muscle physiology & its significance
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orthodonticsTRANSCRIPT
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
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
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
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.
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
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
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.
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
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.
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
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
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.
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.
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
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.
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
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.
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.
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
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
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
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
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
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
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.