Mandibular movements / /certified fixed orthodontic courses by Indian dental academy

MANDIBULAR MOVEMENTS

INDIAN DENTAL ACADEMY
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There are four determinants of
mandibular movements. Two posterior,
one anterior and a neuromuscular
determinant.


POSTERIOR DETERMINANT
The TMJ and its suspensory ligaments,
centres of rotation, axes of rotation,
translation of these centres.
ANTERIOR DETERMINANT - Visible
component
The contacting areas of upper and lower
teeth, inclines of cusps and nature of
occlusion in centric relation and eccentric
movements.

NEUROMUSCULAR DETERMINANT
The role of muscle spindles,
proprioceptive engrain and
neuromuscular response to occlusal
conditions.
The two posterior determinants are
fixed. The third determinant, namely
occlusion can be modified by the
dentist to certain limits.
The fourth neuromuscular determinant
can be reflexly modified by the dentist
indirectly as he alters-the third
determinant, namely, viz. occlusion.

If we modify occlusion (viz. by
restorative procedures, occlusal
equilibration, orthodontic therapy and
extractions.) the fourth determinant,
namely , the neuromuscular
determinant will show a favourable
response by release of inhibited
movement or cessation of bruxism
The ability of dentist to modify the
occlusal contact pattern of teeth to
alter proprioceptive stimuli and muscle
function is known as occlusal
programming .

Example: A case of TMJ
dysfunction with a known
interceptive premature contact
or a slide in centric.
Now, if the dentist corrects the
occlusal discrepancy, then he is
able to alter the proprioceptive
signals received from the teeth,
which resulted in TMJ
dysfunction.

In other words, the dentist was able to
indirectly modify the neuromuscular
response (the fourth determinant) by
modifying occlusion (third determinant)
to alter proprioceptive stimulus.
These altered proprioceptive stimuli
are now able to release the inhibited
movements or spasm of the
musculature.
This ability of the newly created
occlusion (occlusal rehabilitation,
occlusal equilibration, splint, etc) to
Programme muscle function is referred
to as occlusal programming

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.

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

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.

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.

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.

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.

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.

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.

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).
I.O.W External pterygoid moves the orbiting
condyle medially and the rotating condyle
moves out. The bodily shift during laterotrusion
of working condyle is known as Bennett's shift.

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.

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

Working side
Rotating side (rotating condyle)
Ipsilateral side (ipsilateral
condyle)
Laterotrusive side (laterotrusive
condyle)
Pivoting side (pivoting condyle)

Non working/ balancing side /
Idling side
Orbiting side (orbiting condyle)
Contra lateral side (contra
lateral condyle)
Mediotrusive side
(mediotrusive condyle)
Advancing condyle / translating
condyle

Protrusive movement
Sagittal protrusive
condylar path - forward
and downward translation of
mandibular condyle.

Lateral movement
i Sagittal lateral condylar path Medial and downward
movement of the nonworking
condyle. This path is longer
and steeper than sagittal
protrusive condylar path. The
angle between them is the
Fischer angle (5 degrees).

Horizontal lateral condylar path Consists of immediate and progressive
mandibular lateral translation (ISS &
PSS).
ISS occurs when nonworking condyle
moves from centric relation straight
medially (1.0 mm).
PSS occurs during the translitory
forward movement of nonworking
condyle. It is directly proportional to the
forward movement of nonworking
condyle.

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.

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.

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.

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.

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.

The Glossary of
Occlusal Terms,
International
Academy of
Gnathology, 1979:

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.

The Glossary of Prosthodontic Terms, 6 th
edition, the Academy of Prosthodontics, 1994

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.

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

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.

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.

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.

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.

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
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
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.

The combined amount of Bennett
movement (ISS+PSS) is the
Bennett angle of the orbiting
condyle (non-working condyle).
In otherwords, B.A. Is the angle
fon-ned 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).

Hanau (1922), recommended a
formula for Bennett angle
L = h/8+12
Adjustment in articulator from
the sagittal lateral condylar path
obtained by lateral check bites.
Hobo in his studies using
electronic mandibular recording
device showed no significant
correlation between BA and
sagittal lateral condylar path.

Therefore the use of Hanau's
formula for obtaining BA
adjustment in sermadjustable
articulators like (Hanau and
Dentatus) is questionable.
New generation of articulators
such as Hanau radial shift,
Denar Mark II, Pandent,
Panahoby have ISS and PSS
adjustments.

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.

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.

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.

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.

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.

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.

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.

MUSCULATURE
Several muscles are
responsible for mandibular
movements.
These can be grouped into
the muscles of mastication
and the suprahyoid muscles.

Muscles of mastication


The former include the
temporal, the masseter, and the
medial and lateral pterygoids;
the latter are the geniohyoid,
the mylohyoid, and the
digastrics.

Lateral Pterygoid
It is a muscle which runs in a
horizontal direction.
This location make it the chief
muscle for the protraction of
mandible.
As it relaxes, the posterior fibres of
temporalis muscle pull the condyle
back to its centric position.
When it contracts it draws forward
the condyle along with the disc.

This muscle is responsible
for the initial opening of
hinge movement.
If the external pterygoids on
one side contracts and the
other remains relaxed, then
the mandible will be moved
laterally to the other side.

External ptergyoid is not a muscle used for
chewing.
It only places the mandible to open into any
position forward so that incision of food can
be made with anterior teeth by the
contraction of masseter and temporalis.
It can also place the mandible into lateral
position, so that the same muscles can
permit chewing at the molar and bicuspid
region.
It guides the mandible into lateral position
and keeps it steady when chewing take
place in lateral www.indiandentalacademy.com
position.

Functions of superior and inferior
heads of lateral pterygoid.
A.
Harmonious contraction of
both the heads of muscle
There is synchronization of superior
and inferior head during protraction
thus permitting the condyle and disc to
move forward in unison.
Simultaneous relaxation of these two
heads of the muscle permit the condyle
disc assembly to go back to centric
position.

B. Independent functions of
the two heads of muscle
The superior and inferior heads of
the muscle function as two
different muscles.
The superior head is active only
on closing.
It braces the disc ,against the
posterior slope of the eminentia .
The inferior head is active on
mouth opening.

Medial Pterygoid –
Helps in lateral positioning of mandible.
The external pterygoid moves the
condyle forwards while the internal
pterygoid on one side moves the body of
the mandible laterally to the opposite
side.
It thus contributes to Bennett movement.
Acting together it elevates the mandible.
Acting alone it draws the mandible
laterally.

Masseter
The superficial portion of
masseter elevates the
mandible.
Deep fibers of masseter run
more horizontal in direction
and they assist in retraction of
mandible.

Temporalis
Since the posterior fibres are directed
forwards and towards the ascending
ramus when they contract, they retrude
the jaw.
The middle fibers run almost vertical and
their contraction elevates the mandible.
The anterior fibers run backward and their
contraction protrudes the mandible.
When all the - fibres of temporalis contract
simultaneously they close the mandible.

Temporalis and masseter
muscles are closing muscles of
the mandible. They also retrude
the mandible and are partners
in action.
It is interesting to observe that
the temporalis is attached to the
upper part of the ascending
ramus, while masseter is
inserted down below in the
ramus.

Further, the temporalis is
inserted on the medial surface,
while the masseter is inserted
on the outer surface of the
ramus of the mandible.
As a result of this pattern of
insertion, the simultaneous
contraction of these muscles
helps to position the mandible
without unstabilising it during
function.

Temporalis and lateral pterygoid are
antagonistic in their function.
It should be noticed that there is no muscle
to oppose the action of lateral pterygold
(protraction) to retract the mandible from
behind.
There is no muscle inserted into the
posterior aspect of the condyle to retract the
condyle or the articular disc.
This function of retrusion is performed by
the temporalis muscle attached to the
coronoid process.
The simultaneous contraction of middle and
posterior fibres of temporalis assisted by
deep fibres of masseter and posterior belly
of digastric retrude the mandible.

MANDIBULAR RETRUSION
PROTOGONIST (mover
muscle)
Ternporalis + Digastric +
Deep fibres
ANTAGONIST MUSCLE
Lateral Pterygoid of masseter

MANDIBULAR PROTRUSION
PROTOGONIST
Lateral Pterygoid
ANTAGONIST
Temporalis + Digastric + Deep
fibres of masseter

HINGE CLOSURE
Opening on retrusive arc-Digastric, geniohyoid
Closure on retrusive arc --Post fibres of temporalls +
Deep fibres of masseter
exerting a backward pull
Depression Lateral --Elevation

BENNETT SHIFT
Masseter on one side with
the contraction of
pterygoids of opposite side.


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.

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.

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.

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.

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.

This movement occurs in two phases:
The lower portion co-sists 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.

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.

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
concavities of the maxillary anterior teeth.

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.
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.

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 pressoreceptors play
an important role in the development of
functional chewing cycles."

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.

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”.

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.

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.

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.

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.

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.

Therefore, it can be difficult to detect,
but it should be suspected in any
patient exhibiting abnormal tooth
wear or pain.
The prevalence of bruxism is about
10% and is less common with age."
The etiology of bruxism is often
unclear.
Some theories relate bruxism to
malocclusion, neuromuscular
disturbances, responses to emotional
distress, or a combination of these.
factors."

A study on cohort twins has
demonstrated substantial
genetic effects, the condition
has been related to sleep
disturbance and the
symptoms of bruxism are
three times more common in
smokers.

Altered mastication has been observed in
subjects who Brux and may be due to an
attempt to avoid premature occlusal
contacts. ( occlusal interferences).
There may also be a neuromuscular
attempt to "rub out" an interfering cusp.
The fulcrum effect of rubbing on
posterior interferences will create a
protrusive or laterotrusive movement that
can cause overloading of the anterior
teeth, with resultant excessive anterior
wear.

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.

The causes of bruxism are difficult to determine.

One theory states that bruxism is
performed on a subconscious reflexcontrolled level and is related to
emotional responses and occlusal
interferences.
In certain malocclusions, the
neuromuscular system exerts fine
control during chewing to avoid
particular occlusal interferences.

As the degree of muscle activity
necessary to avoid the
interferences becomes greater, an
increase in muscle tone may
result, with subsequent pain in the
hyperactive musculature, which in
turn can lead to restricted
movement.
The relationship, if any, between
bruxism and temporomandibular
disorders is still unclear."

Patients who brux can exert
considerable forces on their teeth,
and much of this may have a lateral
component.
Posterior teeth do not tolerate lateral
forces as well as vertical forces in
their long axes. Buccolingual forces,
in particular, appear to cause rapid
widening of the periodontal ligament
space and increased mobility

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.

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.

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.

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.

Parallelogram of forces.
From the standpoint of the
prosthodontist, the skull presents some
interesting facts that need to be taken
into consideration.
The factor of muscle pull in relation to the
direction and strength of each muscle
used in positioning the mandible after the
loss of teeth is an important
consideration.
The parallelogram of forces can be
studied only in relation to the entire skull.

The direction of these forces has
much to do with the seating or
unseating of dentures. The
occlusal vertical dimension
affects this direction of forces, a
fact that makes positioning of
the mandible after the loss of
teeth so important

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.

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

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
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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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
Rest.

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


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