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Oral Biology > Enamel > Flashcards

Enamel Flashcards

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1
Q

What are the physical properties of Enamel?

A
  • Hardest tissue
  • Withstands shearing and impact forces & has a high resistance to abrasion
  • Thickness varies from up to 2.5mm over cusps to feather edges at cervical margins
  • Cannot be repaired or replaced
  • Brittle - requires support from underlying dentine
  • Low tensile strength but high modulus of elasticity
  • Surface and subsurface enamel (surface is harder and denser and less porous)
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2
Q

How does the hardness and density vary in enamel?

A

The hardness and density decrease from the cusp tips to the cervical margins

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3
Q

What’s the chemical composition of enamel?

A
  • 96% inorganic components
  • 2% organic
  • 2% water by weight
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4
Q

What’s the inorganic composition of enamel?

A
  • Calcium hydroxyapatite is the principal mineral component of enamel

Present in the form of crystallites

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5
Q

What’s the organic composition of enamel?

A

Free amino acids, small molecules, peptides and large protein complexes (amelogenins and non-amelogenins)

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6
Q

What are the characteristics of the hydroxyapatite crystals?

A
  • 70nm in width
  • 25nm thick and of great length
  • Most are hexagonal in cross section
  • Cores of crystals are richer in magnesium and carbonate in comparison to the peripheries
  • Each crystal unit has a hydroxyl group surrounded by 3 calcium ions which are surrounded by 3 phosphate ions
  • 6 calcium ions in a hexagon enclose the phosphate ions!!!
  • The crystal is made of a repetition of those planes of ions side by side in stacked layers!!!
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7
Q

Substitutions in the hydroxyapatite crystals?

A
  • The main and most important substitute is fluoride as it makes the crystal more stable and reduces acid wear
  • It replaces the hydroxyl group in the centre and enlarges the crystal size
  • Carbonate incorporation into the crystal promotes the carious attack
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8
Q

What’s the role of water in enamel?

A
  • 2% by weight, 5-10% by volume
  • Water presence is related to the porosity of the tissue
  • Can be present between the crystals surrounding the organic component
  • Fluoride ions travel through the water component
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9
Q

What’s the composition and function of the organic matrix?

A
  • 50-90% small molecules (peptides and free amino acids; particularly glycine and glutamic acid)
  • Larger protein complexes of amelogenins and enamelins - contain components rich in carbs
  • Highest conc. of proteins in the enamel are located in tufts at the Dentine-enamel junction
  • Lipid content 1% by weight of enamel - remnants of cell membranes
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10
Q

How does the % of proteins and peptides differ in relation to weight in mature and early enamel?

A
  • Account for less than 2% of mature enamel weight

- Account for 25-30% of early enamel weight

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11
Q

What’s the bulk material of developing enamel matrix?

A

90-95% amelogenin protein produced by ameloblasts

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12
Q

What’s the function and characteristics of Amolegenins?

A
  • Hydrophobic and aggregate into clumps
  • Spread throughout the whole developing enamel = a gel matrix through which molecules and ions spread readily
  • This aids the formation of large crystals
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13
Q

What’s the function and characteristics of non-amelogenins in the organic matrix?

A
  • E.g. Tuftelin
  • May be derived from plasma albumin
  • Contain distinct components secreted by ameloblasts
  • Unknown role; but may have a role in mineralisation
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14
Q

Enamel Histology: How is enamel studied?

A
  • Ground sectioning
  • In demineralised sections, enamel is totally lost due to its high mineral content
  • Immature enamel can be studied in demineralised sections due to its high protein (25-30%) and low mineral content
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15
Q

Enamel Prisms: What are enamel prisms?

A
  • Basic structural units of enamel (prism/rod)
  • Each prism is composed of several million hydroxyapatite crystals packed into a long thin rod 5-6µm in diameter and up to 2.5mm in length
  • These prisms run throughout the entire enamel
  • Have a scale like appearance
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16
Q

Enamel Prisms: What are the characteristics of enamel prisms within enamel?

A
  • Prisms are separated by INTER-ROD substance which are crystals with a different orientation (deviated by 40-60 degrees)
  • Only visible via scanning electron microscope
  • Prisms have a head and tail region - the tail of one prism lies between the heads of the two adjacent prisms
17
Q

What are Hunter-Schreger bands?

A
  • Every 10-13 layers of prisms follow the same orientation; but blocks above and below follow paths of different directions
  • Cases a banding pattern called Hunter-Schreger bands
  • Outer 1/4 of enamel doesn’t have an Hunter-Schreger bands as the enamel prisms run in the same direction
18
Q

How does Prism arrangement differ at the cusps?

A
  • Prisms over the cusps appear twisted around each other in a complex arrangement = gnarled enamel
  • Evolved to resist the greater loads and therefore shearing forces at the susp
19
Q

What’s Aprismatic enamel?

A
  • Outer surface layer of enamel is aprismatic
  • Crystallites are aligned at right angels to the surface and parallel to each other
  • Enamel is more highly mineralised than the rest of the enamel - no prism boundaries where organic material is located
20
Q

What are Incremental lines?

A
  • Enamel is formed in increments: Periods of activity alternating with inactivity
  • They reflect different activity levels

2 different types:

  • Cross striations (short period)
  • Enamel striae (long period)
21
Q

How do Cross Striations appear?

A
  • Appear as lines crossing the enamel prisms at right angels to their long axes
  • Reflect daily increments of growth (diurnal rhythm)
  • Closer to each-other near the enamel-dentine junction
22
Q

How do Enamel Striae appear (including cross section)?

A
  • Enamel striae run obliquely across the prisms - known as Striae of Retzius
  • Don’t reach the surface
  • In cross-sections, the SOR run circumferentially like rings of a tree
  • Weekly intervals
23
Q

Enamel Striae: Striae on the surface?

A
  • On the surface, the Stria appear as fine grooves running circumferentially around the crown (lateral lines)

The lines are termed:
- Perikymata grooves with Perikymata ridges in between

24
Q

Surface Enamel: How does it differ from subsurface enamel?

A
  • Physically & chemically, surface enamel differs from subsurface enamel

Surface enamel is:

  • Harder
  • Less porous
  • Less soluble
  • More radio-opague (appears whiter)
  • Rich in trace elements
  • Aprismatic (highly mineralised)
25
Q

What are the traits of surface enamel?

A
  • Have pits within the perikymata which mark the ends of ameloblasts
  • Focal holes are depressions on the surface - result in the loss of enamel caps (happens through abrasion or attrition)
26
Q

What are Enamel brochs?

A
  • On pre-molars, they are elevations on the enamel surface
  • 30-50μm in diameter
  • Radiating groups of crystals
27
Q

What are the features of the Enamel-Dentine junction?

A
  • Clear distinction between the 2 tissues
  • The boarder has a scalloped pattern where shearing forces would be high (beneath cusps and incisal edges)
  • Junction is smooth in lateral surfaces
28
Q

What are Enamel Spindles?

A
  • Narrow, round tubules 8μm in diameter
  • Extend up to 25µm into the enamel
  • Typically beneath cusps/areas of crowding
  • Projections from odontoblasts which have remained in the enamel + died
29
Q

What are Enamel Tufts?

A
  • Junctional structures in the inner-third of enamel which resemble tufts of grass
  • Run in the same direction as enamel prisms
  • Hypomineralised (have the highest conc. of in-organic material)
30
Q

What is Enamel Lamellae?

A
  • Structural faults/defects that run through the entire thickness of the enamel
  • Hypomineralised area
  • Incomplete maturation of groups of prisms

WATCH OUT - SHOULDN’T BE CONFUSED WITH CRACKS PRODUCED DURING GROUND SECTION PREP.

31
Q

Enamel Microporosity: How porous is enamel?

A
  • There are water filled spaces between the crystallites
  • Enamel porosity makes up 3-5% by volume
  • Large pores at prism boundaries
32
Q

Cement-Enamel Junction (CEJ): What are the 3 possible arrangements?

A

REMEMBER: enamel can never overlap cementum as it develops afterwards

REMEMBER: All 3 patterns can occur on a single tooth

Pattern 1: Cementum overlaps the enamel (60%)

Pattern 2: Cementum & enamel meet at butt joint (30%)

Pattern 3: Cementum & Enamel fail to meet and the dentine between them is exposed (10%)

33
Q

What happens if you have Pattern 3?

A

Sensitivity can occur due to exposed dentine

34
Q

How does Enamel change with age?

A
  • Darkens in colour with age + staining; reduced translucency of the tooth as secondary dentine forms and enamel thins
  • Comp. of surface enamel changes due to exchanges with oral fluid
  • Decrease in caries due to enhanced mineralisation/loss of tooth structure/lower intake of carbs/smoothening of plaque retaining areas by attrition
35
Q

Clinical considerations: How prevalent are enamel defects?

A
  • 68-95% of the population have developmental defects present
36
Q

Clinical considerations: How does Hypoplasia present itself?

A

Pits and grooves

37
Q

Clinical considerations: How does Hypomineralisation present itself?

A

Opaque patches (white)

Oral Biology (33 decks)

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