# Biphasic Injection for Masseter Muscle Reduction with Botulinum Toxin
###### Abstract  
Maseter Muscle Hypertrophy (MMH) is a well-known clinical benign condition that is not gender-specific and it can be monolateral or bilateral. Botulinum Toxin type A (BoNTA) injection has been widely described for MMH treatment and non-surgical facial slimming. BoNTA masseter injections have high efficacy and safety profile, but the risks of side effects remain. Muscular bulging during mastication is a complication due to the superficial overcompensation of masseter fibers in response to neurotoxic weakening of the deep masseter. We present a biphasic-injection technique for BoNTA administration, based following anatomical concept and developed in order to prevent paradoxical bulging. A total of 98 treatments from 2015 to 2020 were performed with this technique. No remarkable complications occurred in our study. No cases of loss of full smile, difficulty in mouth opening, dizziness, headache, neurapraxia, and rescotonia were reported. A case of asymmetric smiling was self-resolved within a week. No patient claimed transient muscle weakness as distressing. No cases of paradoxical bulging were observed. Extensive knowledge of muscular anatomy and appropriate injection technique are key factors in achieving the desired result and avoiding complications. We feel that sharing this tip could be helpful for all the physicians involved in MMH treatment with BoNTA. msceter muscle hypertrophy; botulinum toxin type A; biphasic injection technique; paradoxical bulging; muscular bulging; non-surgical facial slimming; lower face reshaping; neurotoxic weakening; complications; neurotoxic weakening +
Footnote †: journal: Medical and Surgical Sciences  
Maseter muscle hypertrophy (MMH) is considered to be a well-known clinical, long recognized, and benign condition, in spite of the growing aesthetic concern manifested around it [2]. It is not gender-specific and it can be monolateral or bilateral. The etiology of masester muscle hypertrophy has been attributed to a number of factors such as emotional stress, bruxism, clenching, temporomandibular joint disorders, masticatory muscles hyperactivity, massoteric hyperfunction and parafunction, and microtrauma as well as dietary habits [2; 3; 4; 5; 6; 7]. Continuous gum, ethnic tough-food diets [2; 8], and hard chewing can also lead to MMH [8; 9]. Furthermore, malocclusion, masticatory muscle hyperactivity and mandibular retrogntahia have been postulated as possible causes [1]. The origin of this condition is controversial because masticatory muscle hyperactivity or dysfunction and parafunction in stomatognathic system may not occur in all MMH cases [1; 2; 3; 4; 5; 6; 7]. Compensatory and stress hypertrophy could be assumed in many patients [4; 5; 6]. Modifications in proprioceptors have been debated [5; 6]: cases of MMH in connection with neuroleptically-induced facial dystonia suggest that the masseter hypertrophy could be attributed to a disturbance of the balance between neurotransmitter as acetylcholine and dopamine [7]. Emotional disturbances and psychological disorders impact the ability to keep muscular tone and proprioception, thus patients that suffer from those diseases are at a higher risk of developing MMH [5; 6; 7]. Various surgical and non-surgical treatments have been introduced for lower face reshaping. In the non-surgical therapeutic approach, effort has been made toward reducing the muscular hyperactivity by using occlusal splint or prescribing muscle relaxants [10]. Occlusal splints are easy to fabricate, widely used in the diagnosis and treatment of bruxism and temporomandibular disorders, and considered as the first choice for protecting teeth and prostheses from damages [11; 12; 13; 14]. Moreover, [PERSON] et al. describe that neither occlusal splint or modified nociceptive trigeminal inhibition splint device affected the masseteric muscle asymmetry or postural activity/maximum voluntary contraction ratio after treatment [11]. Nevertheless, occlusal devices can be effective in altering pressure pain threshold in patients with diagnosed bruxism [12].