Tag Archives: orthodontic materials





The majority of impressions taken by orthodontists are used for pre- and post- records of dental arches.


“Restorations can be as true as your impressions”.


The underlying principle emphasizes the need for complete and accurate impression and highlights the importance of clinical impression procedures.


Purpose of impression is the formation of ‘positive’ model of the proper physical dimension, shapes and spatial relationship of these structures.




Impression – 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 impression or negative reproduction.


-GPT 8




As early as the 17th century G. PURMAN of Breslau was said to have used wax for taking dental impressions.


During the late 1790s plaster was used to take impression for the famous dentures of Sir George Washington. (Dr. John Greenwood—a New York dentist)


In 1844-Westcott Dwinelle and Dunning started the use of PLASTER OF PARIS for taking impression for denture fabrication.


  • Elastic Impression first introduced to the dental profession in 1925 in the form of AGAR HYDROCOLLOID  called ‘Reversible Gel’. Since temperature changes its physical state.
  • Since then, search for the ideal impression material has continued.


Historically, impression making was accomplished with inelastic materials for both soft and hard tissues.


Hydrocolloids ware initially introduced to make impression of hard tissues in place of inelastic materials.


After World War II, advances in polymer technology brought to the dental profession a group of synthetic rubbery materials called ‘ELASTOMERS’


They are capable of making impression of both hard and soft tissue.


In 1950s the rubber base material first in the form of Mercapton Polysulfide and later the Silicon began to be used as dental impression materials.


In 1994, Mitchell described a technique of occlusal registration for functional appliances using elastomeric bite registration materials.


The colloidal material used for making impression are either —-Agar or Algi dissolved in water –Hence the term Hydrocolloid impression materials.


Hydrocolloid impression materials can be classified as:-


  1. REVERSIBLE              eg. Agar
  2. IRREVERSIBLE         eg. Alginate




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


It was later described by Skinner as colloidal sols of emulsified type.


Sears introduced reversible hydrocolloids in 1937




 Supplied as gel in collapsible tube or as Cylinders in a jar.




Agar 13-17 To provide dispersed phase
Borate 0.2-0.5 Provides strength
Sulfate 1-2 Gypsum hardener
Wax 0.5-1 Filler
Thixotropic materials 0.3-0.5 Thickener
Water Balance Reaction medium




The ADA specification No 11 sets standard properties required by agar hydro colloidal material.




    The material should be able to record the fine details


Gelatin temperature:-


After boiling for 8 min, the material should be fluid enough to be extruded from container. After tempering the solution should be homogenous and should set between 37 and 45 when cooled.


The ADA specification requirement for flexibility allows a range of 4% to 15% at a stress of 14.2 psi.


Elasticity and elastic recovery:


Agar hydrocolloids are highly elastic in nature and elastic recovery occurs to the extent of 98%


Accuracy and dimensional stability


Agar impressions are highly accurate at the time of removal from mouth but shrink when stored in air and expand when stored in water.  The least dimensional change occurs when stored in 100% humidity but immediate pouring of cast is recommended.


Working and setting time:-


Working time ranges between 7-15 minutes and setting time is about 5 minutes.  They can be controlled by regulating flow of the water through cooling tubes.




Tear and Compressive strength


Tear strength is 4 psi (psi = pound per square inch)


Compressive strength is 800 gm/cm2.




Conditioning unit is required for manipulation of agar. The Conditioner consists of


  • Boiling section
  • Storage section
  • Tempering section.

In boiling section, the material is kept for 10 min for liquefaction.

Then material is stored in storage section at 65deg.cel. in sol form until it is required.

Material is tempered at 45deg.cel. For 3 to 10 min so as to be tolerated by the patient.

Just before the tempering process for the tray material is completed, the syringe material is directly taken from the storage compartment and applied to teeth.  The water soaked on the layer of the tray hydrocolloid is removed from the container and gauge is removed.

The tray is immediately brought into position and seated with tight pressure and held with a very light force.

Gelatin is accelerated by circulating cool water (18-21deg.cel.) through the tray for 3-5 min.  After the gelatin is over, the tray is removed suddenly with a snap.

—  Advantages:

  1. Accurate dies can be prepared if handled properly.
  2. Good elastic property and reproduces undercuts  properly
  3. Well tolerated by patient
  4. Used for duplicating models.

—  Disadvantages:

1)      Flow is less Compared to recent elastic impression materials

2)      Gelatin may be painful to the patient.

3)      Tears relatively easily.

4)      Extensive equipment is required

Colloids exist as sol or gel.

A sol has the appearance and many characteristics of a viscous liquid.

A gel is a jelly-like elastic semisolid and is produced from a sol by a process called gelation.

It occurs in two ways

1) By temperature changes

2) By chemical means


Gelation in this case is a reversible process

Eg. Agar

The fibrils are held together by secondary molecular forces— they break at slightly elevated temperature and become re-established as the hydrocolloids cool to room temperature.

The temperature at which these changes occur is GELATION TEMPERATURE.

Gelation occurs at approx 37 deg. C temperature whereas liquifaction occurs at a higher temperature, i.e. 60-70 deg C higher than the gelation temperature.

This temp lag between liqueifaction and gelation is known as hysteresis.



  • Conversion of sol           into       gel
  •  The fibrils formed are held together by primary bonds And is unaffected by  temperatures.




A majority of gel volume in hydrocolloid is made of water.

Gel may lose water by:-

a)Evaporation- from the surface

b)Syneresis- Exudation of fluid on the surface.

The exudate is not pure water, may be alkaline or acid depending on the composition of gel, both these process leads to SHRINKAGE.

Gel may absorb water if placed in water by a process called IMBIBITION causing gel to swell.

Both SYNERESIS and IMBIBITION should be avoided

As former can cause SHRINKAGE and later EXPANSION.

The dimensional changes are important in dentistry as any changes in dimension of impression after it has been removed from the mouth is a source of error.



It is extracted from certain brown seaweed.  The substance is called anhydro-beta-d mannuronic acid or alginic acid.


1) Type I – Fast setting (1-2 min)

2) Type II- Normal setting ( 2-4.5 min)


Potassium alginate       15% Dissolves in water and  reacts with calcium ions
Zinc Oxide        4% Acts as filler
Potassium titanium fluoride       3% Accelerator
Diatomaceous earth       60% Filler
Sodium phosphate        2% Retarder
Coloring  and Flowering agents      TRACES



The chief ingredient of irreversible hydrocolloid is one of the soluble alginates. Reaction occurs by a chemical reaction.  Soluble alginates reacts with Calcium sulfate to produce insoluble calcium alginate as a gel.


The production of calcium alginate is delayed by the addition of a third soluble salt to the solution with which calcium sulfate will react in preference to the soluble alginate to form an insoluble calcium salt.

Thus, the reaction between the calcium sulfate and the soluble alginate is prevented as long as any of the added salt is left.  The added salt is called retarder.

The reactions that occur are

2Na3 PO4 + 3 CaS04 à Ca3 (Po4)2 +  3 Na2 So4  When the supply of retarder ( trisodium phosphate) is exhausted, Calcium ions begins to react with potassium alginate to produce calcium alginate.

Sod. Alg + n CaSo4 + H2Oà nNa2So4 + Cal Alg


The gel structure consists of a brush heap of calcium alginate fibril network enclosing unreacted sodium alginate sol, excess water, filler particles and reaction by products.

It is a cross linked structure where calcium is responsible for cross linking.


Setting time can be altered by altering  the amount of water.

As the temperature increases, the setting time decreases.

For 10 rise of temperature 1 min reduction in setting time occurs.

Bowl and spatula also can be cooled to increase setting time.


Gypsum can cause acceleration of setting of alginate

Alginate contamination can decrease strength of casts

For manipulations a clean bowl and metal spatula.

The First step in the manipulation is to prepare a proper mixture of water and powder.  Water is poured in to bowl and specified weight of powder  is added and powder is carefully incorporated into water with spatula.

A vigorous figure 8 motion is best with the mix being stopped against the sides of bowl with intermittent rotations of the spatula to press out air bubbles.  The final mix should be smooth creamy mixture that does not drip off the spatula when it is raised.  Mechanical devices are also available for mixing.  Their advantages are speed convenience and eliminating human errors.


Alginate impression develops effective seal which can be freed by running the finger around periphery.  Impression should be removed suddenly with a jerk. Torquing or twisting should be avoided. After removal impression should be washed with water to remove saliva, covered with wet gauge to prevent synerisis and cast should be poured as soon as possible.


ADA specification No. 18 for alginate sets following standards

Mixing time:

Fast setting– 45 Sec

Normal setting – 60 Se

Working time:

Fast setting – 1.2 to 2minutes

Normal setting – 2-4.5minutes

Setting time:

Fast setting – 1.2 minutes

Normal setting – 2-4.5minutes

Over mixing Causes:

Reduction in final strength as gel fibrils are destroyed

Reduction in working time.

Under mixing Causes:

Lack of homogeneity and reduced strength


—  The compressive strength of alginate ranges from 5000-8000gm/cm2.

—   The tear strength ranges between 350-700gm/cm2.

—              Both these properties are time dependent. If removal is delayed, strength increases.

—       The final strength depends on

—  W/P ratio: Too much or too little water reduces strength

—  Mixing time: Over and under mixing reduces strength


—  Alginate impression lose water by evaporation and shrink in air. If placed in water it absorbs and swells. So cast should be poured immediately




Contact Angle 37 degrees (which means they will displace blood and saliva & pour well with gypsum stones)

Easy manipulation

Minimum equipment

Low cost

Accurate if properly handled


Low tear strength

Poorer reproduction of surface details

Not Dimensionally stable on storage due to syneresis


Zhermack advertised that the alginate impression can be kept up to 48 hours prior to pouring in their long life Hermetically sealed bags

The Vanilla flavoured Orthoprint is Anti Nausea.

Vival NF: Dimensionally stable for up to 100 hours.

High tear strength alginates: Silicone reinforced two paste system one with alginate, other with calcium reactor. It has higher tear strength

Self disinfecting alginates containing iodophor.

Dust free alginates: Organic glycol is used to agglomerate the dust particles


In this technique agar in the syringe is injected on to the area to be recorded, an impression tray containing chilled alginate is placed over it. 

The alginate sets by chemical reaction, where as agar gels by means of contact with cool alginate. Since agar is in contact with teeth maximum detail can be obtained.


Traditionally alginate is supplied in two component system, powder and water.  But other forms are also available.

In the form of a solution containing water and a reactor of plaster of Paris

As a two paste system one containing solution and other calcium reactor. These materials contain silicone and are supplied in tray viscosity and syringe viscosity.


Rinse the impression

Place in zip lock bag, pour in disinfectant to cover all surfaces

Seal for 10 minutes

Remove and rinse prior to pouring stone

Chlorhexidine has also been incorporated to limit cross infection (Hydrogum + CHX- Zhermack)



Condensation polymerizing silicone

Addition polymerizing silicone


Each type is further divided into four viscosity classes

Light body

Medium / regular body

Heavy body



Excellent reproduction of surface details. The low viscosity is able to record fine details

They are hydrophobic. So oral tissues in the area of impression should be dry.

Co-efficient of thermal expansion of elastomers is high.  So thermal contraction of set material occurs when transferred to room temperature

Tear strength is excellent. So impressions can be removed from thin areas.

The elastomeric materials can be copper/silver plated

Shelf life of elastomers is good

Dimensional stability

For visco-elastic material slow elastic recovery may control for sometime after withdrawal of the impression, producing dimensional change. In this case the dimensional change results in more accurate impression.

But many impression materials contain volatile substances either as primary components or as by products of setting reaction. Loss of such volatile materials results in shrinkage of impression material resulting in decrease in accuracy. So for majority of materials, accuracy can be maintained by pouring the cast soon after the removal of impression.

Curing shrinkage

Loss of byproducts

Polyether being hydrophilic absorbs water and loses soluble plasticizers causing change in dimension

Thermal contraction when transferred from mouth to room temperature

Incomplete elastic recovery after deformation during removal

Amount of filler

Uniform thickness of material gives more accurate impression

Time of pouring of impression. Impression should be powered after elastic recovery and before dimensional change


Also called as Mercaptan or Thiokol


It is supplied in tubes as base and accelerate in paste form

Available in three viscosities

Light bodied

Medium bodied

Heavy bodied



Polysulfide polymer – 80-85% This is further polymerized and cross linked to form rubber
Filler (Titanium dioxide zinc sulfate) – 16-18% To provide required strength
Plasticizer (Dibutyl Phthalate) To provide appropriate viscosity
Small quantity of sulphur – 0.5% To accelerate the reaction


Accelerator paste

Lead dioxide – 60-68% To react with thiol group causing setting and give brown colour
Filler and plasticizer To provide strength and to give adequate viscosity
Oleic / stearic acid Act as retarder to control rate of setting reaction



The main component of polysulfide is a multifunctional mercaptan (-SH) or polysulfide polymer. These linear polymer contains pendent –SH groups

Reaction yields water as by product


Unpleasant odour and colour

It has a long Setting time of 8 to 12minutes

Mixing time is 45 seconds

Excellent reproduction of surface detail

Dimensional stability:

The curing shrinkage is high (0.45%)

Has the highest permanent deformation (3.5%)

This improves with time.  So pouring of model should be delayed by half an hour

Tear strength is 4000 gms/cm2

It has good flexibility.  A 2mm spacing in the tray is sufficient to make the impression.

It is hydrophobic.  So impression area should be dry

Shelf life is good i.e. 2 years



            Two types of silicone impression materials are available based on the type of polymerization reaction occurring during setting

Condensation silicone

Addition silicone


Available as base and catalyst in paste form. The condensation silicone catalyst can also be supplied as a liquid. The putty is supplied in jars.

Condensation silicone is available in three viscosities.

Light bodied

Medium bodied




Hydroxyl terminated polydimethyl siloxane Undergoes cross linking to form rubber
Colloidal silica Filler


Tetraethyl orthosilicate Cross linking agent
Stannous octate Catalyst



It is a condensation reaction

Polymerization occurs as a result of cross linkage between orthoethyl silicate and terminal hydroxyl group of dimethyl siloxane to form a three dimensional network

Stannous octate acts as a catalyst

Ethyl alcohol is by product. Its evaporation probably accounts for much of the condensation taking place in set silicone impression


Pleasant colour and odour

Setting time is 6-8 minutes, mixing time is 45 seconds

Excellent reproduction of surface details

Dimensional stability is low due to high curing shrinkage (0.4-0.6%) and shrinkage due to evaporation of ethyl alcohol. To avoid this cast should be poured immediately.

Tear strength is 3000gm/cm2

It is hydrophobic.  So impression field should be dry

Can be electroplated with silver or copper

Evaporation of alcohol can lead to dimensional instability

Production of hydrogen can lead to pitting of dental stone surfaces.


Superior to condensation silicones

Also called polyvinyl silioxane impression materials.



Polymethyl hydrogen siloxane

Other siloxane prepolymers



Divinylpolydimethyl  siloxane

Other siloxane prepolymers


Platinum salt-catalyst


It is an addition reaction

In this case polymer is terminated with vinyl groups and it is cross linked with silane (hydride group). The reaction is activated by platinum salt

No reaction byproduct develops as long as correct proportions of vinyl siloxane and silane siloxane are maintained

-Sulfur compounds retard the setting of addition silicone.


Pleasant colour and odour

Good reproduction of surface details

Setting time is 4-7 minutes and mixing time is 45 seconds

It has the best dimensional stability among all elastomers. It has low curing shrinkage (0.17%) and low permanent deformation (0.05-0.3%)

Good tear strength (3000gm/cm2)

It is hydrophobic with contact angle 80deg. – 100deg. so impression field should be dry

Can be electroplated with copper/silver

Shelf life ranges from 1-2 years


Mode of Supply:

Available as base and accelerate paste in tubes

Available in three viscosities

Light bodied

Medium bodied

Heavy bodied



Polyether polymer                   Cross linked to form rubber

Colloidal silica                        Acts as filler

Glycol ether (or) phthalate     Plasticizer

Accelerator Paste:

Alkyl aromatic sulfonate         Initiates cross linking

Colloidal silica                        Filler

Phthalate                                 Plasticizer


Pleasant odour and taste

Setting time is short 3.5minutes, mixing time is 30 seconds

Good dimensional stability

Curing shrinkage is 0.24%

Polyether absorbs water and can change in dimension. So should not be stored in contact with water or in humid climates

It is extremely stiff

Tear strength is 3000gm/cm2

Hydrophilic, has good compatibility with stone

Can be electroplated

Good shelf life i.e., more than 2 years.


It involves five steps:::

  1. Preparing a tray
  2. Preparing the material
  3. Making impression
  4. Removing the impression
  5. Preparing stone casts and dies

            PREPARING A TRAY:

Impressions are usually made with custom trays. Perforated stock trays are used for making impression in putty.

To prepare a custom tray, an alginate impression is made and stone cast is constructed

Teeth are covered with modeling wax (1 or 2 sheets approximately 3-4mm) to act as spacer and stops are provided

Custom tray is prepared with tray material

Slightly roughened surface of tray inside improves retention.


Materials are provided in two paste tubes

Same length of materials should be dispensed on to mixing pad

Catalyst paste is first collected on to spatula and then distributed over the base

Mixing should be thorough, uniform in colour with no streaks of base of catalyst

If one of the components is in liquid form such as catalyst for condensation silicone, a length of base is dispensed from tube onto the mixing pad and a drop of catalyst is added for each unit length of base.

Putty consistence are dispensed by volume using equal number of scoops and kneaded.

Automatic mixing and dispensing devices are available used for light, medium and heavy body consistencies.

It consists of a double barrel gun with mixing tip. The tip consists of spiral inside.   The mixing tips vary in diameter length and size of the tip opening for specific consistency.


It has the advantages over hand mixing

Less air bubbles

Reduced working time

More uniform mix

A recent development in the automatic mixing and dispensing device is a dynamic mechanical mixer. Materials are supplied in plastic bags housed in a cartridge. Device uses a motor to drive parallel plunger that force the materials into a mixing tip and the spiral inside the mixing tip rotates as the materials are extruded through the tip.  With this uniform mix of even higher viscosity material can be achieved

       Making of impression

Initially tray is coated with adhesive that forms bond between tray and impression material. Slightly roughened surface of tray increases adhesion.


Three techniques

Single mix technique

Multiple mix technique

Reline technique

Single Mix technique:

Viscosity used is regular body


The paste is mixed and part of it is loaded into tray and part into syringe.  The syringe material is then injected on to the prepared area.  Then tray is seated over it.

Multiple mix technique

Viscosity used is heavy body and light body


            The two viscosities are mixed simultaneously on separate pads. Heavy body is loaded onto the tray and light body in to syringe. The syringe material is injected into area of preparation. Tray with heavy body is seated over it.

Reline technique:      

Viscosity used is putty and light body

Two approaches are present

Two stage procedure

Single stage procedure

—  For two stage procedure, thick putty material is placed in stock tray and a preliminary impression is made.  This forms an intra oral custom tray.  Space for light body or was is provided whether by cutting away some of the putty or by using thin polyethelene sheet as a spacer between putty and prepared teeth.

—  A mix of thin wash material is placed into putty and putty with combination tray is seated finally to make impression.

For single stage procedure,  material is syringed into place and then unset putty is seated over light body but the disadvantage is that putty may displace light body.

Removal of the impression:

The material is checked for set by probing with the blunt instrument.  Then impression is dislodged from the mouth with a steady pressure


  1. Impressions for fixed partial dentures
  2. Impressions for removal partial dentures
  3. Impressions for complete dentures
  4. polyether is used for Border moulding
  5. To make impressions for implant procedure
  6. In maxillofacial prosthesis
  7. To make impression in orthodontic cases for study model


# Andree, Alfred and Christoph evaluated the dimensional accuracy of monophase elastic impression making with addition silicone and polyether and found no significant change.

 int J Prosthodont . 2002;15:168–174

# Steven O Hondrum assessed the shelf life of different elastomers like polyether, polysulfide, additive silicone and found all the materials were fairly efficacious. But addition silicone showed better value during 72 month period and polysulfide with a little change.

Journal of prosthodontics 2001 Jan;85(1):73-81.

#  The influence of mixing methods and disinfectant on the physical properties of alginate impression materials.

The European Journal of Orthodontics Advance Access published May 10,2012

# Ohn Chai, Yutaka investigated modulus of elasticity, strain and tear energy of VPS, polysulfide and polyether and concluded that high strain tolerance of PVS allows their removal without distortion from undercuts. The high tear energy of polysulfides indicates their superior resistance to tear in thin sections

Volume 15, Number 2, 2002 183 The International Journal of Prosthodontics.

# Alvin G. Wee evaluated the accuracy of solid implant casts fabricated from different impression materials (Polyether, AS, CS, polysulfide) and concluded that polyther (medium) was recommended for direct implant impression

# Shirley, John H Park and Daneil E. Tira compared  the accuracy of one step putty wash with two step putty wash of addition silicone impression. They found different was not significant.

J Prostho Dent 2010;103:228-239;

# Kern and Reinhold tested the influence of disinfectants (MD/520 and impresept) on the accuracy of reversible hydrocolloid and found accuracy change was not significant.

j.of prostodontic 1993 Nov;70(5):449-56

# Willium Heisler and Anthony evaluated for dimensional accuracy and bond strength of irreversible hydrocolloid with reversible hydrocolloid system and found that accuracy was suitable for clinical use and bond strength was similar to tear strength of reversible hydrocolloid.

J Prosthet Dent. DOI:10.1016/0022-3913(92)90369-L

# L.W. Carlyle evaluated compatibility of irreversible hydrocolloid with three dental stones (Die keen, Quick stone, Hemihydrate). Results showed that Die keen was most compatible.

J Prosthet Dent. 1983 Mar;49(3):434-7.

# Paul, Gardener and Steven conducted a study to evaluate the effect of storage time in a 100% humid environment on the accuracy of gypsum casts poured from reversible hydrocolloid impression and concluded that reversible hydrocolloid can be stored in 100% humidity for 60 minutes before pouring.

(J Prosthet Dent 2001;86:244-50.)






  • Anusavice. Dental Material, 11th edition
  • Robert G. Craig. Restorative Dental Material
  • Smith. The clinical handling of Dental material
  • International Journal of Prosthodontics, 2002; 15: 168-174.
  • Journal Prosthet Dent, 2000; 83: 161-165
  • Journal Prosthet Dent, 2001; 85: 73-81.
  • International Journal of Prosthodontics, 1998; 11: 219-23
  • Journal Prosthet Dent, 2000; 83: 323-31.
  • Journal Prosthet Dent, 2003; 90: 354-64.
  • International Journal of Prosthodontics, 1992; 5: 55-8.
  • Journal Prosthet Dent, 1990; 63: 12-15
  • J Prosthet Dent, 1993; 70: 449-53
  • BJO mayed.1998VOL25 NO.2


Metallurgy of Stainless Steel and Cobalt – Chromium alloys


Several different metals are used in orthodontic appliances, and their physical properties and mechanical behavior are the very life blood of orthodontic therapy. Recent advances in orthodontics have resulted in a varied array of wires that exhibit a wide spectrum of properties. Up until the 1930’s the only orthodontics wires available were made of gold. Austenitic stainless steel with its greater strength, higher modulus of elasticity, good resistance to corrosion, moderate costs was introduced as an