Impression Material

Impression Material

First impressions count

Purpose

To prepare accurate and true replicas of oral

structures (teeth, mucosa)

The replica is prepared in 2 steps:

Step 1. Making a negative (impression)

Step 2. Prepare a model or a cast -

accurate positive

Working time – a period from the start of mixing and ends

 just before the elastic properties have developed ( the final time at which the impression can be seated in the mouth without its distortion)

Setting time – a period from the start of mixing until the curing process has advanced sufficiently so that the

impression  can be removed from the mouth with a minimum distortion.

Hydrocolloids

q Gelatin particles suspended in water (dispersed particle)

q Water is dispersion medium

Classification of Impressions

Preliminary Impressions 

Use to make a reproduction of the teeth and surrounding tissues. 

Uses

 (1) diagnostic models

(2) custom trays

(3) orthodontic appliances

(4) pretreatment and post‑treatment records.

Final impressions

—  Used to make the most accurate reproduction of the teeth and surrounding tissues.

—  Used to make indirect restorations, partial or full dentures, and implants. 

          Bite registrations

—  Make a reproduction of the occlusal relationship between the maxillary and mandibular teeth.

—  Provide an accurate registration of the patient’s centric relationship between the maxillary and mandibular arches. 

Characteristics of Impression Trays

¡  Quadrant tray

§  Covers one half of the arch.

¡  Section tray

§  Covers the anterior portion of the arch.

¡  Full arch tray

§  Covers the entire arch.

¡  Perforated tray

§  Holes in the tray create a mechanical lock to hold the material in place.

¡  Smooth tray

§  Interior of the tray is painted or sprayed with an adhesive to hold the impression material.

Stock tray

Examples of quadrant, section, and full-arch impression trays. 

The Ideal Impression Material

q Easy to mix and handle

q  Suitable working time

q Suitable setting time

q Compatible with die and stone

q Not toxic or allergenic to the patient

q Dimensionally stable on setting

q Accurate to record the fine details of the prepared tooth

q Has acceptable odor and taste

q  Adequate strength

q  Adequate shelf life can be  disinfected without loss of accuracy.

q  Fluid or plastic when inserted into the mouth.

q Biocompatibility

q Cost effective

q High dimensional stability

q High elastic recovery

Impression Materials

¡  Non-elastic/Rigid

¡  Elastic/Non-Rigid

IMPRESSION MATERIALS

Key Properties

a.     Accuracy = ability to replicate the intraoral surface details.               

b.  Dimensional Stability = ability to retain its absolute dimensional size over time.

     c.  Tear Resistance = ability to resist tearing in thin sections (such as through the feather-edged material within the gingival sulcus.

II.  By setting mechanism

A.   Chemical             reaction       (irreversible)	By elasticity and use
	Inelastic or rigid		Elastic
	Materials	Use	Material	Use
	1.Plaster of           Paris 2.Zinc-oxide        eugenol	Edentulous       ridge Interocclusal     records	1. Alginate 2. Non-aqueous        elastomers -      Polysulfide -       Polyether -       Condensation  silicone -  Addition silicone	Teeth and soft tissue
B. Physical reaction (reversible)	3.Impression Compound	Preliminary impression	3.Agar hydrocolloid	Teeth and soft tissue

Impression materials are classified in different ways

1)    Based on mode of setting & elasticity :

I) Impression which set by chemical reaction(irreversible or thermoset) are
A) Rigid Impression Materials :

                         Impression Plaster

B) Elastic Impression Materials :

Hydro colloid

 Alginate

Non aqueous Elastomers

II) Impression which set by temperature change(reversible or thermoplastic) are

A) Rigid Impression Materials :

Impression Compound

Silicons

II) Impression which set by temperature change(reversible or thermoplastic) are

A) Rigid Impression Materials :

Impression Compound

3) Based on their use in dentistry :

I) Impression material used for Edentulous mouth are :

•         Impression Plaster

•         Impression Compound

•         Impression Paste

II) Impression materials used for Dentulous mouth are

•         Alginate

•         Rubber Base impression Material

4) Based on the amount of pressure applied or tissue displacement   during impression :
          Mucostatic   

                   Ex: Impression plaster

                         ZOE impression paste

  

Mucocompressive

                   Ex: Elastomers

          III) Selective Pressure

•         Impression Wax

5) Based on the manipulation :

          I) Hand mixing and Kneading

•         Impression compound

•         putty consistency Elastomers

          II) Circular motion

•         Zoe impression paste

•         Polysulfide

          III) Vigorous mixing

•      Alginate

Based on the tray used for impression

•         Stock tray

Type of tray

Rim locked tray

e.g. alginate, elastomer

water cooled tray

agar

perforated

alginate

elastomer

non perforated

impression compound

•         Special tray

•      Zno eugenol impression paste

•      Medium body elastomer

Impression plaster
ADA type I gypsum product

Uses

For edentulous impression

Old fashioned – not frequently used

Zinc oxide-eugenol

Irreversible Hydrocolloid (Alginate)

Most widely used impression material

q  Indications

q study models

q removable fixed partial dentures

q framework

q Examples

q Jeltrate (Dentsply/Caulk)

q Coe Alginate (GC America)

Elastomeric Impression Materials

Polysulfide

q First dental elastomers

q Indications

q complete denture

q removable fixed partial denture

q tissue

q crown and bridge

q Examples

q Permlastic (Kerr)

q Omni-Flex (GC America)

Condensation Silicone

q Indications

q complete dentures

q crown and bridge

q Examples

q Speedex (Coltene/Whaledent)

q Primasil (TISS Dental)

Addition Silicones

q Indications

q crown and bridge

q denture

q bite registration

q Examples

q Extrude (Kerr)

q Express (3M/ESPE)

q Aquasil (Dentsply Caulk)

q Genie (Sultan Chemists)

q Virtual (Ivoclar Vivadent)

Polyether

q Indications

q crown and bridge

q bite registration

q Examples

q Impregum F (3M/ESPE)

q Permadyne (3M/ESPE)

q Pentamix (3M/ESPE)

q P2 (Heraeus Kulzer)

q Polygel (Dentsply Caulk)

*The dental impression is one of the ways by which pathogens can leave the operatory and spread their risk abroad         

          The impression must be rendered harmless before being passed on to another person who will work with it or with the gypsum cast made from it, outside the dental operatory

¡  Chemical disinfectants used for this purpose

§  Chlorine compounds

§  Synthetic phenolic compounds

§  Glutaraldehydes

§  Iodophors

§  Phenolic/ alcoholic combinations

Material	Method	Disinfectant
l Alginate l Agar-agar	Disinfectant with short-term exposure time (<10min )	Chlorine compounds or iodophors
l Polysulfide l Silicone	Immersion time should not exceed the recommended pouring time	Glutaraldehydes, Chlorine compounds, iodophors, phenolics
l Polyether	Disinfectant with short-term exposure time (<10min )	Chlorine compounds or iodophors

§  An alternative method for the hydrophilic materials:

▪      Disinfectant can be sprayed on the impression - wrapped in a disinfectant soaked paper towel and placed in sealed plastic bag for 10 min

§  Long immersion time may cause the surfactant in hydrophilic PVS to leach out and render the impression less hydrophilic

Alginate

   It is easy to manipulate

   It is comfortable for the patient

   It is relatively inexpensive and does not require

elaborate equipment

Uses of alginate

   Impression making in complete denture prosthesis and orthodontics.

   In undercuts and in excessive saliving flow

   For impressions for mouth protectors

   For impression in study models and working cost

Component	Function	Weight percentage
Potassium alginate	Soluble alginate	15
Calcium sulfate	Reactor	16
Potassium titanium           Fluoride	Accelerator	3
Zinc oxide	Filler particles	4
Diatomaceous earth	Filler particles	60
Sodium phosphate	Retarder	2

q Reactants: sodium or potassium alginate = comes from sea weed .used as thickening agent.

q calcium sulfate  - reacts with potassium alginate to form gel.

q  Retarder: sodium phosphate

(sodium phosphate content adjusted by manufacturer

to make either regular or fast-set alginate)

q  Reinforcing filler: diatomaceous earth

(for strength and body)

(controls the stiffness of the set gel)

q  Potassium titanium  fluorides

. Added so as not to interfere with the setting and surface strength.

(provides good surfaces on gypsum dies)

q  For esthetics: coloring and flavoring agents (traces)

Gelation process

Soluble alginate + calcium sulfate           Insoluble calcium                                                                                  alginate gel

                       trisodium phosphate

                               (retarder)

Na alginate + CaSO4 --------> Cas alginate + Na2SO4

(powder)                                                   (gel)

¡  Mixing time

Time to complete uniform blending of components

¡  Working time

¡  Time from beginning of the mix until the setting reaction

dominates

¡  Setting time

¡  Time from beginning of the mix until the setting reaction

is over

Type 1: Fast set

Type II: Normal set

Types	Mixing time	Working time	Setting time
I- Fast set	45 sec	1.25 mins	1-2 mins
II- Normal          set	60 sec	2 mins	2 - 4.5 mins

Problems with Alginate Impressions

Dimensional stability:

Because it is a gel, it undergoes shrinkage or expansion

upon loss or gain of water.

If an alginate impression is stored in water or in a very wet paper towel, the alginate will absorb additional water and expand. This condition is called imbibition.

If an alginate impression remains in the open air, moisture will evaporate from the material, causing it to shrink and distort. This condition is called syneresis.

For least dimensional change/avoid distortion:

Store impressions in 100% relative humidity

Pour quickly after removal from mouth

-         Colloids – a system consisting of a poorly  

                solvated phase in a medium

    - Two phases

           Dispersed

           Dispersion phases

     - Hydrocolloids - agar or algin, dissolved in  

                               water

¡  Manipulation

§  The measured powder is sifted into pre-measured water

§  A vigorous figure – 8 motion, with the mix being stropped against the sides of rubber mixing bowl

§  Mixing time between 45 secs – 1 min                     

         

               smooth creamy mixture

§  All colloidal dispersions are termed sols

§  Sol-gel transformation

§  In the gel state, the dispersed phase agglomerates, forming chains or fibrils called micelles. The fibrils may branch and intermesh to form a brush-heap structure

Advantages of Alginate

Economical

Easy to use

Quick setting

Fair taste

Hydrophilic

Disadvantages of Alginate

Limited detail reproduction

Low tear resistance

Single pour only

Quick pouring required

Low dimensional stability

Troubleshooting

-Inadequate working or setting time:

– temperature of the water, incomplete spatulation

– W/P too low

– improper storage of alginate powder

- Distortion:

– tray movement during gelation or removed from mouth prematurely

– weight of tray compressing or distorting alginate

– impression not poured up immediately

- Tearing:

– removing impression from mouth before adequately set

– thin mixes (high W/P ratio)

– presence of undercuts (blocking out these areas before an impression

may help)

– inadequate amount of impression material in tray (avoided by

minimum 3 mm of impression material between tray and oral tissues)

- Loss of detail:

– removed from mouth prematurely

- Consistency:

– preset mix is too thin or thick

– the W/P ratio is incorrect (avoid by fluffing powder before measuring; do

not overfill powder dispenser)

– inadequate mixing (avoided by vigorous spatulation and mixing for recommended time)

– using hot water: grainy and prematurely thick mix

*Dimensional change:

– delay in pouring alginate impression stored in air: results in distorted,

undersized cast due to alginate impressions losing water when stored in air

*Porosity:

– whipping air into the mix during spatulation (proper mixing: after initial

wetting of powder by the water, mix alginate so as to squeeze the material

between the spatula blade and the side of the rubber bowl)

Poor stone surface (of cast)

– set gypsum remaining in contact with the alginate for too long a period of  time.

¡  elastomers

¡  Introduced in late 1950’s – popular

        drawbacks of hydrocolloids

           - poor dimensional stability

           - inadequate tear resistance

¡  Setting occurs through a combination of  chain lengthening polymerization and chemical cross linking

            - condensation reaction

            - addition reaction

   Uses

¡  1.in FPD.

¡  2.for RPD.

¡  3. impression of edentulous mouth for CD.

¡  4.border molding.(polyether).

¡  5.for bite registration

¡  6.silicone dublicating material for making refractory cast

kinds of elastomers

I. 

Chemically four

§  Polysulfide

§  Condensation polymerizing silicone

§  Addition polymerizing silicone

§  Polyether

Based on the viscosity  

¡  Type

          1       -Low viscosity (light body) /syringe consistency.

          2       -Medium viscosity (medium body) /regular consistency.

          3       -High viscosity (heavy body)/tray consistency

          4       -Very high viscosity (Putty)

Polysulfide                       

Setting reaction

                                             The terminal  thiol (SH ) group of each molecule is oxidized by the addition of oxidizing agent present in the reaction paste

        Lengthening of polymer chain by oxidation of terminal thiol (SH ) groups and cross linking by oxidation of  thiol (SH ) groups

¡ Mercapten   +lead dioxide = polysulphide+ water

¡  Condensation type  of reaction - water  byproduct

¡ Moisture and temperature -> decreases working time.

                                            

¡ Available in three consistencies

§  Low

§  Medium

§  High

¡ Packaged as two pastes

      Base      (white)

      Catalyst (brown)

Tray adhesive

Butyl rubber or styrene dissolved in volatile solvent such as  chloroform or ketone .

properties

1.Unpleasant odour and colour.

2.Difficult to work and stains linen.

3.Extremely viscous and sticky to manipulate.

4.Pseudoplasticity i.e if sufficient speed and force is used for spatulation ,the material seem easier to handle.

5.Mixing time is 45 second.

6.Long setting time of 12.5 min.heat and moisture accelerate the rxn.

7.Excellent reproduction of surface details.

8. Curing shrinkage is high and continues even after setting.highest permanent deformation among elastomers.

9.Pouring should be delayed by  half an hour.further delayed should be avoided to minimize curing shrinkage. Loss of by product also causes shrinkage.

High tear strength.

                                              

       

Advantages	Disadvantages
- High tear strength	-  Dimensionally unstable
- Long working time	- Unacceptable odor
-  Established precision	- Untidy and stains clothing
-  Economic	-  Long setting time
- Extensive shelf life	- Least elastic recovery
- Less hydrophobic	-  Subsequent pours are less accurate.

CONDENSATION SILICONE
(SILICONE, POLYSILOXANE ORGANO-TIN SILICONE,conventional silicone)

Composition	Components	Function
Base paste	polydimethyl siloxane  prepolymer	undergoes cross linking to form rubber
	Silica	Filler
Reactor	Alkyl Silicate	Cross linking agent
	Stannous octate	Catalyst

Properties of condensation silicone

¡  Pleasant color and odor.

¡  Mixing time 45 sec.setting time is 8-9 min.

¡  Excellent reproduction of surface details.

¡  High curing shrinkage.also due to evaporation of ethyl alcohol by-product.

¡  Stiffer and harder than polysulphide.

¡  Hydrophobic

¡  Shelf life is less than that of polysulphide.

Advantages	Disadvantages
-  Agreeable odor & stain free	-      Inadequate dimensional    stability
-      Satisfactory working & setting       time	- Inadequate shelf life
-  Enhanced elastic recovery	- Hydrophobic
-  Adequate tear strength

ADDITION SILICONE
{POLYVINYL SILOXANE, VINYL(POLYSILOXANE)}

l Available as two consistencies

l           Putty          (75%filler)

l           Light body  (35%filler)

	Components	Function
Base paste	Hydroxyl  terminated polymethyl siloxane  prepolymer	Undergoes cross linking to form rubber
	Colloidal silica	Filler
Reactor	Polyvinyl silicone prepolymer
	Colloidal silica	Filler
	Chloroplatinic acid	Catalyst

Setting reaction

¡  Reaction occurs by cross-linking

      

    Polyvinyl silicone +Polymethyl siloxane                                           Rubber

¡  No  by product formed

¡  Imbalance between  base and reactor- hydrogen gas        

¡  Hydrogen gas scavenger - platinum or palladium

¡  Available in 4 consistencies

§  Light body

§  Medium body

§  Heavy body

§  Putty

Advantages	Disadvantages
- Excellent dimensional accuracy	-   Hydrophobic
- Long term dimensional stability	-   Expensive
	-   Hydrogen gas evaluation in some materials
-  Pleasant to use
-  Short setting time	-  Hydrophilic formulations     imbibe moisture
-  Auto mix available
-  If hydrophilic, good compatibility with gypsum	- Sulfur contamination by latex glove

Composition

Base  Paste   cc	Imine terminated prepolymer	Becomes cross linked to form rubber
	Silica	Filler – to control viscosity
	Pthalate	Plasticizer
Reactor paste	Alkyl  aromatic sulfonate	Initiates cross linking
	Silica	Filler
	Pthalate	Plasticizer

Setting reaction

¡  Cross linking of a difunctional epimine terminated prepolymer catalyzed by an alkyl benzene sulfonate catalyst

¡  Reaction involves ring opening - no by products

§  Stiffness reduced - addition of thinner

§  Absorbs moisture - dimensional change

Available in 3 consistencies

▪      Light body

▪      Medium body

▪      Heavy body.

Advantages	Disadvantages
Dimensional stability	-  Set material very stiff
Accuracy	-  Imbibition
Shorter setting time	-  Short working time.
Automix available	- Allergic hypersensitivity in some cases.