Enamel Properties & Composition

Enamel Properties and Composition

• Enamel is the hardest tissue in the human body
• It covers the external tooth surface and is the only visible part in the oral cavity

Physical Properties

• Enamel withstands shearing, impact, and compressive forces
• It has a high resistance to abrasion
• Enamel thickness varies: up to 2.5 mm on permanent teeth cusps and 1.3 mm on primary teeth cusps
• It thins to a feather edge at cervical margins
• Enamel cannot repair or replace itself because ameloblasts die after formation
• It is brittle with low tensile strength
• It needs the support of resilient dentin to withstand multidirectional forces
• Surface enamel is harder, denser, and less porous than subsurface enamel
• Hardness and density decrease from the cusp tip to the cervical margin and from the exterior to the interior
• Inorganic material increases from the DEJ to the surface
• Young enamel appears white due to crystal reflection
• It becomes more yellow with age due to increased translucency, exposing underlying yellowish dentin

Chemical Composition

• Enamel consists of 96% inorganic components, 2% organic components, and 2% water by weight

Inorganic Composition

• Calcium hydroxyapatite (Ca10(PO4)6(OH)2) is the main mineral component, found as crystallites
• Inorganic composition of all hard tissues is calcium hydroxyapatite
• Hard tissues differ in crystal size, arrangement, and organization
• Hydroxyapatite crystals are about 70 nm in width and 25 nm in thickness
• They extend nearly the full thickness of enamel
• Most crystallites have a hexagonal cross-section
• Crystal cores contain higher levels of magnesium and carbonate, making them more soluble
• Each crystal unit features a hydroxyl group surrounded by three calcium ions
• These are further surrounded by three phosphate ions and enclosed by six calcium ions in a hexagonal structure
• Crystal structure forms from stacked layers of ion planes
• Each hexagonal enamel crystallite consists of hydroxyapatite molecules in a lattice pattern

Substitutions in Hydroxyapatite Crystals

• Main substituents in human apatite are carbonate, magnesium, and fluoride
• Carbonate replaces phosphate or hydroxyl sites
• Magnesium replaces calcium ions
• Fluoride replaces hydroxyl ions, increasing stability and resistance to acidic dissolution
• Fluoride stabilizes the crystal and increases resistance to acid and caries
• Present fluoride integrates into enamel crystals, increasing resistance to acid attacks and caries
• Fluoride levels decrease from the outer surface toward the dentin
• Other substitutions include chloride, lead, zinc, sodium, strontium, and aluminum ions

Water Content

• Enamel contains 2% water by weight and 5-10% by volume
• Water presence is related to tissue porosity
• Water may be present between crystals or trapped in crystalline defects as a hydration layer
• Ions like fluoride travel through water component

Organic Matrix

• Mature enamel contains 1-2% organic matrix, varying with crystal regularity
• Organic matrix consists of proteins found exclusively in enamel
• It contains 90% amelogenin and 10% nonamelogenins
• Organic Matrix does not contribute to the enamel structure

Amelogenins

• Hydrophobic, low molecular weight, and aggregate into clumps
• Produced by ameloblasts
• They result in a gel matrix for molecule and ion spread, aiding large crystal formation

Non-Amelogenins

• Examples are tuftelin, ameloblastin, and enamelin
• They have a role in mineralization along with amelogenins
• They are derived from plasma albumin and contain distinct components secreted by ameloblasts
• High mineral (96%) content results in enamel loss in demineralization sections
• Therefore, enamel structure is studied mostly in ground sections
• Immature enamel can be studied in demineralization sections due to higher protein content (25-30%)
• It starts as an organic matrix that mineralizes in the 2nd stage

Enamel Prisms

• The basic structural unit is called a rod or prism
• Each prism consists of millions of hydroxyapatite crystals in a long, thin rod
• The diameter of the rod is 5-6 micrometers and a length of up to 2.5 mm
• The prism extends from the DEJ to the surface
• Prisms appear passage-like in longitudinal section
• Prisms separate via an inter-rod substance, where crystals deviate by 40-60 degrees
• The prism path is slightly undulating, reflecting ameloblast movement
• Enamel between prisms is inter-prismatic enamel
• The composition of enamel between prisms is similar but has different optical effects

Enamel Prism in Cross Section

• Enamel prisms can have different shapes, the keyhole pattern is most common
• Abrupt crystal orientation change creates an optical effect
• The prism boundary appears darker

Enamel Prisms Structure

• Each prism has head and tail regions
• The tail of one prism lies between the heads of two adjacent prisms
• Crystal orientation varies within the prism
• Parallel crystals run to the long axis in the head and middle
• Crystals diverge at an angle of 65-70° to the long axis in the tail
• Divergences cause keyhole pattern boundaries
• In a demineralized section, prisms appear as keyhole-shaped structures
• The demineralized section's prisms appear empty inside, interrod crystals deviate, leaving organic material and water

Additional Info Regarding Enamel Strength

• Prisms follow a non-straight path due to undulating ameloblasts
• The zigzagging mechanism enhances fracture resistance
• The sectioning angle determines prism appearance
• At 90°, a perfect keyhole pattern is observed
• As the angle decreases pattern gradually becomes circular patterns until it disappears

Prism Patterns

• The four prism patterns are: almost circular, stacked, keyhole (most common), and no pattern

Hunter-Schreger Bands

• Every 10-13 layers of prisms run in the same direction
• The layers above and below deviate, creating Hunter-Schreger bands
• Bands are approximately 50 µm wide and visible due to light reflection
• They are an optical phenomenon, only visible under a microscope
• All the prisms run in the same direction in the outer ¼ of enamel,
• Longitudinally cut prism bands appear light (parazones)
• Transversely cut bands appear dark (diazones)
• The angle between these cuts is 40°
• This pattern enhances enamel strength and resistance to fractures

Gnarled Enamel

• In cusp areas, space for enamel prisms is more limited
• Prisms appear twisted and interwoven in a complex arrangement, forming gnarled enamel

Aprismatic Enamel

• The outer surface layer of enamel (20-100 um primary teeth and 20-70 um secondary teeth) is aprismatic
• Crystallites are aligned at right angles to the surface and parallel to each other
• The surface layer is more highly mineralized
• This is due to the absence of prism boundaries

Histology of Enamel

• The following structures are examined beginning at the dentino-enamel junction (DEJ) and moving toward the surface:
  • Dentino-Enamel Junction (DEJ)
  • Incremental Lines
  • Surface Features of Enamel
  • Cemento-Enamel Junction (CEJ)

Dentino-Enamel Junction (DEJ)

• Has unique structural features that help retard crack propagation between enamel and dentin
• Exhibited scalloped pattern is subjected to high shearing forces
• The pattern enhances mechanical interlocking between enamel and dentin
• Lateral surfaces exhibit smooth pattern is less stress is applied
• Less mineralized than both enamel and dentin
• It contributes to distributing forces effectively

Structures at the DEJ

• Enamel spindles are 8 µm in diameter and extends up to 25 µm into the enamel
• They don't have the same color, structure, or direction of enamel, instead dentine has the same features
• Actually odontoblastic processes extend from dentine to enamel
• Most commonly seen beneath cusps as crowding of odontoblast processes
• They are odontoblastic processes among ameloblasts remnants of dead odontoblasts, dentinal collagen
• Enamel tufts (“Tuft” means grass-like projections) is located near the DEJ, specifically in the inner third of enamel
• Orientation follows the same direction as enamel prisms
• The location recurs at 100 µm intervals
• They are taller than enamel spindles
• The matrix protein composition are hypomineralized regions with high organic material
• Protein Content: tuftelin is a minor non-amelogenin protein, plays a role in enamel formation
• Enamel lamellae run the entire enamel thickness from the surface to the DEJ
• They are commonly caused by an incomplete maturation of groups of prisms
• Hypomineralized areas
• Distinguish them from cracks produced during ground secretion preparation, disappears due to demineralization

Incremental Lines

• Observed lines in tooth sections representing the periodic deposition of dentin, enamel, and cementum during tooth development
• Enamel forms in increments due to alternating periods of activity and inactivity
• Ameloblasts leave behind visible lines, marking resting phases

Cross Striations

• Indicate shorter periods of activity, reflecting daily enamel deposition
• They apear as fine lines crossing the enamel prisms at right angles to their long axis
• Striations reflect the diurnal rhythm daily enamel growth increments
• They are spaced 2.5–6 µm apart and are closer to each other near the DEJ
• Possible causes: variations in organic matrix, crystal orientation, composition, or prism width during enamel formation
• Require high magnification, electron microscope

Enamel Striae (Striae of Retzius)

• Represent longer periods of activity, corresponding to weekly enamel deposition
• Represent incremental lines and are known as the Striae of Retzius
• Enamel striae run obliquely across the prisms
• The mechanism is cells rest at these lines, then resume secretion for a week before resting, the "weekend" of the cells
• Early Formation & Immature Enamel shows ameloblasts are still visible, indicating early enamel formation
• The enamel has a high organic matrix content, making it immature enamel
• Striae run circumferentially, resembling the growth rings of a tree
• There are 7–10 cross striations between two adjacent enamel striae
• In longitudinal Sections, the striae appear oblique to the enamel rods
• At cusp tips, striae do not reach the surface; instead, they form circular patterns, moving up and back
• Ground section striae are become visible due to differential light-scattering effects at their boundaries
• Caused by: slight changes in prism direction or thickness, differences in crystallite composition and orientation or variations in organic content
• Demineralized sections: site of striae has a higher carbonate content, greater solubility of crystals and porosity
• They are hypomineralized areas

Neonatal Line

• A prominent incremental line that marks physiological stress at birth
• One of the incremental lines, shows the difference between enamel formation before and after birth
• A particularly marked stria is formed at birth and reflects metabolic changes at birth
• Prisms appear to change both direction and thickness
• Enamel striae are present throughout the entire enamel and actually reach the enamel surface
• These striae are called perikymata
• Perikymata are lost after eruption by attrition and abrasion

Additional Info

• Surface enamel physically and chemically differs from sub-surface enamel
• Enamel is harder, less porous, less soluble, and more radio-opaque
• Rich in fluoride and less carbonate, aprismatic, highly mineralized, and resistant to carries
• Small pits are the end of ameloblast which range from 1-1.5 micrometers in depth
• Small elevations 10-15 micrometers across resulting from enamel deposition on top of debris, lateral surfaces
• Focal holes are depressions caused by the loss of enamel caps with the underlying material and seen through abrasion and attrition, on lateral surfaces
• Enamel brochs are elevations of the surface (30-50 micrometers in diameter), crystals radiate and are more common in premolars

Cemento-Enamel Junction (CEJ)

• Relates cementum and enamel to each other
• The cementum overlaps the enamel (60% of cases)
• The cementum and enamel meet at butt joint (30%)
• The cementum and enamel fail to meet is exposed (10%)
• Enamel overlaps the cementum (very rare 1.6%)
• All these patterns may be present in a single tooth

Age Changes

• Enamel wears slowly with age
• Can also depend on diet and habits
• Reduced translucency results as secondary dentine forms and enamel thins
• Accumulation of surface coatings and stains
• There’s a decrease in carries due to enhanced mineralisation

Enamel Pearls

• Small droplets of enamel on the root near the furcation
• During tooth formation, some cells form enamel or "differentiate ameloblasts"
• Then slip down into the root and continue to form enamel
• Hertwig's root sheath