OE L32 Demineralisation and Remineralisation of Enamel

Question 1

What is the mechanism behind dental caries?

Answer 1

Dissolution of the enamel surface due to acid production in plaque fluids by bacterial fermentation of carbohydrates.

Question 2

How does early caries present?

Answer 2

As a white lesion

Question 3

How can caries lesions be examined to determine porosity and crystallinity?

Answer 3

Polarised light microscopy:

• 2 polarises used
• Analyser with 90 degree orientation to polariser used

Question 4

How does a caries lesion appear in a polarised light micrograph?

Answer 4

• Hydroxyapatite is a birefringent with 2 refractive indices, which can change light polarisation
• The body of the lesion will be dark and amorphous (no arranged crystal orientation) indicating a large amount of mineral loss

Question 5

Describe the 5 layers in a white spot lesion and the mineral content of each layer.

Answer 5

• Surface layer: re-precipitations (10% mineral loss)
• Subsurface layer: body of lesion (24% mineral loss)
• Zone where carious lesion is in progress (6% mineral loss)
• Translucent zone (1-2% loss)
• Normal enamel

Question 6

What is the crystalline material of enamel composed of?

Answer 6

Predominantly hydroxyapatite and some other calcium phosphates.

Question 7

What is the molecular formula of hydroxyapatite?

Answer 7

Ca10(PO4)6(OH)2

Question 8

What is the molecular formula of fluoroapatite?

Answer 8

Ca10(PO4)6F2

Question 9

Name 5 calcium phosphates found in enamel.

Answer 9

• Monocalcium phosphate
• Dicalcium phosphate
• Tricalcium phosphate
• Tetracalcium phosphate (not found in teeth but found in some toothpastes)
• Octacalcium phosphate

Question 10

Order the minerals found in enamel from most to least soluble.

Answer 10

Most soluble

• Monocalcium phosphate
• Dicalcium phosphate
• Tricalcium phosphate
• Octacalcium phosphate
• Hydroxyapatite
• Fluoroapatite

Least soluble

Question 11

What happens when HAP is exposed to an acidic pH?

Answer 11

It is converted to a more soluble salt with a lower calcium/phosphate ratio

Question 12

What is the solublility product?

Answer 12

Ksp
The equilibrium constant for a solid substance dissolving in an aqueous solution
More soluble = higher Ksp

Question 13

What happens when HAP is exposed to an alkaline environment?

Answer 13

Causes the formation of a new calcium phosphate species which are less soluble.

Question 14

What do calcium phosphates dissociate into?

Answer 14

• Calcium
• Phosphate
• Hydroxyl ion

Question 15

What does the relationship between ionic acitivity product (IP) and Ksp demonstrate?

Answer 15

• IP=Ksp: solution is saturated, crytsal is in equilibrium with medium
• IP>Ksp: solution is super saturated, salt precipitation will occur (remineralisation)
• IP

Question 16

What are the axis labels for solubility phase diagrams?

Answer 16

X-axis: pH

Y-axis: Free calcium concentration (mM)

Question 17

What does the region above the line in a solubility phase diagram show?

Answer 17

Represents conditions of supersaturation -> mineral precipitates

Question 18

What does the region below the line in a solubility phase diagram show?

Answer 18

Represents conditions of undersaturation -> dissolution of solid

Question 19

What is the main acid produced by bacteria during fermentation of carbohydrates?

Answer 19

Lactic acid

Question 20

What is the pellicle?

Answer 20

A protective protein layer composed mostly of saliva components.

• Hydrophobic
• Semipermeable
• Facilitates mineral precipitation at enamel surface

Question 21

What type of species is the pellicle layer permebale to?

Answer 21

Uncharged species.

Electroneutral

Question 22

Explain the interchange of calcium phosphate salts between saliva, the pellicle and the enamel.

Answer 22

• Uncharged species pass through pellicle
• In the enamel layer, this electro-neutral salt reforms its equilibrium between the salt and ions
• HAP dissocaites into calcium, phospahte and hydroxyl ions
• Electroneutral salts are reformed that can travel back into the external environment

Question 23

What does the progression of demineralisation depend on?

Answer 23

• Rate of acid produced externally
• Production of net-neutral species which diffuse more readily across pellicle layer
• Rate of inward diffusion of unionised form of acids through pellicle layer
• Rate of apatite dissolution
• Outward dissolution of calcium phospahte in the form of uncharged aqueous complexes

Question 24

What is the dark zone of the caries lesion also known as?

Answer 24

The positively birefringent zone

Question 25

How does the crystalline state of HAP change throughout the carious lesion?

Answer 25

Becomes disordered in body of lesion, ordered near surface.

Question 26

How do biogenic apatites differ from HAP?

Answer 26

• Calcium in HAP can be substituted e.g. with magnesium, sodium, potassium, lead, strontium or barium
• Hydroxyl or phosphate in HAP can be subsituted with carbonate (destabilising)
• Hydroxyl in HAP can be substituted with fluoride (reduces solubility)

Overall, the incorporation of impurities tend to increase solubility (except fluoride).

Question 27

What are the main strategies for prevention of demineralisation?

Answer 27

• Plaque removal through mechanical action
• Frequent rinsing
• Toothpaste ingredients: fluoride, zinc, antibacterial agents (e.g. chlorhexidine), anionic detergents
• Fissure sealants to prevent diffusion of neutral salts in and out of enamel surface