Enamel Flashcards
What are the physical properties of Enamel?
- 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)
How does the hardness and density vary in enamel?
The hardness and density decrease from the cusp tips to the cervical margins
What’s the chemical composition of enamel?
- 96% inorganic components
- 2% organic
- 2% water by weight
What’s the inorganic composition of enamel?
- Calcium hydroxyapatite is the principal mineral component of enamel
Present in the form of crystallites
What’s the organic composition of enamel?
Free amino acids, small molecules, peptides and large protein complexes (amelogenins and non-amelogenins)
What are the characteristics of the hydroxyapatite crystals?
- 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!!!
Substitutions in the hydroxyapatite crystals?
- 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
What’s the role of water in enamel?
- 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
What’s the composition and function of the organic matrix?
- 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
How does the % of proteins and peptides differ in relation to weight in mature and early enamel?
- Account for less than 2% of mature enamel weight
- Account for 25-30% of early enamel weight
What’s the bulk material of developing enamel matrix?
90-95% amelogenin protein produced by ameloblasts
What’s the function and characteristics of Amolegenins?
- 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
What’s the function and characteristics of non-amelogenins in the organic matrix?
- E.g. Tuftelin
- May be derived from plasma albumin
- Contain distinct components secreted by ameloblasts
- Unknown role; but may have a role in mineralisation
Enamel Histology: How is enamel studied?
- 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
Enamel Prisms: What are enamel prisms?
- 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
Enamel Prisms: What are the characteristics of enamel prisms within enamel?
- 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
What are Hunter-Schreger bands?
- 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
How does Prism arrangement differ at the cusps?
- 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
What’s Aprismatic enamel?
- 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
What are Incremental lines?
- 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)
How do Cross Striations appear?
- 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
How do Enamel Striae appear (including cross section)?
- 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
Enamel Striae: Striae on the surface?
- 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
Surface Enamel: How does it differ from subsurface enamel?
- 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)
What are the traits of surface enamel?
- 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)
What are Enamel brochs?
- On pre-molars, they are elevations on the enamel surface
- 30-50μm in diameter
- Radiating groups of crystals
What are the features of the Enamel-Dentine junction?
- 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
What are Enamel Spindles?
- 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
What are Enamel Tufts?
- 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)
What is Enamel Lamellae?
- 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.
Enamel Microporosity: How porous is enamel?
- There are water filled spaces between the crystallites
- Enamel porosity makes up 3-5% by volume
- Large pores at prism boundaries
Cement-Enamel Junction (CEJ): What are the 3 possible arrangements?
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%)
What happens if you have Pattern 3?
Sensitivity can occur due to exposed dentine
How does Enamel change with age?
- 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
Clinical considerations: How prevalent are enamel defects?
- 68-95% of the population have developmental defects present
Clinical considerations: How does Hypoplasia present itself?
Pits and grooves
Clinical considerations: How does Hypomineralisation present itself?
Opaque patches (white)
