The overall benefits of dental amalgam to the U.S. population, either from a health or other perspectives, have never been formally evaluated before. Custom and pragmatism have reinforced amalgam's continuing central role in the practice of dentistry. However, we are at a critical juncture in both the history of the public's improving oral health and the broadening of clinical choices available to dentists to either prevent or treat dental caries. The confluence of these factors justifies an objective and comprehensive assessment of amalgam's beneficial role vis-a-vis other dental restorative materials.

At the same time, the general increase in public awareness and concern about the potential health risks associated with a wide variety of consumer products, medical procedures, pharmaceuticals, and the environment in general reinforces the rationale for a formal consideration of the benefit of continuing to use dental amalgam.

This chapter provides a discussion of the numerous factors that must be considered in assessing the benefits of amalgam and other dental restorative materials. Additionally, several quantitative models are presented that estimate the value of retaining dental amalgams already placed in the teeth of Americans or the value of using amalgam in restorations that may be required in the future. First, costs are estimated for the complete removal and replacement of all existing amalgams with alternative materials in the permanent posterior teeth for the entire U.S. population, schoolchildren only, and for a typical adult patient. Second, an estimate is presented of the additional national costs that would have been realized had alternative materials to dental amalgam been used to restore all teeth treated by U.S. dentists in 1990. Third, several models are presented that compare the relative costs of treating a single tooth with various restorative materials over a lifetime.

The number of studies documenting the longevity of dental restorations made of various materials is not extensive (Anusavice, 1989; Boyd, 1989; Maryniuk, 1989). Long-term prospective studies following the course of events in the life of individual teeth or restorations have been reported even less frequently. Nevertheless, the available longevity studies combined with the large number of practicing dentists in the U.S. (128,000) (Nash, 1991, personal communication) routinely treating patients affected by dental caries, and the availability of periodic epidemiological surveys of dental caries provide ample opportunity to understand the general picture of dental caries and the durability of dental restorations in clinical use.

Teeth are susceptible to dental caries from the moment they empty into the mouth. At the tooth surface level, bacteria responsible for dental caries are found in the dental plaque that adheres to tooth surfaces. They utilize food sources in the mouth and, in the process, produce acid byproducts. These acids have the ability to demineralize tooth enamel, dentin, and cementum. At the same time, certain salivary constituents, such as calcium and phosphate, have the ability to be deposited in these demineralized areas, particularly in the presence of fluoride ions, thus reversing the process and, perhaps, making the tooth even more resistant to caries than initially. This process, known as remineralization, may result in an arrested carious lesion that will not develop any further. The demineralization and remineralization process continues throughout life as long as teeth are present in the mouth. If the balance of this activity is shifted toward demineralization, frank caries occurs and removal of the caries and restoration of the tooth become necessary. Often, the initial caries is small, although, if left for months or years, it can become extensive, destroy significant tooth structure and eventually threaten the vitality of the blood supply and nerve of the tooth.

Typically, an initial restoration for posterior (back) teeth is made with dental amalgam and involves substantial cutting of tooth structure, even when the extent of dental caries is minimal. The cavity is designed to assure strength and retention of the restorative material, to eliminate caries-prone pits and fissures on the tooth surface, and to extend the cavity preparation into "self-cleansing areas" of the tooth. Thus, the classical cavity design with "extension for prevention" involves the removal of obviously carious tooth structure, possibly carious tooth structure, and some healthy tooth structure, often at the expense of long-term strength and functioning of the tooth (Sturdevant, 1968). In general, the more tooth structure that is removed, the weaker the remaining tooth structure becomes (Anusavice, 1989). Restored premolars are no more than one-half as strong as sound premolars (Reel and Mitchell, 1989).

Ironically, the concept of "self-cleansing areas" of the tooth was adopted intuitively. Research demonstrated, however, that normal oral functioning, including eating, does not produce self cleansing of the oral cavity or tooth surfaces Löe et al., 1965; Theilade et al., 1966; van Der Fehr et al., 1970). In fact, without regular oral hygiene, plaque that contains bacteria implicated in dental caries and gingival diseases accumulates on the tooth surface.

Over time, there are several possible outcomes for an original restoration: It may remain sound; it may develop recurrent caries; it may become defective due to the breakdown of marginal integrity (e.g., fracture or corrosion); the surface of the restoration may corrode or discolor, it may fracture and partially, or completely, fall out; or, the tooth structure around the restoration may fracture. The tooth/restoration margins are sites of potential carious attack, and certain types of restorations, particularly when located near the gingiva, increase the potential for plaque retention and gingival inflammation, especially when oral hygiene is not performed effectively (Waerhaug, 1956; Larato, 1972; Goldberg et al., 1981; Erickson et al., 1986; van Dijken and Sjostrom, 1991). Likewise, caries may develop on other surfaces of the tooth and require extension or replacement of an existing restoration. Such episodes of replacement could occur over short periods of time, such as 1 year, or over much longer periods of time. Throughout a person's lifetime, however, the continued replacement and extension of dental restorations and/or additional carious attack on a tooth can continue to compromise the integrity of the tooth until eventually root canal treatment becomes necessary and/or restoration by a full crown or extraction is required.

This single-tooth scenario could occur with any, or all, teeth in the mouth. It is, therefore, easy to see how complex the oral cavity is and why regular restorative care is needed for most individuals desiring to retain a functional set of teeth. Of course, this scenario does not apply to all individuals or all teeth. Some individuals are little affected by dental caries. But, historically, dental caries has affected most people to some degree, with many individuals having virtually all of their posterior teeth affected by caries. More recently, dental caries in school-aged children have been declining dramatically (Carlos and Wolfe, 1988; NIDR, 1989), and there is early evidence of a decline in younger adults as well (Brown and Swango, 1991). This trend could mean that the "typical life of a tooth" in the future will be improved over its life in the past.

Dental amalgam is seldom used to treat caries of the anterior (front) teeth, due to its nonesthetic characteristics. However, amalgams have been used on the lingual (tongue) side of anterior teeth, the distal sides of canine teeth, and, infrequently, near the gingival (gum) margins where esthetics have not been considered of prime importance. For the most part, restorations on anterior teeth during the past 25 to 30 years have been accomplish ad using plastic materials (e.g., unfilled resins and composites) of various physical and chemical composition which, although not offering the strength and durability of amalgam, do provide acceptable esthetics.

A visit today to virtually any dental office in the United States would provide evidence that dental amalgam is still a prominent material for restoring tooth structure in children and adults. Such a visit, however, might mislead an observer in terms of the extent of dental amalgam use compared to previous times. For example, as recently as 1971-73, the average 17-year-old in the United States had nearly 17 tooth surfaces (out of a total of 128) affected by dental caries (NIDR, 1981). Most of these surfaces were filled, and most of the fillings were made with dental amalgam. By 1979-80, slightly more than 11 surfaces were affected and by 1986-87 the number had declined to about 8 surfaces (NIDR, 1989). Thus, over a 16-year-period, the average 17-year-old experienced a 53 percent decline in dental caries. Similar declines in caries experience have occurred in virtually all age groups of schoolchildren.

The total number of amalgam restorations provided by U.S. dentists in 1979 is estimated at 157 million (Nash, 1991). By 1990, the total estimated was 96 million, a decline of 38 percent. The greatest declines have occurred in one- and two-surface restorations (i.e., those most commonly provided for children), while three- and four-surface restorations (more commonly encountered in adults) have declined to a lesser degree. The rate at which crowns are provided has also increased by 60 percent. Apparently, the overall change in restorative dentistry reflects the declining levels of dental caries in children, and the increased availability of full crowns (probably due to the increased number of teeth being retained by adults and an increase in dental insurance coverage during the 1980's). Both factors are consistent with the declining use of dental amalgam. Still, since today's adults have amalgams that were placed 10 to 70 years ago, when caries rates were much higher, they will need repair and/or re-restoration in future years because of recurrent decay or failed restorations. Up to 60 to 70 percent of the restorative dentistry performed on adults is to re-treat previously restored teeth (Matynink, 1989; Mjor, 1989). It will take decades for age-specific rates of restoration to decline markedly in older age groups.

The use of dental amalgam has permitted the preservation of teeth in both children and adults for decades. In the late 19th through mid-20th century, amalgam was the only material other than gold that could be employed to treat caries in the posterior teeth effectively. Direct filling gold is limited practically to situations of conservative cavity design. Cast metal inlays, onlays, three-quarter crowns, full metal and metal ceramic crowns, and ceramic crowns and inlays generally can be used in situations where amalgam has been placed. Direct filling gold, cast metal, and ceramics, however, are not appropriate for primary teeth or for permanent teeth in children up to about 18 years of age. The amount of tooth structure removed and potential trauma to pulpal tissues contraindicates such uses in children. Cost, alone, prohibits widespread use of these materials both because of the amount of dentist's time involved and the cost of the materials. Thus, until the 1950's, for most persons, extraction was the only feasible alternative therapy to amalgam restorations for posterior teeth.

In the 1950's, early formulations of plastic restorations were introduced. These proved to be of poor clinical value even when used in minimal-stress-bearing situations in anterior teeth (Phillips, 1981). Clearly, they were not suitable in most situations where amalgam was employed. Likewise, silicate cements were too fragile and soluble to be used in such situations and have become obsolete.

The emergence of improved composite materials in the 1970's has made it possible to reasonably consider substituting other materials for amalgam in some posterior restorations (Leinfelder, 1991). As previously discussed, several composite materials are available that have been "accepted" or "provisionally accepted" by the American Dental Association for use in nonstress-bearing areas of posterior teeth (ADA, 1986a, 1986b, 1990). In instances of incipient (small) carious lesions, the use of composite is desirable as it has the advantage of conserving tooth structure. As discussed earlier, however, composite restorations are considerably more technique-sensitive for the dentist, have a lesser longevity than amalgam, and also require additional expense to the patient.

Although glass ionomer cements can be used as direct filling materials for some cavities that traditionally have been filled with dental amalgam, they are technique-sensitive and should not be used in stress-bearing areas, because they are subject to abrasion and fracture.

If one were to consider removing all existing amalgam restorations in the U.S. population, and replacing them with alternative materials such as cast metal, ceramic, and composites, whether for esthetic, functional, or other reasons, the overall cost from such replacement would involve several different direct and indirect costs. Most direct treatment costs would be reflected in the dentist's fee to place the new restorations. This cost would depend on the number of teeth restored, the size of the restorations, and the types of materials used. Direct treatment costs may also include the cost of root canal therapy which is often necessary as a result of having large existing amalgams replaced. Indirect costs include costs of transportation, time lost from work, lost wages, child care expenses, time lost from school, residual serviceable restoration years sacrificed as a result of removing clinically sound amalgams, tooth structure lost as a result of restoration, inconvenience, pain or discomfort, mental anguish for those who fear dental treatment, and increased costs associated with more frequent replacement of subsequent larger restorations. Some of these items are not translated easily into cost units.

Any significant increase in the demand for dental services that would be stimulated by broad-scale replacement of restorations also would create demand for dental services that could not be met by the current supply of dental personnel. This demand would drive up the fees for individual dental services and the costs of dental materials (possibly by a considerable percentage) and dental insurance premiums likely would rise. Many individuals and sponsors of health benefit plans might not choose to retain dental insurance under these circumstances. With the increase in time and dental personnel spent on removing existing amalgam restorations, basic diagnostic and preventive dental services could become less available to the population. The effects would be extremely significant for public programs that are already constrained by resource limitations. The increased costs per treated patient, in the absence of additional resources, would force a reduction in the number of patients served and place increasing strain on an already tenuous system. This could lead administrators of public programs to decide that the delivery of reparative dental services is beyond their means.

Broad societal costs, the quantification of which is beyond the scope of this report, would also be incurred. These include the costs of environmental management of amalgams that are removed and patient/dentist litigation. If millions of older individuals and others who are medically compromised or frail were to subject themselves to the physical stresses of extensive dental treatment, one could also anticipate some deaths and injuries from having their dental amalgams replaced.

A model has been developed for estimating the direct costs of replacing existing dental amalgam restorations with restorations made of alternative appropriate materials. This model utilizes recent epidemiological data and current dental fees (NIDR, unpublished data, 1991a and l991b; ADA, unpublished preliminary data, 1990). Using this model, the total estimated direct costs for one-time replacement of all existing amalgam restorations in permanent posterior teeth of the U.S. population ages 5 and above $248 billion. This cost does not include the costs of orthodontic or periodontic care that might be necessary for some full-coverage restorations.

These direct costs of replacing all existing dental amalgams are problematic from a practical standpoint. One can anticipate that not all individuals will seek replacement insofar as many individuals do not seek regular dental care. Moreover, even if only a small percentage sought to replace their amalgams, adequate numbers of dentists and auxiliaries are not likely to be available to provide the care. Further, small variations in estimates of costs of individual dental restorative services would have little practical impact on the "bottom line," since the factor that most drives the overall cost is the extent of existing dental amalgam restorations in the population, which is estimated to be more than $1 billion.

Because these costs are so difficult to comprehend, one could consider only the replacement costs for a single patient. Using the same basic approach as above, the replacement of existing amalgams with appropriate, alternative materials for an average 40- to 45 year-old person would cost $1,580— a significant cost for most individuals and a cost that would be prohibitive for many families and programs.

Other models, more conservative in scope, would result in smaller overall costs. For example, if only the amalgams in the permanent teeth of schoolchildren were replaced, the estimated overall costs would be $3.1 billion. This figure is much lower than the overall costs for the total population not only because this group represents a small overall population, but also because proportion of the schoolchildren generally have fewer teeth that have been treated with amalgams and generally more expensive restorative alternatives, such as cast metal restorations, would not be used.

Another model relates to the cost of using alternative materials to dental amalgam for new, or replacement, work that is required periodically, such as during annual dental checkups. This model is driven only by those who seek care and by the increase in new dental caries or newly failed restorations. There are no available data on the annual incidence of dental caries in the overall population or on the rate of failed restorations. Still, it has been estimated that about 96 million dental amalgams are placed each year (Nash, 1991). This number is obviously a much smaller number than the estimated 1 billion or more existing amalgam restorations. The increased cost that would have been experienced in 1990 as a result of using alternative restorative materials for the 96 million dental amalgams placed would have been approximately $12.4 billion.

The increase in costs of using an alternative restorative material to dental amalgam on a one-time basis does not represent the total costs of using alternatives. Because no restoration is permanent, the consequences of making the initial material choice over the functional lifetime of a restored tooth becomes relevant. Restorations made with various alternative materials have different, expected serviceable lifetimes and markedly different costs on average, and these need to be figured into any analysis of cost effectiveness. The outcome measure in such an analysis is a tooth restored and likely re-restored on multiple occasions to function over a specified period of years.

Yet, anticipated, clinical life expectancies of different restorative materials are difficult to establish and published figures vary widely (Christensen, 1971; Wilson, McLean, 1988; Anusavice, 1989; Boyd, 1989; Glantz, 1989; Maryniuk, 1989; Moffa, 1989; Mjör, 1989, 1990; Mjör et al., 1990; Qvist J et al., 1990; Qvist V et al., 1990). The longevity of restorations has been reported based on the average age of a restoration at failure, the percentage of restorations lasting for a specified period of time, the median longevity of restorations, etc. This variation makes it difficult to establish with confidence specific figures for the anticipated longevity of a restoration. Additionally, some studies were conducted in well-controlled environments in dental schools with careful case selection and long-term patient follow-up and management (Bayne, 1991). Others were conducted among dental specialists or among the general pool of dental practitioners. Size of the restorations placed and stresses placed on them are not always considered. Also, some materials have improved relatively more than others in recent years (e.g., resin based composite materials more than amalgam and gold alloys) and some are still being improved (e.g., glass ionomes and composites).

Further, the criteria for restoration failure that have been used are subjective, imprecise, defined poorly, and interpreted differently by dentists (Boyd and Richardson, 1985; Maryniuk, Kaplan, 1986; Anusavice, 1989; Elderton, 1989; Leinfelder, 1989; Leitzel et al., 1989; Mjör, 1989; Soderholm et al., 1989). Factors that affect the cost of restorative dentistry and, therefore, the fairness of any data on longevity in the long-term, include the type and location of a dental practice; differences in oral hygiene, diet, and biting forces of patients; and the mode of payment for treatment, such as dental insurance or individual payment. Also, because of the long-standing and widespread practice of replacing amalgam restorations that exhibit "ditching" of the margins, whether or not caries is detectable, the reported longevity of amalgam restorations may be less than could be achieved if replacement criteria were applied more critically. Estimates of the longevity of restorations in the future will probably be longer than those of the past, because of the availability of recently developed dental materials and more conservative restorative techniques.

To model the costs associated with restoring a tooth with various restorative materials, values were selected that represent median values reported in published literature, giving less credence to the extremes reported on both sides. Projections are also made on how enhanced longevity figures for various restorative materials alter overall cost estimates.

With these reservations In mind, the anticipated longevity periods shown in Table 1 (based on past studies) are used in predicting the cost-effectiveness of different restorative procedures in permanent teeth in general practice.

Data on the longevity of restorations in deciduous teeth are too limited to make definitive conclusions, but reported longevity is much less than for permanent teeth (Qvist J et al., 1990; Qvist V et al., 1990).

* For example, class 1, 111, or V

** For example, mesial-occlusal-distal surfaces in a molar

*** Reflects longevity in specialty practices; these procedures are not widely employed in general practice.

Estimates of the costs of various restorative treatment scenarios are open to criticism, not only because they are based on "soft" data, but also because the types of treatment options used in the calculations are selected subjectively and will vary in clinical decision making. Several different models are used below to describe the long-term cost implications of providing restorative care over a 60 year period to an initially carious tooth comparative fees derived from the ADA Survey of Dental Practice, 1990 (unpublished, preliminary data), are used. The relative costs of different scenarios for restoring posterior teeth involve comparative one-time costs (i.e., costs at the time of inserting a restoration) and cumulative, relative costs of selecting a particular clinical course over a 60-year period.

Figure 1 illustrates the costs over a 60-year period by extrapolating the values for amalgam, composite, and gold castings based on the longevity data given above.

Based on this extrapolation of costs, an approximate 2.5-fold increase in cumulative costs would be incurred by selecting composite instead of amalgam as a posterior restorative material. The cumulative costs of cast gold restorations would be about 50 percent greater than composite after 60 years.

Figure 2 is a revised model of anticipated costs that incorporates greater average longevity for dental amalgams (15 years) and composites (10 years) and, thus, fewer required replacements over a lifetime. Although adequate data do not yet exist for substantiating these expected restoration longevities for the future, early reports suggest that such longevities are achievable with improved dental restorative materials, application of more objective replacement criteria, and regular and consistent dental care. The impact of these changes on the model are substantial and suggest that the cumulative cost differential between amalgam and composite would be reduced to about 2:1.

If the relative fees shown in Figure 2 are used in a somewhat more clinically relevant model (Figure 3), the apparent price difference between initial amalgam and composite restorations would be increased beyond that depicted in Figure 2, because a comparatively expensive endodontic procedure will become necessary after three composite restoration replacements. Endodontics is included because of the potential clinical need to resolve expected pulp reactions and/or provide retention for restorations. With amalgam, an endodontic procedure would not be needed theoretically until age 75, the end point for the model. If the endodontic procedure and cast restoration were provided at this point, the relative cumulative costs between the composite and amalgam scenarios would be slightly less than 2:1. However, the endodontic procedure may not be accepted by the patient in this situation.

Figure 3 reflects the "countdown" theory suggested by Lutz et al. (1987) and reinforced by Simonsen (1991). This perspective proposes that teeth that are restored initially will undergo additional restoration over their lifetimes, involving more and more loss of tooth structure—and, eventually, possible root canal therapy and restoration by a crown.

Undoubtedly, much tooth loss will be caused by repeated restoration failures, which may result, in some cases, from a patient's inability to cover the cost of extensive and repeated treatment. In fact, cross-sectional data indicate that regular dental care users aged 35 to 44 lose an average of 4.5 teeth and irregular dental care users lose an average of 8.7 teeth during these 10 years (Kroeze, 1989).

The "countdown" scenario, however, reflects the past and may not hold true for the future. That is, individuals who were regular users of dental services throughout their lives, including the period of high caries activity in this country, and who may not have grown up in fluoridated areas could experience a "countdown" on one, or many, of their teeth. If, however, dental caries rates continue to decline, dental materials continue to improve, and dentists continue to modify practices to conserve more tooth structure, the "countdown" scenario could be altered markedly in the current and future generations of children and young adults.

Conclusions

Over the past century, the use of dental amalgam has provided substantial oral health benefits to the U.S. population. Indeed, the availability of amalgam during this period is perhaps the primary factor in the restoration to health and subsequent long-term retention of hundreds of millions of decayed teeth. Alternative materials are available and are being used increasingly in many situations where amalgam typically has been the material of choice. The combined effects of declining dental caries and use of alternative dental restorative materials have resulted in a dramatic 38 percent decline in the annual placement of dental amalgams by U.S. dentists between 1979 and 1990. There is reason to believe that this overall trend will continue.

All of the alternative materials to dental amalgam are more expensive than amalgam on a one-time basis as well as over the lifetime of an individual, and the general use of these materials instead of amalgam will result in markedly higher treatment costs.

The use of alternative restorative materials rather than dental amalgam to restore teeth in those seeking care would increase the annual national expenditures for dental services by more than $12 billion. The one-time direct costs for replacing all existing dental amalgams in the U.S. population would be enormous and impractical. Additional indirect costs would be substantial.