Mechanism of action of fluoride• Florine is the most electronegative element and therefore highly reactive. It does not occur in the elemental state (F2) in nature but is widespread in nature, occurring in fresh water, vegitables, blood, milk and organic compounds.• Since fluoride can affect both the inorganic tooth structure & the bacterial metaboloims in plaque, several mechanism of action have been proposed.
• Reduced enamel, solubility, improved crystallinity, promotion of reminerlization, lower free surface energy, desorption of protein and bacteria, reduced cariogenic flora & inhibition of bacterial enzymes systems.• Many of these mechanisms probably work simultaneously depending on the cnsentration and form of fluoride but the exact contribution of any one of the total careis reduction is unknown.
Systemic Effect of Fluoride• The main in organic constituent of tooth and bone is hydroxy apatite (HAP)• Hydroxy apatite on addition of fluorine results in the formation of flour apatite (FAP) or fluoridated hydroxy apatite becaue not all the hydroxyl groups are replaced by fluorides.• A pure fluorapatite crystal would contain 38,000 ppm F but enamel form a fluoridated area contain only 500 to 2000 ppm.• And this leads to speculation on several possible mechanims of action of systemically ingested fluoride improved crystalinity, the void theory, FAP V/s HAP solubility in acid & iproved tooth morphology.
• Improved crystallinity :- This theory is explained by x-ray diffraction method - Fluoride increases the crystal size and less produces, less strain in crystal lattice. This takes place through conversion of amorphous calcium phosphate into crystalline hydroxy phosphate.
• Void theory :- void in the crystals decreases the stability and increases chemical reactivity. If fluoride fills these void in the hydroxy apetite crystals it will attain stable from with formation of more and stronger hydrogen bonds. Greater stability will leads to lower solubility and hence greater resistance to dissolution in acids.
• Acid Solubility (FAP V/s HAP) The simplest explanation for the decreased solubility of fluoridated enamel is that fluorapatite (with a solubility product constant of 10-60) is less soluble than hydroxyapatite (whose solubility product is constant between 10-55 and 10-60) 46 However, the amount of fluoride in surface enamel from the teeth of persons living in a fluoridated area is only 500 to 2000 ppm F-. This is but a fraction of the theoretie amount of fluoride in fluorapatite (38000 ppm F in enamel). Obviously little of the enamel is composed of fluorapatite. However the actual difference in the amount dissolving is usually so small that it is not likely to be factor in cariostasis.
• Tooth Morphology:- Quantitative studies on the influence of fluoride in the morphology of human teeth showed that person living in a high fluoride areas shows smaller cusp height and less convexity According to Wallensus, tooth in a high fluoride area were 1.7% wider then control.
Topical Effect Of Fluoride• Topical effect of fluoride may be due to either low or high concentration of fluoride and frequent or sporadic exposure.• Bacterial Metabolism & fluoride:- Once the teeth erupt into oral cavity, the systemic effects of fluoride on enamel ceases fluoride is relieved to have an effect on the glycolates pathway of oral micro organism.
• Enzyme Inhibition
• Fluoride has enolase inhibition effect and it also inhibits glucose transport, enolase is a metallo enzyme that requires adjavalent cation for tis activity., fluoride due to its increased reactivity forms a complex with this cation. Thus inhibiting the enzyme. It also inhibits non-metallo enzyme like phosphatage thus leading to reduce acid production.• Suppression of flora :- Stanous fluoride is a potent suppressor of the bacterial growth because it oxidizes the thiol group present in bacteria thus inhibiting bacterial matabolism.
• Antibacterial action:- The concentration of fluoride above 2 ppm in solution progressively decrease the transport of uptake of glucose into Cells of streptococci and also reduces ATP synthesis. Action of fluoride on tooth surface• Fluoride incorporation in enamel – Incorporation of fluoride into enamel through out development is not a principal mechanism of cario statics effect. It is believed that pre eruptive exposure to fluoride may produce teeth more resistance to caries by making pits and fissures shallower.
• Pre-eruptive in corporation – fluoride gets incorporated in the fluid filled sac, which surrounds the developing tooth. It then enters the developing enamel. Highest concentration of fluoride is seen in enamle crown located at or near the tooth surface.• Post eruptive incorporation – fluoride continuous to enter the enamel surface, causing crystal to change from predominatly carbonated apatite and hydroxy apatite to flour apatite (FAP) and flour hydroxy apatite (FHAP) crystals. These fluoride rich crystals are less acid soluble then the original enamel apatite.
Remenerilization of Acid Dissolved enamelMinerals of tooth enamel are continuously inexchange with the minerals of saliva and thusthe balance is maintained. This Equilibriumcan get disturbed with the organic acidproduced by the metabolism of fermentablecarbohydrates by the microorganism. Thisleads to drop in PH. of the plaque on theenamel surface and in the sub surface.Minerals, particularly calcium and phosphateleave the dissolved enamel in their ionic forman entrace the plaque fluid. This process iscalled deminerilization this get reverrse withthe factor like fluoride and is termsreminerilization.
Specific Plaque Hypothesis• The use of topical fluoride as a cure for infection has been termed the specific plaque hypothesis by hoesche. The primary assumption in this theory is that dental caries results from a specific pathogen, S. mutans. Thus the elimination or reduction of this pathogen with provide a lasting cariostatic effect. Thus the specific palque hypothesis explains a persistant anticaries action of topical fluorides that is not seen with water fluoridation or fluoride dentifrices.
• References:• Shobha TondonText book of pedodontics