# Biphasic Injection for Masseter Muscle Reduction with Botulinum Toxin
###### Abstract
Its effectiveness as an off-label drug in lower face reshaping has been proven by several studies [37; 38; 39; 40; 41; 42; 43; 44; 45; 46]. The mechanism of action of BoNTA in masseter hypertrophy is different from wrinkle and hyperkinetic lines treatment. When injected at high concentrations, BoNTA may cause cell apoptosis, leading to atrophy of the masseter muscle. Moreover, it has been proved that BoNTA injections may prevent muscle fibers from regeneration and therefore muscle atrophy may be semi-permanent or even permanent [47; 48]. Several studies have shown that jaw muscles are very different from other skeletal muscles and their regeneration after atrophy may be limited [47; 48; 49]. The combination of cell apoptosis in a muscle with limited regenerative capacities makes BoNTA an excellent treatment for masseter hypertrophy, without any relevant impact on mastication activities. Various studies demonstrate that botulinum toxin type A provides a long-term effect on the cosmetic reduction of masseter muscle volume to narrow lower face width [40; 41]. As demonstrated by [PERSON] et al., the long-term stability of contouring of the lower face by injection of BoNTA could be achieved according to the occlusal therapy performed after the injection of BoNTA in patients showing widened lower face and malocclusion associated with masseter hypertrophy. The use of the occlusal device was performed to relieve muscle and to give a new guide for masticatory muscle: the new muscular tone is postulated to reduce the incidence of relapse of masseter hypertrophy thus to achieve long-lasting results [45]. Moreover, BoNTA acts as a muscular tension reliever: the retrograde transport of BoNTA and transcytosis block neurotransmitters diffusion across the peripheral nerve. BoNTA acts on nerve endings of the targeted muscle, thus an accurate and extensive anatomical knowledge of nerve endings is essential for achieving a maximum outcome with the minimum toxin concentration [44; 45]. In 1994, [PERSON] and [PERSON] [42] first reported a non-surgical approach for the functional treatment of MMH, performed with BoNTA injections. Later, [PERSON] et al. began to use botulinum toxin type A for aesthetic volumetric reduction of MMH [43]. [PERSON] et al., in 2001, confirmed the efficacy of BoNTA for MMH treatment as alternative treatment to surgery. Compared to invasive surgical techniques, BoNTA injection shows an excellent profile in terms of efficacy, safety, and no recovery time [1]. From 1994 to now, BoNTA injection has been widely described for MMH treatment; moreover, nowadays, non-surgical facial slimming is often performed with multiple procedures such as BoNTA injection into the masseter muscle [44; 45]. Toxin masseter injections have a high efficacy and safety profile but the risks of a variety of side effects or complications. tions such as bruising, headaches, smile limitation, and paradoxical bulging remain [46; 50]. Thus, surgeons should be able to recognize those complications, their causes, and their prevention methods. Several anatomical studies have attempted to determine the most effective BoNTA injection points with the aim to achieve optimal results and minimize complications. Therefore, intramuscular nerve distribution, motor nerve entry point of masseteric nerve and the relationship between marginals mandibular nerve and parotid gland were investigated [44; 51; 52; 53; 54; 55]. [PERSON] et al. reported the occurrence of paradoxical masseteric bulging after BoNTA injection [56]. Muscular bulging during mastication (also known as paradoxical bulging) occurs within a week after injection and is due to a superficial overcompensation of masseter muscle fibers in response to the neurotoxic weakening of the deep masseter [55]. Paradoxical bulging could be distressing for the patient, although it can be resolved, after 1-2 weeks from the previous injection, injecting some more units on the masseter.