Introduction:  What is amalgam?

 

Dental amalgam is an alloy that combines mercury with silver and tin, along with some lesser metals such as zinc and copper.  In many respects, amalgam is an ideal filling material.  As a freshly mixed mass, it has a plasticity allowing it to be easily packed into a prepared cavity in a tooth.  As well as being extremely durable, it expands and contracts with changes in temperature at the same rate as a natural tooth.  Amalgam is very strong, and has been widely used for one hundred and sixty years when restoring posterior teeth.

 

Conventional alloys usually contain the following constituents: silver 67-74%, tin 25-27%, copper 06% and zinc 0-2%.  When mercury is mixed with a powder of amalgam alloy, in a process known as trituration, the alloy dissolves into the mercury.  This process is usually carried out in an amalgamator, a machine that mixes the two components in a capsule. Most alloys can be adequately triturated in seven seconds.  The amalgamation reaction that occurs between mercury and amalgam produces a plastic, condensable mass that solidifies to a hard metal.  This freshly mixed material is placed into the cavity under high pressure.  Condensation, or packing, is achieved using a packing instrument, and applying a load of 4-5 kg to each individual increment.  As this mix is condensed, a proportion of the mercury-rich material rises to the surface.  This can be removed to reduce the final mercury content of the restoration.

 

Once this process has occurred, it is claimed that "for all practical purposes, no free (unreacted) mercury is associated with the amalgam restorations”².  However, this is unrealistic, as there is in fact a small amount of free mercury in every silver amalgam filling, and some is almost certainly released as mercury vapour during chewing, as will be shown later.  It is whether the amount released is significant and capable of producing systemic toxicity that is the contentious issue.

 

Mercury amalgam: an historical background:

Dental amalgam containing both silver and mercury has been used in dentistry as a restorative material since the beginning of the nineteenth century.  However, in the first century AD, the Roman naturalist Pling the Elder noted "that mercury is a poison and has no business being used in medicine”³.  The use of mercury in dentistry is thus difficult to explain, when it was observed so long ago that mercury had detrimental effects on the human body. This did stop many different civilisations experimenting with this highly toxic substance.

 

The earliest record of mercury being used in dental restoration was in 1601 by a German named Tobias Dom Kreilus.  However, his mixture consisted of dissolving copper sulphide with strong acids and then adding mercury.  It was not until 1826, that 0. Taveau of Paris combined mercury with silver to form an amalgam "silver paste".  These early amalgams consisted of filings from silver coins mixed with mercury.  They proved very troublesome, and were often just used to provide more substance to the tooth, for it to be grasped by forceps during extraction.  Those fillings that were used to restore teeth usually expanded too much and cracked the tooth.

 

Even from its earliest beginning as a restorative material, dental amalgam aroused suspicion and debate.  In 1843, the recently formed American Society of Dental Surgeons forbid its members to use silver amalgam containing mercury, leading to the "Amalgam Pledge" being passed by the same society in 1845.  This act of prohibition clouded the introduction of mercury amalgam into dentistry.  This was known as the Amalgam War: "the amalgam war was the war between the craftsman’s ideal of ease of manipulation and the medical ideal of the avoidance of the danger of systemic mercurial poisoning”⁴.  Clearly the use of mercury in dentistry in the 1830's was controversial, and this has continued throughout the development of mercury amalgam.

 

In 1896, Black's detailed work on the properties and manipulation of amalgam allowed the previous technical problems of this restorative material to be overcome.  Certain safety procedures and placement techniques were introduced; many of which are still used today.  Mercury amalgam's effectiveness as a restorative material increased dramatically, and the only criticism available to sceptics was that of mercury being a poison.  The risks of mercury as a toxic substance are well known, but this has not stopped amalgam being used throughout the world for over one hundred and sixty years.  Alternative restorative materials are becoming increasingly available, and general awareness of the potential dangers of mercury amalgam has greatly risen.  Hence the number of amalgam fillings being inserted in the UK today is falling, and surely, it won't be too long before mercury amalgam is removed as a dental material.