Oral Histo 4th Lec

8/2/2019 Oral Histo 4th Lec

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ENAMEL

Enamel is the layer that covers the crown of human teeth.-not all animals have enamel only limited to crown, for example if we take a Rodent(

( you will find enamel on the root also.

On the other hand, cementum which covers the root, it is limited to the root in the human

teeth, but in some animals you find cementum inside the crown

physical properties of enamel:1) Enamel has not the same thickness at different regions of the tooth. You find enamelthickest over incisal edges and cusp tips and this is very important , we have some clinical

importance for this information . It is also the thinnest at cervical margin or near the

crevices of tooth.

the importance of this: when I know the enamel is thick over cusp tips or incisal edges ;

when you for example do a filling , when you can go deep ,you can go deep 2mm without

exposing dentin and we know that enamel is avascular and not innervated

that's why we can drill inside enamel without causing pain to the patient anesthesia

because he/she will feel pain.

Enamel at cervical margin at is thin as knife edge so even if you drill for 0.5 mm then you

reach dentin but if you want to drill over cusp tips or the incisal edges you can go for 2 mm

or 2.5 mm without reaching dentin . So the choice of local anesthesia is related to the area

that you want to drill in the crown ; ex: if you want to make a filling it involve only the

enamel portion at the incisal edge , you don't need anesthesia .because your patient will

not be in pain , but if you want to make any minimal filling even a very shallow filling at the

cervical margin of enamel ; of course you need anesthesia. Because you will reach dentin

after 0.5 mm of drilling (remember there will be Q in the exam)

2) Enamel is the hardest biological object :

it is not the hardest object of course there is object harder than enamel like ( steel,glass)

but it is the hardest biological object( produced by living organism or animal)

3) Enamel doesn't go under replacement or repair

(this is extremely important)it is in contrast to dentin ; dentin go under replacement or repair one you loss some enamel

you can't replace that piece of enamel. Of course we can replace it by a filling but the tooth

itself will not be able to replace by that piece of enamel .why?? The cells that form the

enamel ,once enamel is matured ( after post maturation stage) the cells that form the

enamel which called ameloblast they are lost or at least they will merge with other cells and

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their function will stop, that's why we can't call the ameloblast cell to produce enamel when

we need enamel , in contrast dentin is different to form enamel at this area; because if we

loss dentin ate certain area ,the tooth itself build dentin at that area of course at not that

spot , but underneath that spot to make it simple for you , if we lose 1 mm at surface of

dentin ; the pulp forms 1 mm at the other surface of dentin which is at the pulpal surface of

dentin.this mean amealogensis is different from dentenogensis; the process in which enamel is

formed is different compared to the process in which dentin is formed.

Q: dentin is contain all over the tooth is related to (didn't hear the question!! )

we have 2 type of dentin formation :1-continuous formation of dentin

2- reparative dentin : dentin formation in response to tissue damage ; to build reparative

dentin at the damaged area

Q: The dentenogensis is continuous process does this related to cavity? Dr: no, it is not

related ; the dentin that is related to caries it is called tertiary dentin

3) enamel has lows tensile force which means this peace of enamel if you apply 2 forces to

make tension in that enamel ; apply 2 forces in opposite direction (it is weak) :D

4) enamel has very high compressive strength ; if we apply a force to compress the piece

like this;it decrease resistant ;so enamel is very weak when it comes to tensile forces but it

is strong when it comes to compressive forces.

*** Enamel is brittle ;so it can easily fractured although it is hard but it can easily fractured:

and this is have very important clinical significance

Q: Enamel is brittle then how come we can eat using our teeth without being fractured or

without breaking down

Because enamel is brittle it has to be supported by resilient tissue which is Dentin. For

example, lets imagine that I have a piece of glass between the projector and this computer,

it is very easy to me to break this glass but if I put this piece of glass over a cushion and

apply the same force this piece of glass will not fracture. In similar way if the enamel is

supported by a resilient dentin then it is difficult to break down or fracture, but if enamel

becomes unsupported by dentin it can easily fracture.

In human enamel ;enamel formation has end it doesn't go under replacement on repair ,butif you take an animal like rat, these have continuous formation of enamel through the life of

the tooth.

why do have this feature in Rodin in particular ?

because these Rodin loss their teeth continuously that's why we should have a mechanism

by which enamel form to replace the continuous loss of enamel

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Q: when do we have cases of enamel unsupported by dentin?When we have caries.

-Sometimes if we have caries at the side of the tooth, this caries start in enamel and reach

dentin. So if the caries leave part of enamel unsupported, enamel can easily fractured .

And this explain why sometimes while eating we have piece of our tooth fractured.!!

so any weak force will break it so enamel is resistant to breakdown when supported by

dentin. And because it is very brittle and it can easily fractured when not supported that's

why it is important to go to dentist at periodic intervals ...why?? Because the dentist can

discover the initial caries or initial decay lesion and treat it but if left without treatment it will

enlarge and leave part of enamel unsupported that can be easily fractured.

.

-In dentistry when we do a filling if we feel that piece of enamel will not be supported it hasto be removed as we will see inshallah next year. When we drill a tooth you will notice that

a piece of enamel is not supported this piece must be removed because if we put

restoration under it , this piece of enamel will break easily, because the restoration is not

resilient as dentin, it is harder.

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Notice the upper 2 pictures: the caries start at enamel and reach dentin, so the

enamel become unsupported( the black area under the enamel is the carious dentin,

so the enamel above it become unsupported) and finally will fracture


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Enamel is white in color:But if you look in mirror you see it yellowish, but why??

-Actually enamel is white but what make enamel less white, or yellowish is the reflection of

the dentin, because enamel is translucent so it reflect the shade of dentin.

With age enamel translucency increase that's why the ability of that enamel to show

the color of the underlying dentin is more that's why old people tend to have yellowish

teeth.

Tooth shade is very variable, we have people who have yellow teeth, grayish teeth, ivoryteeth and this is related to Translucency in enamel which is different from one person toanother also genetic role has effect.so the color of the teeth is related to:

1(color of dentin2(thickness of enamel

when enamel is thick its translucency becomes less .

when enamel is thin its translucency becomes more.look at your teeth at mirror you will find the area next to the gingiva are slightly moreyellowish than the area at cuspal tips or incisal edges why?? Because at gingival areaenamel is thinner more translucency yellowish.But at cusp tips or incisal edges, enamel is thick l translucency decreased.

)this is very important there will be 5 or 6 Q in this slide please remember my explanation

for the different properties of ename)l

let's talk about the chemical properties of enamel:

1(made up of inorganic material:the inorganic material (minerals) that form crystals called (hydroxyapatite crystals) so

mineral in enamel not occurring haphazard they are organized in crystals.

Ca10(PO4)6(OH)2

Why do we call them hydroxyapatite crystals??

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Hydro because they have hydroxyl group (OH) water

apatite means it is a composite of calcium phosphorus so we have 10 atoms of Ca , 6

atoms phosphate, 2 ions of hydroxyl group.

What is the volume of hydroxyapatite / inorganic material inside enamel?

-By volume 88-90% of enamel is mineral /inorganic / hydroxyapatite.

-By weight 95-96% of enamel is mineral.Note : here we talk about fully mature enamel, but if we talk about immature enamel then

it is different:

immature enamel is only 25-30% calcified or mineralized.

which is more brittle Mature enamel or immature enamel?

mature enamel is more brittle because it is fully mineralized or almost entirely

mineralized

2(Inorganic material:

-By volume 10-11%

- By weight 4- 5 %

In the form of organic material and water number of protein ,amino acid , water....etc.Mineral content increases from enamel dentin junction to surface.

so if I had a piece of enamel I will find more mineral at the surface of enamel, why?

because our teeth are always in contact with saliva this make enamel accept moremineral from saliva but the deeper area of enamel will not actually accept more mineralbecause they are deep.so that is why enamel has more mineral at surface than the enamel -dentin junction.

when we talk about these percentages of course we talk about the average percentage ofmineral in enamel ex: surface enamel may be 100% mineralized but the deepest layer ofenamel may be only 85% mineralized but the average of mineral in enamel is 88% -90%by volume and 90-96% by weight.

crystallite are hexagonal in shape, 70 nm in width 25 nm thick, and of great length. so the biggest crystalline are the ones found in enamel.Crystallites are much bigger than those in dentine, cementum & bone.

what about the crystalline of dentin? Those hard tissue contain hydroxyappatite crystal butthe size of crystals are smaller so enamel has more mineral also the size of crystals arebigger.

Enamel more mineral + larger crystalDentin less mineral + smaller crystal

The core is more soluble than the peripheriesIf take any crystalline, or if you take a pre enamel prism you find the core more soluble

than the periphery , why? because the organic material tend to accumulate at the peripheryso the core is entirely inorganic, that is why when we apply a solvent like phosphoric acidfor acid etching you will find that the core of enamel prism or the core of enamel crystallinemore soluble than periphery and this is important for attaching the filling that we use indentistry which is composite) because it is restorative material which used in cosmeticdentistry.

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Ion replacement may occurso this crystalline may not remain the same especially at the surface because at the

surface it exchange with the saliva that's why some ion can replace other ions. Examples:-HCO3 for OH

-Mg for Ca ( mg replace the ca(

-F(fluoride) for OH conferring greater stability & resistance to acidic dissolution.And this is has a very significant importance. Fluoride can replace hydroxyl group atsurface enamel.what does this do?This actually gives greater stability and resistance to acidic dissolution.

When do we actually have acidic dissolution of enamel?In case of caries , bacteria that cause caries start to feed on remnant of food and this

produce acidic byproduct the acid will dissolve enamel.so enamel that contain fluoride will be more resistant than enamel contain hydroxylgroup and this is the importance of fluoride in protection.

why do you brush your teeth? Why do you use toothpaste that contain fluoride?Because what we need to do with that fluoride to replace the hydroxyl group and by

this the hydroxyapatite become fluorapatite.Fluorapatite is more resistant to aciddissolution produced by the acids of bacteria than hydroxyapatite.

F level declines from outer to inner layers.Because of that fluoride level declines from outer to inner layer , definite fluorideaccumulated at surface of enamel so fluoroappatite occurs at surface enamel you willnot find too many fluoroappatite crystals at the deepest area of enamel becausefluoride can't reach that deep area.

WaterAbout 2% by weight or 5-10% by volume

Organic matrix-1-2%

-May be more-we find the organic material where crystalline are irregular. The peripheries of prism meetat sharp angel and this meeting make the crystalline irregular that's why anything thatirregular contain many spaces between them these are the spaces where the organicmaterial accumulate.

when the crystalline are packed in a very regular way without irregular arrangementyou will not find organic material or you will find very small amount of organicmaterial........ In the center of the keyhole structure in the prism (in the core) you willnot find too many organic material. You will not find much organic packed parallel to

each other, we may sometimes see some change in the orientation of the crystallinebut this change is very gradual. When the end of one prism meet the end of anotherprism that is the area where the crystalline become irregular and meet at sharp angeland this actually make the area suitable environment for the accumulation of organicmaterial.

-EDJ : also we may find the organic material at area called enamel tufts and at enameldentin junction.

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-Amino acids, peptides, ameloginins & non-amelogenins & lipids. all of these are theorganic material that exist in enamel.

Enamel PrismsBasic structural unit consisting of crystals packed in long & thin rods .

The inorganic material and also the organic material are not haphazard, they are

arranged in a very organized fashion, in what we call (prism.(Run from EDJ to the surface.

They run from enamel dentin junction to the surface of enamel.so each prism it has tohave 2 ends :- one end at the surface of enamel , the other end at enamel- dentinjunction.

Boundaries reflect sudden change in orientation of crystals (40 60) degrees.when the end of one prism meet the end of another prism that is the area where thecrystalline become irregular and meet at sharp angel.

In x-section :the cross section of enamel prisms can have many different shapes but the most

common shape is the key-hole pattern

-Pattern I circular pattern: Near EDJ & surface: We find it near the enamel dentin junction and at the

surface also we find it at the inter prismatic areas or sometimes between prisms wemay find this pattern

Interprismatic areas exist between prisms

-Pattern II _ U shapeParallel rows

-Pattern III keyhole pattern

Most predominant: most predominant pattern and occupy the bulk of enamel

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Keyhole pattern

what is the mean of key - hole ? In cross - section through enamel prism you see a key

hole

can you see this blue zone ; it is a key hole area, it is represent the cross section of

enamel.

Head & tail areas:

The keyhole consist of head and tai

A tail is located between 4 heads:

What is actually produces the prismatic structure of enamel ? the fact that we have

Tomes Processes present during the secretory stage of enamel formation , if our God

created ameloblast without Tomes process this mean that our enamel will be

aprismatic(without prism) the enamel will form of mineral distributed randomly without

this organized structure.

at the beginning of the secretion the first layer it is aprismatic ,which is a thin layer then we

have the full thickness of prismatic enamel and the final layer is aprismatic ; so for this

reason prismatic is present where Toms processes form for the ameloblast.

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Tomes' processes are a histologic landmark identified on an ameloblast, cells involved

in the production of tooth enamel.


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The small black arrow show the aprismatic (prismless) enamel

Change in crystals orientation is gradual within a single keyhole but sudden between 2

keyhole

notice that this is the longitudinal section of (one prism) notice that enamel if you take this

crystal and that crystal they will not be parallel but notice that the change on the orientation

of the crystal is very gradual but if you take this crystal here belonging to this prism and

another crystal belonging to other prism you will final that we have a sudden change in

orientation of crystal , also you find that crystals at the core of the head they run parallel tothe long axis of the enamel prism but as we go away from the core these crystal start to

change their orientation but this change is gradual until they reach the periphery where the

change come suddenly.

In the head, crystals run parallel to prisms long axis

Within the keyhole, crystals diverge in different directions from the heads central area

In the tail crystals are 65-70 degrees from those in the head but divergence is gradual

In longitudinal section, prisms appear to run in straight lines from EDJ to surface,

because of that prism meet enamel surface at different angle.

Prism at the cervical margin they meet enamel surface at right angle.occlusally the surface of enamel make 60 degree with the long axis of enamel prism

at fissure or developmental grooves make 20 degree

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cervically (90(

sorry I did not find a suitable picture to discuss this so I used this picture. Please try toimagine the angle between the prisms and the outer surface of enamel

Now this is important in restorative dentistry, because all the time when we prepare the

cavity we have to leave enamel prisms supported , dont make any cavity leaving some of

enamel prism unsupported, because they can fracture of.

The different angle here produced by the end of enamel prisms when they make angle with

the surface. These are important in restorative dentistry , particularly when it comes to

amalgam restoration.

Terminology

Prism= rod+ interred

Prism = head + tail

Sooo

Head=rod=core

Tail=interred

Is prism equal to hexagonal structure??

In old text books they considered them the same

But now it is not the same the prism consist of the head and the interred area( the

keyhole structure(

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Occlusally (60)


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Hunter-Schreger bands

It is a feature that can be visible in ground section of enamel .You this picture here , it is a ground section of enamel.

in histology we have 2 types of sections:

1) Ground section: only for hard tissue, so if we want to see enamel .

If we decalcify enamel in order to cut it into thin slices by microtome , i will lose enamel

totally and enamel will appear as a space , so the best way to see enamel is to do ground

section:

a)to bring a piece of tooth and to grind that piece of tooth until you get a very thin

section of enamel

b) then put it at microscopic slice and examine the hard tissue

2) Stained histological section:

For example when we discussed amelogensis , dentinogensis, the embryo of Rabbit all

these called stained section because we can see stains and you need to know how these

where prepared .

Stained section generally

a) bring tissue

b) put it in a decalcifier to decalcify or take all the mineral off

c)then we cut very thin slices of that tissue

d) we put it on microscopic slide and examine it under the microscope.

Slide.12 , This Is a "Ground Section" ,In This Ground Section Of Enamel We Can See Dark

And Light Lines "Bands" These Are Called "Hunter Schreger Bands ",Let`s Discuss How

These Show.

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Prisms follow a sinusoidal path in longitudinal sections-We Said, any Prism Runs From Enamel-Dentin Junction To The Surface of Enamel.

Actually This Path Is Not a Straight Path, It is a Sinusoidal Path "In Longitudinal Section ",

Layers in a block of 10 -13 layers follow same direction

-Each 10-13 Layers They Have The Same Direction. If We Take another Layer It`ll Have

Another Direction "But Also Sinusoidal", That`s Why This Difference In The Orientation Of

Different Layers Of Enamel Prisms Present "Hunter Schreger Bands ",You Dont Need To

Know More Than That, Dont Worry About Details.

Blocks above & below follow different direction

-Resistance to fracture

-Fractured enamel has a grinding surface

Periodic changes give Hunter-Schreger bands

Because different bands of prisms transmit light in different directions

Parazones

Areas where bands of prisms are cut longitudinally

Diazone

Areas where bands of prisms are cut tranversely

Angle between parazones & diazones is 40 degrees

Bands in outer run in same direction no HS bands

Gnarled enamel

Underneath cusp tips & incisal edges

Where groups of prisms spiral around others

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Aprismatic enamel

We Said The First Layer of Deposited Enamel Is "Aprismatic, Means That We Cannot SeePrisms", And Also The Last Layer Is "Aprismatic".

Occur in Permanent teeth

the Outer 20 70 m

In Deciduous teeth

the Outer 20 100 m

Crystallite are parallel to each other & at right angle to the surface

In Aprismatic Enamels We Have Crystals But We Dont Have Prisms And These Crystals

Are Parallel To Each Other And At Right Angle To The Surface.

More mineralized due to absence of prism boundaries

Because You Can Have The Parallel Packing of Crystals Without The Meeting At

Peripheries At Sharp Angles Because We Dont Have Prisms, We Dont Have Peripheries

of Prisms,We Dont Have The Meeting of Crystals At Sharp Angles,That`s Why We Dont

Have Organic Material,We Only Have Inorganic Material,That`s Why It`s More Mineralized

Than Prismatic Enamel,It occurs Due To The Absence Of Tome`s Process During Late

Stages of Enamel Deposition.

Occur due to absence of TP at late stage of enamel deposition

Also We Have a Very Thin Aprismatic Layer In Both Deciduous And Permanent Tooth

Just Above The Enamel-Dentin Junction But The Outer Layer Is thicker, which Is The

Layer That Results Following The Loss of Tome`s Process "Projections Surrounded By

The Developing Enamel,Give The Ameloblast Appearance Under The Microscope" ,We

Say We Loose Tome`s Processes But Also We Have Some Deposition of Enamel AndAlso We Have Depositions Of What We Call "Enamel Cuiticle" So This Makes Enamel

"Aprismatic " At The Outer Layer,Which Is Thicker,We Find It Thicker At "Deciduous

Teeth.".

Aprismatic Enamel does not respond to "Etching" like Prismatic enamel, We Said if you

want to utilize enamel for attaching a filling you should do "Etching" and this produces

areas Like cavities or holes and raised areas, but If you do "Etching" For "Aprismatic

Enamel" You just get a smooth surface, that`s Why Aprismatic Enamel is not perfect for

attaching fillings, that`s why if you want to utilize enamel for attaching fillings you have to

go deep, you have to choose deeper layers of Enamel, If You Only Used The Surface You

May Get Attachments But They`ll Actually Be Weaker Than If We Drilled Inside The

Enamel And If W e Used The Layers Just Below To The Aprismatic Layer.

Incremental Lines

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enamel forms incremantally, to simplify it imagine a worker building a wall,this wall has to

be built increment by increment, that`s how enamel is built between one increment and

another there is a line, this line represents the area between one increment and another

increment we call it "incremental line",,

what produces these incremental lines?

they say that Periods of activity alternates with periods of quiescence, Alright, This Isproduced because one ameloblast deposits one layer of enamel and then it gets some

relaxation or let`s say it`s now after having deposited the first or one layer of enamel it

takes a rest and then it starts again and continues to lay down another layer,so because

we have Periods of activity alternates with periods of quiescence that`s why this produces

incremental lines.

Incremental lines are one of two types :

1-short period

2-long period

Short period incremental lines are due to daily rhythm of enamel deposition,

long period incremental lines are related to let`s say from 7-10 days, each 7-10 days we

have one of these incremental lines.

Example to make it easy :

we have one worker,this worker is able to build one line or one layer each day and then he

goes home and take a rest,then the next day he builds another layer so by the end of the

week he has built let`s say 7 layers,then after that,after let`s say one week,this worker says

to you I also want to take Friday off so he takes one long day off,one day in the week,in a

similar way,,a group of ameloblast builds one layer each day but may be after 7 or 10 days

they take a longer period off to relax so the produces the "long period line" and the

relaxation between one day and another or between one layer and another layer it`s the"short period line incremental line cross striation",this is an example of incremental build

up,the "short period incremental line" represents the dayernal rhythm of enamel

deposition ,this means that one ameloblast to build one layer it needs oneday,and in this

example each layer is 4m,so what is the speed of ameloblast in building this layer? 4

m/day so each day they build 4 m of enamel and then they relax and continue the next

day and so on,this is called "short period incremental line" .

we have another aspect which is "long period incremental line",,each 7-10 layers we

have one line,this line is called "long term/period incremental line" or enamel striae

because it reflects the weekly pattern of enamel deposition or the relaxation between one

week of work and another week of work,,the average is 4 m but it`s from 2.5- 6 m apart.

Enamel StriaeAre Structural lines running obliquely across the prisms in longitudinal sections.

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Short Term enamel incremental lines are not visible under compound microscope so we

view it using electron microscope,But Enamel Striae Can Be Visible Under normal

compound microscope,,lines of enamel striae are 7-10 days away from the other one.

for example,the ameloblast took a rest after certain line,then it needed about other 7-10

days to take another weekly rest,But within these two lines we`ll find about 7-10 short term

incremental lines,so enamel striae,they run obliquely across prism in longitudinal sectionsas we see but they run circumferentially in x-sections or cross sections.

Striae overlapping cusps & incisal edges do not reach the surface

Enamel striae are formed because different because layars of enamel are built at different

stage

As you see here this is dentine in white and this is the first layer and this is the second

layer of enamel and each layer caps the other , so this means that enamel striae here

does not reach the surface

Near the cervical margin the striae reach the surfaceNote ; not all enamel striae reach the surface

When any enamel sytriae reach the surface it make a depression small groove these

grooves are called perikymata grooves

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See this pic

The different lines here is the perikymata grooves and the place ptween tow of them is

perikymata ridge , this pic is an electron microscope slide for the surface not a section , this

is a tooth from out side we didn't cat it

You can see it in your incisor if you want see your teeth in the mirror you well see these

lines in cervical 2\3 only but it does not exist in the incisal 1\3 why ? because the enamel

striae which that produced the perikymata grooves do not reach the surface in the incisor

or occlsual 1\3

NotePerikymata grooves & ridges

Occur as enamel striae reach enamel surface

Appear as a series of fine grooves and ridges alternatively running circumferentially

Close together near the cervical margin

In deciduous teeth, only seen in cervical enamel of second molars

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Enamel striae

Structural lines running obliquely across the prisms in longitudinal sections

They run circumferentially in x-sections

Striae overlapping cusps & incisal edges do not reach the surface There are 7 10 cross-striation between 2 subsequent striae

Reflect nearly a weekly intervals

Due to metabolic disturbances during mineralization

Absent in enamel formed before birth

Neonatal line is a marked stria formed at birth reflecting metabolic disturbance at

birth

They are absent in enamel formed before birth so that we can't find them in deciduous

teeth

Enamel dentine junction

JED reflects the boundary between enamel and dentine

Two patterns

Scalloped

Beneath cusps & incisal edges

High shearing forces

Convexities at enamel surfaces

SmoothAt the lateral surface

Low shearing forces

Scalloped (zay el sata2r )

The scalloped junction is more resistant and it provides more attachment

The smooth surface we can easily separate them

Structures visible at EDJ

Enamel spindles

Enamel tuftsEnamel lamellae

Enamel spindleThese process extend up to 25 mm from EDJ into enamel

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Odontoblastic process cross the EDJ into enamel just before mineralization and once

mineralization takes place , those processes got stuck inside enamel and called enamel

spindle

Enamel tuftsThey resemble tufts grass, they consist of hypomineralized enamel rods and they are

several prisms wide

Enamel lamellaeThey are sheet-like structural fault

They run through the entire thickness of enamel

They represent hypomineralized

The different between the cracks and the enamel lamellae

Cracks are limited to some thickness of enamel

Enamel lamellae happened from all the surface of enamel to the EDJ

Causes of enamel lamellae

Developmentally ; maybe due to incompletely of prims

Age changes

Enamel wear

Abrasion

Erosion

Attrition

Darkening in color

Increased thinness

Acquired stains

Composition of surface enamel changes

More Fluoride incorporated

Susceptibility to caries decreases Porosity is reduced

Good luck all

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Done by:

Mai Alsoutari

Heba Da'asGhadeer Afaneh

Aya Shahrori

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Documents

Oral Histo 4th Lec