The function of an impression material is to record accurately the dimensions of oral tissues and their spatial relationships. In making an impression, a material in the plastic state is placed against the oral tissues to set. After setting, the impression is removed from the mouth and is used to make a replica of the oral tissues. The impression gives a negative reproduction of these tissues. A positive reproduction is obtained by pouring dental stone or other suitable material into the impression and allowing it to harden. The positive reproduction is called a model or cast when large areas of the oral tissues are involved or a die when single or multiple tooth preparations are recorded.

The relationships between a tooth, an impression of the tooth, and a die are illustrated in two dimensions in Figure 8-1. When examined from the anterior side, the various parts of the tooth and the impression are in the same relationship to each other, although the highest portions of the tooth are the deepest parts of the impression. When the maxillary die is examined with the occlusal portion downward, the die is a positive reproduction of the tooth preparation. If the die is examined with the occlusal surface upward, the buccal surface (B) is on the left rather than the right. A similar relationship is shown in Figure 8-1 for a cross section of a mandibular left molar, its impression, and the die made from it. The impression of the mandibular molar, when examined from the anterior side, is inverted for ease of examination, and the buccal (B) and lingual (L) surfaces are reversed with respect to the tooth. The cusp (C) is the highest area on the tooth and the lowest area of the impression, and the position of C for the tooth is on the left but on the right for the impression. The die of the mandibular molar, however, is a positive duplication of the tooth.

FIGURE 8-1 Two-dimensional sketches of a maxillary and mandibular left molar and the corresponding impressions and dies.
(B, buccal surface; C, cusp; F, fissure; L, lingual surface.)

Impressions may be taken of portions of a tooth, a single tooth, several teeth, a quadrant of the mouth, or an entire dentulous or edentulous arch. Examples of some of these types of impressions and the corresponding dies or casts are shown in Figure 8-2.

FIGURE 8-2 The prepared cast with its corresponding impression. (From Rosenstiel SF, Land MF, Fujimoto J: *Contemporary Fixed Prosthodontics,* ed 4, St Louis, 2006, Mosby.)

A variety of impression materials are described in this chapter, which is indicative of the fact that no single material is ideal for all applications. The list of properties for an ideal impression material presented in Box 8-1 emphasizes the many demands placed on these materials. Not surprisingly, none of the current materials completely satisfies these requirements.

BOX 8-1

Desirable Properties of Impression Materials

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1. Ease of manipulation and reasonable cost

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2. Adequate flow and wetting properties

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3. Appropriate setting time and characteristics

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4. Sufficient mechanical strength—elastic recovery and resistant to tearing during removal

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5. Good dimensional accuracy—stable over time

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6. Taste and odor acceptable to patient

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7. Safe—not toxic or irritating

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8. No significant degradation of properties as a result of disinfection

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9. Compatibility with all die and cast materials

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10. Good shelf-life

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Impression materials can be classified as those that are flexible and those that are rigid at the time of removal from the mouth. A rigid impression material is restricted to applications in areas where no undercuts exist. A rigid impression material could not be used on the teeth shown in Figure 8-1 because, on setting, it would be locked in place and could not be removed over the bulge of the tooth without fracturing. A rigid material can be removed from a tooth prepared for a full crown or from an edentulous arch, as shown in Figure 8-2, C, because no undercut areas are present. However, a flexible impression material could make these two impressions, in addition to impressions of single teeth or a full dentulous arch. Not surprisingly, impression materials that are flexible when set are used most frequently. Rigid impression materials (dental compound, zinc oxide–eugenol impression paste) are described in Appendix 8-1.

Impression materials discussed in this chapter are listed in Table 8-1 and are classified as rigid or flexible. An estimate of the usage of impression materials in restorative procedures is 51% for addition silicones, 32% for polyethers, 9% for polysulfides, 6% for hydrocolloids, and 2% for condensation silicones.

TABLE 8-1

Classification of Dental Impression Materials

RIGID	FLEXIBLE
DENTAL COMPOUND	AQUEOUS
Zinc oxide–eugenol	Alginate hydrocolloid^∗
	Elastomeric
	Addition silicone (PVS)^∗
	Polyether^∗
	Addition silicone–polyether hybrid
	Condensation silicone
	Polysulfide (rubber base)

^∗Most commonly used impression materials.

Alginate Hydrocolloid Impression Material

Alginate is one of the most widely used aqueous dental impression materials. The wide use of alginates results from (1) the ease of mixing and manipulating them, (2) the minimum equipment necessary, (3) the flexibility of the set impression, (4) their accuracy if properly handled, and (5) their low cost. Their principal disadvantages are that they have low tear strength and they do not transfer as much surface detail to gypsum dies as elastomeric impressions do.

Alginates are used extensively to prepare study models of either the entire dental arch or a segment of it. They also are used to prepare gypsum models for the preparation of athletic mouth protectors. They are not recommended for making impressions of cavity preparations.

Note: Agar hydrocolloid impression materials are described in Appendix 8-1.

Packaging

Manufacturers supply alginate as a powder that is packaged in bulk or in preweighed individual packets. Examples of alginate impression materials are listed in Table 8-2. The bulk material is packaged in a sealed screw-top plastic container; in a hermetically sealed metal can, such as that used to package coffee; or in foil packets as shown in Figure 8-3. The preweighed packages are constructed of plastic and metal foil and contain enough material for a single full-arch impression. These packages minimize moisture contact with the powder and extend the storage life of the alginate.

TABLE 8-2

Examples of Alginate Hydrocolloid and Elastomeric Impression Materials

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PRODUCT	MANUFACTURER	PACKAGING
Alginates
Alginmix	Major Prodotti Dentari (Moncalieri, Italy)	Packet
Hydrogum5	Zhermack (Eatontown, NJ)	Packet
Jeltrate Plus	DENTSPLY Caulk (Milford, DE)	Can, packet
Kromopan 100	Lascod (Firenze, Italy)	Packet
Alginate Substitutes
Alginot	Kerr Corporation (Orange, CA)	AC
Position Penta Quick	3M ESPE (St. Paul, MN)	DMM
Silgimix	Sultan Health Care (Englewood, NJ)	AC, DMM
Status Blue	DMG America (Englewood, NJ)	AC
Addition Silicones
Aquasil Ultra	DENTSPLY Caulk	AC, DMM
Affinis	Coltene/Whaledent (Cuyahoga Falls, OH)	AC
Exafast NDS	GC America (Alsip, IL)	AC
Flexitime	Heraeus (Armonk, NY)	AC
Honigum	DMG America	AC, DMM
Imprint 3	3M ESPE	AC, DMM
Take 1	Kerr Corporation	AC, DMM
Virtual	Ivoclar Vivadent (Amherst, NY)	AC
Polyethers
Impregum Garant	3M ESPE	AC
Impregum Penta Soft	3M ESPE	DMM
Addition Silicone–Polyether Hybrids
EXA’lence	GC America	AC
Bite Registration Materials
Blu-Mousse	Parkell (Edgewood, NY)	AC
Jet Blue Bite	Coltene/Whaledent	AC
Ramitec	3M ESPE	AC, DMM
Regisil Rigid	DENTSPLY Caulk	AC

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AC, Automix cartridge; DMM, dynamic mechanical mixer.

FIGURE 8-3 Alginate powder supplied in bulk.

A plastic scoop is provided for dispensing the bulk powder, and a plastic cylinder is supplied for measurement of the water (see Figure 8-3). A wide-bladed, reasonably stiff spatula is used to mix the powder and water. Mechanically driven mixers (Alginator II; DUX Dental, Oxnard, California) are also available.

Composition

Table 8-3 shows the ingredients in alginate powder and their functions. When water is mixed with the alginate powder, a smooth viscous mass is formed, which becomes an irreversible gel a few minutes after mixing. The overall simplified reaction is as follows:

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Sodium Alginate+CaSo4•H2O→Calcium Alginate↓+Na++SO4−+H2O

TABLE 8-3

Ingredients and Their Function in Alginate Powder

INGREDIENT	FUNCTION
Sodium or potassium alginate salt	To dissolve in water
Calcium sulfate	To react with dissolved alginate to form insoluble calcium alginate
Sodium phosphate	To react preferentially with calcium sulfate and serve as a retarder
Diatomaceous earth or silicate powder	To control consistency of mix and flexibility of impression
Potassium sulfate or potassium zinc fluoride	To counteract the inhibiting effect of alginate on the settling of gypsum model or die material
Organic glycol	To coat the powder particles to minimize dust during dispensing
Pigments	To provide color
Quaternary ammonium compounds or chlorhexidine	To provide self-disinfection
Aspartame	To function as a sweetener

The manufacturer controls the time of setting by the amount of sodium phosphate present in the alginate powder. As long as any sodium phosphate is present, it reacts preferentially with the soluble calcium ions. After all the sodium phosphate has reacted, the soluble sodium alginate reacts with the remaining calcium ions, and calcium alginate precipitates. The sodium phosphate therefore is called a retarder. The calcium alginate precipitates into a fibrous network; water occupies the intervening capillary spaces. This type of structure is called a gel. At least one of the dimensions of the network is colloidal (0.5 µm), and this material traditionally has been named alginate hydrocolloid. The reaction is driven by the lower solubility of calcium alginate compared with sodium alginate. These materials frequently are referred to as irreversible hydrocolloids, because once the paste sets to a gel, the process cannot be reversed.

A gel is a colloidal system in which the solid and liquid phases are continuous.

Properties

American National Standards Institute–American Dental Association (ANSI-ADA) Specification No. 18 (ISO 1563) for alginates established requirements for odor, flavor, lack of irritation, uniformity, mixing and setting times, permanent deformation (alteration in shape) at the time of removal from the mouth, flexibility at the time of pouring the model or die, compressive strength, reproduction of detail, compatibility with gypsum, and deterioration of the packaged powder during storage.

Mixing and Setting Times

Alginates, when properly mixed by hand, should develop a smooth, creamy consistency free of graininess in less than 1 minute for the normal set material and should be suitable for making impressions in the mouth. The setting time of the alginate is indicated as normal or fast by the manufacturer. An alginate sold as a normal-setting material should set in no less than 2 minutes or more than 4½ minutes after the start of the mix and be workable for up to 2 minutes. The setting time of fast-setting alginate is between 1 and 2 minutes and workable for at least 1¼ minutes. The mixing time of fast-setting alginate is 30 to 45 seconds. In general, the setting time should be no less than that listed by the manufacturer and at least 15 seconds longer than the stated working time.

Because the setting occurs as a result of a chemical reaction, an increase in the temperature of the water used to prepare the mix shortens the working and setting times. The proportions of powder and water also affect the setting times. Thinner mixes increase the time required for the material to set. The usual range of setting times for normal-setting mixes of commercial alginate impression materials is from 2½ to 5 minutes.

Elastic Recovery

Because the set alginate is held between the impression tray and the tissues, knowledge of the extent of any permanent deformation during the removal of the impression is important (Figure 8-4). The ANSI-ADA specification requires at least 95% elastic recovery (no more than 5% permanent deformation) when the alginate is compressed 20% for 5 seconds, which simulates removal of the impression from the mouth. Many commercial alginates have actual values of 96% to 98% elastic recovery (2% to 4% permanent deformation). Thus, alginate impression material is flexible but not perfectly elastic (Box 8-2).

FIGURE 8-4 Permanent deformation of alginate impressions as a function of time of testing **(A)**, amount of compression **(B)**, time under compression **(C)**, and consistency of mix **(D)**.

BOX 8-2

Permanent Deformation Affected by Manipulation

Permanent deformation increases:

When the time before testing is shortened

When the amount of deformation during removal increases

When the time that it is held under compression increases

When thinner mixes are used

These effects are shown in Figure 8-4.

Flexibility

The ANSI-ADA specification sets limits of 5% to 20% in compression at the time a model or die is prepared in the impression (10 minutes after the start of mixing). The compression is measured between a stress of 0.01 and 0.10 MPa. Typical values for commercial alginates are between 12% and 18%, but a few manufacturers supply hard-set alginates, which have values of about 5% to 8%.

The relative amounts of water and powder influence the flexibility of the set alginate. Thicker mixes result in lower flexibility.

Strength

The strength of alginates in compression and resistance to tearing are important requirements, although the tear strength is the more critical. The ANSI-ADA specification requires a minimum compressive strength of 0.37 MPa at a time when the material is removed from the mouth. Most commercial products have compressive strengths substantially higher than this limit with values ranging from 0.5 to 0.9 MPa.

The tear strength of alginates varies from 3.7 to 6.9 N/cm. Because many sections of an impression are thin, tearing may result from a rather small applied force.

The strengths of alginates are a function of the rate at which the impression is deformed; higher rates of deformation (removal) result in higher compressive and tear strengths. An example of the effect of the rate of deformation on tear strength is shown in Figure 8-5, which illustrates that alginate impressions are less likely to tear during removal from the mouth when they are removed rapidly.

FIGURE 8-5 Effect of rate of deformation (removal) on the tear strength of alginate impressions.

The strength of alginate impression materials increases if thick rather than thin mixes are used. The advantage of the use of increasingly thicker mixes is somewhat limited because the consistency becomes too thick, and the flow during seating of the impression is so low that an adequate impression cannot be obtained.

The tear and compressive strengths at the time the impression is removed increase if the time of removal is delayed. The effect of the consistency of the mix and of the time of removal on the tear strength is shown in Figure 8-6. Again, only limited advantage can be taken of the increase in tear strength with time because of the inconvenience of leaving the impression in the mouth for longer periods.

FIGURE 8-6 Effect of consistency of the mix of alginate and time of testing on the tear strength of alginate impressions.

Dimensional Change

Syneresis is the formation of an exudate on the surface of the gel and results in continual shrinkage of the gel.

The accuracy of an impression material is important, and alginates are no exception. A problem with alginate impressions is loss of accuracy with increased time of storage (Figure 8-7). The set alginate is a hydrocolloid gel that contains large quantities of water. This water evaporates if the impression is stored in air, and the impression shrinks. If the impression is placed in water, it absorbs water and expands. Therefore, storage in either air or water results in serious changes in dimensions and a loss of accuracy. Storage in humid air approaching 100% relative humidity results in the least dimensional change. Alginate gels, however, shrink even under conditions of 100% relative humidity as a result of a process called syneresis, in which water forms on the surface of the impression. Fortunately, syneresis occurs rather slowly, and alginate impressions prepared from modern products (see Table 8-2) usually can be stored under conditions of 100% relative humidity for up to 5 days without serious dimensional changes.

FIGURE 8-7 Dimensional change of alginate impression materials stored in 100% relative humidity for 100 hours. (Courtesy Lu H, Frey GN, Powers JM: Unpublished data. From Powers JM, Sakaguchi RL, editors: *Craig’s Restorative Dental Materials,* ed 13, St Louis, 2012, Mosby.)

Studies of the dimensional change of alginates during storage emphasize that they should be stored for as short a period as possible and that the preparation of the model or die should proceed directly after the impression has been made. If the immediate preparation of the model or die is not possible, the next most appropriate procedure is to store the impression in an atmosphere of 100% relative humidity for the shortest possible time.

Reproduction of Detail

The impression material must record the detail of the oral tissues, but this detail also must be transferred to the model or die. The ADA specification recommends that alginates must have the minimum capabilities of transferring a line only 0.075 mm wide to a gypsum model or die material. A number of products have properties that exceed this minimum value.

Disinfection

Guidelines recommend that alginate impressions be rinsed and disinfected. Sodium hypochlorite, iodophor, glutaraldehyde, and phenylphenol solutions have been used, and some manufacturers have added disinfectants to the alginate powder. Studies have shown that test viruses have been inactivated in alginate impressions by (1) a 10-minute soak in 0.5% sodium hypochlorite or a 10-minute wait after spraying the impression with this solution; (2) a 10-minute immersion in an iodophor solution diluted 1:213; (3) a 20-minute immersion in 2% glutaraldehyde diluted 1:4; and (4) a 20-minute immersion in phenylphenol diluted 1:32. The incorporation of disinfectants in the alginate powder also was found to be effective.

Measurements of dimensional changes showed that immersion of alginate impressions for 30 minutes did not affect their clinical accuracy.