Enamel PDF

ENAMEL Enamel DR.MH 1 Enamel : 1-is the ectodermal non-collagenous tissue covering the anatomical crown of human teeth. It is the most highly mineralized tissue in the body; it forms a protective covering of the teeth to resist the powerful masticatory forces. 2- is an acellular,...

ENAMEL Enamel DR.MH 1 Enamel : 1-is the ectodermal non-collagenous tissue covering the anatomical crown of human teeth. It is the most highly mineralized tissue in the body; it forms a protective covering of the teeth to resist the powerful masticatory forces. 2- is an acellular, inert, non-vital and insensitive tissue, when destroyed by any means usually wear or caries it cannot be replaced or regenerated. Chemical characteristics: inorganic content organic material 96% 4% a crystalline calcium phosphate (non-collagenous proteins) and hydroxyapatite and various ions, water. strontium, magnesium, lead and fluoride. Physical characteristics Extremely hard tissue. Brittle therefore an underlying layer of resilient dentin is necessary to maintain its integrity. Translucent. Varies in color from light to yellow to grayish white (the underlying yellow dentin is seen through the thinner regions). Varies in thickness from 2.5 mm over the working surfaces to a feather edge at the cervical line. Enamel act as a semipermeable membrane. DR.MH 2 Histological structure of enamel Because of its high crystalline nature, the structure of enamel is extremely difficult to study in decalcified sections, since only an empty space can be seen in areas formerly occupied by enamel, because the minerals have been dissolved and the organic material washed away. 1- Enamel prisms (rods): Number It reaches about five million in the lower central incisor and up to twelve millions in the upper first molar. The direction of the rods in general is perpendicular to the dentin surface. Deciduous teeth Permanent teeth the enamel rods are directed the direction of the enamel roughly vertical in the cusp tip rods is similar to the deciduous or incisal ridge regions, then teeth in the occlusal and they become oblique middle thirds, (towards the occlusal surface) → while in the cervical region in the middle third of the the rods deviated from crown and horizontal in the horizontal to be directed Direction cervical region, where the obliquely rootwise. This is enamel ends abruptly as a because the enamel of thick layer. permanent teeth ends as a knife edge at the cervix DR.MH 3  The course of individual enamel rods is wavy from the amelodentinal junction outwards, but just before they reach the outer surface of the enamel they become straight. Course Under the cusp tips and incisal ridges, the course of the enamel rods is more complicated where they become twisted and braided together to give maximum strength to the areas which are more subjected to the masticatory forces. The enamel in these areas is called gnarled enamel. The diameter of the enamel rods differs from the amelodentinal junction to the outer surface of enamel. The ratio between the diameter of the rods at the inner and Diameter outer enamel surfaces is 1:2 where the diameter at the ADJ is 3-4 microns, it may reach up to 8microns at the outer surface. This is because of the marked difference in the surface area between the inner and outer enamel surfaces. Enamel structure by the light microscope The enamel rods whether cut longitudinally or transversely appear clear and structureless, because they are formed of tightly packed mass of hydroxyapatite crystals that allow light to pass through. The peripheral part of the enamel rod is called prism sheath and forms an incomplete envelope around the prism. It is less calcified than the rod itself i.e. it has a higher organic content. Enamel rods are separated from each other by the interprismatic substance, which is as highly calcified as the enamel rods. When a longitudinally ground section of enamel is treated with mild etching solution, the cylindrical clear crystalline enamel prism appears to be divided horizontally into equal segments by less calcified dark lines, giving the enamel rods a striated appearance. These lines are called cross striation or short increments. DR.MH 4 Hunter Schreger bands The bands of Hunter and Schreger are an optical phenomenon produced solely by changes in rod direction. They are seen most clearly in longitudinal ground sections viewed by reflected light and are found in the inner two thirds of the enamel. These bands appear as dark (diazones) and light (parazones) alternating zones that can be reversed by altering the direction of incidence illumination. →Scanning electron microscopy clearly reveals the difference in orientation of groups of rods within these zones. It may be due to: 1. Change in the direction of enamel rods. 2. Variation in calcification of the enamel. 3. Alternate zones having different permeability and organic material. 4. Optical phenomenon. DR.MH 5