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1
0886-9634/2002-
000$05.00/0, THE
JOURNAL OF
CRANIOMANDIBULAR
PRACTICE,
Copyright © 2002
by CHROMA, Inc.
ABSTRACT: The electromyographic (EMG) potentials of left and right masseter and temporalis anterior
muscles were recorded in 23 healthy young adults during: 1. a 3-second maximum voluntary clench
(MVC) on cotton rolls positioned on the posterior teeth (standardized recording); and 2. a 3-second MVC
in intercuspal position. EMG potentials recorded in intercuspal position were standardized as a percent-
age of the mean potentials of the standardized recording, and the EMG muscle activity was calculated.
The number of occlusal contacts in intercuspal position was assessed by using eight µm thick shim
stocks. Two groups of subjects with either 1. Less than ten occlusal contacts (11 subjects with “few con-
tacts”); or 2. At least ten occlusal contacts (12 subjects with “many contacts”) were selected. The MVC
muscle activity in the “few contacts” group was significantly lower than that recorded in the “many con-
tacts” group (p<0.005). In conclusion, the number of occlusal contacts and masticatory muscular func-
tion are significantly related, at least in young adults with a sound stomatognathic apparatus.
Dr. Virgilio F. Ferrario received his
Ph.D. degree in bioengineering from the
Polictecnico of Milan, Italy in 1969. He
received his M.D. degree from the
University of Milan School of Medicine, in
Italy in 1980. He is a Professor of Human
Anatomy at the Dental School of Milan
University, and Director of the
Laboratory of Functional Anatomy of the
Stomatognathic Apparatus, Department of
Human Anatomy, Faculty of Medicine,
University of Milan, Italy. Dr. Ferrario
has been involved in research on the
anatomical bases of stomatognathic
function and malfunction and is currently
developing functional tests to analyze the
activity of masticatory muscles.
M
ost functional activities of the stomatognathic
apparatus require stable tooth contact between
the maxillary and mandibular dental arches.
This position, namely, the maximum intercuspation,
should be the position of maximum stability for the
mandible.
l-2
Both static (i.e., biting) and dynamic (i.e.,
chewing) activities seem to be more efficient when a suf-
ficient number of dental contacts provide a stable refer-
ence for the contraction of supramandibular masticatory
muscles.
3-4
The literature reports several methods for the detection
and measurement of occlusal contacts from the simpler
assessments made by using articulating paper or occlusal
wax/silicone to the more complex instrumental methods
(occlusal sonography, computerized occlusal contact sen-
sors, pressure sensitive films, photocclusion).
5-11
An
actual contact between opposing teeth can also be mea-
sured with very thin shim stock strips.
2-3,12
This last
method is among the most precise, reliable, and repro-
ducible methods reported in clinical and basic dental lit-
erature.
12-13
Significant correlations between the electromyo-
graphic (EMG) characteristics of masticatory muscles
(amplitude of the electric potentials, duration of contrac-
tile activity) and the number of dental contacts have
Relationship Between the Number of Occlusal Contacts and
Masticatory Muscle Activity in Healthy Young Adults
Virgilio F. Ferrario, M.D., Ph.D.; Graziano Serrao, M.D., Ph.D.;
Claudia Dellavia, D.D.S.; Elisabetta Caruso; Chiarella Sforza, M.D., Ph.D.
OCCLUSION
Manuscript received
July 9, 2001; revised
manuscript received
November 21, 2001;
accepted
December 1, 2001
Address for reprint
requests:
Dr. Virgilio F. Ferrario
Department of Human
Anatomy
Via Mangiagalli 31
I-20133 Milano,
Italy
e-mail: farc@unimi.it
already been found.
3,4,10
In particular, Hidaka, et al.
10
found that the increment in the area of dental contact was
proportional to the increment in masticatory muscle
force with a relatively constant average bite pressure.
Unfortunately, both investigations had some limitations:
Bakke, et al.
3
analyzed women only and Hidaka, et al.
10
studied only twelve subjects. Moreover, the latter authors
quantified dental contacts using an instrument (the Dental
Prescale), which can produce false positive data.
6-l0
In neither of those studies was the masticatory muscle
activity (the integrated areas of the EMG potentials over
time) calculated.
14
This parameter, which integrates the
separate assessments usually made during EMG record-
ings, can be used as a global index of masticatory muscle
activity during dynamic tasks.
14
In the current investigation, the relationship between
EMG muscle activity (area of the EMG potentials over
time) during maximum voluntary teeth clenching and the
number of occlusal contacts in intercuspal position as
detected by shim stocks was quantitatively analyzed in a
group of healthy young adult men and women.
Materials and Methods
Data from 23 white northern Italian men (14 subjects)
and women (nine subjects) were included in this study.
The subjects were selected from a group of dental school
students aged from 20 to 37 years (mean age, 22.7, SD
4.37) according to the criteria described by Ferrario, et
al.
15-l7
Briefly, all subjects had a sound full permanent
dentition (28 teeth at least), with bilateral Angle Class I
first permanent molar and canine relationships (± 2 mm),
overjet and overbite ranging from 2-4 mm, no anterior or
lateral crossbite, no cast restorations or cuspal coverage,
no previous craniofacial trauma or surgery, and no tem-
poromandibular or cranio-cervical disorders.
All subjects gave their informed consent to the experi-
ment. All procedures were noninvasive and performed
with minimal discomfort to the subject. The study proto-
col was approved by the local ethics committee. All EMG
recordings and occlusal contact detection were performed
by a single operator between 9:00 and 11:00 in the morn-
ing. In all subjects, an EMG examination was performed
immediately after occlusal analysis.
Occlusal Contacts
Actual occlusal contacts were tested by using eight
mm wide, and eight µm thick, shim stocks (Hanel, Roeko,
D-89122 Langenau, Germany). The protocol devised and
described by Anderson, et al.
12
was used: in brief, a single
operator positioned the shim stock in relation to the
occlusal surface of each maxillary tooth and asked the
subject to close in intercuspal position using light to mod-
erate force.
2
Teeth holding the shim stock were recorded
as having an occlusal contact with their antagonists. The
test procedure began from the right upper second molar
and proceeded along the dental arch until the upper left
second molar. During testing, the subjects were seated on
a dental chair in an erect position with back straight.
EMG Recordings and Measurements
The masseter and temporalis anterior muscles of both
sides (left and right) were examined. Bipolar surface
electrodes were positioned on the muscular bellies paral-
lel to muscular fibers: temporalis anterior, vertically
along the anterior margin of the muscle (close to the coro-
nal suture); masseter, parallel to the muscular fibers, with
the upper pole of the electrode at the intersection between
the tragus-labial commissura and the exocanthion-gonion
lines
15-18
(Figure 1).
The skin was carefully cleaned prior to placing the
electrodes, and recordings were performed five to six
minutes later, allowing for the conductive paste to ade-
quately moisten the skin surface. During testing, dispos-
able silver/silver chloride bipolar electrodes with a
diameter of 10 mm and an interelectrode distance of 21±1
mm (Duo-Trode; MyoTronics Inc., Seattle, WA, USA)
were used, while a disposable reference electrode was
OCCLUSAL CONTACTS AND EMG ACTIVITY FERRARIO ET AL.
2 THE JOURNAL OF CRANIOMANDIBULAR PRACTICE APRIL 2002, VOL. 20, NO. 2
Figure 1
Bipolar surface electrodes positioned on the masseter (MM) and
temporalis anterior (TA) muscles.
applied to the forehead.
EMG activity was recorded using four of the eight
channels of an instrument (De Gštzen srl; Legnano,
Milano, Italy). The analog EMG signal was amplified
(gain 150, bandwidth 0-10 KHz, peak-to-peak input
range from 0 to 2,000 µV) using a differential amplifier
with a high common mode rejection ratio (CMRR = 105
dB in the range 0-60 Hz, input impedance 10 GΩ), digi-
tized (12b resolution, 2230 Hz A/D sampling frequency),
and digitally filtered (high-pass set at 30 Hz, low-pass set
at 400 Hz, band-stop for common 50-60 Hz noise).
The instrument was interfaced with a computer which
presented the data graphically and recorded it on mag-
netic media for later quantitative and qualitative analyses.
The signals were averaged over 25 ms, with muscle activ-
ity of the four tested muscles assessed as the mean square
root (r.m.s.) of the amplitude (unit: µV). EMG signals
were then recorded for further analysis. Details for the
EMG apparatus and raw data analysis can be found in
Ferrario and Sforza.
18
An initial recording for the standardization of EMG
potentials was made as detailed by Ferrario, et al.
17
Briefly, two 10 mm thick cotton rolls were positioned on
the mandibular second premolar and molars of each sub-
ject, and a three second maximum voluntary clench
(MVC) was recorded. For each muscle, the mean EMG
potential was set at 100%, and all further EMG potentials
were expressed as a percentage of this value (unit: µV/µV
x 100).
EMG activity was then recorded during a maximum
voluntary clench test in intercuspal position lasting five
seconds: the subject was invited to clench as hard as pos-
sible and to maintain the same level of contraction for
duration of the testing. During recording, the subjects sat
with their heads unsupported and were asked to maintain
a natural erect position. Reproducibility of surface EMG
measurements of the same muscles has already been
tested in our laboratory and found to be correct.
17
For each subject, the central three seconds of the max-
imum voluntary clench test were then analyzed, and the
EMG potential was standardized as detailed previously.
Subsequently, the masseter, temporalis, and mean total
muscle activities were computed as the integrated areas
of the EMG potentials over time (unit: µV/µV ¥ s%)
(Figures 2 and 3).
14
Data Analysis and Statistical Calculations
For each subject, the total number of occlusal contacts
in intercuspal position was computed. About half of the
sample (twelve subjects, seven men and five women) had
at least ten dental contacts, while the other half of the
sample (eleven subjects, seven men and four women) had
nine or fewer dental contacts. From the original sample
of 23 subjects, two groups were formed: Òmany con-
tacts,Ó and Òfew contacts.Ó
Within each group, descriptive statistics (mean and
standard deviation) were calculated for each variable
(age, number of occlusal contacts, masseter, temporalis,
and mean total muscle activities).
The mean values computed in the two groups were
then compared using a studentÕs t-test for independent
samples. Chi-square tests were used to compare the dif-
ferent number of teeth with/without dental contacts in the
two groups. For all analyses the level of significance was
set at 5% (p<0.05).
Results
The mean number of dental contacts in intercuspal
position measured in the many contacts group (all sub-
jects with at least ten contacts) was 13 (SD 2.4), while in
the few contacts group, on average only seven contacts
(SD 1.3) were found. Dental contacts were symmetrical
in almost all subjects, and only eight subjects had a dif-
ference of more than one contact between the right and
left side of the mouth.
As far as the first permanent molars and premolars are
concerned, the percentage distribution of dental contacts
was similar in the two groups. Differences were found for
the anterior teeth: while in the many contacts group, sub-
jects had 24% of their contacts on incisors and canines,
the few contacts subjects had only 8% of contacts on the
same teeth (Table 1).
When the number of teeth with or without one occlusal
contact at least was compared between the two groups
(Table 2), significant differences were found for the
canines (p<0.005) and the incisors (p<0.001). In contrast,
molars and premolars had a similar occurrence of con-
tacts in the two groups.
Table 3 shows the mean characteristics of the two
groups of subjects. While age was not significantly dif-
ferent between the two groups, the subjects with many
contacts produced significantly higher muscle activity in
both their masseter and temporalis anterior muscles than
the subjects with few contacts (p<0.005). The total mean
muscle activity was also different between the two
groups. On average, differences around 40 µV/µV¥ s%
were found for all muscles.
Discussion
In the present study, the masticatory muscle EMG
characteristics recorded during maximum voluntary
clench in two groups of subjects with low and high
FERRARIO ET AL. OCCLUSAL CONTACTS AND EMG ACTIVITY
APRIL 2002, VOL. 20, NO. 2 THE JOURNAL OF CRANIOMANDIBULAR PRACTICE 3
number of occlusal contacts in the intercuspal position
were compared. Significant differences were found for
muscle activity computed for both the masseter and tem-
poralis anterior muscle, with a reduced EMG activity in
subjects with a lower number of occlusal contacts than in
subjects with a higher number of contacts (Table 3). This
result agrees with previous investigations performed in
both static and dynamic conditions.
3,4,10
In those studies,
separate parameters obtained from the EMG recordings
(timing, duration, amplitude) were significantly related to
the number of occlusal contacts. Unfortunately, in no
case was a global index of masticatory muscle activity
provided. Moreover, both Bakke, et al.
3
and MacDonald
and Hannam
4
used raw EMG data, without any standard-
ization recording.
Muscle activity, namely the integrated areas of the
EMG potentials over time, has already been used as a
global index of masticatory muscle activity during
dynamic tasks.
14
During free and side-imposed mastica-
tion, both masseter and temporalis muscle activity has
been found to be related to food texture: larger values
were found for foods with a greater mechanical hard-
ness.
14
The muscle activity computed in the present study
differs from that calculated by Mioche, et al.,
14
because
standardized EMG potentials were used. In fact, they
reported a large inter-individual variability for their (raw)
muscle activity and related it to both anthropometric dif-
ferences (thickness of the subcutaneous fat layer, elec-
trode locations relative to active motor units) and var-
iations in the masticatory pattern.
The current standardization method (EMG values as
percentage of a maximum voluntary clenching on cotton
rolls) should neutralize the anthropometric differences
due to skin and electrode impedance, electrode position-
ing, and relative muscular hypo- or hypertrophy.
17
The
relative EMG values found in the experiment should thus
be affected only by the occlusal surfaces.
17
The method used in the present study for the detection
OCCLUSAL CONTACTS AND EMG ACTIVITY FERRARIO ET AL.
4 THE JOURNAL OF CRANIOMANDIBULAR PRACTICE APRIL 2002, VOL. 20, NO. 2
Figure 2
Standardized muscle activity calculated for the masseter and temporalis anterior muscles of one male in the Òmany contactsÓ group while performing
maximum voluntary clenching. Muscle activities: temporalis = 150 µV/µV ¥ s%; masseter = 200 µV/µV ¥ s%.
and location of occlusal contacts (shim stock strips) was
chosen for its simplicity, low cost, and high reproducibil-
ity.
l2
Technology currently supplies several instrumental
methods (occlusal sonography, computerized occlusal
contact sensors, pressure sensitive films, photocclusion)
which can provide information relative to timing and
force characteristics of occlusal contacts which are unde-
tectable by the simpler methods, but their actual cost/
benefit ratio in a clinical setting is yet to be determined.
6
For instance, thickness and rigidity of the sensor interacts
with dental proprioception, and false positive data can
occur.
10
Different methods for the identification of occlusal
contacts produce different numbers and patterns of con-
tacts in intercuspal position.
7,9,19
For instance, silicone
impressions record not only single contacts, but also the
area of Ònear contactÓ because of their increased thick-
ness.
7,9
In contrast, articulation papers identify single
Òspots.Ó
9
Moreover, methods that introduce a thickness of
more than 20 µm between the dental arches are likely to
alter dental proprioception and increase the number of
artifacts.
6,10
Articulation papers thinner than 20 µm are not of prac-
tical to use because the colored spot does not resist saliva.
In contrast, shim stock can be used to test pairs of teeth:
those teeth holding the shim stock are considered to have
occlusal contact with their antagonists.
2,l2
Reproducibility and reliability of the method are
already reported to be suitable for clinical assessments.
l2
Shim stock thickness (8-13 µm) is under the propriocep-
tion threshold of periodontal receptors, and the method is
not likely to record false positive contacts.
2,12,13
Several other factors should be considered in the
analysis of dental contacts in intercuspal position. Their
number may also be a function of the mandibular, head,
and body positions, the applied force, the time of
day.
5,12,13,20-22
In the present study, all measurements were
made in the morning, and with a standardized head and
FERRARIO ET AL. OCCLUSAL CONTACTS AND EMG ACTIVITY
APRIL 2002, VOL. 20, NO. 2 THE JOURNAL OF CRANIOMANDIBULAR PRACTICE 5
Figure 3
Standardized muscle activity calculated for the masseter and temporalis anterior muscles of one male in the Òfew contactsÓ group while performing
maximum voluntary clenching. Muscle activities: temporalis = 75 µV/µV ¥ s%; masseter = 75 µV/µV ¥ s%.
OCCLUSAL CONTACTS AND EMG ACTIVITY FERRARIO ET AL.
6 THE JOURNAL OF CRANIOMANDIBULAR PRACTICE APRIL 2002, VOL. 20, NO. 2
Table 1
Number and Percentage of Dental Contacts in Intercuspal Position
Group 2nd molar 1st molar 2nd premolar 1st premolar Canine Incisors
Contacts N % N % N % N % N % N %
“Many” 23 15 31 20 31 20 31 20 18 12 18 12
“Few” 21 26 19 24 17 21 17 21 4 5 2 3
Many contacts: ≥10 contacts for each subject (n=12)
Few contacts: ≤10 contacts for each subject (n=11)
Table 2
Number of Teeth Without (0) or With (1+) One Occlusal Contact in Intercuspal Position
Group Molars Premolars Canines Incisors
Contacts 0 1+ P 0 1+ P 0 1+ P 0 1+ P
“Many” 3 45 NS 7 41 NS 10 14 0.005 23 15 0.001
“Few” 5 49 12 32 19 3 2 42
Many contacts: ≥10 contacts for each subject (n=12)
Few contacts: ≤10 contacts for each subject (n=11)
Comparison: Chi-square test, one degree of freedom
NS: not significant, p>0.05
Table 3
Muscle Activity (Standardized Potentials) During Maximum Voluntary Clench
As a Function of the Number of Occlusal Contacts
Age Activity TA Activity MM Mean activity
Group (yrs) (µV/µV • s%) (µV/µV • s%) (µV/µV • s%)
“Many” Mean 21.69 130.02 113.51 120.97
(n=12) SD 2.39 30.42 33.71 31.35
“Few” Mean 23.80 91.6 74.73 82.79
(n=11) SD 5.76 16.3 24.2 18.54
Comparison p NS 0.001 0.005 0.002
Many contacts: ≥10 contacts for each subject (n=12)
Few contacts: ≤10 contacts for each subject (n=11)
Comparison: Student’s t-test t for independent samples, 21 degrees of freedom
NS: not significant, p>0.05
TA: temporalis anterior; MM: masseter
body position.
l,5,l2,13
In contrast, it is more difficult to stan-
dardize the amount of occlusal force, unless a simultane-
ous electromyographic recording is made.
l0,23,24
This is a
limitation of the present study, because the number and
area of occlusal contacts depend on bite force.
2,10,22-25
Another limitation of the current investigation may be
the reduced number of analyzed subjects. Nevertheless,
the number of subjects is not very different from that of
other investigations when restricted criteria of occlusal
and dental normality are used.
3-5,l0,20,2l,23
A third limitation
pertains to the sample selection. Only healthy subjects
were included in the study, and at present, not all the cri-
teria used can be extended to patients.
11
The number of occlusal contacts in intercuspal position
was used to divide the present sample of 23 subjects into
two groups, namely those with few contacts and those
with many contacts. The threshold between the groups,
ten contacts, was chosen because it corresponds to the
minimal values reported in the literature.
2,25
The two
groups were homogeneous for number, gender, and age
(Table 3).
In maximum intercuspation, 68% of the teeth of the
many contacts subjects had a contact with at least one
antagonist, while only 49% of the teeth of the few con-
tacts subjects had dental contacts. The difference was due
to both anterior (incisors, canines) contacts, and the pres-
ence of two contacts on each posterior tooth. It seems that
the reduced number of anterior contacts was the main
variation between the two groups. In both groups, the
number of dental contacts along the arch decreased from
posterior to anterior: 94% of the molars of the many con-
tacts group and 89% of the molars of the few contacts
group had at least one contact (Table 2). Premolar teeth
had at least one contact in 85% (many contacts) and 73%
(few contacts) of cases, respectively. Fifty-eight percent
of the canines of the many contacts subjects and 14% of
the few contacts subjects had an occlusal contact, a statis-
tically significant difference (Table 2). Incisors had con-
tacts in 31% (many contacts) and 5% (few contacts) of
cases (p>0.001). The percentage contacts computed for
the incisors in the many contacts group are comparable to
other reports in the literature,
3
while for the other teeth,
the present values are 10-15% lower. It needs to be men-
tioned that Bakke, et al.
3
used 50 µm thick strips, about
six to seven times the thickness of the present shim
stocks. In this case, a certain number of false positive
contacts may have occurred.
2,l2-l3
The number of molars
without occlusal contacts (6% in the many contacts, 11%
in the few contacts groups), as well as the number of pre-
molars without occlusal contacts in the many contacts
group (15%), are comparable to other reports in the liter-
ature.
2
In contrast, the percentage of premolars with no
contacts in the few contacts group (27%) is larger than
previous values.
2
In conclusion, and within the limitations of the present
study, this investigation confirmed that occlusal contacts
and muscular function are significantly related, at least in
young adults with a sound stomatognathic apparatus.
Any alteration to the occlusal surfaces may therefore
modify the actual performance of masticatory muscles,
because these two factors mutually interact, and com-
bined (morphology and function) clinical assessments
should be performed in all patients.
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Dr. Graziuno Serrao received his M.D. degree from the University of
Milan School of Medicine, Italy in 1984, and his Ph.D. in Morphological
Sciences from the same University in 1996. He is currently a staff
member of the Laboratory of Functional Anatomy of the Stomatognathic
Apparatus.
Dr. Claudia Dellavia received her D.D.S. degree from the University of
Milan School of Dentistry, Italy in 1999. She has been working at the
Laboratory of Functional Anatomy of the Stomatognathic Apparatus
since 1994. Dr. Dellavia is currently attending at a postgraduate course
in Morphological Sciences in the University of Milan.
Ms. Elisabetta Caruso is a graduate student of the Dental School of
Milan University. She is preparing her DDS thesis at the Laboratory of
Functional Anatomy of the Stomatognathic Apparatus.
Dr. Chiarella Sforza received her M.D. degree from the University of
Milan School of Medicine, Italy in 1986, and her Ph.D. degree in Sports
Medicine in 1989 from the same University. She is a Professor of Human
Anatomy at the School of Sport Sciences of Milan University. She has
been a staff member of the Laboratory of Functional Anatomy of the
Stomatognathic Apparatus since its foundation in 1989, where she
coordinates research activities.
OCCLUSAL CONTACTS AND EMG ACTIVITY FERRARIO ET AL.
8 THE JOURNAL OF CRANIOMANDIBULAR PRACTICE APRIL 2002, VOL. 20, NO. 2
