Classification of gastrocnemius muscle hypertrophy for personalized botulinum toxin type A treatment

Owing to its safety and convenience, botulinum toxin type A (BoNtA) has become a first‐choice treatment for contouring calf muscle asymmetries or deformities. Different injection methods and dosages have been discussed in the literature, but a standardized BoNtA treatment remains unclear.


| BACKG ROU N D
Physical attractiveness has held societal importance since antiquity.
The rise of the internet and social media, as well as ease and widespread access to plastic surgery has fueled the increase of cosmetic modifications.Cultural, ethnic, and gender variations in aesthetic ideals exist.In female Asians, a body area attracting particular attention for surgical modification is the calf muscles.The desired and most attractive ideal are long, straight, and slim calves.Calve contour is primarily determined by the size and shape of the gastrocnemius muscle (GM). 1 Several invasive treatments for calf hypertrophy have been proposed with positive results for aesthetic purposes over the past few years including liposuction, GM resection and selective neurectomy. 2,3However, such invasive methods have a longterm healing process and can result in several severe complications such as paralysis, muscle dystrophy, and scarring. 4 a nonsurgical therapy, botulinum toxin type A (BoNtA) has become popular among plastic surgeons and patients over the years.
Mechanistically, BoNtA relies on fixation on the SNARE proteins and inhibition of the release of acetylcholine from the terminals 5 to inhibit muscle overactivity and relax target muscle.Due to its safety profile, ease of use, and convenience, BoNtA has become a firstchoice treatment for modifying asymmetries in muscle contour.In 2004, Lee et al. reported for the first time the use of BoNtA injection in six female volunteers with moderate aesthetic GM hypertrophy (GMH). 6All subjects showed reduction of the medial calf which was maintained up to 6 months post-injection.The efficacy of BoNtA in treating GMH was gradually being revealed.
Only sufficient BoNtA can reach the targeted muscles to induce noticeable muscular atrophy. 7Thus, several injection methods and dosages have been proposed for GMH.Injection methods were initially reported to be localized over the medial part of the GM, 6 but are now recommended to be distributed into both the medial and lateral part of the GM.Besides the distribution of the injection sites, various injection dosages were also discussed. 8The recommended injection dosages of BoNtA has varied from 32 to 200 units per leg in prior literature. 9Lee et al. used the gross size of the contracted medial GM to determine the injection dosage. 6i et al. measured the GM thickness using B-mode ultrasound to recommend various BoNtA dosages for different types of GMH. 10 However, there is still no standardized BoNtA protocol for selecting injection methods and dosages in GMH.This may be due to lack of the measurement protocol for GMH categorization.Therefore, it is vital to categorize GMH into different types based on multiple measurements including calf contour, leg circumference, and GM thickness.
In the current study, we first classified GMH based on the multiple measurements from a large-sample study investigating the appearance of calf contour, leg circumference, and GM thickness.The classification then served as the therapeutic protocol for personalized BoNtA injection.The overarching objective of this study was to evaluate the efficacy and safety of BoNtA injection treatment for GMH.

| Study design
To identify the different classifications of GMH, 100 pairs of lower legs from healthy female volunteers were measured using digital photographs and B-mode ultrasound between January 2020 and January 2022.Ten cases had a history of BoNtA injection treatment of the leg, but none had been treated within at least 1-year.

| Clinical examination
Inspection and palpation of the GM was performed both in the standing on a flat floor and tip-toe position.Digital photographs were taken with the camera at the level of the midpoint of the legs on the anterior and posterior views.The length of the leg was measured from the middle of the lateral malleolar protuberance to the lateral end of the popliteal skin crease.The total length was divided into three equal parts, and the maximal leg circumference was measured at the upper level depending on the different cases.
The distance from the max leg circumference to the middle of the lateral malleolar protuberance in standing position (position of max leg circumference) was also measured.B-mode ultrasound was used to measure the thickness of the subcutaneous fat and GM.The patient was standing on the flat floor back to the investigator.The GM muscle was scanned at the cross point of upper third of the lateral malleolar protuberance to the lateral end of the popliteal skin crease and medial third of the calf width.All examinations were performed by the same investigator.

| Evaluation of the BoNtA injection protocol for the treatment of GMH
A total of 40 GMH cases, treated from January 2021 to December 2022, were further analyzed in this study.This retrospective study was approved by the ethical committee of hospital (2022-035-01).
All patients provided written informed consent.One of two injection methods was used, the "Even" method or the "Intense" method.The two patterns of injection were determined based on the bulging types of GMH.(3) Subjects received one of two dosage regimens (150 or 200 units per leg) according to their GMH type classification.

| Clinical evaluation
Patients returned for follow-up at 1, 3, and 6 months after the treatment.At each follow-up time, clinical evaluation comprised of standardized photography, calf circumference measurement and B-mode ultrasound imaging.Patient and doctor satisfaction were graded on a scale of very satisfied, satisfied, no opinion, unsatisfied, and very unsatisfied.Patients were asked to record their satisfaction regarding calf contour, leg circumference and with the overall treatment at each follow-up time.Two surgeons evaluated, under blinded conditions, the improvement in calf contour using digital photographs taken at each follow up time.Complications were also noted during whole study period.

| Statistical analysis
To identify statistically significant differences the data were analyzed with a paired t test using GraphPad Prism (v8.00,MacOS GraphPad Software).Values of  < 0.05 were considered statistically significantly.

| Types of GMH based on GM contour
A total of 100 healthy female volunteers was examined by inspection and palpation on tiptoe position.The mean age of the volunteers was 28.1 years (Range: 18-32 years).Two types of GMH were identified: the Unilateral-Bulging type (UB), characterized by obvious medial or lateral heads of GM contour on the tiptoe position, seen in 69% of patients and the Bilateral-Bulging type (BB), an even GM contraction contour, seen in 31% of patients (Figure 1A, 1B).
The average thickness of the middle head of the GM as measured in 100 volunteers was 21.78 ± 2.85 mm (Range: 16.39-29.80mm).A calf score was also calculated based on GM thickness.A score 1 was assigned to muscle thickness between 15 mm and 20 mm (28%), a score 2 for muscle thickness between 20 mm and 25 mm (60%), and a score 3 for muscle thicknesses greater than 25 mm (12%).The average thickness of the subcutaneous fat was 0.70 ± 0.19 mm (Table 1).

| BoNtA injection protocol
A total of 40 cases enrolled in this retrospective study and completed the entire follow-up schedule.The demographic data in each group are shown in Table 2.The S400 group significantly differed from the M300 group in terms of GMH classification, weight, mean max leg circumference and GM muscle thickness.No significant differences were noted with regards to the other data (Table 2).F I G U R E 1 Appearance of the two types of calf contour on tiptoe position following treatment with one of two injection methods.(Figure 1A) Unilateral-bulging type treated with intense method (Ubi).(Figure 1B) Bilateral-bulging type treated with even method (BBe).

| Determination of injection method
Two injection methods were designed based on the bulging type of GM contour, the "Even" method and the "Intense" method.In the "Intense" method, used in the unilateral-bulging type of GM contour, half of all the injection sites (15 or 20 sites per leg) were intensely distributed on the obvious bulging head of the GM, while the other GM received "half" injections distributed equally across its surface (UBi).In the "Even" method, used in the bilateral-bulging type, BoNtA was distributed evenly across the bulge surface of each leg through 30 or 40 injection sites using 5 units of BoNtA per site (BBe) (Figure 1A and Figure 1B).The unilateral-bulging type and bilateralbulging type were found in 63 and 37 percent, respectively.

| Determination of injection dosage
The injection dosage was determined based on the total calf score calculated from the max leg circumference and GM thickness.A calf score greater than 5 was considered severe GMH and received an injection dosage of 400 units for both legs (S400).A score between 3 and 4 was considered moderate hypertrophy with required a dosage of 300 units for both legs (M300).A score less than 3 indicated mild hypertrophy which did not require BoNtA treatment.Severe GMH and moderate GMH was found in 33 percent and 67 percent, respectively.Mild GMH was excluded from the study.

| Therapeutic efficacy and complications
Follow-up examinations at 1, 3, and 6 months after treatment were completed in 40 cases.Compared to the baseline data, all patients in all four groups showed clinical improvement with a reduction in the mean max leg circumference and GM thickness after BoNtA treatment (Table 3).In the moderate and severe GMH groups treated with the "Even" method (BBe-M300 and BBe-S400), the mean max leg circumference was shown to be significantly decreased at the 1 month follow-up, and gradually returned to baseline at the 3 and 6 month follow-ups (Figure 2A).Besides, the UBi-M300 group, which was treated with the "Intense" method of 300 units of BoNtA, showed the most significant outcome during the 6 months follow up period.The reduction in mean max leg circumference and muscle thickness was recorded at the 1 month follow-up and was maintained up to 6 months after treatment (Figure 2A and Figure 2B).The UBi-S400 group displayed a decrease of max leg circumference and GM thickness at the 1 month follow-up, however, the effect on leg circumference decrease gradually returned to the original baseline by 6 months (Figure 2A and Figure 2B).Moreover, the position of max leg circumference moved significantly upwards from baseline at the 6 month follow-up in all four groups (Figure 2C).
During the follow-up interviews, the patient satisfaction rate and doctor satisfaction rate among the four groups showed no statistically significant differences (Table 3).The majority of patients in all four groups were satisfied (70%-100%).Patient satisfaction rate was found to be higher in the moderate GMH group (BBe-M300 and UBi-M300) than the severe GMH group (BBe-S400 and UBi-S400).
The patients with the UB type of GMH (UBi-M300 and UBi-S400) were more satisfied than the patients with the BB type of GMH contour (BBe-M300 and BBe-S400).According to a blinded evaluator, the GM contour showed improvement at the 1 month follow-up.In the moderate GMH group, the treatment effect was maintained at the 6 month follow-up, while the effect in the severe GMH group decreased from the 3 month to 6 month follow-up after treatment (Figure 3A to Figure 3L).
No severe complications were encountered during this trial.The only complications reported were myalgia (two patients; 5%) and cramping (four patients; 10%) reported at 1 month's post-treatment.
There were no complications reported at the 3 and 6 month follow-up.

| DISCUSS ION
The increasing importance placed on physical attractiveness is evident in the rising rates of cosmetic surgery.A body area that has become a source of perceived unattractiveness in female Asians is the appearance of the calf which may be too large or asymmetrical because of muscle hypertrophy.
To treat GMH, there are several methods to correct calf contour, such as liposuction, selective neurectomy, GM resection and radiofrequency.Liposuction has a strict indication for patients with pinch test of 2 cm on the lower leg. 11Many articles suggested that calves only have one layer of subcutaneous fat which were responsible for skin irregularities postoperatively. 12Apart from liposuction, GM resection, selective neurectomy and radiofrequency were all targeted to muscle reduction.The mean reduction of calf circumference was 2 cm after GM resection. 3However, the complications such as ankle contraction and scar-related complications were unbearable for patients. 3Selective neurectomy achieved muscle atrophy by cutting nerves to GM.The average reduction of calf circumference was 1.0 cm-3.0 cm.The outcome was sustainable but the unpredictable muscle atrophy and unexpected muscle regrowth were also been reported for permanent complications. 13Another calf volume reduction treatment was radiofrequency.Through frictional heating, radiofrequency produced scar tissue and the contraction of the scars resulted in the reduction of muscle volume.Nevertheless, there were several severe incurable ankle equinus cases been reported by Lim. 14 Therefore, even though the outcome of surgical methods for treating GMH were sustainable, the potential risk of functional damaging was unacceptable.Given the accepted safety profile and widespread use of BoNtA, clinical use of BoNtA to treat GMH is increasing.However, previous reports supported that the patient satisfaction rate after BoNtA treatment of GMH were low.
In this retrospective study, four types of GMH were identified based on multiple measurement including the calf contour, max leg circumference and muscle thickness.Two injection patterns and injection dosages of BoNtA were then proposed based on the types of GMH.
Follow-up examinations, satisfaction rate and complications were recorded at 1, 3, and 6 months after treatment.
It is hard for a large muscle like GM to be evenly paralyzed.Before BoNtA treatment, the category of GMH might be the crucial factor for efficacy.type, seen when the patient stands in tiptoe position.When treating masseter hypertrophy with BoNtA, the injection site is determined by identifying the most prominent part of the bulging muscle which outlines the distribution region of the nerve. 16Therefore, the ideal injection method in patients with single dominant hypertrophy of the GM was the "Intense" method.Chong et al. also recommended to identify the muscle type by the thickness of medial head of GM before BoNtA treatment. 15On the other hand, the "Even" method was more appropriate for the bilateral-bulging type, as the BoNtA could be spread equally in the GM to avoid calf asymmetry after treatment.The injection dosage was determined based on the total scores of max leg circumference and GM thickness.The advantages of relying on both measurements to determine the dosage included The treatment efficacy showed that the mean max leg circumference and GM thickness decreased significantly compared to baseline in all treatment groups.However, the max leg circumference in BBe-M300 and BBe-S400 groups did not display the same therapeutic trend.One possible explanation might be the compensatory enlargement of the soleus muscle, which may offset the decrease in leg circumference caused by BoNtA injection. 8,17In addition, the "ideal" appearance of the female leg depends on straightness of the leg as well as the location of the most prominent part of the leg. 1,18Lee et al previously suggested that the location of the most prominent part of the calf moved superiorly up to the three-fourths level of the leg following BoNtA treatment. 6In this article, the position of max leg circumference had moved significantly upwards at the 6 month follow-up following The limitations of this retrospective study should be considered.
Several patients during follow-up were not measured at the same time of the day which could lead to bias.In addition, B-mode ultrasound focuses on GM thickness, whereas compensatory hypertrophy of the soleus muscle could be better measured with an MRI, which is associated with higher costs.

| CON CLUS IONS
To conclude, this study categorized GM contour into unilateral-

R E FE R E N C E S
All patients were treated with BoNtA (Hengli, Lanzhou Institute, China), a therapy considered to have equal efficacy to BOTOX®.Detailed procedure: (1) Each vial of 100 U BoNtA was reconstituted with 2.5 mL of normal saline solution to achieve a concentration of 4 units per 0.1 mL.Distributed the dilution into sterile insulin syringe with needle for single use U-40 (40 units/mL, BD Ultra-Fine 29gauge, 12.7-mm Insulin Syringe, Franklin Lakes, New Jersey, USA) before injection.Each injection sites received 5 units of BoNtA.(2)

( 1 )
minimizing the disturbance of the leg circumference which can be caused by leg edema, (2) avoiding the biases associated with GM thickness measurement with two-dimensional B-mode ultrasound, and (3) choosing a needle size based on the thickness of the subcutaneous fat.

BoNtA treatment ( 2 - 2 .
5 cm) in all groups.Treatment contoured the calf shape closer to an inverted triangle extending the leg visually.In future evaluations of the efficacy of BoNtA treatment of GMH, we should not solely focus on evaluating the changes in leg circumference and muscle thickness, but also evaluate the effectiveness of BoNtA in moving the most prominent location of the calf superiorly.Previous research identified a low patient satisfaction following BoNtA treatment for GMH.10,19Based on our clinical judgment, we separated the satisfaction rate reported with regards to calf F I G U R E 2 Mean maximum leg circumference (Figure2A), mean GM thickness (Figure2B) and change of the position of max leg circumference (Figure2C) as measured at baseline and at 1, 3, and 6 months after treatment.Data are presented as means ± SD (n = 10 per group), (*p < 0.05, ***p < 0.001).F I G U R E 3Posterior view of the four groups on tiptoe position before and 1 and 6 months after treatment.BBe-M300 (Figure3ABefore; Figure 3B 1 month; Figure 3C 6 months); UBi-M300 (Figure 3D Before; Figure 3E 1 month; Figure 3F 6 months); BBe-S400 (Figure 3G Before; Figure 3H 1 month; Figure 3I 6 months); UBi-S400 (Figure 3J Before; Figure 3K 1 month; Figure 3L 6 months).contourand leg circumference for a more objective evaluation of the efficacy of BoNtA for GMH.There are three possible conditions in the severe group, which are thick GM, large leg circumference, or both.The reason for the low satisfaction of the severe group may be due to excessive leg fat thickness or large leg bones in the severe group, which cannot be improved solely by injection therapy and other treatment methods need to be combined.Secondly, a single treatment of BoNtA injection may not be sufficient to reduce GM in the severe group, which can also explain why doctor satisfaction in the severe group rapidly decreases after 6 months.Multiple treatments may improve satisfaction.The results showed that patients were more satisfied with the therapeutic effect on calf contour than on leg circumference.As a result, comprehensive preoperative communication about the advantages and possibilities of improving calf contour and GM appearance after BoNtA injection is critical for improving patient satisfaction.Reports of complications such as myalgia and cramping, although few, were consistent with previous articles.9,20Except for BBe-S400, the max leg circumference and GM thickness maintained treatment efficacy at 6 months in other groups.Approximately 40 percent of patients returned for second BoNtA treatment at 6 months to 12 months.Based on our experience, we recommended the retreatment at 6 months after first BoNtA injection treatment for adjusting the calf contour further and maintaining muscle relaxation continuously.On average, a single BoNtA treatment for GMH costs about $700 (4800 RMB).
bulging and bilateral-bulging types and based the injection method on the type.Deciding the method and dosage of BoNtA based on the type of GMH can improve the therapeutic outcomes and patient satisfaction.Future research is necessary to help accumulate clinical data that will optimize the use of BoNtA injection in GMH.AUTH O R CO NTR I B UTI O N S Drs Liu, Xu, Wu have made substantial contributions to conception and design.Dr Luo, Feng, Ou, Long and Dr Adriana C. Panayi have been involved in drafting the manuscript and revising it critically for important intellectual content.All authors have given final approval of the version to be published.Cui agreed to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.ACK N OWLED G M ENTS This study was supported by the Shenzhen Science and Technology Program (No. JCYJ20220531094004010 and No. JCYJ202 10324105412033), Shenzhen Key Medical Discipline Construction Fund (SZXK026), and Peking University Shenzhen Hospital Research Project Fund (LCYJ2020008).FU N D I N G I N FO R M ATI O NThe authors have no financial interest to declare in relation to the content of this article.DATA AVA I L A B I L I T Y S TAT E M E N TData sharing not applicable to this article as no datasets were generated or analysed during the current study.E TH I C A L S TATEM ENTThis retrospective study was approved by the ethical committee of Peking university Shenzhen hospital (2022-035-01) and have been performed in accordance with the ethical standards as laid down in the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards.All patients provided written informed consent.
Chong et al.measured the thickness of medial, midline Physical measurements, and patient and doctor satisfaction rates among the four groups at 1, 3 and 6 months after treatment.
10Nevertheless, the treatment efficacy and patient satisfactions were both low.Based on our clinical experience, a more personalized BoNtA injection protocol based on multiple measurements, including GM contour, max leg circumference and muscle thickness, is suggested.As a surgeon, according to the morphology of calf contour, the calf muscle can be quickly categorized into two types, unilateral-bulging type and bilateral-bulging TA B L E 3