Effects of a functional appliance on masticatory muscles
of young adults suffering from muscle-related
temporomandibular disorders
T. CASTROFLORIO, F. TALPONE, A. DEREGIBUS, M. G. PIANCINO
& P. BRACCO
Specialisation School of Orthodontics, Department of Biomedical Sciences and Human Oncology, School of Dentistry,
Turin University, Turin, Italy
SUMMARY The aim of this study was to investigate the
effects of an original orthodontic functional appli-
ance [function generating bite for deep bite correc-
tion (FGB-D)] on masticatory muscle activity in
subjects suffering from muscle-related temporoman-
dibular disorders (TMD). Electromyographic (EMG)
analysis was performed on 33 young adults (nine
men, 24 women) to evaluate the contractile symme-
try of the right and left masseter and anterior
temporalis muscles. The subjects were divided into
three groups: a muscle-related TMD group requiring
orthodontic treatment for deep bite correction
(three men, eight women) and treated with FGB-D;
a muscle-related TMD group not requiring orthodon-
tic treatment (three men, eight women) and treated
with a Michigan occlusal splint; and a TMD-free
group (three men, eight women) as a control group.
Records were made by surface EMG of maximum
voluntary teeth clenching, with and without the
functional appliance or occlusal splint in place, before
and after 12 months of therapy. A torque index was
derived from the surface EMG recordings to estimate
lateral displacement of the mandible. The results
show that the FGB-D corrects the torque index and
thus the lateral displacement of the mandible.
KEYWORDS: surface electromyography, functional
appliance, torque index, masticatory muscles
Accepted for publication 28 March 2003
Introduction
The lack of standardized diagnostic criteria was a critical
obstacle towards a further understanding of temporo-
mandibular disorders (TMD). A significant contribution
was made by Dworkin and Le Resche (1) with the
introduction of a set of research diagnostic criteria for
TMD, labelled ‘RDC/TMD’.
According to RDC, TMD can be classified into three
major groups: (1) muscle conditions, (2) disc displace-
ments, and (3) arthralgia, arthritis, and arthrosis, on the
basis of a simple physical examination and case history.
The muscle-related conditions represent the most
prevalent conditions among the TMD, with at least 50%
of cases falling into this category (2). Pain or ache in the
jaw, temples, face, pre-auricular area, or inside the ear
at rest or during function combined with tenderness to
palpation of at least three of 20 palpation sites, with at
least one ipsilateral to the complaint pain, must be
present to be diagnosed as muscle-related TMD (1).
Quantitative surface electromyography (sEMG) of
masticatory muscles is a reliable technique for the
assessment of TMD patients (3, 4). The study of the
electrical performance of masticatory muscles in mus-
cle-related TMD patients, during predefined tasks, can
aid in the development of diagnostic criteria and may
allow the monitoring of the progression of the disease
and the effects of therapy.
Even if occlusion as an etiologic factor of TMD is still
under debate, significant associations have been found
between TMD and several kinds of malocclusion (5–8).
The treatment of TMD by interocclusal devices followed
ª 2004 Blackwell Publishing Ltd
524
Journal of Oral Rehabilitation 2004 31; 524–529
two paths: occlusal splints and functional orthopedic
appliances. Occlusal splints eliminate the potential
triggering factor but do not modify the occlusal
discrepancies.
Functional appliances help in the movement of the
teeth, with the achievement of good facial muscle
function (9–12). This suggested the use of an original
functional appliance called function generating bite for
deep bite correction (FGB-D) in the treatment of
muscle-related TMD patients requiring orthodontic
treatment. This kind of appliance with metallic bite
planes acts simultaneously as an orthodontic corrector
(9, 13–16) and as an occlusal splint.
To evaluate the effects of the FGB-D on the masti-
catory muscles, the EMG activity of left and right
masseter and temporalis anterior muscles during
clenching has been investigated in a group of muscle-
related TMD subjects requiring orthodontic treatment,
just before the therapy and after 12 months of therapy.
Materials and methods
Subjects
Three groups of 11 subjects each (three men, eight
women in each group) were selected for this study. Two
groups were selected from a group of TMD patients
presenting to the Orthodontics and Gnathology Unit,
Turin University, where orthodontic and TMD patients
are treated. The first group (F – functional, mean age
25Æ4 6Æ3 years) was suffering from muscle-related
TMD requiring orthodontic treatment for deep bite
correction. This group was treated with FGB-D. The
second group (S – splint, mean age 27Æ2 4Æ2 years)
was suffering from muscle-related TMD not requiring
orthodontic treatment. This group was treated with a
Michigan occlusal splint (17). Inclusive criteria for both
group F and S were: (1) diagnosis of muscular condi-
tions, according to Dworkin and Le Resche (1); (2)
absence of any kind of fixed or removable prosthetic
restorations; (3) absence of periodontal disease; and (4)
presence of all teeth (with the exception of the third
molars). During the 12 months of therapy the patients
were treated by expert operators.
A control group (C – control, mean age
26Æ3 3Æ1 years) of healthy subjects was selected from
the students of the Orthodontics Specialization School
of Turin University. Furthermore these subjects had (1)
any kind of fixed or removable prosthetic restorations,
(2) no periodontal disease, (3) presence of all teeth
(with the exception of the third molars), and (4) any
kind of orthodontic or occlusal therapy in the previous
12 months. These subjects did not wear any appliance
or splint during the period considered in this study. All
subjects gave their written informed consent.
Instrumentation
The superficial bundle of the right and left masseter
muscles (MM) and the right and left temporalis anterior
muscles were studied. Disposable silver/silver chloride
bipolar surface electrodes DUO-TRODE*, diameter
10 mm and inter-electrode distance of 21 mm, were
positioned on the muscle bellies parallel to the muscle
fibres: for the masseter the belly was palpated during
clenching and the electrodes were fixed parallel to the
fibres 2Æ5 cm above the mandibular angle to avoid
influence from the end-plate location (18); for the
temporalis anterior the belly was palpated during
clenching and the electrodes were fixed along the
anterior margin of the muscle 2 cm above the zygo-
matic arch (19). A disposable ground electrode was
fixed to the right sternocleidomastoid muscle.
Before electrode placement, the skin was carefully
cleansed with abrasive paste to reduce electrode imped-
ance, and recordings were made after 5 min to allow
absorption of the conductive gel by the skin.
Surface EMG recordings were obtained from four
channels of the eight-channel electromyograph
FREELY
†
. The EMG signal was amplified, digitized and
digitally filtered. The instrument was interfaced with a
personal computer (PC) which displayed the data
graphically and stored them on magnetic media for later
analysis (EMA software
†
). The signals were assessed as
the root mean square (RMS) of the amplitude.
Measurement
Surface EMG recordings were obtained during two
sessions: before treatment began (T0) and after
12 months of active treatment, with and without (T1
w
and T1
wo
, respectively) appliance/splint in place. The
control group was monitored at the same time.
The subjects sat upright without head support; they
fixed their eyes on a target (a green rectangle
*Myotronics Inc, Tukwila, WA, USA.
†
DeGoetzen spa, Olgiate Olona, VA, Italy.
MASTICATORY MUSCLES AND RELATED TEMPOROMANDIBULAR DISORDERS 525
ª 2004 Blackwell Publishing Ltd, Journal of Oral Rehabilitation 31; 524–529
30 cm · 5 cm) in front of them at 90 cm. Testing was
performed in a silent and comfortable environment.
In the first session (T0) the subjects were asked to
perform maximum voluntary clenching (MVC) (‘clench
as hard as possible’) in the intercuspal position. In the
second session, after 12 months, they performed maxi-
mum voluntary clenching with FGB-D or splint in place
(T1
w
) and then, after a rest of 30 min without removing
the electrodes and without any appliance or splint in
place, in the intercuspal position (T1
wo
).
At each session, a first recording for the standardiza-
tion of EMG potentials was made as follows. In each
subject, two cotton rolls (diameter 10 mm) were
positioned on the mandibular teeth (second premolar
and molars), and the maximum voluntary clench was
recorded. For each muscle, the maximum potential was
found and set at 100% and all further EMG potentials
were expressed as a percentage of this value (units, lV/
lV%) (20). To avoid muscle fatigue, a rest period of
3 min was allowed between recordings. Each test was
repeated three times at each session. Each recording
lasted for 5 s followed by a rest period of 3 min.
Function generating bite for deep bite correction
The FGB-D was designed over 20 years ago (9, 13); its
orthodontic characteristics will not be discussed here, as
they have been described in detail elsewhere (14–16).
However, the most important characteristic, for the
FGB-D (Fig. 1), is the presence of anterior and posterior
metallic bite planes, which disengage the mandible
from occlusion.
Statistical calculations
The direction of the forces of the masseter and tempo-
ralis anterior muscles may produce a lateral deviation of
the mandible. To evaluate this effect a torque index,
was calculated from the averaged sEMG potentials as
proposed by Ferrario et al. (21):
Torque index ¼
(TAR+MML) (TAL+MMR)
(TAR+MML) þ (TAL+MMR)
%;
where TAR and TAL are the temporalis anterior
muscles RMS values (right and left, respectively) and
MMR and MML are the RMS values of masseter
muscles (right and left, respectively).
This index ranges from )100% to +100%: positive
values indicating a stronger right-side resultant force,
and negative values a stronger left-side resultant force.
According to Ferrario et al. (21), during MVC the
normal value of the torque index is 9Æ47 7Æ19%.
Indices were calculated automatically for each test by
EMA software.
The mean torque index was calculated from the three
tests for each subject at each session. Mean values and
standard deviations were calculated for the torque
indices, and also for the positive and negative values
taken separately. Torque indices before and after
therapy were compared using Student’s t-test for paired
data. The data were also analysed using one-way
repeated measurement analysis of variance (
ANOVA
),
followed by post hoc Tukey–Kramer multiple compari-
sons test when required.
Results
Absolute mean values and standard deviations of
torque indices are reported in Table 1. Results of
Student’s t-test are reported in Table 2.
In the group F, all subjects showed an above-normal
torque index at T0. The values decreased at T1
wo
, after
12 months of therapy with FGB-D (Fig. 2) (P <0Æ05). A
further reduction (Fig. 3) was observed at T1
w
(P <0Æ05). At the beginning of therapy, subjects in
group S had a normal torque index. The value was
Fig. 1. The appliance and a sche-
matic representation of the FGB-D in
place. (1) anterior metallic bite
planes; (2) posterior metallic bite
plane.
T. CASTROFLORIO et al.526
ª 2004 Blackwell Publishing Ltd, Journal of Oral Rehabilitation 31; 524–529
unchanged after 12 months of therapy with the
Michigan occlusal splint.
The control group had normal values of the torque
index at T0 and T1
wo
.
The inter-group analysis showed a highly significant
difference when F group and C group or F group and S
group are compared at T0 (P <0Æ0001) (Table 3).
Discussion
In healthy subjects, a mandibular rotation on the
horizontal axis may be counterbalanced by the actions
of ligaments and other jaw muscles forces. Probably the
lack of this dynamic balance could be an etiologic factor
in TMD. It has been demonstrated that facial asymmetry
is of significant incidence in patients with TMD (22).
Inui et al. (23) reported that facial asymmetry, if present,
was characterized as a mandibular lateral displacement
and a canted frontal occlusal plane ascending towards
the displaced side of the mandible in TMD patients.
It has been reported that a reversible approach (drugs
or splints) is preferable to minimize the risk of iatrogenic
complications and because radical treatments are not
superior to a reversible approach (24). The orthodonthic
treatment of TMD patients is considered to be radical
and to risk iatrogenic complications: there is a significant
risk of failure. It is clear that there is no single approach
to the management of TMD and the rationale underly-
ing orthodontic treatment of TMD patients must be
verified. Previous studies report some specific types of
malocclusion (especially distal molar occlusion, Angle
class III, open bite, extreme maxillary overjet, unilateral
cross-bite and deep bite) to be significantly associated
with signs and symptoms of TMD (5–8, 25). The
orthodontic correction of these occlusal features in
children may reveal a reduction in signs and symptoms
of TMD. An important issue is whether orthodontic
therapy aimed at correcting the occlusion will affect the
neuromuscular system. The torque index proposed by
Ferrario et al. (21) may be helpful in these studies
because it indicates the presence of resultant muscular
forces with a laterodeviating effect on the mandible.
Table 1. Torque index (TOR): mean values and standard devi-
ation at T0, T1
wo
and T1
w
T0 T1
wo
T1
w
Mean s.d. Mean s.d. Mean s.d.
Group F 52Æ27 14Æ24 17Æ05 14Æ96 6Æ06 6Æ58
Group S 10Æ53 7Æ18 12Æ92 11Æ84 9Æ95 12Æ42
Group C 6Æ01 1Æ13 5Æ66 3Æ31
Table 2. Torque index (TOR) at T0 and T1: Student’s t-test results
TOR_T0 v. TOR_T1
wo
TOR_T1
wo
v. TOR_T1
w
Group F 0Æ0003*** 0Æ023*
Group S n.s. n.s.
Group C n.s
***Very significant.
*Significant.
Torque index: effects of FGB therapy
n
= 11
TOR_T0 = Torque index at the beginning of therapy
TOR_T1wo = Torque index after 12 months therapy with FGB
T0 vs. T1wo (Student's
t
-test) =
P
< 0·05
80
70
60
50
40
30
20
10
0
–10
TOR_T1wo
Mean = 52·27
s.d. = 14·24
TOR_T0
Mean = 17·05
s.d. = 14·96
± s.d.
± s.e.
Mean
Fig. 2. MVC in intercuspal position: torque index at the begin-
ning of therapy (TOR_T0) and after 12 months of therapy with
FGB (TOR_T1
wo
).
Torque index: occlusal effects after 12 months
n
= 11
TOR_T1wo = Torque index after 12 months therapy
TOR_T1w = Torque index after 12 months therapy: FGB in place
T1wo vs. T1w (Student's
t
-test) =
P
< 0·05
40
35
30
25
20
15
10
5
–5
0
± s.d.
± s.e.
Mean
TOR_T1w
Mean = 17·05
s.d. = 14·96
TOR_T1wo
Mean = 6·06
s.d. = 6·58
Fig. 3. MVC in intercuspal position after 12 months of therapy
determined without FGB (TOR_T1
wo
) and with FGB in place
(TOR_T1
w
).
MASTICATORY MUSCLES AND RELATED TEMPOROMANDIBULAR DISORDERS 527
ª 2004 Blackwell Publishing Ltd, Journal of Oral Rehabilitation 31; 524–529
The results of this study showed that FGB-D is able to
correct an abnormal torque index after 12 months of
therapy.
Blanskma and Van Eijden (26) reported that the
anterior temporalis muscle is more concerned with
laterodeviation of the mandible than with a closing
action. Our data suggest that the FGB-D probably has
more effect on the anterior temporalis muscle. This may
be due to the stimulus from the periodontal mechano-
ceptors, but considering the change in jaw position, it
may also be the result of a differential output from
other peripheral sensory receptors, such as joint recep-
tors and muscle spindles.
The group S showed a close to normal torque index
before and after therapy with the Michigan occlusal
splint, and similar results were obtained from the
control group. These data suggest that dental deep bite
may be involved in some muscle-related TMD. Further
investigation is necessary before we can fully under-
stand the mechanism of its involvement.
Conclusions
The main conclusions of this study are:
(1) Quantitative sEMG of masticatory muscles cannot
distinguish healthy subjects from those with muscle-
related TMD (group H versus group S);
(2) Deep bite seems to be related to a high torque index
and then to a lateral displacement of the mandible;
(3) The FGB-D reduces the torque index.
These findings suggest that FGB-D may be useful in
muscle-related TMD patients requiring orthodontic
treatment for dental deep bite correction. Because of
the small number of patients investigated, these results
can be considered to be preliminary.
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Table 3. One-way
ANOVA
results: the differences of the torque index between the groups
T0 T1
wo
T1
w
Mean difference q-value P-value Mean difference q-value P-value Mean difference q-value P-value
Cv.F )46Æ66 16Æ018 <0Æ0001 )11Æ62 3Æ321 n.s. )0Æ63 0Æ244 n.s
Cv.S )4Æ92 1Æ692 n.s. )7Æ50 2Æ142 n.s. )4Æ52 1Æ739 n.s
Fv.S 41Æ73 14Æ680 <0Æ0001 4Æ11Æ208 n.s. )3Æ89 1Æ532 n.s
T. CASTROFLORIO et al.528
ª 2004 Blackwell Publishing Ltd, Journal of Oral Rehabilitation 31; 524–529