Enamel

physical and chemical properties, structure, age changes, clinical consideration of enamel. hypocalcified and surf

1. Enamel Dr.madhusudhan reddy

2. Contents Introduction Physical characteristics Chemical characteristics Structure Development Life cycle of ameloblast Amelogenesis Clinical considerations

3. Introduction Enamel is an epithelially derived protective covering of variable thickness over the entire surface of the crown It is the hardest biologic tissue in the body It attains the maximum thickness of about 2-2.5mm on the cusps of molars and premolars . It thins down to almost knife edge at the neck of the tooth

4. Physical characteristics PHYSICAL PROPERTIES 2-COLOR 5-PERMEABILITY 1-THICKNESS 4-BRITTLNESS 3-HARDNESS

5. Physical characteristics 1. Thickness 2.5 mm (thickest) over cusp of molar 2.0 mm over incisal edge Knife edge thickness at the cervical region Thick at maxillary lingual surfaces of molars and mandibular buccal surface

6. 2. Color Depends on thickness and translucency of enamel Grayish white to yellowish white Yellowish – at thin areas underlying dentin Grayish – thick opaque enamel Bluish – incisal area

7. 3. Hardness Enamel is the hardest calcified tissue in the body due to 1) High content of mineral salts . 2) Its crystalline arrangement . Enamel of the permanent teeth is harder than that of the deciduous ones. Enamel microhardness is 1) greater at the surface and decrease towards the DEJ 2) greater at the cusp and incisal ridges and decreases towards the cervical line.

8. 4. Brittleness Its structure and hardness render it brittle, specially when it looses its elastic foundation of healthy dentin 5. Solubility : dissolves in acidic media 6. Permeability : It acts has a semi permeable membrane for certain ions and dye stuffs of small molecular size through pores between the crystals . Permeability is mainly from saliva to outer layers of enamel. But less from the pulp to the inner enamel layers across the dentin. 7. Specific gravity : 2.8 8. Translucency : semi translucent

9. Chemical properties Inorganic content – 96% Organic content – 4% Inorganic content Minerals Hydroxyapatite (calcium phosphate) Ions – strontium, megnesium , lead, fluoride .

10. Organic content : mainly proteins ( Amelogenenins , Non Amelogenins ) Amelogenins 90% Low molecular weight protein Rich in proline , histidin , glutamin , leucine . Non amelogenins 10% High molecular weight Proteins – ameloblastin , tuftelin , enamelin . Rich in glycine , aspartic acid and serine.

11. Structure of enamel Rods Rod sheath Inter-prismatic substance Striations Directions of rods Hunter- schreger bands Incremental lines of ritzius Surface structures Enamel cuticle Enamel lamellae Enamel tufts Dentinoenamel junction Odontoblast processes and enamel spindles

12. Hypocalcified areas of enamel Rod sheath Incremental lines of ritzius Enamel lamellae Enamel tuft Enamel cracks Enamel spindles Neonatal lines

13. Enamel rods Basic structural unit Head of enamel rod is formed by one ameloblast and tail is formed by three ameloblasts Thus each rod is formed by four ameloblasts Shape : hexagonal, oval , fish scale, key hole Starts from DEJ to outer enamel surface

14. Number: Lower lateral incisor - 5 millions Upper 1 st molar - 12 millions Course: Tortous from DEJ Length: greater than thickness of enamel Diameter : 4 µm in light microscopy. Increases from DEJ to outer enamel surface by a ratio of 1:2 since outer surface of enamel is greater than inner surface Appearance : clear crystalline.

15. Light microscopy: Rods appear hexagonal In cross section: fish scale appearance Recently – arcade outline near DEJ and keyhole outline at enamel surface.

16. Ultrastructure / electron microscopy RODS surrounded by rod sheaths separated by interrod substance. KEYHOLE or PADDLE- SHAPED pattern is observed. When cut longitudinally sections passes through heads or bodies of one row and tails of adjacent row. Measures 5 µm in breadth and 9 µm in length . Bodies of rods are towards occlusal / incisal surface and tails point cervically

17. Hydroxyapatite crystals Arranged approximately parallel to the long axis of the rods Length 0.05 to 1µm Width 90 µm Pyramid shape

18. The rod sheath A thin peripheral layer Darker than rod Relatively acid resistant Less calcified and contains more organic matter than the rod itself

19. Inter-prismatic substance Cementing enamel rods together More calcified than the rod sheath Less calcified than the rod itself Appears to be minimum in human teeth

20. STRIATIONS Enamel rods is built up of segments of uniform length of about 4µm , seperated by dark lines that gives it a straited appearance More visible by action of mild acids Appearance is because of formation of enamel matrix in rhythmic manner . More pronounced in hypocalcified areas Cross-striation within a ground, longitudinal section of enamel

21. Direction of Rods ORIENTED at right angles to the dentin surface The bundles of rods seem to intertwine irregularly in the region of cusp or incisal edges . This optical appearance of enamel is called Gnarled enamel . Direction of rods in deciduous teeth Direction of rods in permanent teeth

22. In deciduous teeth , direction of rods is horizontal in cervical and central parts of the crown. Near incisal edge or tip of cusp they gradually increase in oblique direction and almost vertical in the cusp tip region In permanent teeth , in occlusal two third of the crown direction of rods is oblique . In cervical direction rods deviate from the horizontal in apical direction

23. GNARLED ENAMEL Near the dentin in the region of cusps or incisal edge , bundles of rods seem to interwine more irregular , especially in section cut obliquely. This optical appearance of enamel is called gnarled enamel .

24. HUNTER- SCHREGER BANDS. Optical phenomenon seen in reflected light Seen in longitudinal ground section Due to abrupt change in direction of enamel rod Alternating light ( diazones ) and dark ( parazones ) strips of varying widths best seen in longitudinal ground section under oblique reflected light . Originate at the DEJ and pass outward, ending at some distance from the outer enamel surface .

25. Hunter schreger bands are due to Change in the direction of enamel rods Variation in calcification of the enamel Alternate zones having different permeability and organic material

26. INCREMENTAL LINE OF RETZIUS Incremental lines of ritzius , Strae of ritzius Appear as Brownish bands in ground sections of enamel. Incremental pattern of enamel formation. In longitudinal sections - surround the tip of the dentin . In cervical parts of the crown they run obliquely.

27. In transverse section - appears as concentric circles . DEJ to outer surface of enamel Reflects variations in structure and mineralization that occur during growth of enamel. Broadening of incremental lines may reflect metabolic disturbance at the time of matrix formation NEONATAL LINE or NEONATAL RING : Accentuated incremental line of Retzius . Where they end as shallow furrows known as perikymata

28. Neonatal line The enamel of the deciduous teeth and 1 st permanent molar (it is incremental line that is boundary between the enamel formed before and after the birth) The neonatal line is usually the darkest and thickest striae of ritzius Etiology Due to sudden change in the environment and nutrition The antenatal enamel is better calcified than the postnatal enamel

29. SURFACE STRUCTURES : Prismless enamel Perikymata Rod ends Enamel caps Enamel brochs Enamel cracks Enamel cuticle Enamel lamellae

30. PRISMLESS ENAMEL About 30µm thick Present in 70% permanent teeth and all deciduous teeth Found least often over the cusp tips Found commonly in the cervical areas No enamel prisms visible. All the apatite crystals are parallel to one another and perpendicular to the striae of ritzius More mineralized than the bulk of enamel beneath it,

31. PERIKYMATA These are transverse wave like grooves , believed to be the external manifestations of striae of ritzius 30 perikymata in number per mm in the region of CEJ Their concentration gradually decreases near occlusal or incisal surface to about 10 per mm

32. ENAMEL ROD ENDS These are concave and vary in depth They are shallow cervically and deep occlusally / incisally Pits - 1-1.5µm in diameter Enamel caps - 10-15µm elevations Enamel brochs - Larger enamel elevations

33. ENAMEL CRACKS They are actually outer edges of lamellae Extent: They originate from incisal edge and extend to varying distances in enamel in perpendicular direction towards DEJ Length:1mm mostly

34. ENAMEL CUTICLE The delicate membrane covers the crown of newely erupted tooth called Nasmyths membrane or primary enamel cuticle. This is soon removed by mastication This is secreted after epithelial enamel organ retracts from cervical regions during tooth development It protects the surface of enamel from resorptive activity of adjacent vascular tissue

35. PELLICLE Erupted enamel is covered by a precipitate of salivary proteins called pellicle This pellicle reforms within hours after mechanical cleaning . It becomes colonized by microorganisms within a day or two after formation which forms bacterial plaque

36. ENAMEL LAMELLAE Thin leaf like structures that extend from enamel surface towards DEJ Composition mainly organic , little mineral May be confused with cracks. Origin: Develops in planes of tension . When rods cross such a plane, they may not fully calcify . If the disturbance is more severe , a crack may develop Crack is filled either by surrounding cells if it has occurred in unerupted tooth , or by organic material if it has occurred after eruption

37. Types Type A : consists of poorly calcified rod segments Resticted to enamel Type B : Consists of degenerating cells . May reach into dentin Type C : containing organic material , presumably from saliva. If connective tissue invades the cracks in enamel, cementum may be formed

38. Significance: It has been suggested that lamellae may be a site of weakness in a tooth and may form a road of entry for bacteria that initiate caries

39. ` ENAMEL TUFTS Arises from DEJ Thin ribbon like structure , resembling tufts of grass which is created by examining such area under low magnification in thick ground section Tufts consists of hypo calcified enamel rods and interprismatic substance.

40. They arise at DEJ and reach to enamel to about 1/5 to 1/3 rd of its thickness Their presence and their development are consequence of an adaptation to spatial condition of enamel Significance Enamel tuft prevents enamel fractures

41. DENTINOENAMEL JUNCTION Scalloped structure : the surface of the dentin at DEJ is pitted, in shallow depression of dentin, fir rounded projection of enamel. It appears scalloped due to the mixing of crystals of dentin and enamel with each other

42. Significance of scalloping: It ensures firm hold of the enamel cap to dentin

43. ENAMEL SPINDLES Occasionally odontoblastic process passes across DEJ into enamel, since many are thickened at their end , they have been termed enamel spindles. Directions of spindles and rods are divergent as rods are formed at right angle to ameloblast and spindles are parallel to ameloblasts .

44. Hypocalcified areas of enamel Rod sheath Incremental lines of ritzius Enamel lamellae Enamel tuft Enamel cracks Enamel spindles Neonatal lines SURFACE STRUCTURES : Prismless enamel Perikymata Rod ends Enamel caps Enamel brochs Enamel cracks Enamel cuticle Enamel lamellae

45. Age changes Most common change s are – attrition or wear of occlusal and proximal surfaces Loss of verticle dimension of crown and by flattening of proximal contour.

46. Perikymata and rod ends At eruption these are prominent With age they reduce Generalized loss of rod ends Flattening of perikymata . The rate at which structures are lost depends upon Location of surface of tooth location of tooth in mouth

47. WITH AGE Teeth darken Increase in organic content Deepening of dentin colour Decrease in permeability Fluoride ions increase with age Nitrogen increases with age Their resistant to decay may be increased Reduced permeability of older teeth to fluid Enamel may become harder with age

48. Clinical considerations Grooves and fissures on the occlusal surfaces of molars and premolars – weak spots for the action of caries as the maintenance is difficult – use of pit fissure sealants Lamellae, tufs and spindles may facilitate spread of caries . Striae of ritzius are the areas of hypomineralization thus fecilitate the lateral spread of caries . Fluoridation decreases caries .

49. Clinical application Acid etching Removes plaque Increases porosity through selected dissolution