Treating Masseter Hypertrophy

By Dr Souphiyeh Samizadeh / 01 Jan 2016

Dr Souphiyeh Samizadeh details the occurrence and efficacy of treating masseter hypertrophy and bruxism with botulinum toxin

Introduction: The use of botulinum toxin

Botulinum toxin (BTX) has been successfully used for the treatment of many conditions including excessive muscle tone, movement disorders, strabismus, dystonias, blepharospasm, hemifacial spasm, cervical dystonia, juvenile cerebral palsy, and adult spasticity.1,2 Use of BTX for cosmetic purposes has gained popularity in the past two decades and, recently, other therapeutic uses of BTX have been studied.3 A number of potential new cosmetic and therapeutic indications including conditions associated with pain, excessive or dyssynergic muscle contraction and hypersecretion of glands have emerged, which have attracted a significant amount of interest from the scientific community.2

Botulinum toxin A (BTXA) inhibits acetylcholine release from nerve terminals at the neuromuscular junction, resulting in inhibition of muscle contraction.4 In addition, BTXA also produces localised chemical denervation of parasympathetic postganglionic cholinergic fibres, making it effective for the treatment of hyperhidrosis and hypersalivation.1 Studies of conditions such as myokymia, bruxism, and anal fissure have also reported promising results.5-7 A secondary outcome of the BTX injection is pain-relief, suggesting BTX has a direct effect on pain mechanisms, which may be independent of its neuromuscular actions.8 This finding has resulted in new emerging indications, including treatment for conditions associated with pain, such as migraine, headaches, myofascial pain, neuropathic pain and chronic lower-back pain.Current evidence suggests that BTX can selectively weaken painful muscles and interrupt the spasm-pain cycle. The analgesic effects of BTXA were first reported after observation of substantial pain relief in cervical dystonia.

Figure 1: Masseter muscles are located laterally to the mandibular ramus, and therefore play a significant role in facial aesthetics 

Initially the pain reduction was attributed to reduced muscle hyperactivity,8,9 however, a direct analgesic effect has been shown in animal studies.9,10 Various studies have reported positive results for management of conditions associated with pain using BTX,8,11 however further research is necessary. BTXA is very commonly used in Asia for lower facial contouring and to treat masseter hypertrophy, as Asian women tend to dislike the square face shape and favour a delicate ovoid shape instead.12-14 In this article I will discuss masseter hypertrophy and its nonsurgical treatment using BTXA, as well as its role as an adjunct treatment for bruxism.

Masseter muscle hypertrophy

The masseter muscle is one of the four principal muscles of mastication (chewing) and is essential for adequate mastication.15 Masseter muscles are located laterally to the mandibular ramus,16 and therefore play a significant role in facial aesthetics (Figure 1). Enlarged and hypertrophied masseter muscles will alter facial lines and can have a negative aesthetic impact –potentially causing facial asymmetry and a square face shape, which is often considered a masculine trait.17

Masseter hypertrophy was first described by Legg in 1880.18 It is a relatively uncommon condition and is characterised by benign unilateral or bilateral enlargement of the masseter muscles (Figure 2).17,18 Masseter hypertrophy is asymptomatic and the reason patients seek help is predominantly related to aesthetics, in particular if there is noticeable asymmetry due to unilateral presentation.17 Masseter muscle hypertrophy can also be a result of dietary habits, chewing gum habits and bruxism (teeth grinding).18-20 

Figure 2: Masseter hypertrophy, which appears worse on the patient’s left-hand side 

A well-defined jawline is often considered a sign of youth and beauty.21 However, a square jaw shape, which may be out of proportion with other facial features and, in cases of severe masseter hypertrophy and unilateral hypertrophy, can be seen as a disfigurement to a patient. There are also ethnic variations in perception of beauty; Baek et al reported that oriental women greatly desire an oval, delicate and feminine facial shape.22 This is particularly true for Korean women, who tend to have a wide jaw bone15. In addition, as part of their diet, they chew rough and hard food regularly, resulting in well-developed masseter muscles.15 Furthermore, in Korea, the belief that certain facial features bring good or back luck, in addition to the social preferences of slim and smaller faces, has resulted in popular lower face remodelling treatments.14


Prior to treating masseter hypertrophy, it is important to ensure you have the correct diagnosis. Other reasons for swellings at the angle of the mandible could be due to:17,18

  • Compensatory hypertrophy (due to hypotrophy or hypoplasia in the contralateral side)
  • Masseter muscle intrinsic myopathy
  • Masseter tumour
  • Salivary gland disease
  • Parotid tumour
  • Parotid inflammatory disease
  • Odontogenic problems
  • Neoplasms of soft tissues

Masseteric hypertrophy, accompanied by bruxism and frequently clenching the jaw can result in tooth wear, fracture of the teeth and restorations, toothache, masticatory muscle pain, temporomandibular joint pain, and tension-type headache.23

Treating masseter hypertrophy

Treatment of masseter enlargement is usually unnecessary and is carried out for aesthetic purposes.18 Aesthetic contouring of the angles of the jaw can be carried out both surgically and non-surgically. Surgical aesthetic contouring of the prominent mandibular angle through partial excision of the masseter muscle and/or osteotomy (reduction of the thickened bone in the region of the mandibular angle) is well reported in literature.22,24 However, surgical approaches are invasive, have a relatively long recovery time and complications such as hematoma, nerve injury, facial nerve paralysis, infection, trismus, asymmetric resection, uneven contour lines and sequelae from general anaesthesia, can occur.14

Injecting BTX into the masseter muscles to induce ‘disuse atrophy’ is a nonsurgical approach of de-bulking masseter muscles and reducing lower face width, which, as discussed, is very common in Asian countries.25 Numerous studies have highlighted the benefits of this non-invasive approach since Moore and Wood introduced the BTXA injection of the hypertrophic masseter to treat functional problems in 1994.26 Following this, Rijsdijk and colleagues, in 1998, reported the use of BTXA for aesthetic volume reduction of the hypertrophic masseter.27 In 2001, Von Lindern et al published an on article the use of BTX for masseter and temporal muscles.28 Kim et al treated 1021 patients with BTXA for aesthetic purposes to reduce the volume of masseter muscle in 2005. They concluded that BTXA is a simple technique, with few side effects and reduced recovery time, and can also replace surgical masseter resection. They reported five steps for the progression of BTXA in masseter muscles after injection:14

  • Week 1: Muscle softening
  • Week 2-4: Muscles noticeably thinner
  • Week 10-12: Maximum effect
  • Week 12+: Action recurs
  • Week 16+: Muscle volume recurs

Treating bruxism

In addition to reducing the volume of the masseter muscles, injection of BTXA into this area has been shown to reduce pain and grinding episodes. Although scientific evidence to support treatment of bruxism with BTXA is not abundant, current studies and literature reviews have shown reduced frequency of bruxism events, decreased bruxism-induced pain levels and high anecdotal patient satisfaction.5,29,30 Various hypotheses have been described for the mechanism by which BTXA relieves pain. These include direct effects at the neuromuscular junction, direct antiproprioceptive effects on nerves, inhibition of the release of various neuropeptides and neuromodulators, and blockage of the transmission of afferent neuronal signals.31


BTXA injection is a non-invasive, effective and safe treatment for masseter muscle hypertrophy and lower face contouring. It can also be used as an effective adjunct for the treatment of bruxism. Important issues such as immune-resistance from repeated injections and long-term cost effectiveness of treatment should be considered when offering this treatment option to patients.


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