Dental Materials Science: Composition, Properties, Manipulation, Biocompatibility and Testing
Dental Materials Science
This domain tests whether you can predict how a material behaves from its composition and how manipulation changes its properties. Expect questions on gypsum products, impression materials, cements, casting alloys, ceramics, polymers and on how biocompatibility is actually tested.
Rules worth memorising
- Gypsum: the water-to-powder ratio (W/P) is the master variable. Raising W/P increases setting time and porosity, and lowers strength, density and setting expansion. Dental stone is the alpha-hemihydrate (denser particles, lower W/P, stronger); plaster is beta-hemihydrate.
- Elastomeric impressions: addition silicones are the most dimensionally stable; polyethers are stiff and hydrophilic but absorb water and swell; polysulfides have long setting times and poor elastic recovery. Alginate undergoes syneresis (drying, shrinking) and imbibition (water uptake, swelling), so pour promptly.
- Cements: zinc phosphate sets by acid–base reaction with an exotherm; glass ionomer releases fluoride and bonds chemically to tooth structure but is moisture sensitive early; resin cements give the highest bond and lowest solubility.
- Casting alloys: casting shrinkage is about 1.2–1.5% for noble alloys and 2.0–2.3% for base-metal (Co-Cr, Ni-Cr). Total compensation must combine wax expansion, investment setting and thermal expansion.
- Biocompatibility testing follows a hierarchy: in vitro (cytotoxicity, mutagenicity), animal (irritation, sensitisation, implantation), then usage tests in teeth or mucosa.
Worked example
A Co-Cr removable partial denture framework is short and thin at the distal extension. Volumetric shrinkage on solidification is about 2% (roughly 6% linear contraction, offset by feeding). The technician must supply total expansion from three sources: wax pattern expansion (~0.3–0.5%), investment setting expansion (~0.3–0.5%) and investment thermal expansion during burnout (~1.2–1.5%). Summing them shows the margin of error is small - if burnout temperature is too low, thermal expansion is lost and the casting is undersized. The answer: check burnout temperature and investment liquid ratio rather than adding more alloy.
Traps that cost marks
Confusing setting expansion with thermal expansion; assuming a stronger material is always more accurate; forgetting that alginate must not be stored in water; and mis-stating which cements are water-based and therefore desiccate or hydrolyse if left wet.