Practice these 26 MCQs on Dental Amalgam (Part 1). These questions cover composition, phases, properties, and clinical manipulation.
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Correct Answer: B
Explanation: Trituration (or amalgamation) is the process of mixing the alloy powder with liquid mercury to remove the oxide coating on the alloy particles and allow the mercury to wet the surface, initiating the setting reaction.
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Correct Answer: B
Explanation: Creep is the time-dependent plastic deformation of a material under a constant stress. "Dynamic creep" specifically refers to this deformation occurring under cyclic loads, such as mastication, which can lead to marginal ditching.
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Correct Answer: A
Explanation: During condensation, the mercury-rich matrix (which is the weakest part) rises to the surface. If this mercury-rich layer is not adequately carved away or if it pools at the margins, the margins will contain the highest mercury concentration, making them prone to fracture.
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Correct Answer: A
Explanation: Amalgam is a brittle material with high compressive strength (similar to enamel) but very low tensile and shear strength. This necessitates cavity designs that rely on bulk to prevent fracture.
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Correct Answer: C
Explanation: For conventional (low copper) alloys, the classic composition is approximately 65% Silver, 29% Tin, and less than 6% Copper (often with <2% Zinc). High copper alloys have significantly higher copper content (>12%).
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Correct Answer: A
Explanation: Early strength is critical to prevent fracture before the final set. The standard requirement is 80 MPa at 1 hour, though modern high-copper amalgams often exceed 250 MPa within an hour.
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Correct Answer: B
Explanation: The Gamma-2 phase ($Sn_8Hg$) is the weakest and most corrosion-prone phase. High copper amalgams are designed specifically to eliminate this phase.
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Correct Answer: D
Explanation: High copper amalgams must contain enough copper to eliminate the gamma-2 phase (usually >12%). Admixed alloys typically range from 13% to 30% copper (e.g., Dispersalloy is ~12-13%, others go higher).
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Correct Answer: D
Explanation: Spherical alloys have a lower surface area, so they require less mercury (A is false) and are very plastic, requiring less condensation force (B is false). They generally set faster than lathe-cut alloys (C is false).
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Correct Answer: A
Explanation: Polishing creates a smooth surface with less surface area and fewer irregularities for plaque accumulation, thereby significantly increasing resistance to tarnish and corrosion.
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Correct Answer: B
Explanation: This is the FALSE statement. High copper alloys have superior tarnish and corrosion resistance because they lack the highly corrodible Gamma-2 phase.
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Correct Answer: D
Explanation: While manufacturers control particle shape, the clinician can most significantly alter the setting time by changing the trituration time. Over-trituration speeds up the reaction (shortens working time), while under-trituration results in a grainy, slow-setting mix.
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Correct Answer: A
Explanation: Excess residual mercury (>55%) leads to a higher volume of the weak gamma-2 and gamma-1 matrix phases. This drastically reduces strength, leading to fracture, and increases creep and corrosion.
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Correct Answer: C
Explanation: High Copper alloys have LOW corrosion resistance (better properties). Therefore, saying they have "High corrosion" is the exception/false statement.
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Correct Answer: C
Explanation: Inhalation of mercury vapor is the primary route for chronic toxicity in dental settings. Ingested metallic mercury is poorly absorbed, and skin absorption is less significant than vapor inhalation.
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Correct Answer: C
Explanation: According to ADA specifications, a "zinc-free" alloy must contain less than 0.01% zinc. Zinc-containing alloys usually have 0.01% to 1% (often ~1%).
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Correct Answer: A
Explanation: Spherical particles slide over each other easily ("mushy" feel). They require larger condensers and lighter pressure compared to the high force needed to pack irregular lathe-cut particles.
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Correct Answer: A
Explanation: The elimination of the weak Gamma-2 phase and lower creep values result in significantly less marginal breakdown (ditching) over time.
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Correct Answer: D
Explanation: The heat and friction generated during the high-speed oscillation of trituration can cause mercury vapor to leak from capsules if they are not perfectly sealed.
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Correct Answer: A
Explanation: Silver is the primary constituent, typically making up 40% to 70% of the alloy composition depending on the type (High Cu vs Low Cu).
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Correct Answer: B
Explanation: The defining feature of high copper amalgam is the reaction of Tin with Copper to form the Eta phase ($Cu_6Sn_5$), preventing Tin from reacting with Mercury to form the weak Gamma-2 phase ($Sn_8Hg$).
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Correct Answer: B
Explanation: Trituration abrades the oxide layer on the alloy particles, allowing the mercury to wet (coat) the surface of the particles so the chemical reaction can proceed. The alloy does not fully dissolve; it is a surface reaction.
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Correct Answer: C
Explanation: In zinc-containing alloys, moisture reacts with zinc to produce hydrogen gas. This gas pressure causes a delayed, painful expansion of the restoration starting 3-5 days after placement.
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Correct Answer: B
Explanation: The optimal mercury content is generally cited as 48-52% (approx 50%). Levels above 55% significantly degrade properties. (Note: Spherical alloys may use even less, around 42-45%).
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Correct Answer: A
Explanation: Tarnish is a surface discoloration (sulfide layer) that does not necessarily compromise the mechanical integrity. Corrosion is the actual chemical degradation of the body of the material.
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Correct Answer: B
Explanation: The traditional $Ag_3Sn$ (Gamma) phase composition generally requires Tin to be around 26-28%. If it is too high, more Gamma-2 forms; if too low, contraction occurs.
