Clinical Use of Qualitative Electromyography in the

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Clinical Use of Qualitative Electromyography in the Evaluation of
Jaw Muscle Function: A Practitioner's Guide
Sven E. Widmalm, D.D.S., Dr.Odont.
You-sik Lee, D.D.S., M.S., Ph.D.
Duane C. McKay, D.D.S.
I. Introduction
Electromyography (EMG) has a
long history of application in the study
of functional jaw muscle anatomy.1
Normal, healthy jaw muscles are able
to both contract with adequate force
in a coordinated manner and to relax
during mandibular rest. However, the
assessment of these basic functions
through palpation and visual observation is not always sufficient when
rendering a diagnosis of muscular
dysfunction. When a more comprehensive understanding is necessary,
EMG is the only reliable method
available for the objective recording
of a patient's muscular function.
Because dysfunctional muscle activity may be the cause and/or the result
of other factors of investigatory interest, the application of electromyography is appropriate in diagnosis and
treatment.
In clinical dentistry, EMG is used
primarily to evaluate patients with
complaints of temporomandibular
joint dysfunction (TMD), jaw muscle
pain/dysfunction, and tension-type
headaches. In addition, practitioners
apply electromyography to patients in
need of oral rehabilitation.2-8 In these
types of cases, one objective is to estimate the degree of activity in an area
where several muscles act as agonists
or antagonists during mandibular
movements, as demonstrated by the
following muscle sets:
• the temporalis, masseter, and
medial pterygoid muscles during
clenching and lateral movements;
• the lateral pterygoid, deep temporalis, and deep masseter mucles
during lateral movements;
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• the lateral pterygoid during protrusion and the deep masseter
during retrusion;
• the suprahyoid, infrahyoid, and
lateral pterygoid muscles during
jaw opening; and
• the neck/cervical muscles during
head movements.
The diagnostic value of the EMG
recordings increases if considered in
conjunction with simultaneous data
acquisition of both the TMJ sound/
vibrations and translatory movements
of the mandible.9,10 EMG recordings
may also be augmented by an analysis of relative occlusal forces and
additional recordings of a patient's
response to electrical stimulation.
While more encompassing with regard
to muscle function, the EMG evaluation cannot replace traditional examination methods.3 Some authors have
cautioned against the use of electronic
instrumentation, except for biofeedback, emphasizing the potential diagnostic weaknesses while concurrently
advocating that the methods are not
cost effective.11-12 However, the expenses associated with the application and the purchase of equipment
are modest when compared with most
other procedures and equipment in
the dental office. Nonetheless, critics
have focused on errors that properly
trained clinicians do not make. Practitioners who have received EMG
evaluation training avoid the mistakes that occur as the result of applying unfamiliar techniques. The
appropriate use of the instrumentation and a pragmatic interpretation of
the advantages and limitations of
clinical information generated through
calibrated measurement tools are the
products of an educational program
that provides the clinician with adequate preparation in the assessment of
EMG recordings. Therefore, it is unfortunate when misplaced criticisms
lead to ignoring the unique advantages of EMG instrumentation, in
cases where the monitoring of muscle
activity is critical to the development
of a meaningful diagnosis. It is not
possible to calculate, in monetary
terms, the value of increased diagnostic accuracy. Financial assessments
will always differ among practitioners, patients, health care administrators and politicians. Therefore, cost
effectiveness will not be discussed
further.
This article is not a review of EMG
research. The purpose is to give examples of clinical applications wherein
relatively simple EMG recordings of
muscle activity provide valuable additional qualitative information with
regard to diagnosis and treatment
planning that cannot be obtained
through other examination techniques. Diagnosis has often been
based on qualitative data/recordings.
Under such parameters, it is appropriate to study cause-and-effect relationships of phenomena as indicated by
changes in EMG activity level, deviations from normal in jaw movement
patterns and variable observations
of TM joint sounds. Even if often
cited, the statement “diagnosis is
more art than science” is worth remembering. Nonetheless, the value of
qualitative electro-diagnostic data is
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dependent upon an examiner's understanding of functional anatomy and
stomatognathic physiology. Therefore, the practitioner must also have
knowledge regarding the basic limitations and sources of error associated with EMG. When there are
deficiencies in application and understanding, these issues can be readily
addressed as they are well described
in the dental literature.1,13
Before describing the various types
of muscle activity, mention should be
made of surface vs. needle electrodes
and artifact sources. The use of surface electrodes is preferable in most
clinical applications when the principal objective is to evaluate muscular
activity in a broad area. In contrast,
conventional needle electrodes are
applied to evaluate small/localized
areas.1 It is seldom justified to label a
surface EMG (SEMG) recording as
only representative of a single muscle,
as the electrode encompasses an area
large enough to include multiple
muscle groups. Functional activities
of these multiple muscle groups act
as a source of unwanted artifacts.14-15
More specifically, the major masticatory muscles are almost always covered by other superficial muscles, as
clinically demonstrated by: the epicranius over the temporalis; the facial
skin muscles over the jaw muscles;
and, the platysma over the masseter
and digastricus.15
Layered musculature affects the
interpretation of an EMG recording.
For example, the scalp muscle activity level may be high during frowning, as a result of a common reaction
to pain and stress. However, such an
EMG reading is often similar to recordings taken of the anterior temporalis area during clenching activity.
Therefore, to avoid or significantly
reduce muscular artifacts of the scalp
muscles, patients are asked to close
their eyes (Figure 1) and to refrain
from moving their eyeballs during
the recording period.14
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WIDMALM, ET AL.
However, similar admonitions are
not effective for all areas of the head
and neck. For example, SEMG recordings taken in the suprahyoid region
cannot differentiate among the muscular activities of the digastric bellies,
the genioglossus, the mylohyoideus
or the hyoglossus. l6 The ability to
make more refined assessments of
muscle function requires an alteration
in traditionally oriented evaluations.
More specifically, if it is important to
identify an individual muscle group
as the source of dysfunctional activity, then needle electrodes should be
introduced. The precise identification
of such muscle groups is beneficial in
cases where botox injections are used
to treat patients with focal oral mandibular dystonia characterized by
muscle spasms or other TMJ related
dysfunctions/disorders.17-22
The types of muscle activity that
are of primary interest in dental EMG
applications are (1) the ability to voluntarily contract with good coordination of agonists and antagonists during
functional jaw movements such as
chewing, swallowing, speech, etc.;
and (2) the ability to relax completely
between contractions. Of similar interest with regard to muscle activity are
clinically demonstrated (3) non-functional, nonvoluntary conditions such
as jaw muscle hyperactivity during
mandibular rest, nocturnal bruxism,
twitching, and spasms, and (4) habitual nonfunctional activity such as oral
parafunctions, bruxism, clenching,
etc. Furthermore, (5) excitatory and
inhibitory reflex activity (jaw jerk
reflex and silent period) were for a
while subject to extensive clinical
research and will be briefly commented upon. Special applications
of EMG in combination with other
electrodiagnostic methods, such as
jaw tracking and TMJ sound recording, and treatments such as occlusal
splints, are commented on under separate headings, (6) TMD and muscle
dysfunction, and (7) EMG in occlusal
splint treatment.
Voluntary Contraction
There is not a linear relationship
between the level of EMG activity and the force developed during
contraction. However, the relative
Figure 1
The EMG recording was made with a concentric needle in the frontal belly of the epicranius
muscle in a patient with severe tension-type headache. In this patient, the EMG activity and the
headache decreased when the eyes were closed (arrow 1). The EMG activity increased again
(arrow 2) when the eyes were opened. Audio feedback may help in teaching the patient how to
relax the scalp muscles. Horizontal bar = 1 s. Vertical bar = 100 µV.
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WIDMALM, ET AL.
strength of EMG activity during isometric contraction has been found to
approximate the output of mechanical
activity. The loss of teeth reduces the
jaw muscles’ ability to maintain
strength. This phenomenon is clearly
demonstrated in denture wearers.
However, recovery of the masticatory
muscle strength can be achieved as
shown by an increased EMG activity
generated by patients when biting in
the maximal intercuspal position.
This form of treatment in cases with a
complete upper denture (edentulous
maxilla) and lower partial denture
(residual mandibular anterior dentition) has been shown to have a favorable effect on craniomandibular
disorders and masticatory muscle
function.23
The use of SEMG can be beneficial
when establishing an optimal vertical
dimension and a bilateral balanced
occlusion. These goals must be
achieved prior to the placement of a
final restoration.7,24-30 If those goals
are not achievable and a case develops in a manner suggesting a less than
ideal model, then problems can arise.
For example, a decreased vertical
affects the position of the condyle,
creating an excessive load on the
internal TMJ structures thereby causing pathological changes. Such reductions in lower facial height may also
prevent optimal function of jaw muscles, especially that of the masseters
and the medial pterygoids by altering
the distances between origin and insertion sites. Similarly, the jaw-closing musculature is precluded from
efficient function where the distance
between origin and insertion has been
lengthened by a significantly increased vertical dimension. SEMG
can record maximal clenching activity at different levels of vertical by
using occlusal splints, initial treatment dentures or temporary bridges.
Distinct advantages associated with
the use of provisionals include the
direct and rapid modification of a
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patient's vertical dimension (Figure
2). Changes in masticatory muscle
length cause alterations in muscle
contractile properties, as has been
demonstrated with the masseter.26-29
Understanding the principles of occlusion3 relative to managing bite forces
and locating optimal vertical dimension are critical factors when attempting to extend the longevity of dental
implant prostheses.30
When functionally sound, the muscles around a joint provide greater
protection. For that reason, treatment
of TMD patients must include a dynamically oriented evaluation of
muscle strength. Subsequent to such
diagnostic assessments, when a patient's craniomandibular-cervical
muscle activity is found to be minimal, treatment should incorporate a
prescription of physical therapy and/or
exercise to increase strength.31
Coordination
Agonists and Antagonists
Agonists and antagonists must
function together in order to perform
well-coordinated activity. When there
is a lack of coordination between
muscles that move the mandible, a
compromise occurs in the patient's
adaptability to the introduction of any
intraoral appliance (i.e., dentures)
(Figure 3). There are also corresponding alterations in muscle function. Such changes promote tension
-type headaches (Figure 4) and disorders within the mechanics of TMJ
function (i.e., clicking) (Figure 5).
However, this assessment is suspect
Figure 2
Channels are from top to
bottom; left and right anterior
temporalis and left and right
masseter areas. Horizontal bar
= 5 s. Vertical bar = 200 µV.
Surface EMG can be used to
evaluate the effect of changes
in vertical dimension and of
dysfunctional co-contractions.
The EMG recordings were
made from a denture patient
with a decreased vertical
dimension. Recordings were
obtained comparing two
different states while the
patient applied a maximum
clench: 1. with a bite-raising
occlusal splint (lower window); and 2. without a biteraising occlusal splint (upper
window). Possible explanations describing the increased
elevator activity include: 1. an
increased vertical, optimizing
the distances between origin
and insertion sites of the elevator muscles; and 2. improved condylar positions due
to the insertion of an intra-oral
appliance (decompression).
Changes in vertical dimension,
whether by splint or prosthetic
reconstructions, should always
be monitored by TMJ imaging.
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WIDMALM, ET AL.
Figure 3
EMG activity was recorded with surface electrodes
in the left anterior temporal (T), left masseter (M),
and suprahyoid (SH) areas during closing followed
by clenching. Sound from tooth contact was
recorded in a phono channel (P). The patient had
dentures, and the co-contraction in the suprahyoid
area, starting before the activity in the masseter
area, may be from the mylohyoid muscle. This
myalgic co-contraction at times may dislocate
mandibular dentures during function. Audio feedback can be used to teach the patient to avoid such
dysfunctional co-contractions.
Figure 4
In this example, the patient suffered from a strong,
tension-type headache. To alter a patient's perception of pain, biofeedback can be used to change
dysfunctional contraction patterns. In this case,
EMG was recorded from the venter frontalis (F)
with surface electrodes in the forehead, and from
the venter occipitalis with surface electrodes in the
occipital area (O) during voluntary frowning. The
two muscle bellies should not contract simultaneously. However, in this demonstration, the two
upper channels (A) show how the patient made
habitual co-contractions. By looking at the oscilloscope screen and listening to EMG audio from the
loudspeaker, the patient learned to control muscular
activity. The use of EMG provided the patient the
means to alternate muscle function between the two
bellies. This enabled the patient to demonstrate a
more normalized functional pattern of muscular
activity as seen in the two lower channels. As a
consequence of this altered behavior, the degree of
headache pain was significantly minimized during a
follow-up two-month period of observation.
due to the methodologies applied in
defining the origin of a tension-type
headache, possibly the result of a misunderstanding caused by SEMG from
the temporal areas, where the activity
might have been from the epicranius
muscle.
Morphological Differentiation
Bilateral symmetrical activity is
expected in jaw closing muscles
at clenching. It has therefore been
suggested that asymmetry of mas-
66
seter muscle activity during maximal
clenching correlates with the onset
of the stomatognathic dysfunction
syndrome. 32 A certain degree of
asymmetrical EMG activity should,
however, be expected in most subjects because the cranium is seldom
symmetrical. When morphological
differentiation exists within the
anatomical base, muscle activity can
be expected to also be asymmetrical.
Therefore, to compensate for skeletal
imbalances, the muscular system,
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when attempting to generate symmetrical forces bilaterally, must do
so in an asymmetrical manner. Diagnostic instrumentation is necessary
to assess that level of asymmetry,
where gross deviations in muscular
activity occur between sides or between the temporalis and masseteric
areas ipsilaterally. Therefore, the use
of conventional methods for diagnosing and improving occlusal stability needs to be reconsidered in order
to achieve a more comprehensive
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Figure 5
This figure illustrates how dysfunctional muscle activity associated with
jaw clicking can be corrected by
teaching the patient to avoid fast
transitions from opening to closing,
[see recording "A"]. In so doing, the
patient is able to generate a slower
"softer" movement [see recording
"B"]. The complete opening/closing
cycle is illustrated in "A" but not in
"B". The second diagram shows only
the transition and closing phases.
Arrow 1 indicates contraction activity in the left temporalis area (T).
Arrow 2 points at an associated
clicking recorded in the channel P
with a microphone. Evidence of
clicking always appeared upon
abrupt transitions but not during
slower opening/closing movements.
The fourth channel (G) is a recording
of the opening degree, which was
very similar in both the fast and slow
opening/closing movements. EMG
was also recorded from the suprahyoid area (S). The vertical bar
indicates the opening angle in
degrees and the EMG gain in 0.1
mV.
understanding of the interrelationships between occlusal and muscular
function.
Interrelationships Among Head,
Jaw, and Neck Muscles
A greater understanding of jaw
muscle function cannot be achieved
without taking into account the coordination between jaw, neck and shoulder muscles. This was pointed out
decades ago but has not been subject
to more systematic studies until quite
recently,33,34 when studies have shown
concomitant mandibular and headneck movements during jaw openingclosing. Those studies tested the
hypothesis of a functional relationship between the human mandibular
and cranio-cervical motor systems,
(head-neck movements) during voluntary mandibular movements. Their
results give further support to the
concept of a functional trigemino-
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cervical coupling during jaw activities in man.
In studying the effect of jaw clenching on the EMG activities of neck
and trunk muscles, Ehrlich, Garlick,
and Ninio35 often found symptoms of
jaw dysfunction linked to neck muscle
dysfunction or other musculoskeletal problems. The study's conclusions
provided additional support indicating an interrelationship among the
functional activities of the jaw, neck,
and trunk muscles. Co-activation
of the sternocleidomastoid muscle
(SCM) during strong abrupt clenching efforts has been reported in SEMG
studies. 36 The clinical implications
seem to be that the dentist may have
reason to include screening with
SEMG of the neck and shoulder
muscles in the examination13 of some
patients. Treating possible neck
and shoulder muscle dysfunction is,
however, outside the dentists' area
of competence. Such findings should
be referred to orthopedic specialists,
neurologists, and/or physiotherapists.
Relaxation
SEMG has been widely used to
study the effect of various factors on
the jaw muscles' ability to relax2,31,37
and has been accepted as a useful
adjunct in audio-biofeedback techniques.38 Because habitual non-functional tension may be harmful for
muscles and associated jaw joints, a
key goal in treatment should be relaxation between voluntary contractions.4,13 There is a consensus about
the positive effect of relaxation on the
pain threshold. From a clinical point
of view, it is valuable to be able to
monitor a patient's ability to relax and
avoid dysfunctional contractions by
recording SEMG from muscles of
choice using audio feedback. They
may be scalp, jaw, neck, shoulder,
arm, or leg muscles. Generally, it is
considered best to start with a body
part that is most easily under voluntary control—usually the right arm.39
Biofeedback using SEMG is an excellent tool when teaching relaxation.
Stretch-based relaxation procedures
have been shown effective for the
reduction of muscle activity in the
masseter regions of subjects diagnosed with masticatory muscle pain
disorders.40
Excellent methods for relaxation
directed toward a general reduction of
muscle tension are well documented.
Using such relaxation techniques,
major surgeries have been successfully completed without anesthesia.
Ancient yoga techniques, without the
religious attributes, have medical
value in muscle function therapy.
Credible research has shown that most
types of muscle activity—even those
mirrored in heart rate, blood pressure,
and single motor unit activity—can
be controlled by relaxation and/or
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bio-feedback techniques. 39 Simple
relaxation techniques could provide
an alternative to more invasive anesthesia protocols.
Hyperactivity
Consensus has not been reached
concerning the etiology and role of
jaw muscle hyperactivity during
mandibular rest. The rest position of
the mandible is highly variable and
dependent on many factors. 3,41-42
In order to assess the muscular
component of mandibular rest, SEMG
is indispensable when attempting to
localize that range. More specifically, SEMG recordings from the temporal and masseteric areas during
mandibular rest reveal minimal activity in healthy subjects. According to
Yemm,41 if the mouth is closed with
lip seal, the tissue elasticity forces are
sufficient to keep the mandible in its
resting position. According to Møller,42
minimal muscular activity is necessary. Often used in dental EMG literature, the term hyperactivity is applied
WIDMALM, ET AL.
to above-normal EMG levels detected
in the temporal and masseteric areas
during mandibular rest, despite no
observable tooth contacts or clenching. The clinical significance of hyperactivity is unclear due to the difficulty
in localizing, isolating and determining the sources of function.
If the acquired documentation confirms that the mandible's elevator
muscles are the sources of the hyperactivity, it is reasonable to assert that
there must also be corresponding
activity in the antagonistic jaw openers. Otherwise, the mandible could
hardly remain in a "resting position."
The extent of this state can only be
explored by intramuscular EMG
recording. Therefore, if there are areas
of dysfunction occurring in the pterygoids, the deep temporalis, the masseters, and/or in the suprahyoid muscle
groups, then it is unlikely that a noncalibrated approach will sufficiently
describe the extent of disorder.
Opinions differ regarding whether
pain is the cause or the result of such
hyperactivity. In clinics, hyperactiv-
ity is often observed without pain.
However, pain may also exist in the
absence of observed hyperactivity. 4
More specifically, SEMG-recorded
hyperactivity in the temporal areas
was found more often in patients with
acute TMJ dysfunction than in a control group.4 The hyperactivity of the
temporal musculature is often triggered by clenching (Figure 6) and/or
opening-closing (i.e., depression-elevation) movements of the mandible
(Figure 7). However, this hyperactivity can be reduced by repositioning
the mandible through passive depression (Figures 6, 8). When a clinician
measures the extent of muscular activity through SEMG, it is important to
realize that such variations in activity
levels may have their origin in superficial muscles. Therefore, being able
to differentiate the influences between
superficial and deep musculature is a
critical aspect of the diagnostic process, as EMG recordings may result
from the localized hyperactivity of
the pterygoid and/or the deep portions
of the temporalis and masseteric mus-
Figure 6
Recordings made during clenching followed by
mandibular rest. Channels are from top to bottom,
left (LAT) and right (RAT) temporalis, and left
(LM) and right (RM) masseter areas. Horizontal
bar = 5 s. Vertical bar = 50 µV. There is good
relaxation in the masseter areas, while the muscular activity is continuous in the temporal areas.
This level of function was maintained until the
man-dible was passively lowered a few mm. It is
worth noting that the muscle activity recorded in
the temporal areas during mandibular rest may
come from scalp muscles. Therefore, temporalis
hyperactivity should not be diagnosed without first
making controlled recordings with concentric
needles. If the acquired documentation confirms
that the mandible's elevator muscles are the
sources of the hyperactivity, it is reasonable to
assert that there must also be corresponding activity in the antagonists (jaw openers: the lower heads
of the lateral pterygoids). Otherwise, the mandible
could not remain in a "resting position." The extent
of this state can only be explored by intramuscular
EMG recording. Therefore, any observations concerning muscular activity in the masseter and/or
temporalis areas should not be based solely on
SEMG recordings. (See also, Figures 7, 8, and 9).
68
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LAT
RAT
LS
Figure 7
This EMG recording is from a patient where large
jaw opening movements triggered strong activity
in the anterior temporalis when the mandible had
returned to resting position. Window - 19 s.
Channels are from top to bottom left (LAT) and
"right (RAT) anterior temporal areas, left (LS) and
right (RS) suprahyoid areas. Vertical bar = 50 µV
in the two upper and 200 µV in the two lower
channels. (See also comments for Figure 6.)
RS
cles. Further research is needed where
EMG, jaw movement, and TMJ sound
recordings are made simultaneously
to analyze possible connections
between such activities and dysfunctions of the TMJ disk and/or the
mandibular musculature. Presently,
where hyperactivity is triggered in
some areas, as illustrated in Figures
6-8, there is a need to accurately determine the source. To that purpose,
SEMG combined with intramuscular
EMG recordings is recommended.
Stress and anxiety have been shown
to increase EMG activity in masticatory muscles, while relaxation helped
to reduce the stress-induced activity
in relation to baseline levels.41 Subjects
diagnosed with TMD have provided
varying responses to stress, based on
their typical responses to stressful
stimuli. 43 In other words, anxious
patients showed higher levels of muscular activity and emotional irritabil-
LAT
RAT
LS
RS
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ity during stress than did non-anxious
patients.
Parafunctions. Oral parafunctions,
such as day- and night-time bruxism,
thumb sucking, cheek biting, tongue
pressure, and nail biting are of significant interest for dentists, but treatment is not generally taught in dental
schools. For that reason, it appears
that many clinicians are unaware of
effective methods of treating these
issues. There seems to be a consensus
Figure 8
This EMG recording is from a patient who
had continuous hyperactivity in the right
anterior temporal area during habitual
mandibular resting position. It disappeared
every time the mandible was passively lowered about 15 mm (Vi) from the habitual
resting position and returned at closing. The
hyperactivity may have been triggered by a
displaced disc in this patient who had reciprocal clicking. Channels are from top to bottom,
left (LAT) and right (RAT) anterior temporal
areas, left (LM) and right (RM) masseter
areas. (See also comments for Figure 6.)
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WIDMALM, ET AL.
Figure 9
EMG recording with surface electrodes from
the left (upper channel) and the right masseter
(lower channel) areas during mandibular rest.
Involuntary activity started on the right side
every time the patient closed her eyes and
silenced when she opened them. She could
feel the activity as twitching, which was
disturbing when she tried to go to sleep. She
had been through jaw surgery, so a possible
explanation is false re-innervation of motor
units in the masseter area from nerve branches
normally innervating the eyelids. Window = 9
s. Vertical bar = 100 µV.
that biofeedback, using EMG, has
some beneficial effect in patients with
bruxism; however, it is doubtful that
such treatment is more effective than
simple relaxation without EMG.
Muscle hyperactivity and headache. In patients with headache and/
or TMJ dysfunction, higher levels of
activity were reported in the scalp
muscle areas. 44-47 Chronic tensiontype headache sufferers have also
been reported to exhibit higher pericranial muscle tenderness than did
the control group. The association between pericranial muscle tenderness
and chronic tension-type headache
was independent of the intensity, frequency, or chronicity of headaches.48
Some patients feel relief from headache when EMG-observed scalp muscle activity decreases. The dentist can
help such patients by using EMG for
audio feedback.
Headache treatment is not a condition for which the dentist is typically
trained. While some patients suffering with tension-type headache may
benefit from a treatment device such
as an occlusal relaxation splint, the
dentist should never assume sole
responsibility for headache treatment.
Spastic muscle activity. At times,
observable through visual inspection,
brief twitches and prolonged contractions clinically and electromyographically similar to those of hemifacial
spasm and cramps can occur within
70
the head and neck musculature sometimes, but not always, observable by
visual inspection. The patient may
demonstrate dysfunctional activitysuch as pulling the mandible to a position that feels more comfortable while
appearing abnormal from a dental
perspective. In order to more objectively assess this condition, SEMG is
an excellent screening aid in detecting and diagnosing such dysfunctional activity.
Distinct ongoing twitching activity
may be felt by TMD patients in spite
of no visible signs of skin or jaw
movements (Figure 9). In cases where
confirmation through traditional
methods of examination fails to document such reported activity, EMG is
necessary. EMG is the only way to
authenticate the presence of reported
muscular activity. In other cases,
where twitching activity is both felt
and clearly visible, needle EMG
recording is required for accurate
confirmation of the source(s). Even in
cases where such activity cannot be
eliminated, it is important that the
patient knows that there is a physiologic basis for the symptom as documented by EMG, and that it is not “all
in the patient's mind.”
Hemi-masticatory spasm (HMS)
may occur with progressive hemifacial atrophy caused by peripheral
irritation of the trigeminal nerve due
to motor branch entrapment in the
THE JOURNAL OF CRANIOMANDIBULAR PRACTICE
infra-temporal fossa, causing involuntary movement. Local injections of
botulinum toxin Type A into the masticatory muscles have been reported
to be a successful treatment. HMS is
probably the consequence of an abnormal trigeminal hyper-excitability
likely induced by the demyelinating
lesion of its peripheral motor pathway.49-50 Even while spastic activity
and pathologically changed reflex
behavior may not be for the dentist to
treat, it is still important to be able to
detect such signs. The dentist may
help in early detection of neurological
signs and make appropriate referrals.
Reflex Activity
The jaw jerk is a stretch reflex usually elicited during mandibular rest. If
evoked during isometric contraction
of the jaw elevators, it is followed by
one or two periods of inhibition. The
stretch reflex plays an essential role
in motor control of muscular performance. Double periods of inhibition,
early and late silent periods, have
been suggested as signs of TMJ dysfunction.34,51
The duration of the electromyographic silent period (SP) of the masseter and temporalis muscles elicited
by chin-tapping has been subject to
numerous dental studies ever since
Bessette and Shatkin.52 These authors
concluded that measuring the length
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WIDMALM, ET AL.
of the masseteric silent period (SP)
on the EMG was a reliable diagnostic
methodology. They reported that
patients with a lengthened SP had a
93% success rate from treatment with
occlusal splint and occlusal adjustment. In contrast, patients with a
normal masseteric SP on the EMG
had only a 21% success rate from the
same therapy.
Peripheral irritation of the trigeminal nerve due to entrapment of the
motor branches in the infra-temporal
fossa may cause involuntary movement and a disappearance of the masseteric jaw-jerk reflex and SP.38-40,4448,53 The early detection and proper
referral of these abnormal signs are
possible when dentists record jaw
reflex activity.
EMG recordings can be used in
combination with other electrodiagnostic methods, such as jaw tracking
and TMJ sound recording to improve
diagnostic accuracy. The use of calibrated instrumentation in the delivery
of treatment, inclusive of occlusal
splints, is separately addressed below.
TMD and muscle dysfunction. TM
joints with normal function and no
degenerative or arthritic changes are
clinically silent. Audible TMJ sounds
are considered to be cardinal signs of
TMJ dysfunction and pathology. Clinical observations demonstrate that the
method and manner of mandibular
movement contribute to the production of joint sounds (Figure 5).
However, the role of the patient’s
musculature in the performance
of this event is unclear. Isberg,
Widmalm, and Ivarsson l0 examined
patients with internal derangement of
the TMJ clinically, radiographically
and electromyographically. EMG
recordings were also obtained from
subjects without signs or symptoms
associated with their TMJ or masticatory musculature. Slow opening and
closing mandibular movements without clenching could be performed by
healthy persons without noticeable
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EMG activity in the temporalis and
masseter muscles. In association with
disk displacement, EMG activity of
the temporalis and masseter muscles
occurred when the condyle slid over
the posterior band of the disk. This
asynchronous movement could be
interpreted as an arthro-kinetic reflex
caused by distraction.
EMG in occlusal splint treatment.
SEMG can be used in the following
manner: 1. to observe the immediate
effect of occlusal splints and occlusal
stability on the jaw muscles' ability to
contract and to relax; 2. to compare
the effect of hard vs. soft splint material; and 3. to determine the optimal
myalgic consequence associated with
the placement of a new splint.8,54-58
SEMG is also valuable in evaluating
the effect of splint treatment on jaw
muscle activity (Figure 2). More
specifically, electromyography provides clinicians with an accurate
method of assessment regarding the
use of maxillary stabilization occlusal
splints and the changes that can occur
from the placement of said splints
upon the suprahyoid and cranio-cervical relationships.59,60
While occlusal stability has always
been a goal in clinical dentistry,3 opinions differ as to the importance of
occlusal stability in relation to jaw
muscle activity.61 This range of opinion derives from the challenges
associated with the evaluation of
physiologic results following the
adjustment of a patient's occlusion. In
many studies, the authors acted as
both judge and jury in determining
whether splint management was
deemed successful. Under such variable evaluative parameters, it is reasonable to conclude that some may
not have paid as much attention to
those factors as did Ramfjord.24
Conclusions
Although EMG recording has limitations in diagnosis, there are cases
when critical information can be
gained that may otherwise not be
obtainable through other examination
methods. Surface EMG (SEMG) is
valuable in the diagnosis of involuntary abnormal activity, such as hyperactivity and spastic activity. The
in-formation gathered from SEMG is
enhanced when acquired together
with jaw tracking and TMJ sound
recordings. This combination of clinical data can also be used to determine whether reflex activity, activation and inhibition responses are
normal. In therapy and evaluation of
treatment outcome, the calibrated
documentation collected through
SEMG can monitor progress and
results through objective methods of
quantitative and qualitative interpretation. Combined with a sound knowledge of gross and functional anatomy,
clinical data acquired through EMG
provides the practitioner with a more
comprehensive diagnosis of TMJ
muscle dysfunction.
Acknowledgements
The EMG and sound recordings in
Figures 1, 3-5 were made in the EMG
laboratory at the Department of Stomatognathic Physiology, Karolinska
Institutet, Huddinge, Sweden, using
Medelec MS6 System with AA6
MKIII AC amplifier & pre-amplifier
PA62 (Medelec Limited Old Woking
Surrey GU22 9JU England). Lower
frequency cut-off (LF) was 16 Hz and
the higher frequency cut-off (HF) was
3200 Hz. The TMJ sound recording
in Figure 5 was made using a contact
microphone (EMT 25, ElemaSchönander, Sweden) (LF 16 Hz, HF
800 Hz).
The recordings in Figures 2, 6-8
are from one of the author's MS
thesis 4 and were, as was Figure 9,
made in the EMG laboratory at the
School of Dentistry, University of
Michigan, from Dr. Major M. Ash's
patients. The EMG was amplified
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71
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(Grass 7 P 511) and recorded on a
Tektronix Storage Oscilloscope and
photographed with a Polaroid camera.
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Dr. Sven E. Widmalm received his D.D.S.
degree from Karolinska Institutet, Stockholm,
Sweden and his Dr. Odont. from Gothenburg
University, Sweden. He is certified as a specialist of stomatognathic physiology by the Swedish
Board of Health and Welfare. He is an associate professor of dentistry at the School of
Dentistry and an adjunct research scientist at
the College of Engineering, University of
Michigan.
Address for author contact:
Dr. Sven E. Widmalm
1565 Kuehnle
Ann Arbor, MI 48103
E-mail: [email protected]
Dr. You-sik Lee received his D.D.S. degree
from Seoul National University, Seoul, South
Korea and his M.S. degree in oral pathology.
from the Graduate School of Dentistry, Seoul
National University, South Korea. He received
his M.S. in restorative dentistry and occlusion
from the University of Michigan and a Ph.D. in
dental biomaterial from Busan National
University, South Korea. He maintains a private
practice in Busan, South Korea.
Dr. Duane C. McKay is a graduate of the
University of Southern California, School of
Dentistry, where he also received postgraduate
training in prosthodontics and an advanced
specialty degree in oral rehabilitation. He
maintains a private practice in Los Angeles,
California.
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