NEET MDS Lessons
Dental Anatomy
Abnormalities
There are a number of tooth abnormalities relating to development.
Anodontia is a complete lack of tooth development, and hypodontia is a lack of some tooth development. Anodontia is rare, most often occurring in a condition called hipohidrotic ectodermal dysplasia, while hypodontia is one of the most common developmental abnormalities, affecting 3.5–8.0% of the population (not including third molars). The absence of third molars is very common, occurring in 20–23% of the population, followed in prevalence by the second premolar and lateral incisor. Hypodontia is often associated with the absence of a dental lamina, which is vulnerable to environmental forces, such as infection and chemotherapy medications, and is also associated with many syndromes, such as Down syndrome and Crouzon syndrome.
Hyperdontia is the development of extraneous teeth. It occurs in 1–3% of Caucasians and is more frequent in Asians. About 86% of these cases involve a single extra tooth in the mouth, most commonly found in the maxilla, where the incisors are located. Hyperdontia is believed to be associated with an excess of dental lamina.
Dilaceration is an abnormal bend found on a tooth, and is nearly always associated with trauma that moves the developing tooth bud. As a tooth is forming, a force can move the tooth from its original position, leaving the rest of the tooth to form at an abnormal angle. Cysts or tumors adjacent to a tooth bud are forces known to cause dilaceration, as are primary (baby) teeth pushed upward by trauma into the gingiva where it moves the tooth bud of the permanent tooth.
Regional odontodysplasia is rare, but is most likely to occur in the maxilla and anterior teeth. The cause is unknown; a number of causes have been postulated, including a disturbance in the neural crest cells, infection, radiation therapy, and a decrease in vascular supply (the most widely held hypothesis).Teeth affected by regional odontodysplasia never erupt into the mouth, have small crowns, are yellow-brown, and have irregular shapes. The appearance of these teeth in radiographs is translucent and "wispy," resulting in the nickname "ghost teeth"
Formation and Eruption of Deciduous Teeth.
-Calcification begins during the fourth month of fetal life. By the end of the sixth month, all of the deciduous teeth have begun calcification.
-By the time the deciduous teeth have fully erupted (two to two and one half years of age), cacification of the crowns of permanent teeth is under way. First permanent molars have begun cacification at the time of birth. -Here are some things to know about eruption patterns:
(1) Teeth tend to erupt in pairs.
(2) Usually, lower deciduous teeth erupt first. Congenitally missing deciduous teeth is infrequent. Usually, the lower deciduous central incisors are thefirst to erupt thus initiating the deciduous dentition. The appearance of the deciduous second molars completes the deciduous dentition by 2 to 2 1/2 years of age.
- Deciduous teeth shed earlier and permanent teeth erupt earlier in girls.
- The orderly pattern of eruption and their orderly replacement by permanent teeth is important.
- order for eruption of the deciduous teeth is as follows:
(1) Central incisor.........Lower 6 ½ months, Upper 7 ½ months
(2) Lateral incisor.........Lower 7 months, Upper 8 months
(3) First deciduous molar...Lower 12-16 months, Upper 12-16 months
(4) Deciduous canine........Lower 16-20 months, Upper 16-20 months
(5) Second deciduous molar..Lower 20-30 months, Upper 20-30 months
Types of dentitions:
1. Diphyodont. Teeth develop and erupt into their jaws in two generations of teeth. The term literally means two generations of teeth.
2. Monophyodont. a single generation of teeth.
3. Polyphyodont. Teeth develop a lifetime of generations of successional teeth
4. Homodont. all of the teeth in the jaw are alike. They differ from each other only in size.
5. Heterodont. There is distinctive classes of teeth that are regionally specialized.
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.
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
Amelogenesis and Enamel
Enamel is highly mineralized: 85% hydroxyapatite crystals
Enamel formation is a two-step process
The first step produces partially mineralized enamel: 30% (secretory)
The second step: Influx of minerals, removal of water and organic matrix (maturative)
Again, dentin is the prerequisite of enamel formation (reciprocal induction)
Stratum intermedium: high alkaline phosphatase activity
Differentiation of ameloblasts: Increase in glycogen contents
Formation of the enamel matrix
Enamel proteins, enzymes, metalloproteinases, phosphatases, etc.
Enamel proteins: amelogenins (90%), enamelin, tuftelin, and amelin
Amelogenins: bulk of organic matrix
Tuftelin: secreted at the early stages of amelogenesis (area of the DE junction)
Enamelin: binds to mineral
Amelin
Mineralization of enamel
No matrix vesicles
Immediate formation of crystallites
Intermingling of enamel crystallites with dentin
"Soft" enamel is formed
Histologic changes
Differentiation of inner enamel epithelium cells. They become ameloblasts
Tomes' processes: saw-toothed appearance
Collapse of dental organ
Formation of the reduced enamel epithelium
Hard tissue formation (Amelogenesis )
Enamel formation is called amelogenesis and occurs in the crown stage of tooth development. "Reciprocal induction" governs the relationship between the formation of dentin and enamel; dentin formation must always occur before enamel formation. Generally, enamel formation occurs in two stages: the secretory and maturation stages. Proteins and an organic matrix form a partially mineralized enamel in the secretory stage; the maturation stage completes enamel mineralization.
In the secretory stage, ameloblasts release enamel proteins that contribute to the enamel matrix, which is then partially mineralized by the enzyme alkaline phosphatase. The appearance of this mineralized tissue, which occurs usually around the third or fourth month of pregnancy, marks the first appearance of enamel in the body. Ameloblasts deposit enamel at the location of what become cusps of teeth alongside dentin. Enamel formation then continues outward, away from the center of the tooth.
In the maturation stage, the ameloblasts transport some of the substances used in enamel formation out of the enamel. Thus, the function of ameloblasts changes from enamel production, as occurs in the secretory stage, to transportation of substances. Most of the materials transported by ameloblasts in this stage are proteins used to complete mineralization. The important proteins involved are amelogenins, ameloblastins, enamelins, and tuftelins. By the end of this stage, the enamel has completed its mineralization.
Stationary Relationship
a) .Centric Relation is the most superior relationship of the condyle of the mandible to the articular fossa of the temporal bone as determined by the bones ligaments. and muscles of the temporomandibular joint; in an ideal dentition it is the same as centric occlusion.
(b) Canines may also be used to confirm the molar relationships to classify occlusion when molars are missing; a class I canine relationship shows the cusp tip of the maxillary canine facial to the mesiobuccal cusp of the first permanent molar
c) Second primary molars are used to classify the occlusion in a primary dentition
(d) In a mixed dentition the first permanent molars will erupt into a normal occlusion if there is a terminal step between the distal surfaces of maxillarv and mandibular second primary molars; if these surfaces are flush, a terminal plane exists and the first permanent molars will first erupt into an end-to-end relationship until there is a shifting of space or exfoliation of the second primary molar