摘要

Objectives: This study aimed to determine the detailed anatomical structures of the superficial part of the masseter and to elucidate the boundaries, locations, and depths of the deep tendon structure within the superficial part of the masseter through cadaveric dissection and ultrasonography scanning of healthy volunteers. Introduction: 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. Materials / method: Forty-four hemifaces from Korean and Thai embalmed cadavers were used in this study. Thirty healthy volunteers participated in this study. Ultrasonography scanning in both longitudinal and transverse directions was applied to the masseter muscle. Results: The deep tendon structure was located deep in the lower third of the superficial part of the masseter. It was observed in all specimens and was designated as a deep inferior tendon (DIT). In cadavers, the DIT covered most of the inferior part of the masseter muscle in 50% of cases, and in 68.8% of cases for healthy volunteers. 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. Conclusion: The superficial part of the masseter consists not only of the muscle belly but also the deep tendon structure. Based on the results obtained in this morphological study, we recommend performing layer-by-layer retrograde or dual-plane injections into the superficial and deep muscle bellies of the masseter. In addition, 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 depending on the structural pattern of the deep inferior tendon.