16. Enamel Structure

Question 1

Enamel is … of origin

Answer 1

epithelial

Question 2

Chemical properties of enamel

Answer 2

• 96% inorganic - HA
• 4% organic - protein and water

Question 3

Properties of enamel

Answer 3

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

Question 4

Enamel structure

Answer 4

• 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

Question 5

Explain prism direction in enamel

Answer 5

• 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

Question 6

Enamel rods are made of …
Explain structure of them

Answer 6

• 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)

Question 7

Define ‘rod sheath’

Answer 7

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

Question 8

What organic material is present in rod sheath?

Answer 8

non-amelogenin enamel protein

Question 9

3 cross section patterns seen in enamel prism

Answer 9

• circular
• stacked
• keyhole pattern

Question 10

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

Answer 10

• all 3 (stacked, circular and keyhole)
• keyhole

Question 11

Explain keyhole enamel prism pattern

Answer 11

• 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

Question 12

How are crystals orientated within a prism?

Answer 12

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

Question 13

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

Answer 13

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

Question 14

Why is the keyhole prism most common?

Answer 14

• combined with rod decussation
• prevents crack propagation

Question 15

Explain prism decussation

Answer 15

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

Question 16

Advantages of decussation

Answer 16

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

Question 17

Explain Hunter-Schreger bands

Answer 17

• 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

Question 18

What are parazones and diazones?

Answer 18

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

Question 19

Define ‘gharled enamel’

Answer 19

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

Question 20

What shape is seen on the EDJ? Why?

Answer 20

• scalloped
• due to an exaptation of epithelium folding

Question 21

Which teeth are more scalloped? Primary vs permanent?

Answer 21

permanent

Question 22

Explain incremental growth lines in enamel

Answer 22

• 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

Question 23

Explain cross striations

Answer 23

• 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

Question 24

Explain Striae of Retzius

Answer 24

• 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)

Question 25

Give types of accentuated lines

Answer 25

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

Question 26

Explain neonatal line

Answer 26

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

Question 27

Explain perikymata

Answer 27

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

Question 28

Explain enamel tufts

Answer 28

• 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

Question 29

Where are enamel tufts found?

Answer 29

• inner 1/3 of enamel
• from EDJ outwards branching

Question 30

Explain enamel lamellae

Answer 30

• 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

Question 31

Difference between cracks and lamellae

Answer 31

• lamellae contain organic material, cracks don’t

Question 32

Explain enamel spindles

Answer 32

• 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

Question 33

Do prism spindles follow prism direction? Why?

Answer 33

• no
• Tome’s processes produce enamel at an angle

Question 34

3 forces in teeth

Answer 34

• compressive
• tensile
• shear

Question 35

Forces during chewing are mainly … but some …

Answer 35

• compressive
• tensile

Question 36

How do chewing forces affect teeth?

Answer 36

• 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

Question 37

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

Answer 37

• 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

Question 38

Define ‘dental erosion’

Answer 38

dissolution of enamel crystallites