Chemistry of enamel Flashcards
aim: to understand the chemical structure of dental enamel as the background to the aetiology of caries objectives: recognise the structure of calcium hydroxyapatite to understand the apatite structure in terms of foreign ion substitutions the recognise the variations in the chemical composition of the dental enamel in relation to development and eventually disease
what is enamel comprised of
95-96% inorganic mineral (impure calcium hydroxyapatite
4-5% fluid and organic material (usually containing protein)
what is the enamel made from
entirely mineral crystals that are hexagonal
what are the dimensions of the crystals
50nm in width and 30 nm in thickness
what is the length of each crystal
the length is unknown but it is assumed it is almost 1mm
how are the crystals arranged in the structure
they are not arranged randomly
what is the arrangement of the crystals
they are arranged in crystal bundles- called prisms or rods
what are the mineral crystals made up of
inorganic salt
what is the inorganic salt
hydroxyapatite( calcium phosphate salt)
where does calcium hydroxyapatite naturally occur
granite
what else is present in biological systems
many other crystals and apatite that varies in size
what is the smallest unit of a crystal called
the unit cell
what three axis are in the classical view of the crystal
the c axis
a xis
b axis
what is the cell comprised of when we look at it from the edge
one column of hydroxyl groups
what does the unit cell resemble
stacking of plates
what shape is the unit cell actually
rhomboidal in shape
what is the formula for calcium phosphate
Ca10(PO4)6(OH)2
what ions are shared between hexagonal unit cells
OH ions
what surrounds the centre hydroxyl column
a triangle of Ca ions
what is the triangle of calcium ions called
calcium 2
what also surrounds the hydroxyl column
the triangle of phosphate ions
what surrounds the calcium and phosphate triangles
a hexagonal structure of calcium ions called calcium 1
what is the hexagonal structure that surrounds the triangle of phosphate and calcium 2 called
calcium 1
what degree does the hexagonal structures rotate by
60 degrees
if we count the ions in the simplified view why does it not add up to the stoichiometry
as it is only a planar view the real unit cell is rhomboidal
what variations can be seen in apatite enamel
missing ions eg calcium and OH( OH reported to be missing in 20-30% extraneous ions( impurities ) which include Na,Mg,CO3, PO4
what extraneous ions can be found in apatite crystals in enamel
sodium
magnesium
phosphate
carbonate
what % of OH ions can be reported missing from apatite crystals
20-30%
what issues can substituting and missing ions have
massive impact on its solubility at low pH and behaviour
what ions do fluoride substitute
the hydroxy ions
which ions can the apatite crystals take up
many which can result in heteroionic substitution
what type of substitution do apatite crystals undergo
heteroionic
what are the good properties of the fluoride ion
highly symmetrical so fits the hydroxyl ions place better
highly electronegative so pulls ions together stabilising the structure
what are the advantages of fluoride substitution in apatite crystals
makes the crystals more difficult to dissolve
makes it easier for redeposition at lower conc levels
what minerals destabilise the lattice structure
CO3
Mg
which minerals stabilise the lattice structure
F
which ions can carbonate replace
phosphate
hydroxyl
what are the disadvantages of carbonate substituting
poor fit of the carbonate ion in the lattice
less stable
more acid soluble
why does it make the lattice less stable is magnesium substitutes
due to its positive charges its charge density
what happens to the density of the prisms as we advance towards the dentine
the density DECREASES
what happens to the porosity as we advance to the dentine
INCREASES
what happens to the water as we advance to the dentine
INCREASES
what happens to the organic content as we advance to the dentine
INCREASES
where is the densest part (in regards to prisms) of the incisor
the incisal edge
where is the densest part (in regards to prisms) of the molar
the occlusal surface and edge of the molar tooth
what ion does magnesium substitute
calcium
the distribution of calcium changes how
decreases from the surface to the dentine
how does the distribution of the phosphorus change
it decreases as we reach closer to the dentine
what is interesting about the fissure area of premolars and molars in regards to Ca and phosphorus %
low mineral content meaning vulnerable to acid attack
how does the protein distribution change
least protein at the surface and increases at the dentine and cervically
what is carbonate distribution like
high conc near the dentine
what is the % change increases of carbonate ions from surface towards the dentine
from 2%-4-6%
high conc can be found near the fissures in enamel molars
what is magnesium distribution like
increases towards the dentine
what is the % change increases of Mg ions from surface towards the dentine
0.2%- 0.5%
which direction do most carious lesions start
mesial to distal
which layer of the enamel has the most fluoride
the outermost enamel
what is the difference between apatite crystals in enamel and dentine
they are smaller in dentine
what does dentine apatite have higher amounts of
carbonate and magnesium
what forms the organic matrix in dentine
the HA crystals are arranged along and between the collagen fibril meshwork
what % of the organic matrix is formed by HA crystals and the collagen fibril meshwork
90%
how much % of the dentine is organic matrix
20%
