Introduction
The jaws of an infant can accommodate only a few small teeth. Because
teeth, once formed, cannot increase in size ,the larger jaws of the adult
require not only more but also bigger teeth .This accommodation is
accomplished in human beings with two dentition(diphyodont).The first is
the deciduous or primary dentition, and the second is the permanent or
secondary dentition.
For teeth to become functional, considerable movement is required
to bring them into occlusal plane. The movements teeth make are complex
and may be described in general terms as follow: Pre-eruptive tooth movement:-made by the deciduous and
permanent tooth germs within tissues of the jaw before they begin to
erupt.
 Eruptive tooth movement:-made by a tooth to move from its position
within the bone of the jaw to its functional position in occlusion (This
phase subdivided into intra-osseous and extra osseous components.)
 Post-eruptive tooth movement:- maintaining the position of the
erupted tooth in occlusion while the jaws continue to grow and
compensate for occlusal and proximal tooth wear.
Superimposed on these movements is a progression from primary to
permanent dentition, involving the shedding of the deciduous dentition.
This categorization of tooth movement is convenient for
descriptive purposes, what is being described is a complex series of
events occurring in a continuous process to move the tooth in threedimensional space.
Preoruptive Tooth Movement:
These pre-eruptive movements of deciduous and permanent tooth
germs are thought of best as the means by which the teeth are placed in
a position within the jaw for eruptive movement. Analysis has shown that
these pre-eruptive movements of teeth are a combination of two factors
:total bodily movement of the tooth germ and growth in which one part
of the tooth germ remains fixed while the rest continues to grow ,leading
to a change in the center of the tooth germ .This growth explains ,for
example ,how the deciduous incisors maintain their position relative to the
oral mucosa as the jaws increase in height.
Pre-eruptive movements occur in an intra-osseous location and
are reflected in the patterns of bony remodeling within the crypt wall.
During eccentric growth ,only bony resorption occurs, thus altering
the shape of the crypt to accommodate the altering shape of the tooth
germ.
Eruptive Tooth Movement
The mechanisms of eruption for deciduous and permanent teeth are
similar, resulting in the axial or occlusal movement of the tooth from its
developmental position within the jaw to its final functional position in
the occlusal plane.
In pre-emergent tooth eruption, the controlling element is the rate of
resorption of overlying structures. A path is cleared, and hen the erupting
tooth moves along it. In post-emergent eruption, control seems to be light
forces of long duration that oppose eruption, rather than heavy forces of
short duration such as those during mastication. Studies of human
premolars in their passage from gingival emergence to the occlusal plane
show that in this phase eruption occurs only during a few hours in the early
evening. The critical hours for eruption parallel the time that growth
hormone levels are highest in a growing child. In this stage intermittent force
does not affect the rate of eruption, but changes in periodontal blood flow do
affect it.
Histological Features
Many changes occure in association with and for accommodation of
tooth eruption

The collagen fibers of the follicular sac at first run in a circumferential
manner around the tooth term and there is very little attachment to
the adjacent alveolar bone .The periodontal ligament (PDL) develops
only after root formation has been initiated; once established ,the
PDL must be remodeled to accommodate continued eruptive tooth
movement. The remodeling of PDL fiber bundles is achieved by
the fibroblasts, which simultaneously synthesize and degrade the
collagen fibrils as required across the entire extent of the ligament.
 The architecture of the tissues in advance of erupting successional
teeth differs from that found in advance of deciduous teeth. The
fibro-cellular follicle surrounding a successional tooth retains its
connection with the lamina propria of the oral mucous membrane
by means of a strand of fibrous tissue containing remnants of the
dental lamina, known as the gubernacular cord, In dried skull, holes
can be identified in the jaws on the lingual aspects of the deciduous
teeth, these holes, are termed gubernacular canal.in case of the
premolars, however,the openings of their gubernacular canals are
frequently found within the socket of the corresponding deciduous
molar, During tooth eruption the gubernacular cords decrease in
length but increase in thickness,producing awidening of their canals.
 Formation of primary junctional epithelium
The reduced enamel epithelium plays the last role in the life
history
of the enamel
organ. It forms the primary junctional
epithelium by fusing with the basal layer of the oral epithelium. The
epithelium overlying the enamel crown undergoes apoptosis and
is lost.This leaves the occlusal surface exposed to the oral cavity, but
the the lateral surfaces of the enamal remain covered by a tightly
adherent layer of cells. Thus a biological seal is formed that prevents
bacteria and food debris from entering the connective tissue
space.In the case of permanent teeth replacing their deciduous
precursors, the
junctional
epithelium
of
the primary
tooth
proliferates apically to merge with the reduced enamal epithelium
with continued eruption of the tooth the gingival crevice retreat
further root ward on the tooth ,leaving progress more of it uncovered
by epithelium.
The gingival migration proceeds at a fairly rapid rate until the
tooth reaches the plane of occlusion comes into contact with the
opposing tooth or teeth.
At this stage the gingival margin has reached a level so that a
bout two-thirds to three-quarters of the enamel surface is exposed in
the mouth cavity, whereas the remaining one –quarter to one –third is
still covered by the epithelial attachment.
Even after the tooth has reached the occlusal plane the gingival
margin and crevice tend to shift gradually further apically on the
enamel so that eventually they reach the cemento-enamel junction.
 Reduced enamel epithelium: as eruptive movement begins, the enamel
of the crown still is covered by a layer of ameloblasts and remnants of
the other three layers of the enamel organ. These are sometimes
difficulte to distinguish, and together the ameloblasts and adjacent
cells form the reduced enamel epithelium.
The reduced dental epithelium and the oral epithelium fuse
and form a solid mass of epithelial cells over the crown of the tooth.
The central cells in this mass degenerate because they are cut off
from their nutritional supply, forming an epithelial canal through
which the crown of the tooth erupts.
In this way, tooth eruption is achieved without exposing the
surrounding connective tissue and without exposing the surrounding
connective tissue and without hemorrage.
Rate of Eruption:
The rate of eruption depends on the phase of movement; During the
intra osseous phase, the rate averages 1 to 10 mm per day, it increases to
about 75 mm per day once the tooth escapes from its bony cell .This rates
persists until the tooth reaches the occlusal plane, indicating that soft
connective tissue provides little resistance to tooth movement.
Mechanism of Eruptive tooth movement:
Eruptive mechanisms are not understood fully yet, and most reviews
on this subject have concluded that eruption is multifactorial process in
which cause and effect are difficult to separate. Nevertheless, available data
demonstrate that the
mechanism
of eruption
is a property of the
periodontal ligament or its developmental precursor, the dental follicle
and is properly multifactorial in that more than one mechanism may be
involved .the leading candidate theories include:
 Root formation:
The major for the theory of tooth movement involving root
growth is that the intra-osseous phase of eruption does not begin
until root formation has begun .
The crown of the tooth is elevated into the mouth cavity through
the thrust provided by the development of the root. In addition, the
root is only 2/3 formed at the time of emergence into the oral cavity.
However, rootless teeth do erupt and the length of eruption
pathway taken by some teeth is longer than the root itself. Although
root growth can produce a force , it cannot be translated into eruptive
tooth movement unless some structure exists at the base of the
tooth capable of withstanding this force.
In conclusion, root formation is accommodated during tooth
eruption and is a consequence, not a cause, of the eruption process.
 Alveolar Bone Remodeling:
The alveolar process forms during tooth development and is
Bone remodelling of the jaws has been linked to tooth eruption in that, as
in the pre eruptive phase, the inherent growth pattern of the mandible or
maxilla supposedly moves teeth by the selective deposition and resorption
of bone in the immediate neighborhood of the tooth.
The strongest evidence in support of bone remodeling as a cause of
tooth movement comes from a series of experiments i.e. If eruption
prevented by wiring the tooth germ down to the lower border of the
mandible ,an eruptive pathway still forms within the bone overlying the
enucleated tooth as osteoclast widen the eruption pathway.
Rather, alveolar bone growth involving turnover (resorption and
formation) is required during tooth eruption. The relatively demonstration
that bone resorption and bone formation are polarized around erupting
teeth and that these metabolic events depend upon the adjacent parts of
the dental follicle have led to the concept that tooth eruption is a localized,
bilaterally symmetrical event in alveolar bone that is regulated by the
dental follicle proper.
 Periodental ligament formation and renewal:
The third major theory of tooth eruption
involves the
periodontal ligament, and two separate mechanisms have been
proposed .The first is dependent on
the
constant
turnover
(remodeling)of collagen fibers in the ligament .During maturation,
collagen fibers ‘shrink’ in length by about 10%.Because of the
orientation of these oblique collagen fibers, the vector of force
generated in aggregate is directed occlusally.
The second mechanism involves a small, but measurable,
contractile force that can be generated by fibroblasts. Fibroblasts are
the most numerous cell type in the periodontal ligament, and they
can “attach” to collagen type I fibers via fibronectin and integrins.
The difficulty with the periodontal ligament theory is that the
periodontal ligament does not become highly organized until after
the tooth begins to come into functional occlusion .Therefore ,the
periodontal ligament theory is unlikely to explain eruptive phase
of eruption.
 Periodontal ligament hydrostatic pressure:
The theory of tooth eruption has been proposed ,which
involves periodontal /tissue vascular pressure .This theory requires
that eruptive movements are maintained by pressure differentials
along the periodontal ligament space ,and that periodontal tissue
pressures are high. Support for this mechanism includes:
 The predictable effects of vasoactive drugs on eruption
behavior
 The distribution of fenestrations in alveolar bone proper
Changes in the number of fenestrations during different phases of
eruption.