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Table of Contents
The history of occlusion and the objectives of this
page
The most important factors that affect a patient's
occlusion
The relationship of the upper and lower teeth
The components of the temperomandibular joint
The Neuromuscular system
Centric Relation versus centric occlusion
Centric Occlusion
Centric Relation
Discrepancies between centric occlusion and centric relation
Group function versus canine guidance
How a normal jaw opens and closes (the Gothic Arch)
Joint Derangements
The
normal behavior of the articular disc
Internal joint derangements
Anterior disc displacement
The classification of joint derangements
Total
Joint replacement surgery
Other TM Joint derangements
The muscles of mastication
The
temporalis
The masseter and medial pterygoid
The lateral pterygoid
The
Digastric muscle
The complex
relationship between occlusion and facial appearance
My patient
Popeye
| In dentistry, the term occlusion refers to the way that the upper
and lower teeth come together. Originally, the term "occlusion"
meant just the way in which each individual tooth met with it's mate in
the opposite arch. Thus courses in dental school that dealt with
occlusion were most concerned with such concepts as which cusp of an
upper molar occluded (came together) with which particular groove in the
lower molar that it touched when the patient closed his teeth together.
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Occlusion
Indeed, the
very first course in occlusion that I took as a dental student involved
exclusively such minutia as "The mesiobuccal cusp of the maxillary first
molar occludes with the mesial buccal groove of the mandibular first molar...." and so
on throughout the mouth. It is unknown whether anyone ever stayed awake
throughout a full lecture period. This sorry state of dental education did
not begin to change until the late 1970's or early 80's.
Fortunately for the dental students of the world, and especially
fortunately for the dental patients of the world, the study of occlusion came
into its own as research showed that in order for the upper and lower teeth to
meet, the teeth had to be attached to something! And whatever it was
that they were attached to had to have a mechanism that allowed them to come
together. Much more research showed that the teeth were embedded in human
jaws and that the jaws were attached to a joint in front of the ear. (Whoda
thunk it?)
In fact, occlusion is one of the most important factors in
dentistry because the success or failure of practically everything a dentist
does in a patient's mouth depends upon its ability to operate within the
boundaries of the patient's physiologic occlusion. Even a simple filling
that changes the way a patient bites can
cause untold agony for the patient. See my page on
TMJ
for a full rundown of the disease states that are associated with occlusal
factors.
This page is concerned principally with an explanation of the
relationship between the way the teeth come together, and the resulting
configuration of the temperomandibular joint. It is of interest mostly to
dental students, hygienists and assistants who want a simple, practical
straightforward understanding of occlusion as it relates to their patient
without having to deal with the technical minutia and professional infighting
that has become the daily fare of the dentist or physician who wants to
specialize in this field.
|
By the way
For a thorough understanding
of glass and porcelain, Students and dental professionals should
consult my five page course "Dental
Ceramics for the beginner" |
The three most important factors that affect a
patient's occlusion:
1. The first factor is the minute
relationship of the upper and lower
teeth when they come together. This generally coincides with the most
common definition of the patient's "bite", but also includes the
specifics of which cusp on a specific tooth contacts which groove on
the opposing tooth. It is also concerned with how the teeth contact
during lateral excursions (The way that the upper and lower teeth contact
during side to side movements of the lower jaw). This is discussed in
detail
below. 2.
The second factor is the exact relationship of the components of the temperomandibular joint (the TMJ). (See the highlighted spot on the image of
the
skull at the top of this page to get your
bearings before looking at the diagram at the right.) The TMJ is the ball and socket joint that allows the lower jaw
to swing open and closed. The components of the TMJ are as follows :
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The glenoid fossa: The fossa is the
"socket", or depression in which the condyle sits. It is
located in the temporal bone of the skull. The glenoid fossa is
also covered with a layer of cartilage which allows smooth activity in the
joint. The back of the fossa is steep bone, and the condyle of the
mandible sits fairly snugly up against it and can move only slightly backwards from its
normal position in the fossa. The front of the fossa is a more gentle
slope of bone called the articular eminence. The eminence is also
covered with cartilage. The condyle is able to "translate"
forward over this eminence of bone and does so whenever the mouth is opened
wide, moves side to side, or whenever the patient protrudes his jaw.
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The articular disc:
The articular disc is also called the meniscus. It is made
of hyaline cartilage. The meniscus has an indentation
on the bottom side to accommodate the head of the condyle. The
articular disc is really part of a larger structure composed of the
cartilage disc plus fibrous ligaments on either
side and behind it. The ligament behind the meniscus is
called the retrodiscal pad in deference to its function as a
shock absorber for the condyle when the lower jaw is drawn back as far
as it will go. These ligaments are all connected to the condyle
only at their periphery so that there is a thin "potential" space filled with
synovial fluid both above and below the articular disc. (A
potential space is a collapsed space like the one between a rubber glove
and a hand. It is present, but not immediately apparent, and it
could potentially get wider if air or water were introduced under the
glove.) The articular
disc remains between the condyle and the fossa and acts as a shock
absorber. The majority of physical derangements of the TM Joints involve
damage to the articular disc and/or displacements of fragments of
the articular disc.
3.The third factor is the neuromuscular system: This involves the
muscles of mastication which open and close the jaw, as well as the brain
and the cranial nerves which give sensory and motor innervation to the
muscles. The muscles of mastication are discussed later in this piece. The brain is important in the concept of occlusion
because it is the source of both the voluntary muscular activity which
operates the system, as well as unconscious habits such as bruxing (grinding
and clenching) which can lead to some of the most serious disease states of
occlusion. This subject is discussed in detail
below.
Centric Relation vs. Centric Occlusion--How
the position of the teeth determines the position of the components in the
joints.
Centric occlusion is the term used to describe the
position of the lower jaw when the teeth are fully occluded (together).
This varies from person to person depending upon the number and position of
teeth in each jaw. In the image of the skull at the
top
of this page, all 32 teeth are present and occluding (biting) in an absolutely
normal class I relationship. This
position can change throughout a person's life depending on such factors as the
loss of teeth (with the consequent shifting of the teeth that takes place after
teeth are removed), fractures of the jaw, orthodontic movement of the teeth to
new positions, or the shifting of the teeth due to the constant pressures from
bruxing
(generally unconscious habits of grinding of clenching). When the teeth
are fully occluded, the condyle is forced into a specific position within the
glenoid fossa. Note that the term centric occlusion does not take joint
configuration into account. A patient's centric occlusion may be
physiologic, meaning that the joint is placed into a comfortable position, or
pathologic, meaning that the joint is forced into an eccentric position which
may produce organic joint dysfunction.
Centric Relation
 In
the image of the skull at the
top of this page, the TMJ
has been lightened to show the general anatomy. In this case, the condyle
is approximately centered in the glenoid fossa. More specifically, the
condyle is slightly closer to the top and the back of the glenoid fossa. This
relaxed, centralized position of the condyle within the glenoid fossa is called
centric relation.
If the TM Joints are in a state of health, they tend to approximate this
position whenever the teeth are slightly separated and the muscles of
mastication are relaxed. Ideally, this position of the joints should also
be approximated when the teeth are brought together into the patient's centric
occlusion. The image on the right shows another dried skull with the teeth in centric
occlusion and the joint in centric relation. Click on this image to see an
enlargement. In life, the articular disc would be resting in the natural
space between the condyle and the glenoid fossa without much pressure placed
upon it. This is a normal, healthy situation, and in an ideal world,
everyone would have a centric occlusion that would allow the condyle to remain
in centric relation. It is considered the ideal joint configuration, and it
is also the configuration that all dentists strive to produce in patients in
which a new centric occlusion must be recreated from
scratch.
Unfortunately, when a patient places his teeth together in centric
occlusion, the condyles on either side of the jaw do not always line up
within the glenoid fossa in centric relation. Even if the
tooth-to-tooth position is a perfect class I centric occlusion as shown above,
the condyle could be forced into abnormal positions within the fossa.
It could (depending upon the growth patterns the
patient has experienced throughout early life) actually be jammed up hard against the
top of the glenoid fossa. Or it could be located considerably forward of
the ideal position, with a consequent tendency toward sliding backwards up the articular
eminence. It
might be jammed hard against the fibrous connective tissue at the back of the articular disc at the back of the fossa. Situations like these
often
lead to pain in the joint with frequent headaches and
referred
pain that is perceived as earaches and neckaches. They also seem to lead
to bruxing which further exacerbates
the pain.
A discrepancy between centric occlusion and centric relation can
also develop in later life if the patient loses teeth, is injured in an accident
or has orthodontics in which not enough consideration was given to joint
position in the finished case.
Does a discrepancy between centric occlusion and centric
relation always cause trouble for the patient?
NO! Even a centric occlusion that causes serious
misalignment of the TM joint may cause no noticeable joint dysfunction or
pain. The reason that serious misalignment of the joints when the teeth
are closed together may be of no significance is that there is no physiologic
reason for the patient to keep their teeth together at all. When the teeth
are nearly touching, but not occluded, healthy TM joints will automatically fall
into a comfortable configuration approaching normal centric relation, even if
forceful occlusion would ordinarily force them out of this position. Even
while chewing food, the teeth rarely contact at all. (The next time you
eat, take notice.) The only
time during a normal day when the teeth come together for normal physiological
processes is during swallowing, and even then, it is only necessary for light
contact to take place on a relatively few teeth...UNLESS...the patient has a
bruxing habit!
Bruxing is a nervous habit of grinding the teeth
during prolonged periods during the day. To get an idea of the full extent
of the pain and agony caused by bruxing habits, read my page on
TMJ.
If there is no bruxing habit, even seriously dysfunctional occlusions can remain
perfectly physiologic and comfortable.
Group
function versus canine guidance When a person bruxes his
or her teeth side to side, keeping the teeth in constant contact, the cusps of
the upper and lower teeth slide over each other forcing the lower jaw to drop
slightly as they approach a cusp-tip to cusp-tip relationship. The number
of cusps that remain in contact during lateral excursions of the lower jaw vary
from person to person.
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Canine Guidance--In young persons with ideal occlusal
relationships, the upper and lower teeth contact evenly throughout the
entire dental arch when the teeth are fully together in that person's
centric occlusion. However, as soon as he or she begins a lateral excursion,
all the teeth (anterior and posterior alike) lose contact, except for the
upper and lower canines on that side. In other words, the canines are
situated and inclined in such a way that, while they allow full contact of
all teeth in centric occlusion, they force the jaw to open as the upper and
lower canines slide over each other. This
disengages the cusps of all other teeth as the person begins to grind side
to the side. (This phenomenon is called "cuspid rise" in
deference to the fact that most articulators are hinged in such a way that
the upper teeth move instead of the lower. This artificial way of
mounting the models makes the upper canines appear
to rise instead of the lower canines drop, which is what happens in
a real mouth.) In fact, canine guidance is considered the most
physiologic of all occlusal relationships because it protects the
teeth from wear and tends to prevent bruxing in most persons who are likely
to brux only occasionally. In the absence of chronic bruxing habits,
this relationship often persists throughout life.
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Group Function--On the other hand, if a person is a
habitual bruxer, the combination of tooth movement and cuspal wear over a
period of years reduces, and eventually eliminates the prominence of the
canine prematurely. This causes more and more posterior
tooth cusps to remain in contact over more and more of the excursive
movements. The process continues until, eventually, all the cusps of
the back teeth remain in contact throughout the entire lateral excursion.
This "group functioning" of all the posterior teeth now replaces the
original canine rise in causing the lower jaw to drop during excursions.
An occlusion in group function is more prone to perpetuate
the bruxing habit leading to greater and greater wear on all teeth.
Eventually, the occlusion is worn flat, eliminating any tendency of the
lower jaw to drop at all during lateral excursions. In other words,
all, or most of the teeth remain in contact throughout the entire lateral
excursion, and fail to disclude, as they do in canine guided occlusions.
This may cause extreme wearing of the anterior teeth as well as the
posteriors. Tooth wear from bruxing is called
attrition. Continual bruxing
leading to continual wear of the teeth also changes the relationship of the
patient's centric occlusion to their centric relation, causing a slow,
continuing protrusion of the lower jaw bringing about more and more wear on
the anterior teeth. Many dentists believe that by recreating a canine
guided relationship they can stop a severe bruxing habit and save a
dentition otherwise doomed to "death by attrition".
How a normal jaw opens and closes
 When
a person opens his mouth, the lower jaw swings at the TM Joint which is located
just in front of the ear at about the level of the opening of the ear
canal. Place your middle fingers lightly on this spot and you can feel the
condyle as it moves within the joint space. As you begin to open your mouth, at
first you can feel no movement of the joint. During this early part of jaw
opening, the condyle is simply rotating within the glenoid fossa.
But as you continue to open your mouth wider, you can begin to feel the head of the condyle move forward. This forward movement is called translation
and it is a normal part of opening the jaw wide. During translation, the
condyle is slipping forward and downward as it slides over the articular eminence. The
movement of the lower jaw is traced out by the red line in the image to the
right. As the lower jaw begins to swing open, a point on the surface of any
lower tooth traces a smooth radius around the place in the glenoid fossa where
the head of the condyle rotates. As the jaw opens further, the condyle begins
a smooth translation down the slope of the articular eminence. This second
opening component traces out a different radius around the changing position of
the condyle.
The blue arrow traces out the path of the lower jaw on
closing. Notice that the jaw traces out a smooth arc on closing without
the complication of rotational and translational movement seen on opening.
This is because the condyle begins and ends its closing path by smoothly sliding
back up the articular eminence until it comes to rest in centric relation at the
end of its closing cycle. The red "broken" arc combined with the
smooth blue closing arc is often referred to in dentistry as the classic
"gothic arch" due to its similarity with the architectural structure
of the same name.
How the articular disc behaves during normal
jaw opening?
Note the muscle labeled "lateral pterygoid" in
the image on the left above. This muscle has fibers which attach
separately to the front of the articular disc, with the majority attaching to the neck
of the condyle. When they contract, both
the articular disc and the condyle are pulled forward in unison in order to affect
translation of the condyle. When the condyle translates down the incline
of the articular eminence, the articular disc follows. The disc is in red
and the ligaments that attach it to the bony structures are represented in
bright yellow. Notice that the disc is not rigidly attached to the head of
the condyle. It remains on top of the condyle, but moves into new
positions throughout the translation process. The thinnest part of the
disc always remains between the closest points of contact between the articular
eminence and the the condyle.
The damaged TM
Joint (internal joint derangement)
If a patient forcefully bruxes (grinds or clenches the teeth),
the entire masticatory system is placed under great strain. The teeth can
wear. The periodontium (gums and the bone that supports the teeth) may become
inflamed, and in combination with poor hygiene, periodontal disease may
result. The TMJ is always placed under stress during bruxing, even if
centric occlusion coincides with centric relation. See my page on
TMJ
for a full listing of the problems associated with bruxing.
If there is a substantial discrepancy between centric relation
and centric occlusion, bruxing can cause serious long term damage to the TM
Joint. This may include stretching of the ligaments that keep the disc in
place causing a progressive anterior displacement of the disk forward of the
head of the condyle, perforation of the thin area of the disc, tearing of the
disc, or outright fragmentation of the articular disc apparatus into several
small pieces which may seriously interfere with opening and closing.
Anterior disc displacement (What causes popping and jaw
displacement when opening and closing?)
The lateral pterygoid muscle is attached to the condyle and is
responsible for drawing the jaw forward when the right and left joints are
equally active. It is also responsible for shifting the jaw to the right
or left when only one of the two joints are active. For example,
contraction of the right lateral pterygoid shifts the lower jaw to the left.
During unconscious grinding, the lateral pterygoid muscles are extremely active.
 As noted above, some of the fibers of the lateral pterygoid
muscle are attached separately and directly to the anterior of the articular
disc. Over time, constant bruxing can cause the disc ligaments to stretch
displacing the meniscus anteriorly. When this happens, popping noises can
be heard when the patient opens the mouth.
The popping is due to the noise the condyle makes if it moves
under the anteriorly displaced meniscus. The popping is also associated
with deviations in the lateral (side to side) movement of the jaw so the patient
no longer experiences smooth opening and closing jaw movements. As the
condition progresses, the popping and jaw movement deviation may be experienced
by the patient as the jaw closes as well.
Whenever the condyle pops under the firm, elastic, rubber-like
meniscus, the condyle is displaced downward and the jaw is displaced to the
opposite side of the face. These lateral (side to side) jaw deviations can become quite complex if
the disks on both sides are anteriorly displaced or otherwise damaged since the popping and
displacement on either side generally happen at different points in the jaw opening
movement.
It is likely that pain will NOT be experienced during any of
this popping and displacement activity since cartilaginous structures do not
have nerve endings. When pain IS experienced, it is generally due to
capsulitis which means inflammation of the synovial capsule. When pain is
not experienced by the patient, the dentist will generally strive only to treat
the bruxing habit in order to arrest the progressive nature of this
disorder. If pain is experienced by the patient due to internal
joint derangements, the dentist generally combines the bruxing treatment with
NSAID's (non steroidal anti inflammatory drugs).
The classification of joint derangements
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Type IA--popping in the TM Joints without pain: very common:
said to affect as much as 50% of normal subjects.
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Type IB--popping in the TM Joints associated with pain.
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Type II-- similar to type IB but patient experiences
occasional jaw locking with the inability of the
jaw to open or close beyond a certain point. The lock is caused by the
displaced maniscus blocking the path of the condyle during translation. Both types of lock can
generally be reduced by the patient with little difficulty.
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Closed lock--associated with the inability of the
condyle to slide under the displaced meniscus when the patient tries to
open the mouth beyond a certain point
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Open lock-- associated with the inability of the condyle
to slide back under the meniscus when trying to close the
mouth.
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Type III--a persistent lock, usually on trying to
open. Since the patient cannot open the mouth beyond this point, there
is no popping. This condition (unlike all type I and II derangements)
requires aggressive therapy with reduction of the lock under anesthesia and
physical therapy. If no improvement is seen in three weeks, surgery is
generally indicated.
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Serious derangements of the TM Joints are
sometimes treated by total joint replacement. Click the image
below to be directed to a page with images of the surgery as well as
before and after images of the results.
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Other TM Joint disorders
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Torn meniscus--This condition results in free
movement of the anterior fragment of the meniscus which usually moves ahead of
the condyle during translation due to the action of the lateral pterygoid
muscle. The effect of this is generally more
serious displacement of the mandible during opening or closing, as well as
a higher probability of locks. It may also be the cause of the type III
persistent locking noted above. This type of injury also allows the cartilage of both the
head of the condyle and the glenoid fossa to come into forceful contact
without the shock absorbing benefits of an intact meniscus.
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damage to the cartilaginous coverings of the condyle and
glenoid fossa--This can lead to severe bone-to bone contact with
consequent wear of the bone in both structures. This results in
grinding noises in the joint (called crepitus) and results in severe
arthritis and sometimes even a fusing of the bones (called ankylosis) of the
joint.
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The temperomandibular joint is like any other major joint in
the body. It is susceptible to any disease that can affect any other
joint in the body. Thus osteoarthritis is often found in the TMJ in
older persons, although it is generally symptomless. This joint can
suffer traumatic damage which can lead to joint derangements or painful
inflammatory changes in the capsule. It can also suffer dislocations
which can lead to stretched ligaments and a tendency to recurrent
dislocation.
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Dislocation of the TM Joint involves the
displacement of the condyle anterior to the articular eminence.
This generally implies severe stretching of the joint ligaments and is one
of the more severe effects of parafunction. Once the condyle slides
anterior to the articular eminence, reduction is quite difficult owing to
the spasm in virtually all of the muscles of mastication. Spasm in the
masseter, temporalis and medial pterygoid muscles causes them to apply
extreme upward force on the condylar head while spasm in the lateral
pterygoid applies massive anterior force. These forces lock the
condyle into its anteriorly displaced position making self reduction nearly
impossible. A health professional may reduce the dislocation by
placing his/her thumbs well distal and lateral to the lower second molars on
either side (taking extreme care to position the thumbs well out of the way
of the occlusion to avoid injury to his or her own fingers) and the
remaining fingers under the body of the mandible. The trick is to
press down hard with the thumbs while rotating the body of the mandible up
so that the patient's mandible pivots around the thumbs.
Once a patient dislocates one or both condyles, the
ligaments remain stretched out for a long time making further dislocations
all too easy. Patients will frequently "test" whether their jaws will
dislocate again, and find that it happens without too much effort.
Patients should be cautioned not to try to test their jaw. The
ligaments need about a year or sometimes more to heal before they become
reasonably resistant to dislocation, and each time a patient tests out their
jaw, the ligaments are further injured and healing is delayed again. A
soft diet and a bruxing appliance are probably the best recommendations.
The muscles of mastication
The teeth could not occlude or disclude without the five
(paired) muscles of mastication that make it all possible. The relationship
between centric occlusion and centric relation obviously influences the way the
muscles behave (the way the patient moves them during the course of a normal
day). It is very important to note, however, that the reverse is also
true. The use of the muscles may, over time, heavily influence the
relationship between centric occlusion and centric relation. Just as
important, the growth patterns of the structures in the joint, the occlusion
itself, and the shape and length of the muscles of mastication all influence
each other while the child grows. A majority of the pain and
headaches patients experience with temperomandibular dysfunctions (TMD) comes
from muscle splinting (cramps) in these powerful muscles.
 The
Temporalis muscle
The temporalis is one of three muscles that close the jaw and clench
the teeth. It's origin is from the periosteum (covering of the bone)
of the temporal fossa. It forms a thick tendon which passes
under the zygomatic arch and inserts into the the medial surface (the
inside surface) and the anterior border of the coronoid
process. The way the muscle is leveraged gives it a great amount
of power, and splinting in the temporalis can cause serious
headaches. |
 The
Masseter and the Medial Pterygoid muscles
The image on the right is of the masseter muscle. The medial
pterygoid muscle is leveraged in the same way as the masseter, only on
the medial (inside) surface of the mandible.
The masseter has an origin on both the outside and inside of the
zygomatic process of the maxilla and the zygomatic arch. The
masseter inserts into a broad part of the lower jaw, along the lateral
surface of the coronoid process, the ramus and the angle of the
mandible. This is also a powerfully leveraged muscle, and overuse
of this muscle can produce a "square jaw" appearance to the
face.
The medial pterygoid muscle arises from the medial (inside)
 surfaces
of the lateral pterygoid plate which is attached to the undersurface of
the temporal bone. In lay terms, the attachment is on the
undersurface of the skull just behind the last upper tooth. The fibers
of the medial pterygoid are directed downward and backward, just like
the masseter (pictured above), only on the inside of the mandible.
The insertion of this muscle is to the inside of the lower border
and angle of the mandible. Click on the thumbnail to the left to
see a large cutaway diagram of the medial pterygoid.
The masseter and medial pterygoid act like a contractile
"hammock" in which the lower jaw rests. These two
muscles are more or less "twins", the masseter acting on the
outside of the lower jaw and the medial pterygoid on the inside. |
 The
lateral pterygoid muscle
The lateral pterygoid muscle is an incredibly important muscle.
It is responsible for drawing the jaw forward when both the right and left
muscles are equally active. It is also responsible for moving the
lower jaw from side to side when the right or left lateral pterygoid is
active separately. Contraction of the right lateral pterygoid muscle
moves the jaw to the left, and contraction of the left draws the jaw to
the right. It is also responsible, in combination with the
digastric muscle for opening the lower jaw during the
translation phase of opening.
The image to the right shows the lateral pterygoid muscle partially
obscured by the coronoid process and part of the zygomatic arch.
Click on the thumbnail below to see a large cutaway diagram of the
lateral pterygoid muscle. It
is actually shaped a bit like a partly
unfolded fan. The wide end of
the fan, it's origin, originates from a small, finlike projection
under the skull called the lateral pterygoid plate. The narrow end
of the fan inserts into the anterior surface of the coronoid
process. Using your imagination, you can see how contraction of this
muscle draws the condyle--(and the lower jaw)
forward.
This muscle is composed of two parts. The upper belly inserts into
the articular disk inside the TMJ (as noted
above).
The lower belly inserts into the neck of the condyle. The two
bellies may work independently, but usually in concert to keep the articular disk
always situated between the closest points of contact between the condyle
and the glenoid fossa during both the rotational phase of jaw opening and
the translational phase.
Overuse of the lateral pterygoid during bruxism--remember that the
lateral pterygoid is responsible for lateral movements of the lower
jaw--causes stretching of the ligaments that hold the articular disk in
place over the head of the condyle. This in turn can cause the two
heads of the lateral pterygoid to begin to function out of sync which
causes even more stretching of the ligaments. This causes the
articular disc too much latitude and allows the disc to displace
anteriorly. This further exacerbates the asynchronization of the two
heads which causes further anterior displacement...and so on until the
disc becomes traumatized. The huge forces placed on the condyle by
the masseter, temporalis and the medial pterygoid during bruxing will
"mash" the articular disc if it is improperly situated between
the condyle and the glenoid fossa.
Overuse of the lateral pterygoid also causes cramps in the muscle which
manifests as an earache. If the lateral pterygoid is sore, pain can
be stimulated by sticking the fingers in the ear and pressing forward on
the
tragus.
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| The Digastric Muscle
The digastric muscle is the muscle most responsible for opening the
lower jaw (in combination with the coordinated contraction of the lateral
pterygoid muscles). It is actually composed of two muscles connected in the
middle by a strong tendon. The tendon loops under the hyoid bone
which is the only bone in the human body not directly connected to at
least one other bone by ligaments.
 .
The hyoid bone is supported in the neck at the level of the adams apple
by a large number of strap-like muscles, all of which brace each other
using the hyoid bone as an anchor point.
|
 |
| The front half of the digastric is called the
anterior belly. The half of the digastric behind the hyoid bone is
called the posterior belly. The tendon that joins the two bellies
actually slides under the bottom of the hyoid bone. I have
highlighted the tendon in yellow in the illustration above.
The digastric's relatively small bulk, and leveraging under a
bone not directly connected to the rest of the skeleton makes it quite a
weak muscle when compared with the tremendous upward pressure that can be
exerted on the jaws by the combined force of the temporalis, masseter and
medial pterygoid which oppose it. This accounts for the inability of
a patient to open his mouth against spasms of any of the three closing
muscles. The inability to open the mouth is a condition called
trismus.
The digastric muscle is rarely involved in disorders of the TMJ or the
muscular syndromes associated with bruxing. You do not get TMD by
keeping your jaw open, which is the major function of the digastric.
You contract these disorders by overusing the other muscles of
mastication.
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The complex interactions of the joint, the occlusion, the
muscles and facial appearance.
 In
the discussion above, we examined the way in which a poor occlusion, overactivity in the muscles of mastication
and the lack of
canine guidance can cause anatomical changes in the
temperomandibular joint as well as wear on the teeth themselves. This
image shows how serious the wear on the teeth can be. Wear on the
teeth caused by bruxing is called attrition. As the teeth wear
down over time, the lower jaw tends to protrude more and more as well.
Prolonged, forceful bruxing can also cause
tooth movement,
especially if teeth have been extracted in the arch.
Bruxing can, over the years, set up a viscous circle involving
changes in the the shape and position of the teeth, which in turn cause changes
in the muscles of the muscles of mastication. People who chew or grind
their teeth much more on one side than the other will tend to take on an asymmetrical
facial appearance. The muscles on the side of the face in which the
hyperactivity takes place tend to become larger and more bulky, while the muscles
on the underutilized side tend to become atrophied.
As the muscles change in strength and length, the teeth wear
unevenly on both sides causing more and more shifting in the position of the
jaw's centric relation which causes more and more pathological changes in the
anatomy of the TM Joints, more bruxing and more muscle deformation.
 Popeye--An interesting case of muscle/joint/occlusion
interaction.
Early in my career, I had to try to build a complete set of dentures
for an old gentleman from the "old country". He had had
his dentures (the same pair) since the age of 16, and he was now
76. Needless to say, his dentures were very seriously
worn.
As a denture ages, the teeth wear and the bony ridges that support the
denture recede causing the space between the nose and the tip of the chin
to collapse. As this process continues, the back teeth of the
upper and lower dentures no longer make contact, and the patient is forced to protrude his lower
jaw to get them to contact without dislodging the dentures from the ridges
(gums). This can cause quite a bizarre facial
appearance over time, and of course it seriously affects the patient's
ability to function. The most common effect on the wearing of the
denture itself is the fact that the lower front teeth now protrude out well in
front of the upper front teeth, often causing a lot of wear on the buccal
(outside) surfaces of the upper front denture teeth. In my
patient's case, the change in his centric occlusion brought about by the
combination of denture wear and loss of ridge height caused not only this
protrusion of the lower jaw, but a very pronounced shift of the lower jaw
to the patient's right. This caused what would ordinarily be the midlines of
his upper and lower edentulous ridges (toothless gums) to be offset nearly
a half inch. In addition, his nose and his chin were so close
together due to loss of vertical dimension that his mouth had turned into
something of a wide slit that seemed to go from ear to ear. He looked
like Popeye the sailor without the
squinty eye. (Actually, he did smoke a pipe which had worn a large
notch in the front teeth of the denture and may have contributed to the
shift in the lower jaw to the right. The notch coincided with the
area where the upper and lower teeth crossed over into
crossbite.)
In any case, the dentures fit perfectly together in this position. NO
PROBLEMO! I can fix this! So I thought--remember it was early
in my career. So I built him a nice looking upper denture with a
lower denture which occluded in a perfect centric occlusion that coincided
with a normal centric relation. The patient couldn't wear it.
His lower jaw kept protruding and shifting to the right, back into the position his old
denture had forced it into. Nothing I did could correct this
condition. So I rebuilt the denture several more times and ended up
with a new version of his old denture, complete with notch in the front
teeth. The patient was happy, but his wife was not impressed.
Here's what had happened. The slow deterioration of the denture and
ridge height caused a corresponding change in the shape and length of the
muscles of mastication on both sides of the face. Since the back teeth
no longer made contact when the patient closed his teeth together, he began
to protrude his lower jaw farther out to get them to
occlude. This protrusion caused further discrepancies in the ability
of the back denture teeth to make contact, so the patient began to shift his
lower jaw to the right to chew properly. Over the years, the muscles of
mastication changed in shape and length to accommodate this unusual bite. The change in
shape in the muscles was permanent and could not be reversed by a new,
properly built denture. Even when forced to bite in centric relation
with my first attempt at a denture, he retained a lopsided appearance with
the right side of his face larger than the left. I don't know what
his joints looked like, but I suspect that there was some anatomical
change there (ligament stretching and maybe even some minor bony changes)
to accommodate the changed centric occlusion. This patient seemed to
suffer no major joint signs or symptoms other than his pronounced tendency
to shift to the right on closing. Go figure! |
Engrams
Pain in the joint due to disk displacement or other inflammatory changes will
cause the patient to develop muscular engrams. Engrams are unconsciously
memorized programmed muscle movements that happen on opening or closing the
jaws. Their original purpose for the patient is to avoid placing the joint
or muscle in a position which provokes pain or spasm. Thus patients will
sometimes unconsciously cause their lower jaws to deviate to the right or left
during particular points when opening or closing the jaws. These movements
are not caused by mechanical interferences inside the joint as discussed
above. They
are simply reflexive memories that translate into unconscious behavior.
This behavior can be quite complex with the jaw deviating right and left in
complex, but completely reproducible patterns each time the patient opens or
closes his mouth. Furthermore, the engramic behavior persists well after
the pain that originally stimulated them has vanished. Upon
occasion, engrams must be unlearned to promote healing. This is done by
practice sitting in front of a mirror trying to open or close without deviation,
or exercises opening against resistance such as upward pressure placed against
the chin by the heel of the hand.
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