FUNCTIONS OF PERIODONTIUM

Tooth support
Shock absorber
Sensory (vibrations appreciated in the middle ear/reflex jaw opening)

The very first histological evidence of tooth development appear during the second month of intrauterine life. Calcification of deciduous incisors begins at 3-4 months in utero.

Tooth eruption Theories

Tooth eruption occurs when the teeth enter the mouth and become visible. Although researchers agree that tooth eruption is a complex process, there is little agreement on the identity of the mechanism that controls eruption. Some commonly held theories that have been disproven over time include: (1) the tooth is pushed upward into the mouth by the growth of the tooth's root, (2) the tooth is pushed upward by the growth of the bone around the tooth, (3) the tooth is pushed upward by vascular pressure, and (4) the tooth is pushed upward by the cushioned hammock. The cushioned hammock theory, first proposed by Harry Sicher, was taught widely from the 1930s to the 1950s. This theory postulated that a ligament below a tooth, which Sicher observed on under a microscope on a histologic slide, was responsible for eruption. Later, the "ligament" Sicher observed was determined to be merely an artifact created in the process of preparing the slide.

The most widely held current theory is that while several forces might be involved in eruption, the periodontal ligaments provide the main impetus for the process. Theorists hypothesize that the periodontal ligaments promote eruption through the shrinking and cross-linking of their collagen fibers and the contraction of their fibroblasts.

Although tooth eruption occurs at different times for different people, a general eruption timeline exists. Typically, humans have 20 primary (baby) teeth and 32 permanent teeth. Tooth eruption has three stages. The first, known as deciduous dentition stage, occurs when only primary teeth are visible. Once the first permanent tooth erupts into the mouth, the teeth are in the mixed (or transitional) dentition. After the last primary tooth falls out of the mouth—a process known as exfoliation—the teeth are in the permanent dentition.

Primary dentition starts on the arrival of the mandibular central incisors, usually at eight months, and lasts until the first permanent molars appear in the mouth, usually at six years. The primary teeth typically erupt in the following order: (1) central incisor, (2) lateral incisor, (3) first molar, (4) canine, and (5) second molar. As a general rule, four teeth erupt for every six months of life, mandibular teeth erupt before maxillary teeth, and teeth erupt sooner in females than males. During primary dentition, the tooth buds of permanent teeth develop below the primary teeth, close to the palate or tongue.

Mixed dentition starts when the first permanent molar appears in the mouth, usually at six years, and lasts until the last primary tooth is lost, usually at eleven or twelve years. Permanent teeth in the maxilla erupt in a different order from permanent teeth on the mandible. Maxillary teeth erupt in the following order: (1) first molar (2) central incisor, (3) lateral incisor, (4) first premolar, (5) second premolar, (6) canine, (7) second molar, and (8) third molar. Mandibular teeth erupt in the following order: (1) first molar (2) central incisor, (3) lateral incisor, (4) canine, (5) first premolar, (6) second premolar, (7) second molar, and (8) third molar. Since there are no premolars in the primary dentition, the primary molars are replaced by permanent premolars. If any primary teeth are lost before permanent teeth are ready to replace them, some posterior teeth may drift forward and cause space to be lost in the mouth. This may cause crowding and/or misplacement once the permanent teeth erupt, which is usually referred to as malocclusion. Orthodontics may be required in such circumstances for an individual to achieve a straight set of teeth.

The permanent dentition begins when the last primary tooth is lost, usually at 11 to 12 years, and lasts for the rest of a person's life or until all of the teeth are lost (edentulism). During this stage, third molars (also called "wisdom teeth") are frequently extracted because of decay, pain or impactions. The main reasons for tooth loss are decay or periodontal disease.

Classification of Cementum

1. Embryologically

Primary and secondary

2. According to cellular component

Acellular: Thin, Amorphous, First layer to seal the dentin tubules

Cellular: Thick, Better structure, Apical surface

Layers of cellular and acellular cementum alternate (randomly)

3. Based on the origin of the collagenous matrix
Extrinsic
Intrinsic
Mixed

4. Combined classification
a. Primary acellular intinsic fiber cementum
b. Primary acellualar extrinsic fiber cementum
c. Secondary cellular intrinsic fiber cementum
d. Secondary cellular mixed fiber cementum
e. Acellular afibrillar cementum

5. Depending on the location and patterning
Intermediate and mixed stratified cementum

Participating Cells

Cementoblasts

Active
Cells are round, plump with basophilic cytoplasm (rough endoplasmic reticulum)
Inactive
Cells have little cytoplasm
Cementocytes

1. Cementocyte lacuna
2. cementocyte canaliculus

Cells have fewer organelles compared to cementoblasts. They are found in lacunae and have numerous processes toward the periodontal ligament. Eventually they die due to avascularity

Cementicles

a) free
b) attached
c) embedded

Differences Between the Deciduous and Permanent Teeth

1. Deciduous teeth are fewer in number and smaller in size but the deciduous molars are wider mesiodistally than the premolars. The deciduous anteriors are narrower mesiodistally than their permanent successors. Remember the leeway space that we discussed in the unit on occlusion?

2. Their enamel is thinner and whiter in appearance. Side by side, this is obvious in most young patients.

3. The crowns are rounded. The deciduous teeth are constricted at the neck (cervix).

4. The roots of deciduous anterior teeth are longer and narrower than the roots of their permanent successors.

5. The roots of deciduous molars are longer and more slender than the roots of the permanent molars. Also, they flare greatly.

6. The cervical ridges of enamel seen on deciduous teeth are more prominent than on the permanent teeth. This 'bulge' is very pronounced at the mesiobuccal of deciduous first molars.

G. Deciduous cervical enamel rods incline incisally/occlusally.

MANDIBULAR LATERAL INCISORS

The mandibular incisor is a little wider mesiodistal than the mandibular central incisor, and the crown is slightly longer from the incisal edge to the cervical line.

Facial Surface:-The facial surface is less symmetrical than the facial surface of the mandibular central incisor. The incisal edge slopes upward toward the mesioincisal angle, which is slightly less than 90°. The distoincisal angle is rounded. The mesial border is more nearly straight than the distal border.

Lingual Surface:- The incisal portion of the lingual surface is concave. The cingulum is quite large but blends in smoothly with the rest of the surface.

Root Surface:-The root is single and extremely flattened on its mesial and distal surfaces.