Enamel PDF

Enamel Dr.Abdelatif Galal Lecturer of oral Medicine & periodontlogy  Enamel 2  Enamel is the most highly calcified and hardest tissue in the human body due to it’s chemical contents. It cover the anatomical crown of the teeth (It forms a protective covering...

Enamel Dr.Abdelatif Galal Lecturer of oral Medicine & periodontlogy  Enamel 2  Enamel is the most highly calcified and hardest tissue in the human body due to it’s chemical contents. It cover the anatomical crown of the teeth (It forms a protective covering of the teeth to resist the stress during mastication). 3  Unlike dentin, cementum and bone, enamel is produced by cells of ectodermal origin. Enamel tissue is non vascularized , has no blood vessels and no nerves , thus the enamel is considered as a non-vital and non- renewable tissue but it can undergo mineralization changes during the tooth life. Physical Properties: Thickness:- the enamel thickness is variable over the entire surface of the crown Maximum thickness is about 2- 2.5 mm on the cusps. Hardness:- the enamel is considered as the hardest calcified tissue in the human body. Brittleness:- Because of its high mineral content and crystalline arrangement, enamel is brittle and subject to fracture by trauma or heavy sharp-edged forces.. 5  Color:- Thinner areas of the enamel appear yellowish as the underlying yellowish dentin is visible through the enamel, and where thickness of enamel is more, it appears slightly white to grayish. Permeability:- It is semi-permeable which permitscomplete or partial passage of certain molecules ( decreased by age).  CHEMICAL PROPERTIES: Enamel contains about 96% mineral (in organic), 4% organic material and water by weight. The mineral component of mineralized tissues represented by crystalline of calcium phosphate known as hydroxyapatite crystals. The organic content of mature enamel consists mainly of residual enamel matrix proteins or fragments of these proteins, mostly coating the surfaces of the enamel crystals; These proteins are produced by ameloblasts. 7 Structure and organization of enamel: The fundamentals unit of Enamel are 1.Enamel rods or prisms 2.E. rod sheaths 3.cementing inter- rod substance. Enamel rod is a long, thin structure extending from the dentinoenamel junction to the surface of enamel directed at right angles. Each rod is formed by four ameloblasts ; One ameloblast forms the rod head, two ameloblasts form the neck, and the tail is formed by a fourth ameloblast. 8 The rods located in the cusps; the thickest part of the enamel, are longer than those at the cervical areas of the teeth. The number of enamel rods ranges from 5 million in the lower lateral incisors to 12 million in the upper first molars. The course of individual rod is wavy (except in cervical enamel, where Cross section in enamel rod like key- the rods have straight hole course).  cross In sections of human enamel, many rods resemble fish scales. The most common pattern of cross section for enamel rods is a key-hole. 1 0  Rod sheath: It is the peripheral part of enamel rod forms an incomplete envelope around the prisms. stains darker and is more acid-resistant than the rod. It is less calcified and contains more organic substance like enamel protein. 1 1 Inter-rod Substance (inter prismatic) : Light microscope revealed that the rods are cemented together by inter-rod substance, which has slightly higher refractive index than the rods. 1 2  Enamel Structures near D.E.J. : 1- Enamel Spindles: Are short extensions of dentinal tubules across the dentinoenamel junction into the enamel. ▪ It originate from the DEJ,Before enamel forms , some developing odontoblastic process extend into the ameloblast layer ; and when enamel formation begins become trapped to form enamel spindles ▪ Spindles serve as pain receptors. 10 2- Enamel tufts originates at the dentinoenamel junction and extends into enamel for about one-third to one-fifth of its total thickness. They appear branched and contain greater concentrations of enamel protein. may play a role in the spread of dental caries. They are known as enamel tufts because they look like tufts of grass projecting into enamel. 11  3-Gnarled Enamel Over the cusps and incisal edges the rods appear twisted around each other in a complex arrangement known as gnarled enamel. Functionally, changes in rod directions between different layers; may increase the strength of the enamel , less prone to fracutre,and more resistant to wear. 12  3-Hunter-Schreger Bands: These are alternating dark and light bands. are considered as produced by changes in the rod direction. 13  Outer surface structures of enamel: 1-Perikymata: They are transverse, wave like grooves, believed to be the external manifestations of the striae of retzius. They are continuous around a tooth & usually lie parallel to each other. 14 2-Primary Enamel Cuticle (Nasmyth’s Membrane ): Is a non mineralized delicate membrane covers the entire enamel of newly erupted tooth secreted by ameloblasts when enamel formation is completed and is worn away by mastication and cleaning. 15  3- Enamel Pellicle (Salivary Pellicle) Erupted enamel is usually finally covered by pellicle, which appears as precipitated salivary proteins and glycoproteins which having strong affinity for enamel and form a very thin layer called the salivary pellicle. It is less than one micron thick. In the beginning it is bacteria-free and is a dark and amorphous layer. 16  This pellicle is removed by mastication or brushing and is again formed by itself within hours. After 24 to 48 hours of pellicle formation, colonies of microorganisms develop over it. Then it is called bacterial plaque. 4-Enamel lamella : Are very thin, leaf- like structures, sometimes visible to naked eye. They extend from the enamel surface towards the dentinoenamel junction, rarely extending into dentin. The enamel lamellae contain mostly organic material and represent improperly mineralized enamel. It has been observed that enamel lamellae may be sites of weakness in a tooth., They may form a road for entry of bacteria that may initiate caries. 17 18  Dentinoenamel Junction: The dentinoenamel junction is established as soon as the two hard tissues enamel and dentin begin to form. The dentinoenamel junction is a scalloped interface between the enamel and dentin. Dentin has pitted surface, which supports the enamel. The dentinoenamel junction is a hypermineralized zone that is about 30 microns thick. Incremental Lines of enamel 1- Striae of Retzuis: Illustrate the successive apposition of layers of enamel during the formation of crown. When a ground section of a tooth is seen under a light microscope, the incremental lines of Retzius appear as concentric brownish bands. In transverse sections of a tooth,they appear as concentric circles. 20 21  2-Neonatal Line This is one of the brown striae of Retzius which is present only in the deciduous teeth and the first permanent molar. This line separates the enamel which is formed before birth from that formed after birth. It is usually associated with the disturbance in the enamel formation produced at birth, due to the abrupt change in nutrition and environment. 22 Age changes of enamel A- permeability : The enamel becomes gradually less permeable by age, the change is due to alteration in the organic part of enamel crystals may acquire more ions and increase in size by age reducing the size of pores between them this lead to decrease permeability. B- color changes of the tooth Teeth darken with age due to addition of organic material from the environment, in addition to the tobacco stains and that of tea and coffee, also attrition lead to tinning of the enamel surface reflecting the yellow color of underlying dentin. C- Attrition: The most frequent age change of enamel is attrition of occlusal and proximal contact areas as a result of mastication 24 Age changes of enamel 23