IMPRESSION MATERIALS FOR COMPLETE DENTURES

REVIEW OF IMPRESSION MATERIALS FOR COMPLETE DENTURES.

DEFINITION

          A negative imprint of an oral structure used to produce a positive replica of the structure to be used as a permanent record or in the production of a dental restoration or prosthesis.

DESIRABLE PROPERTIES OF IMPRESSION    MATERIALS.

     A. COMPATIBILITY WITH PATIENT: -

1.Pleasant odor.

2.Pleasant taste.

3.Non-toxic.

4.Non-irritant.

5.Decreased setting time.

6.Esthetic color.

     B. EASE OF MANIPULATION: -

1. Minimum equipment.

2. Consistency & Satisfactory texture.

3. Adequate flow property.

4. Readily wets oral tissues.

5. Clinically satisfactory setting characteristics & time.

  6. Dimensional accuracy: Should have elastic properties with    freedom from permanent deformation after strain.

                

7. Adequate mechanical strength to resist tearing.

    C.   STORAGE:-

1.Unused materials should have adequate shelf life for           requirements of storage & distribution.

2.Used/set impression should be dimensionally stable over clinical & lab procedures for a period long enough to permit production of cast/die.

    D.  Impression materials should be economically commensurate with the results obtained.

    E.  They should be readily disinfected without loss of accuracy.

 

CLASSIFICATION OF IMPRESSION MATERIALS:-

Impression materials can be classified into various types based on the  following characters:

1.    Based on rigidity/elasticity: -

                a.  Rigid (non-elastic).

                b. Elastic.

   2.   Based on viscosity: -

                a. Mucostatic.

                b. Mucocompressive.

                c. Pseudoplastic.

   3.  Based on setting of material: -

                1.  a. chemical reaction.

                     b. Physical change of state.

                2.  a. reversible.

           b. Irreversible.

   4.Based on interaction with saliva/water: -

            1. Hydrophobic.

            2. Hydrophilic.

   5.Based on chemistry:-

            1. Impression Plaster.

            2. Impression compound.

            3. Metal oxide (zinc oxide eugenol).

           4. Reversible hydrocolloid.

           5. Irreversible hydrocolloid.

           6. Poly sulfides.

           7. Condensation silicones.

           8. Addition silicones.

           9. Polyether.

         10. Visible light curing polyether urethane  dimethacrylate.

 

   6.Based on use: -

           1.  Primary impression materials.

           2.  Secondary impression materials.

           3.  Duplicating materials.

HISORICAL REVEW OF COMPLETE DENTURE IMPRESSION MATERIALS

w  1756 Bees wax was the first material to be used for the purpose of  impression making.

w  1840 Charles De Loude gave the first references to impression trays.

w  1842 Montgomery discovered Gutta Percha.

w  1847 Desirabode gave references to an impression tray.

w  1848 Gutta percha was introduced as an impression material. High    working temperature and stiffness made it difficult to achieve satisfactory results.

w  1844 Plaster of paris was used for the first time as an impression   material.

w  1862 Franklin first corrected impression, followed by a plaster wash.

w   Until the early 1900s wax or plaster used directly.

w  1857 Modeling plastics were developed by Charles Stens.

w  1874 modeling plastics developed by S.S.White.

w  1900 Green brothers introduced a method for manipulating modeling   plastics.

w  First to use term posterior dam’in describing posterior palatal seal.

w  S.G.Supplee introduced the hot water heater for modeling plastics.

w  1915 Rupert Hall perfected the first moderate heat modeling plastic for making individual impression trays.

w  1925 Poller introduced Agar for impressions.

w  Late 1920s first functional waxes were developed. Waxes used before this  time were paraffin and bees wax .

w  1930 Ward and Kelly first use ZOE for impressions.

w  1939Trapozzano introduced an early technique for using ZOE.

w  1936 Alginate3 impression material introduced.

w  1940 Alginate impression material used first time for corrective wash procedures.

w  1938 mucostatics PASCAL's law –tissue under a mucostatic impressions theory developed.

w  1950 Elastomeric impression materials  were introduced.

w  1955 Pearson reported on polysulfide base materials for use as an impression for inlays, crowns &FPD’s.

   IMPRESSION  PLASTER

wADA specification no –25.

 

Composition:-

 1.Calcined calcium sulfate hemihydrate.

 2.Anti-Expansion agents. - Potassium Sulphate was added which had a tendency to decrease working time.

 3.Accelerators –accelerators that were added were

     Potassium Sulphate.

     Potassium chloride.

 4.Retarders.

 5.Colloidal materials/Gelatin.

 6.Gum – Tragacanth.

 7.Pigments.

Uses:
      1.Primary impression material.                            

 2.Secondary/corrective impression material.

Water-powder ratio:-

      1.60 ml of water for 100 gm of plaster. Fluidity is required for recording finer details.

2.

If water-powder ratio is increased, then the following characters were affected.

a.        It had a thin consistency.

b.       Setting time was increased.

c.        Strength was decreased.

d. 

Mixing time:-  60 seconds.

Strength: -

        Impression plaster had less compressive strength and tensile strength and is considered to be a very brittle material.

Accuracy: -

          Very accurate. It is hydrophilic and has intimate contact with oral tissues by absorbing surface moisture. Plaster undergoes minimal dimensional change on setting.

 

IMPRESSION PLASTER - PRODUCTION

                   These materials are the result of calcining of calcium Sulphate Dihydrate or gypsum. Depending on the method of calcination different forms of hemihydrates can be obtained. Commercially gypsum is ground and subjected to temperatures of 110-120 degrees to drive off part of the water of crystallization. This corresponds to the first stage of the equation. As the temperature is further raised, the remaining water of crystallization is driven off and products are obtained.

          The principal constituent of dental plaster or stone is Calcium Sulphate Hemihydrate. The difference between the two forms is mainly between the crystal size, surface area & degree of lattice perfection.

w        Caso4.2H2OàCaso4.1/2H2OàCaso4

SETTING OF GYPSUM PRODUCTS:

wCalcination Of Calcium Sulphate Dihydrate Forms Calcium Sulphate Hemihydrate

w(Caso4).1/2H2O+3H20     --------->     2caso4.2H20+Heat

The product of the reaction is gypsum and the heat evolved in the exothermic reaction is equivalent to the heat used originally in calcination.

           When hemihydrate is mixed with water, a suspension is formed that is fluid and workable. Hemihydrate dissolves in it until it forms a saturated solution. This saturated hemihydrate solution is supersaturated with dihydrate, so the later precipitates out. As the dihydrate precipitates, the solution is no longer saturated with hemihydrate and so it continues to dissolve. The reaction is continuous and proceeds until no further dihydrate precipitates out of solution.

ADVANTAGES: -

1.Good detail reproduction.

2.Inexpensive.

3.Easy to handle.

4.Viscosity can be altered by minor alterations of w/p ratio.

5.Non-toxic.

6.If stored airtight-increased shelf life.

7.Decreased dimensional change on setting.

8.Setting time can be precisely controlled by use of additives.

  

DISADVANTAGES:-

wRequires separating medium for pouring and removing casts. (Varnish, water glass solution).

wDry sensation in patient’s mouth because it absorbs moisture.

IMPRESSION  COMPOUND

wADA specification no –3

wType I: low fusing.

wType II: high fusing, more viscous when soft & more rigid when hard.

COMPOSITION:-

           Compounds are composed of a mixture of waxes, thermoplastic resins, filler, and a coloring agent. One of the first substances used, as an impression material was bees wax. Because such waxes were brittle, substances such as shellac, Stearic acid and gutta percha are added to improve plasticity and workability.

wRosin:                         30 parts. -Thermoplastic material

wCopal resin:                30 parts.

wCarnauba wax:           10 parts.

wStearic acid:                  5   parts.-increase plasticity.

wTalc/soap stone/wax: 15 parts. -Decrease flow; increase strength.

wColoring agents:         appropriate  amount.

USES:-

       Type-1   -Primary impression, peripheral tracing &tube impression of   single tooth with copper band.

       Type-2   -As a tray to support other materials.

 

MANIPULATION:-

The compound can be softened with either dry heat or oven or in a water bath. Care should be taken when compound is softened with dry heat. The compound should not be over heated since this causes volatility of its constituents. The compound is broken into small pieces. This aids in faster and uniform heating and also a uniform mix. If large amount of compound is heated, it is difficult to heat the compound uniformly. The compound is heated in a water bath with the help of a gauze piece. After the mass is removed from the water bath, it is kneaded. This gives a uniform plasticity to the mass. Prolonged immersion into water has to be avoided since this causes leaching out of low molecular weight ingredients.

FUSION TEMPERATURE: -

1. Approximately 43.5 centigrade.

2. Fusion temperature indicates a definite reduction in plasticity on cooling. 3.Above these temperatures, fatty acids become liquids and plasticised       material softens.

 

THERMAL CONDUCTIVITY:-

w  The thermal conductivity is low. Hence has to be thoroughly cooled before removal.

w  Average linear contraction of compound on cooling from mouth temperature to room temperature of 25 degrees is 0.3 –0.4 percent. This magnitude of contraction is unavoidable.

 FLOW: -

               After the compound has softened, and during the period it is impressed against the tissues, the material should flow easily to confirm to the tissues so that every detail and landmark are recorded accurately. On the other hand, if the amount of flow at mouth temperature is too great, distortion can occur.

 

For type –1:flow at mouth temperature is 6%. When placed against tissues it      is 65%.

For type –2:flow at mouth temperature is 2%. When placed against tissues it is 85%.

If the material is older it is uncomfortable for the patient.

ADA Specifications:-

1. The material should be homogenous.

2. It should have a smooth & glossy appearance on flaming.

3. When trimmed with sharp knife at room temperature margin should be firm & smooth.

GLASS TRANSITION TEMPERATURE.

              The non-crystalline solids do not have a definite melting temperature but rather they gradually soften as the temperature is raised & gradually soften as they are cooled. The structural arrangement of non-crystalline solids does not give a particular melting point to the compound. They gradually soften as the temperature is raised and gradually harden as the temperature is lowered. The temperature at which they form a rigid mass is called the glass transition temperature.

 

             The temperature at which there is as abrupt increase in the thermal expansion coefficient is an indication of formation of short range & is called the glass transition temperature. The glass transition temperature of the compound is 39 degrees or 107 Fahrenheit.

  

 

ADVANTAGES:-

      1. Non-toxic.

      2. Hardens in reasonably acceptable time.

     3. Compatible with gypsum products.

     4. No separating medium required.

     5. Adequate shelf life.

DISADVANTAGES:-

     1. Although plastic not fluid enough to record fine detail.

     2. Distorts over undercuts.

     3. Considerable shrinkage on cooling.

     4. Dimensional change on storage.

 

LOW FUSING COMPOUND

wThey are used to carry the impression material to the depth of the vestibule.

MODELLING COMPOUND:-

wIntroduced by Green brothers.

wCompound softens easily but remains quite hard at mouth temperature.

wThe areas of periphery can be border molded with least possibility of distortion or breakage.

wVery useful for the making of final impressions& when boxing & pouring the cast.

DIFFERENT TYPES :-

1.BROWN (highest working temperature)

wStronger at room temperature.

wSuitable for extending short borders on the custom tray.

wCake form is used for preliminary impressions.

2.GREEN(lowest working temperature)

wIt has easy flow & good handling properties.

wIt is the most popular type.

3.GRAY:-

wAverage working temperature.

wLong period of flow.

wLittle brittle.

GENERAL CONSIDERATIONS.

1.Modelling compound sticks require relatively high working temperature.

2.Great care must be taken not to burn the patient.

3.Working time is limited.

4.The fear of harming the patient and delay in seating the tray may lower the   temperature of the material and result in over extended borders.

METALLIC  OXIDE PASTES.

wADA specification no.-16

w Type-1 (hard)

w Type-2 (soft)

USES:-

          1.Impression paste.

          2.Cement.

          3.Temporary filling.

          4.Root canal filling.

          5.Bite registration paste.

          6.Temporary reline material.

          7.Surgical dressing.

AVAILABILITY:-

     1.Two pastes/tubes: zinc oxide (active ingredient) & eugenol+ rosin.

     2.Powder (ZnO+rosin) & liquid eugenol.

 

COMPOSITION:-

w  Tube-1:

    1.ZnO (French processed or u.s.p)  :             87%

    2.Fixed vegetable oil –olive oil/linseed oil:     13%

           3.Plasticiser acts as the vehicle and forms paste. It also helps to masks irritation effect of eugenol.

w  Tube-2:

      1. oil of cloves (75%-85% eugenol):              12%

        2. Gum/polymerised rosin:                          50%.                            3.Filler(silica/talc/diatomaceous earth):       20%.

     4. Kaolin:                                                       3%.

     5. Resinous balsam (Canada balsam):          10%.

     6. Accelerator solution & color:                       5%.

     7. Retarders –Glycerin & petrolatum.

 

SETTING REACTION: -

           Setting reaction consists of zinc oxide hydrolysis and a subsequent reaction between zinc hydroxide and eugenol to form a chelate. Water is needed to initiate the reaction and it is also the by-product of the reaction. Hence the reaction is called as an autocatalytic reaction. Hence the reaction occurs more rapidly in humid environment. The setting reaction is accelerated by the presence of zinc acetate dihydrate, which can supply zinc ions more readily. Acetic acid is a more active catalyst for the reaction. High atmospheric temperatures also accelerate the setting reaction.

 

MIXING: -

              The mixing of the two pastes is generally accomplished on an oil impervious paper. Two strips of the same length are taken from the two tubes and a flexible stainless steel spatula is used for mixing. The pastes are mixed for about a minute till a uniform color is obtained.

SETTING TIME: -

          It should take place within 10 minutes for type 1 paste and within 15 minutes for type 2 pastes.

MIXING TIME: -1 min /30-40 seconds.

          Final set is said to have occurred when metal rod of Krebs penetrometer fails to penetrate more than 0.2mm under 50gm/load.

 Control of setting time:-

1.Setting time decreases with increase in temperature, humidity & addition of  

   accelerators.                                                                                                                                                                         2.Setting time increases with addition of retarders.

 

CONSISTENCY & FLOW:

Material should be
     1. Homogenous.

2. Flow uniformly.

3. Mucostatic.

4. Flow related to setting time.

DIMENSIONAL STABILITY: -

          The dimensional stability of impression pastes is quite satisfactory. A negligible shrinkage (<0.1%) may occur during hardening. Impressions can be preserved indefinitely without change in shape. 

REMOVAL OF SET MATERIAL FROM GLASS SLAB/SPATULA.

1.Solvents such as Naphtha and oil of orange can be used.

2. The instrument can be heated and the material can be wiped off.

 

SURGICAL PASTES:

1.they are less brittle.

2.material is weaker after hardening.

3.it takes a longer time for setting.

4.it should be capable of being formed into a rope for dressing.

5.material contains more eugenol.

ADVANTAGES:-

1. Adheres well to tray.

2. It is sufficiently fluid to record fine details of tissues.

3. Does not undergo any dimensional change during setting process.

4. Has adequate working time & setting time.

5. Sufficient resistance.

6. It is compatible with gypsum products.

7. No separating medium required for gypsum products.

8. It has a satisfactory shelf life.

9. It is Non-toxic.

DISADVANTAGES:-

1. Burning/tingling sensation of eugenol is a major disadvantage.

2. Persistent taste of eugenol added disadvantage.

3. As it adheres to tissues, lips should be coated with petrolatum jelly.

 

NON EUGENOL PASTES: -

One of the chief disadvantages of zinc oxide eugenol pastes is the possible stinging or burning sensation caused by eugenol when it contacts soft tissues. Moreover ZoE reaction is never completed with the result that the eugenol may leach out. Some patients find the taste of eugenol extremely disagreeable, and in patients who wear surgical pastes for a long time may develop gastric disturbances.

 

If zinc oxide reacts with carboxylic acid, the reaction is

w  ZnO+2RCOOHà(RCOO)2Zn+H2O

w  Most commonly used carboxylic acid is ortho ethoxy benzoic acid.

w  It is not greatly affected by temperature or humidity.

w  Bactericides and other medicaments can be incorporated without interfering with the reaction.

 

ALGINATE(Irreversible Hydrocolloid)

w   ADA specification no.-18

DEFINITONS OF COLLOID:-

1. Colloid represents a soluble particle distribution quantitatively similar to

Molecular liquid in the solvent.

    2. A material in which is suspended a constituent in a finely divided state.

3. A colloid system in which water is the dispersion medium; those

       materials described as colloid sols in water.

          HYDROCOLLOID is a material consisting of a sol of alginic acid having a   physical state that is changed by an irreversible chemical reaction forming insoluble calcium alginate.

COMPOSITION:-

1. ALGINIC ACID:                                                    15%

2. SOLUBLE SALTS: -Na/k/ammonium salts-             18%.

3. Calcium sulfate dihydrate- (reactor)-                        14%.

4. Potassium sulfate/Potassium titanium fluoride/

    Silicates/ Borates-                                                   10%.

5. Na/K Oxalates/Carbonates-                                    2%.

6. Diatomaceous earth-ZnO/Silicate powder-FILLER-  56%.

7. GLYCOLS/DIHYDRIC ALCOHOL-                       small %.

8. WINTER GREEN/PEPPERMINT-                         small %.

9. PIGMENTS.

10.REACTION INDICATOR.

             The chief ingredient of irreversible hydrocolloid is one of the soluble alginates such as sodium, potassium, or triethanolamine alginates.

CHEMISTRY OF HYDROCOLLOID:-

Colloid state represents the highly dispersed system of fine particles of one phase in another. The colloidal state of system has dispersed and dispersion phase. If this has water as the dispersion phase it is called as a HYDROCOLLOID.

 In alginate the dispersion phase is water and the dispersed phase is alginic acid. The molecular weight of alginic acid is greater than that of water, hence it does not dissolve in water and forms a gel. If the concentration of dispersed phase in the hydrocolloid is of the proper amount, sol changes into gel when temperature is decreased. The temperature at which this change occurs is called as the GELATION TEMPERATURE and is in the range of 18-20 degrees. The fibrils of the gel are formed chemically by primary bonds. Hence these are not affected by temperature changes. They can be returned to the sol state by reversal of the reaction and not by heat. Hence these materials   are called as irreversible hydrocolloid.

GELATION PROCESS: -

w  Soluble alginate + CaSo4à Insoluble Ca alginate.

The typical sol-gel reaction can be described as a reaction of soluble alginate with calcium Sulphate and the formation of insoluble calcium alginate gel.  Calcium Sulphate reacts rapidly to produce the insoluble calcium alginate from the potassium or sodium alginate in an aqueous solution. The production of calcium alginate is so rapid that it does not allow sufficient working time. Thus, a third water-soluble salt, such as trisodium phosphate, is added to the solution to prolong working time. Calcium sulphate reacts with this in preference to soluble alginate & hence increases the working time.  Thus, the reaction between the calcium Sulphate and the soluble alginate is prevented as long as there is unreacted trisodium phosphate. When the supply of trisodium phosphate is exhausted, the calcium ions begin to react with the potassium alginate to produce calcium alginate.

w  Insoluble alginate reacts with Ca Sulphate & forms insoluble     calcium alginate gel in the aqueous medium.

w  2Na3PO4 +3Caso4  -> Ca3 (Po4) 2+3Na2So4

 GEL STRUCTURE: -

1.Sodium Or Potassium alginate is formed. The cation is attached to the carboxyl group.

2 .when the insoluble salt formed by the reaction of the sodium alginate in solution reacts with the calcium salt, calcium ions may replace the sodium ions in two adjacent molecules to produce cross-linking between them.

3. As the reaction progresses, cross-linking becomes more extensive and a complex polymer network is formed. This constitutes the brush heap structure of the gel.

  GEL STRENGTH: -

1.Stiffness and strength of the gel are directly related to the brush heap     structure.

2.Greater the concentration of the dispersed phase, greater will be the fibrils formed on gelation.

3.Temperature also affects the gel strength of the material in the case of reversible hydrocolloid. But temperature does not show any effect on irreversible hydrocolloid.

4.Lower the temperature, stronger the gel.

5.Gel strength also depends on the presence of modifiers such as fillers and other chemicals.

6.The gel strength of alginate is 343 Mpa.

7.The type and amount of alginate used also influence the strength.

8.Manipulative factors that affect the strength are

        1.too much or too little water affect the strength.

        2.insufficient spatulation.

        3.overspatulation.

MIXING TIME: -

w   Type-1 (fast) – 45 seconds.

w   Type-2(slow)– 4 minutes.

SPATULATION: -

The measured powder is sifted into premeasured water that has been placed in a clean rubber bowl. Powder is incorporated into the water by careful mixing with a metal spatula. Care should be taken to prevent whipping air into the mix. A vigorous figure- eight motion is best, with the mix being swiped or stropped against the sides of the rubber-mixing bowl with intermittent rotations (180 degrees). Mixing time of 45seconds to 1 minute is generally sufficient. A smooth creamy mix should be obtained.

Clean equipment is important because many of the problems and related failures are attributed to dirty contaminated mixing or handling devices. The bowl should be free of any previous mix of plaster or alginate.

WORKING TIME: -
 Type-1: 1.5-2 minutes.
 Type-2:  3-4 minutes.
        

45 secs of mixing time + 30-75secs working time à acc.to ADA  sp.No. 18 not  <1.5min.   60 secs mixing time + 2-3.5 minutes working timeàaccording to ADA sp.no.18 not  < 2 min.  

           

CONTROL OF GELATION TIME: -

1. Altering water: powder ratio.

2. Alteration of temperature of water.

STRENGTH: -

1. Maximum gel strength is required to prevent fracture and to ensure recovery of the impression on its removal from the mouth. All manipulative factors that are under the control of the clinician affect gel strength.

2.  Manipulative factors that are under the control of the clinician are:

          a. Proper water: powder ratio.

          b. Insufficient spatulation.

          c. Over mixing.

VISCOELASTICITY: -

Hydrocolloids are strain rate dependent. Thus the tear strength is increased when the impression is removed with a snap. Usually an alginate impression does not stick to the oral tissues as strongly as some of the non-aqueous elastomers, so it is easier to remove alginate impressions rapidly. It is always best to avoid torquing/tearing the impression.

ACCURACY: -

Most alginate impressions are not capable of reproducing the finer details that are observed in impressions with other Elastomeric impression materials. Increase in alginic acid results in increased roughness & does not improve dimensional stability. The roughness of the impression material is sufficient to distortion at the margins of prepared teeth. Surfactants can be added to produce a smooth surface, but the addition of a layer of solutin also obscures the accuracy.

 

DIMEMSIONAL STABILITY: -

1. Dimensional stability is very poor.

2 .If exposed to air at room temperature, shrinkage occurs due to processes       such as syneresis and evaporation.

3 .If immersed in water, imbibition of water takes place.

4. 2% potassium sulphate or 100% relative humidity are suggested to solve the problem.

5. Exertion of pressure during gelation process results in the production of internal stresses. Relaxation of such internal stresses also results in syneresis & dimensional changes.

6.Thermal changes also contribute to thermal changes because of changes in room and mouth temperature.

GENERAL CONSIDERATIONS:

1.RETENTION OF MATERIAL TO TRAY:

       Perforated trays or tray adhesive like molten sticky wax.

2.OPTIMUM BULK/THICKNESS OF MATERIAL: 

        3-6 mm to reduce the chance of tearing.

3.TRAY EXTENSION / BORDERS: 

         Refined trays.

4.Less material is loaded posteriorly,patient upright/leaning forward to   

   prevent posterior flow of material & minimize gagging.

5.Loaded material smoothened with moist finger.

6.REMOVAL: - Break peripheral seal with tissues; then single firm    rapid movement to avoid tearing.

7.Chance for permanent deformation under 10%.

8.Compressive strength – 3500 gm/Sq .cm.

9.Tearing strength – 300-700 gm/cm.

COMPATIBILITY WITH GYPSUM PROCUCTS: -

                 The surface of a gypsum cast obtained from the hydrocolloid impression material may sometimes be too soft for waxing procedures. To avoid this,

  1.Impression is immersed in a solution containing an accelerator for setting of gypsum products.

 2. By incorporating a plaster hardener or accelerator in the material by the manufacturer.

 3. The surface of the impression should not be dried completely.

a.    This causes the gel to adhere to the cast on its removal.

b.   Many commercially available products give a satisfactory surface for the stone cast without using any hardeners.

c.    After the impression is removed from the mouth it is rinsed under water to remove oral fluids from the surface.

d.   Surface of the impression should be shiny but there should not be any visible moisture.

e.    Pouring of the cast should be done from one end to the other of the impression.

f.     The stone should be kept in contact with the impression for a minimum of 30 minutes & a maximum of 60 minutes.

g.    If the cast is allowed to remain in contact with the impression overnight, a chalky stone surface may be produced. 

       

 

             

FAILURES: -

1. GRAINY: -

       a. Improper mixing.

       b. Prolonged mixing.

       c. Under gelation.

       d. W: P ratio too low.

2. TEARING: -

            a. Inadequate bulk.

            b. Moisture contamination.

            c. Premature removal.

            d. Severe undercuts.

            e. Thin mix.

            f.  Slow removal.

3.BUBBLES: -

            a. Air incorporation during mixing.

            b. Undue gelation preventing flow.

           

4. ROUGH/CHALKY STONE CAST: -

             a. Inadequate cleaning of impression.

             b. Excess water left in the impression.

             c. Premature removal of cast.

             d. Cast left too long.

             e. Improper manipulation of stone.

 

5. DISTORTION: -

            a. Impression not poured immediately.

            b. Movement of tray during gelation.

            c. Premature removal from mouth.

            d. Improper removal from mouth.

            e. Tray held in mouth for too long.

6.DECREASED WORKING TIME/ SETTING TIME: -

             a. Non homogenous mix.

             b. Temperature.

             c. Contamination- set plaster left in the mixing bowl.

ADVANTAGES: -

1. Non-toxic, non-irritant.

2. No special equipment required.

3. Acceptable odor, taste.

4. Sufficiently elastic to be used in undercut area.

5. Sufficiently fluid to record fine detail.

6. Compatible with gypsum products/no-separating medium required.

DISADVANTAGES: -

1. Dimensionally unstable.

2. Tears if undercuts   are severe.

3. Difficult to sterilize.

4. Poor shelf life if stored. 

ELASTOMERIC IMPRESSION MATERIALS

INTRODUCTION TO ELASTOMERS: -

These materials are classified as synthetic rubbers but mimic natural rubber. Hence they are called as rubber base materials or rubber base impression materials or elastomers or Elastomeric impression materials.

1.   Due to the elastic properties, they are called as elastomers.

2.   These materials consist of large molecules or polymers that are joined by small amount of cross-linking.

3.    The amount of cross-linking determines the stiffness and the elastic behavior of the material. 

4.   The first synthetic rubber materials were developed by the process of vulcanization.

5.   Setting of these materials occurs through a combination of chain lengthening polymerization or cross-linking or either condensation or addition reactions.

HANDLING: -

      The working time of an acceptable material must exceed the time required for mixing and loading the syringe and the tray.

1.   The material distorts soon after removal from the mouth.

2.   If the material is not adequately set, the material will not have sufficient elastic properties to respond to the strain that occurs when removing it from the mouth.

3.   The setting reaction converts them into a visco-elastic solid.

4.   The flow behavior of the material is important to obtain an accurate impression.

5.   The ideal impression material accurately records the oral structures, releases from the mouth undistorted.

6.   The impression material should be removed from the mouth rapidly.

7.while removing the impression, the seal has to be broken   and remove the impression rapidly.

       8. Loss of reaction by products and the imposed loads of   stone or plastic used to make cast results in the distortion of the impression.

DESIRABLE PROPERTIES OF IMPRESSION MATERIALS.

     1.should have an infinite shelf life.

     2. Be non-toxic.

     3. Should have acceptable odor, taste & color.

     4. Should have suitable working & setting times.

     5.should have strength to resist tearing.

     6. Should be compatible with model & die materials.

     7. Should be inexpensive.

     8. Should be easy to clean up.

     9. Should be easy to dispense, proportion& mix.

   10. Permit multiple die pours.

   11. Facilitate visualization of the finish line.

   12. Facilitate the clinical identification of beginning and end of cure.

   13. Completely plastic before cure.

   14. Sufficient fluidity to record final detail.

   15. The ability to wet oral tissues.

   16. Dimensional stability.

   17. Complete elasticity after cure.

   18. Optimal stiffness.

 

wSOLOMON E G R 1973 used silicone material for complete denture impression – a high viscosity material for border molding and low viscosity material for secondary impression. He concluded that silicone impression material was preferable to conventional low fusing impression compound. 

POLYSULFIDES

  

 COMPOSITION: -

Base paste: -

1. Polysulfide polymer: - 80-85%

2. Filler: - Lithopone/Titanium dioxide: 16-18% - for strength.

3. Plasticizer- Di-n-Butyl phthalate- for appropriate viscosity.

4. Sulfur- 0.5% - to enhance reaction.

Reactor paste: -

1.Cross-linking agent.

        a. Lead oxide.

        b. Organic hydro peroxides-T-Butyl hydro peroxides.

        c. Inorganic hydroxides-Hydrated copper hydroxide.

2.Inert oil or Plasticizer.

3.Filler.

4.Oleic/Stearic acid.

CHEMISTRY

The basic ingredient of polymer paste is polyfunctional mercaptan or polysulfide polymer. This polymer contains approximately 1 mol% of branches to provide enough pendant mercaptan groups as chain cross-linking sites. This polymer s usually cross-linked with an oxidizing agent such as lead dioxide. During the condensation reaction of the lead dioxide with the polysulfide polymer two reactions take place. They are

      1. Chain lengthening polymerization and

      2. Cross linking reaction.

Because the pendant group comprises only a small percentage of available –SH groups, initially, the reaction results in chain lengthening, which causes viscosity to increase.  The subsequent cross-linking reactions tie the chains together, forming a three dimensional network.

Curing reaction starts at the beginning of mixing and reaches its maximum after spatulation is complete, at which stage a resilient network has started to build. This gives adequate elasticity and strength to be removed over undercuts readily. The polymerization reaction is exothermic. Hot, humid conditions accelerate the setting reactions.

MANIPULATION: -

With the proper length of the two pastes squeezed on to a glass slab or a mixing pad, the catalyst paste is first collected on a stainless steel spatula and then distributed over the base paste and the mixture is spread over the mixing pad. This procedure is continued till the mix is of uniform color with no streaks of the base or the catalyst paste appearing in the mixture. If mix is not homogenous, curing will not be uniform.

MIXING TIME: 45 seconds – 4 minutes.

SETTING TIME: 8 minutes.

WORKING TIME: 5-7 minutes.

WORKING AND SETTING TIMES: -

1. Measured by oscillating rheometer.

2. Increase in temperature decreases both working & setting times.

3. Cooling the material is a practical method of increasing the working time & when the material is carried to the mouth, setting time is decreased by higher oral temperature.

4. Adding a drop of water accelerates curing time.

ELASTICITY: -

Elastic properties of these materials improve with curing time. Longer the impression can remain in the mouth before removal, greater the accuracy. The impression material must undergo some distortion as it is removed from the mouth, but the elastic properties of the impression material help minimize this distortion. Distortion can occur if the tray is torqued. Recovery of elastic deformation after strain rate is less rapid for polysulphides than for other kinds of materials. Polysulphides exhibit the most permanent deformation following strain in compression compared with the other materials. Polysulphides also sustain more distortion when the strain rate is slow.

 

RHEOLOGY

Polysulfide is one of the least stiff of the Elastomeric impression materials. This flexibility allows the set material to release from undercut areas with minimum stress. Despite the lack of stiffness the unset material has high level of viscosity. This thick consistency of the uncured material helps displace any unwanted fluid present while seating the impression. Due to the high level of viscosity the material does not flow out of the tray when it is placed in the mouth.

TEAR SRENGTH: -

Polysulphides have highest tear resistance – About 4000 gm/cm (8 times that of hydrocolloid).        

DIMENSIONAL STABILITY

      The stone cast must be poured immediately since the impression is most accurate immediately after removing it from the mouth. Sources of dimensional change are

w During setting, most polymers contract slightly due to cross-linking.

w  After setting, the by-product of condensation reaction (water) is lost which causes shrinkage.

w  After setting, there is incomplete recovery of deformation because of the visco-elastic properties.

w  Dimensional changes are greater for polysulfide than for Polyether & for addition silicone.

w  If maximum accuracy is to be maintained, the stone die or cast should be constructed within 30 minutes. Although the material is fluid repellant, it can absorb water, disinfectant etc. when exposed for a long time.                 

BIOCOMPATIBILITY: -

1. Probability of allergic or toxic reactions.

2. Cytotoxic.

 

HANDLING OF TRAY: -

One way to minimize the effects of polymerization shrinkage, loss of by products, and deformation associated with distortion is to minimize the amount of material that is used to make the impression. The most accurate polysulphides impressions are made by using a custom acrylic tray, because uniform thickness of material can be obtained.

A stone cast is constructed from an impression of the tissues and a custom tray is fabricated. Important parts of the cast, such as prepared teeth are covered with one or two layers of base plate wax & tin foil to act as spacer for the impression material. Chemical curing or light curing resin is used to prepare the tray. Adhesion can be obtained by the application of minimal, uniform thickness of adhesive to prepared tray, before the insertion of the impression material. The adhesive then forms a tenacious bond between the rubber material and the tray.

 

HANDLING TECHNIQUES.

The impression material is currently available in two consistencies:

wAvailable in 2 consistencies:

1. Tray material.

2. Syringe material.

Syringe material may have longer working & setting time. Syringe material contains lesser filler particles, hence has greater polymerization shrinkage and more thermal contraction. Hence it is not advisable to use syringe material alone.

wTHE TECHNIQUE OF USING BOTH TRAY AND SYRINGE MATERIAL IS CALLED “MULTIPLE MIX”

1. Mix the tray material first & fill the tray with a uniform thickness of the material & set it aside.

2.  Second person begins mixing &filling the syringe.

3.  The material is injected from the filled syringe within around the prepared teeth.

4. The filled tray is then placed over the syringe material so that both materials cure together.

DISINFECTION OF IMPRESSIONS: -

Polysulphides can be disinfected by most of the various anti microbial solutions without adverse dimensional changes, provided the disinfection’s time is short. Prolonged immersion may produce minimal distortion. One recommended procedure is a 10-minute immersion in a 10% solution of sodium hypochlorite.

SHELF LIFE: -

1. The material does not deteriorate appreciably when stored     under normal environment.

2. The tubes should be kept tightly closed when not in use.

3. Storage in a cool environment is advisable.

 

ADVANTAGES: -

1. No special equipment required.

2. Superior strength in deep sulcus.

3. Finish line can be easily read.

4. Cast pouring can be delayed up to one hour.

5. Can be poured more than once.

6. Adequate shelf life.

DISADVANTAGES: -

1. Custom trays required.

2. Hydrophobic.

3. Sensitive to heat & humidity.

4. Severe undercuts must be blocked.

5. Objectionable odor.

6. Long setting time.

7. Moderately high shrinkage.

8. Fairly high permanent deformation.

RECENT DEVELOPMENTS.

wPOLYETHER PUTTY MATERIAL + POLYSULPHIDES ARE USED AS   WASH MATERIAL.

CONDENSATION SILICONES.

wCOMPOSITION:

             The condensation silicone impression materials are supplied as a base paste and a low viscosity liquid or catalyst paste. Because the silicone polymer is a liquid, colloidal silica is added as a filler to form a paste. The particles should be within the optimum range of 5-10 microns.

BASE PASTE:

1.Poly dimethyl siloxane/Liquid silicone polymer.

2.FILLER-Colloidal silica/Micronised metal oxide.

REACTER PASTE:

1.Tri/Tetra functional alkyl silicates.

2.Tin compound – Stannous Octoate.

wProperties of impression material are influenced by properties of filler, according to Law Of Mixtures.

wDifferent colors are available: Pastel pink, blue, Green, Purple.

CHEMISTRY: -

1.The polymer consists of a hydroxy terminated polydimethyl siloxane. Condensation polymerization reaction of this material involves a reaction with trifunctional and tetra functional alkyl silicates, commonly tetraethyl ortho silicate, in the presence of stannous octoate. These reactions can take place at room temperature & hence these materials are called as “ROOM TEMPERATURE VULCANISATION” silicones.

            Formation of the elastomer occurs through a cross-linking between terminal groups of the silicone polymer & the alkyl silicate to form a three dimensional network. Ethyl alcohol is the reaction by product. Its evaporation accounts for the shrinkage seen in the set polymer.

MANIPULATION: -

1. Supplied as base paste & liquid catalyst.

  2.  A length of the base paste is dispensed onto a graduated mixing pad.

    3.  One drop of liquid catalyst is added for each unit length of base.

    4.  Both pastes are mixed till a uniform color is obtained.

      5. Putty material – very thick paste and a liquid accelerator.         Manufacturer’s directions are followed to mix the material.

     6. Two-paste putty system-best mixing technique is to knead the   material with the fingers.

    

7. Wearing gloves adds another complication; some latex gloves contain sulfur component & this inhibits setting.

 

ELASTICITY: -

Condensation silicones impression materials are more ideally elastic than polysulphides. They exhibit minimal permanent deformation and recover more rapidly when strained.

RHEOLOGY: -

The material is more likely to respond as elastic if it is strained rapidly. Hence, impressions must be removed quickly so that the deformation is elastic and recoverable.   

          

TEAR STRENGTH: -

Tear resistance is low. They must be handled carefully to avoid ruining a margin of a crown preparation. Applying a force rapidly ensures the highest tear resistance. 3000 gm/cm.

WORKING TIME: -up to 5 minutes

SETTING TIME: -10-12 minutes.

Chilling the material or mixing on cool slab slows reaction rate.

DIMENSIONAL STABILITY:

1. Material exhibits excessive polymerization shrinkage. Hence a putty-wash technique is used.

  2. The amount of linear contraction is 2-4 times greater than others.

  3. This is caused by release of ethyl alcohol as an end product.

  4. Polymerization reaction continues after material is clinically set.

5.Accurate model is obtained by pouring up the impression immediately-within 30minutes.

HANDLING TECHNIQUE: -

               Because the putty wash impression technique is used with this material, custom tray fabrication is not necessary. Disposable stock trays can be used to support the putty material.

1.  Thick putty material placed in the tray & preliminary impression is made (Intra oral custom tray).

2.  Space for light body wash material is provided by scraping the tray putty or polyethylene sheet used as a spacer.

3. Thin consistency wash impression material is placed over the putty impression.

This is called as TWO STAGE PUTTY WASH TECHNIQUE OR RELINE  TECHNIQUE.

ADVANTAGES: -

1. No special equipment.

2. Finish lines easily read.

3. Pleasant odor & appearance.

4. Adequate shelf life.

DISADVANTAGES: -

1. Requires special care in pouring.

2. Should be poured immediately after removal.

3. Easily distorted.

 

POLYETHER

               INTRODUCED IN GERMANY DURING LATE 1960’S

COMPOSITION: -

1. Low molecular weight Imine terminated prepolymers.

2.Inert filler.

3.plasticiser-Glycol ether phthalate.

4.Ester derivative of aromatic sulphonic acid.

              Polyether is supplied as two pastes. Base paste contains Polyether polymer, colloidal silica as filler, and a plasticizer such as glycolether or phthalate. The accelerator paste contains the alkyl aromatic suldonate in addition to the aforementioned filler and plasticizer.  

WORKING TIME: 5-7 minutes.

SETTING TIME: 5-6 minutes.

        The curing time of Polyether is less sensitive to temperature changes.

MIXING TIME: 30-45 seconds.

Working time can be altered by Base: accelerator ratio.

ELASTICITY:

Polyethers have been considered to be the stiffest of all the materials. The original material was extremely difficult to remove from undercut areas because of the high modulus of elasticity.

TEAR STRENGTH: -

Tear resistance is better than that of condensation silicones. But Polyether is more prone to tearing than polysulphides. Because of this, the margin should be carefully inspected immediately after removing the impression.

DIMENSIONAL STABILITY: -

The dimensional changes of Polyether are relatively small. They have no reaction by product. Although the residual polymerization continues beyond the clinical time, it is much shorter than that of polysulphides. The stiffness of the material means that the force needed to remove the impression is greater for Polyether but the recovery is nearly complete.

BIOCOMPATIBILITY: -

1.Hypersensitivity.

2.Contact dermatitis.

3.High cell toxicity.

SHELF LIFE: -

Good shelf life: - Storage at room temperature.

Storing in a cool dry environment prolongs shelf life.

ADVANTAGES: -

1. No special equipment required.

2. Finish line easily read.

3. Superior Dimensional stability.

4. Fast setting.

5. Cast can be poured 1-7 days later.

6. Pleasant odor & appearance.

7. Good shelf life.

DISADVANTAGES: -

1. Custom tray required.

2. Very stiff.

3. Short working time.

4. Least tear strength.

5. More expensive.

6. Aromatic sulphonic acid ester catalyst – Skin irritant.

ADDITION SILICONES

COMPOSITION: -

w BASE: - polyvinyl siloxane or vinyl poly siloxane.

wACCELERATOR: -

   1.divinyl siloxane

   2.Platinum salt-catalyst (chloroplatinic acid)

   3.Palladium- hydrogen absorber.

   4.Retarders.

   5. Fillers.

 

Both the base paste and the catalyst paste contain a form of vinyl silicone. The base paste contains polymethyl hydrogen siloxane as well as other siloxane prepolymers. The catalyst paste contains divinyl polydimethyl siloxane other siloxane polymers. If the catalyst contains the platinum salt activator, then the paste labeled base must contain the hybrid silicone.

CHEMISTRY: -

Addition silicones are hydrophilic in contrast to all other silicone impression materials. In contrast to the condensation silicones, the addition polymer is terminated with vinyl groups and is cross-linked with hybrid groups activated by platinum salt- catalyst. Actually, there will not be any by-products as long as correct proportions of vinyl silicones and hybrid silicones are present. If the proportions are out of balance then hydrogen gas is produced as the by-product. The hydrogen gas that evolves from the set material can result in pinpoint voids in the stone cast.  

 

MANIPULATION: -

The light body and medium body Vinyl poly siloxane are supplied as two pastes and the Putty is supplied as two jars of high viscosity base and catalyst materials. These materials are suitable for an automatic dispensing & mixing device. With the mechanical mixing device there is greater uniformity in proportioning & mixing, less air is incorporated into mix and mixing time is reduced. The mixed impression material is ejected directly into adhesive coated tray or onto the prepared tooth. The basic automatic mixers sold by manufacturers are interchangeable. The tips vary in diameter, length & perhaps more importantly in number of spiral units within the tips. More units provide more thorough mixing. Thus an impression material that is adequately mixed in a spiral unit may be inadequately mixed with another spiral tip.  

 

WORKING AND SETTING TIMES.

More sensitive to temperature than polysulphides.Adding suitable retarders & cooling the mixing slab can alter both. Working and setting times can be extended by the addition of a retarder and by cooling the mixing slab.

ELASTICITY: -

The vinyl polysiloxane impression materials are the most ideally elastic of all the currently available materials. Distortion on removal from undercuts is virtually non-existent. Material is extremely difficult to remove from undercut areas because of high molecular elasticity. stiffness is proportional to consistency of the material.

RHEOLOGY: -

As one of the most pseudoplastic impression materials, the effect of increased strain rate on the unset material is quite pronounced for vinyl polysiloxane.  This large discrepancy between flow properties of the material under strong force such as syringing, and light force such as during seating the tray can be used for advantage as a one step material. Medium body material is used to capture the fine detail and is available as a one-stage material. The basic difference between the material used for injection and tray is the viscosity.

TEAR ENERGY: -

The resistance to tearing is adequate. These materials if not handled properly will tear rather than stretch like polysulphides. Polyether is more prone to tearing than polysulphides.

DIMENSIONAL STABILITY: -

The vinyl polysiloxane impression materials are the most dimensionally stable of all the current materials. No volatile by product is released to cause shrinkage of the material. The clinically set material is close to being completely cured, so that little residual polymerization occurs later to contribute to polymerization shrinkage. Impression does not have to be poured immediately.

BIOCOMPATIBILITY: -

The danger of leaving a piece of the material during removal of the impression can be avoided by proper handling. A foreign body of impression material can cause severe gingival inflammation.

HANDLING OF THE TRAY: -

Stock tray is used for reline material. The primary putty impression actually serves as a custom tray for wash or reline material.

HANDLING TECHNIQUE: -

Putty wash technique has been popular for vinyl siloxane impression material. Putty wash technique is more convenient and the bulk of the material is formed by the highly filled putty material that has relatively low polymerization shrinkage and a low thermal contraction coefficient. Supplied as single phase or monophase.

 

DISINFECTION: -

These impression materials are easily disinfected by immersing it in either of the following solutions.

1.10% Hypochlorite.

2.  2% glutaraldehyde.

SHELF LIFE: -

Material has a shelf life of about 2 years. The containers must be tightly closed. Viscosity not affected by temperature. shelf life can be prolonged by storing it in a cool, dry environment.

 

ADVANTAGES: -

1. Superior dimensional stability.

2. Can be poured more than once.

3. Pleasant odor and appearance.

4. Good shelf life.

5. Shorter setting time.

6. Adequate tear strength.

7. Extremely high accuracy.

8. Less distortion on removal.

DISADVANTAGES: -

1. Custom tray required for double mix technique.

2. Hydrogen gas may evolve from some materials.

3. Most difficult to pour, requires special care.

4. Expensive.

RECENT ADVANCES

wVisible light cured polymer urethane dimethacrylate.

wThe advantages of this material are that its working time is controlled by the operator.

wComposition: -

  1.polyether urethane dimethacrylate.

  2. Photo initiators.

  3. Photo accelerators.

  4.silicone dioxide fillers.

SETTING PROPERTIES OF ELASTOMERS

Material	Consistency	Viscosity (in Cp)	W.T (min)	S.T (min)	Dimensional change in 24 hours
Polysulfides	Low	60,000	4-7	7-10	0.40
	Medium	110,000	3-6	6-8	0.45
	High	450,000	3-6	6-8	0.40
Condensation silicones	Low	70,000	2.5-4	6-8	0.60
	High	70,000	2-2.5	3-6	0.38
Addition silicones	Low	150,000	2-4	4-6.5	0.17
	Medium	150,000	2-4	4-6.5	0.15
	High	150,000	2.5-4	4-6.5	0.14
	Very high	150,000	1-4	3-5	0.15
Poly ether	Low	130,000	2.5	4.5	0.23
	Medium	130,000	2-3	3.4-5	0.24
	Medium+ thin	130,000	3-4	4.5-5	0.23
	High	130,000	2.5	4-5	0.19

MECHANICAL PROPERTIES OF ELASTOMERS

Material	Consistency	Permanent deformation	Flow	Hardness	Tear strength (gm/cm)
Polysulfides	Low	3-4	0.5-2	20	2500-7000
	Medium	3-5	0.5-1	30	3000-7000
	High	3-6	0.5-1	35	----
Condensation silicones	Low	1-2	0.05-0.1	15-30	2300-2600
	Very high	2-3	0.02-0.05	50-65	----
Addition silicones	Low	0.05-0.4	0.01-0.03	35	1500-3000
	Medium	0.05-0.3	0.01-0.03	50	2200-3500
	Very high	0.2-0.5	0.01-0.1	75	2500-4300
Polyether	Low	1.5	0.03	35-40	1800
	Medium	1-2	0.02	35-60	2800-4800
	Medium+ thin	2	0.04	30-50	2500
	High	2	0.02	40-50	3000