Definition: A substance that sets and hardens independently and can bind other materials together.
In a deep MOD cavity on a lower molar, a clinician must think about the pulp, the eventual restoration, and how to keep that tooth comfortable and functional for years. Between tooth and restoration sits a very thin layer: the cement. Sometimes it’s a base protecting a near-exposed pulp, sometimes a luting layer holding a crown in place, sometimes a sealer locking an endodontically treated canal, sometimes the adhesive under an orthodontic bracket.
Cements in dentistry make more sense if you imagine them as the “quiet middle layer” in almost every restorative or indirect procedure. They sit between tooth and restoration, often invisible once the work is finished, but they have a disproportionate influence on comfort, longevity and success.
Requirements of cavity lining and base materials
Cavity lining or base materials are placed between the restorative material and dentine, particularly in deep cavities where the pulp is at risk. Their primary purpose is to act as a barrier and a support.
Protection of the pulp
Dentine is not inert; through its tubules it provides a direct pathway from the cavity floor to the pulp. A lining or base therefore serves to shield the pulp from thermal changes, chemical irritants and electrical currents, and to reduce movement of dentinal fluid, which is a key component of dentine sensitivity. In deep cavities, where only a thin layer of dentine remains, this protective role is crucial. In shallow cavities, where residual dentine is thicker, the need for a substantial lining is much less and a thin varnish or adhesive may be enough.
Thermal insulation
Metallic restorations such as amalgam conduct heat very efficiently. Their thermal diffusivity is tens of times higher than that of dentine. If a deep cavity is restored with metal and no insulating base, the pulp will be exposed to rapid temperature changes from hot and cold food and drink, leading to “thermal shock” and pain. A thermally insulating base, of sufficient thickness, reduces the rate at which temperature changes reach the pulp. In shallow cavities, the dentine alone can often provide adequate insulation, and an unnecessarily thick base may simply weaken the overlying amalgam by reducing its bulk.
Varnishes, solutions of natural or synthetic resins in volatile solvents, are sometimes used in shallow cavities under amalgam. Once the solvent evaporates, they leave a very thin resin film that helps seal dentinal tubules but is too thin to offer meaningful thermal protection.
Chemical protection and sealing
Historically, cavity linings were seen largely as chemical shields protecting the pulp from irritants such as phosphoric acid in older materials or unreacted monomer in resin-based materials. It is now clear that dentine and pulp can tolerate brief exposure to even relatively strong acids, provided the tooth is then effectively sealed and microleakage is minimised.
This has shifted the emphasis away from “neutralising” acids and towards creating an effective, long-term seal at the tooth–restoration interface. A lining may still act as a chemical barrier, but – especially in adhesive techniques – its role is often to support or complement the adhesive system rather than to act as a sacrificial shield.
Electrical insulation
When two different metals are present in the mouth, for example amalgam opposing gold, a galvanic cell can form, with currents flowing through the saliva and the tooth. This can produce sharp pain and accelerate corrosion.
An electrically insulating lining can reduce this effect, although many water-based cements are not perfect electrical insulators. Non-polar resins, sometimes used as varnishes, can offer better electrical resistance and may be painted onto metallic restorations to give temporary relief from galvanic pain.
Mechanical strength and resistance to flow
Linings and bases must be strong enough and set quickly enough to withstand the placement of the overlying restoration. This is most demanding when amalgam is condensed: high condensation pressures can cause a partially set lining to flow or fracture.
The behaviour of a lining under load depends on:
how far it has set at the time of restoration placement
its inherent strength and thickness
the cavity design and how well supported the lining is by surrounding tooth structure
the magnitude and direction of the forces applied.
In a Class I cavity, the lining is usually surrounded by dentine and well supported; even a relatively weak set cement is unlikely to fracture. In a Class II cavity with a proximal box, the axial wall of the lining is partially unsupported and more vulnerable. If amalgam is condensed directly against this unsupported surface, the lining can fracture at the corner and voids may be left.
Correct technique minimises these problems. Amalgam should be condensed into the proximal box first, supporting the axial wall, before filling the occlusal portion. And in all cases the lining should be allowed to set sufficiently before heavy condensation forces are applied.
Radiopacity and compatibility
Ideally, linings should be radiopaque, so that they can be identified on radiographs and distinguished from recurrent caries or voids. Radiolucent linings make interpretation difficult and can delay diagnosis of caries around a restoration.
Compatibility with the overlying material is equally important. A lining containing eugenol, for example, may inhibit the polymerisation of resin composites and resin-based luting agents. Some acid-base materials may also interact with resin systems if placed incorrectly. The lining should not adversely affect the setting reaction or properties of the final restoration.
Requirements of luting materials
Luting cements are used to fix indirect restorations and appliances (crowns, bridges, inlays, onlays, posts) in place. Their task is delicate: the layer is very thin, yet it must provide retention, sealing, and some degree of cushioning and insulation.
Handling and rheology
A luting cement must provide enough working time to mix the material, apply it to the restoration and/or tooth, and seat the restoration. During seating, it needs to flow readily so that the restoration can reach its final position and the cement layer is thin and uniform.