Leverett DH. (1991). Appropriate uses of systemic fluoride: considerations for the '90s. Journal of Public Health Dentistry 51: 42-7.
Systemic vs Topical Effects of Ingested Fluorides (p. 44-45)
The relationship between fluoride concentration in drinking water and enamel mottling was established during the 1930s. Further, it was shown that exposure to the fluoride had to occur during the mineralization of the enamel. Dental fluorosis was clearly a systemic condition. Changes in the appearance of the enamel after eruption of teeth were caused by abrasion and extrinsic staining and were not related to posteruptive presence or absence of fluoride. Since this adverse phenomenon was systemic in origin, it was assumed that the associated favorable phenomenon, caries inhibition, was also of systemic origin. Based on this assumption, many studies over the years have attempted, mostly unsucessfully, to demonstrate a relationship between fluoride concentration of enamel and dental caries prevalence.
Only later, on the basis of randomized clinical trials, was it well established that fluoride also has a topical effect. As the topical effect gained respect in the scientific community, there was a corresponding concession that the systemic effect may be less important than originally thought. However, there has never been a randomized clinical trial testing the null hypothesis that fluoride does not have a systemic effect.
...As mentioned previously, the evidence for, and the nature of, a topical effect of fluoride has grown stronger in recent years. Dental caries has been shown to be a dynamic process in which intraoral - i.e., local - circumstances contribute both to demineralization and remineralization of enamel. If the balance tips in the direction of demineralization, the process can pass through the phase of the so-called white-spot lesion and result ultimately in frank cavitation. If the balance tips in the direction of remineralization, the early carious lesion can remineralize, in effect healing itself. The presence of fluoride in minute concentrations at the site of the early carious lesion enhances the ability of the enamel to remineralize and, alternatively, inhibits the demineralization process. This mode of action of fluoride has been documented in several more recent studies. There is some uncertainty whether salivary or plaque fluoride is primarily responsible, but the fact remains that the effect is topical and, although systemic fluoride may be available via the saliva, only local fluoride is necessary for the success of this process.
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