Fluoride & Caries

• What is the principal mineral of enamel and dentine?

  Hydroxyapatite.
• What is hydroxyapatite precipitated from and what is required?

  Ca2+ and (PO4)3- from supersaturated solutions.

  Energy is required.
• What is the chemical formula of hydroxyapatite?

  Ca10(PO4)6(OH)2
• When does fluoride get incorporated and how?

  Pre-eruptive:

  1. During mineralisation- where it is INCORPORATED INTO THE BULK OF THE TOOTH.
  2. Topically from tissue fluid on maturing enamel surface- A SURFACE EFFECT.

  Post-eruptive:

  1. Topically from:

  Saliva

  Fluoride containing products
• What does pre-eruptive enamel consist of?

  Substituted hyroxyapatite:

  Hydroxyapatite with many ionic impurities such as FLUORIDE and CARBONATE.
• What are the 3 ways fluoride is retained in the oral mucosa?

  1. Firmly bound onto the tooth

  Fluorohydroxyapatite- fluoride incorporated directly into crystals

  
  

  2. Loosely bound onto the tooth

  Calcium fluoride (CaF2)

  OR

  Fluoride is absorbed to apatite- as opposed to being intrinsic within its structure (firmly bound fluorohydroxyapatite)

  
  

  3. Within plaque (5-10ppm)

  Reversibly bound to free calcium (dispersed CaF2)

  It is 95% bound.
• What is the critical pH and what occurs at this stage?

  5.5

  Hydroxyapatite crystals on the enamel dissociate.
• What is the equation for hydroxyapatite dissolution?

  Ca10(PO4)6(OH)2 --(H+)--> Ca^2+ + PO4^3- + H20

  
• How is critical pH measured from individual to individual?

  Via the levels of calcium ions in saliva:

  • The point at which dissociation occurs is the critical pH.
  • If the Calcium ion level goes higher, the critical pH LOWERS.
  • Therefore if the hydroxyapatite crystal is saturated with calcium ions, the critical pH lowers, and the tooth can therefore HANDLE HIGHER LEVELS OF ACIDITY.

  
• What are the 5 steps in the acid cycle?

  1. Fermentable carbohydrates

  If the biofilm is cariogenic, it processes these f. carbohydrates leading to:

  2. pH falling

  3. Deminaralisation at the critical pH

  4. Saliva, OH- and (PO4)3- ions return the pH to neutral

  5. Remineralisation- mineral ions reassociate with the enamel.
• Why is the acid cycle essential?

  1. Essential to post-eruptive enamel maturation.
  2. Loss of carbonate from demineralisation and then incorporation of fluoride into the hydroxyapatite from pre-eruptive enamel can occur.
• What is the equation for fluoride incorporation into hydroxyapatite?

  Ca10(PO4)6(OH)2 + 2F^- ----> Ca10(PO4)6F2 + 2OH^-

  
• How does the production of fluorohydroxyapatite from the acid cycle aid the tooth?

  1. Fluorohydroxyapatite has a lower solubility than hydroxyapatite.
  2. This means the critical pH shifts as the enamel can tolerate more acid:
  3. It therefore strengthens the tooth against the caries process.

  
• What is the role of loosely bound fluoride and how is it formed?

  1. It acts as a reservoir of Ca and F (both important in tooth protection)
  2. it is only formed with HIGH CONCENRATION FLUORIDE SOLUTIONS:

  Results in globules of CaF2

  CaF2 globules have protein and phosphate-rich surface. This protects the surface of enamel and therefore reduces rate of dissolution.
• What are the 2 mechanisms of loosely bound fluoride?

  1. Fluoride binds Ca2+ and (PO4)3- ions dissolving from acid and promotes REPRECIPITATION.

  Reprecipitation narrows pores in enamel- deterring acid diffusion into enamel and the loss of dissolved ions.

  
  

  2. Fluoride release is pH dependant- released when needed (low pH).
• What 2 factors determine the amount of mineral lost and how?

  1. pH
  2. Fluoride concentration.
  3. At low pH, fluoride concentration is an important determinant of rate of mineral dissolution.
• How does fluoride induce antibacterial effects, and what are the 2 conditions?

  It can inhibit the carbohydrate metabolism of oral streptococci and

  1. The fluoride must enter the bacteria
  2. It must effect the enzymes and transport systems involved in carbohydrate metabolism.
• How is HF formed?

  Low external pH (lots of H+) promotes formation of more HF; H+ combines with F-.
• How does fluoride enter bacteria?

  Diffuses into the bacteria by the glucose-H+ symporter as HF.
• How does HF dissociate in bacteria and what does this also lead to?

  High intracellular pH of bacteria favours dissociation of HF to H+ AND F-.

  As there is a reduction of HF in the bacteria, there is a concentration gradient for MORE HF TO ENTER.
• What are the 4 main effects of fluoride on bacteria? (1)

  1. Enolase inhibition

  Fluoride/acidification of the cell inhibits enolase, which prevents glycolysis and therefore prevents LACTIC ACID PRODUCTION, so bacteria stops producing acid that causes demineralisation.

  2. PEP phosphotransferase system (glucose and PEP pump)

  As enolase is inhibited, PEP levels decrease.

  Less glucose intake and glycolysis fails to work.

  Less lactic acid production = less demineralisation.
• What are the 4 main effects of fluoride on bacteria?(2)

  3. Acidification of bacteria:

  Due to HF dissociation into H+ and F.

  Fluoride also inhibits the ATPase-dependent H+ pump- so less H+ is pumped out meaning there is:

  4. A high intracellular H+ concentration.

  This further inhibits enolase and other glycolytic enzymes.

  Also reduces the pH gradient between the outside and intracellular- this reduces the H+ sugar transport, therefore regulating the production of lactic acid and reducing it.
• What 2 things does fluoride have no effect on in terms of plaque?

  1. Bacterial adherence
  2. Initial plaque formation
• What effect does fluoride have on plaque?

  1. Reduces acid production in plaque.

  The effect depends on if it is tightly or loosely bound.
• How can fluoride be detrimental to the tooth?

  They can disrupt the mineralised structure in enamel causing fluorosis by impairing the growth of apatite crystals via:

  1. RETENTION OF MODULATING PROTEINS through over-enhanced binding of mineral to matrix proteins.

  AND/OR

  1. Over-enhanced binding of the PROTEASES RESPONSIBLE FOR PROCESSING prior to matrix removal.

  Also too much fluoride during amelogenesis can be toxic to ameloblasts that can also affect enamel production.