Basic mechanism of craniofacial growth /certified fixed orthodontic courses by Indian dental academy

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CONTENTS
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Introduction
Mechanisms of growth:
- Bone growth
- Growth processes
- Growth fields
- Enlow’s V- principle
- Growth pattern
- Growth movements
Changing concepts and hypotheses of
craniofacial growth.
Conclusion.

INTRODUCTION
The fully developed cranium represents the sum of its
separate parts, in which growth is highly differentiated
and occurs at different rates and in different directions,
and is thus a complex concept.
By birth the craniofacial skeleton has undergone
between 30% and 60% of its total growth. Although
this reflects the early development of the skull, the
remaining increase in size is not equal in all parts of
the cranium.
Whereas the size of the neuro-cranium increases by
about 50% after birth, the facial skeleton grows to
more than twice the size, the increase in height being
the greatest, that in depth somewhat smaller, and that
in width smallest.

The old theory on facial growth, introduced by
Brodie, that the skull increases in size by direct
symmetric expansion of all surfaces and contours
is an antiquated statement.
It is now accepted that the fully grown skull is not
simply a larger version of the infant form and that
the adult skull differs not only in size but also in
shape from that of the child, depending on a
process of differential growth in various parts of the
cranium.

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Craniofacial growth may be divided into four
components:
growth mechanism (how new bone is formed).
growth pattern (change in size and shape of the
bone).
growth rate (speed at which bone is formed).
the regulation mechanism, which initiates and
directs those three factors.


MECHANISMS OF GROWTH
All bone growth is a complicated mixture of two
basic processes, deposition and resorption,
which are carried out by growth fields comprised
of the soft tissues investing the bone. Because
the fields grow and function differently on
different parts of the bone, the bone undergoes
remodelling (i.e. shape change). When the
amount of deposition is greater than the
resorption, enlargement of the bone
necessitates its displacement (i.e. the physical
relocation) in concert with other bone
displacement.

BONE GROWTH

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Tissue growth generally connotes an increase in size.
At the cellular level, there are three possibilities for
growth:
Increase in the size of individual cells
hypertrophy.
Increase in the number of cells
hyperplasia.
Secretion of extracellular material.
Growth of soft tissues occurs by a combination of
hyperplasia and hypertrophy. These processes go on
everywhere within the tissues, resulting in interstitial
growth, which means that it occurs at all points within
the tissue.

Bone cannot enlarge by proliferation and/or
hypertrophy of existing cells or intercellular
material because of its calcified, rigid nature. Its
cells, which are encased in a hard matrix, have
no space to divide.
Therefore, the calcification process which
imparts to bone its unique and structural
characteristics also compels bone to grow by
specifically adapted growth mechanisms which
do not involve interstitial expansion.

Two distinct growth methods exist: an
intramembranous and an endochondral bone
growth mechanism.
Another unusual characteristic of bone growth is
that the increase in size is accompanied by a
remodelling of the existing structure to adjust the
bone's shape and dimensions as it enlarges.
The remodelling activity entails localized
apposition and resorption of bone. Therefore,
bone growth is not totally an additive procedure.
In some areas, bone is lost.

Intramembranous bone formation
Undifferentiated cells in a connective tissue
membrane form a cluster.
Primary center of ossification – small spicules of
bone are formed. (Site of initial ossification)
Osteoblasts – organic matrix which subsequently
ossifies.
Meshwork of delicate bony trabeculae.
continued activity
of osteoblasts

Formation of osteoid which rapidly calcifies.

The inner surfaces of the bone are lined by the
endosteal membrane, which has osteogenic
and/or osteoclastic potential. Bone produced by
this membrane is called endosteal bone tissue.
When it is produced by apposition, the
mechanism is intramembranous.
Intramembranous bone tissue is widely
distributed in the prenatal as well as the
postnatal skeleton and is a particularly fast
growth mechanism.

Endochondral bone formation
Begins within cartilage tissue which is
surrounded by its perichondrium.

Within the primary ossification center, the
chondrocytes hypertrophy.

The matrix between these cells becomes
calcified and small blood vessels from the
perichondrium erode into this area.

Spontaneous resorption occurs in the old
calcified matrix and the lacuna spaces are
created in this zone.

Connective tissue accompanying the blood
vessels is osteogenic – cells differentiate into
osteoblasts and produce osteoid tissue directly
on the cartilage spicules.

A thin crust of bone is formed when the matrix
becomes calcified.

Endochondral bone tissue, therefore, is formed
within cartilage by a process involving partial
calcification of a cartilaginous matrix, partial
removal of calcified cartilage and its replacement
by bone which has formed according to the
conventional appositional (intramembranous)
pattern.
The bone increases in thickness by deposition on
its growing surfaces, and the lumina of the
original resorptive spaces are progressively
reduced by the continuing process of bone
formation.

In the skull, some bones form by the process of
endochondral ossification. These are parts of
the sphenoid and occipital bones which partly
form as a result of activity in the sphenooccipital
synchondrosis. In addition, the endochondral
growth process occurs in the mandibular
condyle.


The intramembranous and endochondral
processes represent the main growth
mechanisms of bone. For this reason, bones are
characteristically classified as either
membranous or endochondral. Some bones
such as the mandible contain both mechanisms.
Since most of the endochondral bone is
ultimately resorbed and replaced by endosteal
bone which forms according to the
intramembranous pattern, few scattered
remnants of endochondral bone survive in the
adult skeleton.

GROWTH PROCESSES
Deposition & Resorption.
Bones grow by adding new bone tissue on one
side of a bony cortex and taking it away from the
other side. The surface facing toward the
direction of progressive growth receives new
bone deposition (+). The surface facing away
undergoes resorption(-). This composite process
is termed "drift." It produces a direct growth
movement of any given area of a bone.

GROWTH FIELDS
The outside and inside surfaces of a bone are
completely blanketed by a mosaic-like pattern of
"growth fields." About half of the periosteal
surface of a whole bone has an arrangement of
resorptive fields and the other half is covered by
depository fields.
If a given periosteal area has a resorptive type of
field, the opposite inside (endosteal) surface of
that same area has a depository field, and vice
versa. These combinations produce the drift of
all parts of an entire bone.

Darkly stippled areas
Lightly stippled areas

Resorptive fields
Depository fields


The operation of the growth fields covering and
lining the surfaces of a bone is carried out by the
membranes and other surrounding tissues rather
than by the hard part of the bone. Thus, growth is
produced by the soft tissue matrix that encloses
each whole bone.
The genetic and functional determinants of bone
growth reside in the soft tissues such as the
muscles, tongue, lips, cheeks, integument,
mucosae, connective tissues, nerves, blood
vessels, airway, pharynx, the brain as an organ
mass, tonsils, adenoids, and so forth.

All the various resorptive and depository growth
fields throughout a bone do not have the same
rate of growth activity. Some depository (or
resorptive) fields grow much more rapidly or to
a much greater extent than others.
Fields that have some significant role in the
growth process are often termed growth
sites.


Some growth sites are called “growth
centers ”. They are considered to be areas that
somehow control the overall growth of the bone.
This term also implies that the “force”, “energy”
or “motor” for a bone resides primarily or solely
within its growth center.
The concept of growth centers finds support in
relation to the growth of the epiphyseal plates of
the long bones, but is no longer considered
important in the growth of the craniofacial region.


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Thus, bone growth is now considered to be
controlled by growth sites, not active growth
centers as believed earlier. The following basic
phenomena are involved in the growth
mechanisms:
conversion of cartilage (synchondroses, nasal
septal cartilage, condylar cartilage).
sutural deposition.
periosteal remodeling.


SYNCHONDROSES
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Displacement growth in the cranial base is made
possible mainly by the synchondroses.
Only a few persist postnatally in the region of the
cranial mid base, the spheno-occipital
synchondrosis being the most important one.
The synchondroses of the cranial base may be
regarded as special joints enabling growth to take
place at younger ages. They contribute to the
growth of the skull in all three dimensions.
It is considered that this cartilage plays a
relatively greater role in the adjustment changes
in cranial base flexure than in its linear growth
(Bjork, 1955 ; Scott, 1962).

NASAL SEPTAL CARTILAGE
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The nasal septum is thought to play an important
part in the prenatal and very early postnatal
growth of the middle face.
According to Scott, the septal cartilage occupies a
unique location for pushing the whole maxilla
forward and downward.
The opposing view, commonly termed the
functional matrix by Moss, suggests that the nasal
septal cartilage is a locus of secondary,
compensatory, and mechanical growth. Growth of
the nasal septal cartilage is secondary to and
compensatory for a prior passive displacement of
the midfacial bones but plays a significant
biomechanical role in maintaining normal midfacial
form.

CONDYLAR CARTILAGE

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This is a secondary type of cartilage. It
participates in growth early in human life and
absorbs pressure forces later in life.
The condyle and its cartilage participate in
regional adaptive growth and are thus not a
major growth center for the whole mandible, as
was believed earlier.
The condyle has a great capacity to adapt to
mandibular displacement during growth. As the
condyle is also part of the ramus the fibrous
layers of condylar cartilage are continuous with
the periosteum of the ramus, and remodeling
processes are seen in all components of the
joint.

SUTURES
Displacement growth is made possible by the
craniofacial sutures, which have a dual function of
permitting growth movement and uniting the bones
of the cranium. When cranial growth ceases, most
sutures ossify.

1.
2.
3.

The main biologic function of the sutural tissue,
besides being an articulation, includes:
To unite bones, while allowing minor movement.
To act as areas of growth; and
To absorb mechanical stress, thus protecting the
osteogenic tissues of the bone.

1.

2.

The movements that takes place between
bones at suture sites are of two types:
The first type is the displacement of bones,
which together with an intrinsic deformation of
the bones enables a 'molding' of the skull
when the head is passing through the birth
canal.
The second type of movement occurring at
suture sites is displacement of bones relative
to each other as a part of skull growth.

PERIOSTEUM
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A periosteal cell layer is established with the
initiation of the intramembranous ossification of
the bone, and the surrounding mesenchymal
cells aquire the character of osteoblasts.
Bone growth involves a continuous replacement
of the matrix- producing cells via cell division in
the cambium layer. Owing to their location, both
matrix-producing and proliferating cells are
subject to mechanical influence.

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If the pressure exceeds a certain threshold level,
so that the blood supply to these cells is
reduced, osteogenesis ceases and osteoclasts
appear leading to resorption, until a biochemical
equilibrium is restored. If, on the other hand, the
periosteum is exposed to tension, it responds
with bone deposition.
The periosteum continues to function as an
osteogenic zone throughout life, but its
regenerative capacity is extremely high in the
young child.
The influence of the periosteum is of greatest
significance for the change in size and shape of
the bones.

GROWTH PATTERN
Growth pattern refers to the change in the size
and shape of the bone.
Bone grows by two fundamental physiologic
processes - modeling and remodeling.

MODELING.
Modeling is a surface-specific activity (apposition
and resorption) that produces a change in the
size and shape of the bone.

REMODELING.
Remodeling is a basic part of the growth process.
A bone remodels during growth because its
regional parts become moved ("drift“) from one
location to another as the whole bone enlarges.
This requires sequential remodeling changes in
the shape and size of each region.


For example, the ramus moves progressively
posteriorly by a combination of deposition and
resorption. As it does so, the anterior part of the
ramus becomes remodeled into a new addition for
the mandibular corpus. This produces a growth
elongation of the corpus.
This progressive, sequential movement of
component parts as a bone enlarges is termed
relocation . Relocation is the basis for
remodeling.
The whole ramus is thus relocated posteriorly, and
the posterior part of the lengthening corpus
becomes relocated into the area previously
occupied by the ramus.

In the maxilla, the palate grows downward by
periosteal resorption on the nasal side and
periosteal deposition on the oral side. This growth
and remodeling process enlarges the nasal
chambers.
The bony maxillary arch and palate of early
childhood are thus remodeled into the nasal
chambers of the adult.


In summary, the process of growth remodeling
is paced by the composite of soft tissues
housing the bones, and the functions are to:
(1) progressively enlarge each whole bone;
(2) sequentially relocate each of the component
parts of the whole bone to allow for overall
enlargement;
(3) shape the bone to accommodate its various
functions in accordance with the physiologic
actions exerted on that bone; and
(4) carry out regional structural adjustments so
that a functional fitting of all the separate bones
to each other and to their soft tissues is
achieved.

Four different kinds of remodeling occur in bone
tissues:
 Biochemical remodeling , taking place at the
molecular level. This involves the constant
deposition and removal of ions to maintain blood
calcium levels and carry out other mineral
homeostasis functions.
 Secondary reconstruction of bone by haversian
systems and also the rebuilding of cancellous
trabeculae.
 Regeneration and reconstruction of bone during
or following pathology and trauma.
 Growth remodeling – remodeling process in
facial growth.

ENLOW’S V - PRINCIPLE
One of the basic concepts in
facial growth is the "V" principle.
Many facial and cranial bones, or
parts of bones, have a V-shaped
configuration. Bone deposition
occurs on the inner side of the
"V“ and resorption takes place on
the outside surface. The "V"
thereby moves from position A to
B and simultaneously increases
in overall dimensions. The
direction of movement is toward
the wide end of the "V."

Thus, a simultaneous growth movement and
enlargement occurs by additions of bone on the
inside with removal from the outside.


The diameter at A is reduced because the broad
part of the bone is relocated to position B.
This is a remodeling
change that converts
a wider part into a
more narrow part, as
both become
sequentially
relocated. Periosteal
resorption and
endosteal deposition
of bone tissue carry
this out.


 A transverse histologic section of the bone at A shows
that the periosteal surface is resorptive; bone-removing
osteoclasts blanket this surface during the active period
of bone growth.
The depository endosteal surface is
lined with bone-producing
osteoblasts.
 A transverse section at B shows
new endosteal bone added onto
the inner surface of the cortex.
 A transverse section made at C
shows an endosteal layer that was
produced during the inward growth
phase. This is covered by a
periosteal layer of bone following
outward reversal, as this part of the
bone now increases in diameter.

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A transverse section at
D shows a cortex
composed entirely of
periosteal bone. The
outer surface is
depository, and the
endosteal surface is
resorptive.


GROWTH MOVEMENTS
Two kinds of growth movements are seen during
the enlargement of craniofacial bones:
-

Cortical drift.
Displacement.


Cortical Drift.
Drift encompasses both relocation and shifting
of an enlarging portion of the bone by the
remodeling action of its osteogenic tissues.
The continuous remodeling maintains the shape
and proportions of the bone throughout the
growth period. As bone deposition occurs during
a simultaneous breakdown of opposing bone
surfaces, the bone will migrate in relation to a
fixed structure. This migration through
remodeling is known as drift .

As a general rule, the surface towards which
growth occurs is appositional, whereas the
surface facing away from the direction of growth is
resorptive.
The two processes do not always occur with the
same intensity. Rather, appositional activity
normally exceeds resorption during the growth
period.


Due to new bone deposition on one surface, all
other parts of the structure will undergo shifts in
relative position, a movement that is termed
relocation . As a result of this process, further
adaptive bone remodeling has to take place, to
adjust shape and size of the bone to its new
position.
An example of such passive drift in the facial
region is the hard palate, which subsides in
relation to the overlying structures, due to
resorption of the nasal floor and concomitant
deposition on the roof of the palate. Relocation
and structural remodeling thus are closely
related to each other.

Displacement.
Displacement is the movement of the whole bone as a
unit. It is of two types – primary and secondary
displacement.

Primary displacement.
As a bone enlarges, it is simultaneously carried away
from other bones in direct contact with it. This creates
the "space" within which bony enlargement takes
place. The process is termed primary
displacement (sometimes also called "translation").

It is a physical
movement of a whole
bone and occurs
while the bone grows
and remodels by
resorption and
deposition. As the
bone grows by
surface deposition in
a given direction, it is
simultaneously
displaced in the
opposite direction.

Thus, primary displacement is associated with a
bone's own enlargement, and it always takes place in
the direction opposite to the vector of bone growth.
The process of new bone deposition does not cause
displacement by pushing against the articular contact
surface of another bone. Rather, the bone is carried
away by the expansive force of all the growing soft
tissues surrounding it. As this takes place, new bone
is added immediately onto the contact surface, and
the two separate bones thereby remain in constant
articular junction.

For example, the nasomaxillary complex is in
contact with the floor of the cranium. The whole
maxillary region is displaced downward and
forward away from the cranium by the expansive
growth of the soft tissues in the midfacial region.
This then triggers new bone growth at the various
sutural contact surfaces between the
nasomaxillary complex and the cranial floor.
Displacement thus proceeds downward and
forward as growth by bone deposition
simultaneously takes place in an opposite upward
and backward direction (that is, toward its contact
with the cranial floor).

Similarly, the whole mandible is displaced
"away" from its articulation in each glenoid fossa
by the growth enlargement of the composite of
soft tissues in the growing face. As this occurs,
the condyle and ramus grow upward and
backward into the "space" created by the
displacement process.
The ramus also remodels as it relocates
posterosuperiorly. It also becomes longer and
wider to accommodate:
(1) the increasing mass of masticatory muscles
inserted onto it;
(2) the enlarged breadth of the pharyngeal space;
and
(3) the vertical lengthening of the nasomaxillary
part of the growing face.

Secondary displacement.
Secondary displacement is the movement of a whole
bone caused by the separate enlargement of other
bones, which may be nearby or quite distant.
The secondary displacement is not associated with
growth of the bone itself but initiated by enlargement
of adjacent bones and soft structures and transferred
to adjacent bones.

For example, increases in size of the bones
that compose the middle cranial fossa (in
conjunction with growth of the brain) result
in a marked displacement movement of the
whole maxillary complex anteriorly and
inferiorly.
This is independent of the growth and
enlargement of the maxilla itself.


(1)
(2)

In summary, the overall skeletal growth process
(displacement and remodeling) carries out two
general functions:
It positions each bone, and
It designs and constructs each bone and all of
its regional parts to carry out that bone's
multifunctional role. The functional input to the
membranes of the bone from the aggregate of
soft tissues causes a bone to develop into its
definitive morphologic structure and to occupy
the location it does.


CHANGING CONCEPTS AND
HYPOTHESIS OF CRANIOFACIAL
GROWTH

Craniofacial morphology is now considered to be
multifactorial; that is, facial development is influenced
by several genes together with various environmental
factors.

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Sicher’s hypothesis (Sutural Dominance).

Sicher (1940) claimed that craniofacial growth as a
whole was the result of innate genetic formation in the
skeletal tissues. The importance of environmental
factors, such as pressure from adjacent organs, was
reduced to a certain influence on the shape of the
bone during development.

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Scott’s hypothesis (Nasal septum)
Scott limited the heredity and expansive growth of the
osteogenic tissues to the periosteum and chondral
structures. In contrast to Sicher, he considered suture
growth to be a response to growth in adjacent
structures, which carried the genetic information
(epigenetic regulation).
He considered the displacement of the bones of the
cranium to be secondary to the morphogenetic
requirements of the brain mass, while the growth of the
middle face was mainly the result of growth of the
chondrocranium - above all the nasal septum - which
pushed the bones away from the structures in the
cranial base. Similarly, the growth of the mandible was
considered to be the result of the autonomic expansive
growth of the condylar cartilage.

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Moss’ hypothesis (Functional Matrix).

He hypothesised that the osteogenic tissue is
deprived of all innate genetic control (bone has no
genes). The craniofacial complex is regarded as a
structure with certain functions, classified as
functional cranial components. These consist of a
functional matrix, comprising the tissues and
cavities that carry out the function as such, and a
skeletal unit, consisting of bone, cartilage, and
tendons, which protects and supports this matrix.

Parts of the functional matrix can be shown to
have direct influence on the bone through the
periosteum - for example, muscle function in
muscle insertions and the teeth in the alveolar
process - and are therefore referred to as the
periosteal matrix. This control of osteogenesis is
a local process comprising remodeling and drift
and is limited to changes in the size and shape
of small skeletal units.
A broader effect is achieved by the tissues and
functional cavities surrounded by capsules,
summarized by the term capsular matrix - for
example, the brain mass and respiratory function
- which produce the movement of the whole
bone classified as displacement.

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van Limborgh’s theory.

Postnatal facial growth is controlled by a
multifactorial system that is influenced by intrinsic,
genetic, and local factors. According to van
Limborgh, craniofacial morphogenesis is
controlled by five different factors: Intrinsic genetic
factors, local and general epigenetic factors and
local and general environmental factors.
According to this theory, both local and
general factors can cause anomalies.

The intrinsic genetic factors exert their influence within
the cells in which they are contained and determine
the characteristics of cells and tissues (cranial
differentiation). Epigenetic factors are those that are
determined genetically but are effective outside the
cells and tissues in which they are produced.
According to van Limborgh, these factors can have an
effect on the adjacent structures such as local
epigenetic factors (for example, embryonic induction
influences), or have a distant influence such as
general epigenetic factors (for example, sex and
growth hormones). The local environmental factors
(such as muscular force) are of much greater
relevance to the postnatal craniofacial growth control
than the general factors (for example, food, oxygen
supply).

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Petrovic’s hypothesis (Servosystem)

Petrovic et al. (1990) developed a cybernetic model
(direction and control of a course of events),
illustrating the complexities of multifactorial
relationships involved in the growth process. In
sum, the physiologic effect of factors controlling the
facial growth is not limited to simple commands but
includes relays, implying interactions and feedback
loops as follows:
1)All of them form a structured system, a
servosystem, in which the position of occlusal
adjustment plays the role of the peripheral
'comparator';

2)The sagittal position of the upper dental, arch is
the 'constant changing reference input', controlled
by somatotrophin and somatomedin and by septal
cartilage growth and by tongue growth;
3)The sagittal position of the lower dental arch is
cybernetically, the controlled variable; and
4)Signals originating from the 'peripheral
comparator' of the servosystem produce an
increased postural activity of the lateral pterygoid
muscle and of some other masticatory muscles,
enabling the lower dental arch to adjust to the
optimal occlusal position. The increased muscle
activity hence induces a posterior growth rotation
of the mandible and, secondly, a supplementary
growth rate of the condyle.

CONCLUSION
Malocclusion and craniofacial deformity arise
through variations in the normal developmental
process, and so must be evaluated against a
perspective of normal development. Because
orthodontic treatment often involves
manipulation of skeletal growth, clinical
orthodontics requires an understanding of the
growth of the craniofacial skeleton. Planned
changes of bone growth and morphology are a
fundamental basis of orthodontic treatment.

The vectors of growth can be modified and
manipulated for treatment during the growing
years.
Thus, a knowledge of the basic concepts of
craniofacial growth is an essential for sound
treatment planning, and goes a long way in
achieving the desired treatment outcome.


REFERENCES
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Gianelly A., Goldman H.: Biologic basis of
orthodontics. 2nd Edition, 1971.
Enlow D.H.: Handbook of facial growth.
2nd Edition.
Proffit W.R.: Contemporary orthodontics.
3rd Edition, 2000.

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Moyers R.E.: Handbook of orthodontics.
4th Edition.
Enlow D.H., Harris D.B.: A study of the
postnatal growth of the human mandible.
Am J Orthod. 1964; 50: 25-50.
Thilander B.: Basic mechanisms in
craniofacial growth. Acta Odontol Scand
1995; 53: 144-151.

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Persson M.: The role of sutures in normal
and abnormal craniofacial growth. Acta
Odontol Scand 1995; 53: 152-161.
Ronning O.: Basicranial synchondroses
and the mandibular condyle in craniofacial
growth. Acta Odontol Scand 1995; 53:
162-166.


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