16. Enamel Structure Flashcards
Enamel is … of origin
epithelial
Chemical properties of enamel
- 96% inorganic - HA
- 4% organic - protein and water
Properties of enamel
- hardest substance in human body
- brittle
- thicker at cusp, thinner at border with cementum
- light yellow to greyish
Enamel structure
- 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
Explain prism direction in enamel
- 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
Enamel rods are made of …
Explain structure of them
- 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)
Define ‘rod sheath’
- boundary between prism and inter prismatic enamel
- contains organic material (non-amelogenin enamel proteins)
What organic material is present in rod sheath?
non-amelogenin enamel protein
3 cross section patterns seen in enamel prism
- circular
- stacked
- keyhole pattern
How many of the enamel prism patterns are seen in humans?
Which is most common?
- all 3 (stacked, circular and keyhole)
- keyhole
Explain keyhole enamel prism pattern
- 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
How are crystals orientated within a prism?
- parallel to long axis of prism in head
- oblique to long axis of prism in tail (angled)
Change in crystallite orientation in prism and inter prismatic enamel help to what?
- differentiate between individual prisms
- and between head and tail within them
- different refraction of light, prism sheaths clear in polarised light
Why is the keyhole prism most common?
- combined with rod decussation
- prevents crack propagation
Explain prism decussation
- groups of enamel prisms follow sinusoidal path
- bundles of enamel rods cross each other as they travel from EDJ to surface
- this is decussation
Advantages of decussation
- strengthens enamel structure
- prevents propagation of cracks into deeper areas of enamel
- improves resistance to fracture
- role in Hunter-Schreger bands
Explain Hunter-Schreger bands
- 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
What are parazones and diazones?
- the light reflective and dark transparent respectively bands in Hunter Schreger bands
- due to prism decussation
Define ‘gharled enamel’
an area with exaggerated prism decussation (extremely angular)
- over cusp tips
What shape is seen on the EDJ? Why?
- scalloped
- due to an exaptation of epithelium folding
Which teeth are more scalloped? Primary vs permanent?
permanent
Explain incremental growth lines in enamel
- 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
Explain cross striations
- 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
Explain Striae of Retzius
- 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)
Give types of accentuated lines
- neonatal line
- in linear enamel hypoplasia
- Wilson lines in enamel
- Contour lines of Owen in dentine
Explain neonatal line
- incremental line that occurs at birth
- results from stress
- only teeth developing at birth can exhibit these
- line is darker than other incremental lines
Explain perikymata
- outward aspect of internal growth increments
- normal transverse wavelike grooves or lines on external surface of teeth
- can be lost through tooth wear
Explain enamel tufts
- 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
Where are enamel tufts found?
- inner 1/3 of enamel
- from EDJ outwards branching
Explain enamel lamellae
- 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
Difference between cracks and lamellae
- lamellae contain organic material, cracks don’t
Explain enamel spindles
- 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
Do prism spindles follow prism direction? Why?
- no
- Tome’s processes produce enamel at an angle
3 forces in teeth
- compressive
- tensile
- shear
Forces during chewing are mainly … but some …
- compressive
- tensile
How do chewing forces affect teeth?
- 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
‘The strategy of tooth survival is one of damage containment rather than avoidance’
Explain
- 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
Define ‘dental erosion’
dissolution of enamel crystallites
