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"
HISTOLOGY OF THE ODONTOBLAST
Formation of Dentin
Mantle dentin: First formed dentin
Type I collagen with ground substance
Formation of the odontoblast process
Matrix vesicles
Appearance of hydroxyapatite crystals
Predentin
Primary physiologic (circumpulpal) dentin
All organic matrix is formed from odontoblasts
Smaller collagen fibers
Presence of phosphophoryn
Mineralization
Globular calcification
Interglobular dentin: Areas of incomplete calcification
Incremental lines of von Ebner: Daily, 4mm of organic matrix is deposited. Also every 5 days the arrangement of collagen fibers changes. This creates the incremental lines of von Ebner.
Intratubular dentin
Dentin tubules
S-shaped in the coronal aspect, straight in root dentin
Von Korff fibers
They are an artifact
Root Formation and Obliteration
1. In general, the root of a deciduous tooth is completely formed in just about one year after eruption of that tooth into the mouth.
2. The intact root of the deciduous tooth is short lived. The roots remain fully formed only for about three years.
3. The intact root then begins to resorb at the apex or to the side of the apex, depending on the position of the developing permanent tooth bud.
4. Anterior permanent teeth tend to form toward the lingual of the deciduous teeth, although the canines can be the exception. Premolar teeth form between the roots of the deciduous molar teeth
Crown stage
Hard tissues, including enamel and dentin, develop during the next stage of tooth development. This stage is called the crown, or maturation, stage by some researchers. Important cellular changes occur at this time. In prior stages, all of the inner enamel epithelium cells were dividing to increase the overall size of the tooth bud, but rapid dividing, called mitosis, stops during the crown stage at the location where the cusps of the teeth form. The first mineralized hard tissues form at this location. At the same time, the inner enamel epithelial cells change in shape from cuboidal to columnar. The nuclei of these cells move closer to the stratum intermedium and away from the dental papilla.
The adjacent layer of cells in the dental papilla suddenly increases in size and differentiates into odontoblasts, which are the cells that form dentin. Researchers believe that the odontoblasts would not form if it were not for the changes occurring in the inner enamel epithelium. As the changes to the inner enamel epithelium and the formation of odontoblasts continue from the tips of the cusps, the odontoblasts secrete a substance, an organic matrix, into their immediate surrounding. The organic matrix contains the material needed for dentin formation. As odontoblasts deposit organic matrix, they migrate toward the center of the dental papilla. Thus, unlike enamel, dentin starts forming in the surface closest to the outside of the tooth and proceeds inward. Cytoplasmic extensions are left behind as the odontoblasts move inward. The unique, tubular microscopic appearance of dentin is a result of the formation of dentin around these extensions.
After dentin formation begins, the cells of the inner enamel epithelium secrete an organic matrix against the dentin. This matrix immediately mineralizes and becomes the tooth's enamel. Outside the dentin are ameloblasts, which are cells that continue the process of enamel formation; therefore, enamel formation moves outwards, adding new material to the outer surface of the developing tooth.
INNERVATION OF THE DENTIN-PULP COMPLEX
- Dentine Pulp
- Dentin
- Nerve Fibre Bundle
- Nerve fibres
The nerve bundles entering the tooth pulp consist principally of sensory afferent fibers from the trigeminal nerve and sympathetic branches from the superior cervical ganglion. There are non-myelinated (C fibers) and myelinated (less than non, A-delta, A-beta) fibers. Some nerve endings terminate on or in association with the odontoblasts and others in the predentinal tubules of the crown. Few fibers are found among odontoblasts of the root.
In the cell-free zone one can find the plexus of Raschkow.
Permanent teeth
1. The permanent teeth begin formation between birth and 3 years of age (except for the third molars)
2. The crowns of permanent teeth are completed between 4 and 8 years of age, at approximately one- half the age of eruption
The sequence for permanent development
Maxillary
First molar → Central incisor → Lateral incisor → First premotar → Second pmmolar → Canine → Second molar → Third molar
Mandibular
First molar → Central incisor → Lateral incisor → Canine → First premolar → Second premolar → Second molar → Third molar
Permanent teeth emerge into the oral cavity as
Maxillary Mandibular
Central incisor 7-8 years 6-7 years
Lateral incisor 8-9 years 7-8 years
Canine 11-12 years 9-10 years
First premolar 10-Il years 10-12 years
Second premolar 10-12 years 11-12 years
First molar 6-7 years 6-7 years
Second molar 12-13 years 11-13 years
Third molar 17-21 years 17-21 years
The roots of the permanent teeth are completed between 10 and 16 years of age, 2 to 3 years after eruption
Interarch relationship can be viewed from a stationary (fixed) and a dynamic (movable ) perspective
1.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
Centric occlusion is habitual occlusion where maximum intercuspation occurs
The characteristics of centric occlusion are
(1) Overjet: or that characteristic of maxillary teeth to overlap the mandibular teeth in a horizontal direction by 1 to 2 mm the maxilla arch is slightly larger; functions to protect the narrow edge of the incisors and provide for an intercusping relation of posterior teeth
(2) Overbite or that characteristic of maxillary anterior teeth to overlap the mandibular anterior teeth in a vertical direction by a third of the lower crown height facilitates scissor like function of incisors
(3) Intercuspation. or that characteristic of posterior teeth to intermesh in a faciolingual direction The mandibular facial and maxillary lingual cusp are centric cusps yhat contact interocclusally in the opposing arch
(4) Interdigitation, or that characteristic_of that tooth to articulate with two opposing teeth (except for the mandibular central incisors and the maxillary last molars); a mandibular tooth occludes with the same tooth in the upper arch and the one mesial to it; a maxillary tooth occludes with the same tooth in the mandibular arch and the one distal to it.
2. Dynamic interarch relationshjps are result of functional mandibular movements that start and end with centric occlusion during mastication
a. Mandibular movements are
(1) Depression (opening)
(2) Elevation (closing)
(3) Protrusion (thrust forward)
(4) Retrusion (bring back)
(5) Lateral movements right and left; one side is always the working side and one the balancing or nonworking side
b. Mandibular movements from centric occlusion are guided by the maxillary teeth
(1) Protrusion is guided by the incisors called incisal guidence
(2) Lateral movments are guided by the Canines on the working side in young, unworn dentitions (cuspid rise or cuspid protected occlusion); guided by incisors and posterior teeth in older worn. dentition (incisal/group guidance)
c. As mandibular movements commence from centric occlusion, posterior teeth should disengage in protrusion the posterior teeth on the balancing side should disengage in lateral movement
d. If tooth contact occurs where teeth should be disengaged, occlusal interference or premature contacts exist.