cons sheet # 2

بسم الله الرحمن الرحيم

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On this link :

http://www.mediafire.com/?tu8bnzbx5jj0ylj
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Tooth structure
Enamel and Dentin

Enamel:
-Enamel is formed by Ameloblasts which originate from the ectoderm. Ameloblasts have short extensions that run toward the dentinoenamel junction (DEJ) called “Tomes Processes”.
-Enamel Varies in thickness in different areas of the tooth:
 At the incisal edges of the incisors = 2ml
 At the occlusal area of the premolars = 2.3-2.5 ml
 At the occlusal area of the molars = 3ml
 Near pits and fissures it is nearly zero ( this is the main reason why pits and fissure are highly susceptible to caries )

Chemical composition:
-it is highly mineralized
- 95-98% inorganic material, the main mineral component is Calcium Hydroxyapatite
- 1-2 % organic material
- 4% water

Structure:
1-Enamel rods or prisms, which are the main structural unit of enamel
2- Rod sheath
3- Inter-rod cement

- The alignments of the enamel rods are not the same around the tooth; they curve toward the DEJ and start to straighten up toward the surface of the tooth. The alignments of rods in permanent teeth differ from that in primary teeth; in permanent teeth (in the apical area) they curve downwards or apically. However, in primary teeth, they are perpendicular to the DEJ and the external tooth surface.
- Each enamel rod shows a head portion and a tail portion. This is because the Hydroxyapatite crystallites change direction (each rod is composed of millions of Hydroxyapatite crystallites). In the head portion, the crystals run parallel to the long axis of the rod, while in the tail portion they diverge from this to become angled at 65° to the rod and that’s why we see two different areas.

- As we go from the outer surface of the enamel to the inner surface of the enamel towards the DEJ, there are three things that decrease:
 Hardness of the enamel
 Density
 Diameter of the enamel rods ( the diameter at the surface is about 8 microns , and 4 microns near the DEJ )

- Enamel rods in the first third (1/3) near the DEJ follow an initial curving path, and then they straighten up toward the surface.

Gnarled enamel
-It is a group of rods that spiral around each other.
- It is present near the cervical, incisal and occlusal areas.
- It is hard to cut (it is not as easy to cut with hand instruments as natural or regular enamel because of the spiraling)

Enamel Tufts and Enamel Lamellae
-Enamel tufts: a group of enamel rods that are demineralized. They project from the DEJ toward the surface of the enamel
-Lamella: are thin leaf like folds inside the enamel structure, which might extend all the way to the external enamel surface, and might also extend to the dentin
-Since they are hypomineralized, it makes them more prone to caries.

Dentinoenamel Junction (DEJ):
-It is the structure between enamel and dentin.
-It is scalloped in shape, i.e. there is interdigitation between enamel and dentin, which leads to a firm attachment between them. If it was flat, the enamel layers would slip over the dentin by the occlusal forces (the crisps of the scallops are toward the enamel).
-It is 30 micrometer in thickness (or width)
-It is a hypomineralized structure.

Enamel permeability
-Although enamel is the hardest structure in the tooth, it is permeable to small ions and molecules. They can partially or totally enter into the enamel structure.
-Hypomineralized structures, such as enamel tufts and lamellae, facilitate the inward and outward movements of ions and molecules.

Pulp-dentin complex:
-Odontoblasts, which originate from the mesoderm, are considered part of the pulp-dentin complex; since their cell bodies are present in the pulp and their cytoplasmic processes (Tomes Fibers) extend into the dentinal tubules.


Dentin:
- It is the largest structure in the tooth. It occupies the largest portion because it’s found in the root and the crown.
- The most newly formed dentin is toward the pulp (the newly formed enamel is toward the DEJ).

Chemical composition:
- 75% inorganic material, (Calcium Hydroxyapatite crystals)
-20% organic material (collagen)
-5% water

Structure of the dentin:
-If we take a longitudinal section in the dentin, we can see the lumens of the dentinal tubules.
-Peritubular dentine (or intertubular dentin): is the hardest part of the dentinal tubules. It is highly mineralized and it surrounds these tubules.
-The size and number of the dentinal tubules per surface area varies according to the area of the dentine we’re looking at; the diameter of the dentinal tubules at the surface is about 0.8-0.9 microns, but near the pulpal surface it is 2-3 microns. The number of the dentinal tubules at the DEJ surface is less than that at the pulpal surface. We can count up to 5-10 thousand tubules per mm² at the outer surface, while there is 45-65 thousand per mm² at the inner surface. The number of tubules per 1 mm² differs, but overall it’s the same number of tubules, and what makes the difference is the larger surface area at the outer surface, so the number of tubules is seems less.

-According to the Hydrodynamic theory, external stimuli (for example, hot or cold) cause movement of the fluid inside the dentinal tubules. This movement stimulates the subodontoblastic layer (nerve plexus of Rashkow), which in turn stimulates pain. When the diameter of the tubules increases, the fluid flow inside the dentinal tubules increases to the fourth power, which means increase in pain. A small change in the diameter causes significant increase in the flow causing more pain.

-The tubules in the root dentine are less in number and more widely spread than the ones in the coronal dentine



Types of dentin:
1- Predentin:
-the very first layer of dentine to be formed, which is located near the odontoblasts in the pulp
2- Primary dentin:
- forms the initial shape of the tooth
- continues 3 years after the tooth eruption
3- Secondary dentin:
-it forms continuously, throughout life, with or without external stimuli. It is an aging process.
-it is present all around the pulp chamber, but in multi-rooted teeth, it is present more on the floor and the roof of the pulp chamber of the molars
-the secondary dentin is straighter than the primary dentin (the primary takes the S curve)

4- Reparative dentin (Tertiary dentin):
-it is irregular, atubular, and highly mineralized
-it is only formed if there is a stimulus
-it is considered a defense mechanism toward caries or any external stimulus that is provoking the tooth
-formed by replacement odontoblasts (not the original odontoblasts, because they are already dead because of the stimuli)
-the irritant’s speed must be at a moderate level, if it was too fast there wouldn’t be enough time to form tertiary dentine
(Examples of moderate-level irritants: abrasion, erosion,..)

Dead Tracts:
-with any external stimuli, the odontoblastic processes retract from the dentinal tubules toward the pulp, leaving behind them empty tracts which appear as dark areas when seen under the light microscope, because they are filled with air.
Sclerotic dentin:
-it happens due to:
1- Aging, it’s called physiological dentine sclerosis
2- mild-irritants as a reactive process, it’s called reactive dentine sclerosis
-the peritubular dentine closes the lumens of the dentinal tubules

Smear Layer:
-any processes we do on the dentin (cutting, cavity preparation, etc…) will result with surface debris. It’s a layer composed of denatured collagen. This layer is attached to the surface dentin, but not firmly so it can be removed by acids.
-In case it was not removed, it prevents the material binding to the tooth structure (I think the Dr. is referring to the bonding of the composite restoration to the tooth structure) note: sometimes we have to remove it, sometimes we have to keep it.
-in a longitudinal section, we can see something called smear plug.
-clinical significance of the smear layer: it decreases the sensitivity of the dentinal tubules, because it prevents the external stimuli from triggering a flow of fluids inside the tubules, thus eliminating pain.


How to differentiate Enamel from Dentin Clinically:

Enamel Dentine
Color translucent light-white yellow
Reflectance shinning dull
Hardness harder softer
Sound highly pitched no sound
Notes:
-reflectance: dentine is dull because it doesn’t reflect as much light as enamel does
-hardness : the instruments we use to cut the enamel are different from ones we use to cut the dentine, if we used the same instruments , it will be an X speed with enamel and 4 X speed with dentine so it’s faster to work on dentine because it’s softer.


Lect. 2
27/9/2010
AMAL ABU AWWAD