Unilateral masseteric hypertrophy is a relatively rare anomaly, which follows a slow and progressive pattern of growth [2,9], affecting men and women in a 1:1 ratio, especially between the second and third decade of life [7,9,10].

This hypertrophy causes changes in the facial lines that can generate asymmetries or produce an appearance of a “square” face [2,8]. These characteristics are not well accepted aesthetically, since they cause discomfort and negative impacts for many patients [2,11]. In the present case, the patient's main complaint was aesthetics.

The diagnosis of this anomaly is made by associating the clinical examination with the results of imaging tests, especially computed tomography and magnetic resonance imaging [1]. In these exams, masseteric hypertrophy appears in a similar way, demonstrating muscle enlargement. In addition, in computed tomography, it is possible to observe, in a posteroanterior view, the presence of an abnormal lateral projection at the angle of the mandible, called bone spur [1,6]. In the present case, after the clinical examination, the strongest diagnostic hypothesis was masseteric hypertrophy. Thus, only frontal teleradiography was necessary to confirm the diagnosis, since it was possible to observe a large soft tissue asymmetry, as well as the presence of the bone spur. With the diagnosis already defined, it was not necessary to request other complementary exams, such as computed tomography and magnetic resonance imaging, considered today the gold standard of diagnosis, however, more expensive.

Masseter muscle hypertrophy has, as a differential diagnosis, diseases of the parotid glands, parotid inflammatory disease, unilateral compensatory hypertrophy, benign or malignant muscle tumors, vascular tumors, among others [1,10]. The diagnosis is more challenging when it comes to unilateral masseteric hypertrophy [10]. However, as much as in this case the hypertrophy was unilateral, there was no difficulty in closing the diagnosis, since all the characteristics, both clinical and radiographic, were compatible with masseteric hypertrophy.

There are several treatment options for the management of masseteric hypertrophy, which can be classified into surgical and non-surgical [2]. Non-surgical methods include prescription of anxiolytics, antidepressants and muscle relaxants, dental restorations and occlusal adjustments, radiofrequency and application of type A botulinum toxin. Surgical methods include mandibular angle removal, masseter neurectomy and intra reduction or extra-oral masseter muscle [7].

It is necessary to consider some factors to define the best treatment management for the patient, with the first analysis focusing on the severity of hypertrophy. Surgical intervention is capable of reducing two-thirds or more of the masseter muscle volume, while botulinum toxin, at its maximum dosage, can reduce only one-third of the muscle volume. Thus, in cases where masseteric hypertrophy is severe, the surgical option should be used [6].

Another criteria to be analyzed is related to the longevity and predictability of the treatment [6]. It is well known that the injection of botulinum toxin type A is a safe procedure that produces good results for the treatment of mesenteric hypertrophy [3,12,13], however it has many adverse effects such as edema, bruising, dizziness, headache, weakness in chewing, asymmetry, xerostomia, among others [3,4,14]. In addition to these factors, another limitation of the use of this technique is its temporary pharmacological effects, which require several reapplications due to an approximate duration of six months [2,3,15]. On the other hand, the surgical option is capable of providing a more predictable and definitive treatment [6], even with the possible complications, such as postoperative hemorrhage, edema, hematoma, facial asymmetry and damage to noble structures. Taking into account the severity of hypertrophy, predictability and, consequently, the effectiveness of the treatment, we opted for the surgical procedure using the modified PARY technique, already described.

The traditional treatment for hypertrophy of the masseter muscle involves its partial surgical resection, associated or not with mandibular angles, which can be performed intra or extra-oral [3]. In the past, the headgear technique was better accepted and indicated, since it offered a better visualization of the operative field. However, with the evolution of surgical materials, as well as the development of new techniques, the procedure through the intraoral route has become a good option [2]. In this case, we opted to perform partial resection of the masseter muscle by intraoral route, since we wanted to minimize the risks of facial nerve damage, as well as avoid noticeable scarring in the patient, whose main complaint was aesthetics.

In the present study, surgical treatment proved to be efficient, achieving satisfactory and stable results after 4 years of the surgical procedure, reducing the patient's facial asymmetry, without the need for new interventions and with no sequela and recurrence.