ENAMEL

Hard protective substance that covers the tooth surface and is considered the hardest biologic tissue in the human body.

Provides the shape and contour of the crowns of teeth and covers that part of the tooth exposed to the oral environment and is also a poor conductor of both heat electricity.

Thickest at the crest of cusps or incisal edges (up to 2.5 mm) and becomes thinner within the fissures and pit of multicusp teeth and over the facial, lingual and interproximal surfaces, where it tapers to a minimal thickness (less than 100 micrometers) at the cervical margin.Non-vital tissue and is incapable of regeneration.

PHYSICAL CHARACTERISTICSExtremely hard because of its high mineral content, a property that enables it to withstand mechanical forces applied during its functioning.It is a tightly packed mass of hydroxyapatite crystals.Harder or more mineralized at the surface layer than closer to the dentin.Hardness of enamel also makes enamel brittle; therefore an underlying layer of more resilient dentin is necessary to maintain its integrity.

Without dentin, enamel is subject to fracture which occurs specially if the supportive layer of dentin is destroyed by either caries or improper cavity preparation.Tooth color is determined by the enamel thickness and is white to grayish-white but appears slightly yellow because it is translucent . Underlying dentin is yellowish, giving the yellow tint to enamel.Enamel is permeable to a limited extent stains can enter it from both the outside and from the pulp through the dentino-enamel junction.

Permeability studies in 8 to 10 year old children showed that after eruption, enamel becomes noticeably less permeable, loses its porosities and increases in density.The fact that water and alcohol can flow through enamel relatively free (the flow rate of water measures about 4 mm³/cm² of enamel surface in a 24 hour period) indicates that enamel can function as a molecular sieve and ion exchanger.Specific gravity is 2.8The texture of clinically sound enamel is smooth and glossy.

COMPOSITION of EnamelInorganic material (96%)HYDROXYAPATITE CRYSTALS (90%)

chief inorganic contentVarying amount of carbonatesTrace elements (vanadium, manganese, selenium, molybdenum & strontium)

Organic substances (4%) – soluble & insoluble proteins

Amino acid – enamel proteins, (immature enamel) predominantly of tyrosine-rich AMELOGENNINS-glutamic acid, proline & histidineENAMELINS (mature enamel) aspartic acid, serine & glycine

Water

STRUCTURES OF ENAMELENAMEL RODS

Basic structural unit of enamelIt as an average width of 5µm.First described as hexagonal and prism-like and the term enamel prism was frequently used; but since the basic unit does not have a regular geometry and does not in any way resemble a prism---

Enamel rod is the more appropriate termOriginate at the dentino-enamel junction and extend through the thickness of the enamel to the surfaceIn sections cut along the longitudinal axis of enamel rods, appear segmented because of the dark lines crossing the rods known as TRANSVERSE STRIATION

In cross-section it would appear oval or round or fish-scale like

The basic unit of enamel is more appropriately described as a cylindrical rod that has a specific spatial relation to the interred region directly cervical to it and therefore, the rod has a pattern of “horse-shoe” or “paddle-shaped” or “racquet-shaped” with a head and tail.

The rods run in a direction generally perpendicular to the surface or the dentin (or at right angle to the dentino-enamel junction), with a slight inclination toward the cusp as they pass outward.Near the cusp tip, run more verticallyUnder pits and fissures, the enamel rods are arranged in tent-like manner, that is, rods converge toward each other to the bottom of the pit or fissure.

On the proximal or sides, the rods are horizontal. As they approach the cusp or incisal, the rods are in vertical directionIn cervical enamel, the rods run mainly horizontal or obliquely inclined (apically for permanent teeth and coronally for deciduous)The arrangement of rod rows has a clinical importance because enamel fractures occur between adjacent rows

ENAMEL ROD SHEATHIts core center and is the rod’s surfaceThe rod sheaths contain more enamel protein or organic matrix and recystallized hydroxyapatite crystals which are more acid-resistant than other regions do, because crystal meeting at different angles cannot be packed together tightly.The consistent arrangement of rod sheaths, with their greater protein content accounts for the fish-scale appearance of enamel matrix seen in sections of demineralized developing enamel or in etched ground sections.

The boundary between rod and interrod enamel is marked by a narrow space filled with organic materials known as rodsheath

GNARLED ENAMEL (GNARLED ENAMEL ROD)

which offers the greatest resistance to the cusp and incisal areas where most of the forces are applied.These rods that form enamel are woven during formation into a mass that resists an average masticatory impact of 20 to 30 pounds per tooth.Enamel is thickest over the areas of greatest impact.

INTERROD ENAMELIs the substance that cements the rods togetherThis is an area surrounding each rod in which crystals are oriented in a different direction from those making up the rod

Enamel rod and Interrod enamel

STRUCTURAL CHARACTERISTICS OF ENAMEL

INCREMENTAL LINES (STRIAE) OF RETZIUS (Anders Retzius 1756-1860, anatomist in Stockholm)Also called growth lines, are found in a repeating pattern throughout the entire enamelThese are lines, which appears brownish in transilluminated enamel ground section running from the dentino-enamel junction, in an oblique direction, toward the occlusal surface

Near the incisal edges or cusp tips, they never reach the outer enamel surface, but form horse-shoe shaped structures over the dentinal coreNear the axial surfaces and neck of tooth, they reach the outer tooth surface in a quasirhythmical orderThe distance between lines vary from 4 to 150 micrometers, becoming progressively less in an occlusal to cervical directionIn a cross section, the lines of Retzius appear as concentric rings, an arrangement similar to the annual rings of a tree

Striae of Retzius

Prominent in most human permanent teeth, less prominent in post-natal deciduous enamel, and rare in prenatal enamel.In longitudinal section, they are seen as a series of dark bands reflecting successive enamel-forming fronts.Retzius lines can be recognized on microradiographs as zones that are mostly hypomineralized.It seems that they are formed as a result of a temporary constriction of Tomes processes associated with a corresponding increase in the secretory face forming interrod enamel.

As a result, enamel structure is altered along the lines.It is also believed to represent the physical record of a period of rest between two active secretory phases during amelogenesis.If one observes the enamel surface of sound, freshly erupted tooth, a pattern of line is recognizable to the unaided eye, which can be studied in detail with scanning electron microscope.These pattern comprises wavy bands that are separated by fine furrows or shallow horizontal grooves.

Because of their shingle-like overlapping arrangement, they are called PERIKYMATA or IMBRICATION LINES OF PICKERILLThese grooves represent the lines of Retzius as they meet the surface enamel.The perikymata are closely spaced near the cervical margin, become further apart in the mid-coronal region, and are absent at the cusp tips and incisal edges.Perikymata are shallow furrows and run in circumferentially horizontal lines across the face of the crown.

Part of the enamel of deciduous teeth is formed both before and after birth.This abrupt change in environment and nutrition at birth is recorded in the enamel as a more pronounced incremental or brownish line, termed NEONATAL LINE.The enamel matrix internal to this line represents the enamel formed before birth and the enamel matrix external to it is formed after birth.

Enamel formed before birth is whiter and contains fewer defects than those after birth.The neonatal line is an accentuated line that apparently reflects the marked physiologic changes occurring at birth, therefore most primary teeth and permanent central incisors have these.

Neonatal lines

TRANSVERSE STRIATIONS

These are periodic bands or cross striations occurring at a rate of approximately 4 micrometer per dayScanning electron microscope reveals alternating constrictions and decreasing expansions of the rods in some regions of enamel which may account for this banded appearance in ground section

BANDS OF HUNTER-SCHREGER (J. Hunter 1729-1793, surgeon in London; D Schreger, 1766-1825, anatomist in Erlangen)

Are an optical phenomenon produced solely by changes in rod directionThey are seen most clearly in longitudinal ground section viewed by reflected light and are found in the inner four-fifths of the enamelThey appear as dark and light alternating zones and reflect the wave-shaped course of enamel rods in three-dimensional space and are thought to be functionally adapted structures for the prevention of enamel cracking during chewing

Hunter and Schreger Bands

ENAMEL TUFTSOriginate at the dentino-enamel junction distributed somewhat irregularly around the junction to about 1/5 to 1/3 of the thickness of the entire enamel from the junction to the surface of the tooth.They resemble tufts of grass growing out of a root, form garlands running in an occlusocervical direction.They appear to be branched and contain greater concentrations of enamel protein than the rest of enamel.They are composed of hypomineralized enamel rods, interrod enamel and organic material—enamelin.

Enamel tufts

ENAMEL LAMELLAEAre visible cracks on the surface of the enamelLamellae extend from varying depths from the enamel surface toward the dentino-enamel junctionSome lamellae develop during enamel formation and some are created during tooth functionCracks may also appear in enamel because enamel is a highly mineralized tissue, stress from breathing cold air or drinking cold beverages may cause small cracks to occur in enamel, especially where enamel is weakened by underlying cariesLamellae are not tubular defects but appear leaf-like, extending around the crownLamellae are important as a pathway through enamel and function as a possible avenue for dental caries

Enamel lamellae

Enamel lamellae

ENAMEL SPINDLEExtensions from dentinal tubules that penetrate the dentino-enamel junction into the enamel.Seen as black or dark irregular or spiral-shaped or club-shaped structures and are normally filled with dentinal fluid.Their dark appearance is due to the presence of air and debris resulting from the preparation of the section.They occur most often along the dentino-enamel junction under the cusp tip or incisal edge where the junction is sharply bent.In most cases, they are not aligned along the same path as the enamel rods.This is one of the factors that cause the hypersensitivity of the dentino-enamel junction.

Enamel spindle

Enamel spindle

Dentino-Enamel Junction (DEJ)

It is the junction between the enamel and dentinIt is established as these two hard tissues begin to form and is seen as a scalloped profile in cross-section

Scalloped nature of DEJ as seen with SEM

AGE CHANGES IN ENAMELWith age, enamel becomes progressively worn away in regions of masticatory attrition.Wear facets are increasingly pronounced in older people, and in some cases substantial portions of the crown become eroded.Teeth darken with age.It can be due to the addition of organic material to enamel from the environment, it also may be due to a deepening of dentin color seen through the progressively thinning layer of translucent enamel.Enamel becomes less permeable with age.With age the pores in the enamel diminish as the crystals acquire more ions and increase in size.Because the bulk of the water lies in the pores, it follows that the water content of enamel also decreases with age.

CLINICAL IMPLICATIONFluroridation

The amount of fluoride must be carefully controlled, however, because of the sensitivity of secretory ameloblasts to the fluoride ion and the possibility of producing unsightly mottling.The presence of fluoride enhances chemical reactions that lead to the precipitation of calcium phosphate.An equilibrium exists in the oral cavity between calcium and phosphate ions in the solution phase (saliva) and in the solid phase (enamel), and fluoride shifts this equilibrium to favor the solid phase.

Acid etchingThere are several types of concentrations of acid are used to alter the enamel surface; most etch the surface to a depth of only about 10µm.E.g. 30% to 40% phosphate acid applied to enamel for 60 seconds provide and adequate etch for retention of sealants.There are three etching patterns predominate:

Type IThe most common, characterized by preferential removal of the rod core.

Type IIIs the reverse; the rod periphery is preferentially removed and the core remains intact.

Type IIIOccurring less frequently; which is irregular and indiscriminate

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