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 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.

History of use

There are, according to Geir Bjørklund, indications that dental amalgam was used in the first part of the Tang dynasty in China, and in Germany by Strockerus in about 1528. Evidence of a dental amalgam first appears in the Tang dynasty medical text Xinxiu bencao written by Su Gong 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, 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, 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 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.

Low copper to high copper amalgam alloy

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 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. 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.

Composition

Dental amalgam is produced by mixing liquid mercury with an alloy made of silver, tin, and copper solid particles. Small quantities of zinc, mercury and other metals may be present in some alloys. This combination of solid particles is known as amalgam alloy. The composition of the alloy particles are controlled by the ISO Standard for dental amalgam alloy in order to control properties of set amalgam such as corrosion and setting expansion. It is important to differentiate between dental amalgam and the amalgam alloy that is commercially produced and marketed as small filings, spheroid particles, or a combination of these, suitable for mixing with liquid mercury to produce the dental amalgam. Amalgam is used most commonly for direct, permanent, posterior restorations and for large foundation restorations, or cores, which are precursors to placing crowns.
The reaction between mercury and alloy when mixed together is termed an amalgamation reaction. It will result in the formation of a silver-grey workable mass which can be condensed into cavities. After condensing, the dental amalgam is shaped to generate the required anatomical features and then hardens with time. The standard composition of alloy prior to 1986 is referred to as conventional amalgam alloy. More recently, there has been a change in the compositional standard of the alloy due to better understanding of structure-property relationships for the materials. Conventional amalgam alloy commonly consists of silver, tin, copper and other trace metals; current amalgam alloy consists of silver, tin, copper and other metals.
Alloy powder is then mixed with liquid mercury to produce dental amalgam. Low-copper amalgam commonly consists of mercury, silver, tin, zinc and other trace metals.

Metallurgy

To fabricate an amalgam filling, the dentist uses a mixing device to blend roughly equal parts of shavings of a silver-base alloy with mercury until the shavings are thoroughly wetted. The silver alloy is typically 40–70% Ag, 25-29% Sn, 2–40% Cu and 0–2% Zn. The dentist packs the plastic mass, before it sets, into the cavity. The amalgam expands ≈0.1% over 6–8 hours on setting.
The final structure is a metal matrix composite, where γ₁, η and γ₂ phases, are a matrix for unreacted original alloy, minus the fast-reacting β-phase and excess Sn.

Properties

Amalgam is a mixture of two or more metals with mercury which has been purified first by distillation to remove impurities. Major components of the alloy are silver, tin, and copper. The composition of the alloy powder is controlled by ISO standard for dental amalgam alloy to control the properties of amalgam.

Plastic deformation (creep)

Creep or plastic deformation happens when subjected to intra-oral stresses such as chewing or grinding. Creep causes the amalgam to flow and protrudes from the margin of the cavity forming unsupported edges. "Ditch" is formed around the margins of the amalgam restoration after fracture due to amalgam creep at the occlusal margins. The γ2 phase of amalgam is primarily responsible for high values of creep.

Corrosion

Corrosion occurs when an anode and cathode are set up in the presence of electrolytes, creating an electrolytic cell. The multiphase structure of dental amalgam can contribute as an anode or cathode with saliva as electrolytes. Corrosion may significantly affect the structure and mechanical properties of set dental amalgam. In conventional amalgam, γ2 phase is the most reactive and readily forms an anode. It will break down releasing corrosion products and mercury. Some of the mercury will combine rapidly with unreacted alloy and some will be ingested. The chances of ditching are further increased. Copper-enriched amalgams contain little or no γ2 phase. The copper–tin phase, which replaces γ2 in these materials, is still the most corrosion-prone phase in the amalgam. The corrosion however is still much lower than conventional amalgam.
In spite of that, it is thought that corrosion actually offers a clinical advantage. The corrosion products will gather at the tooth-amalgam interface and fill the microgap which helps to decrease microleakage. Even so, there are no reports of increased marginal leakage for the copper-enriched amalgams indicating that sufficient quantities of corrosion product are produced to seal the margins.
Microleakage is the leakage of minute amounts of fluids, debris, and microorganisms through the microscopic space between a dental restoration and the adjacent surface of the cavity preparation. Microleakage can risk recurrent cavities.

Strength

An amalgam restoration develops its strength slowly and may take up to 24 hours or longer to reach a reasonably high value. At the time when the patient is dismissed from the surgery, typically some 15–20 minutes after placing the filling, the amalgam is relatively weak. Therefore, dentists need to instruct patients not to apply undue stress to their freshly placed amalgam fillings.
In addition, amalgam restorations are brittle and susceptible to corrosion.