Fluoride in dental decay prevention

FLUORIDE IN DENTAL DECAY PREVENTION

Dr A. DELAMARE*, Dr J.MOREAU
*Chirurgien dentiste / Odontologie pédiatrique /
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Key words: fluoride, decay, enamel, prevention


ENAMEL AND DENTAL DECAY

Enamel, with its epithelial structure is the body’s most mineralized tissue. Apatite crystals represent 96% of the 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 us 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 an mineralization cycles that in normal conditions, lead to a decrease in porosity and surface irregularity.

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

5.5 is the critical pH value under which hydroxyapatite dissolution is irreversible. The result is 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.



Demineralization:

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


Remineralization:

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



Decay process:

Tooth decay begins on the subsurface of enamel 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 that of the bacteria.

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 a complete remineralization is an exception. A demineralized subsurface under a mineralized surface remains.

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




FLUORIDE AND DECAY PREVENTION


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

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

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



There are two ways to administer fluoride:

-Taking 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 tabletform will integrate into apatite crystals during amelogenesis in order to increase their density and thus to produce a better protection against tooth decay.

However, this prevention is more theorical than real.

If oral hygiene is defective and the patient’s diet 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.



Let’s keep in mind that fluoride is  naturally part of food :it can be found in spring waters, green tea, salmon, spinach and lettuce.

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

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



So, 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:

-       6 -> 24 months_: ▪Without fluoridated salt : 0,05 mg/kg/day.

                                         ▪With fluoridated salt: 0,025 mg/kg/day_

-       2 -> 4 years_: 0,05 mg/kg/day__

-       4 -> 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 a white spot on the enamel.

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

At a further stage, the enamel has disappeared and the coronary morphology is 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 including 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 prophylaxy 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.

With this type of fluorine supply, overdosing is avoided in decay prophylaxy.



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 insures 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 will be carefully washed 30mn after the end of the application.



For children, this prescription will be adapted with a tooth paste whose fluoride concentration is higher than the recommended standard.

If the child is between 6 and 10 years old, the application will 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 have no 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

- after 6: 1250 ppm.



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

Comparative studies between the use of pastes with doses up to 1000-, 1500- and 2500 ppm have not enabled it to be determined which concentration offers the best protection against tooth decay.


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.