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

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 4μm 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 often extend from the DEJ to the outer surface of the enamel, where they end in shallow furrows know as perikymata (or imbrication lines)
Striae of Retzuis

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. With age enamel becomes worn out because of masticatory attrition
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