Chemistry of enamel Flashcards
(24 cards)
What is the general structure of enamel?
composed of millions of crystals
with a hexagonal outline in cross-section
crystals are organised into enamel prisms
alternative prisms interweave in opposing directions
prisms have a column of hydroxyl groups along its centre
- hydroxapatite crystal (inorganic calcium phosphate salt)
Where does calcium hydroxyapatite occur in non-biological systems?
granite
difference is there are others crystals present, apatite varies in size and morphology
What other ions are present in calcium hydroxyapatite leading to its impurity?
carbonate
magnesium
fluoride
What shape is a apatite unit cell?
rhombohedron
has 3 axis - a, b, c
What are the variations of apatite in enamel compared to stoichiometric formula?
missing ions
extraneous ions substituted
What are the missing ions from the calcium hydroxyapatite?
calcium
hydroxyl (20-30% lower in enamel than stoichiometric formula)
What is hetero-ionic substitution?
when an endogenous ion is substituted
Describe fluoride ion substitution
highly symmetrical
fits hydroxyl position better than itself
high electronegative
lowers lattice energy
stabilises crystal structure
fluoridated crystals are more difficult to dissolve
What ions does carbonate replace?
hydroxyl
phosphate / acid phosphate
Describe carbonate ion substitution
has a poorer fit in lattice
less stable
more acid soluble apatite phase
accounts for higher solubility product of enamel compared to stoichiometric hydroxyapatite
Describe magnesium substitution
substitutes calcium but to limited extent
has destabilising effect due to its charge density - like carbonate
has positive synergistic effect - due to its incorporation into lattice and ability to increased acid solubility of apatite mineral (like carbonate)
What decreases towards dentine?
density of apatite crystals
What increases towards dentine?
porosity
water
organic content
Where in the tooth is there low mineral density?
occlusal fissures
cervical regions of crown
Why is there a low density near occlusal fissures?
more porosity
- due to poorer prismatic packing
more water, space and protein in inner regions of enamel
Where is there higher mineral density?
cusps
What is the % concentration of carbonate at enamel and at dentine?
enamel - 2%
dentine - 4-6%
increases
has pockets of high concentration in areas like fissures of molars
What is the % concentration of magnesium at enamel and dentine?
enamel - 0.2%
dentine - 0.5%
has isolated pockets of high concentration close to dentine - less smooth gradient than carbonate
What minerals are higher in concentration in low density areas compared to where there is higher mineral density?
magnesium
carbonate
Where does fluoride have the highest density?
in outermost enamel and falls dramatically towards tissue interior
due to accumulation on surface and during development
unerupted enamel will scavenge fluoride ions
What happens if there is excessive chronic ingestion of fluoride?
occurs during amelogenesis
fluorotic enamel
What is seen in enamel fluorosis?
bands with higher fluoride may be seen along hypomineralised bands
more resistant to caries
in severe cases, enamel may be pitted due to loss of fragile areas as tooth erupts
How does apatite in dentine differ from that of enamel?
similar chemistry
similar shape
but much smaller
much higher levels of carbonate and magnesium
crystals arranged along and between collagen fibril meshwork
Describe collagen in dentine
90% of organic matrix of tissue
highly cross-linked
extremely stable
20% weight of dentine
