Growth and Development of Mandible www.indiandentalacademy.com
INDIAN DENTAL ACADEMY Leader in continuing dental education www.indiandentalacademy.com
The mandible is derived from ossification of an osteogenic membrane, first appearing as a mesenchymal condensation at 36-38 days of development.
The intramembranous bone lies lateral to Meckelâ€™s cartilage of the mandibular(1st) Branchial arch.
A single ossification centre for each half of the mandible arises in the 6th week i.u in the region of the bifurcation of Inf Alv N and Art into mental and incisive branches.
The ossifying membrane is attached laterally to Meckel’s cartilage.
Scheme of centre of ossification of mandible lateral to Meckelâ€™s cartilage at bifurcation of Inferior alveolar nerve www.indiandentalacademy.com
The intramembranous ossification stops dorsally at the site which will later become the mandibular lingula. From this point it continues into middle ear where the two post. elements later become the malleus and incus These two bones function in the articulation of the mandible in lower animals and are known as the articulare and quadrate. www.indiandentalacademy.com
There is some evidence that malleus and incus function to provide a movable joint until the mandibular condyle develops in relation to glenoid fossa of the temporal bone. Then these two cartilages ossify and function as middle ear bones.
The first branchial arch core of Meckel’s cart meets its fellow of opposite side ventrally. The dorsal end of Meckel’s cart ossifies to form two of auditory ossicles, MALLEUS INCUS Stapes ---- Reichert’s cartilage. www.indiandentalacademy.com
The major portion of Meckel’s cart disappears, leaving the Mylohyoid groove of mandible in which it lay. The fibrous sheath surrounding the cartilage remains as the Sphenomandibular Sphenomalleolar ligaments.
So the Meckelâ€™s cart:in all gives rise to Mental ossicles Incus and Malleus Spine of Sphenoid Sphenomandibular ligament Sphenomalleolar ligaments
Accessory cartilages appear between the 10th and 14th weeks i.u to form Head of condyle Coronoid process Mental protuberance
Accessory cartilages of foetal mandible www.indiandentalacademy.com
The condylar process
At about the 5th week of i.u life an area of mesenchymal condensation can be seen above the ventral part of the developing mandible. This is primordium of future condyle By 10th week- it develops into cone shaped cartilage. 14th week– ossification starts. It then migrates inferiorly and fuses by about 4 months www.indiandentalacademy.com
This condylar cart serves as an important centre of growth for ramus and body of mandible.
Much of the cone shaped cartilage is replaced by bone by middle of fetal life but its upper end persists into adulthood acting both as a growth cartilage and an articular cartilage.
Growth at this site normally ceases at about the 20th year of life. www.indiandentalacademy.com
CORONOID PROCESS •
Secondary accessory cartilages appear in the region of the coronoid process by about 10-14th week of i.u life.
This secondary cartilage of coronoid process is thought to grow as a response to the developing temporalis muscle.
This cartilage becomes incorporated into the expanding intramembranous bone of the ramus and disappears before birth. www.indiandentalacademy.com
Mental region ď Ž
In mental region, on either side of the symphysis, one or two small cartilages appear and ossify in the 7th month of i.u life to form variable numbers of mental ossicles in the fibrous tissues of the symphysis. These ossicles become incorporated into the intramembranous bone when the symphysis ossifies completely during first yr of post-natal life. www.indiandentalacademy.com
Post natal growth of mandible
Mandible undergoes the largest amount of growth post-natally. Exhibits largest variability in morphology. It is developmentally and functionally divisible into several skeletal sub units. www.indiandentalacademy.com
The basal bone forms one unit, to which are attached The alveolar pr The coronoid pr The condylar pr The angular pr The ramus The lingual tuberosity The chin www.indiandentalacademy.com
Each of these subunits is influenced by a functional matrix that acts upon the bone. Teeth –the alv unit. Temporalis –the coronoid pr Masseter &Medial Pterygoid muscles –angle of mandible Lateral pterygoid –the condylar pr
The main sites of post-natal mandibular growth are at condylar cart post border of ramus alveolar ridges These areas account for increases in the height, length, and width of the mandible
Enlow’s expanding ‘V’ principle
Many facial bones or parts of bone have a ‘V’ shaped pattern of growth. The growth movements and enlargement of these bones occur towards the wide ends of ‘V’ as a result of differential deposition and selective resorption of bone. Deposition –on the inner side of wide end of ’V’ Resorption –on the outer surface. www.indiandentalacademy.com
also takes place at the ends of the 2 arms of the ’V’ resulting in growth movement towards the ends. www.indiandentalacademy.com
The ramus moves post by a combination of deposition and resorption. Resorption –on anterior part of ramus Deposition –on posterior region. Result –drift of ramus in post direction. The proportion of ramus is maintained. www.indiandentalacademy.com
This deposition and resorption balance extends up to the coronoid involving the mandibular notch, and progressively repositions the mand foramen in a posterior direction.
The attachment of elevating muscles of mastication to the buccal and lingual aspects of ramus are influential in determining the ultimate size and proportion of these mand elements. www.indiandentalacademy.com
Relocation www.indiandentalacademy.com of mandibular ramus
While the whole ramus grows posteriorly and superiorly, the mandibular foramen drifts backward and upward by deposition on the anterior and resorption from the posterior part of its rim. Maintains a constant position about midway b/w ant & post borders of ramus.
Functions of remodelling of ramus:
To accommodate the increasing mass of masticatory muscles inserted into it. To accommodate the enlarged breadth of the pharyngeal space. To facilitate lengthening of the mandibular body, which in turn accommodate the erupting molars. www.indiandentalacademy.com
Corpus or body of mandible ď Ž
The displacement of the ramus results in the conversion of former ramal bone into the posterior part of the body of mandible.
So the body lengthens by the posterior molar region becoming relocated anteriorly into premolar and canine regions.
So additional space is provided for the later erupting molar teeth, all of which develop in the ramus-body junction.
Their forward migration and post ramal displacement both lengthen the molar region of the mandible.
Resorption on anterior border of ramus, thus making space for the molars www.indiandentalacademy.com
Downward & forward transposition of md. Upward & Backward growth Adult md angle Fetal md angle
Growth of mandible with fetal md superimposed on adult md www.indiandentalacademy.com
Angle of mandible ď Ž
The growth and remodelling changes occurring during infancy and childhood serve to alter the angle b/w the ramus and body of mandible from about 140 to nearly 90 in adulthood. Resorption of alveolar bone with tooth loss results in the angle formed by the ramus and body, again increasing in senility. www.indiandentalacademy.com
On lingual side of the angle of mandible, resorption –on the post-inf aspect while deposition –on the ant-sup part. On buccal side of angle of mandible, resorption –on the ant-sup part while deposition –on the post-inf part. Result –flaring of the angle of the mandible as age advances. www.indiandentalacademy.com
The lingual tuberosity
The lingual tuberosity is a direct equivalent of max tub, which forms a major site of growth for the lower bony arch. It forms the boundary b/w the ramus &the body. It protrudes noticeably in a lingual direction. Lies towards the middle of ramus. www.indiandentalacademy.com
Moves post by deposition on its posteriorly facing surface. The tub is prominent due to presence of large resorption field just below it. Resorptive field produces depression lingual fossa The combination of resorption in the fossa and deposition on the medial surface of the tub itself accentuates the prominence of ling tub. www.indiandentalacademy.com
The post growth of the tub is accomplished by continued new deposits of bone on its posterior facing exposure.
Part of ramus just behind the tub grows medially.
And thus becoming a part of corpus.
Tub growing in an almost directly post direction. www.indiandentalacademy.com
The alveolar process
Alv pr develops in response to the presence of tooth buds. As teeth erupts the alv pr develops and increases in height by bone deposition at the margins. It adds to the height and thickness of the body of mandible. Absence of teeth – pr fails to develop Tooth extraction –pr undergoes resorption www.indiandentalacademy.com
A specific human characteristic Found in its fully developed form in recent man only. In infancy chin is usually underdeveloped. The growth of chin is influenced by sexual and specific genetic factors. Usually males have prominent chins compared to females. www.indiandentalacademy.com
The mental protuberance forms by bone deposition during childhood.
Its prominence is accentuated by bone resorption that occurs in the alv region above it, creating a concavity known as Point B.
Underdevelopment of chin is known as microgenia
The mental neurovascular bundle emanates from mandible at almost right angles at birth. In adulthood the foramen is characteristically backwardly directed. This is due to forward direction of growth that occurs in the body of mandible, while the neurovascular bundle drags along. www.indiandentalacademy.com
Clinical implication In administration of L.A to the mental foramen. In Infancy &childhood the syringe needle is placed at right angles In case of adults needle has to be placed obliquely from behind to achieve entry. www.indiandentalacademy.com
The location of mental foramen also alters its vertical position within the body of mandible from infancy to childhood and old age. When teeth are present –the foramen is located midway b/w the upper and lower borders of the mandible. In edentulous mandible –foramen is located near the upper margin of the thinned mandible www.indiandentalacademy.com
The coronoid process ď Ž
The coronoid pr has a propeller like twist, so that its lingual side faces three general directions all at onceâ€“ posteriorly superiorly medially. When bone is added to the lingual side of coronoid pr, its growth thereby proceeds superiorly, and this part of ramus becomes increased in vertical dimension www.indiandentalacademy.com
Each coronoid pr lengthens vertically, even though additions are made on the medial (lingual) surfaces of the right and left coronoid pr. This is an e.g. of “enlarging V principle”, with the V oriented vertically.
enlarging V principle www.indiandentalacademy.com
The same deposits of bone on the lingual side also bring about a posterior direction of growth movement. This produces a backward movement of the two coronoid pr, even though deposits are added on the lingual surface. This is an e.g. of “expanding V principle”, with the V oriented horizontally. www.indiandentalacademy.com
expanding V principle www.indiandentalacademy.com
These same deposits of bone on the lingual side also function to carry the base of the coronoid pr and the anterior part of ramus in a medial direction in order to add this part to the lengthening corpus, which lies well medial to the coronoid pr. This, again is an e.g. of V principle.
The mandibular condyle is considered as a major site of growth. Initially it was believed to be the ultimate determinant of rate & amount of mandibular growth, growth direction, overall mandibular size and shape. It is no longer believed to represent a pace setting “master center”. www.indiandentalacademy.com
During mandibular development, the condyle functions as a regional field of growth. The condylar cartilage of the mandible serves the unique dual roles of an articular surface cartilage analogous to the T.M joint, and as a growth cartilage analogous to the epiphyseal plate in a long bone. www.indiandentalacademy.com
The formation of bone within the condylar head results in the mand rami growing in an upward and backward direction, with consequent displacement of whole bone in an opposite downward and forward direction. Bone resorption adjacent to the condylar head accounts for narrowed condylar neck. www.indiandentalacademy.com
Direction of condylar growth
Mandibles of a neonate, a 4 yr. Child and an adult www.indiandentalacademy.com
The attachment of the lateral pterygoid muscle to the condylar neck, and growth and action of the tongue and masticatory muscles, are functional forces implicated in this phase of mandibular growth.
Any damage to the condylar cartilage will restrict the growth potential and normal downward and forward displacement of the mandible, unilaterally or bilaterally, according to the sides damaged.
Due to posterior divergence of the two halves of the mandible (in a V shape), growth at the condylar heads of the increasingly more widely displaced rami results in an eventual total widening of the mandibular body. www.indiandentalacademy.com
AGE CHANGES IN MANDIBLE,
1) at Birth At birth mandible is in two separate halves united in the median plane by fibrous tissue -symphysis menti.
A trace of separation near the alveolar margin may still be visible at the beginning of the 2nd year. www.indiandentalacademy.com
The body elongates more especially behind the mental foramen.
2) 1st and 2nd year During the 1st and 2nd year as the prominence of chin develops the mental foramen alters in direction.
Proliferation of condylar cartilage leads to increase in vertical height of ramus and downward and forward growth of mandible. www.indiandentalacademy.com
Also the distance between the condyle increases as the base of skull widens.
As the mandible increases in size bone is laid down along the posterior borders of mandibular ramus and the coronoid process
and resorption of bone occurs along their anterior borders.
This remodelling goes on until the bone has reached the adult size and it enables the alveolar part to lengthen sufficiently to provide the necessary space for the permanent molar teeth.
3) Adults The angle of mandible diminishes
as the height of ramus increases with age but contour of the angle of mandible remains unaltered.
4) Old age Bone is decreased in size as the teeth are lost.
Following the loss of teeth the alveolar part is absorbed and consequently the mandibular canal and mental foramen are close to the alveolar border. www.indiandentalacademy.com
Ramus is oblique.
Angle is 140 degrees and neck of mandible is bent backwards.
The process of absorption affects chiefly the thinner of the two alveolar walls .
As alveolar ridge resorbs the mandible widens.
References: 1.Human embryology : Sperber
2.Comtemporary orthodontics :Profitt
3.Handbook of orthodontics : Moyers
4.Facial growth : Enlow
www.indiandentalacademy.com Leader in continuing dental education