Dental Materials IV: Main Exam, 9 December 2018

Mount Kenya University · Department of Dental Sciences · Diploma in Dental Technology · BDS2204

Dental Materials IV past exam 2018-2019 from Mount Kenya University with model answers for BDS2204.

Instructions: ANSWER ALL QUESTIONS IN SECTION A AND ANY TWO QUESTIONS IN SECTION B

Section A ANSWER ALL QUESTIONS (40 marks)

1. Define anmpression material. (2 marks)

Model answer
  • Impression material: a substance used to record the shape of oral tissues for prosthetic fabrication.

2. Why is an expansion a pre-requisite property of casting in an investment material? (2 marks)

Model answer
  • Expansion ensures that the investment material can accommodate the shrinkage of the metal during cooling, maintaining dimensional accuracy of the cast.

3. Other than water, name other liquid that may be added to phosphate manipulation. (2 marks)

Model answer
  • Ethanol
  • Isopropyl alcohol
  • Glycerol

4. What in hysteresis. (2 marks)

5. Define an impression material. (2 marks)

6. Gelation temperature of a reversible hydrocolloid is dependent on true factors. Name them. (4 marks)

Model answer
  • True factors:
  • - Temperature of the liquid
  • - Concentration of the polymer
  • - Presence of additives (e.g., plasticizers)
  • - Viscosity of the liquid
  • - Ambient humidity

7. Give TWO properties of alginate impression materials that makes it the most successful of all impression materials. (2 marks)

Model answer
  • Rapid setting time
  • - Good dimensional stability
  • More answers: low cost, ease of use, acceptable esthetics

8. Why does ongar ongar impression material include gypsum hardener? Name it and how does it work. (3 marks)

Model answer
  • Onger ongar contains a gypsum hardener to improve dimensional stability and reduce shrinkage.
  • The hardener is a calcium sulfate dihydrate (gypsum).
  • It reacts with the alginate to form a rigid matrix, anchoring the impression.

9. Give FIVE factors that contribute to an increase in hygroscopic expansion. (5 marks)

Model answer
  • High water absorption
  • - Low polymer cross‑linking
  • - Presence of hydrophilic groups
  • - Low polymer density
  • - Inadequate curing time
  • More answers: use of plasticizers, high temperature, low pH

10. What are elastomers? (2 marks)

Model answer
  • Elastomers are flexible, rubber‑like materials that can undergo large reversible deformations under stress.

11. Give TWO advantages and TWO disadvantages of alginate impression material. (4 marks)

Model answer
  • Advantages
  • Quick setting time
  • Good dimensional stability
  • Disadvantages
  • Poor tear strength
  • Sensitive to temperature changes

Section B (30 marks)

12.

  1. a) Name an investment material that does not require a metal costing ring and explain why the metal ring is not necessary. (3 marks)
    Model answer
    • Answer: Zirconia
    • Reason: Zirconia is a high‑strength ceramic that can be cast without a metal ring because its high modulus of elasticity and strength allow it to retain shape during firing.
  2. b) Write down the composition of silica bonded investment material. (5 marks)
    Model answer
    • Silica bonded investment composition
    • 70–80 % silica (SiO₂)
    • 10–15 % alumina (Al₂O₃)
    • 5–10 % zirconia (ZrO₂)
    • 5–10 % glass phase (e.g., CaO, Na₂O)
    • 1–3 % stabilisers (e.g., TiO₂, MgO)
  3. c) Discuss the factors that influence the stability of alginate impression material 2. a) Discuss the basic composition of irreversible hydrocolloids. b) Write down the uses of irreversible hydrocolloids. 3. Write in details the composition properties and manipulation of any two ecostomeric impression materials. 4. Discuss the composition, ideal requirements and properties of gypsum bonded investment material. (20 marks)

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