NEET MDS Lessons
Dental Anatomy
Enamel
Structural characteristics and microscopic features
a. Enamel rods or prisms
(1) Basic structural unit of enamel.
(2) Consists of tightly packed hydroxyapatite crystals. Hydroxyapatite crystals in enamel are four times larger and more tightly packed than hydroxyapatite found in other calcified
tissues (i.e., it is harder than bone).
(3) Each rod extends the entire thickness of enamel and is perpendicular to the dentinoenamel junction (DEJ).
b. Aprismatic enamel
(1) The thin outer layer of enamel found on the surface of newly erupted teeth.
(2) Consists of enamel crystals that are aligned perpendicular to the surface.
(3) It is aprismatic (i.e., prismless) and is more mineralized than the enamel beneath it.
(4) It results from the absence of Tomes processes on the ameloblasts during the final stages of enamel deposition.
c. Lines of Retzius (enamel striae)
(1) Microscopic features
(a) In longitudinal sections, they are observed as brown lines that extend from the DEJ to the
tooth surface.
(b) In transverse sections, they appear as dark, concentric rings similar to growth rings in a tree.
(2) The lines appear weekly during the formation of enamel.
(3) Although the cause of striae formation is unknown, the lines may represent appositional or incremental growth of enamel. They may also result from metabolic disturbances of ameloblasts.
(4) Neonatal line
(a) An accentuated, dark line of Retzius that results from the effect of physiological changes
on ameloblasts at birth.
(b) Found in all primary teeth and some cusps of permanent first molars.
d. Perikymata
(1) Lines of Retzius terminate on the tooth surface in shallow grooves known a perikymata.
(2) These grooves are usually lost through wear but may be observed on the surfaces of developing teeth or nonmasticatory surfaces of formed teeth.
e. Hunter-Schreger bands
(1) Enamel rods run in different directions. In longitudinal sections, these changes in direction result in a banding pattern known as HunterSchreger bands.
(2) These bands represent an optical phenomenon of enamel and consist of a series of alternating dark and light lines when the section is viewed with reflected or polarized
light.
f. Enamel tufts
(1) Consist of hypomineralized groups of enamel rods.
(2) They are observed as short, dark projections found near or at the DEJ.
(3) They have no known clinical significance.
g. Enamel lamellae
(1) Small, sheet-like cracks found on the surface of enamel that extend its entire thickness.
(2) Consist of hypocalcified enamel.
(3) The open crack may be filled with organic material from leftover enamel organ components, connective tissues of the developing tooth, or debris from the oral cavity.
(4) Both enamel tufts and lamellae may be likened to geological faults in mature enamel.
h. Enamel spindle
(1) Remnants of odontoblastic processes that become trapped after crossing the DEJ during the differentiation of ameloblasts.
(2) Spindles are more pronounced beneath the cusps or incisal edges of teeth (i.e., areas where occlusal stresses are the greatest).
Introduction. The Jaws and Dental Arches
The teeth are arranged in upper and lower arches. Those of the upper are called maxillary; those of the lower are mandibular.
- The maxilla is actually two bones forming the upper jaw; they are rigidly attached to the skull..
- The mandible is a horseshoe shaped bone which articulates with the skull by way of the temporomandibular joint the TMJ.
- The dental arches, the individual row of teeth forming a tooth row attached to their respective jaw bones have a distinctive shape known as a catenary arch.
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.
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.
FUNCTIONS OF PERIODONTIUM
Tooth support
Shock absorber
Sensory (vibrations appreciated in the middle ear/reflex jaw opening)
The following cells can be identified in the periodontal ligament:
a) Osteoblasts and osteoclasts b) Fibroblasts, c) Epithelial cells
Rests of Malassez
d) Macrophages
e) Undifferentiated cells
f) Cementoblasts and cementoclasts (only in pathologic conditions)
The following types of fibers are found in the PDL
-Collagen fibers: groups of fibers
-Oxytalan fibers: variant of elastic fibers, perpendicular to teeth, adjacent to capillaries
-Eluanin: variant of elastic fibers
Ground substance
PERIODONTAL LIGAMENT FIBERS
Principal fibers
These fibers connect the cementum to the alveolar crest. These are:
a. Alveolar crest group: below CE junction, downward, outward
b. Horizontal group: apical to ACG, right angle
c. Oblique group: numerous, coronally to bone, oblique direction
d. Apical group: around the apex, base of socket
e. Interradicular group: multirooted teeth
Gingival ligament fibers
This group is not strictly related to periodontium. These fibers are:
a. Dentogingival: numerous, cervical cementum to f/a gingiva
b. Alveologingival: bone to f/a gingiva
c. Circular: around neck of teeth, free gingiva
d. Dentoperiosteal: cementum to alv. process or vestibule (muscle)
e. Transseptal: cementum between adjacent teeth, over the alveolar crest
Blood supply of the PDL
The PDL gets its blood supply from perforating arteries (from the cribriform plate of the bundle bone). The small capillaries derive from the superior & inferior alveolar arteries. The blood supply is rich because the PDL has a very high turnover as a tissue. The posterior supply is more prominent than the anterior. The mandibular is more prominent than the maxillary.
Nerve supply
The nerve supply originates from the inferior or the superior alveolar nerves. The fibers enter from the apical region and lateral socket walls. The apical region contains more nerve endings (except Upper Incisors)
Dentogingival junction
This area contains the gingival sulcus. The normal depth of the sulcus is 0.5 to 3.0 mm (mean: 1.8 mm). Depth > 3.0 mm is considered pathologic. The sulcus contains the crevicular fluid
The dentogingival junction is surfaced by:
1) Gingival epithelium: stratified squamous keratinized epithelium 2) Sulcular epithelium: stratified squamous non-keratinized epithelium The lack of keratinization is probably due to inflammation and due to high turnover of this epithelium.
3) Junctional epithelium: flattened epithelial cells with widened intercellular spaces. In the epithelium one identifies neutrophils and monocytes.
Connective tissue
The connective tissue of the dentogingival junction contains inflammatory cells, especially polymorphonuclear neutrophils. These cells migrate to the sulcular and junctional epithelium.
The connective tissue that supports the sulcular epithelium is also structurally and functionally different than the connective tissue that supports the junctional epithelium.
Histology of the Col (=depression)
The col is found in the interdental gingiva. It is surfaced by epithelium that is identical to junctional epithelium. It is an important area because of the accumulation of bacteria, food debris and plaque that can cause periodontal disease.
Blood supply: periosteal vessels
Nerve supply: periodontal nerve fibers, infraorbital, palatine, lingual, mental, buccal
Angle classified these relationships by using the first permanent molars
Normal or neutral occlusion (ideal):
Mesiobuccalgroove of the mandibular first molar align with the mesiobuccal cusp of the max laxy first permanent molar
ClassI malocclusion normal molar relationships with alterations to other characteristics of the occlusion such as versions, crossbites, excessive overjets, or overbites
Class II malocclusion a distal relation of the mesiobuccal groove of the mandibular first permanent molar to the mesiobuccal cusp of the maxillary first permanent molar
Division I: protruded maxillary anterior teeth
Division II: one or more maxillary anterior teeth retruded
Class III malocclusion a mesial relation of the mesiobuccal groove of the mandibular first permanent molar to the mesiobuccal cusp of the maxillary molar
Genetics and Environment: Introduction
The size of the teeth and the timing of the developing dentition and its eruption are genetically determined. Teeth are highly independent in their development. Also, teeth tend to develop along a genetically predetermined course.: tooth development and general physical development are rather independent of one another. Serious illness, nutritional deprivation, and trauma can significantly impact development of the teeth. This genetic independence (and their durability) gives teeth special importance in the study of evolution.
Teeth erupt full size and are ideal for study throughout life. Most important, age and sex can be recorded.
When teeth erupt into the oral cavity, a new set of factors influence tooth position. As the teeth come into function, genetic and environment determine tooth position.
In real life, however, girls shed deciduous teeth and receive their permanent teeth slightly earlier than boys, possibly reflecting the earlier physical maturation achieved by girls. Teeth are slightly larger in boys that in girls
FUNCTIONS OF PERIODONTIUM
Tooth support
Shock absorber
Sensory (vibrations appreciated in the middle ear/reflex jaw opening)