Fluoride metabolism and fluorosis
Introduction
The benefits of water fluoridation in controlling dental caries are well documented. Fluoride was first used in water for caries control in 1945 and 1946 in the United States and Canada, respectively. The fluoride concentration in the water supply to four communities was adjusted.1, 2 Each study was designed to be of 10-year duration but after 5 years it became so apparent that the trial cities would duplicate the caries reductions seen in cities of similar natural fluoride concentration that the US Public Health Service gave its endorsement stating that ‘communities desiring to fluoridate their communal water supply should be strongly encouraged to do so’.3
The British government sent a mission to the United States and Canada in 1952. It reported back to the UK government and suggested that studies to demonstrate the dental benefits of water fluoridation should be carried out. These studies began in 1955 in Kilmarnock, Anglesey and in Watford. When these investigations were completed, the findings were in accordance with those of the American studies. Water fluoridation was introduced in Dublin in 1964 and over the next 10 years most urban communities in the Republic of Ireland were fluoridated. In 1961–1963, a baseline epidemiological study was carried out to determine the prevalence of dental caries in the school population of the Republic of Ireland. At this time, the percentage of children with no caries in their permanent teeth was 34, 6 and 2% in 8-, 12- and 15-year-olds, respectively. A further study on Children's Dental Health in Ireland was conducted in 1984.4 The authors reported a substantial decrease in the caries level in permanent teeth in both fluoridated and non-fluoridated groups. In the 1984 survey, the ‘Full Fl’ groups contained 69, 23 and 12% of children with no caries in the 8-, 12- and 15-year-old age groups, respectively, and 56, 15 and 8% in the ‘Non Fl’ group.
All US residents are exposed to fluoride to some degree today and widespread use of fluoride has been a major factor in the decline in the prevalence and severity of dental caries in the US and other economically developed countries.5 The effectiveness of drinking fluoridated water on coronal decay in children, adolescents and adults has been studied and proven in numerous community trials and economic evaluations in Canada.6 Approximately 317 million people in 39 countries currently benefit from artificially fluoridated water.7 An additional 40 million benefit from water supplies that are naturally fluoridated. Community water fluoridation schemes have been in existence in the United States for over 50 years and are employed in 39 countries throughout the world including Spain, Switzerland, Canada, the United kingdom, Israel, Singapore and New Zealand. The success of fluoride in controlling dental caries has led to the production of fluoride-containing products including toothpaste, mouthrinses and professionally applied gels or varnishes.
It was previously believed that fluoride needed to be present systemically to be effective in caries prevention. It was assumed that the method of action was due to the incorporation of fluoride into the enamel during enamel formation. This in chemical terms involves the substitution of the hydroxyl ion with the fluoride ion in hydroxyapatite leading to the formation of fluorapatite.8 It was believed that in order for fluorapatite to be formed it was necessary for the fluoride ion to be present during amelogenesis and hence systemic fluoride was essential. However, later work using sophisticated enamel biopsy and fluoride analysis techniques revealed no simple relationship between enamel fluoride levels and caries experience. This view was upheld by further epidemiological evidence in that caries reductions were found in teeth already erupted at the start of fluoridation programmes.9, 10 It became apparent that reduced enamel solubility is not the sole factor involved in the cariostatic action of fluoride.11
At about this time understanding of how a carious lesion develops began to change. It is now known that the early white spot lesion can progress to a cavity, remain static or reverse (remineralise). The presence of fluoride has been shown to promote the remineralisation of white spots and the ‘healed’ lesion has been shown to be more resistant to caries attack than a similar unchallenged site.12 Fluoride is also known to have antibacterial effects.13 It inhibits the process by which cariogenic bacteria metabolise carbohydrates to produce acid. When a low concentration of fluoride is constantly present, Streptococcus mutans produce less acid.14, 15 These advances in knowledge have changed thinking on how fluoride works in caries prevention and thus have an impact on the rational use of fluoride in the community.16 Its method of action is mainly topical but fluoride from water supplies also functions systemically when absorbed and incorporated into developing teeth. Thus, fluoride results in benefits in caries reductions that start in childhood and extend throughout life.
Since the onset of water fluoridation over 50 years ago, there have been numerous claims of harm arising from ingestion of fluoridated water. These have varied from allergic reactions, cancer, birth defects and genetic disorders. The Knox Report 198517 concluded that there was no evidence that fluoride occurring either naturally in water or added to water supplies, was capable of inducing cancer. In 2000 Hillier et al.18 published the results of a population-based case control study, exploring the relationship between fluoride ingestion and the risk of hip fractures. It was concluded that water fluoridated to 1 mg/l does not pose any greater risk of hip fracture. Kaminsky et al. (1990)19 recorded no evidence of skeletal fluorosis among the general US population exposed to drinking water fluoride concentrations less than 4 mg/l. They also found no evidence of increased renal disease or dysfunction in humans exposed to up to 8 mg fluoride per litre in drinking water. The York Review,20 concluded that, other than enamel fluorosis, there was no clear evidence of other potential adverse effects associated with water fluoridation. The Forum of Fluoridation 20027 also reiterated that long-term exposures to fluoride does not have an adverse effect on bone strength, bone mineral density or fracture incidence.
The other risk associated with water fluoridation is enamel fluorosis. The original studies conducted by Dean21 found that the maximum caries reduction in a community served with naturally-occurring fluoride in domestic water supplies was observed at 1 ppm. At this level, however, it was reported that one would expect to see 1% mild fluorosis, 19% very mild and 31% with questionable fluorosis. This gives cumulative total of 51% with some degree of fluorosis and 49% with no change in the appearance of the tooth enamel. At the time of Dean, it was decided that this level of risk (fluorosis) was acceptable taking into account the reduced caries levels. As a result, fluoride was added to the water supplies in 1945 including Grand Rapids, Michigan, Newburgh and New York with Muskegon and Kingston designated as controls. The results of these studies are well known and water fluoridation has subsequently become a widely accepted public health strategy for the control of dental caries. Recently, however, there is increasing evidence from many communities throughout the world that the prevalence of enamel fluorosis is increasing and that in many cases the levels are above those reported by Dean. In the recent systematic review of water fluoridation, the ‘York Review’,20 it was concluded that dental fluorosis of aesthetic concern affected 12.5% of residents of fluoridated communities. This was based on a survey of 12-year-old children in the UK.
This paper is primarily concerned with enamel fluorosis. Its purpose is to review current methods of measuring enamel fluorosis, to assess its prevalence and to address its aetiology and metabolism. In doing so, it will be possible to identify risk factors and employ methods to manage such risk factors and thus, to reduce the prevalence of enamel fluorosis.
Section snippets
Defining and measuring enamel fluorosis
Enamel fluorosis is a hypomineralisation of enamel characterised by greater surface and subsurface porosity than in normal enamel as a result of excess fluoride intake during the period of enamel formation.22 It has also been defined as being ‘a dose response effect caused by fluoride ingestion during the pre-eruptive development of teeth’. This change in the enamel is characterised by altered appearance of the tooth ranging from fine white lines to pitting or staining of enamel.
There are two
Prevalence of enamel fluorosis
In recent years it has been noted that the level of enamel fluorosis is increasing. A study recording the level of enamel fluorosis in the Eastern Health Board in Ireland between 1993 and 1997 was conducted amongst 12-year-olds (Table 1).33 A decrease in the percentage of 12-year-olds categorised as having no dental fluorosis and an increase in the percentage categorised as having very mild, mild or moderate fluorosis was recorded. In the Eastern Health Board the prevalence of fluorosis has
Fluoride metabolism and enamel fluorosis
It is necessary to look at the method by which fluoride acts on enamel to gain an understanding of the way it induces enamel fluorosis. Fluorosed enamel is characterised by a retention of amelogenins in the early maturation stage of development and the formation of a more porous enamel with a subsurface hypomineralisation.45 Secretory enamel is believed to be more susceptible to acute fluoride exposure. The transition/early maturation stage of enamel formation is the most susceptible to chronic
Risk factors for developing enamel fluorosis
A certain degree of enamel fluorosis is inevitable with water fluoridation. Dean regarded an increased prevalence of enamel fluorosis as an acceptable risk when compared to the benefits to oral health that would result from the introduction of this public health measure. However, other fluoride-containing products, such as toothpastes, have become available to the public for consumption since the time of Dean and now there is clear evidence that fluorosis is increasing in the US and worldwide.
Fluoride toothpastes
Fluoridated toothpaste, since its introduction into the European market in the 1970s, now occupies over 95% of the toothpaste market. It has led to a marked decrease in caries in all countries.24 EU guidelines state that fluoride toothpastes sold over the counter should contain no more than 1500 ppm.7 Over the last 10 years it has been noted that an increasing number of infants and very young children have tended to swallow toothpaste and this is likely to be contributing to the increasing level
Fluoride supplements
Fluoride supplements are a risk factor for fluorosis in young children when used inappropriately and not conforming to appropriate dosing schedules. The use and availability of fluoride supplements in Ireland does not appear to be an issue of concern.
Before the American Dental Association (ADA) supplementation schedule, American infants (less than 6 months), who were breast fed, were often prescribed fluoride tablets containing 0.25 mg fluoride per day.55 Mothers would sometimes finish
Infant formula
Larsen et al.68 compared the prevalence of fluorosis in primary and permanent teeth of a population of Danish children who had either been breastfed or fed on cow's milk during the first year of life with that of children from Greenland who had been fed with infant formula reconstituted with fluoridated water containing 1.1 mg/l fluoride. The Greenland children examined (exposed to fluoridated water in infancy) had a higher prevalence of fluorosis in primary teeth, whereas the Danish children
Risk management for enamel fluorosis
It can be seen from the above that a major risk factor in enamel fluorosis is inappropriate use of fluoride toothpaste at a young age. In the US non-compliance to an appropriate dosing schedule of fluoride tablets can lead to an increased prevalence of enamel fluorosis. The fluoride level in drinking water in Ireland should be less than 1 ppm to comply with the Drinking Water Regulations 2000. Having reviewed data from more recent studies, Heller et al.,69 found that little decline in caries
Conclusion
When used appropriately, fluoride is a safe and effective method of reducing dental caries. It is needed throughout life to prevent and control dental decay. To ensure continued use of fluoride toothpaste and water fluoridation, the risks associated with excessive ingestion of fluoride need to be monitored. Use of toothpaste from a young age would seem to be a greater risk for enamel fluorosis than use of infant formula diluted with water. Further research is required to establish the relative
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