Enamel

Question 1

What is the composition of enamel by weight?

Dentine?

Cementum?

Bone?

Answer 1

Mineral: 96% Matrix: 2% Water: 2%

Dentine: 70%, 20%, 10%

Cementum: 65%, 23%, 12%

Bone: 60%, 25%, 15%

Question 2

List some features of enamel:

Answer 2

• epithelial product
• 96% inorganic - hydroxyapatite
• 2mm thick max
• translucent
• non-vital
• hard (KHN 360-390)
• brittle

Question 3

What is the formula of hydroxyapatite?

Answer 3

Ca₁₀(PO₄)₆(OH)₂

• OH ion surrounded by 3 Ca ions, surrounded by 3 PO₄ ions, enclosed by 6 Ca ions

Question 4

Answer 4

Question 5

What is a ground section?

Answer 5

• cut hard tissues, by a saw
• 150 microns thick
• not stained
• light refracts
• light reflects internally
• hypomineralised areas (less mineralised) appear dark

Question 6

What does the enamel structure consist of?

Answer 6

• tightly packed hyydroxyapatite crystals
• basic unit = enamel prism
• prisms visible in: ground sections, scanning electron micrographs of acid etched enamel

Question 7

How are keyhole shapes formed in enamel prisms?

What do they dots represent on a diagram?

Dashes?

Answer 7

• due to the different orientations of hydroxyapatite crystals

Dots represent the prism core: tightly packed hydroxyapatite crystals

Dashes = prism shealth: boundary of clearly different crystal orientations, crystals less tightly packed, more space for organic components

Question 8

Interesting features of the enamel prisms:

Answer 8

• follow path of the ameloblasts ~ 90 degrees to ADJ
• undulating/weaving course –> increases strength
• cross striations, interval ~ 5 microns

Question 9

Name this feature of the prisms?

Where is it most commonly seen?

Answer 9

Gnarled enamel: at the cusps/incisal edges, the prisms appear twisted around each other, giving more strength, minimising risk of fracture

Question 10

What is the purpose of acid etching?

Answer 10

• Gives micro-mechanical retention for bonding of resin materials

Question 11

Where are enamel prisms? Are there any exceptions?

Answer 11

• enamel prisms exist for the majjority of the enamel thickness

2 regions are aprismatic:

• 1st formed (innermost) 5 microns
• last formed (outermost) 30 microns

Bond strength of outer surface may be less than that of the enamel prism surfaces

Question 12

Name this feature?

What does this feature look line as a cross section?

What can cause these lines to be exaggerated?

Answer 12

Incremental growth lines - ‘‘Striae of Retzius’’

• appear as brown lines in enamel
• represent where forming cells were at different times (roughly weekly growth)

Accentuated lines result from:

• systemic disturbance e.g. illness
• birth - Neonatal line, crowns forming at birth e.g. primary teeth and crown of 6s may be forming at birth

Question 13

What happens when incremental line reach the surface?

Answer 13

• Perikymata: shallow furrow where the striae reach the surface, plaque retentive factor
• Imbrication Lines of Pickerill: ridges between perikymata

Question 14

What are these dark lines?

Answer 14

Enamel spindles - extension of dentinal tubules into enamel, so found at ADJ, appear dark as full of water or air

Question 15

What are these black lines?

Answer 15

Enamel tufts - hypomineralised local areas

Enamel lamellae - run from ADJ out to surface

• developmental
• aquired e.g. crack
• preparation artefact e.g. tooth cracks when being cut onto slide

Question 16

Where is enamel caries more likely to deleop in temr sof the enamel structure?

Answer 16

Prism shealth - periphery

• less well packed crystals
• space = pores containing water and organic material
• allows easier diffusion of acid –> where demineralisation starts

Question 17

What is the histology of enamel caries?

Answer 17

Use of quinoline which shows up a number of zones:

• translucent zone (deepest)
• dark zone
• body of lesion (corresponds to radiographic extent of lesion)
• surface zone

Radiographs underestimate amount of caries

Question 18

Answer 18

Question 19

What is the transllucent zone?

Answer 19

• 1st carious change
• loss of ~1-2% mineral
• few large pores due to loss of prism periphery
• penetrated by quinoline (resin): appears traslucent ad quinoline fills large prism spaces

Question 20

What is the dark zone?

Answer 20

• dark brown with quinoline
• porosity 5-10%

Consists of: large and small pores

small: not penetrated by quinoline, therefore appear dark as they are filled by air or water
- demineralisation and remineralisation occurring

Question 21

What is the body of the lesion?

Answer 21

• largest part and centre of the lesion
• 25-50% porosity (half crystals are gone)
• resin penetrates easily giving translucent appearance
• corresponds to radiographic appearance

Question 22

What is the surface zone?

Answer 22

• relatively intact
• 30 microns thick
• highly mineralised - high F content
• porosity of ~1-2%: an area of redeposition of mineral dissolved from deeper layers, traps calcium phophate for environment of redeposition, oresence of fluoride promotes process of remineralisation

Question 23

What causes caries to arrest?

Why is fissure caries a problem?

Answer 23

If caries is not cavitated, it can arrest:

• changes in environment: plaque control and fluoride, altered diet
• can be brown: exogenous stains
• histologically - wide, well-developed dark zone

Fissure caries results in 2 lesions, follows prisms, widening lesion, hitting ADJ and very hard to detect