15: Impression materials

Chapter 15 Impression materials

Introduction

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.

Impression Trays and Tray Selection

The impression material must be adequately supported during the impression process by an impression tray. There are several types of tray available, and these fall into two broad categories:

Stock trays

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.

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.

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).

Triple Tray

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.

Special trays

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.

Tray Adhesives

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.

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.

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.

Table 15.1 Chemical constituents of tray adhesives with their corresponding impression materials

Adhesive 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.

Non-Rigid Impression Materials

Hydrocolloids

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.

Reversible hydrocolloid

Reversible hydrocolloids are based on agar. As well as being used as a dental impression material, agar is used as a base for jellies and microbiological cultures. It is non-toxic and non-irritant.

Chemical constituents

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.

Table 15.2 Chemical constituents of agar impression materials

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
Wax Filler 0.5–1.0
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

* Active ingredient.

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.

Setting

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.

Irreversible hydrocolloid

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.

Chemical constituents

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.

Table 15.3 Typical chemical constituents of an alginate impression material

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

* Active ingredient.

Properties

Mixing

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.

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.

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.

Jan 31, 2015 | Posted by in Dental Materials | Comments Off on 15: Impression materials
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