Dental amalgam

Dental amalgam is a filling material used in dental restorations. It consists of:

• Mercury - The main component is liquid elemental mercury, usually around 44-54% by weight.

• Metal alloys - The main metals used are silver, tin, and copper, which are mixed in powdered alloy form. 

• Other components - Sometimes zinc or palladium are added in small amounts.

When dental amalgam is mixed, the liquid mercury binds to the metal powder through an amalgamation reaction, forming an alloy with a property called "gamma 1 intermetallic compound". This compound is malleable and can be shaped as a dental filling. 

Once placed in the tooth, the amalgam further hardens due to loss of mercury and corrosion within the mouth. This makes it fairly durable for restoring damaged teeth.

However, there are some concerns about dental amalgam:

• Mercury exposure - There is debate about whether low levels of mercury vapors from dental amalgam can cause health issues over time, especially for susceptible individuals. However, most studies find no clear health risks.

• Long-term durability - Over many years, dental amalgam fillings may fracture or leak, requiring replacement. Composite resin fillings may last longer in some cases. 

• Aesthetics - The dark color of dental amalgam is less desirable aesthetically compared to white composite resin fillings.

• Dental waste - Amalgams contain mercury that must be captured and disposed of properly, unlike resin composites.

So in summary, dental amalgam has been a useful and fairly durable tooth filling material for decades. However, concerns about mercury exposure, aesthetics and environmental impact have led many dentists and patients to prefer mercury-free composite resin alternatives when possible.

Amalgam fillings are still appropriate for restoring large cavities or back teeth in situations where durability, ease of use and cost may outweigh aesthetic considerations. The choice of filling material depends on many factors for each individual patient.

There are various classifications of dental amalgam alloys based on their composition:

1. High-copper amalgam: These contain 10-20% copper and have a ratio of 1 part copper to 2 parts silver. They have high strength, corrosion resistance and compressive creep resistance. However, they are darker in color and have lower ductility.

2. Low-copper amalgam: They contain 5-10% copper and have a ratio of 1 part copper to 4 parts silver. They are more ductile and lighter in color but have lower strength and resistance to wear. 

3. Tin amalgam: These contain high amounts of tin, in the range of 15-20%. The tin improves plasticity, mercury pickup and adaptation to cavity walls. But they have lower mechanical properties and darker color.

4. Non-Gamma 2 amalgam: These contain compositions that avoid forming the brittle Gamma 2 phase during setting. They have high ductility, strength and corrosion resistance. Palladium and indium are often included in these amalgams.

5. High-gold amalgam: Gold in amounts up to 10% is added to improve plasticity, strength and corrosion resistance. But they are more expensive.

6. Beryllium amalgam: Adding 2-3% beryllium helps reduce corrosion and decreases the mercury content needed. But they pose health risks due to beryllium sensitivity.

7. Gallium amalgam: The inclusion of 2-5% gallium lowers the setting temperature and increases strength, plasticity and corrosion resistance. But gallium is toxic and relatively expensive.

8. Hybrid amalgam: These contain a mixture of fine and coarse alloy particles to optimize strength, creep resistance and adaptation. The properties can be tailored for specific applications and locations in the mouth.

So in summary, dental amalgam alloys are formulated with varying proportions of silver, copper, tin, zinc and other elements to achieve different combinations of properties and performance. The ideal composition depends on the clinical situation, cavity location and other factors for each patient.

The setting reaction of dental amalgam involves several steps and chemical changes:

1. When the liquid mercury is combined with the powdered alloy particles, the mercury wets the surface of the alloy particles. 

2. The mercury then begins to dissolve atoms from the surface of the alloy particles. This forms an intermetallic amalgam phase called γ1. 

3. As more mercury dissolves alloy atoms, the γ1 phase grows along the edges and surfaces of the particles, joining them together.

4. As the amalgam sits in the tooth, two things happen:

- More mercury gradually evaporates from the amalgam. This leaves behind a higher proportion of alloy particles, making the amalgam harder.

- Corrosion occurs as the amalgam interacts with substances in the mouth. This changes the microstructure of the alloy-mercury mixture.

5. Depending on the composition of the alloy particles, a secondary phase called γ2 may form within the amalgam during setting. This brittler γ2 phase can compromise the long-term strength and durability.

6. Over months to years, the corrosion and evaporation of mercury continue, further concentrating the metal alloy particles within the amalgam. This makes the filling harder and less able to adapt to stress over time.

7. As the amalgam ages, corrosion products may slowly leach out of the filling into surrounding tissues. However, studies debate whether this metal release poses health risks.

So in summary, the setting reaction of dental amalgam involves the dissolution and diffusion of mercury into the alloy particles, forming an amalgam with γ1 and sometimes γ2 phases. Over time, mercury loss and corrosion alter the microstructure and properties, making the amalgam harder and less able to withstand stress. The products of these ongoing chemical reactions may have health implications that are still being studied.