5.6 Bases, Liners, Matrices, and Wedges

Cavity protection: varnishes, liners, and bases

Under a restoration the dentin and pulp must be protected from bacteria, thermal shock, and chemical irritation. Three categories of intermediary materials do this, distinguished mainly by thickness and function:

Calcium hydroxide is the classic therapeutic liner: placed on the deepest dentin near the pulp, it is alkaline, antibacterial, and stimulates the formation of reparative (tertiary) dentin. Glass ionomer is a versatile base/liner that chemically bonds to tooth structure and releases fluoride, helping resist recurrent caries. Zinc oxide-eugenol is sedative to the pulp but eugenol can interfere with resin polymerization, so it is avoided directly under composite. Cavity varnish is reserved for amalgam because its solvent and film are incompatible with bonded resin.

The general placement order in a deep prep is therapeutic liner first (over the deepest dentin only), then base if needed, then the restoration — never covering the entire prep with a thick base that weakens retention. 1).

How depth of the preparation drives material choice

The deeper the cavity, the more pulpal protection is needed. A shallow prep in enamel and superficial dentin may need only the bonding system; a moderate prep benefits from a liner or thin glass-ionomer layer; a deep prep near the pulp calls for a therapeutic calcium-hydroxide or resin-modified glass-ionomer liner on the deepest spot, sometimes with a base to insulate against the metal of an amalgam. Because metal conducts heat, thermal insulation matters more under amalgam than under composite, which is itself an insulator. This is why varnish and ZOE pair historically with amalgam, and bonded liners pair with composite.

The RDA who understands this logic can anticipate which protective material the dentist will request based on the depth and the planned restoration, and can have it dispensed and ready inside its working time.

Matrix systems and wedges for Class II restorations

A Class II preparation removes one or both proximal (mesial/distal) walls, so the missing wall must be rebuilt temporarily to confine the restorative material and form a proper contour and contact with the adjacent tooth. A matrix supplies that wall; a wedge seals the gingival margin and separates the teeth. Placing, wedging, and removing matrices for restorative procedures is an allowable RDA duty in California (BPC §1750.1), so candidates must know the system precisely.

The Tofflemire (universal) matrix has three parts: the retainer (a holder with a locking vise and an adjusting knob/spindle and a guide knob), a thin metal or clear matrix band, and a wedge. Key facts and steps:

1. Band shape: the band is wider on one side; its smaller circumference faces gingivally and the larger circumference faces occlusally, matching the tooth's natural taper.
2. Loading the retainer: form the band into a loop, slide it into the retainer with the slotted (guide) channels facing the gingiva and the diagonal slot opening toward the retainer; tighten the spindle to lock the band.
3. Burnish a slight curve/contour into the band so it reproduces proximal anatomy.
4. Seat the loop over the prepared tooth with the retainer toward the buccal; the band should extend about 1–1.5 mm above the cavosurface margin occlusally and slightly below the gingival margin.
5. Wedge: insert a wooden or plastic wedge from the lingual (larger) embrasure into the gingival embrasure so it presses the band against the gingival margin.
6. Tighten the band, then loosen slightly after wedging so the band can be condensed against the adjacent tooth for a tight contact.

Why the wedge matters

The wedge does two jobs: it adapts the band to the gingival margin, preventing a gingival overhang of excess material, and it separates the teeth slightly to compensate for the band's thickness so that, once the band is removed, a tight proximal contact remains. A missing or poorly placed wedge produces an open contact or an overhang — both common exam distractors. On the exam, recognizing that a Class II needs the full retainer + band + wedge system, and that the smaller band circumference goes gingivally, separates strong answers from weak ones.

After the restoration sets, the retainer is unscrewed and removed buccally, the band is teased out, and the wedge is removed, leaving the rebuilt proximal wall intact.

Other matrix systems and removal sequence

The Tofflemire is the universal system, but candidates should recognize alternatives. Sectional matrices use a small curved metal band plus a separating ring (G-ring) and are favored for composite because they create a tighter, more anatomic contact; clear plastic (Mylar) strip matrices are used for anterior Class III/IV composites because light cures through them and they leave a smooth surface; automatrix bands are retainerless. Whatever the system, the RDA's allowable role is the same: assemble it, place and wedge it under the dentist's direction, and remove it cleanly.

Removal order matters and is a classic ordering item. For a Tofflemire: (1) loosen and remove the retainer first, sliding it toward the occlusal/buccal; (2) gently tease the band out in a buccolingual direction so the new restoration is not fractured; (3) remove the wedge last. Reversing the order — pulling the band while the retainer still locks it — can dislodge the fresh restoration or open the contact. Throughout, the RDA confirms the gingival margin is smooth with no overhang, an inspection point the dentist relies on before dismissing the patient.