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

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

FUNCTIONS OF PERIODONTIUM

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

Dentin

1. Composition

a. Inorganic (70%)—calcium hydroxyapatite crystals.

b. Organic (30%)—water and type I collagen.

2. Types of dentin

a. Primary dentin

(1) Dentin formed during tooth development, before completion of root formation.

It constitutes the majority of dentin found in a tooth.

(2) It consists of a normal organization of dentinal tubules.

(3) Circumpulpal dentin

(a) The layer of primary dentin that surrounds the pulp chamber.  It is formed after the mantle dentin.

(b) Its collagen fibers are parallel to the DEJ.

b. Secondary dentin

(1) Dentin formed after root formation is complete.

(2) Is deposited unevenly around the pulp chamber, forming along the layer of dentin closest to the pulp.

It therefore contributes to the decrease in the size of the pulp chamber as one ages.

(3) It consists of a normal, or slightly less regular, organization of dentinal tubules. However,

as compared to primary dentin, it is deposited at a slower rate.

(4) Although the dentinal tubules in secondary dentin can be continuous with those in primary

dentin, there is usually a tubular angle change between the two layers.

c. Tertiary (reparative, reactive) dentin

(1) Dentin that is formed in localized areas in response to trauma or other stimuli such as caries, tooth wear, or dental work.

(2) Its consistency and organization vary. It has no defined dentinal tubule pattern

d. Mantle dentin
 

(1) The outermost layer of dentin
(2) Is the first layer of dentin laid down by odontoblasts adjacent to the DEJ.

(3) Is slightly less mineralized than primary dentin.

(4) Has collagen fibers that are perpendicular to the DEJ.

(5) Dentinal tubules branch abundantly in this area.

e. Sclerotic (transparent) dentin

(1) Describes dentinal tubules that have become occluded with calcified material .

(2) Occurs when the odontoblastic processes retreat, filling the dentinal tubule with calcium phosphate crystals.

(3) Occurs with aging.

f. Dead tracts

(1) When odontoblasts die, they leave behind empty dentinal tubules, or dead tracts.

(2) Occurs with aging or trauma.

(3) Empty tubules are potential paths for bacterial invasion.

3. Structural characteristics and microscopic features:
 

a. Dentinal tubules

(1) Tubules extend from the DEJ to the pulp chamber.

(2) The tubules taper peripherally (i.e., their diameters are wider as they get closer to the pulp). Since the tubules are distanced farther apart at the periphery, the density of tubules is greater closer to the pulp.

(3) Each tubule contains an odontoblastic process or Tomes’ fiber.

Odontoblastic processes are characterized by the presence of a network of microtubules, with

Occasional mitochondria and vesicles present.

Note: the odontoblast’s cell body remains in the pulp chamber.
 

(4) Coronal tubules follow an S-shaped path, which may result from the crowding of  odontoblasts as they migrate toward the pulp during dentin formation.

b. Peritubular dentin (intratubular dentin)

(1) Is deposited on the walls of the dentinal tubule, which affects (i.e., narrows)the diameter of the tubule .

(2) It differs from intertubular dentin by lacking a collagenous fibrous matrix. It is also more mineralized than intertubular dentin.
 

c. Intertubular dentin

(1) The main part of dentin, which fills the space between dentinal tubules

 (2) Is mineralized and contains a collagenous matrix.

d. Interglobular dentin

(1) Areas of hypomineralized or unmineralized dentin caused by the failure of globules or calcospherites to fuse uniformly with mature dentin.

(2) Dentinal tubules are left undisturbed as they pass through interglobular dentin; however,

No peritubular dentin is present.

(3) Interglobular dentin is found in the:

(a) Crown—just beneath the mantle dentin.

(b) Root—beneath the dentinocemental junction, giving the root the appearance of a granular

layer (of Tomes).

e. Incremental lines

(1) Dentin is deposited at a daily rate of approximately 4 microns.

(2) As dentin is laid down, small differences in collagen fiber orientation result in the formation of incremental lines.

(3) Called imbrication lines of von Ebner.

(a) Every 5 days, or about every 20 µm, the changes in collagen fiber orientation appear more

accentuated. This results in a darker staining line, known as the imbrication line of von

Ebner.

(b) These lines are similar to the lines of Retzius seen in enamel.

f. Contour lines of Owen

(1) An optical phenomenon that occurs when the secondary curvatures of adjacent dentinal tubules coincide, resulting in the appearance of lines known as contour lines of Owen.
 

(2) Contour lines of Owen may also refer to lines that appear similar to those just described; however, these lines result from disturbances in mineralization.

g. Granular layer of Tomes

(1) A granular or spotty-appearing band that can be observed on the root surface adjacent to the dentinocemental junction, just beneath the cementum.

Disturbances to interarch alignment are

a. Excessive overbite where the incisal edge of the maxillary incisors extend to the cervical third of the mandibular incisors

b. Excessive overjet where the maxillary teeth overjet the mandibular teeth by more than 3mm

c. End-to-end relationship: edge-to edge bite where the anterior teeth meet at there incisal edge with  no overjet or overbite; cusp-to bite where the posterior teeth meet  cusp to cusp with no interdigitation

d. Crossbite where the normal faciolingual relationship of the maxillary to the mandibular teeth is altered for the anterior.teeth. the mandibular  tooth or teeth are facial  rather than lingual to the maxillary teeth for the posterior teeth, normal inercuspaton is not seen

Gingiva

The connection between the gingiva and the tooth is called the dentogingival junction. This junction has three epithelial types: gingival, sulcular, and junctional epithelium. These three types form from a mass of epithelial cells known as the epithelial cuff between the tooth and the mouth.

Much about gingival formation is not fully understood, but it is known that hemidesmosomes form between the gingival epithelium and the tooth and are responsible for the primary epithelial attachment. Hemidesmosomes provide anchorage between cells through small filament-like structures provided by the remnants of ameloblasts. Once this occurs, junctional epithelium forms from reduced enamel epithelium, one of the products of the enamel organ, and divides rapidly. This results in the perpetually increasing size of the junctional epithelial layer and the isolation of the remenants of ameloblasts from any source of nutrition. As the ameloblasts degenerate, a gingival sulcus is created.