NEET MDS Lessons
Dental Anatomy
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.
Embryonic development
The parotid derives from ectoderm
The sublingual-submandibular glands thought to derive from endoderm
Differentiation of the ectomesenchyme
Development of fibrous capsule
Formation of septa that divide the gland into lobes and lobules
The parotid develops around 4-6 weeks of embryonic lofe
The submandibular gland develops around the 6th week
The sublingual and the minor glands develop around the 8-12 week
TYPES OF TEETH
The human permanent dentition is divided into four classes of teeth based on appearance and function or position.
Incisors, Canines, Premolars & Molars
MANDIBULAR THIRD MOLAR
Facial: The crown is often short and has a rounded outline.
Lingual: Similarly, the crown is short and the crown is bulbous.
Proximal: Mesially and distally, this tooth resembles the first and second molars. The crown of the third molar, however, is shorter than either of the other molars
Occlusal: Four or five cusps may be present. Occlusal surface is a same as of the first or second molar, or poorly developed with many accessory grooves. The occlusal outline is often ovoid and the occlusal surface is constricted. Occasionally, the surface has so many grooves that it is described as crenulated--a condition seen in the great apes
Contact Points; The rounded mesial surface has its contact area more cervical than any other lower molar. There is no tooth distal to the third molar..
Roots:-The roots, two in number, are shorter in length and tend to be fused together. they show a distinct distal curve
ARTICULAR SURFACES COVERED BY FIBROUS TISSUE
TMJ is an exception form other synovial joints. Two other joints, the acromio- and sternoclavicular joints are similar to the TMJ. Mandible & clavicle derive from intramembranous ossificiation.
Histologic
- Fibrous layer: collagen type I, avascular (self-contained and replicating)
- Proliferating zone that formes condylar cartilage
- Condylar cartilage is fibrocartilage that does not play role in articulation nor has formal function
- Capsule: dense collagenous tissue (includes the articular eminence)
- Synovial membrane: lines capsule (does not cover disk except posterior region); contains folds (increase in pathologic conditions) and villi
Two layers: a cellular intima (synovial cells in fiber-free matrix) and a vascular subintima
Synovial cells: A (macrophage-like) syntesize hyaluronate
B (fibroblast-like) add protein in the fluid
Synovial fluid: plasma with mucin and proteins, cells
Liquid environment: lubrication, ?nutrition - Disk: separates the cavity into two comprartments, type I collagen
anterior and posterior portions
anetiorly it divides into two lamellae one towards the capsule, the other towards the condyle
vascular in the preiphery, avascular in the center - Ligaments: nonelastic collagenous structures. One ligament worth mentioning is the lateral or temporomandibular ligament. Also there are the spheno- and stylomandibular with debatable functional role.
Innervations
|
Ruffini |
Posture |
Dynamic and static balance |
|
Pacini |
Dynamic mechanoreception |
Movement accelerator |
|
Golgi |
Static mechanoreception |
Protection (ligament) |
|
Free |
Pain |
Protection joint |
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.
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"