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.

Fig. 15.3 Inadequate buccal extension on the side of the tray.

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).

Fig. 15.4 (A) Tray touching the soft tissue on the lingual side (B) Absence of impression material at the site where the tray contacts the tissue leads to inaccuracies in the impression.

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).

Fig. 15.6 A plastic stock tray with reinforcing ribs to prevent distortion during the impression process. Note also the rigid handle and extensive retentive features.

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

^* 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.

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).

Fig. 15.12A–D (A) Hydrocolloid water bath, (B) tubing for water cooling and (C, D) upper and lower water cooled trays for specific use with agar-based impression material.

Fig. 15.13A–C Dispensing tubes and syringe cartridges for a reversible hydrocolloid system (CartriLoid, Dux Dental).

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.

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.

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).

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.

Fig. 15.14A,B Two species of seaweed from which alginic salts may be derived. (Photos courtesy of Michael D. Guiry, The Seaweed Site) http://www.seaweed.ie/ Seaweed Site from Michael Guiry.

Table 15.3 Typical chemical constituents of an alginate impression material

^* Active ingredient.

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.

Fig. 15.15 Diatomaceous earth in its original form of soft siliceous sedimentary rock before being ground. (Photo courtesy of Mineraly a horniny Slovenska) http://www.mineraly.sk/

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.

The advantages and disadvantages of alginate impression materials are shown in Table 15.4.

Table 15.4 Advantages and disadvantages of alginate impression materials

Alginate is commonly used to make impressions where the reproduction of detail and level of accuracy is not of the highest requirement. This means they are frequently used for:

Alginate is sufficiently fluid to record detail but it is unsuitable for recording fine detail such as that required for a fixed indirect cast restoration.

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).

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.

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.

A number of commercial disinfectants are available specifically for this purpose. They/>