# Biphasic Injection for Masseter Muscle Reduction with Botulinum Toxin
## 2 Materials and Methods  
From 2006 to 2014, with a total of 43 treatments performed on 21 patients, the senior author ([PERSON]) managed MMH with injection on 4 sites as already published in 2008 [44]. In 2014, for the first time, the senior author had a patient who experienced post toxin paradoxical bulging (Figure 1, Supplementary Materials: Video S1). Since that moment, the author modified the injection technique in order to avoid this kind of complication. From the end of 2014, we started to deliver BoNTA into masseter muscle with a \"biphasic\" injection technique in order to avoid masseter paradoxical bulging. A Bobotulinum toxin A (DYSPORT; Ipsen Ltd., Slough, UK), incobotulinum toxin A (BOCOUTURE; Merz Pharmaceuticals, Frankfurt, Germany) or onabotulinum toxin A (BOTOX; Allergan, an AbbVie company, North Chicago, IL, USA) have been used with no preferences: 0.8 mL of 0.9% saline were used for the reconstitution of 125 speywood units of Abobotulinum or 50 units of Inco- or Ona-botulinum. Injections were performed using a 1-mL syringe with a 29 G 4 cm-long needle (Rays Health & Safety s.p.a., Osimo, AN, Italy). Because of variations of masseteric size and shape, we relied on palpation to delineate the anterior and posterior borders of the muscle, by asking the patient to bite down. The zygomatic bone and mandibular border were  
Figure 1: Morphologic aspect of bilateral masseter bulging on resting (**A**) and clenching (**B**) of the same patient showed in Video S1. Detail of masseteric belly affected by paradoxical bulging (**C**). marked. A zone of 1-cm intervals was drawn on the area overlying the low and lateral side of the jaw and a grid of 2-cm up from zygomatic arch to identify the area to be treated. Injections were performed on the crossed points, thus four site injections per side were given. According to the biphasic injection technique, 0.1 mL were released into the deep belly and 0.1 mL on the superficial belly of the masseter muscle at the same site injection. (Figures 2 and 3, Supplementary Materials: Video S2). Figure 3: Biphasic injection technique. (**A**) 0.1 mL were released into the deep belly of the masseter muscle (red star). (**B**) 0.1 mL were released on the superficial belly (yellow star) of the masseter muscle at the same site injection. The deep inferior tendon (green star) divides the masseter muscle into two layers. Figure 2: Injection site zone. A grid of 1-cm intervals is drawn on the area overlying the low and lateral side of the jaw and 2-cm from the zygomatic bone. Injections were performed on the crossed points. Four site injections per side were given. A total of 98 treatments, performed in 42 patients (23 men and 19 women; mean age, 40.5 \(\pm\) 19.5 year), from 2015 to December 2020, were performed with this technique (Figure 4). Patients were included in the study if they presented a widened lower facial profile with hypertrophic massteric muscles searching for a slimmer, more aesthetic lower face. Exclusion criteria for this study included pregnancy or pregnancy attempt, breastfeeding, neuromuscular disorders (such as myasthenia gravis or Eaton-Lambert disease), peripheral neuropathy as in case of diabetes mellitus or alcoholism, concurrent administration of calcium-channel blocker, quinine, anticoagulant drugs, penicillin, or aminoglycosoide antibiotics. Written consent was obtained from all the enrolled patients. All data considered for investigation in this study were captured during pre-treatment investigations, treatment course, and post-operative visits. Visual analog scale (VAS) was used to assess patient satisfaction: 100 represented the best possible aesthetic outcome and 0 was the worst. In order to evaluate the BoNTA injection effect, we took periodic pictures of the patients and measured the masseter muscle reduction according to morphometric lines. The camera settings used were standardized to ensure the reproducibility of the photographs, patient after patient.