DMD Dental Materials Science 2 — Questions and Answers
Question 1: Which ADA Type of gypsum product has the highest compressive strength and is most suitable for fabricating dental dies?
- Type I — Impression plaster
- Type II — Model plaster
- Type III — Dental stone
- Type IV — Die stone (Correct answer)
Correct answer: Type IV — Die stone
Type IV die stone (high-strength dental stone) has the highest compressive strength due to its low water-to-powder ratio and is the preferred material for accurate dies.
Question 2: The elastic modulus (Young's modulus) of a dental material represents:
- Maximum stress the material sustains before fracture
- The ratio of stress to strain within the elastic range (Correct answer)
- The material's resistance to surface abrasion
- Energy absorbed per unit volume before fracture
Correct answer: The ratio of stress to strain within the elastic range
Young's modulus equals stress divided by strain within the elastic (proportional) range, and it quantifies the stiffness or rigidity of a material.
Question 3: Glass ionomer cement achieves adhesion to tooth structure primarily through which mechanism?
- Micromechanical interlocking with etched enamel
- Chemical ionic bonding with calcium in hydroxyapatite (Correct answer)
- Van der Waals intermolecular forces
- Covalent bonding with collagen fibers
Correct answer: Chemical ionic bonding with calcium in hydroxyapatite
Glass ionomer cement bonds chemically through exchange of ions between the polyalkenoic acid and calcium ions in the hydroxyapatite of tooth structure.
Question 4: What is the primary clinical significance of fluoride release from glass ionomer cements?
- Increases compressive strength over time
- Provides secondary anticariogenic protection at restoration margins (Correct answer)
- Improves translucency and esthetics
- Accelerates final set of the cement
Correct answer: Provides secondary anticariogenic protection at restoration margins
Fluoride released from glass ionomer cements inhibits bacterial metabolism and promotes remineralization, providing secondary caries prevention at the margins.
Question 5: In porcelain-fused-to-metal (PFM) restorations, which type of ceramic is layered directly onto the metal coping?
- Feldspathic porcelain (Correct answer)
- Leucite-reinforced pressed ceramic
- Lithium disilicate glass-ceramic
- Monolithic zirconia
Correct answer: Feldspathic porcelain
Feldspathic porcelain is used in PFM restorations because its thermal expansion coefficient is compatible with dental metal alloys, ensuring a durable bond.
Question 6: The setting expansion of dental stone during gypsum product fabrication is clinically important because it:
- Compensates for the solidification shrinkage of gold casting alloys (Correct answer)
- Improves the surface hardness of the die material
- Prevents internal porosity during mixing
- Reduces water evaporation during storage
Correct answer: Compensates for the solidification shrinkage of gold casting alloys
The slight setting expansion of dental stone compensates for the thermal contraction (solidification shrinkage) that occurs when molten gold alloys solidify in the casting mold.
Question 7: Which elastomeric impression material offers the greatest dimensional accuracy and long-term stability for fixed prosthodontic impressions?
- Alginate (irreversible hydrocolloid)
- Zinc oxide eugenol impression paste
- Polyvinyl siloxane (addition silicone) (Correct answer)
- Polysulfide rubber base
Correct answer: Polyvinyl siloxane (addition silicone)
Polyvinyl siloxane (addition silicone) provides the highest accuracy and dimensional stability because it undergoes minimal dimensional change over time with no byproducts released.
DMD Dental Materials Science 2
Which ADA Type of gypsum product has the highest compressive strength and is most suitable for fabricating dental dies?