muscle physiology & its significance

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MUSCLE PHYSIOLOGY AND MUSCLE SIGNIFICANCE IN ORTHODONTICS INTRODUCTION Development of muscle and muscle changes during growth Muscle physiology and methods of studying muscle activity ORO facial muscles - Facial muscle - Jaw muscle - Portal muscles - -Basic concepts of neuromuscular system -- -Role of muscle in functional jaw orthopedics Role of muscle in temporo mandibular dysfunctions Role of muscle in malocclusion: 1. Functional slides into occlusion due to occlusal interferences 2. Detrimental sucking habits 3. Abnormal patterns 4. Incompetent normal reflexes (lip posture)

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Page 1: Muscle Physiology & Its Significance

MUSCLE PHYSIOLOGY AND MUSCLE SIGNIFICANCE IN

ORTHODONTICS

INTRODUCTION

Development of muscle and muscle changes during growth

Muscle physiology and methods of studying muscle activity

ORO facial muscles

- Facial muscle

- Jaw muscle

- Portal muscles

- -Basic concepts of neuromuscular system

-- -Role of muscle in functional jaw orthopedics

Role of muscle in temporo mandibular dysfunctions

Role of muscle in malocclusion:

1. Functional slides into occlusion due to occlusal interferences

2. Detrimental sucking habits

3. Abnormal patterns

4. Incompetent normal reflexes (lip posture)

5. Abnormal muscle contractions

Role of muscle in orthognathic surgery

Role of muscle in retention and relapse

Page 2: Muscle Physiology & Its Significance

In Orthodontics it is necessary to view the orofacial musculature in a different

context to understand its effects on growth of the face and the effects of

malfunction of jaws and facial structure on muscle activity.

DEVELOPMENT OF MUSCLE AND MUSCLE CHANGES DURING GROWTH

Prenatal muscles grow by increase in size and amount of fibrous tissue

surrounding the muscle bundles as well as by cell division. Striped muscle

differentiation begins in the 7th week of intrauterine life and typical muscle fibers

are seen in the 22nd week. Normal muscular activity begins at the end of the 7 th

month and is not complete in the extremities until after birth. Muscles of

mastication at first develop in relation to Meckel’s cartilage but are independent

of the insertions and are attached only to the forming mandible. Increase in bulk

of a muscle is due to activity. Atrophy results from disease. During infancy and

childhood, gain in muscle tissue is essentially the result of hypertrophy.

Between the 4th fetal month and birth the muscular system increases by 50 fold.

It increases 40 fold between birth and middle of the 3rd decade of post natal life.

POST NATAL GROWTH: Muscle growth is rapid in infancy and childhood,

slower and regular in the middle of childhood and again more rapid preceding

and during adolescence. The muscles of the head show the smallest relative

increment of growth. The weight of the facial musculature increases 4 fold

between birth and age 20 years, which that of the mandible alone increases

almost 7-fold by age 20 years.

MUSCLE CHANGES DURING GROWTH

Friel (1926) has shown correlation between the growth of the muscles of

mastication and development of the dentition. The muscles develop most

rapidly after puberty when the deciduous teeth are replaced by permanent

Page 3: Muscle Physiology & Its Significance

dentition. A general correlation exists between growth of the muscles of

mastication, development of the dentition and strength of the mandible. Growing

bone is susceptible to deformation resulting from muscle forces acting upon it

provided these forces are strong and continuous enough to overcome its

inherent growth vector. Abnormal force during growth period can produce

abnormal form.

Since deformity can be produced by pressure, removal or paralysis

of muscles. Deformities are the result of change in direction of growth of the

developing skulls.

Continued adjustments in muscle attachments occur during skeletal growth.

Muscles can be divided into two groups with respect to their attachments.

1. Periosteal the fibrous layer of the periosteum

2. Tendinous – a tendon which cannot be removed from the bones without

some destruction of the surface of the bone.

The first group can shift its attachments by growth changes of the periosteum.

Different rates of lengthening at different regions allow the periosteum to shift

relative to the bone, carrying the muscle attachments with it thus maintaining the

constant spatial relationship of the muscles.

In the second type of muscle attachments a mechanism exists to break down or

alter the attachment so that the muscles may shift. In muscles attached by

tendons the change is made by bone resorption and apposition, which carries the

tendinous attachments with it. The insertion of the suprahyoid and external

Pterygoid muscles into the mandible belong to the second group and to certain

extent also the internal Pterygoid and temporal muscles since their insertions or

partly tendinous.

Where bone resorption is found in relations to the tendinous attachment of a

muscle, resorption frees the muscle from the bone. Muscles can become

temporarily periosteal in attachment and can shift relative to bone growth

Page 4: Muscle Physiology & Its Significance

maintaining their normal position. This is particularly true of muscles attached at

the growing ends of the mandible. When bone resorption causes, the muscles

may become reattached directly to the bone by tendinous fibers.

Growth at the anterior end of each half of the mandible until the symphyseal

suture is obliterated in the latter part of the 1st year. Gradually tends to separate

the anterior belly of the diagnostic and the geriohyoid muscles.

The tendinous insertion of the temporal muscle is gradually fixed from the bone

of the anterior border of the ramus of the mandible which is resorbed to make

room for permanent molar eruption and the development of the alveolar process

around these teeth.

The attachment of internal Pterygoid shifts during the growth of the mandible and

expands as the ramus increases in size by bone deposition along its posterior

border.

MICROSCOPIC ANATOMY OF SKELETAL MUSCLE

Microscopic examination of a typical skeletal muscle reveals hundreds or

thousands of very long cylindrical cells called muscle fibers or myofibers. The

muscle fibers lie parallel to one another and range from 10 to 100 µm in

diameter. The plasma membrane of a muscle cell is termed the sarcolemma and

it surrounds the muscle fibers cytoplasm which is called sarcoplasm. The nuclei

are at the periphery of the cell next to the sarcolemma conveniently out of the

way of contractive elements. The mitochondria lie in rows throughout the muscle

fiber strategically close to muscle proteins that use ATP during contraction. At

high magnification the sarcoplasm appears stuffed with little threads. These small

structures are the myofibrils. Although the myofibrils extend lengthwise within

the muscle fiber their prominent alternating light and dark bands make the whole

muscle cell look striated or striped. The bands are called cross striations.

MYOFIBRILS

Myofibrils are the contractive elements of skeletal muscle. They are 1 – 2 um in

diameter and contain three types of small structures called filaments. The

filaments are thick filament (16 mm), thin filament (8 mm) and elastic filament.

Page 5: Muscle Physiology & Its Significance

Depending or whether the muscle is contracting or relaxing the thick and thin

filaments overlap one another to a greater extent.

The filaments inside a myofibril do not extend the entire length of the muscle

fiber. They are arranged in compartments called Sarcomeres which are the

basic functional units of striated muscle fiber. Narrow plate shaped regions of

dense material called Z discs separate one Sarcomere from the next. Within

each Sarcomere is a darker area called the A band. It consists mostly of thick

filaments and includes portions of the thin filaments where they overlap the thick

filaments. A lighter, less dense area called the I bands contains the rest of thin

filaments but no thick filaments .The Z disc passess through the center of each I

band. The alternatively darker A bands and lighter I bands give the muscle fiber

its striated appearance. A narrow H zone in the center of the A band contains

thick but not thin filament. Dividing the H zone is the M line formed by a protein

molecule that connect adjacent thick filaments.

(Z – ½ I + A + ½ I) (H = Mid A (thick region) (M = line in mid of H zone)

The two contractive proteins in muscle are myosin and actin. About 200

molecules of the protein myosin form a single thick filament. Each myosin

molecule is shaped like two golf clubs twisted together. The myosin tails point

towards the M line in the center of the Sarcomere. The projections called myosin

heads or cross bridges extend out towards the thin filaments. Tails of

neighboring myosin molecules lie parallel to one another forming the shaft of the

thick filament. The heads project from all around the shaft in a spiraling fashion.

The filaments extend from anchoring points within the Z discs. Their main

component is actin. Also present in the think filament are smaller amounts of two

regulatory proteins Tropomyosin and troponin. Individual actin molecules have an

irregular shape. They join to form an action filament that is twisted into a helix.

On each actin molecule is a myosin binding site, a location where a myosin head

(cross bridge) can attach. In relaxed muscle Tropomyosin causes the myosin

binding sites on actin and thus blocks attachment of myosin heads to actin.

The elastic filament, component of Sarcomere is composed of the protein titin

(connectin) the third most plentiful protein in skeletal muscle (after actin and

Page 6: Muscle Physiology & Its Significance

myosin) Titin anchors thick filaments to Z discs and thereby helps stabilize the

position of thick filaments. It may also play a role in recovery of the resting

Sarcomere length when a muscle is stretched or during relaxation.

SARCOPLASMIC RETICULUM AND TRANSVERSE TUBULES

A fluid filled system of cisterns called the sarcoplasmic reticulum encircles

each myofibril. In a relaxed muscle fiber the sarcoplasmic reticulum stores ca++.

Release of ca++ from the sarcoplasmic reticulum into the sarcoplasm around the

thick and thin filaments triggers muscle contraction. The calcium ions leave the

sarcoplasmic reticulum through channels in its membrane called ca++ release

channels.

The transverse tubules (T tubules) are tunnel like infoldings of the sarcolemma..

They penetrate towards the center of the muscle fiber at right angles to the

myofilaments. There are two transverse tubules in each Sarcomere. One at

each A-I band junction T tubules are open to the outside of the fiber and are filled

with extra cellular fluid .On both sides of a transverse tubule are dilated end sacs

of the sarcoplasmic reticulum called terminal cisterns. The tern triad refers to a

transverse tubule and the terminal cisterns on either side of it.

CONTRACTION OF THE MUSCLE

In the mid 1950s Jean Hauson and High Huxley had a revolutionary

insight into the mechanism of muscle contraction. Previously scientists had

imagined that muscle contraction must be a folding process, some what like

closing an accordion. Hauson and Huxley proposed, however that skeletal

muscle shortens during contraction because the thick and thin filaments slide

past one another. Their model is known as sliding filament mechanism of muscle

contractions.

SLIDING FIMALENT MECHANISM

During muscle contraction, myosin heads pulls in the thin filaments, causing

them to slide increased the H zone at the center of Sarcomere. The myosin

cross bridges may even pull the thin filaments of each Sarcomere so far inward

that their ends overlap in the centre of the Sarcomere. As the thin filament slide

inward, the Z discs come toward each other, and the Sarcomere shortens but the

Page 7: Muscle Physiology & Its Significance

lengths of thick and thin filaments do not change. The sliding of the filaments

and shortening of the Sarcomeres cause shortening of the whole muscle fiber

and ultimately the entire muscle.

ROLE OF CA++ AND REGULAR PROTEIN

The sliding filament model explains the mechanism of contraction, but what starts

and stops sliding of the filaments? As increase in ca++ concentration in the

sarcoplasm starts filament sliding while a decrease turn off the sliding process.

When a muscle fiber is relaxed, the concentration of Ca++ in its sarcoplasm is

low. This is because the SR membrane contains ca++ active transport pumps

that move ca++ from the sarcoplasm into the SR. ca++ is stored inside the SR.

As the muscle action potential travels along the sarcolemma and into the

transverse tubule system, however ca++ release channels open in the SR

membrane. As a result ca++ floods into the sarcoplasm around thick and thin

filaments. The ca++ released from the SR combine with troponin, causing it to

change shape. This shape moves the troponin and Tropomyosin complex away

from myosin binding sites on action.

THE POWER STROKE AND ROLE OF ATP

As we have seen, muscle contraction requires ca++. It also requires energy, in

the form of ATP.

1. While the muscle is relaxed, ATP attaches to ATP binding sites on the

myosin cross bridges (heads) A portion of each myosin head acts as

on ATPASE, enzyme that splits the ATP into ADP (phosphate group)

through a hydrolysis reaction. This reaction transfers energy from ATP

to the myosin head, even before contraction begins. The myosin cross

bridges are thus in an activated (energized state)

2. When the SR release CA++ and CA++ level rises in the sarcoplasm,

Tropomyosin moves away from its blocking position.

3. The activated myosin heads spontaneously bind to the myosin binding

sites on action.

Page 8: Muscle Physiology & Its Significance

4. The shape change that occurs as myosin heads binds to action

produces the power stroke of contraction. During the power stroke the

myosin heads swivel toward the centre of the Sarcomere, like the Oars

of a boat during rowing. This action draws the thin filament past the

thick filaments towards the H zone as the myosin heads swivel, they

release ADP.

5. Once the power stroke is complete, ATP again combines with the ATP

binding sites on the myosin heads. As ATP binds the myosin head

detaches from actin

6. Again the myosin ATPASE splits ATP, transferring its energy to the

myosin head, which returns to its original upright position.

7. The myosin head is then ready to combine with another myosin

binding site further along the thin filaments.

The cycle of steps 3 through 7 repeats over and over as long as ATP is

available and the ca++ level near the thin filaments is high. The myosin

heads keep rotating back and forth with each power stroke, pulling the thin

filaments towards the H zone. At any one instant about ½ of the myosin

heads are bound to actin and are swiveling. The other half are detached and

preparing to swivel again. Contraction is analogues to running on a non

motorized tread mill. One foot (myosin head) strikes the belt (thin filament)

and pushes it backward outward the H zone. Then the other foot comes down

and imparts a second push. The belt soon moves smoothly while the runner

(thick filament) remains stationary. And like the legs of a runner, the myosin

heads need a constant supply of energy to keep going!

This continual movement of myosin head applied the force that draws the

Z discs towards each other, and the Sarcomere shortens. The myofibrils thus

contract, and the whole muscle fiber, shortens. During a maxillary muscle

contraction the distance between Z discs can decrease to ½ the resting

length. But the power stroke does not always result in shortening of the

Page 9: Muscle Physiology & Its Significance

muscle fibers and the whole muscle. Contraction without shortening is called

on isometric contraction. For example, trying to lift a heavy object the myosin

heads (cross bridges) swivel and generate force, but the thin filaments do not

slide inward.

FACIAL MUSCLES

Primary function of facial muscles is expression of emotions. The capacity for

expressing effective states is highest developed in the human.

COLEMAN – contents the human is capable of 7000 possible facial expressions.

In addition to expression of emotions, these muscles are important in

maintenance of posture of facial muscles. According to Proffit the lip and

buccinator muscles opposed by the tongue contribute to a postural equilibrium of

the teeth. The facial muscles also contribute to stabilization of the mandible

during infantile swallowing and in chewing and swallowing in the edentulous and

occlusally compromised adult. It is quite possible that postural alternations in

the facial muscles may contribute to structural changes in the jaws.

Frankel has speculated that the buccinator muscles exert a constraining force on

the maxillary alveolar process as well as the teeth. Form also dictates function:

patients with short upper lips or excessively proclined maxillary incisors

compensate by elevation of the lower lip through action of the mentalis muscle,

to establish an anterior seal during swallowing. Facial muscles also play an

important role in both visual and spoken communication. Lips and cheeks are

essential as well for bolus control in mastication.

JAW MUSCLES

Jaw muscles are often designated as elevators and depressors or

protractors and retractors but this classification of muscles acting as synergist or

antagonists can be handicap to a better understanding of their roles in posture

and jaw muscle synergies.

The simplest concept of neural control of mandibular posture is of the mandible

maintained against gravity by the stretch reflex in mandibular elevators. EMG

studies of postural position have shown that inframandibular group of muscles

are more active than the elevator muscles.

Page 10: Muscle Physiology & Its Significance

While the mandible is capable of a pure rotational movement early in opening

and late in closure, studies have shown that normal opening and closing are

never pure rotational. The actual rotational centers are closer to the mandibular

ramus and shift during opening and closing. This means that the changes the

environment of both the mandible and maxilla and alters the way they grow. The

long face syndrome which Linder arson associates with mouth breathing is a

good example.

PORTAL MUSCLES

The term portal muscles were coined by “BOSMA” to denote the upper

alimentary and respiratory tracks. The muscles of portal area serve the multiple

functions of posture, respiration and feeding. Postal muscles include the

muscles of tongue, soft palate; the pharyngeal pillars the pharynx proper and

larynx. It is the crossing of these tracts in the pharynx that requires special

reflex controls for maintenance and protraction. The two portal reflexes of

greatest significance to orthodontics are pharyngeal airway maintenance and

swallowing.

Basic concepts of orofacial neuromuscular physiology

Active and Passive tension

When a muscle is stretched the tension in that muscle increases. This

increase in tension may be the result of reflex contraction of the muscle. If the

muscle contains sensory organs called muscle spindles the elongation of the

spindle excites the spindle afferents, which synapse on molar neurons

innervating the gross muscle resulting in their contraction. This is the classic

stretch reflex. The spindle afferents can be segregated into at least two types

(nuclear bag or primary and nuclear chain or secondary endings) which are

preferentially sensitive to a sudden stretch or a prolonged stretch. Since all the

mandibular elevator muscles possess spindles this mechanism can occur

opening and closing of the mandible can no longer be conceived as largely an

interplay between elevators and depressors. Mandibular movement is more

Page 11: Muscle Physiology & Its Significance

accurately perceived as that of free body manipulated in an intrinsic muscular

web with the teeth and joints acting as stops and guides. If it were not this way,

patients with bilateral condylectomies can never chew.

In this interactive web muscles serve various functions movements for the two

heads of lateral Pterygoid muscle at rest and open positions completed about the

instantaneous centers of rotations of the mandible. At postural position the

superior head has a closing movement while the inferior head has an opening

moment. At the open position both heads have opening normal.

The tension resulting from contraction of muscle tissue is called active tension.

Since the facial muscles possess no spindles stretching of facial muscle will not

elicit a stretch reflex. Nevertheless the tension in these muscles will increase

with elongation because of the elastic properties of muscle and its investing

tissues. Tension which result from the physical properties of the tissue is called

passive tension.

In many muscles elongation will result in an increase in both active and passive

tension. The sum of both tensions is appropriately called total tension. Below a

specific level all the tensions of the muscle are zero. As the muscle is stretched

the active tension increases. In a muscle containing spindles this has been

attributed to the stretch reflex. Initially there is no passive tension so that total

tension is equal to active tension. As the stretching increases the muscle begins

to behave elastically. Passive tension now adds to total tension. As the muscle

is elongated further active tension is inhibited while passive tension continues to

increase. On further stretching active tension is suppressed or passive tension

rises exponentially. At this length total tension is the same as passive tension

The active tension may be due in part to extent to which the actin and myosin

filaments overlap. The generation of active tension falls off if the overlap is

excessive or inadequate. The decline in active tension with increasing muscle

length is due to initiate inhibitory reflexes suppressing the contraction brought

about by excitation of spindle afferents.

Many procedures result in elongation of jaw and facial muscles. Expansion of

dental arches stretches the cheeks or lips and increases tension in the

Page 12: Muscle Physiology & Its Significance

buccinator and orbicularis oris muscle. Increasing the vertical dimension is

closed – bite malocclusions will stretch in the elevator muscles. Appliances such

as bite planes and activators which increase vertical dimension and / or advance

the mandibles increase tension in both elevator and retractor muscles. Habits

such as mouth breathing which increase the postural vertical dimension increase

tension in elevator muscle.

The contribution of active tension to total tension in facial muscles would be

different from that in jaw muscles since facial muscles contain no spindles.

Characterization of the active and passive tension curves in lips and cheeks

might be indicative of the extent to which expansion could be used in treatment.

Role of the muscle in functional jaw orthopedics:Ever since Andersen and Haupl introduced Functional Jaw Orthopedics

(activator) in 1936, diverse views have been presented regarding the

neuromuscular responses brought about with activator treatment.

Andersen and Haupl claimed that the activator, which stimulates the

protractor muscles and inhibits the retractor muscle of the mandible, produces

myotactic reflexes leading to isometric contractions from the activities of the jaw

closing muscles.

Petrovic in his study of the condylar cartilage came to similar conclusion that

functional requirement for condylar growth stimulation is activation of lateral

Pterygoid muscle (LPMs)

Eschler supported Andersen and Haupl, but claimed that the retractor muscles are

stimulated, not inhibited by the activator. He attributed the muscle contraction to

proprioceptive stretch reflexes and observed the occurrence of both isotonic and

isometric contraction with use of the activator. He described the cycle as at insertion

of the appliance the mandible is elevated by isotonic muscle contractions the

mandible assumes a mucostatic position in contact with appliance, isometric

contractions arise.

According to Woodside, a stretch of the soft tissues primarily requires dislocating

the mandible anteriorly or opening beyond the postural rest vertical dimensions.

Page 13: Muscle Physiology & Its Significance

Between two extremes exemplified by Andersen and Haupl versus Selmer Olsen,

Witts supported a combination of isometric muscle contractions and viscoelastic

properties being responsible for the forces delivered by the activator.

Ahlgren’s electromyography research (1970) shows that activator function as

interference in producing new contraction patterns in jaw muscles. The innervation’s

pattern can be adjusted after a while and the mandible repositioned forward. He

reported that during day time wear of an activator, there was an increased postural

activity in Masseter and suprahyoid muscles but not in the temporalis

Role of LPM

Lateral Pterygoid traction regulates the growth of the mandibular condyle. Lateral

Pterygoid traction on the head of condyle seems to produce increased &

proliferation in the pre chondroblastic layer of condylar cartilage. Petrovic suggested

repeated modulation of condylar growth by lateral Pterygoid activity constitutes on

important element in a feed back mechanism that serves to maintain a stable

occlusion in the face of varying rates of maxillary growth.

Temporal Muscle activity

EMG Recording shows temporal muscle activity in the rest position

constant during 1st year of activator treatment. So, in Maximal bite (in inter Cuspal

position), the temporal muscle activity is decreased with large protrusions in the

construction bite. The decrease was considered to be an effect of occlusal instability

brought about by the activator treatment. There is no evidence of decrease in

postural (rest) activity of the posterior temporal muscle during treatment, although

such decrease has been described as a sign of forward displacement of the

mandible.

McNamara noted decreased postural activity of the posterior temporal muscle but

increased activity of the lateral Pterygoid muscle. This is so called “Pterygoid

response” was thought to lead to a forward repositioning of the mandible.

Page 14: Muscle Physiology & Its Significance

Role of Lateral Pterygoid muscle and Meinsco Temperomandibular Frenum:

Study in young rat of J.J.Stuzmann and A.G. Petrovic, shows the role of LPM, TMF

control the growth rate, growth direction and growth amount of cartilage. Four line

of evidence, that the lateral Pterygoid muscle (LPM) plays a role in this physiologic

control of the condylar cartilage growth rate.

1. After surgical resection of LPM, relative decrease in the growth of the

condylar cartilage seen either in treated or untreated growing rate with functional

appliance.

2. EMG record of LPM in monkey treated with a functional appearance shows

increased electrical activity.

3. Micro electronic stimulation of LPM in young rats produces an increased

rate of condylar cartilage growth.

After treatment with hyperpropulsor, significant increase in proportion of fast non

fatigable fibers in young rats LPM is seen and a significant decrease in the number

of serial sarcomeres in the same muscle.

Harvold (1974) and wood side (1973) do not accept the theory that myotactic reflex

activity with isometric muscle contraction induces skeletal adaptation. According to

their views, the viscoelastic properties of muscle and stretching of soft tissues are

decisive for activator action.

During each application of force, secondary forces are in the tissues, introducing bio

elastic process, thus not only the muscle contractions but also the viscoelastic

properties of the soft tissues are important in stimulating the skeletal adaptations.

According to him, eliciting a stretch of soft tissues primarily requires dislocating the

mandible anteriorly or opening beyond the postural rest vertical dimensions

Page 15: Muscle Physiology & Its Significance

MECHANISM OF CLASP KNIFE REFLEX ON AUTOGENIC INHIBITION: Muscle

first resists, then relaxes. This resembles that of a spring – loaded folding knife

blade this phenomenon is called the “clasp – knife” reaction. The excessive or rapid

stretch of the muscle brings in to play some inference that annuls the stretch reflex

and allows the muscle to be lengthened with little or no tonic resistance. Thus the

stimulus necessary to elicit the clasp knife reflex is excessive stretch and when

elicited it inhibits muscular contraction, thus causing the muscle to relax. The

receptors for the clasp knife reflex are the Golgi tendon organs located in the

tendon of the muscle. The impulses are conducted by group 1B sensory nerve

fibers the impulses act on the motor neuron of alpha efferent supplying the

stretched muscle. However it is a disynaptic reflex arc because an interneuron is

interposed between the sensory neuron and the motor neuron. It follows during the

muscle stretch; the motor neurons supplying the stretched muscles are bombarded

by impulses delivered over two competing pathways one facilitating and other

inhibiting muscle contraction. The output of the motor neuron poll depends upon

the balance between the two antagonists inputs. The functional significance of the

clasp knife reflex is to protect the over load by preventing damaging contraction

against strong stretching forces. The proponent of this concept content that these of

myotactic reflex along with attempts to increase the frequency of biting and

swallowing should be largely ignored, letting passive tension (viscoelastic

properties) in the stretched labial and oral musculature deliver the primary force of

the appliance thus, the power to produce alveolar remodeling is obtained from the

inherent elasticity of muscle, tendinous tissues and skin without motor stimulation

muscle spindles have not been clearly demonstrated in the labial muscles and

therefore there seems to be no mechanism for turning off reflex muscle activity

through a modification of the myotactic reflex. Thus, more these muscle are

stretched, greater is the force delivered to the activator. The forces generated by

this extreme bite registration (10-15 mm) represent combination of forces generated

by swallowing, biting, activation of the myotactic reflex in the stretched muscles of

mastication and the power delivered through the viscoelastic properties of stretched

muscles, tendon tissue, Skin and musculature.

Page 16: Muscle Physiology & Its Significance

The reason that the bite registered for 3mm to 4 mm distal to the most protruded

position is to avoid the possibility of initiating Golgi tendon organ activity and thus

eliminate any undesirable myotactic reflex.

Twin block

The clinical responses are observed after fitting twin blocks, is closely analogous to

the changes observed and reported in animal experiments using fixed inclined

planes by McNamara.

With in few days of fitting the appliances, immediate change in the neuromuscular

proprioceptive response is seen provided that all phasic and tonic muscle activity is

affected. This results in position of muscle balance, which is altered, so that it

becomes painful for the patient to retract the mandible. This has been described as

the “Pterygoid response” by McNamara or the formation of a “tension zone” distal to

condyle by Harvold. This kind of response is rare in other functional appliances that

are not worn full time. The rapid clinical response confirms the adaptive response in

functional protrusion experiments with fixed inclined planes by McNamara.

Structural alterations are more gradual and are measured in months,

whereby dento alveolar skeletal structures adapt to restore a functional equilibrium

to support the altered position of muscle balance.

Role of Lateral Pterygoid muscle

The position of the mandible did not change significantly after fatiguing the

protrusive muscle. Authors agreed that change in muscle activity diminished shortly

after appliance insertion and before correction of the jaw relationship was achieved.

Morphologic change in jaw relationship appeared that the lateral Pterygoid muscle

might not be responsible for the new position of the mandible after treatment with

twin block appliance. The Temperomandibular joint adapted to displacement of the

mandible by condylar growth and surface apposition in the fossa

Growth Relativity hypothesis

Page 17: Muscle Physiology & Its Significance

Growth relativity refers to, growth that is relative to the displaced condyle from

actively relocating fossae. Viscoelasticity is conventionally applied to elastic tissue

primarily muscles i.e. non calcified tissues, specifically addresses to the viscosity

and flow of the synovial fluids, the elasticity of the retrodiskal tissues, the fibrous

capsule and other nonmuscular tissues including LPM perimysium, TMJ tendons

and ligaments, other soft tissues and bodily fluids.

Wolff’s law states that bone architecture is influenced by neuro musculature.

This law may now be extended for orthopedically displaced condyle. With

orthopedic advancement of the mandible, the law of growth relativity states

that bone architecture is influenced by the neuromusculature and the

contiguous, nonmuscular, viscoelastic tissues anchored to the glenoid fossa and

the altered dynamics of the fluids enveloping bone.

Electromyogram activity

Insertion of the twin block appliance in the mouth cause a change in the EMG

pattern of both the Masseter and anterior temporalis during 6 months observation

period

EMG Shows increase in postural and maximum clenching in Masseter, whereas

during the act of swallowing there was no change in EMG activity. The increased

postural activity of the Masseter is explained as a balancing contraction as a result

of the protrusion of the mandible imposed by the Twin block. These findings are in

confirmation with the anatomic function of the Masseter, which plays a dominant

role in elevation when the mandible is protracted.

Muscle activity during maximal voluntary clenching immediately on insertion of the

Twin block appliance in the mouth was lower in both anterior temporalis and

Masseter than without the appliance. This can be accounted for by the fact that

when the muscle is lengthened and isometrically contracted, the EMG activity falls,

although the tension is greater. This is in accordance with the active muscle activity

in the isometric length –tension curve.

Page 18: Muscle Physiology & Its Significance

This can also be interpreted as an effect of reciprocal innervations, temporalis

muscle being an antagonistic muscle to a protrusive movement of the mandible,

which agrees with the results of Ahlgren who reported a decrease in electrical

activity during biting contractions. From study increase in EMG activity of Masseter

and numeric increase of anterior temporalis deduce that active contraction plays a

more important role in treatment with Twin block than passive tension associated

with viscoelastic properties of soft tissues unlike the activator. This increase in

postural EMG activity may reflect an adaptation to a new mandibular position during

active phase of treatment with twin block.

Philosophy of the Frankel appliance

A major tenet of Frankel philosophy is that dentition are heavily influenced by

functional matrix; buccinator mechanism and the orbicularis oris complex.

Compared to other removable functional appliances, the FR is largely confined to

the oral vestibule, and holds away the buccal and labial musculature from the

dentition in those areas in which pressure on the dentoalveolar structures has

restricted the outward development of these structures during the critical

transitional phase of dental development.

Frankel conceives the vestibular constructions as an artificial “ought to be” matrix that

allows the muscles to exercise and adapt. Thus the fundamental phenomenon of

physiology is adaptation or homeostasis. Concurrent muscle adaptation to the new

position through the exercise role of the appliance enhances the stability of the result.

Mode of action of the Frankel appliance

A) FR is not a tooth-moving appliance (i.e. FR is a tissue borne appliance)

B) FR withholds muscle pressure from the developing jaws and surrounding area

having its arena of operation largely in the vestibule surrounding the alveolar bone.

C) Changes with FR in transverse dimensions is achieved by relief of force from the

neuromuscular capsule (the buccinator mechanism)

D) Changes with FR in sagittal posturing is an entirely tissue borne manner.

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ANALYSIS OF TEMPEROMANDIBULAR JOINT DYSFUNCTION

Each muscle involved in mandibular movements should be routinely palpated at

rest and in isometric contractions in an attempt to reduce reflex responses to

pain, often, Unbeknownst to the patient, muscles or parts of the muscles are

painful upon palpation. The Masseter, lateral Pterygoid and temporalis are those

which most frequently demonstrate myalgia in patients with temporo mandibular

dysfunctions associated with malocclusion.

Patients with TMD symptoms can be divided into two large groups

1. Those with general joint pathology including displacement of

destruction of the intra articular disc

2. Those with symptoms primarily of muscle origin caused by spasm

and fatigue of the muscles that position the jaw and head.

Myo functional pain develops when muscles are over fatigued and tend to go into

spasm, it is all but impossible to overwork the jaw muscle to this extent during

normal occlusal function. To produce myo facial pain the patient must be grinding

or clenching the teeth for many hours per day presumably or a response to

stress and interval joint pathology or an occlusal discrepancy.

ROLE OF MUSCLE IN MALOCCLUSION

DETERMINENTAL SUCKING HABITS

All habits are learned patterns of muscle contraction of a very complex nature.

Certain habits serve as a stimuli to normal growth of the jaws for example normal

lip action and mastication. Abnormal habits which may interfere with the regular

pattern of facial growth must be differentiated from the desired normal habits that

are a part of normal oropharyngeal function and thus play an important role in the

craniofacial growth and occlusal physiology. Detorius habitual patterns of

muscular behaviouring often are associated with prevented or impeded osseous

growth, tooth malpositions disturbed breathing habits, difficulty in speech, upset

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balance in the facial musculature and physiological problems. Malocclusion

cannot be corrected without involvement in such reflex activity.

It has been suggested that thumb sucking in one of the earliest examples of the

neuromuscular learning in the infant and that it follows all the laws of the learning

problems.

For us the most important question is , does digital sucking cause malocclusion?

Many children who practice digital sucking habits have no evidence of

malocclusion, however popovietch and Thompson have reported a high

association of abnormal sucking habits in the malocclusion sample. They found

three distinctively different patterns of force application during sucking, all

utilizing forces sufficiently strong to displace teeth or deform growth bone.

Melsen et al found the both digital sucking and pacifier sucking increased the

tendency toward abnormal swallowing. Sucking habits were related to an

increase in severe malocclusion symptoms apart from the type of swallow

presented. Sucking habits were strongly correlated with disto occlusion and

open bite and with cross bite and maxillary over jet.

It should be remembered that the type of malocclusion that may develop in the

thumb sucker depend on a number of variables the position of the digit,

associated orofacial muscle contractions, the position of mandible during

sucking, the facial skeletal morphology, duration of sucking and so forth. An

anterior open bite is the most frequent malocclusion. Protraction of the anterior

maxillary teeth will be seen, particularly if the hand is held upward against the

palate. Mandibular postural retraction may develop if the weight of the hand or

arm continually forces the mandible to assure a retruded position in order to

practice the habit. Consistently the mandibular incisors may be tipped lingually.

When the maxillary incisors have been tipped labially an open bite has

developed, it becomes necessary for the tongue to thrust forward during

swallowing. During thumb sucking buccal wall contractions produce in some

sucking patterns a negative pressure within the mouth with resultant narrowing of

the maxillary arch. Within upset in the force system in an around maxillary

complex, it goes in impossible for the nasal floor to drop vertically to its expected

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position during growth. Therefore thumb suckers may be found to have a

narrower nasal floor and a high palatal vault. The maxillary lip becomes

hypotonic and the mandibular lip becomes hyperactive, since it must be elevated

by contractions of the orbicularis muscle to a position between the malposed

incisors during swallowing. These abnormal muscle contractions during sucking

and swallowing stabilize the deformation.

ABNORMAL PATTERNS

Characteristics of infantile swallow

Jaws apart with the tongue between the gum pads

Mandible is stabilized by the contraction of the muscles of the 7th cranial

nerve and the interposed tongue

The swallow is guided and to a greater extent controlled by interchange

between lips and the tongue

Characteristics of retained infantile swallow

This is the persistence of the infantile swallowing reflex even after the

arrival of the permanent teeth

Very few people have this type of swallow

Teeth occlude on only one molar in each quadrant

They demonstrate violent contractions of 7th cranial nerve musculature

during swallowing and tongue is markedly protruded between all teeth

during initial stages of swallow

The patients will have an expression less face since facial muscles are

used for stabilizing the mandible

TONGUE THRUSTING

Tongue thrust swallow that may be etiologic to the malocclusion are of two types.

- Simple tongue thrust swallow

- Complex tongue thrust swallow

The child normally swallows with the teeth in occlusion the lips lightly closed and

the tongue held against the palate behind the anterior teeth. The simple tongue

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thrust swallow usually in associated with a history of digit sucking even though

the sucking habit is no longer been practiced, since it is necessary for the tongue

to thrust forward into the open bite to maintain an anterior seal with the lip during

swallow. Complex tongue thrusts on the other hand are far made likely to be

associated with chronic naso respiratory distress, mouth breathing, tonsillitis or

pharyngitis. When the tonsils are inflamed the root of the tongue may encroach

on the enlarged facial pillars. To avoid this encroachment the mandible reflex

drops, separating the teeth and providing more room for the tongue to be thrust

forward during swallowing to a more comfortable position. Pain and lessening of

space in throat precipitate a new forward tongue posture and swallowing reflex

while the teeth and growing alveolar processes accommodate themselves to the

attendant upset in neuromuscular forces. During chronic mouth breathing a large

freeway space is seen, since dropping the mandible and protruding the tongue

provides a more adequate airway. Because maintenance of the airway is a more

primitive and demanding reflex than the mature swallow the latter is conditioned

to the necessity for mouth breathing. The jaws are thus held apart during the

swallows in order that the tongue can remain in a protracted position.

Melsen et al in one of the studies stated that both tongue thrust swallow and

teeth apart swallow favors the development of disto occlusion, extreme maxillary

over jet and open bite. There is an increase in tongue thrust swallowing (simple

tongue thrust) seen with both pacifier sucking and digit sucking.

ROLE OF MUSCLE IN ORTHOGNATHIC SURGERY:

The suprahyoid musculature has repeatedly been suggested as a primary cause

of relapse after mandibular advancement surgery. Based on clinical

investigations, it has been hypothesized that when lengthened the suprahyoid

musculature exerts posteriorly directed forces on the advanced mandible as a

result of active muscle contraction, recoil of stretched elastic connective tissue

elements, or both. There are numerous related adaptations that can take place

within muscle in response to an increase in muscle length. Immediately after

muscle lengthening, two specific morphologic changes take place. First, the

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parallel and serial connective tissues within the muscle become stretched.

Second, once the connective tissues have reached the limit of their extensibility,

the muscle fibers themselves become stretched, resulting in an elongation of the

sarcomeres and a decrease in the overlap of actin and myosin filaments.

Effects on lip pressure and different patterns of post surgical changes

When incisors are moved within the sphere of influence of the lips after

previously being outside of it, as when vertically prominent maxillary

incisors are elevated to a new position beneath the lip, lip pressures will

increase and the incisors will tend to move lingually post surgically;

When soft tissues are relaxed by the surgical treatment, as when the

mandible rotates upward and forward following maxillary intrusion, lip

pressures will decrease and the incisors will move labially.

In patients in whom the soft tissues are stretched at surgery, as in

mandibular or maxillary advancement, lip pressures will increase and the

incisors will move lingually post surgically;

Three principles that influence the post surgical stability can be proposed:

1 The suprahyoid musculature has repeatedly been suggested as a primary

cause of relapse after mandibular advancement surgery. Moving the maxilla

upwards relaxes the tissues moving the mandible forward stretches the tissues,

but rotating it upward posteriorly and downward anteriorly decreases the stretch.

It is not surprising that the least stable mandibular advancements are those that

lengthen the ramus and rotate the chin up, while the most stable advancements

rotate the mandible in the opposite direction. The least stable orthognathic

surgical procedure is widening of the maxilla that stretches the heavy, inelastic

palatal mucosa.

2. Neuro muscular adaptation is an essential requirement for stability.

Repositioning of the tongue to maintain air way dimensions,(a change in tongue

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posture)occurs as an adaptation to changes produced by mandibular osteotomy

these adaptations of the tongue, and adaptation in lip pressures that also occur

post surgically, contribute to the stability of tooth positions. In contrast, a neuro

muscular adaptation does not occur when the pterygomandibular sling is

stretched during mandibular osteotomy as when the mandible is rotated to close

an open bite. If then neuro muscular system reacts to change in vertical position

of maxilla, adjustment in muscle length should occur when the maxilla is moved

downward just as it does when the maxilla is moved upward. Even if the muscles

adapt, however, the stretch of other soft tissues apparently can lead to the

instability that is observed when the maxilla is moved down ward and the

mandible is forced to rotate down ward and backward.

3. Neuromuscular adaptation affects muscular length and not muscular

orientation. If the orientation of the muscle group such as the mandibular

elevators is changed, adaptation cannot be expected. This concept is best

illustrated by the effect of changing the inclination of the mandibular ramus when

the mandible is set back or advanced.

Role of muscle in retention and stability

Alfred Coleman (1865) was the first person who claimed that muscular pressure

is responsible for relapse .

According to MOYERS primary cause of relapse is specifically that abnormal

seventh nerve action as it affects the facial muscles, especially abnormal

functioning of the mentalis muscle, is one of the most frequent causes of relapse

of incisor correction.

Stedman (1961,1967), in a comprehensive approach to retention, referred to an

enlarged pharyngeal space, emotionally initiated mentalis or mimetic muscle

hypertension, and anterior component of force of mandibular third molars

because of insufficient growth as factors in bringing about undesirable post

treatment changes or relapse.

Strang theorized that the mandibular inter canine and inter molar arch widths are

accurate indicators of the individual's muscle balance and dictate the limits of

arch expansion during treatment. Weinstein et al and Mills stated that the lower

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incisors lie in a narrow zone of stability in equilibrium between opposing

muscular pressure, and that the labio lingual position of the incisors should be

accepted and not altered by orthodontic treatment. Reitan claimed that teeth

tipped either labially or lingually during treatment are more likely to relapse.