ENAMEL DENTINAND CLINICAL SIGNIFICANT

INTRODUCTION

Enamel is a hard mineralized tissue that covers the anatomic crown of a tooth.

Tooth enamel is unique in many ways. Mature enamel is the only tissue that is totally acellular, In addition enamel is a mineralized epithelial tissue, while dentin, bone and cementum are mineralized connective tissue

The study of enamel requires special tissue preparation, as it is every heavily mineralized tissue.

PHYSICAL CHARACTERISTICS The high mineral content makes enamel the hardest tissue

in the human body. This hardness enables it to withstand the heavy forces applied during mastication.

The nature of its structure and the hardness make enamel very brittle having a low tensile strength. Thus the underlaying layer of resilient dentin helps in maintaining the integrity of the enamel.

Enamel is translucent and its color varies from light yellow to grayish white.

The thickness of enamel varies from a maximum of 2.5 mm at the cusp tips to less than 100 um at the neck of the tooth and along the pits and fissures.

The color is influenced by the thickness of enamel. Thinner areas appear more yellowish as the underlying dentine is seen through the enamel.

The density of enamel is 2.8 –3.0 grams/ ml and has a knoop hardness number of 343.

CHEMICAL PROPERTIES

Enamel is composed of both inorganic and organic substance.

Mature human enamel is made of 96% inorganic and 4% organic material and water.

The inorganic content is basically hydroxyapatite crystals and various ions such as strontium, magnesium , lead and fluoride.

The organic content forms a fine network between the inorganic crystals. The organic material is mostly tyrosine rich amelogenin protein and non-amelogenin proteins.

THE STRUCTURE OF ENAMEL

RODS The basic structural unit of enamel is the enamel

rod [prism] The enamel rod is a very long, thin, structure extending from the DEJ to the surface of the enamel.

The enamel rod follow a tortuous course thus the length of an enamel rod may be greater than the thickness of enamel.

Each enamel rod appears to be encased in a rod sheath the sheathed rods are cemented together by an interred substance.

Of these three structures, the enamel rod is the most highly mineralized and the rod sheath the least mineralized. However all the three tissues are extremely hard.

The enamel rods have an average diameter of 4-5um DEJ towards the enamel surface.

The number of enamel rods vary from 7.5 million in the upper molars.

The average length of the rod in the mid crown region is 0.2mm. The rods are larger at the region where the enamel is thick i.e. at cusp tips and are shorter at the cervical region.

In cross section the enamel rods may appear round or oval or hexagonal in shape.

Each rod has a head and a tail. The tail resides between two heads of adjacent road. The bodies of the rods are closer to the occlusal or incisal

surface while the tail points in a cervical direction. The enamel rods are arranged perpendicular to the DEJ,

except in the cervical region where they are inclined towards the gingival. In case of deciduous teeth, the rods are arranged parallel to the incisal or occlusal area. Thus the rod forms a right angle with a tangent drawn to the tooth surface.

The enamel rod is made up of numerous crystals. These crystals are arranged with their long axis parallel to that of the longitudinal axis of the rod. This pattern of arrangement is true for crystals along the central axis of the rod. Crystals that are more peripherally placed flare laterally as they approach the periphery. The crystals are arranged in a different direction in the inter rod region. The boundary where the crystals of the rod meet those of the interred substance is known as rod sheath. At the rod sheath the crystals meet at right angles and the region contains more enamel proteins.

The enamel crystals are 30 um thick by 65 um wide, and several micrometers in length. The enamel crystals are slightly deficient in calcium and show minor ion substitution in the crystal lattice such as strontium magnesium, lead and fluoride.

STRUCTURAL FEATURES OF ENAMEL

The enamel may exhibit a number of features that characterize the tissue as more complex than the schematic view of enamel rods and crystals presented thus far. The following are some of the features of enamel

INCREMENTAL LINES OF RETZIUS The Lines of Retzius seen in ground sections are comparable to the

concentric growth rings seen in the cross-section of tree trunks. In longitudinal sections of the teeth, these lines are seen as brownish

bands that surround the tip of dentin. In cross section they appear as concentric rings.

The lines of Retzius represent the incremental nature of enamel depositions.

A number of possible explanation have been put forward to explain the formation of these line:

1. According to same workers these lines represent a hypomineralized or rhythmic formation of the enamel.

2. According to another, these lines represen tareas of periodic bending of the enamel rods.

3. Another possible reason for this appearance is the temporary constriction of the Tome’s processes. This is associated with a corresponding increase in the secretory phase of the ameloblasts this results in alteration of enamel structure along the line.

4. Variations in the basic organic structure could be another factor that is responsible for these lines  

The incremental lines are more frequently seen in permanent teeth and less frequently in deciduous and prenatal enamel. The occurrence of a few lines are considered normal, but when they are present in grate numbers or as a broad band it indicates periods of metabolic disturbance or disturbance in amelogenesis.

HUNTER SCHREGER BANDS The Hunter schreger bands are alternating dark [diazones]

and light zones [para zones] seen in longitudinal ground sections when viewed under-reflected light. Their appearance gets reversed when viewed under transmitted light.

These bands originate at the DEJ and pass outward traversing more than half of the enamel. They are not seen towards the enamel surface.

A number of possible explanation are given for the appearance of these bands. The following are some of them:

1. These bands occur due to changes in rod direction i.e, one group of rods extends towards the surface with a mesial dirft while the adjacent group might show a distal drift.

2. Some investigators suggest that they are composed of a slightly different content of organic material.

3. They may occur due to difference in permeability.

GNARLED ENAMEL

The enamel rods below the cuspal and incisal region appear irregular, twisted and intertwised, such a kind of enamel is called gnarled enamel.

Unlike the Hunter schreger bands, gnarled enamel extends through out the thickness of the enamel at the cusp tips and incisal edges. This arrangement of enamel rods is believed to aid in resisting the high masticatory loads that the cusps have to bear.

ENAMEL LAMELLAE Enamel lamellae extend from the enamel surface

towards the DEJ. The enamel lamellae are primarily organic and

represent improperly mineralized enamel, which is a failure of the process of removal of organic matrix and water during mineralization.

The enamel lamellae can extend in a longitudinal or in a radial direction. They can be seen running vertically on the crown from the incisal or cuspal area to the cervical region.

These areas may develop into a crack that is filled with surrounding cells in case the crack occurred prior to eruption. Cracks that occur after tooth eruption are filled with organic material from the oral cavity.

The enamel lamellae are classified into three types They are:

Type A: They are lamella composed of poorly calcified rod segments.

Type B: They are lamellae consisting of degenerated cells.

Type C: They are cracks seen in erupted teeth that are filled with organic matter or debris from saliva.

ENAMEL TUFTS Enamel tufts are organic structures that originate at

the DEJ and extend into enamel for about one third of its thickness.

The enamel tufts resemble tufts of grass and are areas where young enamel proteins are not completely transformed during maturation.

Thus in these areas there is slightly more matrix at the prism borders and possibly more matrix between the crystals of the enamel rods.

Developmentally, they are formed due to the abrupt changes in the rod direction which lead to different ratio of interred and rod enamel, creating less mineralized and weakened planes.

Enamel spindles are odontoblastic processes that went astray, crossed the DEJ.

DENTINOENAMEL JUNCTION

The DEJ is a scalloped interface between the enamel and dentin small curved projections of enamel fit into small concavities of the dentin.

The DEJ in a fully developed tooth separates the mineralized layers of enamel and dentin.

During tooth development, a basal lamina separates the secretary fronts of the odontoblasts and the ameloblasts. This layer is lost as the tissues undergo mineralization.

Very rarely the odontoblastic process may cross the DEJ and get embedded in the enamel . These are called enamel spindles.

APRISMATIC ENAMEL

The outer most and the inner most enamel is rodless as it is produced without the Tome’s processes.

AT the surface it is 30um thick. The crystals are oriented at right angles to

the line of Retzius. This kind of enamel is more mineralized.

ENAMEL CUTICLE

The enamel cuticle or primary cuticle is a structure less membrane seen on the crown of tooth, adhering firmly to its surface.

It is mostly seen in newly erupted teeth and is lost due to mastication.

It is about 0.5 to 1.5 mm thick. Enamel cuticle is actually made of two cuticles.

Primary enamel cuticle and secondary enamel cuticle.

Primary enamel cuticle is the last product of the enamel forming ameloblasts and it becomes mineralized the secondary enamel cuticle covers the primary cuticle and is a product of the reduced enamel epithelium and is not mineralized.

DENTINE

INTRODUCTION

Dentine is a mineralized tissue that forms the bulk of the tooth.

In the Crown it is covered by enamel, in the root by cementer

It is rigid but elastic tissue consisting of large numbers of small parallel tubules in a mineralized collagen matrix.

The combine of enamel & dentine provides a rigid hard s

Structure suitable for tearing & chewing that resist both abrasion & fracture

PHYSICAL PROPERTIES

1. Fresh dentine is pale yellow in color and contributes to appearance of the tooth through the translucent enamel.

2. Harder than bone and cementum but softer than enamel. KHN is 68.

3. Greater compressive, tensile and flexural strength than enamel:-

Compressive strength – 262MN/m2

Tensile strength 33MN/m2 Stiffness – 12GN/m24. Density of 22.1gm/ml5. Dentine is permeable, depends on the size and

patency of the tubules which decline with age.6. Dentine is distinguish with enamel by two major

properties-a. Dentin is sensitiveb. Dentin is formed through out life.

COMPOSITION

The gross composition of dentin is approximately 

70% inorganic 30% Inorganic 20% organic 10% Water OR BY WEIGHT 30% Organic 50% Inorganic 20% Water

Inorganic compoundsi. Calcium 26.9 wt%ii. Phosphorus 13.2 wt %iii. Carbonate 4.6 wt %iv. Sodium 0.6 wt %v. Magnesium 0.8 wt % Organic compoundsvi. 90% of the organic matrix is collagen mainly Type I

collagen is present but Type III & V also traceable.vii. Some amount of citrate, other, protein-

carbohydrate complexes insoluble protein & lipids are also present.

viii. Some growth factors like, insulin like growth factor & transforming growth factors are also present.

STRUCTURE OF DENTIN

DENTINAL TUBULES

Dentinal tubules are sigmoid shaped curved structure which run perpendicularly from the pulp towards the periphery.

There “S” shaped curves preprimary curve turns. At the tooth tip incisor edge & cup tips the

dentinal tubules are almost straight with no primary curveting.

The first curvature of these dentinal tubules from the pulp is towards the apex of the root.

This primary curvature represent the pathway traversed by Odon oblasts & are produced because of he crowding of Odon oblast as they care near the pulp.

Along the course of the dentinal tubules at every few incisors small curvature are seen which are called secondary curvature

The dentinal tubules are thicker near the pulped side that at DEJ

Toward the pulp it is 2.5 micrometer, where as near DEJ 1-1.2 m

As the surface of dentin is more at the DEJ that the pulped side the tubules are widely separated at the DEJ where as they crowded are pulped side.

The density at pulped side is 4-5 time more than near DEJ side

The root dentin has lesser tubules that crown dentin.

The buccal & lingual dentin has more tubules than mesial & distal

The dentinal tubules show breaching at the terminal part, this increase the number of tubules at DEJ which responsible in increased sensibility at this area.

This dentinal tubules also show branching & Loop formation near the cementum.

This area is seen as a granular layer in ground section The dentinal tubules contain the Odontoblastic

process which is layers called Lamina laminate. The Odontoblastic process contain microfilaments

vesicles & ribosome’s in the DEJ toward the pulpal side endoplasmic restriction & Mitochondria are more.

Large vacuoles containing hydrolytic enzymes are seen in center of the process.

Odontoblastic cell bodies measure 7nm diameter & 90nm in length.

DENTIN ENAMEL JUNCTION.

The junction between the enamel & dentin is irregular & is scribed as scalloped having convexities & concavities

The convexities fuse the dentin where as concavities face the enamel

During tooth development the membrane perforation ( I e based lamina which separate the enamel & dentin) sparks the Odon lobules & ameloblastes.

When this basal lamina lost the cells secrete their matrices along this scalloped junction, which get mineralized forming the structure lisle ridges & valleys along DEJ.

INTRA TUBULAR (PER TUBULAR DENTIN)

The dentinal tubules have a hyper mineralized area of dentin on their or walls this dentin is called intra tubular.

This dentin is 4micrometer wide near the DEJ The intra tubule dentin has more mineral cement

&less collagen that the inter tubular dentin. The continuous deposition of intra tubular dentin

causes reduction in the size of the tubular lumen When the intra tubular dentin obliterates several

dentinal tubules it causes the formation of sclerotic dentin.

The intra tubular dentin is usually not formed in young developing teeth

INTER TUBULAR DENTIN

This is dentin which present between the dentinal tubules

The inter tubular dentin contains greater amount of organic matrix causing of tightly interwove net work of type I collagen, phosphoprotein proteoglycans glycoprotein & plasma proteins

The inter tubular dentin form before intra tubular dentin.

PRE DENTIN

Pre dentin is the inner most portion of dentin that is not mineralized & is present adjacent to the pulped tissue

The pre dentin represents the first formed dentin that can be compared to the steroid that forms during osteogenisses

The thickness of pre dentin is 10 – 47 nm. As the collagen fibers present in the pre

dentin undergo mineralization. The pre dentin gets converted to dentin & thus a new layer of pre dentin forms around the pulp tissue.

GRANULAR LAYER OF TOME’S

This is granular layer seen in ground sections of roof dentin.

It appears dark in transmitted light & lighter under reflected light.

The amount of granules increase from the CET to the apex of the roof.

The ground layer represents the looped terminal portion of the dental tubules in the root dentin.

Between the tome’s granular layer & the cement issue is formed a structure less hyaline layer of 15 m width.

This layer is called as nope well layer & thought to be enameled.

TYPES OF DENTIN

PRIMARY DENTIN Most of the tooth is formed by primary dentin

which outlines the pulp chamber & is referred to as “Circumpolar dentin”.

The outer layer is called “ Mantle dentin” & differs from the rest of the primary dentin in the way it is mineralized & in the structural interrelation be of the collagenous & non collagenous matrix components.

SECONDARY DENTIN Secondary dentine develops after root promotions has been

completes & represents the continuity but mach slower deposition of delivery by odontoblasts.

Secondary dentin has a tubular structure that through less regular is for the most part continuous that of the primary dentine.

The ratio of mineral to organic materials the same as for primary dentine.

Secondary sent in not deposit every day around the periphery of the pulp chamber especially in the molar teeth

The operator deposits of the secondary dentine on the roof & floor of the pulp chamber lead to an asymmetric reduction in its size & Shape.

These change in the pulp space clinically referred to as pulp recession can be detected on radiograph.

Scene evidence suggest that the tubules of secondary dentin scleroses (flick calcified material) more readily that those of primary dentin.

This process tends to reduce the overall permeability’s of the dentin thereby protecting the pulp.

TERTIARY DENTIN (ALSO REFERRED RELATIVE OR RE PURCHASE DENTIN)

Tertiary dentin is produced in reaction to various structure such as attrition, caries or a restorative dental procedure.

Unless primary or secondary tertiary dentin pulp along the entire pulp dentin border

Tertiary dentin is only procedure by more cells directly affected by the stimulus.

The quality or quantity of tertiary dentin produced are reputed to the cellular response initiated which depends on the intensely & duration of the stimulus.

Tertiary dentin may resemble secondary dentin when it have a regular tubular structure.

It may have few &/or irregularly arranged tubules. The term reactionary dentin refers to the dentin forming, in response to

an insult in which although some damage has been sustained & serve Odontoblast die the existing Odontoblast recover & the tissue to form dentin.

The term reparative dentin refers to dentin forming after a stimulus in which the original Odontoblasts in the associated region have been disrobed & New calcified tissue has been formed by Odontoblast - like cells.

CLINICAL SIGNIFICANCE OF ENAMEL Many of the structure features of enamel are

actually 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 much more conservative approach by utilizing the phenomenon of remineralization and reducing the need for the removal of sound tissue.

ACID ETCHING

Acid etching or conditioning, has been an important technique in clinical practice. It is involved in the bonding of restorative material to enamel.

It achieves the direct effect in two stage-1. First it remove plaque and other debris along

with a thin layer of enamel.2. Second, it increases the porosity of the exposed

surface through selective dissolution of crystal which provides a better bonding.

Sever types of concentration of acid are used to ulter the enamel.

Most etch the surface to depth of 10micro meter, like 30% to 40% phosphoric acid.

Three etching patterns are predominant 1. The most common is type I, characterized

by preferential of rod core.2. In the reverse, type II, rod periphery is

preferential is removed.3. Occurring less frequently, type III, which is

irregular and indiscriminate. There is still debate, why acid etching produce

different surface patterns.The most commonly view is that the etching

pattern depends on crystal orientation.

In Summary, “ACID CONDITIONING OF ENAMEL

SURFACES IS NOW AN ACCEPTED PROCEDURE FOR OBTAINING IMPROVED BONDING OF RESTORATION TO ENAMEL.”

ENAMEL ADHESION 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.

a. The concepts of large preparations and extension for prevention, proposed by Black in 1917, have been replaced by smaller preparation and more conservative technique.

Advantages of adhesionb. Reduces microleakage at tooth interface.c. Prevention of microleakage or the ingress of oral fluids and

bacteria along the cavity wall.d. Reduces clinical problem like, postoperative sensitivity,

marginal staininge. Adhesive techniques allow deteriorating restorations to be

replaced with minimal or no additional loss of tooth material.

ADHESION TO ENAMEL

Enamel acid etching techniqueAcid etching transforms the smooth surface with a

high surface free energy more than unetched enamel.

Acid etching removes about 10micrometer of enamel surface and creates a micro porous layer from 5-10 micrometer.

Two types of resin tags have been described-1 Macrotages, are formed circularly between

enamel prism peripheries.2 Microtages, are formed at the core of the enamel

prisms, where the monomers cures into a multitude of individual crypts of dissolved hydroxyapatite.

The effect of acid etching depends on several perameters-

1. The kind of acid used2. The acid concentration3. The etching time 4. The form of the acid (gel, semi gel, aqueous solution)5. whether the enamel is instrumented before etching The acid gel is preferred over liquid, because its

application is more controlled. Because phosphoric acid is relatively aggressive etchant,

other demineralizing agents are also used, these are,EDTAPyruvic acid,10%Nitric acid2.5%, organic acid such as citric acid and maleic

acid10%

CONFIGURATION AND CORRELATION OF ENAMEL

THE CONFIGURATION OF THE ENAMEL WALLS IS THE SHAPE, DIMENSION, LOCATION AND ANGULATION OF ENAMEL COMPONENT IN THE FINAL TOOTH PREPARATION.

The correlation is the relationship of the enamel configuration to supporting tooth preparation and restoration details.

It should be emphasized that although the enamel is the hardest tissue in the body, it comprises one of the weakest point in the preparation wall, especially when it losses its dentinal support.

STRUCTURE REQUIREMENT

1. The enamel wall must rest upon sound dentin. All carious dentin should removed and the enamel cut

until it is supported by sound tooth structure.2. The enamel rods which forms the cavosurface

angle must have there inner ends resting on the sound dentin.

Noy suggests that when this condition is established, the dentine provides its elasticity to enamel, which is important at the margins of the restoration.

3. The rods which forms the cavosurface angle must be supported or be resting on sound dentin and there outer ends must be covered by the restorative materials.

It can only produced by bevel of the cavosurface margin.

4. The cavosurface angle must be so trimmed or beveled that the margin will not be exposed to injury during condensing the restorative material.

This rule apply particularly to class I cavity and occlusal portion of class II cavity.

GENERAL PRINCIPLES FOR FORMATION OF ENAMEL WALLS

The direction of the enamel rods 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 component.

There are some additional guidelines:-1. The enamel portion should be the smoothest portion of the

preparation anatomy, if it is not going to be etched.2. Junction between different enamel walls should be very

rounded, even if the junction between the inner walls are angular., this provide adaptability of the restorative material.

3. When the preparation margins come to an area of abrupt directional changes of rods or an area where no rules for enamel rods direction exist, this area should included in the preparation.

SO GOOD KNOWLEDGE OF ENEMAL PATTERN AND DIRECTION OF ENAMEL RODS IS VERY IMPORTANT FOR PREPARATION WITH PROPER FORMULATION.

ENAMEL WALL DESIGNS

OCCLUSAL ENAMEL WALL DESIGNIt is essential for restorative materials in the preparation

of occlusal cavity, that the enamel walls parallel to enamel direction leaving no unsupported enamel.

When following this principle, the variation 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.

Modification of the resulting enamel cavity walls by tapering or cavosurface angles by beveling may indicated for adapting and finishing certain materials.

The angle of the cuspid incline is an additional factor to be concidered when beveling.

PROXIMAL ENAMEL WALL

Basic design is same for proximal-occlusal cavities, as occlusal area of class I.

i. The buccal, lingual and gingival walls of the proximal box also required special attention to confirm with the principle of paralleling enamel rods.

ii. In cervical region rods direction varies, enamel rods can be directed occlusally, horizontally or apically as one progresses from the occlusal surface to the CEJ.

Modification of the proximal margins by increasing the cavosurface angle may be change.

FACIAL LINGUAL ENAMEL WALL DESIGN ( CLASS V)

Enamel wall design for class V cavity requires similar consideration in angulation of both occlusal and cervical enamel walls to that given the gingival wall of class II preparation.

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 walls of a class V cavity may require an increase to 120-130degree in order to assure a sound enamel margin.

Testing such margins with hand instruments for the friability of the enamel is a useful clinical guide.

CLINICAL SIGNIFICANCE OF DENTIN

DENTIN PERMEABILITY Although functional in forming and maintaining dentin, the

open tubular channels of dentin compromises its function as a protective barrier, When the external covering of enamel of cementum is removed from dentin through cavity preparation.

The exposure of he tubules with cavity preparation is somewhat offset by a layer of tenacious grinding debris, the smear layer, which adheres to the surface and plugs the tubular orifices.

For optimum Success, dentin bonding systems must remove or penetrate this organic- inorganic barrier to facilitate resin diffusion and micromechanical bounding with the demineralized dentinal substrate

When injury or active caries affect dentin, the immediate inflammatory response is pulpal vasodilatation increased blood flow, and increased interstitial fluid pressure, which result in an increased outward flow rate of tubular fluid.

In addition, vasodilation and temporary gaps between the junctional complexes of adjacent Odon oblast cells accommodate the passage of plasma proteins, such as albumin and immunoglobuline, into the dentinal fluid.

These components agglutinate within the tubules to limit the diffusion to the pulp of exogenous stimuli and possibly to provide a direct immune response to bacteria .

Thus, with exposure of the tubules, a vascular response and accelerated outward flow of the tubular fluid constitutes an immediate protective response.

Nonetheless, tubules that are blocked or constricted provided the pulp with better protection from the permeation of noxious substances

The diffusion gradient is reduced by both smaller tubular diameters and greater tubular lengths, ie, greater remaining dentinal thickness RDT indeed, the functional diameter of the tubule is only a fraction of the anatomic lumen because intratubular cellular, collagenous and mineral inclusion re-strict flow through the tubular channels .

Furthermore, the long 3,000+um length of tubules and inherent buffering capacity of a full thickness of dentin create an effective bio filter of diffusion products .

There are also regional differences in dentin permeability.

The coronal occlusal dentin [pulpal floor of a cavity preparation] is inherently less permeable than is the dentin around the pulp horns or axial surfaces.

DENTIN ADHESION

Challenges in dentin bonding substrateBonding to enamel is relatively simple process without major

requirements or difficulties. Bonding to dentin presents a mush greater challenge. Several factors account for this difference between enamel and

dentin bonding although enamel is a highly mineralized tissue composed of more that 90% (by volume) hydroxyapatite.

Dentin contains a substantial proportion of water and organic material primarily type I collagen, dentin also contains a dense net work of tubules that connect the pulp with the dentin enamel junction.

A cuff of hyper mineralized dentin called peritubular dentin lines the tubules.

The intertubular dentin is penetrated by submicron channels, which allow the passage tubular liquid and fibers between neighboring tubules .

Dentin is an intrinsically hydrated tissue penetrated by a maze of 1 to 2.5 mm diameter fluid filled dentin tubules movement of fluid from the pulp to the DEJ is a result of a slight but constant pulpal pressure.

Dentinal tubules enclose cellular extensions from the Odontoblasts and are in direct communication with the pulp Inside the tubules lumen, other fibrous organic structures are present such as the lamina limiting which substantially decreases the functional radius of the tubule.

The relative area occupied by dentin tubules decreases with increasing distance from the pulp

Adhesion can be affected by the remaining dentin thickness after tooth preparation bond strengths are generally less in deep dentin than in superficial dentin nonetheless some dentin adhesives including somewhat contain the 4 META monomer do not seem to be affected by dentin depth

Whenever tooth structure is prepared with a bur to other instrument residual organic and inorganic components from a smear layer and decreases dentin permeability by 86%.

The composition of the smear layer is basically hydroxyapatite and altered denatured collagen.

This altered collagen can acquired a gelatinized consistency because of the friction and heat created by the preparation procedure and heat created by the preparation procedure diffusion of dentinal fluid.

The removal of the smear layer and smear plugs with acidic solutions result in an increase of the fluid flow onto the exposes dentin surface.

ROLE OF HYBRID LAYER The role of the hydride layer in dentin bonding is controversial. Data from one in vitro study indicated that resin infiltration or

“hybridization” of the dentinal tubules and intertubular dentin accounts for a substantial pro-portion of the bond of resin to dentin

Date from another study suggested that the collagen layer offers no quantitative contribution to the interfacial bond strength.

A study with prime & Bond 2.1 indicated that the removal of collagen fibers could increase bond strengths of resin to dentin.

studies with one-step indicated that the hybrid layer might not play any important role in the establishment of bound strengths.

For a multibottle adhesive system, one study reported that the presence or absence of the hybrid layer did not affect fracture toughness of resin-dentin interfaces.

In the case of All-Bond 2, a different mechanical behavior of the adhesive interface would be expected.

Young’s modulus of the adhesive resin is 1.8 GPa whereas young’s modulus of the All-Bond 2-infiltrated hybrid layer was estimated to be 3.6 GPa

Dentin has a young’s modulus in the range of 11 to 18 GPA12 the presence of the collagen layer presumably would allow for the establishment of a stress-relieving layer at the interface.

A study using a SEP showed that dentin bond strengths did not vary from 1 day to 6 months to 1 year in teeth subjected to excusal function. It also showed that porosity in the hybrid layer increased significantly at 1 year, owing to loss of resin between the collagen fibers because these result were obtained with a hybrid layer being created by a SEP, they cannot be generalized to total-etch adhesives. They do support the theory, however, that collagen may play and important role in the strength of the resin-dentin interface.

For most total-etch adhesives, the ultra morphologic characterization of the transition between the hybrid layer and the unaffected dentin suffuses that there’s an abrupt shift from hybrid tissue to mineralized tissue, without any empty space or pathway that could result in leakage.

The demarcation line seems to consist of hydroxyapatite crystals embedded in the resin from the hybrid layer for self-etch adhesives, the transition is more gradual, with a superficial zone of resin-impregnated smear residues and a deeper zone of resin-impregnated smear residues and a deeper zone, close to the unaffected dentin , rich in hydroxyapatite crystals

MICROLEAKAGE

Micro leakage is defined as the passage of bacteria and their toxins between restoration margins and tooth preparation walls.

clinically microleakage becomes important when one considers that pulpal irritation is more likely caused by bacteria than by chemical toxicity of restorative materials .

An adhesive restoration might not bond sufficiently to etched dentin to prevent Gap formation at margins.

The smear layer itself can serve as a pathway for micro leakage through the Nan channels within it core.

A useful property of dentin adhesives that remove the smear layer is that they might seal the resin-dentin interface and prevent exposure of the pulp-dentin complex to bacteria and their toxins.

Several in vitro reports have shown such sealing capacity for total-etch dentin adhesive systems, but other studies have shown the occurrence of severe leakage associated with similar dentin adhesives

More recent research also has shown the presence of nanometer-sized pores underneath or within the resin-dentin interdiffusion area, despite the presence of gap-free restoration margins.

It is debatable whether the absence of marginal openings would result in a perfect seal between resin and dentin 287, Bonding the resin to a preparation with cavosurface margins in enamel is still the best way to prevent microleakage 323.

CLINICAL FACTORS IN DENTIN ADHESION Several clinical factors may influence the success of an

adhesive restoration. The mineral content of dentin increases in different

situations, including aged dentin; dentin beneath a carious lesion; and dentin exposed to the oral cavity in noncarious cervical lesions, in which the tubules obliterated with tricalcium phosphate crystals.

The dentin that undergoes these compositional changes is called sclerotic dentin and is much more resistant acid etching than “normal” dentin.

consequently, the penetration of a dentin adhesive is limited, Additionally, the clinical effectiveness of dentin adhesives is less in sclerotic cervical lesions than in normal dentin, Nevertheless, some specific dentin adhesives may perform better in sclerotic dentin than in normal dentin.

There is some evidence that masticatory forces not only might cause noncarious cervical lesions, but also might contribute to failure of class V restorations.

Bruxism or any other eccentric movement may generate lateral forces that cause concentration of stresses around the cervical area of the teeth.

Although this stress may be of very low magnitude, the fatigue caused by cyclic stresses may cause failure of bonds between resin and dentin.

DESENSITIZATION Dentin hypersensitivity is a common clinical condition that is

difficult to treat because the treatment out come is not consistently successful most authorities agrees tha the hydrodynamic theory best explains dentin hypersensitivity.

The equivalency of various hydrodynamic stimuli has been evaluated from measurements of the fluid movement induced in vitro and relating this to the hydraulic conductance of the same dentin specimen.

Patients may complain of discomfort when teeth are subjected to temperature changes, osmotic gradients such as those caused by sweet or salty foods, or even tactile stimuli.

It has been calculated that 40 million Americans have some degree of dentin hypersensitivity at some point in their life whereas in other regions of the should the prevalence of dentin sensitivity approaches 50% of the population.

The cervical area of teeth is the most common site of hypersensitivity cervical hypersensitivity may be caused not only by chemical erosion, but also by mechanical abrasion or even occlusal stresses.

CONCLUSION

Reliable bonding of resins to enamel and dentin has been revolutionized the practice of operative dentistry.

Improvements in dentin bonding materials and techniques are likely to continue.

Even as the materials themselves become better and easier to use, however, proper attention to technique and a good understanding of the bonding process remain essential for clinical success.

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