amalgam 2 l Preclinical Operative MCQs

 

Dental Amalgam  ; describing composition , Cavity preparation feature and classification of amalgam l Preclinical Operative MCQ for dental student

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Dental amalgam

• Dental amalgam is a direct restorative material composed of a mixture of silver-tin-copper alloy and mercury.
• The unset mixture is condensed into a cavity prepared on the tooth after removal of the carious/other structural defect in order to restore the tooth's form and function.
• The cavity preparation for amalgam restoration require special consideration to the properties of the amalgam in order to be more resistive and retentive in nature.

 COMPOSTION DENTAL AMALGAM

• Dental amalgam is an alloy of mercury with a silver-tin alloy. Varying amount of copper and zinc is added to influence the quality of the silver amalgam.

Low-copper amalgam alloys: 

• Or conventional amalgam contains 2 to 5% copper. The low-copper amalgam alloy may contain 69.4% Ag, 26.2% Sn, 3.6% Cu, and 0.8% Zn.

Highcopper amalgam alloys:

• Contain 12 to 30% copper. The high-copper amalgam alloy may contain 60% Ag, 27% Sn, 13% Cu, and 0% Zn. High copper amalgam alloy have better corrosion resistance than low copper amalgam alloy.

Indication of amalgam restorations

1. Moderate to large Class I, Class II cavities and Class V cavities.
2. As a provisional caries control restorations.
3. As a core or foundations.

Contraindications of dental amalgam restoration:

• Class III and class IV restoration in anterior teeth
• Highly mutilated teeth requiring full coverage crown restoration.

Advantages dental amalgam restoration

• Its easy manipulation.
• High tensile strength.
• Excellent wear resistance.
• It is ecnomical.
• Minimal  postoperative sensitivity.

Disadvantages dental amalgam restoration

1. Lack of esthetic.
2. It is non insulating.
3. require more extensive preparation.
4. Is technique sensitive and Weakens tooth structure.
5. It has low early strength and initial microleakage.

Cavity preparation feature for amalgam

1. The external wall of the cavity preparation should be slightly convergent occlusally.
2. The cavosurface margin angle should be 90° because amalgam has low edge strength. This angle is known as Butt joint.
3. Amalgam restoration should have a minimum thickness of 1.5 to 2 mm of amalgam in order to prevent the fracture of the restoration because of its lack of compressive strength. Facial and lingual margins should not be extended more than half way between central roove and cusp tips.
4. Any two outlines that are less than 0.5 mm apart should be joined.
5. Marginal ridges should be supported by sound dentin. And any weakened tooth structure should be removed.
6. Pulpal floor is kept relatively flat.
7. The marginal amalgam angle should not be less than 80° to prevent chipping of the amalgam at the margin of the restoration.
8. External walls of preparation should be parallel to the enamel rods. Facial and lingual walls are parallel or slightly convergent.
9. For class II cavities, occlusal step should have slightly convergent wall.
10. Proximal box should have slightly divergent wall.
11. Isthmus width should be as conservative as possible and should be rounded. The width of isthmus should be ideally 4^th  of the intercuspal distance.
12. Gingival wall or seat is beveled.
13. The basic steps of cavity preparation for amalgam restorations are same as explained under principle of cavity preparation

CLASSIFICATION OF DENTAL AMALGAM

According to Its Content

• Low copper amalgam-contains 2-4% of Cu
• High copper alloy-contains 13-30% of Cu
• Admixed alloy-contain 1/3rd low copper and 2/3rd Ag-Cu eutectic alloy
• Single composition or unicompositional alloy
• Zinc containing alloy-contain more than 0. 01% of zinc.
• Zinc free alloy-contain less than 0.01% of zinc
• Preamalgamated alloy-contain less than 3% of Hg
• Noble metal amalgm alloy-contains Au and or Pd

On the basis of number of metal

1. Binary alloy-contains Ag and Sn
2. Ternary alloy-Ag, Sn and Cu
3. Quarternary alloy-Ag, Sn, Cu and In

On the basis of Shape of alloy particle

1. Spherical-smooth surface spheres
2. Spheroidal-Irregular surface spheres
3. Lathe cut-Irregular shaving or filling (Microcut, fine cut and Coarse cut)

On the Basis of the Development of Amalgam Alloy

1. 1st generation amalgam alloy-G. V. Black's formulation of 3 parts Ag and 1 part Sn.
2. 2nd generation amalgam alloys-Addition of 4% Cu and upto 1% Zn.
3. 3rd generation amalgam alloys-Admixed alloys.
4. 4th generation amalgam alloys-Ternary alloys - Addition of Cu to Ag and Sn to form Ag, Cu, Sn.
5. 5th generation amalgafh alloys-Quarternary alloys - Ag, Sn, Cu, and Indium. Almost no Sn available to react with Hg.
6. 6th generation amalgam alloys-Ag-Cu-Pd eutectic alloy (62%, 28%, and 10% respectively) is added in a ratio of 1:2 to low Cu alloy.

Effect of Various Component on the Properties of Dental Amalgam

• Silver: Increases strength, expansion and corrosion resistance. It decreases flow and setting time

• Copper: increases strength, expansion, corrosion resistance, hardness and brittleness. It decreases flow, setting time and plasticity.

• Tin: Increases flow, setting time and plasticity. It decreases strength, expansion and corrosion resistance.

• Zinc: Increases strength, expansion, setting time, flow and plasticity. It decreases hardness, brittleness and corrosion resistance.

• Indium: Increases strength and decreases creep, surface tension and marginal breakdown.

• Gold: Increases strength and corrosion resistance. Mercury-decreases setting time.

MECHANICAL PROPERITIES OF AMALGAM

Dimensional Change:

• Amalgam can expand or contract depending on its manipulation.

• Expansion that occurs due to reaction of Hg with alloy components is termed primary expansion or mercuroscopic expansion.

• Expansion that occurs after 1 to 7 days due to moisture contamination during trituration or condensation before the amalgam mass is set, is termed secondary expansion or delayed expansion.

• Factors affecting dimensional Change-Expansion increases with increased g phase or b phase. Decreased particle size result in initial contraction (Increased surface tension) and then expansion (Out ward thrust).

• Increased Hg/Alloy ratio result in Increased expansion (mercuroscopic). Rapid trituration and longer trituration within limits results in contraction.

• Increased condensation pressure result in contraction.

Strength

• The compressive strength of low Cu dental amalgam is 145 Mpa at 1 hour and 343 at 7 days. The compressive strength for admix alloy is 137 at 1hr and 431 at 7 days.

• The compressive strength value for single composition is 262 at 1 hour and 510 at 7 days.

• The tensile strength of low Cu is 60 and single composition is 64. The strength of amalgam increases with increased g phase, g1 phase, ç phase.

• Increased trituration within limit increases strength, beyond limit it decreases.Increased condensation pressure increases strength.

• Strength of amalgam decreases with increase in temperature. Reduced corrosion, increases strength.

Creep:

• Creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. h Phase is associated with decrease in creep as it prevents grain boundary sliding.

• Creep occurs because of grain boundary sliding. Creep increases with higher g1 volume fraction and decreases with larger g1 grain size.

• The higher g2 phase is associated with higher creep.

• h Phase is associated with decreases in creep as it prevents grain boundary sliding.

• Increased Hg/Alloy ration, Increased h2 fraction, Increased grain size of h1, overtrituration are all the factors which increase creep.

• Increased condensation pressure decreases creep.

Resistance to corrosion

• g phase is most resistance to corrosion followed by g1, silver cu-eutectic phase, e phase, h phase and g2 pahse (Least resistance)

Mercury free amalgam

• Has also been marketed due to increasing concern and risk of mercury toxicity.
• These mercury free filling may contain same composition of silver alloy but do not contain mercury.
• Gallium or some other noble metal may be mixed with Ag-Sn alloy to form a similar mixture as the mercury silver mixture.