teeth are composed - lecture-notes.tiu.edu.iq

Teeth are composedof enamel, the pulp–dentin complex, andcementum

enamel

dentin

pulp

cementum

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Dentino-enamelJunction (DEJ)

Enamel Formation Amelogenesis Ameloblasts

Embryonic germ layer known as ECTODERM

Enamel covers the anatomic crown of the toothThickness of enamel

• Varies from tooth to another

Incisal edges of incisors 2mm

Cusps of premolars 2.3-2.5 mm

Cusps of molars 2.5-3 mm

• Varies in the same tooth from one place to another

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Thickness decreases gradually from cusps orincisal edges to cemento-enamel junction.

The ameloblasts, are lost as the tooth erupts into the oralcavity, and hence enamel cannot renew itself

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Thickness of enamel

The enamel is thicker in the palatalsurfaces of maxillary molars and in thebuccal surfaces of mandibular molars.WHY?

As these are supporting cusps, it issuggested that the increased thicknessin these areas may be viewed as anadaptation to functional demands.

Structural composition of enamel

Millions of rods or prisms Head

Tail

Rod sheath

Inter rod substance

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In transverse sections, enamel rods appear as Key holeshaped ,paddle shaped or fish scales.

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Model indicating packing of keyhole-shaped rods in human enamel.Various patterns can be produced bychanging plane of sectioning.

Number of rods

≈5 Millions

≈12 Millions

Mandibular incisors

Molars

The rods are densely packed and intertwined in awavy course, and each extends from the Dentino-enamel junction (DEJ) to the external surface of thetooth

DEJ

Enamel RodsAn ameloblast is responsible of the formation ofone rod

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• The rods are aligned perpendicularly to theDEJ and the tooth surface in the primary andpermanent dentitions except in the cervicalregion of permanent teeth, where they areoriented outward in a slightly apical direction.

Alignment of enamel rods

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Near the dentinal borders the diameter is about 4 μm and about 8 μm near theouter surface of enamel.

Diameter of enamel rods

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This difference accommodates thelarger outer surface of the enamelcrown compared with the dentinalsurface at the DEJ.

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1. Color of underlying dentin2. Thickness of enamel3. Amount of stains in enamel.

Physical properties of enamel

Color

Enamel is translucent in nature. Color of tooth mainly depends upon threefactors:

Translucency of enamel is directly related to the degree of mineralizationand homogenicity.

Anomalies occurring during developmental and mineralization stage,antibiotic usage and excess fluoride intake, affect the color of the tooth.


Color

The white color of deciduous teeth in comparison to permanent teeth is duethe opacity of their enamel covering

The yellowish tinge in permanent teeth is due to the translucency andthinness of enamel in addition to color and thickness of underlying dentin

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Hardness

Enamel is the hardest tissue of the body

Hardness can vary over the external tooth surface according to the location;also, it decreases inwardly, with hardness lowest at the DEJ. The density ofenamel also decreases from the surface to the DEJ


Enamel is a rigid structure that is both strong and brittle (high elastic modulus, highcompressive strength, and low tensile strength) and requires a dentin support to withstandmasticatory forces. Dentin is a more flexible substance that is strong and resilient (low elasticmodulus, high compressive strength, and high tensile strength), which essentially increasesthe fracture toughness of the more superficial enamel

Enamel rods that lack dentin supportbecause of caries or improper preparationdesign are easily fractured away fromneighboring rods. For optimal strength intooth preparation, all enamel rods shouldbe supported by dentin

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Undermined enamel should be removed when preparing cavities in posterior teeth

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If the supportive layer of dentin isdestroyed by caries or impropercavity preparation, theunsupported enamel fractureseasily.

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Diagram of course of enamelrods in molar in relation to cavitypreparation. 1 and 2 indicatewrong preparation of cavitymargins. 3 and 4 indicate correctpreparation.

Crystalline Component of Enamel

Human enamel is composed of rods that, in transversesection, have a rounded head or body section and a tailsection, forming a repetitive series of interlockingprisms. The rounded head portion of each prism (5 μmwide) lies between the narrow tail portions (5 μm long)of two adjacent prisms

B

TGenerally, the rounded head portion is oriented in theincisal or occlusal direction; the tail section is orientedcervically.

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The structural components of the enamel prismare millions of small, elongated apatitecrystallites that vary in size and shape. Thecrystallites are tightly packed in a distinctpattern of orientation that gives strength andstructural identity to the enamel prisms. Thelong axis of the apatite crystallites within thecentral region of the head (body) is alignedalmost parallel to the rod long axis, and thecrystallites incline with increasing angles (65degrees) to the prism axis in the tail region.

Crystalline Component of Enamel

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Drawing of keyhole pattern of human enamelindicating orientation of apatite crystals withinindividual rods. Crystals are oriented parallel tolong axes of “bodies” of rods and fan out at anangle of approximately 65 degrees in “tails” ofrods.

The interface of enamel and dentin (dentino-enamel junction, or DEJ) is scalloped or wavy inoutline, with the crest of the waves penetrating toward enamel

Dentino-enamel junction DEJ

The rounded projections of enamel fit into the shallow depressions of dentin. Thisinterdigitation may contribute to the firm attachment between dentin and enamel.

Enamel

DEJDentin

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Structural Features of Enamel

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1. Enamel spindle

Structures associated with DEJ & initial formation

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Enamel spindles are extensions ofdentinal tubules that pass through thejunction into enamel during its initialstage of formation. They may serve aspain receptors, explaining the enamelsensitivity experienced by somepatients during tooth preparation.


Structures associated with DEJ & initial formation

2. Enamel Tufts

• So called because they are similar to tufts ofgrass

• Originate from DEJ & extend to 1/3-1/2 thethickness of enamel

• Rich in protein & represent areas ofweakness of enamel

• Have no clinical significance

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3. Enamel Lamellae and cracks


• Hypominerilized areas of enamel that extend to considerable distances in it

• Represent significant weaknesses of enamel andmake enamel susceptible to cracking

DEJEnamel

Dentin

Enamel lamella

Dentinal part of enamel lamella

• Lamellae are formed as a result of failure of maturationprocess of enamel

• They contain water & enamel matrix remnants

• Lamellae are commonly found at the base of occlusal pitsand fissures

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Those related to appositional growth

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Areas of cyclical variation in organic &/or mineral content or density of the enamel rod


Those related to appositional growth

1. Cross striation

2. Striae of Retzius

Result from variations in structure andmineralization

Can be considered as growth rings

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Those related to the change of rod orientation

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1. Gnarled enamel

Occurs as a result of twisting of enamel rods aboutone another through their wavy course

Present in cusps of molars

Strengthen enamel making it more resistant tofracture

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2. Hunter-Shreger bands

They are seen in ground section of enamel asalternating curved light & dark bands extendingfrom DEJ to enamel surface

They are seen as a result to the way in whichsectioned enamel rods reflect light

Dark bands represent cross sectioned rods whilelighter band represent longitudinally sectioned rods.

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Those related to Surface of tooth

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Those related to tooth surface


Acquired coatings

Salivary pellicle

A thin film of organic material consisting of salivary proteins such as mucoproteins &sialoproteins

Salivary pellicle is converted to dental plaque by the embedment of microorganisms in thepellicle

Dental plaque if not removed and when acidogenic will result in dental caries andperiodontal diseases

Dental plaque in turn when become calcified will change in to calculus or tarter

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Solubility of enamel

Enamel is soluble when exposed to an acid medium, but the dissolution is not uniform

Solubility of enamel increases from the enamel surface to the DEJ. WHY?

Because the density of enamel also decreases from the surface to the DEJ

(Demineralization of enamel)

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Demineralization of enamel

Pathological process i.e.

Bacteria

Intentional i.e. Application of acid

for acid etching

Demineralization of enamel has a very important clinical implication which is:

ACID ETCHING OF ENAMEL

By acid etching, dentist can gain micro holes or empty space inside enamel called enamel tags

Dentists make use of these tags by filling them with bonding agent (Adhesive ) before filling teeth withcomposite materials

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Scanning electron micrograph (SEM) of enameletched with 35% phosphoric acid for 15 seconds.

Acid etching After etching

Can we reverse pathological demineralization ?

By the application of topical &/or intake of systemic fluoride the acid sensitive apatite crystals areconverted to fluorapatite crystals which are more resistant to acid attack

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Demineralization of enamel

Pathological process i.e.

Bacteria

Intentional i.e. Application of acid

for acid etching

teeth are composed - lecture-notes.tiu.edu.iq

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