Introduction :

Teeth contain two major calcified tissues, enamel and dentin, that are

joined by an interface known as the dentin-enamel junction (DEJ). Enamel

is the hard and brittle outer portion of the tooth that cuts and grinds food and

dentin in composed of a tougher biological composite, that can absorb and

distribute stresses. The DEJ is a complex and critical structure uniting these

two dissimilar calcified tissues and acts to prevent the propogation of cracks

from enamel into dentin. The DEJ has a three level structure, 25-100 m

scallops with their convexities directed toward the dentin and concavities

toward the enamel; 2-5 m micro scallops and a smaller scale structure.

The dentinoenamel junction appears to have unique qualities that

permit the joining of highly dissimilar calcified tissues in the teeth. Today is

an going to talk in detail about the dentinoenamel junctions and its clinical

significance.

Formation of DEJ :

To understand the formation of dentinoenamel junction it is necessary

to have a knowledge of the development of the tooth.

The dental lamina contributes for the formation of the tooth. The

enamel organ in the bud stage appears as a simple, spherical to ovoid

epithelial condensation that is poorly morphodifferenciated and

histodifferentiated. It is surrounded by the mesenchyme. By the eleventh

week, morphogenesis has progressed, the deeper surfaces of the enamel

organ invaginating to form a cap shaped structure. In the late cap stage by

twelth week, the central cells of the enlarging enamel organ have been

separated and is termed as stellate reticulum. The peripheral cells are

arranged to form the external enamel epithelia and internal enamel epithelia.

The part of the mesenchyme lying beneath the internal enamel epithelium is

termed as the dental papilla. By fourteenth week, further

morphodifferentiation and histodifferentiation of the tooth germs leads to

the early bell stage where enamel organ shows four distinct layers i.e.

external enamel epithelium, stellate reticulum stratum intermedium an

internal enamel epithelium.

The late bell stage of the tooth development is associated with the

formation of the dental hard tissues, commencing at about eighteenth week.

Enamel and dentin formation takes place in this stage.

The internal enamel epithelium forms a basal lamina (membrane

preformata) separating it from underlying mesenchyme. This basal lamina

marks the position of the future dentinoenamel junction.

Under the inductive influence of the developing ameloblasts (pre

ameloblasts), the adjacent mesenchymal cells of the dental papilla become

columnar and differentiate into odontoblasts. Once the odontoblasts of the

dental papilla become columnar and differentiate into odontoblasts. Once

the odontoblasts of the dental papilla have differentiated, the basal lamina

(future DEJ) separating them from the internal enamel epithelium

disappears as the first layer of dentin matrix is laid down the pre ameloblats

release enzymes by exocytosis that degrade the basal lamina and then resorb

the degradation products by endocytosis.

For a brief period following the degradation of the basal lamina, the

future ameloblasts and odontoblasts are in intimate contact, allowing

inductive signaling to occur between them. The formation of thin layer of

enamel matrix begins as the columnar ameloblasts retreat from the

dentinoenamel junction.

The basal lamina disappears after first layer of dentin is formed and

leads to the formation of the junction by enamel formation over the dentin.

This junction is known as DEJ.

Appositional growth of enamel and dentin is a layer like deposition

of extracellular matrix and this is additive.

Micro structure of DEJ :

DEJ forms a complex interface with atleast three levels of microstructures :

Scallops of varying size

Microscallops within each scallop.

Finer nanolevel structures within each microscallop.

The dentinoenamel junction is not straight but appears as a scalloped

line. The convexities of the scallops are directed towards the dentin. The

concavities are directed towards the enamel.

The surface of the dentin at DEJ is pitted. Into this shallow

depressions of the dentin, the rounded projections of the enamel cap is

fitted. It looks very much like a surface with ridges and valleys. This

relation assures the firm hold of the enamel cap on the dentin. The pitted

DEJ is preformed even before the development of hard tissues and is

evident in the arrangement of the ameloblasts and the basement membrane

of dental papilla.

- Each scallop appears to have a substantial range of microstructure.

- Proximal surfaces are more scalloped than buccal or lingual surfaces.

- Symons reported more scalloping near the cusps.

- Lin et al studied the DEJ using high resolution SEM and immuno-

labelling to identify collagen. They found that the scalloped structures

contained microscallops as well as collagen type I fibrils. These

fibrils appeared to emanate from the dentin and coalesce to form

fibrils approximately 100nm in diameter that crossed DEJ and

inserted into the enamel mineral.

- DEJ is less mineralized than either enamel or bulk dentin, contains

higher organic matrix and is probably associated with the first formed

mantle dentin.

- It is more prominent before mineralization is complete.

Imaging :

1. SRCT : Synchrotron radiation computed tomography (SRCT) images

of tooth specimens bonded to composite restorative material with 3.33

m resolution clearly shows DEJ scalloped structure.

2. SEM : Incisors and molars are sectioned bucco-lingually and enamel

was ground to less than 1mm.

- Then this remaining enamel is removed by the 0.5 M EDTA

(pH 7.4), until the DEJ was revealed, 7-10 days.

- The enamel was chalky in appearance when treated with EDTA

and disappearance of this chalky appearance was indication that the

enamel was gone.

- Samples were fixed in gluteraldehyde and dehydrated in a

graded ethanol series followed by drying.

- SEM images were collected at occlusal, middle and cervical

thirds. There were no differences with intra tooth location, but the

differences between the tooth type with the average scallop size in

incisors 29.4 5.5m and molars 42.3 8.5 m.

Dentin at DEJ :

- The DEJ has scalloped structure.

- A band of predentin is present at the DEJ.

- The outer surface of dentin is approximately five times larger

in surface area than the inner surface.

- The tubular diameter is only 1 m near the DEJ.

- The tubules are farthest apart at this junction and much closer

at the pulpal surface.

- The osmotic pressure is less in the dentinal tubules at DEJ

rather than at pulpal area.

- Branching of the dentinal tubules may be noticed at DEJ.

- The electron microscopes reveals that the crystals dentin and

enamel inter mix. 9.20 c (164-big tencates).

- A granular layer can be seen beneath the DEJ in the malformed

tooth. This is not seen in the normal tooth. This layer is due to

odontoblasts becoming embedded during initial dentin formation.

Enamel near DEJ :

Enamel matrix is noticed at DEJ.

- Enamel forms the convex surfaces at DEJ.

- Enamel rods tends 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 DEJ; where as at the cusp tip they run more vertically.

- In cervical region the rods run mainly horizontally, only a few

rows are tilted apically.

- The first row of the rods run from DEJ to tooth surface at right

angles to the DEJ. The different angulation of rows of the enamel rods

are shown in the picture.

- Enamel spindles, enamel teeth and enamel lamella all noticed

at the DEJ.

- The diameter of a rod is smallest at the dentinoenamel junction

and increases towards the surface. The average diameter of the rod is

about 4 m.

- The number of rods at DEJ is estimated at 44,000/mm2 while

there are about 40,000/mm2 at the surface.

- The rods are divided into two parts i.e. rod head and rod tails.

The rod tails are towards DEJ.

Mechanical properties of DEJ :

Recent work using AFM-based nano in dentition suggests that enamel

has elastic modules and hardness values that vary from about 75 and 90

GPa. Where as the inter tubular dentin has values of about 20 and 1 GPa for

reduced elastic modulus and hardness respectively.

Enamel Dentin DEJ in enamel

Elasticmodulus 75 GPa 20 GPa Dentin by 1 GPa.

The DEJ as noted above is a complex region of small size and

irregular geometry that probably forms a graded interphase. This makes it

especially difficult to study using conventional mechanical testing methods.

simple tensile, comprises or shear testing methods are difficult because of

the complex geometry that results in the imprecise and non uniform stress

distribution.

Rasmussen et al used a work of fracture approach to determine the

fracture characteristics of enamel, dentin and DEJ; and found that enamel is

more resistant to fracture perpendicular to the prims (200 J/m2) than parallel

to prisms (13 J /m2).

Dentin was more resistant to fracture (with values of 550J/m2) parallel

to the tubules (391 J/m2) than perpendicular to the tubules (221 J/m2). When

force applied in the region near to the DEJ. Fracture resistance :

Parallel Prependicular

Enamel 13 J / m2 200 J /m2

Dentin 550 J/m2 270 J/m2

Near DEJ 391 J/M2 221 J/m2

It should be noted that positioning the mandrel within 0.2mm of DEJ,

minimal failure actually occurred at the DEJ, and it is concluded that the

work of fracture of dentin increases in the vicinity of the DEJ, and it is

concluded that the work of fracture of dentin increases in the vicinity of the

DEJ.

Mechanical properties have only been explored in limited fashion,

because of the very small size and difficulty in isolating the DEJ for testing.

However the fracture characteristics indicate that the DEJ differs

substantially from either enamel or dentin, and probably provides a critical

link that preserves the physical integrity of the tooth.

Stress distribution in teeth :

Strain distribution within the tooth is related to its structure.

- Enamel acts as a stress distributor, transferring the compression

load vertically to the root; and horizontally via DEJ to dentin of the

crown.

- In this transfer, a stress concentration occurs at DEJ as it

converts the vertical load in enamel into horizontal load in the dentin. So

shear load is created near DEJ.

- A thick zone (200 m) in the dentin at DEJ under goes greater

stress than the central coronal dentin.

- Differences I elastic modulus between the enamel and the

underlying dentin could cause vertical delamination and fracture along

the DEJ. But between them lies the DEJ due to its high collagen content

has a low elastic modulus in comparison to both dentin and enamel.

Related structures

A number of structures can be seen at the DEJ extending from the

dentin surface into the enamel. These include

- Enamel spindles.

- Enamel tufts.

- Enamel lamellae.

1) Enamel spindles :

- The terminal ends of some dentinal tubules extends across the

DEJ and into enamel for a short distance i.e. up to 25 m into the

enamel.

- They are narrow up to 8 m in diameter, round, sometimes

club shaped.

- They are seen as small dark structures due to presence of air an

debris resulted from section preparation.

- Each represent an odontoblast process that extended between

the immature ameloblasts before the initiation of enamel formation and

remained there until after enamel was formed.

- They are normally filled with dentinal fluid.

- They occur most often along the DEJ.

- These structures do not follow the direction of the enamel rods.

- Tufts are believe to occur developmentally because of abrupt

changes in the direction of groups of rods that arise from different

regions of the scalloped DEJ.

2) Enamel tufts :

- Enamel tuft is a term given to junctional structures in the inner

third of the enamel.

- Enamel teeth resemble as the gross in the ground section.

- They appear to travel in the same direction ad the prisms and in

thick sections, undulate with sheets of prisms.

- They are hypomineralised and recur at approximately 100 m

intervals along the junction.

- Each tuft is several prisms wide.

- Its appearance is resulted from protein, presumed to be residual

matrix, at the prism boundaries of hypomineralised prisms.

- The tuft protein, however is not amelogenin but the minor non-

amelogenin fraction.

- They are of no known clinical significance and do not appear to

be sits of increased vulnerability to caries attack.

3) Enamel lamella :

- Enamel lamellae are sheet like apparent structural faults that

run through the entire thickness of enamel.

- They are hypomineralized and narrower, longer.

- They are less common than tufts.

- Lamella may arise developmentally due to incomplete

maturation of groups of prisms (or) after eruption it may arise as cracks

during function.

- In routine ground sections, many lamellae like structures are

simply cracks produced during sectioning. This can be confirmed by

demineralising the section; where cracks will disappear (not true

lamellae).

DENTINE TUBULES :

- Dentine is permeated by the dentinal tubules.

- The dentinal tubules run from the pulpal surface to the DEJ and

cemento-dentin junctions.

- The dentinal tubules follow a curved, sigmoid course – the

primary curvatures. The convexity of the primary curvatures nearest the

pulp chamber faces towards root.

- The dentin inbetween tubules is the intertubular dentine.

- The tubules taper from approximately 2.5 m in diameter at

their pulpal ends to 1 m or less peripherally (DEJ).

- During formation of dentinal tubules by the odontoblasts the

cells migrate inwards and occupy a smaller surface area.

- The cross sectional area of dentin near DEJ is composed of

only about 2.5% where as near the pulp is 22%.

- The tubules also show changes in the direction of much

smaller. They are known as secondary curvatures.

- Dentinal tubules branch. The most profuse branching is in the

periphery near the DEJ. Many small side branches appear to end blindly

but some may unite with branches and the branches loop.

- Dentinal tubules contain odontoblastic processes that are

responsible for their formation. The spaces are thought to be filled with

extra cellular dentinal fluid.

- Microtubules and intermediate filaments run longitudinally

throughout the odontoblastic process. Mictochondria, endoplasmic

reticulum, vesicles and golgi apparatus are present in the processes.

CLINICAL CONSIDERATIONS :

1) Permeability of dentin :

If there is any trauma, caries attrition or abrasion beyond DEJ the

tubular structure of dentin allows for the possibility of substances applied to

its outer surface being able to reach and affect the dental pulp.

This depends on no of factors :

i) Dentin surface exposed by caries, attrition, abrasion or trauma.

ii) The tubules may be occluded physiologically by peritubular dentin

or by exogenous material precipitated in them peripherally. They may

also be seated off from pulp by tertiary dentin.

iii) The outward movement of interstitial dentinal fluid does not wash

them out of the tubule.

iv) They are able to pass through the odontoblast layer which presents

a barrier to molecules of higher molecular weight.

Most significant materials to travel down the tubules are the

bacteria of dental caries and toxins they produced.

Sensory nerves in the pulp may follow this route and induce pain.

Components of dental materials or etchants used may pass

through the dentin and kill or damage the dental pulp.

2) Response to external stimuli :

- The response to outside stimuli comes from the dental pulp but

is manifest in the structure of dentin it produces.

- If the external stimuli like caries and attrition may lead effect

to dentins enamel functions and when stimuli reaches dentin, the

deposition of tertiary dentin provides a barrier to the progress, of

caries and toxins.

- The presence of secondary dentin and the continuous

deposition through out life, although not a response to external

stimuli, contributes to the barrier function of the dentin.

DEJ in endodontics :

Any trauma, caries, attrition or abrasion beyond DEJ

The dentin surface is exposed

The continuing deposition of secondary dentin throughout life takes

place.

The development of tertiary dentin in response to caries or restorative

procedures

Can lead to reduction in size obliteration of the pulp chamber and root

canals.

When the canals are small and hard to locate, effective treatment

becomes difficult and prognosis is poor

Dentin sensitivity :

Exposed is often (but not always) sensitive these main hypotheses

have been put forward to account for its sensitivity.

i) Nerves in dentin

ii) Odontoblast process

iii) Fluid movements in dentinal tubules

i) Nerves in dentin :

- Pain is due to direct stimulation of nerves in the dentin

- Nerves appear to the absent in the outer parts of the dentin

(near to DEJ)

- The application of local anesthesia to the surface of dentin does

not abolish the sensitivity.

ii) Odontoblastic process :

- The odontoblasts can similarly like nerves conduct impulses

pulpwards

- These process may not extent to the DEJ.

- Odontoblasts have not been shown to be synoptically

connected to nerve fibers.

iii) Fluid movements in dentinal tubules :

The stimuli applied to dentin cause fluid movement through the

dentinal tubules.

Some stimuli such as heat, osmotic pressure and drying would

tend to cause fluid movement outwards, while others such s cold

would cause movements inwards.

Movements in either direction would mechanically distort the

terminals

This movement is sufficient to depolarize nerve endings in the inner

parts of tubules at the pulp predentin function and in sub odontoblastic

neural plexus.

DEJ and dental caries :

Histological evidence shows that the first atteration in dentin is a

hyper mineralize zone that develops even before the enamel lesions reach

the dentino enamel function subsequent demineralization of the dentin is

initiated when the enamel lesions reaches the DEJ.

- When demineralization of the enamel reaches DEJ,

demineralization of dentin starts. This particular reaction pattern has

resulted in the notion that there is a lateral spread of caries lesions at

DEJ.

- The concept of spread of caries lesions is accentuated becuae

the mantle dentins normally has a relatively lower degree of

mineralization at the DEJ. The progress of occlusal enamel lesions at

DEJ in basically similar that of lesions on flat surfaces.

- Lesion originating in a pit and fissure affects a greater area of

DEJ than does a comparable smooth surface lesion.

Fracture and DEJ :

- The hardest substance of the human body is enamel.

- Hardness and density of enamel decreases from the out a

surface to DEJ, with lowest hardness at DEJ.

- Enamel is very brittle, have high elastic modulus and low

tensile strength which indicates a rigid structure.

- Dentin acts as a cushion and with stand the masticatory forces.

- Enamel rods that fail to posses a dentin base because of caries

or improper preparation design are easily fractured away from

neighbouring rods.

- For maximal strength in tooth preparation all enamel rods

should be supported by dentin.

Related structure of DEJ and its significance :

Enamel tufts are projections araise in dentin extend into enamel in

direction of long axis of the crown and play a role in the spread of dental

caries.

Enamel laellae are thin, leaf like faults extend from enamel towards

DEJ.

It contain organic material, which is weak area predisposing a tooth to

the entry of bacteria and dental caries.

Enamel spindles : Odontoblastic process sometimes cross DEJ into enamel.

Their ends are thickened, and serve as pain receptors, there by

explaining the enamel sensitivity experienced by some patients during tooth

preparation.

Physiologic enamel cracking and the DEJ :

The assembly of two tissues with distinctly different elastic moduli

requires a complex fusion for long term functional success.

Stress transfer in simple bilaminated structures with divergent

properties usually induces increased focal stresses at the interface.

If enamel and dentin at the functional surfaces of a tooth comprised

such a simply bonded bilaminate, then enamel-initiated cracks would easily

cross the DEJ and propagate into dentin. In reality the situation seems to be

quite different. It is a complex fusion at the DEJ which can be regarded as

fibril-reinforced bond.

- DEJ is a moderately mineralized interface between two highly

mineralized tissues. parallel, course collagen bundles form massive

consolidations that can divert and blunt enamel cracks through

considerable plastic deformation.

- SEM examination of DEJ specimens have demonstrated crack

deflections to another fracture plane when forced through the DEJ.

- The scalloping structure of DEJ increase the effective

interfacial area and strengthen the bond between dentin and enamel.

The scalloping is most prominent where the function is subject to the

most functional stresses.

- Due to the inherent brittleness of enamel and collagenous

consolidation of DEJ, enamel cracking should be considered a normal

aging process.

- Stress in the enamel is redistributed around the cracks, through

DEJ, which creates a stress concentration at the crack tip and leaves

the tooth surface in the area of crack relatively quiescent.

Thus, enamel cracks can be considered an acceptable enamel

attribute, and the DEJ plays a significant role in assisting stress transfer and

in resisting enamel crack propogation.

The fascinating properties of the DEJ must serve as a reference for

the development of new dentin bonding agents, which should allow for the

recovery of the biomechanical integrity of the restored crown.

Tooth preparation and DEJ :

All initial depths of a tooth preparation for amalgam relate to the

DEJ.

Class I :

- The initial depth pulpally will be 0.2 mm inside DEJ which

ever result in the greatest thickness of amalgam due to its lack of

compressive strength and to provide resistance to fracture.

- The initial depth of the axial wall will be 0.2 mm inside the

DEJ when retention lacks are not used and 0.5 mm inside DEJ when

retention lack are used because lack of bonding to the tooth structure.

Class II :

- Extension should ensure that all caries in removed from the

periphery of DEJ.

- For initial tooth preparation the Pulpal floor should remain at

the initial ideal depth (0.2 mm inside DEJ), even of restorative

material or caries remains. The remaining caries will be removed

during final tooth preparation.

- This provides the adequate thickness of the restoration

providing resistance to fracture and minimize Pulpal initiation by

leaving the remaining tooth crown as strong as possible.

In occluso-lingual class I cavities the Pulpal floor should follow the

contour of the occlusal surface and DEJ, which usually rises occlusally as

the bur moves lingually. The axial wall should follow the contour of the

lingual surface of the tooth. An axial depth of 0.5mm inside the DEJ in

indicated if retentive lacks are required, and an axial depth of 0.2 inside DEJ

is permissible of retentive locks are not required.

The Pulpal floor of the prepared tooth should be uniform that is

usually flat or should follow the slight rise and fall of the DEJ along the

central fissure in teeth with prominent triangular ridges.

Class III :

In class III cavities the initial axial depth i.e. 0.5-0.6 mm inside the

DEJ.

The axial depth should be 0.75 mm 0.8 mm when the gingival margin

will be on root surface which allows 0.25 distance between the retention

groove and gingival cavosurface margin.

Infected dentin that is deeper than this limited initial axial depth is

removed later during final tooth preparations.

Class –V :

- The initial axial depth is 0.5 mm inside DEJ

- The depth is usually 1-25 mm total axial depth depending on

the inciso gingival location

- If the preparation is on the root surface, the axial depth is

approximately -.75mm

The depths will permit the placement of the retention grooves without

undermining the enamel and increases thickness of the remaining dentin in

the gingival aspect of the preparation to aid in protecting the pulp.

Pin hole placement and DEJ :

Caputo and standee state that pin hole should be located halfway

between the pulp and DEJ.

Ditts and associated have reported that pin holes should be placed at

0.5 mm onside the DEJ.

The pin hole should be positioned no closer than 0.5-1m to the DEJ or

no closer than 0.5-1mm to the DEJ or no closer than 1-1.5 mm to the

external surface of the tooth is more practical.

In all the cases the Pulpal floor or axial wall or pin hole placements

are placed mortly in the area near to the DEJ. This is because as the

superficial dentin near DEJ consists of more collagen fibers and has more

elasticity and less elastic modulus and acts as a cushion for the restorative

materials which in turn reduces the fracture of then tooth structure

restorative material during insertion or function.

And this placement near to DEJ minimize the Pulpal irritations by

leaving the remaining tooth structure as strong an possible.

The cavity Pulpal floors are not seated on the DEJ as it is very

sensitive where maximum inter connection of the dentinal tubules exist

which may lead to dentinal sensitivity.