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After studying this chapter, the student will be able to do the following:
1. Describe the use of impression materials during indirect restorative procedures.
2. List the oral structures from which impressions are made.
3. Differentiate between a model, a cast, and a die.
4. Describe the various types of impression trays.
5. List the desirable qualities of an impression material.
6. Differentiate between the following types of impression materials:
- Elastic and inelastic
- Reversible and irreversible
7. Describe the composition and setting mechanism of:
- Wax and impression compound
- Zinc oxide–eugenol (ZOE)
- Agar or reversible hydrocolloid—optional
- Condensation silicones
- Addition silicones
8. Compare the properties, use, and cost of the above impression materials.
9. Describe the effect of water temperature on the setting rate of alginate.
10. Describe the effect of water and heat on the setting rate of polysulfides.
bite registration tray
zinc oxide–eugenol (ZOE)
Taking impressions may be one of the responsibilities of the dental hygienist in a busy practice. It is important to have an understanding of the various types of impression materials, their specific use, and their handling characteristics. Should you make athletic mouth guard or whitening trays, an impression will be a part of the appliance fabrication process. And it is not unlikely that you may be asked to assist with a “final impression” appointment should the dental assistant not be present that day. Taking impressions can be a welcome change from the typical daily routine of the hygienist.
I. Impression Materials
A. General Comments
Impression materials are used to make replicas (models or casts) of teeth and other oral tissues. In dentistry, we take impressions of teeth and their supporting structures. These supporting structures include gingiva, alveolar bone or residual ridge, hard and soft palate, and frenums, which are muscle attachments. The replicas are used to construct restorations and other appliances. The impression is a negative reproduction, whereas the replica (model or cast) is a positive reproduction, as illustrated in Figure 8.1. The impression must be an accurate duplication of the hard and soft tissues of interest and be stable enough to allow disinfection and production of an accurate model.
FIGURE 8.1. Photograph of A. a crown preparation, B. an impression, and C. a cast.
Not all impression materials are compatible with all model materials. Because impression materials are used for many purposes, a wide variety of products are available to make impressions of oral tissues. Some uses are simply to produce a physical model of the oral tissues for study, called a study model. Study models are used in diagnosis and treatment planning. Other uses require very exact (within 0.1%) replication of the size and shape of a preparation for the construction of a restoration or appliance. These replicas are called casts. A replica of a single tooth is called a die.
B. Impression Material Systems
Impression materials are supplied in a variety of forms. Some are powders that are mixed with water; others are paste–paste systems. Several materials are softened or melted by heating. Regardless of their form, impression materials are mixed (or heated) to make a thick paste or liquid. They are then loaded into an impression tray, placed in the mouth, and seated onto the tissues of interest. The tray functions as a carrier and can stabilize the set impression material. Several impression trays are shown in Figure 8.2. It is interesting to note that dental impression materials are also used to make impressions for medical prostheses (such as artificial eyes) and forensic investigations (bite marks).
FIGURE 8.2. Several impression trays (from upper left, moving clockwise): a metal perforated tray for partially edentulous impressions, a solid metal tray for edentulous impressions, a metal tray for dentulous impressions, a bite registration tray, a plastic tray for dentulous impressions, and a custom tray.
Paste–paste impression materials (and many other dental materials) come in tubes, much like toothpaste tubes. The orifice of each tube is sized to provide the proper ratio of the two pastes when equal lengths are dispensed. Thus, if the size of the two orifices is the same, one should dispense equal lengths. If the orifice of one tube is larger than that of the other tube, then again, one should dispense equal lengths. Examples are shown in Figure 8.3.
FIGURE 8.3. Polysulfide (lower) and polyether (upper) impression materials. Note that the lengths dispensed, but not the volumes, are equal.
The cost of impression materials varies greatly, from pennies per impression to as much as several dollars. An accurate cost analysis needs to include the percentage of first impressions that are acceptable and the number of restorations that need to be remade. Unacceptable restorations made on casts from cheap impression materials can make dentistry a nonprofit enterprise.
D. Impression Trays
1. Use of Impression Trays
Impression trays are used to carry the impression material into the mouth, and the handle of the tray is used to remove the impression. The tray can also support the material impression and improve accuracy. Trays are supplied in a variety of shapes and sizes and are made from several materials. Plastic disposable trays are very popular and work well with current infection control practices. Plastic trays are inexpensive and convenient, but they do not support the impression as well as metal trays do. Metal trays are more expensive but reusable. In addition, metal trays are stiffer and, thus, are less likely to distort when removed from the mouth. Cleaning and sterilizing metal trays adds to the cost of their use.
2. Stock Trays
Stock trays are “off-the-shelf” items that come in a variety of materials, shapes, and sizes. The different-shaped trays are designed to take impressions of different oral conditions: edentulous mouths, partially edentulous mouths, and mouths with a full complement of teeth. Such trays are illustrated in Figure 8.2. Stock trays also come in a range of sizes, from very small trays for pediatric patients to very large trays for large adult mouths. Stock trays can be metal or plastic.
3. Custom Trays
The most accurate impressions are made with custom trays. Custom trays are made on a model of the patient’s arch with acrylic or other resin. Two impressions are necessary when a custom tray is used. A preliminary impression is taken with a stock tray and an inexpensive material. A gypsum product is poured into the impression, and the resulting model is used to construct the custom tray. The custom tray is then used to take a final impression.
Custom trays are used to take final impressions for full dentures, inlays, crowns, bridges, and some removable partial dentures. Custom trays use less impression material than do stock trays because they fit around the teeth better than a stock tray does. Some say that custom trays are cost-effective because they use less material. Others say that custom trays are cost-effective because they make the impressions easier to take and accurate impressions result more frequently than with stock trays. Two techniques for the fabrication of custom trays for crown and bridge impressions are presented in detail in Chapter 29, Fabrication of a Custom Impression Trays. It is important to note that proper mixing and handling of the impression material are the factors most critical for success.
4. Special-Use Trays
A variety of special-use trays have been developed. Bite registration trays, as shown in Figure 8.2, record the occlusal surfaces of both arches. Bite registrations are used to relate the upper and lower casts in the dental laboratory in precisely the same manner as they come together in the patient’s mouth. The use of a bite registration without a tray is shown in Figure 8.4.
FIGURE 8.4. Models mounted on a simple articulator with a bite registration on the mandibular right quadrant made of impression material. (Courtesy of GC America, Inc., Alsip, IL.)
Another popular impression tray is the triple tray, which is used with the dual-arch or closed-mouth impression technique. The triple tray is a quadrant tray with a “J” shape that curls around the posterior of the most distal molars (Fig. 8.5A). With the dual-arch technique, the loaded tray (on both sides) is seated on the arch with the prepared tooth. The patient then closes the mouth to his or her normal bite, and the tray records an impression of the prepared tooth, an impression of the opposing teeth, and a bite registration, all at the same time (Fig. 8.5B–D).
FIGURE 8.5. A. Reusable metal and disposable plastic triple trays. B–D.Three views of a triple-tray impression. B. Impression of the opposing arch. C. Impression of the preparation. D. Backlit view showing areas with teeth in contact during impression.
E. Classification of Impression Materials
1. Chemical Reaction or Physical Change
Impression materials set either by a chemical reaction or by a physical change. Impression materials that set by chemical reactions to form elastic rubber materials are called thermoset. The chemical reaction involves chain lengthening, cross-linking, or both. Other impression materials set by a physical change when they cool, either by solidification or by gelation. Solidification occurs when molten wax cools and goes from a liquid state to a solid state. Gelation is the process by which gelatin, such as Jell-O, changes from a liquid state to a semisolid state when it cools. Impression materials that undergo a physical change when they cool are called thermoplastic. In general, thermoplastic materials are not as stable as are thermoset materials.
Because impression materials are used for a variety of purposes, the requirements of a particular procedure determine which impression materials can be used. Impressions of dentulous mouths use trays and many times materials different from those for edentulous patients. Teeth typically have undercuts (nooks and crannies) that require an elastic material to be used. Edentulous impressions may use elastic or inelastic impression materials. Inelastic impression materials set hard and rigid compared with elastic impression materials and would “lock” around teeth if used on a dentulous patient.
The accuracy required by the restoration or prosthesis determines which impression materials can be used. Not all impression materials have sufficient accuracy for crown and bridge impressions. Study models are not considered to be highly accurate reproductions of oral tissues. Therefore, alginate is an acceptable impression material for study models, but it is an unacceptable impression material for the fabrication of bridges, crowns, and inlays. Nearly all impression materials are acceptable for full-denture impressions.
c. Flow and Detail Reproduction
Many kinds of impression materials come in a variety of viscosities. Viscosity is a measure of a liquid’s ability to flow. There are (from thinnest to thickest consistency) light-body, medium-body, monophase, heavy-body, and putty materials. Light-body materials are typically used with an impression syringe and are injected around preparations. They are the most “runny” of the impression materials and best record the details of tooth preparations (small grooves, edges, and margins).
Many times, a retraction cord is placed in the gingival sulcus to facilitate crown and bridge impressions. In most instances, the patient will have had local anesthesia to ensure his or her comfort. The retraction cord pushes the gingiva away from the tooth, as shown in Figure 8.6. Frequently, the cord will contain a hemostatic or astringent medicament to control bleeding. After the cord has been in place for several minutes, it is usually removed, just before the impression is taken. There are exceptions, however, as the cord may be kept in place during the impression. The use of a retraction cord allows the low-viscosity material to flow to, and thus reproduce, areas of a preparation that are difficult to reach. The cord moves the gingival tissue slightly away from the tooth and also controls the moisture in the area.
FIGURE 8.6. Retraction cord being placed in the sulcus during crown preparation. (Courtesy of Ultradent, Inc., South Jordan, UT.)
Putty materials are the thickest impression materials, but they can still record the details of a fingerprint. Heavy-body and putty materials are placed in an impression tray, and their high viscosity prevents running and dripping of the impression material out of the tray and onto the operator or patient. They are typically used with a light-body material. The light-body material records the preparation and its margins, while the thicker material becomes the bulk of the impression. The impression tray, which is filled with the high-viscosity material, is immediately seated over the light-body material. The materials then set together into one mass and are removed as one impression.
Medium-body impression materials may be injected or used in the tray depending on the dentist’s preferences. Monophase materials are designed to be used for both injection and in the tray. Therefore, only one mix of monophase material is needed.
3. Types of Impression Materials
Each impression material has its own advantages and disadvantages for use in dentistry. The following sections will focus on each type of impression material and its use (Table 8.1).
TABLE 8.1. Classification and Use of Impression Materials
aA variety of impression techniques are used for partial denture construction. This table does not address all possible techniques.
bUse of a custom tray is recommended.
cEdentulous trays for hydrocolloid use are not common.
dThese materials could be used, but their use is not economical.
a. Inelastic Impression Materials
2. Wax and impression compound
3. Zinc oxide–eugenol (ZOE)
b. Aqueous Elastomeric Impression Materials
1. Alginate (irreversible hydrocolloid)
2. Agar (reversible hydrocolloid)
c. Nonaqueous Elastomeric Impression Materials
2. Condensation silicones
4. Addition silicones
Plaster would seem to be an unlikely impression material. However, impression plaster is rarely used but still sold. It has the same setting reaction and properties as the gypsum products that are used for models and casts. Gypsum products will be discussed in Chapter 9.
III. Wax and Impression Compound
Wax was probably the first impression material used in dentistry. It is cheap, clean, and easy to use. A multitude of waxes are used in dentistry. Some are hard, much like paraffin wax that is used in canning jellies and jams. Others are soft and moldable at room temperature, similar to Play-doh. Wax used for taking impressions is solid at mouth temperature but is moldable at a temperature that is tolerated by oral tissues. Wax comes in many forms (sticks, strips, tubes, etc.). Wax can be thought of as a low molecular weight polymer. It is a thermoplastic material. It is also very weak, and the technique must compensate for wax’s poor mechanical properties. Wax is used by some dentists to take impressions for full dentures. Wax is often used to extend tray borders or adapt a stock tray when taking impressions.
More commonly, a variety of waxes are used as adjunctive materials in the dental laboratory for the fabrication of crowns, bridges, and other restorations. Wax is softened or melted and then formed into the desired shape of the restoration. Then, the wax piece is surrounded by a mold material. Later, the wax is melted away and the mold is filled with a restorative material, such as gold. These processes are described in more detail in subsequent chapters.
B. Impression Compound
Impression compound is wax with added filler to improve handling and stability. It is stronger and more brittle and, when softened, flows much less compared with wax. Impression compound is supplied as sticks or cakes of material, as shown in Figure 8.7A. It is softened in a warm water bath, as shown in Figure 8.7B. Because the thermal conductivity of impression compound is low, time and patience are required to soften the material properly. The heated, softened, and moldable material is placed in an impression tray, resoftened, and then seated in the mouth. When the material has cooled to mouth temperature, it returns to a rigid state and is removed. Impression compound is a stiff, thermoplastic material and is used by many dentists to make preliminary impressions for complete dentures, as shown in Figure 8.7C. Other impression materials that record fine details are better suited for final impressions.
FIGURE 8.7. A. Impression compound sticks and cakes. B. Tray with impression compound in a water bath. C. Preliminary impression for a complete denture. (Courtesy of Dr. James Overberger, Morgantown, WV.)
IV. Zinc Oxide–Eugenol (ZOE)
Zinc oxide–eugenol (ZOE) has been formulated for a wide variety of uses in dentistry, including as an impression material. The chemistry of ZOE materials is covered in Chapter 7, Dental Cements.
A. Form of the Material
The ZOE materials come as two pastes. One paste contains eugenol and inert fillers; the other is formed by using zinc oxide powder mixed with vegetable oil. Eugenol is a major component in oil of cloves. Therefore, ZOE materials smell and taste like cloves. For some patients, this is unpleasant. The two pastes come in tubes, much like toothpaste, as shown in Figure 8.8. Equal lengths of material from each tube are dispensed. Typically, the two pastes of ZOE (and other materials that come in tubes) are of different colors. The pastes are swirled, stropped, and scraped together during mixing until one homogeneous color is obtained.
FIGURE 8.8. ZOE impression paste.
The ZOE impression materials set to a hard and brittle mass, which limits their use to impressions of edentulous ridges for removable dentures. They are inexpensive and easy to use and were once very popular. They are commonly used in a custom tray for the final impression for a complete denture. Today, however, ZOE materials have been replaced by newer materials, such as addition silicones. Although the newer materials have little advantage in terms of performance and are much more expensive, the simple advantage of stocking one less impression material in the dental office is likely the reason for the declining use of ZOE impression materials.
V. General Aspects of Hydrocolloid Impression Materials
Two impression materials are considered to be hydrocolloid materials because their major component is water. Both materials change from a viscous liquid state, called the sol, to a semisolid, rubbery state, called the gel. The sol state is a solution of one material dissolved in another. In the gel state, two phases exist, much like a foam of soap bubbles. The first phase is a solid carbohydrate polymer network, like the soap used to create a foam. The second phase is water trapped in very small pockets of the material, like the air trapped in the foam formed by soap. Colloids are discussed in Chapter 2, Materials Science and Dentistry.
Hydrocolloid materials that set via a chemical reaction are called irreversible hydrocolloids, or, more commonly, alginate. Heating alginate that was set results in warm alginate; it does not reverse back to the sol (fluid) state. Hydrocolloid materials that gel by a physical change (cooling) are called reversible hydrocolloids. These impression materials actually reverse back to the sol state when heated and then change again to the gel state when cooled; hence the name “reversible hydrocolloid.” Reversible hydrocolloid is also called agar or agar-agar (or sometimes simply hydrocolloid).
VI. Alginate (Irreversible Hydrocolloid) Impression Materials
Alginate materials are termed “irreversible impression materials” because they will not reverse to the sol state once they react and become a gel. They have advantages and disadvantages similar to those of reversible hydrocolloid materials because both types of materials are predominantly water. Alginate materials are not as accurate as reversible hydrocolloid materials, but they are much easier to use. Alginate materials are supplied as powders that are mixed with water as shown in Figure 8.9.
FIGURE 8.9. Alginate impression materials in a bulk container, premeasured envelope, scoop for bulk powder, water measure, mixing bowl, and spatula.