Amalgam (dentistry)

In dentistry, amalgam is an alloy of mercury used to fill teeth cavities. It is made by mixing a combination of liquid mercury and particles of solid metals such as silver, copper or tin. The amalgam is mixed by the dentist just before use. It remains soft for a short while after mixing, which facilitates it being snugly packed into the cavity and shaped before it sets hard.

Dental amalgams were first documented in a Tang dynasty medical text written by Su Gong (苏恭) in 659, and appeared in Germany in 1528. In the 1800s, amalgam became the dental restorative material of choice due to its low cost, ease of application, strength, and durability.

There are, according to Geir Bjørklund, indications that dental amalgam was used in the first part of the Tang dynasty in China (AD 618–907), and in Germany by Strockerus in about 1528. Evidence of a dental amalgam first appears in the Tang dynasty medical text Xinxiu bencao (新修本草) written by Su Gong (苏恭) in 659, manufactured from tin and silver. Historical records hint that the use of amalgams may date to even earlier in the Tang dynasty. It was during the Ming dynasty that the composition of an early dental amalgam was first published, and a text written by Liu Wentai in 1505 states that it consists of "100 shares of mercury, 45 shares of silver and 900 shares of tin."

Ever since its introduction in the Western world from 1818 and into the 1830s, amalgam has been the subject of recurrent controversies because of its mercury content. Early amalgam was made by mixing mercury with the filings of silver coins. In 1833, Polish dentists from London, Edward Crawcour and his nephew Moses Crawcour (incorrectly referred to as "the Crawcour Brothers"), brought amalgam to the United States; but they had to flee back to Europe one year later, leaving "a long trail of victimized patients and exasperated dentists" due to their malpractices.

However, the use of amalgam caught on in the following years, and in 1844 it was reported that fifty percent of all dental restorations placed in upstate New York consisted of amalgam. The same year, the use of dental amalgam was declared to be malpractice by the American Society of Dental Surgeons (ASDS), the only US dental association at the time, who forced all of its members to sign a pledge to abstain from using the mercury fillings. This was the beginning of what is known as the first dental amalgam war. The dispute ended in 1856 with the disbanding of the old association. The American Dental Association (ADA) was founded in its place in 1859, which has since then strongly defended dental amalgam from allegations of being too risky from the health standpoint.

The controversy about amalgam fillings continued throughout the rest of the nineteenth century, with regional dentist societies condemning them, such as the St. Louis Odontological Society did as early as 1867.

Amalgam has been used for many years for restorations, commonly known as fillings. Prior to 1900 many compositions were tried but few were successful when placed in the oral environment. Around 1900, small amounts of copper and occasionally zinc were added. Zinc acts as a scavenger because it prevents oxidation of the other metals in the alloy during the manufacturing process. Zinc accomplishes this by combining readily with oxygen to form zinc oxide. Amalgam restorations made from this balanced formula were reasonably successful and its longevity increased. However, one disadvantage that remained was fracture at the tooth-amalgam interface commonly called marginal fracture. Sn₈Hg (γ₂ phase) was considered to be responsible for this problem. This phase has been shown to be the weakest phase in the set amalgam and is subject to corrosion, particularly at the tooth-amalgam interface.

In 1962 a new amalgam alloy, called Dispersalloy, was introduced by William and Ralph Youdelis of Edmonton, Alberta, Canada. William was a metallurgist in the Faculty of Engineering at the University of Alberta. His younger brother Ralph was a 1955 graduate of the Faculty of Dentistry at the same university. William Youdelis added a spherical silver-copper eutectic particle to the traditional lathe-cut Ag₃Sn particle in a ratio of 1:2. The mixture of these two types of particles is known as admix alloy. This alloy strengthened the set amalgam and reduced the γ₂ phase (Sn₈Hg). The increased copper in the silver-copper eutectic reacted preferentially with tin so that Sn₈Hg could not form. Early results from the clinical use of this new amalgam showed an improvement in marginal integrity. Faculty members of the Department of Operative Dentistry at The University of Alberta Faculty of Dentistry conducted clinical trials on the new material. It was produced by a local Edmonton company, Western Metallurgical. Johnson & Johnson pharmaceuticals eventually purchased the patent from the Youdelis brothers. 10 years later, another alloy, called Tytin, was introduced by adding significant amount of Cu₃Sn together with Ag₃Sn, in the form of a unicompositional spherical particle to eliminate the γ₂ phase. Both of these relatively new alloys raised the copper content from 5%, present in the older balanced composition alloy, to about 13% for the newer alloys.