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 Âľm scallops with their convexities directed toward the dentin and concavities toward the enamel; 2-5 Âľm 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 immunolabelling 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 µm resolution clearly shows DEJ scalloped structure.
2.
SEM : Incisors and molars are sectioned bucco-lingually and enamel was ground to less than 1mm.
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Then this remaining enamel is removed by the 0.5 M EDTA (pH 7.4), until the DEJ was revealed, 7-10 days.
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The enamel was chalky in appearance when treated with EDTA and disappearance of this chalky appearance was indication that the enamel was gone.
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Samples were fixed in gluteraldehyde and dehydrated in a graded ethanol series followed by drying.
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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 ± 5.5µm and molars 42.3 ± 8.5 µm.
Dentin at DEJ : -
The DEJ has scalloped structure.
-
A band of predentin is present at the DEJ.
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The outer surface of dentin is approximately five times larger in surface area than the inner surface.
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The tubular diameter is only 1 µm near the DEJ.
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The tubules are farthest apart at this junction and much closer at the pulpal surface.
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The osmotic pressure is less in the dentinal tubules at DEJ rather than at pulpal area.
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Branching of the dentinal tubules may be noticed at DEJ.
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The electron microscopes reveals that the crystals dentin and enamel inter mix. 9.20 c (164-big tencates).
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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.
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Enamel rods tends to be maintained in rows arranged circumferentially around the long axis of the tooth.
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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.
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In cervical region the rods run mainly horizontally, only a few rows are tilted apically.
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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.
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Enamel spindles, enamel teeth and enamel lamella all noticed at the DEJ.
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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 Âľm.
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The number of rods at DEJ is estimated at 44,000/mm 2 while there are about 40,000/mm2 at the surface.
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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. Elasticmodulus
Enamel
Dentin
DEJ in enamel
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/m 2) 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 Enamel Dentin Near DEJ
2
13 J / m 550 J/m2 391 J/M2
Prependicular 200 J /m2 270 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 Âľm) 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 Âľm into the enamel.
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They are narrow up to 8 Âľm in diameter, round, sometimes club shaped.
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They are seen as small dark structures due to presence of air an debris resulted from section preparation.
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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.
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They are normally filled with dentinal fluid.
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They occur most often along the DEJ.
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These structures do not follow the direction of the enamel rods.
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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.
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Enamel teeth resemble as the gross in the ground section.
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They appear to travel in the same direction ad the prisms and in thick sections, undulate with sheets of prisms.
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They are hypomineralised and recur at approximately 100 Âľm intervals along the junction.
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Each tuft is several prisms wide.
-
Its appearance is resulted from protein, presumed to be residual matrix, at the prism boundaries of hypomineralised prisms.
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The tuft protein, however is not amelogenin but the minor nonamelogenin fraction.
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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.
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The dentinal tubules follow a curved, sigmoid course – the primary curvatures. The convexity of the primary curvatures nearest the pulp chamber faces towards root.
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The dentin inbetween tubules is the intertubular dentine.
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The tubules taper from approximately 2.5 µm in diameter at their pulpal ends to 1 µm or less peripherally (DEJ).
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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%.
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The tubules also show changes in the direction of much smaller. They are known as secondary curvatures.
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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.
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Dentinal tubules contain odontoblastic processes that are responsible for their formation. The spaces are thought to be filled with extra cellular dentinal fluid.
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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.
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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.
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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)
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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
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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.
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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.
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Enamel is very brittle, have high elastic modulus and low tensile strength which indicates a rigid structure.
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Dentin acts as a cushion and with stand the masticatory forces.
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Enamel rods that fail to posses a dentin base because of caries or improper preparation design are easily fractured away from neighbouring rods.
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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.
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SEM examination of DEJ specimens have demonstrated crack deflections to another fracture plane when forced through the DEJ.
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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.
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Due to the inherent brittleness of enamel and collagenous consolidation of DEJ, enamel cracking should be considered a normal aging process.
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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 窶天 : -
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.