teeth are composed - lecture-notes.tiu.edu.iq
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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.
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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.
Physical properties of enamel
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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Physical properties of enamel
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
Physical properties of enamel
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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Structural Features of Enamel
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Structural Features of Enamel
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.
Structural Features of Enamel
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
Structural Features of Enamel
• 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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Structural Features of Enamel
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Structural Features of Enamel
Those related to appositional growth
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Areas of cyclical variation in organic &/or mineral content or density of the enamel rod
Structural Features of Enamel
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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Structural Features of Enamel
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Structural Features of Enamel
Those related to the change of rod orientation
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Structural Features of Enamel
Those related to the change of rod orientation
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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Structural Features of Enamel
Those related to the change of rod orientation
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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Structural Features of Enamel
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Structural Features of Enamel
Those related to Surface of tooth
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Those related to tooth surface
Structural Features of Enamel
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
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