Development of dentition /certified fixed orthodontic courses by Indian dental academy

INDIAN DENTAL ACADEMY
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Embryology is the study of formation and development of
the embryo. The development of the human embryo from the
time of fertilization through the birth week is an important
period for human appearance .
Every individual spends the first nine months of its life
within the womb.
During this period it develops from one-celled structure to an
organism having billions of cells.
An embryo has no face as such
Webster delimits the face as the front part of the head
comprising the nose,cheeks,jaws,mouth,forehead and the eyes.

The key primordial begin to gather,and form slight
swelling,depression and thickening that are rapid which
undergo a series of mergers, and enlargements that will
transfer them,forming a cluster of separate masses into a
Face.
Prenatal life may be arbitrarily divided into three periods:
The period of the ovum
(from fertilization to the end of the fourteenth day)
The period of the embryo
(from the fourteenth day to about the fifty-sixth day)
The period of the fetus
(from about the fifty-sixth day until the two hundred and seventieth
day-birth)

Understanding the development of the structures of the
face requires knowledge of the pharyngeal or branchial
arches. These arches form on either side of the foregut
and correspond to the primitive branchial arches.
The pharyngeal arch consists of a core of mesenchyme
covered externally by ectoderm and covered internally by
endoderm.
Pharyngeal(branchial ) apparatus
The pharyngeal (brachial)arches begin to develop during the
fourth week in utero.

The pharyngeal apparatus consists of a series of bilaterally
paired arches,pouches(clefts)grooves and membranes
This apparatus gives rise to a significant number of
structure of the head and neck.
The structure of the pharyngeal apparatus are numbered
sequentially.
The first four paired arches make up the lateral wall of the
primordial pharynx,which develop from the foregut.


The pharyngeal arch apparatus
Cranial
Endoderm

Pharyngeal pouch
Mesenchyme

Pharyngeal
membrane

Pharyngeal cleft
Nerve
Artery
Ectoderm

Caudal


Weeks 2 through 8 are especially important because
the tissues and organ system are developing from the original
three germ layers.
Formation of Germ layers
At a very early stage in development,the embryo proper
acquires the form of a three-layered disc.,called the
embryonic disc.
The three layers that constitute this embryonic disc are
Endoderm(endo=inside)
Ectoderm(ecto=outside)
Mesoderm(meso=in the middle)

Ectoderm
The ectoderm contributes to the formation of the face is
well around the stomodeum by the fourth week of embryonic
life. It is evident that the ectodermal structures bounding
the stomodeum participate not only in the formation of the
face, but also in the formation of the nasal and oral cavities.
Mesoderm
The mesenchyme that fills the pharyngeal arches is derived
from three sources.. The
Paraxial

Lateral plate

musculature

Neural crest
skeletal

(this musculature is innervated by one nerve that innervates all the
muscles of that particular arch)

Finally each arch has its own artery.

Folding of the Embryo
Beginning in the fourth week of development the flat
trilaminar embryonic disc folds in two planes to form a
more typical-appearing,cylindric,C-shaped embryo.
Folding in the cranial –caudal plane is mainly a result of
rapid longitudinal growth of the central nervous system.
Growth of the somites account for much of the lateral
folding.


Folding of the Embryo……
Folding brings the endodermal-lined yolk sac into the
embryo and creates the primordial gut;the
foregut,midgut,and the hindgut.
At the early stages of embryonic development, the
vertebrate face has a common plan. A series of small buds
of tissue called the facial primordial forms around the
stomodeum, which forms the primitive mouth. The facial
primordial are made up mainly of neural crest cells that
have migrated from the cranial crest and settled.

A summary of the derivatives of the first and second
pharyngeal (ie, branchial)arches is as follows:
·

Pharyngeal arch I

Arch I-Meckels cartilage
(Mandibular arch)
This pair of arch has distinct maxillary and mandibular
prominence
Its major contribution is in the development of face
o Cranial nerve - Maxillary and Mandibular division of
trigeminal nerve (cranial nerve V)
o Artery - Maxillary (terminal branch)

o Muscles - Muscles of mastication (ie, temporalis,
masseter, pterygoids), mylohyoid, anterior belly of
digastric, tensor tympani, tensor veli palatini
o Skeleton - Maxillary cartilage (incus, alisphenoid),
mandibular or Meckel cartilage (malleus), arch dermal
mesenchyme (maxilla, zygomatic, squamous portion of
temporal bone, mandible)

Pharyngeal arch II (hyoid arch) Reichert’s cartilage
Its major contribution is the development of hyoid bone,the lesser
horn and the superior portion of the body

o Cranial nerve -Facial nerve VII
o Artery - Stapedial
o Muscles - Muscles of facial expression (ie, orbicularis
oculi, orbicularis oris, risorius, buccinator, platysma,
auricularis, frontalis), stapedius muscle, posterior belly of
digastric, stylohyoid muscle
o Skeleton - Stapes, styloid process, stylohyoid ligament,
lesser cornu of hyoid, upper part of the body of the hyoid
bone


Derivatives of pharyngeal pouches,Grooves &
membranes

Pouch 1-Tympanic cavity,auditory tube,mastoid antrum
Pouch 2-Crypt lining of palatine tonsils
Pouch 3-Inferior parathyroid glands,thymus

Pouch 4-Superior parathyroid glands,Ultimobranchial body
Groove1-External auditory meatus
Groove2 & 4-Cervical sinus
Membrane 1-Tympanic membrane


Development of the face
Facial development result mainly from enlargement and
movement of the frontonasal prominence and four
prominences from pharyngeal arch1,the paired maxillary
prominences ,and the mandibular prominence.These
structures surround the stomodeum.
The upper jaw develops from 5 main buds of tissue: a
single median frontonasal mass ,the 2 lateral nasal
prominences on both sides,, the 2 maxillary prominences
The lower jaw develops from the paired mandibular
primordia (mandibular prominences).

Paired maxillary and mandibular prominences are
derivatives of the first pair of branchial or pharyngeal
arches. All these prominences are produced by the
proliferation of the neural crest cells that migrated
into the arches from the neural crest during the
fourth week.
The neural crest cells give rise to the connective
tissue components
Frontal prominence
Nasal placode
Maxillaryprominence


Mandibular prominence
Arch I

·

Fourth week of
development

o Primordia of the face appear at the
cephalic end of the embryo.
o   Two nasal placodes cap the bulbous
frontal prominence.
o   The optic discs appear
posterolateral to the frontal
prominence.
O Three paired branchial arches have
formed.
o The first arches split into maxillary
and mandibular prominences. The hyoid
arches are the second pair.

o Between the first arches and frontal prominence, the
buccopharyngeal membrane becomes fenestrated.
Fifth week of
· development
o Nasal pits develop in the
nasal placodes, and the rims
of the placodes differentiate
into medial and lateral nasal
prominences.
o The lens vesicles
invaginate and close within
the optic discs.

o  The mesenchyme of the mandibular arch fills in across
the midline.
o   The caudal end of the medial nasal prominences begins
to fuse with the maxillary prominences.
·                     At the beginning of the sixth week of
development
o   The nasals have shifted to a more ventral, central
position.
o    Growing and shifting subectodermal mesenchyme
smooths out the furrows between prominences and
arches, and the second arch becomes more massive.


o   Six auricular hillocks, which will become the pinna of
the ears, form on the mandibular and hyoid arches.
By the end of the sixth week of
·
development
o Medial and lateral nasal
prominences fuse.
o  Maxillary prominences begin the
formation of the upper jaw.
o  The midline approximation of
the medial nasal prominences forms
the nasal septum.

· At the beginning of the seventh week of
development
o The tip of the nose is elevated between the medial nasal
prominences and is visible in profile.
o Eyelids become prominent.
o The pinna of the ear takes shape.
· End of the seventh week of development
O The pattern of facial features has taken on a human
appearance. However, facial proportions develop during
the fetal period.

· The fusion of the medial nasal prominences is complete,
forming the central axis of the nose and the philtrum of
the lip.
From the beginning of the eighth week of development to
birth, final development of the face occurs slowly and
results mainly from changes in the proportion and relative
positions of the facial components.
At the end of the 8 week the face has taken on a human
appearance.


Developmental
Defects
1. Cleft (hare) lip and
cleft jaw: the medial
nasal processes do not
fuse with the maxillary
processes.
2. Cleft palate and/or
uvula: the palatine
shelves fail to fuse with
each other and/or with
the primary palate.

3.Median cleft of upper lip:
The 2 medial nasal processes
fail to fuse. This is often
accompanied by mental
retardation.
4.Median cleft of lower lip:
Failure of fusion of 2
mandibular arches.

5. Oblique facial cleft: The

nasolacrimal duct remains
exposed due to the lateral
nasal process failing to fuse
with the maxillary process It
can occur along with a cleft
lip.

Development of Dentition
The development of dentition is an integral part of craniofacial
growth and an Unique structure of body
Tooth is derived from ectoderm & mesoderm
The first indication of tooth formation is seen at about sixth
week of intrauterine life
Life cycle of the tooth
Histo
differentiation

Initiation
Proliferation


Apposition

Morphodifferentiation

Initiation
The epithelium overlying the convex border of the alveolar
process becomes thickened and projects into the underlying
mesoderm.
This epithelial thickening is called the dental lamina.
Specific cells within the dental lamina have the potential to
form the enamel organ of certain teeth by responding to
those factors that initiate or induce tooth development.
A lack of initiation result in absence of teeth(anodontia)most
frequently the permanent upper laterals,third molars,and lower
second premolars.
Abnormal initiation result in development of supernumerary
teeth


Proliferation
Enhanced proliferation activity ensures at the point of initiation
and results successively in the Bud,Cap,and Bell stage of the
odontogenic organ.
Proliferative growth causes regular changes in size & proportion of
the growing tooth germ
As the enamel organ grows downwards into the mesenchyme (of
the alveolar process)its lower end assure a cup-shaped
appearance.The cup comes to be occupied by a mass of
mesenchyme called the dental papilla.
The enamel organ and the dental papilla together constitute the
tooth germ.At this stage the developing tooth looks like like a
cap:it is ,therefore ,described as the cap stage of tooth
development.


Histo differentiation
It succeeds the proliferative stage which undergoes definite
morphologic and functionally changes.
The cells of the enamel organ that line the papilla become
columnar.These are called ameloblasts.
Mesodermal cells of the papilla that are adjacent to the
ameloblasts arrange themselves as a continuous epitheliumlike layer.The cells of this layer are called odontoblasts
Ameloblast lay down enamel on the superficial(outer)surface of
the basement membrane.The odontoblasts lay down dentine
on its deeper surface.As layer after layer of enamel and
dentine are laid down,the layer of ameloblasts and the layer
of odontoblasts move away from each other.


The ameloblasts and odontoblasts are separated by a
basement membrane.The remaining cells of the papilla
form the pulp of the tooth.The developing tooth now
looks like a bell (Bell stage of tooth development )

Morphodifferentiation & Apposition
Morphodifferentiation is impossible without proliferation.
The basic form and relative size of the tooth is established
by morphodifferentiation.


In advanced bell stage ,the boundary
between inner enamel epithelium and
odontoblast outline the future dentinoenamel
junction.In addition,the cervical portion of
the enamel organ gives rise to the epithelial
root sheath of Hertwig.
Apposition is the deposition of the matrix of the hard dental
structures.
Appositional growth is characterized by regular and rhythmic
deposition of the extracellular matrix,which is of itself
incapable of further growth.


Development of Permanent tooth

The Permanent tooth bud appears in the 4th & 5th intrauterine months,
at about the same age at which mineralization of the deciduous teeth
commences.
The dental lamina gives off series of buds, one of which lies on the
medial side of each developing milk tooth.
These buds from enamel organ give rise to the permanent teeth
The permanent molars are formed from bud that arise from the
dental lamina posterior to the region of the last milk tooth
At birth the germs of all the deciduous teeth,and of the
permanent incisors,canine and first molars show considerable
development.
The germs of premolar and of the
permanent second molars are
.
rudimentary.
The germ of third molar is formed
after birth.

The stage of dentitions
Gum
pads

Primary
dentition

Mixed
dentition


Permanent
dentition

Gum pad stage:It extends from birth until the eruption of the
first primary tooth in the oral cavity. (The pre-dentition period.)
-This is from birth to six months.
-At this stage, there are no teeth. Clinically, the infant is
edentulous
-Both jaws undergo rapid growth; the growth is in three planes of
space: downward, forward, and laterally (to the side). Forward
growth for the mandible is greater.
Greater the anteroposterior dimension of the gumpads,greater the
possibility of the child developing malocclusion
--occasionally, there is a neonatal tooth present at birth.


Primary dentition stage:
(first transitional period /Deciduous dentition period. )
It extend from the time of eruption of the primary teeth until
the eruption of the first permanent tooth around 6yerars of
age. –
The dentition of this period manifests several important
characteristics.
1.There is either spacing between the teeth or there is not.
2.There is no sagittal and transversal dimensions alteration,only
vertical changes are seen.
3.The distal surface of the maxillary & mandibular second
primary molars maintain vertical relationship.


-The tooth buds anticipate the ultimate occlusal pattern.
-Mandibular teeth tend to erupt first.
--Eruption times can be variable.
Spaced dentition is supposed to be good as space in between the
teeth is utilized for adjustment of permanent successors
Primate spaces occur in about 50% of children. They appear in
the deciduous dentition. The spaces appear between the upper
lateral incisor and the upper canine. They also appear between
the lower canine and the deciduous first molar.


Flush terminal plane
This dentition is completed after the eruption of the second
primary molars,indicating the location for permanent
teeth.Therefore it’s the key factor that influences the future
occlusion of the permanent dentition.
The second primary molar relation is described in terms of
relationship between terminal planes.
In flush terminal plane :The distal surface of upper and lower
teeth are in straight line.It’s a favourable relationship to guide
the permanent molars.


Mesial step:In mesial step the distal surface of the lower molar
is more mesial to that of the upper.Invariably it is favorable to
guide the permanent molar into a class I relationship.
Distal step:The distal surface of the lower molar is more distal
to that of upper.Its unfavorable as it guides the permanent
molars into distal occlusion.
flush terminal plane

Distal step

Mesial step
ClassIII
ClassI(with late mesial shift)
End-end
ClassIII(normally)
ClassI(desirable)


ClassII

Incisor liability:
Mayne has coined the term incisor liability
Permanent incisors are larger than primary incisors,this difference
in size is termed incisal liability.
Eruption of permanent incisors make the
permanent arch circumference wider,normally the interincisal
angle is 150 degree in primary dentition and 120 degree in
permanent dentition
The period of transition determine the successful alignment of
the permanent teeth toward an ideal arch form


Anterior teeth relation
Over bite:The average over bite in primary dentition is 2mm
Overjet :The average over jet is 1-2mm with a normal range of
2-6mm.
With the excessive wear of the primary canines and molars,the
whole lower dental arch may move anteriorly,the incisors may
assume an edge-to-edge interrelationship by the age of six

Sequence of eruption
Deciduous

Eruption Time

Central
Incisor

6-12 months

Lateral
Incisor

9-13 months

Canine

16-23 months

First Molar

13-19 months

Second
Molar

23-33 months


Exfoliation of Primary teeth
The replacement is a multifactorial process.
When the permanent tooth begins to erupt,it is apparently guided
by the gubernaculum..
The pathway for the erupting anterior teeth is essentially lingual
to their predecessors resulting in an oblique lingual pattern of
resorption.
Resorption is not a continuous process,but is rather one of
alternating period of destruction and apposition


Mixed dentition stage:
(second transitional period)
Starts with the eruption of the first permanent tooth,usually
the mandibular central incisor,and is normally completed at the
time the last primary tooth is shed.
During this period ,the vertical dimension of the
face is increased,allowing for a heightening of the alveolar
ridges to accommodate continual root growth of the cuspids and
premolars.


-If the deciduous dentition is spaced dentition with flush
-terminal plane of second deciduous molars,the eruptive force
-cause a closing of of the existing space causing a decrease in
- arch length-(Early mesial shift)
-

-

When no space exist,the erupting first permanent molars is not
able to close spaces.In these case when the primary molars
exfoliates the permanent molar migrates to use up the “leeway
space”-(Late mesial shift)

The leeway space.
-The deciduous anteriors--incisors and canines are
narrower than their permanent successors mesiodistally.
-The deciduous molars are wider that their permanent
successors mesiodistally.
-This size difference has clinical significance. The difference is
called the leeway space.
-The leeway space in the lower arch is approximately 3.4 mm.
-The leeway space in the upper arch is approximately 1.8 mm.
In normal development, the leeway space is taken up by the mesial
migration of the first permanent molars.


Since there are no premolars in the primary dentition, the
primary molars are replaced by permanent premolars. If any
primary teeth are lost before permanent teeth are ready to
replace them, some posterior teeth may drift forward and
cause space to be lost,] This may cause crowding and/or
misplacement once the permanent teeth erupt, which is usually
referred to as maloclussion.


'Ugly duckling stage.:
' On occasion, the permanent incisors 'spread out' due to
spacing. In the older literature, is called by the 'ugly
duckling stage.' With the eruption of the permanent canines,
the spaces often will close.

This is the norm between ages 7 through 12 years of age, and
usually is not connected with a permanent space between the
teeth. This stage of development needs special comment because
its often mistaken as anwww.indiandentalacademy.com
orthodontic problem.

Permanent dentition
Maxillary / mandibular occlusal relationships are established when
the last of the deciduous teeth are lost.
The adult relationship of the first permanent molars is established
at this time.
The growth of the jaw continues during this period,developing room
for the third molars


Sequence of eruption
Permanent
Teeth

Eruption
Time

Central
Incisor

6-8 years

Lateral
Incisor

6.5-9 years

Canine

9-12 years

First
Premolar

8-12 years

Second
Premolar

8.5-13
years

First Molar

6-7 years

Second Molar

11-14
years

Third
Molar
(Wisdom
Tooth)

17-25
years


Physiology of the Stomatognathic
system
• Stomatognathic- of or relating to the jaws and the
mouth :webstar’s medical dictionary
• As a dentist and especially as an orthodontist,it is
essential that we have a sound knowledge of and
understand the importance of the stomatognathic
system
• Until we know what is normal we will not be able to
recognize aberrancy


Functional Osteology

• Bone is one of the hardest tissues of the human body
and it’s most adaptive and responsive to functional
forces
• Meyer -1864:proposed the trajectorial theory of bone
formation
• 1870:Julius Wolff said that the trabecular
arrangement was due to functional forces and
any change in direction or intensity of the forces would
change the internal architecture & external form of
the bone
• It was expressed by a mechanical mathematical law=
law of orthogonality

• Tajectories need not pass at
right angles to each other—even
their course may be wavy
• Increased function increases
bone density while decreased
function decreases .
• Muscles have a great influence
over bones and may even change
their shape
• Thus the timed manipulation of
the musculature may be
beneficial to the orthodontist
• It is established that
cartilaginous and membranous
bones react differently to forces

Beninghoff’s trajectories
He Concluded saying that trajectories can be
demonstrated in the maxillo-facial area
• Lines pass through both the spongy and compact bone
• They obey no individual bone margins
•

Maxilla
Transfers the whole stress to the cranium which
has as thin cortices that are interconnected by
trabeculae
Stress trajectory can be considered as the craniofacial
unit

3 main pillars of trajectories:
• All arise from the alveolar bones
and end in the base of the skull
The canine pillar
The zygomatic pillar
The pterygoid pillar
• Curve around the sinuses,nasal
and orbital opening
• The supra and infra orbital
ridges and the zygomatic
buttress reinforce these pillars


Mandible
• Absorbs all the stresses

Thick cortices and more radially arranged
trabecuae
Stress trajectories
• From beneath the teeth-join together
in comman pillar—ends at the condyle

•Mandibular canal and nerve are protected-unloading
of nerve
•Thick cortical layer at the base provides resistance
to bending forces
•Trajectories from the symphysis, gonial angle and
coronoid process join this main pillar

Myology

Property of muscles
(that are important for help in kinetic activity are)
Elasticity= the ability to return to original length and
shape after contraction or extension
Normally relaxed muscle withstand only a certain
amount of elongation
Contractility=the ability to contract under innervational
impulse.
Strength of contraction depends on
Frequency of stimuli
Number of fibers involved

• The greatest of contraction is seen when the muscle
approximates its resting position


Other principles of muscle physiology
• All or none law
Intensity of contraction is independent of the
strength of the action potential as long as the
threshold is crossed
Seen only when the muscle is not fatigued
• Muscle tonus
State of slight constant tension and form the basis of
reflex posture
• Resting length
Constant and Permits maintenance of postural
relationship
• Reciprocal innervations and inhibition
Inhibition of contraction may be brought about by
excitation of the antagonist.It is through this that
movements are coordinated

Buccinator mechanism
There is strong inter relationship between bone and muscle
Muscles are a potent force-at rest or when active
Teeth and supporting tissues are influenced by the contiguous
musculature
The balance between the tongue and the peri-oral musculature
influences the morphology of the dental arches and teeth
The shape and size of the tooth root and the periodontal
spaces also helps in the final adaptation


• During swallowing and mastication- tongue exerts 2-3
times more force than the peri oral muscles
• Counter acted by the tonal contraction,atmospheric
pressure,peripheral recruitment from the buccal
musculature
• Even though doubts still exist about such a balance of
force-we must admit that aberration of muscles can and
do lead to marked malocclusion
• The continuous band of muscle that encircle the dentition
and attach to the pharyngeal tubercle comprises the
buccinator mechanism


Tongue
• Opposing the buccinator mechanism is a very powerful
structure- the tongue
• It has multiple functions
• Most developed in a new born
• In infancy extrinsic suspensory muscles attach the
tongue to the various osseous structures and are
responsible for gross movement in the horizontal
direction(suckle-swallow)


• The tongue has amazingly versatile functional possibilities
by virtue of the fact it is anchored at only one end.
• This freedom permits the tongue to deform the dental arch.
•

It may Leads to proclination of the maxillary
incisors,constriction of buccal segment and may create an
open bite.

• Compensating action of mentalis causes retraction of the
lower incisors
• Abnormal position and size of the tongue is often seen in
the presence of enlarged tonsils and adenoids.


Muscles of the Tongue

INTRINSIC

Superior longitudinal-Shortens the tongue & makes the dorsum concave
Inferior longitudinal-Shortens the tongue & makes the dorsum convex
Transverse-Makes the tongue narrow & elongated
Vertical-Makes the tongue broad & flattened
EXTRINSIC

Genioglossus-

Protrudes the tongue out of the mouth by pulling the
posterior part forwards

Hyoglossus-depresses the tongue
Styloglossus -pulls it upward & backward
Palatoglossus-brings the palatoglossal arches

together, thus shutting the oral

cavity from the oropharynx

Temperomandibular Joint

• The TMJ is the ball and socket joint that allows the lower
jaw to swing open and close. It’s a Compound –movable joint
It articulation with the mandibular condyle and the
inferior surface of the sqamous portion of the temporal
bone
•The condyle: This is the "ball" in the joint. It is a part of
the mandible (lower jaw), and is covered in a layer of
cartilage which allowswww.indiandentalacademy.com
for smooth motion within the joint
assembly

•The glenoid fossa: The fossa is the
"socket", or depression in which the
condyle sits. It is located in the
temporal bone of the skull.

The front of the fossa is a more gentle slope of bone called the
articular eminence. The eminence is also covered with
cartilage.
The articular disc: The articular disc is also called the
meniscus. It is made of hyaline cartilage.
The ligament behind the meniscus is called the retrodiscal pad
in deference to its function as a shock absorber

•The joint capsule is the covering of the TM joint .It
isolates the contents of the joint and allows free movement
of the condyle and articular disk within a small "swimming
pool" of synovial fluid. The capsule has lots of blood vessels
and nerves as well as connective tissue.
• It is lined by synovial membrane.

Head of the condyle is tubular or ellipsoidal
Unique feature of the joint is that there
are actually 2 joint spaces- separated by
the disc
Thus the joint is capable of dual function
Lower cavity—rotary/hinge movement
Upper cavity—gliding/translatory movement

• The rotary movement occurs by itself when opening from
occlusion to physiologic resting after this even gliding
motion takes place
• When opened in his manner the articular disc glides over
the articular eminence
• The condyle rotates against the inferior surface of the
disc.
• Lateral pterygoid helps to move the disc anteriorly

In the lateral shift/bennet movement the condyle on
the working side rotates and moves laterally.
• The condyle on the other side moves forwards and
mesially in an arc
•

• Posterior limit formed by the squamo-tympanic fissure—
medially by the petro-tympanic fissure

• Due to over closures-the disc is moved forward
and the condyle rides over the posterior border
of the disc. Impinges on the retrodiscal soft
tissues-clicking an discomfort to the patientmisdiagnosed as arthritic changes ,pain may be due
to impingement or due to pterygoid muscle
spasm(mpd)
• The main reason for the mpd is neurogenic or
psychological. The TMJis beautifully engineered to
handle multiple tasks endowed to it
• To maintain it-proper occlusal management an
maintenance of vertical dimensions is essential


Functional movements of the mandibl
•The mandible is the only movable bone in the head and face.
•It has 13 muscles attached to it
•Due to limitations of morphology and structure of the TMJ
it can be moved only in certain directions
•It has postural stability along with primary movements


Forced retrusion

Posterior fibres of the
temporalis,masseter,suprahyoid muscles

Opening

lateral pterygoid-main. Supra,infra hyoidgenio,mylo hyoids,digasric>stabilising
Temporalis,masseter,medial pter.>controlled
relaxation

Closure

Temporalis, masseter >main action medial ptery.
>part action
Lateral ptery >cntrolled relaxation

Protrusion

Lateral and medial pter.

Lateral movement

Temporalis + lat. pter

COMPENSATORY MUSCLE
FUNCTIONS
• While mastication may call for most potent effort
from the associated muscles, the most frequent
demands are made by deglutition, respiration,
speech and posture.
• At times two or more of these functions are
carried on simultaneously.
• where there is malocclusion or abnormal
morphologic relationship certain compensatory or
adaptive muscle functions may arise, either to
restrain the dental malocclusion or to actually
increase the discrepancy.

Positions of the mandible
•
•
•
•
•
•
•
•

Basic positions are
Postural resting position (Physiologic rest)
Centric relation
Initial contact
Centric occlusion
Most retruded position (terminal hinge position)
Most protruded position
Habitual resting position

• Habitual occlusion position


POSTURAL RESTING POSITION
It is one of the earliest posture positions to be
developed. Mandible is suspended from the cranial base
by the cradling musculature. Here the jaws are
separated by a constant distance.
Factors influencing the postural position are the
following:
• Body and head posture
• Sleep
• Psychic factors influencing muscle tonus
• Age
• Proprioception from the dentition and muscles
• Occlusal changes such as attrition
• Pain
• Muscle disease and muscle spasm and
• Temporomandibular joint disease.

CETRIC RELATION
• As far as muscle physiology is concerned, centric
relation may be defined as unstrained, neutral
position of the mandible in which the anterio
superior surfaces of mandibular condyles are in
contact with the concavities of the articular disks
as they approximate the postero inferior third of
their respective articular eminent.
•

This means that the mandible is deviating neither
to the right nor to the left and is neither protruded
no retruded.

•

Such a relation can be the same as the postural
resting position, the point of initial occlusal contact
and centric occlusion.

INITIAL CONTACT
• As the mandible moves from physiologic rest or the postural
resting position toward occlusion of teeth, if all is normal it
maintains a centric relation position as far as the articular
fossae are concerned.
•

If there is normal occlusion the point of initial contact
produces no change in the function of temporomandibular
joint and all inclined planes are brought together
simultaneously in the maxillary and mandibular teeth.

•

Premature contacts are seen quite frequently, they can
initiate deflections in the mandibular path of closure.

•

This causes traumatic forces to be exerted on the teeth
and severe cases will produce temporomandibular joint
problems

CENTRIC OCCLUSION
• Centric occlusion is a static position
• It is harmonious with the centric relation
• Teeth brought into contact with unstrained relation
of the condyles
• Few patients can show centric occlusion
• Premature contact,loss of teeth,Overeruption of
teeth,overextension of artificial
restorations,malpositions of individual teeth-all these
mitigate against the establishment of a centric
occlusion


MOST RETRUDED POSITION (Terminal
Hinge Position)
• It’s a Reproducible retruded mandibular position
with the teeth in occlusion
• It has become a common starting point in occlusal
analysis and rehabilitation
• Some patients can easily retrude a few mm. while
others find it difficult
• Mandible should not be guided or forced beyond the
unstrained position of the mandible-unbiologicalwould compress the tissues.


MOST PROTRUDED POSITION
• This position is variable from individual to individual
than the retruded position,however,it is
reproducible
• Shows the range of movement of the mandible
• Flaccidity of capsular ligament allows condyle to
over ride the anterior margin of the
eminence,leading to dislocation.

HABITUAL RESTING POSITION

There are certain types of malocclusions that
prevent the patient from achieving a physiologic
resting position.


• Eg.,in class II div 2 malocclusion with maxillary incisors
inclined lingually there is a tendency to force the
condyles posteriorly and superiorly in the articular
fossae
• The physiologic resting positions can be changed due to
mental disturbances, enlarged adenoids,
temporomandibular joint pathology, psychic trauma,
selective paralysis by poliomyelitis and confirmed
mouth breathing etc.


HABITUAL OCCLUSAL POSITION
• In normal occlusion the centric occlusion and habitual
occlusion should be the same. But the occlusal
relationship can be changed when there is an
environmental imbalance induced by improper
restoration, tooth loss etc.
• It is vitally important that the dentist make sure that
the habitual occlusal position and the centric occlusal
position are the same and are in harmony with centric
relation and the postural resting position of the

mandible.


Functions of the system
•
•
•
•

Deglutition
Mastication
Respiration
Speech


DEGLUTITION

Infantile swallow ( visceral swallow)

Mature swallow ( somatic swallow)


CHARACTERISTICS OF INFANTILE SWALLOW

The jaws are apart, with the
tongue between the gum pads
The mandible is stabilized by
contraction of the muscles of
the 7th cranial nerve & the
interposed tongue.
The swallow is guided, & to a
great extent controlled by
sensory interchange between
the lips & the tongue.

TRANSITION PERIOD
At about the 5 to 6th month of age, as the incisors
begin to erupt, certain proprioceptive impulses come into
play & the peripheral portions of the tongue starts to
spread laterally.
An average infant would show a dominant & exclusive
thrusting swallow for the first 6 months of life,a
transitional thrusting & lateral spread of tongue during the
next year & a dominant somatic swallow thereafter.


CHARACTERISTICS OF
MATURE SWALLOW

The teeth are together

The mandible is stabilized by
contraction of the mandibular
elevators,which are primarily 5th
cranial nerve muscles
The tongue tip is held against the
palate, above & behind the
incisors
There are minimal contractions of
the lips during the mature
swallow.

DEGLUTITION CYCLE
FLETCHER gave 4 phases
1.Preparatory swallow
2.Oral phase of swallowing
3.Pharyngeal phase of swallowing
4.Esophageal phase of swallowing


PREPARATORY
SWALLOW
• Starts as soon as
liquids are taken in, or
after the bolus has
been masticated
• The liquid or bolus is
then in a swallowpreparatory position
on the dorsum of the
tongue.
• The oral cavity is
sealed by lip & tongue.

Oral phase of
swallowing moves
• Soft palate

upward & the tongue
drops downward &
backward
• Larynx & hyoid bone
move upward
• Smooth path for the
bolus as it is pushed
from the oral cavity by
the wave like rippling of
the tongue
• Oral cavity is stabilized
by the muscles of
mastication, &
maintains the anterior

Pharyngeal phase of
swallowing
• Begins as the bolus passes
through the fauces
• The pharyngeal tube is raised
upward en masse
• Nasopharynx is sealed off by
closure of the soft palate
against the posterior
pharyngeal wall
• Hyoid bone & base of tongue
move forward as the pharynx
& tongue continue their
peristaltic – like movement of
the bolus of the food

Esophageal phase of
swallowing
• Commences as the food
passes the
cricopharyngeal
sphincter
• While peristaltic
movement carries the
food through the
esophagus, the hyoid
bone , palate & tongue
return to their original
positions

Mastication
• In the infant food is taken by suckling-unlearned
or automatic reflex
• Rhythmic caving in of the cheeks, bobbing of the
hyoid bone,snake like movement of the tongue,
anterior movement of the tongue and nodding
movement of the head
• Mandible is protruded
• During deglutition rhythmic contraction of the
tongue and facial muscles aids in stabilizing the
mandible


6 phases outlined by Murphy
• 1)preparatory phase=> food in mouth-tongue
positions it-mandible moves to working side
• 2)food contact=> characterized by a pauseprobably to determine food consistency
• 3)crushing phase=> starts with high velocity and
slows down as food is crushed
• 4)tooth contact=> takes place with slight change in
direction-muscles are already ready for this
• 5)grinding phase=> transgression of the mandible
across the maxillary teeth-most patients chew
unilaterally-cyclic event
• 6)centric occlusion=> mandible back to terminal
position-next cycle begins

Respiration
Inherent reflex activity
Demand on muscles are more subtle and more difficult to
appreciate
Posture has a significant effect on respiration
Oral musculature is responsible for vital positional
relationships-maintains the airway
All the learned jaw functions are built around and
accommodated to the tongue and mandibular positions which
make clear airway possible
Maintenance of the respiratory spaces and airway is
significant in facial growth
Respiration in the child helps keeping the pharynx patent as
well in the development of speech .

Speech
Learned activity –depends on maturation of organism
Makes use of muscles that have other functions
WEST lists the “other than speech functions” as follows
1. innate,automatic,vegetative-swallowing,breathing,gagging
2. learned,automatic,vegetative-biting,chewing,sucking
3. learned,automatic,emotional-grimaces,mannerisms
4. innate,automatic,emotional-laugh,smile,sobbing
5. learned,non automatic,discriminatory,voluntary-movement
of with tongue,spreading lips,kissing,blowing
6. learned,automatic practical reactions-whistling,playing
instrument,humming

Muscles involved
Walls of the torso,the respiratory tract, the pharynx,
the soft palate,the tongue,the lips and face and nasal
passageways are all concerned in the production of
speech.
Simultaneous breathing is necessary to provide vibrations
for sound
Tongue,lips and velopharyngeal closure modify outgoing
stream to produce sound .
Normal structure is necessary for production of sound-in
cleft palate cases its not possible even with muscular
compensation
Speech mechanism acts on the breath streams in a
number of ways,controlling the air mechanism
direction,flow,release,pressure,general and lingual air
path

With respect to tongue
• The lips and the tongue undergo maturational changes
• Degree of lip protrusions varies the length of the vocal
tract
• With the reduction of suckle swallow activity,more
delicate peripheral lip movements are noted.
• In infancy-suckle reflex is active and extrinsic muscles
are well developed-only later the intrinsic muscles capable
of speech develops

Velopharyngeal valve(closure)
Very important for the dentist-in conditions of cleft
palate-inadequate valving is seen-even in rehabilitated
cases
• Upward and backward movement to contact the post
pharyngeal wall is important for certain sound production

Speech difficulties related to
Malocclusion

Speech sound

Problem

S/z(sibilants)
T/d

(linguoalveolar stops)

lips

(labiodental fricatives)

Th/sh/ch

gap between incisors,Anterior open bite

difficulty in production

F/v

Related malocclusion

Irregular incisors,lingual position
of maxillary incisors

distortion
distortion

(linguodental fricatives)


Skeletal classIII
Anterior open bite

Conclusion…
• The study of the intra oral and peri oral structurestheir normal morphology and activity is very
important for us.
• Unless we know what is normal it is difficult to
recognize and correct abnormalities
• Also, the structures are all inter related ::follow
scientific principles and order in their development.
• It becomes important to follow the functions of the
structures which ultimately leads to changes in the
morphology..

Bibliography
• Orthodontics:Principles and practices-T.M
Graber
• Orthodontics:Graber and Vanarsdal
• Contemporary Orthodontics: William.R.Proffit
• Handbook Of Facial Growth :Donald.H.Enlow


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