mandible - development and growth

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DEVELOPMENT AND GROWTH OF MANDIBLE

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Learning Outcomes

At the end of the lecture, students should be able to:a. Describe the prenatal growth of the mandibleb. Explain the ossificaton of the mandible.c. The timing of the growthd. The muscle attachment of the mandiblee. Age changes in the mandiblef. Clinical implication during the growth of the

mandible

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Mandible is

-largest & lowest bone of face.

Horseshoe shaped body which is curved

horizontally.

Two ramii vertically with two processes one condylar & other is coronoid process.

INTRODUCTION

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PRE NATAL GROWTH

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The basic growth of the mandible starts in the 7th week of fertilization.

It is formed from the tissues of the 1st brachial arch.

The cartilage of the 1st arch (Meckel’s cartilage) forms the lower jaw in primitive vertebrates.

But in humans the Meckel’s cartilage has a close potential relationship to the developing mandible but makes no contribution to it.

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These arches appear in the fourth and fifth weeks of development and contribute to the characteristic external appearance of the embryo.

At the end of the fourth week, the centre of the face is formed by the stomodeum, surrounded by the first pair of pharyngeal arches.

The first four branchial arches are well developed in humans.

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The first arch is also called the MANDIBULAR ARCH.

The cartilage of the first arch is called the MECKEL’S CARTILAGE

Nerve - Mandibular nerve

Muscles of Arch – medial and lateral pterygoids , masseter , temporalis , mylohyoid , anterior belly of diagastric , tensor tympani , tensor palati

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The mandibular arch forms lateral wall of the stomodeum .

It gives a bud from its dorsal end called the MAXILLARY PROCESS.

It grows ventro-medially cranial to the main part of the arch which is called the MANDIBULAR PROCESS.

The right and left mandibular processes meet in the midline and fuse. They form the lower lip and lower jaw

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The mandible is derived from the ossification of an osteogenic membrane formed from the ectomesenchymal condensation at around 36 to 38 days IU

Some mesenchymal cells enlarges , acquire a basophilic cytoplasm and form osteoblasts

These osteoblasts secrete a gelatinous matrix called osteoid and result in ossification of an osteogenic membrane

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The resulting intramembranous bone lies lateral to meckel’s cartilage.

In the sixth week of the intrauterine life a single ossification centre for each half of the mandible arises in the bifurcation of inferior alveolar nerve into mental and incisive branches .

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During seventh week of IU bone begin to develop lateral to meckel's cartilage & continues until the posterior aspect is covered with bone

Between eight & twelfth week of IU life, mandibular growth accelerate , as a result mandibular length increases.

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Ossification stops at a point , which later become mandibular lingula, the remaining part of meckel’s cartilage continues to form sphenomandibular ligament & spinous process of sphenomandibular ligament & spinous process of sphenoid.

Secondary accessory cartilage appear between 10th and 14th week of intrauterine life to form head of condyle , part of coronoid process & mental protuberance.

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Both endochondral and intramembranous type.

ENDOCHONDRAL OSSIFICATION The bone formation is preceded by

the formation of a CARTILAGENOUS MODEL Which is subsequently replaced( not converted into) by bone.

OSSIFICATION

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Endochondral bone formation is seen only in 3 areas of mandible:

1.The Condylar Process 2.The Coronoid Process 3.The Mental Region

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Intramembranous ossification Intramembranous ossification is the formation of bone

directly from or within fibrous connective tissue membranes.

It occurs in

1)Whole body of mandible except the anterior part

2)Ramus of mandible as far as mandibular foramen

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RAMUS

The ramus of the mandible develops by a rapid spread of ossification backwards into the mesenchyme of the first branchial arch diverging away from Meckel’s cartilage.

This point of divergence is marked by the mandibular foramen.

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CONDYLAR PROCESS

At about 5th week of IUL, An area of mesenchymal condensation can be seen above the ventral part of the developing mandible.

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Then this condensation develops into a cone-shaped cartilage by about 10th week called the condylar cartilage.

Ossification starts by 14th week

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Then it migrates inferiorly and fuses with the mandibular ramus by about 4 months.

Much of cone-shaped cartilage is replaced by bone by the middle of fetal life but its upper end persists into adulthood acting both as a growth cartilage and an articular cartilage

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CORONOID CARTILAGE

Secondary accessory cartilage appear in region of coronoid process at about 10th --14th week of intrauterine life.

This cartilage become incorporated into expanding intramembranous bone of ramus & disappear before birth.

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MENTAL REGION In mental region, on either side of symphysis ,

one or two small cartilage appear and ossify in seventh week of IUL to become mental ossicles.

These ossicles become incorporated into intramembranous bone when symphysis ossify completely.

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THE ALVEOLAR PROCESS It starts when the

deciduous tooth germs reach the early bell stage.

The bone of the mandible begins to grow on each side of the tooth germ.

By this growth the tooth germs come to be in a trough or groove of bone, which also includes the alveolar nerves and blood vessels.

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Later on, septa of bone between the adjacent tooth germs develop, keeping each tooth separate in its bony crypt.

The mandibular canal is separated from the bony crypts by a horizontal plate of bone.

The alveolar processes grow at a rapid rate during the periods of tooth eruption.

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REMNANTS OF MECKEL CARTILAGE Greater part of Meckel's

cartilage disappears without contributing to the formation of mandible.

The most posterior extremity forms the malleus of the inner ear & the spheno alveolar ligament.

From the sphenoid to the division of the mandibular nerve into its alveolar & lingual branches , the cartilage is lost totally, but its fibrocellular capsule persists as the sphenomandibular ligament.

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From the lingula forward to the division of the alveolar nerve into its incisor & mental branches , Meckel’s cartilage is resorbed completely.

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Growth in width is completed 1st then growth in length and finally growth in height (W>L>H).

Mandibular intercanine width is more likely to decrease than increase after age 12.

Intercanine width is essentially completed by the end of ninth year in girls and the tenth year in boys.

Both molar and bicondylar widths show small increases until the end of growth in length .

Timing of Growth in Width Length and Height:

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continues at a relatively steady rate before puberty.

On the average, ramus height increases 1-2 mm/year.

body length increases 2-3 mm/year.

In girls growth in length of the jaw has ceased by age 14-15 years.

In boys, it does not decline to the basal adult level until 18 years.

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This process occurs with:

1. Growth by secondary Cartilage.

2. Growth with the alveolar process

3. Subperiosteal bone apposition and bone resorption.

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Growth by secondary CartilageIt occurs by secondary cartilages (mainly condylar cartilage), this helps in:

Increase in height of the mandibular ramus

Increase of the inter condylar distance

Increase in the overall length of the mandible

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• Dual in function a) Articular b) Growth

• Not a primary centre for growth, but• Secondary in evolution• Secondary in embryonic origin• Secondary in adaptive responses to

changing developments.

SECONDARY CARTILAGE

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GROWTH WITH THE ALVEOLAR PROCESS• Due to the increase in the space between the upper and lower jaws, a space created between the opposing teeth to erupt.

• At the same time bone apposition occurs at the crest of the alveolar process and the fundus of the alveolus.

• The deposited bone at the fundus of the alveolus counts later to the body of the mandible.• Responsible for the distance between the mandibular

canal and the apices of the premolars and first two molars.

• Bone deposition contributes to the growth of the body of the mandible in height.

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Subperiosteal bone apposition and bone resorption

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Mandibular growth is combination of

morphologic effect of both capsular & periosteal matrices.

Capsular matrices growth causes expansion of orofacial capsule.

Enclose macroskeletal unit (mandible) passively & secondarily translated in new position.

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Periosteal matrices related to mandibular microskeletal units responds to this volumetric expansion.

Such alterations in their spatial position causes them to grow.

Both translation & change in form comprises totality of mandibular growth.

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MUSCLE ATTACHMENTS

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MUSCLES ATTACHMENT ON LATERAL SURFACE

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MUSCLES ATTACHMENT ON MEDIAL SURFACE

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AGE CHANGES IN THE MANDIBLE

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• The body of the bone is a mere shell, containing the sockets of the two incisor, the canine, and the two deciduous molar teeth.

• The angle is obtuse (175°)• Condyloid portion is nearly in line with the body. • Coronoid process is of comparatively large size, and

projects above the level of the condyle.

AT BIRTH

• The mandibular canal runs near the lower border of the bone.

• The mental foramen opens beneath the socket of the first deciduous molar tooth.

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CHILDHOOD• The body becomes elongated in its whole length.

• The depth of the body increases owing to increased growth of the alveolar part• The mandibular canal ,is situated just above the level of the mylohyoid line.

• The mental foramen occupies the position usual to it in the adult

• The angle becomes less obtuse

• About the fourth year it is 140°.

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• The alveolar and subdental portions of the body are usually of equal depth.

ADULTHOOD

• The ramus is almost vertical in direction.

• The angle measuring from 110° to 120°.

• The mental foramen opens midway between the upper and lower borders of the bone.

• Mandibular canal runs nearly parallel with the mylohyoid line.

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OLD AGE• The bone becomes greatly reduced

in size, for with the loss of the teeth the alveolar process is absorbed.

• The chief part of the bone is below the oblique line.

• The ramus is oblique in direction

• The angle measures about 140°

• The mandibular canal, with the mental foramen opening from it, is close to the alveolar border.

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In old age the bone is greatly reduced due to loss of teeth .

Remodelling is important in the areas with thin cortical bone ie: lingual parts of the mandible .

The annual rate of reduction in height is about 0.1 to 0.2 mm .

Loss of sulcus width and depth

Loss of vertical dimension of occlusion

Reduction of lower facial height

Changes in alveolar ridge relationship

Anterior rotation of mandible (becomes wider and inclines outwards)

Clinical considerations

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Mylohyoid ridge : after resorption , it lies flush with the superior

surface of ridge posteriorly .

Relief is must as the mucosa can get easily traumatized .

Mental foramen : As resorption takes place , mental foramen comes to lie closer to the crest of the ridge .

Mental nerve and vessels may be compressed by the denture base unless relief is provided .

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Genial tubercles : Usually lie away from crest of the

ridge but with resorption can become increasingly prominent .

If activity of genioglossus muscle displaces the lower denture or if the tubercle cannot tolerate the pressure , the genial turbercle is removed and muscle detached .

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ANOMALIES OF

DEVELOPMENT

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- Mandible grossly deficient or absent.

- deficiency of neural crest tissue in lower part of face.

AGNATHIA

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Also called goldenhar syndrome

Due to lack of mesenchymal tissue or neural crest cells

Underdeveloped mandible

Unilateral and asymmetrical

Hemifacial Microsomia

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also called Treacher-collins syndrome

Due to disturbance in origins, migration & interaction of neural crest cells.

Prevelance 1:25000

Hypoplasia of mandible

Mandibular Dysostosis

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Prevelance 1: 8500

Mandible is underdeveloped

Small body

Obtuse antigonial angle

Posteriorly placed condyle

Cleft palate

Pierre Robin syndrome

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Produce prognathism usually inherited Abnormal growth

phenomenon – hyperpituitarism.

Anterior teeth setting-edge to edge/negative overjet

Posterior teeth setting-crossbite

Macrognathia

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Mandibular toriBony enlargements on the lingual aspect

Premolar region

Provide adequate denture relief

If relief cannot be anticipated, surgical removal is indicated