OE L11 Chemistry of Hydroxyapatite

Question 1

What are the origins of enamel defects?

Answer 1

Enamel defects may be of genetic or environmental origin.

Question 2

What 2 types of defects lead to enamel malformation?

Answer 2

• Distrubances in initial matrix deposition = hypoplasia

- Disturbances in enamel maturation = hypomineralisation

Question 3

How does hypoplastic amelogenesis imperfecta present?

Answer 3

Pits and grooves on enamel surface.

Secretory phase defect.

Question 4

What are the 2 types defects have a hypomineralised phenotype?

Answer 4

Hypomineralised phenotype:

• Hypocalcified or
• Hypomature

Question 5

Describe hypocalcified amelogenesis imperfecta.

Answer 5

Fully mature tissue, but mineral deposition in end stages of amelogenesis not complete. Normal shaped crowns.

Question 6

Describe hypomature amelogenesis imperfecta.

Answer 6

Patchy appearance, enamel opaque instead of translucent.

Question 7

What are 3 type of defects which cause AI and what do they affect?

Answer 7

• ADJ defects: results in enamel layer that shears easily
• Secretory stage defects: results in insufficient crystal elongation, leaves enamel layer thin and disorganised
• Maturation stage defects: deficient matrix degradation, produces thick but soft enamel

Question 8

Give examples of components that may be mutated in hypoplastic AI.

Answer 8

• Amelogenin
• Enamelin
• Ameloblastin
• Acid phosphatase
• Laminin a-3 and b-3 chains
• Collagen 17a1-chain

Question 9

Give examples of components that may be mutated in hypomaturation AI.

Answer 9

• Kallikrein 4
• Enamelysin
• Amelotin
• WDR72
• SLC24A4

Question 10

What protein mutation causes the most prevalent form of AI in the US?

Answer 10

Mutations in FAM83H, which controls epithelial maintenance through keratin filament regulation.

Question 11

Can AI exist in syndromic form?

Answer 11

Yes, AI usually exists in synromic form as the genes involved are not specific to hard tissue formation.

E.g. Jalili synrome (disease of retina) - CNNM4
E.g. Renal disease - Claudin 16 and 19

Question 12

Which protein was first linked to AI, describe the mutation.

Answer 12

Amelogein

• Tri-tyrosine motif normally mediates interaction of nanospheres with enamelin
• Mutations prevents cleavage by MMP-20 thus preventing formation of TRAP fragment
• Retention of protein in tissue
• Causes hypomaturation

Question 13

How does hypomaturation AI differ between males and females?

Answer 13

Hypomaturation AI related to mutation inherited on X chromosome.

• Men only have 1 X chromosome so more severely effected.
• Women carry 2 but only 1 chromosome is expressed in given group of cells, inactivation of 1 X chromosome is spontaneous, leads to banding pattern- alternative stripes of normal and hypomature tissue.

Question 14

Name 5 defects with environmental causes.

Answer 14

• Infectious disease e.g. measles during tooth development
• Fluorosis, mottled appearance
• Incorporation of ingested supplements into mineralised matrix e.g. tetracycline causes extreme brown pigmentation
• Dietary defincies
• Trauma

Question 15

How does pH differ during maturation stage?

Answer 15

• Ruffled border ameloblasts introduce enzymes and ions into the matrix for degradation lower pH (5.5)
• Smooth border ameloblasts resorb protein fragments raise pH (7-7.5)

pH cycling is essential to forming mature, mineralised crown.

Question 16

How are ions transported from ameloblasts to the enamel space during matrix degradation?

Answer 16

Ions initially accumulate in stellate reticulum, then actively transported across ameloblast layer to reach enamel space.
Terminal bar apparatus prevents ions travelling back up across ameloblasts.

Question 17

Why is an acidic environment needed for mineralisation?

Answer 17

For every unit of HAP, 8 protons are produced- creates acidic environment.

Low pH leads to dissolution of less stable mineral.

EMSP1 works optimally at lower pH.

Question 18

How is the acidic environment created during mineralisation neutralised?

Answer 18

Carbonic anyhrase II is expressed by smooth ended ameloblasts at end of secretory stage and during maturation.
It produces bicarbonate.

Question 19

Describe surface enamel.

Answer 19

Non-prismatic, harder and less porous. More mineralised.

Question 20

Describe the molecular structure of hydroxyapatite.

Answer 20

Crystal lattice:

• Central OH
• Inner ring of 3 Ca ions
• Middle ring of 3 P ions
• Outer ring of 6 Ca ions

Question 21

What is the stoichiometric formula of HAP?

Answer 21

Ca10(PO4)6(OH)2 or Ca5(PO4)3OH

Question 22

What are the 2 forms of apatite growth?

Answer 22

• Rapid growth along C-axis: elongated hexagonal crystals

- Thickening on the side prism places: slow rate

Question 23

Name 3 types of substituted apatite.

Answer 23

• Fluorapatite
• Carbonate
• Magnesium

Question 24

Describe fluorapatite.

Answer 24

• Fluoride replaces central hydroxyl ion
• Stabilising effect on crystal lattice: symmetrical, higher charge density so creates a flatter plane
• Prevents caries

Question 25

Describe carbonate substituted apatite.

Answer 25

• Destablising effect

- Replaces either hydroxyl or phosphate ion

Question 26

Describe magnesium substituted apatite.

Answer 26

• Destabilising effect
• Replaces calcium
• Other ions such as sodium and rubidium can also substitute calcium

Question 27

Do we want enamel to have a high or low solubility product constant?

Answer 27

Low.
Smaller solubility product constant = more stable salt.

Most stable salt is a mixture of hydroxyapatite and fluorapatite.

Question 28

Describe the mineral composition changes during development.

Answer 28

• Hydroxyl increases
• Carbonate and acid phosphotase decrease

Net effect: mineral more stable and less prone to dissolution

Question 29

How does acid affect hydroxyapatite?

Answer 29

• Acid generated in oral plaque biofilms demineralises the tooth’s calcified tissues.
• If the ionic activity product (IP) is less than solubility product it causes ion dissolution (caries)

Question 30

What is acid etching?

Answer 30

Removes the smear layer and causes small amount of mineral dissolution.
This creates surface roughness so materials bond better to the tooth.