My 3rd Seminar - Dentin

DENTIN 

INTRODUCTION



DEVELOPMENT



BASIC STRUCTURE OF DENTIN



PHYSICAL PROPERTIES



CHEMICAL PROPERTIES



TYPES OF DENTIN



HISTOLOGY OF DENTIN



INNERVATION OF THE DENTIN



DEVELOPMENTAL DISTURBANCES OF DENTIN



AGE AND FUNCTIONAL CHANGES



CLINICAL CONSIDERATIONS



REFERENCES



CONCLUSION

Submitted by: Dr. Anshuman Khaitan Dept. of Cons. and Endo. College of Dental Sciences Davanagere INTRODUCTION 1

Dentin provides the bulk and general form of the tooth and is characterized as a hard tissue with tubules throughout its thickness. It forms slightly before the enamel; it determines the shape of the crown, including the cusps and ridges and the number and size of the roots. Along the crown, the dentin is covered by enamel, along the root by cementum. It encloses the dental pulp, with which it shares a common origin from the dental papilla. The dentin and pulp can be considered as a single developmental and functional unit, often described as pulpo dentinal complex. DEVELOPMENT Dentinogenesis : It is the process of dentin formation. The dentin formation begins in late-bell stage. Dentine is formed by the odontoblast cells that differentiate from ecto-mesenchymal cells of dental papilla. Thus dental papilla is a formative organ of the dentin. The dental pulp also develops from dental papilla which is mesodermal in origin. Dentinogenesis occurs in the following 3 stages: A)

Cyto-differentiation.

B)

Matrix formation.

C)

Mineralization.

Cytodifferentiation: This step describes how odontoblasts differentiate from undifferentiated ectomesenchymal cells. During early tooth development of cells of dental papilla resembles the typical undifferentiated cells. At the Bell stage of tooth development a differentiation of odontoblasts takes place, starting at cuspal or incisal region. This differentiation is under the control of inner enamel epithelium by expressing various growth factors. The cell changes their shape from short cuboidal to tall columnar. The number of cell organelles markedly

2

increases. The nucleus moves away from basement membrane. It reverses the polarity of cells. These cells are called true odontoblasts which secrete predentin. Once the formation of predentin at cuspal/ incisal region begins the differentiation of new odontoblasts takes place further apically in dental papilla. Initially daily increments of approximately 4 µm/day of dentin formed. Once crown formation completes the dentin formation slows down to 1µm / day. The root dentine formation resembles the cyto-differentiation of crown but it requires proliferation of epithelial sheath (Hertwig’s epithelial root sheath). Matrix formation : The differentiated odontoblasts will have all features of the secretory cell i.e. an abudance of rough endo-plasmic reticulum, a welldeveloped golgi apparatus, mitochondria and secretory granules. The procollagen synthesized in the rough endoplasm reticulum is transferred to golgi-apparatus, and finally appear in secretory granules. The matrix mainly consists of collagen (Type I), proteoglycans and glycoproteins. As each increment of pre-dentine formed it remains for a day before calcified layer of predentine is formed. The initial dentin deposition along the cusp tip is called VonKorff’s fiber. These are argyrophilic fiber (Stains black with silver). As the matrix formation continues, the odontoblast leaves an extension called the odontoblastic process. Mineralization : The mineralization occurs in a globular pattern. The earliest crystal deposition is in the form of very fine plates of hydroxyapatite on the surface of collagen fibrils and in the ground substances. Subsequently HA crystals are laid down within the collagen fibrils. These crystals are arranged

3

with their long aixs is parallel to the fibril axis. This crystal deposition appears to takes place from a common center in a spherulite form. The peritubular region becomes highly mineralized at a very early stage. The final crystal size remains very small about 3nm in thickness and 100nm in length. The HA crystals of dentin resembles that of cementum and bone and are 300 times smaller than the enamel crystals. STRUCTURE OF DENTIN The dentinal matrix of collagen fibers is arranged in a random network. As dentin calcifies, the hydroxyapatite crystals mask the individual collagen fibers. The bodies of the odontoblasts are arranged in a layer on the pulpal surface of the dentin and only their cytoplasmic processes are included in the tubules in the mineralized matrix.

Each cell gives rise to one process, which traverses the

predentin and calcified dentin within one tubule and terminates in a branching network at the junction with enamel or cementum. Tubules are found throughout normal dentin and are therefore characteristic of it. PHYSICAL PROPERTIES 

It is light yellow in colour and becomes darker with age and less translucent It is harder than bone and cementum but softer and less brittle than enamel.



Dentine has greater compressive strength and tensile strength than enamel because it is traversed by tubules.



The dentin is readily permeable.



Dentin is elastic and subjected to slight deformation.



The lower mineral salt content in dentin renders it more radiolucent than enamel. 4



Compressive strength of dentin – 40,000 – 50,000 PSI



Modules of resilience of vital dentin – 100-140 PSI

MATERIAL

DENSITY

THERMAL

COEFFICIENT OF THERMAL

G/CM3

CONDUCTIVITY

EXPANSION

W/MK

ENAMEL

DENTIN

2.14

0.57

0.75

GIC

2.19

0.51-0.72

0.96

ENAMEL

2.47

0.93

1

COMPOSITE

1.6-2.4

1.09-1.37

1.2-4.4

*

RELATIVE

Less thermal conductive & shows LOWER COEFFICIENT OF THERMAL EXPANSION THAN ENAMEL

CHEMICAL PROPERTIES 70% - Inorganic material 20% - Organic Materials 10% - Water

5

TO

The inorganic substance consists of hydroxyapatite crystals and small amount of phosphate, carbonates and sulfates. The organic substance consists of type-1 collagen containing 20% of matrix with proteoglycans between the fibres. Non collagenous such as Phosphoprotein and Proteoglcans. Others are Growth factors , lipids and acidic glypoproteins. NOTE:-Growth factors in dentinal matrix play important role in tissue repair TYPES OF DENTIN Primary dentin The dentin that forms the initial shape of the tooth is called primary dentin. It is usually completed three years after tooth eruption. Mantle dentin Mantle dentin is the name of the first formed dentin in the crown underlying the D.E. Junction. It is thus the outer (or) most peripheral part of the primary dentin and it is about 20mm thick. The fibrils formed in this zone are perpendicular to the D.E. junction and the organic matrix is composed of the collagen fibrils. Circumpulpal Dentin Circumpulpal dentin forms the remaining primary dentin or bulk of the tooth. It is circumpulpal dentin that represents all of the dentin formed prior to root completion. The fibrils in circumpulpal dentin are much smaller in diameter and are more closely packed together. The circumpulpal dentin may contain slightly more mineral than mantle dentin. Secondary dentin Secondary dentin develops after root completion. It is narrow band of dentin bordering the pulp. It contain fewer no. of tubules than primary dentin and usually there is a bending of tubules where primary and secondary dentin interface. Secondary dentin formation takes place without any external stimuli.

6

In secondary dentin, the tubules take a different directional pattern in contrast to primary dentin. Tertiary dentin It is also referred as a reactive, reparative, irregular secondary dentin. It produced in reaction to various stimuli like caries, attrition, cavity preparation etc. It is produced by cells which are directly affected by the stimulus.

 Reactionary (from existing odontoblasts) 

Reparative (from differentiated odontoblasts) Within 15 days, the odontoblasts differentiate from the undifferentiated

mesenchymal cells. (Also called secondary odontoblasts). The quality and quantity depend on: a.

Intensity of stimulus.

b.

Duration of stimulus.

c.

Vitality of pulp.

The tertiary dentin may or may not have a regularly arranged of tubules or no tubules at all. Sometimes the odontoblasts are trapped in the dentin and they are called osteodentin. It differs from the primary dentin by having both type I, Type II collagen fibers. Usually there is no continuity between tertiary and 1° and 2° dentin tubules. This helps in minimizing the dentin permeability and protecting underlying dental pulp.

7

HISTOLOGY OF DENTIN Dentinal tubules: The dentinal matrix contains tubules, each or which ranges from about 1 to 2µm in diameter at its outer end and 3 to 4µm at its pulpal side. The number of tubules is about 15,000 /mm 2 near the dentin enamel junction and it is 65,000mm 2 near the pulpal surface. The ratio between the outer and inner surfaces of dentin is about 5:1. Accordingly tubules are farther apart in periphery and more closely packed near the pulp (3-4 m) and smaller at their outer ends (1 m). The ratio between the number of tubules per unit area on pulpal and outer surface of dentin is about 4:1. The dentinal tubules are fine canals that extend across the entire width of the dentin. They contain odontoblastic process. The course of the dentinal tubules follows a gentle curve, which is “S” Shaped. They show two curvatures primary curvature and secondary converters. Primary curvature start at right angle from the pulpal surface, the convexity of this curved course is directed towards the apex of the root and the curvature in the outer half is directed towards the occlusal or incisal surface. These tubules end perpendicular to the D.E. Junction and D-C junction. It is almost straight at the root apex, incisal edges and cusps. Over their entire, length, the tubules exhibit minute relatively regular secondary curvatures. Canaliculi or microtubules: The dentinal tubules have lateral branches throughout the dentin termed as canaliculi. These canaliculi are1µm or less in diameter and originate more or less at right angle to the main tubule.

8

Enamel spindles: Near the DEJ, the dentinal tubule divides into several terminal branches and forms an intercommunicating and anatomizing network. Some dentinal tubules extend into the enamel for several millimeters. These are termed as enamel spindles. Peritubular dentin: The dentin that immediately surrounds the dentinal tubules is called peritubular dentin. This dentin forms the walls of the tubules. It is more highly mineralized than the intertubular dentin. It is completely broken down and disappears on being subjected to routine decalcification methods. Intertubular dentin: The main body of the dentin is composed of intertubular dentin. It is located between the dentinal tubules or between the zones of peritubular dentin. Although it is highly mineralized, this matrix, like bone and cementum is retained after decalcification. About one half of its volume is organic matrix, especially collagen fibres that are randomly oriented around the dentinal tubules. The fibres have a lattice like arrangement coursing in gentle curves between the tubules and their peri-tubular zones. Hydroxyapatite crystals are formed along the fibres. Provides tensile strength to dentin. Inter globular dentin: Sometimes mineralization of dentin begins in small globular areas that fail to fuse into a homogenous mass. This results in zone of hypo-mineralization between the globules. These zones are known as interglobular dentin. Inter globular dentin forms in the crown of teeth in the circumpulpal dentin just below the mantle dentin, and it follows the incremental pattern. The dentinal tubules pass uninterruptedly through interglobular dentin, thus demonstrating defects of mineralization and not of matrix formation.

9

Pre dentin: Predentin is located adjacent to the pulpal tissue and is 2µm to 6µm wide. It is the first formed dentin and is not mineralized. As the collagen fibres undergo mineralization at the pre-dentin front, the predentin then becomes dentin and a new layer of predentin forms circumpulpally. NOTE:-loss of predentin is one of the factors for internal resorption. Granular layer: When dry ground section of the root dentin is visualized in transmitted light, there is a zone adjacent to the cementum that appears granular. This is known as tomes granular layer or hyaline layer of Hopewell smith. This zone increases slightly in amount from the Cementoenamel junction to the root apex and is believed to be caused by a coalescing and looping of the terminal portions of the dentinal tubules. The cause of development of this zone is probably similar to the branching and beveling of the tubules at the dentinoenamel junctions. It Serves to bond cementum to dentin.

Incremental Lines: The incremental lines von ebner or imbrigation lines or Stria of Retzius appear as fine lines (or) striations in dentin. They run at right angles to the dentinal tubules and correspond to the incremental lines in enamel (or) bone. These lines reflect the daily rhythmic, recurrent deposition of dentin matrix as well as interruption in the daily formative process. The distance between lines varies from 4 to 8 µm in the crown to much less in the root. The course of the lines indicates the growth pattern of the dentin.

10

Contour lines of owen: Occasionally some of the incremental lines are accentuated because of the disturbances in the matrix and mineralization process. Such lines are readily demonstrated in ground sections and are known as contour lines. The most consistently seen contour lines are at the junction of the primary and secondary dentin. Neonatal lines: In the deciduous teeth and in the first permanent molars, where dentin is formed partly before and partly after birth, the prenatal and postnatal dentin are separated by an accentuated contour line. This is termed as neonatal line and is seen in enamel and as well as dentin. This line reflects the abrupt change in the environment that occurs at birth. The dentin matrix formed prior to birth is usually of better quality than that formed after birth and neonatal line may be a zone of hypo calcification. Odontoblasts: The cells, which are related to the deposition of dentin, are the odontoblasts. The odontoblasts are a layer of specialized cells, which lie on the surface of the pulp against the internal surface of the dentin. In a fully formed tooth, the odontoblasts are arranged at a single layer of closely packed cells, which are pyriform in shape. Each odontoblast possesses a long process (Tome’s Fibres), which passes from the distal end of the cell into the substance of the dentin where it is housed in a fine canal, the dentinal tubules. The odontoblastic processes are largest in diameter near the pulp (3 to 4µm) and taper upto 1µm further into dentin.

11

INNERVATIONS OF DENTIN Intertubular nerves: Dentinal tubules contain numerous nerve endings in the predentin and inner dentin no further than 100 to 150 µm from the pulp. Most of these small vesiculated endings are located in the tubules in the coronal zone, specifically in the pulp horns. The nerves and their terminals are found in close association with the odontoblast process within the tubule. It is believed that most of these are terminal processes of the myelinated nerve fibres of the dental pulp. DEVELOPMENTAL DISTURBANCES OF DENTIN: 

Dentinogenesis imperfecta



Dentin dysplasia



Regional odontodysplasia

Dentinogenesis imperfecta (hereditary opalescent dentin) This anomaly occurs when mesodermal portion of the odontogenic apparatus is affected. It is classified by Shields and his coworkers into: Type I – It is always associated with osteogenesis imperfecta. It is an autosomal dominant trait, but some time recessive also. Type II – It never occur in association with OI. It is also called hereditary opalescent dentin. It is an A.D. trait. Type III – It is “Brandy wine” type. It is autosomal dominant. This anomaly mainly affects deciduous teeth than the permanent, in Type I. But in Type II, Type III both the dentitions are affected. The colour of the teeth ranges from gray to yellowish brown. These teeth show unusual translucency or opalescent hue. The enamel fracture occurs early due to 12

the abnormal DEJ, where normal scalloping is absent. The dentin undergoes rapid attrition (occlusal, incisal surface). These teeth are less suitable to caries than the normal teeth. Radiographically there is a partial obliteration of pulp chambers. There is a continued formation of dentin in root canals. In type III the important radiographic feature is shell teeth. Here the enamel appears normal and dentin is very thin with enormous pulp chambers. This large pulp chamber is due to defective dentin and enamel formation. Dentin-dysplasia (root less teeth) It is rare dental anomaly characterized by normal enamel, abnormal dentin with atypical pulp morphology. Witkop classified it into following types: Type I – Radicular dentin dysplasia. Type II – Coronal dentin dysplasia. Type I is more common. Both are hereditary in origin and A.D. type.

Type I (radicular): Both dentitions are affected. Teeth exhibit normal morphology with slight Amber Translucency. The teeth show mobility and premature exfoliation due to short root. Type II (Coronal): It affects both the dentitions. The roots are short, blunt and conical. The teeth have grey to yellowish brown colour. Radiographically:

13

Type I Radicular: The roots are short and blunt. In deciduous dentition there is a complete or partial obliteration of pulp chamber. In permanent teeth crescent shaped pulp chamber is seen. Type II Coronal: The pulp chambers are obliterated in deciduous tooth as in type I. The permanent teeth show abnormal large pulp chamber in coronal portion described as thistle tube appearance. Small pulp stones are also seen in the chamber. Histological features: It shows tubular dentin, osteodentin and fused denticles. The pulp chamber is obliterated. Normal dentin formation is blocked so the dentin formation occur around the obstacles giving characteristic appearance called “lava flowing around boulders”. Regional odontodysplasia or Ghost teeth The maxillary anteriors are more commonly involved here. It affects both dentition. The etiology of disease is unknown but it is thought that it could be due to a latent virus in odontogenic epithelium, which become active in subsequent period. The teeth affected shows delay or failure of eruption. The shape of teeth is altered, with irregular mineralization. Radiographically teeth show marked reduction in radiodensity gives “Ghost teeth appearance”. The enamel and dentin is very thin and pulp chamber is usually large. Histologically characterized by a marked reduction in the amount of dentin, widening of predentin layer and large areas of interglobular dentin with irregular tubular pattern. Due to poor cosmetic appearance extraction of teeth and restoration with prosthetic appliances is usually indicated. AGE AND FUNCTIONAL CHANGES i.

Vitality of dentin.

14

ii.

Reparative dentin.

iii.

Dead tracts.

iv.

Sclerotic or transparent dentin.

Vitality of dentin: Since the odontoblast and its process are an integral part of the dentin, there is no doubt that dentin is vital tissue. If vitality is understood to be the capacity of the tissue to react to physiologic and pathologic stimuli, dentin must be considered a vital tissue. Dentin is laid down throughout life, although after the teeth have erupted and have been functioning for a short time, dentinogeneis slows and further dentin formation is at a much slower rate. Reparative dentin: If by extensive abrasion, erosion, caries or operative procedures, the odontoblast process are exposed or cut, the odontoblasts die or if they live, deposit reparative dentin. The odontoblasts that are killed are replaced by the migration of undifferentiated cells arising in deeper regions of the pulp. Reparative dentin is characterized as having fewer and more twisted tubules than dentin. Dead tracts: In dried ground sections of normal dentin the odontoblast processes disintegrate and the empty tubules are filled with air. They appear black in transmitted light and white in reflected light. They extend from the DEJ to the corresponding area of dentin pulp interface. In most instances, the dead tracts are sealed at their pulpal aspect by the formation of reparative dentin. These areas demonstrate decreased sensitivity and appear to a greater extent in older teeth. Sclerotic / transparent dentin: 15

In case of caries, abrasion, erosion, cavity preparation, sufficient stimuli are generated to cause collagen fibres and appatite crystals to fill the tubules with a fine meshwork, thus obliterating it. Such calcified tubular space assumes a difference in refractive index becoming transparent which is observed in the teeth of elderly people especially in the roots. It is also found under slowly progressing caries reducing the permeability of dentin and may prolong pulp vitality. CLINICAL CONSIDERATIONS Dentinal caries Progression of caries in dentin is different from progression in the overlying enamel (because of the structural difference of dentin). There is much less mineral in dentin and it possesses microscopic tubules that provide a free way of ingress of acids and egress of minerals. The D.E.J. has the least resistance to caries attack and allows rapid lateral spreading once caries has penetrated the dentin. Because of these characteristics, dentinal caries is “V” shaped in cross section with a wide base at the DEJ, and the apex directed pulpally. Caries produces a variety of responses in dentin, including pain, demineralization and remineralization. Caries advancement in dentin proceeds through three changes: 1) Weak organic acid demineralize the dentin; 2) The organic material of dentin, particularly collagen, degenerate

and

dissolves; 3) The loss of structural integrity is followed by invasion of bacteria. The five different zones are most clearly distinguished in slowly advancing lesions: 1) Zone 1: Normal dentin

16

2) Zone 2: Subtransparent dentin 3) Zone 3: Transperent dentin 4) Zone 4: Turbid dentin 5) Zone5: Infected dentin Affected and infected dentin Affected dentin is softened, demineralized dentin that is not yet invaded by bacteria. The affected dentin is capable of re-mineralization. It may be left in place during operative procedure. Infected dentin is both softened and contaminated with bacteria. It should be compulsorily removed during operative procedure. Dentine permeability The structure of dentine is tubular, as previously stated, and it is this characteristic that provides the channels for the permeation of solutes and solvents across dentine. Dentine permeability can be sub divided into two broad categories (Pashley 1996): 1: Trans dentinal movements of substances through the entire thickness of dentine via dentinal tubules (such as fluid shifts in response to hydro dynamic stimuli) and 2: Intra dentinal movement of exogenous substances such as the infiltration of hydrophilic adhesive resins into demineralized dentine surfaces during resin where the material enters the tubules but does not travel across the tubules. Dentine permeability (Tran dentinal or intratubular) is not uniform across the tooth. Coronal dentine permeability is much higher than that of the root. This can be attributed to the convergence of tubules towards the pulp chamber, the tubule density increases about four fold in coronal dentine, but only two fold in root dentine. (Foegel et al. 1988). The density of tubules per mm square varies from 15,000 at the DEJ to 65,000 at the pulp boundary thus within any location on the tooth peripheral dentine has a lower permeability than deeper dentine.

17

Permeability of occlusal dentin is higher over the pulp horns than at the center of the occlusal surface, proximal dentin is more permeable than occlusal dentin, and the coronal dentin is more permeable than root dentin. Protective function of dentin A key function of enamel and dentin is thermal insulation of pulp. Most restorative materials are not as insulating as dentin and thermal insults may occur during intraoral temperature changes. The need for thermal insulation is greatest for metallic restorations. Thermal insulation is proportional to the thickness of insulating material. Approximately 2mm of dentin or an equivalent thickness of material should exist to protect the pulp. Bacterial toxins, strong drugs, undue operative trauma, unnecessary thermal changes or irritating restorative materials should not insult the cells of the exposed dentin. It should be remembered that when 1 mm 2 of dentin is exposed, about 30,000 living cells are damaged. It is advisable to seal the exposed dentin surface with a non-irritating insulating substance. LINERS: liners are thin layers of material used primarily to provide a barrier to protect the dentin from residual reactants diffusing out of a restoration and or oral fluids, which may penetrate leaky tooth restoration interfaces. they also contribute initial electric insulation and generate some thermal protection. the need for liners is greatest with metallic restorations that are not well bonded to tooth structure and which are not insulating, such as amalgam and cast gold. BASES : they replace the missing dentin and provide thermal insulation to the pulp. They are placed in a thickness of 0.5 to 2 mm and they have sufficient strength to support the overlying restoration. Pulp – capping Pulp capping is the procedure by which a layer of calcium hydroxide is placed over a thin layer dentin remaining over the pulp.

18

Indirect pulp capping If a thin layer of dentin still remains then the procedure is called indirect pulp capping. Direct pulp capping When calcium hydroxide is placed directly over the exposed pulpal tissue and the surrounding deeply excavated dentinal area. The technique of indirect and direct pulp capping is carried out to induce reparative dentin formation by the vital pulp tissue. Calcium hydroxide transmits a local state of alkalinity that is necessary for dentin formation. The calcium for the dentin bridge formation comes from the blood stream. Dentinal hypersensitivity The dentin is highly sensitive tissue that differs from other sensitive tissues in the body. In dentin, different stimuli cause only pain. Exposure of hypersensitive dentin arises from either the removal of the coronal enamel or a denudation of the root surface by the loss of cementum and the overlying periodontal structures. It is noteworthy that not all exposed dentinal surface exhibit hypersensitive symptoms. Factor such as age, rate of exposure, and the effect of naturally occurring or environmental desensitizing mechanisms affects the nature of the sensitivity. Theories of dentin sensitivity: 

DIRECT NEURAL STIMULATION THEORY



TRANSDUCTION THEORY



HYDRODYNAMIC THRORY

Direct neural stimulation theory

19

It was proposed by Scott Stella. It states that stimuli in some manner (which is unknown) reach the nerve endings of inner dentin. There is little scientific support for this theory. Hydrodynamic Theory Most popular theory proposed by Gysi, supported by Branstorm. Various stimuli such as heart, cold, air, blast dessication or mechanical pressure affect fluid movement in the dentinal tubules. The fluid movement, either inward or outward stimulates pain mechanism in the tubules by mechanical disturbance of the nerves closely associated with odontoblast and its processes. Thus, these endings may act as mechanoreceptors as they are affected by mechanical displacement of tubular fluid. Transduction theory According to this theory, the odontoblast is the primary structure excited by the stimulus and that the impulse is transmitted to the nerve endings in the inner dentin. This theory is not popular since there are no neurotransmitter vesicles in the odotoblast process to facilitate the synapse. Treatment The treatment options are

 Topical application of varnishes and DBA.  Iontophoresis using 2 % neutral sodium fluoride.

20

 Use of topical agents containing potassium nitrate, silver nitrate, CPP – ACP, fluorides, oxalates etc.  Placement of restorative material such as glass ionomer cement. Dentin bonding While bonding to enamel is predictable, bonding to dentin is not so easy. Dentin bonding is difficult due to the following reasons:  Dentin is a dynamic tissue that shows changes due to aging, caries or restorative procedures.  It has considerable amount of organic material and water when compared to enamel  Dentinal tubules are filled with dentinal fluid.  Cut dentin surface is covered by a smear layer that blocks the dentinal tubules and reduces its permeability.  Dentin is in close proximity to the pulp, so different chemicals used for etching and bonding may irritate the pulp. Role of smear layer in dentin bonding: The smear layer is an amorphous, relatively smooth layer of microcrystalline debris, which appears on all cut surface of the dentin regardless of the cutting procedure used. It has 2 components: 1.

The superficial smear layer

2.

Smear plugs that occlude the dentinal tubules.

21

It cannot be seen with the naked eye. Whether or not the smear layer should be left in place is a matter of controversy. One view is that the smear layer should be left in place because it effectively seals the dentinal tubules. Removal of the smear layer by acid treatment would open and widen the tubules about three fold at the surface. This would greatly increase dentin permeability. The other view is that the smear layer contains microorganism, which would multiply and produce infection if left alone. Moreover the smear layer would prevent proper bonding of restorative material to the dentinal wall by serving as a barrier to the penetration of resin to the underlying dentin substrate. Recent generations of dentin adhesives involves modification of the smear layer to improve the bond strength to dentin. Conditioning of dentin: Conditioning of dentin is defined as any alteration of dentin done after the creation of dentin cutting debris, termed the smear layer. The objective of dentin conditioning is to create a surface capable of micro -mechanical and possibly chemical bonding to a dentin-bonding agent. Conditioning of dentin may be done by several means : 

Chemicals -

Acids like phosphoric acid, maleic acid, citric acid etc.

-

Calcium Chelators like EDTA



Thermal conditioning using lasers



Mechanical conditioning using abrasives.

22

Acid etching of dentin: Earlier acid treatment was employed only on enamel and not on dentin because of the fear of pulpal damage. After Fusayama’s research on total etching with 37% phosphoric acid, the protocol for simultaneous etching on enamel and dentin is being widely accepted. Goals of acid etching of dentin: 1. Modification of the smear layer to permit bonding to underlying dentin. 2. Demineralize the superficial dentin matrix to permit resin infiltration into surface. 3. Uncover both intertubular and peritubular dentin. 4. Clean the dentin surface free of any biofilms . Hybrid layer : Concept of hybrid layer was proposed by NAKABAYASHI. This is the zone where the adhesive resin of the dentin bonding agent micromechanically interlocks with the intertubular dentin and the surrounding collagen fibers. The hybrid layer is formed in the following manner:

 Etching removes the smear layer and exposes the collagen fibrils.  Primers penetrates the collagen network.  Adhesive resins along with the primers form resin microtags within the intertubular dentin and surround the collagen fibers upon curing. The hybrid layer is also called as the ‘resin reinforced layer’ or the ‘resin dentin interdiffusion zone’ Moist vs dry dentin surfaces : Vital dentin is inherently wet; therefore, complete drying of dentin is difficult to achieve clinically. The use of adhesive systems on moist dentin is made possible by incorporation of the organic solvents acetone or ethanol in the primers or adhesives. Because the solvent can displace water from both the dentin surface and the moist collagen network, it promotes the infiltration of resin monomers 23

throughout the collagen network. The "wet bonding" technique has been shown repeatedly to enhance bond strengths because water preserves the porosity of collagen network available for monomer interdiffusion.

If the dentin surface is

dried with air, the collagen undergoes immediate collapse and prevents resin monomers from penetrating it. Thus excess water after acid etching and rinsing should be removed with a damp cotton pellet, bush or a tissue paper and air drying should be avoided.

REFERENCES Orbans: Oral Histology and Embrology Oral Anatomy Histology and Embryology: Berkovitz Oral Histology: James K Avery Oral Histology: Tencate Dental Pulp: Seltzer and Bender Operative Dentistry: Sturdevant Operative dentistry - Modern theory and practice: Marzouk Text book of operative dentistry - Vimal Sikri

24

CONCLUSION The complexity of dentinal structures makes restorative and rehabilitative procedures highly challenging. Thus it is imperative that every clinician should assimilate and apply the knowledge of dentinal histology and morphology to obtain successful clinical result.

25