INBDE practice questions: FK3 — Biomaterials and dental technology

Answer B: The newer high-copper alloys essentially eliminate the weak tin-mercury gamma-2 phase

In old low-copper alloys the setting reaction produced a tin-mercury phase called gamma-2 — weak, corrosion-prone, and the first thing to fail. High-copper amalgams, containing more than about 6% copper, essentially eliminate gamma-2 because copper ties up the tin instead, and every clinical advantage of modern amalgam — strength, less creep, less marginal breakdown — traces to that substitution. A misattributes the change to mercury content, which is roughly half of any set amalgam old or new, and misreads corrosion, whose products gradually seal the margin rather than dissolve it. C describes something no amalgam does: the mass hardens around unreacted alloy cores by design. D reverses the comparison, since amalgam's expansion is about twice enamel's.

Common trap: Crediting the improvement to mercury content or to corrosion instead of the phase that copper removed.

Source: FDA — Dental Amalgam Fillings

Answer C: Leave the intact restorations and restore tooth #31 with a non-amalgam material

Both halves of the FDA's 2020 position sit in the same document. People who are pregnant or planning pregnancy are named among the higher-risk groups advised to avoid amalgam where possible and appropriate, so the new restoration should use one of the alternatives the FDA itself names — composite resin or glass ionomer. But the FDA does not recommend removing intact amalgam restorations unless medically necessary, and states that the evidence does not show harm in the general population. A and B act on the first half only and disturb material that is currently intact; B compounds the error by placing new amalgam in a patient advised to avoid it. D reads the second half and ignores the first.

Common trap: Applying one half of the FDA's amalgam recommendation and behaving as though the other half did not exist.

Source: FDA — Dental Amalgam Fillings

Answer A: The office must operate and maintain an amalgam separator and never discharge scrap amalgam

The EPA's Dental Office Category pretreatment standards at 40 CFR Part 441 apply to offices that place or remove amalgam: those offices must operate and maintain an amalgam separator, must not discharge scrap amalgam, and must avoid certain line cleaners. B mistakes a federal pretreatment standard for a licensing matter; state boards regulate the practice of dentistry, not effluent. C inverts the rule twice, since scrap amalgam is exactly what may not go down the drain and certain line cleaners are themselves prohibited. D invents a patient-volume threshold the rule does not contain. For a candidate trained where amalgam is being phased out, or where dental wastewater is unregulated, this is a US-specific obligation worth recognizing on sight.

Common trap: Assuming that a material's clinical acceptability settles how its waste may lawfully be handled.

Source: EPA Dental Office Category pretreatment standards, 40 CFR Part 441

Answer D: Glass ionomer, which bonds chemically, releases fluoride, and tolerates a moist field

Glass ionomer sets by an acid-base reaction and bonds chemically to tooth structure, because carboxyl groups bond ionically to calcium in hydroxyapatite, so it seals a margin without an adhesive. It also releases fluoride and can be recharged topically, and its coefficient of thermal expansion is close to tooth structure. High caries risk, difficult moisture control, a root-surface lesion, and limited access to care all push toward that family, and its weaker mechanical properties matter least on an unloaded root surface. C picks the properties that lose this trade-off, since composite needs an adhesive and a controllable field. B misapplies margin sealing by corrosion as a caries strategy. A puts a brittle glass ceramic where caries control is the problem.

Common trap: Ranking materials by strength and polish when caries risk and moisture control are what the case turns on.

Source: Phillips' Science of Dental Materials

Answer C: A eugenol-free provisional cement, since eugenol inhibits the polymerization of resin cements

Eugenol inhibits resin polymerization, so a zinc-oxide-eugenol temporary cement or sedative dressing left on a preparation compromises the resin cement or composite that follows. When a resin-bonded definitive restoration is planned, the provisional cement must be eugenol-free — this is one interaction the module singles out as worth memorizing. A states that interaction backwards and presents a known inhibitor as an advantage. B credits zinc phosphate's acidity with a conditioning role it does not have; the material is strong and proven but bonds only mechanically and is strongly acidic while setting, which is a pulpal caution. D assumes mechanical cleaning cures a chemical problem, when contamination of a bonding surface is something to prevent and then clean per the manufacturer.

Common trap: Choosing a provisional cement for the provisional phase alone, without regard to the definitive cementation it precedes.

Source: Phillips' Science of Dental Materials

Answer B: Cement film thickness and an unrelieved internal contact are preventing full seating

For luting, film thickness decides whether the crown seats, and the specification caps a type-I luting cement at about 25 micrometres; an internal high spot that traps cement holds the restoration open in the same way. The module's rule for this presentation is to think film thickness, cement selection and an unrelieved internal contact before blaming the laboratory. D is refuted by the vignette itself: the crown seated fully with a closed margin at try-in, so its geometry was acceptable and nothing has been remade. A misuses gypsum behaviour, since setting expansion occurs while the die sets, long before the try-in. C invents water uptake in a fired ceramic; it is resin-modified glass ionomer that takes up water and expands afterwards.

Common trap: Reaching for a laboratory error when the crown demonstrably fitted before any cement was introduced.

Source: Phillips' Science of Dental Materials

Answer A: Sulfur from the latex gloves and retraction chemicals poisoned the platinum catalyst

Addition silicone has the best dimensional stability and elastic recovery of the elastomers, which is why it dominates crown-and-bridge work, but it comes with two cautions, and the first is that sulfur-containing latex gloves and some retraction chemicals poison its platinum catalyst so the material will not set. The unset zone is precisely where the contaminated gloves worked. B names the second caution — hydrogen evolution, which may require a delay before pouring — but that happens in material that has already set. C would under-polymerize the entire mix rather than one band over one tooth. D borrows alginate's problem: imbibition is water uptake by a stored alginate impression, which distorts a set impression rather than preventing one from setting.

Common trap: Explaining a strictly local handling error with a whole-mix or whole-material property.

Source: Phillips' Science of Dental Materials

Answer D: Modulus, since cobalt-chromium is roughly twice as stiff, giving thin rigid connectors

Base-metal alloys are stiff and strong, and cobalt-chromium has roughly twice the modulus of elasticity of a gold alloy — near 200 GPa against about 90–100 GPa — which is exactly what a removable partial denture framework wants: thin connectors that do not flex under load. C names the right property and the wrong material, reversing the comparison. A reverses the classification itself: nobility describes resistance to intraoral oxidation and corrosion, high noble means at least 60% noble metal with at least 40% gold, and cobalt-chromium is a predominantly base alloy. B is not the deciding property for an all-metal framework; expansion matching matters where alloy and porcelain must cool together.

Common trap: Reading "noble" as a synonym for "better" instead of as a compositional description with specific consequences.

Source: Phillips' Science of Dental Materials

Answer B: Zirconia contains no glass phase, so etching and silanating achieved essentially nothing

The divide that governs ceramic bonding is whether the material contains glass. Feldspathic porcelain, leucite-reinforced ceramic and lithium disilicate do, so they are etched with hydrofluoric acid at the concentration and time specified for that material and then silanated, which chemically links silica to the resin cement. Zirconia has no glass to etch; hydrofluoric acid does essentially nothing to it, and it is prepared instead by airborne-particle abrasion plus a primer containing a phosphate monomer such as 10-MDP, which bonds to zirconium oxide. A assumes the acid worked too well when it barely worked at all. C invents a furnace step and would condemn every correctly bonded glass ceramic. D is refuted by the well-tapered preparation described.

Common trap: Running the glass-ceramic surface protocol on a polycrystalline ceramic because both are called ceramic.

Source: Phillips' Science of Dental Materials

Answer C: Translucent grades add cubic grains that cannot transform, so much of the toughening is lost

3 mol% yttria tetragonal zirconia polycrystal is uniquely tough because a crack tip triggers a tetragonal-to-monoclinic transformation in nearby grains, which expand and squeeze the crack shut, giving a flexural strength around 900–1,200 MPa. Raising yttria to 4 or 5 mol% adds cubic phase and translucency for anterior work, but cubic grains do not transform, so translucent zirconia surrenders much of that transformation toughening and its strength falls toward the glass-ceramic range. A 5Y anterior bridge is not a 3Y posterior crown. A states the mechanism backwards. B treats a compositional change as a cosmetic one. D compounds two errors, since zirconia of any yttria content has no glass to etch, and cementation cannot restore lost flexural strength.

Common trap: Treating "zirconia" as one material because the brand name and the milling block look the same.

Source: Phillips' Science of Dental Materials