Fluoride

• Fluoride acts on the hydroxyapatite of enamel by decreasing solubility, improving crystallinity and remineralising
• Fluoride replaces hydroxyl group in hydroxyapatite to make fluorapatite
• Highest concentration of fluoride (up to 5000ppm) is present in the outermost micrometers of enamel
• Fluoride improves enamel crystallinity by filling occasional void in the apatite lattice and stabilising the crystal structure
• Remineralises enamel as the fluoride favors precipitation and there is an increased tendency to precipitate as apatite rather than other less crystal/weaker forms
• When carious lesions remineralise in the presence of fluoride, the average crystal diameter increases significantly, providing a more favorable surface area to volume ratio to reduce subsequent dissolution
• Fluoride acts on bacteria in plaque by inhibiting glycolysis, interfering with enzymatic regulation of carbohydrate metabolism and reducing intracellular and extracellular polysaccharide accumulation
• Fluoride levels have a suppressing effect on S. mutans
• Fluoride has the greatest cariostatic action on the smooth surfaces of the teeth, however pits and fissures are protected to a much lesser degree
• Fluoride acts on enamel surface by desorbing proteins and/or bacteria, lowering the free surface energy, and altering tooth morphology
• Water fluoridation is the controlled adjustment of fluoride in the communal water supply to achieve maximum caries reduction and a clinically insignificant level of fluorosis (will invariably have some mild fluorosis, which will only make the teeth appear whiter with no adverse aesthetic effects)
• In 1928 McKay noted that mottled teeth (i.e. teeth with fluorosis) were less susceptible to dental caries
• Controlled water fluoridation was first initiated in Grand Rapids, Michigan, in January 1945
• Water fluoridation can reduce caries by 50-60% in permanent teeth and 40-50% in primary teeth, and these cariostatic benefits are found to persist into adulthood
• Temperature affects water fluoride concentration as it is an independent environmental variable that influences water consumption (i.e. higher mean temperature should correspond to a lower fluoride concentration)
• Previously, Hong Kong’s water fluoridation levels were different in the summer and winter for 1961-1967, but are now constant (0.5ppm year round from 1988 to now)
• Excessive fluoride inhibits the enzymatic function of ameloblasts, resulting in defective matrix formation and subsequent hypomineralisation, which easily becomes hypoplastic due to abrasion and wear
• A 1986 study found that 18% of 12 year old Chinese children in Hong Kong had enamel fluorosis, discoloration and severe hypoplasia that were aesthetically objectionable
• High sources of fluoride in the Cantonese diet include seafood, soup using bone stock, and a variety of dried seafood
• Water fluoridation alternatives include fluoride supplements, school water fluoridation, fluoridated salt and fluoridated milk
• Fluoride supplements include daily sodium fluoride drops, tablets, lozenges and fluoride vitamin preparations, and can be administered to children, preferably before bedtime to maximise exposure time and for reduced saliva flow
• Fluoride supplements provide a topical effect on the enamel of erupted teeth, as well as a systemic effect on the developing enamel after swallowing
• Fluoride supplements should ideally begin shortly after birth and continue until 12-14 years old (when all permanent teeth have emerged)
• Fluoride tablets should not be prescribed for individuals living in areas where water fluoride levels exceed 0.7ppm
• School water fluoridation is beneficial as children attend school for 200 days of the year, and fluoridation can reduce caries DMFT by 33-35%
• Studies suggest that the effectiveness of fluoridated salt in inhibiting caries is substantial, of the same order as that of fluoridated water when appropriate concentration and use are achieved
• Fluoridated milk can also be an option given that the community has a well-developed milk distribution system, since the technical procedures for producing fluoridated milk are straightforward (but more studies needed)
• Topical fluoride aims to promote and increase fluoride concentration in the surface layer of enamel; since enamel is relatively porous and immature upon eruption, the posteruptive accumulation of fluoride by the enamel surface is an important part of maturation
• Topical fluorides include fluoride gel, fluoride varnish, and fluoride prophylaxis paste
• Two forms of fluoride gel are primarily used: acidulated phosphate fluoride (APF) (more widely used) and sodium fluoride (NaF)
• When fluoride gel is applied, calcium fluoride is formed, which coats the teeth and acts as a potential reservoir of calcium and fluoride ions for remineralisation
• APF formula contains 1.23% fluoride acidulated with hydrofluoric or orthophosphoric acid; has a low pH (3.2-3.5) that produces some dissolution of the tooth surface so that the calcium fluoride can be formed
• APF gel can also be used to etch composite and porcelain restorations
• Thixotropic APF gel is high viscosity under normal conditions but becomes low viscosity when under high pressure (e.g. biting forces), can therefore adhere to the tooth surface for longer and a thin coat under biting forces can penetrate into pits, fissures, and interproximal sites
• Sodium fluoride gel contains 0.9% fluoride and are used when patients have restorations, veneers or crowns as it does not contain acid and will not etch
• Fluoride gel should be left in mouth for 4 minutes and be repeated every 6 months
• However, clinical studies have shown that as much as 78% of topically applied fluoride can be ingested if carelessly applied
• Duraphat fluoride varnish contains 5% sodium fluoride and 2.2% fluoride ion, sets in the presence of moisture and remains on the tooth for up to 12 hours; patient should not eat food for at least 1 hour and not brush teeth or eat abrasive foods for the rest of the day
• Whenever teeth are polished, a thin layer of enamel is abraded and fluoride prophylaxis paste may replenish the fluoride ions lost in the polishing process
• Fluoride mouthrinse provides 35% protection against caries and is recommended for individuals with high caries susceptibility, patients with orthodontic/prosthetic appliances, and patients with medical or physical disabilities