Dental Materials Science
Dental materials science tests selection and handling, not just names. The board asks why a cement, ceramic, or impression material fails in a specific clinical situation.
Core rules
- Glass-ceramics are etched with hydrofluoric acid and silanated; zirconia is not etchable and needs airborne-particle abrasion plus an MDP-containing primer.
- Resin cements bond chemically and are preferred for zirconia and thin ceramics; RMGI is useful for metal and some zirconia when moisture control is difficult.
- Composite polymerization shrinkage increases with cavity C-factor; use incremental placement and adequate light exposure.
- Addition silicone is accurate and dimensionally stable; polyether is hydrophilic but can absorb water and is stiff.
- Gypsum strength and expansion depend on water-to-powder ratio; too much water lowers strength and increases setting time.
Worked example
A zirconia molar crown is to be cemented. You avoid HF acid and silane alone; instead you sandblast the intaglio with 50 µm alumina at low pressure, clean it, apply an MDP primer, and use resin cement. Reasoning: zirconia lacks a glass phase for etching, and MDP provides chemical bonding to its oxide surface.
Traps
Etching zirconia with HF acid does nothing useful. Using silane alone on zirconia gives weak bond strength. Incorrect water-to-powder ratio changes gypsum expansion and fit. A contaminated impression causes voids. Overthinning ceramic leads to fracture regardless of cement.