# Biphasic Injection for Masseter Muscle Reduction with Botulinum Toxin
## 3 Results
We confirm to the evidence already published in the literature assessing that this could more frequently occur when masseteric volume is thick, the skin is thin but especially when the technique and the point of injection is not appropriate. After the introduction of our biphasic technique, no masseteric bulging was described. To prevent overcompensation complications, a combination of superficial and deep injections is recommended, particularly in patients with a history of paradoxical bulging after receiving masseter toxin injections. Thus, the most common functional complications after BoNTA lower face reshaping are compensatory hypertrophy or masseteric bulging and many patients experience compensatory and stress hypertrophy symptoms [50]. Botulinum toxin masseter injection is an effective technique, commonly performed with pleasant results and good safety profiles, nonetheless an understanding of the muscular anatomy is vital to minimize undesirable complications. In fact, complications could result from uneven injections into a large area or a single layer of the muscle. Several surrounding structures should be considered: zygomaticus major and minor muscles might not to be considered as significant during BoNTA treatment, if the surgeon performs the procedure based on an accurate anatomical knowledge of the masseteric location and depth. Moreover, even if it is described that isorius muscle originates from the surface of the masseter, several studies have shown that this muscle is limited to within an anterior area that do not need to be injected [51]. We suggest the lower part of the masseter as optimal for BoNTA treatments because it constitutes the largest part of masseteric prominence, preventing the targeting of the parotid gland and thereby avoiding the risk of xerostomia. Moreover, we exclude the cheliion-tragus line due to the risk of parotid duct lesion [44; 45; 46; 47; 48; 49; 50; 54; 51; 54]. Thus, a wide anatomical knowledge, appropriate dosing, injection location, and depth are all key factors in achieving the desired result with minimal complications. Dose and injection site-related complication as asymmetry, sagging, or sunken lateral cheeks could be avoided according to an appropriate marking of the injection site as described by [PERSON] et al. [44; 45]. Nevertheless, it is still poorly described in the literature how to prevent level-related complication as paradoxical bulging of the muscle [55; 56]. Muscular bulging during mastication, also known as paradoxical bulging, is a rare complication that can arise after BoNTA injection in order to get a slimmer face and could occur within a week after injection. This clinical condition is related to a superficial overcompensation of the muscle as physio-pathological reaction to the neurotoxic weakening of the deep layer fibers [55; 56]. If it is only the deep layer to be injected, it is expected that the superficial layer increases in strength, possibly resulting in hypertrophy and bulging, a bothersome condition for a patient that is looking for a procedure without any recovery time. Based on this clinical evidence, several anatomical studies propose the mechanism underlying paradoxical masseteric bulging after BoNTA injection. In 2012, [PERSON] et al. reported that there are more aponeuroses in the deeper part of the masseter [68]. In 2017, [PERSON] et al. investigated the deep inferior tendon (DIT) originated deep to the aponeurosis of the superficial layer of the masseter muscle. This cadaveric study showed the presence of this tendinous structure located in the deeper part of the superficial masseter layer, which may block toxin diffusion from the deep layer to the superficial layer, making overcompensation more common [47]. Based on these observations, it was hypothesized that a broad tendon structure located within the superficial belly could act as a window that confines the injected toxin within the deeper muscle belly and avoid BoNTA spreading into the entire superficial layer.