1. ENAMEL
Dr. Saleem Shaikh

2. Introduction Enamel is the hardest tissue of the body
It forms the outer covering of the crown
It varies in thickness having a maximum thickness of mm at the cusp tips of molars and is almost as thin as a knife edge at the cementoenamel junction.

3. Physical properties of enamel
Average knoop hardness number for enamel approximately 343.
Forms a resistant covering (suitable for mastication).
Brittle.
Forms a protective covering
Acts as semipermeable membrane.
The colour of the crown appears yellowish white to grayish white, the enamel is translucent and the colour depends on the thickness and homogenecity of enamel.
Indentation test have showen that the average knoop hardness number for enamel approximately 343. (table page 155)
Five times harder than the dentin
A material is brittle if, when subjected to stress, it breaks without significant deformation (strain)

4. Physical properties of enamel

5. Chemical properties of enamel
By weight:
Inorganic components (96%)
Organic and water components: 1% and 3% respectively.
Inorganic material of enamel is made up of hydroxyapatite crystals.
The chemical formula is Ca10(PO4)6 (OH4)2
The crystals are 30 nm in width, and may extends several millimeters.
Organic components consists unique proteins and lipids. The proteins are divided into two main groups – Amelogenin and Nonamelogenins (Enamelin, Ameloblastin and tuftelin).
The presence of carbonated appetite in the crystal core makes the crystal more susceptible to dissolution from the central region than from its sides.

6. The presence of carbonated appetite in the crystal core makes the crystal more susceptible to dissolution from the central region than from its sides.
Enamel crystals in the head of the enamel rod are oriented parallel to the long axis of the rod. When found in the tail of the enamel rod, the crystals' orientation diverges slightly(65 degrees) from the long axis

7. Structure of Enamel Enamel is made up of enamel rods (prisms).
These rods are covered by rod sheath.
The number of rods may be around 5 million in lower central incisor to 12 million in molars.
The rods start from the dentinoenamel junction and extend upto the surface of the tooth in a wavy (tortuous) course.
Measuring 4 μm wide to 8 μm long

8. The surface of enamel also looks like fish scale appearance.
When the rods are seen under high magnification the rods appear to have a ‘key hole’ pattern appearance.
Consisting of a head and tail
When enamel is cut longitudinally it may pass through the head of one rod and tail of the adjacent rod.
This gives the appearance of rod and inter rod.
The head of the rod is towards occlusal surface and tails are towards the cervical.

9. A thin peripheral layer, seen around the rod.
Enamel rod sheath
A thin peripheral layer, seen around the rod.
Darker than the rod.
Less calcified and contains more organic matter than the rod itself.
The presence of carbonated appetite in the crystal core makes the crystal more susceptible to dissolution from the central region than from its sides.

10. Striations
Each enamel rod is made up of segments made up of dark lines – these are known as cross striations.
In humans these segments are of 4µm width and represent daily rythmic deposition of enamel
More prominent when enamel is not calcified properly.
These are different from striae of retzius.

11. Direction of rods
The rods are oriented at right angle to the dentinoenamel junction.
They are horizontal in the center part
In the incisal and cuspal regions they are almost vertical
In the cervical region the rods are horizontal in deciduous and oblique in permanent teeth.
Follow a wavy course in clockwise and anticlockwise deviation