DEVELOPMENT & GROWTH OF MANDIBLE

CONTENTS

Introduction Prenatal Embryology Of Mandible Meckel’s Cartilage Endochondral Bone Formation Post-natal Growth Of Mandible

INTRODUCTION Definition of GROWTH:“ The self multiplication of living substance” (J.S. Huxley)“An increase in size” (Todd)“Quantitative aspect of biological development per unit of

time” (Moyers)

Definition of DEVELOPMENT:“Development is progress towards maturity” (Todd)

PRENATAL EMBRYOLOGY OF MANDIBLE

About the 4th week of Intrauterine life, the developing brain and the pericardium form 2 prominent bulges on the ventral aspect of embryo.

These bulges are separated by the primitive oral cavity or the stomatodeum.

The floor of the stomatodeum is formed by the bucco-pharyngeal membrane, which separates it from the foregut.

The pharyngeal arches are laid down on the lateral and ventral aspect of the cranial most part of the foregut that lies in close approximation with the stomadeum.

Initially there are 6 pharangeal arches, but the 5th one usually disappears as soon as it is formed leaving only 5.

They are separated by 4 branchial grooves.

The 5th arch is called the mandibular arch and the 2nd arch, hyoid arch

Each of these 5 arches contain:1. A central cartilage rod that forms the skeleton of the

arch.2. Muscular components termed as branchiomere.3. A vascular component.4. Neural element. The mandibular arch form the lateral wall of the

stomodeum. It gives off a bud from its dorsal end, bud is called

maxillary process. It grows ventro-medially, cranial to main part of the

arch, which is now called a mandibular process. The mandibular process of both sides grow towards

each other and fuse in the midline. They now form the lower border of the stomodeum i.e.

the lower lip and the lower jaw.

MECKEL’S CARTILAGE It is derived from the 1st branchial arch around the

41st-45th day of IUL. It provides a template for guiding the growth of the

mandible. A major portion of the meckel’s cartilage disappears

during growth and the remaining part develops into:1. The mental ossicles.2. Incus and malleus.3. Spine of the spenoid bone.4. Anterior ligament of the malleus.5. Spheno-mandibular ligament.

The ossifying membrane is located lateral to the meckel’s cartilage and its accompanying neurovascular bundle.

From this primary centre, ossification spread below and around the inferior alveolar nerve and its incisive branch and upwards to form a trough for a accommodating the developing tooth bud.

Spread of the intramembraneous ossification dorsally and ventrally forms the body and ramus of the mandible.

ENDOCHONDRAL BONE FORMATION CONDYLAR PROCESS: About the 5th week of IUL, an area of the

mesenchymal condensation can be seen above the ventral of developing mandible.

This develops into a cone-shaped cartilage by about 10th week and starts ossification by 14th week.

Then migrates inferiorly and fuses with the mandibular ramus by about 4th month.

CORONOID PROCESS: Secondary accessory cartilages appear in the region of

the coronoid process by about 10-14th week of IUL.

MENTAL FORAMEN: In this region, on the either side of the symphysis, one or

two small cartilages appear and ossify in the 7th month of IUL to form a variable numbers of mental ossicles in the fibrous tissue of the symphysis.

POST-NATAL GROWTH OF MANDIBLE

While the mandible appears in the adult as single bone, it is developmentally and functionally divisible into a several skeletal sub-unit.

Basal bone forms one unit, to which is attached the alveolar process, coronoid process, condylar process, angular process, the ramus, the lingual tuberosity and the chin.

RAMUS: It moves progressively posterior by a

combination of deposition and resorption. Resorption occurs on anterior part of ramus while bone deposition occur on the posterior region.

This result in a “drift” in posterior direction.

Functions of the remodelling of ramus are:

1. Accommodate the increasing mass of the masticatory muscles inserted into it.

2. To accommodate the enlarged breadth of the pharyngeal space.

3. To facilitate the lengthening of the mandibular body, which in turn accommodates the erupting molars.

CORPUS OR THE BODY OF THE MANDIBLE

Displacement of the ramus results in the conversion of the ramal bone into the posterior part of the body of the mandible.

In this manner, it lengthens. Thus additional space made available by means of resorption of the anterior border of the ramus is made use of to accommodate the erupting molar.

ANGLE OF THE MANDIBLE

On the lingual side of the angle of the mandible, resorption take place on the posterio-inferior aspect while deposition on the anterio-superior aspect.

On the buccal side, resorption occur on the anterio-superior part while deposition takes place on poserio-superior part.

This results in flaring of the angle of mandible as age advances.

LINGUAL TUBEROSITY

It moves posteriorly by deposition on its posteriorly facing surface.

It protrudes noticeably in a lingual direction and that it lies well toward the midline of ramus.

The prominence of the tuberosity is increased by the presence of the large resorption field just below it.

ALVEOLAR PROCESS: It develops in response to the presence of tooth

buds. As the teeth erupt, it develops and increases in

height by bone deposition at the margins.

THE CHIN: As the age advances the

growth of chin becomes significant.

It is influenced by sexual and specific genetic factors.

Usually males are seen to have prominent chin as compared to females.

Mental tuberosity forms by bone deposition during childhood.

THE CONDYLE:

The head of the condyle is covered by the thin layer of cartilage called the condylar cartilage.

The presence of conylar cartilage is an adaptation to withstand the compression that occurs at the joint.

It is believed that the growth of the soft tissues including the muscles and the connective tissues carries the mandible forward away from cranial base. Bone growth follows secondary at the condyle to maintain constant contact with cranial base.

The condylar growth rate increases at puberty reaching a peak between 121/2-14 years. The growth ceases around 20 years of age.

THE CORONOID PROCESS

The growth follows the enlarging ‘V’ principle. Viewing, the logitudinal section of coronoid

process from posterior aspect, it can be seen that deposition occurs on the lingual (medial) surfaces of the left and right coronoid process .

Although additions take places on the lingual side, the vertical dimension of the coronoid process also increases. This follows the ‘V’ principle.

Viewing from the occusal aspect, the deposition on lingual of coronoid process brings about posterior growth movement in ‘V’ pattern

REFERENCE

1. The textbook of Orthodontics S.I. Bhalajhi (Fourth edition)

2. Internet