Development of dentition :

Phases of teeth development :

• Pre-eruptive phase : from the initiation of tooth development to completion of the crown.

• Per-functional phase : begins once roots begin to form .

• Functional phase : after teeth have emerged , concerned with development and maintenance with occlusion.

• Eruption is the process a tooth moves from its developmental position into its functional position .

• Permanent 2nd molar has two stages of active eruption :- 1st stage Occurs 6-12 yrs tooth is emerging into mouth.- 2nd stage 16 years in association with the adolescent growth

spurt.• Eruption rates are greatest at the time of crown emergence .

rates differ according to tooth type.• Main direction is axial . movement occurs in other planes.• As the tooth erupts : the outer cells of the reduced enamel

epithelium proliferate into connective tissue , secret enzymes that degrade collagen.

• Reduced enamel epithelium may also be concerned with the removal of breakdown products resulting from resorption of C.T .

Development of dentogingival junction :

Layer of reduced enamel epithelium and the basal cell layer proliferate and unite .

Epithelium covering the tip of the tooth degenerate , enabling the crown to emerge .

Active eruption movement and passive seperation of oral epithelium from the crown result in further emergence .

Epithelial seal – the junctional epithelium: reduced enamel epithelium attached to the unerupted part of the crown .

• Permanent tooth germ situate lingually to the deciduous tooth …. With continued growth permanent tooth comes to lie near the root apex

• Gubernacular canal :through which the permanent tooth germ communicates with the overlying oral mucosa .

contains the gubernacular cord , composed of central strand epi. From dental lamina and surrounded by connective tissue organized to inner and outer layer .

• This cord will increase in thickness and decrease in length .

• Resorption of primary roots :-Inherent developmental process-Pressure from the permanent .-Increased masticatory loads.• Resorbing of dentine by multinucleated osteoclast like cells

(odontoclast ) lie within resorption lacunae ( howship’s lacunae ).• Resorption of dec. teeth is not continous . there is rest periods reparative tissue may be formed. If

reparative > resorption tooth may become ankylosed . and then may cause submerged teeth .

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MECHANISM OF TOOTH ERUPTION :

• The rate of eruption represents the balance between eruptive force and resistive force .

• In brief ; Eruptive mechanism is - A property of PDL ( the dental follicle).- Doesn’t need a tractional force pulling the tooth out.- Is probably multifactorial - Involve a combination of the force produced by the

activity periodontal fibroblast through their contractility and motility ,and vascular and tissue hydrostatic pressures in and around the tooth are responsible for eruption .

• Initially , there is a period of slow eruption when the crown is carried to the oral mucosa .it takes 2-4 years in perm. teeth . then tooth erupts more rapidly as it enters the oral cavity( the length of the root is about 2/3 ) . and once the tooth emerge into oral cavity it takes 1-2 years to reach occlusal plane .

• Dental age is a useful index of maturity . estimated clinically and radiographically

Panoramic radiograph of a child about 7 years of age. This type of examination is of great value in registering an

overall record of development.

DEVELOPMENT OF OCCLUSION :

• AT BIRTH , thickened oral mucosa forms mand. And max. gum pads (rarely come into occlusion ).

• During the first year of life the gum pads grow rapidly in lateral directions and provide space for the developing teeth.

• Occasionally a natal tooth is present . its usually supernumerary tooth.

• The deciduous teeth start to erupt at the age of 6 mnths and dentition is complete by the age of 3 yrs .

• Occlusion of dec. dentition differs from perm. Dentition in the followings :

1. The incisors are more vertically positioned within the alveolus and are often spaced.

2. The overbite is usually greater .3. Significant spacing distal to the mandibular canines and

mesial to tha maxillary canines ( primate spaces).4. Anteroposterior relationships appears that the distal

edges of the maxillary and mandibular deciduous molars are flush , and the mesiobuccal cusps of upper D and E occlude in the buccal grooves of the lower D and E .

• The dental arches become wider and longer so deciduous teeth become more spaced.

• There is a greater forward growth of the mandible than maxilla ,then the lower arch moves forwards so an edge to edge incisor relationship is obtained. And distal surfaces of upper and lower E’s show a slight mesial step.

• Flush terminal plane is the usual relationship in deciduious dentition.

• Molar relationship tends to shift after E’s are shed and adolescent growth spurt occurs.

• Change in molar relationship depends upon the leeway space and mandibular growth .

• Ugly duckling stage : when the permanent incisors incline distally as a result of the pressure on the roots from the developing canines . the diastema usually closes after permanent canines eruption .

• By the age of 12 yrs all primary teeth have been shed and the occlusion appears similar to that in adults. Space is provided for permanent molars by continued growth of mandible and maxilla .

• Primate spaces , developmental spaces (increase as child grows ), generalized spacing ---- requirement for proper alignment of permanent incisors.

• Once a tooth reaches its functional position, it is occupy a position of equilibrium between the soft tissue of the cheeks and lips and the tongue .

• Mesial drift : During functional phase . and may involve bodily movement of the

tooth . The 6 drifts 4mm in a mesial direction between 6-18 years. Mechanisms are multifactorial and involve the periodontal tissue . Four hypotheses : I. The mesial inclination of teeth favours mesial drift .II. Action of certain jaw muscles , as buccinator .III. Bone deposited preferentially on the distal surface of the socket .IV. Contraction of gingival connective tissue .