History

Amalgam was introduced to the United States in the 1830s. Initially, amalgam restorations were made by dentists filing silver coins and mixing the filings with mercury, creating a putty-like mass that was placed into the defective tooth. As knowledge increased and research intensified, major advancements in the formulation and use of amalgam occurred. Concerns about mercury toxicity in the use of amalgam were, however, expressed in many countries; concerns reached major proportions in the early 1990s. The American Dental Association (ADA) and the U.S. Public Health Service have issued many statements expressing their support for the use and safety of amalgam as a restorative material.1,4

Current Status

Today, the popularity of amalgam as a direct restorative material has decreased.5,6 This decline is attributed, in part, to the reduction in caries rates and to esthetic concerns. However, the primary cause of the reduction in the use of amalgam is the recognition of the benefits and improved esthetics of composite as a restorative material. Thus, concerns about the use of amalgam restorations relate primarily to poor esthetics and the greater potential for the weakening of the tooth structure: Dental amalgams inherently require greater removal of tooth structure to accommodate its strength requirements.

Because of environmental concerns about mercury contamination, the use of amalgam as a restorative material already has decreased in many countries. Legislation restricting and, in some cases, phasing out the use of amalgam has been implemented in Japan, Denmark, Canada, Sweden, and Germany.

Even with the concern about the disposal of mercury, this textbook advocates the continued use of amalgam as a direct restorative material, especially in light of the finding that bonded composite resin restorations have an increased risk of development of secondary caries.7,8 Research repeatedly has shown the safety of amalgam and the success of restorations made from amalgam. Although the scope of the clinical uses of amalgam presented in this book are narrower than in the past, amalgam still is recognized as an excellent material for restoring many defects in teeth.

Low-copper amalgams were prominent before the early 1960s. When the setting reaction occurred, the material was subject to corrosion because a tin–mercury phase (gamma-two) formed. This corrosion led to the rapid breakdown of amalgam restorations. Subsequent research for improving amalgam led to the development of high-copper amalgam materials. Low-copper amalgams are used very seldom in the United States.

Important Properties

The linear coefficient of the thermal expansion of amalgam is 2.5 times greater than that of tooth structure, but it is closer than the linear coefficient of thermal expansion of composite.17–19 Although the compressive strength of high-copper amalgam is similar to tooth structure, the tensile strength is lower, making amalgam restorations prone to fracture.^8,20,21 Usually, high-copper amalgam fracture is a bulk fracture, not a marginal fracture. All amalgams are brittle and have low edge strength. The amalgam material must have sufficient bulk (usually 1 to 2 mm, depending on the position within the tooth) and a 90-degree or greater marginal configuration.

Creep and flow relate to the deformation of a material under load over time. High-copper amalgams exhibit no clinically relevant creep or flow.22,23 Because amalgam is metallic in structure, it also is a good thermal conductor. An amalgam restoration should not be placed close to the pulpal tissues of the tooth without the use of a liner or base (or both) between the pulp and the amalgam.

Amalgam Restorations

Amalgam functions as a direct restorative material because of its easy insertion into a tooth preparation and, when hardened, its ability to restore the tooth to proper form and function. The tooth preparation form not only must remove the fault in the tooth and remove weakened tooth structure, but it must also be formed to allow the amalgam material to function properly. The required tooth preparation form must allow the amalgam to (1) possess a uniform specified minimum thickness for strength, (2) produce a 90-degree amalgam angle (butt-joint form) at the margin, and (3) be mechanically retained in the tooth (Fig. 13-2). Without this preparation form, the amalgam possibly could be dislodged or could fracture. After desensitizing the prepared tooth structure, mixing, inserting, carving, and finishing the amalgam are relatively fast and easy (Fig. 13-3, A). For these reasons, it is a user-friendly material that is less technique sensitive or operator sensitive compared with composite.

Some practitioners have continued to use bonded amalgam restorations in their practice (see Fig. 13-3, B). As noted previously, this book no longer promotes the use of bonded amalgams.^24–27 The mechanism of bonding an amalgam restoration is similar to that for bonding a composite restoration in some aspects, but it is different in others. A bonded amalgam restoration, done properly, may seal the prepared tooth structure and may strengthen the remaining unprepared tooth structure. The retention gained by bonding, however, is minimal; consequently, bonded amalgam restorations still require the same tooth preparation retention form as do nonbonded amalgam restorations.^28,29 Isolation requirements for a bonded amalgam restoration are the same as for a composite restoration.

Another amalgam technique uses light-cured adhesive to seal the dentin under the amalgam material (see Fig. 13-3, C). This procedure, as is true of all procedures that use adhesive technology, requires proper isolation. The prepared tooth structure is etched, primed, and sealed with adhesive. The adhesive is polymerized before insertion of the amalgam. (Usually, a one-bottle sealer material that combines the primer and the adhesive is used.) This technique seals the dentinal tubules effectively.^30,31

Uses

Because of its strength and ease of use, amalgam provides an excellent means for restoring large defects in non-esthetic areas.³² A review of almost 3500 4-surface and 5-surface amalgams revealed successful outcomes at 5 years for 72% of the four-surface and 65% of the five-surface amalgams. This result compared favorably with the 5-year success rates for gold and porcelain crowns, which were 84% and 85%, respectively.³³ Generally, amalgams can be used for the following clinical procedures:

1. Class I, II, and V restorations (Fig. 13-4)

2. Foundations (Fig. 13-5)

3. Caries-control restorations (see Chapter 2)

Handling

Because of the concern about mercury, amalgam restorations require meticulous handling to avoid unnecessary mercury exposure to the environment, the office, the personnel, or the patient. Proper mercury hygiene procedures are described in Online Chapter 1 and in the subsequent chapters on amalgam restorations.

Amalgam has greater wear resistance than does composite.2,34 It may be indicated in clinical situations that have heavy occlusal functioning. It also may be more appropriate when a restoration restores all of the occlusal contacts of a tooth.

Contraindications

Amalgams are contraindicated in patients who are allergic to the alloy components. The use of amalgam in more prominent esthetic areas of the mouth is usually avoided. These areas include anterior teeth, premolars, and, in some patients, molars. Occasionally, a Class III amalgam restoration may be done if isolation problems exist. Likewise, in rare clinical situations, Class V amalgam restorations may be indicated in anterior areas where esthetics is not an important consideration. Amalgam should not be used when composite resin would offer better conservation of the tooth structure and equal clinical performance.

Advantages

Some of the advantages of amalgam restorations already have been stated, but the following list presents the primary reasons for the successful use of amalgam restorations for many years:

Disadvantages

The primary disadvantages of amalgam restorations relate to esthetics and increased tooth structure removal during tooth preparation. The following is a list of these and other disadvantages of amalgam restorations:

Local Anesthesia

Because most amalgam tooth preparations are relatively more extensive, local anesthesia usually is necessary. Profound anesthesia contributes to a comfortable and uninterrupted operation and usually results in a marked reduction in salivation.

Isolation of the Operating Site

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