1. INTRODUCTION

2. DEVELOPMENT OF TOOTH

3. DEVELOPMENT OF ENAMEL

4. HISTOLOGY

a) Physical Properties Of Enamel

b) Chemical Properties Of Enamel

5. STRUCTURE OF ENAMEL

6. EXTERNAL SURFACE OF ENAMEL

7. AGE CHANGES IN ENAMEL

8. CLINICAL CONSIDERATIONS

a) Defects In Enamel

b) Enamel Structure And Dental Caries

c) Enamel Adhesion

d) Acid Etching Of Enamel

e) Configuration And Correlation Of Enamel Walls.

f) Designs Of Different Carious Lesions.

g) Recent Advancements.

9. CONCLUSION

10. REFERENCES.

Introduction:

Development of tooth:

The primitive oral cavity or stomodeum, is lined by stratified squamous

epithelium called the oral ectoderm. The oral ectoderm contacts the endoderm of the

foregut to form the buccopharyngeal membrane. At about the 27 day of gestation this

membrane ruptures and the primitive oral cavity establishes a connection with the

foregut.

The first indication of the formation is seen at about 6th week of intrauterine

life, and horse – shoe shaped proliferation of the oral epithelium of both maxilla and

mandible along the outline of future alveolar processes. This is known as primary

epithelial band.

At about 7th week both the upper and lower epithelial bands divide into a

buccal band known as vestibular band or lip furrow band and an inner band called

dental lamina.

By the 8th week of intrauterine life, the dental lamina of each jaw shows ten

areas of thickening. These epithelial thickening along with underlying mesenchyme

are called the tooth buds. The lingual extension of the dental lamina is named the

successional lamina. Total activity of the dental lamina extends over a period of at

least 5 years.

Stages of tooth development :

The tooth germs undergo through a series of more or less well – defined

morphologic and histologic stages.

They are as follows :

Morphologic stages Histogenic or physiologic

plases

1. Dental lamina Initiation

2. Bud stage Proliferation

3. Cap stage – early – advanced Histo – differentiation

4. Bell stage – early – advanced Morpho – differentiation

5. Formation of enamel and dentin matrix Apposition.

1. Bud stage : The epithelium of the dental lamina is separated from the

underlying ectomesenchyme by a basement membrane. Simultaneous with the

differentiation of each dental lamina, round or ovoid swellings arise from the

basement membrane at 10 different points, corresponding to the future positions

of the deciduous teeth.

2. Cap stage : The tooth germ continues to proliferate at different rates in

different parts of the tooth bud. The epithelial portion assumes a cap stage. There

is a shallow mesenchymal invagination on the deep surface of the bud. The tooth

bud, now called enamel organ or dental organ at this stage consists of distinct cell

layers.

a) The peripheral cells consists of one row cuboidal cells and is called

outer enamel epithelium.

b) Inner layer of cells are tall, columnar cells are called inner enamel

epithelium.

The outer enamel epithelium is separated from the dental sac, and the inner enamel

epithelium from the dental papilla, by a delicate basement membrane.

c) Polygonal cells located in the center of the epithelial enamel organ,

between the outer and inner enamel epithelia. These intermediate cells are called

stellate reticulum.

d) Two temporary structure are seen, enamel knot and enamel cord. The

cells in the centre of the dental organ proliferate and project towards dental papilla

like a knob. This is called enamel knot. A vertical extension of the knot in the

dental organ is called enamel cord.

e) The mesenchyme in the invaginated portion of the inner dental

epithelium proliferates under the influence of epithelium. It condenses and is

called dental papilla which later on forms dentin and pulp. This a marginal

condensation of the mesenchyme around the dental organ and dental papilla. This

condensation becomes fibrous and is called the primitive dental sac.

3. Bell stage : During histodifferentiation some cells of dental organ

differentiates into specific form and shape. This is seen in early bell stage.

In late bell stage, dental organ assumes the characteristic shape of the tooth

and is called morphodifferentiation. The invagination of the epithelium deepens and

its margins continue to arrow and the enamel organ assumes a bell shape. This is

known as bell stage.

In early bell stage, the odontoblasts are differentiated from mesenchyml cells.

With formation of dentin the cells of inner dental epithelium transform into

ameloblasts and enamel matrix is laid down opposite dentin.

Presence of dentin is absolutely essential for laying down of enamel.

Future dentino – enamel junction is outline and the form of crown is

established.

During this stage a tooth germ shows the following structures :

I. Dental organ

II. Dental papilla

III. Dental sac

I) Dental organ :

1. Outer dental epithelium : A single row of cuboidal cells are

thrown into folds and contain blood vessels at late bell stage.

2. Stellate reticulum : There is increase in intercellular fluid

and the layer expands. The cells assume star shape with long processes that

anastomose with adjacent cells.

3. Stratum intermedium : Several layers of sq. cells appear

between stellate reticulum and inner dental epithelium and called stratum

intermedium. This layer is essential for enamel formation. It helps in calcification

of enamel and is a reserve source for new ameloblasts.

4. Inner dental epithelium : This consists of a single layer of

cells that differentiates into tall columnar cells, the ameloblasts. These cells

influence the underlying mesenchymal cells which differentiates into

odontoblasts.

II. Dental papilla : It later on forms dentin and pulp.

III Dental sac : This forms cementum, alveolar bone and periodontal ligament.

5. Apposition : The tooth germ form calcified tissues of the

tooth, the enamel, the dentin, and the cementum. There is a layer like deposition

of an extra cellular matrix resulting in additive growth.

ROOT FORMATION :

The development of the roots begins after enamel and dentin formation has

reached the future cementoenamel junction. The enamel organ plays an important part

in root development by forming Hertwigs epithelial root sheath, which molds the

shape of the roots and initiates radicular dentin formation.

DEVELOPMENT OF ENAMEL (Amelogenesis) :

Enamel is ectodermal in origin. It is a hard tissue and follows the same pattern

of formation as is shown by other mineralized tissues of the body. That is, first there

is laying down of a matrix by cells and then that is mineralized with hydroxyapatite.

Life Cycle of Ameloblasts :

There are 6 stages

1. Morphogenetic stage

2. Organizing stage

3. Formative stage

4. Maturative stage

5. Protective stage

6. Desmolytic stage.

1. Morphogenetic stage : Before the ameloblasts are fully differentiated and

produce enamel, they interact with the adjacent mesenclymal cells, determining

the shape of the dentinoenamel junction and the crown.

During this morphogenic stage the cells are short and

columnar, with large oval nuclei that almost fill the cell body.

The Golgi apparatus and the cantrioles are located in the

proximal end of the cell, where as the mitochondria are evenly dispersed

throughout the cytoplasm.

Terminal bars appear when the mitochondria migrate

peripherally.

Inner dental epithelium is separated from the dental

papilla by a delicate basement membrane.

The zone next to basement membrane is cell free zone.

2. Organizing stage :

This stage is characterized by a change in the appearance of the cells of the

inner enamel epi. The cells become longer

The centrioles and golgi apparatus migrate towards basement membrane.

Mitochondria concentrates near this end and the clear cell free zone near

basement membrane disappears due to lengthening of the cells.

The epithelial cells come in close contact to the connective tissue cells of the

dental papilla and induce them to differentiate into adontoblasts o dontoblasts

starts forming dentin

As soon as first layer of dentin is laid down the blood supply of inner dental

epi. from dental papilla in cut - off. New blood supply is established through

outer dental epithelium. Gradually the stellate reticulum disappears.

3. Formative stage : After the first layer of dentin is formed, formation of

enamel starts. Presence of dentin is essential for enamel formation, just as it was

necessary for the epithelial cells to come into close contact with the connective

tissue of the pulp during differentiation of the odontoblasts and the beginning of

dentin formation.

4. Maturation stage : Enamel maturation occurs after most of the thickness of

the enamel matrix has been formed in the incisal or occlusal area.

Enamel matrix is still being formed in cervical region.

During this stage the ameloglasts are reduced in length and are closely

attached to enamel matrix.

During maturation, ameloblasts display microvilli at their distal extremities,

and they produce enamel cuticle.

5. Protective stage : When the enamel has completely developed and has fully

calcified, the ameloblasts cease to be arranged in a well defined layer, means these

cells lose their separate identity.

These cells then form a stratified epithelial covering layer, the reduced enamel

epithelium.

The layer protects enamel against connective tissue till it erupts into oral

cavity.

If connective tissue comes in contact with enamel before eruption, enamel is

either resorbed or is covered with cementum.

6. Desmolytic stage : The reduced enamel epithelium proliferates and seems to

induce atrophy of the connective tissue separating it from the oral epithelium, so

that fusion of two epithelia can take place.

Premature degeneration of the reduced enamel epithelium may prevent the

eruption of a tooth.

Amelogenesis :

Amelogenesis or formation of enamel may be described in two stages:

1. Formation of matrix

2. Maturation (calcification)

1. Formation of Matrix :

STEP I : Ameloblasts deposit a layer of matrix extracellularly over the first layer of

dentin which has started to calcify. This is called dentino- enamel membrane.

This prevents direct contact of enamel rods and dentin.

This contributes to the formation of dentino – enamel junction and

forms the foundation of which the rods will rest.

STEP 2 :

First stage : Formation of Tome’s process :Tome’s process is really a finger like

distal terminal of ameloblast. There is deposition of matrix between the distal ends of

ameloblasts and completely surrounds the distal end of ameloblasts. These are known

as Tome’s processes of enamel globule.

Adjacent Tome’s processes are separated from each other by terminal

bass formed by thickening of cell membrane. This stage is absent in prenatal

enamel formation in deciduous teeth.

Second Stage : Formation of Rod Space :

After Tome’s process formation is completed, the ameloblasts retreat

and a space is created. This space will be filled with secretory products of the

cell. This enamel matrix is formed extracellularly.

Third stage : Formation of Rod segment :

The space created by withdrawal of ameloblasts are filled up by an amorphous

organic matrix

The matrix is deposited progressively from periphery inwards through

the rod segment.

While a given segment of rod is being filled in, new segments are

being outlined at the distal end of the cell. That is repeated and the rod increases

in length.

The rhythmic deposition of rod segment is responsible for the cross –

striation seen in mature rods.

The ameloblasts are at an angle to the developing rod segments. They

may bend first to one side and then to the other giving a wavy course to the rods.

Generally one rod is produced from one ameloblast. After laying down

of the rods the ameloblasts secrete an organic material of 1um thickness, known

as primary enamel cuticle. After this, ameloblasts, along with the remaining cell

layers of the dental organ form Nasmyths membrane.

2. Maturation (minerlization) :

Phase I : Initial or primary phase :

In this stage an immediate partial mineralization occurs in the matrix segments

and the inter-prismatic substance as they are laid down.

First mineral is in the form of a crystalline appatite.

Phase II : Maturation phase :

It is the gradual completion of mineralization.

The process of maturation starts from the height of the crown and progresses

cervically.

Maturation begins before the matrix has reached its full thickness

Each rod matures from dentino – enamel junction towards surface and the

sequence of maturing rods is from the cusps or incisal edge towards cervical

line, that is rods near the incisal mature before those near cervical region. The

loss in volume of the organic matrix shows the loss of volume is caused by

withdrawl of a substantial amount of protein as well as water.

Clinical considerations :

Clinical interest in amelogenesis is centered primarily on the perfection of

enamel formation.

The principal expressions of pathologic amelogenesis are hypoplasia, which

is manifested by pitting, furrowing, or even total absence of the enamel, and

hypocalcification, in the form of opaque or chalky areas on normally contoured

enamel surfaces.

The causes of such defective enamel formation can be generally classified as

Systemic

Local

Genetic

Systemic influences are

Dentist should exert his or her influence to ensure sound nutritional practices

and recommended immunization on procedures during periods of gestation and

postnatal amelogenesis.

Chemical intoxication of the ameloblasts is limited essentially to the ingestion

of excessive amounts of water – borne fluoride. Where the drinking water

contains fluoride in excess of 1.5 parts per million, chronic endemic fluorosis

may occur as a result of continuous use throughout the period of amelogenesis.

If matrix formation is affected, enamel hypoplasia will ensue. If maturation is

lacking or incomplete, hypocalcification of the enamel results.

Endocrinopathies

Chemical intoxications

Febrile diseases

Nutritional deficiency.

In case of hypoplasia a defect of enamel is found. In case of hypocalcification

a deficiency in the mineral content of the enamel is found.

Hypoplasia as well as hypocalcification maybe caused by

Systemic :

Hypoplasia of systemic origin is termed chronological hypoplasia because the

lesion is found in the areas of those teeth where the enamel was formed during

the systemic disturbance.

Since the formation of enamel extends over a longer period and the systemic

disturbance is, in most cases, of short duration, the defect is limited to a

circumscribed area of the affected teeth.

A single narrow zone of hypoplasia may be indicative of a disturbance of

enamel formation during a short period in which only those ameloblasts that at

that time had just started enamel formation were affected.

Multiple hypoplasia develops if enamel formation is interrupted on more than

one occasion.

The systemic influences causing enamel hypoplasia are, in the majority of

cases, active during the first year of life. Therefore the teeth most frequently

affected are the incisors, canines, and first molars.

The upper lateral incisors are sometimes found to be unaffected because its

development starts later than that of the other teeth mentioned.

Local :

Local factors affect single teeth, in most cases only one tooth.

The cause of local hypoplasia may be an infection of the pulp with

subsequent infection of the periapical tissues of a deciduous tooth if the

irritation occurred during the period of enamel formation of its permanent

successor.

Hereditary factors :

It is generalized disturbance of ameloblasts. Therefore the entire enamel of all

the teeth, deciduous as well as permanent, is affected rather than merely a belt

like zone of enamel of a group of teeth, as in systemic cases.

Systemic

Local

Hereditary Factors

In a rare hereditary disturbance of the enamel organ called

odontodysplasia ,both the apposition and maturation of the enamel are disturbed

such teeth are having a irregular pains more – eaten, poorly calcified enamel.

The discoloration on of teeth from administration of tetracyclines during child

hood is a very common clinical problem. Whereas usually this discoloration is

because of deposition of tetracycline in the dentin, a small amount of drug may

be deposited in the enamel.

Histology

Chemical and physical properties of enamel :

The anatomic crown of a tooth is composed of an acellular calcified material

known as enamel. It has several distinguishing characteristics.

1. It is the hardest tissue of the body.

2. It is the only calcified tissue arising from ectoderm.

3. It contains the largest crystals among the mineralized tissues. Its matrix is a

gel like structure secreted in organic form by ameloblasts.

PHYSICAL CHARACTERISTICS:

1. Density : Density decreases from the surface of the enamel to the dentino – enamel

junction.

It varies from 3.0 to 2.84 gm/ml.

Permanent teeth have more density than the deciduous teeth. The density of

enamel increases progressively during development.

The final value is reached after eruption.

The permanent upper incisors have maximum density and premolars and

lower incisors have the least.

2. Thickness :

Enamel covers the crown of the tooth.

It is thickest over cusps and incisal edges and thinnest at the cervical margin.

Over the cusps of unworn permanent teeth it is 2.5 mm thick (over the cusps

of deciduous teeth 1.3 mm), and on lateral surfaces up to 1.3 mm.

The thickness declines gradually to become a very thin layer at the cervical

margin.

3. Colour :

Enamel is bire fringent crystalline material, the crystals refracting light

differently in different directions.

The color of enamel ranges from yellowish white to grayish white.

Color is determined by differences in the translucency of enamel.

Yellowish teeth having a thin, translucent enamel through which the yellow

color of the dentin is visible and grayish teeth having a more opaque enamel.

Grayish teeth frequently show a slightly yellowish color at the cervical areas,

bez the thinness of the enamel permits the light to strike the underlying yellow

dentin and be reflected.

Incisal areas may have a bluish tinge where the thin edge consists only of a

double layer of enamel.

4. Hardness : Enamel is the hardest structure of the body.

KHN of enamel is 296.

The peripheral regions of the tooth are harder than the deeper regions.

This variation depends on the degree of calcification, prism orientation, and

distribution of metallic ions.

5. Tensile Strength and Compressibility :

The elastic modulus of enamel is 19 x 106 psi, which indicates it’s a rigid

structure.

Tensile strength is 46 MN m-2 or 11,000 psi which indicates its brittle (mega

newtons) N x 106 compressive strength 76 MN m-2

6. Solubility : Enamel dissolves in acidic media.

The solubility rate is influenced by certain ions and molecules such as

fluorides, silver nitrate, zinc chloride, indium nitrate, stannous sulfate,

carbonates, organic matrix etc.

The surface enamel is less soluble in acidic than deeper enamel.

7. Permeability : Enamel is permeable for varying degrees.

The route of passage occurs mainly via the rod sheath, enamel lamellae and

enamel tufts, possesses a submicroscopic pore system similar to that of a

molecular sieve.

Chemical properties :

Weight Volume

Organic 0.5 % 1.0 %

Inorganic 95.5 % 87.0 %

Water 4.0 % 12.0 %

Organic Composition :

The organic matter increases towards dentino – enamel junction and is least at

the surface.

The organic content of deciduous teeth is slightly higher than that of

permanent series.

The principal organic content is protein. A wide variety of organic molecules

occur in enamel, ranging from free amino acids to large unique proteins

complexes. These proteins are the amelogenins and non – amelogenins.

Smaller amounts of carbohydrates, 0.6 % lipids, citrates are also seen.

It contains glucose, galactose, mannose, and traces of xylose, glucosamine,

galactosamine.

Inorganic composition :

It is the main constituent of enamel.

Calcium hydroxyapatite Ca10 (PO4)6 (OH)2 is the principal mineral component

of enamel. Other apatites like fluorapatites are also seen. Calcium and

phosphorus form the major constituent of enamel. These two elements along

with hydroxyl ions are present in apatite.

Strontium, radium, vanadium and carbonate can replace phosphate to modify

the apatite crystal. These ions modify the properties of enamel.

Fluoride makes enamel more resistant to acid and is anti – cariogenic.

On the other hand presence of carbohydrates makes the tooth more

susceptible to acid and thus more prone to cariogenic process.

Besides the hydroxyapatite other minor constituents include fluoride, silver,

aluminium, barium, copper, magnesium, nickel, selenium, strontium, titanium,

vanadium and lead.

Most of these are trace elements except fluoride and zinc. These are not

uniformly distributed.

Water : The presence of water is related to the porosity of the tissue.

Some of the water may lie between crystals and surround the organic

material, some may be trapped with in defects of the crystalline structure and the

remainder forms a hydration layer coating the crystals.

As ions such as fluoride would travel thro the water component its

distribution is of clinical importance.

Structure of enamel :

The three – dimensional architecture of enamel is extremely complex and

varies.

Most of the studies, were based on ground sections, which are thin slices of a

tooth ground and polished to a thickness of less than 50 m to permit transmission of

light and resolution of structure for proper study of enamel different planes of section

are used like sagittal , a longitudinal plane passing from the facial to lingual surface

of the tooth.

Transverse : a horizontal plane at any level of the crown

Facial : a longitudinal plane that is tangent to the facial surface, passing through the

tooth from mesial to distal.

A. Saggital Section :

1. Dentino enamel junction

2. Rods

3. Striae of Retzius

4. Neonatal line

5. Cross striations

6. Hunter – Schreger Bands

7. Spindles

B. Transverse section

1. Path of Rods from the dentinoenamel junction.

2. Relationship between Hunter – Schreger bands and enamel rods.

3. Lines of Retzius

4. Enamel tufts.

5. Enamel lamellae

C. Facial Sections :

1. Light microscopic observations

2. Electron microscopic observations

3. Crystalline substructure

4. Rod sheath

A. Saggital Section :

1) D – E J : (Amelodentinal junction)

Examination of a tooth sectioned longitudinally reveals

a line of junction between enamel and dentin that grossly follows the external

contours of the crown.

Because enamel thickness varies, the contour of the

dentinoenamel junction differs slightly from the external contour.

At higher magnifications, the d-EJ appears scalloped

with the concavities of the scallop facing the enamel. Each concavity measures

roughly 70 m in diameter, and the convexities on the dentinal surface.

2) Rods : (enamel prism or rod)

The basic structural component of enamel is called an enamel rod.

Rods originate at the dentinoenamel junction and extend through the width of

the enamel to the surface.

The crown of an incisor contains over 8 million rods and that of a molar, over

12 million.

A rod is narrowest at its point of origin and widens gradually as it approaches

the surface. It has an average diameter of 4 m.

Enamel rods consists several million hydroxyapatite crystallites packed into a

long thin rod 5 – 6 micro m in diameter and up to 2.5 m in length.

In cross section the shape of an enamel prism approximates to one of three

main patterns.

In pattern I enamel the rods are circular.

In pattern II enamel the prisms are aligned in parallel rows.

In pattern III enamel the prisms are arranged in staggered rows such that the

tail of a rod lies between two heads in the next row, giving key hole appearance.

All the three patterns are present is human, but the pattern III keyhole pattern,

predominates .

In pattern I enamel the prisms appear circular. The enamel between the prisms

had been termed ‘interprismatic’. Its composition is similar to that inside the

prisms but it has a different optical effect because the crystals deviate by 40 -

600 from those in the prism.

The keyhole shape of pattern III enamel shows clear ‘head’ and ‘tail’ regions,

the tail of one prism lying between the heads of the adjacent prisms and pointing

cervically.

In the head of the prism the crystals run parallel to the long axis of the prism.

In the tail the crystals gradually diverge from this to become angled 65 - 700 to

the long axis.

Complex pattern of prisms makes enamel resistant to fracture and when

exposed on the surface, leads to micro ridged grinding surface.

As prisms are arranged in a spiral pattern, in some areas beneath the cusps and

incisal edges the changes in direction of the prisms appeal more marked and

irregular. Groups of prisms seem to spiral around others,giving the appearance

of gnarled enamel.

Enamel is formed incrementally, periods of activity alternating with periods of

quiescence. This results in structural markings known as incremental lines.

There are two types : short period (cross striations) and long period (enamel

striae).

Rod Interrelationships :

Rods tend to be maintained in rows arranged circumferentially around the long

axis of the tooth.

The rods in each row run in a direction generally perpendicular to the surface

of the dentin, with a slight inclination toward the cusp as they pass outward.

Near the cusp tip the rows have a small radius, and the rods run more

vertically.

In cervical enamel the rods run mainly horizontally only a few rows are tilted

apically.

The arrangement of rod rows has clinical importance because enamel fractures

occur between adjacent rows.

3. Striae of Retzius : (Incremental lines )

When a ground section of a tooth is seen under a light microscope concentric brown

lines are seen in the enamel. These are called lines of Retzius.

They form concentric arcs at the cusps and incisal edges they terminate at

various levels along dentino – enamel junction.

Near the cervical region these parallel lines fan out towards enamel surface

and don’t complete the arc. These lines of Retzius, which terminate at the

enamel surface and don’t complete the arc, form a series of grooves known as

the imbrication lines of pickerill.

The elevations between the grooves are known as peri kymata.

Lines of Retzius are due to any or all of the following.

1. Variations in organic structure.

2. Disturbances in rhythm of mineralization

3. Intermittent alteration of rod’s course.

In human teeth there are 7-10 cross – striations between adjacent striae in any one

individual. The striae are therefore formed at about weekly intervals.

As the average distance between two cross – striations is about 4 m, enamel

striae in the middle portion of enamel are about 25 – 35 m apart.

1. In cervical enamel, where enamel is formed more slowly and cross – striations

may be only about 2 m apart, the striae are closer together and may be separated

by only 15 – 20 m.

2. Accentuated striae may be due to metabolic disturbances occurring during the

time of mineralization.

3. The structural basis for the production of striae of Retzius has recently been

examined ; it seems that they are formed at the result of a temporary constrition of

Tome’s 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 possible to use the incremental markings in enamel (cross – striations, enamel

striae and perikymata) to assess the time taken to form the crown of the tooth, and

to help age material.

4. Enamel striae are less pronounced or absent from enamel formed before birth.

5. A particularly marked striae is formed at birth – this is the neonatal line, and

reflects the metabolic changes at birth.

Cross striations :

1.Human enamel is known to form at a rate of approximate 4 m per day.

2.Cross – striations are seen as lines traversing the enamel rods at right angles to

their long axes.

3.These lines are a result of the crystals with in the prism following a spiral path.

4.At low power cross – striations appear as line 2.5 – 6 m apart, being closer

together near the enamel – dentin junction.

5.It has also been suggested that cross – striations are the result of subtle changes

in the nature of the organic matrix and / or crystallite orientation and / or

composition (especially in the carbonate component).

Neonatal line :

The enamel of the deciduous teeth develops partly before and partly after

birth.

The boundary between the two portions of the enamel in the deciduous teeth is

marked by an accentuated incremental line of Retzius, the neonatal line or neonatal

line.

It appears to be the result of the abrupt change in the environment and

nutrition of the newborn infant.

The prenatal enamel usually is better developed than the postnatal enamel.

This is explained by the fact that the fetus develops in a well protected

environment with an adequate supply of all the essential materials, even at the

expense of the mother.

Because of the undisturbed and even development of the enamel pri or to

birth, perikymata are absent in the occlusal parts of the deciduous teeth, where

as they are present in the postnatal cervical parts.

Hunter – Schreger Bands :

The change in the direction of rods is responsible for the appearance of the

Hunter – Schreger bands.

These are alternating dark and light strips of varying widths that can be best

seen in a longitudinal ground section under oblique reflected light.

They originate from the dentino enamel junction toward surface.

They are not sharply delineated, but tend to merge with one another and

gradually disappear in the outer third of the enamel.

In a transverse plane, enamel rods follow a wavy path as they course from

dentino enamel junction. Thus, when longitudnal ground sections are prepared,

the plane of section passes through these undulations, cutting some rods as they

pass into the specimen and others as they emerge from the specimen.

The shift of rod direction between adjacent bands is gradual. Bands in which

the rods are sectioned more transversely are called diazones ; those in which the

rods are sectioned more longitudinally are termed parazones.

When light is reflected from the surface of ground sections, the rod orientation

causes alternate bands to absorb or reflect light in the characteristic Hunter –

Schreger pattern. The diazones appear dark and the parazones light.

It is due to difference in degree of mineralization.

It is the result of variation in permeability and organic content.

Enamel Spindles : These make the area hypersensitive to pain

Narrow (up to 8 m in diameter), round, sometimes club shaped tubules – the

enamel spindles – extend up to 25 m into the enamel.

These are tiny blind canals filled with air and debris in the process of grinding

and preparing the slide.

In sections in which the spindles and dentinal tubules lie in the same plane,

one can see that two are continuous.

At the tips of the cusps, the direction is similar to that of the rods.

Along the cusp slope, however, the spindle angle cervically.

Some investigators believe that with the formation of enamel the terminals of

the odontoblasts become trapped in the enamel matrix.

Fixed boundaries for enamel and dentin don’t exist. Thus transgression of the

processes of both ameloblasts and odontoblasts can occur.

Others have noted that the processes of the ameloblasts may project well into

the dentin matrix. In this case, enamel matrix is deposited in the territory of the

dentin and will, therefore, surround the processes of the odontoblasts.

Transverse Section :

1. Path of rods from the D E J :

The transverse plane reveals the path of enamel rods to be more complex than

suggested in a sagittal section.

With in the inner half of the enamel, the rods weave from side to side.

The weaving is superimposed on a gradual lateral deviation of about

10 degrees, with the result that the rods pursue a step like course to one side

before resuming a straight parallel path in the outer half of the enamel.

There is gradual and alternating shift in the weaving pattern when

focusing thro’ successive layers. This architectural pattern is responsible for the

appearance of Hunter – Schreger bands.

2. Relationship between Hunter – Schreger bands and enamel rods :

To understand the relationship of Hunter – Schreger bands to enamel rods, the

sagittal and transverse views must be merged to form a three – dimensional picture.

In longitudinal sections Hunter – Schreger bands don’t always parallel enamel

rods. The bands tend to bow to ward the cervix, the curvature being more

pronounced in teeth with ovoid contours (e. g. molars).

The bands and rods, therefore, take independent paths, the bands crossing and

recrossing the rod.

Where the rods intersect a Hunter – Schreger band, each rod assumes the

directional orientation of that band. As a rod courses toward the surface, it may

pass through several bands, alternately assuming the orientation of each band.

This is directly responsible for the weaving of rods seen in transverse sections.

The fact that this is most obvious in the inner half of the enamel correlates well

with the observation that the sharpest curvature of the Hunter – Schreger bands

is also in the inner half. Thus, each rod intersects a greater number of bands in

this region.

The no. of bends in a rod equals the no. of Hunter – Schreger bands it crosses on

its path to the surface.

3. Lines of Retzius :

Brown striae of Retzius may be seen in transverse as well as sagittal sections. They

appear as a series of rings concentrically arranged around the dentin core and are

analogous to growth rings in a tree.

4. Enamel Tufts :

Transverse ground sections exhibit structures that resemble tufts of grass and

which, therefore, have been named enamel tufts.

They are long ribbons of organic material that grow out from the

dentinoenamel junction and are oriented longitudinally along the crown.

They are hypermineralized and secure at approximately 100 m intervals

along the junction.

Each tuft is several rods wide.

It has been suggested that this appearance results from protein, presumed to be

residual matrix, at the rod boundaries of hypomineralized rods. Tuft proteins

however is not amelogenin, the major developmental enamel protein, but the

minor non – amelogenin fraction.

5. Enamel Lamellae :

Most teeth, when examined closely, have cracks in the enamel extending in a

longitudinal direction. The cracks are more easily seen if the tooth surface is

stained, they appear to be most numerous in the cervical half of the crown.

These cracks, termed lamellae usually extending thro’ the enamel from the

dentinoenamel junction to the surface.

These are sheet – like apparent structural faults that run thro’ the entire thickness

of the enamel. In ground sections these structures may be confused with cracks

caused by grinding of specimens.

Careful decalcification of ground sections of enamel makes possible the

distinction between cracks and enamel lamellae. The former disappear, where as

the latter persist.

Lamellae may develop in planes of tension. Where rods cross such a plane, a short

segment of the rod may not fully calcify.

If the disturbance is more severe, a crack may develop that is filled either by

surrounding cells, if the crack occurred in the unerupted tooth, or by organic

substances from the oral cavity, if the crack developed after eruption.

Three types of lamellae can thus be differentiated.

Type A : Lamellae composed of poorly calcified rod segments.

Type B : Lamellae consisting of degenerated cells.

Type C : Lamellae arising in erupted teeth where the cracks are filled with organic

matter, presumably originating from saliva.

Last type may be more common than formerly believed.

Lamellae of type A are restricted to enamel, those of type B and C may reach in to

dentin.

Lamellae extend in the longitudinal and radial direction of the tooth, from the tip

of the crown toward the cervical region. This arrangement explains why they

can be observed better in horizontal sections.

Enamel lamellae may be a site of weakness in a tooth and may form a road of

entry for bacteria that initiate caries.

C. FACIAL SECTIONS (Tangential Long Sections) :

1. Light Microscopic Observations :

When enamel is studied from the facial aspect, the rods are seen in cross section.

Some early investigators interpreted the architecture as consisting of horizontal

rows of interlocking hexagons, which they named prisms.

Each prism was believed to be surrounded by a sheath and joined to its neighbors

by an interprismatic cementing substance, which was resistant to decalcification.

Other investigators interpreted the prisms as resembling fish scales, this concept is

more close to the modern view.

2. Electron microscopic observations :

In 1965 undecalcified enamel led to a new concept of enamel rod structure.

The term rod is used currently more frequently than prism.

Electron microscopic observations suggest that each rod consists of an are shaped

head and a tail which is interposed between subjacent rod heads. This

interpretation suggests a key hole configuration for the rod unit.

It was shown that the tail portions, which occupies the area previously known as

inter prismatic substance, is also crystalline. It differs from the head portion only

in the orientation of its crystals.

The interpretation currently accepted is one of arcade – shaped rods arranged

roughly in parallel horizontal rows, situated in a continuous network of inter rod

(inter prismatic) enamel.

Rod and interod enamel vary only in crystal orientation. Each rod is partially

surrounded by a thin sheath, which faces the cervix of the crown and apex of the

tooth, the crystallites of the rod are confluent with those of the inter rod area.

3. Crystalline Substructure :

Mineralization of the enamel matrix occurs immediately after its deposition by the

ameloblasts. The process involves seeding of matrix with apatite crystals, which

initially resemble long ribbons.

Crystal length has been impossible to determine because true longitudinal section

are difficult to establish.

In addition, crystals invariably show apparent breaks in linear continuity which

may represent either the ends of individual crystals or fracture points of longer

units.

Microspaces exist between crystals. With in the rod proper the crystals are

oriented parallel to the rod length.

4. Rod Sheath : The true nature of the sheath can be seen only by electron

microscopy.

It is a thin structure which is essentially an inter crystalline space consisting of

organic matrix.

In untreated sections it is delineated by a marked difference in crystal orientation

between the rod and interrod regions.

When sections of enamel are treated with weak acids, the sheath is more resistant

to dissolution because of its higher organic content.

THE EXTERNAL SURFACE OR SURFACE ENAMEL

The outer layer and surface of enamel may exhibit features such as

Aprismatic enamel

Cuticles

Perikymata

Lamellae

Pit and fissures.

1. Aprismatic enamel :

Many teeth exhibit a superficial layer of enamel that is devoid of typical rod

architecture.

It is found in most deciduous teeth and occurs near the cervical regions of many

permanent teeth.

The outer 20 – 100 m of enamel of newly erupted deciduous teeth and the outer

20 – 70 m of newly erupted permanent teeth is aprismatic (prismless).

It is believed to be caused by cessation of secretary activity of ameloblast and the

retraction of Tomes process. Because the extension of tomes’ process in thought

to influence crystal orientation, and in the absence of the process, the

characteristic shift in crystal orientation found in the interrod areas will be

lacking.

This layer is more heavily mineralized.

They may contribute to the adherence of plaque material with a resultant caries

attack, especially in young people.

It is this shift in orientation that is primarily responsible for rod or prism

morphology.

Aprismatic enamel may also be observed at the dentinoenamel junction. It is

presumed that aprismatic enamel at the DEJ is secreted by the preamelo blast

prior to the for mation of Tome’s process.

2. Enamel Cuticle : At the completion of enamel formation the crown is covered by a

very thin layer of organic material. Nasmyth’s membrane this may be referred to as

the primary cuticle. It is intimately associated with the crystals of the enamel surface.

It resembles an epithelial basal lamina.

As the tooth erupts, forces of abrasion and attrition wear away the pricuticle

except in shetered areas such as the cervix adjacent to the gingival sulcus.

With in hrs after eruption, a secondary deposit may be found on all exposed parts

of the crown. This organic layer is believed to be derived from the saliva and,

unlike the primary cuticle can be continuously replenished. It is called the

pellicle.

Finally, a thick microbial plaque is added, consisting of oral flora, cellular debris,

and food remains. The action of normal chewing wears away both salivary

pellicle and microbial plaque except in the more sheltered areas.

3. Perikymata and Imbrication Lines of Pickerill :

The enamel surfaces, particularly those that have not been exposed to abrasive

forces, appear corrugated. The corrugations consist of alternating horizontal

ridges and roughs.

They are collectively referred to as perikymata, although some authors have reserved

this term for the ridges alone.

The troughs are the surface terminations of the lines of Retzius, which in

longitudinal section resemble over lapping shingles for this reason, some authors

refer to the grooves as imbrication lines.

Perikymata are most numerous and are more closely spaced near the cervix, but

are absent over the cusps and incisal edges where the lines of Retzius don’t

reach the surface.

(At high magnification) with age, the perikymata are worn smooth in areas

exposed to abrasion.

4. Lamellae : It is discussed earlier. At the enamel surface, they resemble cracks

extending longitudinally and penetrating the enamel perpendicular to the surface.

5. Pits and Fissures : Pits and fissures are defects in the enamel surface usually

associated with developmental grooves that mark the lines of fusion between cusps

and other major divisions of the crown.

Multicuspid teeth develop from several growth centers, each of which represents a

cusp or division of the mature tooth. These growth centers are located at crests

of the dental papillae and correspond to the prospective cusptips.

As enamel formation proceeds from these centers, it progresses over the medial

inclines of the cusps toward the center of the tooth. Ultimately, adjacent cusps

coalesce and fuse. When the inclines are steep and the developmental centers are

close together, the potential for strangulation of ameloblast exists.

Strangulation occurs when ameloblasts from adjacent cusps literally collide as

they retreat from DEJ. Because secretary activity ceases in these compressed

cells, a fissural defect in the enamel results.

Pits are similar manifestations found at the ends of developmental grooves or at

the intersections of two or more grooves.

Although pits and fissures are commonly observed in multicuspid teeth, they are

also frequently seen on the lingual surface of max lateral incisiors.

Termination of pits and fissures may vary from

1. A shallow groove

2. to complete penetration of the enamel

3. The end of the fissure may end blindly

4. Or open into an irregular chamber.

Pits and fissures are often the primary sites for carious invasion because of the

inaccessibility of the areas for cleaning.

Dentists may employ plastic sealants to close this potential pathway to invading

micro – organisms. Because pits may be very small, the detection of early caries

at thick bases may be quite difficult.

Cemento – Enamel Junction :

Both cementum and enamel lie external to dentin. Three possible variations

may exist at the cemento –enamel junction :

1. The cervical enamel is covered by cementum. This is the most frequent variety

(65 5)

2. There is no overlapping but a simple contact of the terminal portions of the

enamel and cementum (25 %)

3. Enamel and cementum are separated exposing dentin. This is the least frequent

variety found (10 %)

AGE CHANGES :

Enamel is a nonvital tissue that is incapable of regeneration.

1. With advancing age enamel becomes darker. This is due to increased

pigmentation of organic part and increased thickness of dentin.

2. Gradually, enamel becomes less permeable.

3. There is loss of enamel due to attrition on occlusal and proximal surfaces. This

is due to mastication. The vertical dimension of crown shortens and the

proximal contour flattens.

4. Changes on enamel surface are :

a) Perikymata disappears.

b) There is localized inc of some elements eg. sodium, fluoride.

c) Permeability to fluids is reduced.

Clinical Consideration :

Defects in enamel

Enamel structure and dental caries

Enamel structure and restorative dentistry.

Defects in enamel

a) Amelogenesis imperfecta : (Hereditary enamel dysplasia)

It represents a group of hereditary defects of enamel. It is entirely an

ectodermal disturbance

The development of normal enamel occurs in three stages

1) The formative stage, during which there is deposition of the

organic matrix.

2) Calcification stage, during which this matrix is mineralized.

3) Maturation stage, during which crystallites enlarge and

mature.

Accordingly, three basic types of amelogenesis imperfect are recognized.

Hypoplastic type : in which there is defective formation of matrix.

Hypocalcification type : in which there is defective mineralization of the formed

matrix.

Hypomaturation type : in which enamel crystallites remain immature.

CLINICAL FEATURES :

1. Hypoplastic type : The enamel has not formed to full

normal thick ness on newly erupted developing teeth.

2. Hypocalcified type : The enamel is so soft that it can

be removed by a prophylaxsis instrument.

3. Hypomaturation type : The enamel can be pierced by

an explorer point under firm pressure and can be lost by chipping away from the

underlying normal appearing dentin.

R / F : The enamel may appear totally absent or when present may appear as a very

thin layer, chiefly over the tips of the cups and on the inter proximal surfaces.

T / t : There is no treatment except for improvement of cosmetic appearance.

ENVIRONMENTAL ENAMEL HYPOPLASIA :

Defects may be environmental or genetic in origin.

Enamel hypoplasia may be defined as an incomplete or defective formation of

the organic enamel matrix of teeth.

There are no. of conditions, each capable of producing injury to the

ameloblasts like.

1) Nutritional deficiency (vit A, C and D)

2) Exanthematous diseases (e.g. measles, chicken pox, scarlet

fever).

3) Congenital syphilis

4) Hypocalcemia

5) Birth injury, prematurity, Rh hemolytic disease.

6) Local infection or trauma

7) Ingestion of chemicals (chiefly fluoride)

8) Idiopathic causes.

Hypoplasia results only if the injury occurs during the time the teeth are

developing or more specifically, during the formative stage of enamel

development. Once the enamel has calcified, no such defects can be produced.

Generalized intrinsic enamel stain : Two substances which may be infested during

tooth formation are of particular importance. These are the fluoride ion and drug

tetracycline.

Mottled Enamel :

Mottled enamel is a type of enamel hypoplasia that was first described under

that term in this country by G.V. Black and Frederick S. Mckay in 1916.

Etiology : The ingestion of fluoride – containing drinking water during the time of

tooth formation may result in mottled enamel. The severity of mottling increases with

an increase in amount of fluoride in the water. If level is more than 1 parts per million

then signs of mottling are present.

Cl / F : Depending upon the level of fluoride in the water supply, there is a wide

range of severity in the appearance of mottled teeth like.

a) White flecking or spotting of the enamel.

b) Mild changes manifested by white opaque areas

involving more of the tooth surface area

c) Moderate and severe changes showing pitting and

brownish staining of the surface.

d) Corroded appearance of the surface.

T / t : For removing the stains bleaching is a effective t / t.

ECTODERMAL TUMORS :

1. Enameloma (Enamel Drop;Enamel Pearl) :

It is not a true neoplasm and may be classified as a tumor only by virtue of the

fact that it constitutes a small, focal excessive mass of enamel on the surface of the

tooth.

The enamel pearl is most frequently found near or in the bifurcation or trifurcation

of the roots of teeth, or an the root surface near the CEJ.

It appears as a tiny globule of enamel, firmly adherent to the tooth, which arises

from a small group of misplaced ameloblasts.

Enamel pearl may predispose to plaque accretion following gingival recession.

Dental fluorsis : Dental fluorsis may occur when the total daily in take of the fluoride

ion from source such as H2O, tooth paste, drops, and tablets is high while the enamel

is undergoing preruption formation and maturation.

Most severe cases are seen in areas where the fluoride content of the natural

H2O supply is high, such as part of Africa and India.

Regressive Alterations of the teeth :

Regressive changes in the dental tissue include :

1) Attrition

2) Abrasion

3) Erosion.

1. Attrition : It is a mechanical wear of the incisal or occlusal tooth structure as a

result of functional or parafunctional movements of the mandible.

Certain degree of attrition is expected with age, it is important to note abnormal

advanced attrition.

If significant abnormal attrition is present, the patients functional movements must

be evaluated and inquiry made about any habits creating this problem such as

tooth grinding, or bruxism, usually due to stress.

2. Abrasion : Abrasion is abnormal tooth surface loss resulting from direct friction

forces between the teeth and external objects, or from frictional forces between

contacting teeth components in the presence of an abrasive medium.

Abrasion may occur from

a) Improper brushing teeth

b) Habits such as holding a pipe stem by the teeth.

c) Tobacco chewing

d) Vigorous use of tooth picks between adjacent teeth.

3. Erosion : Erosion is the wear or loss of tooth surface by chemicomechanical

action. Regurgitation of stomach acid can cause this condition on the lingual surface

of maxillary teeth.

ENAMEL STRUCTURE AND DENTAL CARIES :

Dental caries is a microbial disease of the calcified tissues of the teeth,

characterized by demineralization of the inorganic portion and destruction of the

organic substance by the tooth.

Caries of the enamel :

Caries of the enamel is to be preceded by the formation of a microbial (dental)

plaque.

Process varies slightly, depending upon the occurrence of the lesion on smooth

surfaces or in pit or fissures.

1. Smooth Surface caries :

The earliest manifestation of incipient caries of the enamel is the appearance beneath

the dental plaque of an area of decalcification which resembles a smooth chalky white

area.

Scott has revealed that the first change is usually a loss of the interprismatic or

interrod substance of the enamel with increased prominence of the rods.

According to wislocki the work on mucopolysaccharide present in the

interprismatic organic substance of the enamel revealed that the degradation of

this substance occurred very early in the caries process.

Another change in the early enamel caries is the accentuation of the incremental

striae of Retzius. This conspicuous appearance of the calcification lines is an

optical phenomenon due to loss of minerals which causes the organic structures

to appear more prominent.

As this process advances and involves deeper layers of enamel, it will be noted

that smooth surface caries, particularly of proximal surfaces, has a distinctive

shape.

It forms a triangular or actually a cone shaped lesion with the apex toward the D-E-J,

and the base toward the surface of the tooth.

There is eventual loss of continuity of the enamel surface.

There are some implications that enamel lamellae play a role in caries but Scott

reported that there is no direct relation between the occurrence of enamel

lamellae and smooth surface caries on the basis of electron microscope studies.

2. Pit and Fissure Caries : The carious process in pits and fissures doesnot

differ in nature from smooth surface caries except as the variations in anatomic

and histologic structure dictate.

Here too the lesion begins beneath a bacterial plaque with decalcification of

the enamel.

Pit and fissures are of ten of such depth that food stagnation with bacterial

decomposition in the base is to be expected.

As told previously termination of pit and fissures may vary from

a) A shallow groove

b) To complete penetrate on of the enamel.

c) The end of the fissure may end blindly.

d) Or open into an irregular chamber.

Furthermore, the enamel in the bottom of the pit or fissure may be very thin,

so that early dentin involvement frequently occurs.

Enamel rods flare laterally in the bottom of the pits and fissures.

When caries occurs here, it follows the direction of the enamel rods and

characteristically forms a triangular or cone – shaped lesion with its apex at the

outer surface and its base toward the D-E-J. The general shape of the lesion here

is just opposite of that occurring on smooth surfaces.

The carious lesion is more apt to be stained with a brown pigments in pits and

fissures

1. Clinical features of incipient smooth – surface lesion how to detect

1. There are white spots usually observed on the facial and lingual surfaces of the

teeth.

2. White spots are chalky white, opaque areas that are revealed only when the

tooth surface is desiccated, and are termed incipient caries.

These areas of enamel lose their translucency because of the extensive

subsurface porosity caused by demineralization.

3. Care must be exercised to distinguish white spots of incipient caries from

developmental white spot hypoclcifications of enamel.

Incipient caries will partially or totally disappear visually when the enamel is hydrated

(wet), while hypocalcified enamel is relatively unaffected by drying and wetting.

4. The surface texture of an incipient lesion is unaltered and is undetectable by

tactile examination with an explores.

5. Softened chalky enamel that can be clipped away with an explores is a sign of

active caries.

6. Similar incipient lesions occur on the proximal smooth surfaces but usually

are undetectable.

Radiographically:

1. Sometimes it can be seen on radiographs as a faint radiolucency, limited to the

superficial enamel.

2. When a proximal lesion is clearly visible radio graphically the lesion may

have advanced significantly and histologic alteration of the underlying dentin

probably has already occurred.

Treatment :

1. Incipient caries of enamel can remineralize.

2. Non cavitated enamel lesions retain most of the original crystalline framework

of the enamel rods and the etched crystallites serve as nucleating agents for

remineralization. Ca++ and phosphate ions from saliva can then penetrate the

enamel surface and precipitate on the highly reactive crystalline surfaces in the

highly reactive crystalline surfaces in the enamel lesion. The supersaturation of

the saliva with Ca++ and phosphate ions serves at the driving force for the

remineralization process.

3. Further more, the presence of trace amounts of fluoride ions during this

remineralization process greatly enhances the precipitation of Ca++ and phosphate,

resulting in the remineralized enamel becoming more resistant to subsequent

caries attack because of the incorporation of more acid – resistant fluorapatite.

2. Clinical features of pits and fissure caries and how to detect :

1. Caries cavitation is difficult to detect in pits and fissures because it is difficult

to distinguish from the normal anatomic form of these features.

2. Cavitation at the base of a pit or fissure sometimes can be detected tactilely as

softness or by binding of the explores tip.

However mechanical binding of an explores in the pits or fissures may be due to

noncarious causes, such as the shape of the fissure, sharpness of the explores, or

force of application. Thus, explorer tip binding is not, by itself, a sufficient

indication to make a caries diagnosis.

3. There are some factors for pit and fissure caries

a) Softening at the base of the pit or fissure.

b) Opacity surrounding the pit or fissure, indicating

undermining or demineralization of the enamel ;

c) Softened enamel that may be flaked away by the explores.

4. Discolored enamel due to under mining caries is easily distinguished from

superficial staining because it is more diffuse and doesn’t affect the surface of the

enamel.

Radiographically :

On bitewing radiographs, evidence of dentinal caries may be seen as a

radiolucent area spreading laterally under the occlusal enamel from a pit or

fissure.

C. Enamel Structure and Restorative Dentistry :

Many of the structural features of enamel are acutely relevant to restorative

dentistry. The understanding of the initiation and progress of dental caries has

been based on a knowledge of enamel composition and morphology and has led

to a much more conservative approach by utilizing the phenomenon of

remineralization and reducing the need for the removal of sound tissue.

This reduced sacrifice of sound tooth structure has also been brought about by

the development of adhesives that will bond to enamel, a development that is

based on an understanding of the prismatic st. of enamel and the controllable

effects of acids on it.

Different acids at different concentrations can produce a variety of patterns of

partial prism dissolution to provide a roughened surface suitable for adherence of

restorative materials (acid conditioning). This reduces or eliminates the need for

mechanical retention cut into sound tissue. For agents mechanically binding to

enamel, it is necessary to produce micro-porosities in the surface by acid – etch tech.

Thus when bonding agents are applied to such as surface, microscopic tags can be

seen invaginating into the roughened surface.

When cavities are prepared a knowledge of the microanatomy of enamel,

particularly in terms of prism orientation, is essential to conserve as much as

possible of the original strength of the tissue. Cutting cavities into enamel with

rotary instruments will inevitably lead to subsurface cracking. Fortunately, some

of the adhesive materials are capable of reinforcing this weakened substrate.

ACID ETCHING :

Acid etching of the enamel surface, or enamel conditioning, has become an

important technique in clinical practice. It is involved in the use of fissure sealants, in

the bonding of restorative materials to enamel, and in the cementing of orthodontic

brackets to tooth surfaces.

It achieves the desired effect in 2 stages :

1. First it removes plaque and other debris, along with a thin layer of enamel.

2. Second, it increases the porosity of exposed surfaces through selective

dissolution of crystals, which provides a better bonding surface for the restorative

and adhesive materials.

Several types and concentration of acid are used to alter the enamel surface.

Most etch the surface to a depth of only about 10 m e.g. 30 % to 40 %

phosphoric acid applied to enamel for 60 seconds provides an adequate etch for

retention of sealants.

The scanning electron microscope beautifully demonstrates the effects of acid

etching on enamel surfaces.

Three etching patterns predominate

1. The most common is type I, characterized by preferential removal of the rod

core.

2. In the reverse, type II, the rod periphery is preferentially removed and the core

remains intact.

3. occurring less frequently is type III, which is irregular and indiscriminate.

There is still some debate as to why acid etchants produce differing surface

patterns.

The most commonly held view is that the etching pattern depends an crystal

orientation.

Ultrastructural studies of crystal dissolution indicate that crystals dissolve

more readily at their ends than on their sides. Thus crystals lying perpendicular

to the enamel surface are the most vulnerable.

The type of pattern also depends on nature of the etching agent.

In summary, acid conditioning of enamel surfaces is now an accepted

procedure for obtaining improved bonding of resins to enamel. Retention

depends mainly on a mechanical interlocking.

ENAMEL ADHESION :

Bounocore’s, in 1955, applied acid to teeth to render the tooth surface more

receptive to adhesion. Buonocore’s pioneering work led to major changes in the

practice of dentistry. Today, we are in the age of adhesive dentistry. Traditional

mechanical methods of retaining restorative materials have been replaced, to a large

extent, by tooth conserving adhesive methods. The concepts of large preparations and

extension for prevention, proposed by Black in 1917, have gradually been replaced by

smaller preparations and more conservative techniques.

Advantages of Adhesive Techniques :

1. Adhesion reduces microleakage at the tooth interface.

2. Prevention of microleakage, or the ingress of oral fluids and bacteria along the

cavity wall, reduces clinical problems such as postoperative sensitivity, marginal

staining, recurrent caries, all of which may jeopardize the clinical longevity of

restorative efforts.

3. Adhesive restorations better transmit and distribute functional stresses across

the bonding interface to the tooth, and have a potential to reinforce weakened

tooth.

Structure :

Adhesive techniques allow deteriorating restorations to be replaced with

minimal or no additional loss of tooth material.

Indications :

Adhesive teeth with resin composites were initially employed to replace

carious and fractured tooth st. or to fill erosion or abrasion defects in cervical areas.

Principles of Adhesion :

The word adhesion is derived from the Latin word adhaerer, which is a

compound of ad, or to, and haerere, or to stick.

The adhesive in dental terminology the bonding agent or adhesive system,

may be defined as the material that, when applied to surfaces of substances, can join

them together, resist separation, and transmit loads across the bond.

Adhesion refers to the forces or energies between atoms or molecules at an

interface that hold two phases together,.

If the bond fails at the interface between the two substrates, the mode of

failure is referred to as adhesive.

If failure occurs in one of the substrate, but not at the interface then it is

termed as cohesive. The mode of failure is often mixed.

There are four different theories for the observed phenomena of adhesion.

1. Mechanical theories state that the solidified adhesive interlocks

micromechanically with the roughness and irregularities of the surface of the

adhered.

2. Adsorption theories encompass all kinds of chemical bonds between the

adhesive and the adherend, including primary (ionic and covalent) and

secondary (hydrogen, dipole interaction, and London dispassion) valence forces.

London dispassion forces are almost universally present, because they arise

from and solely depend upon the presence of nuclei and electrons.

3. Diffusion theories propose that adhesion is the result of bonding between

mobile molecules. Polymers from each side of an interface can cross over and

react with molecules on the other side. Eventually the interface will disappear

and the two parts will become one.

4. Electrostatic theories state that an electrical double layer forms at the

interface between a metal and a polymer, making a certain, yet obscure,

contribution to the bond strength.

5. An important requirement for any of these interfacial phenomena to take

place is that the two materials being joined must be in sufficiently close and

intimate relation.

Besides an intimate contact, sufficient wetting of the adhesive will only occur

if its surface tension is less than the surface free energy of the adherend.

Wetting of a surface by a liquid is characterized by the contact angle of a

droplet placed on the surface. If the liquid spreads out completely on the solid

surface, this indicates complete wetting or a contact angle of 0 degrees.

According to this theory of wetting and surface free energies, adhesion to

enamel is much easier to achieve than is adhesion to dentin.

Enamel contains primarily hydroxyapatite, which has a high surface – free

energy, where as dentin is composed of two distinct substrates hydroxyapatite

and collagen, which has a low surface free energy.

In the oral environment the tooth surface in contaminated by an organic saliva

pellicle with a low critical surface tension of approach 28 dynes / cm, which

impairs adequate wetting by the adhesive.

Likewise, instrumentation of the tooth substrate during cavity preparation

produces a swear layer with a low surface free energy. Therefore, the natural

tooth surface should be thoroughly cleaned and pretreated prior to bonding

procedures to increased its surface free energy and hence to render it more

receptive to bonding.

ADHESION TO ENAMEL

Enamel acid – etching teeth :

Enamel etching transforms the smooth enamel surface into an irregular surface

with a high surface – free energy of about 72 dynes / cm. more than twice of

unetched enamel.

An unfilled liquid acrylic resin with low viscosity, the enamel bonding agent

wets the high energy surface and is drawn into the microporosities by capillary

attraction.

Acid etching removes about 10 m of enamel surface and creates a

microporous layer from 5 to 50 m deep.

Two types of resin tags have been described :

1) Macrotags : Are formed circularly between enamel prism peripheries.

2) Microtags : Are formed at the cores of enamel prisms, where the

monomer cures into a multitude of individual crypts of dissolved

hydroxyapatite crystals. These probably contribute most to the bond

strength because of their greater quantity a large surface area.

The effect of acid etching on enamel depends on several parameters.

o The kind of acid used.

o The acid concentration.

o The etching time.

o The form of the etchant (gel, semigel, or aqueous solution).

o The rinse time.

o The way in which etching is activated (rubbing, agitation, and or

repeated application of fresh acid).

o Whether enamel is instrumented before etching.

o The chemical composition and condition of enamel.

o Whether enamel is on primary or permanent teeth.

o Whether enamel is prism structure or primless.

o Whether enamel is fluoridated, demineralized, or stained.

An acid gel is generally preferred over a liquid because its application is

more controllable.

In vitro bond strengths of resin composite to phosphoric acid – etched enamel

typically ava 20 MPa.

Consequently if the preparation is completely bordered by enamel, acid

etching significantly reduces microleakage at the cavo-surface interface.

Alternative enamel etchants :

Because phosphoric acid is a relatively aggressive etchant that removes

substantial amounts of enamel, other demineralizing agents are :

1) EDTA Ethylene diaminetetracetic acid, a strong decalcifying agent,

promotes only low bond strengths to enamel, it doesn’t etch preferentially.

2) Pyruvic acid (10 %), buffered with glycine to a pH of about 2.2,

promotes high bond strengths to enamel, but has been found to be impractical

because of its instability.

3) Nitric acid, usually in a 2.5 % concentration organic acids such as

citric and maleic acid 10 % concentration and oxalic acid in a 1.6 % to 3.5 %

concentration.

CONFIGURATION AND CORRELATION OF ENAMEL WALLS :

The configuration of enamel walls is the shape, dimension, location, and

angulation of enamel components in a final tooth preparation.

The correlation is the relationship of the enamel configuration to surrounding

tooth preparation and restoration details.

It should be emphasized that although enamel is the hardest tissue in the

human body, it comprises one of the weakest points in a preparation wall,

especially when it loses its dentinal support.

The enamel rods (prism) are stronger than the interprismatic enamel, so when

ever enamel is stressed, it tends to split along the length of the rods.

Splitting is easier if the enamel rods are parallel to each other, if the rods are

interlaced and twisted together, this splitting will be somewhat difficult.

Fortunately, the enamel prisms are interlaced and twisted upon each other in the

inner one half to 2/3 of their thickness, while in the remaining outer portion of

the enamel, they are parallel. For an ideal enamel wall, Noy devised certain

structural requirements. These requirements tend to take full advantage of the

enamels hardness and strength and avoid the disadvantages of the enamel’s

splitting characteristics.

A) STRUCTURE REQUIREMENTS :

1) The enamel wall must rest upon sound dentin :

All carious dentin must be removed and the enamel cut back until it is

supported by sound tooth structure.

Otherwise, there would be some portion of the enamel left

standing that has been weakened by the dissolution of its minerals in backward

caries.

This enamel would most likely break down under the stresses of mastication

after a restoration was placed.

2) The enamel rods which form the cavosurface angle must have their inner ends

resting on sound dentin.

Noy suggests that when this condition is established, the dentin, which is elastic, gives

the enamel, which is brittle, a certain degree of elasticity, which is very important at

the margins of a restoration.

3) The rods which form the cavosurface angle must be supported, or be resting,

on sound dentin and their outer ends must be covered by the restorative

material.

This provides the strongest wall possible but it can only be produced by a

bevel of the cavosurface angle.

The second strongest configuration is when the plane of the enamel wall is

made parallel to the length of the rods.

The first situation can only be applied when the restorative materials

are stronger and tougher than tooth structure.

The second situation is the maximum that can be expected when the

restorative material is weaker and more brittle than the tooth structure.

4)The cavosurface angle must be so trimmed or beveled that the margins will

not be exposed to injury in condensing the restorative material against it.

This rule is applied particularly to class I cavities and to occlusal portion of

class II. i.e., in narrow occlusal cavities where the enamel rods are inclined

inward so that the cavosurface margin will be very sharp.

The latter pattern can make the enamel rods extremely susceptible to injury, so

they have to be trimmed or beveled to prevent this type of marginal failure.

B) GENERAL PRINCIPLES FOR FORMULATION OF

ENAMEL WALLS :

The direction of the enamel rod is one of the most influential factors in

dictating the number of planes, angulation, configuration, and correlation of a wall

which has enamel as part of its structural components.

There are some additional guidelines :

1) The enamel portion of a wall should be the smoothest

portion of the preparation anatomy, if it is not going to be etched.

Any roughness, besides interfering with the proximity of tooth to the

restorative material, will increase the possibilities of frail, loosely attached

enamel rods. Such frail enamel rods will be detached during function, increase

the leakage space in this critical marginal area.

2) Junctions between different enamel walls should be

very sounded, even if the junctions between the inner parts of the wall are

angular. Thus will improve adaptability of the restorative material at the

preparation corners, in addition to decreasing stress concentration there.

Furthermore, such rounding decreases the possibility of any frail, unsupported,

easily detachable enamel rods at the corners.

3) If inclining a preparation wall to follow the direction of

the enamel rods will nullify its resistance and retention capabilities, different

planes for that wall should be established.

4) When the preparation margins come to an area of abrupt

directional changes of enamel rods or an area where no rules for enamel rods

direction exist, thus area should be included in the preparation, and the margins

placed in areas of a more predictable rod pattern.

So good knowledge and understanding of the enamel pattern, especially

the direction of enamel rods in three dimensions, is very important for a proper

formulation of cavity details containing enamel.

C) ENAMEL PATTERN :

1) The direction of the enamel rods (prism) in different parts of the

crowns of the teeth.

Generally, the enamel rods (prisms) in the outer one – third to

one – half project in straight lines from the D EJ to the enamel surface.

For a clear, understandable illustration of the direction of the enamel

rods, one should have a reference of a fixed plane. Thus can either the long axis

of the crown, or the enamel surface itself, i.e., the tangent of the surface at the

point of the examination.

Directions of the enamel rods relative to the long axis of the crown

may be described as follows :

The rods at the center of the occlusal surface always lean to

pits or fissures towards the long axis of the crown.

The stronger the inclination of cusps, the greater the degree of

such slants.

On the periphery of the occlusal surfaces, i.e., near the tips of

the cusps and crests of the marginal ridges, the rods are inclined toward those

tips and crests, away from the long axis of the tooth crown. In between the rods

are parallel to this axis.

The rods on the axial surfaces are perpendicular to the long

axis of the crown only in the middle third. They incline incisally or occlusally in

the making an average of plus 360 occusally in the incisal or occlusal third.

In the gingival third, they incline by an average of minus 130

(toward the gingival) from the perpendicular to the long axis of the crown in this

area.

The rods stand perpendicular to the tangent of the enamel

surface in only two areas : the middle third of all axial surfaces and the middle

third of the distance from the tips of cusps and crests of ridges to their adjoining

fossae occlusally.

The rods in occlusal enamel generally radiate their heads

toward cusps tips and ridge crests at the outer third.

At the center of the tips of the cusps and crests of ridges they

open their head making an onion like section.

So the change in enamel rod direction from the occlusal surface to the

axial surface is abrupt at the intervening cusp tip or ridge crest.

In Anterior teeth :

The rods change their direction suddenly on the incisal edge, from labial to

lingual surface, showing onion like sections.

The change in direction takes place rather gradually on the

incisal angles from incisal to proximal, if the incisal angle is round.

In square incisal angles, the change in direction will be abrupt

from proximal to incisal surfaces.

These changes in direction can also be noticed over pronounced marginal

ridges and pointed cuspid tips.

On the axial surface :

The rods will make a right angle to the tangent of the enamel surface just at

their middle thirds.

In their incisal or occlusal thirds, they lean incisally or

occlusally, making an angle on the average of plus 240 with the perpendicular to

the tangent of the enamel surface in this area.

In the gingival third :

The enamel rods lean gingivally, making an angle of – 20, on the average, with

the perpendicular to the tangent of the enamel surface at this area.

2) Thickness pattern of enamel in different areas of the tooth crown :

The enamel will have its maximum thickness at the tips of the

cusps, and at the crests of triangular, marginal, and crossing ridges. It diminishes

in thickness going occlusally to the depth of the pits, fissures and grooves.

At the inner one third of the occlusal inclined plane it may

have a thickness of 0.2 – 0.5 mm. At the depth of the pits and fissures there is no

enamel coverage in most cases.

Enamel also diminishes in thickness going gingivally on the

axial surfaces, with its least thickness at the CEJ. At the cervical third of the

axial surfaces, the thickness may be as low as 0.2 mm.

For anterior teeth, the maximum dimension of enamel is at

the incisal ridge (slopes in the canine) and it diminishes gingivally to the CEJ on

the axial surfaces.

The lingual enamel plates are generally thinner than the

facial, a pattern more apparent in anterior teeth than in posterior ones.

With increasing age, enamel is decreased in thickness at the

occluding areas as a result of attrition. Also, enamel mineralization and

dehydration increased by age, increasing the brittleness and crazing tendency of

enamel.

DESIGNS OF DIFFERENT CARIOUS LESIONS :

Cavities are not all of the same shape. They are prepared in very definite ways

depending upon the location of the lesion. The various types of prepared cavities and

the general directional paths of the rods involved.

Some of the non carious enamel and dentin is removed with

the diseased material so that the restorations will rest on sound tissue.

In thus way not does the restoration have a firm base, but the possibility

for reinfection is lessened.

1) Enameloplasty :

It is the judicious reduction of the surface of the enamel, thereby converting a

shallow fissure into a smooth – based groove.

When such fissures are shallow, penetrating perhaps less than

one half the enamel thickness, removal may be accomplished without further

extension by enameloplasty.

Enamel thus removed is not covered by the restorative

material, but becomes a part of the functioning occlusal or axial surface.

Such a procedure is frequently done when the shallow fissure

encroaches upon a marginal ridge area.

2) Management of enamel crazing (microcracking)

If it is confirmed that a tooth demonstrates the signs and symptoms of cracked

tooth syndrome, and / or if cracks are numerous and have an unlimited extent,

care should be taken not to involve them in the tooth preparation. The tooth in

these cases will need an amalgam foundation, then restoration with a reinforcing

or protecting type of cast restoration, such as an onlay or full veneer casting, to

splint together the separated portions of the tooth.

If cracks are limited in number and penetrate enamel only, with no traces in

dentin, enamelectomy may be performed and the reduced areas to be involved in

the final cavity preparation.

If cracks are limited in number, and only part of the enamel is involved,

enameloplasty can be tried until the cracks are eradicated.

Then the thickness and nature of the remaining enamel should be evaluated

relative to the future restoration.

If this enamel is of sufficient dimension and maintains the capacity to support

marginal tooth structure, it should be left untouched. Otherwise the area should

be included in the final cavity preparation.

3) Enamel wall designs :

a) Occlusal enamel wall design :

In the preparation of occlusal cavities it is

essential for many restorative materials that the enamel walls parallel enamel

rod direction leaving no unsupported enamel.

When following this principle, the variations in enamel rod direction can

result in cavity wall angulation ranging from.

Convergent, to parallel, to divergent depending

upon the buccal- lingual width of the prepared cavity.

Studies has shown that cavity preparations with an intercuspal distance

of ¼ to 1/3 could utilize any of the three cavity design concepts and still result in

sound enamel at the cavosurface margin.

Knowledge of the ever changing pattern of

enamel rod direction from the central sulcus outward is helpful to the clinician

in preparing occlusal walls, testing of the enamel immediately adjacent to the

cavosurface angle using moderate force on a sharp chisel or hatchet is a practical

means of determining enamel integrity.

Thus, the intercuspal width of the cavity as

influenced by a variety of factors determines the convergent, parallel or

divergent angle of the occlusal walls.

Certainly this angulation will relate to the extent of the tooth tissue lost due to

caries and the type of restorative material selected.

Modification of the resulting enamel cavity

walls by tapering or the cavosurface angles by beveling may be indicated for

adapting and finishing certain materials.

The angle of cuspal incline is an additional

factor to be considered when beveling.

b) Proximal enamel wall design :

The basic design principles for preparation of enamel walls for proximal –

occlusal cavities is similar in the occulusal area to that of a class I cavity.

The buccal, lingual and gingival walls of the proximal box also require special

attention to conform with the principle of paralleling enamel rods.

In cervical region enamel rods direction varies, enamel rods can be directed

occlusally, horizontally or apically as one progresses from the occlusal surface

to the cementoenamel junction.

In the case of a cavity prepared for amalgam, an enamel hatchet, or a GMT

should be used to plane the gingival wall perpendicular to the outer tooth surface

or at an apically declined angle depending upon its approximation to the CEJ.

In cast gold restorations placement of a bevel with a rotary instrument would

not only assure adequately supported enamel, but also facilitate restoration

finishing.

In box form cavity design the buccal and lingual proximal walls of a prepared

proximal – occlusal cavity should meet the cavosurface at 90 degrees to assure

supported enamel rods.

Further modification to the proximal margins by increasing the cavosurface

angle may be made for certain restorative materials. e.g., changing the

angulation of the proximal enamel walls by slicing, beveling or merely

enhancing flare is used to provide convenience in adapting cast gold margins to

the enamel.

Facial – lingual enamel wall design – class 5 :

Enamel wall design for class 5 cavities requires similar consideration in the

angulation of both occlusal and cervical enamel walls to that given the gingival

wall of the class 3 preparation.

There is similar wide variation of enamel rod direction the facial and lingual

surfaces as is noted on the proximal surfaces.

The cavosurface angulation of the occlusal wall must become more obtuse as

the position of this margin approaches the occlusal surface of the tooth.

The cavosurface angle of the occlusal wall of a class 5 cavity may require an

increase to 120 to 130 degrees in order to assure a sound enamel margin.

Testing such margins with hand instruments for the friability of the enamel is

again a useful clinical guide.

c) Cuspal protection :

Cavity designs that include cuspal protection must

demonstrate should enamel cavosurface margins regardless of whether the finish

line is on the outer or inner cuspal inclines.

Enamel rod direction in cusp tip areas varies dramatically.

The determinant for protection of a cusp should be based on

the amount of tooth structure lost and the dentinal support of the remaining

cusps.

Tetracycline staining :

This group of broad spectrum antibiotics has an affinity for calcified tissue. It

can cross the placental barriers to affect the deciduous teeth of the drug is taken by

infants and young children the developing permanent teeth will usually be discolored

treatment of developmental defects.

The aim of treatment is entirely cosmetic, where molar

hypomineratization results in chipping of the enamel and plaque stagnation, a

restoration may be needed so that the patient can clean.

Bleaching may be effective in some cases of tetracycline

stain, but in most cases composite restorations or veneers of composite or

porcelain, often with little or no tooth preparation, are the treatment by choice.

Crown may be necessary in most severe cases.

Fluoridation (Process of mineralization) :

If the fluoride ion is incorporated into or adsorbed on the

hydroxyapatite crytal, the crystals become more resistant to acid dissolution.

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 unightly mottling.

The semipermeable nature of enamel enables topical

fluorides, fluoridated tooth pastes, and fluoridated drinking water to provide a

increase concentration of fluoride in the surface enamel of erupted teeth.

Also the fluoride in surface enamel reduces the free energy in thus

region and thus lessens the adsorption of glycoproteins from the saliva.

The presence of F enhances chemical reactions that lead to

the precipitation of calcium phosphate.

An equilibrium exists in the oral cavity between Ca and

phosphate ions in the solution phase (saliva) and in the solid phases (enamel),

and fluoride shifts this equilibrium to favour the solid phase.

Clinically when a localized region of enamel has lost mineral

(e.g., a white spot lesion), it may be remineralized if the destructive agent dental

plaque) is removed. The remineralization reaction is greatly enhanced by

fluoride.

Emdogain :

Enamel matrix protein furcal repairs, bone loss.

There are no limits to science. Each advance merely widens the sphere of

exploration.

Enamel margin design.

The cavosurface angle of tooth structure formed by free junction of a prepared

walls the external surface of due tooth.

Enameloplasty, sealants and preventive resin or conservative composite

restoration prophylactic odontomy.

ENAMEL BONDING WITH SELF – ETCH RESINS:

Many studies have shown that enamel bond strengths don’t depend on the

aggressiveness of the conditioner or the length of the resin tags.

Bond strength also depends on the strength of adhesive resins and the tooth

substrates.

Un-ground enamel surfaces are hyper mineralized and contain more fluoride

than does ground enamel, and they appear to be poor substrates for self –

etching adhesive systems.

Caries in enamel :

Dental caries is a dynamic process of alternating demineralization and

remineralization.

According to researcher kidd, “sound enamel may become carious in time if

plaque bacteria are given the sugary substrate they need to produce acid.

However, saliva is an excellent remineralization yield particularly if it

contains the fluoride ion.

1) Use magnifying loupes or intraoral camera magnification.

2) Use excellent lighting (operatory head lamp or fiber optic)

3) Clean and dry the teeth (ideally with an air abrasion cleaning device or

a water abrasion device such as prophy Jet or prophy flex.

4) Use caries indicators and caries detection devices.

5) Transilluminate teeth, especially along the dentin –enamel junction.

6) Use accurate bitewing radiographs and view them with magnification.

7) To avoid binding in the grooves, use only light pressure with an

explorer. An explorer is best used as a diagnostic device to determine only

the width and depth of a fissure orifice.

The smith and knight index.

This requires the surface of each tooth to be given a score between

0 and 4 according to its appearance.

Score Surface Criterion

0 BLO1 No loss of enamel characterize

C No changes of contour

1 BLO1 Loss of enamel surface characterize

C Minimal loss of contour

2 BLO Enamel loss just exposing dentin < 1/3 of the surface

1 Enamel loss just exposing dentin

C Defect less than 1 mm deep

3 BLO Enamel loss just exposing dentin > 1/3 of surfaces

1 Enamel loss and substantial dentin loss but no pulp exp.

C Defect 1-2 mm deep

4 BLO Complete enamel loss, or pulp exposure or 20 dentin exposure.

1 Pulp exposure, or 20 dentin exposure

C Defect more than 2 mm deep, or pulp exposure or 20 dentin

exposure.

Acid etching :

60 – second etch with 30 to 50 % non buffered phosphoric acid achieves the

strongest bond between enamel and resin. In general, lower concentration of acid

selectively remove inorganic material from the organic matrix of the enamel surface,

there by creating faster and deeper channels. Higher concentration are less effective

since they are nonselective and are more likely to denude the surface.

Oral development and histology :

Structural features of enamel :

Structure features Developmental origin Clinical relation

1) Enamel rod Secretory product of one

ameloblast from the distal or

interdigitating portion of tomes

process

Confers strength of the

enamel, paths are

important in cavity

preparations

2) Enamel spindle Extension of an odontoblast

process and tubule across the

basal lamina during the initial

stage of matrix formation

No major clinical

significance but may

confer additional

permeability to the deeper

layers of enamel

3) Enamel tufts Hypomineralized areas of

enamel (rich in enamielin) near

the DEJ formed during the

initial stages of matrix secretion

; resemble “tufts of grass”

No major clinical

significance, but represent

areas of enamel weakness

4) Enamel lamellae Hypomineralized areas of

enamel extending from the DEJ

for considerable distances into

the enamel

Represent a significant

weakness in the structure

of enamel and is

susceptible to cracking

5) Cracks May occur naturally, especially

in hypomineralized areas

between enamel rods ; may be

the result of lamellae ; may be

distinguished from lamellae in

that they arise from the enamel

surface and contain salivary

proteins

Significant weakness in

enamel ; prone to breaking

and caries

6) Hunter –schreger

bands

Viewed in ground sections with

incident light and represent

differences in the pattern of

sectioning of enamel rods

Of no clinical significance

7) Gnarled enamel Twisting of enamel rods in the

cusps of teeth due to the small

radius of rotation of

ameloblasts during secretion

May confer some strength

to the enamel

8) Enamel pits Found between cusps ;

represent thin areas of enamel

matrix due to the crowding of

ameloblasts during

development

Significant area of caries

development ; difficult To

Whom It May Concern:

clean ; areas are often

treated with sealants

9) Incremental lines

a) Neonatal line

b) Retzius strial

c) Cross striations

All are formed due to the

cyclical activity of

ameloblasts ; represent

hypomineralized areas or are

due to small variations in rod

orientation ; during significant

physiologic changes (birth and

illness) these lines are

accentuated or hypomineralized

; cross striations have been

explained as being due to

sectioning of enamel rods

across rows

Banding patterns formed

during illnesses will show

upon contralateral teeth

which are developing at

the same time patterns of

enamel hypoplasia on a

single tooth or on one side

indicate trauma or a

localized rather than

systemic infection

10) Perikymata Represent the external

boundary of Retzius strial

No real clinical

significance

11) External layer of

prism less enamel

Formed during the latter stages

of enamel secretion by the

proximal part of Tomes process

after the distal portion is lost ;

this layer in thicker on prior

This layer must be

removed by acid etching to

create “tags” prior to the

application of orthodontic

appliances or bonding

teeth. agents

12) Enamel cuticle Formed by the remnants of the

reduced enamel epi and its

secretory products ; it is quickly

lost

Of no major clinical

significance

13) Enamel pellicle Formed after the tooth is in oral

cavity ; acquired from saliva

and the oral flora

May contain factors which

hinder the attachment of

bacteria to tooth surfaces

Clinical application of cracks :

The development of cracks or fractures can be a very serious dental

complication requiring a crown and endodontic treatment. It is important to recognize

fractures before they progress to a level below the gingival or through the pulpal

floor.

Symptomatically, patients will experience pain upon biting on hard objects

cracks or lamellae can often be seen in the dental office by transillumination with the

use to fiber optics.

It is also important to distinguish cracks from crazed enamel.

Crazed enamel is minute cracks most often seen in anterior teeth, which are of

little consequence.

Clinical application for strial of retzius :

Caries spreads more rapidly in dentin than enamel. The reason for this spread

is not strictly the mineral content, because in acid solutions enamel demineralizes

more quickly than dentin. The low organic content of enamel doesn’t provide a

nutrient source for the growth of bacteria or production of acid. In dentin, exposed

and degraded collagen can serve this purpose.

Additionally, in enamel the Retzius strial (or lines) are planar structure of

increased organic content. These strial have been proposed as preferential areas for

the spread of caries with in the enamel. However the change in crystal direction at

these site may actually impede the progress of caries.

Enamel margin design :

Bonding to enamel changes cavity preparation design because less tooth

reduction in necessary.

Enamel discing :

Freshly cut enamel provides a better bonding surface than non prepared

enamel.

The outer layer of all deciduous teeth and 70 % of permanent teeth contains

aprismatic enamel (i.e., enamel that lacks uniform prisms) and provides less

mechanical retention when etched.

In permanent teeth, this layer is usually 30 m thick and is most prevalent in

gingival areas.

Discing off 0.1 mm of the enamel removes this layer, which improves bond

strength 25 to 50 %, depending on the amount of aprismatic enamel present.

Non prepared enamel also often contains fluoride, which makes it acid

resistant.

Preparation angles :

Etching the ends of enamel rods produces the greatest benefit. Exposing cross

– sectional areas of enamel allows the formation of longer tags than does

exposing longitudinal sections.

A 900 exit angle is useful when it is desirable to maintain maximum tooth

structure. However, thus exit angle does not expose the ends of the enamel rods

is less retentive.

A 450 bevel 15 most commonly used finish line. This design conserve the

tooth structure and expose the rods.

Compared with the 900 exit, the 450 bevel provides a superior seal for enamel,

particularly at the gingival margins. Less microleakage compared with

preparations in which the sides of enamel rods are etched.

A concave bevel (changer), made by exposing the maximum surface area of

enamel rod ends, is the most retentive finish line. The changer also allows a 90

degree angle of exit for restoration, making it more durable margin, particularly

at occlusal contact points.

However it is the least conservative design, the concave bevel should be used

only when maximum retention from acid etching is necessary (e.g., the class IV

fracture).

Some research demonstrates that a joining convex bevel seals better than the

three conventional designs. This design may not always be clinically practical,

but it illustrates that rounded areas of unsupported enamel provide excellent

exists for bonded resins but make it more different to remove carious dentin

from D.E.J.

CONCLUSION :

Every preventive measure should be taken to prevent loss of this precious

component of teeth.

REFERENCES:

1)Oral histology and embryology.10th edition

ORBANS

2)Dental anatomy,histology and development.

BHALA JI

3)Oral anatomy, histology and embryology.

B.K.B BERKOVITZ

4)Oral histology, inheritance and development.2nd edition

D.VINCENT PRRROVENZA

5) Oral histology, development, structure and function.5th edition

RICHARD TENCATE

6)Operative dentistry, modern theory and practice

M.A MARZOUK

7) Principles and practice of operative dentistry.3rd edition

CHARBENEAU

8)Art and science of operative dentistry.4th edition

STURDEVANT

9)Oral pathology

SHAFER

10)Oral development and histology.3rd edition

JAMES K.AVERY

11)Restorative dentistry

A DAMEIN WALMSELY

12) Tooth colour restorative

ALBERTS

13) Studies on the structure of human enamel by the replica method.

J DENT RES 1970.

14) Ultrastructure of organic films on enamel surfaces.

CARIES RES;10(1):19:1976.