dental impression materials

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Materials in common use can be classified as elastic or non-elastic according to the ability of the set material to be withdrawn over undercuts.
The main properties to consider are discussed below.
Ideal Properties of Impression Materials
(1) Non toxic and non irritant
(2) Acceptability to the patient: (a)Setting time,
(b)Taste,
(c)Consistency
(3) Accuracy : both (a) Surface reproducibility
(b) Dimensional stability
(4) Use of material : (a) Ease of mixing
(b) Working time
(c) Setting time
(d) Handling of the material
(5) Compatible with model materials
(6) Economics of material (a) Cheap
(b) Long shelf life
(c) Accuracy (save redoing impression)
Mucostatic vs Mucodisplacive
Impression materials, in the fluid or plastic state, are carried to the mouth in a suitably sized tray. Hardening of the material takes place either on cooling or through a chemical reaction.
There is no clear line between mucostatic and mucodisplacing impression materials.
Mucodisplacive : compound, high viscosity alginates, high viscosity elastomers.
These viscous impressions materials are called mucocodisplacive impression materials. These record an impression of the mucosa under load. This results in a wider distribution of load during function, making it more stable it also compensates for the differing compressibility of the denture bearing area reducing the risk of a fracture due to flexion. However the retention of the dentures may be compromised as the soft tissues wish to return to their original position at rest.
Mucostatic : plaster, zinc oxide eugenol, low viscosity alginates. Materials which are more fluid displace the tissues less - these are known as mucostatic impression materials. These essentially record an impression of the undisplaced mucosa. This results in better retention of the denture because of closer adaptation to the mucosa at rest. However instability of the denture may occur during function as the tissues distort.There is no clear line between mucostatic and mucodisplacing impression materials.

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.
The main non-elastic impression materials are:
Impression compound
Impression Plaster
Zinc Oxide Eugenol
Impression Waxes
Impression Compound

The compound disc is completely immersed in a water bath at 55-60oC 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 wen 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 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 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

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
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
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.
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, overdentures, 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.
The following hydrocolloids are discussed in this section :
Agars
Alginates
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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