NEET MDS Lessons
Dental Anatomy
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
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Ruffini |
Posture |
Dynamic and static balance |
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Pacini |
Dynamic mechanoreception |
Movement accelerator |
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Golgi |
Static mechanoreception |
Protection (ligament) |
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Free |
Pain |
Protection joint |
Cementum
Composition
a. Inorganic (50%)—calcium hydroxyapatite crystals.
b. Organic (50%)—water, proteins, and type I collagen.
c. Note: Compared to the other dental tissues, the composition of cementum is most similar to bone; however, unlike bone, cementum is avascular (i.e., no Haversian systems or other vessels are present).
Main function of cementum is to attach PDL fibers to the root surface.
Cementum is generally thickest at the root apex and in interradicular areas of multirooted
Types of cementum
a. Acellular (primary) cementum
(1) A thin layer of cementum that surrounds the root, adjacent to the dentin.
(2) May be covered by a layer of cellular cementum, which most often occurs in the middle and apical root.
(3) It does not contain any cells.
b. Cellular (secondary) cementum
(1) A thicker, less-mineralized layer of cementum that is most prevalent along the apical root and in interradicular (furcal) areas of multirooted teeth.
(2) Contains cementocytes.
(3) Lacunae and canaliculi:
(a) Cementocytes (cementoblasts that become trapped in the extracellular matrix during cementogenesis) are observed in their entrapped spaces, known as lacunae.
(b) The processes of cementocytes extend through narrow channels called canaliculi.
(4) Microscopically, the best way to differentiate between acellular and cellular cementum is the presence of lacunae in cellular cementum.
Mixed Dentition Period.
-Begins with the eruption of the first permanent molars distal to the second deciduous molars. These are the first teeth to emerge and they initially articulate in an 'end-on' (one on top of the other) relationship.
-On occasion, the permanent incisors spread out due to spacing. In the older literature, is called by the 'ugly duckling stage.' With the eruption of the permanent canines, the spaces often will close.
-Between ages 6 and 7 years of age there are:
20 deciduous teeth
4 first permanent molars
28 permanent tooth buds in various states of development
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Maxillary (upper) teeth |
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Primary teeth |
Central |
Lateral |
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First |
Second |
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Initial calcification |
14 wk |
16 wk |
17 wk |
15.5 wk |
19 wk |
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Crown completed |
1.5 mo |
2.5 mo |
9 mo |
6 mo |
11 mo |
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Root completed |
1.5 yr |
2 yr |
3.25 yr |
2.5 yr |
3 yr |
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Mandibular (lower) teeth |
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Initial calcification |
14 wk |
16 wk |
17 wk |
15.5 wk |
18 wk |
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Crown completed |
2.5 mo |
3 mo |
9 mo |
5.5 mo |
10 mo |
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Root completed |
1.5 yr |
1.5 yr |
3.25 yr |
2.5 yr |
3 yr |
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1. Errors in development. These are usually genetic.
a. Variability of the individual teeth. In general, the teeth most distal in any class are the most variable.
b. Partial or total anodontia. missing teeth in children,
c. Supernumerary teeth.
d. Microdontia
e. Macrodontia
F. Microdontia
2. Errors in skeletal alignment. Malpositioned jaws disrupt normal tooth relationships.
3. Soft tissue problems.
-Ocasionally, the proper eruption of a tooth is prevented by fibrous connective tissue over the crown of the tooth.
-In the mixed dentition, the deciduous second molars have a special importance for the integrity of the permanent dentition. Consider this: The first permanent molars at age six years erupt distal to the second deciduous molars.
-Permanent posterior teeth exhibit physiological mesial drift, the tendency to drift mesially when space is available. If the deciduous second molars are lost prematurely, the first permanent molars drift anteriorly and block out the second premolars.
An incisor diastema may be present. The plural for diastema is diastemata.
-Important: The deciduous anteriors--incisors and canines are narrower than their permanent successors mesiodistally.
-Important: The deciduous molars are wider that their permanent successors mesiodistally.
-This size difference has clinical significance. The difference is called the leeway space.
The leeway space in the lower arch is approximately 3.4 mm.
-The leeway space in the upper arch is approximately 1.8 mm. In normal development, the leeway space is taken up by the mesial migration of the first permanent molars.