Introduction to Impression Materials

1. Ideal Properties of Impression Materials

2. Mucostatic vs Mucodisplacive

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Materials in common use can be classified as elastic or non-elastic according to the ability of the set material to be withdrawn over undercuts.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

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

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

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 detail2. Reusable and easily sterilised

DISADVANTAGES

1. Need special equipment (water bath) and special technique2. 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

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.

Main types of elastomers are:

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. Accuracy3. Comes in a number of different viscosity's4. 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 effects2. Staining of clothes due to the Lead oxide3. Messy to work with - unpleasant rubbery smell4. Can only be used in a special traY

5. Silicones6.

7. 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

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. Accurate2. Ease of use3. Can be used on severe undercuts

DISADVANTAGES

1. Hydrogen evolution2. 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