GROWTH THEORIES

It is strongly believed and understood that growth is

strongly influenced by genetic factors. Although genetic factor

is one of the main factors affecting growth a variety of other

factors like environmental factors. In the form of nutritional

status, degree of physical activity, health or illness etc. may

also contribute normal growth. Great studies have been made in

recent years in improving the understanding of growth control.

What exactly determines the growth of the jaws however

remains unclear and continues to be a subject of intensive

research.

A number of theories have been explained in connection

with growth of craniofacial structures. These include:

Level of growth control: sites Vs centers of growth.

Cartilage as a determinant of craniofacial growth.

The functional matrix concept.

Genetic theory.

Sutural theory.

Van Cimborgh’s theory.

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The major difference in the first three theories mentioned

in the location at which control is expressed.

In the first theory, it implies that genetic control is

expressed directly on bone and therefore its focus should be

periosteum. The second theory states a genetic influence on

cartilage, thus bone playing a passive role. This indirect

growth pattern is called as epigenetic. The third theory states a

genetic influence to a larger extent on tissues outside the

skeletal system, thus bone and cartilage growth occurring

mainly epigenetically in response to a variety of other factors.

GENETIC THEORY

The first factors of consideration always when growth

control is considered is the role of genetic factors. Although

role of genetics in growth and development control has long

been performed by many to have fundamental and overriding

influence its exact application is not understood fully.

Contemporary researchers have failed to accept the idea that

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simply gives are the exclusive determinants of all growth

parameters including regional growth amounts, velocities and

minute details of regional configuration. Fully understood of

course is that genes are basically the participants in

maintaining the operation of any given cells organells loading

to the expression of that cells particular function. For example,

functions of fibroblasts chondroblasts etc. which does their

function when needed and once saturation is reached the

function classes. Thus the genes donot play any role as a

“starter or terminator”, it is the requirement of a particular

situation which makes the cells to performed all these

functions.

LEVEL OF GROWTH CONTROL SITE VERSUS CENTERS

OF GROWTH

A clear distinction between site and centers of growth is

necessary. The site of growth refers merely to a location at

which growth occurs and center of growth is a location at

which independent (genetically controlled) growth occurs. All

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the centers of growth are also the sites but not the reverse is

true.

The major support for the theory that the tissues form

bone carry their own stimulus with them came from the

observation that, the all pattern of craniofacial growth is

relatively constant. This constancy of growth was interpreted

to mean that major sites of growth were also the centers.

Particularly the sutures between the membranes bone of

cranium and jaws were considered growth centers along with

sites of endochondral oscification in the cranial base and at

mandibular condyle.

Growth in this view, was thus the result of expression at

all these sites determined genetically. Thus growth at maxilla

was explained to be the result of pushing apart due to growth in

the midline along the sutures.

If this theory was correct, growth at the sutures should

occur largely independently of the environmental factors and it

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would not be possible to change the expression of growth at

sutures very much.

This theory was a dominant theory of growth till 1960’s

and a variety of drawbacks were explained.

It is known, at present that sutures and periosteum to a

larger extent are not the primary determinants of growth, in

craniofacial region.

This was concluded by the fact that:

a) When an area of sutures between two facial bones is

transplanted to another location (ex: to a pouch in the

abdomen) the tissues does not continue to grow. This

indicating lack of innate growth potential in sutures.

b) Secondly, growth at sutures is influenced by a number of

outside factors. Ex: if cranial or facial bones are pulled

apart, the new bone fills in and thus enlargement of bone

is seen. If the sutures are compressed, the growth at

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sutures is impeded thus reducing the bone size than

normal.

This implies that the sutures are the areas that react to

and not the primary determinants of growth.

c) Growth takes place in untreated cases of cleft palate even

in the absence of sutures.

d) Microcephaly and hydrocephaly raised doubts about

intrinsic, genetic stimulus of sutures.

CARTILAGENOUS THEORY CARTILAGE AS A

DETERMINANT OF CRANIOFACIAL GROWTH

This theory was put forward by James Scott in 1969.

according to this theory the primary determinant of growth of

craniofacial structures is the growth of cartilage. The fact

which made this theory attractive was that for many bones,

cartilage makes the growing part whereas bone merely replaces

the grown cartilage.

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If the cartilage growth is of primary influence the growth

of condyle in mandible acts as a pacemaker for growth of that

bone and remodeling of ramus and other surface changes could

be viewed as secondary to cartilage formation.

Growth of mandible in this regard can be explained by

imaging it like a diaphysis of long bones bent into horse shoe

shape with cut epiphysis at the ends exposing the cartilage that

represents condyles.

Epiphysis

Diaphysis bent into horse shoe shape

If this is a true situation then condyles should act as

growth centers behaving basically as a epiphyseal growth

cartilage.

Growth of maxilla although is difficult to explain is not

impossible. Although maxilla does not contain any cartilage.

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The nasal septum contains cartilage and nasomaxillary complex

grows as a single unit.

Proponents of this theory according to the study of R.D.

Latham (1969) hypothesize that cartilaginous nasal septum acts

as a pacemaker for the growth of other aspects of maxilla.

The location of nasal septal cartilage is such that its

growth brings about a forward and downward pushing of

maxilla thus opening the sutures. Now the secondary (passive)

growth of bone at sutures occurs thus resulting in net increase

in the size of maxilla.

Two types of tests have been carried out to test the

effectiveness of this theory. One by transplanting the cartilage

to some other part of the body and secondly by removing the

cartilage and then assessing the growth occurring in bones.

Coprey and Duterloo (1986) stated that not all the

cartilages act similarly when transplanted. If a piece of

epiphyseal plate is transplanted to a new state or culture, it

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continues to grow normally indicating presence of an innate

potential in the cartilage to grow. It seems that it is very

difficult to get cartilage from synchondrosis at cranial base

during their developmental stage. Thus the results of growth of

cartilage from synchondrosis is not as good as the growth of

cartilage in case of epiphysis transplantation.

Ronning (1966) observed a very little or no growth of

mandibular condyle cartilage when transplanted to other areas

of the body or culture media.

The second group of experiment were to test the removal

of cartilages from their developing sites and testing for growth

disturbance. Many studies have shown that the removal of

cartilage from their sites of function drastically affects the

growth of related bones. Sarnet (1976) in his experiment on

rats has shown a significant reduction in the growth of maxilla.

Another support to this experiment is a reduced midface

development as a whole due to surgical removal of nasal

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septum in humans at an earlier stage (due to a variety of

reasons like severe trauma).

A depressed growth of mandible in case of children

suffered from trauma during childhood is next supporting

factor due to impaction condylar cartilage.

Thus is appears of that epiphyseal cartilage and the

cranial base synchondrosis can act as independently growing

centers as can nasal septum. Thus proving cartilage to be one

of the primary determinant of growth.

FUNCTIONAL MATRIX THEORY OF GROWTH

This theory of growth was put forward by Melvin Moss in

early 1960’s. The concept of this theory is that if neither bone

not cartilage were the determinants for growth of craniofacial

complex then it would appear that the control would have to be

lying in adjacent soft tissues. According to this theory neither

mandibular condylar cartilage, nor nasal septal cartilage are the

determinants of jaw growth. Instead he theorizes that growth of

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facial skeleton occurs in response to functional needs and is

mediated by soft tissues in which the jaws are embedded. In

this conceptual view soft tissues grow and both bone and

cartilage react.

The theory can also be explained by the growth of

cranium in response to growth of brain. The pressure exerted

by growing brain causes saparation of sutures of the cranium

thus inducing growth at the sutures thereby enhancing the

increase in the size of cranial bones. The classical examples

being size of cranium in case of microcephaley and

hydrocephaly. In microcephaley the growth of brain being less

than normal resulting in its smaller size.

This inturn results in a disturbed growth of cranium

resulting in smaller cranium than normal. The opposite can be

explained in case of hydrocephaly where the reabsorption of

C.S.F. is disturbed resulting in accumulation of CSF in an

abnormal amount in cranium. This in turn causes on increased

intracranial pressure leading to suppuration of cranial bones.

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This also induces thus the bone deposition at the sutures. Thus

although brain size is small, the fluid collected along with

brain acts as functional matrix for the development of a larger

cranium.

The next example is the size of orbit relating to the size

of the eyes. In case of large eyes the corresponding orbital size

is increased and opposite for smaller eyes. Thus eyes act as

functional matrix for the growth of orbits of particular size.

In general the functional components according to this

theory are divided into:

1. Functional matrix component.

2. Skeletal unit.

All the tissues organs and spaces taken as a whole,

comprise the functional matrices whereas the skeletal tissues

associated with a particular matrix forms skeletal unit.

The changes in their size, shape including operational

activity is due to the activity of their functional matrix.

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SKELETAL UNIT

All the skeletal tissues associated with a single function

are called as “The skeletal unit”. This may be comprised of

bone, cartilage and tendenous tissue, when a bone is comprised

as a number of continuous units, these units are called as

“microskeletal units”.

Example, mandible has several microskeletal units like

coronoid, angular, alveolar, condylar process ramus and body.

Maxilla is composed of orbital pneumatic alveolar palatal,

basal microskeletal units.

When adjoining portions of a number of bones unit to

perform a single function they are called as macroskeletal

units. Ex: entire enoberanial surface of cranium.

FUNCTIONAL MATRIX

The functional matrix is composed of muscles, glands,

vessels, nerves and functioning spaces. This matrix is divided

into:

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a. Capsular matrix.

b. Periosteal matrix.

a. Periosteal Matrix : Periosteal matrices act directly and

actively on their related skeletal components. Alterations

in their functional demands produces a secondary

transformation of size and shape of their skeletal units.

This is brought about by selective resorbtion and

deposition. The periosteal matrices include muscles,

vessels, glands nerves etc.

b. Capsular Matrix : According to capsular matrix theory

the capsular matrices act indirectly and passively on their

related skeletal structures thereby bringing about the

necessary changes in these skeletal components which are

embedded, grow and maintained in them.

The facial bones move and expand in response to the

growth of their capsule. This kind of growth is not brought

about by deposition and resorbtion.

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The two capsules of important are neurocranial capsule

and orofacial capsule. Ech of these capsules is an envelop

which contains a series of functional cranial components

(skeletal units and related skeletal matrix) which as a whole

are sandwiched between two covering layers. In case of

neurocranial capsule the layers include skin and dermates and

in case of orofacial capsule, it includes skin and mucosal

membranes.

The neurocranial capsular surround and protects brain,

leptmininger and C.S.F. which are the functional matrices of

cranium whereas the orofacial capsule surrounds and protects

esopharyngeal structures.

Thus the function matrices have an major role in

predicting the growth of craniofacial skeleton.

VAN LIMBORGH’S THEORY

A multifactorial theory was performed in 1970 by Van

Limborgh. According to him the factors explained by all

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previous theories were insufficient yet contain elements that

cannot be denied. Van Limborgh explained the growth process

in a view that combines all three existing theories.

He suggested five factors that he believed to control the

growth.

a. Intrinsic genetic factors:

They are the genetic control of skeletal units themselves.

b. Local epigenetic factors:

Bone growth is determined by genetic control originating

from adjacent structures like brain eyes etc.

c. General epigenetic factors:

They are the genetic factors determining growth from

distant structures e.g. growth hormone, sore hormone.

d. Local environmental factors:

They are non genetic factors from local area caries –

habits, muscle forces etc.

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e. General environmental factors:

They are general non genetic factors influencing growth

which include – nutrition, oxygen etc.

The views expressed by Van Limborghs can be

summarized as:

a. Chondrocranial growth is mainly controlled by intrinsic

genetic factors.

b. Dimensional growth is controlled by any few intrinsic

genetic factors.

c. Cartilagenous part should be considered as a growth

center.

d. Sutural growth is controlled mainly by influences arising

from skull cartilages and other adjacent skull structures.

e. Periosteal growth largely depends upon growth of

adjacent structures.

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f. Sutural and periosteal growth are additionally governed

by local non-genetic environmental influence.

Thus a variety of factors other than genetic factor as a

major determinant play important role in basic growth is

development.

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