OE L32 Demineralisation and Remineralisation of Enamel Flashcards
What is the mechanism behind dental caries?
Dissolution of the enamel surface due to acid production in plaque fluids by bacterial fermentation of carbohydrates.
How does early caries present?
As a white lesion
How can caries lesions be examined to determine porosity and crystallinity?
Polarised light microscopy:
- 2 polarises used
- Analyser with 90 degree orientation to polariser used
How does a caries lesion appear in a polarised light micrograph?
- 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
Describe the 5 layers in a white spot lesion and the mineral content of each layer.
- 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
What is the crystalline material of enamel composed of?
Predominantly hydroxyapatite and some other calcium phosphates.
What is the molecular formula of hydroxyapatite?
Ca10(PO4)6(OH)2
What is the molecular formula of fluoroapatite?
Ca10(PO4)6F2
Name 5 calcium phosphates found in enamel.
- Monocalcium phosphate
- Dicalcium phosphate
- Tricalcium phosphate
- Tetracalcium phosphate (not found in teeth but found in some toothpastes)
- Octacalcium phosphate
Order the minerals found in enamel from most to least soluble.
Most soluble
- Monocalcium phosphate
- Dicalcium phosphate
- Tricalcium phosphate
- Octacalcium phosphate
- Hydroxyapatite
- Fluoroapatite
Least soluble
What happens when HAP is exposed to an acidic pH?
It is converted to a more soluble salt with a lower calcium/phosphate ratio
What is the solublility product?
Ksp
The equilibrium constant for a solid substance dissolving in an aqueous solution
More soluble = higher Ksp
What happens when HAP is exposed to an alkaline environment?
Causes the formation of a new calcium phosphate species which are less soluble.
What do calcium phosphates dissociate into?
- Calcium
- Phosphate
- Hydroxyl ion
What does the relationship between ionic acitivity product (IP) and Ksp demonstrate?
- IP=Ksp: solution is saturated, crytsal is in equilibrium with medium
- IP>Ksp: solution is super saturated, salt precipitation will occur (remineralisation)
- IP
What are the axis labels for solubility phase diagrams?
X-axis: pH
Y-axis: Free calcium concentration (mM)
What does the region above the line in a solubility phase diagram show?
Represents conditions of supersaturation -> mineral precipitates
What does the region below the line in a solubility phase diagram show?
Represents conditions of undersaturation -> dissolution of solid
What is the main acid produced by bacteria during fermentation of carbohydrates?
Lactic acid
What is the pellicle?
A protective protein layer composed mostly of saliva components.
- Hydrophobic
- Semipermeable
- Facilitates mineral precipitation at enamel surface
What type of species is the pellicle layer permebale to?
Uncharged species.
Electroneutral
Explain the interchange of calcium phosphate salts between saliva, the pellicle and the enamel.
- 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
What does the progression of demineralisation depend on?
- 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
What is the dark zone of the caries lesion also known as?
The positively birefringent zone
How does the crystalline state of HAP change throughout the carious lesion?
Becomes disordered in body of lesion, ordered near surface.
How do biogenic apatites differ from HAP?
- 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).
What are the main strategies for prevention of demineralisation?
- 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
