DENTIN INTRODUCTION Dentin is the hard connective tissue that forms the bulk the tooth and is located in both the crown and root of the tooth. It consists of tubules throughout its thickness. Since it forms slightly before enamel, it determines the shape of the crown, including the cusps and the ridges. Physically and chemically the dentin closely resembles bone. [The main difference is that some of the osteoblasts that form bone become enclosed within its matrix substance as osteocytes, whereas the dentin contains only the processes of the cells that form it]. Dentin is considered as a living tissue because it contains within its tubules the processes of the specialized cells, the odontoblasts – the cells that produce dentin. PHYSICAL PROPERTIES OF DENTIN 1. Dentin is yellowish in colour (In the teeth of young individuals the dentin is usually light-yellowish in colour, becoming darker with age) because light can readily pass through thin, highly mineralized enamel
and be reflected by the underlying dentin, the crown of a tooth has a yellowish appearance. Thicker / hypomineralized enamel does not permit light to pass through as readily and in such teeth the crown appears whiter. 2. Dentin is elastic and subject to slight deformation. This provides flexibility to prevent # of the overlying brittle enamel. 3. Dentin is somewhat harder than bone, but considerably softer than enamel (because of inorganic content). 4. The dentin, in radiographs appears more radiolucent (darker) than enamel and more radiopaque (lighter) than pulp (because of the lower content of mineral salts). CHEMICAL COMPOSITION OF DENTIN Dentin consists of 35% organic matter and water 65% inorganic material. Organic Matrix: Consists mainly of collagenous (Type I) fibrils and a ground substance of mucopolysaccharides (proteoglycans and glycos aminoglycans). Inorganic Component: Consists mainly of hydroxyapatite crystals (i.e. calcium phosphte the final form of this mineral salt is crystalline hydroxyapatite). 2
The hydroxyapatite crystals found in dentin are small, slender and needle – like, unlike those found in enamel. Dentin also contains small amounts of phosphates carbonates and sulfates. Types of Dentin In human teeth, 3 types of dentin can be recognized. a.
Primary Dentin forms most of the tooth and outlines the pulp chamber of the fully formed tooth.
The outer layer of the primary dentin is called as the Mantle dentin. It is located immediately subjacent to the enamel or cementum It differs for the rest of the primary dentin. Width of mantle dentin is 80-100 µm.
This layer is the 1st layer formed by newly differentiated odontoblasts.
It has an organic matrix consisting of ground substance and loosely packed thick, fan shaped collagen fibres. Spaces between fibres are occupied by smaller collagen fibrils lying more or less parallel to the DEJ or DCJ.
The matrix is slightly less mineralized than the rest of the primary dentin. 3
Circumpulpal Dentin: Forms the remaining primary dentin or bulk of the tooth. It is formed after the layer of mantle dentin is deposited. It represents all of the dentin formed prior to root completion. The collagen fibrils in circumpulpal dentin are much smaller in diameter and are more closely packed together and form an inter-woven network. They are oriented at right angles to the long axis of the tubules. The circumpulpal dentin may contain slightly more mineral than mantle dentin. b.
Secondary Dentin : (Diagram)
Secondary dentin is a narrow band of dentin bordering the pulp and is formed after root formation has been completed.
It was once thought that 2o dentin was formed only in response to functional stimuli, but it has been shown that it is formed in unerrupted teeth as well.
Thus 2o dentin represents the continuing, but much slowed deposition of dentin by the odontoblasts after root formation has been completed.
2o dentin has an incremental pattern and a tubular structure. It contains fewer tubules than primary dentin. 4
2o dentin although deposited around the periphery of the pulp space is not deposited evenly, especially in the molar teeth. There is a greater deposition 2o dentin on the roof and the floor of the pulp chamber. This leads to an asymmetrical reduction in the size and shape of the pulp chamber and the pulp horns. Clinically this process is referred to as pulp recession, can be readily detected in radiographs and are important in determining the form of cavity preparation in certain dental restorative procedures. For e.g.: Preparation of tooth for a full crown restoration in young patients presents a substantial risk of involving one of the pulp horns and of mechanically exposing the dental pulp. In older patients, the pulp horn has receded, presenting less danger. There is also evidence that 2 o dentin sclerosis more readily than primary dentin. This tends to reduce the overall permeability of the dentin, thereby protecting the pulp. c.
Tertiary Dentin : Is also referred to as reactive / reparative / irregular 2o dentin. The formation of reparative dentin is induced by certain (noxious
stimuli) stimuli such as the following : 1. temperature (extreme heat and
extreme cold) 2. carious lesions 3. chemical agents (calcium hydroxide and sodium fluoride) 4. demineralized tooth matrix. ďƒ˜ Unlike primary / 2o dentin, which is formed along the entire pulp dentin border, tertiary dentin is produced only by the odontoblasts directly affected by the stimulus. The quality and quantity or the degree of tertiary, dentin produced is related to the intensity and direction of the stimulus. For example: The stimulus of an active carious lesion cause extensive destruction of dentin and considerable pulp damage. In such instances, tertiary dentin is deposited rapidly and displays a sparse, irregular tubular pattern with frequent cellular inclusions. Tertiary dentin with such cellular inclusions is sometimes called â€œosteodentinâ€?. On the other hand, if the stimulus is less active, tertiary dentin is deposited less rapidly, its tubular pattern is more regular and there are fewer, if any, cellular inclusions. The main function of the reparative dentin is to protect the pulp from the inward spread of noxious materials along the dentinal tubules (like bacterial toxins etc.).
This protection is accomplished by the “sealing off” of those involved dentinal tubules so that their potentially harmful contents do not reach the dental pulp. Because of frequent insults that the teeth are routinely exposed to, the pulp chambers from aged teeth normally show multiple focii of osteodentin. Pre-Dentin The pre-dentin is located adjacent to the pulp tissue i.e. it lines the innermost (pulpal) portion of the dentin. It is the first formed dentin and is not mineralized. It consists of collagen and proteoglycans. As the collagen fibres undergo mineralization at the pre-dentin front, the predentin then becomes dentin and a new layer of pre-dentin forms circumpulpally. Pre dentin is thickest where active dentinogenesis is occurring and its presence is important in maintaining the integrity of dentin, since its absence appears to leave the mineralized dentin vulnerable to resorption by odontoclasts (from the pulp).
HISTOLOGY OF DENTIN When the dentin is viewed microscopically, several structural features can identified. These include : 1) Dentinal tubules, 2) intra and intertubular dentin, areas of deficient calcification called 3) interglobular dentin, 4) increment lines and an area seen solely in the root portion of the tooth known as the 5) granular layer of tomes and finally cells of the dentin – odontoblasts. 1.
Dentinal Tubules The course of the dentinal tubules follows a gentle curve in the crown and less so in the root, where it resembles on S in shape. They start at right angles from the pulpal surface and end angular to the dentinoenamel and dentinocementum junctions. Near the root tip and along the incisal edges and cusps, the tubules are almost straight. Over their entire lengths, the tubules exhibit minute, relatively regular 2 o curvatures. The ratio between the outer and inner surfaces of dentin is about 5 : 1 accordingly, the tubules are farther apart in the peripheral layers and are more closely packed near the pulp.
They are larger in diameter near the pulpal cavity (3-4 m) and smaller at their outer ends (1 m). The ratio between the nos of tubules / unit area on the pulpal and outer surfaces of dentin is about 4:1. A few dentinal tubules extend through the dentinoenamel junction into the enamel for several millimeters. These are termed as enamel spindles. Dentinal tubules make the dentin permeable, providing a pathway for the invasion of caries. Microscopic examination of infected dentin shows that the dental tubules are packed with micro-organisms well ahead of the decalcified intertubular dentin. Drugs and chemicals present in a variety of dental restorative materials can also diffuse through the dentin and create pulpal injury. 2.
Peritubular Dentin / Intratubular Dentin Around the dentinal tubule is a hypermineralized ring of dentin. This peritubular dentin is the best calcified of all the mineralized
components of dentin. For this reason, it is clearly demonstrated in cross sections of ground sections of undecalcified dentin with the light microscopic (because it is so highly mineralized, it is lost in decalcified sections).
This can also be demonstrated by electron microscopy. The organic matrix of peritubular dentin is sparse and its collagen components virtually absent. The hydroxyapatite cryptals in the peritubular dentin are extremely overall and very tightly and closely packed. (Age change) The formation of intratubular dentin is a slow, continuing process, which can be accelerated by ext – stimuli; This process causes a progressive reduction in the size of the tubule lumen, and on occasion, eventually obliterates the tubule space. When this occurs in several tubules in the same area, the dentin assumes a glassy appearance - this dentin is known as Sclerotic dentin – which shall be dealt with later. Strictly speaking the term ‘Peritubular dentin’ is incorrect because this dentin forms within the dentinal tubule (nor around it), narrowing the lumen of the tubule and is because of more accurately referred to as intratubular dentin. 3.
Intertubular Dentin The main body of the dentin is composed of intertubular dentin. It is
located between the dentinal tubules.
Although, it is highly mineralized, this matrix like bone and cementum, is retained after decalcification, whereas peritubular dentin is not. About one half of its volume is organic matrix, specifically collagen fibres, which are randomly oriented around the dentinal tubules. Hydroxyapatite crystals are formed along the fibres with their long axis oriented parallel to the collagen fibres. Note : Sheath of Newmann The Junction of P.T. Dentin and I.T. dentin reacts differently to stains, acids and alkali treatments. Even in ground section a distinct difference is noted. Because of these differences, it was once believed that the intertubular matrices were separated by a kind of membrane called the “Sheath of Newmann”, Electron microscopic studies however do not confirm the existence of junctional sheath. 4.
Globular Dentin Results
calcospherules. The calcospherules eventually form spheroids or globules of crystalline hydroxyapatite. These calcospherules continue to grow appositionally, resulting in the formation of the large calcified globules.
Interglobular Dentin Interglobular dentin is the term used to describe areas of unmineralized or hypomineralized dentin that persist within mature dentin. The name describes areas where the globular areas of mineralization (calcospherites) have failed to fuse into a homogenous mass. They are especially prevalent in human teeth in which there has been a deficiency in vit. D or exposure to high levels of fluoride at the time of dentin formation. Interglobular dentin is seen most frequently in the circumpulpal dentin just below mantle dentin, where the pattern of mineralization is largely globular. Because this irregularity of dentin is a defect of mineralization and not of matrix formation, the architectural pattern of the tubules remains unchanged, and they run uninterruptedly through the interglobular areas. Incremental Lines The incremental lines (Von Ebner / Imbrication lines) 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. 12
These lines reflect the daily rhythmic, recurrent deposition of dentin matrix as well as hesitation in the daily formative process. The distance between lines varies from 4-8 m in the crown to much less in the root. The daily increment decreases after a tooth reaches functional occlusion. The course of lines indicates the growth pattern of the dentin. Contour Lines (Owen): Occasionally some of the incremental lines are accentuated because of disturbances in the matrix and mineralization process. Such lines are readily demonstrated in ground sections and are known as contour lines and (owen). Analysis with soft x-ray has shown these lines to represent hypocalcified bands. Neonatal Line: In the deciduous teeth and in the 1st permanent molars, where dentin is formed partly before and partly after birth, the pre-natal and the postnatal dentin are separated by an accentuated contour line. This is termed as the ‘neonatal line’ and is seen in enamel as well as dentin. This line represents the abrupt change in environment and nutrition.
The dentin matrix formed prior to birth is usually of better quality than that formed after birth, and the neonatal line may be a zone of hypocalcification. Granular Layer of Tomes When dentin is viewed under transmitted light in ground sections, a granular layer of Tomes can be seen just below the surface of the dentin where the root of the tooth is covered by cementum. A progressive increase in the so-called granules occurs from the cementoenamel junction to the apex of the tooth. These granules were once thought to be minute foci of hypocalcified dentinal matrix (small foci of interglobular dentin). However, electron microscopic studies have determined that no collagenous matrix is present in the granules and hence are not small foci of interglobular dentin. Most contemporary reports advocate that “Tomes granular layer is a series of small air spaces probably caused by looping of dentinal tubules in this region. Dentino-Enamel Junction The surface of the dentin at the dentinoenamel junctions is pitted.
Into the shallow depressions of the pit rounded projections of the enamel. This relation assures the firm hold of the enamel cap on the dentin. In sections, the dentinoenamel junction appears not as straight but as a scalloped line. The convexities of the scallops are directed toward the dentin. The pitted d.E junction is preformed even before the development of hard tissues and is evident in the arrangement of the ameloblasts and the basement membrane of the dental papilla. In microradiographs of ground sections, a hypermineralized zone about 30 m thick can sometimes be demonstrated at the d.E junction. It is most prominent before mineralization is complete. INNERVATION AND SENSITIVITY The question of dentin innervation and dentin sensitivity to a variety of stimuli has not been resolved in spite of numerous studies. Many oral histologists are of the opinion that the microspace between the odontoblast process and the tubule wall is not large enough to accommodate nerves. Yet
electron micrographs have provided substantial
evidence for their presence in the tubule areas, especially close to the pulp.
Intertubular Nerves: Dentinal tubules contain numerous nerve endings in the pre-dentin and inner dentin no farther than 100-150 Âľm for the pulp. Most of these small vesciculated endings are located in tubules in the coronal zone, specifically in the pulp horn (because the tubule diameter is the largest here and the peritubular dentin is very little or almost absent). The nerves and their terminals are found in close association with the odontoblast process within the tubule. The nerve endings interdigitate with the odontoblast process, indicating an intimate relationship to this cell. It is believed that most of these are terminal process of the myelinated nerve fibres of the dental pulp. Three theories might explain dentin sensitivity, they are : 1. Direct neural stimulation: Stimuli in some manner, reach the nerve endings in the dentin and the dentin is stimulated. There is little scientific support of this theory. 2. Fluid or hydrodynamic theory: Various stimuli such as heat, cold, are blast desiccation or mechanical pressure affect fluid movement in the dentinal tubules. ďƒ˜ The fluid movement either inward / outward, stimulates the pain mechanism in the tubules by mechanical disturbance of the nerves closely associated with the odontoblast and its process. ďƒ˜ Thus, these endings may act as mechanoreceptors as they are affected by mechanical displacement of the tubular fluid. 16
3. Transduction theory: This theory presumes that the odontoblast process is the primary structure excited by the stimulus and that the impulse is transmitted to the nerve endings in the inner dentin. ďƒ˜ This
not a popular theory since there
neurotransmitter vesicles in the odontoblast process to facilitate the synapse. In summary, no single proposed mechanism fully explains all the facts related to dentin sensitivity. It may well be that more than one mechanism operates at any one time. AGE CHANGES IN DENTIN Changes in the structural features of dentin occur with age. These are : 1.
Reparative dentin or Tertiary dentin which has been discussed earlier.
Sclerotic / Transparent dentin.
1. Reparative dentin: If by extensive abrasion, erosion, caries, or operative procedures, the odontoblast processes are exposed or cut, the odontoblasts die, or, if they line, deposit reparative dentin. 17
The majority of odontoblasts in this situation degenerate, but a few may continue to form dentin. Reparative dentin is characterized as having fewer and more twisted tubules than dentin. 2. Dead Tracts: Another age related change in dentin structure, which may be related to pathologic process is the formation of dentin dead tracts. Here, the odontoblastic cell processes in the involved dentinal tubules are degenerated, leaving behind empty, air-filled tubules. The emptied tubules in these areas and the dentin are referred to as “dead tracts”. They are expectedly less sensitive than those in which the processes are present in the tubules. When ground sections are examined with reflected light, the air-filled tubules are light and unaffected tubules are dark. With transmitted light, however, the tubules are dark and the remaining dentin light. Dead tracts generally extend from the dentinoenamel junction to the corresponding area of the dentin-pulp interface. In most instances, the dead tracts are sealed at their pulpal aspect by the forming of reparative dentin. 18
Dead tracts are often encountered on the tips and cusps that have been subjected to abrasive forces sufficient in intensity to cause attrition and they appear to a greater extent in older teeth. ďƒ˜ Dear tracts are probably the initial step in the formation of sclerotic dentin. 3. Sclerotic / Transparent Dentin: It is suggested by some researchers that the drying or dead processes induce, or otherwise enhance mineralization resulting in the formation of collagen fibres and apatite crystals within the dentinal tubules. The calcified tubular space assumes a different refractive index becoming transparent. The calcified predentin and process space of the tubule is known as sclerotic or transparent dentin. Hardness tests and Roentgen Ray studies indicate these areas to be more highly mineralized than the other regions of the dentin. ďƒ˜ Sclerotic dentin, because it is characterized physically by increased transparency with transmitted light, increased hardness and density and decreases permeability. Sclerotic dentin is frequently found beneath worn enamel such as occurs in the incisal area of anterior teeth of the teeth in elderly people.
ďƒ˜ Sclerotic dentin may also be found under slowly progressing caries. In such cases blocking of the tubules may be considered or defensive reaction of the dentin, sclerotic dentin is also found beneath tomes granular layer in the cervical area of older teeth where the cervical cementum has been exposed to the oral cavity as a result of recession of the gingival. DEVELOPMENT Dentin former begins during the late bell stage of development in the papillary tissue adjacent to the tip of the folded internal dental epithelium. Dentin is formed by cells, the odontoblasts, which differentiate from the ectomesenchymal cells of the dental papilla foll on organizing influence emanating for the cells of dental epithelium. The actual development of dentin begins at the cusp tips after the odontoblasts have differentiated and begin collagen production. As the odontoblasts differentiate, they change from ovoid to columnar shape. Nuclei become basally oriented at this early stage of development. ďƒ˜ One or several processes arise from the apical end of the cell in contact with the basal lamina.
The length of the odontoblast then increases to approx 40 µm (width7µm remains the same). Proline appears in the rough surface endoplasmic reticulum and golgi apparatus. The proline then migrates into the cell process in dense granules and is emptied into the extracellular collagenous matrix of the pre-dentin. As the cell recedes, it leaves behind a single extension and the several initial processes join into one, which becomes enclosed in a tubule. As the matrix formation continues, the odontoblast process lengthens, as does the dentinal tubule. Initially daily increments of approximately 4 m of dentin are formed. This continues until the crown is formed and the teeth erupt and move into occlusion. After this time dentin production shows to about 1 m day. After root development is complete, dentin formation may decrease further.
Dentinogenesis is a 2-phase sequence Collagen matrix is 1st formed
then calcification occurs
As each increment of pre-dentin is formed along the pulp border, it remains a day before it is calcified and the next increment of pre dentin is formed. Korffs’ Fibres: These fibres were found during the initial stages of dentinogenesis. Early investigations of dentinogenesis used a method of silver impregnation to demonstrate argyrophilic (“Silver–loving”) fibres. Because of their presence only during early dentinogenesis, the concept arose of a dual origin of dentin matrix collagen, whereby mantle dentin collagen arose from Von Korffs’ fibres while circumpulpal dentin collagen was formed by odontoblasts. Recent electron-microscope studies have indicated that there are no collagen fibrils between the differentiating odontoblasts and that the phenomenon originally described by Von Korffs’ is an artifact caused by the binding of silver to the ground substance between the odontoblasts. Mineralization Mineralization of dentin follows different patterns.
Basically mineralization occurs by : 1. Globular 2. Calcospheric calcification. Which involves the deposition of crystals in several discrete areas of matrix at any one time. ďƒ˜
With continued crystal growth these crystals form globular masses, which continue to enlarge and eventually fuse to form a single calcified mass.
This pattern of mineralization is best seen in the circumpulpal dentin formed just below mantle dentin, where a few, but large, globular masses form and coalesce. In the rest of the circumpulpal dentin, the size of the globules progressively decreases until the mineralization front appears linear. The size of the globules seems to depend on the rate of dentin deposition, with the larger globules occurring where dentin deposition is fastest. Root-dentin Formation: Only slightly different from coronal dentinogenesis in that its rate of deposition is slower. The differentiation of odontoblasts that form root dentin is initiated by epithelial cells of Hertwigâ€™s root sheath.
CLINICAL IMPORTANCE The structure of the dentin influences both the pattern of a carious lesion and the speed which dental caries destroys a tooth; and it accounts for the sensitivity frequently experienced by patients during the performance of an oral prophylaxis or during the eating of hot or cold foods. When at any point the caries process has prevented the enamel as far as the d.E junction, the carries producing bacteria will also reach this depth and will come in contact with the peripheral ends of the dentinal tubules.
Since the bacteria are smaller than the tubular they enter the tubules. The odontoblastic processes which occupy the tubules are destroyed, the bacteria travel pulpward in the opened tubules and the dentin is slowly destroyed.
Because the bacteria follow the course of the dentinal tubules, a carious lesion originating around a contact area or in the cervical area of a tooth, extends in an apical direction as it approaches the pulp.
The horizontal spread of caries is considerably more rapid in dentin than in enamel. A tooth with only a small area of caries visible on the surface may be so extensively carious in the dentin the clinical restoration is impossible.
The progress of dental caries is often retarded, but not stopped, by defense reactions that take place in the pulp. One such reaction is the production of sclerotic dentin. Another defense reaction against caries is the formation of reparative dentin (at the location whre the bacteria – filled tubules reach the pulp).
Another characteristic of dentin of particular interest to the dentist is the location of the “Tomes granular layer” and its effect on the comfort of the patient. As we have seen the Tomes granular layer consists of a narrow band of unmineralized areas in the root dentin immediately beneath the cementum. A natural aging process which occurs in nearly all mouths is the gradual recession of the gingival and a resulting exposure of the cementum at the necks of the teeth. In the performance of oral prophylaxis it is necessary to cleans this exposed cervical cementum. This means working very close to Tomes granular layer, which being unmineralized and in close contact with the odontoblastic processes, causes this area to be very sensitive to hot or cold foods. But after a few weeks or months, the dentin beneath the exposed surface usually becomes sclerotic and the patient ceases to notice discomfort. Dentin Anomalies: Most dentin anomalies such as the dentin dysplasias and dentinogenesis imperfecta, involve autosomal patterns of inheritance. 25
It is now understood with some certainty that dentin can respond to calcium regulating hormones (such as vit D) and adverse changes in serum calcium levels. For e.g. patients with parathyroid gland adenomas frequently show a widened predentin zone and enlarged dentinal tubules. Both are indicative of dentin resorption and probably demonstrate that the odontoblasts, active in dentiongenesis throughout life, can function in dentin resorption as well. Both dentin resorption and apposition are indicative of dentin remodeling throughout the life of the tissue. Dentinogenesis Imperfecta : (Hereditary opalascent dentin) 3 Types: Type I – always occurs in association with osteogenesis imperfecta * deciduous teeth more affected. Type II – This type is not associated with osteogenesis imperfecta unless by chance * both dentitions are equally affected. Type III – (Brandywine type) is a racial isolate and is characterized by the same clinical appearance as I and II but with multiple pulp exposures of the deciduous teeth * both dentitions are affected.
Clinically, they exhibit a characteristic unusual translucent line. Because of an abnormal d.E junction, the scalloping is lost leading to early loss of enamel by #ing away after which the dentin undergoes rapid attrition because the teeth are severely flattened. Radiographically obliteration of the pulp chambers and root canals by continud forming of dentin. In type III, there is a great variability in the deciduous teeth they appear as “Shell teeth”, the enamel of the tooth appears normal but the dentin is extremely thin and the pulp chambers are enormous, this large size of the pulp chambers is not due to resorption, but rather due to insufficient and deflection dentin formation. So, on the radiographs, the teeth appear as shells of enamel and dentin surrounding extremely large pulp chambers and root canals. Histologically – Irregular tubules often with areas of uncalcified matrix. In some areas three may be complete absence of tubules. [ Treatment – cast metal crowns on posterior teeth and J. C. on anterior teeth, fillings are not usually permanent because the dentin is soft].
CONTENTS INTRODUCTION PHYSICAL PROPERTIES OF DENTIN CHEMICAL COMPOSITION OF DENTIN STRUCTURE OF DENTIN HISTOLOGY OF DENTIN INNERVATION AND SENSITIVITY AGE CHANGES IN DENTIN DEVELOPMENT CLINICAL CONSIDERATIONS CONCLUSION
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