Restorative Dentistry and Dental Materials
Restorative dentistry and materials questions join operative technique with polymer and ceramic science, usually asking why a restoration failed.
Adhesion
- Enamel bonding: 37% phosphoric acid for 15-30 seconds, then hydrophobic resin infiltration into the etch pattern.
- Dentine: etch-and-rinse leaves a collagen network that must remain moist; self-etch systems demineralise and infiltrate in one step with milder acidity, giving less postoperative sensitivity but a weaker enamel bond.
- Configuration factor is bonded surface divided by unbonded surface; a high factor with shrinkage causes debonding and cuspal flexure, so composite is placed in increments.
Materials
- Composites shrink about 2-4% by volume; bulk-fill reduces layering time but not all stress.
- Glass ionomer releases fluoride and bonds chemically to dentine but is brittle; resin-modified versions are stronger with less fluoride release.
- Amalgam: the gamma-2 phase is weakest and corrodes; creep and marginal breakdown follow poor condensation.
- Zinc phosphate is acidic and irritating but proven; resin cements are strongest but technique-sensitive; eugenol cements inhibit resin polymerisation.
- Ceramic strength depends on surface flaws, so etching, sandblasting and glazing must match the material.
Worked example
A mesio-occlusal composite in a mandibular molar has a high configuration factor because five walls are bonded and only the occlusal surface is free. Shrinkage stress concentrates at the gingival margin, so a flowable liner or horizontal increments reduce the bonded-to-unbonded ratio, and finishing is delayed until stress relaxes. The reasoning is stress distribution, not brand choice.
Traps
Overdrying etched dentine; using total-etch adhesive on sclerotic dentine without adjusting; selecting zinc phosphate under a ceramic veneer; ignoring coefficient of thermal expansion mismatch; forgetting that sealant retention depends on isolation.