It has become popular to routinely place a flowable composite (e.g., Filtek Flow, Flow-It ALC, Tetric Flow, Revolution Formula 2) on the pulpal floor and axial wall of a Class II preparation prior to restoring the tooth with a packable resin composite (e.g., Pyramid, SureFil, Solitaire 2, Prodigy Condensable).
In fact, some manufacturers of packable and flowable composites include recommendations in their instructions to do so. Clinicians usually place a flowable liner because it reduces the bulk of packable composite that has to be placed. This makes it easier and less time consuming to restore the tooth. Others believe it helps reduce leakage at the tooth/resin interface because the liner is flexible and absorbs some of the packable composite’s shrinkage as it cures.
This, at least theoretically, may result in a better bond between the resin and tooth with little or no gap being formed. There is some evidence supporting this theory. Finally, some users place a flowable because it contains fluoride, and they believe that the fluoride release will have a anti-cariogenic effect.
If you routinely place a flowable composite as a liner before restoring a tooth with a resin composite, be it a microhybrid or packable, you should be aware of some precautions to take. First, the flowables are essentially “thinned down” composite resins, which accounts for their appealing characteristic of easy placement. The thinning down process is accomplished, at least in part, by incorporating fewer filler particles into the resin. As a result, physical properties such as strength and resistance to fracture are lower.
So we should be mindful of the need to place a flowable in a relatively thin layer. Also, a study published a few years ago found that a number of then currently-available flowable composites lacked a sufficient degree of radiopacity. This means that on radiograph the flowable would appear as a thin, radiolucent line extending from the margin to the axial wall. Without a well-documented record, a clinician could misinterpret this as caries, possibly secondary to microleakage. Unfortunately, cases have been reported where the otherwise acceptable resin composite restoration has been removed only to find that the radiolucent “line” was a non-radiopaque flowable resin.
Perhaps the best reason for using a flowable resin as a liner beneath a packable composite is to make it easier to pack the composite into the preparation. Packables are thick, and it can be difficult to place them in a preparation (especially one that is irregular with undercuts) without producing voids. By placing a flowable resin liner into areas of the preparation that are difficult to access, the potential for producing voids is reduced.
The bottom line is not that we shouldn’t use flowable resins as liners, but that we need to be aware of their limitations, so that we choose the right flowable product and use it sparingly so that its lesser physical properties do not compromise the clinical success of the packable resin restoration.
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