# INBDE practice questions: FK3 — Biomaterials and dental technology Ten original INBDE practice questions on FK3 — Biomaterials and dental technology, each answered on this page with a rationale and a source. Last updated: 2026-08-10. ## Question 1 A 68-year-old woman has amalgams placed 25 years ago with heavily ditched, broken-down margins, alongside two placed last year that remain sharp and intact. She asks why the old ones failed and the new ones have not. Which metallurgical difference best accounts for it? - A. The older alloys contained more mercury, whose corrosion products dissolved the margins - B. The newer high-copper alloys essentially eliminate the weak tin-mercury gamma-2 phase - C. The newer alloys set with no unreacted alloy particles left in the mass - D. The newer alloys have a thermal expansion far below the surrounding enamel's **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](https://www.fda.gov/medical-devices/dental-devices/dental-amalgam-fillings) ## Question 2 A 29-year-old woman who is ten weeks pregnant has four intact, asymptomatic amalgam restorations and one new carious lesion in tooth #31 that needs restoring. Having read about mercury, she asks whether her existing fillings should be replaced now. What should the dentist recommend? - A. Replace all four amalgams now, since pregnancy places her in a higher-risk group - B. Replace the four after delivery, and restore tooth #31 with amalgam in the meantime - C. Leave the intact restorations and restore tooth #31 with a non-amalgam material - D. Leave everything unchanged, and restore tooth #31 with amalgam at today's appointment **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](https://www.fda.gov/medical-devices/dental-devices/dental-amalgam-fillings) ## Question 3 A dentist trained abroad is opening a US practice that will both place and remove amalgam restorations. Reviewing the plumbing plan with her contractor, she asks what federal law requires of the office's wastewater. Which statement is correct? - A. The office must operate and maintain an amalgam separator and never discharge scrap amalgam - B. No federal rule applies; amalgam waste is governed only by the state dental board - C. Scrap amalgam may be discharged provided an oxidizing line cleaner is used daily - D. A separator is needed only once the office removes amalgam from fifty patients yearly **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](https://www.epa.gov/eg/dental-effluent-guidelines) ## Question 4 A 79-year-old man taking several xerostomic medications has four shallow root-surface lesions at the gingival margins of the mandibular premolars. Isolation is difficult, his dexterity is poor, and he has developed three new lesions each year. Which material family best fits this case? - A. Feldspathic porcelain inlays, whose glassy surface resists acid dissolution in a dry mouth - B. Amalgam, whose corrosion products will seal these margins in a high-caries-risk mouth - C. Conventional composite, whose polish and wear resistance protect root surfaces best here - D. Glass ionomer, which bonds chemically, releases fluoride, and tolerates a moist field **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 ## Question 5 A 50-year-old woman has tooth #14 prepared for a lithium disilicate onlay that will be adhesively cemented with a resin cement in three weeks. The assistant asks which cement to use for the provisional restoration in the meantime. What should be chosen, and why? - A. Zinc-oxide-eugenol, because eugenol sedates the pulp and improves the later resin bond - B. Zinc phosphate, because its acidity conditions the dentin ahead of the resin cement - C. A eugenol-free provisional cement, since eugenol inhibits the polymerization of resin cements - D. Any provisional cement, because air abrasion at delivery removes all chemical residue **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 ## Question 6 A 55-year-old man's metal-ceramic crown on tooth #19 seated completely at try-in with a closed margin. At cementation it stands visibly high, the margin is open, and the occlusion is premature. The laboratory has changed nothing since the try-in. What should be considered first? - A. The die stone expanded after the try-in, enlarging the internal surface of the crown - B. Cement film thickness and an unrelieved internal contact are preventing full seating - C. The porcelain absorbed water from the cement and expanded during the seating procedure - D. The laboratory built the crown to the wrong occlusal scheme before the try-in appointment **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 ## Question 7 A 47-year-old woman's addition-silicone impression for a crown on tooth #3 has set everywhere except in a tacky, unset band directly over the prepared tooth, where the assistant had packed retraction cord while wearing latex gloves. What is the most likely cause? - A. Sulfur from the latex gloves and retraction chemicals poisoned the platinum catalyst - B. Hydrogen gas evolved by the set silicone produced the unset band at that surface - C. The base and catalyst were dispensed in the wrong ratio, under-polymerizing the material - D. Moisture in the sulcus caused imbibition, which softened the material at the margin **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 ## Question 8 A 66-year-old woman is having a mandibular distal-extension removable partial denture made. The laboratory asks whether to cast the framework in a high-noble gold alloy or in cobalt-chromium. Which property most directly decides that choice for the major connector? - A. Nobility, since cobalt-chromium resists intraoral corrosion better than a gold alloy does - B. Thermal expansion, since gold's lower value keeps the framework from distorting in service - C. Modulus, since gold's higher stiffness lets connectors be made thinner without flexing - D. Modulus, since cobalt-chromium is roughly twice as stiff, giving thin rigid connectors **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 ## Question 9 A 43-year-old man's 3Y-TZP zirconia crown on tooth #19 debonds intact four months after cementation. The preparation is well tapered with adequate axial height, the crown is undamaged, and laboratory records show the intaglio was etched with hydrofluoric acid and silanated before resin cementation. What explains the failure? - A. Hydrofluoric acid over-etched the zirconia and removed the retentive surface it needed - B. Zirconia contains no glass phase, so etching and silanating achieved essentially nothing - C. Silane cannot couple to any ceramic unless the surface is first heat-treated - D. The preparation lacked the taper and height that any cement chemistry would have required **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 ## Question 10 A 38-year-old woman needs a full-coverage restoration on tooth #30 and asks for the highly translucent zirconia her friend received for anterior crowns. What should the dentist explain to her before she decides? - A. Translucent grades are stronger, because added yttria stabilizes more of the tetragonal phase - B. Translucent and conventional zirconia differ only in the shade the laboratory applies afterwards - C. Translucent grades add cubic grains that cannot transform, so much of the toughening is lost - D. Translucent zirconia can be hydrofluoric-acid etched, which offsets any loss of strength **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 ## Next step [Take the free INBDE practice test](https://dentovio.com/inbde/free-practice-test) Official reference: [JCNDE — Integrated National Board Dental Examination](https://jcnde.ada.org/inbde). Original exam-style questions written for study, never recalled exam content. Independent educational preparation, not clinical advice, and not affiliated with or endorsed by the Joint Commission on National Dental Examinations.