Dentures

Impression Materials are used to record the shape of the teeth and alveolar ridges.

Classified as:

1. Elastic
2. Non-elastic

Elastic Impression Materials

Impression Materials which can be stretched and bent to a fairly large degree without suffering any deformation.
These are used for recording oral structures where undercuts are present.

The Elastic Impression Materials are:

1. Hydrocolloids
2. Elastomers

Hydrocolloid

Colloid is a state of matter in which individual particles of one substance, are uniformly distributed in a dispersion medium of another substance.

Hydrocolloid – dispersion medium is water.
1. Agar-agar
2. Alginate

Agar-agar

A gel in its natural state, but on heating becomes a sol.
Non toxic and non irritant.
Slow setting time.
Poor tear resistance.

Brands Names
Acculoid by Van R
Cohere/Superbody/Supersyringe by Ghingi-Pak
Indentic by Cadco

Advantages
Good surface detail
Reusable and easily sterilised
Accurate
Low cost
Hydrophillic

Disadvantages
Dimensional instability
Need special equipment (water bath) and special technique
Tears easily
Difficult to see margins

Alginate

On mixing the powder with water a sol is formed, a chemical reaction takes place and a gel is formed.
Powder contains
1. Alginate salt (e.g. sodium alginate)
2. Calcium salt (e.g. calcium sulphate)
3. Trisodium phosphate
Powder and water should be measured to manufactures instructions.
Faster or slower setting times can be achieved by using warm or cold water.
Pour as soon as possible.

Advantages
Easy to use and mix.
Non toxic and non irritant.
Good surface detail.
Cheap and good shelf life.
Setting time can be controlled with temperature of water used.

Disadvantages
Poor dimensional stability.
Setting time very dependent on operator handling.
Messy to work with
Not accurate.
Tears easily

Elastomers

Main Types

Polysulphides
Silicones
Polyethers

Polysulphides

Thiokol rubbers, rubber base or mercaptan.
Supplied as two pastes (base and activator) mixed in a 1:1 ratio.
Very small setting contraction (0.3-0.4% over the first 24 hrs).
The active constituent in the base paste is the polysulphide and the active constituents in the activator paste is lead oxide and sulphur which cause further polymerisation of the polysulphide.

Advantages
Dimensional stability
Accuracy
Comes in a number of different viscosity’s
Long working time (although this may be a disadvantage in some clinical situations)
Long shelf life

Disadvantages
Lead oxide in base paste may have toxic effects
Staining of clothes due to the Lead oxide
Messy to work with – unpleasant rubbery smell
Can only be used in a special tray

Silicones

Two Types
Addition Silicone
Condensation Silicone

Addition Silicone

These materials are often termed vinyl polysiloxanes.
Supplied in 2 pastes or in a gun and cartridge form as light, medium, heavy and very heavy bodied.
One paste contains a polydimethylsiloxane polymer in which some methyl groups are replaced by hydrogen.
The other paste contains a pre-polymer in which some methyl groups are replaced by vinyl groups, this paste also contains a Chloroplatinic acid catalyst

Advantages
Accurate
Easy to use
Fast setting
Wide range of viscosity’s
Pour repeatedly
Stable (pour can be delayed)
Easily seen margins

Disadvantages
Hard to mix
Sometimes difficult to remove the impression from the mouth
Too accurate in some circumstances (cast produced is not sufficiently oversized)
Low tear strength

Condensation Silicone

Supplied as a paste and liquid or two pastes, in light, medium, heavy or very heavy bodied (putty).
polysilixone
Hydrophobic
Moderate tear strength
Good surface detail
Very elastic

Brand names
Accoe by GC America
Cuttersil by Heraeus Kulzer
Speedex by Coltene Whaledent

Advantages
Accurate
Easily seen margins
Ease of use
Can be used on severe undercut
Good working time

Disadvantages
Hydrogen evolution
Liquid component of paste/liquid system may cause irritation
High polymerization shrinkage
Low tear strength
Hydrophobic

Polyethers

Mixed in a 1:1 ratio until homogeneous colour, the amount of catalyst used can be used to control the setting time.
Used in special or stock trays with an adhesive.
Supplied in two pastes
Base – Polyether, Filler
Catalyst – Sulphonic acid ester, inert oils
When mixed the polymer and sulphonic acid ester react to form a stiff polether rubber.

Brand Names
Impregum F by 3M
Permadyne by 3M
Polyjel NF by Dentsply Caulk

Advantages
Accuracy
Good on undercuts
Ease of use
Least hydrophobic
Good stability
Delay pour

Disadvantages
May cause allergic reaction due to the sulphonic acid ester
Bitter taste
Poor tear strength
Rapid setting time (ie short working time)
Stiff set material (sometimes hard to remove from mouth)

Setting Time 23-30C

Polysulfide WT 4.3-6 m ST 12.5-16 m
Condensation WT 2.5-3.3 m ST 8.9-11 m
Addition WT 1.8-3.1 m ST 5.9-8.9 m
Polyether WT 2.3-3.3 m ST 8.3-9m

Non-Elastic Impression Materials

These materials are rigid and therefore exhibit little or no elasticity.
Any significant deformation produces a permanent deformation.
They are used where there are no undercuts and are mainly used for edentulous patient cases.

Impression compound
Impression Plaster
Zinc Oxide Eugenol
Impression Waxes

Impression Compound

It is a thermoplastic compound – softens when heated and hardens when cooled
The compound disc is completely immersed in a water bath at 55-60oC for about 4-5 minutes to ensure complete softening.
Poor surface detail
High coefficient of thermal expansion (contraction of up to 0.3% when removed from mouth to room temperature)
Distorts wen removed over undercut areas
Mucocodisplacive
Poor dimensional stability
Can be modified by re-heating
Non toxic and non irritant

Advantages
Non irritant and non toxic
Reusable (but with re-use the constituents are leached out)
Can be reheated and readapted
Can support other materials for wash impressions
Mucocompressive

Disadvantages
Poor dimensional stability
Poor surface detail
Expansion coefficient
Will distort if removed from undercuts

Impression Plaster

Impression plaster is based on calcined calcium sulphate hemihydrate, which reacts with water to form a hard mass of calcium sulphate dihydrate.
Dimensionally stable if anti expansion solution used
Fractures if undercuts present
Mucostatic
Needs to be treated with a separating medium (e.g. varnish or soap solution) before being cast in stone or plaster
Non toxic

Advantages
Good surface detail
Excellent dimensional stability
Rate of the setting reaction can be controlled by the clinician

Disadvantages
Cannot be used for mucodisplacive impressions
Cannot be added to
Properties affected by operator handling technique
Taste and roughness may cause the patient to vomit

Zinc Oxide-Eugenol Impression Materials

This material is used for recording edentulous ridges in a close fitting special tray or the patients existing dentures.
The material is mixed in a 1:1 paste ratio and used in thin sections only (2-3mm) as a wash impression. Vaseline is used as a separating agent on those areas requiring protection (soft tissues, teeth etc).
Base paste and reactor paste come in contrasting colours and are dispensed in a 1:1 ratio. They are mixed to give a paste of even colour.

Non toxic
Adherence to tissues
Good surface detail in thin section
Good dimensional stability (little or no dimensional change on setting, 0.1% dimensional change during setting)
Can be added to with fresh zinc oxide eugenol

Advantages
Dimensional stability
Good surface detail
Can be added to
Mucostatic or mucocodisplacive

Disadvantages
Cannot be used in very deep undercuts
Only sets quickly in thin section
Eugenol allergy in some patients

Impression Waxes

They are thermoplastic materials, which flow at mouth temperature and are soft a room temperature.
They do not set by chemical reaction.
Normally used to correct small imperfection (e.g. airblows) in other impressions, especial zinc oxide impressions.
They consist of a combination of a low melting paraffin wax and beeswax in a ratio of approximately 3:1 to ensure the wax flows at room temperature.
A cast should be poured up immediately after taking the impression to avoid distortion which readily occurs in wax.
Not commonly used.

References

Givney GP and Brown D : Mc Craken’s Removable Partial Prosthodontics. Eleventth ed, The CV Mosby Company. St Louis 2005
Keyf F : Some Properties of Elastomeric Impression Materials Used in Fixed Prosthodontics. Journal of Islamic Academy of Sciences 7:1, 44-48, 1994
Craig R, O’Brien W, Powers J : Dental Materials. Mosby-Year Book, Inc , 1996
Anusavice K: Phillips’s Science of Dental Materials, Saunders, Elsevier Science, 2003
Harcourt JK: A review of modern impression materials. Aust Dent J 23:178, 1978
Webpage – http://www.dentistry.bham.ac.uk