Dental Enamel

Question 1

Provide a basis for understanding their importance in disease progression (dental caries).

Question 2

Describe the composition and structure of dental enamel, at both the light microscope and ultrastructure level, in relation to the function of the tissue

Question 3

Enamel is thickest _ and thinnest at the _

Answer 3

over cusps and incisal edges, cervical margin

Question 4

Over the cusps of unworn permanent teeth it is about ? thick and on lateral surfaces it is up to ? thick

Answer 4

2.5 mm, 1.3 mm

Question 5

It differs from all other calcified tissues in the body by

Answer 5

lacking cells

Question 6

The hydroxyapatite comprises about _% of the tissue by weight, the remainder consisting of

Answer 6

95, organic matrix and water

Question 7

Mature enamel contains only _% by weight of organic matrix (mainly protein but not collagen). It may contribute to

Answer 7

1–2%, some of the mechanical properties of enamel, such as fracture toughness.

Question 8

Water constitutes about % by weight of enamel. The presence of water is related to the porosity of the tissue.

Answer 8

2, the porosity of the tissue.

Question 9

Hydroxyapatite crystallites are long, ribbon-like, measuring _ in width and _ in thickness. It has been suggested that each crystal may stretch from the

Answer 9

70 nm, 30 nm, amelodentinal junction (ADJ) to the tooth surface. Length unknown

Question 10

mature enamel crystals are no longer perfectly hexagonal but rather exhibit an irregular outline because

Answer 10

they press against each other during the final part of their growth (enamel maturation).

Question 11

The hydroxyapatite crystallites in enamel contain impurities such as _ and _. These impurities make these mineral phases

Answer 11

carbonate and magnesium, more soluble in acid.

Question 12

In early developing enamel, enamel proteins account for 25–30% of composition

Answer 12

25–30%

Question 13

immature enamel comprises several proteins, of which are the most abundant.

Answer 13

amelogenin (90%), ameloblastin (5%) and enamelin

Question 14

is one of the enamel proteins that persists in mature enamel;

Answer 14

Tuftelin

Question 15

Enamel crystallites are grouped together as _ and some form the _

Answer 15

prisms (rods), interprismatic enamel (interrod enamel).

Question 16

The enamel prism or rod consists of several million crystallites packed into a long thin units approximately _ in diameter and up to _ in length:

Answer 16

5μm, 2.5 mm

Question 17

In longitudinal sections of the tooth the prisms are approximately parallel/perpendicular to the ADJ

Answer 17

perpendicular

Question 18

‘keyhole’ shape.

Answer 18

‘head’ and ‘tail’ regions, the tail of one prism lying between the heads of the adjacent prisms.

Question 19

Interprismatic enamel composition is similar, but has a different optical effect, because

Answer 19

the crystals here deviate by 60 degrees from those in the prism cores

Question 20

The boundaries of the prisms reflect

Answer 20

the sudden changes in crystallite orientation that give an optical effect different from that of the prism cores.

Question 21

The level of organic material and water is likely to be higher/lower at the prism boundary regions because of _.

Answer 21

higher, the larger pores produced by the abutment (support) of hydroxyapatite crystallites at this junction

Question 22

Describe crystal orientation changes in prisms

Answer 22

In the head of the prism the crystals run parallel to the long axis of the prism. In the tail, the crystals gradually diverge from this to become angled 60 degrees to the long axis, no clear division between head and tail of the same prism.

Question 23

The difference in orientation of crystallites and the prismatic structure is due to

Answer 23

the secretion of enamel matrix and mineral by ameloblasts Tomes processes which have two secretory surfaces.

Question 24

each prism (head) is formed by _ but _ contribute to each interprismatic region(tail).

Answer 24

a single ameloblast, four ameloblasts

Question 25

how many prismatic structures are there

Answer 25

3

Question 26

circular pattern

Answer 26

clear distinction between prisms and interprismatic enamel.

Question 27

Offset keyholes are designed to

Answer 27

break up lines of shear within the enamel and dissipate the forces applied to the tissue.

Question 28

‘gnarled’ enamel.

Answer 28

groups of prisms show regular changes in direction. Helps enamel over cusps and incisal edges better resist occlusal masticatory forces.

Question 29

ADJ marks the site of

Answer 29

initial deposition of enamel and dentine.

Question 30

ADJ concave + convex structure. convexities are on the _ and the corresponding concavities are _

Answer 30

enamel surface, on the dentine surface can be seen following removal of enamel by demineralisation.

Question 31

Tooth surfaces which _ have more/less scalloping of the ADJ beneath them compared to _ which have more/less scalloping

Answer 31

face high shearing masticatory forces, such as cusps and incisal edges, more, smoother on lateral surfaces, less

Question 32

Enamel spindles are

Answer 32

dark, club-shaped structures which penetrate up to 25μm into the enamel

Question 33

Enamel spindles arise at _, particularly _

Answer 33

the ADJ, around the cusps of teeth where most crowding of odontoblasts would have occurred

Question 34

Enamel tufts

Answer 34

look like tufts of grass, appearance represents areas between prisms where enamel protein has collected, presumed to be residual organic enamel matrix/proliferate 1/3 into enamel, contain ‘tuftelin’ enamel protein, highest protein content of mature enamel

Question 35

Enamel lamellae are_. They form _

Answer 35

fine cracks in the surface of enamel that may be visible to the naked eye, during enamel development due to incomplete maturation of groups of prisms, in which case they would contain enamel proteins. Spaces between groups of prisms are another example of lamellae and may be caused by stress cracks

Question 36

Hunter-Schreger bands

Answer 36

sinusoidal path with groups of prisms above and below them following paths in different directions.

Question 37

areas where the prisms are section transversely are called _ and areas where they are longitudinal are called _.

Answer 37

diazones, parazones

Question 38

The outer 20–70 μm of newly erupted permanent teeth is _. Here the enamel crystallites are all aligned _

Answer 38

nonprismatic, at right angles to the surface and parallel to each other.

Question 39

Why is surface enamel nonprismatic

Answer 39

more highly mineralised than the rest of the enamel because of the absence of prism boundaries, not subject to abrasion, are less porous and less susceptible to acid

Question 40

Why are there striations in enamel

Answer 40

The incremental lines seen in histological sections of mature enamel are the result of the rhythmic recurrent deposition of the enamel.

Question 41

2 types of striations

Answer 41

There are two types of incremental line: short period (cross-striations. circadian rhythm) and long period (enamel striae, also known as the striae of Retzius).

Question 42

describe striae of Retzius.

Answer 42

prominent lines that run obliquely across the enamel prisms to the surface. In transverse sections they form concentric rings. represent the successive positions of the enamel-forming front

Question 43

striation intervals

Answer 43

there are generally 7 to 9 cross-striations between adjacent striae in enamel. The striae are therefore formed at about weekly intervals.
It has been suggested that there may be a rhythm with a 27-hour cycle in addition to a diurnal daily rhythm. The two rhythms would coincide approximately every 7 days, producing a fault in the developing enamel.

Question 44

perikymata grooves

Answer 44

wavelike features that are normally found in the surface enamel of teeth, related to striations

Question 45

Neonatal line

Answer 45

This is a particularly marked line in enamel is formed at birth and reflects the metabolic changes at birth.
Prisms appear to change both direction and thickness at the time of this event.

Question 46

enamel over time

Answer 46

becomes less permeable with age as its microporosity reduces, becomes thinner

Question 47

Why does an early carious legion look like a white spot

Answer 47

The early caries lesion in enamel appears as a ‘white spot’ due to subsurface mineral loss with partial dissolution of crystals so that there is an enhanced spacing between crystals.

Question 48

Acid etching

Answer 48

allows micro-mechanical retention for adhesive bonding to tooth tissue. increase surface area, phosphoric acid is used

Question 49

Microporosity of enamel

Answer 49

Ions and small molecules from the oral fluids can enter interprismatic enamel regions. enamel and dentine change colour as a result of the passage of the bleaching agents through the tooth