Chapter 13 Impression materials and techniques
Impressions of teeth and related structures are taken for making implants, crowns, bridges and dentures, and the material selected will partly depend on the personal choice of the clinician and the use. The choice of impression tray used will also be dictated by the impression material and its viscosity. Impression materials used for advanced operative procedures such as crown and bridgework must have elastic properties to allow the removal from undercut areas in the mouth without creating permanent distortion; these can be either water based (hydrocolloids) or synthetic elastomers.
Agar is an example of a reversible hydrocolloid impression material. It consists of long chain polysaccharide molecules and water. When cool, the polysaccharide chains are linked together by hydrogen bonds between hydroxyl groups on the polysaccharide chains, resulting in the characteristic gel. When the temperature is raised, the hydrogen bonds are broken and the gel liquefies; on cooling again the gel state reforms, hence the name ‘reversible hydrocolloid’. Whilst it is a hydrophilic material and gives an accurate impression, it suffers from a number of disadvantages, namely it is dimensionally unstable and needs to be cast up within 1 hour, it has low tear resistance, is hot and uncomfortable on insertion and requires additional equipment such as specially designed water baths, impression trays and tubing (for the water coolant). The use of a water bath is a major drawback in relation to contamination and problems with cross infection control; as such, the material is rarely used in contemporary practice.
Alginate is an irreversible hydrocolloid material whose main ingredients are sodium alginate, calcium sulphate dihydrate and inert filler particles. Alginates are long polysaccharide chains derived from a marine plant (seaweed). It is presented as a powder which is mixed with water. When this is done a setting reaction takes place which involves the calcium ions irreversibly cross-linking the polysaccharide chains, so converting the sol into a gel. This would be a rapid reaction if a retardant was not added. Sodium phosphate is therefore added which preferentially reacts with the released calcium ions forming calcium phosphate; the calcium ions initially are therefore not available to cross-link the polymer chains. Once all of the sodium phosphate has been used up in this reaction, the remaining calcium released is used in forming the cross-linkages and the gel. The filler particles give the set material its rigidity.
Alginate is a commonly used material but suffers from the same disadvantages of the reversible hydrocolloids, that is, they have poor tear resistance and are dimensional unstable. If alginate impressions are stored dry, they dry and shrink; if stored under water, they imbibe water (imbibition) and swell. In addition, the impression is not as detailed as that taken with a synthetic elastomer and therefore not suitable for impressions of crown and bridge preparations; its main use is for study casts and opposing models.
Before taking the impression an appropriately sized impression tray should be selected (Figure 13.1). A perforated plastic stock tray is ideal for this as the material is not too viscous and should not distort the tray on seating. Such trays are for single use only and would distort if heated during a decontamination process. Although a metal tray could be used as an alternative (Figure 13.2), thorough cleaning and decontamination of the trays between patients is time consuming and, as such, disposable single-use plastic trays are more popular and cost effective. A tray should be selected that seats fully over the dentate arch with its flanges not bedding into the soft tissues of the buccal, labial and lingual sulci. Occasionally, the stock tray needs modifying with impression compound to increase its extension to completely cover the dentition and areas of interest.
Figure 13.1 Selection of impression stock trays. Light blue plastic (opaque) trays are suitable for alginate impressions. Clear coloured polycarbonate trays are more rigid and are suitable for more viscous elastomeric impression materials. Three colour-coded sizes are seen.
Despite the fact that stock trays have perforations and ridges to retain the set impression material in the tray, a tray adhesive should also be used. Alginate adhesive is supplied either in a bottle for use with a brush or in an aerosol can (Figure 13.3). Brushing the adhesive on the fit surface of the tray and just over its flanges ensures an even thin layer of adhesive; however, the adhesive needs to be dispensed into a disposable dappens pot first and applied with a single-use brush to prevent contamination of the adhesive in the bottle. To simplify matters and avoid problems with cross-infection control, aerosol adhesives can be used. The work surface should be covered with a paper napkin and the fit surface sprayed with an even thin application of adhesive. Before use the adhesive should be dry and tacky; if the adhesive is applied too thickly it will not dry and will act as a lubricant, with the risk of the impression material pulling away from the tray on withdrawal from the mouth.
Figure 13.3 Selection of tray adhesives. The alginate adhesive is provided as either paint on (top right) or aerosol (bottom). A universal adhesive can be used for silicone impression materials (top left) and a specific polyether adhesive (top centre) should be used for polyethers such as Impregum (3M ESPE).
When mixing alginate it is important to use the correct proportion of powder to water. To achieve this, manufacturers provide a measuring scoop for the powder and a measuring cylinder for the water. Before use the powder should be stirred in its container to thoroughly mix the ingredients and to ensure the material has not become compacted during storage. The number of scoops of powder will be dictated by the size of the impression tray and the patient’s mouth. The powder is mixed with the correct amount of water and evenly combined in a mixing bowl. Most alginate materials have a mixing time of about 1 minute. The mixed alginate is then loaded into the impression tray to the level of the flanges and posterior extremity using a spatula. Overloading the tray should be avoided as this will lead to difficulty in seating the impression in the mouth and will be unpleasant for the patient, with a risk of obstructing the airway.
When taking alginate impressions the patient should be seated upright in the dental chair with their head at about the elbow height of the operator. Once the tray has been loaded with alginate, a portion of alginate can be scooped up on the operator’s finger and smeared over the occlusal surfaces of the teeth and any areas of importance. Rubbing the material into fine recesses such as pits and fissures reduces the risk of incorporating any air bubbles. The loaded impression tray is then seated before the smeared alginate starts to set (single-stage technique). For the lower impression, if right handed, the operator should stand on the patient’s right-hand side facing the patient. The impression tray is then rotated into the mouth, with that part of the tray on the patient’s left entering first. Once the tray is over the dentate arch the lips should be retracted before seating the tray, allowing the alginate material to flow into the sulci. The patient should then be asked to raise the tongue over the tray to mould the lingual sulcus and the material in the buccal and labial sulci can be moulded with external light pressure on cheeks and lips. If the operator is left handed they should stand on the patient’s left-hand side and carry out the procedure in a similar mirror fashion.
The upper impression is taken in a similar way, with the exception that the operator now stands behind the patient’s right shoulder (if right handed) and the tray is rotated into the mouth with the patient’s right side of tray first. Once positioned over the dental arch, the tray should be seated posteriorly first, lifting the lip up and out when seating the tray anteriorly. This ensures that the impression material flows forwards and not backwards towards the patient’s throat. If the patient retches it is important not to remove the tray when the material is unset, but ask the patient to lean forwards and breathe deeply through their nose. This should effectively close the oral cavity from the oropharynx and prevent material flowing posteriorly.
The speed with which the material sets can be altered by the manufacturer, and regular and rapid-set materials are available with working and setting times ranging from 1 to 4 minutes depending on the material chosen. The setting time can also be influenced by the temperature of the water; using warmer water will speed up the setting reaction. Alginate when initially mixed is alkaline (pH ≈11) but when set is near neutral. Some manufacturers have taken advantage of this and include a pH indicator which changes colour with the pH. Once set, the tray should be removed from the mouth with a rapid technique. This is because the material is viscoelastic and if removed rapidly elastic recovery should take place; slow removal from undercuts would lead to distortion of the material.
Once removed from the mouth any saliva should be rinsed off the surface and the impression should be decontaminated; prolonged immersion in a disinfectant solution leads to distortion of irreversible hydrocolloid impression materials. Ideally, if there is a laboratory facility, the impression should be cast within 30–60 minutes. If not, the impression should be stored at 100% humidity by wrapping the impression in wet gauze or a napkin and placing it in a sealable plastic bag during transit to the laboratory.
The impression provides the technician with a negative mould into which stone can be poured. This is done by holding the impression over a vibrator so that air bubbles in the stone rise to the surface as the material flows into all recesses. Once the impression is filled to the level of the sulcus, it is inverted and based onto stone mounded up on the bench top. Once the stone has set the impression can be removed and the excess stone trimmed. The alginate impression should not be left on the cast for any period after the stone has set as it dries out and becomes difficult to remove.
Impressions for indirect restorations need to be accurate to record fine detail of tooth preparations and they need to be dimensionally stable with time. Usually there is a delay in taking the impressions and casting them up as most practices do not have a laboratory on site and have to send impressions through the post. An ideal impression material should also have good elastic recovery when it is used in undercut situations and should have good tear resistance. The synthetic elastomeric materials in general meet these high demands. The elastomeric impression materials available are silicones (addition and condensation cured) and polyethers. Historically, polysulphide impression materials were included in the list of elastomeric impression materials and favoured by some for their long setting time; however, they are rarely used in contemporary practice due to a list of disadvantages which include an unpleasant smell, shrinkage on storage resulting in distorted dies, and the need to use it in conjunction with a laboratory-made special tray.
The most commonly used elastomeric impression material is the addition cured silicone. These materials consist of polyvinyl siloxane polymer chains which have terminal vinyl groups that are cross-linked by a silanol (present in the base paste) in the presence of a platinum catalyst (present in the catalyst paste). No by-products are produced during the setting reaction and, as such, the material is dimensionally stable.
Manufacturers usually produce addition cured silicone base and catalyst in putty, heavy bodied, medium bodied and light bodied forms dependent on the amount of filler content. Base and catalyst with the same viscosities are mixed together. Usually addition cured silicones are used in a two-phase, single-stage technique using a stock tray.
The two-phase (or twin mix) refers to the fact that two viscosity materials are used to take a single impression, with a light body wash being syringed around the tooth preparation to record the fine detail and either a heavy body or putty material loaded into the tray to take up the bulk of space between the preparation and the tray. The wash is syringed around the tooth preparation whilst the dental nurse is mixing the heavy body or putty and loading it into the stock tray (Figure 13.4); the heavier bodied materials have the benefit that they do not flow from the tray. Before the wash begins to set, the loaded tray is seated over the teeth, with the heavier bodied material forcing the low viscosity wash into the intricate recesses around the tooth preparation (Figure 13.5). Both wash and heavier bodied materials set at the same time, hence the term single-stage technique.