Summary and conclusion
Use of silver amalgam for filling molars is still allowed in Denmark, but it is
believed that resin composites will replace amalgam in the near future. This will end the
ongoing amalgam discussion, but of greater importance it will reduce the mercury burden of
our environment. To day, the dentist uses resin composites in situations where amalgam was
the only choice just a few years ago. It is estimated that relatively small improvements
of the existing resin composites will make products, which may completely replace amalgam
as tooth filling material.
Introduction and purpose
Mercury is processed in the aquatic food chain in such way that the concentration is
increased about 100.000 times from the level of algae to fish such as pike, tuna and
sharks. For this and other reasons, the Danish Ministry for Environment and Energy wish to
ban the use of mercury including its use for dental amalgam. Nevertheless, the Ministry
has allowed the use of amalgam for dental fillings in molars that are exposed to extensive
abrasion. This decision was based on recommendations from national health authorities
claiming that a sufficient replacement material for amalgam has not yet been developed.
The Danish Ministry for Environment and Energy has from 1994 to 1998 financially
supported a project with the goal of developing a sufficient replacement material for
amalgam. The project has been conducted as a joint project between (1) Wolff & Kaaber
A/S in Farum, Denmark, represented by graduate engineer Robert Lessèl and (2) Dr. E.
Christian Munksgaard, Department for Dental Materials, Royal Dental College, University of
Copenhagen in Denmark. The following is a summary of the results from the project
including a survey of the existing alternatives to amalgam materials.
Existing alternatives to amalgam
For many purposes, amalgam can be replaced with cast restorations made of gold alloy,
ceramic, or metal fused ceramic. As cast restorations are costly, their is a need for a
more inexpensive and direct filling material which, as amalgam, is plastic and can be
formed in a dental cavity before it sets to a hard material. Certain alloys based on
gallium has been marketed as a replacement for amalgam, but unsuccessfully due to their
relative low corrosion resistance giving insufficient biocompatibility. A number of other
direct filling materials have been marketed including resin composites, glass ionomer
cements, resin-modified glass ionomers, and compomers. Except for the resin composites,
relatively low abrasion resistances make these materials insufficient as all round
replacement materials for amalgam.
Resin composites as alternative?
Around 75% of amalgam fillings in molars are still in use after 13 years. Some clinical
investigations have shown a comparable survival ratio of molar fillings made of resin
composites, but such comparisons have been questioned. It has thus been mentioned that the
comparisons are not adequate due to 1) differences in sizes and types of fillings in the
compared groups, 2) only patients with a good oral hygiene were selected for the
investigations, 3) the filling procedure were performed with greater care than usually in
average dental practise, and 4) the observation period for the resin composite fillings
were often relatively short.
For extended fillings in the molar region, dentists are generally warned against using
resin composites in stead of amalgam. For newer resin composites the warnings refer to 1)
a relatively short period of experience, 2) a time consuming and difficult procedure, and
3) a sensitive technique. People do not deny that resin composites can be used with
success in the molar region. The concerns refer to lack of information suitable for
judging the consequences of a general replacement of amalgam with resin composite as
filling material.
How to improve resin composites
Previously, resin composite fillings in the molar region were subjected to extensive wear,
but measurements on some of the materials developed within the last 5-15 years have shown
a wear resistance comparable to that of amalgam. Wear resistance of resin composite
materials can be improved by increasing the mechanical properties and by using a
relatively high concentration/loading of ceramic fillers in the material. Most researchers
within the field points out that the tendency of the materials to contract during and
after setting is the major reason for failure of the materials when used for fillings. The
contraction creates gap between material and tooth structure and such gaps are seat for
bacteria that might damage the tooth. By developing materials with small or no
tendency to form gaps, a decisive step will thus have been taken in order to improve the
characteristics of the resin composites.
Another disadvantage connected to the use of resin composites is the risk of developing
contact allergic eczema. Between 1 and 2% of the dental personnel have developed such
allergy, and this ratio is feared to increase in the future. The development of resin
composites with low or no allergic potential is thus desirable.
Results within the project
It was found that spherical fillers of well-defined sizes and size distribution make it
possible to increase filler loading. Higher filler loading gave a reduced tendency for gap
formation and an increase in mechanical properties. A number of selected or newly
synthesised monomers were evaluated. It was found that certain carbonate-containing
monomers used to formulate resin composites resulted in an increase in strength.
Furthermore, composites formulated with methacrylate-modified dendrimers showed an
increase in fracture strength. By use of a light curing initiator, Irgacure 369® in the
resin composite formulations, a 30% increase in stiffness could be obtained. In addition,
composites formulated with the light curing cationic initiator, CIBA 1700® showed reduced
tendency to gap formation, compared to conventional formulations.
In order to reduce the allergenic potential of the resin composites, investigations
with sulphur-containing monomers were performed. These monomers were thioanalogs to
monomers used in commercial composites. One of these, thioTEGDMA could replace the
allergic TEGDMA without loss of mechanical properties. It was speculated that thioTEGDMA,
due to its low solubility in water, might be less allergenic than TEGDMA.
The project has added to the development of a resin composite that might replace
amalgam, for filling of the apical part of the root of a tooth while still in its position
in the jaw. The material, Retroplast® is now commercial available, and clinical tests
have shown a 50% better performance compared to root-end fillings with amalgam.
Furthermore, the technique can be used for repair of root perforations and fractures.
Conclusion
Results from a number of investigations points out that resin composites generally can
replace amalgam for tooth filling, if the resin composites undergo minor developments. The
results from the project propose ways that can be followed in order to obtain such
improvements. An alternative for amalgam as filling materials for cavities in the end of
roots has been developed. The material, Retroplast® is now commercial available.