Long-Term Fluoride-Release Restorative Materials

Barry L. Musikant, D.M.D., F.A.C.D.; Brett I. Cohen, Ph.D.; Allan S. Deutsch, D.M.D., F.A.C.D.
Long-Term Fluoride-Release Restorative Materials

Barry Musikant

OMPOSITE RESINS have replaced many of the materials that were formerly used in dentistry. Posterior and anterior composites and resin cements used with the appropriate bonding agents are now substitutes for amalgam, glass ionomers, and zinc phosphate. An important element associated with long-term success of these newer materials is the release of fluoride. Fluoride is dynamically incorporated into the hydroxyapatite matrix forming the more acid- resistant fluorapatite and thereby rendering the tooth structure less susceptible to subsequent decay. Fluoride has also been shown to reduce the cariogenic potential of bacteria by inhibiting their metabolism.¹
Cohen, et al., recently reported in Oral Health a ten-year fluoride release of four reinforced composite resins; theirs is the only study found in the literature for this duration. The composite resins included in the study are Ti-Core and Ti-Core Natural (Essential Dental Systems, South Hackensack, New Jersey), two core materials that are approximately 80 percent filled and have incorporated titanium and lanthanides respectively to produce compressive strength equivalent to that of dentin.²
The other two composite resins were Flexi-Flow and Flexi-Flow Natural (Essential Dental Systems), cements that are 60 percent filled to allow for greater flow and also incorporate titanium and lanthanide respectively. The four composite resins have continued to demonstrate release of fluoride over a ten-year period. The higher filled Ti-Core samples released fluoride in a burst effect over the first two years and then settled down to a lower level of continuous release. The Flexi-Flow composite resins, on the other hand, released fluoride at a low level from the beginning. The different release patterns make sense because the more densely filled Ti-Core resins leave less room for the efficient incorporation of fluoride while the less densely filled Flexi-Flows have room to incorporate the fluoride more efficiently, consequently releasing it over time in smaller quantities. (See the graphs.)

Graph 1

Graph 2

    Although the mechanics of fluoride release are interesting, the important fact is that fluoride release does not weaken the Ti-Core and Flexi-Flow polymer structures over time. In contrast, glass ionomers and resin-modified glass ionomers weaken as the fluoride is released over time. In real-world dentistry, the fluoride release should occur only when the polymer is exposed to moisture; such exposure occurs at the composite-dentin interface, because dentin is 30 percent water. The amount of fluoride released in these situations is far less than the amount released when the composite resins are completely immersed in water for test purposes.
    Ti-Core core material and Flexi-Flow cement will release their fluoride only at the composite-dentin interface if there is no marginal leakage under the crown. If leakage does occur, more fluoride will be released at the very time when it is most needed. Since we do not expect leakage to occur on the first day of post-and-crown placement, the effective release of fluoride should last far longer than the ten-year release period reported in this study, which subjected the composite resins to an immersion bath. Because the incorporation of the fluoride into dentin is dynamic, meaning that it is a reversible reaction, the fluoride must be released continuously to prevent the eventual loss of the fluoride from tooth structure and the conversion of the fluorapatite matrix back to the hydroxyapatite matrix—and, of course, to maintain its inhibitory effect on bacterial metabolism. Hence, the significance of this long-term fluoride release study.
    Fewer people are exposed to fluoride because many people now drink bottled water, which does not contain fluoride, rather than public water, which does. Despite the beneficial effects of the low-level mass exposure to fluoride in many public water systems, a number of people avoid municipal water supplies for fear of exposing themselves to lead and arsenic. Given this reality, using long-term fluoride-releasing restorative materials in areas of restoration that abut dentin is essential.

References:

Nouri M-Reza, Titley KC. A Review of the Anitbacterial Effect of Fluoride. Oral Health 2003;93(1):8-11.
Cohen, BI, Musikant BL, Deutsch S. Ten Year Fluoride Release from Four Reinforced Composite Resins. Oral Health 2002;92(9):44-52.

February-March 2003

Because many people now drink bottled water, which does not contain fluoride, . . . using long-term fluoride-releasing restorative materials in areas of restoration that abut dentin is essential.

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