Operative (Restorative) Dentistry and Dental Materials
Adhesion
Enamel bonding is micromechanical: 37% phosphoric acid for 15–30 seconds creates microporosity that resin tags fill. Dentin is wetter and more organic, so bonding depends on a hybrid layer. Etch-and-rinse (three-step is the gold standard) removes the smear layer entirely; self-etch primers leave it partly in place. Over-drying etched dentin collapses the collagen network and overwetting dilutes hydrophobic resin - both cause nanoleakage and post-operative sensitivity.
Shrinkage and C-factor
Composites shrink about 2–3% volumetrically on polymerisation. The C-factor (bonded ÷ unbonded surface area) is highest in a Class I box, where shrinkage stress can deform cusps and debond margins. Control it with increments of 2 mm or less, oblique or gingival-first layering, and a flowable liner.
Material selection
- Amalgam: strong and cheap, no adhesive bond, subject to corrosion and creep; avoid where aesthetics matter.
- Conventional glass ionomer: chemical bond and fluoride release, but weak and brittle - suited to primary teeth and non-stress-bearing Class V lesions.
- Resin composite: aesthetic but technique sensitive.
- Glass ceramics: etch with 5–9.5% hydrofluoric acid and silanate; zirconia cannot be etched - use airborne-particle abrasion plus an MDP-containing primer.
Worked example
A deep Class II composite on a first molar: use a sectional matrix for a tight contact, place a flowable liner on the gingival floor (low C-factor), then two oblique increments of 2 mm or less, curing each through the cusp. Shrinkage vectors are directed toward bonded walls and the cervical margin is protected.
Traps
- Prescribing hydrofluoric acid etching for zirconia.
- Confusing a base (thermal and pulpal insulation, e.g. resin-modified glass ionomer) with a liner (thin, e.g. calcium hydroxide).
- Forgetting that eugenol-containing temporary cements inhibit resin polymerisation.
- Leaving unsupported enamel at the cavosurface margin.