Fluoridation

5. What role does fluoride play in preventing tooth decay?

The SCHER opinion states:

Mechanism of fluoride action in caries prevention

Fluoride treatment regimens have been developed to prevent dental caries. Systemic fluoride is easily absorbed and is taken up into the enamel during the period of pre- eruptive tooth formation. The predominant beneficial cariostatic effects of fluoride in erupted teeth occur locally at the tooth surface. This could be achieved by fluoridated toothpaste, fluoride-containing water, fluoridated salt, etc. maintaining elevated intra- oral fluoride levels of the teeth, dental biofilm and saliva throughout the day.

Dental health and fluoridation

Figure 2 indicates that independent of the fluoridation policies across the EU Member States, there has been a consistent decline over time in tooth decay in 12 year old children from the mid-1970s, regardless of whether drinking water, milk or salt are fluoridated.

Figure 2 – Trends in tooth decay in 12 year olds in European Union countries (from Cheng et al. 2007).

It should be noted that there is a probable error regarding the figures from Germany because the data were collected during the unification period. Moreover water fluoridation was not practised in West Germany, and in East Germany only in certain regions and intermittently. Therefore, Germany should be placed under “no water- fluoridation”.

A vast number of clinical studies have confirmed that topical fluoride treatment in the form of fluoridated toothpaste has a significant cariostatic effect. Other preventive regimens include fluoride supplement and fluoridated salt given during the period of tooth formation. In the 1970s, fluoridation of community drinking water, aimed at a particular section of the population, namely children, was a crude but useful public health measure of systemic fluoride treatment. However, the caries preventive effect of systemic fluoride treatment is rather poor (Ismael and Hasson 2008).

In countries not using water fluoridation, improved dental health can be interpreted as the result of the introduction of topical fluoride preventive treatment (fluoridated toothpaste or mouth rinse, or fluoride treatments within the dental clinic). Other preventive regimens include fluoride supplements, fluoridated salt, improved oral hygiene, changes in nutrition or care system practices, or any change that may result from improved wealth and education in these countries. This suggests that water fluoridation plays a relatively minor role in the improvement of dental health.

The role of fluoride on dental health has been demonstrated by comparing the efficiency of naturally occurring low and high fluoride concentrations in tap water to prevent dental caries. A recent study showed an inverse association between fluoride concentration in non-fluoridated drinking water and dental caries in both primary and permanent teeth in Denmark. The risk was reduced by approximately 20% at the lowest level of fluoride exposure (0.125-0.25 mg/L) compared to less than 0.125 mg, and the reduction was approximately 50% at the highest level of fluoride exposure (more than 1.0 mg/L) (Kirkeskov et al. 2010). The data were adjusted for socio-economic factors.

Water fluoridation

Water fluoridation was considered likely to have a beneficial effect, but the range could be anywhere from a substantial benefit to a slight risk to children's teeth with a narrow margin between achieving the maximal beneficial effects of fluoride in caries prevention and the adverse effects of dental fluorosis (McDonagh et al. 2000).

The available evidence suggests that fluoridation of drinking water reduces caries prevalence, both as measured by the proportion of children who are caries free and by the mean change in dmft/DMFT score (decayed, missing and filled deciduous –dfmt– or permanent –DFMT– teeth)1. The studies were of moderate quality (UK-CRD 2003), supported by a Canadian review (Locker 1999), with the addition that the effect tends to be more pronounced in the deciduous dentition. The few studies of water fluoridation discontinuation do not suggest significant increases in dental caries.

The effect of water fluoridation tends to be maximized among children from the lower socio-economic groups, so that this section of the population may be the prime beneficiary. There appears to be some evidence that water fluoridation reduces the inequalities in dental health across social classes in 5 and 12 year-olds, using the dmft/DMFT measure. This effect was not seen in the proportion of caries-free children among 5 year-olds (McDonagh et al. 2000). In a recent review, Health Canada has concluded that the optimal concentration of fluoride in drinking water for dental health was 0.7 mg/L (http://www.hc-sc.gc.ca/ewh-semt/alt_formats/hecs- sesc/pdf/consult/_2009/fluoride-fluorure/consult_fluor_water-eau-eng.pdf).

In a study of students (16-year olds) living on the border between the Republic of Ireland (fluoridated water) and Northern Ireland (non-fluoridated water) it was found that some of the variance in decay experience among the adolescents was explained by parental employment status. The higher decay experience in lower socio-economic groups was more evident within the non-fluoridated group, suggesting that water fluoridation had reduced oral health disparities (CAWT 2008). Similarly, Truman et al. (2002) and Parnell et al. (2009) concluded that water fluoridation is effective in reducing the cumulative experience of dental caries within communities, and that the effect of water fluoridation tends to be maximized among children from the lower socio-economic groups. Furthermore water fluoridation offers additional benefits over alternative topical methods because its effect does not depend on individual compliance.

The benefits of water fluoridation for adult and elderly populations in terms of reductions in coronal and root decay are limited (Seppä et al. 2000a, Seppä et al. 2000b).

Fluoridated foods and dietary supplements

There is no consistent information on the efficiency of fluoridated milk compared with non-fluoridated milk on dental health. For permanent teeth, after 3 years there was a significant reduction in the prevalence of DMFT (78.4%, p<0.05) between the test and control groups in one trial, but not in the other. The latter study only showed a significant reduction in the prevalence of DMFT until the fourth (35.5%, p<0.02) and fifth (31.2%, p<0.05) years. For primary teeth, again there was a significant reduction in the DMFT (31.3%, p<0.05) in one study, but not in the other. The studies suggest that milk fluoridation is beneficial in the prevention or reduction of caries especially in permanent dentition, but the available data are too limited to reach a conclusion (Yeung et al. 2005). However, recent studies have concluded that milk fluoridation may be an effective method for preventing dental caries. (AU-NHMRC 2007).

The effectiveness of fluoride supplemented foods has not been investigated systematically. The effectiveness of salt fluoridation at reducing dental caries has been assessed in cross-sectional studies in Mexico, Jamaica and Costa Rica. These studies are all considered of simplistic methodological quality. However, the data suggest that salt fluoridation reduces caries in populations of children aged 6-15 years (AU-NHMRC 2007).

Several studies from Switzerland suggest that the decline in caries after introduction of fluoridated salt is not drastically different from the one obtained by introducing dental hygiene in schools (Marthaler 2005).

The benefits of preventive systemic supplementations (salt or milk fluoridation) are not proven. There is also only weak and inconsistent evidence that the use of fluoride supplements prevents dental caries in primary teeth. Available evidence indicates that such supplements prevent caries in permanent teeth, but mild to moderate dental fluorosis is a significant side effect. (Ismail and Hasson 2008).

Topical fluoride treatments

Topical application of fluoride in the oral cavity has two advantages: a) application at the site of action; and b) reducing the systemic exposure since in subjects with an adequate spitting response, only a percentage (adults 10%, young children 40%) of the fluoride applied becomes systemically available.
The effectiveness of topical fluoride treatments (TFT), i.e. fluoride varnish, gel, mouth rinse, or toothpaste on dental health have been compared (Marinho et al. 2002, Marinho et al. 2003a, Marinho et al. 2003b, Marinho et al. 2003c, Marinho et al. 2004a, Marinho et al. 2004b, Salanti et al. 2009). Comparisons were made with a placebo treatment in children between 5 and 16 years old for at least 1 year. The main outcome was caries increment measured by the change in decayed, missing and filled tooth surfaces. There was substantial heterogeneity, but the direction of effect was consistent. The effect of topical fluoride varied according to the type of control group used, the type of TFT used, mode/setting of TFT use, initial caries levels and intensity of TFT application, but was not influenced by exposure to water fluoridation or other fluoride sources. Supervised use of self-applied fluoride in children increases the benefit. The relative effect of topical fluoride may be greater in those who have higher baseline levels of D(M)FS. These results are clearly in favour of a beneficial effect of topical fluoride treatment. There was no evidence of adverse effects of topical fluoride treatments (Marinho et al. 2003b). The authors did not consider analyses on specific time-windows or by regions.

The same authors also found that the combined regimens achieved a modest reduction (10%; 95% CI: 2-17%) of dental caries compared with toothpaste used alone (Marinho et al. 2004a). There was no clear evidence that any topical fluoride modality is more effective than any other (Salanti et al. 2009). The AU-NHMRC (2007) and a group of Swedish scientists (Petersson et al. 2004, Twetman et al. 2004) carried out additional reviews on the topic. The results do not challenge the above conclusions.

However, Twetman et al. (2004) point out that long-term studies in age groups other than children and adolescents are still lacking.

Conclusion

Water fluoridation as well as topical fluoride applications (e.g. fluoridated toothpaste or varnish) appears to prevent caries, primarily on permanent dentition. No obvious advantage appears in favour of water fluoridation compared with topical prevention. The effect of continued systemic exposure of fluoride from whatever source is questionable once the permanent teeth have erupted.
SCHER agrees that topical application of fluoride is most effective in preventing tooth decay. Topical fluoride sustains the fluoride levels in the oral cavity and helps to prevent caries, with reduced systemic availability. The efficacy of population-based policies, e.g. drinking water, milk or salt fluoridation, as regards the reduction of oral-health social disparities, remains insufficiently substantiated.