NEET MDS Lessons
Dental Anatomy
CONTACT POINT.:-The point on the proximal surface where two adjacent teeth actually touch each other is called a contact point.
INTERPROXIMAL SPACE.:-The interproximal space is the area between the teeth. Part of the interproximal space is occupied by the interdental papilla. The interdental papilla is a triangular fold of gingival tissue. The part of the interproximal space not occupied is called the embrasure.
EMBRASURE. :-The embrasure occupies an area bordered by interdental papilla, the proximal surfaces of the two adjacent teeth, and the contact point (fig 4-18). If there is no contact point between the teeth, then the area between them is called a diastema instead of an embrasure.
OCCLUSAL
The occlusal surface is the broad chewing surface found on posterior teeth (bicuspids and molars).
OCCLUSION.:-Occlusion is the relationship between the occlusal surfaces of maxillary and mandibular teeth when they are in contact. Many patterns of tooth contact are possible. Part of the reason for the variety is the mandibular condyle's substantial range of movement within the temporal mandibular joint.
Malocclusion occurs when any abnormality in occlusal relationships exist in the dentition. Centric occlusion, is the centered contact position of the chewing surfaces of mandibular teeth on the chewing surface (occlusal) of the maxillary teeth.
OCCLUSAL PLANE.:-Maxillary and mandibular teeth come into centric occlusion and meet along anteroposterior and lateral curves. The anteroposterior curve is called the Curve of Spee in which the mandibular arch forms a concave (a bowl-like upward curve). The lateral curve is called the Curve of Wilson . The composite (combination) of these curves form a line called the occlusal plane, and is created by the contact of the upper and lower teeth
VERTICAL AND HORIZONTAL OVERLAP. :-Vertical overlap is the extension of the maxillary teeth over the mandibular counterparts in a vertical direction when the dentition is in centric occlusion Horizontal overlap is the projection of maxillary teeth over antagonists (something that opposes another) in a horizontal direction.
KEY TO OCCLUSION.:-The occlusal surfaces of opposing teeth bear a definite relationship to each other. In normal jaw relations and when teeth are of normal size and in the correct position, the mesiofacial cusp of the maxillary first molar occludes in the facial groove of the mandibular first molar. This normal relationship of these two teeth is called the key to occlusion.
PERMANENT DENTITION
The permanent dentition consists of 32 teeth. Each tooth in the permanent dentition is described in this section. It should be remembered that teeth show considerable variation in size, shape, and other characteristics from one person to another. Certain teeth show a greater tendency than others to deviate from the normal. The descriptions that follow are of normal teeth.
Tooth development is commonly divided into the following stages: the bud stage, the cap, the bell, and finally maturation. The staging of tooth development is an attempt to categorize changes that take place along a continuum; frequently it is difficult to decide what stage should be assigned to a particular developing tooth. This determination is further complicated by the varying appearance of different histological sections of the same developing tooth, which can appear to be different stages.
Bud stage
The bud stage is characterized by the appearance of a tooth bud without a clear arrangement of cells. The stage technically begins once epithelial cells proliferate into the ectomesenchyme of the jaw. The tooth bud itself is the group of cells at the end of the dental lamina.
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
Disturbances to interarch alignment are
a. Excessive overbite where the incisal edge of the maxillary incisors extend to the cervical third of the mandibular incisors
b. Excessive overjet where the maxillary teeth overjet the mandibular teeth by more than 3mm
c. End-to-end relationship: edge-to edge bite where the anterior teeth meet at there incisal edge with no overjet or overbite; cusp-to bite where the posterior teeth meet cusp to cusp with no interdigitation
d. Crossbite where the normal faciolingual relationship of the maxillary to the mandibular teeth is altered for the anterior.teeth. the mandibular tooth or teeth are facial rather than lingual to the maxillary teeth for the posterior teeth, normal inercuspaton is not seen
FORMATION OF THE ROOT AND ITS ROLE IN ERUPTION
- As dentin and enamel is deposited the shape of the future crown appears.
- The cells just superficial to the horizontal diaphragm start to proliferate and grow pushing the horizontal diaphragm down into the mesenchym.
- This forms a tube.
- This tube is the epithelial root sheath of Hertwig's.
- The mesenchym cells lying inside the tube nearest to the epithelial root sheath are induced to differentiate into odontoblasts, which then start to deposit dentin.
- After the first dentin of the root has been laid down the inner epithelial cells of the sheath start to deposit an enameloid substance called intermediate cementum.
- The root sheath cells then separate from the intermediate cementum and breaks up in a network of epithelial strands.
- The mesenchym on the outside comes into contact with the intermediate cementum and differentiate into
cementoblasts, which will deposit the cementum.
- This cementum traps the collagenic fibres, of the periodontal ligament, which are also formed.
- Epithelium of the root sheath persists as epithelial rests of Malassez. Because the epithelium of the root sheath forms from enamel epithelium it can develop into ameloblasts which will deposit enamel pearls.
- There is little space for the root to develop.
- To create space the crown is pushed out.
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).
FORMATION OF THE PERMANENT DENTITION
Twenty deciduous tooth buds are formed initially.
Proliferative activity of the dental lamina during the bell stage that leads to formation of permanent tooth buds (cap stage) lingual of each deciduous tooth germ.
Molars have no predecessors; they are formed by posterior proliferation of the dental lamina.
HARD TISSUE FORMATION
Hard tissue formation starts at the late stages of the bell stage.
Differentiatioin of cells into odontoblasts and ameloblasts.
The cells of the inner dental epithelium will become ameloblasts.
The cells of the dental papilla opposite to the inner dental epithelium will become odontoblasts.
Dentin is formed before enamel.
Dentin initiates the formation of enamel.
ROOT FORMATION
The root of the tooth is composed by dentin and cementum.
Dentinogenesis is initiated by the odontoblasts.
Odontoblasts are formed as epithelial cells continue to proliferate from the cervical loop as a double layer of cells known as Hertwig's root sheath.
TOOTH SHAPE
The shape of the crowns results from the interaction of inner dental epithelium and the dental papilla.
The cells of the inner dental epithelium have a programmed proliferation.
This internal program determines the tooth form.
The fate of the dental lamina
Rests of Serres
The rest of Serres are rests of the dental lamina identified in the gingival soft tissues.
They are round to ovoid aggregates of epithelial cells that have clear cytoplasm (glucogen rich).
They result from early breakup of the dental lamina during bell stage.
Rests of Malassez
The rests of Malassez result from breakup of the Hertwig's root sheath during root formation.
They can be identified in the periodontal ligament and are responsible for the development of radicular cysts.