Impression Materials

Impression

Negative likeness or copy in reverse of the surface of an object

An imprint of the teeth and adjacent structures for the use in dentistry

Preliminary Impression

Diagnosis

Treatment planning

Final Impression

To re-create existing tooth structure and surrounding gingival structures into stone casts for fabrication of indirect restorations

Features of Impression

Detailed occlusal surfaces

Gingival margins are clear and defined

No bubbles, voids, and thin walls

Intact material

Biocompatibility Requirements of Impressions

Setting time

Working time

Pleasant taste, odor, and esthetic color

Adequate strength - no break or tear while in use

Shelf life and storage

Dimensional stability

Easy to use

Economical

Elastomeric Properties of Impressions

No permanent deformation once strain is removed

Sufficient viscoisty for surface details

Compatible with die and cast materials

Classification of Impressions

Based on setting reaction

Based on elasticity on setting

Non-Aqueous Elastomers

Polysulfides

Polyethers

Additional silicon (PVS)

Condensation silicone

Viscosity

Fluid’s resistance to flow

Measure of internal friction

Temperature dependence

Shear Viscosity

Shear stress / shear strain rate

Newtonian

Viscosity independent of shear rate

Non-Newtonian

Viscosity depends on shear rate

Shear Thickening (Dilatant)

Viscosity increases with increase shear rate

Shear Thinning (Pseudoplastic)

Viscosity decreases with increase shear rate

Shear-Thickening Molecules

Clump up together through movement causing increase viscosity

Shear-Thinning Molecules

Spread apart through movement causing decreased viscosity

Shear-Thinning Impression

Less viscous when stressed during injection and then recovers its viscosity when it rests on the tissue or in the tray

Material should flow freely and wet the tissue as its being injected to achieve adaption

Then resist flow away from the intended SA

Hydrophilic Angle

Lower contact angle

Hydrophobic Angle

Higher contact angle

Significance of Hydrophilicity

Impressions of teeth in presence of saliva, blood, and sulcular fluid

Elastic Behavior

Ability o the material to resist being stretched or compressed without permanent distortion

Should be flexibility enough to not break upon retrieval

Elastic Modulus

Effects the ease of impression removal from the mouth

Viscoelastic Materials

Solids yet they respond to forces by behaving like both an elastic solid and viscous liquid

Viscoelasticity Depends On

The rate at which a force is applied to a material

How long a force/deformation is applied to a material

How many times that force is applied and removed

Temperature

Forces on Viscoelastic Material

Able to distort material and can cause permanent or plastic deformation

Even after the force is removed, the material may never recover its original dimensions

Tear Strength

Measures the resistance of an elastomeric material to fracture when subjected to a tensile force acting perpendicular to a surface flaw

Amount of force needed to tear a specific test specimen divided by the thickness of the specimen is called the tear strength

Tear Strength

F / t

Force divided thickness of the specimen

Elastic Recovery

Initially the impression material is stretched and if the pressure is released, it will spring back to its original shape

Yield Point

If stretching continues, it is stretched to a point of no return

Releasing tension does not cause the impression to return to its original length

Permanent distortion

Tear Strength

Stretching continues past the yield point

Polyaddition

Polymer chains are formed by the successive addition of monomers

Polycondensation

Polymer chains formed by successive addition of monomers but polymerization is accompanied by the release of low mW products (water, alcohol, salt)

Polyaddition Materials

PVS

Polyether

Polycondensation Materials

Polysulfides

Condensation silicone

Polysulfide Base Paste

Low mW -SH

Inert filler (such as lithopone or titanium dioxide) - strength

Viscosity modifier/plasticizer (Dibutyl phthalate)

Polysulfide Catalyst Paste

Reactor (catalyst accelerator - lead dioxide)

Filler

Retarder (oleic or stearic acid)

Dibutyl pthalate

Polymerization of Polysulfides

Mercaptan + lead oxide catalyst → polysulfides + H2O

Releases H2O causing decreased dimensional stability - needs to be poured by 30 mins

Condensation reaction

Polysulfides Used

For edentulous final impressions

Advantages of Polysulfides

High tear resistance

Moderate cost

Long working time

Compatible with stone

Different viscosities

Long shelf life

Disadvantages of Polysulfides

Custom tray needed

Long setting time

Stretching leads to distortion

Odor and stains

Pour within 1 hour

Hydrophobic

Polyethers Base Paste

Low mW polyether - terminal imine groups

Filler (colloidal silica)

Viscosity modifier/plasticizer (dibutyl phthalate)

Polyether Catalyst Paste

Reactor (aromatic sulfonic acid)

Filler

Viscosity modifier/plasticizer

Polymerization of Polyether

Polyether prepolymer + sulfonic acid → crosslinked polymer

Addition reaction

No by-product

Polyether Properties

Material sets by addition reaction - no by product therefore more dimensionally stable

High crosslinking means that the set material is very stiff

Very hydrophilic - must be stored in dry environment to preserve accuracy

Advantages of Polyethers

Fast setting

Clean

Easy to dispense

Hydrophilic

Good stability

Long shelf life

Disadvantages of Polyether

Brittle

Stiff and high modulus

Absorbs water

Bitter taste

Costly

Need to block undercuts

Polyvinyl Siloxane (PVS) Base Paste

Polymethylhydrosiloxane (PMHS) and divinylpolysiloxane (DVPS)

Filler (lithopone or titanium dioxide)

Viscosity modifier/plasticizer (dibutyl phthalate)

PVS Catalyst Paste

Divinylpolysiolxane

Catalyst (platinum salt)

Scavenger (palladium)

Viscosity modifier (Dibutyl phthalate)

PVS

No reaction by products if used correctly

Residual base paste can lead to secondary reaction with each other or with moisture, to produce hydrogen gas

H2 Gas in PVS

Results in pinpoint voids in the gypsum casts poured soon after removal

Palladium is often added as a scavenger for this

Wait 60 mins before pouring impression

Properties of PVS

Hydrophobic

Nonionic surfactant can be added for hydrophilicity

Susceptible to contamination - latex gloves

Latex Gloves in PVS

Touching the tooth with this before seating the impression can inhibit the setting of the critical surface next to the tooth

PVS Based on Viscosity

Light body (low viscosity)

Medium body

Heavy body

Putty consistency

Advantages of PVS

Multiple viscosities available

Multiple pours possible

Good dimensional stability

Can wait before pouring

No unpleasant odors

Exhibit better elastic recovery and less permanent deformation than other elastomers

Disadvantages of PVS

Hydrophobic

Sulfur contamination with latex gloves

H2 gas can cause voids

Polymerization of Condensation Silicones

Hydroxyl terminated PDMS + tri and tetra alkyl silicates + stannous oxide catalyst → crosslinked polymer

Condensation reaction

Release ethanol

Condensation Silicone Properties

Compression and shrinkage cannot be predicted

High curing shrinkage

Evaporation of the ethanol

Condensation Silicone Used In

Putty stents

Advantages of Condensation Silicone

Reproduction of detail

Compatibility with die

No unpleasant odors

Disadvantages of Condensation Silicone

Less dimensional stability

Hydrophobic

Dimensional stability

Loss of a small molecule from the reactants such as water or alcohol

Thermal contraction (oral to room temp.)

Absorption of water or disinfectant

Polymerization shrinkage

Plastic deformation - incomplete recovery