Electrophysiologic Change and Facial Contour following Botulinum Toxin A Injection in Square Faces

Systematic review of the histological and functional effects of botulinum toxin A on masticatory muscles: Consideration in dentofacial orthopedics and orthognathic surgery

INTRODUCTION

Botulinum toxin type A causes muscle paralysis and is widely used in the masticatory muscle for stomatognathic diseases, such as temporomandibular disorder, bruxism, or masseteric hypertrophy. Nonetheless, its muscular effect remains unclear. Better understanding could aid improved use and perhaps new indications, particularly in dentofacial orthopaedics and orthognathic surgery.

METHODS

This systematic review explored the histologic and functional effects of botulinum toxin in animal and human masticatory muscles and was conducted in accordance with the recommendations of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement. The MEDLINE, Web of Science, and Cochrane Library electronic databases were searched for relevant articles. The inclusion criteria were human or animal masticatory muscle analysis after botulinum toxin injection(s) AND histological structural/ultrastructural analysis by optical or electronic microscopy OR functional effect analysis by bite force evaluation (occlusal force analyzer) and muscle activity (electromyography).

RESULTS

Of an initial 1578 articles, 44 studies were eventually included. Botulinum toxin injection in the masticatory muscle altered its histological structure and functional properties. The human and animal studies revealed ultrastructural change, atrophy, and fiber type modifications of the masticatory muscles after one injection. Botulinum toxin decreased bite force and muscle activity, but recovery was uncertain.

CONCLUSIONS

Muscle forces applied on the skeleton is a key feature of facial growth. Masticatory muscle paralysis changes mechanical stress on bones, which rebalances the force applied on facial bones. This new balance could benefit dental deformity or surgical relapse. Therefore, botulinum toxin could limit the orthognathic effect of the masticatory muscles in such patients. Given the uncertain recovery, multiple injections should be avoided, and usage should not deviate from established consensus.

Effect of Botulinum Toxin Injection on EMG Activity and Bite Force in Masticatory Muscle Disorder: A Randomized Clinical Trial

Botulinum toxin type A (BoNT-A) is increasingly used in treating masticatory muscle pain disorder; however, safe doses and reinjection intervals still need to be established. The purpose of this randomized clinical trial was to evaluate the degree and duration of the impairment of masticatory muscle performance. Fifty-seven subjects were randomly divided into two groups: one of which received BoNT-A first (n = 28) while the other received saline first (n = 29), with the cross-over being in week 16, and a total follow-up period of 32 weeks. A total dose of 50 U of BoNT-A was injected in the masseter and temporal muscles bilaterally. Electromyographic (EMG) activity and bite forces were assessed. A significant reduction in EMG activity was observed up to week 18 (p ≤ 001), with total recovery at week 33. A significant reduction in maximum bite force was observed up to week 11 (p ≤ 005), with total recovery at week 25. In conclusion, when treating masticatory muscle pain disorder with 50 U of BoNT-A, a reinjection interval of 33 weeks can be considered safe since the recovery of muscle function occurs by that time.

Effects of Two Botulinum Toxin Type a Evaluated by Shear Wave Elastography and Electromyographic Measurements of Masseter Reduction

ABSTRACT

The purpose of this study was to evaluate the efficacy and safety of onabotulinumtoxin A and Chinese botulinum toxin type A (CBA) for masseter reduction using elastography and electromyographic measurement. Female subjects aged 21 to 38 years with benign masseter hypertrophy received 1 treatment of either 50 units onabotulinumtoxin A or 50 units CBA in a double-blind clinical trial. The study enrolled 102 subjects (204 sides of masseters); 51 subjects per group. The thickness and stiffness of the masseter muscle and the electromyographic changes were evaluated before and 1, 4, 12, and 24 weeks after injection. One week after injection, the thickness of the masseter muscle did not change significantly, but the stiffness was reduced by 5% to 9%. After 4 weeks of injection, masseter muscle thickness, stiffness and strength decreased significantly compared with before injection. The changes were significantly at 12 weeks. Muscle stiffness measured by Shear-wave elastography was significantly reduced by 20% to 32% in the relaxed state and 25% to 47% in the contractile state. The electromyography showed that masseter muscle strength changed consistently with Shear-wave elastography value before and after injection. Six months after injection, ultrasound and electromyography showed that the masseter muscle thickness and stiffness began to recover. Between the 2 groups, there are no significant difference in thickness, stiffness and muscle strength reduction of masseters after treatment (P > 0.05), as well as in side effects (P > 0.05). Onabotulinumtoxin A and CBA were comparable in the efficacy and safety for masseter reduction.

High Precision Use of Botulinum Toxin Type A (BONT-A) in Aesthetics Based on Muscle Atrophy, Is Muscular Architecture Reprogramming a Possibility? A Systematic Review of Literature on Muscle Atrophy after BoNT-A Injections

Improvements in Botulinum toxin type-A (BoNT-A) aesthetic treatments have been jeopardized by the simplistic statement: “BoNT-A treats wrinkles”. BoNT-A monotherapy relating to wrinkles is, at least, questionable. The BoNT-A mechanism of action is presynaptic cholinergic nerve terminals blockage, causing paralysis and subsequent muscle atrophy. Understanding the real BoNT-A mechanism of action clarifies misconceptions that impact the way scientific productions on the subject are designed, the way aesthetics treatments are proposed, and how limited the results are when the focus is only on wrinkle softening. We designed a systematic review on BoNT-A and muscle atrophy that could enlighten new approaches for aesthetics purposes. A systematic review, targeting articles investigating BoNT-A injection and its correlation to muscle atrophy in animals or humans, filtered 30 publications released before 15 May 2020 in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Histologic analysis and histochemistry showed muscle atrophy with fibrosis, necrosis, and an increase in the number of perimysial fat cells in animal and human models; this was also confirmed by imaging studies. A significant muscle balance reduction of 18% to 60% after single or seriated BoNT-A injections were observed in 9 out of 10 animal studies. Genetic alterations related to muscle atrophy were analyzed by five studies and showed how much impact a single BoNT-A injection can cause on a molecular basis. Seriated or single BoNT-A muscle injections can cause real muscle atrophy on a short or long-term basis, in animal models and in humans. Theoretically, muscular architecture reprogramming is a possible new approach in aesthetics.

Ultrasound-guided five-point injection of botulinum toxin for patients with trapezius hypertrophy

OBJECTIVES

Our study aimed to explore the clinical therapeutic effects of ultrasound-guided five-point injection of botulinum toxin type A for patients with trapezius hypertrophy.

METHODS

Twenty female patients diagnosed with trapezius hypertrophy were enrolled in this study. The thicknesses of the trapezius muscle were measured by using the ultrasound scanner to locate the thickest point of trapezius, followed by labelling the other four points around the first point. Botulinum toxin type A was injected bilaterally (50 IU/side, 5 points/side) in the trapezius muscle of these patients. The surgery effects were evaluated by thicknesses of the trapezius muscle, intramuscular needle electromyographic and electroneurographic examinations, appearance changes and patients' satisfactions.

RESULTS

Statistically significant differences in thicknesses of the trapezius muscle were observed at 4 weeks (p < 0.001), 12 weeks (p < 0.001), 20 weeks (p < 0.001), 28 weeks (p = 0.011), 36 weeks (p = 0.022), and 44 weeks (p = 0.032) after surgery. The latencies of trapezius muscle became longer at 12 weeks after surgery (left: 2.40 ms, right: 2.53 ms vs. left: 1.75 ms, right: 2.00 ms). Electroneurographic results showed amplitude reduction of compound muscle action potentials (CMAPs) at 12 weeks after surgery (left: 1.91 uV, right: 3.10 uV vs. left: 15.00 uV, right: 15.40 uV). Obvious appearance changes were revealed at 12 weeks after surgery. All of 80% patients were very satisfied, 15% patients were relatively satisfied, and 5% patients were not satisfied with the surgery.

CONCLUSION

Ultrasound-guided five-point injection of botulinum toxin type A might be effective for patients with trapezius hypertrophy.

Botulinum toxin type A injections for masticatory muscles hypertrophy: A systematic review

The aim of this study is to compare the efficacy of single-point injection of botulinum toxin A versus multi-point operative protocols found in the literature in reducing hypertrophy in patients with masticatory muscle hypertrophy. A systematic review was performed in accordance with the Park et al., 2018 guidelines, selecting articles from PubMed, Google Scholar, Web of Science and Ovid databases up to July 8, 2020.28 studies met the eligibility criteria. 748 patients were treated for masseter hypertrophy (MH) and 4 patients for temporal muscle hypertrophy. As for MH: in 45.2% of cases 3 injection sites (IS) were used, in 18.8% 1 IS, in 16.2% 2 IS, in 13% 5 IS, in 4.1% 6 IS, 2.7% 4 IS. At three months, the mean reduction in masseter muscle thickness is 26-31% when 1 IS was used, 28% in 2 IS, 12-27% in 3 IS and 22-30% in 6 IS. Heterogeneity of results, high bias level and selective reports led to a difficult efficacy comparison of the injection techniques described. Data suggest that the lowest number of IS possible should be used until stronger evidences are presented. Homogeneity in pre- and post-operative protocols is needed to establish a reliable setting for the condition under study.

Botulinum toxin in the masseter muscle: Lingering effects of denervation

Botulinum neurotoxins (BoNTs) are paralytic agents used to treat a variety of conditions in jaw muscles. Although their effect is considered temporary, there are reports of persistent functional changes. Using rabbits that received BoNT injection in one masseter muscle, the recovery of neuromuscular connection was investigated using nerve stimulation to evoke an electromyographic (EMG) response, and the recovery of muscle fibers was investigated using histological morphometry and bromodeoxyuridine (BrdU) immunohistochemistry. One month after treatment, evoked EMG was greatly reduced in both amplitude and duration, indicating that little reinnervation had taken place. Muscle fibers were atrophied and collagenous tissue was increased. Three months after treatment, evoked EMG duration was normal, indicating that at least some neuromuscular junctions were functional. Histologically, some muscle fibers were hypertrophied, some were still atrophied, and some appeared to have died. Fibrosis was still apparent amid slight increases in dividing cells and regenerating fibers. The histological effects of BoNT were evident although attenuated at a distance of about 1 cm from the injection level, but no regional differences could be discerned for the evoked EMGs. In conclusion, there were persistent muscular deficits seen 3 months after BoNT treatment that may have been caused by the failure of some affected muscle fibers to become reinnervated.

Anatomical Considerations When Treating Compensatory Hypertrophy of the Upper Part of the Masseter after Long-Term Botulinum Neurotoxin Type A Injections

The masseter is the most targeted muscle when treating hypertrophy to produce a smooth face shape. Compensatory hypertrophy is a well known clinical sequela that occurs in botulinum neurotoxin (BoNT) treatments and is limited to the lower part of the masseter. Based on the masseteric hypertrophy procedure, which targets a confined area, we predicted the possibility of compensatory hypertrophy occurring in the upper part of the masseter. If the patient complains about an unexpected result, additional injections must be performed, but the involved anatomical structures have not been revealed yet. The aim of this study was to identify the morphological patterns of the masseter. Deep tendons were observed in most specimens of the upper part of the masseter and mostly appeared in a continuous pattern (69.7%). The superficial and deep tendons could be classified into a simply connected form and forms surrounding part of the muscle. In 45.5% of cases there were tendon capsules that completely enclosed the muscle, which can interfere with how the injected toxin spreads. Interdigitation patterns in which the tendons could be identified independently between the muscles were present in 9.1% of cases. The present findings provide anatomical knowledge for use when injecting BoNT into the masseter.

The effect of botulinum toxin injections on bruxism: A systematic review

OBJECTIVE

To systematize evidence on the efficacy of botulinum toxin type A (BTA) in the treatment of bruxism measured through bite force or electromyography (EMG) at the masseter muscle.

METHOD

Identification of relevant articles through databases PubMed, Web of Science, SCOPUS, Ovid and EBSCO and manual search were performed for sources from review articles. Studies scoring less than 3 on the Jadad Scale were excluded.

RESULTS

Four articles were included after an exclusion of 333 articles. 3 articles measured EMG and 1 bite force. 1 article did not record a significant drop of activity, 1 article recorded reduction midway and at final endpoint. 2 articles recorded initial reduction, but a non significant difference at later follow up.

CONCLUSION

The available research is inconclusive and does not show enough evidence that bruxism can be treated with BTA injections. However, promising results have been shown in individual studies and further research in this area is needed.

Masseter Muscle Volume Changes Evaluated by 3-Dimensional Computed Tomography After Repeated Botulinum Toxin A Injections in Patients With Square Facial Morphology

INTRODUCTION

Botulinum neurotoxin A (BoNT-A) is a minimally invasive and technically straightforward treatment of masseter muscle (MM) volume reduction and facial contouring, but the literature on its long-term effect on MM volume remains unclear.

OBJECTIVE

This study aimed to assess quantitatively for progressive volume changes of lower facial contour after 3 BoNT-A injections in patients with bilateral MM hypertrophy causing square facial morphology using 3-dimensional computed tomographic scans.

MATERIALS AND METHODS

Ten female patients with square facial morphology due to bilateral MM hypertrophy were recruited to, and 6 completed, this clinical study. Each received 24 U of BoNT-A into the inferior portion of each MM on both sides, repeated 6 monthly to complete 3 treatments. Masseter muscle volume changes were assessed using 3-dimensional computed tomography at pretreatment (before injections) and posttreatment (1 year after the third injection).

RESULTS

Mean MM volume significantly reduced from 26.39 ± 4.18 cm before treatment to 23.26 ± 4.31 cm 1 year after treatment (P = 0.002).

CONCLUSION

Three consecutive 6-monthly BoNT-A injections into the MMs reduced their volume by 12% when assessed 1 year after completion of treatment.

Use of a botulinum toxin A in dentistry and oral and maxillofacial surgery

Botulinum toxin (BT) was the first toxin to be used in the history of human medicine. Among the eight known serotypes of this toxin, those currently used in medicine are types A and B. This review article mainly discusses BT type A (BTA) because it is usually used in dentistry including dental anesthesiology and oral and maxillofacial surgery. BTA has been used mainly in the treatment of temporomandibular joint disorder (TMD) and hypertrophy and hyperactivity of the masticatory muscles, along with being a therapeutic option to relieve pain and help in functional recovery from dental and oral and maxillofacial surgery. However, it is currently used broadly for cosmetic purposes such as reducing facial wrinkles and asymmetry. Although the therapeutic effect of BTA is temporary and relatively safe, it is essential to have knowledge about related anatomy, as well as the systemic and local adverse effects of medications that are applied to the face.

Mechanism of motor coordination of masseter and temporalis muscles for increased masticatory efficiency in mice

The demand for the use of mice as animal models for elucidating the pathophysiologies and pathogeneses of oral motor disorders has been increasing in recent years, as more and more kinds of genetically modified mice that express functional disorders of the stomatognathic system become available. However, the fundamental characteristics of mouse jaw movements during mastication have yet to be fully elucidated. The purpose of this study was to investigate the roles of the masseter and temporalis muscles, and the mechanisms of motor coordination of these muscles for increasing masticatory efficiency in the closing phase in mice. Twenty-two male Jcl:ICR mice were divided into control (n = 8), masseter-hypofunction (n = 7) and temporalis-hypofunction groups (n = 7). Botulinum neurotoxin type A (BoNT⁄A) was used to induce muscle hypofunction. The masticatory movement path in the horizontal direction during the occlusal phase became unstable after BoNT⁄A injection into the masseter muscle. BoNT⁄A injection into the temporalis muscle decreased antero-posterior excursion of the late-closing phase corresponding to the power phase of the chewing cycle. These results suggest that the masseter plays an important role in stabilizing the grinding path, where the food bolus is ground by sliding the posterior teeth from back to front during the occlusal phase. The temporalis plays a major role in retracting the mandible more posteriorly in the early phase of closing, extending the grinding path. Masticatory efficiency is thus increased based on the coordination of activities by the masseter and temporalis muscles.

Early effect of Botox-A injection into the masseter muscle of rats: functional and histological evaluation

Background

The purpose of this study was to evaluate the change of food intake after different dosages of botulinum toxin A (BTX) injection in the animal model. Additionally, the dimensional and histological change at 14 days after BTX injection was also evaluated.

Methods

The comparative study was performed using the BTX injection model in rats (n = 5 for each group). Group 1 was the saline-injected group. Group 2 was the 5-unit BTX-injection group to each masseter muscle. Group 3 was the 10-unit BTX-injection group to each masseter muscle. Food intake rates and body weight were checked daily before and after BTX injection until 10 days. All animals were sacrificed at 14 days after BTX injection, and the specimens underwent hematoxylin and eosin stain and immunohistochemical staining for myosin type II (MYH2).

Results

The recovery of food intake in groups 2 and 3 decreased significantly compared with group 1 from day 2 to day 7 and day 9 after injection (p < 0.05). The BTX-treated masseter muscles were significantly smaller than those in group 1 (p = 0.015). The immunohistochemical findings demonstrated that the expression of MYH2 was significantly higher in group 3 compared to groups 1 and 2 (p < 0.001).

Conclusions

BTX injection to the masseter muscle in rats demonstrated short food-intake-rate reduction with recovery until 10 days after injection. The thickness of the masseter muscle and MYH2 expression were significantly changed according to the injected dose of BTX.

Differential response of pig masseter to botulinum neurotoxin serotypes a and b

INTRODUCTION

Pigs respond to direct administration of botulinum neurotoxins (BoNTs), although they are resistant to botulism. The human masseter is frequently targeted for BoNT therapy. We aimed to understand how BoNT affects chewing by injecting porcine masseters.

METHODS

One masseter of minipigs was injected with BoNT serotype A or B at doses comparable to those used in humans. Masticatory function was evaluated electromyographically. Muscle force was measured during tetany. Four weeks after injection, strain gauges affixed to the mandible assessed bone strain during chewing. Masseter mass and fiber diameter were measured after euthanasia.

RESULTS

BoNT-A had no measurable effect. In contrast, BoNT-B reduced electrical activity and muscle force, producing substantial asymmetry between injected and uninjected muscles.

CONCLUSIONS

The pig masseter is highly resistant to direct injection of BoNT-A, but it is affected by BoNT-B.

Lower facial remodeling with botulinum toxin type A for the treatment of masseter hypertrophy

BACKGROUND

Masseter hypertrophy has been treated with botulinum toxin injections because of esthetic complaints especially in Asians.

OBJECTIVES

The goal of the present study was to evaluate the efficacy of abobotulin toxin use in masseter hipertrophy treatment in Brazilians.

METHODS

Ten Brazilian female patients with masseter hypertrophy were subjected to injections of 90U of abobotulinum toxin A applied on each side respecting the safety zone stabilished in literature and were followed up for 24 weeks.

RESULTS

When analyzing the coefficients between measures of middle and lower third of the face obtained from standardized photographs, an increase was observed, with statistical significance at 2 weeks (p=0.005) and 12 weeks (p=0.001). The progression of lower third reduction was 3.94%, 5.26%, 11.99%, and 5.47% (2, 4, 12, and 24 weeks respectively). All patients showed improvement in bruxism after treatment. Observed adverse effects were masticatory fatigue, smile limitation, and smile asymmetry.

CONCLUSION

The use of abobotulinum toxin A for masseter hypertrophy is effective in Brazilians and reached its maximum effect of facial thinning at 12 weeks. Smile limitation had a higher incidence compared to that reported in the literature and may result from risorius muscle blockage caused by toxin dissemination. Despite its side effects, 80% of the patients would like to repeat the treatment.

Effects of a botulinum toxin type A injection on the masseter muscle: An animal model study

BACKGROUND

The aim of this study was to investigate the effect of a botulinum toxin type A (BTX-A) injection in the masseter muscle using electromyography (EMG) in an animal model.

METHODS

Ten male adult (>3 months of age) New Zealand white rabbits were used. Muscle activity was continuously recorded from 8 hours before to 8 hours after BTX-A injection. The rabbits received unilateral BTX-A injections of either 5 units (group 1, n = 5) or 20 units (group 2, n = 5).

RESULTS

The masseter muscle activity of the rabbits was significantly reduced immediately after BTX-A injection (P < 0.05 for both groups). When the results from group 1 were compared with those from group 2, only the peak voltage was significantly decreased in group 2 (P = 0.013).

CONCLUSION

Masseter muscle activity measured by EMG was immediately decreased after a BTX-A injection.

Changes in masticatory function after injection of botulinum toxin type A to masticatory muscles

This study examined changes in masticatory function after botulinum toxin type A (BTX-A) injection using objective and subjective tests during 12 weeks. Also, we compared differences in masticatory function between group in which only masseter muscle (M group) was injected and group in which masseter and temporal muscle (M-T group) were injected. Forty subjects were assigned into two groups; M group (n = 20) and the M-T group (n = 20). The Meditoxin(®) was used as BTX-A injection. The mixing ability index (MAI) was used as the objective indicator, and visual analogue scale (VAS) and food intake ability (FIA) index were used as subjective indicators. Overall, the masticatory function drastically declined after 4 weeks and gradually recovered with time. Compared with the pre-injection state, the masticatory function decreased by 89·2% (MAI), 12·2% (FIA) and 32·2% (VAS) 4 weeks after the injection (P < 0·05). When the results between M group and M-T group were compared, scores of VAS and FIA were significantly different 4 weeks after the injection (P < 0·05), but the MAI score showed no significant difference between two groups. In conclusion, this study showed that masticatory function was significantly decreased after BTX-A injection into the masticatory muscle after 4 and 8 weeks from injection. However, masticatory efficiency measured using MAI could completely recover after 12 weeks. Furthermore, after 8 weeks from the injection, the masticatory function measured after injection into only the masseter muscle was similar to that measured after injection into both masseter and temporal muscle.

Botulinum toxin for masseter hypertrophy

BACKGROUND

Benign masseter muscle hypertrophy is an uncommon clinical phenomenon of uncertain aetiology which is characterised by a soft swelling near the angle of the mandible. The swelling may on occasion be associated with facial pain and can be prominent enough to be considered cosmetically disfiguring. Varying degrees of success have been reported for some of the treatment options for masseter hypertrophy, which range from simple pharmacotherapy to more invasive surgical reduction. Injection of botulinum toxin type A into the masseter muscle is generally considered a less invasive modality and has been advocated for cosmetic sculpting of the lower face. Botulinum toxin type A is a powerful neurotoxin which is produced by the anaerobic organism Clostridium botulinum and when injected into a muscle causes interference with the neurotransmitter mechanism producing selective paralysis and subsequent atrophy of the muscle.This review is an update of a previously published Cochrane review.

OBJECTIVES

To assess the efficacy and safety of botulinum toxin type A compared to placebo or no treatment, for the management of benign bilateral masseter hypertrophy.

SEARCH METHODS

We searched the following databases from inception to April 2013: the Cochrane Central Register of Controlled Trials (CENTRAL); MEDLINE (via PubMed); EMBASE (via embase.com); Web of Science; CINAHL; Academic Search Premier (via EBSCOhost); ScienceDirect; LILACS (via BIREME); PubMed Central and Google Scholar (from 1700 to 19 April 2013). We searched two bibliographic databases of regional journals (IndMED and Iranmedex) which were expected to contain relevant trials. We also searched reference lists of relevant articles and contacted investigators to identify additional published and unpublished studies.

SELECTION CRITERIA

Randomised controlled trials (RCTs) and controlled clinical trials (CCTs) comparing intra-masseteric injections of botulinum toxin versus placebo administered for cosmetic facial sculpting in individuals of any age with bilateral benign masseter hypertrophy, which had been self-evaluated and confirmed by clinical and radiological examination were considered for inclusion. We excluded participants with unilateral or compensatory contralateral masseter hypertrophy resulting from head and neck radiotherapy.

DATA COLLECTION AND ANALYSIS

Two review authors independently screened the search results. For future updates, two authors will independently extract data and assess trial quality using the Cochrane risk of bias tool. Risk ratios (RR) and corresponding 95% confidence intervals (CI) will be calculated for all dichotomous outcomes and the mean difference (MD) and 95% CI will be calculated for continuous outcomes.

MAIN RESULTS

We retrieved 683 unique references to studies. After screening these references 660 were excluded for being non-applicable. We assessed 23 full text articles for eligibility and all of these studies were excluded from the review.

AUTHORS' CONCLUSIONS

We were unable to identify any RCTs or CCTs assessing the efficacy and safety of intra-masseteric injections of botulinum toxin for people with bilateral benign masseter hypertrophy. The absence of high level evidence for the effectiveness of this intervention emphasises the need for well-designed, adequately powered RCTs.

Botulinum neurotoxin type A in the masseter muscle: effects on incisor eruption in rabbits

INTRODUCTION

Botulinum neurotoxins are responsible for the paralytic food poisoning, botulism. Commercial formulations such as botulinum neurotoxin type A are increasingly used for various conditions, including cosmetic recontouring of the lower face by injection of the large masseter muscles. The paralysis of a major muscle of mastication lowers occlusal force and thus might affect tooth eruption. The purpose of this study was to investigate the effects of unilateral masseter muscle injection of botulinum neurotoxin type A on the rate of eruption of incisors in a rabbit model. We hypothesized that the teeth would overerupt in an underloaded environment.

METHODS

Forty rabbits were injected with either botulinum neurotoxin type A or saline solution in 1 masseter muscle. Mastication and muscle force production were monitored, and incisor eruption rate was assessed by caliper measurement of grooved teeth.

RESULTS

The injection of saline solution had no effect. The masseter muscle injected with botulinum neurotoxin type A showed a dramatic loss of force 3 weeks after injection despite apparently normal mastication. Incisor eruption rate was significantly decreased for the botulinum neurotoxin type A group, an effect attributed to decreased attrition.

CONCLUSIONS

This study has implications for orthodontics. Although findings from ever-growing rabbit incisors cannot be extrapolated to human teeth, it is clear that botulinum neurotoxin type A caused a decrease in bite force that could influence dental eruption.

Electromyographic differences between normal upper and lower facial muscles and the influence of onabotulinum toxin a

IMPORTANCE

Empirically determined doses of onabotulinum toxin A for aesthetic treatments are as much as 5 times higher for the upper than for the lower facial muscles.

OBJECTIVE

To use electromyography (EMG) to determine objectively whether the disparity between doses is due to intrinsic differences between the muscle groups' responses to onabotulinum toxin A or to variable amounts of paralysis required to achieve the desired aesthetic outcomes.

DESIGN

We collected EMG data before and at 2 to 4 weeks and 3 months after 8- and 2-U onabotulinum toxin A injections to the corrugator and depressor anguli oris muscles, respectively.

SETTING

A private oculofacial plastic surgery practice.

PARTICIPANTS

Twenty-six subjects recruited from February 1 through April 1, 2009.

INTERVENTIONS

Electromyography recordings and cosmetic onabotulinum toxin A injections.

MAIN OUTCOME MEASURES

Mean motor unit (MU) durations and maximal amplitudes at baseline and 2 to 4 weeks and 3 months after injection.

RESULTS

Baseline mean MU amplitudes were similar for the corrugator and depressor anguli oris muscles. At 2 to 4 weeks after injection, 78% MU and 64% maximal amplitude reduction for the corrugator muscle were detected, but only 54% MU and 18% maximal amplitude reduction for the depressor anguli oris (P = 2.7 × 10-8 and P = 1.3 × 10-14, respectively). At 3 months, function was partially recovered for both muscle groups.

CONCLUSIONS AND RELEVANCE

Onabotulinum toxin A causes a similar dose-dependent reduction in MU and maximal voluntary amplitudes for muscles of the upper and lower face. The dose disparity appears to result from differences in the amount of paralysis required to achieve desirable aesthetic results.

LEVEL OF EVIDENCE

2.

Botulinum toxin in masticatory muscles: short- and long-term effects on muscle, bone, and craniofacial function in adult rabbits

Paralysis of the masticatory muscles using botulinum toxin (BTX) is a common treatment for cosmetic reduction of the masseters as well as for conditions involving muscle spasm and pain. The effects of this treatment on mastication have not been evaluated, and claims that the treatment unloads the jaw joint and mandible have not been validated. If BTX treatment does decrease mandibular loading, osteopenia might ensue as an adverse result. Rabbits received a single dose of BTX or saline into one randomly chosen masseter muscle and were followed for 4 or 12 weeks. Masticatory muscle activity was assessed weekly, and incisor bite force elicited by stimulation of each masseter was measured periodically. At the endpoint, strain gages were installed on the neck of the mandibular condyle and on the molar area of the mandible for in vivo bone strain recording during mastication and muscle stimulation. After termination, muscles were weighed and mandibular segments were scanned with micro CT. BTX paralysis of one masseter did not alter chewing side or rate, in part because of compensation by the medial pterygoid muscle. Masseter-induced bite force was dramatically decreased. Analysis of bone strain data suggested that at 4 weeks, the mandibular condyle of the BTX-injected side was underloaded, as were both sides of the molar area. Bone quantity and quality were severely decreased specifically at these underloaded locations, especially the injection-side condylar head. At 12 weeks, most functional parameters were near their pre-injection levels, but the injected masseter still exhibited atrophy and percent bone area was still low in the condylar head. In conclusion, although the performance of mastication was only minimally harmed by BTX paralysis of the masseter, the resulting underloading was sufficient to cause notable and persistent bone loss, particularly at the temporomandibular joint.

Mandible changes evaluated by computed tomography following Botulinum Toxin A injections in square-faced patients

BACKGROUND

A facial contour that is oval is more pleasing in Asian women. Patients with a square face often seek facial contouring procedures to improve their appearance. Treatment often involves various combinations of Botulinum NeuroToxin A (BoNTA) injections into the masseters and/or mandibular angle resection. Many physicians claim that muscle paralysis with injections alone will decrease pulling on the underlying bone and also treat underlying bony flaring when present. Muscular changes after BoNTA injections have been well documented. However, the effect of BoNTA injections on the underlying mandibular bone morphology has not been studied to the best of the authors' knowledge. The goal of this study was to determine whether there are mandibular changes after masseter injection with botulinum toxin.

METHODS

In this retrospective study of ten female patients seeking treatment for a square face, three-dimensional CT scans were taken before and 3 months after standardized BoNTA injections in bilateral masseters. Mandibular cortex thickness, mandibular bone thickness, and mandibular volume were measured.

RESULTS

Soft-tissue changes were observed but no bony changes were observed 3 months after injections.

CONCLUSIONS

In this study of adult patients, there were no statistically significant mandibular changes 3 months after BoNTA injection. The current theory of mandibular flaring resolution after partial muscle paralysis is not supported by our findings. Therefore, a patient presenting both masseteric hypertrophy and bony flaring will most likely require a combined muscular and bony procedure.

Comparison between neurectomy and botulinum toxin A injection for denervated skeletal muscle

Neurectomy and botulinum toxin A (BoNT-A) injection cause denervated muscle atrophy, but questions remain about their clinical utility. We investigated time-series alterations of rat muscle weight, functional deficits, signaling pathways, and microscopic structures, to gain an understanding of the clinical implications. Between 2008 and 2009, the maximal calf circumference of patients for calf reduction either by neurectomy or BoNT-A injections were recorded for study. A rat skeletal muscle model was established through repeated or dose-adjusted BoNT-A injections and neurectomy. The survival, apoptosis pathways, functional deficits, and microscopic structures were investigated using Western blot, sciatic functional index (SFI), and transmission electron microscopy (TEM), respectively. The rat muscle weight ratio of the BoNT-A group had recovered to 89.3 +/- 3.8% by week 58, but it never recovered in the neurectomy group. Muscle weight reduction by BoNT-A not only depended on the dose, but additive effects were also obtained through repeated injections. Rat SFI demonstrated rapid recovery in both groups. Molecular expressions showed a coherent and biphasic pattern. p-Akt and apoptosis-inducing factor (AIF) were upregulated significantly, with a peak at 8 weeks in the neurectomy group (p < 0.01), but cleaved caspase-9 and caspase-3 showed no significant changes in either group. TEM findings showed irreversible and reversible inner-structure disruption and sarcomere discontinuity in the neurectomy and BoNT-A groups, respectively. We demonstrated that denervation induced lasting muscle weight and structural changes of different degrees. Muscle weight reduction by BoNT-A was related to frequency and dose. AIF-mediated caspase-independent apoptosis was significantly different for neurectomy and BoNT-A injection.

Electromyographic activity of the masseter muscle after radiofrequency therapy in an animal model

PURPOSE

The purpose of this study was to examine changes in the electromyographic (EMG) activity of the masseter muscle after radiofrequency therapy (RF).

METHODS

Twelve rabbits were used in this study: four in each group according to the number of RF applications. Preoperative EMG in the masseter muscle was used as the control. EMG was recorded at 1, 2, 3, and 4 weeks after RF in each rabbit. The recorded data were analyzed in terms of voltage and frequency, and changes in recorded variables were compared among the groups. The relative activity in peak voltage, root mean square of the action potential, area of voltage, and area of frequency were investigated.

RESULTS

When compared to preoperative values, the variables at 3 or 4 weeks after RF application were significantly different in the single and quadruple therapy groups (P<0.05). There was no significant difference in the other groups (P>0.05). When the samples were regrouped as two groups like small number of application group (one or two point) and large number of application group (three or four points), the area of voltage and the area of frequency were significantly different between the groups at 4 weeks (P<0.05).

CONCLUSIONS

Masseter muscle activity after RF was significantly decreased compared to its preoperative state. The decreased activity was related to the number of applications and time elapsed after RF.

The adaptive response of jaw muscles to varying functional demands

Jaw muscles are versatile entities that are able to adapt their anatomical characteristics, such as size, cross-sectional area, and fibre properties, to altered functional demands. The dynamic nature of muscle fibres allows them to change their phenotype to optimize the required contractile function while minimizing energy use. Changes in these anatomical parameters are associated with changes in neuromuscular activity as the pattern of muscle activation by the central nervous system plays an important role in the modulation of muscle properties. This review summarizes the adaptive response of jaw muscles to various stimuli or perturbations in the orofacial system and addresses general changes in muscles as they adapt, specific adaptive changes in jaw muscles under various physiologic and pathologic conditions, and their adaptive response to non-surgical and surgical therapeutic interventions. Although the jaw muscles are used concertedly in the masticatory system, their adaptive changes are not always uniform and vary with the nature, intensity, and duration of the stimulus. In general, stretch, increases neuromuscular activity, and resistance training result in hypertrophy, elicits increases in mitochondrial content and cross-sectional area of the fibres, and may change the fibre-type composition of the muscle towards a larger percentage of slow-type fibres. In contrast, changes in the opposite direction occur when neuromuscular activity is reduced, the muscle is immobilized in a shortened position, or paralysed. The broad range of stimuli that affect the properties of jaw muscles might help explain the large variability in the anatomical and physiological characteristics found among individuals, muscles, and muscle portions.

Botulinum toxin for masseter hypertrophy

BACKGROUND

Benign masseter muscle hypertrophy is an uncommon clinical phenomenon of uncertain aetiology which is characterised by a soft swelling near the angle of the mandible. The swelling may on occasion be associated with facial pain and can be prominent enough to be considered cosmetically disfiguring. Varying degrees of success have been reported for some of the treatment options for masseter hypertrophy, which range from simple pharmacotherapy to more invasive surgical reduction. Injection of botulinum toxin type A into the masseter muscle is generally considered a less invasive modality and has been advocated for cosmetic sculpting of the lower face. Botulinum toxin type A is a powerful neurotoxin which is produced by the anaerobic organism Clostridium botulinum and when injected into a muscle causes interference with the neurotransmitter mechanism producing selective paralysis and subsequent atrophy of the muscle.

OBJECTIVES

To assess the effects of botulinum toxin type A in the management of benign bilateral masseter hypertrophy.

SEARCH STRATEGY

We searched the following databases in August 2008: the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library 2008, issue 3); MEDLINE (via PubMed) (1950 to August 2008); EMBASE (via embase.com) (1980 to August 2008); and LILACS via BIREME. We searched two bibliographic databases of regional journals which may be expected to contain relevant trials (IndMED and Iranmedex) using free text terms appropriate for this review.

SELECTION CRITERIA

Randomised controlled clinical trials (RCTs) and controlled clinical trials (CCTs) comparing intra-masseteric injections of botulinum toxin versus placebo administered for cosmetic facial sculpting in individuals of any age with bilateral benign masseter hypertrophy, which had been self-evaluated and confirmed by clinical and radiological examination. We excluded participants with unilateral or compensatory contralateral masseter hypertrophy resulting from head and neck radiotherapy.

DATA COLLECTION AND ANALYSIS

Two review authors conducted screening of studies in duplicate and independently, and although no eligible trials were identified, the two authors had planned to extract data independently and assess trial quality using standard Cochrane Collaboration methodologies.

MAIN RESULTS

We retrieved 167 references to studies, none of which matched the inclusion criteria for this review and all of which were excluded.

AUTHORS' CONCLUSIONS

We were unable to identify any randomised controlled trials on the efficacy of intra-masseteric injections of botulinum toxin for people with bilateral benign masseter hypertrophy. The absence of high level evidence for the effectiveness of this intervention emphasises the need for well-designed, adequately powered, randomised controlled clinical trials (RCTs) and controlled clinical trials (CCTs).