Dentin 2020 PDF

Dentin Shamel & Ankily 2020 Dentin Shamel & Ankily Dentin Dentin is the mineralized connective tissue that forms the bulk of the tooth. It surrounds and protects the dental pulp. In the cro...

Dentin Shamel & Ankily 2020 Dentin Shamel & Ankily Dentin Dentin is the mineralized connective tissue that forms the bulk of the tooth. It surrounds and protects the dental pulp. In the crown it is covered by enamel, in the root by cementum. Unlike enamel, dentin is sensitive and is formed throughout life, giving rise to secondary dentin. Physical properties: 1. Dentin is yellowish in color. 2. Dentin is highly elastic and this allows the impact of mastication to occur without fracturing the brittle overlying enamel. This resilience is partly the result of the presence throughout the matrix of tubules, which extend from the dentino- enamel junction to the pulp. 3. Dentin is harder than cementum and bone but less hard than enamel. 4. By X-ray, dentin is more radiolucent than the enamel and more radio opaque than cementum and bone. Page 1 of 12 Dentin Shamel & Ankily Chemical composition. 70 – 75 % inorganic substance 25 – 30 % organic substance ( appetite crystals) ( 18% collagen (type I). 0.9% citric The crystallites are smaller than acid, 0.2% insoluble proteins, lipids enamel. and carbohydrates and 10% water ) Histological structure: Dentin can be examined using either decalcified or ground sections.  The basic repeatable unit in dentin is the dentinal tubule. This runs through the dentin from the pulp to the enamel. Page 2 of 12 Dentin Shamel & Ankily Odontoblasts: Dentinal tubules contains the odontoblasts which are closely packed together and arranged in what is called palisading manner. These odontoblasts have different shapes according to the site: o In crown (pulp chamber) the cells are pseudostratified columnar. o Near to the beginning of root canal the cells are columnar. o In the mid portion of the root the cells are cuboidal. o In the apical part the cells are flat.  The odontoblasts have a process which traverse the whole thickness of dentin and are called Tomes' fiber.  The odontoblastic process gives off along its course lateral branches which unit with the lateral branches of the adjacent process.  At the outer surface of dentin, the odontoblastic process and dentinal tubules will terminate by dividing into two terminal branches which will unit with the terminal branches of the adjacent process to form plexus beneath the outer dentin surface near D.E.J.  If Tomes' fiber passes the D.E.J. to enamel (before enamel formation) will be called enamel spindle. Dentinal tubules: o Closely backed together at the pulpal surface than the outer surface with a ratio per unit area 4:1. o The diameter of dentinal tubules is narrower at the outer surface (1 micron) than the pulpal surface (2-3 micron). o The course of dentinal tubules from the pulp surface to the outer surface takes an S shape which is called primary curvature (the first convexity facing root ward). o In the root portion and under the tip of the cusp or incisal edge, the dentinal tubules take a straight course. o Each dentinal tubule shows small secondary curvature along its course which Page 3 of 12 Dentin Shamel & Ankily resulted from the spiral track taken by the odontoblastic process during its course from the outer surface of dentin to the pulp surface. Periodontoblastic space:  It is the space between the odontoblastic process and the wall of dentinal tubules.  It is filled with tissue called (dentinal lymph or liquor) and few collagen fibers.  Tissue changes in dentin occur in this location.  The odontoblastic process and periodontoblastic space are the soft tissue of dentin. Peritubular dentin:  It is the wall of dentinal tubules surrounding the odontoblastic process.  It contains more minerals than the rest of dentin (highly calcified) so, in decalcified section the odontoblastic process will appear shrunken with ring shaped space around it.  By T.S. ground section the peritubular dentin appears as translucent rings around the odontoblastic space. Inter tubular dentin:  It forms the main body of dentin and present in between the tubules.  The first formed layer of dentin is termed mantle dentin.  The remaining part of dentin is termed circumpulpal dentin Page 4 of 12 Dentin Shamel & Ankily Interglobular dentin.  The calcification of dentin starts in the form of small globules which will fuse together and form the homogenous dentin.  In some cases, the globules do not fuse together and remain desecrate so, the area of organic matrix which remain uncalcified is called interglobular dentin. Granular layer of Tomes': It is a continuous layer of granular appearance which is always present adjacent to cementum layer. Page 5 of 12 Dentin Shamel & Ankily Incremental lines: All dentin is deposited incrementally, which means that as a certain amount of matrix is deposited daily, a rest in activity follows. This rest in formation results in an alteration of the matrix known as incremental lines:  Daily rest lines are termed incremental lines of von Ebner: In L.S. ground section the incremental lines appear as cones on each other.  If disturbance of calcification occurs, this will lead to accentuation of some incremental lines which are called contour lines of Owen.  Neonatal line which separates dentin formed before birth and that formed after birth where there is disturbance of nutrition and sudden change of environmental condition of the child at birth. Incremental lines represent hypocalcified areas. Dentin sensitivity One of the most important clinical aspects of the dentin/pulp complex relates to the intense pain that can be generated from this site when given appropriate stimuli. Three main hypotheses have been put forward to account for dentin sensitivity: Direct neural stimulation : One view states that stimuli are transmitted directly via nerves in dentin. However, arguing against this view is the relative scarcity of nerves and their apparent absence in the outer parts of dentin. In addition, the application of local anaesthetics to the surface of dentin does not abolish the sensitivity. Odontoblastic transduction theory :A second hypothesis states that stimuli are transmitted via odontoblast processes. However, there is no physiological evidence to date that indicates that the odontoblast process is analogous to a nerve fibre and can similarly conduct impulses pulpwards (i.e. it has a low membrane potential). Furthermore, like nerves in dentin, the process may not extend far into dentin; nor is the application of substances designed to prevent Page 6 of 12 Dentin Shamel & Ankily transmission of such impulses effective.In addition, odontoblasts have not been shown to be synaptically connected to nerve fibres. Fluid or hydrodynamic theory :The third and most plausible hypothesis to explain the transmission of sensory stimuli suggests that all effective stimuli applied to dentin cause fluid movement through the dentinal tubules, and that this movement is sufficient to depolarize nerve endings in the inner parts of tubules, at the pulp–predentin junction and in the subodontoblastic neural plexus. Movement in either direction would mechanically distort the terminals. Page 7 of 12 Dentin Shamel & Ankily Life cycle of odontoblasts An odontoblast is a biological cell of neural crest (ectomesenschyme) origin that is a part of the outer surface of the pulp and whose biological function is dentinogenesis, which is a creation of dentin. Stages of odontoblasts development: In cap stage : the dental papilla is formed of spindle shaped cells & loose intercellular substance and fibers. There is cell free zone between dental papilla and enamel organ.  In early bell stage: under the influence of inner enamel epithelium (induction) the U.M.Cs of the dental papilla (U.M.Cs.) will de differentiated into short columnar cells called odontoblasts.  After odontoblasts start function, they will be separated from the dental papilla by a cell free zone again.  As the differentiation progresses, odontoblasts (40 microns in length) are closely packed together.  By the formation of predentin the odontoblasts are protein forming cells, which have some criteria: 1. Large open face nucleus. 2. Rich in rough endoplasmic reticulum. 3. Rich in Golgi apparatus. 4. Rich in mitochondria. 5. Rich in ribonucleic acid. 6. Rich in alkaline phosphatase especially the cell membrane. 7. Presence of secretory vesicles. 8. Rich in cytoplasm. With further dentin formation the odontoblasts decrease in size until primary dentin is formed. Then they will enter acquiescent state, in which there is decrease in organelles, that form secondary dentin but in a very slow way, unless they are stimulated by external influences to produce reparative dentin. Page 8 of 12 Dentin Shamel & Ankily Dentinogenesis During dentinogenesis two types of dentin are deposited: Dentin is formed in two steps: 1- Matrix formation: which is the formation of predentin. 2- Maturation (mineralization): Mineralization has three patterns: a- Linear (occur in mantel dentin). b- Globular pattern (in circumpulpal dentin just below the mantel dentin) as globules which enlarge and fuse together to form a calcified mass. c- Combination (in the rest of the circumpulpal dentin). Page 9 of 12 Dentin Shamel & Ankily Age changes of dentin Once the tooth is erupted and fully formed, dentin can undergo a number of changes that are either related to age or occur as a response to a stimulus applied to the tooth, such as caries or attrition. Primary Dentin: Dentin which forms the crown and the root of the tooth. Secondary dentin: Once the crown has erupted into the mouth and root completion has occurred, dentin continues to form throughout life, but at a slower rate. This dentin is termed secondary dentin. Types of secondary dentin: A- Regular secondary dentin B- Irregular secondary dentin Rate of Slow rate Rapid rate. formation Stimulus Mild stimuli e.g. attrition or slowly Severe stimuli e.g. erosion, progressing caries severe attrition or deep caries. Site Formed all around the pulp chamber especially at its floor Dentin formation localized under and roof in multirooted teeth. In the area of exposed dentinal the root canal it is formed tubules uniformly. (so, this type of dentin can be called reparative dentin). Histology The dentinal tubules 1. They may not follow wavy 1. Have wavier course. course but, they will be 2. Fewer in number; because irregular (twisted). continuous apposition of 2. Odontoblasts in severe cases secondary dentin will diminish the may be completely area of the pulpal surface degenerated or injured. This and so; the odontoblasts will be will lead to areas of dentin crowded then degeneration of become free of dentinal some of them and the remaining tubules (this type is called Page 10 of 12 Dentin Shamel & Ankily cells will rearrange themselves. atubular). 3. Often there is a line of 3. Line of demarcation may or demarcation which appears as may not be present. dark line between the primary * Osteodentin is another type of and secondary dentin. At that line secondary dentin which is caused the dentinal tubules change their by: Some odontoblasts of will be direction. included in the rapidly formed dentin matrix. So, they will be away from nutrition, and then they will die and leave empty spaces. Result of This type of dentin will lead to : This type of dentin barrier will formation 1. Decrease the size of the pulp increase caries resistance. cavity. 2. Obliteration of the pulp horns this; will leads decrease the ability of pulp exposure during operative procedures. 3. Total or partial obliteration of the root canals may occur. Page 11 of 12 Dentin Shamel & Ankily Age changes of the primary dentin A- Translucent (sclerotic) B- Dead tracts. dentin. Cause Normal aging process or mild Severe stimuli e.g. stimuli (as before) erosion, severe attrition or deep caries. 1. Injury of odontoblasts then, Stimulus leads the odontoblastic will undergo fatty to degeneration then, calcification (the 1. Death of the odontoblasts source of Ca is the dentinal lymph). and their processes. 2. The dentinal tubules will be 2. The empty and widen occluded by widening of peritubular dentinal tubules will be filled dentin zone (intra-tubular dentin). with air. The Tom's fiber becomes smaller. The dentin will have uniform refractive index. Sclerotic dentin will appear In ground section Dead tracts translucent when viewed by the Appearance appear black by transmitted light. transmitted light. By reflected light appear -By the reflected light will white. appear black. 1. Decrease the conductivity Result of of the Tomes' fibers. formation 2. Decrease the permeability 1. Surrounded by narrow of dentin. zone of sclerotic dentin. 3. Increase the caries 2. Under the dead tract from resistance (It slows down the pulpal side, reparative progressing caries). dentin is formed. 4. This type of dentin becomes brittle. Page 12 of 12