Placing a liner or base has been a time-honored technique to protect and insulate the pulp from the onslaught of materials being placed to restore cavity preps. However, the necessity of placing a liner or base has become questionable. Certainly, a frank pulp exposure needs some type of covering to at least protect it from the mechanical insults of placing different restorative materials over it. Even the very deep preps where you may see “pink” underneath the thin dentin layer still covering the pulp probably need some type of liner.
But the standard of care has changed. There is widespread belief that leaving a layer of deep carious dentin so as not to take a chance in exposing the pulp in an asymptomatic tooth with no clinical or radiographic signs of irreversible pulpal disease gives the pulp the best chance to heal without the rigors of endodontic therapy. In these instances, it is prudent to place some type of bioactive liner that is not offensive to pulpal tissues and, at the same time, encourages the formation of reparative dentin.
The big question, of course, is what is the best type of liner? Or do we need a base, which is really just a thick liner? While calcium hydroxide has been the gold standard, it has largely been replaced by a new generation of calcium-releasing materials. Some of these newer materials are light-cured for clinical expediency, although the resin component in these light-cured versions have come under criticism as not being pulp-friendly.
And what about ionomers? Although dismissed as having low bond strength, possessing poor esthetics, and being old-fashioned, the ionomer family of materials continues to be used by many dentists. This group of materials basically consists of glass ionomers, which have been with us for many decades, and resin ionomers, the somewhat more recent addition to the clan. Both types presumably form a chemical bond to tooth structure and release more fluoride than any other class of materials. On the other hand, the amount of fluoride released by a liner or base buried under a composite material or a crown may be too minimal to have any efficacy.
While all glass ionomers are self-cured, their resin cousins are available in both self-cured and dual-cured versions and form a chemical bond with overlaid composite when used as a liner or base. Etching a resin ionomer to increase its bond strength to resin is neither necessary nor advantageous. On the other hand, inadvertent etching will not interfere with the bond of a resin ionomer to composite.
However, no ionomer is recommended to be placed directly on a pulp exposure. So, if an ionomer liner or base is placed, another, more pulp-accommodating material needs to be placed first and the ionomer is layered on top of it to aid in protecting the pulp. But is this really necessary with the current generation of pulp protective materials? The answer is doubtful, but there is enough uncertainty that keeps some dentists playing it safe and continuing to use these products.
Powder/Liquid (hand-mixed) This is the old standby for ionomers, but it also applies to some MTA products. It allows you to vary the viscosity, does not require any special equipment, any amount can be mixed at the same time, and it is typically still the least expensive alternative. However, it can be very messy, the consistency of the mixes depends on the expertise of the person doing the mixing, and the cleanup can be a nuisance unless you are using disposable spatulas.
Powder/Liquid (capsule-mixed in a triturator) At least one, tricalcium silicate product uses this mixing protocol, but it still needs to be placed manually.
Paste/Paste (hand-mixed) Resin-containing materials, usually dual-cured. Much less messy than powder/liquid, dispensing is typically calibrated, and mixing is fairly easy, taking the human factor somewhat out of the equation. And you can still mix any volume of material that is needed. However, cleanup is still a nuisance, some type of instrument is usually needed to extrude the material, and the cost is higher. Nevertheless, this option is a step up from powder/liquid.
Paste (single-component) Light-cured, syringe dispensed. By far, the easiest of all types of liners to use. All contain resin.
Flow Graded on a grid going from 0-5, with 0 being very runny and 5 having virtually no flow.
Hardness Discs of material are fabricated and then placed in a digital hardness tester to determine the Knoop hardness (KHN). Harder materials may resist occlusal forces better than softer ones.