Enamel is the hardest substance in the body, composed primarily of hydroxyapatite crystals (96% inorganic). It is acellular and highly mineralized. Enamel is formed by ameloblasts and is composed of enamel rods oriented perpendicular to the dentin-enamel junction. The rods are composed of tightly packed hydroxyapatite crystals in a protein matrix that directs their growth. Enamel provides protection and strength to teeth.

1. Enamel Dr. Syed Sadatullah

2. Introduction Enamel is an epithelially derived hard, protective covering of teeth Fully formed enamel is the most highly mineralized extracellular matrix known It is highly brittle yet exhibits certain degree of resistance to withstand fracture

3. 1 Physical characteristics of enamel Enamel is the hardest substance of the body, its hardness is comparable to mild steel Average knoop hardness number for enamel is approximately 343 Surface of enamel is more mineralized and hard than deeper enamel Unlike other calcified structures in the body enamel is unique as it is totally acellular

4. 2 Composition of enamel Enamel consists of approximately 96% of inorganic material and 4% of organic material and water by weight It consists of approximately 12% of inorganic material and 88% of organic material and water by volume The organic component forms the matrix and the inorganic component comprises of various minerals

5. Percentage of dental tissue components by weight Tissue Inorganic Content Organic Content + Water Enamel 96% 4% Dentin 70% 30% Cementum 45-50% 50-55%

6. The organic matrix of enamel is made from non-collagenous proteins and enzymes Of the enamel proteins 90% are amelogenins and 10% are non-amelogenins The different types of nonamelogenins associated with formation of enamel are ameloblastin, enamelin and tuftelin The primary function of the organic material is to direct the growth of enamel crystals

7. The inorganic component of enamel is comprised almost entirely of hydroxyapetite crystals Enamel hydroxyapetite crystals are the largest hydroxyapetite crystals of all the calcified tissues in the body In addition to hydroxyapetite crystals enamel also contains carbonates and trace elements These crystals are susceptible to dissolution by acids and hence provides the basis for dental caries

8. Enamel is translucent and varies in colour from light yellow to whitish It varies in thickness, with maximum over cusps (2.5 mm) to a feather edge at the cervical line Thickness of enamel in primary teeth is nearly half than that in permanent teeth

9. Although enamel is an extremely hard tissue it is partially permeable to some fluids, bacteria and other products of the oral cavity The permeability of enamel is due to the presence of cracks and microscopic spaces on the surface of enamel which allows penetration of fluids The permeability of enamel decreases and hardness increases with age

10. 3 Structure of enamel Rod and interrod enamel The fundamental units of enamel are rods and interrod enamel The rod and interrod enamel is built from closely packed and long ribbon like hydroxyapetite crystals The rod is shaped like a cylinder with a wide head portion, a neck and a thinner tail portion Each rod is formed by four ameloblasts

11. Enamel rods are connected to one another in such a way that they appear like ‘keyholes’. Head of one rod nestling against the necks of two neighbouring rods Enamel rods appear like keyholes

12. Rods are formed nearly perpendicular to DEJ and curve slightly towards the cusp tip The follow a wavy course as the traverse from the DEJ to the surface of the crown The length of most rods is much longer than the thickness of enamel

13. The diameter of the rod at the outer surface is double the diameter at DEJ Crystals that surround each rod are called interrod enamel Rod and interrod enamel is formed from the Tomes process of Ameloblasts

14. The crystals making up the rod and interrod enamel have same composition but are oriented in different direction Enamel rod and Interrod enamel

15. The boundary between rod and interrod enamel is marked by a narrow space filled with organic materials known as rod sheath Immunocytochemical preparation showing rodsheath

16. Dentino-enamel junction (DEJ) DEJ represents the interface between dentine and enamel It appears scalloped which increases the surface area and enable the two dissimilar matrices to interlock DEJ

17. Scalloped nature of DEJ as seen with SEM

18. 3 Histological features of enamel Enamel spindles Enamel spindles originate from the DEJ Before enamel forms, some developing odontoblasts process extend into the ameloblast layer, and when enamel formation begins become trapped to form enamel spindles Enamel Spindles

19. Enamel tufts Enamel tufts also originate from the DEJ, run a short distance in the enamel or sometimes to one half of the thickness They represent protein (enamelin) rich areas in the enamel matrix that fail to mature They are formed during the formative stages of enamel They are considered to be ‘faults’ by some researchers while others consider them to be necessary to anchor dentine to enamel

20. Enamel Tufts

21. Enamel lamellae Enamel lamellae extend from the surface to varying depths of the enamel They are faults that develop as a result of failure of maturation process They are filled with organic material and water There are three types of lamellae Type A – composed of poorly calcified rod segments Type B – filled with degenerated epithelial cells and formed before tooth eruption Type C – filled with organic matter and formed after eruption

22. Enamel Lamella

23. Cross striations Cross striations are periodic bands that appear along the full length of enamel rod . Because of this the enamel rod appears like a ladder with cross striations being the rungs of the ladder They appear at regular intervals that is in agreement with the rate of enamel deposition (which is approximately 4 μ m per day) Cross Striations

24. Striae of Retzuis Striae of Retzuis also represent incremental growth In ground cross sections they appear like concentric growth rings similar to those found in trees In ground longitudinal sections they appear to be dark line extending from the DEJ to the tooth surface Along the Retzuis striae fewer enamel crystals are found and this is related to physiologic disturbances in the body Neonatal line is a Striae of Retzuis that forms at birth

25. Striae of Retzuis Striae of Retzuis often extend from the DEJ to the outer surface of the enamel, where they end in shallow furrows know as perikymata (or imbrication lines)

26. Perikymata

27. Gnarled enamel Most enamel rods follow an undulating pathway from DEJ to the tooth surface But in the cusps tips of molars groups of enamel rods twist about one another. This twisting pattern of enamel rod is known as Gnarled enamel Gnarled enamel makes the enamel strong and more resistant to fracture

28. Hunter-Schreger bands Hunter-Schreger bands are an optical phenomena and are seen in reflected light They can be seen in ground longitudinal sections as alternating dark and light bands

29. Hunter Shcreger bands

30. The dark bands correspond to the cross sectional enamel rods (diazones) and the light bands represent the longitudnally sectioned interrod enamel (parazones) Hunter Shcreger bands

31. Age changes in enamel With age enamel becomes worn out because of masticatory attrition Age also causes a decrease in the permeability of enamel Other characteristics of aging of enamel are discoloration and a change in the surface layer Enamel attrition and discoloration

32. Defects of enamel formation Generally three conditions effect enamel during its formative stages Defects caused by febrile disease Defects caused by tetracycline Finally defects caused by excess fluoride Dental Fluorosis (mottled enamel)

33. Clinical Implications of enamel Fluoridation If fluoride ion is incorporated into the hydroxyapetite crystals then it becomes more resistant to acid dissolution The amount of fluoride must be controlled because high fluoride can cause mottled enamel (in excess of 5ppm)

34. Acid etching Acid etching of enamel is a very important technique for conditioning enamel for many clinical procedures Acid etching is used when doing fissure sealants, restoration, cementing orthodontic bands etc. It is carried out by using a mild acid like orthophosphoric acid on the enamel surface for a controlled period of time Applying acid on tooth surface for etching

35. Enamel before acid etching Enamel after acid etching

36. Acid etching results in three predominant patterns Type 1 Type 1 is characterized by removal of rods Type 2 Type 2 is characterized by removal of inter rod enamel and the rods remain intact Type 3 This type is characterized by irregular and random removal of enamel. Type 3 is less frequent