Fluoride in Caries Control

The incorporation of fluoride in a dentifrice has proven to be one of the most effective prevention and intervention strategies toward dental caries.

Commonly termed cavities or tooth decay, caries is the localized destruction of tooth tissues over time by acid, such as lactic acid. This acid is produced in the mouth when cariogenic bacteria in biofilm, such as Streptococcus mutans, ferment dietary carbohydrates.² In the caries process, particular attention is paid to the enamel—the hard, outermost layer—because it is the primary contact with cariogenic bacteria, and it is where caries develops. Enamel derives its hardness from a calcium and phosphate crystal matrix called hydroxyapatite. When acidity reaches a pH of 5.5—called the critical pH—saliva and plaque fluid cease to be saturated with calcium and phosphate, thereby permitting the hydroxyapatite in dental enamel to dissolve in a process called demineralization. If demineralization continues over time, enough mineral content may be lost so that the soft organic material left behind disintegrates, forming a cavity.³

Caries Epidemiology

It is clear from the most recent National Health and Nutrition Examination Survey (NHANES) that oral health has improved since the 1980s for most Americans. However, prevalence is still high.¹ Among children aged 2 to 11, 41% have caries of the primary dentition, and 21% have untreated caries. Caries prevalence increased with age: In the 1999 to 2004 period, 24.2% of children aged 2 to 5 years had caries; and this rose to 49.9% in 6- to 11-year-olds, 57.3% of 12- to 15-year-olds, and 78.2% of 16- to 19-year-olds. In addition, there was an increase in early childhood caries, when caries occurs between the ages of 2 and 5.¹

According to the latest NHANES data, 91% of American adults between the ages of 20 and 64 had coronal caries, with prevalence increasing with age: 85.6% of 20- to 34-year-olds have caries; and this increased to 94.3% in 35- to 49-year-olds, and 95.6% in 50- to 64-year-olds. There was an effect of gender: men had fewer coronal caries compared to women (90.6% vs 92.7%). In adults aged 20 to 64, 14% had root caries, and this prevalence also increased with age: 8% of 20- to 34-year-olds had caries, and this rose to 14.8% of 35- to 49-year-olds, and 21.6% of 50- to 64-year-olds. There was also a difference based on gender, with men having more root caries (15.8%) than women (12.7%).¹

It is evident from the high prevalence of caries in the United States that interventions that prevent the initiation or progression of caries would be beneficial.

The Role of Floride in Enamel Development

After the ingestion of fluoride from a water, food, or supplement source, 86% to 97% of the element is absorbed in the stomach and small intestine. After fluoride is ingested, it is distributed from the plasma to all tissues and organs of the body, and it gradually becomes incorporated into the crystal lattice structure of teeth in the form of fluorhydroxyapatite.^4,5 In teeth, the fluoride concentration is very high on surface enamel, but falls steeply within the first 100 μm. Then, fluoride concentration remains constant up to the enamel–dentin junction. Fluoride concentration once again increases inside the dentin, increasing deeper into the tooth, with fluoride steadily accumulating over a lifetime at the dentin–pulp surface. There is no homeostatic mechanism that maintains fluoride concentration in the body. Therefore, regular exposure is required to maintain fluoride concentration in enamel, saliva, and in biofilm on dental surfaces.^4-6

Fluoride in Caries Protection

Hydroxyapatite starts to dissolve when pH drops below 5.5.⁷ If pH is higher than 4.5 and fluoride is available in low concentrations, fluorhydroxyapatite forms on the surface layers of enamel even if hydroxyapatite dissolves in the subsurface enamel. The overall effect is reduced demineralization thanks to the protective outer layer of fluorhydroxyapatite. When oral pH normalizes after an acid attack and rises again above 5.5, fluoride enhances enamel–dentin remineralization.⁸ If fluoride is no longer available, the oral environment begins to favor demineralization.

Fluoride also exhibits antibacterial properties. Fluoride ions inhibit the bacterial enzyme enolase, which interferes with the production of phosphoenolpyruvate (PEP). PEP is a bacterial source of energy and a molecule that is necessary for the uptake of sugar, the food source of bacteria. Plaque that contains just 1 ppm to 5 ppm of fluoride (an amount that is reached by using fluoridated toothpaste) is found to inhibit the adhesion, growth, metabolism, and multiplication of caries-linked oral Streptococci. The presence of higher concentrations of fluoride—10 ppm to 100 ppm, which can be obtained in prescription fluoride preparations—has also been found to inhibit acid production by most plaque bacteria.⁹

Systemic Methods of Fluroide Delivery

A primary method of fluoride delivery is systemic, with the mineral being artificially provided in water, milk, salt, or supplements, which must be ingested to be able to have any effect on teeth. In all of these applications, the primary action of fluoride in promoting remineralization and reducing demineralization is due to the presence of fluoride in a beneficial amount and at the right time. Water fluoridation is the primary systemic method of delivery fluoride to the American population.⁴ Fluoride occurs naturally in water supplies, usually at very low concentrations of 0.1 ppm. Community water studies have uncovered a few key findings: Overall, there is a 50% reduction in dental caries rates among children with 1 ppm fluoride in the community drinking water.¹⁰ However, this caries protection occurs only with consistent fluoride exposure. This is evident in studies that found that children who move to a nonfluoridated water community experience an increase in caries rates.^11,12 In addition, adults also benefit from fluoride, with reduced coronal and root caries rates among those residing in fluoridated water communities.⁴

In the United States, it is estimated that more than 184 million people (approximately 61% of the population) are served by fluoridated water supply systems. This is a relatively inexpensive endeavor: The annual cost of fluoridating the drinking water for a community larger than 20,000 people in this country averages 50 cents per person. Just $1 invested in this preventive measure yields approximately $38 savings in dental treatment costs.¹³

The Centers for Disease Control (CDC) currently recommend that oral fluoride supplements be used only in high-risk children residing in non-fluoridated areas. The recommended Supplemental Fluoride Dosage Schedule appears in Table 1.¹⁴

Topical Methods of Fluoride Delivery

Another main method of fluoride delivery is topical, in the form of dentifrice (toothpaste or gel), varnishes, paint-on formulations, and mouth rinses that come into contact with the surface of the tooth. The most popular is fluoridated dentifrice. Today, over-the-counter dentifrice products contain between 500 ppm to 2,500 ppm fluoride. Clinical trials indicate a dose-dependent relationship between fluoride concentration and caries prevention, with a 6% increase in efficacy and 8.6% reduction in caries for every 500 ppm fluoride increase.^4,15 To recap the caries-reducing benefits of fluoridated dentifrice: Research has documented that a regular low-dose source of fluoride is the most efficient means to prevent demineralization of teeth and to enhance remineralization. Fluoride becomes incorporated with the enamel hydroxyapatite apatite structure, rendering the enamel more resistant to acid dissolution. Fluoride in saliva and plaque also promotes remineralization. And finally, fluoride has an antimicrobial effect on plaque bacteria.

Sodium monofluorophosphate (MFP) is the most commonly used form of fluoride used in dentifrice. However, sodium fluoride (NaF), a mixture of NaF and MFP, amine fluoride (AF), and stannous fluoride (SnF₂) are also recognized as safe and effective forms of fluoride in over-the-counter therapeutic dentifrices.

There are also prescription fluoridated toothpastes that contain 5,000 ppm fluoride and are intended for limited use. One 6-month study conducted in adults found that 57% of root caries lesions became hard in subjects using a 5,000-ppm toothpaste, compared to 29% for subjects who used a 1,100-ppm toothpaste.¹⁶

Recommendations for Fluoride Dentifrice Use

Clinical studies have found little association between the amount of toothpaste used and anti-caries efficacy; instead, fluoride concentration is the important determinant of anti-caries efficacy. Therefore, using more toothpaste than is recommended (such as a pea-sized amount for children) does not provide more caries protection.⁴

Brushing twice a day is linked to a 20% to 30% lower likelihood of caries compared to brushing once or less daily.¹⁷ While there has been much debate about whether it is better to brush before or after meals, there is no scientific evidence to indicate the better option. However, data does show that brushing immediately before bed plays an important role in reducing plaque load in the oral environment during sleep, when salivary flow and buffering capacity are naturally reduced. Therefore, the recommendation to brush just before going to bed and at least one other time during the day before or after a mealtime is appropriate for most patients.⁴ Fluoridated toothpaste can also be used therapeutically by asking the patient to apply a dab of paste with a finger or brush directly to a cleaned active lesion immediately before going to bed. This also allows an increased concentration of fluoride in the vicinity of the lesion at a time of day when salivary output is naturally low.⁴

Rinsing behavior is another determinant of anti-caries efficacy. Studies show that people who use a cup to rinse with water after brushing (and so put more water in their mouths) have approximately 20% more caries than those who use the toothbrush or hand to collect water. This is because more fluoride is washed away.⁴

Fluoride Mouth Rinses

These are most commonly available as 0.5% NaF (227 ppm F) for daily rinsing and 0.2% NaF (909 ppm F) for weekly rinsing, and may need a prescription even if they are for home use. Mouthwashes have also been formulated with acidulated phosphate fluoride, stannous fluoride, ammonium fluoride, and amine fluoride, although some of these come with precautions. For example, stannous fluoride can cause discoloration of teeth and tooth restorations, and acidulate phosphate fluoride is contraindicated in people with porcelain or composite restorations because it can cause pitting or etching.⁴

Typically, it is recommended that 10 ml of the fluoridated mouth rinse solution is swirled around in the mouth for 1 minute. Clinical trials of both the daily and weekly regimen show an average caries reduction of 30%.¹⁸ The benefit of daily rinsing is marginally greater but not statistically significant. Overall, fluoride mouth rinses are considered beneficial only if groups of people at high risk of caries are being targeted, because they are not cost-effective in a population with a low incidence of disease.

Professional Delivery of Fluoride

When a patient is at extremely high risk of caries, and appropriate dental care measures are not working or not being followed, a dentist should consider professional forms of fluoride delivery.

Professional Fluoride Gels, Foams, and Solutions

These contain higher concentrations of fluoride than products sold over the counter for home use. They typically contain 5,000 ppm to 12,300 ppm and are applied only in the dental office. They are generally recommended for use twice yearly, although in severe cases, they may be applied more frequently. Significant reductions in dental caries—as much as 41%—have been seen when applied in this way.⁴ However, no benefit has been seen with the use of single applications or infrequent applications.

Professional Fluoride Varnishes

These contain a high level of fluoride (22,600 ppm) and are applied only in a dentist's office. Varnishes are used to deliver fluoride to specific sites or surfaces and are typically applied every 3 months or 6 months. The correct application of a fluoride varnish has been linked with a 38% reduction in dental caries.¹⁹ Varnishes are designed to harden on the tooth, forming a deposit of calcium fluoride that can act as a reservoir for the slow release of fluoride over time.

Professional Slow-Release Fluoride

Methods to deliver small amounts of fluoride throughout the day are still being developed. Currently, materials such as silicate and glass-ionomer cements that contain between 15% to 20% fluoride are being used, and this amount of fluoride is also being added to composite and amalgam fillings. The concept is that these materials could provide a reservoir of fluoride to prevent secondary caries and to help remineralize caries in adjacent surfaces. Fluoride release begins high, but reduces as the available reservoir depletes.²⁰

However, glass-ionomer cements are unique because they also absorb fluoride from other sources, such as toothpaste, and also slowly release this into the oral cavity, long after the fluoride that was originally placed in the glass ionomer has dissipated.²⁰ Clinical data on these methods of slow-release delivery have not yet been collected to get a sense of the extent to which they are able to provide therapeutic benefits.

Conclusion

Fluoride is an effective therapeutic and preventative agent for dental caries. The mineral alters the caries process by interfering with the dynamic of caries development by enhancing remineralization, reducing demineralization, and inhibiting bacteria. While there are many forms of fluoride delivery, the incorporation of fluoride in a dentifrice has proven to be one of the most effective prevention and intervention strategies toward dental caries. However, the tried-and-true public health recommendations of proper oral hygiene, such as brushing teeth at least twice a day, flossing to clean in between teeth, and cutting back on dietary sugar intake, continue to be very important in fighting caries.

References

1. Centers for Disease Control and Prevention. Oral Health: Preventing Cavities, Gum Disease and Tooth Loss. Available at: www.cdc.gov/nccdphp/publications/aag/pdf/doh.pdf . Accessed October 27, 2010.

2. Touger-Decker R, van Loveren C. Sugars and dental caries. Am J Clin Nutr. 2003;78(4):881S-892S.

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4. Fejerskov O, Kidd E, eds. Dental Caries: The Disease and its Clinical Management. 2nd ed. Oxford, United Kingdom: Blackwell Munksgaard; 2008.

5. Ekstrand J. Fluoride metabolism. In: Fejerskov O, Ekstrand J, Burt BA, eds. Fluoride in dentistry, 2nd ed. Copenhagen: Munksgaard, 1996:55-68.

6. Weatherell JA, Deutsch D, Robinson C, Hallsworth AS. Assimilation of fluoride by enamel throughout the life of the tooth. Caries Res. 1977;11(Suppl 1):85-115.

7. Moreno EC, Zahradnik RT. Chemistry of enamel subsurface demineralization in vitro. J Dent Res. 1974;53(2):226-235.

8. Moreno EC, Kresak M, Zahradnik RT. Fluoridated hydroxyapatite solubility and caries formation. Nature. 1974;247(5435):64-65.

9. Baron S. ed. Medical Microbiology. 4th Ed. Galveston, Texas: University of Texas Medical Branch; 1996.

10. Arnold FA, Likins RC, Russell AL, Scott DB. Fifteenth year of the Grand Rapids fluoridated study. In McClure FJ, ed. Fluoride Drinking Waters. Bethesda, MD: National Institute of Dental Research, 1962:253-256.

11. Backer-Dirks O. The relationship between the fluoridation of water and dental caries experience. Int Dent J. 1967;17:582-605.

12. Parnell C, Whelton H, O'Mullane D. Water fluoridation. Eur Arch Paedriatr Dent. 2009;10(3):141-148.

13. Centers for Disease Control and Prevention. Cost savings of Community Water Fluoridation. Last reviewed: September 1, 2009. Available at: www.cdc.gov/fluoridation/ fact_sheets/cost.htm. Accessed October 27, 2010.

14. Centers for Disease Control. Dietary Fluoride Supplement Schedule. Last reviewed: April 2010. Available at: www.cdc.gov/fluoridation/other/spplmnt_schdl.htm. Accessed: October 27, 2010.

15. Stephen KW, Creanor SL, Russell JI, et al. A 3-year oral health dose-response study of sodium monofluorophosphate dentifrices with and without zinc citrate: anticaries results. Community Dent Oral Epidemiol. 1988;16(6):321-325.

16. Baysan A, Lynch E, Ellwood R, et al. Reversal of primary root caries using dentifrices containing 5000 and 1100 ppm fluoride. Caries Res. 2001;35(1):41-46.

17. Ashley PF, Attrill DC, Ellwood RP, et al. Toothbrushing habits and caries experience. Caries Res. 1999;33(5):401-402.

18. Ripa LW. A critique of topical fluoride methods (dentifrices, mouthrinses, operator-, and self-applied gels) in an era of decreased caries and increased fluorosis prevalence. J Public Health Dent. 1991;51(1):23-41.

19. Helfenstein U, Steiner M. Fluoride varnishes: a meta-analysis. Community Dent Oral Epidemiol. 1994;22(1):1-5.

20. Toumba KJ, Al-Ibrahim NS, Curzon ME. A review of slow-release fluoride devices. Eur Arch Paedriatr Dent. 2009;10(3):175-182.

About the Author

Kathy Zwieg, LDA, CDA
Minneapolis, Minnesota