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1. Muscle function &
malocclusion
INDIAN DENTAL ACADEMY
Leader in continuing dental education
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2. 
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Introduction
Physical properties of muscle.
Muscle physiology
Role of muscles in functional mandibular
movements.
Compensatory muscle function:
Class-I malocclusion
Class-II malocclusion
Class-III malocclusion
Electromyographic response of muscles
Conclusion
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3. Introduction
To propel his skeleton ,
man has 639
muscles,composed of 6
billion muscle fibers.each
fiber has 1000 fibrils,which
means there are 6000
billion fibrils at work at one
time or another
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4. Physical properties of muscle :
o
Muscle has 2 physical properties:
Elasticity
Contractility
1.
ELASTICITY:

Normal relaxed muscle withstands only a certain amount of
elongation[about 6/10th of its natural length] before rupturing.

This depends on the muscle involved,the type of stress,the
individual resistance,age & possible pathologic conditions
which have produced fibrotic changes which limit the
extensibility of muscle.
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o
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5. 2. CONTRACTILITY:
 It is the ability of muscle to shorten its length
under innervational impulse.
 Muscle is 1st stimulated by electric action
potential,causing a contraction.energy for the
muscle is provided by breakdown of high
energy ATP.
 Fatigue in a muscle is produced when
lacticacid ,an energy breakdown byproduct
,collects in the tissues,lowering the pH to a
level at which the muscle can no longer
function effeciently.
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6. 
The strength of muscle contaction depends
on the number of fibers engaged in a activity
at a particular time.

Factors on which contraction depends:
Striated or smooth.
Number of fibers.
Cross section.
Frequency of discharge.
Muscle fiber length.
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7. 
ISOMETRIC CONTRACTION:
Isometric contraction occurs when a
muscle is simply resisting an external force
without any actual shortening.
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ISOTONIC CONTRACTION:
Isotonic contraction occurs when there is
actual shortening of muscle.
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8. Strength of muscle
contraction in
various mandibular
positions:
1.Open mouth
2.Postural
restingposition
3.Occlusal position
4.Overclosure
Shaded area is area
of greatest strength.
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9. Muscle physiology

ALL OR NONE LAW :
•
The intensity of contraction of any fiber is independent of strength of
the exciting stimulus, provided that stimulus is adequate.
•
Stimuli below threshold strength do not elicit response;if they are
over threshold strength a contraction of maximal intensity is
made by the muscle fiber.
•
All or none law applies when only muscle is in physiologic reacting
state.
•
Merton noticed that when muscle is fatigued,the action potential no
longer triggers all or none law.
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10. 
MUSCLE TONUS
•
Muscle tonus is a state of slight constant
tension which is a characteristic of all healthy
muscle.
•
Tonus is the basis of reflex posture.
•
It is purposive and coordinated in the
maintenance of various positions.
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11. 
RESTING LENGTH:
•
The resting length of a muscle is rather
constant & predeterminable
relationship,permitting the maintenance
of postural relations & dynamic
equilibrium by contraction of the minimal
number of fibers,consistent with any
particular moment.
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12. STRETCH OR MYOTATIC REFLEX:
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The reflex contraction of a healthy muscle which
results from a pull on its tendon is called a stretch,or
myotatic reflex.
The stimulus of stretch reflex is the stretch of the
muscle The stretch reflex when elicited ,causes
contraction of the stretched muscle.
Muscle stretch receptors are proprioceptive nerve
endings called muscle spindles.
The functional significance of the stretch reflex is that
it serves as a mechanism for upright posture or
standing.natural stretches are usually imposed on
muscles by the action of gravity.
The same stretch reflex acts in mandibular
musculature to maintain the postural rest position of
the mandible in relation to maxilla.
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13. RECIPROCAL INNERVATION AND
INHIBITION
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The inhibition of the tonus or contractility of
the muscle is brought by excitation of its
antagonist.
Without reciprocal innervation & inhibition,
the myotatic or stretch reflexes would make
flexion & extension simultaneously
antagonistic.
The reciprocal innervation helps in control of
primary mover.
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14. ROLE OF MUSCLES IN FUNCTIONAL
MANDIBULAR MOVEMENTS
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Mandible is the only
movable bone in the head
& face region.
The mandible responds to
various muscular stimuli.
The following diagram
shows muscle groups
maintaining the balance of
the head & vertebral
column.
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15. 
1.
2.
3.
4.
5.
Muscles primarily
responsible for
mandibular functional
movements:
Anterior & posterior
fibers of temporalis
Lateral pterygoid
Ant,post,middle
components of
masseter
Suprahyoid
Infrahyoid
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16. 
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During opening of mandible from teeth in
occlusion:
Primary contraction of lateral pterygoid
muscles.
Suprahyoid,infrahyoid,genioglossus,
mylohyoid &diagastric muscles stabilize the
mandible.
Temporal,masseter & medial pterygoid
muscles show relaxation.
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17. During closing of mandible:
 Bilateral activity of masseter & temporalis
muscles assisted by smaller madial pterygoid
muscles.
 More power elicited during closure.
 Lateral pterygoid through their controlled
relaxation,effect smooth &uninterrupted
closure.
 During resistance to closure,gerater activity is
generated in lateral pterygoid ,suprahyoid &
infrahyoid muscles.
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18. 
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During protrusion,lateral & medial pterygoid
contract in unison.
Retrusion is less efficient & less definite.
Retrusion is largely accomplished by
posterior fibers of temporalis muscles,with
assistance from geniohyoid,diagastric &
mylohyoid muscles.
Electromyographic research indicatse role of
deep fibers of masseter muscle in retrusion.
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19. 
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Normal muscle
activity associated
with normal jaw
relationship & normal
occlusion.
Electromyographic
studies show even
distribution of
anterior,posterior,
middle temporalis &
deep &superficial
fiber activity.
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20. 
In classII malocclusion,
mandibular retrusion &
excessive apical base
difference,middle&
posterior temporalis &
deep masseter fibers
show great magnitude of
contraction.

This adapts to &
enhances the
mandibular retrusion.
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21. 
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In classII malocclusion
withdeep overbite,the
functional retrusion
tendency is increased.
Posterior temporalis &
deep masseter activity is
dominant.
Stretch reflex is elicited
for lateral pterygoid
muscle which insert into
articular disk.This serves
to pull the disk forward as
the condyle is functionally
retruded.
Condyle may then
impinge on
retrodiscalpad.
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22. COMPENSATORY MUSCLE FUNCTION
IN DIFFERENT MALOCCLUSIONS
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Class I malocclusion:
Muscle function is usually normal in cases of class I
malocclusion.
The teeth are in balance with environmental forces.
The only exception to normal muscle activity in
classI malocclusion is open –bite problems.
The greatest share of class I openbite problems is
attributed to thumbsucking,retained infantile swallow
or visceral swallow or both.
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23. 
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INFANTILE SWALLOWING
MECHANISM:[1st 6 months]
Plunger like action is
associated with
nursing,unopposed by the
peripheral portions of the
tongue.
Associated with tongue thrust
is the anterior positioning of
the mandible.
There is concavity in the
midline of the tongue &
peripheral portions are raised.
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24. 
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MATURE SOMATIC
SWALLOW:[1 yr after]
The dorsum of tongue is
less concave and
approximates palate
during deglutition.
The tip of the tongue is
contained behind the
incisors.
Peripheral portions flow
between opposing
posterior segments.
Anterior mandibular
thrust has disappeared.
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25. 
Anterior open bite
associated with retained
infantile swallowing habit
& manifest tonguethrusting.
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The peripheral portions
of the tongue do not
overlie the posterior
occlusal surface during
rest.
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Thus,postural resting
position & habitual
occlusion are same,with
no demonstrable
interocclusal clearence
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26. 
FINGER SUCKING
MALOCCLUSION:

Bilateral narrowing of
maxillary arch may be
attributed to tongue
thrusting,lower resting tongue
posture,and excessive buccal
pressures that are a part of
infantile swallowing
mechanism.

Unilateral crossbites are the
result of a “convenience
swing”of mandible to one
side,with tooth guidance from
point of initial contact to
habitual occlusion.
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27. 
Compensatory mechanism:
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Retained infantile swallow results in openbite.
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To close off the oral cavity for normal
deglutition,either a lipseal or a tongue seal is
needed to create negative atmospheric pressure
associated with the swallowing phenomenon.

If the finger displaces the maxillary incisors labially
,the lipseal becomes more difficult & the tongue
thrust forward between the maxillary incisors to
“close-off” the oral cavity.
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28. 
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Open bite accentuates, lips become more
hypotonic as tongue force during function is
greater than opposing lip force & no longer
contact each other at rest.
Mouth breathing is aggravated with each
swallow,the lower lip cushions to lingual of
maxillary incisors & joins the tongue in
natures adaptive attempt to create the
oralseal during swallowing.
Mentalis activity greatly increases &
puckering of chin seen with each swallow.
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29. 
Lip sucking , the
cushioning of the
lower lip to the
lingual aspect of the
maxillary incisors
during both rest&
active function &
hyperactive mentalis
muscle activity
enhance
malocclusion &
prevent normal
deglutition.
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30. Compensatory muscle function in classII div 1
malocclusion:

Class II div1 malocclusion involve abnormal muscle
activity.
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A change in muscle function is a requisite ,expansion
is a t/t objective.
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With hereditary type of class II malocclusion, the teeth
reflect abnormal anteroposterior jaw relationship.
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If such a malocclusion exsists, the muscle function
adapts to this pattern as best it can in line with the
requirements of mastication, deglutition,respiration &
speech.
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31. 
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Class II div 1
relationship:
Lowered tongue posture.
Elongated functional
position.
Narrowed buccal dental
segments in maxillary
arch.
Lower lip cushioning to
lingual aspect of
maxillary incisors during
rest & active function.
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32. 
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Lower lip cushions to the lingual of the
maxillary incisors, both at postural rest &
during active function.
In lip sucking habit lower lip mass almost
constantly thrust into the excessive overjet.
The lip becomes hypertrophic.
The maxillary incisors move further
labially,weakly resisted by hypotonic upper lip
.
The lower incisors buckle as the mandibular
segment is flattened by continuosly abnormal
mentalis muscle activity.
The curve of spee increases.
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33. 
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With compensatory tongue thrust,lower
tongue position & increased buccinator
muscle activity,the maxillary arch narrows &
assumes V shape.
Abnormal muscle activity can create pseudo
class II div 1 problems even in harmonius
antero posterior jaw relationship.
In true classII div 1 to begin with morphology
&jaw relationship are abnormal and muscle
activity has accentuated the exsisting pattern.
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34. Compensatory muscle function in class II
div 2
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Activity of cheek & lip muscles is normal.
Tongue accentuates the curve of spee & the
eruption of the posterior teeth by occupying
the interocclusal space.
Due to lingual inclination of maxillary central
incisors & increased interocclusal clearence
& infraocclusion of the posterior teeth.
Functional guidance of mandible is common.
There is forced retrusion phenomenon.
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35. 
Mandible closes from postural rest position to
point of initial contact.

The lingually inclined maxillary incisors then
guide the mandible into a retruded position
during the balance of closing movement to
full occlusal contact.
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36. Compensatory muscle function in classIII
malocclusion:
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Strong hereditary pattern.
The upper lip is short not necessarily hypotonic.
The lower lip is hypertrophic & redundant & appears passive
during the deglutition cycle.
During swallowing ,there is greater activity of the upper lip.
The tongue is a potent force & lie lower in the floor of the mouth.
The maxillary arch does not have the balancing effect of tongue
mass & since the peripheral portions of the tongue are less
apparent between the occlusal surfaces,the maxillary arch is
usually narrow &the interocclusal space is very small or entirely
absent.
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37. Electromyographic response of muscles:
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Electromyography provides a method of studying
the physiologic basis of mastication, deglutition &
speech.
In classII div 1 malocclusion electrical activity
appears in the masseter muscles before the
temporal muscles.
The temporal muscle ,although more rapid in
action,is relatively weaker than the masseter muscle
in power.
According to moyers the lateral pterygoid preceeds
digastric action in mandibular depression.
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38. 
Ralston states that at present
electromyography is capable of assesing
time only ,duration,and phasic relationship of
muscle contraction,but not of measuring such
function as force ,speed of contraction, and
work produced.
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39. CONCLUSION:

Muscle function can be adaptive to morphogenetic pattern. A
change in muscle function can initiate morphologic variation in
the normal configuration of the teeth and supporting bone, or
it can enhance an already existing malocclusion.

The structural abnormality is increased by compensatory
muscle activity to the extent that a balance is reached
between pattern, environment, and physiology.

It is imperative that the orthodontist appraise muscle activity
and that he conduct his orthodontic therapy in such a manner
that the finished result reflects a balance between the
structural changes obtained and the functional forces acting
on the teeth and investing tissues at that time.
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40. Thank you
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