Oral Biology Enamel PDF

Oral Biology Enamel Professor Name : Dr. Sara elbanna Done by: Menna Hazem ENAMEL ♦Enamel covers the anatomical crown of human tooth. COLOU-1 THICKN-2 R ESS PHYSICA L PROPERT HARDN-3 PERMEA-5 BILITY IES...

Oral Biology Enamel Professor Name : Dr. Sara elbanna Done by: Menna Hazem ENAMEL ♦Enamel covers the anatomical crown of human tooth. COLOU-1 THICKN-2 R ESS PHYSICA L PROPERT HARDN-3 PERMEA-5 BILITY IES ESS BRITTLN-4 ESS  1 - COLOUR Yellowish white to grayish white Depends on : 1- Degree of calcification 2- Homogenisity of the enamel so: Yellowish teeth…. translucent enamel. Grayish teeth …… opaque enamel.  2 - THICKNESS 2 – 2.5 mm at the cusps and incisal edge. Thinning down to almost knife edge at the cervical margin of the tooth  3 – Hardness It is the hardest calcified tissue in the body :due to High content of the mineral salts -1.Its crystalline arrangement -2 Enamel of the permanent teeth is harder than.that of deciduous ones Enamel microhardness- Enamel 1 - Is greatest at the surface and decreased toward DEJ. 2 - It is greater at the cusps and incisal ridge and decreases toward the cervical line. Dentin  4 - Brittleness Its structure and hardness render it brittle, specially when it loses its elastic foundation of healthy dentin or improper cavity preparation results in fracture of the unsupported enamel. unsupported enamel  5- PERMEABILITY It acts as a semipermeable membrane for certain ions and dyestuffs of small molecular size through pores between the crystals. saliva Permeability is mainly from saliva to outer layer of enamel, but less from the pulp to.the inner enamel layer across the dentin  CHEMICAL PROPERTIES INORGANIC ORGANIC ORGANIC INORGANIC BY %4 96% WEIGHT BY EQUAL VOLUME  Enamel histology Ground section Decalcified section the organic substance is inorganic substance burnt and the inorganic dissolve and the organic substance will remain contents remain.  Enamel rod Longitudinal Number Direction Course Diameter Cross section section  Enamel Rod (Enamel Prism) 1- The number of the enamel rod varies from 5 millions in lower lateral incisor to 12 millions in the upper first permanent molar. 2- Direction: The enamel rod is perpendicular to the dentin surface. In permanent teeth In deciduous teeth In incisal ridge & cusp tip: vertical Cervical & middle they are almost horizontal Then change gradually to an oblique direction till: Then change gradually to an increasingly Middle they become horizontal and finally oblique direction till: Cevically they are tilted apically to a knife edge Incisal ridge & cusp tip they become vertical 3- Course: the enamel rod starts straight at dentino-enamel junction (D.E.J.) for about 30 μ then has a wavy course till near the outer surface of enamel where it become straight once more..At the incisal edge or cusp tip the enamel rod has a twisted course and is called gnarled enamel D 4- Diameter: the diameter of the enamel rod increases from the dentino-enamel junction to the outer enamel surface by a ratio of 1:2. Area of enamel surface Area of DEJ Enamel surface 2 1 DEJ 5- Cross section: the enamel rods appear as hexagonal, oval, round or fish scales shape. 6- Longitudinal section: the enamel rods appear as cylinders with cross striations. 4 µ/ day Cross striations  Histological structure of enamel Enamel is essentially a tightly packed mass of apatite crystals and most of its structural features are the results of a highly organized pattern of crystal orientation The appetite crystals at the center of the rod are arranged parallel to the By cross section the rod is longitudinal axis of the rod. represented by a key-hole Then the crystals flare away from the the head toward the occlusal or center till it become perpendicular to the incisal surface where the tail is rod surface and inter-rod substance. cervically. Submicroscopic structures: The enamel rod is somewhat like a cylinder and made up of crystals Crystals: Needle like or ribbon like 300 Ao thickness, 900 Ao width and 0.005-1 μ length For most part of rod (specially near the central axis), the crystals are arranged along the axis of the rod Distant from the center, crystals flare laterally to an increasing degree as they approach the rod boundaries Key hole model of enamel structure (head & tail pattern): One of the cross sectional patterns of enamel rod & inter-rod substance is the key hole Head is the enamel rod (directed occlusally) Tail is the inter-rod substance (directed cervically) When cutting an L.S in enamel: Head of a row, tail of the adjacent row Key hole shape Paddle shape Inter-rod substance: Separates the enamel rods The cervical crystals follow a confluent pattern from the central axis crystals and deviate about 65 0 from the long axis as they fan out into the tails of the rod continuing tilting till being nearly perpendicular to the rod in the associated inter-rod region  Rod sheath: Formed along interface between groups of crystals having markedly different angulations Sheath is seen well surrounding ¾ of each rod (at which crystals of rod meet crystals of neighboring inter-rod region The forth side (facing the cervical inter-rod region) at which no sharp angles but confluent orientation between crystals of rod & crystals of cervical inter-rod region (no rod sheath cervical to rod) This irregular junction accounts for the fish scale appearance in etched ground section or cross section of demineralized developing enamel The sheath contains more enamel proteins because crystals are not tightly packed  Hunter-Shreger bands It is an optical phenomenon caused by changes of rod direction (the wavy course). They are seen clearly by longitudinal ground section viewed by reflected light at cervical 2/3. Transmitted Light Oblique Reflected Light Histology of Enamel Optical Phenomenon.Diazone: Dark Zone.Parazone: Light Zone  Incremental lines Short increments Incremental lines of (cross striations) Retzius  1.Short increments or cross striations: It is the rhythmic apposition of enamel (daily) layers during formation of the crown This daily increment is 4 microns in thickness 4 microns\day Period of rest Period of activity  2.Incremental lines of Retzius The ameloblasts take 4 days to form the enamel between 2 dark lines (so, between 2 dark bands there will be 4 segments of enamel rod of 16 microns length). Incremental lines of Retzius Incremental lines of Retzius  Neonatal line This line separates between enamel formed before birth and enamel formed after birth. PRENATAL POSTNATAL POSTNATAL PRENATAL ENAMEL ENAMEL ENAMEL ENAMEL Longitudinal ground section in Enamel spindle Enamel Neonatal line Incremental lines of Retzius Enamel lamellae The amelo-dentinal junction  T.S ground section in enamel Incremental lines of Retzius Neonatal line Enamel Tuft Enamel lamellae The amelo-dentinal junction Appears as scalloped line where the convex side is toward the dentin.  Enamel lamellae It is a hypocalcified rod and inter-rod substance It appears as a thin leaf or sheet-like structure which extends from the outer surface of enamel to a considerable distance of enamel till the D.E.J. and may extend to dentine.  Enamel lamellae Types of enamel lamellae Enamel lamella type (A) Enamel lamella Enamel lamella {true lamella}. type (B) type (C) It is a type of crack that It is also a type It is due to poorly occurs before eruption of crack which calcified enamel occurs after rod and inter-rod eruption and substance. It is It may be filled It may be filled contains the limited to the with epithelial with connective organic enamel. cells of the tissue of the substance of enamel organ dental sac saliva. lamellae type (B) and (C) may be limited to the enamel or reach the dentine.  Enamel tuft It is a hypocalcified prism and inter-prismatic substance. It always appears in transverse ground section and rarely in longitudinal ground section.  Enamel spindle It is an odontoblastic process which extends in between the cells of inner dental epithelium before the formation of enamel. SURFACE STRUCTURES Outer Structureless Enamel – 1 um thick 30  Outer Structureless Enamel – 1 um thick 30 -Highly mineralized than the rest of enamel. -Thickness is 30 microns from the surface. -It is found in all deciduous teeth and 70% of permanent teeth. -The appetite crystals are arranged parallel to one another and perpendicular to the incremental lines of Retzius. -Commonly they occur at the cervical area more than the cusp tip or incisal edge. REMEMBER: That There Is An Inner Structureless Enamel  2 - Perikymata.Parallel to each other and to C.E.J.Continuous around the tooth.mm – at the region of C.E.J /30 mm – near to the occlusal or incisal /10 edges 3 - Rod Ends Shallower cervically Deeper occlusally 4 - Cracks ENAMEL LAMELLA By careful decalcification crack completely disappears 5 - Afibrillar Cementum ENAM EL DENTI N CEMENTU M  6-Primary Enamel Cuticle (Nasmyth’s Membrane) - 0.2 um thick. - Its structure is similar to the basal lamina of the epithelium. - It is the last product of the ameloblasts. AS G  Age changes of enamel I. Attrition: It is a physiological wear away of the tooth hard substance that mainly occur on the occlusal and incisal surfaces. It is more in permanent teeth than deciduous It is more in men than women due to greater masticatory forces Clinically: It appears as a small polished facets on a cusp tip or ridge Then gradual reduction in cusp height, depth of fissures and consequent flattening of occlusal inclined planes resulting in decrease in the vertical dimensions In case of severe attrition, dentin maybe exposed, loss of surface structures may occur as perikymata and rod ends 2. Color changes Teeth may become darker in color due to: Deepening of dentin color seen through the progressively thinning layer of translucent enamel Addition of organic material from outside on the enamel 3.Alteration of chemical composition of enamel After eruption, ionic exchange between enamel and saliva occurs which may lead to an increase of certain elements such as Fluorine this exchange increase the inorganic content to 98% A decrease in water content of enamel by age was also suggested 4. Permeability Enamel is semi permeable allowing ionic exchange which leads to its surface modification Enamel crystals may acquire more ions and increase in size by age which leads to decrease in permeability or maybe disappears