The mixed dentition

I. Transition dentition between 6 and 12 years of age with primary tooth exfoliation and permanent tooth eruption

2. Its characteristic features have led this to be called the ugly duckling stage because of

a. Edentulated areas

b. Disproportionately sized teeth

c. Various clinical crown heights

d. Crowding

e. Enlarged and edematous gingiva

f. Different tooth colors

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

PULP

Coronal

Occupies and resembles the crown,

Contains the pulp horns

It decreases in size with age

Radicular

Occupies roots

Contains the apical foramen

It decreases in size with age

Accessory apical canals

PULP FUNCTIONS

Inductive: The pulp anlage initiates tooth formation and probably induces the dental organ to become a particular type of tooth.

Formative: Pulp odontoblasts develop the organic matrix and function in its calcification.

Nutritive: Nourishment of dentin through the odontoblasts.

Protective: Sensory nerves in the tooth respond almost always with PAIN to all stimuli (heat, cold, pressure, operative procedures, chamical agents).

Defensive or reparative: It responds to irritation by producing reparative dentin. The response to stimuli is inflammation.

 Histologically the pulp consists of delicate collagen fibers, blood vessels, lymphatics, nerves and cells. A histologic section of the pulp reveals four cellular zones:

Odontoblastic

Cell-free (Weil)

Cell-rich

Pulp core

The Transition from the Deciduous to the Permanent Dentition.

1. The transition begins with the eruption of the four first permanent molars, and replacement of the lower deciduous central incisors by the permanent lower central incisors.

2. Complete resorption of the deciduous tooth roots permits exfoliation of that tooth and replacement by the permanent (successional) teeth

3. The mixed dentition exists from approximately age 6 years to approximately age 12 years. In contrast, the intact deciduous dentition is functional from age 2 - 2 /2 years of age to 6 years of age.

4. The enamel organ of each permanent anterior tooth is connected to the oral epithelium via a fibrous cord, the gubernaculum. The foramina through which it passes can be seen in youthful skulls

The deciduous second molars are particularly important. It is imperative that the deciduous second molars be preserved until their normal time of exfoliation. This prevent mesial migration of the first permanent molars.

Use a space maintainer in the event that a second deciduous molar is lost prematurely

ERUPTION OF THE PERMANENT TOOTH

- At the time at which the deciduous tooth erupts the tooth bud for the permanent tooth has already been building up enamel and dentin.

- When the permanent tooth starts to erupt, pressure on the root of the deciduous tooth causes resorption by the osteoclasts.

- Wolff's law states that when two hard tissues exert pressure on one another the softer of the tissues will be resorbed.

- The dentin and cementum of the root of the deciduous tooth is softer than the enamel of the permanent tooth that is why the root of the deciduous tooth is resorbed.

- Most permanent teeth have erupted and have been in use for 2 years before the root is completely formed.

INNERVATION OF THE DENTIN-PULP COMPLEX

1. Dentine Pulp
2. Dentin
3. Nerve Fibre Bundle
4. 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.