Tooth Eruption It is the axial or occlusal movement of the tooth from its developmental - PowerPoint Presentation

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Tooth Eruption

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It is the axial or occlusal movement of the tooth from its developmental position within the jaw to its functional position in the occlusal plane. However, eruption is only part of the total pattern of physiologic tooth movement, because teeth also undergo complex movements related to maintaining their position in the growing jaws and compensating for masticatory wear.

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PATTERN OF TOOTH MOVEMENT

Pre-eruptive

tooth movement

When deciduous tooth germs first differentiate, they are very small and a good deal of space is between them. This space is soon used because of the rapid growth of the tooth germs, and crowding results, especially in the incisor and canine region. This crowding is then relieved by growth of the jaws in length, which permits drifting of the tooth germs.

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Bony remodeling of crypt wall occurs to facilitate movements of growing tooth germ and its movement.

In

bodily movement in a mesial direction, bone resorbs on the mesial side and forms on the distal side of the crypt.

Permanent

teeth with deciduous predecessors also move before they reach the position from which they will erupt

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The permanent molars, which have no deciduous predecessors, also exhibit movement. For example, the upper permanent molars, which develop in the tuberosity of the maxilla, at first have their occlusal surfaces facing distally and swing around only when the maxilla has grown sufficiently to provide the necessary space.

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Similarly, mandibular molars develop with their occlusal surfaces inclined

mesially

and only become upright as room becomes available.

All

these movements occur in association with growth of the jaws, in order positioning the tooth and its crypt within the growing jaws preparatory to tooth eruption.

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Eruptive tooth movement

During

the phase of eruptive tooth movement the tooth moves from its position within the bone of the jaw to its functional position in occlusion, and the principal direction of movement

is occlusal or axial. However, as in the case of

preeruptive tooth movement, jaw growth is still occurring while most teeth are erupting so that movement in planes other than axial movement is superimposed on eruptive movement.

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Posteruptive

tooth movement

(1) maintain the position of the erupted tooth while the jaw continues to grow

and (2) compensate for occlusal and proximal wear.

The

former movement, like eruptive movement, occurs principally in an axial direction to keep space with the increase in height of the jaws.

It

involves both the tooth and its socket and ceases when jaw growth is completed.

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The movements compensating for occlusal and proximal wear continue throughout life and consist of axial and mesial migration, respectively

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Proximal wear, which can decrease the arch length by as much as 7 mm, is compensated by the mesial drift.

The mesial drift involves the contraction of transseptal

ligament and an anteriorly directed force that is the summation of

mesially

directed occlusal forces along

cuspal

planes.

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HISTOLOGY OF TOOTH MOVEMENT

Preeruptive

phase

Preeruptive tooth movement, whether it involves drifting or growth of the tooth germ, demands remodeling of the bony wall of the crypt. This is achieved by the selective deposition and removal of bone by osteoblastic and osteoclastic activity.

Thus

marrow spaces of consistent configuration develop in bones, and.

The

intraosseous

phase of tooth eruption can be considered as a process of bone modeling.

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Eruptive phase

During

this phase ; significant developmental events occur that are associated with eruptive tooth movement

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They include the formation of the roots, the periodontal ligament, and the

dentogingival

junction

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Root formation is initiated by growth of Hertwig’s epithelial root sheath, which initiates the differentiation of odontoblasts from the dental

papilla

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. The odontoblasts then form root dentin, bringing about an overall increase in length of the tooth that is largely accommodated by eruptive tooth movement, which begins at approximately the same time as root formation is initiated.

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Shortly after the onset of root formation cementum, periodontal ligament, and the bone lining the crypt wall are formed.

In

addition, a number of structural changes are seen within the periodontal ligament, which could be responsible for tooth movement

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Fibroblasts exhibit frequent cell-to-cell contacts of the adherence type and a further specialization involving the cell membrane, the

fibronexus

.

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Finally, the ligament fibroblast has the

ability

to ingest and degrade extracellular collagen while forming new collagen fibrils

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Significant histologic changes also occur in the tissues overlying the erupting tooth. Bone removal is necessary for permanent teeth to erupt

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In the case of those teeth with deciduous predecessors there is an additional anatomic feature, the

gubernacular

canal

and its contents, the

gubernacular

cord

, which may have an influence on eruptive tooth movement.

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When the successional tooth germ first develops within the same crypt as its deciduous predecessor, bone surrounds both tooth germs but does not completely close over them.

As

the deciduous tooth erupts, the permanent tooth germ becomes situated apically and is entirely

enclosed

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by bone except for a small canal that is filled with connective tissue and often contains epithelial remnants of the dental lamina. This connective tissue mass is termed the “

gubernacular

cord” and it may have a function in guiding the permanent tooth as it erupts.

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After removal of any overlying bone there is loss of the intervening soft connective tissue between the reduced enamel epithelium covering the crown of the tooth and the overlying oral epithelium

.

pressure from the erupting tooth causes local ischemia and therefore local necrosis, this connective tissue affect the epithelia it sustains and both the reduced dental epithelium and the overlying oral epithelium begin to proliferate and migrate into the disorganized connective tissue so that eventually a solid plug of epithelium forms in advance of the erupting tooth.

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The central cells of this epithelial mass degenerate and form an epithelium-lined canal through which the tooth erupts without any hemorrhage.

This epithelial cell mass is also involved in the formation of the

dentogingival

junction. Once the tooth has broken through the oral mucosa, it continues to erupt at the same rate until it reaches the occlusal plane and meets its antagonist. Rapid eruptive movement then ceases.

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Posteruptive

phase

is associated with condylar growth, which separates the jaws and teeth. Although bone deposition occurs at the alveolar crest and on the socket floor this is not responsible for tooth movement. The same forces responsible for eruptive tooth movement achieve axial

posteruptive

movement, with bone deposition occurring later.

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Movements are also made to compensate for occlusal and proximal wear of the tooth. It is generally assumed that the continuous deposition of cement around the apices of the roots of teeth is sufficient to compensate for occlusal wear axial movement of the tooth to compensate for occlusal wear.

Histologically

this drift is seen as a selective deposition and resorption of bone on the socket walls by osteoblasts and osteoclasts respectively, and collagen remodeling in both the periodontal and

transseptal

ligaments is seen.

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MECHANISM OF TOOTH MOVEMENT (THEORIES OF TOOTH ERUPTION)

The

mechanism that brings about tooth movement is still

debatable

and is likely to be a combination of a number of factors. Although many possible causes (theories) have been proposed, only four theories merit serious consideration:

(1) bone remodeling theory.

(2) root formation theory.

(3) vascular pressure theory, and

(4) periodontal ligament traction theory.

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The vascular pressure theory supposes that a local increase in tissue fluid pressure in the periapical region is sufficient to move the tooth. The ligament traction theory proposes that the cells and fibers of the ligament pull the tooth into occlusion.

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The various steps in eruption of teeth are the recruitment of the mononuclear cells to the dental follicle, its differentiation into osteoclast and their activation, bone resorption at the coronal half of the dental follicle and bone formation at the basal end. Thus, dental follicle serves not only as a target tissue for mono- nuclear cells but also to regulate cellular events of eruption.

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Eruption is a localized genetically programed event. The dental follicle contains genes concerned with eruption

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The regulatory genes that encode expression of various transcription factors are complex and involve series of signaling interactions between the dental follicle cells and cells of the bony crypt, namely the osteoclasts and the osteoblasts. Paracrine signaling from the stellate reticulum affects gene expression of molecules from dental follicle.

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Various molecules regulate eruption, often there are more than one having similar and overlapping functions (redundancy in function). This, is to ensure that such a critical event like eruption, does not fail in the absence of a single factor.

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CLINICAL CONSIDERATIONS

1-malalignment

of teeth can often be corrected. after assessment of dental age and this along with skeletal age is an index of maturity in development of the individual.

Radiographic

examination of the jaws of children would show the extent of crown formation, amount of root development, all of which aid in a more correct assessment of the dental age.

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2-Premature eruption of teeth occurs infrequently. Sometimes infants are born with “erupted” lower central incisors, but this is an example of gross

maldevelopment

. These teeth are termed

natal teeth

or neonatal teeth if they erupt during the neonatal period.

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Such teeth need to be extracted as soon as possible because they prevent sucking The natal and neonatal teeth showed abnormalities on microscopic examination. The prism structure was absent in cervical region, dentin was

atubular

or having giant tubules in areas and cementum was absent.

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Such teeth need to be extracted as soon as possible because they prevent sucking The natal and neonatal teeth showed abnormalities on microscopic examination. The prism structure was absent in cervical region, dentin was

atubular

or having giant tubules in areas and cementum was absent.

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3- Premature loss of a deciduous tooth without closure of the gap may lead to early eruption of its successor.

4-Dealayed eruption: This may be caused by either local or systemic factors. Systemic factors include nutritional, genetic, and endocrine deficiencies like decreased secretion of hormones influencing eruption— growth hormone, thyroid hormones or bone diseases like

osteopetrosis

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Local factors include such situations as loss of a deciduous tooth and drifting of opposing teeth to block the eruptive pathway. Severe trauma may eliminate the dental follicle, and hence periodontal ligament formation is prevented. When this happens, the bone of the jaw fuses with tooth, a condition known as

ankylosis

,

and eruption is not possible.

Eruption may also be delayed by an increased density of fibrous tissue over the erupting tooth or the development of an eruption cyst from remnants of the dental lamina.

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5-Crowding

:a diminution in the size of the jaws has not been accompanied by a corresponding decrease in the size of the teeth, and as a result crowding is a common occurrence.

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6-impaction: The third molars are the last teeth to erupt, and frequently all the available space has been used. As a result these teeth become impacted. Canines are also often impacted because of their late eruption time. Primary failure of eruption is a condition wherein permanent teeth, especially molars fail to erupt. Family history of eruption problems and hypo-

dontia

(reduction in number of teeth) were identified in many cases.

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It has been shown that the moment a tooth breaks through the oral epithelium, an acute inflammatory response occurs in the connective tissue adjacent to the tooth. Clinically, as teeth break through the oral mucosa, there is often some pain, slight fever, and general malaise, all signs of an inflammatory process. In infants these symptoms are popularly called

“teething.”

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Teeth which have erupted beyond the occlusal plane are referred to as

overerupted

or

supraerupted

teeth. Lack of opposing teeth makes the tooth to overerupt. The periodontal ligament and the bone develop together and therefore the gingival margin follows the tooth. In other cases the gingival margin stays at the original level and the root gets exposed due to

supraeruption

.