Tooth Eruption Theories Tooth eruption is defined as â€œThe axial movement of or occlusal movement of tooth from its developmental position within the jaw to its functional position in the occlusal planeâ€?. 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. 1
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
frequently, but it does not follow that root growth is responsible for that.
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
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. 4
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.
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 â€“ 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.
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.
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.
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.