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Biologic Considerations of Enamel Structure

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الكلية كلية طب الاسنان     القسم ترميم ومعالجة الاسنان     المرحلة 4
أستاذ المادة امير حمدي حكيم العميدي       19/02/2018 09:12:09
Lect.1 2018 - biologic considerations of enamel

1. o The coronal dentin (crown) provides both color and elastic foundation for enamel. Together with radicular dentin (root), dentin forms the bulk of the tooth and protective (cover) for the pulp. o As a vital tissue without vascular supply or innervation, it is nevertheless able to respond to thermal, chemical or external stimuli Function:
2. Support: o Tooth strength and rigidity are provide by intact dentinal substrate. o Resistance of tooth to fracture significantly lowered with increasing depth and width of cavity preparation. Therefore, a conservative initial approach that combines localized removal of carious tooth structure placement of a bonded restoration, and placement of sealant recommended. o If large preparations are required, the dentist should consider placement of onlay or crown.
3. Morphology o Dentin is composed of small apatite crystals embedded in cross-linked organic matrix of collagen fibrils. o The extended cytoplasmic processes of the formative cells (the odontoblasts) form channels or tubules traversing the full thickness of the tissue Dentin contains 45% to 50% inorganic apatite crystals, about 25% to 30% organic matrix, and about 25% water.
4. Dentin distinguished from enamel during tooth preparation by: 1-Color: dentin is normally yellow-white and slightly darker than enamel, in older patients dentin is darker and become brown or black in cases if dentin exposed to oral fluids, old restorative materials or slowly advancing caries. 2-Reflectance: dentin surfaces are more opaque and dull, being less reflective to light than enamel surfaces, which appear shiny.
5. 3-Hardness: dentin is softer than enamel, sharp explorer tends to catch and hold in dentin. 4-Sound: when moving an explorer tip over the tooth, enamel surfaces provide a sharper, higher pitched sound than dentin surfaces. Dentin distinguished from enamel during tooth preparation by:
6. Depth of dentin Outer dentin (The dentin near the DEJ). • The tubules of the outer dentin are relatively far a part of the Intertubular dentin makes up 96% of the surface area.
7. • Inner dentin (The dentin near the pulp). • The dentin near the pulp differs from that near DEJ. these differences affect the permeability and bonding characteristics of inner dentin. • In the inner dentin the tubules diameters are larger and the distance between tubule centers is half that of tubules at DEJ. Thus, the Intertubular matrix area is only 12% of the surface area, and the permeability of inner dentin is about eight times more permeable than the dentin near DEJ.
8. Permeability of dentin • The permeability of dentin directly related to its protective function. When the external cap of enamel and cementum is lost from the periphery of the dentinal tubules through caries, preparation with burs or abrasion and erosion, the exposed tubules become canals between the pulp and the external oral environment.
9. o Restored teeth are also at risk of toxic seepage through the phenomenon of microleakage between the restorative material and the cavity wall, through capillary action differential thermal expansion, and diffusion, fluids containing various acidic and bacterial products can penetrate the gap between the tooth and restoration and initiate secondary caries of the internal cavity walls.
10. Bacterial substances can continue diffusion through permeable dentinal tubules to reach the pulp, putting the tooth at risk for pulpal inflammation and sensitivity. So restorative techniques with varnishes, liners or dentin bonding resin adhesives are effective to provide reliably sealed margins and sealed dentinal surface. oThe remaining dentin thickness is the key determinant of the diffusion of gradient
11. Sensitivity of dentin • Although dentin is sensitive to thermal, tactile and osmotic stimuli across its (3-3.5 mm) thickness. • Dentin is neither vascularized nor innervated; accept for about 20% of tubules that have nerve fibers penetrating inner dentin by few microns. Therefore, odontoblast and its process is the possible stimulus receptor.
12. Theories of thermal sensitivity 1. Theory of thermal shock • This states that sensitivity is the result of direct thermal shock to the pulp via temperature changes transferred from the oral cavity through the restorative material, especially when the remaining dentin is thin. Protection from this insult would be then provide by an adequate thickness of an insulating material.
13. 2. Theory of hydrodynamic • This theory based on the capillary flow dynamics of the fluid-filled dentinal tubule. In a vital tooth with exposed dentin, there is a constant slow movement of fluid outward through the dentinal tubules. the hydrodynamic theory proposed that when a stimulus such as air evaporation, cold or heat (i.e. generated from dental bur) or tactile pressure these stimuli causes the slow fluid movement to become more rapid causing displacement of odontoblast bodies and the nerve endings in the pulp are deformed, a response that is interpreted as pain.
14. As the dentin near the pulp, tubule density and diameter increase also the permeability increase, thus increasing both the volume and flow of fluid. This explains why deeper restorations are associated with more problems of sensitivity. According to this theory, if the tubules could occluded, fluid flow is prevent and temperatures do not induce pain. So the operative factor in reducing sensitivity to thermal changes is by effective sealing of the dentinal tubules rather than placement of an insulating materials.
15. This theory has gained general acceptance in recent years and has changed the direction of restorative procedures away from thermal insulation and toward dentinal sealing. Thus, there is increasing emphasis on the integrity of the interface between restorative material and cavity preparation.
16. 3. Transduction theory: • Which presumes that the odontoblast process is the primary structure excited by the stimulus and that the impulse is transmitted to the nerve ending in the inner dentine through the membrane of the odontoblast process, this is not a popular theory since there are no neurotransmitter viscles in the odontoblast process to facilitate the synapse.
17. 4. Direct conduction theory: • In which the fluid stimulates directly affect the nerve endings in the dentinal tubules.
18. Physiologic dentin 1. Primary dentin: formed relatively quickly until root formation completed by odontoblasts. 2. Secondary dentin: This slowly formed dentin continues to constrict the dimensions of the pulp chamber. In response to mild occlusal stimulus, secondary dentin mainly deposited in the pulp horns and on the roof and floor of the pulp chamber so after many decades the chamber becomes quite narrow occluso-gingivally. The dentist must pay attention for the size and location of the pulp chamber to decide the design of the reparation and placement of retentive features such as pins.
19. Sclerotic dentin (transparent or peritubular dentin) • Results from aging or mild irritation (such as slow caries) and cause a change in the composition of the primary dentin. The tubular content appears to replace by calcified material that obliterates the tubules, progressing from the DEJ pulpaly. These areas are harder, denser, less sensitive & more protective of the pulp against subsequent irritation. • Sclerosis resulting from aging is (physiological dentin sclerosis) and that resulting from mild irritation called (reactive dentine sclerosis).
20. Reparative dentin (tertiary dentin) • Intense traumatic insult (injury) to the tooth, whether caused by bacterial penetration associated with caries, or heat and trauma from a dental bur, may be severe enough to destroy the supporting odontoblasts in the affected location. Within 3 weeks, fibroblasts or mesenchymal cells of the pulp are converted or differentiated to stimulate the activities of original odontoblast, and form irregularly organized tubules. • The rate of formation and the thickness and organization of reparative dentin depend on the intensity and duration of the stimulus.


المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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