In the name of GOD

Development of Occlusion

Presented by:

Dr Somayeh Heidari

Orthodontist

Reference:

* Contemporary Orthodontics

Chapter 3

William R. Proffit, Henry W. Fields, David M.Sarver. Fifth Edition: 2012. Mosby

Stages of dental development

• gum pads

• primary dentition

• mixed dentition

• permanent dentition

Gum pads stage

Gum pads stage

• from birth until eruption of the first primary tooth (6-7 months age)

• elevations and grooves that outline the position of the developing primary

teeth

• the upper and lower gum pads frequently have anterior open bite

relationship while the posterior segments are touching

• the maxillary gum pad often slightly overlaps the mandibular gum pad

both horizontally and vertically

Natal tooth

• the tooth that occasionally is present at birth time

• may be a supernumerary tooth that formed by an aberration in development

of dental lamina

• usually is merely a very early but otherwise normal central incisor

• such a natal tooth should not be extracted casually

Primary dentition

Sequence and timing of primary teeth eruption

Mandible Maxilla Tooth

8 mo. 10 mo. Central

13 mo. 11 mo. Lateral

20 mo. 19 mo. Canine

16 mo. 16 mo. First molar

27 mo. 29 mo. Second molar

• up to 6 months of acceleration or delay is within the normal range

• the eruption sequence, however, is usually preserved

• the primary dentition is usually completed at 24 to 30 months

• spacing is normal throughout the anterior part of the primary dentition

• the space is most noticeable in two

locations called the primate spaces

• in the maxillary arch, the primate space is located between the lateral

incisors and canines

• in the mandibular arch, the space is between the canines and first molars

• the primate spaces are normally present from the time the teeth erupt

• developmental spaces between the incisors become larger as the

child grows and the alveolar processes expand

• generalized spacing of the primary teeth is a requirement for proper

alignment of the permanent incisors

Primary molars relationship

* flush terminal plane

* mesial step

* distal step

Mixed dentition

Eruption of the permanent teeth

Pre-emergent eruption

• the amount of labial or buccal drift of the tooth follicle within the bone is

extremely small, observable only with vital staining

• dental follicle can be used as a natural marker in radiographic studies

• eruption movement begins soon after the root begins to form

•Two processes are necessary for preemergent eruption:

resorption of bone and primary tooth root

eruption mechanism

Defective bone resorption

• in mice: the incisor teeth can not erupt

Ia (Incisors absent)

• in humans: the syndrome of cleidocranial dysplasia

deficient primary teeth and bone resorption

heavy fibrous gingiva

multiple supernumerary teeth

if the interferences are removed, the teeth often erupt

Defective eruption mechanism

• the rate of bone resorption and the rate of tooth eruption are not

controlled physiologically by the same mechanism

• primary failure of eruption:

posterior teeth fail to erupt (defect in the eruption mechanism)

bone resorption proceeds normally

the tooth do not follow the path that has been cleared

:in Primary Failure of Eruption

The teeth do not respond to orthodontic force

• The signal for resorption is activated by the beginning of root formation.

but active formation of the root is not necessary for continued clearance

of an eruption path or for tooth movement along it.

• normally the apical area remains at the same place while the crown

moves occlusally.

• if eruption is mechanically blocked, the proliferating apical area will

move in the opposite direction.

Possible mechanisms of eruption force

• development of cross-links in maturing collagen in the periodontal ligament

after a tooth comes into function

can not be the primary mechanism of preemergent eruption

• localized variations in blood pressure or flow

• forces delivered from contraction of fibroblasts

• alterations in the extracellular ground substances of the PDL

Post emergent eruption

• once a tooth emerges into the mouth, it erupts rapidly until it approaches

the occlusal level: post emergent spurt

• after the tooth reaches to occlusion, its eruption slows: juvenile occlusal

equilibrium

• eruption occurs only during a critical period between 8 PM and midnight

or 1 AM: circadian rhythm

• changing in blood flow in the apical area and shortening of collagen fibers

affect post emergent eruption of the teeth

• during the juvenile equilibrium, teeth that are in function erupt at a rate

that parallels the rate of vertical growth of the mandibular ramus

• the amount of eruption necessary to compensate for jaw growth can best

be appreciated by observing the “submerge” position of an ankylosed tooth

• a pubertal spurt in eruption of the teeth accompanies the pubertal spurt in

jaw growth

• during adult life, teeth continue to erupt at an extremely slow rate:

adult occlusal equilibrium

• the eruption mechanism remains active and capable of producing

significant tooth movement even late in life

Permanent teeth eruption sequence and timing

• the transition from the primary to permanent dentition begins at

about age 6 years

• the permanent teeth tend to erupt in groups

• dental age is determined from 3 characteristics:

which teeth have erupted

the amount of resorption of the roots of the primary teeth

the amount of development of the permanent teeth

Dental age 6

• the most common eruption sequence is the eruption of the mandibular

central incisor, closely followed by the mandibular first permanent molar and

the maxillary first permanent molar

• theses teeth normally erupt at so nearly the same time, that it is quite within

normal variation for the first molars to slightly precede the mandibular central

incisors or vice versa

• usually the mandibular molar will precedes the maxillary molar

Dental age 7

• the maxillary central incisors and the mandibular lateral incisors erupt

• root formation of the maxillary lateral incisor is well advanced

• the canines and premolars are still in the stage of crown completion or

just at the beginning of root formation

Dental age 8

• is characterized by the eruption of the maxillary lateral incisors

• after these teeth come into the arch, there is a delay of 2 to 3 years

before any further permanent teeth appear

Dental age 9

• the primary canines, first molars and second molars are present

• approximately one third of the root of the mandibular canine and the

mandibular first premolar is completed

• root development is just beginning , if it has started at all, on the

mandibular second premolar

• root development has begun on the maxillary first premolar, but is just

beginning, if it is present at all, on both canine and second premolar

Dental age 10

• approximately one half of the roots of the roots of the mandibular

canine, mandibular first premolar and maxillary first premolar have been

completed

• there is a significant root development of mandibular second premolar,

maxillary canine and maxillary second premolar

• completion of the roots of the mandibular incisors and near completion

of the roots of the maxillary laterals

Dental age 11

• is characterized by the eruption of the mandibular canine, mandibular

first premolar and maxillary first premolar

• in the mandibular arch, the mandibular canine most often appears just

ahead of the first premolar

Dental age 12

• the remaining succedaneous permanent teeth (maxillary canine,

maxillary second premolar and mandibular second premolar) erupt

• the second permanent molars in both arches are nearing eruption

• it is usually possible to note the early beginning of mineralization of

the third molars

Dental age 13 - 15

• the extent of completion of the roots of permanent teeth

• by dental age 15, if a third molar is going to form, it should be

apparent on the radiographs, and the roots of all other permanent

teeth should be complete

teeth usually emerge when three fourths of their roots are completed

it takes 2 to 3 years for roots to be completed after a tooth has erupted into

occlusion

like the other developmental ages, dental age correlates with chronologic

age, but the correlation for dental age is one of the weakest

a change in the sequence of eruption is a much more reliable sign of a

disturbance in normal development than a generalized delay or acceleration

Variations in eruption sequence that have clinical significance

• eruption of second molars ahead of premolars in the mandibular arch

→ space deficiency

• eruption of canines ahead of premolars in the maxillary arch

→ labial displacement

• asymmetries in eruption between the right and left sides

→ more than 6 months

Space relationships in replacement of incisors

in both the maxillary and mandibular arches, the permanent incisor

tooth buds lie lingual as well as apical to the primary incisors

the mandibular permanent incisors have a tendency to erupt somewhat

lingually and in a slightly irregular position , even in normal dental arches

with normal spacing

the maxillary lateral incisor is likely to be lingually positioned

the permanent canines are positioned to lie more nearly in line with

the primary canines

permanent canines can be displaced either lingually or labially, but

usually they are displaced labially if there is not enough space for them

Because the permanent incisors are considerably larger than the primary

incisors, so spacing between the primary incisors is not only normal, it is

critically important and necessary for alignment of the permanent teeth

when the central incisors erupt, these teeth use up essentially all of the

excess space in the normal primary dentition

the maxillary arch, on the average has just enough space to accommodate

the permanent lateral incisors when they erupt

in the mandibular arch, when the lateral incisors erupt, there is on average

1.6 mm less space available for the four incisors

the difference between the amount of space needed for the incisors and

the amount available for them is called the “incisor liability”

so, a normal child will go through a transitory stage of mandibular incisor

crowding at age 8 to 9 even if there will eventually be enough room to

accommodate all the permanent teeth in good alignment

Where will the extra space come from?

a slight increase in the inter-canine width

about 2 mm

boys > girls

maxillary arch > mandibular arch

labial positioning of the permanent incisors relative to the primary incisors

approximately 1 to 2 mm

when the mandibular canines erupt, they slightly move back into

primate space

1 mm of space

All these changes occur without significant skeletal growth in the

front of the jaws.

The crowding is likely to persist into the permanent dentition if it

was sever initially.

Ugly duckling stage

The space tends to close as the permanent canines erupt

A maxillary central diastema of 2mm or less will probably close spontaneously

Space relationships in replacement of

canines and primary molars

• the permanent premolars are smaller than the primary teeth they replace

• the mandibular primary second molar is on the average 2mm larger than

the second premolar

• the maxillary primary second molar is 1.5mm larger than the second

premolar

• the primary first molar is only slightly larger than the first premolar (0.5

mm in the mandible)

Each side in the mandibular arch contains about 2.5 mm of what is

called leeway space, while in the maxillary arch, about 1.5 mm is

available on the average.

Normal relationship of the primary molars : flush terminal plane

h

Primary molars relation corresponds to Angle’s class I : mesial step

primary dentition equivalent of Angle’s class II: Distal step

An equivalent of Class III is almost never seen in the primary dentition

At the time the primary second molars are lost, both the maxillary

and mandibular molars tend to shift mesially into the leeway

space, but the mandibular molar normally moves mesially more

than its maxillary counterpart.

Flush terminal plane needs 3.5 mm of mesial movement of the lower

molar for a smooth transition to a class I molar relation :

* Leeway space

* Differential growth of mandible