Tooth filling materials Dental amalgams & alternative materials

1. Introduction

The SCENIHR opinion states:

3. SCIENTIFIC RATIONALE

3.1. Introduction

This Opinion concerns one of the oldest unresolved controversies in medicine, that is whether there is a causal relationship between the use of mercury-containing amalgams for the restoration of teeth and the aetiology of a variety of diseases in individuals with amalgam restorations, in dental professionals and in the general population.

Dental amalgam has been used in various forms for the reconstruction of carious teeth for more than 150 years and became common, especially in the USA, in the latter part of the nineteenth century, its formulation and clinical use being rationalised by G V Black at the end of that century. The use of amalgam was almost wholly predicated on the fact that mercury is one of the very few metallic elements that is liquid at room temperature. As a consequence of this it is able to undergo an alloying reaction with other elements at ambient temperatures to form, in a clinically acceptable time, a customised mass that can be adapted to the size and shape of a tooth cavity, where it should be strong enough to resist the forces of occlusion for many years. At the time of the introduction of amalgam into dentistry, gold could be used in some types of dental restoration, but its cost prohibited widespread use. There were no other synthetic materials that had the combination of the required mechanical properties and ease of intra-oral manipulation. As a metallic alloy, amalgam did not have any aesthetic appeal, but the increasing prevalence of dental caries in the late eighteenth and early nineteenth centuries meant that this was a minor consideration. The even more profound increase in caries throughout the early and middle twentieth century, through the ubiquitous use of refined carbohydrates in foodstuffs, resulted in the increased use of dental amalgam fillings.

The essential metallurgical principles of dental amalgam, discussed in detail below in section 3.3, are fairly straightforward. Liquid mercury is able to react with many other metallic elements to produce a series of multi-phase alloys that are solid at room temperature. The key development was to find an element, or a combination of elements, that would allow the amalgamation reaction to occur in a short space of time, with a rapid rate of solidification and development of strength. Although several metallic elements were tried, it was soon realised that an alloy of silver and tin, essentially Ag3Sn, reacts with liquid mercury to produce a clinically acceptable alloy that would solidify in a few minutes and gradually harden over a few hours.

It had been recognised for a long time that certain forms of mercury and its compounds have toxicological characteristics, and the potential for neurotoxicity had already been discussed at the same time that amalgam was introduced into dentistry. Throughout the twentieth century and even more now at the beginning of the twenty-first century, the potential role of dental amalgam in the causation of disease has been a matter of considerable controversy. The focus has been on the mercury contained within the amalgam, and the potential for it to induce local intra-oral reactions to the amalgam restorations and to cause systemic or remote-site diseases associated with its systemic distribution and accumulation. Both governmental and non-governmental organisations have considered this possibility and many reports have been written on the subject. Many academic studies have been published, including some very recent epidemiological studies, which have attempted to prove conclusively, one way or the other, whether the mercury in amalgam has a causative role in disease, but until now, no clear unequivocal conclusion has been forthcoming. This is of immense importance since, during the last forty years, several types of alternative to amalgam for dental restorations have been developed such that the overall risk - benefit assessments for dental restorations in general have had to be changed. However, it is far from clear whether the use of such alternatives, involving, as they do, their own potentially toxic components, reduces the risk of disease associated with dental restorations.

This Opinion therefore takes into account currently available scientific and clinical evidence concerning mercury and other elements contained within dental amalgam, and also the components of the alternative materials. These alternatives include resin based composite materials, glass ionomer cements and a variety of hybrid structures. In addition, restorations made of gold-based and other alloys are possible alternatives to dental amalgam. These latter types of restoration are considered as custom-made devices in the context of the Medical Device Directive of the European Commission and are produced by indirect techniques in dental laboratories, which are clearly more time consuming and expensive. With each of the different types of alternative material, it is necessary to consider the chemistry and toxicology of all of the components, including monomers, acids, glasses and ions, taking into account the physico-chemical aspects of the setting process, the techniques for promoting adhesion to the tooth substance and the energies of any light sources used in photo-polymerisation. The clinical and epidemiological evidence has to be analysed in relation both to the patients themselves and to dental personnel, taking into account the phases of use, including placement of the filling, corrosion, degradation or wear in clinical service, and the release of materials during the removal of restorations. With respect to amalgam, it is also necessary to consider the exposure of the general population to mercury derived from the use of dental amalgam, placing this in the context of environmental exposure in general, and the contribution that amalgams make over their whole life cycle, including aspects of waste water treatment in dental offices and the release of mercury into the atmosphere in crematoria. With respect to alternatives to dental amalgam, it is also relevant to consider the life cycle of these materials, although very little data is available.

It is also important to examine the pattern of usage of amalgams and alternatives in dental clinics, since perceived benefits and risks, and the trends in these perceptions may change and this should be taken into account in making recommendations for future usage. For example, in some countries, general environmental considerations and public attitudes and expectations have contributed to a decline (sometimes a very substantial decline) in amalgam use and to a reduction in the use of mercury-containing products in general. Furthermore, dental schools are either reducing or have discontinued teaching the use of amalgams in view of the changing attitudes to restorative dentistry (Roeters et al. 2004). These trends must be placed in the context of the overall performance and longevity of amalgam and non-amalgam restorations, taking into account the size and location of the restorations. It is also important to recognise that the perception of patients may differ to some extent from the views of health care professionals.

3.2. Methodology

This Opinion of SCENIHR is concerned with the analysis of the evidence for the potential for either amalgam or alternatives to amalgam to have adverse effects on human health, from the perspectives of both scientific plausibility and clinical and epidemiological data, and to make observations about the future uses of these materials in dentistry.

The Working Group has considered evidence derived from a wide variety of sources, including peer-reviewed scientific and medical literature and published reports of institutional, professional, governmental and non-governmental organisations. In common with the usual practice of SCENIHR Working Groups, no reliance has been made on unpublished work or publicly available opinions that are not scientifically based. Due to the availability of extensive peer reviewed epidemiological and large scale clinical studies with respect to dental amalgam, it has not been necessary to rely on single case or anecdotal reports in establishing this Opinion. The Working Group has been careful, however, to review as much evidence as possible and, especially where the available data on alternatives is limited, attention has been given to some less rigorous studies where no other information was available. During the course of the deliberations of the Working Group, a Call for Information was issued by the Commission and the replies have all been considered.

In a major review of the evidence for or against causation of disease, it is necessary to take into account the generally accepted criteria for causation. The Working Group has therefore been mindful of such criteria, particularly the Bradford Hill Criteria of Causation (Bradford Hill 1965). The main features of these criteria are the paramount need to establish a temporal relationship between exposure and outcome, the strength of any effect or association determined statistically, the evidence of a dose-response relationship, the plausibility and specificity of any association, and the coherence of any putative association with existing knowledge. It will become obvious with respect to both dental amalgam and the alternative restorative materials, that some of these questions are difficult to analyse, for example because of the uncertainty over the exposure of individuals to mercury derived from amalgams compared to their exposure to mercury from other sources. Nevertheless, the Working Group is confident that it has been possible to take all factors into account in producing an Opinion that is consistent with these criteria for causation.

Source & ©: SCENIHR, The safety of dental amalgam and alternative dental restoration materials for patients and users (2008), 3. Scientific rationale, p.13