Anatomy and Structure of Tooth Enamel

Layers of a Tooth

• Enamel: Outermost layer of the crown, 96% mineralized.
• Dentin: Lies beneath enamel, forms the bulk of the tooth.
• Pulp: Surrounded by dentin.

Structure of Enamel

• Enamel Rods:
  • Basic structural unit.
  • Keyhole pattern in cross-section, rod and inter-rod pattern in longitudinal section.
  • Run from the dentino-enamel junction to the enamel surface.
  • Paths are tortuous and undulating, not straight.
  • Functional adaptation to minimize risk of cleavage and cracks.
  • Variation in local orientation despite the general direction being the same.
• Microscopic Structures:
  • Complex orientation refracts light differently.
  • Hypocalcified areas add to varied microscopic structures.

Microscopic Structures in Enamel

Hunter-Schreger Bands

• Alternating light and dark bands.
• Originate from dentino-enamel junction, end before outer surface.
• Diazones: Dark zones.
• Parazones: Light zones.
• Visualized under oblique reflected light.

Gnarled Enamel

• Optical appearance in cusp or incisal ridge.
• Rods show more pronounced undulations in areas of maximum masticatory forces.

Incremental Lines

• Formed due to alternating periods of activity and rest in enamel formation.
• Cross Striations:
  • Dark and light bands perpendicular to enamel rods.
  • Represent diurnal rhythm.
• Incremental Lines of Retzius:
  • Brownish bands in longitudinal section.
  • Illustrate successive oppositional pattern of enamel deposition.
  • May reflect variations in structure and mineralization.
  • Presence of perikymata due to striae reaching enamel surface.
• Neonatal Line:
  • Accentuated striae of Retzius due to birth.
  • Divides enamel into prenatal and postnatal parts.
  • Present in all deciduous teeth, first molars in permanent dentition.

Enamel Lamellae

• Thin, leaf-like structures, extend from enamel surface towards dentino-enamel junction.
• Hypo-calcified, potential pathways for bacteria and caries.
• Types:
  • Type A: Form during tooth development, restricted to enamel.
  • Type B: Form after enamel formation, may cross into dentin.
  • Type C: Form post-eruption, may cross into dentin.

Enamel Tuft

• Ribbon-like, extend from dentino-enamel junction.
• Hypo-calcified rod sections, appear as a tuft of grass.

Enamel Spindle

• Straight vertical lines originating from dentino-enamel junction.
• Perpendicular due to odontoblastic processes.
• Formed during simultaneous amelogenesis and dentinogenesis, more common in cusp areas.