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Craniofacial and Tooth Biology > 16. Enamel Structure > Flashcards

16. Enamel Structure Flashcards

1
Q

Enamel is … of origin

A

epithelial

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2
Q

Chemical properties of enamel

A
  • 96% inorganic - HA
  • 4% organic - protein and water
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3
Q

Properties of enamel

A
  • hardest substance in human body
  • brittle
  • thicker at cusp, thinner at border with cementum
  • light yellow to greyish
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4
Q

Enamel structure

A
  • composed of millions of prisms, each prism is from one ameloblast
  • separated by inter-prismatic region
  • orientation of crystals isn’t uniform and differs
  • prisms grow from EDJ to crown surface in layers/time-lines
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5
Q

Explain prism direction in enamel

A
  • radiate from EDJ to surface like spokes of a wheel but with 3D curve
  • in cervical enamel, enamel of primary is obliquely orientated to oral cavity
  • in permanent, teeth obliquely orientated towards alveolar crest
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6
Q

Enamel rods are made of …
Explain structure of them

A
  • millions of HA crystallites
  • boundaries between these are porous and small
  • changes in crystallite orientation in prism and inter prismatic enamel occur
  • outer rodless enamel layer is more mineralised as lacks rod sheath (organic material)
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7
Q

Define ‘rod sheath’

A
  • boundary between prism and inter prismatic enamel
  • contains organic material (non-amelogenin enamel proteins)
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8
Q

What organic material is present in rod sheath?

A

non-amelogenin enamel protein

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9
Q

3 cross section patterns seen in enamel prism

A
  • circular
  • stacked
  • keyhole pattern
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10
Q

How many of the enamel prism patterns are seen in humans?
Which is most common?

A
  • all 3 (stacked, circular and keyhole)
  • keyhole
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11
Q

Explain keyhole enamel prism pattern

A
  • wide head towards coronal/occlusal
  • narrow tail towards cervical
  • each keyhole rod formed by 4 ameloblasts
  • one ame forms head or rod of keyhole
  • 3 form tail - interrod enamel
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12
Q

How are crystals orientated within a prism?

A
  • parallel to long axis of prism in head
  • oblique to long axis of prism in tail (angled)
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13
Q

Change in crystallite orientation in prism and inter prismatic enamel help to what?

A
  • differentiate between individual prisms
  • and between head and tail within them
  • different refraction of light, prism sheaths clear in polarised light
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14
Q

Why is the keyhole prism most common?

A
  • combined with rod decussation
  • prevents crack propagation
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15
Q

Explain prism decussation

A
  • groups of enamel prisms follow sinusoidal path
  • bundles of enamel rods cross each other as they travel from EDJ to surface
  • this is decussation
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16
Q

Advantages of decussation

A
  • strengthens enamel structure
  • prevents propagation of cracks into deeper areas of enamel
  • improves resistance to fracture
  • role in Hunter-Schreger bands
17
Q

Explain Hunter-Schreger bands

A
  • an optical phenomenon occurring in inner 2/3 of enamel thickness as alternating light and dark bands (not to do with incremental growth)
  • underlying mechanism responsible is prism decussation
  • parazones - light reflective zones
  • diazones - dark transparent zones
18
Q

What are parazones and diazones?

A
  • the light reflective and dark transparent respectively bands in Hunter Schreger bands
  • due to prism decussation
19
Q

Define ‘gharled enamel’

A

an area with exaggerated prism decussation (extremely angular)
- over cusp tips

20
Q

What shape is seen on the EDJ? Why?

A
  • scalloped
  • due to an exaptation of epithelium folding
21
Q

Which teeth are more scalloped? Primary vs permanent?

A

permanent

22
Q

Explain incremental growth lines in enamel

A
  • enamel forms in layers
  • daily enamel secretion rate increases from EDJ to enamel surface in perm and prim teeth
  • rates drop by 0.5 microns per day across neonatal line in primary teeth
23
Q

Explain cross striations

A
  • result of daily variation in ameloblast secretory rate and mineralisation
  • equivalent to von Ebner lines in dentine
  • more weakly defined with closer spacing
  • transverse lines across enamel rod
24
Q

Explain Striae of Retzius

A
  • result of ameloblast position at various points of time during development
  • equivalent to Andresen lines in dentine
  • more sharply defined with wider spacing
  • around 7-10 short period lines/cross striations between 2 on these
  • often from EDJ to outer surface ending in shallow pits called perikymata
  • accentuated striae - neonatal line
  • accentuated incremental lines produced due to systematic disturbance (linear enamel hypoplasia)
25
Q

Give types of accentuated lines

A
  • neonatal line
  • in linear enamel hypoplasia
  • Wilson lines in enamel
  • Contour lines of Owen in dentine
26
Q

Explain neonatal line

A
  • incremental line that occurs at birth
  • results from stress
  • only teeth developing at birth can exhibit these
  • line is darker than other incremental lines
27
Q

Explain perikymata

A
  • outward aspect of internal growth increments
  • normal transverse wavelike grooves or lines on external surface of teeth
  • can be lost through tooth wear
28
Q

Explain enamel tufts

A
  • hypomineralised voids containing organic material - mainly tuftelin (a non-amelogenin protein from prism sheath)
  • located in inner 1/3 of enamel, start from EDJ and project outwards for short distance (branched)
  • orientation follows prism decussation
29
Q

Where are enamel tufts found?

A
  • inner 1/3 of enamel
  • from EDJ outwards branching
30
Q

Explain enamel lamellae

A
  • when enamel tufts pass through entire thickness of enamel, called lamellae
  • appear similarly to cracks but lamellae contain organic material, cracks don’t
  • seen best in transverse sections but visible in lonitudinal sections in cusp tip
31
Q

Difference between cracks and lamellae

A
  • lamellae contain organic material, cracks don’t
32
Q

Explain enamel spindles

A
  • formed from odontoblast processes embedded in first zone of enamel
  • mainly in cusp tips
  • don’t follow prism direction as Tome’s processes produce enamel at an angle
33
Q

Do prism spindles follow prism direction? Why?

A
  • no
  • Tome’s processes produce enamel at an angle
34
Q

3 forces in teeth

A
  • compressive
  • tensile
  • shear
35
Q

Forces during chewing are mainly … but some …

A
  • compressive
  • tensile
36
Q

How do chewing forces affect teeth?

A
  • radial cracks can form at contact point on cusp and marginal cracks going upwards from cervical margin
  • difficult for cracks to enter dentine as tougher (absorbs stress before plastic deformation)
  • slowly growing fissures fill with organic/protein fluid gluing them
  • tensile stress on tufts is highest under load but fall off around it so adjacent tufts have less stress
37
Q

‘The strategy of tooth survival is one of damage containment rather than avoidance’
Explain

A
  • fractures initiate easily at tufts
  • hard to continue to failure
  • older people have crack-like defects in teeth that accumulate
  • protein rich fluids (tuftulin) continually repair cracks
  • decussation is higher in region of EDJ where tufts occur to prevent crack propagation
38
Q

Define ‘dental erosion’

A

dissolution of enamel crystallites

Craniofacial and Tooth Biology (25 decks)

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