Chemistry of enamel

Question 1

what does electron microscopy show about enamel?

Answer 1

enamel is highly organised
- shows enamel is composed of millions of individual mineral crystals
- hexagonal outline
-crystals in transverse and longitudinal sections
- individual crystals organised in crystal bundles= enamel prisms
- SEM- intricate way individual prisms are arranged in opposing directions
- gives enamel unique properties

Question 2

what doe prisms of enamel comprise of

Answer 2

bundle of narrow roughly hexagonal crystals

Question 3

what is calcium hydroxyapatite

Answer 3

• occurs in nature in non biological systems
• can vary in size and morphology
• chemical composition can vary

Question 4

how is the atomic reconstruction of apatite crystal best understood

Answer 4

• examining smallest repeating unit of crystal- unit cell

Question 5

describe the view of apatite unit cell

Answer 5

• resembles rhombohedron
• one edge of cell - OH groups (c-axis)
• shows relationship between ions illustrated in hexagonal plate model and true unit cell structure for HAP
• unit cell- rhomboidal in shape
• shares ions form different hexagonal plates in stack

Question 6

what is the formula and constituents of calcium hydroxapatite

Answer 6

10 Ca
6 PO4
2 hydroxyls

Question 7

draw the structure of calcium hydroxyapatite

Answer 7

hydroxyl groups - centre (black on yellow)
surrounded by triangle of calcium ions - Calcium 2
alternating with calcium 2, triangles of phosphate groups
Hexagon of calcium ions called Calcium 1
planar view- unit cell for HAP is rhomboidal

Question 8

how does apatite in enamel vary from theme

Answer 8

1. Missing ions, particularly calcium and hydroxyl
2. Extraneous ions found within crystal structure including carbonate, fluroirde, magnesium and sodium
3. Carbonate and acid phosphate found in significant amounts
4. Fluoride replace hydroxyl to some extent
5. These defects and substitutions have profound effect on behaviour of apatite regards to solubility at low PH

Question 9

what is the important of fluoride in hydroxyl potion of HAP

Answer 9

• take up different ions to their structure by substituting one of the endogenous ions (heteo-ionic substitutions)
• Important substitution is fluoride form hydroxyl ions
• Profound effects on properties of apartheid
• Highly symmetrical and fits hydroxyl position better than hydroxyl itself
• Highly electronegtiave - pulls other ions together , lowers lattice energy and stabilisies crystal structure
• Fluoridated crystals mroe difficult to dissolve in acid and makes easier for redeposition at lower solution concentrations
• Accounts for part of caries preventive effect of fluoride

Question 10

what is the location of fluoride in Hydroxyl column of apatite

Answer 10

sense of hydroxyl groups is reversed at fluroide location
- fluoride substituation - causes change in hydroxyl column due to electronegatvity
- hydroxyl hydrogen bones point towards fluoride ion
- stabilising stucture

Question 11

what other ions can substitute to HAP lattice?

Answer 11

• carbonate can replace hydroxyl or phosphate - poorer fit of carbonate in lattice
• less stable and more acid soluble apatite phase
• carbonate substitutions along with ion vaccines in lattice account for much higher solubility product of enamel compared to stoichiometric HAP
• Magnesium can some extent substitute calcium but is limited due to charge density
• destabilising effect on apatite lattice than carbonate

Question 12

which ions have positive syngergistic effects?

Answer 12

carbonate and magnesium

Question 13

where can some impurities such as non apatitiee phases be present in enamel

Answer 13

soem mineral phases in enamel

Question 14

how is density distributed in natural human enamel

Answer 14

• density of crystal prisms which determines mineral content is not uniform in general
• decreases from tooth surface towards dentine
• porosity, water and organic content increase in this direction
• low mineral density at cervical regions of crown and higher density in certain areas of cusps in this sample.

Question 15

how is protein distributed in human molar enamel

Answer 15

• water,space and protein exists in inner regions of enamel
• due to poorer prismatic packing

Question 16

how is carbonate distributed in human molar enamel?

Answer 16

exclusively during tissue development
- concentration rise form enamel surface from about 2% towards dentine where concentration 4-6% are found
- pockets of high conc can be found in certain areas such as fissure enamel molars

Question 17

How is magnesium distributed in mature human enamel?

Answer 17

Mg incorparted during enamel formation
- conc are 1/10 of those carbonate
- show similar distribution pattern on increases towards dentine surface
- 0.2% at enamel surface to 0.5% close to dentine
- Gradients are smooth than those for carbonate
- isolated pockets of high conc close to dentine
- high mg patches near surface could be important; less resistant to acid solutions

Question 18

how is fluoride distributed in human molar enamel?

Answer 18

• opposite to carbonate and magnesium
• outermost enamel- most fluoride
• fluoride conc falls towards tissue interior
• at surface of tissue - high conc reflects fluoride accumulation during development by unerupted enamel scavenging fluoride ion form tissue fluids
• form more stable fluoridated mineral at enamel surface; mops up fluoride entering tissue
• restricting passage of fluoride to deeper layers of tissue
• high conc of fluoride at surface of teeth

Question 19

What is the distribution of magnesium in normal and fluroidised human enamel

Answer 19

• conc of mg increase towards dentine surface( isolated pockets of high conc)
• excessive chronic ingestion of fluid - lead to formation of fluoride enamel- fluorisis
• Bands with higher fluoride- seen along hypominerlasied bands
• mineral phases with higher conc of magnesium located closer to dentine
• isolated regions of high and low magnesium found at surface
• mild and moderate fluorosis- enamel resistant to caries due to high fluoride content
• enamel may be pitted due to loss of fragile area as tooth erupts

Question 20

what crystals have similar chemistry to enamel

Answer 20

apatite
smaller than enamel crystals

Question 21

how are HAP crystals arranged

Answer 21

along and between collagen fibril meshwork which forms 90% of organic matrix of tissue

Question 22

what percentage by weight does the organic matrix form of dentine

Answer 22

20%