Botulinium toxin applications in the lower face and neck: A comprehensive review

Botulinum toxin has been widely and mainly used for the treatment of conditions affecting the upper and middle face; however, recent efforts have expanded the indications of botulinum toxin injection to the lower face and neck areas for cosmetic and medical purposes.


| INTRODUC TI ON
Botulinum neurotoxin (BNT) has been a popular medication used for the management and treatment of several medical conditions and a number of cosmetic indications.Being non-invasive and accessible, BNT's interest and clinical use have been increasing among physicians and surgeons. 1e toxin is produced by a spore-forming, gram-positive, anaerobic bacterium called Clostridium botulinum.Several subtypes, namely from A to G are available for this product; however, Types A and B are the subtypes with clinical indications. 2Onabotulinum toxin A is the most popular subtype, being used in aesthetics and clinical practice.The toxin has been used for therapeutic purposes since the 1980s.The United States FDA has approved the use of botulinum toxin mainly for the treatment of spasmodic torticollis, focal dystonia (i.e.[5][6][7] The toxin is also being injected for the treatment of several off-label indications.][10] The mechanism of action is temporary muscle paralysis by blocking acetylcholine release in the neuromuscular junction.There is a higher rate of toxin uptake at the neuromuscular junctions of hyperactive muscles; therefore, higher clinical efficacy is visible at these sites.Almost all of the abovementioned conditions can improve after muscle relaxation caused by the injection of botulinum toxin. 11 date, botulinum toxin has been widely and mainly used for the treatment of conditions affecting the upper and middle face; however, recent efforts have expanded the indications of botulinum toxin injection to the lower face and neck areas for cosmetic and medical purposes.The lower face is referred to the area below the corners of the mouth.In this paper, we have reviewed the latest updates on using botulinum toxin in the lower face and neck focusing on cosmetic purposes and have discussed the existing gaps and concerns as well as the adverse sequelae of these newer indications.

| ME THOD
A comprehensive literature search was performed through databases to incorporate the maximum number of relevant articles.The search was accomplished in databases including PubMed, Embase, Google Scholar and the gray literature using the following keywords:

[[botulinum] AND [[Toxin] OR [Neurotoxin]]] AND [[Lower AND
Face] AND/OR [Neck]].After the initial search, a total of 602 articles were included based on their title and abstract.Thereafter, duplicates were removed and a total of 350 articles underwent full-text screening.Exclusions comprised case reports, abstracts, and clinical trials on animals or cadavers.Finally, 84 papers were included for the final review (Figure 1).Then, an overview has been provided on the medical and cosmetic indications of botulinum toxin injection in the lower face and upper neck.We have also reported and discussed the adverse reactions, side effects and future concerns regarding the use of botulinum toxin in the lower face and upper neck.

| Lower face and neck scars
Scars are a common concern among dermatology and surgery patients.Many surgical procedures are associated with scar formation, which is subject to removal or revision due to medical indications or appearance-related concerns.3][14][15] For example, cleft lip surgery is associated with hypertrophic scarring in many cases.Injecting botulinum toxin into the orbicularis oris muscle prior to cheiloplasty has decreased this unpleasant outcome after surgery.This is mainly due to the decreased activity of the muscle resulting in diminished tension at the site.Using this method, Chang et al have reported narrower scars with better appearance without any additional benefits in terms of scar pigmentation, vascularity, pliability, or height after cheiloplasty surgery in these patients. 12udies have reported that botulinum toxin injection prior to scar revision surgery is highly effective in the stabilization of the wound, improvement of wound healing, and prevention of wound widening during healing.Using fractional CO 2 laser (starting 6 weeks postsurgery) and anti-scar creams (starting 2 weeks post-surgery) has also improved the clinical outcome in these studies.The injection was performed 2 weeks prior to the surgery around the scar to prevent muscle contractions and tension at the site. 13terestingly, Bae et al. have reported that intra-incisional injection of botulinum toxin after thyroidectomy surgery in 20 patients is associated with a significantly lower rate of postoperative scar F I G U R E 1 Flow chart of the number of initial searched, excluded and final articles.
formation, as compared with the control group who did not receive the injection after thyroidectomy. 16Similarly, during scar revision surgery, injection of botulinum toxin at the incision site has been associated with significant clinical improvement. 172][23] Chang et al. have reported a favorable long-term efficacy for this method with a 12% reduction of masseteric muscle volume after three consecutive injections, 1 year after completion of the treatment. 24,25The minimum recommended dosage for toxin injection into the masseters is 20 U 25 on each side with two injection sessions at 4-month interval. 26e upper platysma is another muscular region in which toxin injection is indicated for lower face contouring.Although the muscle is mainly located in the neck, it is closely related to lower facial structures and muscles, the modulation of which improves the shape of the lower face 27 (Table 1).

| Rejuvenation
BNT alone or in combination with other aesthetic procedures such as hyaluronic acid (HA) filler injection has been of specific importance for facial rejuvenation.Studies have demonstrated the superiority of combination therapy with BNT and HA filler compared with BNT injection alone. 32 a study, injection of a mean dose of 124.9 U of BNT per patient along the inferior border of the mandible and into the platysmal bands resulted in statistically significant improvement in platysmal bands at rest and with maximal tension.Patient satisfaction was achieved at 96.6%.This method is more effective in younger patients with platysma muscle hyperactivity and retained skin elasticity. 30

| Drooping mouth corners
The drooping mouth corner is the sodded appearance of the mouth by the downward inclination of mouth corners, oral commissures, and/or marionette lines.It can be either a congenital or an age-related disorder.The condition is non-surgically treatable with a combination of soft-tissue filler and botulinum toxin injections.
In a study by Bae et al., the toxin was injected into three sites of the lower face, two were injected intradermal on each side of the upper one-third of the depressor anguli oris muscle and one was injected intramuscularly into the mentalis muscle.The treatment was associated with monophasic HA injection and resulted in satisfactory and good clinical outcomes within 3 months. 33Similar results are reported by Qian et al., who have injected botulinum toxin into the lateral aspect of the depressor anguli oris muscle and observed significant clinical improvement lasting 6-9 months.This method is associated with satisfactory clinical outcomes and little or no adverse events.Adverse events mainly include asymmetry, which was resolved with further injections at the next visits. 34The dosage, sites of injection and adverse effects are summarized in the Table 2.

| Gummy smile
Gummy smile, characterized by excessive gum exposure during laughter, is considered a cosmetic concern causing distress and low levels of self-esteem.This condition is caused by the excessive elevation of the upper lip and/or anatomical variations such as altered lip length or abnormal crown exposure.Treatment mostly involves surgical intervention, which results in long-lasting outcomes. 37wever, less invasive methods including botulinum toxin injection have been utilized as a safe and effective treatment for gummy smiles.9][40][41] A treatment is considered successful when less than 2-3 mm of the gingiva appears while smiling.The injection site described in the earlier studies is as follows: at the nasolabial fold, 1 cm below the alar nasi, at the convergence of levator labii superioris alaeque nasi and zygomaticus minor, 5 mm deep the skin surface. 42Cengiz et al. compared two different injection sites of botulinum toxin and reported that injection into the levator labii superioris alaeque nasi muscle results in greater improvement of gummy smile than injection into the orbicularis oris muscle.The treatment remained successful for more than 6 months, similarly in both groups. 43A study has introduced four subtypes for the gingival smile, based on the underlying and/or affected muscles respectively as anterior, posterior, mixed and asymmetric.This is important to determine the involved muscles in order to determine the injection site and obtain optimum results 38 (Table 3).

| Microinjection/Microbotulinum
The term microbotulinum or intradermal injection of the toxin is accomplished by several injections of microdroplets of low-dose BNT as superficially as possible between the dermis and the superior layer of the muscle.This method is invented to prevent and treat skin flaws.In this method, the fine lines and wrinkles on the face are resolved by targeting and paralyzing the superficial fibers of the TA B L E 1 Botulinum toxin injection sites, dosage, and adverse reactions for contouring of the face and neck.One horizontal line at the mandibular border with 3 sites of injection at each side of the face with 2 cm intervals.One horizontal line 2cms below the mandibular border with 4 sites of injection with 2 cm intervals.

28-36 U total
In the lower face the skin was attached better to the bone and the loose appearance was reduced after injection.
The wrinkles and lines of the lower face were decreased.
facial muscles and weakening their insertion into the lower layers of the skin.High patient and physician satisfaction as well as highefficacy and safety of the method is proved in studies.The injection is commonly performed using a 32-gauge or 30-gauge needle with a regular 1-cm grid all over the targeted areas of the skin. 44crobotulinum is highly effective for lower face and neck rejuvenation and for prevention of aging and ptosis.For example, after applying this technique to these areas, platysmal bands with contraction, jowls and neck volume significantly improved.The findings are comparable with other methods such as the Nefertiti technique with high rates of patient satisfaction. 45This method aims at decreasing the effect of the superficial muscle fibers attached to the skin, causing the development of skin rhytides.Furthermore, the microinjection decreases the activity of sweat and sebaceous glands and therefore improves skin texture. 46

| Neck area
There are several indications for injecting BNT into the neck area.
8][49][50] For instance, therapeutic BNT injection is the first-line and long-term treatment for cervical dystonia, and the treatment response is highly dependent on the method and the site of the injection. 48,50Ultrasound-guided approaches have been introduced to improve the accuracy of injection and increase the clinical outcome in such cases. 50Some patients develop neutralizing antibodies after one or two successful BNT treatments and thereafter, they are less responsive to this medication than they did earlier. 49r cosmetic purposes, BNT injection into neck muscles for rejuvenation especially into the superficial skin has been recommended in several studies.This method is usually accompanied by filler injection to increase the clinical outcome in rejuvenation and cosmetic purposes. 47 a study by Jabbour et al., BNT has been injected according to the 'Nefertiti lift'.In this method, BNT is injected into the platysmal bands and the inferior border of the mandible.Final results indicated high patient and physician satisfaction as well as significant clinical improvement, especially at the platysmal band.This method is specifically recommended for younger patients with platysma muscle hyperactivity and retained skin elasticity. 30e study compared the injection of BNT with combination therapy of HA injection with mesotherapy for the treatment of horizontal neck lines.Results indicated significantly higher rates of clinical improvement among those treated with combination therapy rather than patients who received BNT injection alone.Although these methods are being combined in many cosmetic procedures, their efficacy has been interestingly compared in this study.Treatment of a wide lower face and masseteric hypertrophy includes BT injection into the masseteric muscle and more invasive methods such as bone resection.[58][59][60][61][62] Interestingly, this method has been associated with low rates of side effects and a desirable clinical outcome.Therefore, most patients desire to repeat this treatment.Studies have reported some limitations associated with this method and have attributed this issue to the dissemination of the toxin to other muscles such as risorius. 63e medial part of the masseter muscle is a hazard zone that BT injection at this site can affect the risorius muscle, causing iatrogenic unnatural facial expressions.Therefore, BNT injection at this site can be prevented to decrease the side effect. 64BNT injection into masseteric muscle has also decreased the stiffness of masticatory muscles, which needs to be considered in long-term treatment. 65reover, this safe treatment only decreases muscle thickness and does not affect subcutaneous thickness. 66ximum clinical efficacy in this method is observed between 1 and 6 months in different studies, while the treatment remains effective within 2 years after the injection.In a study, patients who received three injections experienced higher efficacy and longer duration of treatment maintenance compared with those who received two injections. 67Transient buccal weakness and decreased bite force are reported as additional side effects of this treatment method. 68,69The adequate dose of BNT in this method is suggested to be above 20 units. 25thod and appropriate site of injection are key factors in clinical outcome.Chirico et al. have invented a biphasic-injection method that is associated with minimum self-resolving side-effect, and the result was satisfactory in almost all cases.Their study proves the role of extensive knowledge of muscular anatomy and appropriate injection technique as key factors in desirable clinical outcomes and decreasing side effects. 70However, comparing two methods of multi-injection and single injection showed no superiority of the methods over each other. 71Research has shown that radiofrequency ablation of masseter muscle results in a more significant reduction of muscle volume as compared with BNT injection. 72mparing different BNT type A formulations, no significant difference in terms of mean masseter volume was observed. 73A summary of the injection sites and dosage has been reported in Table 4.

| DISCUSS ION
The cosmetic and therapeutic effects of BNT injection are mostly achieved due to several reasons.First, the relaxation of a muscle causes an increase in the volume of that muscle and subsequently, the occupied space increase, stretching the skin.This results into a skin with fewer wrinkles and rhytides. 40udies have widely assessed the short-term and long-term uses of different types of BNT for lower-face cosmetic and therapeutic purposes and have invented newer methods.Some of these new methods are superior and some are non-superior to previously existing approaches, however, most of them have been reported in a few studies and on a limited number of patients.Moreover, injection doses as well as the site of injection have been reported differently in various studies.While using techniques such as ultrasound-guided injection is reported to be more effective and accurate in the literature.Therefore, more comprehensive research is highly required to investigate and compare these variables in a controlled, long-term, and large-scale setting.
Interestingly, there are very few adverse events associated with BNT injection into the lower face, most of which have been selflimited.Allergic reactions, bruising, edema, or pain at the injection site as well as paralysis of adjacent muscles have been more frequently reported among the adverse events. 6,79However serious rare adverse reactions may occur due to the increased use of botulinum toxin worldwide including angioedema, cutaneous vasculitis, and drug eruption. 80T injection for cosmetic and medical purposes is an affordable, less-invasive, and effective anti-aging and therapeutic procedure, which is highly appreciated by many patients and physicians in an increasing amount.However, regulations for the administration of BNT are variable or even absent in different countries.Moreover, this is more applicable in cases of cosmetic uses of BNT in the lower face, while many sites and methods of injection have no strictly de-

3. 2 |
ContouringAesthetic contouring of the lower face using botulinum toxin is increasing.Lower facial contouring is typically accomplished by injecting the masseter and other lower facial muscles.One common clinical complaint for which patients seek treatment is a 'square face', and the solution is the injection of botulinum toxin into the above-mentioned muscles to decrease muscle hypertrophy and/or bony flaring.
sites are indicated with 1.5 cm intervals with different patterns for each patient based on their clinical appearance.3-5 U for the main site, 2-4 U for the site lateral to the main site, 2 U for the other sites each, 1 U for the sites on the neck near the clavicle Transversal bands were improved in patients.Neck weakness (10%, n = 4) Prager, W. et al 29 2014 Incobotulinum toxin A Platysma: 6 sites for each band with 2 cm intervals 15 U for each platysmal band The mean change score from baseline was −1.696 (p < .001) 2 weeks after injection.Hematoma (8.7%, n = 2), neck weakness (4.34%, n = 1) Jabbour, S. F. et al 30 2017 ABO, Dysport® One line below the mandibular border with 4 sites on each side of the face with 1-2 cm intervals.Platysma: Vertical lines with 2-4 sites with 2 cm intervals on each band 5 U for each site (Maximum total of 125 U) marionette lines (p = .096) and platysmal bands at rest(p = .009)and with contraction(p < .001)were improved significantly.93.3% of patients are improved when the pre-injection and post-injection photographs are evaluated.Ecchymosis in injection site (16.6%, n = 5), dysphagia and mild neck weakness (3.3%, n = 1) de Almeida, A. R. T. et al 27 2017 OBA Mentalis muscle: 2 sites.

Acne ( 5 .
2%, n = 10), fat-apple phenomenon (1.56, n = 3), disconnected jaw (2.6%, n = 5) TA B L E 2 Botulinum toxin injection sites, dosage and adverse reactions for Downturned mouth corner. in the upper 1/3 and 1 site in the lower 1/3.Mentalis muscle: 1 site in the middle of lower 1/3.The First session; 4 U for each DAO muscle, 8 U for the mentalis muscle The Second session; 4 U for each DAO muscle, 4 U for the mentalis muscle The median degrees of baseline down turning of the mouth corners on each side of the mouth: −4° (IQR: −7° to −2°).The median degrees of down turning of the mouth corners on each side of the mouth 2 weeks after injection: −1° (IQR: −3° to 1°) The value difference between baseline and 2 weeks after injection was significant (p < .001).Minor discomfort during speaking (12.5%, n = 2 patients), a minor problem during playing the saxophone (6.25%, n = 1 patient), herpes labialis (6.25%, n = 1 patient) Fabi, S. G. et al 36 2015 ABO, OBA DAO: 1 cm lateral to the lateral oral commissure, superior to the angle of the mandible 10 U of ABO in DAO in one site 4 U of OBA in DAO on the other side of the face The mean melomental fold severity score at maximal contraction decreased from 2.3 to 0.55 (ABOinjected; p = .01),and from 2.5 to 0.625 (OBA-injected; p = .002)There were no differences statistically between the outcome of the two types of botulinum toxin.3 sites 8-15 mm below and 8-10 mm lateral to the mouth corner 2-4 U in each site The degree of downturned mouth corner before and 1 month after the injection was different statistically (p < .01).

3. 8 |
Masseteric hypertrophyStandards of beauty are commonly different among cultures and each culture appreciates specific shapes for a face such as triangular TA B L E 3 Botulinum toxin injection sites, dosage and adverse reactions in gingival smile.for mild gingival smile, 10 U total for moderate gingival smile, 5,10 or 15 U total for severe gingival smile Mean gingival exposure ±SD (mm) decreased in all groups 12 weeks after injection.The patients with 5 U of injection: 3.5 ± 1.2-2.4± 1.6 (p = .0012)The patients with 10 U of injection: 4.0 ± 1.0-1.6 ± 1.9 (p < .0001)The patients with 15 U of injection: 5.2 ± 0.8-3.3 ± 1.5 (p = .0014)No adverse effects Han, Y. et al 40 2021 Botulinum toxin A, Botox® 4 sites, between the peak of the lip and the corner 1-2 U for each site Mean gum exposure above the central incisor tooth decreased from 3.75 ± 0.68 mm to 1.00 ± 1.13 mm, and mean gum exposure above the canine tooth decreased from 4.15 ± 0.71 mm to 0.9 ± 1.13 mm (P < 0.05) No adverse effects and heart-shaped.In most cultures, a wide lower face is not desirable and people seek cosmetic interventions to decrease the volume of mandibular prominence as well as masseter muscle and subcutaneous tissue.Masseteric hypertrophy usually occurs between 20 and 40 years and is more common among the Asian population. 13,52-55

83 5|
termined guidelines.BNT injection for cosmetic purposes is limited to glabellar lines, lateral canthal lines (crow's feet), and forehead lines, and FDA has approved the injection of incobotulinum toxin A, abobotulinum toxin A, and prabotulinum toxin A only for the treatment of glabellar lines.Therefore, other cosmetic uses of BNT for lower face areas are off-label.81-CONCLUS IONBNT injection has been widely used for the treatment of different medical and cosmetic purposes.Low rates of side effects, which were self-limited in most cases, have been reported in the literature, making BNT a safe therapeutic medication in most cases.However, regulatory status needs to be updated and more accurately revised in many countries and more comprehensive research is required to address the existing gaps in this area including the site, dosage, and method of injection in each case.AUTH O R CO NTR I B UTI O N SMartinKassir conceived the research idea and revised the manuscript.Mahsa Babaei and Saba Hasanzadeh collected data and prepared the first draft.Mostafa Rezaei Tavirani revised the final draft TA B L E 4 Botulinum toxin injection sites, dosage, and adverse reactions for Masseter hypertrophy.in a rectangular safety zone (between four lines including a line connecting the ear lobe to the corner of the mouth, the line along the mandible angle, and two vertical lines along the anterior and posterior margins of the masseter) for injection Temporalis: -injection technique = about 1 cm above and 1 cm anterior of the mandibular angle.multi-injection technique = 5 sites with 1 cm intervals.40 U in each muscle The median prominence of the masseter muscle was decreased from 6.0 to 3.5 (p = .005).There was not any statistically significant different outcome between the two techniques.in the upper, middle, and lower part with 1 cm intervals 30 U in each muscle Patients with 2 sessions of injection: muscle size reduced by 13.62% (p < .0001)Patients with 3 sessions of injection: Muscle size reduced by 43.42% (p < .0001) in 1/3 lower part with perpendicular puncture 25 U in each muscle 63.34% of patients were very satisfied.33.33% of cases were satisfied and 3.33% were unsatisfied.Facial paralysis (5.00%, n = 3), muscle atrophy (8.33%, n = 5), infections (6.67%, n = 4), toxic reaction (3.33%, n = 2), and anaphylactic reactions (5.00%, n = 3) Nikolis, A. et al 72 technique = 1 site in the overlap of three heads of the muscle, multi-injection technique = 5 sites with 1 cm intervals 40 U in each muscle The midpoint of the intercanthal distance and the maximal masseteric protrusion point to midline distance decreased significantly (right masseter (p ≤ .005,left masseter (p ≤ .001).There was not any statistically significant different outcome between the two techniques.The adverse events reported were pain, edema, hematoma, redness, and bruising.Cha, Y. R.et al 62 2013 Botulinum toxin A, Botox® Masseter: 2 sites of injection in 1/3 lower part with 1 cm intervals at the bulging region during clenching.25 U on one side The mean change in masseter muscle volume was −1153.8(p = .004)Not mentioned.Chang, C. S. et al 18 2010 Botulinum Toxin A, Dysport® Masseter: in 6 sites with 1 cm intervals 120 U for each muscle There were soft tissue changes after injection but not assessed as the goal of this study was to evaluate the effect of BT injection on mandibular bone.The injection had no effect on mandibular volume and thickness.Masseter: 3 sites in the central region of the 1/3 lower part with 1 cm intervals 24 U for each muscle The mean ratio of bigonial distance to total facial height decreased from 0.599 to 0.569 (P < 0.01).No adverse effects Chang, C. S. et al 24 2019 Botulinum toxin A, Botox® Masseter: 3 sites in the 1/3 lower part with 1 cm intervals (a vertical line from lateral orbital wall to the antegonial notch and an oblique line from oral commissure to ear lobe were drowned.The injection was done in the region inferior to the oblique line and posterior to the vertical line.24 U for each muscle Mean masseter muscle volume decreased from 26.39 ± 4.18 cm3 to 23.26 ± 4.31 cm3, (p = .002)No adverse effects Gaofeng, L. et al 56 2009 Botulinum toxin A Masseter: 3 sites with 1.5 cm intervals.30-50 U for each muscle The indices of the distance between gonions and intergonial width/ the distance between the zygions and bizygomatic facial width decreased from 0.8281 ± 0.0209 to 0.7925 ± 0.0206 (p < .01)Swelling in masseter region (15.6%, n = 5), weakness of bite force (12.5%, n = 4) Kim, J. H. et al 76 2005 Botulinum toxin A Masseter: 2 sites in the center of the lower part of the muscle with 1 cm intervals 25 U for each muscle in one group of patients 35 U for each muscle in another group Mean decrease in masseter muscle thickness was 18.2 ± 0.07 (in the 25 U group) and 20.4 ± 0.07 (in the 35 U group) There was no statistically significant difference between masseter volume reduction in the two BT dose groups.In 25 U group: headache (18.75%, n = 3), swelling (6.25%, n = 1), pain (18.75%, n = 3), muscle weakness (25%, n = 4) In 35 U group: headache (12.5%, n = 2), bruise (6.25%, n = 1), pain (12.5%, n = 2), muscle weakness (25%, n = 4), dry mouth (6.25%, n = 1) Nam-Ho Kim et al 77 2010 Botulinum Toxin A, Dysport® Masseter: 3 sites in the lower part of the muscle, under the line connecting the earlobe and the oral commissure.100-140 U for each muscle Mean masseter volume muscle was decreased from 13.32 to 9.94 mm Not mentioned.Lee, H. J. et al 78 in the center of the lower part of the muscle 25 U for each muscle The masseter muscle volume was decreased 6 months after injection significantly (p < .001)Not mentioned.Park, G. et al 67 2017 Botulinum toxin A, Meditoxin® Masseter: 2 sites in the middle 1/3 of the muscle with 1 cm interval 25 U for each muscle The masseter thickness decreased after 12 weeks by about 2.92 mm at rest and 3.34 mm during contraction (p = .001)each muscle The masseter volume decreased significantly (p < .001).Muscle weakness (35.7%, n = 5)