Tooth eruption is the movement of teeth from developmental positions within the jaw to functional positions in the occlusal plane, influenced by various mechanisms including root formation, hydrostatic pressure, bone remodeling, and periodontal ligament traction. While root formation is often seen as a key factor in eruption, studies indicate other mechanisms also contribute significantly, such as the pressure from erupting permanent teeth affecting deciduous root resorption. The precise mechanisms behind these processes remain complex and multifactorial, with ongoing research needed to fully understand tooth eruption.

1. Tooth Eruption Theories
Tooth eruption is defined as “The axial movement of or
occlusal movement of tooth from its developmental position within
the jaw to its functional position in the occlusal plane”.
The eruption is only a part of total pattern of tooth movement
because teeth also undergo complete movements related to
maintaining their position in the growing jaws and compensating
for occlusal wear.
Mechanisms of Eruptive Movements
The exact mechanism of tooth eruption is still a matter of
controversy. Even so, most of the mechanisms are still not
understood, majority of the studies agree that the eruption is a
multifactorial process in which cause and effect are difficult to
separate.
Four theories of mechanism of tooth formation have been put
forward and accepted. These include:
1. Root formation.
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2. 2. Hydrostatic pressure.
3. Selective deposition and resorbtion of bone around the roots
or bone remodeling.
4. Periodontal ligament traction.
Root formation:
According to root growth theory it is the formation of root
and increase in length of root that causes occlusal movement of the
crown. Since it definitely cause an overall increase in length of the
tooth, this increase in length should have to be accommodated in
the bone. This can occur by:
a. Roots growing into the jaw and hence its accommodation by
increase in jaw height or
b. By occlusal movement of crown portion when the two above
given chances are considered the layer occurs more
frequently, but it does not follow that root growth is
responsible for that.
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3. Many experimental studies deny the above given mechanism.
It has been observed that in rodents and guinea pigs the teeth
continue to erupt. If these teeth are prevented from erupting by
pinning the crown to the bone it has been found that the roots cause
resorbtion of bone in the apical portion and continue to lengthen in
size. Since it has been demonstrated that pressure causes bone
resorption by stimulating osteoclastic activity, the pressure created
by the growth of root might be sufficient to produce such a
resorbtion. If the tooth has to be moved occlusally by root
lengthening, then there should be an object or tissue capable of
resisting such a force.
It has been shown that surgical resection of growing root and
associated structures eliminates the vascular supply in periapex
without stopping eruption.
This means that local vessels are not absolutely necessary for
tooth eruption.
Bony Remodelling
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4. Bony remodeling of the jaws has been linked with tooth
eruption. It is suggested in this theory that inherent growth pattern
in maxilla and mandible moves teeth by selective deposition and
resorbtion of the bone in the immediate neighborhood of tooth.
Studies with tetracyclines and bone markers have disproved
this suggestion. Tetracyclines get incorporated actively in newly
formed bone and identified by their florescent properties with the
onset of tooth eruption bone is resorbed at the base of socket. Later
occurs bone deposition at the floor of socket. Measurements have
shown that the amount of bone deposited plus amount of root
formation together equals the distance that tooth moves which is
surprising. However some workers have proved that, bone
formation increase as the teeth move occlusally, deposit the initial
resorbtion by tooth eruption. This suggests that the deposition of
bone occurs as an infilling effect in resorbed area.
Another factor resisting the root growth theory is eruption of
rootless teeth. Some teeth in whom roots are formed incompletely
show active eruption.
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5. The next factor to consider is certain teeth erupt from a path
that is greater than overall root length. The few teeth show eruption
even after completion of their root development.
Experimentally when root forming tissues are removed the
tooth does not show ceasation of eruption which is the next
drawback of the study.
Thus formation of root may not alone be a factor in
producing active tooth eruption. It could / may of course contribute
in the occlusal movement but is not the only factor to do it.
Hydrostatic Pressure
The hydrostatic pressure theory requires high pressure
systems either within or around the base of the tooth. It is known
that teeth move in their sockets in synergy with arterial pulse. Thus
local volume changes may produce tooth movement.
It has been demonstrated that ground substance can swell up
to 30-50% by retaining additional water and presence of fenestrated
capillaries in PDL suggests capability of rapid fluid adjustment.
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6. This suggests that bone deposition occurs as a result at eruption
rather than acting as a cause.
It has also been shown that when the enamel organ is
removed leaving the follicle behind, the path of eruption is still
formed by osteoclastic activity. This suggests that the remodeling
may occur in response to follicles activity rather than occurring on
its own to cause tooth eruption.
Periodontal Ligament Traction
There is a good deal of evidence that eruptive forces reside in
periodontal ligament – dental follicle complex. Experiments in
rodents designed to retard root development as well as vascular
supply show an undisturbed eruption as long as periodontal tissue
is available.
Certain drugs like vitamin C which is essential for collagen
formation and latharytic agent which prevents cross-linking
between collagen aggregates has shown positive results in
preventing tooth eruption by decreasing growth of collagen fibers.
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7. Experiments have shown that ligament fibroblasts are able to
contract a collagen gel which in turn brings about movement disc of
root tissue attached to that gel.
Thus there is no doubt that P.L fibers play essential role in
bringing tooth eruptive movement. Thus fibroblasts possess
contractile element, that are in contact with one another to permit
summation of contractile forces and exhibit fibronases by which
such forces are transmitted to collagen bundles. These collagen
fiber bundles not only remodel but also are arranged in a way of
eruption of tooth. This angulation of periodontal ligament fibers in
a prerequisite for proper tooth movement and the orientation is
believed to be established by developing root.
The follicle before ligament formation also plays an
important role in tooth eruption even though it may not provide
actual eruptive force. It has been demonstrated that the removal of
germ by leaving follicle in place forms a path of eruption.
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8. It has also been shown that when a tooth is removed and
follicle with ligament is left behind and removed tooth is replaced
by silica, replica the replica erupts in a manner similar to normal
tooth thus proving importance of ligament follicle complex for
eruption.
Thus eruptive movements are brought about by combination
of events involving a force initiated by fibroblasts. This force is
transmitted to extracellular compartment via fibronexuses and to
collagen fiber bundles which are aligned at a proper angulation
brought about by root development bring about tooth movement.
These fibre must have the ability to remodel for eruption to
continue and interference with this remodeling affects eruption
process.
Thus eruption process is an active tooth movement process
brought about by a multiple number of factors.
Shedding of Teeth
The exact mechanism causing resorbtion of deciduous roots
and their subsequent shedding is not known yet. But the most
commonly accepted mechanism is pressure created by erupting
permanent successor.
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9. The pressure created by erupting permanent successor play
an important role in determining the pattern of deciduous root
resorbtion and shedding. This is best explained by the fact that in
cases where permanent successor is congenitally missing the
resorbtion of deciduous tooth is slowed down markedly and these
teeth exfoliate at a later date than normal. Resorbtion is also
initiated by increasing the amount of forces applied. Growth of
jaws, muscles progresses as the age is advanced and subsequently
there will be an increased force on primary teeth causing their early
resorbtion.
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10. The pressure created by erupting permanent successor play
an important role in determining the pattern of deciduous root
resorbtion and shedding. This is best explained by the fact that in
cases where permanent successor is congenitally missing the
resorbtion of deciduous tooth is slowed down markedly and these
teeth exfoliate at a later date than normal. Resorbtion is also
initiated by increasing the amount of forces applied. Growth of
jaws, muscles progresses as the age is advanced and subsequently
there will be an increased force on primary teeth causing their early
resorbtion.
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