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

MAXILLARY SECOND BICUSPID

smaller in dimensions. The cusps are not as sharp as the maxillary first bicuspid and have only one root.

Facial: This tooth closely resembles the maxillary first premolar but is a less defined copy of its companion to the mesial. The buccal cusp is shorter, less pointed, and more rounded than the first.

Lingual: Again, this tooth resembles the first. The lingual cusp, however, is more nearly as large as the buccal cusp.

Proximal: Mesial and distal surfaces are rounded. The mesial developmental depression and mesial marginal ridge are not present on the second premolar.

Occlusal: The crown outline is rounded, ovoid, and is less clearly defined than is the first.

Contact Points; When viewed from the facial, the distal contact area is located more cervically than is the mesial contact area.

Enamel

Structural characteristics and microscopic features

a.  Enamel rods or prisms

(1) Basic structural unit of enamel.

(2) Consists of tightly packed hydroxyapatite crystals. Hydroxyapatite crystals in enamel are four times larger and more tightly packed than hydroxyapatite found in other calcified

tissues (i.e., it is harder than bone).

(3) Each rod extends the entire thickness of enamel and is perpendicular to the dentinoenamel junction (DEJ).
 

b. Aprismatic enamel

(1) The thin outer layer of enamel found on the surface of newly erupted teeth.

(2) Consists of enamel crystals that are aligned perpendicular to the surface.

(3) It is aprismatic (i.e., prismless) and is more mineralized than the enamel beneath it.

(4) It results from the absence of Tomes processes on the ameloblasts during the final stages of enamel deposition.

c. Lines of Retzius (enamel striae)

(1) Microscopic features

 (a) In longitudinal sections, they are observed as brown lines that extend from the DEJ to the

tooth surface.

 (b) In transverse sections, they appear as dark, concentric rings similar to growth rings in a tree.
 

(2) The lines appear weekly during the formation of enamel.
 

(3) Although the cause of striae formation is unknown, the lines may represent appositional or incremental growth of enamel. They may also result from metabolic disturbances of ameloblasts.

(4) Neonatal line

(a) An accentuated, dark line of Retzius that results from the effect of physiological changes

on ameloblasts at birth.

(b) Found in all primary teeth and some cusps of permanent first molars.

d. Perikymata

(1) Lines of Retzius terminate on the tooth surface in shallow grooves known a perikymata.

(2) These grooves are usually lost through wear but may be observed on the surfaces of developing teeth or nonmasticatory surfaces of formed teeth.
 

e. Hunter-Schreger bands

(1) Enamel rods run in different directions. In longitudinal sections, these changes in direction result in a banding pattern known as HunterSchreger bands.

(2) These bands represent an optical phenomenon of enamel and consist of a series of  alternating dark and light lines when the section is viewed with reflected or polarized

light.

f. Enamel tufts

(1) Consist of hypomineralized groups of enamel rods.

(2) They are observed as short, dark projections found near or at the DEJ.

(3) They have no known clinical significance.

g. Enamel lamellae
 

(1) Small, sheet-like cracks found on the surface of enamel that extend its entire thickness.

(2) Consist of hypocalcified enamel.

(3) The open crack may be filled with organic material from leftover enamel organ components, connective tissues of the developing tooth, or debris from the oral cavity.

(4) Both enamel tufts and lamellae may be likened to geological faults in mature enamel.
 

h. Enamel spindle
 

(1) Remnants of odontoblastic processes that become trapped after crossing the DEJ during the differentiation of ameloblasts.
 

(2) Spindles are more pronounced beneath the cusps or incisal edges of teeth (i.e., areas where occlusal stresses are the greatest).
 

HISTOLOGY OF THE ODONTOBLAST

Formation of Dentin

Mantle dentin: First formed dentin
Type I collagen with ground substance
Formation of the odontoblast process

Matrix vesicles
Appearance of hydroxyapatite crystals
 

Predentin
Primary physiologic (circumpulpal) dentin
All organic matrix is formed from odontoblasts
Smaller collagen fibers
Presence of phosphophoryn

Mineralization
Globular calcification
Interglobular dentin: Areas of incomplete calcification
Incremental lines of von Ebner: Daily, 4mm of organic matrix is deposited. Also every 5 days the arrangement of collagen fibers changes. This creates the incremental lines of von Ebner.
Intratubular dentin

Dentin tubules
S-shaped in the coronal aspect, straight in root dentin

Von Korff fibers
They are an artifact

ERUPTION

. Root completion (approximately 50% of the root is formed when eruption begins)

Generally mandibular teeth erupt before maxillary teeth,

Primary teeth

I. Emerge into the oral cavity as follows:

           Maxillary                       Mandibular

Central Incisor                          7½ months                     6 months

Lateral incisor                           9 months                       7 months

Canine                                     18 months                      16 months

First Molar                               14 months                     12 months

Second Molar                          24months                       20 months

The sequence of  primary  tooth development is central incisor, lateral incisor, first molar, second molar

3. Hard tissue formation begins between 4 and 6 months in utero

4. Crowns completed between 1½ and 10 months of age

5. Roots are completed between I½ and3 yearsof age 6 to 18 months after eruption

6. By age 3 years all of the primary and permanent teeth (except for the third molars) are in some stage of development

7. Root resorption of primary teeth is triggered by the pressure exerted by the developing permanent tooth; it is followed by primary tooth exfoliation in sequential patterns

8. The primary dentition ends when the first permanent tooth erupts

Time for tooth development

Entire primary dentition initiated between 6 and 8 weeks of embryonic development.
Successional permanent teeth initiated between 20th week in utero and 10th month after birth Permanent molars between 20th week in utero (first molar) and 5th year of life (third molar)