Operative Dentistry and Dental Materials
Cavity design and Black's classification
Class I occlusal/pits and fissures, Class II proximal of posteriors, Class III proximal of anteriors without incisal edge involvement, Class IV with incisal edge, Class V cervical, Class VI cusp tip. Design principles: resistance form (adequate depth, rounded internal line angles, no unsupported enamel) and retention form (convergence, grooves, bonding). Bevel enamel for cast metal and ceramic margins; do not bevel for amalgam or for a butt-joint ceramic margin.
Materials
Amalgam is roughly 50% mercury; high-copper alloys reduce the weak, corrosive gamma-2 (Sn-Hg) phase. Zinc-containing amalgam exposed to moisture shows delayed expansion - avoid contamination. Amalgam is not adhesive and relies on mechanical retention. Composites (Bis-GMA/TEGDMA with filler) bond micromechanically to etched enamel; polymerisation shrinkage creates stress, and a high C-factor (bonded/unbonded ratio) - as in a Class I box - makes that stress worse. Cure in increments no thicker than about 2 mm. Glass ionomer releases fluoride and bonds chemically to tooth but is brittle and moisture-sensitive; resin-modified glass ionomer improves handling. Zinc oxide-eugenol is sedative but inhibits resin polymerisation. Calcium hydroxide and MTA are the pulp-capping materials of choice.
Impressions and gypsum
Alginate is an irreversible hydrocolloid prone to syneresis and imbibition - pour promptly. Addition silicone (polyvinyl siloxane) is the most dimensionally stable elastomer. For gypsum, a lower water-to-powder ratio gives a stronger, denser cast.
Rule of thumb
When a question asks for the strongest and most durable bond, the answer usually involves etched enamel plus a hydrophobic resin - enamel bonding always outperforms dentine bonding.