Masseter muscle hypertrophy is a condition that can negatively affect both the appearance and function of an individual’s face. This condition is characterized by the enlargement of the masseter muscle, leading to a squared jawline, facial imbalance, and discomfort associated with bruxism.
While traditional surgical interventions were once the primary treatment modality, advancements in non-invasive procedures, particularly the utilization of botulinum toxin type A (Botox), have significantly revolutionized treatment options. Botox injections offer an effective means to relax the masseter muscle, alleviating symptoms and enhancing facial aesthetics. This article provides a comprehensive examination of the utilization of Botox injections for the treatment of masseter muscle hypertrophy, encompassing its mechanism of action, injection techniques, expected outcomes, and potential complications.
Masseter muscle
The masseter muscle is one of the four primary muscles of mastication. This complex muscle consists of three heads – superficial, deep, and coronoid – that originate from the zygomatic arch. The superficial layer attaches to the external surface of the mandible at the angle and to the inferior half of the lateral surface of the ramus, while the deep layer attaches to the superior half of the ramus almost as high as the coronoid process.
The muscle’s primary function is to elevate the mandible and facilitate some protraction of the jaw. Additionally, the coronoid head is believed to aid in mandibular retraction.
Masseter Muscle Hypertrophy (MMH)
Masseteric hypertrophy is a medical condition initially identified by Legg in 1800, characterized by the overgrowth or enlargement of the masseter muscle, resulting in a more prominent, square-shaped appearance of the jawline.1 Although it can affect individuals of any age, gender, or ethnic background, studies indicate that it is more prevalent among young adults and adolescents, particularly males of Asian descent.2 Asymmetric presentations may occur, primarily in cases of one-sided chewing or clenching habits.
Masseter muscle hypertrophy can cause facial asymmetry, discomfort, and negative cosmetic effects. It can also lead to functional difficulties such as bruxism, prognathism, clenching, and trismus. These issues can adversely affect mandibular movement and overall oral health. Despite its prevalence, the exact etiology of this condition remains unclear, although it is generally believed to be a result of a combination of genetic and environmental factors.
The diagnosis of masseteric hypertrophy relies primarily on a comprehensive clinical examination. This examination includes palpation of the masseter muscle during forceful clenching and observation of facial disfigurement, particularly in the lower third of the face. Differential diagnosis may include other pathologies such as parotiditis, parotid tumors, lipomas, or benign and malignant muscle or mandibular tumors. Imaging techniques such as USG and computed tomography (CT) scans may be employed to confirm the diagnosis and exclude other soft tissue lesions. Early detection and timely treatment can prevent the progression of masseteric hypertrophy. Therefore, accurate diagnosis is imperative to ensure prompt intervention and prevent long-term complications.3
The treatment options for MMH vary depending on the severity and impact on the individual’s quality of life. Conservative measures such as lifestyle modifications can be helpful in some cases, while others may require surgical interventions such as muscle reduction procedures.
Role of botox in MMH
The use of botulinum toxin type A (BTA) as a minimally invasive approach to address masseter hypertrophy was first introduced in 1994 by Smyth, Moore, and Wood.4 BTA acts by inhibiting the release of acetylcholine at the neuromuscular junction, leading to temporary flaccid paralysis of the muscle. This procedure has demonstrated efficacy in reducing muscle bulk, reshaping the lower face, and alleviating symptoms of pain, grinding, or clenching.
While nerve terminals regenerate approximately three months after the procedure, repeated sessions can maintain complete paralysis. The chemo-denervation effect of BTA results in temporary atrophy of the muscle, which persists for a duration of 3-6 months post-treatment. Therefore, it is recommended that the interval between each session be approximately 3-6 months to sustain the desired outcomes.
Despite its generally safe profile, the administration of BTA can occasionally produce adverse effects. These effects are primarily associated with excessive infiltration or inaccurate placement of the neuromodulator. Some of the reported adverse effects include limitations in smiling or asymmetry, reduced chewing force, difficulties with wide mouth opening, mild bruising, local pain at the site of injection, mild headache, awkward facial expressions, speech disturbances, uneven bulging of muscles, sunken cheeks, and herniation of the parotid gland through the attenuated overlying muscle.
It is important to note, however, that the adverse effects are generally mild and short-lasting. Despite these occasional side effects, BTA is widely considered to be a safe and effective neuromodulator.
Treatment procedure
The administration of Botox for masseter muscle relaxation necessitates a comprehensive understanding of facial anatomy and precise injection technique. The procedure entails multiple injections into predetermined sites along the masseter muscle. The dosage and injection technique may vary based on factors such as muscle thickness, hypertrophy severity, and patient preferences.
The Moore & Wood et al (1994) technique was the initial method proposed, which entailed injecting at the most prominent point in the masseter muscle. Since then, various techniques have been proposed, including Kim NH, Park RH et al (2010), Shim et al (2010), Bae J.H et al (2014), Wei J et al (2015), Quezada-Gaon et al (2016), and Hong JY et al (2021), which have undergone modifications and improvements.3
The improved technique, which includes the Kim NH, Park RH et al (2010) technique, establishes a safety zone by marking the boundaries of the masseter muscle. The anterior and posterior borders of the muscle, the earlobe-mouth corner line superiorly, and the mandibular border inferiorly mark the safety zone. In this technique, 50% of BTA is injected into the center of the safety zone, and 25% is given at two other points within the safety zone.
Conclusion
The utilization of botulinum toxin for masseter muscle relaxation is a sophisticated and accessible remedy for the treatment of hypertrophy and its related issues. This method enables healthcare providers to obtain natural-looking outcomes that promote facial harmony and elevate the quality of life for their patients. However, the efficacy of this treatment is contingent upon an intricate patient selection process, meticulous treatment planning, and a precise injection technique. By adhering to these guidelines, professionals can ensure that patients derive the full benefits of this transformative treatment. 
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References
- Legg, J. W. (1880). Enlargement of the temporal and masseter muscles on both sides. Trans Pathol Soc (Lond), 31, 361-366.
- Sannomya, E. K., Gonçalves, M., & Cavalcanti, M. P. (2006). Masseter muscle hypertrophy: case report. Brazilian dental journal, 17, 347-350.
- Kundu, N., Kothari, R., Shah, N., Sandhu, S., Tripathy, D. M., Galadari, H., … & Goldust, M. (2022). Efficacy of botulinum toxin in masseter muscle hypertrophy for lower face contouring. Journal of cosmetic dermatology, 21(5), 1849-1856.
- Smyth, A. G. (1994). Botulinum toxin treatment of bilateral masseteric hypertrophy. British Journal of Oral and Maxillofacial Surgery, 32(1), 29-33.
About the Author
Dr. Pratyusha Kondath is a dental surgeon and writer from India, currently based in Canada. Drawing on her experience from both countries, she offers a global perspective and shares insights on enhancing the dental experience and the latest trends in the field. Through her writing, she aims to educate and bridge gaps in dental care.