Dental Materials
Dental materials: what is tested
The exam tests composition, setting reactions, physical properties, and clinical failure modes of restorative, impression, and laboratory materials.
Core rules
- Amalgam is silver-tin-copper alloy with mercury; high-copper mixes reduce gamma-2 phase and creep. Corrosion releases mercury, but systemic risk is low.
- Composite resin has organic matrix (Bis-GMA, TEGDMA), filler, and silane coupling. Polymerization shrinkage is about 2-3 percent by volume.
- C-factor is bonded surface area divided by unbonded surface area; high C-factor increases contraction stress, as in Class I cavities.
- Glass ionomer sets by acid-base reaction, releases fluoride, bonds chemically to tooth, and is moisture-sensitive early.
- Zinc phosphate is strong but acidic and brittle; polycarboxylate bonds chemically but has weak strength.
- Addition silicone is the most accurate elastomer and has low shrinkage; alginate is an irreversible hydrocolloid and dimensionally unstable.
- Gypsum: hemihydrate plus water forms dihydrate; setting expansion is greater for plaster than for die stone.
- Zirconia toughens by tetragonal-to-monoclinic transformation.
Worked example
A large Class I composite restoration develops marginal leakage. Reasoning: the bonded floor and walls restrict shrinkage, producing high C-factor and stress at the margins. If the resin cannot flow, stress exceeds bond strength. Incremental placement, a low-shrinkage base, and careful light curing reduce the problem.
Traps
- Confusing syneresis (alginate loses water) with imbibition (alginate absorbs water).
- Assuming more powder always means less expansion in gypsum; the ratio affects strength and expansion in specific ways.
- Claiming composite bonds to enamel without acid etching.
- Treating glass ionomer as a permanent high-stress restoration.