Development of Dentition - A Complete Guide

DEVELOPMENT OF DENTITION

INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

Contents
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Evolution of human dentition
Prenatal dental development
The predental period
The deciduous dentition
The mixed dentition
The permanent dentition
Dental arch development
References

EVOLUTION OF HUMAN
DENTITION
During evolution several changes took place in
jaws and teeth. When the reptilian dentition
evolved to mammalian, dentition went from
“polyphyodont” (many sets of teeth), to
“diphyodont” (two sets of teeth), and then to
“heterodont” (different types of teeth).


There are four stages of tooth evolution:
1.Reptilia stage (Haplodont).
This type of dentition is depicted by the simplest form
of teeth – a single cone. It usually includes many teeth
in both jaws. Jaw movements are limited to opening
and closing only. No occlusion of teeth is seen in this
class.
2.Early mammalian stage (Triconodont).
This exhibits three teeth in line in the posterior teeth.
Anthropologically, the largest cusp is centered, with a
smaller cusp anteriorly and another posteriorly.

3.Triangular stage (Tritubercular stage).
The three triconodont lines are changed to
three cone-shaped structures, with the teeth
more or less by-passing each other when the
jaws are opened or closed.
4.Quadritubercular stage.
The next stage of development created a
projection on the triangular form that finally
occluded with the antagonist of the opposing
jaw.During this time, as an accommodation to
changes in tooth form and anatomy, the
articulation of jaws changed accordingly.

Human tooth size has undergone a clear cut
reduction during the Upper Paleolithic Age, and
the rate of that reduction has accelerated since the
end of the last Ice Age.
Beginning about 10,000 years ago, the rate of
reduction seems to have doubled to about 1%
every 1,000 years.
Associated with the overall dental reduction is a trend
for substantial decrease in sexual dimorphism in tooth
size.

Common evolutionary trends in primates.
1.

2.

3.

4.
5.
6.
7.

There was shortening of jaws due to decrease in size of
olfactory organs, upright body position and wide angle of
head to body.
Decrease in tooth size occurred, so as to accommodate the
teeth into the smaller jaws, with subsequent elimination of
some teeth from the dentition.
There was progressive shortening and relative widening of
the dental arches.
The canines reduced in size.
The lower premolars became more symmetrical from oval.
The first molars became the dominant cheek teeth.
The third molars, which were larger than the first molars,
were reduced in size www.indiandentalacademy.com
and often eliminated.

Characteristics of the human dentition.
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Acrodont – teeth attached to the jaws by a
connective tissue.

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Pleurodont – teeth set inside the jaws.
Thecodont – teeth inserted inside a bony socket.
Diphyodont – two sets of teeth.


PRENATAL DENTAL
DEVELOPMENT
The embryonic development of both deciduous and
permanent teeth proceeds in four stages:
1. Initiation
2. Bud stage
3. Cap stage
4. Bell stage.


a.) INITIATION OF ODONTOGENESIS.
The first sign of tooth development appears late in
the third embryonic week when the epithelial lining
of the oral cavity begins to thicken in broad zones.
The epithelial thickenings occur on the inferolateral
borders of the maxillary and on the superolateral
borders of the mandibular arches where the two join
to form the lateral borders of the mouth. At 6 weeks,
the four maxillary odontogenic zones coalesce to
form a continuous dental lamina, and the two
mandibular odontogenic zones fuse at the midline.
The teeth begin with invagination of the dental lamina
into the underlying mesenchyme at specific locations
along the free border of each arch.

1.

2.

3.

Morphologic changes in the dental lamina begin at
about 6 weeks in utero and continue beyond birth to
the fourth or fifth year. This occurs in three main
phases:
Initiation of the entire deciduous dentition occurs
during the 2nd month in utero.
Initiation of the permanent teeth occurs by the
growth of the free distal end of the dental lamina
into the surrounding connective tissues, giving rise
to the successional lamina.
The dental lamina elongates distal to the second
deciduous molar and gives rise to the permanent
molar tooth germs.

b.) BUD STAGE.
Soon after dental lamina formation, a vestibular
furrow divides the cheeks and lips from the dental
arches. Subsequently, the dental lamina shows specific
sites of increased mitotic activity which produce knoblike tooth buds corresponding to the ten deciduous
teeth in each jaw. The first buds to form are the
mandibular anterior teeth, at about the 7th week. By
the 8th week, all maxillary and mandibular deciduous
tooth buds are present.

c.)CAP STAGE.
The growth rate at the periphery of the bud is
greater. By the end of the 8th week, there
appears a concavity on the deep surface of the
bud. The tooth is now in its cap stage. As the
epithelium of the cap-shaped organ enlarges and
proliferates into the deeper ectomesenchyme,
there is increased activity in cells contiguous
with the ectodermal tooth bud. At this time, the
essential parts of the tooth – enamel organ,
dental papilla and dental follicle – are
identifiable. Collectively they are called the tooth

germ.


d.)BELL STAGE.
Enlargement of the overall size of the tooth germ and
deepening of its undersurface occurs. Epithelial cells next to
the papilla develop into an enamel-producing layer of cells, the
inner enamel epithelium; epithelial cells along the leading edge
of the germ form the outer enamel epithelium, which eventually
gives rise to the dental cuticle.
The differentiation of dentin producing odontoblasts in the
dental papilla is initiated by the neighboring cells of the inner
dental epithelium. Neighboring cells of the two epithelia
progressively constrict around the dental papilla to leave only
a small opening, which will become the apical foramen. At this
time, the dentin which forms the tooth root is first laid down.
The germ loses its connection with the oral epithelium and the
inner dental epithelium begins to fold, making it possible to
recognize the crown shape of specific morphologic classes of
teeth.

PRENATAL ARCH SHAPE.
The prenatal dental arch progressively changes
shape; at 6 to 8 weeks it is anteroposteriorly
flattened. By the bell stage of the tooth germs,
the anterior segment of the dental arch has
elongated and approaches the form of a catenary
curve by the beginning of the fourth month.


THE PREDENTAL PERIOD.
This period lasts from birth to the eruption of the first
deciduous tooth into the oral cavity, at about 6 months of age.
The Gum Pads.
The alveolar arches at the time of birth are termed gum pads,
and they are firm and pink. They develop in two distinct parts,
a labio-buccal and a lingual portion. The labio-buccal portion
is differentiated first and grows more rapidly. It is divided by
transverse grooves into ten segments, each corresponding to a
deciduous tooth germ. Of these grooves, those between the
canines and first deciduous molar segments are called the
lateral sulci , and they extend to the buccal side. The lingual
portion remains smooth. These portions are separated by the
dental groove , which is the site of origin of the dental lamina.
The lingual portion is limited lingually by the gingival groove.
In the upper jaw, the gingival groove separates the gum pad
from the palate.

Relationship of gum pads.
At rest the gum pads are separated by the
tongue, which protrudes over the lower gum
pad to lie immediately behind the lower lip, and
may even protrude a little between the lips. The
anteroposterior movement vary greatly, while
the lateral movements are very much limited.
The upper gum pad is wider than the lower, and
when the two are approximated there is
complete overjet of the upper over the lower
gum pad, with a considerable anterior overjet.
The lateral sulcus of the lower gum pad is
usually posterior to that of the upper.

At birth, the gum pads are not sufficiently wide to
accommodate the developing incisors, which are
crowded and rotated in their crypts. During the first
year of life the pads grow rapidly. The growth is most
marked in the lateral direction. This increase in width
permits the incisors to erupt in good alignment and to
be spaced. Also during this period, there is a rapid
increase in the labio-lingual dimensions of the gum
pads. The length of the gum pads increases more
moderately.


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The size of the gum pads at birth may be
determined by any one of the following
factors, according to Leighton :
the state of maturity of the infant at birth.
the size at birth as expressed by birth weight.
the size of the developing primary teeth.
genetic factors.


THE DECIDUOUS
DENTITION
The deciduous dentition period begins with the
eruption of the first primary tooth at about 6 months
of age, and lasts till the eruption of the first permanent
molars.


DEVELOPMENT OF THE PRIMARY TEETH
1.Calcification.
The sequence of initial calcification of the primary
teeth is central incisors (14 weeks), first molars (15 ½
weeks), lateral incisors (16 weeks), canines (17 weeks)
and second molars (18 weeks). The crowns of the
teeth continue to grow in width until there is
coalescence of the calcifying cusps, at which time
most of the crown diameter has been determined. The
crown morphology, rate and sequence of growth,
pattern of calcification and mineral content are under
genetic control.

2. Eruption.
Eruption of the primary teeth begins in a variable
fashion but not until root formation has begun.
Occasionally, a “natal tooth” is present at the time
of birth. The natal tooth may be a supernumerary
one but usually is a very early erupted normal
primary central incisor. For this reason such a
tooth should not be extracted casually.


Chronology of tooth development, Primary
Dentition.
Calcification Crown
Tooth
begins
completed

Eruption

Root
completed

Max. Mand. Max. Mand. Max. Mand Max. Mand
Central 14 wk 14 wk 1 ½ 2 ½ 10
8
1½ 1½
I.U. I.U.
Mo.
Mo. Mo.
Mo.
Yr. Yr.
Lateral 16 wk 16 wk 2 ½
I.U. I.U. Mo.

3
11
Mo. Mo.

Canine 17 wk 17wk 9 Mo. 9Mo. 19
Mo.

13 2 yr. 1 ½
Mo.
yr.
20 3 ½ 3 1/4
Mo. Yr. Yr.

1st Molar 15wk 15wk 6
Mo.
I.U. I.U.

5 ½ 16
Mo. Mo.

16
2½ 2¼
Mo. Yr.
Yr.

2nd
Molar

10
29
Mo. Mo.

27
Mo.

19wk 19wk 11
I.U. I.U. Mo.


3 Yr. 3 Yr.

FEATURES OF PRIMARY
DENTITION.
Spacing.
In the primary dentition stage a child may have
generalized spaces between the teeth, localized spaces,
no spaces, or a crowded dentition. The presence of
spacing in the deciduous dentition is a common
occurrence.
Generalized spacing occurs in nearly 2/3 of the
individuals in the primary dentition stage and is a
requirement for the proper alignment of the
permanent incisors.

In addition to the generalized spacing, localized
spacing are often present and are referred to as
‘primate spaces’. Such spaces are present in
87% of the maxillary arches between the lateral
incisor and canine. In the mandibular arch, their
incidence is 78% and they occur between the
canines and first molars. The primate spaces are
normally present from the time the teeth erupt.


Spacing is normal throughout the anterior
part of the primary dentition also. Spaces
develop between the deciduous incisors
subsequent to their eruption, but become
somewhat larger as the child grows and the
alveolar processes expand. Failure of incisor
spacing to appear before 5 years of age occurs in
about 20% of cases and usually indicates
crowding in the permanent dentition.


Overbite.
Overbite is the vertical overlap between the
maxillary and mandibular central incisors. The
overbite in the deciduous dentition varies between
10% and 40%. Foster in a study of 100 British
children between the ages of 2 and 3 years
described the overbite relationship as ideal (19%),
reduced (24%), and excessive overbite (20%). The
fact that more than 60% of the children in this
population have a reduced overbite or an open
bite is attributed to the effects of the various oral
habits (finger or pacifier sucking) that are
common in this age group.

Overjet.
Overjet is the horizontal relationship or the
distance between the most protruded
maxillary central incisor and the opposing
mandibular central incisor. The normal range
of overjet in the primary dentition varies
between 0 and 4 mm. In the same study by
Foster, the overjet was ideal in 28% of the
cases and excessive in 72% of the cases.
Again, this feature was attributed to the
effects of the oral habits

Molar relationship.
The anteroposterior molar relationship in the primary
dentition is described in terms of the terminal planes.
The terminal planes are the distal surfaces of the
maxillary and mandibular second primary molars. The
two terminal planes can be related to each other in
one of three ways.
In the flush terminal plane relationship, both the
maxillary and mandibular planes are at the same level
anteroposteriorly.
In the mesial step relationship, the maxillary terminal
plane is relatively more posterior than the mandibular
terminal plane.
In the distal step relationship, the maxillary terminal
plane is relatively more anterior than the mandibular
terminal plane.

In a study of 121 Iowa children at age 5 years, the distribution
of the terminal plane relationships of the primary second
molars were found to be as follows:
Distal step
10%
Flush terminal plane
29%
Mesial step of 1.0 mm
42%
Mesial step >1.0 mm
19%
Determining the terminal plane relationships in the primary
dentition is clinically important because the erupting first
permanent molars are guided by the distal surfaces of the
second primary molars as they erupt into occlusion.
At the late primary dentition stage of development, the maxilla
and mandible are housing the greatest number of teeth ever,
including 20 erupted primary teeth and at least 28 unerupted
but partially forming permanent teeth.

Anomalies of primary dentition
Anomalies of crown development are seen less
frequently in primary than in permanent dentition.
Primary teeth are rarely congenitally missing, the
incidence being 1%.Most frequently missing teeth are
the maxillary lateral incisors, maxillary central incisors
and the first primary molars in that order.

Ankylosis of primary teeth
Primary teeth are more likely to be ankylosed than
permanent teeth and lower teeth twice as often as
upper. Ankylosis occurs during the normal
physiological resorption of teeth. The majority of
ankylosed primary teeth are observed in the late
primary and the mixed dentitions. The condition is
often bilateral and a posterior open bite appears as the
occlusal level of the ankylosed fails to keep up with
the vertical development of adjacent teeth. Ankylosed
teeth often are referred to as “submerged teeth”.

THE MIXED DENTITION
The mixed dentition period begins at approximately 6
years of age with the eruption of the first permanent
molars, and is normally completed at the time the last
primary tooth is shed. During the mixed dentition
period, the deciduous teeth along with some
permanent teeth are present in the oral cavity.


DEVELOPMENT OF PERMANENT
DENTITION
I. Calcification
Nolla arbitrarily divided the development of each
1.
2.
3.
4.
5.
6.

tooth into ten stages:
Absence of crypt
Presence of crypt
Initial calcification
One third of crown completed
Two thirds of crown completed
Crown almost completed

7. One third of root completed
8. Two thirds of root completed
9. Root almost complete, open apex
10. Apical end of root completed
Girls are more advanced in calcification of
permanent teeth than are boys at each stage and
more so in the later stages.


II. Eruption.
(a) Interrelationships between calcification and

eruption.

Eruption is the developmental process that moves a
tooth from its crypt position through the alveolar
process into the oral cavity and to occlusion with its
antagonist.
Those permanent teeth that follow into a place in the
arch once held by a primary tooth are called
successional teeth (e.g. incisors, cuspids and
bicuspids). Those permanent teeth that erupt
posteriorly to the primary teeth are termed

accessional teeth.


During eruption of succedanous teeth, many activities occur
simultaneously: the primary tooth resorbs, the root of the
permanent tooth lengthens, the alveolar process increases in
height, and the permanent tooth moves through the bone.
Permanent teeth do not begin eruptive movements until after
the crown is completed. They usually emerge when threefourths of their roots are completed. They pass through the
crest of the alveolar process at varying stages of root
development. It takes from two to five years for the posterior
teeth to reach the alveolar crest following completion of their
crowns and from 12-20 months to reach occlusion after
reaching the alveolar margin. It takes about 2-3 years for the
roots to be completed after the tooth has erupted into
occlusion.

Developmental processes during eruption of succadaneous teeth. Aelongation of permanent root. B-resorption of primary predecessor. Cmovement of permanent tooth occlusally. D-growth of alveolar process. Einferior border of mandible, which shows much less growth activity than
the www.indiandentalacademy.com
other four processes.

Chronology of Tooth Development,
Permanent Dentition
Calcification Crown
Tooth
begins
completed

Eruption

Root
completed

Max. Mand. Max Mand Max Mand Max

Mand

Central 3 mo. 3 mo. 4 ½ 3 ½
Yr. Yr.

7¼
Yr.

6¼
Yr.

10 ½
Yr.

Lateral 11
mo.

8¼
Yr.

7½
Yr.

11 yr. 10 yr.

3 mo. 5 ½ 4 yr.
Yr.

Canine 4 mo. 4 mo. 6 yr. 5 ¼
Yr.

11 ½ 10 ½ 13 ½
Yr. Yr.
Yr.


9½
Yr.

12 ¾
Yr.

1st
20
Premolar Mo.

22
Mo.

7 yr.

6¾
Yr.

10 ¼ 10 ½ 13 ½ 13 ½
Yr.
Yr.
Yr. Yr.

2nd
27
Premolar Mo.

28
Mo.

7¾
Yr.

7½
Yr.

11 yr. 11 ¼ 14 ½ 15
Yr.
Yr. Yr.

1st
Molar

32
Wk
I.U.

32
Wk
I.U.

4¼
Yr.

3¾
Yr.

6¼
Yr.

2nd
Molar

27
Mo.

27
Mo.

7¾
yr.

7½
Yr.

12 ½ 12
Yr.
Yr.

3rd
Molar

8 yr. 9 yr. 14 yr. 14 yr. 20 yr. 20 yr. 22
yr.

6 yr.

10 ½ 10 ¾
Yr. Yr.
15 ¼ 16
Yr. Yr.
22
yr.

The physiologic principles underlying tooth eruption are
the same for both primary and permanent teeth.
Pre-emergent Eruption.
Eruptive movement of the tooth follicle begins soon
after the root begins to form. Two processes are
necessary for pre-emergent eruption. First, there must
be resorption of the bone and primary roots overlying
the crown of the erupting tooth. Second, the eruption
mechanism itself must then move the tooth in the
direction where the path has been cleared.
Failure of tooth eruption due to failure of bone
resorption occurs in the case of cleidocranial dysplasia.

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The precise mechanism through which eruptive force is
generated is still not entirely understood. Various theories
have been put forward over the years:
Lengthening of the root within its crypt was initially
considered to be the mechanism which caused the tooth to
erupt. However, eruption of teeth even after removal of their
apical area rejected this hypothesis.
Localized variations in blood pressure or flow in the vessels
surrounding the developing tooth was another theory.
Forces derived from contraction of fibroblasts were thought
to constitute the eruptive force.
Alterations in the extracellular ground substance of the
periodontal ligament similar to those that occur in thixotropic
gels were thought to be the driving force behind eruption of
teeth.

From animal studies, it presently seems clear that
the major eruption mechanism is localized
within the periodontal ligament. It is theorized
that the cross-linking of maturing collagen in the
periodontal ligament provides the eruption
force. This is supported by the fact that eruptive
movements begin when root formation starts
and a periodontal ligament begins to develop.


Post-emergent Eruption.
Once a tooth erupts into the mouth, it erupts rapidly until it
approaches the occlusal level and is subjected to the forces of
mastication. At that point, its eruption slows and then as it
reaches the occlusal level of other teeth and is in complete
function, eruption all but halts. The stage of relatively rapid
eruption from the time a tooth first penetrates the gingiva
until it reaches the occlusal level is called the post-emergent spurt,
in contrast to the following phase of very slow eruption,
termed the juvenile occlusal equilibrium. During the juvenile
occlusal equilibrium, teeth that are in function erupt at a rate
that is parallel to the rate of vertical growth of the mandibular
ramus. As the mandible continues to grow, it moves away
from the maxilla, creating a space into which the teeth erupt.
Due to this, a pubertal spurt in the eruption of teeth
accompanies the pubertal spurt in jaw growth. When the
pubertal growth spurt ends, a final phase in tooth eruption
called the adult occlusal equilibrium is achieved.

The amount of tooth eruption after the teeth
have come into occlusion equals the vertical
growth of the ramus. Vertical growth increases
the space between the jaws, into which the
upper and lower teeth erupt.

During adult life, teeth continue to erupt at an
extremely slow rate. If its antagonist is lost at any age,
a tooth can again erupt more rapidly, a condition
called supraeruption.

b.) Variability in eruption timing.
Eruption timing varies with racial differences, lineages
and within a dentition, that is, those children who
erupt any tooth early or late tend to acquire other teeth
similarly early or late. Sex differences are also seen in
eruption. Except for third molars girls erupt their
permanent an average of approximately five months
earlier than boys.

c.) Sequence of eruption
In the maxilla the sequences 6-1-2-4-3-5-7 and 6-1-2-4-5-3-7
account for almost half of the cases, whereas in the mandible
the sequences (6-1)-2-3-4-5-7 and (6-1)-2-4-3-5-7 include
more than 40% of all children.
Normal variations in eruption sequence having clinical
significance include
Eruption of second molars ahead of premolars in mandibular
arch – this tends to shorten the arch perimeter and may create
space difficulties for the second premolar and may lead to its
being partially blocked out of the arch.
Eruption of canines ahead of premolars in maxillary arch – this
forces the canine labially out the arch especially when there is
an overall lack of space in the arch.
Asymmetries in eruption between right and left sides

Favorable eruption sequence

d) Factors regulating and affecting eruption
Both the sequence and timing of eruption seemed to
largely genetically determined. Racial differences,
socioeconomic status, nutritional influences,
mechanical disturbances and localized pathosis all
influence eruption of teeth. If the primary tooth is
extracted after the permanent successor has begun
active eruptive movements, the permanent tooth will
erupt earlier. If the primary is extracted prior to the
onset of permanent eruptive movements, the
permanent tooth maybe delayed in its eruption.
Crowding of permanent teeth has been shown to
affect their rate of calcification and eruption.

e.) Ectopic development
Ectopic teeth are teeth developing away from
their normal position. The most common found
in ectopy are the maxillary first permanent molar
and the maxillary cuspid followed by the
mandibular cuspid, maxillary second molar,
other premolars and maxillary lateral incisors.
Girls show more tooth germs in ectopy than
boys.

Ectopic eruption of maxillary first molars is associated with
(1) large primary and permanent teeth, (2) a diminished
maxillary length, (3) posterior positioning of the maxilla and
(4) an atypical angle of eruption of the first molar. The
treatment for this problem is best begun early in dental
development in order to utilize the natural forces of eruption.
Surgical uncovering and repositioning are required before
orthodontic treatment.
Impacted teeth are ones that cannot erupt because of
impingement. Third molars and maxillary cuspids are most
commonly impacted. Transposition is a very rare form of
ectopy, which involves exchanged positions between cuspids
and first premolars or cuspids and lateral incisors.

f.)

Factors determining the tooth’s position during
eruption.
During eruption, the tooth passes through four distinct phases
of development – preeruptive, intraalveolar, intraoral and
occlusal.
During intraalveolar eruption, the tooth’s position is affected
by the presence or absence of adjacent teeth, rate of
resorption of the primary teeth, early loss of primary teeth,
localized pathologic conditions and any factors that alter the
growth or conformation of the alveolar processes. There is a
strong tendency of the teeth to drift mesially even before they
appear in the oral cavity. This phenomenon is called mesial

drifting tendency.

Stages of eruption. 1. Pre-eruptive
2. Intra-alveolar 3. Intra-oral 4.Occlusal

Once the oral cavity has been entered, the tooth can be
moved by the lip, cheek and tongue muscles, or by extraneous
objects brought into the mouth(e.g. thumb, fingers, pencils).
In the occlusal stage of eruption, the muscles of mastication
exert an influence through the interdigitation of cusps. The
upward forces of eruption and alveolar growth are countered
by the opposition of the apically directed force of occlusion.
The periodontal ligament disseminates the forces of chewing
to the alveolar bone. The axial inclination of the permanent
teeth is such that some of the forces of chewing produce a
mesial resultant through the contact points of the teeth, the
“anterior component of force”. This is countered by the
approximal contacts of the teeth and by the musculature of
the lips and cheeks.

The anterior component of force

SPACE RELATIONSHIPS IN
REPLACEMENT OF INCISORS.
The permanent incisors are considerably larger than
the primary incisors they replace. Due to this, spacing
between the primary incisors is critical to
accommodate the former. Spacing in the primary
dentition is distributed among all incisors, in addition
to the primate spaces present in both arches.


When the central incisors erupt, they use up almost the entire
space available in the primary dentition. With the eruption of
the lateral incisors, the space situation in both arches becomes
tight. The maxillary arch usually has just enough space to
accommodate the permanent lateral incisors when they erupt.
However, in the mandibular arch, when the lateral incisors
erupt, there is an average deficit of 1.6mm space to align the
four permanent incisors. This difference between the amount
of space needed for the incisors and the amount available for
them is called the “incisor liability”. Due to this, a child goes
through a transitory stage of mandibular incisor crowding at
age 8 to 9. Continued development of the arches improves the
spacing situation, and by the time the canines erupt, space is
once again adequate.

The extra space to overcome the incisor liability and to
accommodate the incisors comes from three sources:
1. A slight increase in the inter-canine width of the dental
arch. As growth continues, the teeth erupt upward and slightly
outward. This increase is only about 2mm. but it contributes
to the resolution of early incisor crowding. More width is
gained in the maxillary arch than in the mandible, and more is
gained by boys than by girls.
2. Labial positioning of the permanent incisors relative to the
primary incisors. This contributes 1 to 2mm. of additional
space in the arch, and thus helps resolve crowding.
3. Repositioning of the canines in the mandibular arch. As the
permanent incisors erupt, the canine teeth widen out slightly,
and also move slightly back into the primate spaces.
These changes occur without significant skeletal growth
in the anterior part of the jaws.

Mandibular anterior crowding is identified as the
discrepancy between the mesio-distal tooth widths of
the four permanent incisors and the available space in
the alveolar process. However, incisor crowding is not
merely a tooth-arch size discrepancy but a discrepancy
among many variables. Several factors can be assumed
to affect the development and severity of crowding,
such as the direction of mandibular growth, early loss
of deciduous molars, mesio-distal tooth widths and
arch dimensions, oral and perioral musculature and
incisor and molar inclination.

In a study by Turkkahraman & Sayin (Angle
Orthod., 2004) , it was determined that patients
with crowding had smaller lower incisor to NB
angles, maxillary skeletal length, mandibular
skeletal length and mandibular dental
measurements. They also had greater interincisal
angles, overjet, overbite and Wits appraisal
measurements and FMIA. Thus the study
concluded that crowding of mandibular incisors is
not only a tooth size- arch length discrepancy.
Dentofacial characteristics also contribute to this
malalignment.

Another study by Sayin & Turkkahraman (Angle
Orthod.. 2000) showed that crowded dentitions had
significantly smaller mandibular deciduous intercanine width, mandibular deciduous inter-molar
width, mandibular permanent inter-molar width and
mandibular inter-alveolar width. The space available
for the mandibular permanent incisors was also less in
crowded dentitions, as was the total arch length.
However, the total width of the four permanent
incisors did not vary greatly between crowded and
non-crowded dentitions.

Ugly Duckling Stage
Sometimes a transient malocclusion is seen in the maxillary
incisor region between 8-9 years of age. This is particularly
seen during the eruption of the canines. As the developing
permanent canines erupt, they displace the roots of the lateral
incisors mesially. This transmits the force to the roots of the
central incisors, which also get displaced mesially. A resultant
distal divergence of the two central incisors causes midline
spacing. This condition has been described by Broadbent as
the ugly duckling stage, as the appearance of the teeth is not
very aesthetic.
The spaces tend to close as the canines erupt. The greater the
amount of spacing, the less the likelihood that a maxillary
central diastema will totally close on its own. Generally, a
diastema of 2mm. or less will probably close spontaneously,
while total closure of a diastema greater than 2mm. is unlikely.

SPACE RELATIONSHIPS IN
REPLACEMENT OF CANINES AND
PRIMARY MOLARS.
The permanent premolars are smaller than the primary
teeth they replace. The combined mesio-distal widths
of the permanent canines and premolars are usually
less than that of the deciduous canines and molars.
The surplus space is called the leeway space. It
amounts to a total of about 1.8mm. in the maxillary
arch and 3.4mm. in the mandibular arch.

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 the maxillary molar. This differential
movement contributes to the normal transition
from a flush terminal plane relationship in the
mixed dentition to a Class I relationship in the
permanent dentition. Also, differential growth of
the mandible more than the maxilla carries the
lower molar more mesial than the upper molar and
helps to establish a Class I relationship in the
permanent dentition.

MOLAR RELATION IN PERMANENT
DENTITION.
The occlusal relationships in the mixed
dentitions determine the molar relation in the
permanent dentition. The transition in molar
relation from the mixed dentition to the early
permanent dentition is usually accompanied by
a one-half cusp (3 to 4mm.) relative forward
movement of the lower molar, accomplished by
a combination of differential growth and tooth
movement

•

A child’s distal step relation may change to an end-to-end
relationship in the permanent dentition, but it is not likely to
change all the way to a Class I relation. It is also possible that
the pattern of growth may not lead to greater prominence of
the mandible, in which case the molar relation in the
permanent dentition will remain a full cusp Class II.

•

Similarly, a flush terminal plane relation in the mixed dentition
can change to a Class I relation in the permanent dentition or
can remain end-to-end if the growth pattern is not favorable.

•

A mesial step relation in the primary molars may produce a
Class I permanent molar relation at an early age. It can
proceed to a half-cusp Class III during the molar transition
and progress further to a full Class III relationship with
continued mandibular growth.

THE PERMANENT DENTITION.
•

•

•

Characteristics of the “normal” occlusion in the
permanent dentition stage include the following:
Overlap: in a normally occluding dentition, the
maxillary teeth are labial/buccal to the mandibular
teeth.
Angulations: in the primary dentition stage, the teeth
are vertically positioned in the alveolar bone. In
contrast, the teeth in the permanent dentition stage
have buccolingual and mesiodistal angulations.
Occlusion: with the exception of the mandibular
central incisors and the maxillary second molars, each
permanent tooth occludes with two teeth from the
opposite arch.

•

•

•

Arch Curvatures: the anteroposterior curvature in the
mandibular arch is called the curve of Spee. The
corresponding curve in the maxillary arch is called the
compensating curve. The buccolingual curvature from one
side to the other is called the Monson curve or Wilson curve.
Overbite and overjet: the overbite often ranges between 10 %
and 50%, and the overjet ranges between 1 mm and 3 mm.
Posterior relationships: the maxillary and mandibular molars
are in Class I occlusion( i.e. the mesiobuccal cusp of the
maxillary first molar is in the buccal groove of the mandibular
first molar). In addition the whole posterior segment needs to
be well interdigitated.

•

•

LATE CHANGES IN THE PERMANENT
DENTITION STAGE
After the eruption of the permanent teeth, the dentition is
relatively stable when compared with the cascade of changes
observed in the mixed dentition stage.
Changes considered to be of clinical importance are :
In both males and females the lips become more retruded
relative to the nose and chin between 25 and 45 years of age.
The implication is that orthodontic treatment at earlier ages
should not result in an overly straight soft tissue profile and
overly retrusive lips because the expected changes in the
relative positions of the nose, lips and chin may exaggerate
these characteristics.
In both males and females, interincisor and intercanine arch
widths decreased. Also, total arch lengths decreased and , as a
result, anterior crowding increased.

Richardson (Dent Update, 2002) reviewed the
causes of crowding that commonly occurs in the lower
arch after the eruption of the second permanent
molars. It was concluded that the factors responsible
for late lower arch crowding included:
a.) Mesially directed forces – the mesial migration of
teeth may be due to the physiological mesial drift, the
anterior component of force of occlusion on the
mesially inclined teeth, the mesial vectors of muscular
contraction, the contraction of the trans-septal fibres
of the periodontal ligament and the presence of a
developing third molar.

b.) Distally directed forces – which may cause
retroclination of the lower incisors, with reduction in
arch length and consequent crowding. These forces
may be due to incisor uprighting, growth patterns,
skeletal structure or soft tissue maturation.
c.) Occlusal factors – which may produce a different
pattern of masticatory forces or an occlusion with
premature contacts. Such occlusal changes may be due
to tooth loss, restorations, development of
parafunctional habits or orthodontic treatment.

d.) Direction of eruption – mesially inclined molars and
distally inclined incisors that continue to erupt in the
same direction would result in reduction in arch depth
and increased crowding.
e.) Tooth morphology – well aligned lower incisors are
smaller mesiodistally and larger labiolingually.
f.) Degenerative tissue changes – gingival recession and
bone loss are likely causes of late crowding.
g.) Orthodontic treatment – teeth that have been moved
orthodontically have a tendency to return to their
original (crowded) positions.

•

•

•

Both overbite and overjet decrease throughout the
second decade of life, due to relatively greater forward
growth of the mandible.
Third molar development: third molars show more
variability in calcification and eruption than do any
other teeth. Impaction of third molars is a frequent
and serious problem in modern man. Mandibular third
molar impactions, which are usually more serious, are
seen more often with skeletal Class II particularly
when the body of the mandible is short and acutely
angled.
By the end of the second decade most persons display
idiopathic resorption of one or more teeth. Nearly
90% of all teeth show some evidence of resorption by
the time a person is 19 years of age.

DENTAL AGE
On the basis of dentition, 3 related estimates of dental
age can be made:
1. From the number and type of teeth visible in the
oral cavity.
2. Based on the schedule of calcification of permanent
mandibular first molar. However, this is a limited
estimate.
3. Based on schedule of calcification of the dentition as
a whole.
Dental age is highly correlated with body height and
chronologic age.

DENTAL ARCH DEVELOPMENT
DIMENSIONAL CHANGES IN THE
DENTAL ARCH
Three sets of measurements are taken for the
dental arches:
(1)The combined widths of teeth (2) the
dimensions of the dental arch in which the teeth
are arrayed (3) the dimensions of the mandible
or maxilla proper i.e. the so called basal bone.

The usual arch dimensions measured are: (1)
widths at the canines, primary molars
(premolars), and first permanent molars; (2)
length (or depth) and; (3) circumference.


Arch dimensions. A, arch length. B1, bicanine
diameter. B2,bimolar diameter. C-C, arch
perimeter/circumference

I. Width.
Dental arch width increases correlate highly with
vertical alveolar process growth, whose direction is
different in the two arches. Maxillary alveolar
processes diverge, due to vertical growth of the
alveolar processes (which also coincides with
eruption of teeth), while the mandibular processes
are more parallel. As a result maxillary width
increases are much greater and can be more easily
altered in treatment.

The crowns of the first molars erupt tipped
somewhat lingually and do not upright fully
until the time of the eruption of the second
molars. As the first molars upright, they cause
an increase in the bimolar width. Furthermore,
both first molars move forward at the time of
the late mesial shift to use up any remaining
leeway space and thus assume a narrower
diameter along the convergent dental arch.

The only postnatal mechanism for widening the basal
bony width of the mandible is deposition on the
lateral borders of the corpus mandibularis. Such
deposition occurs only in small amounts. The maxilla,
in contrast, widens with vertical growth because the
alveolar processes diverge; therefore, more width
increase is seen and more can be procured during
treatment. Furthermore, the midpalatal suture can be
reopened with “rapid palatal expansion” to acquire
large amounts of actual widening of the maxilla.


A study by Marshall,

Dawson, Southard et.

al (AJO DO, 2003) showed that (1)
maxillary molars erupt with buccal crown torque
and upright with age, whereas mandibular
molars erupt with lingual crown torque and
upright with age, and (2) molar crown torque
changes are accompanied by concurrent
increases in maxillary and mandibular intermolar width.


II. Length or Depth
Dental arch length or arch depth is measured at
the midline from a point midway between the
central incisors to a tangent touching the distal
surfaces of the second primary molars or second
premolars. It does not have the clinical
importance of the circumference. Any changes
in arch length are but coarse reflections of
changes in perimeter.


III. Circumference or Perimeter
It is measured from the distal surface of the second primary
molar (or mesial surface of the first permanent molar) around
the arch over the contact points and incisal edges in a
smoothed curve to the distal surface of the second primary
molar (or first permanent molar) of the opposite side.
The reduction in mandibular arch circumference during the
transitional and early adolescent dentition is the result of (1)
the late mesial shift of the first permanent molars as the
“leeway space” is preempted, (2) the mesial drifting tendency
of the posterior teeth throughout all of life, (3) slight amounts
of interproximal wear of the teeth, (4) the lingual positioning
of the incisors as a result of the differential
mandibulomaxillary growth, and (5) the original tipped
positions of the incisors and molars.

•

•

Northway in 1977 reported that moderate caries,
severe caries, and early loss of primary molars
caused dramatic increases in the amount of
perimeter loss.
Hunter and Smith in 1972 noted that children
with crowded arches in the early mixed dentition
showed less arch perimeter loss by the time of
the completed permanent dentition and more
continued crowding.


•

Moorrees in 1969 reported that the arch length and width
affect arch circumference, or the space available for the
alignment of teeth. Between the ages of 5 and 18 years,
maxillary arch circumference increases slightly in the average
boy (1.3mm.) and in the average girl (0.5mm.), while in the
mandible, a mean decrease of 3.4 and 4.5mm. occurs in boys
and girls, respectively. The individual variations in the changes
in arch circumference are considerable and are explained by
differences in the amount of interdental space in the
deciduous dentition, the changes in arch breadth and arch
length, the ratio of mesiodistal crown diameters of deciduous
teeth and their permanent successors, and the sequence of
shedding and emergence of the permanent posterior teeth.

•

A study by Slaj et. al (Angle Orthod., 2003) suggests
that dental arch dimensions are more defined by tooth
eruption and less so by the growth of the supporting bone
during the mixed dentition. In the early mixed dentition, intercanine relations are primarily defined by the early onset of
mandibular growth. However, the skeletal growth of the
maxillofacial complex in the late mixed dentition is not always
predictable. The period between the early and late mixed
dentition is suitable for environmental factors to disrupt the
pattern of ideal symmetrical development of ideal arch form.
Since a number of orthodontic treatments may be planned or
applied in the period of early or late mixed dentition, this
factor should be kept in mind for deciding upon and
administering the appropriate orthodontic therapy.

IV. Overbite and Overjet.
Overbite (vertical overlap of the incisors) and overjet
(horizontal overlap) undergo significant changes during
the primary and transitional dentitions. During the
primary dentition, the overbite normally decreases a
slight amount, and the overjet often is reduced to zero.
From the early mixed dentition to the completion of
the permanent occlusion the average overbite increases
slightly and then decreases, but there is great variability
in its behavior. Overbite is correlated with a number of
vertical facial dimensions (e.g., ramus height), whereas
overjet usually is a reflection of the anteroposterior
skeletal relationship. Overjet is also sensitive to
abnormal lip and tongue function.

REFERENCES
1. Gardiner J.H., Leighton B.C., Luffingham J.K,
Valiathan, A.: Orthodontics for Dental Students.
Oxford University Press, 1998; 4th Edition.
2. Moyers Robert E.: Hand book of Orthodontics.
Year book Medical publishers, Inc, 1988; 4th
Edition.
3. Bishara Samir E.: Text book of Orthodontics.
Saunders 2003.
4. Nanda Surender K.: The developmental basis of
occlusion and malocclusion. Quintessence
Publishing Co. Inc. 1983.

5. Proffit W.R.: Contemporary Orthodontics.
Mosby 2000. 3rd Edition.
6. Salzmann J.A.: Practice of Orthodontics Vol. 1.
J.B. Lippincott Co. 1966.
7. Orban’s Oral Histology and Embryology.
Harcourt Asia Pvt. Ltd. 11th Edition.
8. Tencate R.: Oral histology – development,
structure and function. Mosby 2001. 5th
Edition.


9. Tibana H.W., Palagi L.M., Miguel J.A.M.: Changes in
dental arch measurements of young adults with
normal occlusion-a longitudinal study. Angle
Orthod. 2004;74:618-623.
10. Sayin, Turkkahraman: Factors contributing to
mandibular anterior crowding in the early mixed
dentition. Angle Orthod. 2004; 74:754-758.
11. Turkkahraman, Sayin: Relationship between
mandibular anterior crowding and lateral dentofacial
morphology in the early mixed dentition. Angle
Orthod. 2004; 74:759-764.
12. Slaj M., Jezina M.A., Lauc R., Rajic-Mestrovic S.,
Miksic M.: Longitudnal dental arch changes in the
mixed dentition. Angle Orthod. 2003;73:509-514

13. Marshall S., Dawson D., Southard K.A., Lee
A.N., Casko J.S., Southard T.E.: Transverse
molar movements during growth. Am J Orthod
Dentofacial Orthop 2003; 124:615-24.
14. Richardson M.E.: Late lower arch crowding: the
aetiology reviewed. Dent Update 2002; 29: 234238.
15. Moorrees C.F.A.: Growth studies of the
dentition-a review. Am J Orthod Dentofacial
Orthop 1969 55:600-616.

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