Dental Anatomy: Enamel Properties

Enamel

• Enamel is a hard, mineralized tissue covering the crown of a tooth.
• It's the hardest tissue in the body and is totally acellular once formed.
• It's an ectodermal derivative.

Physical Properties

• High mineral content makes it the hardest tissue, resistant to mastication forces.
• Hardness is greatest at the surface, decreasing towards the DEJ and cervically.
• It's brittle with low tensile strength, relying on dentin for integrity.
• Shows some permeability to small molecules (e.g., dyes, isotopes).
• Color varies from light yellow to grayish-white, influenced by thickness.
• Deciduous teeth enamel is generally whiter due to less mineralization.
• Thickness ranges from 2.5 mm at cusp tips to less than 100 µm at the cervical area.
• Density/specific gravity is 2.8-3.0 g/ml, knoop hardness number of 343.

Chemical Properties

• Composed of 96% inorganic and 4% organic material (plus water) by weight.
• Inorganic content is mostly hydroxyapatite crystals (92-98%).
• Ions like strontium, magnesium, lead, and fluoride are also present.

Histological Structure

• Basic unit is the enamel rod (prism).
• Extends from the dentino-enamel junction to the surface.
• Rods are long, thin, and tortuous, often thicker than their diameter.
• Rods have a sheath, less mineralized.
• Average diameter is 4-5 µm, increasing towards the surface.
• Numbers vary, from 12.5 million in upper molars to a lesser number in incisors.
• Average rod length is 0.2 mm.
• Rods show various shapes in cross-section (round, oval, hexagonal, keyhole).
• Rods are arranged perpendicular to DEJ, but more horizontal in cervical region.
• Their arrangement is gnarled (twisted) at pits and fissures.

Structural Features of Enamel

• Incremental lines of Retzius: Represent enamel deposition, seen as brownish bands in longitudinal sections. More frequent on permanent teeth than deciduous.
• Neonatal line: A noticeable, accentuated stria that marks the distinction between prenatal and postnatal enamel formation.
• Hunter-Schreger bands: Alternating light and dark zones, primarily seen in longitudinal sections.
• Enamel gnarls: Irregular, twisted rods near cusps and incisal edges, resisting masticatory forces.
• Enamel lamellae: Thin, leaf-like structures extending from surface towards DEJ. Often caused by incomplete mineralization.

Enamel Tuft Structure

• Organic structures originating near the DEJ, reaching halfway into the enamel.
• Possibly areas of higher organic matrix content.

Enamel Spindles

• Odontoblastic processes that have extended across the DEJ.
• Club-shaped structures within enamel, randomly located.
• Potentially structural reinforcement.

Dentino-enamel Junction

• Scalloped interface between enamel and dentin.
• Enamel projections fit into dentin concavities.
• Crucial area for attachment and stress transmission.

Surface Structures

• Perikymata: Transverse grooves representing striae.
• Enamel cuticles (Nasmyth's membrane): Thin membrane, present on newly erupted teeth; quickly lost.

Amelogenesis

• Distinct stages of enamel formation.
• Early stage: Matrix secretion.
• Late stage: Enamel maturation.