muscle function & malocclusion / orthodontic courses by indian dental academy

Muscle function & malocclusion INDIAN DENTAL ACADEMY

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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


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


Physical properties of muscle : Muscle has 2 physical properties:o Elasticityo 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.


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.


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.


ISOMETRIC CONTRACTION: Isometric contraction occurs when a

muscle is simply resisting an external force without any actual shortening.

ISOTONIC CONTRACTION: Isotonic contraction occurs when there is

actual shortening of muscle.


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.


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.


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.


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.


STRETCH OR MYOTATIC REFLEX: 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.


RECIPROCAL INNERVATION AND INHIBITION 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.


ROLE OF MUSCLES IN FUNCTIONAL MANDIBULAR MOVEMENTS 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.


Muscles primarily responsible for mandibular functional movements:

1. Anterior & posterior fibers of temporalis

2. Lateral pterygoid3. Ant,post,middle

components of masseter

4. Suprahyoid5. Infrahyoid


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.


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.


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.


Normal muscle activity associated with normal jaw relationship & normal occlusion.

Electromyographic studies show even distribution of anterior,posterior, middle temporalis & deep &superficial fiber activity.


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.


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.


COMPENSATORY MUSCLE FUNCTION IN DIFFERENT MALOCCLUSIONS 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.


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.


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.


Anterior open bite associated with retained infantile swallowing habit & manifest tongue- thrusting.

The peripheral portions of the tongue do not overlie the posterior occlusal surface during rest.

Thus,postural resting position & habitual occlusion are same,with no demonstrable interocclusal clearence


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.


Compensatory mechanism:

Retained infantile swallow results in openbite.

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.


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.


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.


Compensatory muscle function in classII div 1 malocclusion: Class II div1 malocclusion involve abnormal muscle

activity.

A change in muscle function is a requisite ,expansion is a t/t objective.

With hereditary type of class II malocclusion, the teeth reflect abnormal anteroposterior jaw relationship.

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.


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.


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.


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.


Compensatory muscle function in class II div 2 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.


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.


Compensatory muscle function in classIII malocclusion: 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.


Electromyographic response of muscles: 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.


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.


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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