Fluoride and dental caries

Question 1

What is the main mineral of enamel and dentine?

Answer 1

Hydroxyapatite

Question 2

How is hydroxyapatite made?

Answer 2

It is precipitated from calcium ions and phosphate ions from supersaturated solutions (some energy required)

Question 3

How may fluoride get incorporated into the teeth pre-eruption?

Answer 3

During mineralisation - incorporated into bulk of tooth

Topically from tissue fluid on maturing enamel surface - surface effect

Question 4

How may fluoride be incorporated into the tooth post-eruption?

Answer 4

Surface effect only: topically from saliva or fluoride containing products

Question 5

Why may pre-eruptive enamel not be described as ‘pure hydroxyapatite’?

Answer 5

As it contains other ions, such as fluoride or carbonate or magnesium.
Contains ionic impurities

Question 6

What is created in fluoride is firmly bound?

Answer 6

Fluorohydroxyapatite

Question 7

What is produced if fluoride is loosely bound?

Answer 7

Calcium fluoride

Fluoride absorbed to apatite

Question 8

How much fluoride can be found within plaque and how is it bound?

Answer 8

5-10 ppm
Reversibly bound to free calcium (dispersed CaF2)

95% bound

Question 9

What is the critical pH for enamel (and hydroxyapatite)?

Answer 9

5.5

Question 10

What cycle is an essential element of post-eruptive enamel maturation?

Answer 10

Acid cycle

Question 11

What chemical difference is relevant when comparing pure hydroxyapatite with fluorohydroxyapatite? Also how does this affect the critical pH?

Answer 11

Fluorohydroxyapatite has a Lower solubility than hydroxyapatite.

This lowers the critical pH - the tooth is more susceptible to acid attack

Question 12

What is the effect of calcium fluoride (i.e. loosely bound fluoride)?

Answer 12

Acts as a reservoir of calcium and fluoride.

Question 13

Why is dissolution rate reduced with CaF2 (calcium fluoride)?

Answer 13

Because they have protein- and phosphate rich surface

Question 14

What is fluoride release dependent on?

Answer 14

pH

Question 15

What are the 2 mechanisms of loosely-bound fluoride in remineralisation?

Answer 15

1. Binds calcium and phosphate ions dissolving from acid attack and promotes reprecipitation
2. Reprecipitation narrows pores in enamel - affects acid diffusion into enamel, affects loss of dissolved ions.

Question 16

The amount of mineral loss is a consequence of what?

Answer 16

pH

Fluoride concentration

Question 17

At low pH, what is an important determinant of rate of mineral dissolution?

Answer 17

Fluoride concentration

Question 18

How is fluoride antibacterial?

Answer 18

It can inhibit carbohydrate metabolism of oral streptococci and lactobacilli

Question 19

For the fluoride to exert it’s antibacterial effect what must happen?

Answer 19

It must enter the bacteria.

It must effect enzymes and transport systems involved in carbohydrate metabolism

Question 20

Outline how fluoride enters bacteria

Answer 20

• Fluoride diffuses into cells as HF
• Low external pH promotes formation of more HF
• High intracellular pH of bacteria favours dissociation of HF to hydrogen and fluoride ions
• this means more HF can enter

Question 21

What are the 4 effects fluoride has on bacteria?

Answer 21

1. ENOLASE INHIBITION:
   May be direct action of fluoride.
   May be due to acidification of the cell.
2. PEP PHOSPHOTRANSFERASE SYSTEM:
   Needs PEP from the glycolysis to work.
   PEP levels reduced by enolase inhibition
   Results in less sugar uptake
3. ACIDIFICATION OF BACTERIA DUE TO:
   HF dissociation
   Inhibition of ATPase-dependent H+ pump by fluoride
4. HIGH INTRACELLULAR H+ CONCENTRATION
   Inhibits enolase and other glycolytic enzymes
   Reduces pH gradient between outside of cell and intracellular (reduces H+ sugar transport that occurs normally at pH 5.5)

Question 22

What does fluoride have no effect on?

Answer 22

Bacterial adherence

Initial plaque formation

Question 23

What does fluoride reduce?

Answer 23

Acid production in plaque

effect depends upon nature of fluoride application

Question 24

How does fluoride effect apatite crystal growth?

Answer 24

Appears to impair growth of apatite crystals.
Retention of modulating matrix proteins through enhanced binding of mineral to matrix protein and/or

enhanced binding of the proteases responsible for processing prior to matrix removal