Amalgam Flashcards
What is amalgam?
Dental amalgam is an alloy made by mixing mercury with silver tin alloy.
Amalgam consists of?
Mercury combined with powdered silver-tin alloy
Why mercury suitable to be used in dentistry?
Liquid at room temperature and is able to form workable mass when mixed with alloy. This behaviour makes it suitable
What is amalgamation reaction?
- Reaction between mercury and alloy which follows mixing
- It results in formation of hard restorative material of silver grey appearance
- The colour generally limits its use in those cavities where appereance is not of primary concern
Why mercury in dental amalgam should be purified?
- it is purified by distillation
- to ensure elimination of impurities so that it does not affect setting characteristics and physical properties of amalgam
Composition of amalgam?
- MAJOR COMPONENTS: Silver, tin, copper
* MINOR COMPONENTS: zinc, mercury, palladium
What is silver tin compound and its importance?
- Ag and Sn react to form Ag3Sn intermatellic compound
- this compound is known as Y phase
- its advantage is, it readily undergoes amalgamation reaction with mercury.
Role of copper in amalgam?
- most conventional alloys contain arround 5% copper
* it gives strengthening effect on set amalgam
Role of zinc in amalgam?
- is a scavenger during production of alloy
- alloy is formed by melting of all constituent metals together at elevated temperature so for this purpose there is tendency for oxidation to occur
- oxidation of Sn,Cu or silver would affect properties of alloy and amalgam
- zinc reacts rapidly with oxygen and forms slag of ZnO which is easily removed
How oxidation is prevented in zinc free alloys?
By carring out procedure in inert atmosphere
What are pre amalgamated alloys?
Alloys which contain3% mercury
Which 2 methods are used to produce powder particles?
1) lathe cut: amalgam powder made by grinding and milling.
* Size~ 1-40um
* irregular shape
2) atomization: molten alloy is sprayed into column filled with inert gas
* equal size particles
* spherical shape
Which 2 heat procedures carried out on lathe cut alloys?
•Before lathe cutting:
1) homogenisation heat treatment:
*alloy ingot is heated at 420C for several hours
*the resulting alloy contains homogeneous grains of Ag3Sn compound
* large grains of Y phase
•After lathe cutting 2nd heat treatment
2) alloy ageing:
* heat the alloy at 100C for 1 hour
* it removes residual stress introduced during cutting
Classification of amalgam according to number of alloy metals?
1) Binary alloys: silver-tin
2) ternary alloys: silver-tin-copper
3) quaternary alloys: silver-tin-copper-indium
Classification of amalgam to whether the powder consists of unmixed or admixed alloys?
Certain amalgam powders are only made up of one alloy. Other have one or more alloys or metals physically added to the basic alloy. Eg adding copper to binary silver tin alloy
Classification of amalgam according to shape of powdered particles?
1) spherical shaped (smooth surfaced spheres)
2) lathe cut (irregular spindles)
3) admixed (combination of lathe cut and spherical)
Classification of amalgam according to powder particle size?
1) micro cut
2) fine cut
3) coarse cut
Classification of amalgam according to copper content of powder?
1) low copper content (1-6%)
2) high copper content (more than 6%)
Where amalgam is used?
1) class 1 and class 2 cavities
2) cuspal restorations (with pins usually)
3) cavities in posterior teeth, used in combination with composite resins
4) as a die material
5) class 3 in unaesthetic areas
Class 5 lesions in non aesthetic areas
Where amalgam should not be used?
1) anterior teeth where aesthetics is of primary concern
2) prominent areas of posterior teeth
What are the advantages of amalgam used in dentistry?
1) easy to use, easy to manipulate
2) relatively inexpensive
3) excellent wear resistance
4) Restoration is completed within one sitting
5) good compressive strength
6) sealing ability improves with age by formation of corrosion products at tooth amalgam interface
7) bonded amalgam has less micro leakage
Disadvantages of amalgam?
1) non aesthetic appearance
2) tarnish and corrosion
3) metallic taste and galvanic shock
4) Discolouration of tooth structure
5) lack of chemical and mechanical adhesion to tooth structure
6) mercury toxicity
7) promotes plaque adhesion
