AGD Dental Materials and Biomaterials Questions and Answers — Questions and Answers
Question 1: A dentist is taking a final impression for a crown on a maxillary molar. The gingival margins are slightly subgingival, and maintaining a completely dry field is proving difficult despite best efforts. Which impression material would be the most suitable choice in this situation due to its inherent properties?
- Addition silicone (PVS)
- Polyether (Correct answer)
- Condensation silicone
- Impression compound
Correct answer: Polyether
Polyether is the most hydrophilic of the elastomeric impression materials. This property gives it a greater tolerance for residual moisture, allowing it to displace moisture and capture accurate marginal detail more reliably in a damp environment compared to the more hydrophobic silicone materials.
Question 2: The primary difference that accounts for the superior strength and abrasion resistance of a Type IV dental stone (die stone) compared to a Type III dental stone (model stone) is the:
- Chemical composition of the calcium sulfate hemihydrate.
- Use of a different accelerator in the powder.
- Shape, size, and porosity of the hemihydrate particles. (Correct answer)
- Higher water-to-powder ratio required for mixing.
Correct answer: Shape, size, and porosity of the hemihydrate particles.
The manufacturing process for Type IV stone creates dense, cuboidal, and less porous hemihydrate particles. This regular particle morphology allows for a lower water-to-powder ratio, which results in a set gypsum product that is significantly denser, stronger, and more resistant to abrasion than the more porous, irregular particles found in Type III stone.
Question 3: In the formulation of modern high-copper dental amalgams, what is the primary function of the increased copper content compared to traditional low-copper alloys?
- To improve the polishability and luster of the final restoration.
- To eliminate the weak and corrosion-prone Gamma-2 (Sn-Hg) phase. (Correct answer)
- To increase the setting expansion for a tighter marginal seal.
- To accelerate the setting reaction, reducing clinical chair time.
Correct answer: To eliminate the weak and corrosion-prone Gamma-2 (Sn-Hg) phase.
High-copper amalgams contain sufficient copper (typically 12% or more) to react preferentially with the tin (Sn). This reaction prevents the formation of the Gamma-2 (Sn-Hg) phase, which is the weakest and most corrosion-susceptible component of low-copper amalgams. Eliminating this phase significantly improves the amalgam's marginal integrity, strength, and clinical longevity.
Question 4: A 55-year-old patient with a history of severe bruxism requires a three-unit fixed partial denture to replace tooth #19. Strength and fracture resistance are the highest priority. Which all-ceramic material would be the most appropriate choice for the framework of this prosthesis?
- Feldspathic porcelain
- Leucite-reinforced glass-ceramic
- Zirconia (Correct answer)
- Lithium disilicate
Correct answer: Zirconia
Zirconia (specifically, yttria-stabilized tetragonal zirconia polycrystal) possesses the highest flexural strength and fracture toughness of all currently available dental ceramics. This makes it the ideal choice for multi-unit posterior bridges, especially in high-stress situations such as in patients with parafunctional habits like bruxism.
Question 5: Which of the following dental cements is unique in its ability to form a true chemical bond to tooth structure via an ionic interaction between its polyacid component and calcium in the hydroxyapatite?
- Glass ionomer cement (Correct answer)
- Zinc phosphate cement
- Zinc oxide-eugenol (ZOE) cement
- Self-adhesive resin cement
Correct answer: Glass ionomer cement
Glass ionomer and resin-modified glass ionomer cements contain polyacrylic acid. This acid component is able to chelate with calcium ions in the enamel and dentin, forming a true, continuous ionic bond. This chemical adhesion is a distinct advantage of this class of cements, in addition to their fluoride release.
Question 6: The excellent biocompatibility and long-term clinical success of commercially pure titanium as a dental implant material is primarily attributed to which of the following phenomena?
- Its modulus of elasticity being identical to cortical bone.
- Its inherent bacteriostatic properties that inhibit plaque.
- Its low thermal conductivity, preventing thermal irritation.
- The formation of a stable, inert, and self-passivating oxide layer. (Correct answer)
Correct answer: The formation of a stable, inert, and self-passivating oxide layer.
When titanium is exposed to oxygen (in air or bodily fluids), it instantly forms a very thin, stable, and highly adherent layer of titanium dioxide (TiO2) on its surface. This 'passivation layer' is extremely inert and corrosion-resistant, preventing the release of metal ions and allowing bone cells to directly attach to the implant surface, a process known as osseointegration.
AGD Dental Materials and Biomaterials Questions and Answers
A dentist is taking a final impression for a crown on a maxillary molar.
The gingival margins are slightly subgingival, and maintaining a completely dry field is proving difficult despite best efforts.
Which impression material would be the most suitable choice in this situation due to its inherent properties?