Chemistry of enamel

Question 1

What does enamel consist of?

Answer 1

An acellular tissue, composed of 95% inorganic impure calcium hydroxyapatite and the remaining fluid and organic proteinaceous material.

Question 2

How are enamel crystals organised?

Answer 2

Into bundles called prisms/rods and arranged interweaving relative to one another and alternative prisms in opposing directions.

Question 3

How are the long axes of prisms arranged?

Answer 3

Parallel

Question 4

What is the difference between hydroxyapatite in biological and non biological systems?

Answer 4

In non biological hydroxyapatite is present with other crystals and varies in size and morphology. In skeletal systems size and arrangement are carefully controlled but chemical composition can vary.

Question 5

What is the smallest repeating unit in calcium hydroxyapatite called?

Answer 5

The unit cell

Question 6

How many axes does a unit cell have?

Answer 6

3. • a, b and c

Question 7

What does the unit cells shape resemble?

Answer 7

A rhombohedron

Question 8

Where is the column of hydroxyl cells from which hydroxyapatite obtains it’s name?

Answer 8

C- axis

Question 9

Which model do we see if we place the column of hydroxyl groups at the centre of view?

Answer 9

Hexagonal plate model

Question 10

What does the unit cell share from different hexagonal plates in the stack?

Answer 10

Ions

Question 11

What is the stoichiometry of calcium hydroxyapatite?

Answer 11

Ca10(PO4(6(OH)2

Question 12

What is substituted/carbonated hydroxyapatite?

Answer 12

Not pure hydroxyapatite containing small amounts of other ions (magnesium, fluoride etc)

Question 13

What is the structure of calcium hydroxyapatite?

Answer 13

Hydroxyl groups in the middle, surrounded by a triangle of calcium (calcium 2’s) alternating with a triangle of phosphate. Surrounding the triangles is a hexagon of calcium (calcium 1’s)

Question 14

How does apatite in enamel vary from calcium hydroxyapatite?

Answer 14

Missing ions (calcium and hydroxyl)
Extraneous ions impurities found (magnesium, carbonate, fluoride and sodium)
Carbonate and acid phosphate found in significant amounts
Fluoride replaces hydroxyl to a certain extent.

Question 15

How does the substitution of hydroxyl ions for fluoride have a caries preventative effect?

Answer 15

Fluoride is highly electronegative so pulls ions together, lowering lattice energy and stabilises crystal structure. It makes fluoridated crystals more difficult to dissolve in acid and makes it easier for re-deposition at lower solution concentrations.

Question 16

What happens to the sense of hydroxyl groups at a fluoride location?

Answer 16

It is reversed, hydroxyl hydrogen bonds point towards the fluoride ion stabilising the structure further.

Question 17

What does carbonate substitute and what is it’s effect?

Answer 17

Either hydroxyl or phosphate and has a destabilising effect and more acid soluble apatite phase.

Question 18

What does magnesium substitute and what is it’s effect?

Answer 18

Calcium to a limited extent and has a destabilising effect and increases acid solubility of apatite.

Question 19

How does the density of enamel prisms vary in the tooth?

Answer 19

High at the tooth surface and decreases towards the dentine, whereas porosity, water and organic content increase towards dentine.

Question 20

How does fissure enamel vary from the rest of the tooth?

Answer 20

Low density of mineral near the fissures, has a complicated prismatic structure due to poorer packing.

Question 21

Where is there a high amount of protein in enamel?

Answer 21

Inner regions due to poorer prismatic packing.

Question 22

Where is concentration of carbonate highest and when is it incorporated?

Answer 22

Incorporated in tissue development and is found in dentine at highest concentrations.

Question 23

Where is concentrations of magnesium highest?

Answer 23

Dentine

Question 24

Where in the tooth contains the most fluoride?

Answer 24

Outermost enamel, accumulates during development by unerupted fluoride scavenging enamel ion from tissue fluid, mops up any fluoride entering the tissue.

Question 25

How does dentine apatite differ from enamel?

Answer 25

Similar shape and chemistry but a lot smaller in enamel, contain more carbonate and magnesium and are arranged alongside collagen fibril meshwork which is highly stable and cross linked. Organic matrix makes up 20% of the weight of dentine.