Introduction

What is Orthodontics?

Ortho means straight upright or correct.

Dontics referres to teeth.

Definition of Orthodontics:

It is the art and science of studying normal growth and development of the

dentofacial complex and correction of malocclusion and other deviations in the

the dento-facial region.

Types of Orthodontics:

Preventive Orthodontics:

In this type, Orthodontic intervention is required to prevent the deviation before

it starts. In addition, it deals with the problem when it starts unnoticeably. It

involves the use space maintainers.

Interceptive Orthodontics:

In this phase, a deviation had started and if it is ignored, it will complicate the

case. Accordingly, orthodontic intervention is required to eliminate causative

factors and their deleterious effects. Example of this type is the active space

maintainers.

Corrective orthodontics:

This is the phase where the dental problem had been already established.

Abnormal growth pattern may be also detected. This type of Orthodontics

comprises the use of removable or fixed appliances.

Methods of time analysis:

Chronological age:

It is a convenient universal measure. It is calculated from the date of birth.

Although it is precise and scientifically controllable, it does not point precisely

nor correlate to a particular maturational stage.

Biologic (skeletal) age:

Each individual has a unique and specific timetable of biologic maturation. The

bone tissue was selected as an indicator of the metabolic processes leading to

maturation. The best information could be derived from the hand since it has a

large number of bones and epiphysis. Their degree of calcification, size and

shape are used to establish the biologic age. An x-ray film of the left hand wrist

is taken and is compared to available atlases that permit easy and precise

estimate of the skeletal age.

Dental age:

Since the skeletal age does not correlate well with the chronology of eruption,

dental development was used for prediction. Tables of eruption stages and

formative stages were made available for radiographic comparisons and

correlation to the chronological age.

Patterns of Growth throughout the body (Scammon Curves)

Scammon illustrated four types of growth changes of various organs in relation

to adult weights as 100%:

Somatic (general) Growth Pattern:

This is the growth of the body as a whole: the external dimensions, respiratory

and digestive organs, kidneys, muscles, and skeleton. They increase in size

throughout the growth period until 18-20 years. It is accelerated after birth to

the age of 4 years. Then it slowly decelerates till the pubertal growth spurt

whereas the growth accelerates rapidly reaching a peak at 11-13 years in girls

and 13-15 years in boys. Then the growth decelerates till it virtually stops at

19-20 years. The principle tissues related to the craniofacial skeleton showing

this somatic growth pattern are those of the maxilla, mandible, cartilage and

other supporting connective tissues.

Neural Growth Pattern:

This is the growth of the brain and its parts, dura, spinal cord, optic apparatus.

They grow rapidly in prenatal and early postnatal periods before the rest of the

body, and reach 90% of the adult size by the age of 6 years. Their development,

however, continues for much longer time. There is no evidence of a growth

spurt in the neural growth pattern.

Lymphatic Growth Pattern:

The lymphatic tissues (thymus and lymph nodes) grow rapidly in early life and

enlarge at 11-12 years, and then decline.

Genital Growth Pattern:

The primary and secondary sexual organs remain small until puberty and then

grow rapidly to their final adult size.

Of these four patterns, the first two are seen in the growing skull. The

neurocarnium and orbital cavities form a protecting box for the brain and

eyeball and their growth accordingly follow the neural pattern.

Growth & development

The nature of skeletal growth:

At the cellular level, there are three possibilities for growth:

1- Hypertrophy: increase in the size of individual cell.

2- Hyperplasia: increase in the number of cells

3- Secretion of extra-cellular material; thus contributing to an increase in

size independent of number or size of the cell.

Growth of the soft tissues occurs by a combination of hyperplasia and

hypertrophy. These processes go on everywhere within the tissues and the result

is what is called interstitial growth which simply means that it occurs at all

points within the tissue.

When mineralization takes places, hard tissue is formed and interstitial growth

becomes impossible. Hypertrophy, hyperplasia and secretion of extra-cellular

material are still possible. But in mineralized tissue, these processes can occur

only on the surface not within the mineralized mass. Direct addition of new

bone to the surface of existing bone occurs through the activity of the cells in

the periosteum: the soft tissue membrane covering the bone. Formation of new

cells occurs in the periosteum and extra-cellular material secreted is mineralized

and becomes new bone. This process is called direct or surface apposition of

bone.

Variability of the growth pattern:

In fetal life, at about the third month of intrauterine development, the head takes

up almost 50% of the total body length. At this stage, the cranium is large

relative to the face and represents more than half of the total head. The limbs are

rudimentary and the trunk is underdeveloped.

By the time of birth, the trunk and limbs grow faster than the head and face. So,

the proportion of the entire body devoted to the head decrease to about 30%.The

overall pattern of growth thereafter follows this course with a progressive

reduction of the relative size to about 12%.

Schematic representation of the changes in overall body proportions during

normal growth and development. After the third month of fetal life, the

proportion of total body size contributed by the head and face steadily declines.

(Robins WJ et al)

Concepts of the craniofacial growth:

Concept 1:

Bones grow by adding new bone tissue on one side of the cortex (deposition,

+ve) and the other side undergoes resorption (-ve). This process is termed drift.

Concept 2:

- The outside and inside of bone surfaces are blanketed by growth fields.

- The external surface (periosteum) is not all depository.

- If a given periosteal area has a resorptive type, the opposite inside

(endosteum) of that same area has a depository field and vice versa.

Concept 3:

- Periosteal bone comprises about half of all cortical bone.

- Bone laid down by the lining membrane (endosteum) making up the

other half.

Concept 4:

Bone does not grow itself; growth is produced by the soft tissue matrix. The

operation of the growth fields is carried out by the membranes and other

surrounding tissues rather than by the hard part of bone.

Concept 5:

Growth fields do not have the same rate of growth activity. Fields having

significance in growth are termed growth sites e. g. condyle.

Concept 6: Remodeling & relocation:

It leads to movement of the regional part of bone. Each part is moved from one

location to another as the whole bone enlarges. The ramus is an example. It

moves posteriorly by a combination of resorption and deposition. Elongation of

the corpus is carried out by adding bone to the anterior part. The progressive

sequential movement as bone enlarges is termed relocation. The ramus is

relocated posteriorly and what was used to be part of the ramus became part of

the corpus.

Concept 7: Primary displacement:

- As a bone enlarges, it is carried away from other bones in direct contact

with it.

- This creates the space within which bony enlargement will take place.

- It is a physical movement of the whole bone and occurs by deposition and

resorption.

- As bone grows by surface deposition in a given direction, it is displaced

in the opposite direction.

- Example: The condyle and ramus grow upward and backward into the

space created by the displacement process. The ramus becomes longer

and wider to accommodate:

a- The increasing mass of the masticatory muscles inserted into it.

b- The enlarged breadth of the pharyngeal space.

c- The vertical length of the naso-maxillary complex.

Concept 8: Secondary displacement:

It is the separate movement of bone caused by the separate enlargement of other

bones.

Example: Increase in the size of the bone that composes the middle cranial fossa

results in a downward and forward movement of the naso-maxillary complex.

Concept 9: Posterior growth anterior displacement:

The whole maxillary region is displaced downward and forward away from the

cranium by the expansive growth of the soft tissues in the mid-facial region and

bone deposites in the sutural surfaces between them.

Drift versus displacement:

Drift is the deposition process while displacement is the push contribution.

Drift occurs by deposition on one side and resorption on the other side.

Concept 10: The V principle:

- Enlow had observed that many surfaces are better expressed as Vs.

- The inside of the V is where the new tissue is formed; it also represents

the surface facing the direction of growth.

- Growth of the head of a long bone can be expressed as a series of ever-

increasing Vs.

Concept 11: The law of electrogenesis:

- Bone is moldable under certain external forces.

- It reaches a normal configuration on the physiologic loads and muscle

pulls.

- When sufficient load is applied to a bone, bending occurs.

- A differential pattern of electrical activity elicites osteoblastic activity on

the surface.

- Accordingly, the concave surface becomes positively charged and the

convex one is negatively charged.

- Example: The direction of the mandibular angle due to contraction of the

attached muscle.

Types of bone formation:

Endochondral bone formation:

In the long bones, areas of ossification appear in the center of the bone and at

the ends producing central shaft: diaphysis and a bony cap on each end called

epiphysis. Between epiphysis and diaphysis is a remaining area of uncalcified

cartilage called the epiphyseal plate. The epiphyseal plate is responsible for

almost all growth in length of long bones. The periosteum on the bone surface

plays an important role in adding thickness and in reshaping the external

contours.

Near the outer end of each epiphyseal plate is a zone of actively dividing

cartilage cells. Some of these undergo hypertrophy, secrete an extra-cellular

matrix and eventually degenerate as the matrix begins to mineralize and then is

rapidly replaced by bone.

Intramembranous bone formation:

In this type of formation, it is possible for bone to form by secretion of bone

matrix directly within the connective tissue without any intermediate formation

of cartilage. This type of ossification occurs in the cranial vault and both jaws.

Early in embryonic life, the mandible develops in the same area as the cartilage

of the first branchial arch: Meckel's cartilage. Development of the mandible

begins as a condensation of the mesenchyme just lateral to Meckel's cartilage

and proceeds entirely. Meckel's cartilage disintegrates and then disappears as

the bony mandible develops.

The condylar cartilage develops initially as an independent secondary cartilage,

which is separated by a considerable gap from the body of the mandible. Early

in fetal life, it fuses with the developing mandibular ramus.

The maxilla forms initially from a center of mesenchymal condensation in the

maxillary process. This area is located on the lateral surface of the nasal

capsule.

Whatever the location for intramembranous bone formation, interstitial growth

within the mineralized mass is impossible and the bone must be formed entirely

by apposition of new bone to free surfaces. Its shape can be changed through

removal (resorption) in one area and addition (apposition) of bone in another.

This process is called remodeling.

Site and types of growth in the craniofacial complex:

To understand growth in any area in the body, it is necessary to understand:

1- The site or location of growth.

2- The type of growth occurring at that location.

3- The determinants or controlling factors in that growth.

Accordingly, it is convenient to divide the craniofacial complex into four areas

that grow differently:

1- Cranial vault: The bones that cover the outer and upper surface of the

brain.

2- Cranial base: the bony floor under the brain.

3- Naso-maxillary complex.

4- The mandible.

Cranial vault:

The cranial vault is made up of a number of flat bones that are formed directly

by intramembranous bone formation without cartilaginous precursors. The

growth process is entirely the result of periosteal activity at the surface of the

bones. Remodeling occurs at the contact areas between adjacent bones i.e. at the

sutures.

At birth, the flat bones of the skull are widely separated by loose connective

tissues. These open spaces are the fontanelles. After birth, apposition of bone

along the edges of the fontanelles eliminating them. However, the bones remain

separated by a thin periosteum-lined sutures for many years. Some of these

latters fuse in adult life. Despite their small size, apposition of new bone at these

sutures is the major mechanism for growth of the cranial vault: Sutural growth.

Cranial base:

In contrast to the cranial vault, the bones of the cranial base are formed initially

from cartilage; hence endochondral ossification takes place. Centers of

ossification appear early in the embryonic life in the chondrocranium. They

indicate the location of basioccipital, sphenoid and ethmoid bones that form the

cranial base. As ossification proceeds bands of cartilage called synchondrosis

remain between the centers of ossification.

There are four synchondrosis:

1- Spheno-ethmoidal: It closes about 5-7years.

2-Inter-sphenoidal: It persists till the beginning of adult life.

3- Spheno-occipital: It persists to 18-20 years of age.

4- Intra-occipital: It closes 3-5 years.

A synchondrosis looks like two-sided epiphyseal plate. It has an area of cellular

hyperplasia in the center with bands of maturing cartilage cells extending in

both directions, which will be eventually replaced by bone. Endochondral

ossification occurs at both margins. Growth of the synchondrosis lengthens the

cranial base.

A significant difference from the bones of the extremities is that immovable

joints develop between the bones of the cranial base in contrast to the highly

movable joints of the extremities. In addition, the periosteum-lined sutures,

containing no cartilage, are quiet different from the cartilaginous synchondrosis.

Naso-maxillary complex:

The maxilla develops entirely by intramembranous ossification. Since there is

no cartilage replacement, growth occurs in two ways:

1- By apposition of bone at the sutures connecting the maxilla with the

cranium and the cranial base.

2- By surface remodeling.

Growth of the surrounding tissues translate the maxilla downward and forward

opening up spaces at the superior and posterior sutural attachments. New bone

is added on both sides of the sutures. Accordingly, the bone to which the

maxilla is attached becomes larger.

Part of the posterior border of the maxilla is a free surface: in the tuberosity

region. Bone is added at this surface to create space for the erupting primary

and permanent teeth.

Interestingly, as the maxilla grows downward and forward, its front surfaces are

remodeled. Almost the entire anterior surface is an area of resorption not

apposition!!! This can be considered as a contradiction to the concept that

deposition occurs at the direction of growth.

To understand this paradox, Enlow illustrated his cartoon where he represented

as a platform on wheels being rolled forward while at the same time its surface,

represented by the wall, is being reduced from its anterior side and built up

posteriorly, moving opposite to the growth direction.

Conversely, the roof of the mouth is carried downward and forward along with

the maxilla. Bone is removed on the nasal side and added on the oral side

translating the roof of the mouth downward and forward. Since the anterior part

of the alveolar process is resorptive, some of the forward growth is cancelled.

The same process of bone remodeling also widens the maxilla.

Mandible:

In contrast to the maxilla, both endochondral and periosteal activity are

important in mandibular growth. Cartilage covers the condyle at the TMJ.

Although this cartilage is different from cartilage at the epiphyseal plate and

synchondrosis, hyperplasia, hypertrophy and endochondral replacement occur.

All other areas of the mandible are formed and grow by direct surface

apposition and remodeling.

The overall growth of the mandible can be represented in two ways depending

on the frame of reference (both are right):

1- If the cranium is the reference area:

The chin moves downward and forward. The principal sites of growth are the

posterior surface of the ramus and the condyle and coronoid processes. The

body of the mandible grows longer by apposition of bone on its posterior

surface. The ramus grows higher by endochondral replacement at the condyle

accompanied by surface remodeling ie growing upwards and backwards. The

upward and backward growth occurs in response to the downward and forward

translation to maintain the contact with the skull.

2- If mandibular growth in length is reference:

Instead of the backward movement, resorption and deposition explain the

mandibular growth. The mandible grows longer by deposition of new bone on

the posterior surface of the ramus. At the same time large quantity of bone is

removed from the anterior surface of the ramus (resorption). This gives a room

for the erupting posterior teeth.

Accordingly, what was the posterior surface at one time becomes the center at a

later date and eventually may become the anterior surface as remodeling

proceeds. Ex. In infancy, the ramus is located at about the spot where the

primary first molar will erupt. Progressive posterior remodeling creates space

for the second primary molar and then for the sequential eruption of the

permanent molars. However, if this growth ceases before enough space has

been created for the eruption of the third molar, the latter will become impacted.

Determinants of growth:

Three major theories had attempted to explain the growth determinants:

1- Bone is the primary determinant of its own growth.

2- Cartilage is the primary determinant of skeletal growth, while bone

responds secondarily and passively.

3- The soft tissue matrix is the primary determinant and both cartilage and

bone are secondary followers.

The contemporary thought believes in the second and third theories; while the

first, that was dominant until the 1960s, had been discarded.

Sites versus centers of growth:

Site is merely a location at which growth occurs; while a center is a location at

which independent (genetically controlled) growth occurs. All centers of growth

are sites but the reverse is not true.

As a simple explanation if part of the center is transplanted in a distant area,

growth will occur but the opposite is not true. Example of growth sites:

- Condyle

- Sutures

- Tuberosity

Example of growth centers:

- Tibia

- Iliac crest

- Ribs

- Synchondrosis

Cartilage as a determinant of craniofacial growth:

This can be applied to the mandible by visualizing it as along bone that had

been modified by removal of epiphysis leaving the epiphyseal plate exposed

and bending the shaft into a horse shoe shape. So, the cartilage at the distal ends

of the bone should behave like true growth cartilage. However, if this could be

applied to the mandible, it could not be applied to the maxilla.

Transplantation experiments demonstrated that not all skeletal cartilage act the

same when transplanted ie if a piece of epiphyseal plate of long bone is

transplanted, it will continue growth in the new location or culture; while little

or non growth was observed when the mandibular condyle was transplanted in

another area. From this experiment the other cartilages appeared to be capable

of acting as growth centers but mandibular condyle does not.

Functional matrix theory:

Neither bone nor cartilage were the determinants for growth of craniofacial

skeleton, it would appear that the control would have to lie in the adjacent soft

tissue.

In this concept, soft tissues grow and both cartilage and bone react to this

growth. As excellent examples:

1- The relationship between the size of eye and the orbit.

2- Mechanical restriction caused by scar tissue in the TMJ due to condylar

fracture is the reason for growth deficiency causing hemifacial

microsomia.

3- If there is a growth deficiency in the brain, the brain case will be small

causing microcephaly.