Dental Materials I: Supplementary Exam, August 2018

1.

1. a) Describe how to test the following in gypsum products;
   1. i) Initial setting.
   2. ii) Final setting.
   3. iii) Setting time.
   4. iv) Loss of gloss.
2. b) Illustrate the stress - strain curve showing the slope of modulus of elasticity.
   Model answer

   • Stress‑strain curve: Draw a straight line from the origin with a gentle slope representing elastic deformation. Label the slope as the modulus of elasticity (E). After the linear region, show a gradual curve indicating plastic deformation until failure.
3. c) Outline four factors which may influence setting mechanism of calcium sulphate hemihydrates.
   Model answer

   • Water availability: Insufficient water retards hydration.
   • Particle size: Finer particles increase surface area, speeding setting.
   • Impurities: Presence of salts or organic matter can inhibit crystal growth.
   • Temperature: Higher temperatures accelerate setting; lower temperatures slow it.

2. Describe the process of manufacturing dental stone from gypsum rock. (20 marks)

3.

1. a) Explain five physical properties applicable to dental materials. (10 marks)
   Model answer

   • Hardness: Resistance to indentation.
   • Fracture toughness: Ability to resist crack propagation.
   • Elastic modulus: Stiffness under load.
   • Thermal conductivity: Heat transfer capability.
   • Biocompatibility: Non‑toxic interaction with tissues.
2. b) Explain the rationale of studying dental materials. (10 marks)
   Model answer

   • Clinical relevance: Determines suitability of materials for patient care.
   • Safety: Ensures materials do not harm tissues.
   • Performance: Predicts longevity and function.
   • Innovation: Drives development of improved materials.
   • Regulation: Supports compliance with standards and guidelines.

4. Describe five non-metallic denture base materials. (20 marks)