Oral Biology Lecture Notes on Enamel Structure (Fall 2024) PDF

Course Title: Oral Biology Course Code: BDS123 Topic of Lecture: Biological concepts of Enamel structure Lecturer: Prof Samah Mehanny F A C U L T Y O F D e n t i s t r y T h e F u t u r e S t a r t s H e r e F a l...

Course Title: Oral Biology Course Code: BDS123 Topic of Lecture: Biological concepts of Enamel structure Lecturer: Prof Samah Mehanny F A C U L T Y O F D e n t i s t r y T h e F u t u r e S t a r t s H e r e F a l l 2 0 2 4 Prepared by Samah Mehanny Prepared by Samah Mehanny Biological concepts of enamel Prof Samah Mehanny Prepared by Samah Mehanny Enamel covers the anatomical crown of a human tooth. Prepared by Samah Mehanny  1-COLOR 2-THICKNESS PHYSICAL PROPERTIES 5-PERMEABILITY 3-HARDNESS 4-BRITTLNESS Prepared by Samah Mehanny  1 – Natural color The naturality of enamel Depends on: Yellowish white Grayish white 1- Degree of calcification. 2- Translucency of the enamel. so: Yellowish teeth have translucent enamel. Translucent Obaque White or Grayish white teeth have opaque enamel. Prepared by Samah Mehanny Shade selection should be on wet teeth and should consider the different thicknesses of enamel occluso- cervically. Prepared by Samah Mehanny Prepared by Samah Mehanny  2 - Thickness 2 – 2.5 mm at the cusps of the molars and premolars. Thinning down to almost a knife edge at the cervical margin of the tooth. Prepared by Samah Mehanny Awareness of the thickness is of great benefit in the treatment plans as in case of Partial denture Prepared by Samah Mehanny  3 – Hardness It is the hardest calcified tissue in the body due to: 1- High content of the mineral salts 2- Its crystalline arrangement. Enamel surface - Scanning electron microscopic picture The enamel of the permanent teeth is harder than that of deciduous ones. Prepared by Samah Mehanny -Enamel micro-hardness: 1- It is greatest at the surface and decreased toward DEJ. 2 - It is greater at the cusps and incisal ridges and decreases toward the cervical line. + ++ + Prepared by Samah Mehanny  4 - Brittleness Its structure and hardness render it brittle, especially when it loses its elastic foundation of healthy dentin. Prepared by Samah Mehanny  To avoid fracture of enamel in thin cusps should perform inlay- onlay restorations Prepared by Samah Mehanny  5- Permeability It acts as a semipermeable membrane for certain ions and dyestuff of small molecular size through pores between the crystals. -Permeability is mainly from saliva to the outer layer of enamel, but less from the pulp to the inner enamel layer across the dentin. Prepared by Samah Mehanny  Chemical properties Human enamel Inorganic Rodent enamel Inorganic Prepared by Samah Mehanny Chemical properties Inorganic ** Organic * BY 96% 4% WEIGHT BY 88-90 % 12-10 % VOLUME Prepared by Samah Mehanny Inorganic material of enamel constitutes 96% of enamel weight Hierarchical structure of enamel 1-The smallest inorganic structural unit of enamel is formed of Calcium phosphate crystalline (Hydroxyl apatite crystals) Ca10(PO4)6(OH)2 Prepared by Samah Mehanny 2- At the next hierarchical structural level up is the Enamel prism after about 30 micron from dentin surface. Prepared by Samah Mehanny  Enamel prism Calcium Calcium phosphate crystal phosphate crystal Calcium Calcium Calcium phosphate phosphate phosphate Prepared by Samah Mehanny The probability of the replacement of the Hydroxyl radical with various ions as Fluoride ions during the development or after eruption gives the basis for national fluoridation of drinking water or topical fluoride application. Fluro-apatite crystals (3 Ca3 (PO4)2. Ca (F)2) are less soluble so show more resistance to caries, inhibit demineralization and enhance re-mineralization. -Their color is variable, including green, brown, blue, yellow, and sometimes colorless. Prepared by Samah Mehanny Prepared by Samah Mehanny Organic material of enamel are enamel proteins (4%) All over the enamel structure, the organic materials form a fine network between the crystals or are compressed forming an envelope surrounding each crystal. A higher percentage of organic materials exists in the inter-rod region and increasingly in the rod sheath (surrounding each rod where no crystals can be detected). Inter-rod region Rod sheath Prepared by Samah Mehanny Enamel rod Inter-rod region Rod sheath Prepared by Samah Mehanny Prepared by Samah Mehanny The enamel histology can be studied by both light and electron microscopes. For a light microscope, only ground sections can be used. For electron microscopic examination, the enamel tissue is partially mineralized (transmission E M) or examined with its original mineralization (Scanning E M). Prepared by Samah Mehanny  Histological structure of enamel: 1- Enamel Rod (Enamel Prism) 2- Incremental lines of enamel 3- Amelo-dentinal junction 4- Enamel lamellae 5- Enamel tuft 6- Enamel spindle 7- Surface structure Prepared by Samah Mehanny  1- Enamel Rods (Enamel Prisms) The initially formed enamel (30 microns) as well as finally formed layer (20-30 microns) are in the form of parallel arranged crystals and secreted by smooth ended surface ameloblasts. In between these layers, the ameloblasts develop Tomes’ process that is responsible for enamel rod formation. The rods are formed of parallel hydroxyapatite crystals that is covered by the rod sheath (enamel protein) and in between the inter- rod regions exist (flared crystals with higher organic content). Prepared by Samah Mehanny Scanning electron microscopic picture of the enamel rods’ cross section showing oval-shaped enamel rods or take a keyhole, racket shaped, or fish scale patterns. oval-shaped fish scale patterns Prepared by Samah Mehanny Keyhole or racket shaped The longitudinal sections of the enamel rods appear as cylinders with cross striations. Scanning electron microscopic picture of the enamel rods showing both cross-sectional and longitudinal enamel rods. Prepared by Samah Mehanny Scanning electron microscopic picture of the enamel rods showing longitudinal enamel rods with cross striations. Prepared by Samah Mehanny  Scanning electron micro- scopic picture of the enamel rods showing both cross- sectional and longitudinal enamel rods. Prepared by Samah Mehanny The rods’ ends appear on the external surface of the enamel as shallow concavities that are getting deeper occlusally. Scanning electron microscopic picture of enamel surface showing the enamel rods’ ends. Prepared by Samah Mehanny Direction: The enamel rod is perpendicular to the dentin surface. (detected by both light and scanning EM) In permanent teeth In deciduous teeth Vertical Oblique Horizontal Obliquely root wise Prisms end as a knife edge. Prisms end horizontally. Prepared by Samah Mehanny Knowing the orientation of enamel rods is very important in: *Conservative dentistry because unsupported enamel by the underlying dentin is prone to fracture and usually must be avoided. *Detection of the pattern of caries penetration. Prepared by Samah Mehanny The walls of the prepared cavity must follow the rod directions avoiding undermined enamel. Undermined enamel Prepared by Samah Mehanny  The course of the enamel rod: the enamel rods have a wavy course. Initial rodless enamel Outer rodless enamel Prepared by Samah Mehanny Aprismatic D Scanning electron microscopic picture of the enamel Prepared by Samah Mehanny  At the incisal edge or cusp tip, the enamel prism has a twisted course and is called gnarled enamel. (detected by both light and scanning E M) D This increase the strength of the enamel. Prepared by Samah Mehanny  Gnarled enamel Ground section of the enamel Prepared by Samah Mehanny  Hunter-Shreger bands It is an optical phenomenon caused by changes in the rod course (the wavy course). They are seen clearly in the longitudinal ground section viewed by reflected light. Ground section Prepared by Samah Mehanny  Ground section Prepared by Samah Mehanny Explanation for the Hunter Shreger bands: Parazone (the light is reflected so appeared parazone white band) Diazone (the light is transmitted so appeared dark band) Prepared by Samah Mehanny  Hunter Shreger bands could explain the fact that enamel in spite of being translucent structure, it doesn’t entirely reflect the yellowish color of dentin. Some of the light rays passed through the enamel and others reflected giving natural appearance of the tooth. If the enamel rods passed in a straight course, the teeth would clearly reflect the dentin color that gives an unaccepted esthetic appearance. Prepared by Samah Mehanny The diameter of the enamel rod increases from the dentino-enamel junction to the outer enamel surface by a ratio of 1:2. 2 1 Prepared by Samah Mehanny Prepared by Samah Mehanny Prepared by Samah Mehanny  2- Incremental lines of enamel: Enamel formation is not a continuous process but showed alternating formative and resting periods. Prepared by Samah Mehanny  A– Short incremental lines (cross striation): Long increments Short increments Represent daily rate of secretion. Prepared by Samah Mehanny B- Incremental lines of Retzius (long increments): Represent weekly rate of formation Prepared by Samah Mehanny Long increments Short increments Prepared by Samah Mehanny Incremental lines of Retzius Prepared by Samah Mehanny Incremental lines of Retzius Prepared by Samah Mehanny  C-Neonatal line This line separates the enamel that is formed before birth and that formed after birth. PRENATAL POSTNATAL POSTNATAL PRENATAL ENAMEL ENAMEL ENAMEL ENAMEL Exist in all primary teeth and may in the permanent first molar. Prepared by Samah Mehanny 3- Amelo-dentinal junction Prepared by Samah Mehanny Prepared by Samah Mehanny The bond between enamel and dentin occurs chemically not mechanically (through their organic structures). Prepared by Samah Mehanny  4- Enamel lamellae They are hypocalcified enamel structures. they appear as thin leaves or sheet-like structures which extend from the outer surface of the enamel to a considerable distance of enamel till the D.E.J. and may extend to dentin. Prepared by Samah Mehanny  5- Enamel tuft It is a hypocalcified enamel structure. It always appears in the transverse ground section and rarely in the longitudinal ground section.  6- Enamel spindle (Enamel pain receptors) It is an odontoblastic process that extends in between the cells of the inner dental epithelium before the formation of enamel. Best demonstrated: In L.S and may be in T.S of enamel. They are most common beneath cusp tips and incisal edges (why?). Prepared by Samah Mehanny Prepared by Samah Mehanny Prepared by Samah Mehanny  7- Surface structure A– Outer rodless (aprismatic) Enamel Remember that there is an inner Prismless Enamel 30 um thick Prepared by Samah Mehanny  B- Perikymata (micro-texture) It is the outer manifestation of the incremental lines. Prepared by Samah Mehanny They are transverse wave-like grooves that run horizontally and circumferentially across the surface of the crown. They are parallel to each other and to the cervical line. Their numbers are numerous in the cervical region and decrease occlusally. Prepared by Samah Mehanny Prepared by Samah Mehanny C- Rod Ends They are shallow cervically while deeper occlusally, why?. Prepared by Samah Mehanny  D- Cracks Prepared by Samah Mehanny  ENAMEL LAMELLA Prepared by Samah Mehanny E- Afibrillar Cementum ENAMEL DENTIN CEMENTUM Prepared by Samah Mehanny Acid-etching techniques: Acid-etching is used frequently when bonding dental restoration to teeth. This is important for long-term use of some materials, such as composites and sealants. By dissolving minerals in surface enamel, etchants remove the outer 10 micrometers on the enamel surface and makes a porous layer 5–50 micrometers deep. This roughens the enamel microscopically and results in a greater surface area on which to bond. Prepared by Samah Mehanny There are three patterns formed by acid-etching: Type 1 is a pattern where predominantly the enamel rods are dissolved. Type 2 is a pattern where predominantly the inter-rod enamel is dissolved; Type 3 is a pattern where there is no evidence left of any enamel rods. It is concluded that type 1 is the most favorable pattern and type 3 the least. The explanation for these different patterns is not known for certain but is most commonly attributed to different crystal orientation in the enamel Prepared by Samah Mehanny  Tooth whitening Tooth whitening or teeth bleaching are procedures that attempt to lighten a tooth color in either of two ways: by chemical or mechanical action. Whitening keeps the natural color shade of the tooth while bleaching whitening beyond the natural shade. Prepared by Samah Mehanny  Working chemically, a bleaching agent is used to carry out an oxidation reaction in the enamel and dentin. The agents most used to change the color of teeth are hydrogen peroxide and carbamide peroxide. A tooth whitening product with a low pH can put enamel at risk for decay or destruction by demineralization. Consequently, care should be taken, and risk evaluated when choosing a product which is very acidic. Prepared by Samah Mehanny  - Tooth whiteners in toothpastes work through mechanical action. They have mild abrasives that aid in the removal of stains on the enamel. Although this can be an effective method, it does not alter the intrinsic color of teeth. Prepared by Samah Mehanny Prepared by Samah Mehanny