NEET MDS Lessons
Dental Anatomy
AGE CHANGES
Progressive apical migration of the dentogingival junction.
Toothbrush abrasion of the area can expose dentin that can cause root caries and tooth mobility.
Histology of the alveolar bone
Near the end of the 2nd month of fetal life, mandible and maxilla form a groove that is opened toward the surface of the oral cavity.
As tooth germs start to develop, bony septa form gradually. The alveolar process starts developing strictly during tooth eruption.
The alveolar process is the bone that contains the sockets (alveoli) for the teeth and consists of
a) outer cortical plates
b) a central spongiosa and
c) bone lining the alveolus (bundle bone)
The alveolar crest is found 1.5-2.0 mm below the level of the CEJ.
If you draw a line connecting the CE junctions of adjacent teeth, this line should be parallel to the alveolar crest. If the line is not parallel, then there is high probability of periodontal disease.
Bundle Bone
The bundle bone provides attachment to the periodontal ligament fibers. It is perforated by many foramina that transmit nerves and vessels (cribiform plate). Embedded within the bone are the extrinsic fiber bundles of the PDL mineralized only at the periphery. Radiographically, the bundle bone is the lamina dura. The lining of the alveolus is fairly smooth in the young but rougher in the adults.
Clinical considerations
Resorption and regeneration of alveolar bone
This process can occur during orthodontic movement of teeth. Bone is resorbed on the side of pressure and opposed on the site of tension.
Osteoporosis
Osteoporosis of the alveolar process can be caused by inactivity of tooth that does not have an antagonist
The very first histological evidence of tooth development appear during the second month of intrauterine life. Calcification of deciduous incisors begins at 3-4 months in utero.
CEMENTUM vs. BONE
Cementum simulates bone
1) Organic fibrous framework, ground substance, crystal type, development
2) Lacunae
3) Canaliculi
4) Cellular components
5) Incremental lines (also known as "resting" lines; they are produced by continuous but phasic, deposition of cementum)
Differences between cementum and bone
1) Cementum is not vascularized
2) Cementum has minor ability to remodel
3) Cementum is more resistant to resorption compared to bone
4) Cementum lacks neural component
5) Cementum contains a unique proteoglycan interfibrillar substance
6) 70% of bone is made by inorganic salts (cementum only 46%)
Relation of Cementum to Enamel at the Cementoenamel Junction (CEJ)
"OMG rule"
In 60% of the teeth cementum Overlaps enamel
In 30% of the teeth cementum just Meets enamel
In 10% of the teeth there is a small Gap between cementum and enamel
Age changes in the dentition
I. After the teeth have reached full occlusion, microscopic tooth movements occur to compensate for wear at the contact area (Mesial Drift) and occlusal surfaces (by Deposition of cementum at the root apex)
2. Attrition of incisal ridges and cusp tips may be so severe that dentin may become exposed and intrinsically stained
3. Secondary dentin may be formed in response to dental caries, trauma, and aging and result in decreased pulp size and tooth sensation
FORMATION OF THE ROOT AND ITS ROLE IN ERUPTION
- As dentin and enamel is deposited the shape of the future crown appears.
- The cells just superficial to the horizontal diaphragm start to proliferate and grow pushing the horizontal diaphragm down into the mesenchym.
- This forms a tube.
- This tube is the epithelial root sheath of Hertwig's.
- The mesenchym cells lying inside the tube nearest to the epithelial root sheath are induced to differentiate into odontoblasts, which then start to deposit dentin.
- After the first dentin of the root has been laid down the inner epithelial cells of the sheath start to deposit an enameloid substance called intermediate cementum.
- The root sheath cells then separate from the intermediate cementum and breaks up in a network of epithelial strands.
- The mesenchym on the outside comes into contact with the intermediate cementum and differentiate into
cementoblasts, which will deposit the cementum.
- This cementum traps the collagenic fibres, of the periodontal ligament, which are also formed.
- Epithelium of the root sheath persists as epithelial rests of Malassez. Because the epithelium of the root sheath forms from enamel epithelium it can develop into ameloblasts which will deposit enamel pearls.
- There is little space for the root to develop.
- To create space the crown is pushed out.
INNERVATION OF THE DENTIN-PULP COMPLEX
- Dentine Pulp
- Dentin
- Nerve Fibre Bundle
- Nerve fibres
The nerve bundles entering the tooth pulp consist principally of sensory afferent fibers from the trigeminal nerve and sympathetic branches from the superior cervical ganglion. There are non-myelinated (C fibers) and myelinated (less than non, A-delta, A-beta) fibers. Some nerve endings terminate on or in association with the odontoblasts and others in the predentinal tubules of the crown. Few fibers are found among odontoblasts of the root.
In the cell-free zone one can find the plexus of Raschkow.
PULP
Coronal
Occupies and resembles the crown,
Contains the pulp horns
It decreases in size with age
Radicular
Occupies roots
Contains the apical foramen
It decreases in size with age
Accessory apical canals
PULP FUNCTIONS
Inductive: The pulp anlage initiates tooth formation and probably induces the dental organ to become a particular type of tooth.
Formative: Pulp odontoblasts develop the organic matrix and function in its calcification.
Nutritive: Nourishment of dentin through the odontoblasts.
Protective: Sensory nerves in the tooth respond almost always with PAIN to all stimuli (heat, cold, pressure, operative procedures, chamical agents).
Defensive or reparative: It responds to irritation by producing reparative dentin. The response to stimuli is inflammation.
Histologically the pulp consists of delicate collagen fibers, blood vessels, lymphatics, nerves and cells. A histologic section of the pulp reveals four cellular zones:
Odontoblastic
Cell-free (Weil)
Cell-rich
Pulp core