chapter 13 Impression Materials, Concepts, and Techniques for Dental Implants
The art and science of impression making is an ancient endeavor, with the earliest dentures carved from ivory by hand. The fit was essentially determined by sight and palpation, with some results considered works of art. However, historical accounts described the discomfort and suffering that the average patient had to endure. Later, waxes, plaster, and other compounds were used with varying degrees of success.
With the development of new instrumentations and new techniques, the availability of better quality impression materials became imperative. For example, although it was possible to produce extremely fine detail as well as maintain good dimensional stability with plaster, the fact that it was inelastic limited its use in impression making. Thus, the goal of researchers and clinicians was to develop materials that had elastic properties, were easy to handle, and still retained good accuracy and dimensional stability.1–10
Today’s advancements in the science of dental materials and the development of modern impression materials are generally considered to have had their origins in the mid-1920s, coinciding with the inception of the American Dental Association (ADA) Council on Dental Materials and Devices. Over time, the ADA directly and indirectly improved the quality of dental technology and materials, which had previously been left in the hands of any individual or company who had the means of developing and marketing such to highly receptive dental professionals, whose field was experiencing significant growth, both scientifically and intellectually.
Today’s dentist has a number of different choices of impression materials, including agar-agar, alginates, polysulfides, silicones, and polyethers. Each has its advantages and disadvantages. The correct choice is dictated by the particular prosthodontic case and depends on a number of different factors, such as accuracy (immediate and time-dependent), dimensional stability, handling properties, hardness, elasticity, working time, esthetic appearance, odor, shelf life, cost, and acceptance by the dentist and patient (Figure 13-1).
FIGURE 13-1 ▪ The correct choice of an impression material is dictated by the particular situation and depends on a number of different factors.
(Courtesy of Dentsply.)
Although impression materials can be classified by a number of different criteria, such as by their generic chemical name, a more general method involves consideration of the properties of the materials before or after setting.
Before setting, the property most often used to characterize impression materials is viscosity. The viscosity of the material can affect the time that the detail can be recorded in hard tissue impressions and can influence the extent to which tissue compression or displacement can be achieved in impressions of soft tissue. Viscosity often varies, however, with the applied stress. Material that appears viscous under low stress conditions may become more fluid during the recording of the impression, when it is placed under greater stress. When a substance reacts in this manner, the reaction is often due to the spacing of the impression tray. A relatively fluid impression material confined in a close-fitting tray will be compressed to a greater degree than the same material in a loose-fitting tray. Thus, classifying impression materials by their viscosity is more difficult than it may seem at first.
A more widely used classification system uses the properties associated with impression materials after setting. Of these, the two most significant are elasticity and rigidity. Impression materials that are elastic possess the ability to change their shape in direct response to a force (such as compression) and to recover their original form to a measurable degree after removal of the force. Nonelastic impression materials may clearly be plastic, or they may be very rigid but show little evidence of plastic deformation (e.g., waxes, impression plasters). The degree of elasticity and rigidity is important because it determines whether the material can be used to record undercuts. When elastic impression materials are removed from the undercut areas, they often are put under significant tensile stress. If a patient has deep soft tissue undercuts, the set impression material must be flexible enough to pass the undercut and must have enough elasticity to allow for proper recovery and to provide an accurate impression.
Accuracy: To record the fine detail of hard or soft tissue, the impression material should be fluid when inserted into the patient’s mouth. This requires the material to have a low viscosity and a degree of pseudoelasticity. The manner in which the material interacts with moisture and saliva also affects fine detail reproduction. Some impression materials are compatible with moisture and saliva and require no special precautions. Others, however, are hydrophobic and may be repelled by moisture and saliva in a critical area of the impression, causing formation of a “blow-hole” in the material.
Dimensional changes that generally occur during the setting of impression material (which sometimes involves a chemical reaction or simply a physical change of state) can also affect accuracy. These dimensional changes can involve contraction or expansion. Materials that contract during setting are firmly attached to the impression tray, resulting in expansion of the impression space and oversized dies or casts, whereas materials that expand during setting produce undersized dies or casts. How this affects the fit of the resultant restoration depends on the type of restoration being made. In the case of implants, excessive space results in loose fitting of the impression plate and subsequent incorrect construction of the framework.
Thermal contraction occurs because of the difference in temperature between the patient’s mouth (32°C to 37°C) and the operating area (23°C), and is a result of the value of the coefficient of thermal expansion of the impression material and the tray to which it is attached. Thus, it is crucial that the impression material stays attached to the tray while the impression is being recorded. Impression trays are often provided with adhesives to enhance bonding.
Dimensional Stability: Dimensional stability, defined as the change in accuracy with respect to time, is an important factor when a particular impression material is chosen. For most impression materials, accuracy is best maintained by pouring the cast as soon as possible after recording the impression. However, when it is not convenient for the dentist to cast the model, the impression has to be sent to a dental laboratory. If the laboratory is not on the same premises or is a long distance from the dentist’s office, the delay between recording the impression and fabricating the cast can be several hours or even days (Figure 13-2).
The “ideal” impression material would have perfect dimensional stability, such that the impression would keep its original accuracy indefinitely. Various factors can contribute to dimensional changes during storage or transformation of impressions. When elastomers are bound with adhesive to a custom tray, the dimensional stability of the impression materials is significantly improved. Continuation of the setting reaction past the expected setting time can result in dimensional changes. When viscoelastic materials exhibit continued elastic recovery for some time after the impression has been taken, the dimensional changes result in a more accurate impression. However, many impression materials contain volatile substances, which may be primary components or by-products of the setting reaction. When these volatile materials are lost during storage, the impression material shrinks, with a resultant decrease in accuracy.
Manipulative Variables: Impression materials can be dispensed in a number of different ways. Some do not require any mixing, whereas others involve the mixing of powder and water, paste and liquid, or paste and paste. When materials need to be mixed, the two-paste system (usually supplied in toothpaste-like tubes) makes proportioning easier. One simply squeezes out equal lengths of paste from each tube onto a paper mixing pad or a glass plate. In contrast, the point at which mixing has been satisfactorily completed with the paste/liquid or powder/water systems is not as clearly defined.
Impression materials that soften when warmed and set when cooled are sometimes difficult to control. The setting characteristics are completely out of the control of the operator because they depend on the temperature at which the material is heated and the time at which it is maintained at that temperature before the impression is recorded. For impression materials that set through a chemical reaction, the working time for the material extends from the beginning of mixing until the material is no longer suitable to record the impression. This time period is usually defined as how long it takes for the materials’ viscosity to increase by a given amount above that of the freshly mixed material.
Whereas the working time of the impression material is determined at room temperature, the setting time is usually determined at mouth temperature. The setting time of an impression material can be defined as the time required to complete the setting reaction. For the convenience and comfort of both the operator and the patient, the ideal handling properties of an impression material are, naturally, short working and setting times. This can be realized with materials that set through chemical reactions, provided that the reaction rate is much faster at mouth temperature than at room temperature.
Other Variables: Impression materials should be nontoxic, nonirritating, clean, easy to use, and esthetic, and should have an acceptable odor and taste. It also should be possible to decontaminate the material to make it safe for further handling.
Before the development and availability of complex polymer impression materials, nonelastomeric impression materials, such as gypsum, impression compounds, and zinc oxide/eugenol-based impression materials, were used by the dentist. Although these materials enabled the dentist to reproduce oral structures with greater accuracy and detail, they have their limitations.
Gypsum: The setting expansion, setting time, and consistency of gypsum are well controlled. The material assumes an early set, is easily read, and, in general, makes a good mucostatic impression that does not disturb loose or fibrous oral tissue. In addition, it is relatively esthetic in appearance and has a satisfactory taste for the patient. However, as an impression material, gypsum has many characteristics that make it undesirable and limit its use, such as difficulty separating it from a working cast. Thus, when it is used in the preparation of fixed or removable partial dentures, it is used mainly as an indexing material that can be mixed with starch to provide an index that dissolves in hot water.
Impression Compound: Impression compound, composed mostly of natural products (e.g., resin, copal resins, carnauba wax, stearic acid), is usually limited to primary impressions for edentulous patients. Although the impression compound can sometimes be removed from the undercuts, it usually distorts when removed. In addition, the material can be unesthetic and may have a disagreeable taste. Impression compound can be used in individualized copper bands for single-tooth impressions; however, when used in this manner, it demonstrates undesirable temperature-related expansion and contraction, as well as deleterious flow and distortion after hardening. Thus, similar to gypsum, impression compound has limited use in fixed partial dentures.
Zinc Oxide/Eugenol Pastes: Zinc oxide/eugenol (ZOE) pastes, developed for making impressions of the tissues of edentulous patients, are good as a secondary or wash impression. Like plaster, they are capable of producing fine detail and maintaining excellent dimensional stability, and they have a reasonable setting time. Zinc oxide/eugenol paste is usually used for a final impression in full denture construction. The material can also be used as corrective lining in preparatory impressions, as a liner for full denture bases to create a tissue-based contact, and as an interocclusal recorder in conjunction with a bite registration tray. However, because ZOE is brittle, it cannot be used where undercuts are present. It also has an unesthetic appearance, has a bad odor and taste, and, in some patients, causes tissue reactions due to sensitivity.
Two types of hydrocolloids (a colloid with water as a dispersion medium) can be used to make direct impressions. One is the agar-agar type of material, which is reversible from liquid to solid and from solid to liquid, and the other is alginate, which is irreversible.
Agar-Agar: Agar-agar, a complex polysaccharide extracted from seaweed, was the first elastic impression material that could be removed from undercut areas without fracturing. Despite some drawbacks, the material was considered to represent an important development in removable prosthodontics. Two initial limitations—a retarding effect on gypsum products that resulted in casts with chalky or soft surfaces, and rapid shrinkage of the impression after removal from the patient’s mouth—have essentially been overcome, the first by adding strong gypsum-product accelerators, and the second by promptly filling the impression with the cast material.
The steps involved in making an impression with agar require a team effort and should not be attempted by a single individual. The materials are usually supplied as a gel in a flexible, toothpaste-like tube or syringe. The gel is composed primarily of a 15% colloidal suspension of agar-agar in water, with small quantities of borax and potassium sulfate present. Agar-agar sets at a temperature at or greater than mouth temperature. Three baths (i.e., boiling, storage, and tempering) are needed to prepare the material. The impression material first is liquefied in boiling water, in a syringe or a tube, and then is stored in a chamber at 145°F to 150°F until it is used. Bef/>