Abstract
There are still concerns about masseteric bulging due to a lack of knowledge about the internal architecture of the masseter muscle. Further investigations are therefore required of the most-effective botulinum neurotoxin (BoNT) injection points and strategies for managing masseteric bulging. The purpose of this study was to identify safer and more effective botulinum neurotoxin injection points and strategies by using ultrasonography to determine the structural patterns of the deep inferior tendon. We also measured the precise depths and locations of the deep inferior tendon of the masseter muscle. Thirty-two healthy volunteers participated in this study, and ultrasonography was used to scan the masseter muscle both longitudinally and transversely. Three structural patterns of the deep inferior tendon were identified: in type A, the deep inferior tendon covered the anterior two-thirds of the masseter muscle (21.8%); in type B, the deep inferior tendon covered the posterior two-thirds of the masseter muscle (9.4%); and in type C, the deep inferior tendon covered most of the inferior part of the masseter muscle (68.8%). Depending on the ultrasonography scanning site, the depth from the skin surface to the mandible in the masseteric region ranged from 15 to 25 mm. The deep inferior tendon was typically located 2 to 5 mm deep from the mandible. Ultrasonography can be used to observe the internal structure of the masseter muscle including the deep inferior tendon in individual patients. This will help to reduce the side effects of masseteric bulging when applying retrograde or dual-plane injection methods depending on the structural pattern of the deep inferior tendon.
Highlights
There is a worldwide tendency of smooth facial lines and a slim jawline being desirable in females, which has resulted in botulinum neurotoxin (BoNT) injections for facial contouring being widely performed ever since this procedure was introduced by Moore in 1994 [1,2,3]
The present study demonstrated that the internal architecture of the masseter muscle (i.e., deep inferior tendon (DIT), which is helpful when treating BoNT injections in young adults aged approximately 20 to 30), further studies with a larger sample size of various age groups are required to build patient-customized treatment strategies for each age group
The DIT was observed at a similar depth in the anterior-to-posterior direction, and from the superior to inferior directions it gradually attached to the bone and to the inferior border of the mandible
Summary
There is a worldwide tendency of smooth facial lines and a slim jawline being desirable in females, which has resulted in botulinum neurotoxin (BoNT) injections for facial contouring being widely performed ever since this procedure was introduced by Moore in 1994 [1,2,3]. A hypertrophied masseter muscle is commonly treated in Asians due to the face being more angular than that of Caucasians. The shape of the Caucasian face is narrow compared to that of Asians. A squared jaw is considered more of a major aesthetic issue among Asians than among Caucasians. Angled jawlines are not considered attractive among Asians, especially in young women [4]. For this reason, many previous studies have conducted research to demonstrate the effectiveness of botulinum toxin injections into the masseter muscle in young Asians aged between 20 and 30 [5,6]
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