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 three-dimensional 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.
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.
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:
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.