Сraig. Dental Materials

342 Chapter I 2 IMPRESSION MATERIALS

CLINICAL MANIPULATION OF THE SOL-GEL

Clinically, the tray type agar can be liquefied conveniently by immersion in boiling water, usually 8 to 12 minutes, depending on the bulk of material. If the material is to be used immediately after boiling, the tube is immersed in water at 43" to 49" C and manipulated to ensure even cooling. The tube is then opened and a tray filled. The filled tray is finally tempered for a minimum of 2 minutes in water at 46" C rf:l o .Before the tray is placed in the mouth, a thin layer of material that has been in direct contact with the water in the bath is removed with a suitable instrument.

When the material is liquefied, it can be stored for several hours and kept ready for use by immersing the container in water at 63' to 66" C. When needed, the material is taken from the storage bath and placed immediately in a warmed tray. The filled tray is then tempered at 46" C f 1" for a minimum of 2 minutes before it is inserted into the mouth. Tempering is necessary to cool the material to a temperature that is compatible with the oral tissues, and this also serves to develop a heavier consistency.

A slightly more fluid agar hydrocolloid material is made for use in injection syringes for inlay, crown, and bridge impressions. The increased fluidity is achieved by decreasing the agar content and increasing the water content. Usually this material is supplied in small cylinders of the correct size to fit the syringe. The syringe, loaded with a cylinder, is placed in boiling water for 10 minutes and then stored at 63' C until needed. No tempering is required before use; the syringe is taken from the storage bath and the agar hydrocolloid injected directly into the tooth preparation. The thin strand of material passing down the needle rapidly cools to a temperature compatible with the oral tissues. These procedures may vary from one product to another; manufacturers' directions should be followed carefully.

After the impression is placed in the mouth, the agar is cooled to obtain a set condition. Cool tap water is circulated around tubes built into agar impression trays to hasten setting. After removal the impression is rinsed, disinfected, su-

perficially dried, and poured in dental stone. After the initial setting of the stone, the gypsum model and impression should be stored in a humidor to prevent drying and shrinkage of the impression before the model is removed.

PROPERTIES

Typical properties of the tray type of agar hydrocolloid impression lnaterials are listed in Table 12-2.

Gelation Temperature After boiling for

8 minutes, the material should be fluid enough to be extruded from the container. After tempering, the sol should be homogeneous and should set to a gel between 37" and 45" C when cooled, as required by ANSI/ADA Specification No. 11 (IS0 1564) for dental agar hydrocolloid impression material.

Permanent Deformation Permanent deforn~ationis determined in the same manner as for alginates and at the time the material is removed from the mouth. The ANSVADA Specification requires that the recovery from deformation be greater than 96.5% (permanent deformation be less than 3.5%) after the material is compressed 20% for 1 second, Most tray types of agar hydrocolloid impression materials readily meet this requirement with recovery values of about 99%. However, a reasonable thickness of impression material should be present between the tray and the undercut areas so compressions higher than 10% do not occur, because higher compression results in higher permanent deformation. As for alginates, the magnitude of the permanent deformation depends on the time under compression, and impressions should be removed rapidly.

Flexibility The ANSVADA Specification requirement for flexibility allows a range of 4% to 15%; most agar hydrocolloid impression materials meet this requirement. Materials with low flexibility can be accommodated in areas of undercuts by providing somewhat more space

for the impression material so it is subjected to a lower percentage of compression during removal.

Strength The compressive strength of a typical agar hydrocolloid impression material is 8000 g/cm2. The tear strength of agar hydrocolloid impression materials is about 800 to 900 g/ cm, which is higher than the ANSI/ADA Specification requirement of 765 g/cm:Because agar hydrocolloid impressions are viscoelastic, the strength properties are time dependent, and higher compressive and tear strengths occur at higher rates of loading. These properties again emphasize the importance of removing the impressions with a snap, because such a procedure minimizes the chances of rupture or tearing of the impression.

Compatibility with Gypsum Not all agar hydrocolloid impression materials are equally compatible with all gypsum products, and the manufacturer's suggestions should be followed. The ANSI/ADA specification requires

manufacturers to list compatible model materials. Agar hydrocolloid impression materials are more compatible with gypsum model materials than alginates. The impression should be washed of saliva and any trace of blood, which retard the setting of gypsum. After the impression is rinsed with water and disinfected, the excess liquid should be carefully blown from the impression with an air syringe to avoid dehydrating the surface of the agar hydrocolloid impression.

If the agar hydrocolloid impression must be stored in a humidor, it should be rinsed with cool water to remove any exudate formed from syneresis before pouring up the gypsum model.

Dimensional Stability When stored in air, agar hydrocolloid gels lose water and contract. The extent of contraction varies from product to product, as shown in Fig. 12-9.After 1 hour in air, one product shrank only 0.15%, whereas another shrank about 1%.Replacing the agar in water resulted in absorption and swelling. After an hour the materials had almost retained their original dimensions, although one was

0.05% larger, and two were 0.1% smaller. Continued storage in water resulted in continued swelling.

As with alginate impressions, agar hydrocolloid impressions are best stored in 100% relative humidity if the gypsum models cannot be prepared immediately. Even in 100% humidity they can be stored for only limited times, such as 1 hour, without shrinkage of the impression material caused by syneresis. The best procedure is to pour up the impression immediately after removing, rinsing, disinfecting, and superficial drying.

The suggestions for disinfection of alginates should also be followed with agar hydrocolloid impressions.

AGAR-ALGINATE COMBINATION

IMPRESSIONS

The equipment needed for taking an agar hydrocolloid impression can be minimized with an agar-alginate, syringe-tray combination impression. In this procedure, a syringe type of agar hydrocolloid in a cartridge is heated in boiling water for 6 minutes and stored in a 65" C water bath 10 minutes before use. Several products and a simple heater are shown in Fig. 12-10. The tray alginate of the regular set type is mixed and placed in a tray. The agar hydrocolloid is injected around the preparation, and the mixed alginate is promptly seated on top of the agar hydrocolloid. The alginate sets in about 3 minutes, and the agar gels within this time as a result of being cooled by

the alginate. During the setting of the alginate and gelling of the agar hydrocolloid, a bond forms between them. The impression may be removed in about 4 minutes. Cross-sections of impressions of a laboratory model are shown in Fig. 12-11. The surface of the impression is in agar hydrocolloid backed by the alginate. The same precautions used in preparing stone models in alginate or agar hydrocolloid impressions should be obsen~ed.

For effective bonding between agar and alginate, the materials must be placed when both are in a flowable state; some combinations of agar hydrocolloid and alginate bond together better than others, with tensile bond strengths ranging from 600 to 1100 g/cm2.Values at the high end of the range result in cohesive failure of the agar

hydrocolloid, whereas those at the low end produce adhesive failure between the agar hyclrocolloid and alginate. Therefore following the manufacturer's suggestions for appropriate combinations is important.

The accuracy of the agar-alginate impressions was determined with a laboratory model shown in Fig. 12-12. Impressions were taken, and models poured in high-strength stone. The accuracy of (1) the interpreparation distance, (2) buccolingual diameter, and (3) preparation height of the models was measured and compared with values obtained with polysulfide, condensationsilicone, polyether, and addition-silicone impression materials. These values are listed in Table 12-4. Except for the interpreparation distance, the agar-alginate system had the same

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346 Chapter 12 IMPRESSION MATERIALS

order of accuracy as the elastomeric impression materials.

In summary, the advantages of the agaralginate combination impression compared with the agar hydrocolloid system alone are the simplification of heating equipment, the elimination of water-cooled impression trays, and the overall simplification of the procedure. In addition, the agar hydrocolloid is more compatible with gypsum model materials than alginates, making them useful for crown and bridge impressions; the accuracy is acceptable, and the cost of materials is low.

DUPLICATING IMPRESSION MATERIALS

In preparing partial dentures, a duplicate should be made of the plaster or stone cast of the patient's mouth. This duplicate is required for two reasons: (1) the cast on which the wax pattern of the metal framework is to be formed must be made from a refractory investment, because it must withstand the casting temperatures required for gold or base metal alloys; and (2) the original cast is needed for checking the accuracy of the metal framework and for processing the plastic portion of the partial denture.

A duplicate refractory cast is obtained by making an impression of the original cast in an elastic duplicating material. The most common duplicating materials are agar hydrocolloid compounds. Their composition is quite similar to the agar hydrocolloid impression compounds, but a greater proportion of water is used with the duplicating compounds. For example, an impression compound may be diluted with as much as one to three times its weight of water and used as a duplicating compound.

Agar hydrocolloid duplicating materials have many advantages. They are reversible, and the material may be reused a number of times. This is particularly important in duplication procedures, because 200 to 400 ml of the material may be needed for each duplication. The agar hydrocolloid duplicating materials may be continuously stored in the sol state at 54" to 66" C and

used when needed without converting the material from the gel to the sol state each time it is required. After a duplication procedure, the gel is chopped up, reheated until in the sol condition, and added to the material being stored at 54" to 66" C.This procedure may be repeated about 20 times before the material is discarded. Of prime importance is that the agar hydrocolloid duplicating materials have adequate strength and elastic properties to duplicate undercut areas. The accuracy of the agar hydrocolloid duplication compounds is also quite satisfactory if proper techniques are followed.

The disadvantages of agar hydrocolloid duplicating materials are similar to those of agar hydrocolloid impression compounds. The set material is a gel and therefore is subject to dimensional changes if stored in air or water. Generally, the best storage condition is 100%relative humidity. The best procedure is to pour the duplicate refractory cast as soon as possible. The agar is a polysaccharide and gradually hydrolyzes at storage temperatures. Accompanying this hydrolysis is a loss of elasticity and strength, which eventually renders the agar hydrocolloid duplicating material useless. While in use, the duplicating material is contaminated by components of the stone, investment, hardening solutions, separators, and others. Indications are that some of these components accelerate the degradation of agar hydrocolloid.

Other types of materials, such as alginate hydrocolloids, reversible plastic gels, silicones, and polyethers, have been used as duplicating materials. Obviously, the major objection to the alginate type is that the material is irreversible. However, its use does not require heating and storage equipment, as do the reversible agar hydrocolloid and plastic duplicating compounds. The reversible plastic gel is a polyvinylchloride gel that is quite fluid at 99" to 104" C . The main advantages of this material are its high-strength properties and high chemical stability, which permit a large number of duplications before replacement. Silicones and polyethers that set at room temperatures are examples of the nonreversible

non-aqueous type. The principal problem with them has been their cost, and numerous techniques have been developed to use minimum amounts in a duplicating procedure. At present, agar hydrocolloid duplicating materials are the most common type used in dental laboratories.

Properties ANSI/ADA Specification No. 20 for dental duplicating materials includes two types: thermoreversible and nonreversible. Within these two types are the hydrocolloid and non-aqueous classes.

The specification requires these materials to be free from foreign agents and impurities and to be suitable for taking impressions of plaster, stone, or investment casts of the oral tissues.

Pouring temperature and the temperature of gel formation are defined for the thermoreversible products. Working and setting times are specified for the nonreversible materials. Compatibility with at least one type of investment and the ability to reproduce detail satisfactorily are required. The duplicating material may be compatible with a silicateor phosphate-bonded investment but not with a gypsum-bonded investment. Fig. 12-13 shows the surface reproduction and detail when a single agar hydrocolloid duplicating material was poured up in A, a silicatebonded investment; B, a phosphate-bonded investment; and C, a gypsum-bonded investment. High-quality surfaces were obtained with silicateand phosphate-bonded investments, but a poor-quality surface was found with gypsumbonded investment. This incompatibility is often caused by the addition of glycerin or glycols to the duplicating material to reduce the loss of water from the gel; these compounds interfere with the setting of the gypsum matrix.

Type I products are required to show no mold growth after inoculation under controlled conditions. Requirements for permanent deformation

recovery from deformation), strain in 'Ompression, and resistance to tearing are described for each type and class; and the acceptable values and ranges are listed in Table 12-5. Aging tests are described, and permissible changes

Fig. 12-13Surface quality of, A, silicate-bonded,

B, phosphate-bonded, and C, gypsum-bonded investments formedagainstthe same agar duplicating material,

(From Craig RG, Dootz ER: Ann Arbor, 1965, University of Michigan school of Dentistry,)

"Minimum elastic recovery of 97%.

in physical properties defined. Packaging must include instructions that indicate the type of investment that can be used with the material and, for Type I products, must also include

(1) method of liquefying,(2) tempering or storing temperature, and (3) pouring temperature.

Four types of synthetic elastomeric impression materials are available to record dental impressions: polysulfides, condensation silicones, addition silicones (polyvinylsiloxanes),and polyethers. Although polysulfides were the first synthetic elastomeric impression material introduced (1950), the latter three types form the vast majority of elastomeric impressions used worldwide today. Condensation silicones were made available to dentists in 1955, polyether in 1965, and addition silicones in 1975. Changes in recent years have provided greater choice of consistency and new mixing techniques.

CONSISTENCIES

Elastomeric impression materials are typically supplied in two to four consistencies (viscosities) to accommodate a range of impression techniques. Polysulfide impression materials are sup-

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plied in three consistencies: low (syringe or wash), medium (regular), and high (tray). Addition silicones are available in these three consistencies plus an extra-low and putty (very high) type, whereas condensation silicones are usually supplied in low and putty consistencies. The catalyst of the condensation silicone can be supplied as a putty or a liquid. The first polyether impression materials were medium consistency, but they are now available in low, medium, and high consistencies.

MIXING SYSTEMS

Three types of systems are available to mix the catalyst and base thoroughly before taking the impression: hand mixing, static automixing, and dynamic mechanical mixing. All three systems are illustrated Fig. 12-14and are described below. Impression pastes are most commonly dispensed from collapsible tubes, as shown in Fig. 12-14,A. Equal lengths of catalyst and base are dispensed on a paper pad, as shown in Fig. 12-15, A. Initial mixing is accomplished with a circular motion, as shown in Fig. 12-15, B, and final mixing to produce a mix free from streaks is done with broad strokes of the spatula, as shown in Fig. 12-15, C. Mixing is readily accomplished within 45 seconds, although the low-consistency material is easier to mix than high-consistency mate-

rials. When the catalyst is supplied as a liquid, a specified number of drops per unit of length is indicated in the instructions, and mixing is accomplished in a manner similar to that of the two-paste systems. All four types of impression materials are available for mixing in this fashion.

One variation in hand mixing is with the twoputty systems, offered both with condensation and addition silicones. Scoops are supplied by the manufacturer for dispensing, and the putties are most often kneaded with fingers until free from streaks. The putty materials that have a liquid catalyst are initially mixed with a spatula until the catalyst is reasonably incorporated, and mixing is completed by hand. It should be noted that latex gloves may interfere with setting of additionsilicone impression materials, as discussed later.

A very popular means of mixing the catalyst and base is with a so-called automixing system, as illustrated in Fig. 12-14, B. The base and catalyst are in separate cylinders of the plastic cartridge. The cartridge is placed in a mixing gun containing two plungers that are advanced by a ratchet mechanism to extrude equal quantities of base and catalyst. The base and catalyst are forced through the static-mixing tip containing a stationary plastic internal spiral; the two components are folded over each other many times as they are pushed through the spiral, generally resulting in a uniform mix at the tip end. Because one cylinder may be filled slightly more that the other, the first part of the mix from a new cartridge should be discarded.

The mixed material can be extruded directly into an injection syringe or into the impression tray. Intraoral delivery tips can be placed on the end of the static mixing tip, as shown in Fig. 12-14, B, and the mixed material can be injected into and around the cavity preparation. The tip can be removed, and additional mixed material can be extruded into the impression tray. The automixing systems have been shown to result in mixes with many fewer voids than hand mixes. Although for each mix the material left in the mixing tip is wasted, the average loss is only 1 to 2 ml, depending on the manufacturer's tip, whereas three to four times this much is wasted in a hand mix as a result of overestimating the

amount needed. Initially, automixing was used for low consistencies, but new designs of guns and mixing tips allow all consistencies except putty to be used with this system. Addition silicones, condensation silicones, and polyethers are available with this means of mixing.

The third and newest system is a dynamic, mechanical mixer, illustrated in Fig. 12-14, C.The catalyst and base are supplied in large plastic bags housed in a cartridge, which is inserted into the top of the mixing machine. A new, plastic mixing tip is placed on the front of the machine, and when the button is depressed, as shown in the figure, parallel plungers push against the collapsible plastic bags, thereby opening the bags and forcing material into the dynamic mixing tip. This mixing tip differs from automixing in that the internal spiral is motor driven so it rotates. Thus mixing is accomplished by this rotation plus forward motion of the material through the spiral. In this manner, thorough mixing can be ensured and higher viscosity material can be mixed with ease. The advantage of this system is ease of use, speed, and thoroughness of mixing, but more must be invested in the purchase of the system compared with hand and automixing. In addition, there is slightly more material retained in the mixing tip than with automixing, but less than that wasted when mixed by hand. Polyether and addition-silicone impression materials are available for mixing with this system.

IMPRESSION TECHNIQUES

Three common methods for making crown and bridge impressions are a simultaneous, dualviscosity technique, a single-viscosity or monophase technique, and a putty-wash technique. In nearly all cases, impression material is injected directly on and into the prepared teeth and a tray containing the bulk of the impression material is placed thereafter. After the impression is set, the tray is removed.

The simultaneous, dual-viscosity technique is one in which low-consistency material is injected with a syringe into critical areas and the highconsistency material is mixed and placed in an impression tray. After injecting the low-viscosity