Setting Mechanism

There are two basic setting mechanisms: reversible and irreversible. Irreversible implies that chemical reactions have occurred and that the material cannot revert to a previous state in the dental office. For example, alginate, zinc oxide–eugenol (ZOE) impression paste, impression plaster, and elastomeric impression materials, which set by chemical reactions, are irreversible. On the other hand, reversible materials, such as agar and impression compound, soften upon heating and solidify slightly above body temperature with no chemical change taking place.

Mechanical Properties

The set impression materials can be rigid (inelastic) or elastic. A set rigid material is highly resistant to flexure, and it fractures suddenly when stressed, in a manner similar to that of chalk. Material is not flexible and will fracture when deformed, like chalk. ZOE impression paste, impression plaster, and impression compound are inelastic impression materials. The term elastic means that the material is flexible and can be deformed and still return to its original form when unstressed. Examples include agar, alginate, and elastomers.

Clinical Application of Impression Materials

Elastic impression materials can be stretched or compressed slightly, and they then rebound when the impression tray is removed from the mouth. They are capable of accurately reproducing both the hard and soft structures of the mouth, including the undercuts and interproximal spaces. The extent of the rebound determines the accuracy of the material.

Inelastic impression materials, such as ZOE paste and plaster, are ideal for making impressions of edentulous jaw structures or soft tissue because, in the proper consistency, they do not compress the tissue during seating of the impression tray. Impression compound is often used to make trays for the construction of full dentures.

Polysulfide

The base paste, is a polysulfide polymer that contains a multifunctional mercaptan (-SH) called a polysulfide polymer, a suitable filler (such as lithopone or titanium dioxide) to provide the required strength, a plasticizer (such as dibutyl phthalate) to confer the appropriate viscosity to the paste, and a small quantity of sulfur, approximately 0.5%, as an accelerator. The catalyst (or accelerator) paste contains lead dioxide, filler, and plasticizer as in the base paste, and oleic or stearic acid as a retarder to control the rate of the setting reaction. Lead dioxide is the component that gives polysulfide impression material its characteristic brown color. The terms catalyst and accelerator used here and with other impression materials are actually misnomers. Reactor is a more appropriate term for the reactions associated with polysulfide and other types of impression materials.

Each paste is supplied in a dispensing tube with appropriately sized bore diameters at the tip so that equal lengths of each paste are extruded from each tube to provide the correct ratio of polymer to cross-linking agent. Since the composition of the material in the tube is balanced with that of the accelerator, the matched tubes supplied by the manufacturer should always be used.

The reaction starts at the beginning of mixing and reaches its maximum rate soon after spatulation is complete (Figure 8-3). At this stage, a resilient network has started to form. During the final set, a material of adequate elasticity and strength is formed that can be removed past undercuts quite readily. Moisture and temperature have a significant effect on the course of the reaction. In particular, hot and humid conditions will accelerate the setting of polysulfide impression material. The reaction yields water as a by-product. Loss of this small molecule from the set material has a significant effect on the dimensional stability of the impression.

Condensation Silicone

The materials are supplied as a base paste and a low-viscosity liquid catalyst (or paste catalyst), a two-paste system, or a two-putty system. The putty can be used as the tray material in conjunction with a low-viscosity silicone, that is referred to as the putty-wash technique.

The base paste consists of α-ω-hydroxyl-terminated polydimethyl siloxane (Figure 8-4). The curing of this material involves a reaction of tri- and tetra-functional alkyl silicates in the presence of stannous octoate as a catalyst. The material sets by cross-linking between terminal groups of the silicone polymers and the alkyl silicate to form a three-dimensional network (Figure 8-4). Ethyl alcohol is a by-product of the condensation setting reaction. Its subsequent evaporation accounts for much of the contraction that takes place in the setting impression.

Addition Silicone

This material is often called a polyvinyl siloxane (PVS) or vinyl polysiloxane (VPS) impression material. In contrast to the condensation silicone, the addition silicone is based on addition polymerization between divinylpolysiloxane and polymethylhydrosiloxane with a platinum salt as the catalyst (Figure 8-5). The base paste contains polymethylhydrosiloxane, as well as divinylpolysiloxane. The catalyst (or accelerator) paste contains divinylpolysiloxane and a platinum salt. The platinum salt and polymethylhydrosiloxane are separated before mixing. Both pastes contain fillers.

No reaction by-products are formed as long as the correct proportions of divinylpolysiloxane and polymethylhydrosiloxane are used and there are no impurities. However, the residual polymethylhydrosiloxane in the material can lead to a secondary reaction with each other or with moisture, to produce hydrogen gas. Technically, hydrogen gas is a reaction by-product that does not affect the dimensional stability of the impression. Nonetheless, the hydrogen gas evolved can result in pinpoint voids in the gypsum casts poured soon after removal of the impression from the mouth. Manufacturers may add a noble metal, such as palladium, as a scavenger for the released hydrogen gas. The impression should be left overnight if epoxy will be used for pouring models.

One of the disadvantages of the silicone impression materials (including condensation silicones) is their inherent hydrophobic nature. A nonionic surfactant can be added to the paste in the manufacturing process to render a degree of hydrophilicity to the surface of the material. This surfactant migrates toward the surface of the impression material and has its hydrophilic segment oriented toward the surface—a phenomenon that makes the surface more wettable by water. These impression materials still require a dry field for impression making. Pouring the set impression with a gypsum-forming mixture is facilitated because the wet stone has a greater affinity for the hydrophilic surface. The clinical significance of hydrophilic additives is discussed in subsequent sections.

Sulfur contamination from natural latex gloves inhibits the setting of addition silicone. Some vinyl gloves may have the same effect because of the sulfur-containing stabilizer used in the manufacturing process. The contamination is so pervasive that touching the tooth with latex gloves before seating the impression can inhibit the setting of the critical surface next to the tooth.

Medium-body addition silicone has also been formulated for making impressions for diagnostic purposes, as a substitute for alginate impression material (discussed later). The advantage of these so-called alginate substitutes is the ability to make multiple, accurate diagnostic casts from one impression. Laboratory studies have reported that they exhibited better detail reproduction and less variability in linear dimensional change than irreversible hydrocolloid.

Polyether

There are two types of polyether impression materials. The first is based on the ring-opening polymerization of aziridine rings, which are at the end of branched polyether molecules (Figure 8-6, left). The main chain is probably a copolymer of ethylene oxide and tetrahydrofuran. Cross-linking and setting are promoted by an initiator and an aromatic sulfonate ester (Figure 8-6, top), where R is an alkyl group. This produces cross-linking by cationic polymerization via the imine end groups (Figure 8-6). The material is supplied as two pastes. The base paste contains the polyether polymer, colloidal silica as filler, and a plasticizer such as glycol ether or phthalate. The accelerator paste contains an alkyl-aromatic sulfonate in addition to the filler and plasticizer. The ether-dominated polymer backbones make this group of materials the most hydrophilic of all elastomeric impression materials.

The second type is based on an acid-catalyzed condensation polymerization of polyether prepolymer with alkoxysilane terminal groups. The mechanism is similar to that observed in condensation silicones having low-molecular-weight alcohols as by-products. This material is often called a hybrid. Since the ether-linkages constitute the main component of the polymer chains, these materials behave very much like the first type of polyether impression material.

Impression Trays

The use of a custom tray (Figure 8-7, A) is recommended to reduce the quantity of material required to make impressions; thus, any dimensional changes attributed to the materials are minimized. A custom tray allows a uniform distribution of impression material between the tray and the object, which also improves accuracy. This is especially true for polysulfide impression material. The use of custom trays for polyether and addition silicone impressions is not critical, since these materials are stiffer and have less polymerization shrinkage than the polysulfide material. In addition, disposable stock trays (Figure 8-7, B) work satisfactorily. Note that the use of less material in a custom tray reduces the compressibility of the impression, which can make removal of the impression tray more difficult. When severe undercuts are present, the use of a custom tray should be avoided. The stock tray should be rigid, thereby minimizing flexure of the tray during impression making. Disposable stock trays are also used to support the putty when the putty-wash technique is used for making impressions. Prior to making an impression, a uniform thickness of tray adhesive is applied on the tray surface, extending over its edge, and it is allowed to dry (through evaporation of solvent). A slightly roughened surface on the tray will increase adhesion. Tray adhesives are not interchangeable among different types of materials.

When the material is used correctly, either custom or stock trays yield clinically acceptable impressions.

Tissue Management

The margins of tooth preparations for fixed prostheses often extend to or below the free margin of the gingiva. To ensure access for the tooth preparation and for making the impression, it is necessary to displace the gingival tissues, control gingival hemorrhage, and control sulcular fluids. Among the most popular methods of gingival displacement is the use of gingival retraction cord. An electrosurgical unit or a soft tissue laser can also be used.

The objective of placing a retraction cord is to displace the gingival tissue laterally away from the margin of the preparation. One or two gingival retraction cords are placed under the margin around the tooth for at least 5 min before making impressions. The double-cord technique is used when the margin is very close to the gingival attachment. A fine cord is placed at the base of the crevice to facilitate moisture control, with a larger cord placed on top of the first and near the coronal extent to displace the gingiva laterally. The latter, outermost cord is removed, leaving the fine cord within the crevice while the impression is made. A single cord is sufficient to deflect the soft tissue around the margin when the preparation margin is at or slightly above the gingival crest.

Retraction cords may be impregnated with a hemostatic agent by dipping the cord in a hemostatic solution prior to placement. These agents can have unintended side effects and should be used with caution. For example, epinephrine, which is used widely, is of particular concern in patients with cardiovascular disease. In addition, agents with a low pH can remove the smear layer and superficial dentin apical to the margins of the preparation, possibly leading to postoperative sensitivity of some teeth.

Manipulation of Impression Materials

Currently, elastomeric impression materials are supplied for three modes of mixing: hand mixing, static mixing, and dynamic mechanical mixing (Figure 8-8).

Static Mixing

This technique transforms two fluid (or paste-like) materials into a homogeneous mixture without mechanical mixing. The device used to accomplish this mixing is a gun for compressing materials in a two-cylinder cartridge, which contains the base and catalyst separately, as well as a mixing tip (Figure 8-8, B). The mixing tip is made of helical mixer elements in a cylindrical housing (Figure 8-9). The mixer elements are a series of alternating right- and left-turn 180° helixes positioned so that the leading edge of one element is perpendicular to the trailing edge of the next (Figure 8-9, B). The length of each element is about the same as the inner diameter of the cylindrical housing.

The base and catalyst are pressed from the cartridge into the mixing tip as one stream of a two-layer material. The leading edge of the first element splits the material entering the mixer into two streams. The streams that flow in either side of the helix will make a 180° turn (Figure 8-9, A) when they reach the second element. Both streams are split by the leading edge of the second element, and two substreams (one from each original stream) combine into two new streams entering the second element. This process is known as flow division. Assuming that there is no intermixing between the two substreams as they merge, the new stream will have a two-layer structure. After flowing through “n” elements, the number of layers in the stream of material increases to 2ⁿ. The most common mixing tips for impression material mixing have 11 or 12 elements. The stream of material that exits the mixing tip will have a 2048- or 4096-striation structure, which can be treated as a uniformly mixed stream of material. In addition, as the materials make turns along the helix, the rotational circulation causes a radial mixing of the materials. Thus, mixing between substreams occurs before the next flow division that further increases the uniformity of the mixture. Since there is no mechanical mixing, porosity caused by mixing with air is avoided. The mixing tips vary in their diameter, length (number of helical mixer elements), and the size of openings for a specific consistency.

The mixed impression material is injected directly into the adhesive-coated tray or, if the “syringe tip” is in place, onto the prepared teeth. Static mixing provides greater uniformity in proportioning and mixing, yields fewer voids in the mix, and reduces the mixing time. In addition, there are fewer possibilities for contamination of the material. One precaution that should be taken in using these automixing devices is to make sure that the openings of the tubes that dispense the pastes remain unclogged. Problems can be avoided if one expresses a small amount of material from the cartridge before attaching the mixing tip. This type of device has also been adapted to mix and dispense temporary crown and bridge acrylic materials and cements that are used for luting and for producing restorations (Chapter 14).

Making an Impression

Elastomeric impression materials are typically supplied in several viscosities to accommodate different techniques for impression making. Three techniques for making impressions are discussed in this section.

Multiple-Mix Technique

A syringe material (light body) and a tray material (heavy body) are used in this technique. Usually, the two groups of materials are mixed simultaneously, each by a different person. With the mechanical devices described earlier, the materials now can be mixed as needed by one individual. The lighter material is injected from the filled syringe or directly from a static mixing gun within and around the tooth preparation. The filled tray is then inserted in the mouth and seated over the syringe material, which has been extruded on hard and/or soft tissue. The tray material will force the syringe material to adapt to the prepared tissues. The two materials should bond together upon setting. If either material has progressed past its working time when the two materials are brought together, the bond between them will be compromised. If a partially set material is seated, it will be compressed elastically. Once removed from the mouth, the impression will “spring back” or relax, and the dies from this impression will be too narrow and too short, as illustrated in Figure 8-10.

In rare cases a clinician may attempt to repair an impression that has small defects or that lacks sufficient detail. This is usually performed by cutting away the interproximal and gingival areas of the impression. Even with proper relief of the initial impression, it will be difficult to reseat the tray precisely. Entrapment of a minute fragment of impression material or debris will eliminate any chance of a successful repair. The impression material’s surface must be roughened to ensure that the new material bonds to the set impression. The safest method is to make a new impression when bubbles or similar defects are detected in critical areas.

Removal of the Impression

Under no circumstances should the impression be removed until the curing has progressed sufficiently to provide adequate elasticity, so that distortion will not occur. One method for determining the time of removal is to inject some of the syringe material onto a space that is not in the field of operation before inserting the impression tray. This material can be probed with a blunt instrument from time to time; when it is firm and returns completely to its original contour, the impression is sufficiently elastic to be removed. When a multiple mix technique is used, it is advisable to test both the syringe and the tray materials in this manner. Typically, the impression should be ready for removal within at least 10 minutes from the time of mixing, allowing 6 to 8 minutes for the impression to remain in the mouth. Manufacturers usually provide the optimal time for removal after mixing.

The mechanics of removing the impression involves separation at the impression/tissue interface and stretching of the impression. The first step is to break the physical adhesion between the tissue and the impression; therefore, an impression material, such as polyether, that wets the tissue well will require extra effort to break the seal for the removal. The second step stretches the impression enough to pass under the height of contour of the hard tissue to remove the impression; therefore, using a material of higher rigidity will require a greater force to stretch the impression to facilitate removal. Polysulfide has the lowest viscosity and ranks as one of the least stiff of the elastomeric impression materials of a similar consistency. This flexibility allows the set material to be easily removed from undercut areas and from the mouth with a minimum of stress.

In addition, all elastomeric impression materials are viscoelastic, and it is necessary to use a quick snap to minimize plastic deformation of the impression during the final step of the removal process. The phenomenon of viscoelastic behavior is discussed in subsequent sections.

Preparation of Stone Casts and Dies

The hydrophobic characteristics of silicone impression materials make them suitable for pouring of epoxy resin to produce dies. However, this hydrophobicity makes pouring with gypsum products challenging, as it increases the potential of forming voids in gypsum dies and casts. There are a number of surfactant sprays, also known as debubblizers, that can improve the surface wettability of the silicone impression material for the stone slurry. Only a thin layer of surfactant should be applied to the impression surface. A dilute solution of soap is also an effective surfactant. An alternative to the use of a surfactant is to select a hydrophilized addition silicone (discussed later). Pouring of a stone cast in a polyether or polysulfide impression does not require the aid of a surfactant.

The excellent dimensional stability of addition silicone and polyether impression materials makes it possible to construct two or three casts or dies from these materials. It is also possible to construct successive stone dies or casts from polysulfide impressions when duplicate stone dies are needed. However, each successive die will be less accurate than the first die constructed from the material. The time interval between impression pours should not be greater than 30 minutes. To minimize tearing and gross distortion after the first pour, the clinician should remove the excess gypsum-forming mass from undercut areas along the periphery of the tray. Be aware that the stiffness of the impression material makes it difficult to remove the stone cast from the impression. A weak stone cast may fracture during removal.

Working and Setting Times

Table 8-3 lists working and setting times for the various kinds of elastomeric materials as measured by an oscillating rheometer. The working time, which begins at the start of mixing and ends just before the elastic properties have developed, must be greater than the time required for mixing, filling the syringe and/or tray, injecting the material on tooth preparations, and seating the tray. The setting time is the time that has elapsed from the beginning of mixing until the curing process has advanced sufficiently that the impression can be removed from the mouth with no distortion. Remember, however, that polymerization may continue for a considerable time after setting. An increase in temperature accelerates the rate of polymerization of all elastomeric impression materials; therefore, the effect of temperature on working and setting time should be taken into consideration.

Working time and setting time decrease as the filler content in the materials increases. Altering the base/catalyst ratio will change the curing rate of these impression materials. Normally, having more base materials in the mixture tends to increase the working and setting times. One should be aware that this is not economical, as a portion of the paste is not used. Moreover, since the accelerator paste contains a retarder as well as a reactor, increasing the base/accelerator ratio may not produce a predictable change in the polymerization rate.

Reproduction of Oral Structure Detail

A surface reproduction test is a requirement of national standards for elastomeric impression materials. There is little doubt that these elastomers can record detail to the finest degree. When dental stone is poured on the surface of such test impressions, the finest detail is not always reproduced. The reason for this situation is that the elastomeric impression materials are capable of reproducing detail more accurately than can be transferred from the stone die or cast, which may not be capable of such accuracy.

The clinical significance of the surface reproduction tests is not entirely evident. It is possible that the detail obtained from the elastomeric impression materials under in vitro test conditions might be greater than that obtained in the mouth because of the hydrophobicity exhibited by some of these materials.

Rheological Properties

Impression materials are introduced into the mouth as viscous pastes with precisely adjusted flow properties. The viscosity and flow behavior of the unmixed components are also important in regard to the ease of mixing, air entrapment during mixing, and the tendency for the trapped air to escape before the impression is made.

Ideally, the impression material should flow freely and wet the tissue as it is being injected to achieve adaptation, and then resist flow away from the intended surface areas. The same procedure will facilitate spreading of heavy-body material on the impression tray and retain it in the tray. This phenomenon is called shear thinning (Chapter 3). Essentially, a stress-thinning material becomes less viscous when stressed as during injection and then recovers its viscosity when it rests on the tissue or or in the tray. All elastomeric impression materials exhibit shear-thinning characteristics before setting.

There are two categories of shear-thinning phenomena, pseudoplasticity and thixotropy, depending on how the material responds to the applied stress and how it behaves at rest. A pseudoplastic material displays decreasing viscosity with increasing shear stress, and recovers its vis/>