Keywords
hyaluronic acid - lips - orbicularis oris muscle - facial harmonization - EMG
Introduction
Aging affects facial harmony and symmetry due to cellular damage and genetic predispositions.[1] Over time, changes such as reduced lip volume, sagging, uneven skin tone, and bone
support loss impact both function and aesthetics, influencing quality of life.[2] The lips, located at the center of the lower third of the face, are essential for
facial harmony, beauty, and aesthetic appeal. This area is the most expressive in
terms of movement and facial dynamics and experiences morphofunctional changes as
time passes.[3] Full, well-defined lips with a reddish hue and hydration, along with elevated lip
corners and a smooth texture, are associated with beauty and youth.[4]
The global demand for solutions to combat aging has been increasing, leading to a
notable 54.4% growth in nonsurgical aesthetic procedures over the last 4 years.[5] Among these procedures, facial fillers—particularly those based on hyaluronic acid—play
a fundamental role in addressing tissue aging by restructuring and restoring volume
to key areas of the face, such as the lips. As a leading nonsurgical solution for
facial rejuvenation and beautification,[6]
[7] hyaluronic acid is naturally present in the extracellular matrix of various tissues.
It is highly hydrophilic, hypoallergenic, and biocompatible, providing essential hydrodynamic
properties that support hydration, tension, and tissue integrity.[8]
[9]
Beyond its volumizing effects, hyaluronic acid may influence muscle function by altering
tissue biomechanics and neuromuscular response. In the perioral region, it can modify
muscle tone, elasticity, and contractile patterns, influencing facial expressions.[10] Understanding these effects is essential for optimizing aesthetic and functional
outcomes, emphasizing the need for muscular assessments.[11] In this context, electromyography serves as a valuable tool for analyzing neuromuscular
changes, providing deeper understanding of the functional impact of refining treatment
approaches.
Moreover, this filler not only enhances lip volume but also interacts with underlying
muscular structures like the orbicularis oris muscle. This muscle, consisting of superficial
and deep fibers around the lips, is important for mouth-related functions, underscoring
the interdependence between the structural and functional aspects of the face in the
facial beautification process.[12]
Despite the growing use of hyaluronic acid fillers, their functional impact on the
orbicularis oris muscle remains understudied. Given its role in speech, mastication,
and oral competence, filler-induced changes in tone, elasticity, and resistance could
have effects beyond aesthetics. The lack of objective studies highlights the need
for further research on these potential impacts.
Therefore, the objective of our longitudinal study was to evaluate the electromyographic
(EMG) activity of the orbicularis oris muscle in adult women after hyaluronic acid
lip filling. The null hypothesis states that no differences exist in the EMG of the
orbicularis oris muscle 60 days post-lip filling.
Materials and Methods
Study Design and Sample Selection
This longitudinal study was approved by the ethics committee (protocol #10589419.0.0000.5419).
Informed consent was obtained from all subjects.
The sample size was calculated using G*Power 3.1.9.2 (Franz Faul, Kiel University,
Germany) for an a priori test (pilot project). With a significance level of α = 0.05, an effect size of 1.26 and a power of 84%, the calculation, based on the
EMG of the upper orbicularis oris muscle during lip protrusion, determined a minimum
of 10 subjects.
Among 50 women evaluated, 22 were selected based on the inclusion and exclusion criteria.
Their ages ranged from 20 to 59 years (mean age 35.4 ± 12.3), normal occlusion, excluding
third molars. Women exhibiting temporomandibular dysfunction, mental or physical discomfort
during evaluations, mouth breathing, lip incompetence, use of muscle relaxants, speech
therapy, otolaryngological treatment, recent orthodontic treatment (less than a year),
or prior history of lip filling were excluded from the study.
Hyaluronic Acid Injection
The procedure was conducted by a dental surgeon specialized in orofacial harmonization,
utilizing the Restylane Kysse product, which comes in a syringe containing 20 mg/mL
of hyaluronic acid (Galderma SA, Lausanne, Switzerland). Given the high mobility of
this region, the use of low molecular weight hyaluronic acid was chosen to minimize
interference with the natural biomechanics of lip movement.[13]
The protocol followed Blandford et al's technique,[14] with the only change being the use of a 25-gauge microcannula. This method was chosen
for its ease of reproducibility and ability to reduce edema and bruising, common postprocedure
symptoms in the labial area after dermal fillers.
After application of operational eligibility criteria, participants underwent the
following procedures before hyaluronic acid application: facial antisepsis, biosafety
protocol (gown, cap, mask, and gloves), and local anesthesia, including infraorbital
and mental nerve blocks. In this study, the block did not interfere with the EMG,
as the assessment was conducted before the procedure and again after 30 and 60 days,
ensuring accurate measurements.
Hyaluronic acid was deposited in the superficial compartments above the orbicularis
oris muscle ([Fig. 1]). A total of 1 mL was injected, with 0.6 mL in the upper lip and 0.4 mL in the lower
lip. This distribution is based on aesthetic principles and the physiological changes
associated with aging. Over time, the upper lip loses more volume due to a reduction
in collagen, elastin, and bone resorption, leading to thinning, elongation, and reduced
vermilion exposure. Adding volume to the upper lip aims to restore its youthful proportions,
balance it with the lower lip, and compensate for the loss of projection and support,
resulting in a more harmonious outcome.[4]
Fig. 1 Topographic anatomy of hyaluronic acid application.
EMG Analysis Orbicularis Oris Muscle
EMG recordings of the orbicularis oris muscle (upper and lower) were captured using
a wireless surface EMG system (Trigno EMG sensor, Delsys Inc., United States). The
wireless Trigno mini surface sensor, equipped with a smaller 25 mm EMG sensing head
designed for small muscles like the orbicularis oris, was used ([Fig. 2]).
Fig. 2 Trigno mini sensor location.
Following the Surface EMG for Non-Invasive Assessment of Muscles Project (SENIAM)
guidelines, a mini surface sensor was placed on the orbicularis oris muscle (upper
and lower).[15] The reference sensor was adhered in the region of the upper trapezius muscle. Following
the EMG protocol guidelines, each participant sat comfortably in a chair with hips,
knees, and ankles at a 90-degree angle, maintaining an upright posture. Skin preparation
involved alcohol-soaked gauze to reduce impedance and remove fat or dirt before sensor
application.[16]
[17]
EMG of the orbicularis oris muscle (upper and lower) was recorded during rest (4 seconds)
and sustained isometric exercises, each lasting up to 4 seconds. These exercises included
lip protrusion (pouting), lip compression (pressing the lips firmly together, activating
the muscle toward the inside of the mouth), and bilateral cheek inflation (inhaling
and holding air in the mouth to distend both cheeks symmetrically). Additionally,
right and left cheek inflation were performed by holding air specifically on one side
of the mouth. The amount of air used for cheek inflation was minimal. Tasks were repeated
three times, averaging the data for final results.
The lip tasks analyzed in this study were selected for activating different portions
of the orbicularis oris muscle, reflecting the motor function and physiological capabilities
of this muscle. Lip protrusion assesses the activation of the fibers responsible for
projecting the lips, which is essential for sound articulation and facial expressions.
Lip compression activates the deep portion, important for oral control and food manipulation.
Cheek inflation involves both the orbicularis oris and other facial muscles, being
useful in evaluating muscle function in the context of oral dynamics.
The analysis of the EMG focused on the signal amplitude, measured in microvolts (μV),
reflecting the recruitment and synchronization of motor units (motor unit action potentials).
After acquiring the EMG signal data, it underwent bandpass filtering from 25 to 250 Hz
and notch filtering at 50 Hz to reduce noise. Following this, the signal underwent
full-wave rectification and then low-pass filtering. The evaluator, a dental surgeon,
was trained in the EMG protocol.
Statistical Analysis
The data were tested for normal distribution using the Shapiro–Wilk test. Statistical
analyses were conducted with SPSS version 20.0 (SPSS Inc., Chicago, Illinois, United
States). The results were analyzed using a t-test (p < 0.05) to compare the mean EMG of the orbicularis oris muscle (upper and lower).
Repeated measures analysis of variance was conducted to compare data across these
time points, with Bonferroni correction (p < 0.05).
Results
[Table 1] shows the EMG of the upper and lower orbicularis oris muscle before, 30 days, and
60 days after hyaluronic acid lip filling. There were significant differences in the
EMG of the upper orbicularis oris muscle before versus 30 days (p = 0.04) and lower orbicularis oris muscle before versus 30 days (p = 0.0006) during bilateral cheek inflation after 30 days. Additionally, a decrease
in EMG of the upper and lower orbicularis oris muscle was observed in all tasks at
30 days. After 60 days, the upper orbicularis oris muscle showed increased EMG in
most tasks, while the lower orbicularis oris muscle showed decreased EMG in cheek
inflation tasks (concurrently or alternately). Significant differences in the upper
orbicularis oris muscle were also found between 30 and 60 days at rest (p = 0.05).
Table 1
Differences in mean values (±standard error) of EMG of the orbicularis oris muscle
before, 30 days, and 60 days after hyaluronic acid lip filling
|
EMG (µV)
OOM
|
Periods
|
p-Value
|
p-Value (Bonferroni)
|
|
Before
|
30 d
|
60 d
|
(Before) vs. (30 d)
|
(Before) vs. (60 d)
|
(30 days) vs. (60 d)
|
|
Upper
|
|
|
|
|
|
|
|
|
RE
|
4.06 ± 0.18
|
4.04 ± 0.25
|
4.73 ± 0.24
|
0.03
|
–
|
–
|
0.05
|
|
LP
|
96.95 ± 12.97
|
72.70 ± 9.47
|
87.70 ± 10.94
|
0.22
|
–
|
–
|
–
|
|
LC
|
97.60 ± 9.19
|
87.09 ± 12.98
|
103.85 ± 13.42
|
0.38
|
–
|
–
|
–
|
|
BCI
|
127.30 ± 8.21
|
101.52 ± 11.03
|
110.03 ± 11.66
|
0.05
|
0.04
|
–
|
–
|
|
RCI
|
120.15 ± 10.27
|
105.17 ± 15.53
|
100.88 ± 9.27
|
0.18
|
–
|
–
|
–
|
|
LCI
|
121.51 ± 8.68
|
114.49 ± 14.72
|
106.26 ± 10.37
|
0.36
|
–
|
–
|
–
|
|
Lower
|
|
|
|
|
|
|
|
|
RE
|
4.98 ± 0.17
|
4.96 ± 0.21
|
5.30 ± 0.22
|
0.32
|
–
|
–
|
–
|
|
LP
|
148.94 ± 16.40
|
133.56 ± 13.31
|
134.41 ± 11.58
|
0.61
|
|
|
|
|
LC
|
99.40 ± 10.64
|
83.16 ± 8.30
|
93.64 ± 12.90
|
0.45
|
–
|
–
|
–
|
|
BCI
|
178.40 ± 18.49
|
119.34 ± 11.91
|
114.92 ± 11.00
|
0.000
|
0.006
|
–
|
–
|
|
RCI
|
131.38 ± 14.56
|
109.43 ± 15.49
|
103.38 ± 10.67
|
0.08
|
–
|
–
|
–
|
|
LCI
|
126.60 ± 12.62
|
111.06 ± 11.59
|
107.31 ± 11.22
|
0.17
|
–
|
–
|
–
|
Abbreviations: BCI, bilateral cheek inflation; EMG, electromyographic activity; LC,
lip compression; LCI, left cheek inflation; LP, lip protrusion; OOM, orbicularis oris
muscle; RCI, right cheek inflation; RE, rest.
Note: Significant differences measured with repeated measures with Bonferroni correction
(p < 0.05).
[Table 2] shows the EMG of the orbicularis oris muscle, comparing tasks between the upper
and lower lips before, 30 days, and 60 days after lip filling. The lower orbicularis
oris muscle had significantly higher EMG than the upper in nearly all tasks (p = 0.000).
Table 2
Differences in mean values (±standard error) of EMG between the upper and lower orbicularis
oris muscle before, 30 days, and 60 days after hyaluronic acid lip filling
|
Period/Task
|
EMG (µV)
Orbicularis oris muscle
|
p-Value
|
|
Upper
|
Lower
|
|
Before
|
|
|
|
|
Rest
|
4.06 ± 0.18
|
4.98 ± 0.80
|
0.000
|
|
Lip protrusion
|
96.55 ± 12.97
|
158.94 ± 16.40
|
0.000
|
|
Lip compression
|
97.60 ± 9.19
|
99.40 ± 10.64
|
0.000
|
|
Bilateral cheek inflation
|
127.30 ± 8.21
|
178.40 ± 18.49
|
0.000
|
|
Right cheek inflation
|
120.15 ± 10.27
|
131.38 ± 14.56
|
0.000
|
|
Left cheek inflation
|
121.51 ± 8.68
|
126.60 ± 12.62
|
0.000
|
|
30 days
|
|
|
|
|
Rest
|
4.04 ± 0.25
|
4.96 ± 0.21
|
0.000
|
|
Lip protrusion
|
72.70 ± 9.47
|
133.56 ± 13.31
|
0.000
|
|
Lip compression
|
87.09 ± 12.09
|
83.16 ± 8.30
|
0.000
|
|
Bilateral cheek inflation
|
101.52 ± 11.03
|
119.34 ± 11.91
|
0.000
|
|
Right cheek inflation
|
105.17 ± 15.53
|
109.43 ± 15.49
|
0.000
|
|
Left cheek inflation
|
114.49 ± 15.00
|
111.06 ± 11.59
|
0.000
|
|
60 days
|
|
|
|
|
Rest
|
4.73 ± 0.24
|
5.30 ± 0.22
|
0.000
|
|
Lip protrusion
|
87.70 ± 10.94
|
134.41 ± 11.58
|
0.000
|
|
Lip compression
|
103.35 ± 13.42
|
93.64 ± 12.90
|
0.000
|
|
Bilateral cheek inflation
|
110.03 ± 11.66
|
114.92 ± 11.00
|
0.000
|
|
Right cheek inflation
|
100.88 ± 9.27
|
103.38 ± 10.67
|
0.000
|
|
Left cheek inflation
|
106.26 ± 10.37
|
107.31 ± 11.22
|
0.000
|
Abbreviation: EMG, electromyographic activity.
Note: Significant differences, t-test (p < 0.05).
Discussion
The null hypothesis of this study was rejected due to significant differences in the
EMG of the upper and lower orbicularis oris muscle between different time periods,
after lip augmentation with hyaluronic acid.
Lip volume augmentation using hyaluronic acid can influence the function of the orbicularis
oris muscle, a strong sphincter muscle linked to the dermis of the upper and lower
lips via a thin, superficial muscle-aponeurotic system. The procedure can modify the
soft tissue structure around the mouth, causing tissue tension or stretching and possibly
affecting the muscle's natural function. Additionally, it may trigger neuromuscular
adaptation, potentially impacting EMG readings.[18]
[19]
[20]
[21]
[22]
EMG differences were noted in the upper orbicularis oris during rest and bilateral
cheek inflation and in the lower orbicularis oris during cheek inflation. The use
of hyaluronic acid on the lip may impact the EMG of the superior and inferior orbicularis
oris muscle due to local inflammation. This minimally invasive procedure triggers
symptoms such as redness, swelling, pain, and warmth as part of the body's natural
response.[22] These inflammatory signs are essential for healing and tissue adaptation, involving
the release of inflammatory mediators and increased blood flow, which supports tissue
expansion.[23]
Inflammation can temporarily reduce nerve excitability or impair nerve signal transmission
to muscles. Muscle spindles, sensory receptors that facilitate motoneuron activity,
can influence muscle function when their transduction and transmission properties
are altered.[23]
[24] Thus, the decrease in EMG of the orbicularis oris muscle 30 days after lip augmentation
is likely due to inflammation and changes in the electrophysiological and biochemical
properties of the facial muscles. Hyaluronic acid aids tissue repair and regeneration
by enhancing cellular signaling and modifying chemical and mechanical properties.[25]
Over time, the body is expected to adapt to hyaluronic acid, leading to a reduction
in symptoms. A possible explanation for the changes in the orbicularis oris muscle's
EMG after 60 days could be the impact of hyaluronic acid on muscle dynamics.[26] It can alter the structure of lip tissue, affecting the alignment and function of
muscle fibers. The interaction between the contractile proteins actin and myosin in
the cross-bridge cycle promotes movement, but increased tissue volume and stretching
can change sarcomere length and contractile efficiency, affecting the muscle's ability
to generate and control force.[27] Additionally, inflammation and tissue remodeling may disrupt excitation-contraction
coupling, causing temporary functional changes as the muscle adapts.[28]
The impact of hyaluronic acid application on EMG signal detection is influenced by
its absorption over time, which can interfere with signal capture. This interference
occurs due to the increased volume conductor effect, as the geometry and conductivity
of physiological tissues surrounding the sensors can alter signal conduction. For
instance, the moisturizing effect of hyaluronic acid can raise the volume conductor,
affecting signal capture by changing local tissue impedance and conductivity.[29] This study did not account for electrostatic factors affecting human tissues in
relation to the volume conductor.
After 60 days from the lip augmentation procedure, an increase in EMG of the orbicularis
oris muscle was observed in almost all tasks compared with the first month postprocedure.
However, exceptions were noted during the action of inflating the cheeks to the right
and left sides, where there was a gradual decrease over time.
The increase in EMG in the orbicularis oris muscle may suggest an adaptation to the
structural change resulting from lip volume augmentation, leading to mucosal distension.
The application of substances such as hyaluronic acid, which alters the structure
of the lip mucosa, influences muscular biomechanics. These biomechanical adaptations
result in slightly different lip movements, implying adjustments in tissue force that
contribute to increased muscle activity.[30]
However, muscle activity during the act of inflating the cheeks showed a gradual decrease
over time, possibly due to neural reorganization in response to structural changes
in the lips over time. So, how do we explain this decrease in muscle activity when
inflating the cheeks, contrasting with the EMG of the orbicularis oris muscle in other
tasks such as rest, lip protrusion, and lip compression, observed over time?
After lip augmentation, the variation in facial muscle activity can be influenced
by different factors, including muscular anatomy, patterns of muscle fiber recruitment,
sensory sensitivity, and neuromuscular interconnection.[31] These elements could explain the differences in muscle response between inflating
the cheeks and other tasks after the lip aesthetic procedure. The orbicularis oris
muscle controls the protrusion and compression of the lips, with the arrangement of
fibers modulating the intensity of contraction.[32] The fibers are recruited uniformly during sustained protrusion and compression but
are more localized when inflating the cheeks.[21] Sensory feedback adjusts contraction based on the perception of stretch and pressure.
Coordination with other facial muscles is essential for efficient contraction and
for maintaining unilateral cheek puffing, demonstrating differentiated and controlled
neuromuscular coordination.[33]
The results of the comparison analysis of the EMG of the upper and lower orbicularis
oris muscle revealed significant differences in all tasks across the time periods.
The differences in EMG between the upper and lower orbicularis oris muscles can be
explained by the specific anatomy and functions of each lip region.
The lower lip plays a key role in actions like lip compression, smiling, and maintaining
lip closure. These complex functions, which involve more intense muscle activity,
are essential for the stability and proper functioning of the oral region. On the
other hand, the upper lip performs more delicate functions such as lip pursing and
fine movements,[12] which could result in lower EMG readings compared with the lower lip. Over time,
neuromuscular adaptations may improve the efficiency of the lower orbicularis oris
muscle, leading to higher EMG readings in that area.[30]
This study had limitations. Longer longitudinal studies with extended follow-up periods
are necessary for a more comprehensive evaluation of muscle adaptations over time.
The age range was not controlled, and lip fillers can either enhance or limit muscle
function in older individuals, depending on the technique, injection site, and rheological
characteristics of hyaluronic acid. Additionally, the findings may only apply to the
hyaluronic acid lip augmentation technique used and should not be generalized to other
facial filling methods or materials. Finally, electrostatic properties of human tissues
as a volume conductor were not considered. Furthermore, other factors that influence
EMG activity, such as diet, hydration, and the menstrual cycle, were not controlled.
The study highlights the need for health care professionals to monitor and adjust
lip augmentation treatments, stressing the importance of assessing changes in muscle
function, in addition to facial aesthetics, within 60 days postprocedure. While further
research is needed, it is essential to examine the residual effects of hyaluronic
acid on muscle function before subsequent applications, especially between 6 months
and 2 years after the initial treatment. These findings are vital for orofacial harmonization,
helping to optimize treatment protocols for more natural aesthetic results.
Understanding facial and lip muscle dynamics can customize approaches and ensure safe,
effective procedures. By linking the physiological changes caused by hyaluronic acid
to the EMG activity of the orbicularis oris muscle, clinicians can predict functional
adaptations, optimize injection techniques, and adjust treatment intervals to minimize
adverse effects, ultimately improving patient outcomes and satisfaction by ensuring
more natural results and reducing complications.
Conclusion
This study identified distinct EMG changes in the upper and lower orbicularis oris
muscles following hyaluronic acid lip augmentation. At 30 days, both muscles showed
decreased EMG, indicating initial relaxation. By 60 days, the upper muscle displayed
increased activity, suggesting adaptation, whereas the lower muscle remained less
active during cheek inflation tasks, indicating a different pattern of adaptation.
Throughout most tasks, the lower orbicularis exhibited higher EMG than the upper muscle.
These results suggest that the upper muscle experienced relaxation followed by adaptation,
while the lower muscle maintained reduced activity during specific tasks after 60
days. These findings suggest that clinical and functional reassessment of the orbicularis
oris muscle by clinicians should be considered before touch-ups or new applications
of hyaluronic acid. Neuromuscular adaptation occurs over 60 days, indicating that
this period may be an appropriate interval for reinjections, minimizing interference
with muscle recovery and optimizing both aesthetic and functional outcomes.
