Enamel: Properties and Characteristics

Physical Properties of Enamel

• Enamel is the hardest tissue in the human body
• Enamel covers the exterior surface of the tooth and is only part of the tooth that is visible in the oral cavity
• Enamel can withstand:
  • Shearing forces (two forces working in opposing directions)
  • Impact forces (pressure during occlusion)
  • Compressive forces
• Enamel has a high resistance to abrasion
• Enamel thickness varies:
  • Up to 2.5 mm over the cusps in permanent teeth
  • 1.3 mm in primary teeth
• Enamel gradually thins to a feather edge at the cervical margins
• Enamel is thinner in deciduous teeth compared to permanent teeth
• Enamel cannot be repaired or replaced
• Enamel is secreted by ameloblasts, which move to the surface and die after formation
• Enamel has low tensile strength - it is brittle and does not handle tension well
• Enamel can't withstand forces applied in different directions
• Support of resilient dentin (softer than enamel but can withstand multidirectional forces) is required

Subsurface Enamel

• Surface enamel is harder, denser, and less porous than sub-surface enamel
• Surface enamel is more mineralized at the cusp tip and on the outer surface
• Hardness and density gradually decrease from the cusp tip to the cervical margin and from the exterior to the interior
• The percentage of inorganic material increases from the DEJ to the surface
• Crystals in young enamel reflect almost all the light, causing it to appear white
• Enamel takes on a more yellow appearance with age as translucency increases
• Translucency increases as enamel wears down, exposing the yellowish dentin underneath

Chemical Composition of Enamel

• Enamel is composed of:
  • 96% inorganic components
  • 2% organic components
  • 2% water by weight

Inorganic Composition of Enamel

• Calcium hydroxyapatite (Ca10(PO4)6(OH)2) is the main mineral component of enamel, mainly found in the form of crystallites
• Calcium hydroxyapatite is the inorganic composition of all hard tissues (enamel, dentin, cementum, and bone)
• Hard tissues differ in the size, arrangement, and organization of their crystals, which affects their hardness and properties

Organic Composition of Enamel

• Enamel contains free amino acids, small molecules, peptides, and large protein complexes
  • Amelogenins (mainly)
  • Non-amelogenins

Hydroxyapatite Crystals Structure

• Hydroxyapatite crystals are approximately 70 nm in width, 25 nm in thickness
• Crystals extend nearly the full thickness of enamel
• Most crystallites have a hexagonal cross-section
• Crystal cores contain higher levels of magnesium and carbonate compared to their peripheries
• Higher levels of magnesium and carbonate in crystal cores make them more soluble
• Each crystal unit has a hydroxyl group surrounded by three calcium ions, which are further surrounded by three phosphate ions
• The phosphate ions are enclosed by six calcium ions in a hexagonal structure
• Repeated layers of these ion planes, stacked side by side, form the crystal structure
• Each hexagonal enamel crystallite consists of multiple hydroxyapatite molecules arranged in a lattice pattern

Substitutions in Human Apatite

• The main substituents include:
  • Carbonate, replacing phosphate or hydroxyl sites
  • Magnesium, replacing calcium ions
  • Fluoride, replacing hydroxyl ions

Fluoride Incorporation

• Fluoride is a smaller molecule that fits in place of the hydroxyl ion at the center of the crystal structure
• Fluoride stabilizes the crystal, increasing makes it more resistant to acidic dissolution and to caries
• Fluoride present in water or applied to teeth integrates into enamel crystals
• Fluoride increases enamel's resistance to acid attacks and caries formation
• Fluoride levels (unlike magnesium and carbonate) decrease from the outer enamel surface toward the dentin
• Fluoride is acquired during enamel maturation
• Other possible substitutions in the apatite lattice include chloride, lead, zinc, sodium, strontium, and aluminum ions

Water

• Water makes up 2% by weight, 5-10% by volume of enamel
• Water presence is related to the porosity of the tissue
• Water might be present between the crystals surrounding the organic component or trapped within crystalline defects
• Trapped water forms a hydration layer
• Ions such as F travel through the water component

Organic Matrix Components

• Mature enamel is 1-2% organic material, varying from 0.05% to 3% depending on the regularity of the crystals
• Matrix consists of proteins exclusively located in enamel, including:
  • 90% amelogenin
  • 10% nonamelogenins

Amelogenins

• Hydrophobic
• Low molecular weight
• Tend to aggregate into clumps
• Produced by ameloblasts
• Spread throughout the developing enamel, resulting in a gel matrix through which molecules and ions spread readily
• Helps in the formation of large crystals

Non-Amelogenins

• Components include tuftelin, ameloblastin and enamelin
• Low molecular weight
• May be derived from plasma albumin
• Contain distinct components secreted by ameloblasts
• Role in mineralisation alongside amelogenins

Histology of Enamel

• Enamel is mainly studied in ground sections due to its high mineral content (96%)
• Enamel is totally lost in demineralization sections
• Immature enamel, with high protein content (25-30%), can be studied in demineralization sections
• Organic matrix of immature enamel is mineralized in the second stage, allowing it to be seen in decalcified section

Enamel Prisms

• The basic structural unit of enamel is called the rod or prism
• A prism contains several million hydroxyapatite crystals packed into a long, thin the with a diameter of 5-6 micrometers and a length of up to 2.5 mm
• Prisms extend from the DEJ to the surface
• In the longitudinal section, prisms appear as passage-like structures
• Prisms are separated by an inter-rod substance where crystals change direction and deviate by 40-60 degrees, creating space for organic material
• Path of prisms is slightly undulating, reflecting the movement of ameloblasts during secretion
• Enamel between prisms is known as inter-prismatic enamel
• Inter-prismatic enamel has a different optical effect due to the deviation in crystal arrangement, but has similar composition as inside the prisms

Enamel Prisms in Cross Section

• Enamel prisms can have different shapes (patterns)
• Keyhole pattern (Pattern III) is the most common
• Abrupt change in crystal orientation at the prism boundary creates an optical effect
• Optical effect makes the boundary appear slightly darker than the inside of the prism

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
• In the head and middle, crystals run parallel to the long axis
• In the tail, crystals gradually diverge at an angle of 65-70° to the long axis
• Divergence happens gradually within each prism, the tail of one prism shows a sudden divergence from the head of an adjacent prism
• These divergences create the keyhole pattern boundaries

Demineralized Cross Section

• Prisms in cross-section appear as keyhole-shaped structures in demineralized sections
• Prisms appear empty inside
• Interrod crystals deviate, leaving some space for organic material and water, which remain visible after demineralization

Prism Orientation and its Role in Enamel Strength

• Prisms don't move in a straight line
• Zigzagging mechanism enhances enamel's resistance to fractures
• Non-linear path is due to ameloblasts following an undulating course during enamel secretion

Sectioning Angle on Prism Shape

• The angle at which the section is cut determines how the prisms appear
• At 90°, perfect keyhole pattern
• As an angle decreases from 90° to 0°, a shape gradually becomes more circular until the pattern disappears
• There are four prism patterns:
  • Almost circular
  • Stacked
  • Keyhole (most common)
  • No pattern

Hunter-Schreger Bands

• Enamel prisms do not follow a straight path
• Every 10-13 layers of prisms run in the same direction, while the layers above and below deviate, creating a banding pattern known as Hunter-Schreger bands
• Bands are approximately 50 μm wide and become visible due to light reflection in different directions
• Hunter-Schreger bands are an optical phenomenon, not an actual structural feature, and are only visible under a microscope
• All prisms run in the same direction in the outer ¼ of enamel, so no banding is present
• Longitudinally cut prism bands appear light (parazones), while transversely cut bands appear dark (diazones)
• The angle between these cuts is 40°
• This banding pattern enhances enamel strength and resistance to fractures

Gnarled Enamel

• The space available for enamel prisms is more limited in cusp areas compared to other regions
• Prisms appear twisted and interwoven in a complex arrangement, forming what is known as gnarled enamel

Aprismatic Enamel

• The outer surface layer of enamel (20-100 μm primary teeth and 20-70 μm secondary teeth) is aprismatic
• It is where crystallites are aligned at right angles to the surface and parallel to each other
• It is more highly mineralised in the surface layer than the rest of the enamel
• Attributed to the absence of prism boundaries where organic material is located

Histology of Enamel

• The study will begin at the dentino-enamel junction (DEJ) and progress toward the surface, examining sections to explain its features
• The study will examine:
  • Dentino-enamel junction (DEJ)
  • Incremental lines
  • Surface features of enamel
  • Cemento-enamel junction (CEJ)

Dentino-Enamel Junction (DEJ)

• DEJ has unique structural features that help retard crack propagation between enamel and dentin
• DEJ exhibits a scalloped pattern in areas exposed to high shearing forces, such as beneath cusps and incisal edges
• Scalloped pattern enhances the mechanical interlocking between enamel and dentin
• In contrast, the DEJ is smooth in lateral surfaces, where less stress is applied
• It is less mineralized than both enamel and dentin, which contributes to its ability to absorb and distribute forces effectively

Structures at the DEJ

• Scientists have identified three distinct structures at the DEJ, each with specific characteristics and functions:
  • Enamel spindles
  • Enamel tufts
  • Enamel lamellae

Enamel Spindles

• Narrow and round tubules (8 μm in diameter) that extend a very short distance (up to 25 μm) into the enamel
• They don't have the same color, structure, or direction of enamel but actually have the same color as dentine
• Odontoblastic processes that are extending from dentine to enamel
• Most commonly seen beneath cusps due to crowding of odontoblast processes
• Processes are odontoblastic processes among ameloblasts remnants of dead odontoblasts, dentinal collagen

Enamel Tufts

• Found near the DEJ, specifically in the inner third of the enamel
• Follow the same direction as enamel prisms and do not resemble dentin
• Enamel tufts are part of the enamel structure
• Tufts recur at 100 μm intervals and are taller than enamel spindles
• Hypomineralized regions that contain residual matrix proteins, primarily at prism boundaries
• Lead to a higher percentage of organic material
• Contain tuftelin, a minor non-amelogenin protein, which plays a role in enamel formation

Enamel Lamellae

• Structural faults run the entire enamel thickness from the surface to the DEJ
• They're caused by an incomplete maturation of groups of prisms
• Hypomineralized areas
• Cracks produced during ground Section preparation disappear with demineralization

Incremental Lines

• Incremental lines are lines observed 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 during its development
• Each time the ameloblasts pause and resume activity, they leave behind visible lines, marking their resting phases
• Types of Incremental Lines:
  • Cross striations
  • Enamel striae (striae of Retzius)

Cross Striations

• Indicate shorter periods of activity, reflecting daily enamel deposition
• Appear as fine lines crossing the enamel prisms at right angles to their long axis
• Reflect the diurnal rhythm, indicating daily enamel growth increments
• Spaced 2.5–6 μm apart and are closer to each other near the DEJ
• May result from variations in the organic matrix, crystal orientation, composition, or prism width during enamel formation
• Cannot be seen under a light microscope
• Electron microscope (high magnification) is required for their visualization

Enamel Striae (Striae of Retzius)

• Represent longer periods of activity, corresponding to weekly enamel deposition
• Run obliquely across the prisms
• Represent incremental lines
• Cells rest at these lines, then resume secretion for a week beforeresting again
• The cycle can be thought of as the "weekend" of the cells
• Ameloblasts are still visible in early formation and immature enamel, indicating early enamel formation
• Enamel has a high organic matrix content, making it immature enamel
  • Cross Sections: the striae run circumferentially, resembling the growth rings of a tree
  • There are 7-10 cross striations between two adjacent enamel striae
  • Longitudinal Sections: the striae appear oblique to the enamel rodsSurface
• At cusp tips, the striae don't reach the surface, and instead, they form circular patterns, moving up and back
• Striae become visible due to differential light-scattering effects at their boundaries
• May be caused by slight changes in prism direction or thickness, differences in crystallite composition and orientation or variations in organic content
• The site of striae has a higher carbonate content in demineralized sections, which causes greater solubility of the crystals and greater porosity
• They are hypomenerlised areas

Neonatal Line

• A prominent incremental line that marks the physiological stress at birth
• Shows the difference between enamel formation before and after birth
• A particularly marked stria is formed at birth and reflects the metabolic changes at birth
• Prisms appear to change both direction and thickness at the time of this event
• These enamel striae are present throughout the entire enamel and actually reach the enamel surface, known as perikymata
• Lines are called perikymata grooves, with perikymata ridges in between
• Grooves run circumferentially around the crown, and they are removed after eruption by attrition and abrasion
• High areas represent perikymata ridges, while low areas correspond to perikymata grooves

Surface Enamel

• Surface enamel differs from sub-surface enamel physically and chemically
• Surface enamel is harder, less porous and less soluble more radio-opaque
• Richer in trace elements especially Fluoride, less carbonate
• Aprismatic, therefore highly mineralized, more resistant to carries

Common Phenomena on the Surface

• Small pits, seen within the perikymata, mark the end of the ameloblast and measure 1-1.5 micrometers in depth
• Enamel caps, small elevations of 10-15 micrometers across, result from enamel deposition on top of debris late during tooth development, and they are frequent on lateral surfaces
• Focal holes, depressions on the surface caused by the loss of enamel caps with the underlying material through abrasion and attrition, and they are frequent on the lateral surfaces
• Enamel brochs, elevations on the surface measuring 30-50 micrometers in diameter, are larger than caps, radiating groups of crystals, and are more common in premolars

Cemento Enamel Junction (CEJ)

• CEJ describes how cementum and enamel correlate to each other
• There are four arrangements between cementum and enamel:
  • Pattern 1: the cementum overlaps the enamel (60% of the cases)
  • Pattern 2: the cementum and enamel meet at butt joint (30%)
  • Pattern 3: the cementum and enamel fail to meet and the dentine, in between them is exposed (10%)
  • Pattern 4: 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 depending on diet and habits
• It darkens in colour
• The composition of surface enamel changes as a result of exchanges with the oral fluids
• Reduced translucency of the tooth as secondary dentine forms and enamel thins
• Accumulation of surface coatings and stains
• There is a decrease in caries due to enhanced mineralisation

Enamel Pearls

• Small droplets of enamel form on the root near the furcation during tooth formation
• Differentiation of ameloblasts slip down into the root and continue to form enamel there
• This phenomenon is called Hertwig's root sheath