Introduction

The ideal female face is perceived as delicate, smoothly contoured & oval shaped. A square jawline seen more commonly in Asians, widens the lower face and is considered masculine [1]. In most patients, the square lower jawline is created by masseteric muscle hypertrophy. This is more common in Koreans, Chinese & other south east Asians as opposed to Caucasians.Caucasians have long, narrow faces, while Asians typically have short, wider faces.

Masseteric muscle hypertrophy may be unilateral or bilateral. It is most common between the ages of 20 and 40 years and is not gender specific [1].

Causes of masseter hypertrophy include

• Idiopathically enlarged masseters
• Asymmetric chewing due to dental problems
• Temporomandibular joint dysfunction
• Focal dystonia
• Genetics – Asians are predisposed to a wider mandibular and
  thicker masseter
• Diet – chewing dried squid, meat, fibrous vegetables, betel nut, etc. can increase masseter bulk and mandibular size
• Personal habits – jaw clenching/grinding, bruxism, gum chewing

In the past, surgery was the only option to reshape the lower jaw. Partial resection of the masseter or bony angle of the mandible was done. Surgery however, had lot of side effects like hematoma formation, pain, facial nerve paralysis, infection, difficulty in opening the mouth and sequelae from general anesthesia. Hence it was not very popular [2]. Botulinum toxin A is a highly efficacious and cost-effective, nonsurgical option for reducing the width and shape of the lower face and jawline. Injection of botulinum toxin type A into the masseter muscle was first introduced by Smyth, Moore and Wood in 1994 and considered a less invasive modality and for cosmetic sculpting of the lower face. This decreased masseteric girth produces a gentler, more rounded lower face and jawline [3, 4].

Before making a diagnosis of benign masseter hypertrophy and going in for botulinum toxin injection, it is very important to investigate and rule out head and neck masses which can mimic masseter hypertrophy [5].

Functional anatomy [6]

The masseter is one of the muscles of mastication. It is a thick, quadrilateral muscle, consisting of two portions, superficial and deep. The superficial portion arises from the zygomatic process of the maxilla, and from the anterior two-thirds of the lower border of the zygomatic arch. Its fibers pass downward and backward, to be inserted into the angle and lower half of the lateral surface of the ramus of the mandible. The deep portion arises from the posterior third of the lower border and from the medial surface of the zygomatic arch. Its fibers pass downward and forward, to be inserted into the upper half of the ramus and the lateral surface of the coronoid process of the mandible. The fibers of the two heads are continuous at their insertion.

Intimate knowledge of the surrounding anatomy is very important before injecting the masseter to prevent inadvertent injections into the surrounding tissues. Most at risk are the superficial mimetic muscles of the face. These are located just below the skin overlying the masseter. Above the masseter and just below these muscles lie their motor nerves. Overlying the masseter posteriorly is the parotid gland. The facial artery and vein cross the masseter at its anteroinferior corner and then course superiorly and anteriorly.

Inection technique

In our study, 25 patients, 20 females and 5 males, all of Indian origin were treated with botulinum toxin. They had bilateral masseter hypertrophy but no history of Temporomandibular joint dysfunction or dental problems. None of them had a bony enlargement of the mandibular angle. There were no contraindications for botulinum toxin injection in any of the patients.

One hundred units of lyophilized onabotulinum toxin was reconstituted with 2.5 mL of sterile normal saline, giving a concentration of 4 units per 0.1 ml. The solution was injected with a one inch long 30-gauge hypodermic needle attached to a 1-mL tuberculin syringe. We used the three point injection technique.

After taking a written informed consent, photographs of the face in frontal view were taken with a standard camera. The face was cleaned to maintain asepsis. A sterile brow pencil was used to draw a line from the corner of the angle of lip to the lower part of the external auditory meatus [6]. All injections were kept below this line. The patient was then asked to clench the teeth tightly. This was done to accentuate the contours of the masseter muscle at the angle of the jaw. The massetter muscle was palpated & the bulkiness of the muscle assessed. The anterior border and posterior border of the muscle were marked with the pencil (as in Fig. 1). The centre of the muscle which is also the point of maximum bulge was marked. The second point was marked lower down, one centimetre lateral to the anterior border of the masseter muscle. The third point was marked a centimeter medial to the lateral border of the masseter muscle in the same line as the second point. Depending on the bulk of the muscle, 5 to 15 units were injected per injection point and a total of 15 to 45 units were injected per side. Deep intramuscular injections were given with the needle perpendicular to the skin surface so that the neurotoxin diffused into both the superficial head and part of the deep head of the masseter muscle.

The injection points two and three were kept as low as possible to the mandibular margin [6].

We made sure the needle used for injection was at least 1inch long so that both the superficial and deep part of the muscle are injected. In case the deep part is left unattended, it bulges out like a chipmunk everytime the patient talks.

The patients were asked to follow up in 6 weeks and additional units of botulinum toxin were injected if the masseter was still palpable.

Results

Patients responded within 4 to 6 weeks of injection. 4 of them needed a touch up of 10 to 15 units after 6 weeks. They found that their face looked slimmer and more oval in shape. The square jaw had disappeared leaving them pleasantly surprised. (Fig. 2 and Fig. 3).

3 of them reported that they were relieved of bruxism. In all, they were happy with the contouring of their face. One patient developed asymmetry of her smile and complained that her mouth wasn’t opening wide enough when she smiled. This effect lasted for almost 4 weeks until it got back to normal.

Two patients complained of prominent jowls which according to them were not present before the masseter injection. This resolved to normal in 6 weeks. No other side effects were seen.

The patients were followed up for up to a year and it was found that the effect of the toxin continued to last for a year. Some of the patients were re-injected after a year.
Discussion

The masseter muscle is one of the four muscles involved in mastication. Hypertrophy of the muscle leads to a square face and a broad jaw which is aesthetically not pleasing to most people. Botulinum toxin is injected into the enlarged masseter muscle to reduce its size giving a more oval and slim look to the face. The best results are seen only 6 to 8 weeks after the neurotoxin is injected unlike the other cosmetic indications where the results may become evident as early as even 24hours.

By binding to the presynaptic cholinergic nerve terminals, onabotulinum toxin blocks the release of acetylcholine. This causes atrophy of the masseter muscle. Although the binding is permanent, muscle contraction usually recovers over 3 to 4 months as new nerve terminal axon sprouts form, restoring neuromuscular transmission. The muscle thus begins to increase in size after 4 to 6 months [7].

In an ultrasound study by Park et al, they found that the maximum reduction in the masseter thickness was seen a month after injection with botulinum toxin. A continued reduction of masseteric muscle thickness was seen on the CT up to 3 months after injection [8].

The effect is seen to last from 4-10months.However, the results are not desirable if the mandibular bone is prominent or there is excessive fat present or there is sagging facial tissue⁶. Botulinum toxin should not be injected in these cases.

Injection techniques have varied from three point technique to five point technique. Ahn et al have described four injection points along the inferior border of the masseter muscle along the jawline. They used the needle tip to palpate the mandible and then retracted the needle 1 to 2 mm prior to injection, thus avoiding the superficial facial musculature. They recommended injection of the deeper portions of the muscle because the surrounding facial musculature, nerves, vessels, and ducts lie superficial to the masseter. Patients were injected up to 3 times at 1-week intervals. 4 Injection points along the border of mandible, 25 units per side were injected first, a week later 25 units more. So total 50 units were injected per side [9].

Lieu and dart described the five injection point technique. This according to them ensured better spread of the toxin to the entire muscle. In their study, they injected 25 to 30 units of botulinum toxin into each masseter muscle in the western group of patients. In East Asian patients, the initial dose was 40 to 45 units on each side. Patients who showed a minimum response on week 8 were offered additional injection of 10 to 20 units on each side [10].

In our study, we found the three point injection technique to be adequate in Indians. We found that 25 to 30 units per side in females and 30 – 35 units in males was adequate in Indian patients for desirable results.(Fig. 2 and 3) In 2 male patients, we did inject 10 units per side more after 6 weeks of the first dose. There was uniform spread of toxin to both superficial as well as deep part of the masseter muscle resulting in a cosmetically acceptable reduction in the muscle.

In order to prevent diffusion into the buccinators or risorius which can lead to smile asymmetry, one must remember to keep the injection point at least 1 cm from the anterior border of the masseter muscle. Injection points should also be kept below the sigmoid notch of the mandible in order to prevent inadvertent paralysis of other muscle of mastication, namely the pterygoid muscles [10]. This can be done by staying below an imaginary line from the auditory meatus to mid-philtrum while injecting.

According to Kaya et al, the ideal site of Botox injection into the masseter is a rectangular area, 5 cm inferior to the Orbito-Meatal Line (OML), 1 cm anterior and posterior to a vertical line which intersects the mid-distance of the OML to the tip of the angle of the mandible, and just above the periosteum. These landmarks will avoid injection into the parotid gland, marginal mandibular nerve, and other branches of the facial nerve such as its buccal branch. They also said that the masseteric nerve can easily be found approximately 1.5 cm inferior to the zygomatic arch, 1 cm medial to the temporomandibular joint capsule and 1 cm superior to mandibular notch which makes its use for facial reanimations more efficient [11].

In an interesting study, Yunxie et al. classified the masseter muscle into five types in order to help in accurate injection and spread of botulinum toxin. This classification depended on clinical and ultrasound examination of the muscle bulge.

Classification of the Contracted Masseter Muscle Regarding Bulging Type [12] :

I Minimal Even masseter contraction, no obvious bulging palpable

II Mono Local single longitudinal bulge

III Double Two separate longitudinal bulges of equal or differing height

IV Triple Three longitudinal bulges

V Excessive Massive single bulge

The doses and injection points of botulinum toxin varied from 20 to 40 units per side and one to three injection points per side depending on this classification.

Complications [1, 9, 10]

Based on a number of studies, it has been seen that in some patients, there may be a temporary inability to chew hard foods from 1 week post-injection, lasting up to 6 weeks. Localised swelling at the injection site due to haematomas may be seen but they resolve on their own within a week. Muscle ache and pain at site of injection is also reported. Minor changes in facial expression can occur. This is due to accidental weakening of the zygomaticus major or risorious muscle. Smile asymmetry & difficulty insmiling from diffusion into zygomaticus major or levator anguli is a temporary complication if botulinum toxin is injected too high into the masseter.

It has also been seen that the cheeks appear sunken following injection. This is because of atrophy of the superior portion of the masseter in patients. The superior portion of the masseter lies just below the bony prominence of the cheekbone, thus thinning the masseter may result in a sharp drop-off between the zygomatic arch and cheek causing the sunken cheek appearance [13].

Sometimes, a minor skin ‘sagging’ has been seen post injection. This is because the muscle shrinks faster than skin contracts. It usually resolves in 6–12 weeks [14].

There have been no permanent complications reported.

Conclusion

Injecting botulinum toxin in patients with Masseter muscle hypertrophy is an excellent and safe nonsurgical option to reduce the size of the muscle and give a chiselled jaw line resulting in a desirable oval face. Some key injections pearls must be kept in mind to avoid untoward effects.

Always stay below the line drawn from lip corner to ear tragus and keep injections below this line. Mark and delineate the muscle with the patient’s teeth clenched. Always inject at least 1 cm away from muscle borders. Lastly, do not be in a hurry to do a touch up before 6weeks of the first injection.