Treating Masseter Hypertrophy

In Asia, it is very common to use botulinum toxin to treat masseter hypertrophy and augment the lower face. Asian women, in general, prefer an ovoid facial shape than the square one. In this article, we will discuss masseter hypertrophy, and the effectiveness of bruxism and masseter hypertrophy treatments. In particular, we will discuss about the role of botulinum toxin in the non-surgical treatment of masseter hypertrophy, and the use of botulinum toxin type A as an adjunctive therapy of bruxism.

Masseter hypertrophy

Being one of the main principle muscles of chewing (mastication), the masseter muscle is vital to adequate mastication. Masseter muscles play an important role in the overall aesthetics of the face. Hypertrophied, enlarged masseter muscles can alter the facial lines, negatively affecting the overall facial aesthetics. Masseter hypertrophy can potentially cause a square facial shape and asymmetry. A square facial shape is often regarded as a masculine feature and generally not desirable in females.

First described in 1880 by Legg, masseter hypertrophy is a relatively rare medical condition characterized by the bilateral or unilateral enlargement of the masseter muscle. While usually asymptomatic, masseter hypertrophy can greatly affect the facial aesthetics. In the case of a unilateral masseter muscle enlargement, there may be significant asymmetry (which is the main reason patients seek aesthetic treatments). In some cases, hypertrophied masseter muscles result from poor dietary habits, teeth grinding (bruxism), and chewing gum habits.

A well-defined jawline is a sign of beauty and youth. A square jaw shape, on the other hand, may be out of proportion with other features on the face, and considered a disfigurement to patients. This is especially common in serious cases of unilateral hypertrophy and masseter hypertrophy. The perception of beauty varies greatly across ethnic groups and cultures.

According to Baek et al, oriental women generally prefer a feminine, delicate oval facial shape. This holds especially true for Korean women, who inherently have a wider jaw bone. Additionally, due to their diet, most Korean women regularly chew hard food. This consequently results in well-developed masseter muscles. Traditionally, Koreans believe that some facial features bring bad or good luck. The belief, in combination with the social preferences for slim faces, has propelled the popularity of lower face remodeling therapies.

Diagnosis of masseter hypertrophy

Before undergoing masseter hypertrophy treatment, it is important to have the correct diagnosis. Sometimes, the swelling at the angle of the mandible can be caused by other factors such as:

  • Parotid tumor;
  • Neoplasms of soft tissues;
  • Odontogenic problems;
  • Salivary gland disease;
  • Masseter tumor;
  • Parotid inflammatory disease;
  • Masseter muscle intrinsic myopathy;
  • Compensatory hypertrophy (caused by hypoplasia or hypotrophy in the contralateral side).

In combination with frequent clenching of the jaw and bruxism, masseter hypertrophy may lead to tension-type headaches, temporomandibular joint pain, masticatory muscle pain, toothache, fracture of the teeth and restorations and tooth wear.

The treatment of masseter hypertrophy

In most cases, the treatment of masseter hypertrophy is performed for aesthetic reasons and is not medically necessary. Doctors may choose to contour the angle of the jaw surgically or non-surgically. The surgical approach involves a partial excision of the masseter muscles. Alternatively, osteotomy can be performed to contour the prominent mandibular angle. Medically, osteotomy refers to the reduction of a thickened bone in the area of mandibular angle. While well reported in literature, these invasive surgical procedures tend to have a long recovery time. More importantly, they can potentially cause serious complications such as uneven contour lines, asymmetric resection, trismus, infection, facial nerve paralysis nerve injury and hematoma. Apart from that, sequelae from general anesthesia may also occur.

In contrast, the non-surgical approach involves the induction of disuse atrophy through the injection of botulinum toxin into the masseter muscle. The non-surgical masseter hypertrophy treatment helps to debulk the masseter muscles and reduce the width of the lower face. In 1994, Moore and Wood introduced the use of botulinum toxin type A as a treatment in hypertrophic masseter to fix functional issues.

A number of studies have described the advantages of using this non-invasive approach. Later in 1998, a study suggested using botulinum toxin for volume reduction in masseter hypertrophy. In 2005, another study used botulinum toxin type A to perform aesthetic treatments on over 1,000 patients, helping them to decrease the volume of masseter muscles. They found that masseter hypertrophy treatment using botulinum toxin type A is associated with a decreased recovery time and fewer adverse effects. Therefore, it is a better alternative to the conventional surgical masseter resection, which tends to be more invasive in nature. Researchers had reported five steps for the progression of botulinum toxin type A after injection into the masseter muscles.

  • Week 1: Muscle softening
  • Week 2 to 4: Muscles become visibly thinner
  • Week 10 to 12: Maximum effect can be seen
  • Week 12 and beyond: Action recurs
  • Week 16 and beyond: Muscle volume recurs

Treatment of bruxism

Apart from volume reduction, botulinum toxin type A injection into the masseter muscles also helps to decrease pain. At the same time, it minimizes grinding episodes. While there is little evidence supporting the use of botulinum toxin type A in bruxism, there are new literature reviews and studies that have demonstrated high anecdotal patient satisfaction, reduced bruxism-induced pain levels and decreased frequency of bruxism events. There are a few theories of how botulinum toxin type A relieves pain. It is believed that it directly affects the neuromuscular junction or exerts an anti-proprioceptive action on the nerves. Also, it could be that botulinum toxin blocks the signal transmission of afferent neurons or inhibits the release of neuropeptides.

Conclusion

Having muscle relaxant and analgesic properties, botulinum toxin injectables are used for a wide range of clinical and cosmetic purposes. In aesthetic medicine, it is commonly used in the treatment of wrinkles, lines and folds. Other than that, botulinum toxin type A injection is a safe and effective way of treating masseter muscle hypertrophy and contouring the lower face. The non-invasive botulinum toxin injection is also used in bruxism as an adjunctive treatment. Prior to treatment, the practitioner should consider important factors such as the long-term cost effectiveness and the potential immune resistance that may occur due to repeated injections.