NEET MDS Lessons
Dental Anatomy
MANDIBULAR CUSPIDS
Mandibular canines are those lower teeth that articulate with the mesial aspect of the upper canine.
Facial: The mandibular canine is noticeably narrower mesidistally than the upper, but the root may be as long as that of the upper canine. In an individual person,the lower canine is often shorter than that of the upper canine. The mandibular canine is wider mesiodistally than either lower incisor. A distinctive feature is the nearly straight outline of the mesial aspect of the crown and root. When the tooth is unworn, the mesial cusp ridge appears as a sort of 'shoulder' on the tooth. The mesial cusp ridge is much shorter than the distal cusp ridge.
Lingual: The marginal ridges and cingulum are less prominent than those of the maxillary canine. The lingual surface is smooth and regular. The lingual ridge, if present, is usually rather subtle in its expression.
Proximal: The mesial and distal aspects present a triangular outline. The cingulum as noted is less well developed. When the crown and root are viewed from the proximal, this tooth uniquely presents a crescent-like profile similar to a cashew nut.
Incisal: The mesiodistal dimension is clearly less than the labiolingual dimension. The mesial and distal 'halves' of the tooth are more identical than the upper canine from this perspective. In the mandibular canine, the unworn incisal edge is on the line through the long axis of this tooth.
LOCATION OF THE TEETH
Normally, a human receives two sets of teeth during a lifetime.
The first (deciduous or primary) set consists of 20 teeth ("baby" teeth).
The second (permanent) set usually consists of 32 teeth. In each quadrant, there are eight permanent teeth: two incisors, one cuspid, two bicuspids, and three molars
The tooth positioned immediately to the side of the midline is the central incisor, so called because it occupies a central location in the arch.
To the side of the central incisor is the lateral incisor. Next is the cuspid, then the two bicuspids (the first bicuspid, followed by the second bicuspid). The last teeth are three molars. After the second bicuspid comes the first molar, followed by the second molar, followed by the third molar or more commonly called the "wisdom tooth."
Another method of describing the location of teeth is to refer to them as anterior or posterior teeth .
Anterior teeth are those located in the front of the mouth, the incisors, and the cuspids. Normally, these are the teeth that are visible when a person smiles.
The posterior teeth are those located in the back of the mouth-the bicuspids and molars.
Classification of Cementum
- Embryologically
Primary and secondary
2. According to cellular component
Acellular: Thin, Amorphous, First layer to seal the dentin tubules
Cellular: Thick, Better structure, Apical surface
Layers of cellular and acellular cementum alternate (randomly)
3. Based on the origin of the collagenous matrix
Extrinsic
Intrinsic
Mixed
4. Combined classification
a. Primary acellular intinsic fiber cementum
b. Primary acellualar extrinsic fiber cementum
c. Secondary cellular intrinsic fiber cementum
d. Secondary cellular mixed fiber cementum
e. Acellular afibrillar cementum
5. Depending on the location and patterning
Intermediate and mixed stratified cementum
Participating Cells
Cementoblasts
Active
Cells are round, plump with basophilic cytoplasm (rough endoplasmic reticulum)
Inactive
Cells have little cytoplasm
Cementocytes
- Cementocyte lacuna
- cementocyte canaliculus
Cells have fewer organelles compared to cementoblasts. They are found in lacunae and have numerous processes toward the periodontal ligament. Eventually they die due to avascularity
Cementicles
a) free
b) attached
c) embedded
Deciduous dentition period.
-The deciduous teeth start to erupt at the age of six months and the deciduous dentition is complete by the age of approximately two and one half years of age.
-The jaws continue to increase in size at all points until about age one year.
-After this, growth of the arches is lengthening of the arches at their posterior (distal) ends. Also, there is slightly more forward growth of the mandible than the maxilla.
1. Many early developmental events take place.
-The tooth buds anticipate the ultimate occlusal pattern.
-Mandibular teeth tend to erupt first. The pattern for the deciduous incisors is usually in this distinctive order:
(1) mandibular central
(2) maxillary central incisors
(3) then all four lateral incisors.
-By one year, the deciduous molars begin to erupt.
-The eruption pattern for the deciduous dentition as a whole is:
(1) central incisor
(2) lateral incisor
(3) deciduous first molar
(4) then the canine
(5) then finally the second molar.
-Eruption times can be variable.
2. Occlusal changes in the deciduous dentition.
-The overjet tends to diminish with age. Wear and mandibular growth are a factor in this process.
-The overbite often diminishes with the teeth being worn to a flat plane occlusion.
-Spacing of the incisors in anticipation of the soon-to-erupt permanent incisors appears late. Permanent anterior teeth (incisors and canines) are wider mesiodistally than deciduous anterior teeth. In contrast, the deciduous molar are wider mesiodistally that the premolars that later replace them.
-Primate spaces occur in about 50% of children. They appear in the deciduous dentition. The spaces appear between the upper lateral incisor and the upper canine. They also appear between the lower canine and the deciduous first molar.
Bell stage
The bell stage is known for the histodifferentiation and morphodifferentiation that takes place. The dental organ is bell-shaped during this stage, and the majority of its cells are called stellate reticulum because of their star-shaped appearance. Cells on the periphery of the enamel organ separate into three important layers. Cuboidal cells on the periphery of the dental organ are known as outer enamel epithelium.The cells of the enamel organ adjacent to the dental papilla are known as inner enamel epithelium. The cells between the inner enamel epithelium and the stellate reticulum form a layer known as the stratum intermedium. The rim of the dental organ where the outer and inner enamel epithelium join is called the cervical loop
Other events occur during the bell stage. The dental lamina disintegrates, leaving the developing teeth completely separated from the epithelium of the oral cavity; the two will not join again until the final eruption of the tooth into the mouth
The crown of the tooth, which is influenced by the shape of the internal enamel epithelium, also takes shape during this stage. Throughout the mouth, all teeth undergo this same process; it is still uncertain why teeth form various crown shapes—for instance, incisors versus canines. There are two dominant hypotheses. The "field model" proposes there are components for each type of tooth shape found in the ectomesenchyme during tooth development. The components for particular types of teeth, such as incisors, are localized in one area and dissipate rapidly in different parts of the mouth. Thus, for example, the "incisor field" has factors that develop teeth into incisor shape, and this field is concentrated in the central incisor area, but decreases rapidly in the canine area. The other dominant hypothesis, the "clone model", proposes that the epithelium programs a group of ectomesenchymal cells to generate teeth of particular shapes. This group of cells, called a clone, coaxes the dental lamina into tooth development, causing a tooth bud to form. Growth of the dental lamina continues in an area called the "progress zone". Once the progress zone travels a certain distance from the first tooth bud, a second tooth bud will start to develop. These two models are not necessarily mutually exclusive, nor does widely accepted dental science consider them to be so: it is postulated that both models influence tooth development at different times.Other structures that may appear in a developing tooth in this stage are enamel knots, enamel cords, and enamel niche.
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
ERUPTION
. Root completion (approximately 50% of the root is formed when eruption begins)
Generally mandibular teeth erupt before maxillary teeth,
Primary teeth
I. Emerge into the oral cavity as follows:
Maxillary Mandibular
Central Incisor 7½ months 6 months
Lateral incisor 9 months 7 months
Canine 18 months 16 months
First Molar 14 months 12 months
Second Molar 24months 20 months
The sequence of primary tooth development is central incisor, lateral incisor, first molar, second molar
3. Hard tissue formation begins between 4 and 6 months in utero
4. Crowns completed between 1½ and 10 months of age
5. Roots are completed between I½ and3 yearsof age 6 to 18 months after eruption
6. By age 3 years all of the primary and permanent teeth (except for the third molars) are in some stage of development
7. Root resorption of primary teeth is triggered by the pressure exerted by the developing permanent tooth; it is followed by primary tooth exfoliation in sequential patterns
8. The primary dentition ends when the first permanent tooth erupts