Dr Vikas Aggarwal

IMPRESSIONS IN FIXED PARTIAL

DENTURES

It is the first step necessary for the indirect

fabrication of a prosthesis .

Fixed prosthodontics is practiced as INDIRECT

RESTORATIONS.

Well-fi tting indirect restorations accurate casts of the oral

tissues available, high quality impressions.

A good quality impression thorough knowledge of materials,

their properties, and techniques for their best manipulation .

INTRODUCTION

IMPRESSION: Negative likeness or copy in reverse of the surface of an object; an imprint of the teeth and adjacent structures for use in dentistry.

IMPRESSION TRAY : A receptacle into which suitable impression material is placed to make a negative likeliness (or)

a device that is used to carry, confine, and control impression material while making impression

DEFINITION

1. It should be an exact duplication of the prepared tooth, including all of the preparation and enough uncut tooth surface beyond the preparation for the dentist and technician to be certain of the location and configuration of the finish line.

2. Teeth and tissues adjacent to the prepared tooth must be accurately reproduced to permit proper articulation of the cast and contouring of the restoration.

3. It must be free of bubbles, especially in the area of the finish line and occlusal surfaces

REQUIREMENTS OF AN IMPRESSION

Impression material

Non Rigid

hydrocolloid

Reversible agar

Irreversiblealginate

Elastomeric

Polysulfide Condensation silicone

Addition silicone Polyether

Rigid

Impression compound

CLASSIFICATION OF IMPRESSION MATERIAL

Basic requirements of impression material

Complete plasticity before use

Suffi cient fluidity to record fine detail

The ability to wet the oral tissues/ hydrophilic

Dimensional accuracy

Dimensional stability

Complete elasticity after cure

Optimal stiffness 

REQUIREMENTS OF IMPRESSION MATERIAL

Have a good shelf lifeBe non-toxic and non-irritatingHave acceptable taste and odourHave suitable working and setting timesHave strength to resist tearingBe compatible with model and die materialFacilitate clinical identification of beginning

and end of curePermits multiple die pouringCost effective

ADDITIONAL REQUIREMENTS

IMPRESSION MATERIALS USED IN FIXED PROSTHODONTICS

Agar

Condensation silicone

polysulfide

polyether addition silicone

Introduced by “Alphous poller of Vienna” in 1925 Adopted commercially as “Dentacol” in 1928  The credit for its fi rst use in United States for

fabricating cast restorations is given to J.D. Hart (1930). Agar is an organic hydrophilic colloid (polysaccharide) extracted from certain types of sea weed. It is sulfuric ester of a linear polymer of galactose.

REVERSIBLE HYDROCOLLOID (AGAR)

Agar

Borates

Potassium sulfate

Diatomaceous earth,Silica

Clay

Alkyl benzoates

Thixotrophic materials

Coloring agents

Water

13 – 17%

0.2 – 0.5%

1 – 2%

0.5 – 1.1%

0.1%

0.3%

Basic constituent

Improves strength of the gel retards plaster or stone

Gypsum hardener

Filler

Preservatives

Plasticizers

-

Dispersion medium

The ideal conditioner for preparing hydrocolloid for use has 3 baths :

Liquefying bath: Tubes of impression material and loaded syringes are boiled at 100º C for 10 minutes in this bath. If the material is being re liquefied, it should be boiled for 12 minutes.

Storage bath: The tubes fi lled with liquefied material are moved to this bath, where they are stored at 65 º C for a minimum of 10 minutes. Heavier bodied materials may require storage at 65 to 68º C. The material can be stored for 5 days. If it has not been used by that time, it should be re liquefied by boiling it for 12 minutes.

Tempering bath: loaded impression trays are tempered in this bath at 440–460 C for 5 to 10 minutes immediately before placing them in the mouth.

BaseLiquid polysulfide polymer  80 – 85% Inert fi llers (TiO, Zn Sulfate, Copper carbonate)  16 –

18%Plasticizers (Di butyl phthalate)

Accelerator pasteLead di-oxide  60 – 68% - ReactorDi butyl phthalate  30 – 35% - PlasticizerSulfur 3% - Retarder

POLYSULFIDE (MERCAPTAN, THIOKOL)

The lead dioxide reacts with the poly sulfide polymer

Chain lengthening by oxidation of the terminal -SH groups

Exothermic reaction - 3 to 40 C rise in temp. It is accelerated by heat and moisture T-butyl hydro peroxide – alternative to PbO2

PbO2 + S HS - R- SH HS - R-S-S-R-SH

+H2O

Mercaptan + Lead dioxide Poly sulfide+Water

CHEMISTRY AND SETTING REACTIONS

1.Unpleasant odor and colour - stains linen & messy to work with

2.Extremely viscous and sticky - mixing is diffi cult

3.Mixing time is 45 seconds

4.Long setting time of 12.5 (at 370C) - Patient discomfort

5.Excellent reproduction of surface detail

6.Dimensional stability:

- Curing shrinkage is high 0.45%.

- It has the highest permanent deformation (3 to 5%)

among the elastomers

7. It is hydrophobic - so the mouth should be dried thoroughly

before making an impression and should be poured with in 1 hour

8. The shelf life is good (2 years)

PROPERTIES

Base pastePoly dimethyl siloxane     25 – 65%Colloidal silica or micronize metal oxide fi ller

(Depending upon the viscosity) 35 – 75%Coloring agentAccelerator pasteOrthoethyl silicate – cross linking agentStannous octate – catalystAvailable as Light body,Regular body, Heavy body

and Putty consistency

CONDENSATION SILICONE

Dimethyl + orthoethyl Silicone + ethyl

siloxane silicate rubber alcohol

SETTING REACTION:

Ethyl alcohol - byproduct - evaporate - shrinkage &

dimensional instability

PROPERTIES

• Pleasant odor and color.• Mixing time of 45sec & setting time of 8-9mins.• Excellent reproduction of surface details and highly elastic.• Lesser dimensional stability

- high curing shrinkage (0.4 - 0.6%) - permanent deformation due to shrinkage caused by the evaporation of ethyl alcohol is also high (1-3%).• Hydrophobic - needs a dry fi eld.• Since ethyl alcohol is the by product they are dimensionally

stability is poor and they should be poured immediately after removal from the mouth.

• They have a very poor shelf life because of the instability of alkyl silicates in the presence of organo-tin compounds, which may result in the oxidation of tin

Base pastePoly methyl hydrogen siloxaneOther siloxane polymersFillersAccelerator pasteDivinyl siloxanePlatinum salt (chloroplatinic acid) catalystPalladium – hydrogen absorberFillers

Available as Light body, Regular / medium body, Heavy body and Putty

ADDITION SILICONE

Vinyl + Silane Siliconesiloxane siloxane rubber

It is addition polymerization reaction terminated with vinyl groups and is cross linked with hydride groups activated by a platinum salt catalyst.

There is no reaction by products as long as correct proportions of vinyl silicone and hydride silicone are manipulated and there are no impurities,

if the proportion is out of balance or impurities are present then side reactions will produce hydrogen gas.

This is avoided by the manufacturer adding noble metals or palladium or platinum to act as scavengers.

• Pleasant odor and color

• Excellent reproduction of surface details

• Mixing time of 45 secs ,setting time of 5-9 mins.

• Best dimensional stability

- low curing shrinkage (0.17 %)

- lowest permanent deformation (0.05 – 0.3

%)

• Stone pouring delayed by 1-2 hours

• Extremely hydrophobic, some manufacturers add a

surfactant (detergent) to make it more hydrophilic

• Good shelf life of 1-2 yrs

PROPERTIES

PVS is the impression material least aff ected by pouring delays or by second pours, and it is still accurate, even when poured 1 week after removal from the mouth.

Early formulations of this material released hydrogen gas from the impression surface, resulting in voids in the surface of the setting stone cast. If the impression was not poured within 15 minutes, then the best results were obtained by waiting 24 hours before pouring.

Modification of the formula by the addition of palladium to absorb the hydrogen has minimized this problem. Pouring should now be delayed for a short time (ie, 15 to 30 minutes) rather than a day

In its unaltered form, PVS is hydrophobic. Surfactants can be incorporated into the material to make it less hydrophobic and easier to pour.

Casts poured in impressions made with altered hydrophilic PVS exhibit 26% to 55% fewer trapped voids than casts poured in unaltered or conventional PVS.

However, casts made in unaltered PVS impressions whose surfaces have been treated with a surfactant at the time of pouring show a reduction in voids of 86%.

Also, casts poured in hydrophilic PVS materials with intrinsic surfactants produce slightly less accurate casts with surfaces that are 14% to 33% softer than those poured in conventional PVS materials. The incorporated surfactant also makes the impression material more sensitive to the retardant action of sulphur. So use of unaltered PVS is suggested.

One disadvantage of these materials is that setting inhibition can occur with latex gloves. Dithiocarbamates, which are used in glove manufacturing as either vulcanizing agents or accelerators, have been implicated as causative agents.

The problem is most apparent if hand mixed putty is used, but problems can occur if the tissues are touched with gloved hands immediately before impression placement.

It has also been shown that sulfide and sulfide-chloride can be transferred from latex gloves to retraction cord, which enables the transfer of these inhibiting agents to the sulcular tissue. The problem can be avoided by using latex free or vinyl gloves

Base pastePolyether polymerColloidal silicadi butyl phthalate

Accelerator pasteAromatic sulfonate ester – cross linking agentColloidal silicaDi butyl phthalate

POLYETHER

Polyether + Sulfonic ester Crosslinked rubberExothermic reaction 4-5 degreeC

Successive pours can be accurate even after 1 week. It is hydrophilic material. Material is stiff and undercuts must be blocked out.

Allergic hypersensitivity has been documented with use of polyether due to aromatic sulfonate catalyst.

Polyether impression should be stored in dry condition, as this material can absorb water from atmosphere.

Die should be carefully removed from impression material as these are more rigid than other materials

SETTING REACTION

1.Pleasant odor and taste

2.Mixing time is 30 secs, setting time of 8 mins

3.Dimensional stability is very good.

Curing shrinkage is low (0.24%)

The permanent deformation is also low (1-

2%).

4.Very stiff (flexibility of 3%), needs extra space, around

4 mm is given.

5.Hydrophilic (moisture control not critical)

6.Shelf life extends upto 2 years

PROPERTIES

Property Polysulfide Condesn. Addn. Polyether

Working time (min) 4-7 2.5-4 2-4 3

Setting time (min) 7-10 6-8 4-6.5 6

Tear strength (N/m) 2500-7000 2300-2600 1500-4300 1800-4800

By product H2O Ethanol - -

Custom tray YES NO NO NO

Unpleasant odour YES NO NO NO

Multiple casts NO NO YES YES

% contraction (24hr) 0.40 – 0.45 0.38 – 0.60 0.14 – 0.17 0.19 – 0.24

Stiffness (1= > stiff) 3 2 2 1

Distortion (1= > Dist) 1 2 4 3

Each impression material has diff erent handling characteristics. Ease of pouring gypsum products varies among diff erent impression materials. Impression materials can be classified as:

Hydrophilic (readily wettable by gypsum)Hydrophobic (resistant to wetting) Irreversible hydrocolloid (alginate), reversible

hydrocolloid and polyether are hydrophilic and are easiest to pour.

Hydrophobic impression material: Polysulfide< Polyvinyl siloxane < Condensation silicones

WETTABILITY

The greater the contact angle, the greater the probability of air entrapment during pouring. Not only incidences of voids are greater also the voids are larger in size.

Material

contact

angle of

water (°)

Polysulfide 82

Condensation

silicone98

Addition silicone  

i) Hydrophobic 98

ii) Hydrophilic 53

Polyether 49

Of clinical significance is the fact that materials exhibiting large contact angles are more readily repelled by hemorrhage or other moisture in the gingival sulcus.

There is also required caution in pouring an impression made of one of the materials whose surface is more diffi cult to wet. The use of 'surfactants' is also eff ective in reducing the contact angle and the number of voids trapped in the resulting cast

Impression materials need to readily flow into the minute details of the cavity preparations and accurately capture grooves, pinholes, and cervical margin detail.

Light-body polysulfide and condensation silicone are the least viscous, and heavy-body polysulfide is the most viscous. Viscosity will increase as time elapses after the start of mixing

These materials exhibit lower viscosity when the shear rate (the speed at which a liquid flows under external forces) increases, which occurs when a material is expressed through a syringe.( thixotropic eff ect)

VISCOSITY

This eff ect, called shear thinning, explains why a single-viscosity monophasic material can be placed in a tray, where false body (a higher apparent standing consistency) permits the material to stay in the tray without sagging or dripping, and yet the same material still can have suffi ciently high fl uidity (low viscosity) to be used in a syringe. ( Thixotropic eff ect)

Early versions of light-body materials possessed excellent fl ow characteristics, but the materials tended to fl ow off of the prepared tooth with time, which posed problems when attempting to make an impression of several prepared teeth at one time.

Most of the newer PVS products and polyether are thixotropic and stay where they are syringed but fl ow readily when the heavier body tray materials are placed over the top of them.

1) UNIFORM BULKDiff erent impression materials require diff erent cross-

sectional thicknesses of impression material to provide optimum accuracy. Water-based impression materials, such as reversible and irreversible hydrocolloid, provide maximum accuracy with a cross-sectional thickness of 4 to 6 mm. This thickness is achieved with the use of a properly sized stock tray.

Elastomeric impressions are most accurate when used with a cross sectional thickness of approximately 2 mm. This thickness is optimally provided with a custom tray.

PRINCIPLES OF IMPRESSION MATERIAL MANIPULATION

It is imperative that the impression material adheres to the tray. With proper adherence, the impression material shrinks toward the tray as it polymerizes. This results in a slightly larger die, which is preferable to a smaller die. Adhesive should be applied at least 7- 15 min before making impression

2) ADHESION OF IMPRESSION MATERIAL TO TRAY

Should be poured with in the prescribed timeWater-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 and polysulfi de should be poured within 30 minutes for maximum accuracy as they release products.

Polyether materials can absorb water from the atmosphere and thus should be poured within 1 hour for maximum accuracy.

PVS impression materials are stable there is no volatile by-product to the reaction also because they do not give off or absorb water. This dimensional stability permits pouring of the impression at the convenience of the operator.

3) POURING OF IMPRESSION MATERIAL

Various viscosities are available for elastomeric impression materials.

Two rules of thumb regarding diff erent viscosity materials are (1) the lower the viscosity, the better the fine detail reproduction and

(2) the lower the viscosity, the greater the polymerization shrinkage during the setting reaction.

Thus, the optimum method of impression making is to use as little low-viscosity material as possible to capture the fine detail of the prepared margin, grooves, box-forms, etc., and the bulk of the impression should be made with high-viscosity material.

The heavy-body material helps push the light-body material into the gingival sulcus and results in minimal distortion due to polymerization shrinkage.

4) VISCOSITY CONTROL

5) ADEQUATE MIXINGThe materials then should be vigorously stropped until

a homogeneous material is obtained. If automix system is available it should be used as

these systems provide optimum mixing of the material with significantly fewer inherent voids, extend the essential working time of the material, and reduce waste of the material.

6) DISINFECTIONMicroorganisms in the oral cavity can be transmitted

from impressions to the dental laboratory. The clinician must disinfect impressions before pouring the cast or sending the impressions to the dental laboratory

Various types of trays are used for fixed partial dentures impression procedures.1. Stock trayMetallic Non metallic2. Custom made traysAutopolymerizing acrylic resinThermoplastic or photo initiated resin3. Water cooled rim lock trays

TRAYS USED IN FPD

Tray space seems to have no eff ect on the dimensional accuracy of monophasic PVS impressions, except for the distance between fixed partial denture abutment preparations.

the inter preparation distance in casts made from polysulfide, PVS, and polyether impressions was 45 to 100 µm greater when stock trays were used instead of custom acrylic resin or thermoplastic trays.

They also reported 260 µm cross-arch discrepancies, which they attributed to stock tray flexibility

GORDON ET AL 1990 JPD

Custom resin trays used for elastomeric impressions as they provide uniform thickness ( 2 to 3 mm).

Custom trays can be made from autopolymerizing acrylic resin, thermoplastic resin, or photo polymerized resins.

With any tray material system, tray rigidity is important, because even slight flexing of the tray leads to a distorted 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.

It should have stops on the occlusal surfaces of the teeth to orient the tray properly when it is seated in the mouth

CUSTOM TRAY

Advantages Improves the accuracy of an elastomeric impression by

limiting the volume of the material, thereby reducing the 2 sources of error Stress during removal Thermal contraction

No need for sterilizationUniform thickness of the impression material minimizes

distortions resulting from curing shrinkageSaves impression materialDisadvantages Time taken for the fabricationAging for 24 hours to minimizes the distortionSensitivity to monomer

Armamentarium

Tray extensions marked 5 mm below cervical line

Base plate wax 2 sheet thickness 2-3 mm is adapted

Three Occlusal stops 3 sq mm are created on non functional cusps ( four according to Tylman)

• Stops are made by removing wax at an angle of 45 degrees to the occlusal surfaces of three teeth that have a tripodal arrangement in the arch.

• This lends stability to the tray, and the 45-degree slope helps center the tray during insertion

Stops can also be prepared at hard palate

Tin foil is adapted over the wax to protect it from melting during exothermic rx

Wooden slab and roller are used.

The resin is gently adapted to the cast, and the excess is trimmed.

Handle formed

2 buccal wings or ridges on either side for easy removal

EVALUATION

The completed custom tray needs to be rigid, with a consistent thickness of 2 to 3 mm.

It should extend about 3 to 5 mm cervical to the gingival margins and should be shaped to allow muscle attachment.

It should be stable on the cast with stops that can maintain an impression thickness of 2 or 3mm.

The tray must be smooth, with no sharp edges. Finally, the handle should be sturdy and shaped to fi t

between the patient’s lips. 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

THERMOPLASTIC TRAY

VACUUM FORMED

VISIBLE LIGHT POLYMERIZED

A liquefaction bath (100C [212F]) for the heavy-bodied tray material and the l ight bodied syringe material ( 10 -12 minutes)

A storage bath (about 65 C [150 F]) for maintaining l iquefi ed materials until needed. (10 minutes) Material can be stored for 5 days.

A tempering bath (about 40 C [105F]) for reducing the temperature of the heavy-bodied tray material enough to avoid tissue damage.(5- 10 minutes)

IMPRESSION MAKING WITH REVERSIBLE HYDROCOLLOID

1. Select the correct size of water-cooled impression

tray.

2. Place small modeling compound or prefabricated stops (tripod fashion) in the tray to prevent over seating.

3. Displace the gingival tissues with the help of retraction cord.

STEP BY STEP PROCEDURE:-

4. Fill the impression tray with heavy-bodied material from the storage bath. Squeeze some wash hydrocolloid onto the tray material & submerge the tray in a tempering bath. Load the syringe and replace it in the storage tank.

5. Remove the cord from the sulcus, fl ood the sulcus with warm water, and inject the light- bodied impression material over the entire surface of the prepared tooth.

6. Remove the impression tray from

the tempering tank, wipe off the

surface layer with a gauze piece

and place it in the patient’s mouth.

7. After seating, cold water is

circulated through the tray

until the impression material

is completely set. (5-6min.)

8 Hold the tray fi rmly in the patient’s mouth while the impression material is setting.

9Remove the tray with a rapid motion, wash it with cold water, disinfect it, immerse it in 2%potassium sulfate solution.

10. Pour immediately with type IV or V stone.

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

Syringe material is taken directly from tempering compartment and added to prepared tooth.

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

Gelation is accelerated by circulating cool water, through the tray for 3 – 5 minutes.

It is postulated that the hydraulic pressure of the viscous tray material forces the fl uid syringe hydrocolloid into the area to be restored.

WET FIELD TECHNIQUE

This technique combining reversible and irreversible hydrocolloid was described in 1951 by Schwartz

A modification to the traditional agar procedure is the combination of agar and alginate impression materials.

The tray hydrocolloid is replaced with a mix of chilled alginate (70 F), that bond with the syringe agar.

The alginate gels by chemical reaction, but agar gels by means of contact with cool alginate rather than water circulating through the tray

Impression is removed from mouth after 3 minutes, which is faster than other elastic material and also less expensive.

LAMINATE TECHNIQUE

Advantages Less preparation time and less complicated when

compared to the wet field technique.

Disadvantages Bond between agar and alginate is not strongHigh viscosity alginate displaces agar during seatingDimensional inaccuracy of alginate can lead to

inaccuracy

Impression technique can be classified A) Based on the tray useda) Stock tray/ putty wash i) double mix ii) Single mix (twin mix/ laminate technique)b) Custom trayc) Closed bite/ dual archB) Based on the number of viscosities useda) Heavy and light body combinationb) Medium body light body combinationc) Single mix technique using monophasic materialC) Based on the number of steps a) One step techniqueb) Two step technique

B) ELASTOMERIC MATERIALS

PUTTY/WASH TECHNIQUEAdvantages Eliminates time and expense of fabricating custom

trayMetal stock trays are rigid and are not susceptible to

distortion.Disadvantages More impression material is required.Must be sterilized

USING STOCK TRAY

Powder from latex gloves can react with impression material. It must be removed to prevent incomplete setting of the impression material.

WASHING HANDS IS A MUST!

An excellent technique for putty/wash impressions is to use the putty material to fabricate a custom tray.

It is fabricated in the same manner as with PMM materials or light-cure materials.

One layer of base plate wax is placed over the diagnostic cast as a spacer, and wax is removed from non-functioning cusps to provide occlusal stops.

A putty impression is made in a stock tray, and a PVS putty custom tray result.

In this tray made up of PVS putty, wash impression is made using light body after removing the wax spacer

THREE APPROACHES TO PUTTY/WASH IMPRESSIONS (CHEE AND DONOVAN JPD 1992)

In this technique, a pre-operative putty impression is made intra-orally.

Plastic sheets may be placed over the teeth to prevent material from entering gingival embrasures. In the area where the teeth are to be prepared, impression material is removed with a bur or scalpel to provide relief, and the impression is ‘‘washed’’ or relined with low-viscosity PVS impression material

A SECOND APPROACH USES A RELIEVED PUTTY IMPRESSION.

This approach can be successful, but there are two potential drawbacks.

It is diffi cult to confine the wash materials to the area of the relieved impression, and some wash material enters the unrelieved impression. This results in an inaccurate occlusal pattern for the resultant cast.

Thus, the entire impression, rather than just the relieved area, should be ‘‘washed.’’ This creates the potential problem of hydraulic distortion of the putty material as the impression is seated in the mouth.

This is impossible to detect on a clinical level but may have a deleterious eff ect on the accuracy if the impression and resulting restoration

With this technique, a stock tray is loaded with putty material, and the syringe material is injected around the prepared tooth or teeth. The tray containing the putty material is squashed over the syringe material, and the impression is made with the putty material and the syringe material setting simultaneously. This approach is unacceptable because it is impossible to control the thickness of impression material and excess bulk is used.

THE THIRD APPROACH ‘‘SIMULTANEOUS’’ OR ‘‘SQUASH’’

TECHNIQUE (ONE STEP TECHNIQUE).

Select a stock tray and coat with adhesive Mix high viscosity putty and roll it in the

shape of the cylinder and load it on the tray, give a space which is a sheet of polyethylene and seat with rocking motion and wait till the initial set (2 minutes) and then remove from the mouth with the minimal sideward movement.

After fi nal set spacer is removed

PUTTY WASH DOUBLE MIX

Remove any excess impression material with sharp knife

Gingival retraction doneNeeded amount of the low viscosity material is mixed

on a pad and either loaded in the syringe by scraping or by making a paper cone and then fi lling the syringe.

Remove the retraction cord gently and syringe inaccessible areas fi rst (e.g.) disto lingual finish lines.

Now insert the tray with low viscosity impression material and position the tray over the arch

Apply force in a vertical direction until further seating is impossible.

After material is set, insert two index fingers under each side of the tray to break the seal.

Remove the tray in the direction parallel to the preparation.

Evaluate the set impressions

Same as the putty wash double mix method, except for the use of polyethylene spacer.

Retract the gingival tissue with the help of gingival retraction cord.

Putty and light-bodied materials are mixed at same time.

Putty material is put onto the stock tray and syringe material is loaded in the syringe.

Remove the gingival retraction cord.Blow dry the area with compressed air & syringe the

low-viscosity material on the preparations.Position the tray over the arch. Wait till it sets.

PUTTY WASH SINGLE MIX

Step-by step Procedure Try the custom tray in the mouth to verify its fi t.

Correct as needed. Apply tray adhesive to extend a few millimetres onto

the external surface of the tray.

2) USING CUSTOM TRAY (HEAVY BODY LIGHT BODY TECHNIQUE)

Isolate the abutment teeth and place gingival displacement cord in the sulcus.

On separate pads (one for the tray and one for the syringe material), disperse equal amounts of base and accelerator.

NOTE: When mixing polysulfi de polymers pick up the brown catalyst fi rst rather than the white base material, because the base will stick to the spatula and make it virtually impossible to incorporate all the catalyst.

Blend the two pastes thoroughly. Initially, the spatula is kept somewhat vertical during mixing, which is changed gradually to a more horizontal position as the two pastes become better incorporated. At this time, the spatula is wiped on a clean paper towel. Mixing continues for another 10 seconds to ensure that the material is homogeneous. Load the syringe.

Remove the displacement cord and gently dry the preparation with compressed air.

Place the tip of the syringe nozzle so that it touches the margin and inject the material slowly. The tip should be inserted into the most distal embrasure fi rst

Heavy body or medium body tray material is loaded without overfi lling the tray

Seat the tray. It must remain immobile while the material undergoes polymerization.

The impression must be inspected for accuracy when it is removed.

If bubbles or voids appear in the margin, the impression must be discarded. An intact, uninterrupted cuff of impression material should be present beyond every margin 0.5 mm

Streaks of base or catalyst material indicate improper mixing and may render an impression use less

There should be no tray show-through in any areas of the impression, except at tissue stops

There should be no shiny smooth areas; if present, they suggest moisture contamination

Disregard small voids in unimportant areas Impression should not get separated from the tray If the impression passes these entire tests, it can be then

disinfected and poured to obtain a die and working cast

EVALUATION

The same steps are performed for the single-mix technique as for the heavy body-light body technique, however, as the name indicates, only one mix is used to load the syringe and fi ll the tray. Most single-mix materials tend to produce a more viscous combination with a slightly shorter working time.

SINGLE-MIX-TECHNIQUE

Most manufacturers off er impression material in prepackaged cartridges with a disposable mixing tip attached

The cartridge is inserted in a caulking gunlike device.

The homogeneously mix can be directly placed on the prepared tooth and impression tray.

Automixing is not available for the polysulfi de polymers because these materials are too sticky for proper combination .

AUTOMIX TECHNIQUE

An alternative method for improving impression mixing is to use a machine mixer. This is system is convenient and produces void-free impressions

MACHINE MIXING TECHNIQUE

Pentamix machine

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

C) CLOSED BITE DOUBLE ARCH METHOD

The impression includes the prepared tooth, the adjacent teeth, and the opposing teeth and records their maximum intercuspation relationship, hence the name “triple tray.

These include the use of only one tray, which captures an impression of the prepared tooth, the adjacent and opposing teeth, and the relationship between them in maximal intercuspation without the need for an interocclusal record.

This type of impression tray produces extremely accurate occlusion in the resultant mounted casts. The average occlusal error for articulated casts made from this type of impression is 5 μm, opposed to an average error of 72 μm for mounted casts made from full-arch impressions. In addition, less impression material is needed

Minimum conditionsEither natural teeth or an incisal pin and table should

be provided in the articulator as vertical stops.Suffi cient space distal to the last tooth should be

present to allow tray approximation. Intact dentition, class I occlusion, bounded on either

side by intact teeth, opposing tooth having intact occlusal contact is ideal.

Advantages Physical deformation of the impression by mandible

during opening is minimized.Seating of teeth during maximum intercuspation is

captured.Less material is needed and patient is more

comfortable.Less gagging may occurDisadvantages Tray is not rigid and it depends on the impression

materials rigidity.Not a functionally generated technique – so limited to

one casting per quadrant.

Diff erent techniques for making a dual arch impression

a) Standard technique

b) Pre impression techniques

Technique Fit of the tray is checked such that the tray extends

distal to the last tooth of the arch by and then asks the patient to close, observe for the bilateral closure and see that the patient is comfortable.

Gingival retraction is done.

A) STANDARD TECHNIQUE

Ready the tray material and the syringe material which is placed on the tooth after cord removal.

Tray placed inside mouth while observing the distal extent and slowly asking the patient to close the mouth.

After waiting for the set (2 minutes), the patient is asked to open the mouth, and then the tray adheres to one arch. After placing fi ngers on either side of the tray it is removed with equal pressure bilaterally to minimize the distortion of the tray.

The handle should not be used for removal of the tray and the material is removed from the sulcus the impression is then washed and checked.

After pouring the impression it is articulated on hinge articulator with incisal pin

Tray is removed from the articulator

Pre-impression techniques involve a 2-step process. The fi rst step is fabrication of a pretreatment matrix followed by either a wash with light-body material or the use of a laminar flow technique.

The pretreatment matrix is made of putty or heavy-body impression material.

IN THE WASH IMPRESSION TECHNIQUE, the operator should confirm that the matrix allows suffi cient space for the wash material. Light-body impression material is placed around the tooth and the tray is reseated. Pressure is used to force impression material into the sulcus around the tooth. It is important not to overfi ll the area and to provide a vent for excess material.

PRE IMPRESSION TECHNIQUES

The laminar fl ow technique also uses a pre-impression matrix followed by injection of l ight-body impression material around the prepared site.

The pre-impression matrix is modifi ed by dril l ing 2 access holes through the impression material on the buccal aspect of the prepared tooth or teeth.

One access hole is made at the mesial l ine angle and the other at the distal l ine angle.

The modifi ed pre-impression matrix is reseated in the patient’s mouth, with the teeth fully occluded into MIP.

The tip of the syringe containing l ight-body impression material is inserted into the mesial hole and material is injected until clean material extrudes from the distal hole

The copper tube or band is used to salvage an impression of multiple preparations when there are only vague margins on one or two preparations that are not adequately replicated in the impression

COPPER BAND IMPRESSION

Select a copper band of correct diameter by trial and error. Deform the tubes to semi ellipsoidal cross section and try in.

Approximate position of the finish line and mark it on the band with an explorer and cut with a scissors. Smoothen the rough edges using a carborundum stone.

Evaluate the fi t of the band such that it extends 1mm beyond the finish line.

Orientation holes are cut on the top one fi fth of facial surface of the tube.

FITTING COPPER BAND TO PREPARATION

A warm red stick compound is inserted in the top one third of the copper tube. The compound should touch the occlusal surface. It is then cooled and removed by Backus towel clamp by grasping top one fi fth of the copper band.

0.2mm of compound is removed from the impressed occlusal surface. This creates a space for the polyvinyl siloxane material.

Use a long shank carbide bur to create a hole (escape vent) through centre of compound plug. This will facilitate seating of copper band

MAKE COMPOUND PLUG

Making an impression Coat the inner surface with adhesive Mix heavy body polyvinyl siloxane Load it in the copper band Position the fi nger and on the top of band, orient and

seat customized copper band Stabilize the band Wait till the fi nal set. Using towel clip remove the band

evaluate the impression

The defective areas is relieved on the original full arch impression & the waxing of the margin created from the copper band die and transferred to the relieved die for adjustment of proximal contacts and occlusion

Duret and Termoz were granted the fi rst US patent for a computer aided design/computer assisted manufacture (CAD/CAM) device for making a dental prosthesis in 1987.

The fi rst commercially available CAD/CAM system for fabricating dental restorations was also introduced in 1987 (CEREC [chairside economical restoration of esthetic ceramics], Sirona).

DIGITAL IMPRESSIONS (CAD/CAM)

This technology uses optical scanning and requires the entire area to be captured in the impression to be coated with a reflective powder.

In 2008 the E4D Dentist system (D4D Technologies) was introduced. This system uses laser scanning and requires no reflective powder. Like the CEREC system, the E4D system can be connected directly to a milling machine to create the restoration.

PROCEDURETooth preparation and gingival retraction is doneRubber dam application is done Imaging powder and liquid is coated on the prepared

area for photo receptivitySymmetrical optic beam is passed3D picture is produced on a computer for milling

procedure

After being removed from the patient’s mouth, the impression is immediately rinsed with tap water and dried with an air syringe.

There are 5 types of chemical disinfectants that can be used for this purpose.Chlorine compoundsCombination synthetic phenolic compoundsGlutaraldehyde IodophorsPhenolic/alcohol combinations

DISINFECTION

The most recent recommendation for the disinfection of impression and casts published by the ADA council on dental materials, instrument and equipment that the immersion of polysulfi de, condensation silicone, polyvinyl siloxane polyether and agar hydrocolloid in ADA ‑ accepted disinfecting solution that require immersion for no longer than 30 minutes.

An alternation technique, SPRAYING can be used on those materials most vulnerable to distortion. This is done by rinsing the impression under running water, trimming excess impression material, spraying the entire impression top and bottom, (including the tray) and then sealing the impression tray in the bag for the time recommended for the disinfectant used. Solutions requiring shorter immersion times should be selected for materials that are prone to distortion when immersed in water.

 Devan stated that the impression should be in the dentists mind before it is in his hand.

A good impression is an exact negative replica of each prepared tooth and must include all of the prepared surfaces as well as an adequate amount of unprepared tooth structure adjacent to the margin.

While making an impression dentist should judiciously select the impression material, achieve adequate gingival retraction, good saliva control and manipulated the impression according to manufacture’s instruction, so that a good impression can be made.

CONCLUSION

THANK YOU