Enamel Structure and Biomechanical Properties

Question 1

What colour is enamel

Answer 1

Greyish or bluish-white but appears yellowish white as it is semi-translucent and reflects colour of the underlying dentine

Question 2

How thick is enamel on different teeth

Answer 2

0-2mm on incisors

2.6mm on molars

Question 3

What is the refractive index of enamel

Answer 3

1.655 (porcelain = 1.5, Quartz = 1.54)

Question 4

Describe the microscopic properties of enamel

Answer 4

1. Needle-like crystals - up to several mm long
2. Cluster into prisms containing containing around 1000 crystallites
3. Prisms arranged into larger arrays with inter prismatic enamel in between

Question 5

What happens to the teeth in patients with mucopolysaccharidosis

Answer 5

Theres a gap between the enamel and dentine surfaces due to defects in protein structure so not well bonded and there is more stress at EDJ = fractures and cracks

Question 6

What can be used to observe enamel at a nano/sub nanoscopic level

Answer 6

Synchrotron x-ray diffraction - not many in the world

Question 7

What direction do enamel crystallites run

Answer 7

Almost perpendicular to the EDJ, also the crystallites are almost parallel to the direction of force at the cusp tips

Question 8

Why is it beneficial for crystallites of enamel to be parallel to the direction of force at the cusp tips

Answer 8

Allows more even distribution of force in the enamel structure and minimises fracture risk

Question 9

What does the lesser crystallite:force alignment at the cervical margins and pits mean for these areas

Answer 9

That these areas have weaker enamel and are more prone to acid dissolution

Question 10

What is the compressive, shear and tensile strength of enamel

Answer 10

Compressive = 350MPa
Shear = 90 MPa
Tensile = 10 MPa

Question 11

What is the hardness of enamel on the Moh scale

Answer 11

5-8

Question 12

In the oral environment what is the normal force on a single tooth

Answer 12

3-18 N

Question 13

In the oral environment what is the maximum biting force on the overall teeth

Answer 13

640N

Question 14

Describe the elastic modulus of enamel at macro, micro and nano scales

Answer 14

Macro scale = lower elastic modulus thats micro and nano

Cross sectional enamel has lower elastic modulus than surface enamel

Higher applied load = lower elastic modulus

Question 15

What is fracture toughness

Answer 15

The ability of a material to resist fracture when a crack is present

Question 16

What is the critical crystallite thickness for optimised stress resistance

Answer 16

30nm

Question 17

How is enamel adapted to fit its function

Answer 17

Minimises Wear - oriented rods to reduce wear by increasing hardness

Minimises Fracture - Inter-rod structure to holt cracks

Nano-scale HAP crystals have higher strength than bulk materials

30nm crystal thickness

Protein Sheath

Question 18

What role does the protein sheath play in enamel

Answer 18

Proteins from amelogenesis act as a cushion where they deform under load and return to normal upon load release as they will be thermodynamically unstable when deformed