# Biphasic Injection for Masseter Muscle Reduction with Botulinum Toxin
## 3 Results
The literature describes a remarkable pattern of dose-related headache; thus, it is suggested to avoid injecting deeply in multiple sites in patients who are prone to headaches [2, 50]. Differently from the reported data, in our 4-point injective biphasic technique, no cases of headache were reported. We suggest the injections should be performed from a zone 2 cm beneath the zygomatic arch to the inferior mandibular border. We believe that injecting right beneath the zygomatic bone may cause a change in facial expressions because it directly affects the mimic muscles attached to the bones, as reported by [PERSON] et al. [1]. Asymmetric facial expression is an undesired complication and could be due to weakened zygomatic major, zygomatic minor, and risorius, which act in lifting the corners of the mouth. This complication may occur when the point of injection is located on areas that could directly affect the major and minor zygomaticus muscles. This can easily be prevented by a proper injection technique, avoiding injection on the zygomatic bone and near the anterior border of the masseter. In fact, according to our injective pattern, we experienced asymmetric smiling in a percentage of only 2.4%, self-resolved within 4 weeks from the injection, and at 1 month follow up, no asymmetry was recorded. The loss of the full smile is due to toxin diffusion that could led to risorius muscle paralysis. In a cadaver study, this muscle attaches to the anterior and middle part of the masseter muscle in 95% of cases [51]. In our experience, no patient reported the loss of full smile; thus, we suggest that to prevent this complication, the physician should be familiar with muscular anatomy, set an injection safe zone at least one centimeter from the anterior border of the masseter, and keep both a superficial but also deep injection. As described in the literature, worsened jowls or sagging [58, 60] could occur in middle-aged or elderly patients with prominent zygoma. As reported in our experience, the inability of the overlying skin to tight and contract fast enough to match the degree of atrophy of the muscle resulted in one case of local sagging. Difficulty in mouth opening is a rare complication caused by a paralysis of the lateral pterygoid muscle. As the masseter originates just below the zygomatic bone, in this location the lateral pterygoid muscles are deeper than masseter muscle. Thus, when performing biphasic injective technique, we must be aware that injecting too highly and deeply could cause an unwanted paralysis of the lateral pterygoid muscles. Xerostonia is a rare complication caused by toxin diffusion into parotid gland, resulting in reducing or thickening salivary secretion [57]. Parotid gland covers the posterior part of the masseter muscle, so surgeons should be aware not to inject too deep during the biphasic injection, keeping 1 cm far from the posterior margin of the masseter. Neurapraxia is an extremely rare complication caused by marginal mandibular nerve paralysis [57]. There were no cases of neurapraxia in the authors' experience with masseter toxin injections. In fact, cadaver studies describe that the marginal mandibular nerve runs about 0.1-1.0 cm from the mandibular border [54]. This area is not affected as the surgeons mark the injective safe zone correctly, even in the deep injection of the biphasic technique. Uneven paradoxical bulging of masseteric muscle during mastication is a common but easily overlooked side effect that occurs because the toxin results in a different effect on the different belly of the muscle, according to the injective technique. Some parts of the masseteric belly may remain contracted while other parts could be affected by a decrease in contraction level. In the zone where the movement remains overactive, the movement appears to be much larger if compared with the region where the movement of the muscle has decreased. Before the introduction of the biphasic technique, in our experience this occurred in percentage of 2.4%, compared to the 5-10% described by [PERSON] [8].