Enamel And Caries

Front 1

Composition of enamel

Back 1

80-90% by vol. hydroxyapatite crystal

20-10% fluid and non-collagenous protein

Front 2

Dimensions of enamel crystal

Back 2

Length: 1-2mm possibly full length of enamel

Width: 25nm

Depth: 50nm

Front 3

Describe enamel prism

Back 3

5um wide, made of approx. 1000 enamel crystals.

Long axis of crystal and prism are parallel

Cross section varies from circular to keyhole

At edge of prism orientation of crystals deviates to create interface with neighbouring prism. (and space for diffusion)

Front 4

Density of enamel

Back 4

In general it decrases from the surface to the dentine although in specific locations the denstiy can be complex.

Front 5

Formula of HAP

Back 5

Ca10(PO4)6(OH)2

Front 6

Structure of HAP crystal

Back 6

Cental column of OH ions, parallel to long axis.

Surrounded by 3 Ca ions (calcium 1) in triangle

Surrounded by 3 P ions in triangle but rotated by 60 degress (Ca1 and P ions form 6 pointeed star)

Surrounded by 6 Ca ions (Ca 2) in hexagon.

Structure of whole can be thought of as a series of hexagonal plates stacked on top of each other and rotated by 60 degrees.

Front 7

Variations in HAP

Back 7

During the formation of HAP ions that are present in the surrounding milieu can substitue the correct ions.

P- <> HCO3-

Ca <> Sr, Pb, Zn, Mg

If the ions are substituted this introduces weakness to the crystal as the substitutes are more susceptible to acid.

This is why newly erupted teeth are more susceptible to caries because the impurities are present whereas with older people the impurities have been removed leaving only the sound enamel.

(once the crystal has formed little substitution takes place)

Front 8

Changes in HAP from surface to dentine

Back 8

Density - more dense on the outside, less dense on the inside

Mineral distribution - no clear pattern?

Calcium and phosphorous - areas of higher (pure HAP) and lower concentration - areas of low concentration coincide with known weak areas.

Protein - negligible in outer layers, concentrated at inner layers - scaffold for enamel construction

carbonate - outer layers carbonate free, inner layers more carbonate

magnesium - similar to CO3 - high conc magnesium <> high incidence of caries

Magensium and density - inverse correlation

effect of lower purity at inner layer - small lesion can spread underneath to much larger lesion

Fluoride - higher conc in outer layers - evidence of ectopic application

Fluoride and magnesium - more fluoridated a tooth less magnesium

Front 9

Dentine structure

Back 9

Contains apatite crystals of similar chemistry to enamel

Differences:

- crystals are much thinner

- contains much higher levels of carbonate and magnesium - softer, easier to demineralise

- apatite is embedded in organic matrix of type 1 collagen

- collagen is highly cross linked and extremely stable

- collagen is more than half of the tissue

- as it is a living structure it can repair

Front 10

Effect of fluoride substitution

Back 10

Flouride can substitue OH as it has the same charge and is of similar size. Fluoride is more electronegative than OH so it pulls the surrounding cations in closer creating a more dense and stable structure.

Fluoride also causes the orientation of the OH ions to reverse. This increases the compactness and makes the OH less susceptible to acid attack.

Front 11

I don't understand the crystallogrphic cell unit or the formula

Back 11

email bonass?

Front 12

Possible substitutions in dental enamel

Back 12

As enamel is formed ions present can substitute themselves for the ions that should be in the enamel structure.

Phosphate- <-> HCO3-

F <> OH-

Mg2+ <> Ca2+

Sr2+, Pb2+, Zn2+ <> 2Ca+

These substitutions alter the structure and shape of the crystals and introduce weakness and susceptibility to acid.

This is the reason that newly erupted teeth are more susceptible to caries, after time this weak enamel will most likely have decayed and been filled (the ions do not substitute out (can't as they are in a solid)).

Front 13

What is supersaturation?

Back 13

Supersaturation is the point of concentration of ions in solution where the ions will precipitate. Below this point any solid ionic material will dissolve.

The value of supersaturation is the same as the solubility product.

Front 14

What is a solubility product Ksp?

Back 14

Solubility product is the same as equilibrium constant. It is the ratio of products/reactant. If the concentration of the ions in solution is lower than the solubility constant then they will stay dissolved and no precipitation will occur. If the solubility product is exceeded then precipitation will occur.

A really low Ksp means the ions are essentially insoluble because very low concentration is required to cause precipitation.

Front 15

What is the solubility product of HAP?

Back 15

Ksp = [Ca]^10 x [PO4]^6x[OH]^2

Front 16

Equation for the dissolution of Phosphate ions and their pKa values

Back 16

PO4^3- <> HPO4^2- <> H2PO4^1- <> H3PO4

pKa 12.3 7.2 2.1

Front 17

What is pKa?

Back 17

pKa is the pH at which there is equal proportions of ions on either side of the reaction

Front 18

What happens as the plaque pH drops?

Back 18

pH 8 - 5

reaction H+ + HPO4^2- <> H2PO4^1-

solution and mineral bind H+

pH <5

reaction H+ + PO4^3- <> HPO4^2-

H+ replaces Na+

H+ occupies Ca2+ vacancies

lesion forms

Front 19

Zones of lesion moving from surface to dentine

Back 19

Surface zone

Body of lesion

Dark zone

Translucent zone

Sound enamel

Front 20

Pore structure of zones of carious lesion

Back 20

Body of lesion - big pores

Dark zone - Medium and small pores

Translucent zone - big pores

Front 21

Explain this table

Back 21

The table shows the composition of the solution that is coming from each of the different zones of the lesion.

The much higher concentration of carbonate lost than is present in sound enamel shows that it is lost preferentially and must be more susceptible to acid attack.

In the dark zone the level of carbonate is much lower because most has been lost in the translucent zone. The magnesium is lost next shown by the higher composition than present in sound enamel.

The composition lost from the body of the lesion is very similar to sound enamel as the impurities have already been lost.

Front 22

Flouride distribution in carious lesion

Back 22

Low levels in sound enamel, translucent zone, dark zone and body of lesion.

Very high in surface zone

Front 23

Explain the reason for the different pore sizes in the zones of the lesion.

Back 23

Translucent zone - some large pore, the acid is attacking the crystals that contain magnesium and carbonate.

Dark zone - small and large pores as most of the Mg and CO3 has been removed in the translucent zone and Mg and CO3 raise the solubility product the solubility product in the dark zone is lower than the translucent zone. This causes a small amount of precipitation of the calcium phosphate creating small pores.

Body - large pores. Here the proton level is high enough to dissolve all the material

Surface zone - small pores - the high level of fluoride lowers the solubility product causing the calcium phosphate to remineralise.

Front 24

What effect do protiens have on the carious process?

Back 24

Enamel contains most protein near the dentine. The protein is extremely insoluble and resides between the prisms. The protein helps to slow the carious process.

Front 25

Lesion zones pore volume and mineral per unit volume lost for enamel caries (no cavitation)

Back 25

Pore volume mineral loss

Surface - <5% 1-10%

Body - 5-25% 24%

Dark - 2-4% 6%

Translucent - 1%

When the body of the lesion reaches 60-70% mineral loss cavitation occurs.

Front 26

Dentine crystal structure

Back 26

Crystals have similar chemistry to enamel crystals

Smaller than enamel 35nm x 10nm

Much higher levels of carbonate and magnesium - less resistant to acid attack

Embedded in matrix of type 1 collagen

Dentine collagen provides a good environment for bacteria