Development of mandible / fixed orthodontics courses for general dentists

INTRODUCTION :
It is the largest and strongest bone of the face; forms the lower jaw.
Consists of a central body and two rami.
Parts :
1. Body : horse-shoe (U) shaped; horizontal, and convex forwards.
2. Rami : one ramus on each side, projecting upwards from the posterior
ends of the body.
DEVELOPMENT OF THE MANDIBLE :
“The human mandible has no one design for life.”
D.E. Poswillo, 1988
Prenatal development :
The cartilages and bones of the mandibular skeleton form from
embryonic neural cells that originate in the mid-and hindbrain regions of the
neural folds. These cells migrate ventrally to form the mandibular (and
maxillary) facial prominences, where they differentiate into bones and
connective tissues.
The first structure to develop in the region of the lower jaw is the
mandibular division of the trigeminal nerve that precedes the
ectomesenchymal condensation forming the first (mandibular) pharyngeal
arch. The prior presence of the nerve has been postulated as requisite for
inducing osteogenesis by the production of neurotrophic factors. The
mandible is derived from ossification of an osteogenic membrane formed
from ectomesenchymal condensation at 36 to 38 days of development this
mandibular ectomesenchyme must interact initially with the epithelium of
the mandibular arch before primary ossification can occur; the resulting
intamembranous bone lies lateral to Meckel’s cartilage of the first
(mandibular) pharyngeal arch. A single ossification center for each half of
the mandible arises in the 6th
week post conception (the mandible and the
clavicle are the first bones to begin to ossify) in the region of the bifurcation

of the inferior alveolar nerve and artery into mental and its accompanying
neurovascular bundle. From the primary center below and around the
inferior alveolar nerve and its incisive branch, ossification spreads upwards
to form a trough for the developing teeth. The spread of the
intramembranous ossification dorsally and ventrally forms the body and
ramus of the mandible. Meckel’s cartilage becomes surrounded and invaded
by bone. Ossification stops dorsally at the site that will become the
mandibular lingua, where Meckel’s cartilage continues into the middle ear.
The prior presence of the neurovascular bundle ensures the formation of the
mandibular foramen and canal and the mental foramen.
The first pharyngeal-arch core of Meckel’s cartilage almost meets its
fellow of the opposite side ventrally. It diverges dorsally to end in the
tympanic cavity of each middle ear, which is derived from the first
pharyngeal pouch, and is surrounded by the forming petrous portion of the
temporal bone. The dorsal end of Meckel’s cartilage ossifies to form the
basis of two of the auditory ossicles (ie, the malleus and the incus). The
third ossicle (the stapes) is derived primarily from the cartilage of the
second pharyngeal arch (Reichert’s cartilage).
Meckel’s cartilage lacks the enzyme phosphatase found in ossifying
cartilages, thus precluding its ossification; almost all of Meckel’s cartilage
disappears by the 24th
week after conception. Parts transform into the
sphenomandibular and anterior malleolar ligaments. A small part of its
ventral end (from the mental foramen ventrally to the symphysis) forms
accessory endochondral ossicles that are incorporation into the chin region
of the mandible. Meckel’s cartilage dorsal to the mental foramen undergoes
resorption on its lateral surface at the same time as intramembranous bony
trabeculae are forming immediately lateral to the resorbing cartilage. Thus,
the cartilage from the mental foramen to the lingula is not incorporated into
ossification of the mandible.

The initial woven bone formed along Meckel’s cartilage is soon
replaced by lamellar bone, and typical haversian systems are already present
at the 5th
month post conception. This remodeling occurs earlier than it
occurs in other bones, and is thought to be a response to early intense
sucking and swallowing, which stress the mandible.
Secondary accessory cartilages appears between the 10th
and 14th
weeks post conception to form the head of the condyle, part of the coronoid
process, and the mental protuberance. The appearnce of these secondary
mandibular cartilages is dissociated from the primary pharyngeal (Meckel’s)
and chondrocranial cartilages. The secondary cartilage of the coronoid
process develops within the temporalis muscle, as its predecessor. The
coronoid accessory cartilage becomes incorporation into the expanding
intramembranous bone of the ramus and disappear before birth. In the
mental region, on either side of the symphysis, one or two small cartilages
appear and ossify in the 7th
month post conception to form a variable
number of mental ossicles in the fibrous tissue of the symphysis. The
ossicles become incorporated into the intramembranous bone when the
symphysis menti is converted from a syndesmosis into a synostosis during
the 1st
postnatal year.
The condylar secondary cartilage appears during the 10th
week post
conception as a cone-shaped structure in the ramal region. This condylar
cartilage is the primordium of the future condyle. Cartilage cells
differentiate from its center, and the cartilage condylar head increases by
interstitial and appositional growth. By the 4th
week, the first evidence of
endochondral bone appears in the condyle region. The condylar cartilage
serves as an important center of growth for the ramus and body of the
mandible. The nature of this growth – as primary (an initial source of
morphogenesis) or secondary (compensating for functional stimulation) – is
controversial, but experimental evidence indicates the need for mechanical

stimuli for normal growth. By the middle of fetal life, much of the cone-
shaped cartilage is replaced with bone, but its upper end persists into
adulthood, acting as both growth and articular cartilage. Changes in
mandibular position and form are related to the direction and amount of
condylar growth rate increases at puberty, peaks between 12 ½ and `4 years
of age, and normally ceases at about 20 years of age. However, the
continuing presence of the cartilage provides a potential for continued
growth, which is realized in conditions of abnormal growth such as
acromegaly.
Postnatal development :
The shape and size of the diminutive fetal mandible undergo
considerable transformation during its growth and development. The
ascending ramus of the neonatal mandible is low and wide, the coronoid
process is relatively large and projects well above the condyle, the body is
merely an open shell containing the buds and partial crowns of the
deciduous teeth, and the mandibular canal runs low in the body. The initial
separation of the right and left bodies of the mandible at the midline
symphysis menti is gradually eliminated between the 4th
and 12th
months
after birth, when ossification converts the syndesmosis into a synostosis,
uniting the two halves.
Although the mandible appears as a single bone in the adult, it is
developmentally and functionally divisible into several skeletal subunits.
The basal bone of the body forms one unit, to which are attached the
alveolar, coronoid, angular, and condylar processes and the chin. The
growth pattern of each of these skeletal subunits is influenced by a
functional matrix that acts upon the bone: the teeth act as a functional matrix
for the alveolar unit; the action of the temporalis muscle influences the
coronoid process; the masseter and medial pterygoid muscles act upon the
angle and ramus of the mandible; and the lateral pterygoid has some

influence on the condylar process. The functioning of the related tongue and
perioral muscles and the expansion of the oral and pharyngeal cavities
provide stimuli for mandibular growth to reach its full potential. Of all the
facial bones, the mandible undergoes the most growth postnatally and
evidences the greatest variation in morphology.
Limited growth takes place at the symphysis menti until fusion
occurs. The main sites of postnatal mandibular growth are at the condylar
cartilages, the posterior borders of the rami, and the alveolar ridges. These
areas of bone deposition largely account for increases in the height, length,
and width of the mandible. However, superimposed upon this basic
incremental growth are numerous regional remodeling changes that are
subjected to the local functional influences involving selective resorption
and displacement of individual mandibular elements.
The condylar cartilage of the mandible uniquely serves as both (1) an
articular cartilage in the temporomandibular joint, characterized by a
fibrocartilage surfaced layer, and (2) a growth cartilage analogous to the
epiphysial plate in a long bone, characterized by a deeper hypertrophying
cartiliage later. The subarticlar appositional proliferation of cartilage within
the condylar head provides the basis for the growth of a medullary core of
endochondral bone, on the outer surface of which a cortex of
intramembranous bone is laid. The growth cartilage may act as a “functional
matrix” to stretch the periosteum, inducing the lengthened periosteum to
form intramembranous bone bone beneath it. The diverse histologic origins
of the medulla and cortex are effaced by their fusion. The formation of bone
within the condylar heads causes the mandibular rami to grow upward and
backward, displacing the entire mandible in an opposite downward and
forward direction. Bone resorption subjacent to the condylar head account
for the narrowed condylar neck. The attachment of the lateral pterygoid

muscle to this neck and the growth and action of the tongue and masticatory
muscles are functional forces implicated in this phase of mandiular growth.
Any damage to the condylar cartilages restricts the growth potential
and downward and forward displacement of mandible, unilaterally or
bilaterally, according to the side(s) damaged. Lateral deviations of the
mandible and the varying degrees of micrognathia and accompanying
malocclusion result.
In the infant, the condyles of the mandible are inclined almost
horizontally, so that condylar growth leads to an increase in the length of the
mandible rather than to an increase in height. Due to the posterior
divergence of the two halves of the body of the mandible (in a v shape),
growths in the condylar heads of the increasingly more widely displaced
rami results in overall widening of the mandibular body, which keeps pace
by (remodeling) with the widening cranial base. No interstitial widening of
the mandible can take place at the fused symphysis menti after the first year,
apart from some widening by surface apposition.
Bone deposition occurs on the posterior border of the ramus, whereas
concomitant resorption on the anterior border maintains the proportions of
the ramus and, in effect, moves it backward in relation to the body of the
mandible.This deposition and concomitant resorption extends up to the
coronoid process, involving the mandibular notch, and progressively
repositions the mandibular foramen posteriorly, accounting for the anterior
overlying plate of the lingula.The attachment of the elevating muscles of
mastication to the buccal and lingual aspects of the ramus and to the
mandibular angle and coronoid process influences the ultimate size and
proportions of these mandibular elements.
The posterior displacement of the ramus converts former ramal bone
into the posterior part of the body of the mandible. In this manner, the body
of the mandible lengthens, the posterior molar region relocating interiorly

into the premolar and canine regions. This is one means by which additional
space is provided for eruption of the molar teeth, all three of which originate
in the junction of the ramus and the body of the mandible. Their forward
migration and posterior ramal displacement lengthen the molar region of the
mandible.
The forward shift of the growing mandibular body changes the
direction of the mental foramen during infancy and childhood. The mental
neurovascular bundle emanates from the mandible at right angles or even a
slightly forward direction at birth. In adulthood, the mental foramen (and its
neurovascular content) is characteristically directed backward. This change
may be ascribed to forward growth in the body of the mandible while the
neurovascular bundle “drags along”. A contributory factor may be the
differing growth rates of bone and periosteum. The latter, by its firm
attachment to the condyle and comparatively loose attachment to the
mandibular body, grows more slowly than the body, which slides forward
beneath the periosteum. The changing direction of the foramen has clinical
implications in the administration of local anesthetic to the mental nerve: in
infants and children, the syringe needle may be applied at right angles to the
body of the mandible to enter the mental foramen whereas the needle must
be applied obliquely from behind to achieve entry in the adult.
The location of the mental foramen also alters its vertical relationship
within the body of the mandible from infancy to old age. When teeth are
present, the mental foramen is located midway between the upper and lower
borders of the mandible. In the edentulous mandible, lacking an alveolar
ridge, the mental foramen appears near the upper margin of the thinned
mandible.
The alveolar process develops as a protective trough in response to
the tooth buds and becomes superimposed upon the basal bone of the
mandibular body. It adds to the height and thickness of the body of the

mandible and is particularly manifest as a ledge extending lingually to the
ramus to accommodate the third molars. The alveolar bone fails to develop
if teeth are absent and resorbs in response to tooth extraction. The
orthodontic movement of teeth takes place in the labile alveolar bone to
both maxilla and mandible and fails to involve the underlying basal bone.
The chin, formed in part of the mental ossicles from accessory
cartilages and the ventral end of Meckel’s cartilage, is very poorly
developed in the infant. If develops almost as an independent subunit of the
mandible, influenced by sexual as well as specific genetic factors. Sex
differences in the symphyseal region of the mandible are not significant
until other secondary sex characteristics develop. Thus, the chin becomes
significant only at adolescence, from the development of the mental
protuberance and tubercles. Whereas small chins are found in adults of both
sexes, very large chins are characteristically masculine. The skeletal “unit”
of the chin may be an expression of the functional forces exerted by the
lateral pterygoid muscles that, in pulling the mandible forward, indirectly
stress the mental symphyseal region by their concomitant inward pull. Bone
buttressing to resist muscle stressing, which is more powerful in the male, is
expressed in the more prominent male chin. The protrusive chin is a
uniquely human trait, lacking in all other primates and in hominid ancestors.
The mental protuberance forms by osseous deposition during
childhood. Its prominence is accentuated by bone resorption in the alveolar
region above it, creating the supramental concavity known as “point B” in
orthodontic terminology. Underdevelopment of the chin is known as
microgenia.
The torus mandibularis, a genetically determined exostosis on the
lingual aspect of the body of the mandible, develops (usually bilaterally) in
the canine-premolar region. This torus is unrelated to any muscle
attachments or known functional matrices.

During fetal life, the relative sizes of the maxilla and mandible vary
widely. Initially, the mandible is considerable larger than the maxilla, a
predominance lessened later by the relatively greater development of the
maxilla; by about 8 weeks post conception, the maxilla overlaps the
mandible. The subsequent relatively greater growth of the mandible results
in the approximately equal size of the upper and lower jaws by the 11th
week. Mandibular growth lags behind maxillary growth between the 13th
and 20th
weeks due to a changeover from Meckel’s cartilage to condylar
secondary cartilage as the main growth determinant of the lower jaw. At
birth, the mandible tends to be retrognathic to the maxilla although the two
may be of equal size. This retrognathic condition is normally corrected early
in postnatal life by rapid mandibular growth and forward displacement to
establish an Angle Class I maxillomandibular relationship. Inadequate
mandibular growth results in an Angle Class II relation (retrognathism), and
overgrowth of the mandible produces a class III relation (prognathism). The
mandible can grow for much longer than the maxilla.
Genetic basis of development of mandible :
The developing mandible is an unique craniofacial bone of profound
importance for facial shape and dental occlusion. The development and
morphogenesis of the mandible is regulated by MSX gene and DLX
gene.
The studies also reveal that special protein integrin and CP27 gene
expression is important for mandibular morphogenesis.
Anomalies of development :
In the condition of agnathia, the mandible may be grossly deficient or
absent, reflecting a deficiency of neural crest tissue in the lower part of the
face. Aplasia of the mandible and hyoid bone (first- and second-arch
syndrome) is a rare lethal condition with multiple defects of the orbit and

maxilla. Well-developed (albeit low-set) ears and auditory ossicles in this
syndrome suggest ischmeic necrosis of the mandible and hyoid bone
occurring after the formation of the ear.
The diminutive mandible of micrognathia is characteristic of several
syndromes, including Pierre Robin and cat’s cry (cri du char) syndromes,
mandibulofacial dysostosis (Treacher Collins syndrome), progeria, Down
syndrome (trisomy 21 syndrome), oculomandibulodyscephaly (Hallermann-
Streiff syndrome), and Turner syndrome (XO sex chromosome
complement).
A central dysmorphogenic mechanism of defective neural crest
production, migration, or destruction may be responsible for the hypoplastic
mandible common to these conditions. Absent or deficient neural crest
tissue around the optic cup causes a “vacuum,” so that the developing otic
pit (normally adjacent to the second pharyngeal arch) moves cranially into
first-arch territory and the ear becomes located over the angle of the
mandible. Derivatives of the deficient ectomesenchyme (specifically the
zygomatic, maxillary, and mandibular bones) are hypoplastic, accounting
for the typical facies common to these syndromes.
In Pierre Robin syndrome, the underdeveloped usually demonstrates
catch-up growth in the child. In mandibulofacial dysostosis, deficiency of
the mandible is maintained throughout growth. In unilateral agenesis of the
mandibular ramus, the malformation increases with age. Hemifacial
microsomia (Goldenhar syndrome) also becomes more severe with retarded
growth.
Variations in condylar form may occur, among them the rare bifid or
double condyle that results from the persistence of septa dividing the fetal
condylar cartilage.
Macrognathia, producing prognathism, is usually an inherited
condition, but abnormal-growth phenomena such as hyperpituitarism may

produce mandibular overgrowth of increasing severity with age. Congential
hemifacial hypertrophy, evident at birth, tends to intensify at puberty.
Unilateral enlargement of the mandible, the mandibular fossa, and the teeth
is of obscure etiology; more common is isolated unilateral condylar
hyperplasia.
ANATOMY OF MANDIBLE :
1. Body :
Possesses : 2 Surfaces : a. External
b. Internal
2 Borders : a. Upper (Alveolar)
b. Lower (Base).
i) Surfaces :
a. External surface : convex in outline.
Presents :
1. Symphysis menti : a faint ridge in the midline in the upper part of the
body ; indicates the line of fusion of two-halves of foetal bone
(mandible).
2. Mental protuberance : a triangular raised area below the symphysis
menti, the apex pointing upwards and the base downwards; forms the
prominence of chin.
3. Mental tubercles : a small tubercle at each lateral angle of the mental
protuberance.
4. Mental foramen : a foramen below the interval between the 1st
and 2nd
premolar teeth or below the 2nd
premolar tooth; faces backwards and
slightly upwards and leads into the mandibular canal.
Give exit to : Mental nerve and vessels.
5. External oblique line : a faint line running upward and backward on
each side from the mental tubercle; very prominent behind and
continuous with the anterior border of the ramus.

Its anterior part gives origin to
i) Depressor labi inferioris : in front.
ii) Depressor angulioris : behind.
Its posterior part (below molar teeth) and the adjoining area
above it gives origin to : Buccinator.
6. Area below the oblique line gives insertion to : Platysma.
7. Incisive fossa : a small shallow depression below the incisor teeth;
gives origin to
i) Mentalis,
ii) Part (mental slips) of Orbicularis oris.
b. Internal surface : concave in outline.
Presents :
1. Mylohyoid line or ridge (internal oblique line) : an oblique ridge
running forwards and downwards on each side from the 3rd
molar
tooth to the symphsis menti; very prominent behind but fades out as it
passes forwards; divides the internal surface into two areas
submandibular fossa below the line and sublingual fossa above the
line.
Gives origin to
i) Mylohyoid muscle : throughout its whole length.
ii) Superior constrictor muscle of pharynx : from its posterior
end.
Pterygomandibular raphe : is attached above and behind its posterior
end.
2. Groove for the lingular nerve : a groove above the mylohyoid line but
below the last molar tooth; often made by the passage of lingual
nerve.
Above this groove, the superior constrictor muscle and the
pterygomandibular raphe are attached.

3. Submandibular fosa : a triangular hollow area below the mylohyoid
line.
Lodges : i) Submandibular salivary gland.
ii) Submandibular lymph nodes.
iii) Facial artery.
4. Sublingual fosa : a triangular concave area above the mylohyoid line.
Lodges sublingual gland.
5. Genial tubercles : 4 or less in no; sharp projections low down in the
mid-line above the anterior ends of mylohoid lines; lie in pairs-2
upper and 2 lower.
Upper tubercles give origin to : Genioglossus muscles, one on
each side.
Lower tubercles give origin to : Geniohyoid muscles, one on
each side.
6. Mylohyoid groove : runs downwards and forwards from the ramus on
to the body below the posterior end of mylohyoid line.
Transmits : i) Nerve to mylohyoid – supplies mylohyoid and anterior
belly of Digastric muscles.
ii) Mylohyoid vessels
ii) Borders :
a. Upper border (alveolar part) :
a) In adults : 16 sockets, 8 on each side, for lodging the roots of teeth,
named from before backwards :
1. Medial or 1st
incisor
2. Lateral or 2nd
incisor
3. Canine
4. 1st
premolar
5. 2nd
premolar
6. 1st
molar

7. 2nd
molar
8. 3rd
or last molar.
Dental Formula (I=incisor; C=canine; P=premolar; M=molar).
b) Alveolar bone around the teeth and the portion of surface adjoining it
are covered by mucoperiosteum to form the gums.
c) Cavity of the socket gives attachment to periodontal membrane.
b. Lower border (base of the mandible) : thick and rounded; becomes
continuous with the lower border of the ramus behind the 3rd
molar tooth.
1) Digastric fossa : a shallow oval depression on either side of the mid-
line.
Gives origin to anterior belly of digastric.
2) Insertion of plastsyma : to the lower border and extending anteriorly
on to the adjoining external surface.
3) Attachment of investing layer of deep cervical fascia, extending from
the mid-line of the angle of mandible.
2. RAMUS :
Flattened, quadrilateral part projecting upwards from the posterior
end of the body on each side; gives insertion to all muscles of mastication;
deep to the ramus lies infra-temporal fossa.
Possesses : 2 Surfaces : a) Lateral (external)
b) Medial (internal)
4 Borders : a) Anterior
b) Posterior
c) Upper
d) Lower
2 Processes : a) Coronoid
b) Condyloid
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i) Surfaces :
a) Lateral (external) surface : flat surface, rough throughout except in its
upper and posterior part which is smooth.
Presents :
1. Insertion of masseter muscle : to the rough area.
2. Relation of parotid salivary gland : the smooth upper and
posterior part is covered by parotid salivary gland.
b) Medical (internal) surface :
Presents :
1. Mandibular foramen – an irregular opening near the center
which leads into the mandibular canal, which in turn leads to
the mental foramen.
Transmits : inferior alveolar (dental) vessels and nerve.
2. Lingula : a sharp thin process of bone projecting from the
medial margin of the mandibular foramen.
Gives attachment to the lower end of the
sphenomandibular ligament; the ligament is pierced by
mylohyoid vessels and nerve.
3. Mylohyoid groove : begins just behind the lingual and runs
downwards and forwards to fade out at the posterior end of
submandibular fossa.
Lodges : i) Mylohyoid vessels : branches of inferior alveolar
vessels.
ii) Mylohyoid nerve L a branch of the inferior alveolar
nerve.
4. Insertion of medial pterygoid muscle : to the rough impression
below and behind the mylohyoid groove.

ii) Borders :
a) Anterior border : sharp and prominent; continuous with the anterior
border of the coronoid process and below with the oblique line.
Gives insertion to some fibres of temporalis.
b) Posterior border : thick and rounded; concave in the middle; extends
from the back of the condyloid process to the midible where it
becomes continuous with the inferior border of the ramus.
Related to parotid salivary gland.
Angle of mandible : is formed where the straight lines drawn along
the posterior and inferior borders meet.
Gives attachment to stylomandibular ligament.
c) Upper (superior) border :
Presents :
1. Mandibular notch : in the middle
2. Coronoid process : in front
3. Condyloid process : behind
Mandibular notch : a wide concave notch in the upper border : its
anterior margin is continuous with the posterior margin of the
coronoid process and its posterior margin curves up to the lateral end
of the head of mandible.
Transmits : Masseteric vessels and nerves.
d) Lower (inferior) border : continuous in front with the base of
mandible and behind it meets posterior border of the ramus at the
angle of mandible.
Gives attachment to the investing layer of Deep cervical fascia.
Related to facial vessels at the anterior border of masseter muscle.

ii) Processes :
1) Coronoid process : flat and triangular with the apex pointing
upwards while the base is fused with the upper and anterior part of
the ramus; its anterior border is continuous with the anterior border
of the ramus and the posterior border forms the anterior boundary of
mandibular notch.
1. Insertion of temporalis muscle : to the apex, margins, whole of
the medial surface and a part of the lateral surface close to the
apex.
2. Relation of masseter muscle : the lateral surface is covered by
the anterior fibres of the masseter muscle.
2) Condyloid process : projects upwards from the upper and posterior
part of the ramus.
1. Head of mandible : thick and expanded upper part, broader
transversely than antero-posteriorly; lined by fibrocartilage.
Articulates with the articular part of mandibular fossa of
the temporal bone and forms temporo-mandibular joint.
The articular margins give attachment to the capsule and
synovial memebrane of the joint.
Presents a rounded tubercle on its lateral asopect which
gives attachment to the lateral ligament of the
temporomandibular joint.
2. Neck of mandible : the constricted part just below the head. Its
lateral and posterior aspects adjoining the tubercle of the head
also gives attachment to lateral ligament of temporomandibular
joint.
The anterior surface of the neck is hollowed out into a
depression, known as pterygoid fovea or pit which gives
insertion to lateral pterygoid muscle.

Related to (i) Parotid salivary gland on the lateral aspect below
the attachment of the ligament.
ii) Auriculo-temporal nerve above and
iii) Maxillary artery below on the medial aspect.
Mandibular canal : runs within the substance of the bone from the
mandibular foramen, first vertically downwards, then obliquely downwards
and forwards in the ramus and then horizontally forwards in the body below
the sockets of teeth with which it communicates by small openings; ends
below the interval between the 1st
and 2nd
premolar teeth by dividing into
mental and incisive canals the mental canal runs up and behind to reach the
mental foramen, the incisive canal rums forwards below the sockets of
incisor teeth.
Transmits : the inferior, alveolar vessels and nerve which divide into
mental and incisive vessels at the point of bifurcation of the canal.
Sex determination of mandible :
In males : the angle of mandible is everted (projected outwards).
In females : the angle of mandible is inverted (projected inwards)
Age determination of mandible :
Mandible undergoes considerable changes with age of which the most
important to determine the age of mandible is :
1. Eruption of temporary and permanent teeth in the alveolar border of
the mandible; the age of eruption is almost fixed in average healthy
individual.
The normal age of eruption of teeth in both jaws, upper and lower is
as given below –

Deciduous or temporary teeth
(10 in each jaw, 5 on each side)
Permanent teeth
(16 in each jaw, 8 on each side)
6th
month – Lower 1st
incisors
7th
month – Upper 1st
incisors
8th
month – Upper 2nd
incisors
9th
month – Lower 2nd
incisors
10th
month – 1st
molars
10th
month – Canines
24th
month – 2nd
molars
Dental formula
6th
year – 1st
molars
7th
year – 1st
incisors
8th
year – 2nd
incisors
9th
year – 1st
premolars
10th
year – 2nd
premolars
11th
year – Canines
12th
year – 2nd
molars
(wisdom teeth)
A lower tooth precedes its corresponding upper number.
1st
permanent molar erupts before any temporary tooth is shed. So at
the age of 6 years, there are 6 teeth on one side – 6 temporary and 1
permanent (1st
molar).
2nd
permanent molar erupts at the age of 12 years. So between 6 to 12
years of age, the 5 temporary teeth in each half-jaw are replaced in order-
first incisors – 1st
and 2nd
, then molars – 1st
and 2nd
and last of all, long-
rooted canine.
3rd
permanent molar (wisdom tooth) erupts between 18-25 years of
age.
2. Condition of sockets of teeth : Healthy teeth do not fall out of the
sockets of dried bone (mandible) because alveolar bone is constricted
somewhat about their necks. After loss of a permanent tooth in life,
the alveolar bone undergoes atrophy and resorption changes and the
bottom of the socket fills up with new bone. Thus, if a tooth has fallen
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out a look at the alveolar border will tell whether the tooth was
broken before or after death.
In old age, because of loss of teeth, the alveolar bone is resorbed and
the alveolar border is reduced to an alveolar ridge.
Besides the above two changes in the alveolar part, other changes in the mandible
to help determination of age are summarized in a tabulated form below –
At birth Adult Old age
1. Mental
foramen
Nearer the lower
border
Midway between
upper and lower
borders.
Nearer the upper
border.
2. Angle of the
mandible
Obtuse (nearer
1800
Right angle (about
900
)
Obtuse (nearer
1400
)
3. Coronoid &
condyloid
processes
Coronoid
process is larger
and above the
level of
condyloid
process
Condyloid process
is above the level
of the coronoid
process
Condyloid process
is above the level of
the coronoid
process but in
extreme old age it is
bent backwards
4. Mandibular
canal
Lies a little
above the level
of the mylohyoid
line.
Runs nearly
parallel with the
mylohyoid line.
Runs close to the
upper or alveolar
border.
5. Symphysis
menti
Present; the bone
remains in two
halves united
together by
fibrous tissue.
Represented by a
faint ridge only in
the upper part
Not recognizable or
absent.
The size of the bone and the depth of its body are also the points to be
added to the list.

Clinical considerations of the mandible :
I. Radiological considerations
II. Anatomical variations
1. Mandibular accessory foramina and canals
2. Bifid mandibular condyles
III. Mandibular endodontic related paresthesia
I. Radiological considerations :
The various radiological views by which the parts of mandible can be
viewed is broadly classified into –
a) Intraoral techniques
b) Extraoral techniques
In intra oral techniques, the following are the views.
i) Intraoral periapical radiograph (IOPA)
ii) Bitewing radiograph
iii)Occlusal radiograph
In extra oral techniques, the following are the views –
i) Oblique lateral radiographs
ii) Cephalometric
iii) Dental panoromic tomography
iv) Computed tomography
v) Transcranial, transpharyngeal, transorbital etc.
vi) MRI
II) Anatomical variations :
i) Mandibular accessory foramina and canals :
Accessory mandibular foramina refer to any openings in the
bone other than the sockets of the teeth, the mandibular foramina, the
mental foramina.

Based on the literature, accessory canals and foramina are
prevalent in the posterior mandible and the area of the symphysis and
more frequently on the internal than the external surface of the
mandible. Bilateral symmetry is common. Variations exist in size and
number. Occurrence may change with age and racial origin. Nerves,
neurovascular bundles, arterioles and venules have been found to
occupy the accessory canals and foramina. No gender differences
have been described.
ii) Bifid mandibular condyles :
A review of the literature showed that only four cases have been
reported previously as incidental findings patients, while another four
cases have been detected in post mortem material. Due to the minimal
symptomatology, the diagnosis of a bifid condyle usually rests on
radiological rather than clinical evidence.
The splitting of the condyles ranges from a shallow groove to two
distinct condyles with a separate neck. The orientation of the head may
be mediolateral or anteroposterior.
It is believed that the bifid mandibular condyle is a development
anomaly and perinatal trauma has been cited as a possible cause. The
genetic origin of this abnormality has been speculated as well as
secondarily from a variety of causes.

III) Mandibular endodontic – related paresthesia :
Paresthesia is an abnormal sensation that patients often describe as a
numb, prickly, or tingling sensation. Possible causes include local
nerve injury, ischemia, pressure on the associated nerves, and toxins.
Pareshtesia of the inferior alveolar nerve is rare because of its unique
intraosseous anatomy and the consequent protection by the mandibular
bone. Oral paresthesia can result from surgical, iatrogenic,
odontogenic, and systemic factors; dentoalveolar surgery is the most
common cause of injury to both the inferior alveolar and lingual
nerves.
Various authors have reviewed the causes of odontogenic-related
sensory disturbances. Possible systemic causes include viral and
bacterial infections, metastatic neoplasms, sarcoidosis, sickle cell
disease, diabetes mellitus, syphilis, systemic sclerosis, bacterial
endocarditis, surgical complications, accidental trauma, anesthetic
injections, surgery, implant placement, orthodontic movement, and
endodontic physical and chemical trauma.
The great majority of endodontic associated paresthesia causes are
diagnosed soon after therapy. They result from overfilling with
obturating material, such as paraformaldehyde paste and resin sealers;
also, the passage of eugenol, sodium hypochlorite, or other chemicals
into the vicinity of the mental or mandibular alveolar nerve has
resulted in paresthesias. Paresthesia caused by bacterial endotoxin
release (associated with late-stage failure of the initial endodontic
therapy) appears to be even more rare.

CONCLUSION :
The sum total of what we know about the body (mandible) to date
represents a confluence of findings in physics, chemistry and biology.
Interestingly, the structure and design of the human body is an
architectural marvel, in which bone is the structural steel and the
reinforced concrete of the human body. In fulfilling its structural
assignments, the human body solves problems of design and
construction familier to the architect and engineer.
“What a piece of work is man”
- Hamlet

“NO KNOWLEDGE CAN BE MORE SATISFACTORY TO A MAN, THAN
THAT OF ITS OWN FRAME, ITS PARTS, THEIR FUNCTIONS AND
ACTIONS”
- THOMAS JEFFERSON

COLLEGE OF DENTAL SCIENCES
DEPARTMENT OF CONSERVATIVE DENTISTRY AND
ENDODONTICS
PRESENTED BY :
Dr. Siddheswaran V.

CONTENTS
1. INTRODUCTION
2. DEVELOPMENT
- PRE NATAL
- POST NATAL
- GENETIC BASIS
- ANAMOLIES
3. ANATOMY
4. CLINCIAL CONSIDERATIONS
- RADIOLOGICAL CONSIDERATIONS
- ANATOMICAL VARIATIONS
- MANDIBULAR ENDODONTIC–RELATED PARESTHESIA
5. REFERENCES
6. CONCLUSION
7. OHP FORMAT OF SEMINAR

Development of mandible / fixed orthodontics courses for general dentists

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