44. Minitutorial. Fluoride

Question 1

History of fluoride use

Answer 1

• 1901-15 Black and McKay saw tooth mottling and low caries incidence in Colorado Springs
• 1931 Churchill observes high F in Colorado Springs
• 1948 Dean starts a long term water fluoridation trial in Grand Rapids, MI

Question 2

How does fluoride prevent caries?

Answer 2

• reduces demineraisation
• promotes remineralisation
• inhibits cariogenic bacteria

Question 3

How does fluoride stop demineralisation?

Answer 3

• mineral phase of tooth enamel and dentine is an impure calcium hydroxyapatite - biological apatite
• hydroxyapatite forms a crystal lattice
• fluoride stabilises this by substituting OH ions by ion exchange and by filling gaps left in crystal structure when OH ions displaced during crystal formation

Question 4

How does OH ion displacement work?

Answer 4

• due to polarity and size, OH ions can’t fit in calcium triangles
• they’re displaced either above or below (Z coordinates of 0.19 or 0.31 or 0.69 or 0.81)
• can lead to exclusion from unit cell

Question 5

How does fluoride affect apatite solubility?

Answer 5

• children in areas of high F have lower tooth decay
• replacing OH in apatite with F to make fluorapatite reduces solubility of crystals but levels of F in enamel are too low to account for this
• F incorporates only into outer layers of apatite - across tooth surfaces it’s uneven and fluorapatite concentrates in stagnant areas (pits/fissures)
• plaque may enhance fluoraapatite formation and dissolution of hydroxyapatite may leave fluorapatite

Question 6

How does fluoride aid remineralisation?

Answer 6

• apatite dissolves in acidified plaque fluid
• raises calcium, phosphate and fluoride local concentrations - fluoride ions in plaque contribute
• fluoridated apatite (FHA) forms - common ion effect
• low solubility and precipitates

Question 7

Practical applications of the remineralisation theory

Answer 7

• use fluoride toothpastes to build up local concentrations of fluoride
• acidulated phosphate fluoride treatment

Question 8

Fluoride inhibits bacteria what?

Answer 8

metabolism

Question 9

How does fluoride inhibit bacterial metabolism?

Answer 9

• fluoride uptake by bacteria higher at acid pH - pK of hydrofluroic acid is 3.45
• pH of plaque fluid during cariogenic challenge is lowered (more acidic)
• low pH favours formation of HF, which is taken up by bacteria by simple diffusion
• bacterial cytoplasm pH is almost neutral which favours dissociation of HF
• causes reduction in cytoplasmic pH and releases fluoride ions which bind to magnesium and inhibit enolase among others

Question 10

Resistance to fluoride

Answer 10

• generally resistant strains of bacteria haven’t been found clinically
• some F-resistanrt S.mutans strains have been selected in lab (resistance mechanism not clear, probably multiple things)