Amalgam
Amalgam being the metallic direct restorative material has its own way of manipulation and properties and this section deals with question and answers covering various aspects of silver amalgam.
1. What is the difference between spherical alloy and admixed alloy?
Spherical alloy has small round alloy particles and because of this, the material can be condensed with little condensation pressure. It is used for large amalgam restorations and for deep restorations close to the pulp. Admixed alloy contains irregularly shaped and sized alloy particles and it requires more condensation pressure.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;654
2. What is high copper amalgam?
When the copper content in amalgam alloy is increased to 12% or more, it is called high copper amalgam. It is the commonly used type of amalgam.The high copper content reacts preferentially with tin and prevents the formation of gamma 2 phase within the amalgam mass which in turn reduces the corrosion effects on the restoration.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;654
3. What is dental amalgam?
It is a metallic direct restorative material composed of a mixture of silver-tin- copper alloy and mercury.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;653
4. What is Delayed Expansion?
In zinc containing low copper or high copper amalgam, when there is a moisture contamination that has occurred during trituration or condensation, an expansion occurs that usually starts after 3-5 days and continues for months. This is called Delayed or Secondary Expansion.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn,317
5. How does delayed expansion occur?
In zinc containing alloys, when moisture contamination occurs, zinc reacts with water and releases hydrogen. This hydrogen collects within the restoration, increasing the internal pressure causing the amalgam to creep. The contamination can occur during the manipulation or insertion into the cavity. So it is always advised to keep the operative area dry.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn,317
6. How does the mercury content affect the strength of the restoration?
For a thorough amalgamation, sufficient mercury should be mixed with the alloy to coat the alloy particles so that each particle of the alloy must be wetted by the mercury. When there is insufficient mercury, a dry,granular mix results leaving a rough pitted surface. Also, excessive amount of mercury reduces the strength.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn,319
7. What is the objective of amalgam condensation?
While condensing, the alloy is compacted in the prepared cavity to achieve a dense filling and the mercury rich amalgam is brought to the top of each increment so that there is a continuous matrix phase for the successive increments to bond. Excess mercury is removed from each increment when it is condensed.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn,334