NEET MDS Lessons
Dental Anatomy
Periodontal ligament
Composition
a. Consists mostly of collagenous (alveolodental) fibers.
Note: the portions of the fibers embedded in cementum and the alveolar bone proper are known as Sharpey’s fibers.
b. Oxytalan fibers (a type of elastic fiber) are also present. Although their function is unknown, they may play a role in the regulation of vascular flow.
c. Contains mostly type I collagen, although smaller amounts of type III and XII collagen are also present.
d. Has a rich vascular and nerve supply.
Both sensory and autonomic nerves are present.
(1) The sensory nerves in the PDL differ from pulpal nerves in that PDL nerve endings can detect both proprioception (via mechanoreceptors) and pain (via nociceptors).
(2) The autonomic nerve fibers are associated with the regulation of periodontal vascular flow.
(3) Nerve fibers may be myelinated (sensory) or unmyelinated (sensory or autonomic).
Cells
a. Cells present in the PDL include fibroblasts; epithelial cells; cementoblasts and cementoclasts; osteoblasts and osteoclasts; and immune cells such as macrophages, mast cells, or eosinophils.
b. These cells play a role in forming or destroying cementum, alveolar bone, or PDL.
c. Epithelial cells often appear in clusters, known as rests of Malassez.
Types of alveolodental fibers
a. Alveolar crest fibers—radiate downward from cementum, just below the cementoenamel junction (CEJ), to the crest of alveolar bone.
b. Horizontal fibers—radiate perpendicular to the tooth surface from cementum to alveolar bone, just below the alveolar crest.
c. Oblique fibers
(1) Radiate downward from the alveolar bone to cementum.
(2) The most numerous type of PDL fiber.
(3) Resist occlusal forces that occur along the long axis of the tooth.
d. Apical fibers
(1) Radiate from the cementum at the apex of the tooth into the alveolar bone.
(2) Resist forces that pull the tooth in an occlusal direction (i.e., forces that try to pull the tooth from its socket).
e. Interradicular fibers
(1) Only found in the furcal area of multi-rooted teeth.
(2) Resist forces that pull the tooth in an occlusal direction.
Gingival fibers
a. The fibers of the gingival ligament are not strictly part of the PDL, but they play a role in the maintainence of the periodontium.
b. Gingival fibers are packed in groups and are found in the lamina propria of gingiva
c. Gingival fiber groups:
(1) Transseptal (interdental) fibers
(a) Extend from the cementum of one tooth (just apical to the junctional epithelium), over the alveolar crest, to the corresponding area of the cementum of the adjacent tooth.
(b) Collectively, these fibers form the interdental ligament , which functions to resist rotational forces and retain adjacent teeth in interproximal contact.
(c) These fibers have been implicated as a major cause of postretention relapse of teeth that have undergone orthodontic treatment.
(2) Circular (circumferential) fibers
(a) Extend around tooth near the CEJ.
(b) Function in binding free gingiva to the tooth and resisting rotational forces.
(3) Alveologingival fibers—extend from the alveolar crest to lamina propria of free and attached gingiva.
(4) Dentogingival fibers—extend from cervical cementum to the lamina propria of free and attached gingiva.
(5) Dentoperiosteal fibers—extend from cervical cementum, over the alveolar crest, to the periosteum of the alveolar bone.
MANDIBULAR SECOND MOLAR
Facial: When compared to the first molar, the second molar crown is shorter both mesiodistally and from the cervix to the occlusal surface. The two well-developed buccal cusps form the occlusal outline. There is no distal cusp as on the first molar. A buccal developmental groove appears between the buccal cusps and passes midway down the buccal surface toward the cervix.
Lingual: The crown is shorter than that of the first molar. The occlusal outline is formed by the mesiolingual and distolingal cusps.
Proximal: The mesial profile resembles that of the first molar. The distal profile is formed by the distobuccal cusp, distal marginal ridge, and the distolingual cusp. Unlike the first molar, there is no distal fifth cusp.
Occlusal: There are four well developed cusps with developmental grooves that meet at a right angle to form the distinctive "+" pattern characteristic of this tooth.
Contact Points; When moving distally from first to third molar, the proximal surfaces become progressively more rounded. The net effect is to displace the contact area cervically and away from the crest of the marginal ridges.
Roots:-The mandibular second molar has two roots that are smaller than those of the first molar. When compared to first molar roots, those of the second tend to be more parallel and to have a more distal inclination.
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
MAXILLARY SECOND MOLAR
The second molars are often called 12-year molars because they erupt when a child is about 12 years
Facial: The crown is shorter occluso-cervically and narrower mesiodistally whe compared to the first molar. The distobuccal cusp is visibly smaller than the mesiobuccal cusp. The two buccal roots are more nearly parallel. The roots are more parallel; the apex of the mesial root is on line with the with the buccal developmental groove. Mesial and distal roots tend to be about the same length.
Lingual: The distolingual cusp is smaller than the mesiolingual cusp. The Carabelli trait is absent.
Proximal: The crown is shorter than the first molar and the palatal root has less diverence. The roots tend to remain within the crown profile.
Occlusal: The distolingual cusp is smaller on the second than on the first molar. When it is much reduced in size, the crown outline is described as 'heart-shaped.' The Carabelli trait is usually absent. The order of cusp size, largest to smallest, is the same as the first but is more exaggerated: mesiolingual, mesiobuccal, distobuccal, and distolingual.
Contact Points; Height of Curvature: Both mesial and distal contacts tend to be centered buccolingually below the marginal ridges. Since themolars become shorter, moving from first to this molar, the contacts tend to appear more toward the center of the proximal surfaces.
Roots: There are three roots, two buccal and one lingual. The roots are less divergent than the first with their apices usually falling within the crown profile. The buccal roots tend to incline to the distal.
Note: The distolingual cusp is the most variable feature of this tooth. When it is large, the occlusal is somewhat rhomboidal; when reduced in size the crown is described as triangual or 'heart-shaped.' At times, the root may be fused.
As root and cementum formation begin, bone is created in the adjacent area. Throughout the body, cells that form bone are called osteoblasts. In the case of alveolar bone, these osteoblast cells form from the dental follicle. Similar to the formation of primary cementum, collagen fibers are created on the surface nearest the tooth, and they remain there until attaching to periodontal ligaments.
Like any other bone in the human body, alveolar bone is modified throughout life. Osteoblasts create bone and osteoclasts destroy it, especially if force is placed on a tooth. As is the case when movement of teeth is attempted through orthodontics, an area of bone under compressive force from a tooth moving toward it has a high osteoclast level, resulting in bone resorption. An area of bone receiving tension from periodontal ligaments attached to a tooth moving away from it has a high number of osteoblasts, resulting in bone formation.
MORPHOLOGY OF THE DECIDUOUS TEETH
Deciduous Anterior Teeth.
-The primary anteriors are morphologically similar to the permanent anteriors.
-The incisors are relatively simple in their morphology.
-The roots are long and narrow.
-When compared to the permanent incisors, the mesiodistal dimension is relatively larger when compared to axial crown length
-At the time of eruption, mamelons are not present in deciduous incisors
-They are narrower mesiodistally than their permanent successors.
The Transition from the Deciduous to the Permanent Dentition.
1. The transition begins with the eruption of the four first permanent molars, and replacement of the lower deciduous central incisors by the permanent lower central incisors.
2. Complete resorption of the deciduous tooth roots permits exfoliation of that tooth and replacement by the permanent (successional) teeth
3. The mixed dentition exists from approximately age 6 years to approximately age 12 years. In contrast, the intact deciduous dentition is functional from age 2 - 2 /2 years of age to 6 years of age.
4. The enamel organ of each permanent anterior tooth is connected to the oral epithelium via a fibrous cord, the gubernaculum. The foramina through which it passes can be seen in youthful skulls
The deciduous second molars are particularly important. It is imperative that the deciduous second molars be preserved until their normal time of exfoliation. This prevent mesial migration of the first permanent molars.
Use a space maintainer in the event that a second deciduous molar is lost prematurely