Changes in lips, cheeks and tongue pressures after upper incisor protrusion in Class II division 2 malocclusion: a prospective study
Partal and Aksu Progress in Orthodontics
Changes in lips, cheeks and tongue pressures after upper incisor protrusion in Class II division 2 malocclusion: a prospective study
Irmak Partal 0
Muge Aksu 0
0 Department of Orthodontics, Faculty of Dentistry, Hacettepe University , Sihhiye, 06100 Ankara , Turkey
Background: The etiology of Class II division 2 (CII/2) malocclusion focuses on heredity; however lip, cheek, and tongue pressures that are associated with the environmental effect are considered to have an effect. The aim of this study was to evaluate the relation between perioral pressures and the upper incisor inclination in CII/2 malocclusion. Methods: Twenty patients (8 females, 12 males; mean age 10.29 ± 0.90 years) with CII/2 malocclusion were included in the study group, and 15 patients (5 females, 10 males; mean age 10.56 ± 1.06 years) with Class I malocclusion were included. The upper incisors were protruded with a utility arch (0.016 × 0.022 in. blue elgiloy wire). Perioral pressure assessment was made with the Iowa Oral Pressure Instrument. Upper lip pressure, lower lip pressure, vertical lip pressure, left-right buccal pressures, swallowing, and maximum tongue pressures were measured. Repeated measure ANOVA was used to test the intragroup differences. Intergroup comparisons were made using two-way repeated measure ANOVA with Bonferroni correction. Relationships between the variables were analyzed using rank correlation (Spearman's rho). The significance for all statistical tests was predetermined at p < 0.05. Results: A significant change occurred in the upper lip pressure, lower lip pressure, and vertical lip pressure; however, significant difference was not found between the groups. Upper lip pressure increased significantly in both groups. In the CII/2 group, lower lip pressure increased after protrusion and decreased after retention, while vertical lip pressure decreased and then increased significantly. Left buccal pressure changes between the groups were not parallel. Right buccal pressure, swallowing, and maximum tongue pressure changes were not statistically significant. Statistically significant correlation was found between U1-NA (mm) and vertical lip pressure (r −0.467). Conclusions: In the CII/2 group, upper lip pressure increased only in retention. Lower lip pressure increased and vertical lip pressure decreased after protrusion. Nevertheless, these changes did not remain stable after the retention period. The difference between groups was not statistically significant at the end of retention.
Class II division 2 malocclusion; Perioral pressure; Incisor position
The identification of the etiology is quite important for
the success of the orthodontic treatment. In the etiology
of Class II division 2 (CII/2) malocclusion, genetics is
accepted to be the most important etiologic factor. In
studies among twins and triplets, strong evidences were
obtained regarding the fact that genetics is the
fundamental etiologic factor in the development of CII/2
], though other etiologic factors are
lip, cheek, and tongue-related environmental factors.
Previous studies have shown that the lips and cheeks,
rather than the tongue, are the most important
environmental factors of teeth position [
]. For stable
treatment results, it is necessary to determine the effects of
those factors in malocclusion.
Lower lip resting pressure is indicated to be more
affective on the position of the upper incisors rather
than the upper lip [
]. In some studies, high lip line
was shown as the reason behind the retroclined position
of the upper incisors [
], while in others, the
hyperactive lip or mentalis muscle were shown as the reason
]. It has been shown that for individuals with CII/
2 malocclusion exposed to significantly higher resting lip
pressure than those with Class I malocclusion, the high
lower lip line and its pressure were found to be related
to the retroclination of the upper incisors in CII/2
]. Oppositely, in another study, it was
mentioned that the pressure from the lips is a result of
the incisor position [
In the early treatment of CII/2 malocclusion,
correcting the molar relationship and inclination of the upper
incisors are recommended for the initial phase of
treatment. Several appliances such as cervical headgear,
removable appliances with anterior bite block, or fixed
appliances can be used to correct the malocclusion. The
study examining the effects of the cervical headgear on
tongue pressure is present in the literature [
However, there are no clinical studies examining the effects
of fixed-treatment mechanics, mentioned above, on
perioral soft tissues yet.
The purposes of our study were to evaluate the
changes in perioral pressures after protruding the
upper incisors and to investigate if there is any
relationship between the upper incisor inclination and
perioral pressure changes. The null hypothesis is that
protruding the upper incisors does not change the
This prospective study was approved by the Ethics
Committee of Hacettepe University with the approval
number KA-15027. The individuals and their parents
were informed about the treatment process, and all of
them signed consent forms voluntarily.
The study group was composed of 20 Caucasian
subjects (8 females and 12 males), with the mean age of
10.29 ± 0.90 years, who have CII/2 malocclusion and
applied to be treated at the Hacettepe University, Faculty
of Dentistry, Department of Orthodontics. The inclusion
criteria for the study group were (1) horizontal growth
direction, (2) deep bite, (3) retroclined upper incisors,
(4) cusp to cusp and/or Class 2 molar tooth relation, (5)
not having congenitally missing upper incisors, and (6)
not having orthodontic treatment before.
A control group was also composed to distinguish the
growth effects on perioral pressures when compared to
the study group. Fifteen Caucasian subjects (5 females
and 10 males) with the mean age of 10.56 ± 1.06 years
who had Class I malocclusion with minimum crowding
were included. The inclusion criteria for the control
group were (1) upper incisors with normal inclination
and (2) not using any fixed or removable orthodontic
appliance during the study.
The exclusion criteria for both of the groups were (1)
having any systemic disease or craniofacial deformity
and (2) having any bad oral habits.
Perioral pressure evaluation
The evaluation of the perioral pressure of the subjects in
the study and control groups was done by using the
Iowa Oral Performance Instrument (IOPI). The
pressures were recorded before the treatment (T0), at the
end of incisor protrusion (T1), and after the retention
period (T2). Control group records were taken
approximately 6 months later from the T1 records, similar to
the study group.
IOPI is a pressure-sensing circuitry and used in
conjunction with a connecting tube and measurement bulb
(Fig. 1). The device measures the maximum pressure of
the lips, cheeks, and tongue in an air-filled bulb. Each
Fig. 1 Iowa Oral Pressure Instrument
measurement was repeated every 10 s, for three times in
total. The pressure values were recorded in terms of
kilopascals (kPa), and the average values were calculated
according to the arithmetic mean.
During the measurement, comfortable, relaxed, and
upright sitting position of the patient was provided to
achieve natural head position. All participants were
trained by verbal training before taking the perioral
The following measurements were done in order to
evaluate the perioral pressure:
Upper lip pressure (ULP): The measurement bulb
was located between the upper lip and upper
incisors, in the midline between the upper central
Lower lip pressure (LLP): The measurement bulb
was located between the lower lip and lower
incisors, in the midline between the lower central
Vertical lip pressure (VLP): The measurement bulb
was located between the upper and lower lips, in the
midline without contacting the teeth.
Left buccal pressure (LBP): The measurement bulb
was located on the premolar teeth, aligned distal to
the left canine tooth.
Right buccal pressure (RBP): The measurement bulb
was located on the premolar teeth, aligned distal to
the right canine tooth.
Swallowing tongue pressure (STP): The
measurement bulb was located on the tongue, and
the patient was asked to swallow.
Maximum tongue pressure (MTP): The
measurement bulb was located on the tongue, and
the patient was asked to squeeze the measurement
cap between his tongue and palate.
In order to determine the changes of upper incisors’
inclination (U1-SN angle, U1-NA angle, U1-NA distance),
digital lateral cephalometric radiographs were taken in
natural head position for the study group at the T0, T1,
and T2 stages. The cephalometric measurements were
performed by using the 11.8 version of Dolphin software
(Dolphin Imaging & Management Solutions, Chatsworth,
Orthodontic treatment protocol
Orthodontic treatment started with application of a
transpalatal arch for increasing the anchorage of the
upper molar teeth. Conventional brackets of 0.018-in.
slots (Gemini, 3M Unitek, Monrovia, CA) were bonded
to the upper incisors. A 0.016-in. and/or 0.016 ×
0.016in. nickel titanium levelling utility arch was applied
according to the crowding. After the levelling process, a
protrusion utility arch was bent out of a 0.016 ×
0.022in. blue elgiloy wire and was applied. The subjects were
observed every 4 weeks. In order to determine if there is
enough protrusion, the inclination of the incisors was
evaluated clinically. Moreover, whether there was an
interference between upper and lower incisors or not
was controlled by bringing the mandibula towards to the
Class 1 molar-canine tooth relation. When enough
protrusion was achieved, a Hawley retainer was applied
during the 6-month retention period.
SPSS Statistics software (version 21, IBM Corp, Armonk,
NY) was used for the analysis of the data. The normality of
the variables is evaluated with the Kolmogorov-Smirnov
test. In repetitious measurements, the intra-observer
reliability was evaluated with intra-class correlation
coefficient. All cephalometric measurements were realized by the
first author (….), and the measurements of five patients
were repeated within 15 days. In order to evaluate the
reliability of the perioral pressures, the measurements that
have been repeated before were used.
A power calculation indicated that the achieved power
for the study was 0.98. The descriptive statistics were
expressed as mean ± standard deviation for continuous
variables. The two groups were evaluated using the
independent sample t test in terms of quantitative variables
and chi-square test in terms of categorical variables. In
order to observe the difference of the variables within
the group according to the time frames, repeated
measure ANOVA was utilized. Two-way repeated measure
ANOVA was used to examine the main effects of the
malocclusion groups within the treatment stages and the
interaction effects between them for the pressure
measurements. The Bonferroni test was used as post
hoc multiple comparisons. Spearman rho correlation
coefficient was calculated to find any correlation
between the upper incisor position and pressure
measurements. The results for p < 0.05 were accepted to be
The data is distributed normally according to the
Kolmogorov-Smirnov normality test. While the
intraclass correlation coefficient for cephalometric
measurements is between 0.972 and 1.000, for the perioral
pressure, measurements are between 0.924 and 0.986.
In terms of sex distribution, there was no significant
difference between the study and control groups
(p > 0.05). At the same time, no statistically significant
difference between the groups in terms of starting age of
treatment could be identified (p > 0.05). The duration of
orthodontic treatment was calculated as 4 months,
retention period as 6 months, and the total duration of
treatment as 10 months.
Cephalometric measurements are shown in Table 1.
All parameters about upper incisor inclination (U1-SN
angle, U1-NA angle, and U1-NA distance) increased
significantly (p < 0.001, T1-T0, T2-T0, Table 2).
There are statistically significant changes observed for
the measurements of ULP (p < 0.001), LLP (p < 0.01),
and VLP (p < 0.001) (Table 3). The interaction between
malocclusion and treatment was seen in LLP, VLP, and
LBP (p < 0.01) (Table 3).
The difference between the groups was observed in
LBP at the beginning of treatment (p < 0.05, T0, Table 4)
and in VLP after the protrusion (p < 0.05, T1, Table 4).
The perioral pressure changes in different treatment
stages between the study and the control group are
shown in Table 5. ULP increased significantly after the
retention period in the study group (p < 0.01, T2-T1)
and also in the control group (p < 0.05, T2-T1). These
increases for both groups remained stable at the end of
the study (p < 0.05 for the study group, p < 0.01 for the
control group, T2-T0). In the study group, LLP increased
after protrusion (p < 0.01, T1-T0) and then decreased
back to the initial values after the retention period
(p < 0.001, T2-T1). Meanwhile, VLP decreased after
protrusion (p < 0.01, T1-T0) and then increased back
significantly after the retention period (p < 0.01, T2-T1).
While, in the control group, VLP increased during the
study and it was observed that at the end of the study,
the value was statistically significant (p < 0.01, T2-T0).
Moreover, LBP decreased significantly after protrusion
in the study group (p < 0.01, T1-T0). The significance
has not been determined at the RBP, STP, and MTP
measurements (p > 0.05).
The relationship between the changes in the
inclination of upper incisors (U1-SN angle, U1-NA angle,
U1NA distance) as well as the changes in the ULP, LLP,
and the VLP were examined after the protrusion and
retention periods. The decrease in the VLP (1.57 kPa) had
a negative and moderate correlation (correlation
coefficient −0,467, p < 0.05) with the increase in the U1-NA
distance (6.29 mm), after the protrusion period (T1-T0,
Table 6). The significance has not been found between
the remaining parameters (p > 0.05).
Perioral structures play important roles in the
development of either a normal occlusion or a malocclusion.
Soft tissues like the lips, cheeks, and tongue affect hard
tissues and orthodontic treatment results by perioral
pressures, muscle forces, and periodontal attachments.
Therefore, soft tissue limitations should be assessed
more precisely by orthodontists, and they should
consider not only the genetics but also the environmental
According to the balance theory defined by Weinstein
et al. [
], the teeth are balanced by the tongue from the
inside and by the lips and cheeks from the outside. At
the same time, even though the magnitude of force is
low, it may cause a movement in the teeth when applied
for a sufficient amount of time [
]. Graber [
stated that the changes observed in the muscle functions
may change the normal morphology or may compound
the current malocclusion where he examined the muscle
morphologies of the Class I, Class II, and Class III
malocclusions. Evaluations regarding the soft tissues are
very important for determining the malocclusion
etiology and the stability of the orthodontic treatment.
Thus, in this study, we aimed for clarifying the
relationship between the upper incisor protrusion and perioral
In the evaluation of the soft tissues, it is possible to
investigate thickness or volume measurements as well as
the electromyographic or electrodynamic measurements.
Applying electrodynamic measurement techniques with
strain gauges is a reliable method for evaluating soft
tissue forces and pressures [
]. Lindeman and Moore
], comparing three different methods of evaluating
the perioral pressure and the force, maintained that the
lips cause fluid-like pressure, and thus, they should be
evaluated using devices that are sensitive to pressure
*p < 0.001
instead of the force. IOPI is a reliable tool that measures
the perioral pressures from the lips, cheeks, and tongue
]. Additionally, there is no need for additional
laboratory processes and no risk of contamination because
of its single use measurement bulbs.
It was mentioned in previous studies that perioral
pressures were affected by the alterations in head
]. According to Thüer et al. , Ingervall
and Thüer [
], and Hellsing and L’Estrange [
perioral pressures were increased with the head
extension than the natural head position. Therefore, in our
study, we paid attention during the pressure records to
the maintenance of natural head position.
Consensus about the relationship between the types of
malocclusion and perioral pressures has not been
achieved yet. Lapatki et al. [
] indicated that the upper
incisors were exposed to more resting lip pressure in
CII/2 malocclusion compared to Class I occlusion.
Oppositely in another study, it has been shown that
maximum lip pressure was the lowest in individuals with
CII/2 malocclusion [
]. Nevertheless, in both studies,
the individuals were older and also the methodology of
these studies was different from our study. Thüer and
] found that the ULP on the upper incisors
was at the lowest in CII/2 malocclusion and at the
highest in Class II division 1 malocclusion. They also
mentioned that the LLP did not show significant difference
between malocclusions. In a study that included both
young and adult individuals, ULP change between Class
I and Class II malocclusions did not show significant
]. Additionally, Lambrechts et al. [
that among the types of malocclusion, no significant
differences could be found in terms of tongue pressure. In
our study, there are no significant pressure differences
except LBP between the study and control groups at the
beginning of the treatment. This difference disappeared
with the decrease of LBP after the protrusion period.
However, this decrease has not been thought to result
from orthodontic treatment. Though a utility arch has
buccal segments, tooth movement apparently occurs at
the anterior region. Additionally, since the utility arch is
a symmetric appliance, we would wait to see the same
effect on the right side. But, a significant change in RBP
has not been determined. It was thought that these
differences might be due to transitions in dentition, and
although attention is shown, bending and inserting the
utility arch may be different from one side to the other.
Mitchell and Williamson [
] and Posen [
stated that the perioral forces increase with age. In the
present study, it was seen that ULP increased during the
retention period in both groups. The retention period
covered a longer phase than the protrusion period. Di
Fazio et al.  found an increase in the upper lip
pressure with age, and he suggested that this increase might
be described by the maturation of the orbicularis oris
muscle due to growth. Additionally, in another study, it
was indicated that the ULP tended to be high in children
with a large overjet [
]. These observations might be
acceptable for our study.
In the present study, LLP increased and VLP
decreased after upper incisor protrusion but these changes
did not remain stable in the study group. Some authors
] stated that the upper incisor position was
*p < 0.05
determined by the increased LLP in CII/2 malocclusion.
In one of the studies that investigated the effect of
increased overjet in perioral pressures, it was observed
that LLP increased , while in another study, VLP
decreased because tightening of the lips became harder
]. These findings are compatible with the findings of
our study. However, these studies had a cross-sectional
study design, and individuals with an increased overjet
were included. Meanwhile, in the control group, VLP
did not increase significantly after protrusion and then
significantly after the protrusion period. Due to these
changes, significant difference occurred at the end of
protrusion between the two groups. In fact, although not
significant, LLP also increased in the control group. All
these lip pressure increases observed in the control
group might be described as mentioned above in ULP.
The only correlation between the incisor inclination and
the lip pressures was found between the increase of the
U1-NA distance and the decrease of VLP. This
correlation, even though statistically significant, was moderate.
*p < 0.05
However, a significant correlation between the increase of
VLP and the decrease of upper incisor inclination was not
observed after the retention. Therefore, VLP which did
not remain stable after retention was not associated with
the mild relapse of upper incisor protrusion. Such as soft
tissue morphology changes, other factors that may be
effective on the perioral pressures need to be investigated
in further studies. Thüer and Ingervall [
] determined that
ULP was correlated with the morphology of the lips, while
Di Fazio et al. [
] found a significant correlation between
ULP and age.
This is the first study to evaluate the perioral pressure
changes in terms of lips by IOPI. Furthermore, IOPI can
be considered as a diagnostic tool for malocclusions and
the progression of myofunctional exercises. Hence,
orthodontists can use IOPI for the malocclusions arising
from bad oral habits in a practical way. The only
limitation of this prospective study was that transitions in
mixed dentition might affect the perioral pressure
It has been known that soft tissues affect skeletal and
dentoalveolar hard tissues during growth. Our findings
showed that changes that occurred in the anterior teeth
had a temporary effect on the soft tissues. Therefore, it
should be noted that permanent changes could not be
achieved in soft tissue pressures after upper incisor
protrusion and relapse resulting from soft tissue pressures
should always be kept in mind. In further studies,
perioral pressure changes can be examined by including
Permanent changes did not occur in perioral pressures with
upper incisor protrusion in CII/2 malocclusion. In addition,
there is a negative moderate correlation between the
protrusion of upper incisors and the vertical lip pressure.
CII/2: Class II division 2 malocclusion; IOPI: Iowa Oral Performance Instrument;
LBP: Left buccal pressure; LLP: Lower lip pressure; MTP: Maximum tongue
pressure; RBP: Right buccal pressure; STP: Swallowing tongue pressure;
ULP: Upper lip pressure; VLP: Vertical lip pressure
The authors would like to thank Jale Karakaya for her assistance with the
This study was supported by Hacettepe University Scientific Research
Coordination Unit (Project Number: TDH-2017-10,747).
Availability of data and materials
The data of this study is not available.
IP participated in treating the patients, collecting and interpreting the data,
and writing the manuscript. MA participated in designing the study protocol
and revising the manuscript critically for intellectual content. Both authors
read and approved the final manuscript.
Ethics approval and consent to participate
The study was reviewed and approved by the Ethical Committee of
Hacettepe University (Ankara, Turkey) with the approval number KA-15027.
All procedures followed adhered to the Declaration of Helsinki guidelines. All
patients and their parents were informed of the purpose of this study and
signed an informed consent form voluntarily.
Consent for publication
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
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