Liners, Varnishes and Bases
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Direct answer
Two thin coats of copal varnish painted on the cavity floor seal dentinal tubules and reduce microleakage and fluid movement under an amalgam restoration — but varnish must never meet a bonded restoration, because it blocks resin and glass ionomer adhesion. Suspension liners such as calcium hydroxide give a thin therapeutic film; cement liners and bases (zinc oxide eugenol, glass ionomer, polycarboxylate, zinc phosphate) supply thickness, insulation and sometimes sedative action under metallic restorations. Each cement has a personality: zinc phosphate mixes on a cool slab with an exothermic acid–base reaction and bites the pulp unless tubules are sealed; polycarboxylate is gentle and actually chelates to enamel and dentine; ZOE is the sedative but has no place beneath composite, which it plasticises.
What you must remember
- Varnish: copal resin in solvent (acetone, chloroform or ether), applied as two thin coats, dried between; it seals tubules, reduces marginal percolation under amalgam and blocks phosphoric acid ingress from zinc phosphate.
- Varnish contraindications: under composite, resin-bonded and glass ionomer restorations (it prevents bonding) and over linings that must set undisturbed; a resin "dentin sealer" replaces it for composites.
- Calcium hydroxide liner: thin (under 0.5 mm), alkaline, antibacterial, placed only at the deepest point; soluble, so a covering layer protects it.
- Zinc oxide eugenol: sedative liner, base and temporary filling (IRM-type reinforced); excellent for deep caries palliation; never under composite, whose resin it plasticises.
- Zinc phosphate: zinc oxide powder with magnesium oxide, phosphoric acid liquid buffered with aluminium and zinc; exothermic, mixed on a cool slab over a large area; putty consistency for a base, string for luting.
- Polycarboxylate (zinc polyacrylate): powder zinc oxide plus polyacrylic acid liquid, mixed on a paper pad with a plastic spatula; bonds to enamel and dentine calcium, pulpal-friendly, weaker and more soluble than phosphate.
- Glass ionomer and resin-modified glass ionomer liners (Vitrebond type) bond to dentine, release fluoride and double as bases — the modern default under amalgam and composite in deep cavities.
- Thermal logic: a base of about 1–2 mm replaces lost dentine as insulation under metal restorations; dentine itself, not a liner thickness, is the tooth's insulator.
- Sequence rule: therapeutic liner first (deepest), then base, then varnish last — but only under non-bonded restorations, painted on walls and margins but not over unset liners.
Mixing zinc phosphate on the cool slab, stepwise
The glass slab is chilled under running water and dried — the point of the cool slab is to slow the exotherm and lengthen working time, allowing a larger number of powder increments into the liquid, which is what gives the set cement its strength. Divide the powder into five or six increments and spatulate each broadly over the slab for 15–20 seconds, letting the acid wet every particle. The mix passes through a glossy, fluid luting consistency — it strings an inch from the spatula — and continues to a heavy putty that rolls and no longer sticks: base consistency, at which it is scooped, pressed into the cavity and carved flat to re-establish depth. If the gloss disappears before placement, the mix has passed working time and must be remade, not rescued. For luting, the restoration is seated during the string stage, before the set advances. In materials practicals, this is where examiners watch timing, increments and consistency.
Where students slip
The punished error is order-of-layers confusion: varnish painted over unset calcium hydroxide (the solvent penetrates and disturbs the liner), or varnish placed first and liner stuck on top of a film that never bonded to dentine. The correct order is liner at the deepest point, base over it, varnish last on the walls for amalgam. The second classic slip is cement chemistry: writing that zinc phosphate "bonds to tooth" (it does not; it is mechanically retained) or that polycarboxylate is "more irritating than phosphate" (it is markedly less, its large polyacrylic molecules barely permeating tubules). The third, most clinically costly, is reaching for ZOE as a base under a composite — eugenol contaminates the bonding interface and softens the resin, so glass ionomer is the base of choice whenever adhesive restorations follow.
Frequently asked questions
What does a cavity varnish do and when is it used?
It seals dentinal tubules and reduces microleakage and acid ingress under non-bonded restorations, chiefly amalgam over zinc phosphate; two thin coats are standard.
Why must varnish not be used under composites?
The varnish film blocks resin infiltration and tooth adhesion, so composite restorations rely on bonding agents, not varnish, for their seal.
Why is zinc phosphate mixed on a cool glass slab?
Cooling dissipates the exothermic heat and slows setting so more powder can be incorporated, improving strength; the wide spatulation area serves the same purpose.
Which base bonds to tooth structure?
Polycarboxylate and glass ionomer cements — both chelate or ionically bond to calcium in enamel and dentine, unlike purely mechanical zinc phosphate.
Why is zinc oxide eugenol unsuitable beneath composite?
Eugenol plasticises and interferes with resin polymerisation at the interface; a glass ionomer base replaces it for adhesive restorations.