A

Seminar

On

Eruption & Shedding of Teeth

Guided by : Submitted by:

Dr. Mrs. Anuradha Pathak Dr. Mahesh Jingarwar

Professor and H.O.D. (MDS I st year)

Deptt of Pedodontics and preventive Dentistry,

Govt. Dental College and hospital,Patiala

i) INTRODUCTION

ii) DEFINITION

iii) PATTERN OF TOOTH MOVEMENT

(a)pre eruptive tooth movement

(b)eruptive tooth movement

(c) post eruptive tooth movement

iv) HISTOLOGICAL CHANGES IN TISSUES OVERLYING ERUPTING TEETH

v) HISTOLOGY OF SURROUNDING TISSUES

vi) HISTOLOGY OF UNDERLYING TISSUES

vii) THEORIES OF ERUPTION

(a)pulp theory

(b)vascular theory

(c) root elongation theory

(d)alveolar bone growth

(e)periodontal ligament theory

(f) genetic input

(g)hydrostatic pressure theory

(h)follicular theory

viii) PATTERN OF ERUPTION OF TEETH

ix) ACTIVE AND PASSIVE ERUPTION

x) SEQUENCE OF ERUPTION

xi) CHRONOLOGY OF HUMAN DENTITION

xii) FACTORS AFFECTING TOOTH ERUPTION

xiii) DISORDERS OF ERUPTION

xiv) TEETHING

xv) SHEDDING OF TEETH

xvi) CLINICAL IMPLICATIONS

xvii) REFERENCES

ERUPTION

Derived from LATIN WORD erumpere , ”to break out”

INTRODUCTION

The timely initiation and their eruption into oral cavity is very important for

healthy dentition . it is the process by which tooth moves within the jaw

bone ,comes out into the oral cavity and comes up to the occlusal contact and

maintains its clinical position.Generally, tooth eruption occurs earlier in females

than in males, and the mandibular teeth tend to erupt earlier than their maxillary

counterparts except the premolars .

DEFINITION

“The axial or occlusal movement of the tooth from its developmental position

within the jaw to its functional position in the occlusal plane is called as

eruption.”(Massler M ,Schour I).

Dentition:

Diphodont: two sets of dentition is present in humans.

Primary vs Secondary dentition

Deciduous vs Permanent dentition

Mixed dentition: presence of two dentition

Teeth in primary dentition are smaller and fewer in number than permanent

dentition to conform to the smaller jaw size:

Primary dentition: ~ 2 to 6 years of age

Mixed dentition: ~ 6 to 12 years

Permanent dentition: > 12 years

PATTERN OF TOOTH MOVEMENT

Has been divided into:

1 PRE ERUPTIVE TOOTH MOVEMENT.

2 ERUPTIVE TOOTH MOVEMENT.

3 POST ERUPTIVE TOOTH MOVEMENT.

during all these 3 stages is the progression that happens from primary to

permanent dentition which involves the shedding (exfoliation) of primary teeth.

PRE ERUPTIVE TOOTH MOVEMENT

Made by deciduous and permanent tooth germs within tissues of jaw before they

begin to erupt. When deciduous tooth germ first differentiate they are very small and

a good deal of space is present between them ,this space is soon used because rapid

growth of the tooth germs and a crowding result in incisor and canine region .This

crowding is then relieved by growth of jaws in length which permits drifting of tooth

germs .

Bone remodelling of crypt wall occurs to facilitate movements of growing tooth

and its movement.in bodily movement in a mesial direction ,bone resorbs on mesial

side and forms on distal side of crypt, the permanent tooth with its deciduous

predecessors also move before they reach the position from which they will erupt.

The permanent molars ,which have no deciduous predecessors ,also exhibit

movement .for eg upper permanent molars ,which develop in tuberosity of

maxilla ,at first have their occlusal surface facing distally and swing around only

when maxilla has grown sufficiently to provide the necessary space.

Why do developing crowns move constantly in the jaws during the

preeruptive phase?

To place teeth in position for eruptive tooth movement

To alleviate the problems of jaw growth which allows second molar to move

backward and anterior teeth to move forward.

Developing crown move constantly during the preeruptive phase as they respond

to positional changes of the neighbouring crowns and to changes in the mandible

and maxilla.

Permanent teeth develop lingual to the incisal level of the primary anterior teeth

and later as primary teeth erupt, the permanent crowns are lingual to the apical 3 rd

of primary roots .

Permanent premolars move from occlusal level of primary molars to a position

enclosed within the primary tooth roots.All movements in the preruptive phase occur

within the crypts of the developing crowns .

Two types of tooth movement in pre-eruptive phase:

1 Total bodily movement

2 Movement where one part remains fixed while the rest continues to grow

leading to change in the center of the tooth germ.

ERUPTIVE TOOTH MOVEMENT

Made by a tooth to move from its position within the bone to its functional

position in occlusion, this phase is sometimes subdivided into intraosseous and

extraosseous components and the principal direction of movement is occlusal or

axial..the term “prefunctional eruptive tooth movement” is used to describe the

movement of the tooth after its appearance in the oral cavity till it attains the

functional position.

major events takes place in eruptive tooth movement

1.Root formation: it is initiated by growth of hertwigs epithelial root sheath

which initiates the differentiation of odontoblasts from dental paplla,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

the same time as root formation is initiated. Shortly after the onset of root formation

cementum,periodontal ligament,and the bone lining the crypt wall are formed.

2.Movement : occur incisally or occlusally ..main reason for the movement is so

that the root formation can take place normally ,reduced enamel epithelium fuses

and contacts the oral epithelium.

3.Penetration of tooth crown tip through the fused epithelial layers allowing

the entrance of crown into the oral cavity

4.Intraoral incisal or occlusal movement of the erupting tooth continues until

clinical contact with opposing crowns occur.

Histology – changes that occur in tissues overlying erupting teeth

1 Degeneration of connective tissue (decrease in blood vessels and

degeneration of nerves) immediately overlying the erupting teeth

2 Eruption pathway – altered tissue area overlying the teeth

3 Macrophages destroy cells and fibers by secreting hydrolytic enzymes

Gubernacular cord: The connective tissue overlying a successional tooth that

connects with the lamina propria of the oral mucosa by means of a strand of fibrous

connective tissue that contains remnants of dental lamina.

Gubernacular canal: Holes noted in a dry skull noted lingual to primary teeth

in jaws that represent openings of gubernacular cord . As the successional teeth

erupt, gubernacular canal widens enabling tooth to erupt.

HISTOLOGY OF SURROUNDING TISSUES

The surrounding fibers change from being parallel to the tooth surface to bundles

that are attached to the tooth surface and extending towards the periodontium

(bone).The periodontal ligament have contractile properties and changes drastically

during eruption.

During eruption, collagen fiber formation and turnover are rapid fibers to attach

and release and attach in rapid succession. Some fibers may attach and reattach

later while the tooth moves occlusally as new bone forms around it and the fibers

will organize and increase in number and density as the tooth erupts .

HISTOLOGY OF UNDERLYING TISSUES

1 As the tooth moves occlusally , it creates space underneath the tooth to

accommodate root formation.

2 Fibroblasts around the root apex form collagen that attach to the newly

formed cementum

3 Bone trabeculae fill in the space left behind as the tooth erupts in the

pattern of a ladder which gets denser as the tooth erupts.

4 After tooth reaches functional occlusion periodontal fibers attach to the

apical cementum and extend into the adjacent alveolar bone

The rate of tooth eruption depends on the phase of movement

Intraosseous phase: 1 to 10 µm/day

Extraosseous phase: 75 μm/day

Environmental factors affecting the final position of the tooth:

Muscular forces

Thumb-sucking

POST ERUPTIVE TOOTH MOVEMENT

Movements to accommodate the growing jaws. Mostly occursbetween 14 and

18 years by formation of new bone at the alveolar crest and base of socket to

keep pace with increasing height of jaws.

Movements to compensate for continued occlusal wear. Compensation

primarily occurs by continuous deposition of cementum around the apex of

the tooth. However, this deposition occurs only after tooth moves.

Similar to eruptive tooth movement

Movements to accommodate interproximal wear. Compensatedby mesial or

approximal drift. Mesial drift is the lateral bodily movement of teeth on

both sides of the mouth. Very important in orthodontics.

Several factors control mesial drift:

A. Contraction of the transseptal fibers: As the proximal tooth surfaces of

adjacent teeth become worn from functional tooth movement, the

transseptal fibers of the periodontal ligament become shorter (due to

contraction) and thereby maintain tooth contact .

B. Adaptability of bone tissue: The side of pressure on PDL fibers causes

bone resorption, whereas pull on the fibers causes bone apposition

(formation). Therefore, as the contact areas of the crowns wear, the teeth

tend to move mesially, thereby maintaining contact

C. Anterior compartment of occlusal force: An anteriorly directed force

is generated when teeth are clenched, due to the mesial inclination of

most teeth and the forward-directed force generated from inter-cuspal

forces. Eliminating opposing teeth results in elimination of biting forces,

causing a slowing down of the mesial migration .

D. Pressure from soft tissues: Buccal mucosa and tongue push teeth mesially

Active eruption: to compensate incisal and occlusal wear

Passive eruption: gradual recession of the gingiva and the underlying alveolar

bone.Both active and passive eruption leads to lengthening of clinical crown.

THEORIES OF TOOTH ERUPTION

The mechanism that brings about tooth movement is debatable. There are

numerous theories of tooth eruption which is usually a reflection of incomplete

understanding. All these theories have contributed to and provoked research into

various aspects, to support or refute hypotheses which are now briefly reviewed.

1 Pulp Theory

This theory suggests that a propulsive force is generated by extrusion of the pulp

through three mechanisms; first growth of dentin, secondly, interstitial pulp growth

and thirdly, hydraulic effects within the vasculature. Perhaps the most damning

evidence against this theory is the work of Herzberg and Schour (1941) who removed

the pulp of rodent incisors and found that its eruption rates were unaffected.

2 Vascular Theory

The mechanisms behind this theory to some extent overlaps the pulp theory. The

force of eruption comes from the pressure in the blood vessels within or below the

tooth. This theory has been discounted by some for the same reasons as the pulp

theory. In addition, use of hypotensive drugs appears to have no effect on the

eruption rates. However, a critical review by Moxham suggests that at least part of

the eruptive force is generated by a non-functional force.

3 Root elongation theory

This theory attributes tooth eruption to elongation of the roots. It suggests that

the tooth erupts as a result of root pushing against an immovable base. Root

formation appears to be the obvious cause of tooth eruption, since it undoubtedly

causes an overall increase in the length of the tooth which must be accommodated

either by the root growing into bone of the jaw, by increase in height of the jaw, or by

crown of the tooth moving occlusally. It is the latter movement, of course that occurs

but it does not follow that root growth is responsible. At one time, it was proposed

that a structure called “Cushion hammock ” ligament was strung across the base of

the socket and when the growing root impinged on it. This structure acted as a sling,

translating downward root growth into eruptive tooth movement. Careful histologic

study has found no such ligament. It must therefore be concluded that some force

other than root growth is moving the tooth to provide room for the newly formed root

tissue.

Furthermore, Marks and Cahill (Arch. Oral Biol.; 1984) using young dogs, took

teeth at the beginning of eruption, removed their pulps and killed the periodontal

ligament cells by freeze thawing. These inert rootless teeth with no periodontal

ligaments were reimplanted and still managed to erupt by compensatory bone

growth. Thus, although root growth can produce a force, it cannot be translated into

eruptive tooth movement unless there is some structure at the base of the tooth

capable of withstanding the force.

4. Alveolar bone growth

The importance of bone growth in tooth eruption was demonstrated with a series

of classical experiments by Brash (1928) using madder fed pigs. Madder is a dye

which binds to newly formed bone and Brash noticed large amounts of bone laid

down between the crypts of erupting teeth. These observations have been confirmed

(Marks and Cahill 1980), but although bone formation is clearly involved in tooth

eruption, cause and effect are still at the phenomenology stage.

5. Periodontal ligament theory

This theory suggests that the impetus for tooth eruption is derived from the

periodontal ligament. Evidence for this came from some brief observations by

Moxham and Berkovitz (Arch. Oral Biol.; 1974) where root transsection failed to

prevent the incisor segment superficial to the transsection from erupting. This

strongly implicates the periodontal ligament in the eruption process, and suggests

that there is little contribution from alveolar bone, root growth and indeed pulp

pressure. Evidence against this theory includes studies with lachyritic compounds,

such as α-aminoproprionitrile. They inhibit intermolecular crosslinking of the

polypeptide chains in the collagen molecule and should therefore inhibit the teeth

from erupting..

In summary, then the force moving the tooth is most likely generated by the

contractile property of the ligament fibroblast. The fibronexus and associated

fibronectin could transmit this force to the collagen fibre bundles, but a number of

other conditions must exist to translate this contraction into tooth movement.

Eruption must therefore be considered a multifactorial phenomenon.

The periodontal ligament theory has also gained some support from tissue

culture experiments. If a fibroblast is cultured on a substrate on which it can move, it

vibrates using contractile mechanisms generated by its cytoskeleton. The actin

molecule has a particularly prominent role. As the fibroblast moves, it elongates on

the leading edge and leaves the trailing end of the cell adherent to the substrate.

Eventually the latter edge will detach.

This model has been adapted to show that periodontal ligament fibroblasts are

capable of generating sufficient contractile force to lift a piece of root, against

gravity, towards the top of a tissue culture well (Arch. Oral Biol.; 1983). Direct

evidence of this tractional effect is not available but these models prove that

periodontal ligament has some role in the process of eruption.

6.Genetic input

If tooth eruption is to be explained at the cellular and molecular level, a degree of

genetic control is highly likely in normal development of occlusion. Incisors erupt

before premolars and this process of eruption is often disturbed in a number of

genetic disorders. A classification of this has been presented by Caulk (1988). These

comprise:

inhibited defects, primarily involving enamel - amelogenesis imperfecta.

syndromes with enamel involvement.

disorders associated with supernumerary teeth and / or crowding of teeth.

growth retardation syndromes.

conditions associated with tissue overgrowth of the gingiva and hyperplastic frenula.

miscellaneous disorders (these include premature exfoliation such as

Hypophosphatasia, Juvenile Periodontosis and Papillon Lefevre Syndrome).

There is no simple explanation of tooth eruption and this biological phenomenon

is a multifactorial event. Biological sciences are more likely to offer clear, rational

approaches to improve our understanding of tooth eruption.

7.Hydrostatic pressure.

This theory requires a higher pressure system, either within or around the base of

the tooth. It is known that teeth move in their sockets in synchrony with the arterial

pulse, so local volume changes can produce limited tooth movement. Ground

substance can swell from 30%-50% by retaining additional water, so this too could

create pressure. But, since surgical excision of the growing root and associated

tissue eliminate the periapical vasculature without stopping eruption, this means that

the local vessels are not absolutely necessary for tooth eruption.

8. Follicular Theory

This theory attributes a critical role to the dental follicle for the eruption of teeth.

It seems unlikely that the dental follicle provides the eruptive force since fibre

transsection fails to prevent eruptive movement. It seems more probable that the

loose connective tissue of the dental follicle is a rich source of factors which are

responsible for bone formation and resorption.

Because of the pioneering experiments of Marks and Cahill, it was established

that, in teeth of limited eruption, a tissue required for eruption is the dental follicle, a

loose connective tissue sac that surrounds the tooth prior to eruption. Their studies

showed that surgical removal of the follicle prevents eruption whereas leaving the

follicle intact but substituting an inert object for the tooth results in eruption of the

inert object (Marks & Cahill; 1984).

At the cellular level there is an influx of mononuclear cells (monocytes) in the

dental follicle which is the onset of active eruption (Marks et al.; 1983 Wise et al.;

1985).

At least 4 molecular signal(s) ultimately initiate the onset of tooth eruption

because of their ability to accelerate eruption, their immuno localization, their gene

expression or a combination of these.

Perhaps the molecule that plays the most direct role in initiating the cellular

events of eruption is colony stimulating factor one (CSF-1). When these were injected

into osteopetrotic (toothless) rats, the incisors erupted (Ilizuka et al.; 1992) and

injection of CSF-1 in normal rats lead to eruption of first molars with increase in

numbers of monocytes and osteoclasts (Cielinski et al.; 1995).

A cascade of molecular signals is probably involved in stimulating the expression

of CSF-1 for the onset of eruption. In particular, interleukin-1α (1L-1 α) enhances the

transcription of CSF-1 gene in rat dental follicle cells (Wise and Lin; 1994).

Immunolocalization studies have shown that 1L-1 α is present in the stellate

reticulum (Wise et al.; 1995), the portion of enamel organ that is immediately

adjacent to the dental follicle. Thus the 1L-1α might diffuse into the dental follicle to

stimulate the dental follicle cells to express the CSF-1 gene.

The expression of the 1L-1α gene may be regulated by epidermal growth factor

(EGF). EGF, long known for its ability to stimulate precocious eruption of incisors in

rodents (Cohen;1962), also increases the amount of 1L-1α in the stellate reticulum

following injection into rats (Wise et al.; 1995).

Another molecule that might be involved in a cascade of signals leading to tooth

eruption is transforming growth factor β1 (TGF-β1). Like 1L-1α , TGF β1

immunolocalizes to the stellate reticulum and in vitro, its mRNA expression is

enhanced by incubation with EGF. Because TGF β1 is a chemo attractant for

monocytes, it is possible that TGF β1 could enter the capillaries adjacent to the

dental follicle and attract monocytes to the follicle.

Based on these above studies, a hypothesis of the molecular events of tooth

eruption can be presented:

If EGF were the first signal there are at least three ways it could initiate eruption.

If EGF were not required, however, eruption could begin with a signal from TGF1.

Should EGF and TGF β1 both not be required, eruption could begin with 1L-1α enhancing CSF-1 mRNA expression.

PATTERN OF ERUPTION OF TEETH

The teeth of the deciduous dentition begin to appear in the mouth at about 6

months of age and the dentition is complete by 3 years. A majority of the

permanent teeth appear in the mouth between 6 and 12 years of age, during this

time teeth from both dentitions are present in the mouth, a phase known as mixed

dentition.

In the deciduous dentition, calcification of the crowns commences about a month

after the completion of cytodifferentiation of the tooth germ. Calcification of all

deciduous teeth begins before birth. Crown formation takes about 6 months to

complete and the tooth appears in the mouth some 6 months after crown formation

is achieved. When the teeth first appear, their roots are incomplete and are not fully

formed until 18 months later.

In the permanent dentition, the tooth germs are fully formed before birth for all

but the second and third molars. Crown formation begins at varying times thereafter.

In general, for the teeth of the permanent dentition, crown formation takes 3 years

and the teeth appear in the oral cavity about 3 years after the crown is complete.

Root completion is achieved about 3 years after eruption.

The sequence of eruption is an important aide memoire; the first permanent

molars erupt first at 6 years of age. The other teeth appear at approximately yearly

intervals corresponding to their sequence of eruption. If the sequence and dates of

eruption are remembered, the timing of other events may be calculated by simple

addition or subtraction.

BIRTH TO TWO YEARS

The permanent incisors and canines first develop lingual to the deciduous tooth

germs at the level of their occlusal surfaces and in the same bony crypt. As their

deciduous predecessors erupt, they move to a more apical position and occupy their

own bony crypts. First teeth to erupt are the mandibular central incisors.

The usual eruption sequence in the primary dentition is as follows. First the

central incisors, followed in order by the lateral incisors, first molars, canines and

second molars. Mandibular teeth usually precede the maxillary teeth. This sequence

is not always followed.

Time of eruption is usually stated as 6 months of age for the maxillary primary

centrals, 7-8 months for the mandibular primary laterals and 8 or 9 months for the

maxillary primary laterals. At about 1 year, the first primary molars erupt. At around

16 months, the primary cuspids appear. Two years is usually given as the age for the

second primary molars to appear.

TWO YEARS TO SIX YEARS

By two and a half years of age, the deciduous dentition is usually complete and in

full function. By three years of age, the roots of all deciduous teeth are complete .

First permanent molar crowns are fully developed and the roots are starting to form.

The crypts of the developing permanent second molars are now definite and can be

seen in the space formerly occupied by the developing first permanent molars.

Between three and six years of age, the development of the permanent teeth

continues, with the maxillary and mandibular incisor teeth more advanced. From five

to six years, just before the shedding of the deciduous incisors, there are more teeth

in the jaws than at any other time. Space is quite critical within both the alveolar

process and the deciduous dental arches themselves. Developing permanent teeth

are shifting close to the alveolar border, the apices of the deciduous incisors are

being resorbed; the first permanent molars are about ready to erupt.

Very little bone exists between the permanent teeth and their crypts and the

“front line” of deciduous teeth. [A cross section of the maxilla and mandible

illustrates this remarkable phenomenon]. The complex interplay of forces makes it

imperative that the integrity of the dental arch be maintained at this time. Loss of

arch length through caries may make the difference between normal occlusion and

malocclusion. It does not take very much to upset the delicate timetable of tooth

formation, eruption and resorption within a viable osseous medium.

SIX YEARS TO TEN YEARS

Between six and seven years of age, the first permanent molars erupt into the

mouth. As the upper and lower first permanent molars erupt, a pad of tissue

overlying them creates a premature contact. Proprioceptive response conditions the

patient against biting on this natural “bite opener” and thus the deciduous teeth

anterior to the first permanent molar area erupt, reducing the overbite. About this

time, the deciduous central incisors are lost and their permanent successors start

their eruptive path toward contact with the incisors of the opposing arch. Usually the

mandibular central incisors erupt first, followed by the maxillary permanent central

incisors. These teeth frequently erupt lingual to their deciduous counterparts and

move forward under the influence of tongue pressure as they erupt. The maxillary

central incisors appear as large bulges in the mucobuccal vestibule above the

deciduous incisors before they erupt.

The period from eruption of the lateral incisors to the eruption of canine is termed

by Broadbent as the “ugly duckling stage”. It is an apt term, implying an unaesthetic

metamorphosis leading to an esthetic result. During this period parents become

worried. A space may develop between the maxillary central crowns. The lateral

crowns may flare. Frenums are often sacrificed in an effort to remove the cause of

the space between the centrals. Margolis has called the alveolar process “the

servant of the tooth”. At this stage the maxilla is bulging in the canine region as

the alveolar process develops around the forming canine. With the further migration

of the canine occlusally, with its servant the alveolar process, the point of influence

of the canine on the laterals shifts incisally so that eventually, the lateral crowns are

driven medially, also effecting closure of the space between the centrals.

Eruption of the incisors is usually completed by eight and a half years of

age. Even though the central and lateral incisors erupt into the normal position, root

formation is not complete. The apices are wide open and do not close for at least

another year. Between nine and ten years of age, the apices in the deciduous

canines and molars begin to resorb. Individual variation is great here. Girls are

usually a year to a year and a half ahead of boys.

AFTER TEN YEARS

Between 10 and 12 years of age, there is considerable variability in the sequence

of eruption of the canines and premolars. In about half the cases, the mandibular

canines erupt ahead of the mandibular first and second premolars. In the maxilla, the

first premolar usually erupts before the canine. The first premolar usually erupts

before the canine. The maxillary second premolar and the maxillary canine erupt at

about the same time. At times, deciduous teeth are retained beyond the time that

they should normally be shed.

Eruption of the second molar teeth usually occur shortly after the appearance of

the second premolars. Since the second premolar and second molar teeth show the

greatest variability in order of eruption of any of the teeth (third molars excepted),

the second molar teeth may be expected to erupt before the second premolar teeth

in 17% of cases in CaucasiansBoth maxillary and mandibular second molars erupt

at about the same time. If the second molars exfoliate before the second premolars,

occasionally the first permanent molars may tip to the mesial.

Radiographs taken shortly after eruption of second molar teeth often show an

image of the developing third molar teeth that are difficult to interpret. This is

especially true of the mandibular third molars. (which are seen to be in the ramus

but actually present lingual to the ramus). Although maxillary second molars erupt in

a downward and forward direction, the maxillary third molars erupt downward

and backwards. To this might be added the term ‘outward’.

ACTIVE AND PASSIVE ERUPTION

According to the concept of continuous eruption (Orbans, Gottlieb J. Dent. Res. 13

; 214 ; 1933), eruption does not cease when the teeth meet their functional

antagonists but continues throughout life. It consists of an active and passive phase.

Active eruption – is the movement of the teeth in the direction of the occlusal

plane.

Passive eruption – is the exposure of the teeth by the apical migration of the

gingiva.Passive eruption is divided into four stages although this was originally

thought to be a normal physiologic process ,it is currently considered a pathologic

process:

Stage one: The teeth reach the line of occlusion. The junctional epithelium and the

base of the gingival sulcus are one the enamel.

Stage two: The junctional epithelium proliferates so that part is on the enamel.

The base of the sulcus is still on the enamel.

Stage three: The entire junctional epithelium is on the cementum, and the base of

the sulcus is at the cementoenamel junction. As the junctional epithelium proliferates

from the crown onto the root, it remains at the CEJ no longer than any other area on

the tooth.

Stage four: The junctional epithelium has proliferated further on the

cementum. The base of the sulcus is on the cementum, a portion of which is

exposed. Proliferation of the junctional epithelium onto the root is accompanied by

degeneration of gingival and periodontal ligament fibres and thin detachment from

the tooth.

The six/four rule for primary tooth emergence

four teeth emerge for each 6 months of age

1. 6 months: 4 teeth (lower centrals & upper centrals)

2. 12 months: 8 teeth (1. + upper laterals & lower laterals)

3. 18 months: 12 teeth (2. + upper 1st molars & lower 1st molars)

Sequence of Tooth EruptionPrimary

CI LI 1MCuspid 2ML U U U L

U L L L UPermanent

U1M LCI UL LCU U1PM U2PM UCU L2M L3M

L1M UCILL L1PM L2PM U2M U3MSource: http://www.columbia.edu/itc/hs/dental/d9903/lectures/lecture4.pdf

4. 24 months: 16 teeth (3. + upper canines & lower canines)

5. 30 months: 20 teeth (4. + lower 2nd molars & upper 2nd molars)

SUMMARY

1 By 5 months in utero, all crowns started calcification

2 By 1 year old, all crowns completed formation

3 By 2.5 years, all primary teeth erupted

4 By 4 years old, all primary teeth completed root formation

Chronology of Human Permanent Dentition

The rules of “Fours” for permanent tooth development (3rd molars not

included)

At birth, four 1st molars have initiated calcification

At 4 years of age, all crowns have initiated calcification

At 8 years, all crowns are completed

At 12 years, all crowns emerge

At 16 years, all roots are complete

Rules of “sixes” in dental development

6 weeks old in utero: beginning of dental development

6 months old: emergence of the first primary tooth

6 years old: emergence of first permanent tooth

FACTORS AFFECTING TOOTH ERUPTION

1. Age

2. Genetics

3. Race:- eruption timing seems to be earlier in the American black and Indian population than in American whites of European origin

4. Sex :- Except for the 3rd molar Females tend to calcify and erupt their permanent teeth about five month earlier than males.

5. Height and Weight :- Taller and heavier have greater number of erupted teeth then shorter and lighter children

6. Environment :-

1. Socio economic group

2.Climate

7. General Factors :-

1.Disease

2.Hormones

3.Metabolism

4.Vit- C deficiency

Local Factors :-

1.Pathology, trauma, ankylosis

2.Early Loss of Primary teeth

3.Delayed Loss of Primary teeth

4.Impaction / Crowding

5.Supernumerary teeth

6.Space Available

7.Degree of Root Formation

8.Growth Factor

DISORDERS OF ERUPTION

Premature & Accelerated Tooth Eruption

All these Conditions are Attributed to a superficial position of forming tooth germ

These may be:-

1.Natal Teeth :-Present at the time of birth.

2.Neo Natal Teeth:-Erupts with in 30 days after birth.

3.Preerupted Teeth :-Appear in 2nd or 3rd Month after birth.

Delayed Tooth Eruption

Situation in which it occurs:

1 Ankylosis

2 Congenital hypothyroidism

3 Juvenile acquired hypothyroidism

4 Cleidocranial dysplasia

5 Down syndrome

6 Gingival fibromatosis

7 Amelogenesis imperfecta

8 Epidermolysis bullosa

9 Mucopolysaccaroidosis

10 Acondroplasia

11 Regional odontodysplasia

TEETHING

Physiological process of eruption of primary dentition through the gingiva .

Clinical features:-

A) Local signs-

1.Hyperemia or swelling of the mucosa overlying the erupting teeth

2. Patches of erythema on the cheeks

3. Flushing in the skin of adjacent cheek

B) Systemic signs:-

1. General irritability & crying

2. Loss of appetite

3. Sleeplessness, restlessness

4. Increase salivation & drooling

5. Insanity

6. Increase thirst

7. Circumoral rash

SHEDDING OF TEETH

The physiologic process resulting in the elimination of the deciduous dentition is

called shedding or exfoliation. The eruptive pathway of the permanent teeth is very

much related to the shedding or exfoliation of the deciduous teeth as pressure from

the erupting successional tooth helps to determine the pattern of deciduous tooth

resorption.

1. Osteoclast/bone remodeling

2. Odontoclast (cementoclast; dentinoclast)

3. Resorption of soft tissues

4. Pressure from successional teeth

Osteoclasts are bone resorbing cells derived form monocyte-macrophage lineage

they are giant multinuclear cells with 4-20 nuclei. Osteoclasts resorb hard tissue by

separating mineral from the collagen matrix through the action of hydrolytic

enzymes...Resorption occurs at the ruffled border which greatly increases the surface

area of the osteoclast in contact with bone .

Hard Tissue resorption:

1. Extracellular phase

2. Intracellular phase

Shedding of teeth can occur due to two factors:

Odontoclasts.: The resorption of the hard tissues of the tooth is achieved by

cells that have an identical histology to osteoclasts but which, because they are

involved in the removal of dental tissue, are sometimes called odontoclasts. The

odontoclasts are capable of resorbing all dental hard tissues, including enamel,

but it is most commonly found on the surface of roots, where it resorbs

cementum and dentin. It is also found on occasion within the pulp chamber,

resorbing coronal dentin. This variation in the pattern of the deciduous tooth

resorption depends very much on the position of the successional tooth in

relation to the deciduous tooth. Thus, since the permanent incisors and canine

develop lingually to the deciduous teeth and erupt in an occlusal and vestibular

direction, resorption occurs at the lingual surface of the root and the tooth is shed

with much of its pulp chamber intact. Permanent premolars, however, develop

between the divergent roots of the deciduous molars and erupt in an occlusal

direction. Hence the resorption of interradicular dentin occurs with the resorption

of the pulp chamber and coronal dentin.

While little is known about resorption of the dental hard tissues, even less is

known about the resorption of the soft tissues associated with them; such as

dental pulp and periodontal ligament. Simple observation of histological sections

shows that the loss of periodontal ligament is abrupt. Electron microscopic

examination confirms this finding and also shows that cell death occurs in this

region in the absence of inflammation.

Pressure : Obviously, pressure from the erupting successional tooth plays a role

in the shedding of the deciduous dentition. For instance, if a successional tooth

germ is congenitally missing or occupies an aberrant position in the jaw, shedding

of the deciduous tooth is delayed. Yet the tooth is eventually shed. Growth of the

jaws and also the muscles of mastication also aid in resorption of deciduous teeth.

PATTERN OF SHEDDING

It has been seen that the pattern of exfoliation is symmetrical for the right

and left sides of the mouth. Girls exfoliate their teeth before boys. The greatest

discrepancy between the sexes is observed for the mandibular canines, the least

for the maxillary central incisors..

TISSUE CHANGES PRODUCED BY ERRUPTIVE GROWTH

Effect of coronal growth on adjacent tissue :-The developing component of

the growing crown and its associated dental sac tissue are protected by

compressing forces. This protection is offered by the tissue of the surrounding

dental sac which is abundantly supplied with intercellular fluid ,thus the growth

pressure of the crown are transmitted through the tissue of dental sac to the

peripheral bony wall. For this reason neither the developing germ nor its adjacent

soft tissue is affected.

The crypt changes: developing crowns produced osteoclastic activity which was

restricted to the lateral walls and the ceiling of bony the crypt .Because of the

high fluid content of the dental sac , the hydraulic phenomenon limit bone

resorption.to the sides and top of the crypt.

Resorption of the roots:- In general the pressure generated by the growing

and erupting permanent tooth dectates the pattern of deciduous tooth resorption.

At first this pressure is directed against the root surface of deciduos teeth

itelf ,because of the developmental position of the permanent incisor and canine

tooth germs and their subsequent physiologic movement in an occlusal and

vestibular direction .Resorption of the roots of the deciduous incisors and

canine begins on their lingual surfaces .Resorption of the of deciduos molars

often first begins on their inner surface because the early developing bicuspids

are found between them.

Resorption occurs on the surface of cementum and dentine . Resorption involves a loss of the organic as well as the mineral constituent of

the matrix . during resorption the process of disorganisation relative to the mineral and the

organic components occurs more or less concomitantly. Resorption of cementum and dentine of deciduous teeth is characterized by

the presence of osteoclasts.

Changes in the Dental pulp:-Dramatic changes in the organization of the

fibrous and the cellular components are not evidenced ,nor are there

conspicuous indication of degeneration. In fact , the dentinogenic layer located

near the ceiling of the pulp chamber may remain active up to the period

immediately preceding exfoliation. Because the integrity of the dental pulp

with the subjacent connective tissue is maintained long after much of the

efficacy of the suspensory apparatus is negated and precocious exfoliation is

avoided, In fact ,belated shedding may be a result of the tenacious connection

between the dental pulp and the underlying connective tissue.

Exfoliation proper:-with progressive resorption of the root and even the

crown and with disorganization of the structural and functional features of the

adnexal tissue , the temporary of primary tooth is loosened more and more

until tooth can be removed from its shallow alveolus with little efforts.

Why there is no pain when deciduous tooth exfoliate?

'spontaneous' degeneration of nerve fibres has been said to occur in the nerves of

the papillae of the organ of Eimer in the growing mole (Boeke, 1940).It may be noted

that this early terminal and preterminal degeneration in the nerves of deciduous

teeth probably accounts for the absence of pain when they are shed.

{THE FATE OF THE NERVES OF THE DECIDUOUS TEETH BY A. MOHIUDDIN

Department of Anatomy, St Mary's Hospital Medical School *}

Why there is no bleeding/less bleeding when deciduous tooth exfoliate?

Clinically it has been noticed that human deciduous teeth are shed with little

bleeding, when the teeth naturally exfoliate. Immediately after teeth are shed,

stratified squamous epithelium present in the dentogingival junction (DGJ) and

gingiva were found in the underlying tissue indicating that DGJ epithelium and

gingival epithelium play an important role in the process of exfoliation.

Furthermore, wound healing after exfoliation is usually more rapid than

after eruption. A study by N. Sahara et al. showed migration appeared to be further

stimulated as a result of chronic inflammation by microorganisms present adjacent to

the DGJ. The most interesting finding of the study was the evidence of the stratified

squamous epithelium of the DGJ and gingiva proliferated and migrated towards the

inside of the crown and eventually ended up under the deciduous crown.

Clinical consideration

Remnants of deciduous teeth :-sometimes parts of the roots of deciduous

teeth are not in the path of erupting permanent teeth and may escape

resorption.They are most frequently found in association with the permanent

premolars, especially in the region of lower second premolars. The reason is that the

roots of lower second deciduous molars are strongly curved or divergent.

Retained deciduous teeth:-deciduous teeth may be retained for long time

beyond their usual shedding schedule. such teeth are usually without permanent

successor, or their successor are impacted. Retained deciduous teeth are most

often the upper lateral incisor and rarely lower central incisor.

Submerged deciduous teeth:- Trauma may result in damage to either the

dental follicle or the developing periodontal ligament. In such case the eruption

ceases and it becomes ankylosed to the bone of jaw. Submerged deciduous teeth

prevents the eruption of their permanent successor or force them from their position.

Such teeth should be removed as early as possible

REFERENCES

1. ORAL HISTOLOGY – A.R. TEN CATE, MOSBY PUBLICATIONS (3RD EDITION)

2. ORBAN’S ORAL HISTOLOGY AND EMBRYOLOGY – MOSBY PUBLICATIONS

(10TH EDITION).

3. ERUPTION OF PERMANENT TEETH – A COLOUR ATLAS – SADAKATSU SATO AND

PATRICIA PARSONS (ISHIYAKU EURO AMERICA INC.).

4. PRINCIPLES OF ANATOMY AND ORAL ANATOMY FOR DENTAL STUDENTS – M.E.

ATKINSON AND F.H. WHITE (1ST EDITION) CHURCHILL LIVINGSTONE

PUBLISHERS.

5. ORTHODONTICS, PRINCIPLES AND PRACTICE - T.M. GRABER (3RD EDITION)

W.B. SAUNDERS PUBLICATION.

6. CLINICAL PERIODONTOLOGY - CARRANZA AND NEWMAN (8TH EDITION)

W.B. SAUNDERS PUBLICATION.

7. MECHANISM OF TOOTH ERUPTION – T.B. KANDOS. B.D.J. VOL. 181, NO. 3 AUG.

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8. THE MOLECULAR BIOLOGY OF INITIATION OF TOOTH ERUPTION - G.E. WISE AND

F LIN. J. DENT. RES. 74 (1) : 303-306 JAN 95.

9. A HISTOLOGIC STUDY OF THE EXFOLIATION OF HUMAN DECIDUOUS TEETH – N.

SAHARA, N. OKAFUJI, Y. ASHIZAWA AND K. SUZUKI. J. DENT. RES. 72 (3) : 634-

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10. THE DAILY RHYTHM OF TOOTH ERUPTION. AJODO 1995;107: 38-47.

11. ERUPTIVE TOOTH MOVEMENT — THE CURRENT STATE OF

KNOWLEDGE .BRITISH DENTAL JOURNAL ;197: 385 – 391,2004.

12. TEETHING AND TOOTH ERUPTION IN INFANTS: A COHORT STUDY.

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