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*Note: The exam will be on the 28th/Monday at 3:15pm, after which you are to return to the hall for the discussion of the exam questions and if there is

enough time a class shall be given*

Physical Properties of enamel

Enamel is the hardest biologically synthesized object. Its thickest parts are over the cusp tips and the incisal edges and can reach to about 2.5mm in thickness. It is thinnest at the cervical margin where enamel ends at the cervical line. This is important to remember because when patients visit the clinic complaining of pointed cusp tips which in turn would injure their gums/soft tissue; the treatment is to smoothen the cusp tips to about 2.5mm hence the importance of the information. But if you go even 0.5mm more than the 2.5 you’re going to expose the dentine so it is of the upmost importance you know the thickness of the enamel.

Enamel does not undergo replacement or repair; which is different than dentine; when the surface dentine is subjected to injury such as caries, trauma or a cavity preparation for a filling, the dentine at the deepest area will be built up as a response to the injury but enamel is irreplaceable, why? Because the cells forming the enamel disintegrate or merge with the enamel immediately after the enamel is formed after which they only serve in the protection of the enamel and not the regeneration. In contrast, dentine is produced by odontoblasts which are still alive after the dentine formation hence they are to protect against any damage.

Enamel has a low tensile strength and is quite brittle. If a force/tension was to be applied upon enamel it would fracture but it is resilient against compression forces. But

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one would ask, ‘if the enamel has a low tensile and is brittle and we have been using our permanent teeth for a long time, how come it is not fractured yet?’, that would be because of the support of the dentine which is resilient hence any pressure applied by the enamel will be absorbed by the dentine and this way enamel fracture will be avoided. When does the enamel get fractured? When the supporting dentine is lost; this is due to several factors the most common one being the dental caries. For example when a person suddenly chips their tooth during mastication, which would be due to the absence of dentine or the presence of a week of dentine which in turn would leave the enamel with no support in absorbing the tension caused by mastication.

Enamel is white and is translucent. The translucency is usually low but when we grow up in age this translucency increases that is why in old people you observe yellowish teeth which are actually the color of the dentine but in young people the teeth will not reflect that much translucency therefore the teeth appear white. Which is another misconception, natural human teeth color is actually ranging between yellowish white and grayish white but never completely white. The ones seen completely white are actually bleached like with celebrity teeth which is not natural.

Chemical Properties of the teeth

The chemical properties of enamel are basically inorganic materials/ minerals are arranged in a crystal form termed as Hydroxyapatite Ca10(PO4)6(OH)2. They form 88-90% by volume and 95-96% of weight of the enamel and this is reason for enamel loss during decalcification (the fact that Hydroxyapatite makes most of the enamel and when it is decalcified we lose almost all the enamel) of teeth in the

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process of stained slide preparation (previously explained); when we make a slide we cut thin sections of the enamel and to do that we need it to be soft therefore we add a decalcifying agent to remove all the minerals leaving us with 5% of weight or 10-20% by volume therefore there is not enough enamel to show hence shows up as a space, this is for mature enamel. Immature enamel however, does not appear as a space because it is low in minerals having only 30% mineralized and 70% organic material and that is what you will see in the slide. That is why the unstained slide known as the ground section is the best way to observe the mature enamel.

The mineral content increases from the Enamel Dentine Junction to surface. As the surface enamel is all the time in contact with saliva and the saliva contains calcium and other minerals, therefore the surface enamel is more mineralized then the deepest areas of the enamel as the deep area is only in contact with the dentine.

Crystallites are much bigger than those in dentine, cementum & bone. Other calcified tissues like the bone, cementum and dentine also possess hydroxyapatite but is much smaller than that of the enamel.

The core is more soluble than the peripheries. The enamel is made of prisms. The core of the enamel is filled with these prisms; as the core is more soluble at the core upon pressure more enamel will be removed from the core of these prisms than the peripheries, that is why after itching of the enamel it will be rough because we have 1-very deep areas where the core of the prisms are and we have 2-very raised areas where the peripheries of the prisms are. This is very important for aesthetic dentistry like composite dentistry (طب االسنان التجميلي) or dentistry of composite

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fillings owes its presence to fact that enamel’s solubility is different from the core than the peripheries.

Ion replacement may occur. Sometimes “HCO3 can replace OH”, “Mg can replace Ca” and most importantly “F can replace OH”. This happens during the brushing of teeth by fluoride enriched toothpaste where the ions of the fluoride replace the ions of the OH in the teeth forming VIP

Fluorapatite instead of hydroxyapatite. Fluorapatite is much better than hydroxyapatite because it is much more resistant against acidic solubility. The enamel is affected by acidic solubility when it is affected by dental caries which are bacteria that accrue their nutrients from the food bits attached to teeth by secreting acids which in turn dissolves the enamel in its way. The Fluorapatite in the teeth being resistant against such acids help in the enamel protection.

Fluoride Level declines from outer to the inner layers. This means that the amount of fluoride on the enamel surface is more than the fluoride present in the inner/core of the enamel. That is due to the fluoride contained within the saliva which is acquired from external sources such as tea, toothpaste, water, etc. People who drink fluoride enriched water such as rain water which carries abnormal amounts of fluoride, have a lesser chance of dental caries but on the other hand this fluorine inters in the composition of the teeth during teeth development which would lead to fluorosis.

Water accounts for 2% by weight and 5-10% by volume. The organic matrix is 1-2% which is variable but the rest of

the enamel is crystals. So recounting; the crystals make about 95-96% of the enamel and the rest 4-5% is made out of water and organic matrix.

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Amelogenins and Enamilens are proteins making up the enamel *will be explained later* and Amino acids, peptides and lipids are the organic materials in the matrix.

Where do we find the organic matrix? Is it evenly distributed within the enamel? The answer is no, they tend to be accumulated where the crystals are irregular because then there will be larger spaces for the matrix to accumulate in but where the crystals are regularly packed and organized there will be less spaces between them for the organic matrix to accumulate in. This is all at the peripheries of the prisms where we have a sudden change in the orientation of the crystals as it is (orientation) gradual from the core to the periphery of one prism of enamel.

Enamel Prisms

The basic structural unit of enamel is prisms. The enamel is not simple calcified or mineralized tissue that is accumulated irregularly but is actually organized in the form of prisms. If you observe a cross of the enamel you would see the prisms in the form of key holes ( مثل رصيف مطعم السهل The crystals are packed in a long think rod like .(!االخضرstructure *will be explained shortly*

These prisms start from the Enamel Dentine Junction. It keeps elevating until it reaches the surface of the enamel.

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The boundaries reflect a sudden change in the orientation of the crystals. The crystals within one prism undergo gradual difference in orientationSLIDE 8 this is a cross section of the enamel prism; observe the part I blue, it looks like a keyhole and that is the enamel prism.

Notice that this is a longitudinal section; the crystals run with the long access of the prism but as we go toward the periphery they tend to change their orientation and this change is gradual within one keyhole pattern. Notice the sudden change in orientation once one keyhole pattern is over within a prism and as it gets to the boundary of the second keyhole. Hence the irregular arrangement of crystals at which we have the organic material.

These sudden changes can range from 40° to 60° (‘°’ means degrees). Even with the orientation the prisms are still closely packed as the orientation is gradual.

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In each cross section we have what we call a head and a tail. The tail is located between four heads.

In the Head the prisms run parallel to the prism’s long axis; within the keyhole crystals diverge in different directions in the mid-section area (this starts when it gets away from the core of the prisms)

Prism = rod + interrod Prism = head + tail = keyhole patternHead = core = rodTail = interrod Prism = Hexagonal>>this is not accepted any more in modern dentistry but was taught previously and can be found in some old books and references. Their thesis was based upon the shape of the core and its borders.

In the tail crystals are running at an angle of 65° to 70° from those in the head but the divergence is gradual

^Patterns of Enamel

Within a cross section of enamel we can observe three different patterns of prisms;

Pattern I – Circular This is not a dominant pattern and is only found near the Enamel Dentine Junction (EDJ) and the surface enamel; Because of this circular pattern the size of the interprismatic areas (the space between prisms) will be large therefore this part of enamel will be occupied by a large amount organic material.

Pattern II – Parallel Here we basically have parallel rows of prisms.

Pattern III – Keyholethis is the most common and the most predominant pattern occupying the bulk of the enamel.

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In longitudinal section, prisms appear to run in straight lines from EDJ to surface. When these prisms reach the enamel surface they would form three different angles with the surface; *At the cervical margin they would form right angles (90°) *At a more occlusal (incisally) part they would form a 60° angle *At fissures it is 20° angleFor the preparation of cavities for amalgam fillings it is very important to remember these angels.

Hunter-Schreger Bands

As we previously mentioned, the prisms run from the enamel dentine junction (EDJ) to the surface and that it generally fallows a straight path; but in fact this path is not 100% straight it’s said to be sinusoidal (as a reference to the sine or sinusoid wave in mathematics).

That is why we have the enamel prisms running in layers; a block of 10-13 layers have the same path/direction but the next block could fallow a different path and therefore not all the prisms layer follow the same sinusoidal path. This difference of paths is what we term as the Hunter-Schreger

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Band. This sinusoidal path gives the enamel; 1- property of having a high resistance to fracture and 2- a rough surface if the enamel is to be fractured.

The periodic change in the path of the enamel layers is actually what gives rise to the Hunter-Schreger bands.

*Ground Section of Enamel; Due to strong calcification of enamel it would be lost in the process of decalcifying for the preparation of a stained slide therefore we

prepare an unstained slide known as the ground section which is made by the grinding of the teeth until a thin layer is achieved which is then put on a slide and

observed. In this technique the tooth is dead hence you cannot see the organic material and can only observe the crystals and inorganic

minerals.

Slide 12 Here you can observe contrast zones, as in some are dark

and some are light; these are actually the Hunter-Schreger Bands and the contrast is due to the light

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reflection off of the different sinusoidal paths of the prism bundles.

Also notice that they run very close to the long axis of the enamel prisms

*Parazones and Diazonnes not included

Gnarled Enamel, (‘G’ is silent); under the cusp tips and incisal edges a group of prisms spiral around each other.

Aprismatic Enamel

Is the enamel entirely prismatic? No. If you recall, the Enamel is prismatic because the ameloblasts forms tomes' processes; after tomes process we have a thin layer of enamel foam laid down without the ameloblasts having tomes’ processes therefore this layer will be aprismatic. In other words you will not see prisms in this layer but only storage of prisms/prisms laid down with no prismatic structure.

In permanent teeth this layer will be 20 – 70μm In deciduous teeth this layer will be 20 – 100μm Crystallites are parallel to each other and are at a right angle

(90°) with the surface VIPIt is more mineralized due to the absence of prismatic

pattern of enamel as in that state the prisms are arranged in a straight from leaving a very small space of organic material and giving more space for the inorganic material. That is also why the surface enamel is more mineralized then deep layers of enamel.

VIP; surface enamel is more mineralized than deep enamel because…

1. It is Aprismatic

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2. It is in constant contact with saliva which contains calcium deposition

Why does Aprismatic enamel occur? It’s because we have an absence of tomes’ process at the late stage of deposition

Incremental lines

Enamel is formed incrementally as in layer by layer. This is due to the periods of activity and periods of Quiescence (periods of lethargy فترة ركود) of the ameloblasts. Between the layer formed at the activity and the layer formed at the quiescence is a line known as the incremental line. To explain this we are going to follow a simple example; to build a house (tooth), we hire workers (ameloblasts). As they start working (activity) they will gradually be tired and need a break (quiescence) after which they will start again. The break period is distinguished as an end time and then the continuation later by a line known as the incremental line. These breaks or incremental lines are of two types;

1. Short period; they are basically the daily rhythm cross-striations which reflect the daily break. They are usually 2.5μm – 6μm apart. In other words the ameloblasts everyday would build a layer of enamel of 2.5μm thickness and from previous information we know that the total enamel thickness is 2.5mm; this means that we need about 1000 days to build the full thickness of enamel theoretically speaking *it is actually the average tome needed to the enamel completion from the tooth emersion to the crown completion*

In the cervical part the enamel is only 2μm apart because the enamel grows slowly.

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2. Long period; they are known as the enamel striae and reflect the weekend holiday.

a. They are structural lines running obliquely across the prisms in longitudinal sections. If a layer of these striae were to be observed even closer we would find even 7-10 smaller lines of the short period of the incremental lines.

b. They also run circumferentially in cross sections. They are similar to the striations found in s tree when cut transversally through its log.

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c. Striae overlapping cusps & incisal edges do not reach the

surface.

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As the enamel is formed layer by layer with the presence of one enamel striae between each layer (long period striation); notice that once the full thickness of the cusp is established further layers are not added on it but are only added to the sides and that is why not all of the enamel striae reach the surface.

Perikymata Grooves & ridges

When one enamel striae reaches the surface it produces a groove known as the perikymata groove which is a depression at the

end of the elevation of the enamel.

VIPAt the cusp/incisal tips we do not have perikymata, why? Because enamel striae do not reach the surface at these areas it only reaches the surface at the sides of the tooth

Between one perikymata groove and another we have a raised area called the perikymata ridge.

They appear as a series of fine grooves and ridges alternatively running circumferentially

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They are close together near the cervical margin as they are built slowly there

In deciduous teeth, only seen in cervical enamel of second molars. VIPThe second deciduous molar is the only tooth that has some part of its crown forming after birth that is why we can observe the perikymata. This means that the perikymata grooves and ridges only accrue in any enamel built post dentally

Enamel Dentine Junction

It is the junction between the enamel and the dentine and it shows two patterns1. Scalloped; this is found where the enamel needs strength

like beneath cusps, incisal edges and convexities2.Smooth; at areas that are not subjected to any forces like

at the lateral surfaces Structures visible at the EDJ are;

o Enamel spindles; they are dentinal tubules extending into the enamel, they should end in dentine but sometimes they extend up to 25μm into the enamel. They are believed to be the odontoblast processes that remain between ameloblasts

o Enamel tufts; they resemble a tuft of grass which travel in the same direction as the prisms. They represent hypomineralized areas formed as a result of residual non-Amelogenins (protein matrix) that stay after the enamel formation.

o Enamel lamellae; They are sheet-like structural fault running from the dentine to the enamel surface. It is also hypomineralized. It causes can be either 1- developmental, as in due to incomplete maturation of a

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group of prisms or 2- because of cracks caused after eruption.

Age Changes

As we grow in age our enamel start to wear out and it does that in three different ways;o Attrition; this is due to constant tooth to tooth contact.

Usually more apparent in people who grind their teeth during tension.

o Abrasion; due to contact with hard objects like with nomads or Bedouins where their food contains a lot of sand particles which are harsh against the teeth or with people who brush their teeth the wrong way could also cause abrasion to their teeth.

o Erosion; enamel loss due to chemicals like in access eating of citrus fruits or pickled vegetable this causes the teeth to have a smooth and glossy surface

Darkening of the teeth is also a side effect of aging due to teeth translucency where the teeth would have increased in its thinness and also because of acquired stains

Composition of surface enamel also changes as in more 1- fluoride will be incorporated in the surface enamel with age therefore 2- dental caries is less common in older people and also the 3- porosity is also reduced (chance of pores)

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Good Luck and sorry for the delay………Mohamed Harun B. Sanoh

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