Mandibular movements / fixed orthodontic courses

MANDIBULAR MOVEMENTS

INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

1

Once it is accepted as it must
be that the movement of the jaw
are complex and variable ,then it
became imperative to learn as
much as possible about jaw
movements in order to reproduce
those aspects of its movements
considered necessary for proper
functioning of the occlusion,
either natural or artificial.

2

There are broad agreements
concerning jaw movements
like the mandible performs
habitual movements and
border movements ,opening
movements and closing
movements, protrusive and
lateral movements .

3

• The main influences on normal jaw
movements are the teeth ,the joints and
the surrounding muscles and ligaments
• Normally the mandible moves in a
habitual manner to accomplish speech,
mastication, deglutition, respiration ,
sucking ,whistling etc.
• Abnormally the jaw moves in a habitual
and often pernicious manner as in
bruxism
• In the edentulous patient the influence of
the teeth is lessened by their movable
relation of the mandible and maxilla.

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5

The mandible, or lower jaw, is the largest
and the strongest bone of the face.
It has a horse shoe shaped body which
lodges the teeth, and a pair of projections
or rami.
The ramus on either side extends
vertically and slightly laterally from the
posterosuperior aspect of the body. The
upper part of the body is continuous as
the alveolar process. It generally
surrounds and supports the teeth, but
when they are lost, it becomes the bony
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base for dentures.

The ramus terminates superiorly in two
processes. Of these, the coronoid
process is anterior to the condyloid
process, which is capped by the
condyle.
The constricted area just inferior to
the condyle is called the neck of the
condyle. Between coronoid process
and condyloid process is the
mandibular notch, which is concave
superiorly.

7

The mandibular foramen, through which
the inferior alveolar nerves and vessels
enter, lies on the medial aspect of the
ramus, approximately midway between
the lowest point of the notch and the
inferior surface of the mandible.
The anterior border of the ramus
presents two ridges. The lateral ridge
continues onto the body as the external
oblique line. The medial ridge is called
the temporal crest and is almost
continuous with the mylohyoid ridge of
8
the body of the mandible.

The mental foramen is located in
the vicinity of the apex of the
premolar teeth. When the teeth are
lost and resorption occurs, it may
progress downward to involve the
mental foramen.
On the lingual surface of the
midline, the genial tubercle may
exhibit prominences on both sides
of the midline.

9

TEMPOROMANDIBULAR
JOINTS
The major components of the
temporomandibular joints are the
cranial base, the mandible, and the
muscles of mastication with their
innervation and vascular supply.
Each joint can be described as
ginglymoarthrodial, meaning that it
is capable of both a hinging and a
gliding articulation.

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11

An articular disk separates the
mandibular fossa and articular tubercle
of the temporal bone from the condylar
process of the mandible.
The articulating surfaces of the
condylar processes and fossae are
covered with avascular fibrous tissue
(in contrast to most other joints, which
have hyaline cartilage). The articular
disk consists of dense connective
tissue; it also is avascular and devoid
of nerves in the area where articulation
normally occurs.

12

Posteriorly it is attached to loose
vascularized connective tissue, the
retrodiscal pad or bilaminar zone
(Called bilaminar because it
consists of two layers: an elastic
superior layer and a collagenous
inelastic inferior layer), which
connects to the posterior wall of
the articular capsule surrounding
the joint.

13

Medially and laterally the disk
is attached firmly to the poles
of the condylar process.
Anteriorly it fuses with the
capsule and with the superior
lateral pterygoid muscle.
Superior and inferior to the
articular disk are two spaces,
the superior and inferior
synovial cavities.

14

These are bordered peripherally by
the capsule and the synovial
membranes and are filled with
synovial fluid.
Because of its firm attachment to
the poles of each condylar process,
the disk follows condylar movement
during both hinging and translation,
which is made possible by the loose
attachment of the posterior
connective tissues.

15

LIGAMENTS

The body of the mandible is
attached to the base of the
skull by muscles and also by
three paired ligaments: the
temporomandibular (also called
the lateral), the
sphenomandibular, and the
stylomandibular.
Ligaments cannot be stretched
significantly, so they limit the
movement of joints.

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The temporomandibular ligaments
limit the amount of rotation of the
mandible and protect the structures of
the joint, limiting border movements.
The spheno-mandibular and
stylomandibular ligaments limit
separation between the condylar
process and the disk.
 the stylomandibular ligaments also
limit protrusive movement of the
mandible.

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19

The Glossary of Prosthodontic Terms, 7 th
edition, the Academy of Prosthodontics, 1999

Bennett movement ( Sir Norman

Godfrey Bennett, British dental
surgeon, 1870- 1947) :.
• Laterotrusion n: condylar movement on
the working side in the horizontal plane.
This term may be used in combination
with terms describing condylar
movement in other planes, for example,
laterodetrusion, lateroprotrusion,
lateroretrusion and laterosurtrusion.

20

Bennett's movement refers to
the condylar movements on the
working side and Bennett's
shift is the bodily side shift of
the mandible on the working
side generally in horizontal
direction.

21

Bennett's movement
(transtrusion, side shift) - The
bodily side thrust or shift of the
mandible regulated by the
anatomical configurations of
the glenoid fossa or the
capsular ligaments.

22

Laterodetrusion n : lateral and downward
movement of the condyle on the working side.

Lateroprotrusion n : a protrusive movement
of the mandibular condyle in which there is a
lateral component.

Lateroretrusion n : lateral and backward

Laterosurtrusion n : lateral and upward
Bennett's movement is composed of two
phases an immediate side shift and a
progressive side shift

23

CONDYLAR MOVEMENTS
During lateral movements of the jaw,
nonworking condyle is drawn inward from
centric position by the lateral pterygoid and as
a result it translates in a forward, downward
and anterior direction. The opposite working of
condyle rotates and moves outward (latero
protrusion - Bennett's movement).


24

Condylar guidance
The Glossary of Prosthodontic Terms, 7 th
edition, the Academy of Prosthodontics
1999:
I Condylar guidance : Mandibular guidance
generated by the condyle and articular disc
transversing the contours of the glenoid fossae.
2. Condylar guidance : The mechanical form
located in the upper posterior region of an
articulator that controls movement of its mobile
member.
Condylar path: That path traveled by the
path
mandibular condyle in the temparomandibular
joint during various mandibular movements. 25

Protrusive condyle path: The path
path
the condyle travels when the
mandible is moved forward from its
initial position.
Lateral condylar path: The path of
path
movement of the condyle disc
assembly in the joint cavity when a
lateral mandibular movement is
made.
Condylar inclination : The direction
of the lateral condyle path

26

The Glossary of Prosthodontic Terms, 7th
edition, the Academy of Prosthodontics 1999:

Mandibular translation:

The translatory (medio-lateral)
movement of the mandible when
viewed in the frontal plane. While this
has not been demonstrated to occur as
an immediate horizontal movement
when viewed in the frontal plane, it
could theoretically occur in an
essentially pure translatory form in the
early part of the motion or in
combination with rotation in the lateral
part of the motion or both.
27

Bennett angle

: The angle
formed between the sagittal
plane and the average path of
the advancing condyle as
viewed in the horizontal plane
during lateral mandibular
movements.

28

Early mandibular translation: The
translatory portion of lateral movement in
which greatest portion occurs early in the
forward movement of the nonworking
condyle as it leaves centric relation.

Immediate mandibular translation:
The translatory portion of lateral
movement in which the non-working
condyle moves essentially straight and
medially as it leaves the centric relation
position.

29

Bennett (1908) studied working condylar
path and called it Bennett movement, now
referred as laterotrusion.
Bennett showed that the working
condyle moved outwards during sideward
movement of mandible in frontal plane,
whereas the non-working condyle moved
inward.
Bennett described this bodily shift of
mandible without having any knowledge
of Balkwill's description in 1866 of the
same side shift.
30

The orbiting condylar path (horizontal
lateral condylar path) consists of two
components namely; an immediate and
progressive mandibular lateral
translation.
Immediate lateral translation - Immediate
side shift ISS, occurs when non working
condyle moves from centric relation
straight inward or medially.
Progressive lateral translation progressive side shift PSS, is the
translatory portion of lateral movement.

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32

Although Bennett has described about this
movement which became popularly known
as Bennett movement, the original
discovery of this movement should go to
BALKWILL.
As early as 1870 Balkwill observed that
the mandible opened and closed on an axis
that runs through the condyles, that the
condyles move downwards and forwards in
protrusion and also the mandible moves
bodily from side to side.

33

His observation was forgotten
and remained in the archives of
London library.
Without being aware of
Balkwills work, Bennett
demonstrated that the TMJ
permitted three kinds of
movement.

34

Progressive mandibular translation
(Guichet)
1: The translatory portion of mandibular
movement when viewed in a specified
body plane.
2 : The translatory portion of mandibular
movement as viewed in a specified body
plane that occurs at the rate or amount
that is directly proportional to the forward
movement of the non-working condyle.

35

Timing of Bennett's movement:
Amount of immediate side shift and
progressive side shift. The rate or
amount of descent of contra lateral
condyle and the rotation and lateral
shift of Ipsilateral condyle

Immediate Side Shift (ISS) Progressive Side Shift (PSS). It is
the bodily shift of mandible in
horizontal direction. This is regulated
by the shape of glenoid fossa,
looseness of capsular ligament and the
36
contraction of lateral pterygoids.

ISS is the first movement the mandible
makes when initiating lateral excursion.
ISS occurs when the non-working
condyle moves medially from its
centric position in the fossa during
lateral movement.
It takes place at the beginning of lateral
movement.
This is not an exact 90' or a right
angled medial movement in horizontal
plane.
This horizontal movement varies
according to the shape of glenoid fossa
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etc.

ISS ranges from 0.2 mm to 2 mm in width,
with a mean 1.0 mm (Lundeen, Wirth).
Using an electronic recording device Hobo
found it to be 0 to 2.6 mm with a mean value
of 0.42 mm.
Beyond this (ISS), the condyle moves
forward, downward and inward or medially.
Guichet referred this movement component
as Progressive Side Shift (PSS). Lundeen
and Wirth found that ISS varies with
individuals, whereas PSS showed a value of
7.5 mm among different subjects.

38

The combined amount of Bennett
movement (ISS+PSS) is the
Bennett angle of the orbiting
condyle (non-working condyle).
In other words, B.A. Is the angle
formed by the orbital condylar path
(horizontal lateral condylar path)
and sagittal plane.
It varies 2- 44 degrees; with a mean
value of 16 degrees (Hobo,
Mochizuki).

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40

Origin: Lateral surface of the skull
Insertion: Coronoid process and anterior
border of the ramus
Function : Elevates and retracts jaw
Assists in rotation
Active in clenching


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42

Origin
: Zygomatic Arch
Insertion : Angle of mandible
Function : Elevates and protracts jaw
Assists in lateral movements
Active in clenching


43

Origin

: Pterygoid fossa and mesial
surface of lateral pterygoid
plate
Insertion: Medial surface of angle of
Insertion
mandible
Function: Elevates jaw, causes lateral
movement and protrusion


44

SUPERIOR LATERAL PTERYGOID
Origin : Infra temporal surface of
greater
wing of sphenoid
Insertion: Articular capsule and disc
,neck
of the condyle
Function: Position disc in closing


45


46

Origin

: Lateral surface of lateral
pterygoid plate
Insertion : Neck of the condyle
Function: Protrudes and depresses jaw
causes lateral movements


47

Origin : Inner surface of the mandible
Insertion: Hyoid and mylohyoid raphe
Function : Elevates and stabilizes
hyoid


48

GENIOHYOID
Origin : Genial tubercle
Insertion: Hyoid
Function : Elevates and draws hyoid
forward


49

Origin : Tendon linked to hyoid
Insertion: Digastric fossa
Function: Elevates hyoid, depresses
jaw


50

MOVEMENT

MUSCLES

Elevation of chin
(closing)

Masseter
Medial pterygoid
Anterior part of
temporalis

Depression of chin
(opening)

Lateral pterygoid
Digastric
Geniohyoid and
mylohyoid with infra
hyoid muscles


51

MOVEMENT

MUSCLES

Protraction

Lateral pterygoid
Medial pterygoid
Masseter

Retraction

Temporalis
Digastric

Chewing

Medial and llateral
pterygoid
Masseter
Temporalis


52

MANDIBULAR MOVEMENT
As for any other movement in
space, complex three-dimensional
mandibular movement can be
broken down into two basic
components:
translation, when all points within a
body have identical motion, and
rotation, when the body is turning
about an axis.

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54

Every possible three-dimensional
movement can be described in
terms of these two components.
It is easier to understand
mandibular movement when the
components are described as
projections in three
perpendicular planes: sagittal,
horizontal, and frontal reference
planes and three axis of rotation.

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56

Sagittal Plane.
In the sagittal plane, the
mandible is capable of a purely
rotational movement as well as
translation.
Rotation occurs around the
terminal hinge axis, an
imaginary horizontal line
through the rotational centers
of the left and right condylar
processes.

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58

The rotational movement is limited to
about 12 mm of incisor separation
before the temporomandibular
ligaments and structures anterior to
the mastoid process force the
mandible to translate.
During translation, the lateral
pterygoid muscle contracts and
moves the condyle-disk assembly
forward along the posterior incline of
the tubercle.
Condylar movement is similar during
protrusive mandibular movement.

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Horizontal Plane

In the horizontal plane, the mandible
is capable of rotation around several
vertical axes. For example, lateral
movement consists of rotation
around an axis situated in the
working (laterotrusive) condylar
process with relatively little
concurrent translation.
A slight lateral translation-known as
Bennett movement, mandibular
sideshift, or laterotrusion is
frequently present.

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62

This may be slightly forward or
slightly backward (lateroprotrusion
or lateroretrusion). The orbiting
(nonworking) condyle travels
forward and medially as limited by
the medial aspect of the
mandibular fossa and the
temporomandibular ligament.
Finally, the mandible can make a
straight protrusive movement.

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Frontal Plane.
When observing a lateral
movement in the frontal plane,
the mediotrusive (or
nonworking) condyle moves
down and medially while the
laterotrusive (or working)
condyle rotates around the
sagittal axis perpendicular to
this plane.

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66

Again, as determined by the anatomy of
the medial wall of the mandibular fossa
on the mediotrusive side, transtrusion
may be observed as determined by the
anatomy of the mandibular fossa on the
laterotrusive side, this may be lateral and
upward or lateral and downward
(laterosurtrusion and laterodetrusion).
A straight protrusive movement observed
in the frontal plane, with both condylar
processes moving downward as they
slide along the tubercular eminences. 67


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69

Mandibular motion consists of
curved, and more often, elliptical
motion.
The related axes of rotation in the
three planes of space are associated
with this three dimensional motion.
Although mandibular motion is
controlled by the neuromuscular
complex, physiologic axes of
rotation exist as an integral part of
motion itself.

70

Transverse Hinge Axis
The transverse hinge axis
which passes through both
condyles is associated with
rotation of the mandible in the
vertical (sagittal) plane. Motion
is always perpendicular to its
axis of rotation by definition.

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Vertical Axis
The physiologic vertical axis
of rotation is associated with
rotation in the horizontal
(transverse) plane and is
located in the working
condyle.

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SAGITTAL AXIS
The physiologic sagittal axis
of rotation is associated with
rotation in the frontal plane.
The balancing condyle rotates
about the sagittal axis which is
located through the working
condyle

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Mandibular movements are
limited by the
temporomandibular joints and
ligaments, the neuromuscular
system, and the teeth.
Posselt was the first to
describe the extremes of
mandibular movement, which
he called border movements.

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Posselt used a three-dimensional
representation of the extreme
movements the mandible is capable
of.
All possible mandibular movements
occur within its boundaries.
At the top of both illustrations, a
horizontal tracing represents the
protrusive movement of the incisal
edge of the mandibular incisors.

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81

Starting at the intercuspal positions in
the protrusive pathway, the lower
incisors are initially guided by the
lingual concavity of the maxillary
anterior teeth.
This leads to gradual loss of posterior
tooth contact as the incisors reach the
edge-to-edge position.
This is represented in Posselt's
diagram by the initial downward slope.

82

As the mandible moves farther
protrusively, the incisors slide
over a horizontal trajectory
representing the edge-to-edge
position (the flat portion in the
diagram), after which the lower
incisors move upward until new
posterior tooth contact occurs.
Further protrusive movement of
the mandible typically takes
place without significant tooth
contact.

83

The border farthest to the right of
Posselt's solid represents the
most protruded opening and
closing stroke.
The maximal open position the
mandible is represented by the
lowest point in the diagram.
The left border of the diagram
represents the most retruded
closing stroke.

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85

This movement occurs in two phases:
The lower portion consists of a
combined rotation and translation, until
the condylar processes return to the
fossae.
The second portion of the most
retruded closing stroke is represented
by the top portion of the border that is
farther to the left in Posselt's diagram.
It is strictly rotational.

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87

Most functional movement of the
mandible (as occurs during
mastication and speech) takes
place inside the physiologic limits
established by the teeth, the
temperomandibular joints, and the
muscles and ligaments of
mastication; therefore, these
movements are rarely coincident
with border movements.

88

Chewing

When incising food, adults open their
mouth a comfortable distance and move
the mandible forward until they incise,
with the anterior teeth meeting
approximately edge to edge.
The food bolus is then transported to the
center of the mouth as the mandible
returns to its starting position, with the
incisal edges of the mandibular anterior
teeth tracking along the lingual
89
concavities of the maxillary anterior teeth.

90
Comparison of border & chewing movements of soft food

The mouth then opens slightly, the
tongue pushes the food onto the
occlusal table, and after moving
sideways, the mandible closes into
the food until the guiding teeth
(typically the canines) contact.'
The cycle is completed as the
mandible returns to its starting
position.

91

This pattern repeats itself until the
food bolus has been reduced to
particles that are small enough to be
swallowed, at which point the
process can start over.
The direction of the mandibular path
of closure is influenced by the
inclination of the occlusal plane with
the teeth apart and by the occlusal
guidance as the jaw approaches
intercuspal position.

92

Chewing pattern observed in children
differs from that found in adults.
Until about age 10, children begin the
chewing stroke with a lateral
movement.
After the age of 10, they start to chew
increasingly like adults, with a more
vertical stroke.
Stimuli from the press receptors play
an important role in the development of
functional chewing cycles."

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Mastication is a learned process.
At birth no occlusal plane exists, and
only after the first teeth have erupted
far enough to contact each other is a
message sent from the receptors to the
cerebral cortex, which controls the
stimulai to the masticatory
musculature.
Stimulai from the tongue and cheeks,
and perhaps from the musculature
itself and from the periodontium, may
influence this feedback pattern.

96

SPEAKING

The teeth, tongue, lips, floor of the
mouth, and soft palate form the
resonance chamber that affects
pronunciation.
During speech, the teeth are
generally not in contact, although the
anterior teeth may come very close
together during "C "CH," "S," and
"Z" sounds, forming the "speaking
space”.

97

When pronouncing the fricative
"F," the inner vermilion border of
the lower lip traps air against the
incisal edges of the maxillary
incisors.
Phonetics is a useful diagnostic
guide for correcting vertical
dimension and tooth position
during fixed and removable
prosthodontic treatment.

98

PARAFUNCTIONAL MOVEMENTS
Parafunctional movements of the mandible
may be described as sustained activities
that occur beyond the normal functions of
mastication, swallowing, and speech.
There are many forms of parafunctional
activities, including bruxism, clenching, nail
biting and pencil chewing.
Typically, parafunction is manifested by
long periods of increased muscle
contraction and hyperactivity.

99

Concurrently excessive occlusal
pressure and prolonged tooth
contact occur, which is
inconsistent with the normal
chewing cycle.

Over a protracted period this can
result in excessive wear,
widening of the periodontal
ligament (PDL), and mobility,
migration, or fracture of the
teeth.

100

Muscle dysfunction such as myospasms,
myositis, myalgia and referred pain
(headaches) from trigger point
tenderness may also occur.
The two most common forms of
parafunctional activities are bruxism and
clenching. Increased radiographic bone
density is often seen in patients with a
history of sustained parafunctional
activity.

101

BRUXISM.

Sustained grinding, rubbing
together, or gnashing of the teeth
with greater-than-normal chewing
force is known as bruxism.
This activity may be diurnal,
nocturnal, or both.
Although bruxism is initiated on a
subconscious level, nocturnal
bruxism is potentially more harmful
because the patient is not aware of it
while sleeping.

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103

It is common for wear on anterior
teeth to progress from initial faceting
on the canines to the central and
lateral incisors.
Once vertical overlap diminishes as
the result of wear, posterior wear
facets are commonly observed.
However, the chewing patterns of
normal subjects can be quite varied,
and the relationship, if any, between
altered mastication and occlusal
dysfunction is not clear.

104

CLENCHING
Clenching is defined as forceful
clamping together of the jaws in a
static relationship.
The pressure thus created can be
maintained over a considerable time
with short periods of relaxation in
between.
The etiology can be associated with
stress, anger, physical exertion, or
intense concentration on a given
task, rather than an occlusal
disorder.

105

As opposed to bruxism,
clenching does not necessarily
result in damage to the teeth
because the concentration of
pressure is directed more or
less through the long axes of
the posterior teeth without the
involvement of detrimental
lateral forces.

106

Abfractions- cervical defects at the
CEJ may result from sustained
clenching.
Also, the increased load may
result in damage to the
periodontium, temporomandibular
joints, and muscles of mastication.
Typically, the elevators will
become overdeveloped.

107

A progression of muscle
splintir myospasm, and
myositis may occur, causing
the patient to seek treatment.
As with bruxism., clenching
can be difficult to diagnose and
difficult if not impossible for
the patient to voluntarily
control.

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109

The muscles that hold move or
stabilize the mandible do so
because they receive impulses from
the central nervous system.
Mandibular motion at conscious
level results in voluntary movement
where as at subconscious level due
to stimulation of oral or muscle
receptors cause involuntary
movement.

110

Receptors in the oral mucous
membrane are stimulated by touch
pain thermal changes or pain and
pressure where as other receptors
are principally located in the
periodontal ligaments, mandibular
muscles and ligaments provide
information as to the location of
mandible in space and thus called
PROPRIOCEPTORS

111

• Impulses form oral receptors
Trigeminal nuclei
• From proprioceptors
Mesencephalic nuclei of the brain
From these 2 receptors
Cerebral cortex

112

• From the cerebral cortex
It comes though three ways
Via the thalamus to the sensoriomotor
cortex (conscious level) to produce
voluntary change in the position of the
mandible
By way of a reflex arc to the motor nuclei of
the Trigeminal nerve to cause involuntary
movement
By combination of the these two ways
through the subcortical areas as the
hypothalamus, basal ganglion.
113

In edentulous patients the periodontal
ligament are lost thus the source of control
in the positioning of the mandible are lost
thus to compensate this centric occlusion
must be in harmony with the centric
relation and meet evenly in the normal
range of functional activity and these
impulses can be generated by voluntary
thought which are transmitted through the
motor nuclei and from there to the muscle
of mastication so the mandible performs
the desired activity

114

Mastication is a programmed event
residing in a chewing centre
located in the brain stem (in the
reticular formation of the pons )
The cyclic nature of mastication
(jaw opening and closure ,tongue
protrusion and retrusion) is a
result of of action of this central
pattern generation.

115

The alteration of the chewing
pattern or character (rate, force,
duration)are related to the
consistency of the bolus of the
food.
The relatively continuous flow of
impulses through the specific
pathway form the receptors to the
CNS and back to the musculature
establishes a memory pattern for
mandibular movements.

116

Thus when natural teeth are
present a individual sub
consciously develops these
memory patterns
But these patterns are disturbed
when the teeth are lost or a new
restoration is placed with an
occlusion which is not in harmony
with mandibular movement leads
to pain ,pathosis and mental stress

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119

In an explanation of the clinical
implications of mandibular
movements, it is helpful to define
the limits of possible motion and
certain mandibular reference
positions.
Recent tests indicate that
edentulous patients can make
reproducible lateral border
movements when stabilized
baseplates are used to support
the pantograph.

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121

Fig shows an envelope of motion (maximum border
movements) in the sagittal plane as described by a
122
dentate subject.]

The tracing was made from motion
picture film when the pathway of a bead
attached to a lower cen-tral incisor was
plotted.
The tracing starts at P,which represents
the most protruded position of the
mandible with the teeth in contact.
As the mandible is moved posteriorly
while tooth contact is maintained, a dip in
the top line of the tracing occurs as the
incisal edges of the upper and lower
anterior teeth pass across one another.
123

CO (centric occlusion) is reached when the
opposing posterior teeth are maximally
intercuspated.
When the mandible is further retruded, as
most people with natural teeth can do, the
most posterior relation of the mandible to
the maxillae is depicted by CR (centric
relation).
Centric relation and the mandibular
position where centric occlusion occurs are
two reference positions that are of extreme
importance in constructing dental
124
restorations. www.indiandentalacademy.com

Single restorations are generally
constructed to be in harmony
with centric occlusion (that is,
with the mandible positioned at
CO).
Multiple restorations, and certainly
complete dentures, are so
constructed that their occlusion
will be in harmony with centric
relation (i.e., with the mandible
positioned at CR).

125

As the teeth separate, the mandible
moves to its most retruded position
from CR and the patient can continue
to open in this retruded position, with
no apparent condylar translation, to
approximately MHO (maximum hingeopening position).
Any opening beyond MHO will force
the condyles to move forward and
downward from their most posterior
position. CR-MHO represents the
posterior terminal hinge movement.

126


127

This movement is used clinically to
locate the transverse hinge axis for
mounting casts on the articulator.
The posterior terminal hinge
movement and centric relation at
the vertical level of tooth contact
coincide at CR.
This terminal hinge movement can
be made only by a conscious
effort.

128

At approximately MHO the patient can no
longer retain the mandible in the most
retruded position; and as further opening
occurs the mandible begins to move
forward with translation of the condyles
in a forward direction. Obviously,
different muscles and impulses come into
play.
At MO (maximum opening) the jaws are
separated as far as possible and the
condyles are in or near their most
anterior position relative to the
mandibular fossae.
129


130

The most forward line on the tracing, running
from MO to P, represents the pathway of the
mandible as it is moved from its most open
position upward to its most protruded position
until the teeth contact at P, which was the
starting point for tracing the envelope of
motion.
Any mandibular movement observed from the
side will fall within this envelope of motion
since it represents all extreme positions into
which the mandible can be moved. However,
few normal mandibular movements follow the
border tracings; normal mandibular
movements occur somewhere in front of the
terminal hinge movement line, CR-MHO.

131

The dotted line beginning with the teeth
in centric occlusion (at CO) and
extending downward and then upward
anterior to the path of the posterior
terminal hinge movement line (CR-MHO)
is a tracing of the masticatory cycle
viewed in the sagittal plane and
superimposed on the envelope of motion.
The arrows pointing downward indicate
the pathway of the bead attached to the
lower central incisor during the opening
part of the chewing cycle, and the arrows
pointing upward indicate the pathway
during the closing part of the cycle.
132


133

Note that the pathways occur anterior to
the line representing the terminal hinge
movement. This holds true for most
persons with natural teeth.
However, if restorations are so
constructed that centric occlusion and
centric relation coincide at CR, many of
the chewing cycles will terminate at CR.
This applies also to people whose
occlusions have been equilibrated for
centric relation. The important point to
remember is that for edentulous patients
the teeth should contact evenly
throughout the normal range of function.
134

When the patient is relaxed and the jaw
is in the resting Position, obviously the
teeth are not in contact.
Mandibular rest position normally
occurs somewhere downward and
slightly forward of CR, as indicated by
Rest,
This is defined as the habitual postural
Position of the mandible when the
patient is at ease and upright.

135

The only muscle activity required
is the minimal tonic contraction
necessary to support the mandible
against the force of gravity.
The rest Position is an important
reference in prosthodontics,
particularly for complete denture
patients, since it is a guide to
reestablishing the proper vertical
dimension of occlusion.

136


137

The envelope of motion as seen in the
frontal plane roughly resembles a
shield. Such an envelope whose
tracing was made from a motion
picture film when the pathway of a
bead attached to the lower central
incisor was plotted. The tracing begins
with the teeth in centric occlusion (at
CO).
As the mandible is moved to the right
with the opposing teeth maintaining
contact, a dip in the upper line of the
tracing is created as the upper and
lower canines pass edge to edge.
138


139

The mandibular movement is continued
as far to the right as possible.
Then the opening movement is started
and continued with the mandible in the
extreme right lateral position until
maximum opening occurs (at MO).
From MO (the position of
maximum opening) the mandible is
moved in an extreme left lateral
excursion as it is closed until the
opposing teeth make contact.

140

Then, with the opposing teeth
maintaining contact, the mandible
is moved from the extreme left
lateral position back to where the
opposing teeth again contact in
centric occlusion, CO.
The dip in the left side of the
superior border movement is made
when the upper and lower left
canines pass edge to edge.

141


142

The dotted line beginning at
approximately the middle of the
tracing and extending upward
(indicated by the upwardpointing arrows) represents the
upward component of the
masticatory cycle as the
subject chews a bolus of food
on the left side.

143

Note that the dotted line contacts the
superior border of the envelope at CO,
indicating that the opposing teeth have
penetrated the bolus and come into
contact with one another. The
masticatory cycle moves to the right
when the subject opens from centric
occlusion as indicated by the
downward dotted line (downwardpointing arrows).
In the frontal view the rest position is
located slightly downward and to the
left for this individual, as indicated by
144
Rest.


145


146

BIBLIOGRAPHY
•
•
•
•
•
•

GPT-7thedition(1999)
BOUCHERS
ROSENSTIEL
HEARTWELL
SHARRY
WEINBERG articles


147

Thank you
Leader in continuing dental education


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