Fluoride in dental decay prevention


Key words: fluoride, decay, enamel, prevention


Enamel, with its epithelial structure is the body’s most mineralized tissue. Apatite crystals represent 96% of the enamel’s weight.

Unlike what they appear to be, they do not constitute an inert and impenetrable structure.

Microscopically, they show a microporous aspect that offers diffusion channels along the sheath of the framework prisms.

A more detailed investigation allows to distinguish the presence of the following components:

  • -hydroxyapatite precursors
  • -calcium phosphate apatite crystals with a composition close to hydroxyapatite
  • -organic material around the sheath of the prisms
  • -fluoroapatites
  • -fluoride hydroxyapatites
  • -impurities: carbonate, carbonated apatites, adsorbed magnesium, adsorbed calcium
  • -diphosphonate dihydrate
  • -octocalcium phosphate.

The newly emerged tooth has not yet reached its definitive degree of mineralization. It presents a more porous enamel surface than a mature tooth.

Moreover, mineral density varies on the same tooth: Cervical enamel is less dense than occlusal enamel.

Through buccal environment, the post eruptive maturation is dependent on buccal fluids and mineral ions (calcium and phosphate).

We can observe a succession of demineralization and mineralization cycles that in normal conditions, lead to a decrease in porosity and surface irregularity.

We should notice that the porous surface of a young enamel is more sensitive to acids produced by bacterian glycolysis.

When the tooth has matured, mineral exchanges continue for its whole life between the enamel surface and ions included in buccal fluids.

pH, phosphates and calcium concentrations regulate these exchanges.

With a 7-pH, a reduced number of calcium ions is sufficient to maintain the stability of enamel.

The more pH decreases, the more calcium ions are needed to avoid apatite demineralization.

Under the critical value of 5.5- pH, hydroxyapatite dissolution is irreversible. This results in a calcium and phosphate leakage around the tooth.

The 5.5 pH does not damage fluoroapatite which begins to dissolve when pH reaches 4.6.

The initial carious lesion is materialized by a break of the balance of the demineralization – remineralization cycle of enamel surface.

Mineral ions are lost by enamel: pH decreases and the buccal environment does not provide enough replacement ions.

Mineral ions precipitate on enamel. There is a sufficient concentration in buccal fluids and pH remains above the critical threshold of apatite dissolution.

Decay process:

Tooth decay begins on the enamel ‘s subsurface with a widening of intercrystal compartments and a dissolution of apatite crystals by acids produced from bacteria.

This is macroscopically materialized by white spots.

Dissolution occurs along the edges of the prisms round the sheaths which favour the acid spread. The progress of the lesion always precedes the bacteria apparition.

At this stage, tooth decay is reversible if the buccal fluids bring enough mineral elements as such as calcium phosphate to the enamel surface.

The lesion can be stopped but it is unlikely to recover a complete mineralization. A demineralized subsurface under a mineralized surface often remains.

In the cement which is less mineralized than the enamel, the evolution of the decay is faster.


The enamel surface can also be enriched with fluoridated calcium phosphate which is more resistant to acids.

Fluoride has a significant function in the prevention of tooth decay.

It also helps increasing mineral density of the tooth during all its construction.

Fluoride can be administered in two forms:

  • Fluoride tablets
  • Topical application on dental surfaces with fluoride gel in a mould (mouth tray) or with tooth pastes.

Fluoride in oral taking

Fluoride taken in tablet form integrates into apatite crystals during amelogenesis. This leads to an increase of crystals’ density thus providing a better protection against tooth decay.

However, it should be noted that this prevention is more theorical than real.

If oral hygiene is defective and the patient’s diet is rich in refined sugar, nothing will stop the development of dental plaque and decay will progress.

The purpose of systemic fluoration is to obtain development of fluoroapatites and fluorohydroxyapatites.

We should keep in mind that fluoride is naturally present in food : it can be found in spring waters, green tea, salmon, spinach and lettuce among other.

For spring waters, the concentrations vary according to the springs.

Additionally, fluoride can be added to food (salt).

Therefore, before any prescription, it is necessary to assess the daily fluoride ingestion, to avoid fluorosis.

Recommended prescriptions in the absence of fluoride contained in food are the following:

–       6 to 24 months:
▪Without fluoridated salt : 0,05 mg/kg/day.
▪With fluoridated salt: 0,025 mg/kg/day

–       2 to 4 years: 0,05 mg/kg/day

–       4 to 8 years: 0,05 to 0,075 mg/kg/day.

Overdose leads to fluorosis whose degree depends on the level of the poisoning. The first sign is the apparition of a white spot on the enamel.

A chalky slit enamel is the effect of the process growth.

At a further stage, the enamel disappears and the coronary morphology becomes atypical.

Finally, poisoning can have serious consequences. The lethal dose is 15 mg/kg for children and 32 to 64 mg/kg for adults.

European authorities do not recommend fluoride systemic between 0 and 6 months.

For spring waters, recommended concentrations are between 0.7 mg/L and 1.3 mg/L.

Fluoride in topical application

Fluoride systemically incorporated is not sufficient to protect surfaces efficiently during the demineralization stages.

Fluoride in the interface enamel-oral environment is more efficient in the remineralization process.

Fluoride in topical application has a triple effect:
– Inhibition of enamel demineralization by acids.
– Disturbance of tooth decay bacterial growth and metabolism.
– Activation of the fluoride crystals precipitation on the enamel surfaces.

We commonly use preparations containing sodium fluoride and calcium fluoride.

The application is carried out after descaling and drying out the surfaces which need to be protected.

The product is applied in a thin layer with a brush or a cotton bud (cotton wad).

The prophylaxis is carried out every 6 months.

It is accepted that the decrease of the tooth decay index in industrialized countries is the outcome of topical fluoride common use.

This form of fluorine administration allows to avoid overdosing in decay prophylaxis.

Topical application after orofacial radiotherapy

A topical fluorine daily use is recommended for patients undergoing head and neck radiotherapy.

This application which lasts five minutes, aims at thwarting the unwanted effects of irradiations on salivary glands. It must be prescribed for life.

It ensures an efficient protection against hyposaliva and acidity. This application must necessarily be carried out with a soft plaster mouth tray.

The molecule which is used, is a blend of sodium fluoride and ammonium bifluoride.

The mouth tray and the oral cavity should be washed carefully 30mn after product application.

For children, this prescription should be adapted with a tooth paste whose fluoride concentration higher than the standard recommendation.

For children between 6 and 10 years old, application should be substituted by a daily fluoride mouth rinse.

Fluoride and tooth pastes

There are 2 types of fluoride molecules incorporated in tooth paste.

Inorganic molecules:
– sodium fluoride (NaF)
– tin fluoride (SnF2)
– sodium mono fluoro phosphate (Na2FPO3)

Organic molecules:
–  Aminfluoride (AmF297)
–  Nicomethanolfluorhydrate

A new evidence based study has concluded that the clinical efficiency of sodium fluoride is undisputable.

Tooth paste dosage

Tooth pastes are not associated with toxicity risks but allergies are possible.

Until the age of 10, children swallow 44 percent of tooth paste. This explains the different dosages in fluoride.

Tooth pastes which benefit from a sale authorization, offer 3 dosages:
– 2 for children: 250 or 600 ppm
– 1 for adults: 2500 ppm

Tooth pastes without sale authorization do not exceed 1500 ppm.

European Directives advise the following dosages for a fluoride tooth paste:
– 2 to 6 years old: 450 ppm
– > 6 years old: 1250 ppm.

It has not been proven that prophylactic efficiency is proportional to fluoride concentration.

Comparative studies between the use of pastes with doses up to 1000-, 1500- and 2500 ppm failed to show a difference of protection against tooth decay between the different doses.

European Directives recommend tooth brushing with an up to 500 ppm fluoride paste as soon as the first temporary molars have emerged, and to reserve fluoride systemic use to patients with a high risk of tooth decay after a fluoride assessment.