NEET MDS Lessons
Dental Anatomy
Enamel
Composition: 96% mineral, 4% organic material and water
Crystalline calcium phosphate, hydroxyapatite
Physical characteristics: Hardness compared to mild steel; enamel is brittle
Support from dentin is necessary
Enamel has varies in thickness
Structure of enamel
Ground sections of enamel disclose the information that we have about enamel
Enamel is composed of rods
In the past we used the term prism (do not use)
Enamel rod
The rod has a cylinder-like shape and is composed of crystals that run parallel to the longitudinal axis of the rod. At the periphery of the rod the crystals flare laterally.
Interrod region: surrounds each rod; contain more enamel protein (fish scale appearance)
Rod sheath: boundary where crystals of rods meet those of the interrod region at sharp angles (We used to describe that as a keyhole configuration)
Each ameloblast forms one rod and together with adjacent ameloblasts the interrod region Very close to dentin there is no rod structure since the Tomes' processes develop after the first enamel is formed.
Striae of Retzius and cross striations
Incremental lines
Enamel structure is altered along these lines
Cross striations are also a form of incremental lines highlighting the daily secretory activity of ameloblasts
Bands of Hunter and Schreger
Optical phenomenon produced by changes in rod direction
Gnarled enamel
Twisting of rods around each other over the cusps of teeth
Enamel tufts and lamellae
They are like geologic faults
Tufts project from the DE junction, appear branched and contain greater concentrations of enamel protein than enamel
Lamellae extend from the enamel surface
Enamel spindles
Perikymata
Shallow furrows on surface of enamel formed by the striae of Retzius
Clinical importance of cementum
1) Deposition of cementum continues throughout life.
The effects of the continuous deposition of cementum are the maintenance of total length of the tooth (good) and constriction of the apical foramen (bad).
2) With age, the smooth surface of cementum becomes more irregular due to calcification of some ligament fiber bundles. This is referred to as spikes.
Behavior of cementum in pathologic conditions
ERUPTION OF THE PERMANENT TOOTH
- At the time at which the deciduous tooth erupts the tooth bud for the permanent tooth has already been building up enamel and dentin.
- When the permanent tooth starts to erupt, pressure on the root of the deciduous tooth causes resorption by the osteoclasts.
- Wolff's law states that when two hard tissues exert pressure on one another the softer of the tissues will be resorbed.
- The dentin and cementum of the root of the deciduous tooth is softer than the enamel of the permanent tooth that is why the root of the deciduous tooth is resorbed.
- Most permanent teeth have erupted and have been in use for 2 years before the root is completely formed.
The periodontium, which is the supporting structure of a tooth, consists of the cementum, periodontal ligaments, gingiva, and alveolar bone. Cementum is the only one of these that is a part of a tooth. Alveolar bone surrounds the roots of teeth to provide support and creates what is commonly called a "socket". Periodontal ligaments connect the alveolar bone to the cementum, and the gingiva is the surrounding tissue visible in the mouth.
Periodontal ligaments
Histology of the Periodontal Ligament (PDL)
Embryogenesis of the periodontal ligament
The PDL forms from the dental follicle shortly after root development begins
The periodontal ligament is characterized by connective tissue. The thinnest portion is at the middle third of the root. Its width decreases with age. It is a tissue with a high turnover rate.
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