Ultrasound-Guided Botulinum Neurotoxin Injection for Masseter Hypertrophy Based on the Structural Pattern of Deep Inferior Tendon and Masseteric Contraction

As angled jawlines and square faces are not considered attractive among Asians, particularly in young women, botulinum neurotoxin A (BoNT-A) treatment for masseter hypertrophy has become popular in the Asian population.1,2 However, iatrogenic adverse events, such as paradoxical masseteric bulging (chipmunk deformity), which occurs in 0.5 percent to 18.8 percent of cases after botulinum neurotoxin A injection, are still frequently reported.3 Previous research has suggested that the deep inferior tendon can affect treatment effectiveness and the incidence of chipmunk deformity by preventing the toxin from dispersing uniformly during injection.4 Lee et al. noted that this structure was located 2 to 5 mm above the mandibular bone, and they recommended the use of retrograde and dual-plane injections with ultrasound to reduce the probability of chipmunk deformity.5 However, the influences of masseter contraction and its morphological changes were not mentioned. To examine the different morphological changes of the masseter in resting and clenching, we recruited 15 healthy volunteers without previous aesthetic treatment to evaluate the thickness of their masseters above and below the deep inferior tendon at the most bulging point while clenching, using a linear 10-MHz ultrasound transducer. The thickness of the deep part was 31.8 percent thicker than the superficial part when relaxing, and 41.8 percent thicker when clenching (Table 1). The total masseteric thickness increased about 30.4 percent after clenching, and the deep part had an incremental share of 65 percent (Fig. 1). According to the results, the muscle belly deep to the deep inferior tendon tends to play a key role in masseter hypertrophy. It is consistent with previous studies, with a low prevalence rate of chipmunk deformity when using a conventional technique of multiple deep injections. However, a few individuals were born with thicker superficial parts, or with thinner deep parts due to having multiple sessions of botulinum neurotoxin A injection in the deep masseter. In the aforementioned case, a regular conventional deep injection method may cause ineffective results and chipmunk deformity due to the paradoxical superficial part of masseteric contraction. Table 1. - Deep Inferior Tendon–Based Evaluation of Ultrasound Muscle Thickness Measurements (n = 30) Masseter (total) Superficial to DIT Deep to DIT Muscle thickness when resting, mm 10.2 ± 2.0 4.4 ± 1.1 5.8 ± 1.3 Muscle thickness when clenching, mm 13.3 ± 2.2 5.5 ± 1.3 7.8 ± 1.4 Incremental muscle thickness, mm 3.1 ± 1.8 (30.4%) 1.1 ± 1.3 (27.86%) 2.0 ± 1.2 (37.91%) Dominant part when resting Muscle belly superficial to DIT: 5Muscle belly deep to DIT: 25 Dominant part when clenching Muscle belly superficial to DIT: 3Muscle belly deep to DIT: 27 DIT, deep inferior tendon. Fig. 1.: The morphological changes of the masseter when resting and clenching. After clenching, the total thickness of the masseter increases by 3.1 mm on average. In particular, the muscle thickness below the deep inferior tendon (deep part) accounts for 65 percent of the incremental share. Orange double arrow indicates the superficial part (muscle belly above the deep inferior tendon); red double arrow indicates the deep part (muscle belly below the deep inferior tendon). DIT, deep inferior tendon.Given our findings, we suggest a dose distribution ratio of 1.32 to 1.42:1 (5.8/4.4 to 7.8/5.5:1) for botulinum neurotoxin A injection into the deep and superficial sections based on the thickness ratio of the deep to the superficial part of a masseter when resting and clenching, and under the hypothesis that the thickness is proportional to the volume of the masseter. To treat masseter hypertrophy, physicians may use our suggested dose distribution ratio or measure each person's masseteric thickness with ultrasound before injection. With consideration of the anatomical structural pattern of the deep inferior tendon and masseteric contraction, as well as ultrasound measurement of masseter thickness, a precise and safe outcome with a lower complication rate can be achieved. A quick ultrasound check before or during injection is recommended to evaluate the structural pattern of the deep inferior tendon and the change in thickness when resting and clenching, as well as to establish an individualized injection plan. The key points of our appropriate, adequate, and accurate injection techniques for masseter hypertrophy are depth control, dose control, multiple injections, retrograding, and dual-plane injections. DISCLOSURE The authors have no financial interest to declare in relation to the content of this article. No funding was received for this article.