Alternatives to Dental Amalgam: What Do We Know About Their Safety?
Ideally, all dental restorative materials
should be harmless to the pulp and soft tissues, should contain no toxic substances that
can reach the circulatory system, should be free of sensitizing agents that could cause an
allergic reaction, and should have no carcinogenic potential.
Mercury amalgam, the most commonly available restorative
material, has been in use for more than 150 years, and its safety is well documented.
There is no direct evidence of adverse health effects from the mercury in dental amalgam,
and the prevalence of allergic reactions in the general population is extremely low (see
Oral Care Report Vol. 9 No. 2).
Alternatives to Mercury Amalgam
The most commonly used alternatives to mercury amalgam are
composite resins and glass ionomers. Composite resins were introduced in the late 1960's
and were initially intended as an esthetic restorative material for anterior teeth. Glass
ionomers are fluoride-containing materials that have been on the market for about 20
years. Data comparing the safety of composites with mercury amalgam are beginning to
emerge. Local adverse reactions are either toxic or allergic, while systemic effects are
usually an allergic reaction to substances leached from the resin. Patient reactions to
resins and glass ionomers, however, are usually not severe.
Composition of Composite Resins and Glass Ionomers
Composite resins are made up of 30-70% methacrylate and/or
polyurethane. They also contain amines, pigments and filler particles. The filler
particles appear opaque on radiographs and are usually made of finely ground mineral or
glass. In the past, composites tended to be soft compared to mercury amalgam, and were
used mostly for restoring anterior teeth. Newer composites, however, are becoming more
suitable for molars and larger cavities because their mechanical properties and resistance
to abrasion are higher. In addition, they are less likely to expand, contract or change
shape on setting. Yet the resistance to wear is still not as good as amalgam, and
composites appear to be less effective than amalgam for restoring large cusp areas or
marginal ridges in areas of high occlusal stress.
Polymerization of composites can be achieved chemically
using benzoyl peroxide as a catalyst, or with light using camphoroquinone for the same
purpose. Composites can be placed directly into the cavity, or inlays can be prepared from
impressions of a cavity and cemented into place.
Glass ionomer can be used as a filling material, base and
liner material, and can be used in fissure sealing and luting. It is formed by an acid
base reaction between a polyalkenoic acid and a fluoro-aluminosilicate glass powder.
Tartaric acid is added to modify the working and setting characteristics and to improve
clinical use. Both self-polymerizing and light-cured glass ionomers are available.
Light-cured ionomers contain 10-30% hydroxyethyl methacrylate.
Glass ionomers are fluoride reservoirs, activating
remineral-ization and inhibiting bacteria colonization. If there is a surfeit of fluoride,
they will absorb it. Glass ionomers are chemically stable and their bond with the tooth
increases over time. They cannot, however, be used in high stress-bearing areas because
they are brittle and do not have the properties required for large restorations. Although
they are tooth-colored, they are considered by many to be less esthetic than composites.
The surface is rough, encouraging plaque formation, although this is mitigated somewhat by
the antibacterial properties of the material. It is difficult to distinguish glass ionomer
restorations or sealants from tooth tissue because the material is not radio-opaque. Glass
ionomers are best used on the primary dentition. Because of these properties, glass
ionomers are employed in some public health programs throughout the world as part of a
caries control procedure called ART Ð Atraumatic Restorative Technique.
Safety of Composites and Glass Ionomers
Resin-based dental composites contain a number of substances
that are known to be allergenic prior to polymerization. Once polymerized, however, they
are much less allergenic. During light curing, only part of the reactive substances in the
resin is polymerized. Consequently, unreacted methacrylate groups can comprise up to 50%
of the total mass of the filling. At the gingival level, these substances are in contact
with epithelial cells and they can be sensitizing. These uncured components also appear to
be more cytotoxic than cured components. In cell culture, they have been observed to
retard cellular growth and interfere with lipid metabolism. Incompletely polymerized
dental restorations are also more likely to degrade, and degradation products like
formaldehyde can have allergenic and toxic potential. Increasing the time of light curing
appears to decrease the observed cytotoxicity.
Pulpal irritation associated with composite fillings may be
due to toxicity of the material or bacterial leakage around margins. Pulpal reactions are
moderate if they appear at all, and tend to decrease with time. There have been reports of
oral lichenoid reactions related to composite restorations. Symptoms were traced back to
formaldehyde, probably originating from the material itself or from plaque deposits on the
rough surface of the material.
Glass ionomer cements appear to have a high level of
bio-compatibility with both the pulp tissue within the tooth and the soft tissue of the
oral cavity. Local or systemic adverse effects have not yet been reported. Glass ionomers
have relatively mild effects on the pulp, appearing to irritate only when used as luting
agents. As a result, calcium hydroxide should be applied for pulpal protection during
extensive crown preparations.
The carcinogenic potential of many of the components of
resins and glass ionomers has not been established. Benzoyl peroxide, a polymerization
catalyst, has been found to promote tumors in mice. Some resin components like methacrylic
acid and the catalyst camphoroquinone have mild genotoxicity. Data so far suggests that
the amount of these materials used in restorations is too low to be either carcinogenic or
mutagenic in humans.
While all dental restorative materials have the potential to
cause bio-compatibility problems, amalgam continues to be the most well-documented. The
allergic potential of the numerous biologically active resin components that may be
released from restorations has yet to be fully documented. A major concern for the use of
these relatively new materials is the risk to practitioners of high occupational exposure.
Occupational Safety
Adverse reactions can be the result of high local
concentrations of irritating substances, or small amounts activating the immune system.
Since many reactions seem similar, diagnosis is usually made after a thorough history is
taken and not simply from clinical appearance. There are five categories of reactions to
dental materials.
1. Irritant contact dermatitis. This type
of reaction can be acute or cumulative. Primary contact with an irritant will produce an
acute reaction, while contact with low doses over time produces a cumulative reaction. The
reaction is localized at the site of contact, and can present as redness to necrosis
depending on the concentration of the irritant and the exposure time. It can also involve
paresthesia, a burning sensation with slight numbness in the fingertips. Paresthesia may
be associated with methylmethacrylate monomer, and may be a direct neurotoxic effect.
2. Allergic contact dermatitis.
The clinical appearance of this type of dermatitis may be the same as contact dermatitis,
although the cause is different. Many dental materials are composed of small molecules
that are not in themselves allergenic, but which can combine with body proteins to form
compounds that are allergenic. Although symptoms will develop at the contact site first,
sensitized individuals can develop a range of symptoms.
3. Contact urticaria. Urticaria is a weal and flare
response elicited by the application of compounds to the skin. It can be immunologic or
non-immunologic. Contact from latex gloves and powder can cause localized and widespread
immunologic reactions, while substances like benzoic acid, a degradation product of the
benzoyl peroxide used in composites and dentures, can cause a localized, non-immunologic
response.
4. Hyper-reactivity. Hyper-reactivity is an
irritation of the mucosal linings. It is not an allergic reaction, although a number of
perfume-like products, such as eugenol and some volatile monomers, can cause rhinitis- and
conjunctivitis-like symptoms that are similar to an allergic response.
5. Photo-related conditions.
While they have yet to be reported among dental professionals, photo-related reactions may
become a problem because of the extensive use of lights in the curing of resins. Reaction
to light can be either toxic or allergic. Toxic reactions involve the skin becoming more
photosensitive because of contact with a chemical resulting in burning, redness and
urticaria. In photo-immune reactions, the skin can become eczematous.
Post-Market Surveillance is Needed
Dental restoratives are complex materials that are necessary
for preserving the natural dentition, but which also can be toxic and can sensitize. The
frequency of adverse reactions among patients has been estimated at anywhere from 1 in
1000 to 1 in 10,000. Clearly, however, reactions among patients are much less frequent
than among dental workers. As yet there is little comparative data on the safety of the
numerous agents. Hence, the World Health Organization has called for an extensive
post-market surveillance on all dental materials so that risks can be assessed and
practitioners can take the relevant precautions to protect both themselves and their
patients. At present, the most thorough review of existing studies, reported by the Oral
Health Division of the World Health Organization, concludes that composites and glass
ionomers are not superior to traditional dental amalgam in their safety. They appear to be
less effective for large restorations in stress-bearing areas, though they have the
obvious advantage of being tooth-colored. Patients, however, can be informed of their
risks and the possible limitations on their longevity when they are recommended.
References
1. Kosella P, Infirri J, Pakhomov G. Direct restorative dental materials in oral health
care amalgam, composites and glass ionomers. In: Proceedings of the WHO consultation on
dental amalgam and its alternatives, Mjor I and Pakhomov G, eds. Oral Health Division of
Non-Communicable Diseases, World Health Organization, Geneva, 1997, pp.19-33.
2. Schedle A, Dahl J, Parzefall W et al. Evaluation of toxic effects of resin based
restorative materials. In: Proceedings of the WHO consultation on dental amalgam and its
alternatives, Mjor I and Pakhomov G, eds. Oral Health Division of Non-Communicable
Diseases, World Health Organization, Geneva, 1997, pp.75-92.
3. Hensten-Pettersen A. Occupational health problems associated with dental materials. In:
Proceedings of the WHO consultation on dental amalgam and its alternatives, Mjor I and
Pakhomov G, eds. Oral Health Division of Non-Communicable Diseases, World Health
Organization, Geneva, 1997, pp.101-104.
Mercury Amalgam Is Safe and Effective
The past three issues of the Colgate Oral Care Report have
pre-sented summaries of the World Health Organization (WHO) report on mercury amalgam.1
The WHO study committee reviewed the scientific literature from many different countries.
The executive summary of this report was pre-sented verbatim as the Clinical Practice
article of Volume 9, No. 1; the safety of dental amalgam and related toxicity issues were
summa-rized in Volume 9, No. 2; and in the present issue as a summary of the current body
of knowledge regarding the safety of alternative restorative materials.
Since the publication of the WHO report, Fung et al of the
University of Nebraska (USA) have reported further evidence that the concern over the use
of mercury amalgam in dentistry may not be warranted. If mercury is neurotox-ic, it should
be associated with neu-rodegenerative disorders such as multiple sclerosis and Alzheimer's
disease. Frozen autopsy specimens from Alzheimer's disease patients, multiple sclerosis
patients, and control patients were assayed for concentrations of mercury using
radiochemical neuron activation analysis. No statistically significant differences were
detected for con-centrations of mercury in the frontal cortex, temporal cortex, occipital
cortex, putamen corona radiata and corpus callosum, and significantly lower concentrations
were found in the hippocampus. Since mercury is not present in ele-vated concentrations in
deceased subjects with Alzheimer's disease or multiple sclerosis, the authors con-cluded
that mercury does not appear to play a major role in the occurrence of these disorders.
A second study that analyzed the relationship between
mercury and brain function showed that existing dental amalgams were not associated with
lower performance on eightdifferent neurophysiologi-cal tests of cognitive function.3
These studies add further weight to the conclusion that mercury does not play a role in
the occurrence of neurological degenerative disor-ders such as Alzheimer's disease or
multiple sclerosis. My own overall conclusion from the well-conduct-ed WHO report as
presented in Volume 9 of the Colgate Oral Care Report is that mercury amalgam is safe and
remains the most cost-effective dental material for posterior tooth restorations.