Elastic Impression Materials-Elastomers-Prosthetic Dentistry Lecture Note

Introduction to Elastic Impression Materials

These materials can be stretched and bent to a fairly large degree without suffering any deformation. These are used for recording the patient's mouth where undercuts are present. Usually used for partial dentures, over dentures, implants and crown and bridge work .The elastic impression materials are:

Introduction to Elastomers

These are used where a high degree of accuracy is needed, especially in crown and bridge work. They have two main advantages over the Hydrocolloids - good tear resistance and dimensional stability.They are mainly hydrophobic rubber based materials. All of these materials come in different viscosity's ranging from low to high viscosity. The light bodied material maybe used as a wash impression over a medium or heavy-bodied material. There are two ways this can be carried out as described below.

ONE STAGE IMPRESSION

Light bodied impression material is placed in a syringe, and placed over the areas where high detail is required (e.g. over a crown preparation). Some is then squirted over the heavy-bodied impression material which has been loaded into an impression tray. The impression is then taken as normal. This technique saves time, but it can be very labour intensive because the two need to mixed at the same time often requiring more than one DSA.

TWO STAGE IMPRESSION

An impression is taken with the heavy-bodied material. This is then removed from the mouth and inspected. The light bodied material is then prepared and again placed in a syringe. This is then squirted over heavy-bodied material and then impression relocated in its original impression.

Polysulphides

Silicones

Polyethers

Polysulphides

CLINICALLY

Used for crown and bridge work mainly, but also used for partial dentures, overdentures and implants. Two equal lengths are mixed together with a spatula for about a minute. The tray needs to be treated with an adhesive (rubber solution in acetone) to provide retention for the polysulphide. Taking the impression is delayed by 5 minutes before the impression is placed in the patients mouth - the final setting time is usually about 10 minutes from the start of mixing - this delay therefore decreases the amount of time the impression tray is in the patients mouth. A one or two stage impression technique may be used. Although dimensionally stable, the impression should be cast within 24 hours.

CHEMISTRY

Other names : Thiokol rubbers, rubber base or mercaptan.
Supplied as two pastes mixed in a 1:1 ratio.

BASE PASTE

· Polysulphide (forms rubber on polymerisation)

· Filler (to give body)

· Plasticiser (control viscosity)

ACTIVATOR PASTE

· Inert oil (forms a paste)

· Sulphur (facilitates the reaction)

· Lead oxide (causes polymerisation and cross-linking)

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.
On mixing crosslinking and chain lengthening causes the material to become an elastic solid after about 5-8 minutes. Setting is more rapid in the presence of moisture.
They come in three types: light, regular and heavy bodied (viscosity increasing from light through to heavy bodied). The light bodied polysulphides are used as wash impressions on heavier bodied impression materials. The medium and heavy-bodied impression materials may be used on their own.

PROPERTIES

• Dimensional stability
• Excellent surface detail (is only used in special trays)
• Viscosity depends on the brand used
• Very small setting contraction (0.3-0.4% over the first 24 hrs)
• Contraction on cooling from mouth to room temperature
• Very good tear resistance
• Good shelf life
• Viscoelastic

ADVANTAGES

1. Dimensional stability
2. Accuracy
3. Comes in a number of different viscosity's
4. Long working time (although this may be a disadvantage in some clinical situations)
5. Long shelf life

DISADVANTAGES

1. Lead oxide in base paste may have toxic effects
2. Staining of clothes due to the Lead oxide
3. Messy to work with - unpleasant rubbery smell
4. Can only be used in a special traY

Silicones

The silicone impression materials are classified according to the type of chemical reaction by which they set.

Addition

Condensation

Addition silicones

Can be used as a one or two stage technique. May be used in special or stock trays. The very heavy bodied materials are measured in scoops and are mixed by hand until homogeneous in colour.

1) An example of an addition silicone - Xantropen	2) An example of an addition silicone - Kerr's Extrude

3) An impression taken in Xantopren Green	4) Xantopren impression with beading

5) Addition silicone impression material being used to take an impression of implants

Properties of Addition Silicones

CHEMISTRY

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.
On mixing, in equal proportions, crosslinking occurs to form a silicone rubber. Setting occurs in about 6-8 minutes.

PROPERTIES

• Good shelf life
• Dimensionally stable
• Moderate tear strength
• Excellent surface detail
• No gas evolution
• Non toxic and non irritant

ADVANTAGES

1. Accurate
2. Ease of use
3. Fast setting
4. Wide range of viscosity's

DISADVANTAGES

1. Hard to mix
2. Sometimes difficult to remove the impression from the mouth
3. Too accurate in some circumstances (cast produced is not sufficiently oversized)

Condensation Silicones

CLINICALLY

Used for crown and bridge work mainly, but also for partial dentures, implants and overdentures. Used in stock trays or special trays. One or two stage impression stage. Although dimensionally stable the impression should be cast within 24 hours.

CHEMISTRY

Supplied as a paste and liquid or two pastes, in light, medium, heavy or very heavy bodied (putty).

BASE PASTE

• Silicone polymer with terminal hydroxy groups
• Filler

CATALYST PASTE

• Crosslinking agent (organohydrogen siloxane)
• Activator (dibutyl-tin dilaurate)

On mixing the two pastes react, cross linking occurs and setting takes about 7 minutes.
The setting reaction is a condensation reaction.
Hydrogen gas is evolved on setting which leads to surface pitting, and a roughened surface to the resulting model.

PROPERTIES

• Hydrophobic
• Hydrogen gas evolution on setting
• Moderate shelf life
• Moderate tear strength
• Good surface detail
• Shrinking of impression over time
• Non toxic and non irritant
• Very elastic (near ideal)

ADVANTAGES

1. Accurate
2. Ease of use
3. Can be used on severe undercuts

DISADVANTAGES

1. Hydrogen evolution
2. Liquid component of paste/liquid system may cause irritation

Polyethers

Used for crown and bridge work, partial dentures, implants and overdentures. 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. A one or two stage technique can be used. Although dimensionally stable the impression should be cast within 24 hours.

1) Polyethers come as a two paste system for mixing

Properties of Polyethers

CHEMISTRY

Based on imine chemistry
Supplied in two pastes

BASE PASTE

· Polyether

· Filler

CATALYST PASTE

· Sulphonic acid ester (enhances further polymerisation and crosslinking)

· Inert oils

When mixed the polymer and sulphonic acid ester react to form a stiff polether rubber.
Setting time occurs in about 6 minutes.
Usually only comes in one viscosity - regular bodied, but can also come as light + heavy bodied (Diulent)
Heat and moisture speed up the setting reaction.

PROPERTIES

• Hydrophillic (ie absorbs water)
• Good shelf life of up to 2 years
• Good elastic recovery
• Non toxic
• Low setting contraction
• Low tear strength
• Excellent surface detail
• Good dimensional stability

ADVANTAGES

1. Accuracy
2. Good on undercuts
3. Ease of use

DISADVANTAGES

1. May cause allergic reaction due to the sulphonic acid ester
2. Poor tear strength
3. Rapid setting time (ie short working time)
4. Stiff set material (sometimes hard to remove from mouth)

Elastic Impression Materials-Hydrocolloids-Prosthetic Dentistry Lecture note

Introduction to Elastic Impression Materials

These materials can be stretched and bent to a fairly large degree without suffering any deformation. These are used for recording the patient's mouth where undercuts are present. Usually used for partial dentures, over dentures, implants and crown and bridge work .The elastic impression materials are:

Hydrocolloids

Elastomers

Introduction to Hydrocolloids

A colloid is a state of matter in which individual particles of one substance are uniformly distributed in a dispersion medium of another substance. When the dispersion medium is water it is termed a hydrocolloid. The colloid is relatively fluid when the solute particles present are dispersed throughout the liquid. This is called a sol. Alternatively the particles can become attached to each other, forming a loose network which restricts movement of the solute molecules. The colloid becomes viscous and jelly like, and is called a gel. Some colloids have the ability to change reversibly from the sol state to the gel state. A sol can be converted into a gel in one of two ways:
1. Reduction in temperature, reversible because sol is formed again on heating (eg agar).
2. Chemical reaction which is irreversible (eg alginates). A gel can lose (syneresis which results in shrinkage) or take up (imbibition which results in expansion) water or other fluids.
Hydrocolloids are placed in the mouth in the sol state when it can record sufficient detail, then removed when it has reached the gel state. Hydrocolloid materials especially the alginates, may display a lack of incompatibility with some makes of dental stones. The resultant model may show reduced surface hardness and possibly surface irregularities and roughness.

Agar Impression Materials

CHEMISTRY

• Agar (colloid)
• Borax (strengthen gel)
• Potassium Sulphate
• Water (dispersion medium)

In its natural state it a gel, but on heating becomes a sol.
PROPERTIES

• Good surface detail
• Can be used on undercuts, but liable to tear on deep undercuts
• Evaporation or imbibition
• Non toxic and non irritant
• Slow setting time
• Poor tear resistance
• Adequate shelf life
• Can be sterilised by an aqueous solution of hypochlorite.

ADVANTAGES
1. Good surface detail
2. Reusable and easily sterilised
DISADVANTAGES
1. Need special equipment (water bath) and special technique
2. Dimensional instability
CLINICAL
Supplied in sealed tubes to prevent evaporation of water. The tubes are heated in boiling water (in a water bath) for 10-45 minutes. Once the impression is taken the tray can be cooled with water to aid gel formation. A higher temperature is needed to convert the gel into a sol. The first material to set is that which is in contact with the tray since it is cooler than the tissues. Thus it is the material in contact with the tissue which stays in the sol state for the longest time. Agars have been largely superseded by alginates and elastomers, although are still used for complex impressions for advanced restorative work. They are often used in labs to duplicate model because they can be reused many times.

Alginate Impression Materials

Container of powder should be shaken before use to get an even distribution of constituents. Powder and water should be measured to manufactures instructions. Water at room temperature should be used, this gives a reasonable working time of a couple of minutes. Faster or slower setting times can be achieved by using warm or cold water respectively. The material nearer the tissues sets first (cf. agar). Retention is needed to the impression tray and is provided by perforations in the tray and/or adhesives. Once removed from the mouth the impression should be rinsed with cold water to remove any saliva or blood. It should then be covered in a damp gauze/napkin to prevent syneresis (not placed in water which would cause imbibition-expansion). The impression should be soaked in hypochlorite for 60 seconds and then cast as soon as possible.

1) An alginate impression of the upper arch in a special tray

2) An alginate impression of the lower arch

Properties of Alginates

CHEMISTRY
On mixing the powder with water a sol is formed, a chemical reaction takes place and a gel is formed.
The powder contains
1. Alginate salt (e.g. sodium alginate)
2. Calcium salt (e.g. calcium sulphate)
3. Trisodium phosphate
The setting reaction is as follows:
On mixing the powder with the water

SODIUM ALGINATE		SODIUM SULPHATE
+	>	+
CALCIUM SULPHATE		CALCIUM ALGINATE

The above reaction occurs too quickly often during mixing or loading of the impression tray. It can be slowed down by adding trisodium phosphate to the powder. This reacts with the calcium sulphate to produce calcium phosphate, preventing the calcium sulphate reacting with the sodium alginate to form a gel.
This second reaction occurs in preference to the first reaction until the trisodium phosphate is used up, then the alginate will set as a gel.
There is a well-defined working time during which there is no viscosity change.
PROPERTIES

• Good surface detail
• Reaction is faster at higher temperatures
• Elastic enough to be drawn over the undercuts, but tears over the deep undercuts
• Not dimensionally stable on storing due to evaporation
• Non toxic and non irritant
• Setting time can depend on technique
• Alginate powder is unstable on storage in presence of moisture or in warm temperatures

ADVANTAGES
1. Non toxic and non irritant
2. Good surface detail
3. Ease of use and mix
4. Cheap and good shelf life
5. Setting time can be controlled with temperature of water used
DISADVANTAGES
1. Poor dimensional stability
2. Incompatibility with some dental stones
3. Setting time very dependent on operator handling
4. Messy to work with

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Non Elastic Impression Materials-Prosthetic Dentistry Lecture Note

Non-Elastic Impression Materials

1. Impression compound

2. Impression Plaster

3. Zinc Oxide Eugenol

4. Impression Waxes

Impression Plaster

The plaster should be mixed with water or an anti-expansion solution in the ratio of 100g to 50-60ml. The mix should be a smooth paste, free of air bubbles, which may appear on the surface of the impression leading to inaccuracy. The tray should be loaded and "puddled" into position, and held in place until set. As the impression is removed from the mouth it is not uncommon for pieces of plaster around the periphery of the impression to fracture off. These pieces should be retrieved and glued back onto the impression before it is cast. Long narrow strips of wax are then fit around the periphery of the impression just below where it ends. This is called beading. The impression is then coated with a thin layer of separating medium and cast in fresh plaster. The beading provides a clear indication of where the impression ends; the creation of this level area prevents over-trimming and over-extension.

1) Plaster powder, anti-expansion solution, mixing bowl and spatula	2) Plaster impression

3) A plaster impression with wax beading around it's periphery

Properties of Impression Plaster

CHEMISTRY

Impression plaster is based on calcined (להישרף) calcium sulphate hemihydrate, which reacts with water to form a hard mass of calcium sulphate dihydrate.
This setting reaction is associated with an expansion of 0.3-0.6%. When this is within the confines of an impression tray it will lead to a significant reduction in accuracy.
Mixing the plaster with anti-expansion solution (containing 4% potassium sulphate and 0.4% borax) will reduce this. The potassium sulphate reduces expansion to 0.05%, but this also accelerates the setting reaction, and borax is added as a retarder, which gives more time to take the impression.

PROPERTIES

• Excellent at recording fine detail (because very fluid when inserted in mouth)
• 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
• Exothermic setting reaction
• On storage dimensionally stable but a small amount of shrinkage may occur
• Non toxic but may be unpleasant due to dryness and heat evolved during setting

ADVANTAGES

1. Good surface detail
2. Excellent dimensional stability
3. Rate of the setting reaction can be controlled by the clinician

DISADVANTAGES

1. Cannot be used for mucocdisplacive impressions
2. Cannot be added to
3. Properties affected by operator handling technique
4. Taste and roughness may cause the patient to vomit

Impression Compound

The compound disc is completely immersed in a water bath at 55-60^oC for about 4-5 minutes to ensure complete softening. Gauze is placed at the bottom of the water bath to prevent adherence. However if it is left in too long some of the constituents may be leached out into the water bath, altering the properties of the material (it is often the plasticiser stearic acid that is leached out). If the compound is kneaded water will become incorporated and act as a plasticiser. The compound is loaded on to the tray and firm pressure is used to seat the tray home in the mouth. After the impression has been taken it is carefully examined, a common mistake is not softening the compound enough. In this is the case the impression can be reheated in a separate bowl of water (cross infection control) and the impression is repeated.

1) An impression compound disc	2) An impression compound impression taken in an upper edentulous stock tray

3) Upper and lower impression compound impressions

Properties of Impression Compound

CHEMISTRY

Made up of 3 constituents

1. Resins (e.g. wax)
2. Filler (e.g. talc or soapstone)
3. Lubricants (stearic acid or stearin)

It is a thermoplastic compound i.e. it softens when heated and hardens when cooled. This process does not involve a chemical reaction.

Impression compound is available in 2 forms:

1. Stick form - used for modifying impression trays. Softening temperature is 70⁰C
2. Sheet form - used to take impressions of the mouth. Softening temp is 55-60⁰C.

PROPERTIES

• Poor surface detail
• High coefficient of thermal expansion (contraction of up to 0.3% when removed from mouth to room temperature)
• Distorts when removed over undercut areas
• Mucocodisplacive
• Poor dimensional stability
• Can be modified by re-heating
• Non toxic and non irritant
• Good shelf life

ADVANTAGES

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

DISADVANTAGES

1. Poor dimensional stability
2. Poor surface detail
3. Expansion coefficient
4. Will distort if removed from undercuts

Impression Waxes

Waxes 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. These materials are not commonly used.

Zinc Oxide-Eugenol
Impression Material

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). Firm pressure is used during impression taking.

1) Zinc Oxide Eugenol impression material	2) An impression taken in Zinc oxide eugenol

Properties of Zinc-Oxide Eugenol

CHEMISTRY

Constituents of a typical zinc oxide eugenol paste are:

BASE PASTE

• Zinc oxide
• Inert oils (plasticiser)
• Hydrogenated resins (increases setting time and improves cohesion)

REACTOR PASTE

• Eugenol
• Zinc acetate (accelerator)
• Fillers (talc or kaolin)

Some pastes contain a substitute for eugenol e.g. a carboxylic acid.
The 2 pastes come in contrasting colours and are dispensed in a 1:1 ratio. They are mixed to give a paste of even colour. The setting reaction is shown below:

ZINC OXIDE (excess)	ZINC OXIDE (UNREACTED)
+	-> +
EUGENOL	EUGENOL

The set material contains both some unreacted zinc oxide and eugenol.
Any movement of the tray as the paste is hardening will lead to a deformed, inaccurate impression.
Setting time depends on:

1. Accelerator additives (e.g. zinc acetate, acetic acid)
2. Exposure to moisture on mixing or the addition of water will accelerate the reaction
3. Increasing temperature causes a faster setting reaction

Setting time is normally 4-5 minutes.

PROPERTIES

• Non toxic
• Adherence to tissues
• Mucostatic or mucocodisplacive (depending on brand used)
• 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
• Stable on storage and good shelf life

ADVANTAGES

1. Dimensional stability
2. Good surface detail
3. Can be added to
4. Mucostatic or mucocodisplacive

DISADVANTAGES

1. Cannot be used in very deep undercuts
2. Only sets quickly in thin section
3. Eugenol allergy in some patients

Classification of Impresssion Materials-Prosthetic Dentistry Lecture Note

Impression Materials
Classification Depend on Viscosity

High Viscosity

• Impression Compund
• Putty Elastomers

Medium Viscosity

• Regular Elastomers

Low Viscosity

• Impression Plaster
• Impression Paste
• Hydro colloids(Reversible,Irreversible)
• Light Body Elastomers

Classification Depend on Elasticity

Elastic Materials

• Synthetic Elastomers(Polysulfides,Silicones,Poly Ether)
  
• Hydrocolloids(Reversible-Agar,Irreversible-Alginate)

Non Elastic Material

• Impression Compound
• Zinc Oxide/Eugenol Paste
• Impression Waxes
• Impression Plaster

Impression materials are used to record the shape of the teeth and alveolar ridges. There are a wide variety of impression materials available each with their own properties, advantages and disadvantages. Materials in common use can be classified as elastic or non-elastic according to the ability of the set material to be withdrawn over undercuts.