURE:

Donovan and Chee (2004) suggested that the clinician is urged to review contemporary

principles of impression materials and to familiarize themselves with various specialty

impression techniques available and use them when indicated.

Gordon, Johnson and Drennon 1990 stated that custom trays produced dies that were

more accurate when compared with stock trays and thermoplastic custom tray produced

dies as accurate as those of the custom acrylic resin tray.

Gus J Livaditis 1998 compares the methods and effectiveness of traditional fixed partial

denture impression systems where several concepts are questioned and alternative

procedures are proposed.

Herfort, Gerberich, macosko and Goodkind (1978) stated that polysulfide rubber was

twice as strong as silicone system which in turn was twice strong as irreversible

hydrocolloid.

Kimoto, Tanaka, Toyoda and Ochiai 2005 identified the particulate contamination of

sulfur and sulfur-chloride compounds that resulted in polymerization inhibition of the

tested VPS dental impression material.

Samet, Shohat, Linvy and Weiss 2005 evaluated the quality of impressions sent to

commercial laboratories for the fabrication of FPD where impressions made with

polyethers had the most detectable errors followed by condensation type silicones.

Tjan, Dent, Whang, Sarkissian 1986 stated that impressions made with addition curing

silicones base or polyether rubber impression material remained accurate even after 1

week when compared with reversible hydrocolloids.

Fusayama, Iwaku, Daito, Kurosaki and Takatsu 1974 stated that the laminated single

impression technique was found to be capable of producing indirect stone models having

adequate dimensional accuracy and surface sufficient reproducibility.

Lacy, Fukui, Bellman and Jendresen 1981 stated that Polyvinylsiloxane( addition

polymerization) silicones are the most stable of elastomers currently available and

accuracy and consistency are best maintained by the use of custom tray and adhesives to

retain polyvinyl siloxanes.

Millar, Dunne and Robinson 1998 reported that monophase addition cured impression

material in stock trays carries an increased risk of void formation on the surfaces of

impression when compared with two phase addition silicone materials in custom trays.

Idris, Houston and Claffey 1995 stated that putty wash one step technique and two step

technique were sufficiently accurate in conjunction with the addition silicone materials

for fixed partial dentures.

Vitre, Galburt and Maness 1985 reported that electrosurgical method showed more tissue

loss at each time interval than the bur method.

Jokstad 1999 stated that knitted gingival retraction cords were ranked better than twined

cords.

Peregrina, Land, Wandling and Jhonston 2005 stated that the universal spray on adhesive

demonstrated significantly low bond strength values compared to conventional paint on

VPS adhesives.

Wilson and Werrin 1983 stated that the dual-arch or double-arch impression technique is

extremely accurate and a viable alternative to full-arch impressions.

Breeding and Dixon 2000 stated that dual arch impression technique provides accurate

and simple method for fabricating restoration using maxillo-mandibular relationship.

Gardner 1981 stated that segmental impression technique offers a procedure that

predictably permits making of successful impression with multiple prepared teeth.

Shafa et al 2008 stated that No significant difference was found among dimensions of

stone casts poured from an impression made using a light-cured tray and

autopolymerizing acrylic trays.

Carrotte 1998 reported that Metal and rigid plastic stock trays give greater accuracy in the

putty/wash silicone twin mix impression technique compared with flexible plastic trays.

DISCUSSION:

An impression or negative likeness of the teeth and surrounding structures is used to obtain a

cast, on which the planned restoration is fabricated. A good impression is an exact negative

replica of each prepared tooth and must include all of the prepared surfaces and an adequate

amount of unprepared tooth structure adjacent to the margin.

Healthy soft tissues and the control of saliva flow are essential for a successful impression.

However, caution must be exercised to prevent injury to the gingiva. Cotton rolls, cards, and

saliva evacuators are needed for adequate moisture control. During the impression procedure,

using a local anesthetic to minimize discomfort and to reduce saliva flow is recommended. Both

mechanical-chemical and surgical methods for enlargement of the gingival sulcus can be used to

obtain access to sub gingival margins of prepared teeth. However, a narrow cord impregnated

with a mild astringent (e.g., AIC13) is recommended. Deformation of gingival tissues occurs

during retraction and impression procedures which involves four forces:

1. Retraction: downward and outward movement of the free gingival margin.

2. Relapse: gingival cuff to go back to its original position

3. Displacement: downward movement of the gingival cuff

4. Collapse: tendency of the gingival cuff to flatten under forces

To protect the smear layer, excessive contact between haemostatic agents and cut tooth structure

should be avoided. A custom acrylic resin tray should be used when making an impression with

any of the elastomeric materials.

This topic outlines the type of impression materials, ideal properties of impression materials and

several impression techniques for fixed partial denture. A number of ideal properties for

impression materials that includes accuracy, elastic recovery, dimensional stability, flow,

flexibility, workability, hydrophilicity, long shelf life, patient comfort and economics.

Impression materials vary considerably in relation to these ideal properties, and these differences

may provide a basis for the selection of specific materials in specific clinical situations. Special

attention is paid to polyvinyl siloxane impression materials because they have become the most

widely used impression materials (49.8%) in dentistry followed by condensation silicone

(24.4%) and Polyether (18.2%).

Hydrocolloids have a high hydrophilic nature (low contact angle of less than 90° but materials

with contact angle more 30° become adhesive in nature) that allows this material to capture

accurate impressions in the presence of saliva or blood. It has low wetting angle so it easily

captures full arch impressions. It has moderate ability to reproduce detail and costs relatively

little compared with other impression materials. It is not accurate enough for fixed partial

dentures. It has poor dimensional stability, must be poured within 10 to 12 minutes and is good

for only one pour per impression.

Polysulfide impression materials are generally low to moderately hydrophilic and make an

accurate impression in the presence of saliva or blood. Because the material has a low wetting

angle it makes a full arch impression than polyvinyl siloxane or polyethers. It reproduces detail

with excellent results but its dimensional stability is only fair. It may allow for more than one

pour and it is not a rigid material and impressions are easier to remove.

Addition silicones involve the linking of a vinyl siloxane in the base material with a hydrogen

siloxane via a platinum catalyst. The reaction produces hydrogen, which is scavenged by the

platinum. Viscosity is altered by changing the amount of silica filler which produces either a

putty or less viscous wash material. Vinyl polysiloxane silicones are considered state-of-the-art

for fixed partial denture impressions because it shows accuracy of 96.86% over other

impressions. They are virtually inert after set, but cannot make impression in a wet environment

and they can be trimmed and poured in any die material. It has a moderately high wetting angle,

which makes it difficult to have an accurate full arch impression. It has an excellent ability to

reproduce detail of 25um or less and is dimensionally stable, which allows multiple pours of

accurate casts for several weeks after impression are made. The material is moderately rigid and

can be more easily removed.

Polyether impression material is moderately hydrophilic and captures accurate impressions in the

presence of some saliva or blood. Because their wetting angle is low, they capture a full arch

impression easier than polyvinyl siloxane. Their ability to reproduce detail is excellent and is

dimensionally stable and allows multiple pours of accurate casts for 1 to 2 weeks after

impression are made. They are rigid materials and may be more difficult to remove but they are

contraindicated in periodontally involved teeth. They do not tear easily which enables the dentist

to get good sub gingival detail without tearing the impression on removal. (DCNA2004; 48: 445-

70)

All impression materials should be rinsed, dried, and disinfected when removed from the mouth.

Impressions made with polyether polymer should be poured within 1 hour. Impressions made

with polyether or addition silicone have high dimensional stability and can be stored

considerably longer before pouring. In all techniques, a good impression is critical for an

accurately fitting restoration.

CONCLUSION:

Dentists have relied on impression materials for fabricating dental prosthesis. The materials that

have received a lot of attention because of their physical and handling properties include

irreversible hydrocolloids, polyethers, polyvinyls and polysulfides.

The polyvinyls and the polyethers account for a major portion of the market used as impression

materials in fabricating fixed partial dentures. The hydrophilic addition silicones and polyethers

flow easily, results in fewer retakes, and produce more bubble- free casts when used under

appropriate guidelines. The polyvinyl siloxane materials are intrinsically hydrophobic by nature,

so they must be made hydrophilic by adding surfactants. When these surfactants come in contact

with moisture, it has to migrate to the surface, which prevents the hydrophilicity from fully

developing during working and setting times and can result in voids and inaccurate impressions.

A dry field is critical for their use. Polyether is hydrophilic by nature of its chemical make up,

and moisture does not interfere as much with achieving void-free impressions.

The condensation silicones, polysulfides and irreversible hydrocolloids have qualities that make

them more sensitive with respect to handling considerations and mix-and-pour techniques

because they exhibit more changes over time after setting, which may affect accuracy in detail

reproduction. The polyvinyls and polyethers are more stable to deformation after setting has

occurred. All have specific protocols for disinfecting that must be followed to prevent distortion

of the material before pouring casts; however the polyvinyls seem to be most impervious to

different disinfection protocols. (DCNA 2007; 51: 629-42)

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