Eruption and Shedding of Teeth_2014

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Professor Dr. Md Nurul Islam1

ERUPTION AND SHEDDING OF TEETH

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Professor Dr. Md Nurul Islam

Active Tooth Eruption

The term active tooth eruption implies the

emergence of a crown of tooth into the oral cavity.

In general, however, the term refers to the total life

span of the tooth, from the beginning of crown

development until the tooth is lost or the individual

dies.

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Active Tooth Eruption

This eruptive process is usually divided into three

stages, and although there may be some difference

in the terminology, they refer to the same

mechanism.

Preeruptive Stage

The preeruptive stage begins as the crown starts to

develop.

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Recall that dental lamina formation - bud, cap, and

bell stages, as well as the calcification of the crown -

takes place in the connective tissue beneath the oral

epithelium.

During this time the bone of the maxilla or mandible

surrounds the developing primary tooth in a U-

shaped crypt or beginning socket.

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The eruptive movement associated with the

preeruptive stage is of two varieties - spatial and

excentric.

In spatial movement the crown develops while the

bottom of the socket fills in with bone, pushing the

crown toward the surface.

A similar facial movement accompanies jaw growth.

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In excentric, or off-center growth, the crown of a

tooth does not grow in a perfectly symmetrical

pattern.

As the crown enlarges, it grows more in one area

than in another, and so the tooth seems to be

moving because the center of the tooth is shifting.

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It appears to have moved because the center point

of the developing crown has shifted.

This is the activity of the preeruptive stage.

It involves crown growth and some movement

toward the surface while the crypt is developing.

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Eruptive Stage

The eruptive stage or prefunctional eruptive stage

begins with the development of the root.

The root develops in a crypt of bone.

As it begins forming, osteoclasts temporarily may

deepen the crypt by resorbing bone at the bottom

to accommodate for the increase in root length.

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While the root continues to lengthen, the tooth

begins to move toward the surface of the oral cavity.

As it approaches the oral cavity, the alveolar bone is

growing to keep pace with it.

However, in time the tooth moves faster than the

growing alveolar bone and approaches the surface

of the oral epithelium and breaks into the oral cavity.

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The crown of the tooth is surrounded by reduced

enamel epithelium.

Around the reduced enamel epithelium there are

cells of the dental sac, or follicle, that covers the

crown.

Cells of the dental follicle form a cord of connective

tissue epithelium.

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This fibrous cord is known as the gubernacular cord

and forms a gubernacular canal that leads the way

and, with the help of macrophages and osteoclasts,

breaks down the bone between the tooth and the

surface oral epithelium for the primary tooth to

erupt.

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As the tooth moves to the surface, the reduced

enamel epithelium moves with it until it compresses

the connective tissue and causes it to disintegrate.

The reduced enamel epithelium then contacts the

oral epithehlium, and these two layers fuse into one

layerthe united oral epithelium.

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The tooth breaks through this layer and emerges

into the oral cavity.

It is believed that this break down of epithelium is

caused by an enzyme probably produced by the

reduced enamel epithelium.

This stage continues until the erupting teeth meet

the opposing teeth.

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For both primary and secondary dentition, tooth

movement in the eruptive stage tends to be occlusal

and facial, more facial in the anteriors than in the

posteriors.

When we think about the pathway for the secondary

teeth, we have to consider their mechanism for

development.

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The successional lamina buds off the dental lamina

and forms a permanent tooth at its end, still partially

attached by the successional lamina.

As the permanent tooth becomes surrounded by

bone, the attachment of the successional lamina of

the anterior teeth forms a gubernacular canal.

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SuccessionalLamina

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This is the gubernacular canal, and under normal

circumstances the erupting permanent tooth follows

the pathway of the gubernacular canal to reach the

surface.

It is seen only lingual to the anterior teeth because

the permanent anteriors erupt slightly lingual to

their primary counterparts, while the permanent

posterior teeth lie below the primary and erupt

through their established alveolus.

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Posteruptive Stage

The posteruptive stage begins when the teeth come

into occlusion and continues until they are lost or

death occurs.

This posteruptive stage functions in several ways.

First, as the mandible continues to grow and

increase the space between the maxilla and

mandible, the teeth will continue to erupt to

maintain a balance in the arches.

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Second, as the teeth wear occlusally because of

prolonged masticatory stress and wear, they will

continue to erupt to maintain tooth contact.

Third, because there is slight interproximal wear, a

slight mesial eruptive force will keep the teeth in

contact.

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This mesial drifting can be caused, particularly in

the mandible, by unerupted third molars that are

pushing against the second molars; however, there is

a physiologic mesial drift inherent to teeth.

Both of these circumstances are referred to as

mesial drift.

Finally, if an opposing tooth is lost, the tooth may

continue to erupt in what is generally referred to as

supraeruption, or hypereruption.

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Supra eruption can cause serious problems in the

replacement of the missing tooth because it makes it

difficult to reestablish the normal occlusal plane.

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Causes of Eruption

What causes tooth eruption? What are the forces

involved? Much research has been done concerning

this question, but much more still needs to be done.

Following are some of the discussions and thoughts.

Root Growth. It has been said that the increase in

root length, or root elongation, forces the tooth into

the oral cavity.

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However, several things would tend to disprove this.

Experiments have been done in which Hertwigs root

sheath has been destroyed and root growth has

been stopped or inhibited, yet the tooth has still

erupted.

Teeth have had their roots cut and a pin placed

through the apical section, but the occlusal portion

continues to erupt.

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On the other hand, third molars have grown roots to

full length, but the teeth have not erupted.

It seems that root elongation by itself is not required

for tooth eruption, but probably has a relationship to

the process.

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Alveolar Bone Formation and Changes. It has been

said that alveolar bone growth, tooth development,

and eruption are interdependent mechanisms.

The alveolar process forms in areas where teeth are

developing and is deficient in areas where teeth fail

to develop.

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Alveolar bone changes involve both formation and

resorption, and these metabolic events are

dependent on the presence of the various parts of

the dental sac or dental follicle.

The dental sac plays a role in the formation of

cementum of the root, the periodontal ligament, and

alveolar bone.

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The presence of the outer layers of the enamel organ

also is important in assisting with the osteoclastic

activity and soft tissue destruction necessary for the

crown to move up through the bone and soft tissue.

This cycle of bone development is rhythmic both in

the crest and crypt of the alveolar bone - instances

of osteoblastic activity followed by osteoclastic

activity or inactivity.

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In multirooted teeth the interradicular bone seems

to have a fairly significant role in the eruptive

process, according to some researchers.

Periodontal Ligament. Even though the periodontal

ligament is less involved in tooth eruption than was

once believed, it still has a role to play, probably

more so toward the end of eruption than at the

beginning.

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Earlier theories of the periodontal ligament having

contractile properties in some of its fibers is now a

thing of the past in most of the literature.

There does still appear to be a remodeling of the

intertwining of periodontal fibers in later tooth

eruption that is part of the process, but teeth

without roots have been shown to erupt in the

absence of the periodontal ligament.

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Vascular Pressure in Dental Tissues. It has been

known for a long time that there are vascular

pressures present in pulpal tissues as well as in the

periodontal ligament.

There is also reasonable agreement that the function

of the periodontal ligament as a shock absorber

for teeth is a result of these vascular pressures.

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These pulsating blood pressures not only enhance

cellular activity but seem to have a direct eruptive

role.

However, it is difficult to assess because removal of

all fluid pressures would mean the elimination of

oxygen and other nutrients from the developing

teeth.

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The role of the tooth itself. The tooth plays little if

any role in eruption because developing teeth have

been surgically removed and replaced by metal or

silicone implants into the dental sac, and these

implants have erupted.

The question is yet to be answered as to the actual

factors in tooth eruption.

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It is obvious from the information presented above

that tooth eruption is definitely multifactorial.

Much research is being focused on the biochemistry

and cell and molecular biology aspects of the

subject, and it seems that these disciplines will begin

to provide more and more answers.

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Shedding of Primary Dentition

As mentioned, the 20 permanent teeth that follow

the primary teeth develop as off shoots of the

primary dental lamina.

Recall that the anterior permanent teeth develop

apically and lingually to the primary teeth, whereas

the permanent premolars develop between the roots

of the primary molars.

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Regardless of its position, the fact that the

permanent tooth is present and is in the eruptive

stage means that the permanent tooth is moving

toward the surface and putting pressure on the root

of the primary tooth.

It is believed that this pressure causes osteoclasts to

form and begin resorbing the primary tooth root.

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This resorption is intermittent and not constant.

This is the usual manner in which resorption occurs,

but other factors may be involved.

Although most primary teeth would be retained if a

permanent tooth did not develop, it is still possible

to see a primary tooth undergo root resorption in

the absence of a permanent tooth and a primary

tooth retained in the presence of a permanent tooth.

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Therefore, although the pressure of a developing

tooth is a major factor in resorption of primary teeth,

it is not the only factor; there is a focus on the role

of the enamel organ of the erupting tooth in the

whole process.

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Retained Primary Teeth

There are several reasons why primary teeth are

retained beyond their normal time for exfoliation.

Here we are not really considering a general delayed

eruption of retarded growth patterns, but rather the

cases in which one or two teeth are retained well

beyond the expected period of time for them to be

lost.

The reasons for this are several. First, there may be

no permanent successor, and the tooth remains.

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Second, there may be ankylosis of the primary

tooth, a condition in which the alveolar crest of bone

fuses in the cervical area with the cementum of a

resorbing root.

Although virtually all the root may have been

resorbed, the tooth remains firmly in place,

preventing the permanent tooth below from

erupting.

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This may remain that way for years, and yet when

the ankylosed tooth is removed the permanent

tooth will generally begin to erupt.

The last reason for a retained primary tooth is that

the permanent tooth does not erupt in its normal

position and therefore does not cause resorption of

the primary tooth root or roots, and the tooth

remains.

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This is frequently seen in the anterior mandibular

area, where the permanent teeth erupt apically,

rather than apically and labially.

In these cases, both sets of anterior teeth will be

seen.

Another problem associated with the shedding of

primary teeth is unresorbed root fragments.

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This condition is usually but not always associated

with a malaligned primary or permanent tooth.

If the root tip of a primary tooth is not in the path of

eruption of a permanent tooth, the cervical portion

of the root may be resorbed, leaving the apical part

still embedded in the jaw.

The fragments may remain there for some time and

eventually may work their way to the surface and be

removed.

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These retained root tips are seen in radiographs

from time to time.

The time schedule of eruption and shedding is

varied.

In general, the posterior teeth go through a slower

process than the anterior teeth do.

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Not only does the length of time for eruption vary,

but also its beginning or ending time varies from

one person to another.

There is a range for normal eruption time, and only

when this period is exceeded is there cause for

concern.

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Development DisturbancesDuring Eruption

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Developmental Disturbances During Eruption

An odontogenic cyst that forms from the REE after

the crown has completely formed and matured is the

dentigerous cyst, or follicular cyst.

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Dentigerous Cyst

Elsevier, Imaging Consult, imaging.consult.com, 2009

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Dentigerous Cyst

Elsevier, Imaging Consult, imaging.consult.com, 2009

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Developmental Disturbances During Eruption

If a dentigerous cyst

appears on a partially

erupted tooth, it is

considered an eruption

cyst and appears as

fluctuant, blue,

vesicle-like gingival

lesion.

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Clinical Considerations forTooth Development

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Nasmyth's Membrane

A residue may form on newly

erupted teeth of both dentitions

that may leave the teeth

extrinsically stained.

This residue, Nasmyth's

membrane, consists of the fused

tissue of the REE and oral

epithelium as well as the dental cuticle placed by the

ameloblasts on the new enamel surface.

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Bohn's Nodules

The white bumps present on the maxillary alveolar

ridge are Bohn's nodules.

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Problems in Spacing

A permanent tooth often starts to erupt before the

primary tooth is fully shed, possibly creating

problems in spacing.

Interceptive orthodontic therapy can prevent some

of these situations.

Thus it is important for children with prolonged

retention of any primary teeth to seek early dental

consultation.

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Root Fragments

Root fragments from primary molars may be left

from the process and create periodontal

complications for the permanent dentition;

Panoramic radiographs of the mixed dentition are

important in order to monitor tooth development.

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RETAINED ROOT TIP Radiopaque area of the

alveolar ridge with the

structural appearance of the

apical part of a tooth root.

A thin periodontal ligament

space is visible on the distal.

Note the similarity of the

radiodensity of the root tip

and the adjacent premolar

root.

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Traumatic Injury Prevention

Prevention of traumatic injury to the

permanent teeth before they are

fully anchored into the jaws is very

important.

Sport bite guards are recommended for children active

in all areas of sport and consist of individually formed

plastic coverings for the teeth.

Any injury to a child's dentition needs to be seen

promptly by a dentist.

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Traumatic Injury Response

Avulsed tooth, a tooth that is completely removed

from the socket, occurs as a result of trauma and

may involve moderate pain or no pain.

Patient needs immediate examination in a dental

setting or emergency hospital unit by a dental

professional.

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Traumatic Injury Response

Ask patient to bring tooth in a wet handkerchief,

milk, or water or in buccal vestibule (preferred mode

of transport because of maintenance of hydration in

saliva);

Purchased transport medium for sport teams is

now available.

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RESEARCH UPDATE

SUMMARY: In comparison with the teeth from the

controls, the teeth from preterm children had a higher

relative value of carbon (C), a lower relative value of

calcium (Ca), a lower ratio of calcium/phosphorus

(Ca/P) and a lower ratio of Ca/C throughout the outer

part of the enamel. In dentin, the relative values for P

were higher, and Ca/P ratio was lower, at the dentin-

pulp junction. The Ca/P ratio indicated normal

hydroxyapatite in the crystals in enamel and dentin.

The lower ratio of Ca/C in the bulk and outer part of the

enamel indicated more porous enamel.

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Eruption and Shedding of Teeth_2014