Chapter 15 Impression materials
For the construction of an indirect restoration or dental appliance (e.g. a fixed restoration such as crown or bridge and removable appliances such as an orthodontic appliance or an occlusal splint), accurate information about the oral cavity or the dental arches needs to be given to the dental technician. This information is usually gathered by making (recording or taking) an impression, which involves placing a material into the mouth in an unset or fluid state and which then hardens (Box 15.1). This impression is then cast, so creating a model of the dental or oral structures. More recently, techniques have been developed which use computer-operated optical imaging to create the impression, and the cast is constructed from this information. The impression is a negative, with the model or cast being the positive, and the technician constructs the device/restoration using this cast.
For the device to fit well, it is necessary that the impression accurately reproduces the fine details and dimensions of the oral structures and their relationship to one another. There are many types of impression material available, with the material choice being based on the type of device to be made and the oral conditions. In general, a more accurate impression is required for the construction of a fixed restoration than for a removable device. This chapter describes and discusses the impression materials available to the dentist in order to construct an indirect restoration or appliance together with the allied materials that are used to facilitate the taking of a good impression.
Desirable Properties of an Impression Material
Inaccuracies can occur at any stage in the process of constructing any dental device. The inaccuracies at each stage must compensate for each other so that the final restoration will fit accurately. Therefore, an impression that is too accurate may not work as the inaccuracies in the other stages in the process are not taken into account. This is called a total process chain.
All the impression materials which are available to the dentist have various degrees of accuracy and the indication of the material will depend on the need for detail on the working cast. All of these impression materials require to be supported in an impression tray and be retained there until after casting.
Impression Trays and Tray Selection
Stock trays are supplied in a range of average sizes and shapes of arch form. They may be made of plastic or metal (Figure 15.1). To obtain a satisfactory impression, a stock tray must:
Plastic stock trays
Plastic impression trays are frequently injection moulded from a high-impact styrene. Good trays have a moulded periphery, which takes account of the anatomy of the mouth. They may be either dentate or edentulous and perforated or non-perforated (Figure 15.2). Perforations aid in the retention of the impression material.
It is important that the stock tray is extended adequately to support the impression material, otherwise distortions are likely in any unsupported regions. Figure 15.3 shows a poorly extended stock tray that would be inadequate if a full sulcular impression is required.
Correct size and shape of the dental arch
The tray also needs to approximate to the size and shape of the dental arch. If the tray selection is incorrect, the ill-fitting tray will be unable to support the impression material properly, leading to inaccuracies in the impression (Figure 15.4).
Importance of rigidity
It is very important that the tray is strong enough to withstand the force of the impression material being placed in the mouth. It must not flex, as this will result in stresses being formed within the impression material, and the clinician will be unaware of this. Flexure of the tray under load causes the tray to distort and the impression to flex. The sides of the impression tray will bow out in the mouth (Figure 15.5). Once the pressure is released as the impression is removed, the tray will return to its original shape. This will mean that the impression will be narrower buccolingually than the actual buccolingual width in the oral cavity.
Fig. 15.5 The effects of pressure on the impression while the tray is in the mouth. (A) A flexible tray will distort. (B) If the tray is too flexible, pressure on the occlusal surface of the tray will lead to the lingual and buccal sides being displaced outwards by the impression material. (C, D) Once the impression is removed and the pressure released the tray returns to its original shape leaving the impression distorted.
Tray distortion during impression making will then lead to the cast model being inaccurate and finally the constructed prosthesis not fitting in the mouth. This is seen especially with the higher viscosity impression materials, such as the putty presentation (see p. 261). Some plastic stock trays have strengthening features to maintain rigidity (Figure 15.6).
One type of plastic tray available is called the Triple Tray (Figure 15.7). This tray system is used when the double arch or dual bite impression technique is being employed to record an impression for a crown. This technique uses an accurate (usually elastomer) impression material to record both the tooth preparation and the opposing teeth. Although this is a popular technique in North America and can provide good results, its success depends on the patient’s ability to close their teeth together without any interference by the tray; thus it can only be used in specific indications. By definition, this technique cannot reproduce the complete dental arch, which can lead to serious occlusal discrepancies. The details of the technique are beyond the scope of this text and the reader is referred to an operative dentistry text for further information.
Metal stock trays
To overcome many of the shortcomings of plastic trays, such as lack of rigidity, many clinicians prefer to use metal trays. These are also supplied as dentate or edentulous and perforated or non- perforated. Non-perforated metal trays have a locking rim peripherally and undercut retention, which holds the impression material in the tray. Metal trays are expensive to purchase but may be disinfected by autoclaving and are therefore reusable.
Dentists should get their details engraved on their metal stock trays so that the trays are correctly returned from the laboratory and not misappropriated elsewhere! It is advisable to use laser etching for this if possible, so that the surface is not damaged, which may make cleaning of the tray more difficult.
A special or customized tray is one which is made specifically for one patient. A preliminary impression (using a stock tray) is made and sent to the dental laboratory. A model is cast and wax is laid down on the model, the thickness of which corresponds to specific spacing. This spacing is determined by the impression material to be used in the final impression. The equal thickness of the impression material used means that dimensional change will theoretically be the same in all directions, so decreasing inaccuracies in the impression. Special trays are usually constructed from polymethylmethacrylate (Figure 15.8).
In addition, special trays may be modified, such as the creation of a window. This is done so that an impression material of a lower viscosity can be injected into the tray for accurate recording of displaceable tissues such as flabby ridges.
The advent of putties has made the use of customized trays much less common. The putty, forming the bulk of the impression, shows very little dimensional change and supports the light-bodied material which gives fine detail. This has led to an increase in the types of stock tray available on the market.
It is very important that the impression material is retained securely in the tray. This may be done mechanically through perforations as described earlier or by the use of an adhesive. Ideally both these means should be used together as the use of each type alone can lead to failure. For example, the addition of a tray adhesive is helpful where the mechanical retention is not so effective. Figure 15.9 shows the modes of failure of a rim-lock tray and of a perforated tray where no adhesive is used. These discrepancies can potentially lead to distortion of the impression material and failure of fit of the prosthesis. The addition of a tray adhesive will reduce this as it addresses the area where the mechanical retention is not so effective.
Fig. 15.9 (A) A rim-lock tray, where the impression has pulled away from the tray as a result of the polymerization shrinkage. (B) A perforated tray, where the impression material has pulled away from the tray wall between and despite the presence of perforations. Additional perforations will reduce this but not eliminate it completely.
All tray adhesives are based on contact adhesive technology. This means that they should be applied to the tray and allowed to dry in advance of the impression being taken. Failure to let the material dry will adversely affect the union between the tray, adhesive and impression material. The adhesive should be applied sparingly to the internal surface of the tray and extended just over the margin of the tray to the external surface to ensure that the periphery of the impression material remains attached to the tray. Pooling of excess adhesive is undesirable as the solution will not dry and will weaken the bond between the impression material and the tray. For the most effective use, two thin coats should be applied with the first coat being allowed to dry before the application of the second. None of the available adhesives is particularly effective and it is unwise to rely on adhesive alone in the tray.
Most tray adhesives are provided in screw-top bottles with a brush affixed to the lid (Figure 15.10). Invariably, as the adhesive lasts for long periods of time, the excess adhesive from each application gets deposited around the neck of the bottle. This means that the seal on the lid becomes less secure with time, leading to evaporation of the solvent and the consequent thickening of the adhesive. Application of adhesive to the tray thus becomes more difficult and the adhesive layer deposited is thicker than desired, leading to reduced performance.
Fig. 15.10 Example of a tray adhesive. Note the brush attached to the lid and a residue of excess adhesive deposited around the screw thread at the neck of the bottle. This compromises the seal of the bottle, causing deterioration of the adhesive with time.
• A paint-on presentation of tray adhesive is preferable to the spray-on presentation. Spraying on the adhesive will result in pooling of the adhesive, which will not dry so decreasing its ability to bond. However, from a cross-contamination perspective, the dental nurse should not dip the brush back into the adhesive if it has been applied to a tray which has been tried into a patient’s mouth. Decanting some adhesive into a dappen dish and using a cotton bud will circumvent this consideration.
• The tray adhesive should be dry before the unset impression material is placed into it. To facilitate this, it is wise to select the impression tray at the start of the appointment. The tray adhesive may then be coated onto the tray. While the preparation is being done, the tray adhesive will dry.
Types of tray adhesive
Most tray adhesives are specific to each generic group of impression materials (Table 15.1). It is important that the correct type of tray adhesive should be used with its corresponding impression material. All adhesives contain a solvent, which evaporates leaving a film of adhesive which will then bond to the impression material.
|Butyl rubber or styrene acrylonitrile dissolved in chloroform or a ketone||Polysulphide|
|Ethyl acetate in propanol or acetone||Polyether|
|Poly dimethyl silicone to react with the impression material and ethyl silicate to bond physically to the tray. Ethyl acetate and naphtha are frequently included||Addition silicone|
|Poly dimethyl silicone to react with the impression material and ethyl silicate to bond physically to the tray||Condensation silicone|
|10–12% toluene in 45–50% isopropanol (isopropyl alcohol acts as a volatile solvent)||Alginate|
While the adhesives are adequate in holding the impression materials firmly in the tray the bond between the tray and the impression material varies with the material used. Figure 15.11 shows the bond strengths between the tray and impression material for the four common elastomeric impression materials. The strongest bond is achieved with the polyether adhesive.
Types of Impression Material
There are a number of types of impression material. They may be broadly divided into non-rigid and rigid. The former group consists of the reversible and irreversible hydrocolloids and the elastomeric materials (which are synthetic rubbers). The rigid impression materials include impression plaster, impression compound (compo) and zinc oxide and eugenol-based impression paste. These are generally restricted in use to record those areas where no undercuts are present, and mainly in the construction of removable dentures.
Non-Rigid Impression Materials
A hydrocolloid is a colloid in which the continuous phase is water. A colloid is a substance which is distributed evenly through another material. A colloid is generally made up of two phases: the dispersed phase, which is distributed in the other phase, the continuous phase. The two phases are not readily detectable even under microscopic examination. The dispersed phase has particles below 300 nm in size. A colloid exists as either a viscous liquid (sol) or as a solid (gel). The hydrocolloid impression materials may either be reversible or irreversible.
Agar is a mixture of polysaccharides, agarose and agaropectin, which are subunits of the sugar galactose. These components are extracted from certain types of seaweed, specifically some red algae. The material used as a dental impression material also has other chemicals added to improve the properties and handling of the material. A generic formulation for these materials is set out in Table 15.2.
|Constituent||Purpose||Per cent of composition|
|Agar*||Disperse phase of the colloid||13–17|
|Potassium sulphate||To counter adverse effect of borax on setting reaction of model plaster||1.0–2.2|
|Borax or borates||To strengthen the gel||0.2–0.6|
|Alkyl benzoate||To prevent mould growth in impression during storage||0.1–0.2|
|Thixotropic materials||Viscosity regulators and thickeners||0.2–0.4|
|Colours and flavouring||To enhance the taste and appearance of the material||<0.1|
|Water*||Provides the continuous phase of the colloid. The amount present determines the flow properties of the sol and the physical properties of the gel phases||79–85|
The gel alone is insufficiently strong to make the material viable as a dental impression material. Borax is therefore added to strengthen the gel. Unfortunately, borax retards the set of the model material (which contains gypsum) and this has an adverse effect when the positive cast is poured from the impression. Potassium sulphate is therefore added in an attempt to compensate for this, and while it reduces the problem it is not eliminated.
The change in the state of the dental hydrocolloid is determined by the temperature of the material. The gel may be converted to its sol state by heating to between 70 and 95°C. This is known as the liquefaction temperature. This is of course far too high a temperature for a material to be placed in the mouth of the patient. Fortunately, the phase transformation back to the gel stage occurs at a much lower temperature of between 35 and 50°C, which is just above mouth temperature. This allows the clinician to take the gel and heat it sufficiently to permit it to be placed in the sol state in an impression tray. The assembly is then tempered, allowing the temperature to be lowered until the patient can tolerate the material being seated in the mouth in a fluid state. At that point, the impression tray may be cooled to lower the temperature of the sol, which then solidifies.
The process requires a number of pieces of hardware, namely a hydrocolloid bath and metal trays incorporating water cooling coils (Figure 15.12). There must also be facilities to pour the models as soon as possible after the impression has been taken. This is because the dimensional stability of the agar is determined by the relative humidity and temperature at the point of pouring the plaster cast. The agar impression material is presented in tubes or cartridges for use in a syringe (Figure 15.13).
The water bath consists of three separate chambers. The first is used to heat the agar and is usually set at a temperature near boiling point. Each cartridge or tube is totally immersed in this bath for a minimum of 8 minutes to liquefy the hydrocolloid. The second chamber is a tempering bath, which is used to cool the material to an acceptable temperature. This is generally set at between 43 and 46°C. It is from this chamber that the material is dispensed either into a tray or the cartridges loaded into a syringe. The third bath is primarily a storage bath which is maintained at between 63 and 66°C. A number of cartridges and tubes may be maintained at this temperature in the sol state for several days so that they may be available for immediate use. The complex nature of the preparation means that this type of impression material is more appropriate to the clinician specializing in extensive advanced restorative dentistry.
Material in tubes or cartridges which have not been used may be allowed to cool down. The content will return to the gel state. They may be reused by replacing in the boiling bath, however, they will require a rather longer time to change to the sol state; also, this reheating process may only be repeated up to four times before the material is discarded, because it becomes increasingly harder to break down the agar structure after reheating several times. The material can be sterilized.
Making the impression
The cartridge of material is removed from the tempering bath, placed in a syringe and injected around the preparation(s), ensuring that the nozzle of the syringe remains within the mass of material being injected. The preparation(s) and the immediate surroundings are covered. While this is being carried out, the dental nurse takes one of the tubes from the tempering bath and fills the selected tray with the material. The adhesion of the agar to the tray is poor, so a perforated tray should be selected. The dental nurse also connects the cooling hose to the tray. The tray is then seated over the syringe material covering the whole of the dental arch. Once the tray has been seated, one end of the cooling system is connected to a cold water supply, and the other is placed to permit the cooling water to drain away to waste. The tray is held steady in the mouth until the mass of hydrocolloid has cooled to below the sol/gel transition temperature. Care is needed at this stage, as the material closest to the tray will cool fastest, with the material at the tooth surface setting last.
The reversible hydrocolloid system is probably the only true hydrophilic impression material. It is also the only impression material where the teeth may be left wet intentionally and is probably the most accurate. It is therefore mainly used when accuracy is very important, such as for fixed indirect restorations (crowns and bridges) and it is also used in dental laboratories to duplicate models. However, it must be handled with care to achieve successful results. The viscosity of the material should be such that it is sufficiently thick that it will be retained in the tray but not so viscous that the material will not flow around the teeth as the tray is seated. The impression must be thoroughly washed and all blood and saliva removed before pouring the cast.
After removal from the mouth there is very little distortion, unless the material is in very thin sections. A generous thickness of material is therefore required to limit the deformation which may arise on removal, especially from an undercut. When the impression has been removed from the mouth, the impression should be kept at 100% relative humidity. Failure to do this will lead to changes in dimension as water may be lost. The amount of water which is lost varies with the material type. Furthermore, if the impression is left for a period of time before a cast is made from it, the gel contracts and beads of moisture appear on the surface (syneresis).
Similarly if the impression is allowed to dry out and is then immersed in water, it imbibes water but it will not necessarily return to its normal dimensions. This is called imbibition. In fact, the impression may take up excess water and swell up, with this swelling being uncontrolled in direction and extent.
Agar has its aficionados, who enthuse about the results obtained. However, unless a large amount of advanced restorative work is contemplated, the complexities of heating and storage of the material, together with the need for the tray to be cooled means that the general dentist may find other elastomeric impression materials more convenient to use. The system has significant start-up costs as the hardware needs to be purchased, reflecting the very few agar products available on the market. An example of one such product is CartriLoid (Dux Dental).
Probably the most commonly used dental impression material is the irreversible hydrocolloid called alginate. Alginate impression materials change from the sol to the gel state by a chemical reaction, which cannot be reversed unlike the agar-based materials. Alginate impression materials are presented as a powder, to which a measured amount of water is added. This is mixed to a paste and loaded in an impression tray. The paste (the sol phase) then sets, with the sol phase converting to the gel phase. The impression may then be removed.
The active ingredients are sodium and potassium salts of alginic acid. Alginic acid was first derived from the mucus that exudes from brown seaweed (Figure 15.14). It is now manufactured from synthetic components. A typical alginate powder formulation is shown in Table 15.3.
|Constituents||Weight percentage (%)||Function|
|Potassium alginate* Sodium alginate*||18||Dissolves in water to form a hydrogel with calcium|
|Calcium sulphate dihydrate*||14||Reacts with soluble alginate to form insoluble calcium alginate|
|Potassium sulphate, silicate or borate||7–10||Reduces inhibition of setting of plaster in poured model|
|Sodium phosphate||2||Acts as a retarder by preferentially reacting with calcium ions. This provides the working time. The material sets once the phosphate ions have been used up|
|Filler (diatomaceous earth or silicate powder)||56||Filler controlling consistency and flexibility of the set material|
|Sodium silicofluoride||0–3||Controls the pH of the material. Usually included at the expense of the potassium sulphate fraction|
|Organic glycols||Small traces||Reduces dustiness of the powder|
|Oil of Wintergreen, peppermint and pigments||Small traces||Provides pleasant taste and colour|
Diatomaceous earth: a soft, siliceous sedimentary rock (Figure 15.15) that crumbles into a fine white to off-white powder. It has a typical particle size between 10 and 200 μm. It is very light as it is highly porous.
The setting reaction of alginates is a simple double decomposition reaction. As well as the alginate salt, the powder also contains hydrated calcium sulphate. On addition of the water, the alginate salt is converted to insoluble calcium alginate and potassium/sodium sulphate. The chemical reaction can be summarized as:
Since the reaction starts immediately once the water is added to the powder, sodium phosphate is added to slow the reaction. It can do this as the calcium ions in solution preferentially react with the phosphate ions, forming (insoluble) calcium phosphate. It is only when all the phosphate ions have been used up that the calcium alginate formation will commence. The amount of sodium phosphate will determine the working time of the commercial material. There is a quite marked change in pH during this setting reaction (11 to 7). Some manufacturers include pH sensitive indictors so that the material changes colour during the setting reaction. This can assist in determining that the material has set completely.
As with the reversible hydrocolloids, the set material will retard the set of gypsum-based die material poured into the impression. To overcome this, additional potassium sulphate, silicate or borate salts are added.
Normally, the mixing time for alginates is between 45 and 60 seconds. Under-spatulation and shortened mixing times will lead to a paste where not all the powder particles have been wetted and consistency will be affected.
The working time for these materials may be regulated by the amount of sodium phosphate incorporated in the powder – the smaller the amount, the shorter the working time. Manufacturers generally produce material with working times between 45 seconds (fast set) and 1 minute and 15 seconds (regular set).
This is manifested by the impression material surface losing it tackiness and showing rebound if indented. The setting times range between 1 and 4.5 minutes, depending on manufacturer and material indication. The very fast setting material can set too quickly. The manufacturer can influence this by increasing the amount of (tri)sodium phosphate added to the powder.
Setting time can also be varied by altering the temperature of the water. Lowering the temperature will slow the set, while increasing the water temperature will shorten the setting time. It is inadvisable to take this to extremes, but is useful clinically for those patients who find impression taking unpleasant or are prone to retching. Using tepid water will increase the setting time and therefore the time the impression is required to remain in the mouth.
It is advisable to retain the impression in the mouth for 2–3 minutes after the initial gel formation has occurred when the material loses its tackiness, as the material increases in elasticity during this period. A longer retention period has adverse effects on the impression and the material may tear. This highlights the importance of following the manufacturer’s instructions as they will have determined the optimum conditions for a satisfactory result.
Any impression will be compressed in certain areas during the removal from the mouth as it is removed from undercuts. Once removed, the impression should ideally return to its original shape with no distortion. However, most impression materials do not recover completely. A standard requirement for an alginate impression material is that if it is compressed by 20% for 5 seconds, the percentage recovery from this deformation should be in excess of 95%. This means that it could have a permanent deformation of 5%. Most commercial alginate impression materials have a recovery from deformation of about 98.5%. Permanent deformation of the impression material will only occur in areas where it was stressed. This, in turn, means that some parts of the cast produced from the impression will be more accurate representations of the mouth than other parts. Permanent deformation may be minimized by:
• Reducing the amount of compression: Increasing the bulk of impression material between the tray and the teeth will reduce the bulk deformation as there will be more material available to be compressed.
• Allowing longer time for recovery by not pouring the model immediately: However, to avoid inaccuracies caused by syneresis and imbibition, the model should be made ideally within a hour of the impression being taken.
The strength of the gel phase of the impression must be optimized to ensure that it will not tear too readily on removal. The clinician has control over many means of altering the gel strength during mixing and manipulation, including:
Advantages and disadvantages
|Easy flow||Poor dimensional stability|
|Reproduction of detail adequate||Poor tear strength|
|Fast set||Distorts if unsupported|
|Minimal tissue displacement||3 mm minimum thickness required – otherwise distortion occurs where thin sections are found, such as interproximally|
|Patient tolerance good||Easy to include air|
Indications and contraindications
Manufacturers usually provide dispensing measures for both the water and powder. These are most commonly a measuring cylinder and scoop, respectively (Figure 15.16) and will ensure that the correct volume of water and powder is used. Box 15.2 demonstrates the correct mixing procedure for an alginate impression material by hand. An alterative method involves using a bespoke alginate mixing machine. However, the powder and water should still be handled as directed in steps 1 to 4 in Box 15.2.
Fig. 15.16 Equipment required to mix an alginate impression material. Note the flexible bowl and straight alginate spatula. A measuring cylinder is used to accurately dispense the water and the scoop is used for the same purpose for the powder. The alginate impression material is Blueprint cremix (Dentsply).
• It is important to exercise care when opening the container as the particulate material creates dust, which if inhaled potentially has adverse side effects. This is because a proportion of the particles are of a similar dimension to asbestos fibres, which may lead to pulmonary fibrogenesis.
• The container containing the alginate powder should have a hermetic seal and should not be left open to the atmosphere for any length of time. Water vapour in the air will affect the powder and therefore the properties of the set material.
• It is unwise to vary the stated proportions between the water and the powder as a small variation can adversely affect the properties. A change of only 15% in proportion of powder to water will markedly change both the setting time and the consistency, although some clinicians prefer a slightly runnier mix when making an impression for construction of a denture.
Once the alginate has been mixed, it is loaded into the impression tray. This should be done without delay as the material will be entering its setting phase. A smooth surface should be achieved, which results in better surface reproduction in the impression (Figure 15.17). Some clinicians like to smear some of the alginate over the surfaces of the teeth using their finger to ensure that the material is properly adapted to the tooth surfaces. In patients with a high palatal vault, the dentist may also place a blob of material in this region prior to the insertion of the tray to ensure that it is recorded properly.
Fig. 15.17 An alginate impression material loaded into a metal stock tray and ready for presentation to the dentist for insertion into the mouth. Note the smooth surface of the impression material, which will result in better surface reproduction in the impression.
If the surface of an alginate impression is smoothed and wetted under the tap, the material will flow better, with improved reproduction of detail. However, excess water will affect the powder/liquid ratio so weakening the material, which will become apparent when the cast is poured.
When an alginate impression is placed into the mouth, the lip should be everted to displace any trapped air, which would prevent the material from flowing fully into sulcus. The clinician should be careful that they do not pull the impression tray towards their dominant side. This will result in overextension of the impression on the dominant side and an underextended impression on the other side. That is, a right-handed clinician would make an impression which would be underextended on the left and overextended on the right.
Disinfection of all impression materials is an essential part of prevention of cross-contamination (Box 15.3). Each impression should be subjected to a disinfection regime before it leaves the surgery. Many dental laboratories will also disinfect all impressions on arrival at the laboratory.
2. The impression tray (including the handle) and material should be immersed in a bath of a suitable water-based disinfecting solution prepared according to the manufacturer’s instructions for the recommended period of time.