What We Know About Fluoride-Releasing Varnish by Clifton Carey, PhD

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What We Know About Fluoride-Releasing Varnish

by Clifton Carey, PhD

Fluoride-releasing varnish (F-varnish) is used for two main purposes: to treat dental hypersensitivity and to prevent tooth mineral loss. The mechanisms for these purposes are related to the composition of the F-varnish.


Application
F-varnish is typically a thick viscous non-aqueous fluid that contains sodium fluoride salt (NaF) suspended within the matrix. The varnish is usually designed to be thick and sticky, to lengthen the contact time of the product on the tooth and allow the maximum uptake of fluoride. The F-varnish is painted onto the cleaned dry tooth, covering the enamel and any exposed dentin surfaces, with a small brush (Fig. 1). Typically, the patient is asked not to close their mouths for a short period of time (15 to 60 seconds) to allow the varnish to make full contact on the tooth and to cover any open tubules. After the patient closes their mouth, saliva contacts the varnish, which in some products stimulates the varnish to harden.
What We Know About Fluoride-Releasing Varnish
Fig. 1: Application of F-varnish


Hypersensitivity
Hypersensitivity occurs when the dentin is exposed and the tubules that penetrate through the dentin to the pulp are open (Fig. 2a). Pain is thought to occur when the movement of fluid through the tubules is increased.1 The treatment of hypersensitivity is simply to plug the tubules, halting the movement of fluid through them and thus stopping the pain (Fig. 2b). The relief of hypersensitivity usually occurs as soon as the F-varnish is applied. The varnish will cover the open tubules and flow into them, resulting in long-term relief of hypersensitivity. The fluoride in the F-varnish will either precipitate as a calcium-fluoride salt or slowly react with the tooth mineral.

Dentin perfusion studies done in the author’s laboratory have found that the fluoride treatment system and all the F-varnishes tested significantly reduced the movement of fluid through the tubules. Scanning electron microscopy verified the open tubules were covered. The thickness of the F-varnish on the surface of the dentin varied greatly from about 1 µm to about 50 µm. The thickness of the residue on the tooth surface could have an effect on how long the applied fluoride will remain on the surfaces.
What We Know About Fluoride-Releasing Varnish
Fig. 2: (a) Open dentin tubules; (b) Treated dentin surface showing covered open tubules and varnish within tubules.


Protection of healthy enamel
Protection of healthy enamel surfaces requires the uptake of fluoride from the F-varnish. Once the F-varnish has been painted on the tooth surfaces, the varnish will absorb water from the saliva and, while hydrating, the NaF dissolves, freeing fluoride ions at the surface of the tooth. The dissolved fluoride reacts with any available dissolved calcium and the tooth mineral. Most of the fluoride rapidly precipitates on the tooth surface as calcium fluoride-like salts, which are moderately soluble. These precipitates, called loosely bound fluoride, will dissolve into the saliva over the next few hours and will not provide any long-term beneficial effects for the tooth other than as a fluoride reservoir during this process until it is dissolved away. The fluoride that does not precipitate reacts with the tooth mineral. Over time that reaction leads to fluoridated mineral, which is less soluble than non-fluoridated mineral. This is called “enamel-bound fluoride” (sometimes “tightly bound fluoride” or “firmly bound fluoride”). It has been shown that enamel-bound fluoride persists for months, resulting in protection of the enamel because of its insoluble nature.2 Non-fluoridated mineral (pre varnish application) is called hydroxyapatite (HAp) and the fluoridated mineral (post varnish application) is called fluoroapatite (FAp). FAp is approximately 100 times less soluble than HAp. The chemical reaction for the production of enamel-bound fluoride is very slow, taking hours to produce FAp (Fig. 3). This is why most fluoride varnishes are designed to adhere tightly to the enamel—to provide fluoride ions during this long process.

Not all F-varnishes produce the same amount of enamel-bound fluoride. Some F-varnishes contain calcium and phosphate salts with the intent that these salts enter the enamel and produce enamel-bound fluoride. While logical, the included calcium-phosphate salts are required to dissolve with an acidic exposure before the fluoride can bond with the calcium. Subsequently, when the calcium salts are dissolved and react with fluoride in solution, most of the resulting calcium-fluoride salts will precipitate on the surface as loosely bound fluoride to be dissolved over the next few hours. Recently, a new fluoride treatment product has been introduced that is water-based and does not produce a sticky coating. It is proposed that it provides fluoride along with calcium and phosphate without the delay needed for hydration, presumably to react with the tooth mineral sooner. As with other F-varnishes, most of the calcium and fluoride precipitates as loosely bound fluoride. The slow reaction for fluoride ions to react with the enamel is not accelerated, and without a varnish layer to hold the fluoride on the tooth surface, the released ions will quickly be washed away by the saliva before any significant enamel-bound fluoride is made. At this time, the formation of additional enamel-bound fluoride to healthy enamel because of the presence of calcium salts included within the F-varnish has not been demonstrated.

A comparison of a few commercial products is given in Figure 4. The enamel-bound fluoride was measured in the same laboratory using human enamel from teeth that had been extracted for other reasons. The amount of time before rinsing the F-product-laden enamel surface followed the manufacturer’s instructions for use. The release time was three hours before extracting the loosely bound fluoride and subsequently the enamel-bound fluoride.
What We Know About Fluoride-Releasing Varnish
Fig. 3: The chemical reaction for the formation of fluorapatite is very slow.


Measurement of enamel-bound fluoride
At this time, the procedures for the measurement of enamel-bound fluoride are not consistent between laboratories. Notable differences are that some laboratories do not distinguish between loosely bound fluoride and enamel-bound fluoride. Others do not normalize the amount of fluoride extracted from the treated enamel surface by the area of the extraction. Some labs do not do either of these important steps. Because human teeth are often not available, some laboratories use bovine teeth as the substrate. Bovine enamel is more porous and therefore takes up more fluoride at a faster rate than human enamel.4 Additionally, it is important to note that the enamel-bound fluoride produced in white spot lesions is often much higher than that for sound enamel.5 Therefore, generally speaking, comparisons of enamel-bound fluoride between laboratories should not be done.

As this data shows, varnishes that adhere to the enamel of the teeth provide significantly greater fluoride uptake than varnishes that do not. The data from our lab indicates that the uptake from more adhesive sticky varnishes is about three times more than fluoride treatments that do not stay on the teeth. While some patients may like that they feel nothing on their teeth, unfortunately they are getting very little of the desired benefits.


Conclusion
Clearly F-varnishes in the marketplace vary greatly in many respects. This includes fluoride concentration, viscosity, supplemental calcium salts, residence time on the tooth, and other features. Nevertheless, the F-varnishes and fluoride treatment tested significantly reduced fluid flow within tubules to treat hypersensitive teeth. Scanning electron microscopy (SEM) studies found the thickness of the treatment residue varied greatly. The thickness of the residue on the tooth surface could influence how long the applied fluoride will remain on the surfaces.

The data presented support the use of F-varnish for long-term protection of healthy enamel from mineral loss with stickier varnish generating significantly greater F uptake than fluoride treatments that do not adhere to the enamel.
What We Know About Fluoride-Releasing Varnish
Fig. 4: The comparison of enamel-bound fluoride for three F-varnishes (Centrix FluoroDose 5% NaF, Colgate Duraphat 5% NaF, and Elevate Oral Care FluoriMax 2.5% NaF) and one fluoride treatment (3M ClinPro Clear Fluoride Treatment 2.1% NaF).

References
1. Addy M. Dentine hypersensitivity: Definition, prevalence, distribution and etiology. In: Addy M, Embery G, Edgar WM, Orchardson R, editors. Tooth wear and sensitivity: Clinical advances in restorative dentistry. London: Martin Dunitz; 2000. pp. 239–48.
2. Weyant RJ, Tracy SL, Anselmo T, et al. Topical fluoride for caries prevention: executive summary of the updated clinical recommendations and supporting systematic review. J Am Dent Assoc 2013;144:1279–91.
3. International Standard ISO 28399 Dentistry — External tooth bleaching products.
4. Lippert F, Hara AT. Fluoride dose-response of human and bovine enamel caries lesions under remineralizing conditions. Am J Dent 2012;25:205–9.
5. Ogaard G, Rolla G, Helgeland K. Fluoride retention in sound and demineralized enamel in vivo after treatment with a fluoride varnish (Duraphat). Eur J Oral Sci 1984;92:190–7.

Author Bio
Clifton Carey, PhD Clifton Carey, Ph.D., is a scientist specializing in fluoride chemistry. He holds a doctorate in analytical chemistry from American University and is a professor at the University of Colorado Anschutz School of Dental Medicine. He has played a leading role in developing standards for fluoride-containing dental products, including serving as chair of the U.S. delegation on Dental Products for ISO Technical Committee 106-SC7 and as convener of work groups on fluoride varnishes and dentifrices. Carey is also fellow of the American Chemical Society, a member of the International Association for Dental Research and consults with Centrix Inc.
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