Oral Health Group
Feature

Fluoride Revisited:

January 1, 2012
by Jessica Dobrinski, DMD, Molly Ehrlich, BSc, DMD and Claire Kanasewich, BSc, DDS


The World Health Organization states “fluoridation of water supplies, where possible, is the most effective public health measure for the prevention of dental decay.”1 Such a compelling statement demonstrates the global impact that water fluoridation has had since its introduction in the early 20th century. Despite this endorsement, water fluoridation remains a controversial topic in Canada. The issue makes its way into the media as cities re-examine the decision of whether or not to fluoridate their water supplies. It is indeed ironic, that the beneficial effects of fluoride in drinking water were discovered while Trendley Dean and his colleagues were studying its negative effects on health. It was, while trying to show that fluoride was the chemical compound responsible for causing unsightly brown spots on tooth surfaces, the telltale signs of dental fluorosis, that the group instead uncovered its beneficial effects in protecting tooth enamel. This true story shows to highlight that, like any drug, fluoride has the potential to do both good and harm. Through in-depth research and questioning into fluoride’s mechanism of action, fluoride has been shown to have safe and beneficial effects for the public.2,3 This article provides answers to some of the commonly asked questions regarding the use of fluoride as a preventative oral health measure.

Q: How Does Fluoride Work?

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A: There are three main mechanisms by which fluoride prevents caries following the eruption of teeth: one, by preventing demineralization; two, by enhancing remineralization; and, three by interfering with bacterial metabolism, which causes a decrease in acid production.4

The mineral component of teeth is comprised of calcium, phosphate and hydroxyl ions arranged in acrystal lattice structure known as hyrdoxyapatite. During the mineralization of enamel and dentin, other ions such as carbonate, fluoride and chloride can be incorporated into the hydroxyapatite structure. The carbonated form of hydroxyapatite is more soluble and thus more easily demineralized by acids. In contrast, the fluoridated form of hydoxyapatite, fluorapatatite, is less soluble and more resistant to demineralization. As enamel is always undergoing demineralization and remineralization, the presence of fluoride in plaque fluid allows the fluoride to be incorporated into the enamel surface to form fluorapatite that is more resistant to demineralization by bacterial acids.

Fluoride promotes remineralization by associating with demineralized enamel and attracting calcium and phosphate ions to the surface. Once there, a new peripheral layer of hydroxyapatite, similar in solubility to fluorapatite, and more resistant to demineralization, is formed.5

Fluoride decreases bacterial acid production by interfering with a key metabolic enzyme, enolase. Fluoride in its ionized form cannot cross bacterial membranes. When bacteria produce acid and lower the pH of dental plaque, fluoride combines with H ions to form HF. In this form, fluoride is able to cross bacterial membranes and enter the cell. Once inside the cell, the fluoride ions separate and inhibit the enzyme enolase, effectively shutting down the metabolic pathway of bacterial acid production.6

Q: How is fluoride delivered?

A: There are multiple modalities by which fluoride is delivered to the general population. In a broad context, these can be classified into mass delivery via water fluoridation programs, in-office preparations, and at home regimens.
Communal water fluoridation is the most effective way to reduce caries in the general population.7 The nature of this delivery medium is such that all residents have access to the benefits of fluoride. The effects of fluoridating the water can extend to communities without a fluoridated water supply through a phenomenon known as the ‘halo effect.’ This effect occurs when food and beverages prepared with fluoridated water are consumed in non-fluoridated communities, sharing the benefits of their fluoridated water.

Fluoride gels, foams and varnishes can all be used for in-office topical fluoride application. They vary in fluoride concentration and in exact application techniques. Fluoride varnishes that have been used in Europe for decades, have recently gained popularity in North America.8 The benefit of fluoride varnish is its ability to set quickly and be retained on teeth for a longer period of time. In addition, the amount required is much less than gels or foams, decreasing the amount swallowed post treatment. Varnishes and gels have been proven to be equally as effective at preventing caries.9

Fluoride supplements in the form of drops or tablets can be prescribed to high-risk patients or to those that do not reside in municipalities with a fluoridated water supply. The amount prescribed is calculated based on patient age and the amount of fluoride found in their drinking water.

A more recent innovation in fluoride delivery is the slow releasing fluoride device.10 This delivery modality is based on the finding that saliva fluoride levels affect the incidence of caries. Fluoride is released at a constant rate from the device into the oral cavity increasing intra-oral fluoride levels. There are three main types of slow releasing devices, copolymer membrane type, glass bead and the NaF/hydroxyapatite/Eudragit controlled type. The devices are bonded to the teeth and can effectively release fluoride into the oral cavity at a constant rate from months to years at a time. Retention of the devices on the teeth can be problematic, especially in children and adolescents. The hope is that the seslow releasing devices can be of great benefit to patients with little compliance for daily oral hygiene regimens.

Daily home use of fluoride toothpastes and mouth rinses are a main delivery modality for fluoride. The widespread use of fluoride toothpastes has largely contributed to the general decline in dental caries.11 In recent years fluoride mouth rinses have become commercially available to the public. A 2009 review found that fluoride mouth rinses reduce caries prevalence in children, with a greater effect in childrenwith high DMFS scores.12

Q: What are the systemic effects of fluoride?

A: It was a long-standing belief that fluoride’s most important effects were pre-eruptive with its incorporation into developing tooth enamel, leading to a less soluble enamel apatite, which is more resistant against acid attack.13,14 This believed mechanism of action lead to the hypothesis that systemic ingestion of fluoride during tooth formation could provide protection against dental caries once teeth were fully erupted.3 The result was that fluoride was introduced to the water supply as a public health measure. Fluoride ingestion during tooth formation can also cause permanent hypomineralization of tooth enamel, known as dental fluorosis, which manifests as an altered clinical appearance on the enamel surface. This can range from un-noticeable changes in the opacity of the tooth, in milder forms of fluorosis, to noticeable dark pigmentation or ameloblastic disruption in moderate to severe cases. 3,15-18 Fluorosis is a dose-dependent relationship, and although threshold levels have been suggested, no data can definitively show a threshold value below which fluoride will not affect dental enamel to some degree.19 From a qualitative standpoint, there is some level of measurable change in amelogenesis and dentinogenesis that can be seen either microscopically or clinically. These changes can be seen when the water fluoride concentration is as low as 0.2mg/L, a concentration that is considered relatively low.14,18,20

As a systemically ingested compound, it is important to understand and address the fact that fluoride affects tissues other than teeth. Evidence of fluoride causing harm when administered at optimal water fluoridation levels does not exist, however, resear
ch studies have investigated the notion that systemically administered fluoride can cause such adverse effects as bone fractures, osteoporosis, cancers of the bone, intellectual deficiencies, senile dementia, Alzheimer’s disease, goiters, and congenital malformations. The quality of evidence for these claims is low, and is unable to show a direct cause and effect relationship between fluoride and any of the above-mentioned ailments.3,16,21-23 These misconceptions should not deter dental providers from advocating for the safety and efficacy of community water fluoridation programs.

Q: Why add fluoride to water if fluoride’s effects are mainly topical and not systemic?

A: Addition of fluoride to water is “a cornerstone of modern, non-invasive dentistry” and is a cost-effective way to provide basic caries prevention to many people.3 While it is true that when incorporated into the crystal lattice during tooth formation, fluoride provides structural stability and decreased solubility, it is now generally accepted that fluoride exerts an even greater effect through post-eruptive mechanisms. Consumption of fluoridated water provides two opportunities for fluoride to exert its anti-caries effects. First, topically when the fluoride comes into direct contact with the tooth surface during ingestion. Second, ingested fluoride is taken up into salivary glands and secreted into the oral cavity as a component of saliva to coat the teeth during and after acid attacks.13

Q: Do cities without water fluoridation have higher caries rates?

A: Numerous studies have repeatedly demonstrated that community water fluoridation reduces the incidence of caries both in the primary and permanent dentitions.23 Consequently, studies have been done worldwide to compare communities with and without water fluoridation. It has been shown that when municipal water fluoridation is discontinued, a rise in caries incidence ensues.7

In their 2010 study comparing three communities in New South Wales, Arora et al. found that children in the non-fluoridated community had higher caries prevalence in the permanent dentition compared to children in the fluoridated communities.24 The mean DMFT of children in the non-fluoridated community was 0.69 versus 0.33 and 0.29 in the two communities with fluoridated water. Attwood and Blinkhorn looked at the dmft and DMFT of children in Stranraer, Scotland five years after their water supply had ceased to be fluoridated.25 Their dmft and DMFT scores were compared with those of children from Annan, a nearby city without water fluoridation. five years after water fluoridation had ceased, Stranraer’s mean dmft had worsened by 24% and it’s mean DMFT by 50%. Moreover, these DMFT and dmft values were found to be very close in value to those for Annan’s children who never had a fluoridated water supply.

The studies demonstrate a clear relationship between water fluoridation and its positive effect on caries prevention. Overall, the evidence shows that communities without water fluoridation have increased caries rates and that removal of fluoride from a community’s water supply greatly impacts the oral health of its residents.

Q: Is the amount of Fluoride in Ontario municipal water supply within the optimal range?

A: The decision whether to fluoridate a city’s water supply is undertaken by the municipal government. There are several municipalities within Ontario that do not fluoridate their water supply. Natural levels of fluoride are often found in lakes, rivers and well water.26 In Canada, the Maximum Allowable Concentration (MAC) for water fluoridation is 1.5mg/L. In Ontario, the Safe Drinking Water Act requires that fluoridated water systems maintain a fluoride level between 0.5 and 0.8mg/L.27 Health Canada recommends a target fluoride concentration of 0.7 mg/L. This concentration is based on the recommendations of a 2008 report from a Fluoride Expert Panel and aims to maximize the preventative benefits of fluoride while minimizing its unwanted effects.28 A study by Kenny et al (2009), examined the levels of water fluoridation in 17 rural communities and 17 urban communities. Of the urban communities surveyed, 82% fluoridated their water. In those communities that fluoridated their water, the fluoride levels ranged from 0.58 – 0.78mg/L, while the non-fluoridated sites ranged from 0.07-0.3mg/L. The study found that 18% of rural communities fluoridated their water. Further, the study discovered that of those communities that fluoridated their water, the concentrations ranged from 0.41-0.58 while the non-fluoridated community levels were from <0.01 – 1.9mg/L.29 This data reveals that in the majority of fluoridated communities, the water fluoride concentrations are within the beneficial range for fluoridation and none are above the limit.

Q: What is the cost- benefit of water fluoridation?

A: The cost of fluoridating municipal water supplies in Canada has been estimated at $0.60 to $1.00 per person.30 While opponents of water fluoridation may argue that this cost seems high, cost-benefit analyses find that community water fluoridation is indeed a cost saving effort. The Centre for Health Economics and Policy Analysis (CHEPA) conducted a review of the literature on the economics of water fluoridation programs and converted the findings to 2003 Canadian dollars. Of the eleven studies evaluated, all concluded positive savings from community water fluoridation programs. The reported benefit-cost ratios varied greatly and were related to the population size being studied, with larger populations having higher ratios. In the analysis by Griffin et al (2001), The CHEPA report found that in communities larger than 20,000 people the advantage of water fluoridation translated to an annual net benefit of just under $24 (Canadian 2003 dollars) per person.31 A more recent study by O’Connell et al (2005) estimates that Colorado community water fluoridation programs saved the community $60.78 (USdollars) per person.32 The cost savings reported by water fluoridation vary greatly, but are substantial enough to justify the continuation of water fluoridation programs. These cost benefits, in conjunction with the treatment’s effectiveness in preventing caries, make water fluoridation worthwhile.  OH

Dr. Jessica Dobrinski is a dental resident in the Department of Dentistry at The Hospital for Sick Children in Toronto, Canada.

Dr. Molly Ehrlich is a dental resident in the Department of Dentistry at The Hospital for Sick Children in Toronto, Canada.

Dr. Claire Kanasewich is a dental resident in the Department of Dentistry at The Hospital for Sick Children in Toronto, Canada.

Oral Health welcomes this original article.

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