chemistry of enamel Flashcards
ca-HAP has a hexagonal cross-section. what is its width, thickness and length
width= 50 nm thickness= 30 nm length = 1mm (corresponds to the belief that HAP runs from the dentine junction to the surface of tooth
what is the structural arrangement of ca-HAP (the simplified unit cell structure) according to the stacking plate model
circle in middle (OH- the c-axis)
triangle (P)
upside down triangle (Ca II)
hexagonal (Ca I)
called the stacking plate model because all stacked on top of each other and rotated by 60•
what vacancy in HAP structure does Fl fill?
OH (the c-axis) vacancy.
outline the key points from the classic crystallographic unit cell of enamel
- HAP structure resembles a rhomboedron
2. there are 3 axis: a,b,c. where c- is the column of OH in the centre
what is the effect of Fl being substituted into HAP and why
- fl is highly SYMETRICAL therefore fits the OH vacancy better than OH itself
- Fl is highly ELECTRONEGATIVE –> changes the SENSE of OH column so the hydrogen bonds point towards the Fl-ion –> stablises the structure.
…therefore Fl is LESS acid soluble, MORE acid resistant, and MORE stable in acid
what vacancy in HAP structure does HCO3 fill?
carbonate can replace either the phosphate group OR the OH from the c-axis
what is the effect of HCO3 being substituted into HAP and why
doesnt fit properly
…therefore MORE acid soluble, LESS stable in acid
what vacancy in HAP structure does Mg fill?
Ca I (the outer calcium on the hexagon)
what is the effect of Mg being substituted into HAP
MORE acid soluble, LESS stable in acid
the amount/ density of HAP on a tooth varies in different regions of that tooth.
Also, the FORMS of HAP present varies e.g. Mg substituted/ carbonated HAP may be found more in certain parts of tooth than others.
in dental enamel of humans, does the density increase or decrease as we move from the surface of tooth/outer enamel to dentine?
decreases
there is more HAP near the surface of tooth.
the distribution in densities of various minerals/ proteins was measured in mature enamel.
what were the results for the general PROTEIN distribution in mature human enamel?
less protein is seen on cusp.
more protein is seen towards dentine and cervically
–> this makes sense because dentine is going to have more collagen/ organic material than ca-HAP mineral
the distribution in densities of various minerals/ proteins was measured in mature enamel.
what were the results for the Ca/ PHOSPHOROUS distribution in mature human enamel?
more ca/p seen in the cusps
less ca/p seen in fissures of molars/pre-molars
the distribution in densities of various minerals/ proteins was measured in mature enamel.
what were the results for the CARBONATE and Mg distribution in mature human enamel?
carbonate and mg are higher near to DENTINE and lower nearer the surface of the tooth.
pockets of high carbonate HAP found in fissures of molars
- wherever there is low density of HAP, there is high Mg and carbonate ions found
the distribution in densities of various minerals/ proteins was measured in mature enamel.
what were the results for the FLUORIDE distribution in mature human enamel?
outermost enamel has the highest Fl and decreases as we get closer to dentine.
ALSO, when the fl conc is high, mg is reduced. the pattern for fl and mg are inverse to each other.
HAP is also found in dentine (its smaller, NOT packed in prisms)
how does the dentine apatite structure differ from enamel one?
dentine apatite has higher levels of carbonate and Mg –> more acid soluble and more acid resistant
