Bone growth

SEMINAR ON
MECHANISM
OF
BONE GROWTH
www.indiandentalacademy.com

CONTENTS
1. INTRODUCTION
2. GROSS STRUCTURE OF BONE
3. HISTOLOGY OF BONE AND TYPES OF BONE
4. COMPOSITION OF BONE
5. FORMATION OF BONE
6. BONE GROWTH
7. GROWTH MECHANISMS
8. BONE METABOLISM
9. CONCLUSION
10. BIBLIOGRPHY

INTRODUCTION
•In orthodontics skeletal growth is emphasized more
than aspects of craniofacial development, perhaps
because the methods for its study were developed
earlier. Knowledge of skeletal morphology and
growth is routinely applied in clinical practice.
•The craniofacial bony skeleton is a composite
structure which supports and protects a series of
vital functions.
• Craniofacial skeletal growth is very important in
orthodontics, since variations in craniofacial
morphology are the source of most serious
malocclusions, and clinical changes of bony growth
and morphology are a fundamental basis of
orthodontic treatment.www.indiandentalacademy.com

GROSS STRUCTURE OF BONE

•If we examine a longitudinal section across a long
bone we see that the wall of the shaft is tubular
and encloses a large marrow cavity.
•The wall of the tube is made up of a hard dense
material which appears, on naked eye
examination, to have a uniform smooth texture with
no obvious space in it.
•This kind of bone is called compact bone. The
compact bone is thickest midway between the two
ends of the bone and gradually tapers towards the
ends. www.indiandentalacademy.com

•When we examine the bone ends we find that
the marrow cavity does not extend into them.
They are filled by a meshwork of tiny rods and
plates of bone and contain numerous spaces.
• The whole appearance resembling that of a
sponge.
•This kind of bone is called spongy or
cancellous bone.
•The spongy bone at the bone ends is covered
by a thin layer of compact bone, thus providing
the bone ends with smooth surface.www.indiandentalacademy.com

•Where the bone ends take part in forming joints they
are covered by a layer of articular cartilage, with the
exception of the areas covered by articular cartilage, the
entire outer surface of bone is covered by a membrane
called the periosteum.
•The wall of the marrow cavity ( inner side) is lined by a
membrane called the endosteum.
•The marrow cavity and the spaces of spongy bone
(present at the ends) are filled by a highly vascular
tissue called bone marrow. At the bone ends the
marrow is red in color. Apart from blood vessels this red
marrow contains numerous masses of blood forming
cells ( haemopoietic tissue).www.indiandentalacademy.com

• In the shaft of the bone of an adult, the marrow is
yellow, this yellow marrow is made up
predominantly of fat cells.
• In bone of a fetus, or of a young child, the entire
bone marrow is red.
• The marrow in the shaft is gradually replaced by
yellow marrow with increasing age.

HISTOLOGY OF BONE
AND TYPES OF BONE

• Bone is modified connective tissue. It consists of
bone cells or osteocytes that are widely separated
from one another by a considerable amount of
intercellular substance.
• Intercellular substance is made up of homogeneous
ground substance or matrix in which collagen fibers
and mineral salts mainly ( calcium and phosphorous)
are deposited.

 In addition to mature bone cells
( osteocytes) two additional types of cells
are seen in developing bone.
 Osteoblasts - bone producing cells
 Osteoclasts – bone removing cells
 Other cells present includes:
osteoprogenitor cells – from which
osteoblasts and osteocytes are derived.
cells lining the surface bone
cells belonging to periosteum
cells of blood vessels and nerves

TYPES OF BONE

•When we examine the structure of any bone of an
adult, we find that it is made up of layers or lamellae.
This kind of bone is called lamellar bone.
•Each lamellus is a thin plate of bone consisting of
collagen fibers and mineral salts that are deposited in a
gelatinous ground substance. Even the smallest piece
of bone is made up of several lamellae placed over one
another. Between adjoining lamellae we see small
flattened spaces or lacunae.
•Each lacuna consists one osteocyte. Spreading out
from each lacuna there are fine canals or canaliculi that
communicate with those from other lacunae.
•The canaliculi are occupied by delicate cytoplasmic
processes of osteocytes.

LAMELAR BONE

Woven bone:-
The newly formed bone is called woven bone.
The woven bone does not have lamellar structure.
The collagen fibers are present in bundles that
appear to run randomly in different directions,
interlacing of fiber bundles this kind of bone is
called woven bone.
It is later replaced by lameller bone.

Structure of Cancellous bone:
The bony plates or rods that form the
meshwork of cancellous bone are called
trabeculae. Each trabeculus is made up of a
number of lamellae between which there are
lacunae containing osteocytes.
•The trabeculae enclose wide spaces which are
filled in by bone marrow. They receive nutrition
from blood vessels in the bone marrow.

Structure of Compact bone:-
•This type of bone is also made up of lamellae, it is
pervaded by lacunae ( containing osteocyte) and by
canaliculi.
•Most of the lamellae are arranged in the form of
concentric rings that surround a narrow Haversian canal
present at the center of each ring.
•The Haversian canal is occupied by blood vessels, nerve
fibers and cells.
•One Haversian canal and the lamellae around it
constitute a Haversian system or osteon.
•Compact bone consist of several such osteons. Between
adjoining osteons there are angular intervals that are
occupied by interstitial lamellae.

•Near the surface of compact bone the lamellae are
arranged parallel to the surface; these are called
circumferential lamellae.
•In longitudinal section through compact bone, we find that
the Haversian canals run and predominantly along the
length of the bone. The canals branch and anastomose
with each other. They also communicate with the marrow
cavity, and with the external surface of the bone through
channels that are called the canals of Volkmann.
•Blood vessels and nerves pass through all these
channels so that compact bone is permeated by a
networks of blood vessels that provide nutrition to it.

 There is a similarity in the structure of
cancellous and compact bone. Both are made
up of lamellae.
 The difference lies in the relative volume
occupied by bone lamellae and by the spaces.
 In compact bone the spaces are small and the
solid bone is abundant; where as in
cancellous bone the spaces are large and
actual bone tissue is less.

CELLS OF BONE:-
Osteoprogenitor cell :
These are stem cells of mesenchymal origin that can
proliferate and convert themselves into osteoblasts
whenever there is need for bone formation. They
resemble fibroblasts in appearance. In the fetus such
cells are numerous at sites where bone formation is to
take place.
In adults, osteoprogenitor cells are present over bone
surface( on both periosteal and endosteal aspects).

OSTEOBLASTS:-
• These are bone forming cells derived from osteoprogenitor
cells. They are found lining growing surfaces of bone.
•The nucleus of an osteoblast is ovoid and euchromatic. The
cytoplasm is basophilic because of the presence of abundant
rough endoplasmic reticulum, presence of well developed
golgi complex- signifies the cell is in synthetic activity.
•Osteoblasts are responsible for laying down the organic
matrix of bone including the collagen fibers. They are also
responsible for calcification of the matrix.
•Alkaline phosphatase present in the cell membranes of
the osteoblasts plays an important role in this function.www.indiandentalacademy.com

OSTEOCYTES:-
•These are the cells of the mature bone. They lie in the
lacunae of bone and represent osteoblast that have
become “imprisoned” or entrapped in the matrix during
bone formation. Delicate cytoplasmic processes arising
from osteocytes establish contacts with other
osteocytes and with bone lining cells presents on the
surface of bone.
•Osteocytes have eosinophilic or lightly basophilic
cytoplasm. This indicate that cells have negligible
secretory activity; and presence of small amount of
endoplasmic reticulum in the cytoplasm.
•Osteocytes are present in greatest number in young
bone, the number gradually decreased with age.

Functions of osteocytes :-
•They maintain the integrity of the lacunae and
canaliculi and thus keep open the channels for diffusion
of nutrition through bone.
•They play a role in removal or deposition of matrix and
of calcium when required.

Osteoclasts:-
 These are bone removing cells. They are found in
relation to surfaces where bone removal is taking
place. At such locations the cells occupy pits called
resorption bays or lacunae of Howship.
 Osteoclasts are very large cells( 20-100µm) or,
they have numerous nuclei up to 20 or more. The
cytoplasm shows numerous mitochondria and
lysosomes containing acid phosphatase.
 At the site of bone resorption the surface of an
Osteoclasts shows many folds which are described
as ruffled membrane.

Bone lining cells:-
These cells form a continuous epithelium like layer on
bony surface where active bone deposition or removal
is not taking place. They present both on periosteal
and endosteal when bone formation is required, i.e,
many of them are osteoprogenitor cells.

ORGANIC AND INORGANIC
CONSTITUENTS OF BONE
MATRIX

The organic matrix:
This consists of a ground substance in which
collagen fibers are embedded.
The ground substance consists of
• Glycosaminoglycans
• Proteoglycans
• Water

Two special glycoproteins osteonectin and
osteocalcin are present in large quantity. They bind
readily to calcium ions and therefore, play a role in
mineralization of bone.
Various other substances including:
• chondroitin sulphates
• phospholipids
• phospho proteins

• Collagen fibers are similar to those in connective
tissue ( Type – I collagen ) they are some time refered
as osteoid collagen. The fibers are usually arranged in
layers the fibers with in layers running parallel to one
another. Collagen fibers of bone are synthesized by
osteoblasts.
• The term osteoid is applied to the mixture of
organic ground substance and collagen fibers ( before
it is mineralized).

The inorganic ions:-
• The ions present are predominantly calcium and
phosphorous ( or phosphate)
• Magnesium, carbonate, hydroxyl, chloride, fluoride,
citrate, sodium and potassium are also present in
significant amounts.
• Most of the calcium, phosphate and hydroxyl ions are
in the form of needle shaped crystals that are given the
name hydroxyapatite ( Ca10
[PO4
]6
(OH)2
). These
crystals lie parallel to collagen fibers and contribute to
the lamellar appearance of bone.

 About 65% of the dry weight of bone is
accounted for by inorganic salts and 35%
by organic ground substance and collagen
fibers. ( In living bone about 20% of its
weight is made up by water).
 About 85% of the total salt present in
bone are in the form of calcium carbonate

The periosteum:-
• External surface of any bone is covered by a
membrane called periosteum (except those areas
where covered with articular cartilage)
• The periosteum consists of two layers, the outer layer
is a fibrous membrane and the inner layer cellular.
• In young bones the inner layer contains numerous
osteoblasts and is called the osteogenetic layer.
• Periosteum is richly supplied with blood. Many
vessels from the periosteum enter the bone and help to
supply it.

Formation of bone

Formation of bone:-
All bones are mesodermal origin
1. The process of bone formation is called ossification
2. The formation of most bones is preceded by the
formation of a cartilaginous model, which is
subsequently replaced by bone.
3. This kind of ossification is called endochondral
ossification and bones formed in this way are called
cartilage bones.
4. When the bone is laid down directly in a fibrous
membrane, this process is called intra membranous
ossification and bone formed in this way are called
membrane bones.

Intra membranous ossification

Intra membranous ossification:-
1. At the site where a membrane bone is to be formed the
mesenchymal cells become densely packed ( i.e.,
mesenchymal condensation is formed ).
2. The region becomes highly vascular
3. Some of the mesenchymal cells lay down bundles of collagen
fibers in the mesenchymal condensation. In this way a
membrane is formed.
4. Some mesenchymal cells enlarge and acquire a basophilic
cytoplasm, and may now be called osteoblasts. They come to
lie along the bundles of collagen fibers. These cells secrete a
gelatinous matrix in which the fibers get embedded. The fibers
also swell up. Hence the fibers can no longer be seen distinctly.
This mass of swollen fibers and matrix is called osteoid.

5. Under the influence of osteoblasts calcium salts are
deposited in osteoid. As soon as this happens the layer of
osteoid can be said to have become one lamellus of bone.
6. Over this lamellus, another layer of osteoid is laid down by
osteoblasts. The osteoblasts move away from the lamellus to
line the new layer of osteoid. However, some of them get
caught between the lamellus and the osteoid. The osteoid is
now ossified to form another lamellus. The cells trapped
between the two lamellae become osteocytes.
7. In this way a number of lamellae are laid down one over
another and these lamellae together form a trabeculus of
bone.

Endochondral ossification:-
1. At the site where the bone is to be formed, the
mesenchymal cells become closely packed to form
a mesenchymal condensation.
2. Some mesenchymal cells become chondroblasts
and lay down hyaline cartilage. Mesenchymal cells
on the surface of the cartilage form a membrane
called the perichondrium. This membrane is
vascular and contains osteoprogenitor cells.
3. The cells of the cartilage are at first small and
irregularity arranged. However, in the area where
bone formation is to begin, the cells enlarge
considerably.

4. The intercellular substance between the enlarged cartilage
cells becomes calcified, under the influence of alkaline
phosphatase, which is secreted by the cartilage cells. The
nutrition to the cells is thus cut off and they die, leaving behind
empty spaces called primary areolae.
a- PERICHONDRIUM
c- CARTILAGE CELLS
m-MATRIX
Pa- Primary areolae
Cd- dead cartilage
cells

5. Some blood vessels of the perichondrium now invade the
cartilaginous matrix. They are accompanied by osteoprogenitor cells.
This mass of vessels and cells is called the periosteal bud. It eats
away much of the calcified matrix forming the walls of the primary
areolae and thus creates large cavities called secondary areolae, also
called medullary spaces.
Sa-secondary areolae
b- calcified matrix

 The walls of secondary areolae are formed
by thin layers of calcified matrix that have
not been dissolved, the osteoprogenitor
cells become osteoblasts and arrange
themselves along the surface of these bars,
or plates of calcified matrix.

7. These osteoblasts now lay down a layer of ossein fibrils
embedded in a gelatinous ground substance, exactly as in
intra membranous ossification. This osteoid is calcified and a
lamellus of bone is formed.
8. Osteoblasts now lay down another layer of osteoid over the
first lamellus. This is also calcified. Thus two lamellae of
bone are formed. Some osteoblasts that get caught between
the two lamellae becomes osteocytes. As more lamellae are
laid down bony trabeculae are formed.

9. Endochondral ossification is exactly the same as in
intramembranous ossification. The calcified matrix of
cartilage only acts as a support for the developing
trabeculae and is not itself converted into bone.

At this stage the ossifying
cartilage shows a central region
where bone has been formed
(1in fig). As we move away from
this area we see
a region where cartilaginous
matrix has been calcified
and surrounds dead and
dying cartilage cells.(2 in fig)
a zone of hypertrophied
cartilage cells in an
uncalcified matrix.(3 in fig)
normal cartilage in which
there is considerable mitotic
activity.(4 in fig)

• Same cartilage some time later, the
ossification has now extended in to zone 2 and
simultaneously the matrix in Zone 3 has
become calcified. The deeper cells of zone 4
have mean while hypertrophied, while the more
superficial ones have multiplied to form zone 5.
• In this way formation of new cartilage keeps
pace with the loss due to replacement by bone,
the total effect is that the ossifying cartilage
progressively increase in size.

Conversion of cancellous bone to compact bone:-
• As newly formed bone is cancellous it is converted
into compact bone as follows.
• Each space between trabeculae of cancellous bone
comes to be lined by a layer of osteoblasts. These
osteoblasts lay down lamellae of bone. The first
lamellus is formed over the inner wall of the original
space, subsequent, concentric lamellae are laid down
inside this ring thus forming an osteon. The original
space becomes smaller and smaller as persists as
Haversian canal.

Bone Growth

Growth of Bones of Vault of Skull
•In the bones of vault of the skull (eg., the parietal
bone) ossification begins in one or more small areas
called centers of ossification. At first it is in the form of
narrow trabeculae or spicules.
•These spicules increase in length by deposition of
bone at their ends. As the spicules lengthen they
radiate from the center of ossification to the periphery.
• Gradually the entire mesenchymal condensation is
invaded by this spreading process of ossification and
the bone assumes its normal shape.

• The mesenchymal cells lying over the developing
bone differentiate to form the periosteum.
• Growth in size of the bone can occur by deposition of
bone on the edges adjoining sutures. Growth in
thickness and size of the bone also occurs when the
overlying periosteum forms bone over the outer surface
of the bone.
• Simultaneously, there is removal of bone from the
inner surface. In this way, as the bone grows in size,
there is simultaneous increase in the size of the cranial
cavity.

Development of a typical long bone:-
• In the region where a long bone is to be formed the mesenchyme first
lays down a cartilaginous model of the bone.
• This cartilage is covered by perichondrium. Endochondral ossification
starts in the central part of the cartilaginous model (i.e., at the center of
the future shaft). This areas is called the primary center of
ossification.

• Gradually, bone formation extends from the primary center towards the
end of shaft. This accompanies by progressive enlargement of the
cartilaginous model.
• Soon after the appearance of the primary center, and the onset of
endochondral ossification in it, the perichondrium ( which may now be
called periosteum ) becomes active. The osteoprogenitor cells in its
deeper layer lay down bone on the surface of the cartilaginous model
by intramembranous ossification. This periosteal bne completely
surrounds the cartilaginous shaft and is, therefore, called the
periosteal collar.

• The periosteal collar is first formed only around the region of the
primary center, but rapidly extends towards the ends of the
cartilaginous model. It acts as a splint, and gives strength to the
cartilaginous model at the site where it is weakened by the
formation of secondary areolae. We shall see that most of the
shaft of the bone is derived from this periosteal collar and is,
therefore, membranous in origin.

• At about the time of birth the developing
bone consists of a part called the diaphysis
(or shaft), that is bony, and has been
formed by extension of the primary center of
ossification, and ends that are
cartilaginous.
• At varying times after birth secondary
centers of endochondral ossification appear
in the cartilages forming the ends of the
bone. These centers enlarge until the ends
become bony. More than one secondary
center of ossification may appear at either
end. The portion of bone formed from one
secondary center is called an epiphysis.

• For a considerable time after birth the bone of the
diaphysis and the bone of any epiphysis are
separated by a plate of cartilage called the
epiphyseal cartilage or epiphyseal plate. This
is formed by cartilage into which ossification has not
extended either from the diaphysis or from the
epiphysis. This plate playas vital role in growth of the
bone. www.indiandentalacademy.com

Diagram showing how a bone grows in thickness

Growth of long bone:-
• To understand how a bone grows in
length, we will now take a closer
look at the epiphyseal plate.
Depending on the arrangement of
cells, three zones can be
recognized.
• Zone of resting cartilage: here the
cells are small and irregularly
arranged.
• Zone of proliferating cartilage: this
is also called the zone of cartilage
growth. In this zone the cells are
larger, and undergo repeated
mitosis. As they multiply, they come
to be arranged in parallel columns,
separated by bars of intercellular
matrix.
• Zone of calcification: this is also
called the zone of cartilage
transformation. In this zone the cells
become still larger and the matrix
becomes calcified. www.indiandentalacademy.com

•Next to the zone of
calcification, there is a zone
where cartilage cells are dead
and the calcified matrix is
being replaced by bone.
• Growth in length of the bone
takes place by continuous
transformation of the
epiphyseal cartilage to bone.
• At the same time, the
thickness of the epiphyseal
cartilage, is maintained by the
active multiplication of cells in
the zone of proliferation.
•When the bone has attained
its full length, cells in the
cartilage stop proliferating.www.indiandentalacademy.com

• The process of
calcification, however,
continues to extend into
it until the whole of the
epiphyseal plate is
converted into bone.
• The bone substance of
the diaphysis and that
of the epiphysis then
become continuous.
• This is called fusion of
the epiphysis.

METAPHYSIS
The portion of the diaphysis adjoining
the epiphyseal plate is called
metaphysis.
It is a region of active bone formation
and, for this reason, it is highly
vascular. The metaphysis does not
have a marrow cavity. Numerous
muscles and ligaments are usually
attached to the bone in this region.
Even after bone growth has ceased,
the calcium turnover function of bone is
most active in the metaphysis, which
acts as a store house of calcium. The
metaphysis is frequently the site of
infection.

Growth mechanisms

Growth mechanisms
• Endosteal and Periosteal Growth
• Cortical drift
• Relocation and Remodeling
• “V” Principle
• Surface principle
• Growth fields
• Displacement

Growth mechanisms:-
• Bone growth takes place according to several basic principles,
which can also be used to explain the growth processes of the
facial skeleton.
The following two mechanisms are important for bone growth in
the facial and cranial regions:
•Direct bone growth by means of deposition and resorption
processes on the bone surfaces, which cause the cortical
plate to drift.
•Displacement of the entire bone due to growth of the bone
itself or expansion of adjacent structures.

ENDOSTEAL AND PERIOSTEAL GROWTH
•Approximately half of the cortical plate of the facial
and cranial bones is formed by the outer surface
(periosteum) and the other half by the inner surface
(endosteum).
•Appositional layers of cortical bone can originate
entirely from the periosteum or the endosteum.

Periosteal and
endosteal bone
formation:-
+ = Bone apposition
- = Bone resorption
•If the direction of growth
remains constant, the
right cortical plate is
formed periosteally and
the left plate endosteally.
Both shift in the direction
of growth, i.e., to the
right. www.indiandentalacademy.com

CORTICAL DRIFT
• All bone structures have one growth principle in
common, which was termed “drift”
• The cortical plate can be relocated by
simultaneous apposition and resorption
processes on the opposing periosteal and
endosteal surfaces ( cortical drift).
• The bony cortical plate drifts by depositing and
resorbing bone substance on the outer and inner
surfaces, respectively, in the direction of growth.

• If resorption and deposition takes place at the
same rate, the thickness of the bone remains
constant.
• If more bone is deposited than resorbed, the
thickness of the bone increases. During the
developmental period, deposition takes place at a
slightly faster rate than resorption, so that the
individual bones slowly enlarge.

Cortical drift : Diagrammatic view
a – cortical plate of bone
b – increase in thickness due to
apposition on one of the surfaces
c – When the resorption process on
one side of the bone exceeds the
apposition process on the opposing
side, the thickness of the bone will be
reduced.
d – When resorption on one side of
the bone corresponds in magnitude to
apposition on the opposing side, the
bone will drift without changing its
size.
e – The cortical plate has drifted
completely to the right when
compared to its original position in (a)
by the process of surface remodeling.

Due to new bone deposition on an existing surface, all other parts
of the structure undergo shifts in relative position; a movement
which is termed relocation.
• As a result of this process, further adaptive bone remodeling is
necessary in order to adjust the shape and size of the area to
the new relationship. Selective resorption and apposition
processes functionally remodel the area to conform to the new
physiological loading.
RELOCATION AND REMODELING

• Relocation and cyclic, structural remodeling are growth
mechanisms which are closely related to one another:
• Remodeling is based on relocation and is a secondary result
of the displacement process.
• When one level passes into the next due to growth, its
Position is taken by the following level which undergoes the
relevant structural changes. The information which initiates
the remodeling processes contained within the various soft
tissues surrounding the bone.

Relocation and
remodeling
(Diagrammatic view)
The position of the gray
zone changes in relation
to the original position (A)
due to bone apposition (+)
and resorption (-). As a
result of the level-by-level
growth process, the
marked area is
translocated from the
posterior to the anterior
border of the ramus,
without changing its own
position.

Primary Displacement
And bone growth
1. From position (1) the entire
mandible is displaced downward
and forward(2),away from its
articular joints, by the growth of the
surrounding soft tissue.
2. This translatory movement
stimulates the enlargement and
remodeling(3) of the condyles and
rami which take place parallel to
the displacement.
3. The bone growth processes are
directed upward and backward by
an amount that equals the
displacement of mandible.
4. The changes resulting from these
combined processes are shown
in(4).

THE “V” PRINCIPLE
•The "V" principle is an important facial skeleton growth
mechanism, since many facial and cranial bones have a “V”
configuration or "V" -shaped regions.
• Such areas grow by bone resorption on the outer surface of the
"V" and deposition on the inner side due to the concept of
surface growth depending on growth direction. The “V” moves
away from its tip and enlarges simultaneously.
The outcome of these growth processes is:
1) Enlargement in overall size of the "V" -shaped area
2) Movement of the entire "V" structure towards its own wide end
3) Continuous relocation
.

The “V” principle – vertical
expansion:-
• According to this growth concept,
bone is deposited on the inner
surface of the "V" shaped bone and
resorbed on the outer surface. Thus,
the "V" moves away from its narrow
end (direction of the arrow) and
enlarges in overall size.
• Longitudinal section through the right
and left coronoid processes of a
mandible. The processes are
enlarged during growth in accordance
with the "V" principle. Bone is
deposited on the lingual surfaces and
resorbed from the opposing buccal
surfaces. The structures increase in
height, the tips of the coronoid
processes diverge further, and their
bony bases converge.

The "V" principle -
Horizontal expansion
• The mandible viewed from
above, including a
horizontal section through
the base of the coronoid
process.
• Bone is deposited on the
lingual side of the
mandibular structures up to
the ramal surface. Thus,
the coronoid processes
move - despite bone
deposition on the inner
surfaces - in backward
direction, and the posterior
parts of the mandible
widens.

SURFACE PRINCIPLE
• The surface principle states that bone sides which face the
direction of growth are subject to deposition and those opposed
to it undergo resorption. These processes always take place on
contralateral bone surfaces so that the cortical plate follows the
course of growth.
• As the individual parts of the bone grow in different directions,
only half of the deposition process is localized on the outer
cortical plate (periosteal bone formation). The other half of the
growth process consists of bone deposition on the inner cortical
surface (endosteal bone formation).

Surface principle
+ = Bone deposition
- = Bone resorption
The areas marked “X” on the outer surfaces of the bone and
those marked “B” on the inner surfaces grow in the direction of
arrow and are depository. Accordingly, areas “A” and “Y” resorb in
the opposite direction.

Direction of growth of individual
areas of the mandible:
Red arrow = Bone deposition
Blue arrow = Bone resorption
•The arrows pointing toward the bone
indicate periosteal bone surfaces
which do not face the direction of
growth and are, therefore resorptive.
Those arrows which point away from
the bone indicate periosteal surfaces
which face toward the direction of
growth and are depository.
•The main direction of growth of the
ramus and the mandibular corpus is
upward and backward.

• Bone growth is controlled by so-called growth fields.
These fields, which are distributed in a characteristic
mosaic-like pattern across the surface of a given bone,
have either depository or resorptive activity.
• If the periosteal growth field is resorptive, the opposing
endosteal field is depository.
• On the other hand, if the endosteal surface undergoes
resorption, the periosteal surface is depository. Bone
growth processes, i.e., bone drift, are based on these
periosteal and endosteal relationships.
GROWTH FIELDS

• Growth fields have a pacemaking function, which is
controlled by the contiguous soft tissue. Each
increase in length of the bone commences with the
growth fields migrating within the respective
connective tissue membranes (e.g., periosteum and
endosteum, sutures, periodontal ligament). The
enclosing soft tissues determine the changes in the
underlying bone parts which are controlled by this
specific growth field.
• Not all growth fields of a bone exhibit the same
amount of activity or velocity of growth.

Arrangement of the growth fields:-
Depository and resorptive fields are
distributed characteristically across the
entire inner and outer surfaces of the
neurocranial and facial skeleton.
The activity of the growth field is not
located in the bone itself. The genetic
information resides within the soft
tissues.
The soft tissue acts as a functional
matrix to control bone growth, whereas
the bone itself only reports – via a
feedback mechanism which is
connected to the connective tissues-
when the shape, size and
biomechanical aspects coincide with
the functional requirements. This
information causes the histogenetic
activity of the osteogenic membranes
to respond.
Red fields-bone deposition.
Blue field-bone resorption.

Growth centers: is a location
at which independent (genatically
controlled) growth occurs.
•This term is often used to describe
very active growth fields which are
significant to the growth processes
such as the cranial and facial
sutures, the mandibular condyles,
the maxillary tuberosities, the
alveolar processes, and the
synchondroses of the cranial base.

DISPLACEMENT
• Apart from direct bone growth due to deposition and resorption, the
process of displacement, i.e., the translatory movement of the whole
bone caused by the surrounding physical forces, is the second
characteristic mechanism of skull growth. The entire bone is carried
away from its articular interfaces ( sutures, synchondroses, condyles )
with adjacent bones.
• Displacement in conjunction with bone’s own growth is termed “ primary
displacement”. Displacement is initiated by the sum of the expansive
forces of the soft tissues in the growing face. It occurs parallel to bone
growth, thus creating a space around the contact surfaces into which
the bone can enlarge. The degree of displacement exactly equals the
amount of new bone deposition, although the direction of displacement
is always opposite to that of the bone deposition.
• Bone displacement due to the enlargement of bones and soft tissue
which are nearby is termed “secondary displacement”.

Primary displacement of the
nasomaxillary complex:
1. The bone structure grows upward
and backward due to deposition
and resorption processes.
Simultaneously, the entire complex
is displaced forward. Thus, the
necessary space for bone
deposition is created around the
articular surfaces. Primary
displacement always takes place in
the opposite direction to the vector
of the bone growth.
2. Events during primary
displacement. The bone is
remodeled and shifted in the
opposite direction simultaneously.www.indiandentalacademy.com

Secondary displacement of the
nasomaxillary complex:
• This process if not associated with
growth of the bone itself. This type of
displacement is initiated by
enlargement of adjacent bones and
soft tissues.
• Its effect is transferred from bone to
bone and develops in relatively distant
areas. Secondary displacement of the
nasomaxillary complex is caused by
growth of the middle cranial fossa and
the temporal lobe, and is directed
forward and downward.

Bone Metabolism
•Orthodontics and dentofacial orthopedics
manipulates bone. The Biomechanical response to
altered function and applied loads depends on the
metabolic status of the patient. Bone metabolism is an
important aspect of clinical medicine that is directly
applicable to orthodontics and orthopedics.
•Bone is the primary calcium reservoir in the body
about 99% of the calcium in the body is stored in the
skeleton. The continual flux of bone mineral responds
to the complex interaction of endocrine, biomechanical,
and cell-level control factor that maintain the serum
calcium level at about 10mg/dl ( 10mg%) .

• Calcium homeostasis Is the process by which
mineral equilibrium is maintained . Maintenance of
serum calcium levels at abot 10mg/dl is essential life
support function.
• When substantial calcium is needed to maintain the
critical serum calcium level, bone structure is
sacrificed.
Calcium homeostasis is supported by three
temporally related mechanism.:
• Rapid ( instantaneous ) flux of calcium from bone
fluid ( which occurs in seconds.)
• Short term response by osteoclasts and osteoblasts
(which extends from minutes to days.)
• Long term control of bone turnover (over weeks to
months) www.indiandentalacademy.com

conclusion
 Various aspects of growth mechanisms areVarious aspects of growth mechanisms are
significant when assessing the etiology ofsignificant when assessing the etiology of
malocclusion and the possible methods ofmalocclusion and the possible methods of
treatment.treatment.
 The development of the craniofacialThe development of the craniofacial
structure is not mearely a symetricalstructure is not mearely a symetrical
expansion of the outer bone contours, but itexpansion of the outer bone contours, but it
is based on different growth mechanisms.is based on different growth mechanisms.

Bibliography
1. Text book of histology by Leeson, 5th
Edn.
2. Handbook of orthodontics – Robert E. Moyers
3. Orthodontic Diagnosis – By Rakosi et al
4. Contemporary orthodontics – Proffit – 3rd
Edn.
5. Orthodontic current principles and techniques. Graber
and Vanarsdall – 4th
Edn.
6. Essentials of facial growth. – Donald Enlow
7. Practice of orthodontics – J.A. Salzmann

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Bone growth

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