Saturday, August 10, 2013





          An imprint or negative likeness of the teeth and/or edentulous areas where the teeth have been removed, made in a plastic material which becomes relatively hard or set while in contact with these tissues.

Impression material
          Any substance or combination of substances used for making a negative reproduction or impression.

Impression tray
          A device which is used to carry, confine and control an impression material while making an impression.

Ideal requirements of impression material (According to Tylman)
Ø Complete plasticity before use
Ø Sufficient fluidity to record fine detail
Ø The ability to wet the oral tissues
Ø Dimensional accuracy
Ø Dimensional stability
Ø Complete elasticity after cure
Ø Optimal stiffness
Ø Have a good shelf life
Ø Be non-toxic and non-irritating
Ø Have acceptable taste and odor
Ø Have suitable working and setting times
Ø Have strength to resist tearing
Ø Be compatible with model and die material
Ø Facilitate clinical identification of beginning and end of cure
Ø Facilitate visualization of the finish line
An impression for a fixed restoration should meet the following requirements
Ø 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.
Ø Teeth and tissues adjacent to the prepared tooth must be accurately reproduced to permit proper articulation of the cast and contouring of the restoration.
Ø It must be free of bubbles, especially in the area of the finish line and occlusal surfaces of other teeth in the arch.
Impression materials that are used in fixed prosthodontics are
Ø Reversible hydrocolloids
Ø Polysulfide
Ø Condensation silicone
Ø Addition silicone
Ø Polyether
Ø  urethane dimethacryalte

According to chemical nature
Ø Hydrocolloid impression material
Ø Elastometric impression material
Each of them are further classified according to the viscosity
      Heavy body
      Regular/ medium body
      Light body
      Very heavy/ putty

Reversible hydrocolloid (Agar)
          The credit for its first use in United States for fabricating cast restorations is given to J.D. Hart (1930). It is a polysaccharide extracted from certain types of seaweed.

Percentage by weight

Potassium sulfate
Wax hard diatomaceous earth
Alkyl benzoates
Thizotrophic materials
Coloring agents
13 – 17%
0.2 – 0.5%

1 – 2%
0.5 – 1.1%

Basic constituent
Improves strength of the gel retards plaster or stone

Gypsum hardener

Dispersion medium

Gellation process
          The setting process of reversible hydrocolloid is called as a gellation process it is due to change in temperature.

          It is available as tray material, or as syringe material as semi-solid gel in polyethylene tubes.

The cycle is
          Gel    ----------à      Sol       -------------à Gel
        (Tube)                 (Conditioner)                    (Tray)

          Agar requires a conditioning unit which has three compartments one for liquefying were the tubes are placed at 1000C as it is too hot for intra oral use it is cooled in two stages: storage and tempering. Addition to lowering the temperature it also increases the viscosity. The storage temperature is 700C and can be kept for 5 days. Tempering is done just before the impression is made that is between 370C – 500C. After loading in the tray and placing in the mouth water is circulated at 170C – 210C until the material is completely gelled. This is done using water cooled rimlock tray

Elastomeric impression materials
Polysulfide (Mercaptan, Thiokol)
Ø Liquid polysulfide polymer à 80 – 85%
Ø Inert fillers (TiO, Zn Sulfate, Copper carbonate) à 16 – 18%
Ø Plasticizers (Dibutyl phathalate)

Accelerator paste
Ø Lead di-oxide à 60 – 68% - Reactor
Ø Dibutyl phathalate à 30 – 35% - Plasticizer
Ø Sulfur à 3% - Retarder

Setting reaction
          This polymer is usually cross linked with an oxidizing agent such as lead dioxide. It is this lead dioxide that gives polysulfide its characteristic brown color. This is a condensation reaction where all the polymer chains grow simultaneously and a reaction by product is formed like water.

They are available as
Ø Light body
Ø Regular body
Ø Heavy body

Polysulfide impression material is hydrophobic and should be poured within 1 hour of removal from the mouth and it is a radio opaque impression material because of the presence of lead dioxide.

Condensation silicone
Base paste
Ø Poly dimethyl siloxane     à 25 – 65%
Ø Colloidal silica or micronize metal oxide filler (Depending upon the viscosity) à 35 – 75%
Ø Coloring agents

Accelerator paste
Ø Orhtoethyl silicate – cross linking agent
Ø Stannous octate – catalyst

Available as
Ø Light body
Ø Regular body
Ø Heavy body
Ø Putty

Setting reaction
          This is condensation polymerization reaction which reacts with trifunctional and tetrafunctional alkyl silicates like orthoethyl silicate in the presence of stannous octate. Ethyl alcohol is a by product which is responsible for shrinkage of the material.

          The condensation silicone impression materials are supplied as a base paste and a low-viscosity liquid or catalyst paste. Since ethyl alcohol is the by product they are dimensionally stability is poor and they should be poured immediately after the removal from the mouth. They have a very poor shelf life because of the instability of alkyl silicates in the presence of organo-tin compounds, which may result in the oxidation of tin.

Addition silicone
Base paste
Ø Poly methyl hydrogen siloxane
Ø Other siloxane polymers
Ø Fillers

Accelerator paste
Ø Divinyl siloxane
Ø Other siloxane prepolymers
Ø Platinum salt (chloroplatinic acid) catalyst
Ø Palladium – hydrogen absorber
Ø Fillers

Available as
Ø Light body
Ø Regular / medium body
Ø Heavy body
Ø Putty

Setting reaction
          It is addition polymerization reaction terminated with vinyl groups and is cross linked with hydride groups activated by a platinum salt catalyst. There is no reaction by products as long as correct proportions of vinyl silicone and hydride 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 another way is to wait for 1 hour before pouring up the impression.

Trays used
Ø Custom tray / resin tray
Ø Stock tray

Base paste
Ø Polyether polymer
Ø Collidal silica
Ø Glycoether or dibutyl phathlate
Accelerator paste
Ø Aromatic sulfonate ester – cross linking agent
Ø Collidal silica
Ø Dibutyl phathalae or glycoether

Ø Single viscosity (both syringe and tray material)
Ø Regular

Setting reaction
It is a addition polymerization reaction with no by products the reaction is between aziridine rings which are at the end of branched polyether molecules. The main chain is a copolymer of ethyle oxide and tetrahydrofuran. Cross linking and setting is brought about by an aromatic sulfonate which Acts as an initiator by releasing a cation R+ an alkyl group. This produce the cross linking by cationic polymerization via the imine end groups.

Trays used
Ø Custom / resin trays
Ø Stock tray

Comparison of advantages and disadvantages
Recommended uses
Reversible and hydrocolloid
Ø Hydrophilic
Ø Long working time
Ø Low cost
Ø No custom tray required
Ø Low tear resistance
Ø Low stability
Ø Equipment needed

Ø Multiple preparations
Ø Problems with moisture

Pour immediately use only with stone
Ø High tear strength
Ø Easier to pour than other elastomers

Ø Messy
Ø Unpleasant odor
Ø Long setting time
Ø Stability only fair
Ø Most impressions
Latex gloves use in contra-indicated for all the elastomers. Pour within 1 hour.
Addition silicone
Ø Dimensional stability
Ø Pleasant to use
Ø Short setting time
Ø Automix available
Ø Hydrophobic
Ø Poor wetting
Ø Some material release hydrogen
Ø Most impressions
Delay pour some materials, care to avoid bubbles when working
Condensation silicone
Ø Pleasant to use
Ø Short setting time
Ø Hydrophobic
Ø Poor wetting
Ø Low stability
Ø Most impressions
Pour immediately, care to avoid bubbles when pouring
Ø Dimensional stability
Ø Accuracy
Ø Short setting time
Ø Automix available
Ø Set material very stiff
Ø Most impressions
Care not to break teeth when separating cast

          Various type of trays are used for fixed partial dentures impression procedures.

1. Stock tray
Ø Metallic
Ø Non metallic

2. Custom made trays
Ø Autopolymerizing acrylic resin
Ø Thermoplastic or photo initiated resin

3. Water cooled rim lock trays

Custom trays
Ø Improves the accuracy of an elastomeric impression by limiting the volume of the material, thereby reducing the 2 sources of error
a.    Stress during removal
b.    Thermal contraction
Ø No need for sterilization
Ø Uniform thickness of the impression material minimizes distortions resulting from curing shrinkage
Ø Procuring of the tray material is not required

Ø Time taken for the fabrication
Ø Aging for 24 hours to minimizes the distortion
Ø Sensitivity to monomer

Steps in fabrication
Ø Soak replicas of diagnostic casts in slurry water for 10 mins.
Ø Tray extensions are marked with the pencil line at either the cervical region of the teeth or 5mm below the cervical line.
Ø Base plate wax is adapted (for 2 sheets thickness – 2-3mm) after softening on the Bunsen burner and trim the excess with the knife until the marked pencil line .(Rigid or stiff material like polyether requires of wax spacer of 4mm thickness).
Ø Cover the wax with tin foil, or aluminium foil for preventing the wax to melt during the exothermic heat produced during the polymerization of the resin (autocuring).
Ø Wax is removed either in 4 or 3 tripodal areas 3 and located not on the prepared tooth but on non-functional or non-centric cusps. If all the teeth are prepared then soft tissues stops either on the palatal area or the crest of the ridge is placed.
Ø Needed amount of polymers and monomers of autopolymerizing resin is mixed as per manufacturer instructions till the dough stage is attained.
Ø Then it is flattened to almost 4mm thick putty. It is then adapted and molded over the tin foil separation and excess is trimmed off.
Ø A handle is formed along with 2 buccal wings or ridges on either side for the easy removal.
Ø After the set it is slowly removed form the cast, check if the stops are proper.
Ø It can then be placed in the water for 9 – 24 hours (Rosenstiel) or 5 minutes in boiling water before use.
Ø A tray adhesive can be applied 15 minutes before placing the impression material and allow it to dry (inside and outside the bond of the tray)

Advantages of stock tray
Ø Eliminating the time and expenses of fabricating a customized tray.
Ø They are rigid metal and susceptible to distortion.

Impression techniques
For reversible hydrocolloid
          It requires a hydrocolloid conditioning unit. A conditioning unit has three units.
Ø Liquefying bath
Ø Storage bath
Ø Tempering bath
1. Liquefying bath
          Tubes of impression materials and syringes are boiled at 2120F (1000C) for 10 – 12 minutes.

2. Storage bath
          Stored at 1500F (650C) at least for 10 minutes. The material can be stored for 5 days.

3. Tempering bath
          Loaded impression trays are tempered in this bath at 1100F to 1150F (about 400C) for 5 – 10 minutes immediately before placing in the mouth. 

          Because only one accurate cast can be made from a hydrocolloid impression, two impressions are made
  1. A sectional impression for making a die (made first)
  2. A full arch impression for the working cast
Ø After selecting a proper tray and placing stops in the proper area.
Ø Tray material is kept on the tray for tempering.
Ø Low viscosity syringe material is placed in the bath which is then removed and applied after the retraction cord is removed and then the tray with heavy body is placed into the mouth and cold water is circulated.
Ø It is then removed after holding without movement with a rapid motion, washed with cold water, inspected, disinfected and immediately type 4 stone is poured.

Wet field technique
Ø The areas of the teeth and tissues are flooded with warm water.
Ø Syringe material is taken directly from tempering compartment and added to prepared cavities, first at base of preparation and then the tooth is covered.
Ø The material used to fill the tray should be cooler or tempered.
Ø Gelation is accelerated by circulating cool water, through the tray for 3 – 5 minutes.
Ø It is postulated that the hydraulic pressure of the viscous tray material forces the fluid syringe hydrocolloid into the area to be restored.

Laminate technique
Ø A recent modification to the traditional agar procedure is the combination of agar and alginate impression materials.
Ø The tray hydrocolloid is replaced with a mix of chilled alginate, that bonds with the syringe agar.
Ø The alginate gels by chemical reaction, but agar gels by means of contact with cool alginate rather than water circulating through the tray

Ø Less preparation time and less complicated when compared to the wet field technique.
Ø Bond between agar and alginate is not strong
Ø High viscosity alginate displaces agar during seating
Ø Dimensional inaccuracy of alginate limits the use to single units.

Techniques for electrometric impression materials
1. Using stock tray
          Synonyms – Putty wash
          Mixing method – double mix and single mix

Ø Eliminates time and expense of fabricating custom tray
Ø Metal stock trays are rigid and are not susceptible to distortion.
Ø More impression material is required.
Ø Must be sterilized

Ø Select a stock tray and coat with adhesive
Ø Mix high viscosity l putty and roll it in the shape of the cylinder and load it on the tray, give a space which is a sheet of polyethylene and seat with rocking motion and wait till the initial set (2 minutes) and then remove from the mouth with the minimal sideward movement.
Ø Gingival retraction done
Ø After lubricating the O-ring of the syringe, the needed amount of the low viscosity material is mixed on a pad and either loaded in the syringe by scraping or by making a paper cone and then filling the syringe.
Ø Remove the retraction cord gently and syringe inaccessible areas first (e.g.) disto lingual finish lines.
Ø Now insert the tray with low viscosity impression material
Ø Position the tray over the arch
Ø Apply force in a vertical direction until further seating is impossible.
Ø After material is set, insert two index fingers under each side of the tray to break the seal.
Ø Remove the tray in the direction parallel to the preparation.
Ø Evaluate the set impressions.

Difference between single mix and double mix technique
Ø In single mix technique one viscosity material (regular body) is used to fill the tray and load the syringe.
Ø In double mix technique, the light body is loaded in the syringe and syringed around the tooth prepared and heavy body is mixed and loaded in the tray.

2. Using custom tray
Ø Do the gingival retraction and Prepare the syringe
Ø In a mixing pad mix the low viscosity impression material as per manufacturer’s instruction. First use circular motion combining the two strands then a finger of eight motion, obtaining a streak free mixture in a less than 1 minute.
Ø Load the syringe as well as the tray.
Ø Syringe the material in the inaccessible area first then subsequently removing the retraction cord gently and then syringe the impression material.
Ø Now insert the tray. Seal from posterior to anterior allowing excess to extrude in an anterior direction.
Ø Continue seating in a vertical direction until the trays stops prevent further progress.
Ø After the material is completely set, remove the impression parallel to the preparation path.
Ø Evaluate the impression.

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

Minimum conditions
Ø Either natural teeth or an incisal pin and table should be provided in the articulator as vertical stops.
Ø Sufficient space distal to the last tooth should be present to allow tray approximation.
Ø Intact dentition, class I occlusion, bounded on either side by intact teeth, opposing tooth having intact occlusal contact is ideal.

Ø Physical deformation of the impression by mandible during opening is minimized.
Ø Seating of teeth during maximum intercuspation is captured.
Ø Less material is needed and patient is more comfortable.
Ø Tray is not rigid and it depends on the impression materials rigidity.
Ø Not a functionally generated technique – so limited to one casting per quadrant.

Types of dual arch trays
Ø Metal
Ø Plastic (both with or without side wall)

Depending on the location
Ø Posterior / anterior sexant
Ø Quadrant
Ø ¾ of an arch
Ø Full arch

Ø Fit of the tray is checked such that the tray extend distal to the last teeth of the arch by and then ask the patient to close, observe for the bilateral closure and see that the patient is comfortable.
Ø Gingival retraction is done.
Ø Ready the tray material and the syringe material which is placed on the tooth after cord removal.
Ø Tray placed inside mouth while observing the distal extent and slowly asking the patient to close the mouth.
Ø After waiting for the set (2 minutes), the patient is asked to open the mouth, and then the tray adheres to one arch. After placing fingers on either side of the tray it is removed with equal pressure bilaterally to minimize the distortion of the tray.
Ø The handle should not be used for removal of the tray and the material is removed from the sulcus the impression is then washed and the retraction cords is removed and checked.

Different techniques for making a dual arch impression
One step technique
          Stock trays are used, light body is injected around the prepared tooth and putty or bite registration paste is inserted on both sides of the tray and the patient is asked to bite in centric occlusion.

Dual arch – hydraulic pressure technique
          Low or medium viscosity impression material is injected on the prepared and preoperative impression of the unprepared tooth. The tray is placed and patient is asked to bite in centric occlusion, the hydraulic pressure created, will force the material into the sulcus and through the vent holes created in the buccal or the lingual sides of the pre-operative impression.

Dual arch – laminar impression technique
          A pre-operative impression is made. The post operative impression of the tooth to be prepared is relieved to a depth of 0.5mm in the cervical area. Two holes are drilled from the buccal surface and one on the mesial and one of the distal.

Auto-mix technique
Ø Several manufacturers often supply impression material in pre-package cartridges to which a disposable mixing tip is attached.
Ø The cartridge is inserted in a caulking-gun like device, and the base and catalyst are extruded into the mixing tip, where mixing occurs as they progress to the end of the tube.
Ø The homogenously incorporated material can be directly placed on the prepared tooth and impression tray.

Matrix impression system
          This is a new system that requires a series of three impression procedures using three types and/or viscosities of impression materials.

Ø A matrix of occlusal registration elastomeric material is made over the tooth preparation.
Ø Matrix is trimmed to prescribed dimension and  retraction  cord is removed.
Ø A definitive impression is made in matrix of the preparation with a low viscosity elastomeric impression material.
Ø After the matrix impression is seated, a stock tray is filled with a medium viscosity elastomeric impression material is seated over the matrix and remaining teeth to create an impression of the entire arch.

Ø This system effectively controls the four forces (relapsing, retraction, displacement and collapsing) that impact on the gingiva during the critical phase of making impression when attempting to register the subgingival margins.
Ø The design of the matrix also gently forces the high viscosity impression materials along the preparations and in to the sulcus where it cleanses the sulcus of unwanted debris and fills the sulcus.
Ø The high viscosity material gently extends into the sulcus and does not permit it to collapse as the medium viscosity material in the stock tray a seated for the pick up impression.
Ø The matrix facilitates the formation of the optimum flange.
Ø Tearing is virtually eliminated because of the improved configuration of the sulcular flange and by the elimination of voids or contaminants in the sulcus.

Copper band impression
Fitting copper band to preparation
Ø Select a copper band of correct diameter by trial and error method. Deform the tubes to semi ellipsoidal cross section and try in.
Ø Approximate position of the finish line and mark it on the band with an explorer and cur with a scissors. Smoothen the rough edges using a carborundom stone.
Ø Evaluate the fit of the band such that it extends 1mm beyond the finish line.
Ø Orientation holes are cut on the top one fifth of facial surface of the tube.

Make compound plug
Ø Fingers are covered with petrolactum jelly
Ø A warm red stick compound is inserted in the top one third of the copper tube. The compound should touch the occlusal surface. It is then cooled and removed by Bachins towel clamp by grasping top one fifth of the copper band.
Ø Excess is relieved by cutting with a no. 6 or no. 8 round bur and under slow speed and by frequently removing debris. 0.2mm of compound is removed from the impressed occlusal surface. This creates a space for the heavy body polyvinyl siloxane.

Making an impression
Ø Coat the inner surface with adhesive
Ø Mix heavy body polyvinyl siloxane
Ø Load it in the copper band
Ø Position the finger and on the top of band, orient and seat customized copper band
Ø Stabilize the band
Ø Wait till the final set. Using towel clip remove the band evaluate the impression and followed the orientation impression.

Special considerations
Pin retained restorations
          Elastomeric impression materials are strong enough to be used in these situations but should be introduced with
Ø Cement tube or Lentulo spiral to avoid any bubbles
Ø Special nylon bristles or Prefabricated plastic pinscan be put in the pin hole and the impression is made

Post and cores
          This procedure involves reinforcing the impression with a plastic pin or suitable wire (e.g. orthodontic wire) – elastomeric impression materials can be used.

Impression technique with the use of preformed crown shells
Ø Select preformed crowns (polycarbonate) adjust the gingival margin slightly apical to finish line.
Ø Adhesive applied to inner side
Ø Regular body is loaded
Ø Then seated properly
Ø Wait till it sets and remove
Ø Then full arch pick up impression made
Impression procedure for a subgingivally prepared shoulder margin
Ø An individual tray is prepared directly with acrylic, which should not cover the shoulder margin. Gingival retraction done.
Ø The tray is rebased with fluid resin and some placed on the teeth and pressed till contact is felt wait till the acrylic becomes hard.
Ø An outline is marked and excess is removed. Space is created for the impression materials except the shoulder margin area.
Ø An adhesive is then applied and the tray filled with elastomer. This impression is then picked up with elastomer or hydrocolloid in a stock tray.

Impressions for CAD/CAM procedures
Prepared tooth
Apply rubber dam
Imaging powder + liquid is coated on the prepared area for photo receptivity
Symmetrical optic beam is passed
3D picture is produced on a computer for milling procedure

This technique is mostly advocated for the CEREC II system. 
Evaluation of impression
Ø Upon removal the impression must be inspected for accuracy.
Ø Bubbles or voids in the margin necessitate discarding the impression and starting over.
Ø An intact uninterrupted cuff of impression material should be present beyond every margin.
Ø Streaks of base and catalyst material indicate improper mixing and may render an impression useless.
Ø Immediately after removal from the mouth impression is rinsed under tap water and dried with an air syringe.
Ø Disinfection is an essential step for prevention of cross infection and exposure of laboratory personnel.
Impression material
Recommended disinfection procedures
1. Reversible hydrocolloid
2.Irreversible hydrocolloid
3. Polysulfide
4. Addition silicone
5. Condensation silicone
6. Polyether
Diluted bleach, Iodophor spray
Diluted bleach, Iodophor spray
Immersion in iodophor or 2% glutaraldehyde
Immersion in 2% glutaraldehyde or hypochloride solution
Immersion in 2% glutaraldehyde or hypochlorite solution
Iodophor spray or chloride dioxide

Ø Done properly it has no clinically significant effects on the accuracy or surface reproduction of the elastomers.

            Devan states that the impression should be in the dentists mind before it is in his hand. Based on this statement I conclude the use of impression material and technique should be purely the dentists choice.


1.    Restorative Dental materials:G Craig & John M Powers-11th edition2002, pg 329-378
2.    Phillips science of dental materials:11th edition
3.    GPT Academy of prosthodontics JPD,july 2005 vol 94
4.    Tylman’s Theory and Practice Of Fixed Prosthodontics, 8th edition, pg 237-254.
5.    Fundamentals Of Fixed Prosthodontics by Herbert T. Shillingburg, 3rd edition, pg 281
6.    Contemporary Fixed Prosthodontics, by Rosenstiel, 4th edition, pg452- 462.
7.    Notes on dental materials by E.C. Combe, 6th edition,pg 115- 126.