Transcript development of the dentition

DEVELOPMENT OF THE
DENTITION
THE DECIDUOUS DENTITION
The stage of deciduous teeth represents an important
part of ontogenetic development when usually an
exact diagnosis can be established as well as a further
prognosis can be assessed.
Existence of the deciduous dentition can be divided
into three stages:
1.the eruptive stage
2.the functional stage
3.the stage of make-ready
for eruption of permanent
teeth
THE DECIDUOUS DENTITION
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Eruption of the lower central incisors begins at about 6 month
of age and usually by the age of 30 month all the deciduous
teeth have erupted. Typical ages of eruption:
central incisor...6-8 month
lateral incisor...8-10 month
first molar ...12-18 month
canine
...18-24 month
second molar ...24-30 month
THE DECIDUOUS DENTITION
• Eruption times vary
considerably. Up to 6
month earlier or later
than the times given is
not unusual.
• There no distinct
difference between the
sexes.
• Root formation is
complete between 12
and 18 month after
eruption.
THE DECIDUOUS DENTITION
• After eruption of deciduous
incisors and first molars
there is still contact between
the distal parts of alveolar
processes.
• Before the eruption of
canines by the further growth
of erupted teeth the
contact between alveolar
processes is interrupted.
It´s the “1th physiological
reduction of overbite”.
THE DECIDUOUS DENTITION
The regular deciduous
dentition has these
characteristic features:
• both arches have
semicircular shape
• the long axises of the
teeth are parallel and
curve of Spee is missing
THE DECIDUOUS DENTITION
• every tooth ocludes
with two antagonists
except lower central
incisors and upper
second molars
• upper arch overlaps
the lower arch, there
is small frontal and
lateral overjet
THE DECIDUOUS DENTITION
• the second deciduous
molars are flush distally. It
occurs in 70-80% cases.
However, the negative step
may be also present if the
upper second deciduous
molar is as wide as lower or
the positive step if the
lower second molar is too
wide in relation to upper or if
there is class II relationship
of the dental arches.
THE DECIDUOUS DENTITION
• there may be spacings distal
to the lower canines and
mesial to the upper canines
called primate spacings
• incisors may be spaced
The second phase is
biologically inactive only
attrition take place there.
THE DECIDUOUS DENTITION
• The third stage is the period when
replacement of the teeth starts to
prepare.
• Prior to eruption of permanent
teeth, the roots of the primary teeth
are resorbed and their crowns
shed. The pressure created by the
erupting permanent tooth is
generally believed to play an
important role in primary tooth
resorption. Dentinoclasts appear
on the apical surface of the roots of
primary teeth probably by similar
mechanism as in the case of
osteoclasts ( It is known that
pressure stimulates differentiation
of osteoclasts from mesenchymal
cells and their action.)
THE DECIDUOUS DENTITION
Resorption of the roots begins :
• for incisors at 4
• for molars at about 6 and
• for canines at 7 years of age.
Also, if a permanent tooth is missing, the
primary predecesser usually undergoes
root resorption, but in the later time.
THE MIXED DENTITION
• At the age of 6 years permanent teeth, usually the first
molars or lower central incisors start to erupt.
• As in the case of the deciduous teeth, eruption times
are very variable and a range of 18 months on either
side is not unusual.
• There are also some sex differences in the age of
eruption. Girls are 1/2 year ahead of boys.
• During this century there has been a tendency
towards earlier eruption of permanent teeth in the
developed countries ( "secular trend" ). It has mainly
been due to the earlier onset of puberty, and indirectly
to better child health and nutrition
THE MIXED DENTITION
Typical ages of eruption:
• first molar ...6-7 years
• central incisor ...6-7 years
• lateral incisor ...7-8 years
• canine
...9-13 years
• premolars
...9-13 years
• second molar ...11-14 years
• third molar ...17-30 years
The permanent teeth erupt first in mandible and then in maxilla
except the premolars which erupt first in maxilla.
THE MIXED DENTITION
THE MIXED DENTITION
• At the time of eruption
2/3 of the root is
generally formed.
Thereafter, 1,5 -3
years are needed to
complete root length
and even longer to
close the root apices
to the mature size.
REPLACEMENT OF THE TEETH
The eruption can be divided into periods:
1. The first period means a replacement of
incisors, after which a break (usually 2
years) follows.
2. At the age of 10-12 years the second
period is performed. So the canines and
premolars erupt.
ERUPTION OF THE FIRST
PERMANENT MOLARS
The stage of first permanent
molars relationship
formation is the most
important stage of
replacement.
In normal occlusion, the
anterior buccal cusp of
upper first permanent molar
should occlude with
intercuspal ( buccal ) groove
of lower first permanent
molar.
This relationship can be
achieved in several ways.
ERUPTION OF THE FIRST
PERMANENT MOLARS
• at the end of dental arches a negative step is
present ( when the upper second deciduous
molar is as wide as lower ). So the first
permanent molars are guided into right cuspal
relationship.
ERUPTION OF THE FIRST
PERMANENT MOLARS
• a negative step may be present as a result of
mesial drift of lower deciduous molars and
closure of the lower primate spacing.
ERUPTION OF THE FIRST
PERMANENT MOLARS
• the second deciduous molars are
flush distally, the first permanent
molars are guided into
a cusp-to-cusp relationship by
their distal surfaces.
• mesiodistal width of the
permanent canines and premolars
is usually less than of the
deciduous canines and molars.
The surplus space is greater in
mandible. Thus when the second
deciduous molars are shed the
greater leeway space in the lower
arch allows the lower first
permanent molars to move
forwards into a correct cuspal
relationship with the upper.
ERUPTION OF THE FIRST
PERMANENT MOLARS
The order of eruption
is important (the lower
molar must be
replaced before the
upper )
ERUPTION OF THE FIRST
PERMANENT MOLARS
• if there is enough space, the maxilla and mandible are
large, the right relationship of the first molars can be
achieved
• the bad situation is,
if the lower second
deciduous molar is
too wide in relation
to upper or there is
a class II relationship
and the positive
step is present.
REPLACEMENT OF INCISORS
REPLACEMENT OF INCISORS
• The permanent incisors
develop lingual and
palatal to the roots of
deciduous incisors
• The upper lateral
incisors in their
developmental position
are overlapted by the
central incisors. They
escape as the central
incisors erupt.
REPLACEMENT OF INCISORS
The average difference in wide of the deciduous and
permanent incisors is 7 mm in maxilla and
5,5 mm in mandible.
Space for these teeth, which are larger then their deciduous
predecessors (particularly the uppers), is provided by:
1. utilization of existing spacing between the deciduous
incisors
2. an increase in arch width takes place during the eruption
of incisors. The amount of it is about 3,5 mm in maxilla and
2,0 mm in mandible.
3. an increase of length of dental arches, 2,5 mm in maxilla
and 2,0 mm in mandible.
4. the permanent incisors are more proclined and thus
form a larger arch than the deciduous incisors.
REPLACEMENT OF INCISORS
REPLACEMENT OF INCISORS
REPLACEMENT OF INCISORS
While the lower
primate spacings are
important to allow
arrangement of the
first permanent
molars in right
relationship, the
uppers provide the
space for upper
permanent incisors.
REPLACEMENT OF INCISORS
• the deciduous dentitions without spacings
(primate and between incisors) are liable to be
replaced by crowded permanent dentition.
REPLACEMENT OF INCISORS
REPLACEMENT OF INCISORS
REPLACEMENT OF INCISORS
• if there is not sufficient
growth in arch width the
lateral incisors may be
trapped in their palatal
position.
• When the upper incisors
erupt they are frequently
distally inclined, there is a
median diastema. This is due
to the incisor roots being
crowded mesially by the
permanent canine crowns
and should not be mistaken
for a malocclusion. When the
permanent canines erupt the
median diastema will usually
close.
REPLACEMENT OF INCISORS
• When the lower incisors
erupt they are often
crowded but it is
improve by further
growth in arch width.
• If the deciduous incisor
root is not resorbed
normally, the
permanent incisor may
be deflected lingually or
palatally.
REPLACEMENT OF INCISORS
The lower incisors erupt before the uppers. It
causes the accelerated growth of mandible and
the “2th physiological reduction of overbite”.
THE SECOND PERIOD OF
REPLACEMENT
After a replacement of incisors is over the
break (usually 2 years) follows. The
deciduous canines and molars are very
important to be retained, because:
• they keep up the position of the erupted
permanent teeth
• they maintain space for permanent
canines and premolars
• they keep up the overbite
THE SECOND PERIOD OF
REPLACEMENT
• the permanent canines are
wider than their predecessors,
the premolars, particulary the
second premolars, are
narrower than the deciduous
molars.
• the combined mesiodistal
width of the permanent
canines and premolars is
usually less than of the
deciduous canines and
molars.
• The the leeway space is
greater in mandible 3,0 mm
than in the maxilla 1,5 mm
THE SECOND PERIOD OF
REPLACEMENT
The order of eruption in mandible is: the canine, the first and
then the second premolar. The canine cannot drift distaly and
the space for it (because it is wider than its predecessor) is
made by accelerated growth of mandible. This is the cause of
the “3th physiological reduction of overbite”. The order of
eruption in maxilla is: the first premolar, the second premolar
and then the canine.
ERUPTION OF THE MOLARS
• The upper permanent molars develop in the maxillary tuberosity
with their occlusal surfaces facing distally and bucally. Posterior
growth in maxillary length is necessary to allow them to rotate
forwards and downwards into the line of the arch.
• The mandibular molars develop under the anterior border of the
ascending ramus of the mandible. Growth in the mandibular
lenght is necessary if the tooth is to have room to erupt.
PERMANENT DENTITION
• The upper arch is shaped like an elipse and that of lower like a parabole.
• The mandibular teeth are set one inclined plane in advance of the maxillary
teeth. This is because the mandibular central incisor is smaller mesiodistally
than the maxillary central incisor. Thus every tooth occlude with two
antagonists except the lower central incisors and the upper third
molars.
• The maxillary teeth are half a cusp to the buccal of the mandibular teeth.
• Angle stated that the mesiobuccal cusp of the upper first molar occludes
with the intercuspal groove of the lower first molar. That relationship of
the first permanent molars is known as a key of occlusion.
• The upper permanent canine falls into the embrasure between the
lower permanent canine and first premolar.
•
The lower incisor edges occlude with the cingulum plateau of the upper
incisors. Provided that the incisor inclinations are average this gives a
normal overbite of about one-third of the height of the lower incisor
crowns and an overjet of about 2 mm.