Enamel

• Physical properties of enamel:

  • Thickness: Enamel varies in thickness from one part of the crown to another
  • Thickest on the cusps (2-2.5 mm) and thinnest near the cervical line
  • Thicker on the lingual surface of maxillary molars and buccal surface of mandibular molars due to adaptation according to function
  • Hardness: Enamel is the hardest tissue of the body due to high content of inorganic salts
  • Enamel without dentin support will easily chip off
  • Hardness varies, with surface enamel being harder than enamel close to DEJ
• • Permeability: Enamel is semipermeable, allowing passage of certain molecules
  • Translucency: Depends on calcification and homogeneity, reversible on rehydration
  • Color: Ranges from yellowish-white to grayish-white, thickness affects color
  • Optical properties: Enamel is birefringent, refractive index is 1.62
• Chemical properties of enamel:

  • Enamel consists of 96% inorganic material, 4% water, and traces of organic material
  • Organic material includes unique proteins found only in enamel
  • Amelogenins (low mol wt proteins) and Nonamelogenins (high mol wt) are the main organic proteins
  • Inorganic material: Major structural component of enamel, made up of hydroxyapatite crystals
  • Structure of hydroxyapatite crystals: Hexagonal in cross-section, arranged around a central core of hydroxyl ion
  • Enamel rods are formed by arrangement of hydroxyapatite crystals
• Histology of enamel:

  • Enamel Rods: Fundamental organizational unit of enamel, cylindrical in longitudinal section
  • Rod sheath separates rod and interrod enamel, filled with organic material
  • Enamel rods run tortuously from DEJ towards the surface of the tooth
  • Enamel rods are longer than the thickness of enamel, straight at cusp tips and incisal ridges
  • Hunter Schreger bands: Optical phenomenon produced by difference in direction between adjacent groups of rods
  • Incremental lines of Retzius: Series of lines extending from DEJ towards the tooth surface, appear as concentric rings
• • Neonatal line: Enlarged striae of Retzius reflecting physiological changes at birth
• Neonatal line:

  • Enlarged striae of Retzius reflecting physiological changes at birth
  • Seen in all primary teeth and permanent first molars
• Accentuated incremental lines:

  • Produced by systemic disturbances like fever
• Cross-striations:

  • Enamel rods exhibit periodic bands at 4µm intervals
  • Two proposed reasons for appearance:
  • Diurnal rhythmicity in rod formation
  • Sectioning artefact with cross striations representing obliquely sectioned rods
  • More accepted reason is diurnal rhythmicity
• Enamel tuft:

  • Structures resembling tufts of grass projecting from DEJ into enamel
  • Higher protein content, called tuft protein
  • Formed due to abrupt changes in direction of enamel rods
• Enamel lamellae:

  • Thin leaf-like structures in enamel extending from surface towards DEJ
  • Formed during enamel development, filled with organic material
  • Classified into 3 types: A, B, C
  • Type C can be differentiated from cracks post-development by careful demineralization
• Dentinoenamel junction:

  • Junction between dentin and enamel, scalloped
  • Scanning electron microscope reveals ridges for increased adherence
• Enamel spindle:

  • Spindle shaped structures at DEJ formed by trapped odontoblast processes
  • Demonstrated in ground section, surrounded by interrod enamel
• Gnarled enamel:

  • Seen at incisal edge or cusp tips due to intertwining of enamel rods
  • Formed by irregular movement of ameloblasts during enamel deposition
• Surface structures of enamel:

  • Surface enamel differs from subsurface enamel in radiopacity, hardness, solubility, and morphology
  • Structures include dental cuticle, salivary pellicle, perikymata
• Age changes in enamel:

  • Attrition leads to loss of hard tissue
  • Enamel becomes harder with fluoride adsorption
  • Permeability reduces, becomes more yellowish, develops cracks with age
• Enamel Lamellae:

  • Thin leaf-like structures in enamel that extend from varying depths from the surface of enamel towards DEJ
  • Visible in ground sections of tooth, better in cross-section
  • Seen in carefully demineralized/ decalcified sections of human enamel due to higher organic content
  • Defects formed during enamel development, filled with organic material
• Classification of enamel lamellae:

  • Type A: Group of poorly calcified enamel rods
  • Type B: Filled with odontogenic degenerated cells
  • Type C: Cracks filled with organic matter
  • Develop in lines of stress where calcification may be incomplete, becoming a site of weakness in enamel for bacterial entry
• Formation of enamel lamellae:

  • Develop in lines of stress where calcification may be incomplete
• Clinical significance of enamel lamellae:

  • Route for bacterial entry due to weakness in enamel
• Differentiation between type C enamel lamellae and crack:

  • Use careful demineralization, crack disappears while lamella is retained due to higher organic content
• Dentino-enamel junction (DEJ):

  • Junction between dentin and enamel, established during formation of dentin and enamel
  • Scalloped appearance, revealed as a series of ridges under scanning electron microscope
  • Ridging more pronounced in coronal region to prevent shearing of enamel during function
• Enamel spindle:

  • Spindle-shaped structures seen in enamel at DEJ
  • Formed by developing odontoblast processes extending into ameloblasts layer before enamel formation
  • Demonstrated in ground sections, surrounded by interrod enamel
  • More spindles in incisal and cuspal regions, appear dark in transmitted light and lighter in reflected light
• Formation of enamel spindles:

  • Developing odontoblast processes extend into ameloblasts layer before enamel formation
• Gnarled enamel:

  • Structure seen at incisal edge or cusp tips, formed by intertwining of enamel rods
  • Arrangement withstands masticatory forces, formed due to irregular movement of ameloblasts during enamel deposition
• Formation of gnarled enamel:

  • Irregular movement of ameloblasts during enamel deposition due to overcrowding
• Amelogenesis is the process of formation of enamel by ameloblasts
• Ameloblasts secrete matrix proteins and create an environment favorable for mineral deposition
• Ameloblasts help in the transport of minerals to the site of mineralization
• Enamel formation begins in the bell stage with the differentiation of inner enamel epithelial cells to ameloblasts
• Formation & deposition of partially mineralized enamel matrix is the first stage of amelogenesis
• Enamel is only 30% mineralized at the time of deposition
• Complete mineralization and maturation of enamel is the second stage of amelogenesis
• Reciprocal induction is when ameloblasts signal odontoblasts to secrete dentin
• Tome's process is a cytoplasmic extension of ameloblasts that leaves a picket-fence or saw-toothed appearance between enamel and ameloblasts
• Tome's process has two portions: distal gives rise to enamel rod, proximal gives rise to interrod enamel
• During maturation of enamel, crystals increase in size, matrix is gradually removed, and organic components are broken down
• Enamel proteins deposited on the crystal limit their growth, then are cleaved by proteolytic enzymes allowing crystals to grow freely
• Modulation is the process of removal of organic matrix and growth of HA crystals in small increments of enamel, mediated by ameloblasts
• Ameloblasts undergo modulation, a process of cyclic creation, loss, and re-creation of a ruffled apical surface
• When the apical surface of cell turns ruffled, the proximal junction becomes leaky