Objectives: Paradoxical masseteric bulging is a rare sequela, clinical trials focused on its anatomical clarification need to be performed. The aim of this study
was to determine the detailed anatomical structures of the superficial part of the masseter and to elucidate the boundaries and locations of the deep tendon structure within the superficial part of the masseter.
Introduction: Various anatomical studies have attempted to determine the most effective BoNT-A injection points and obtain optimal results while minimizing complications. Several of these studies have investigated the intramuscular nerve distribution, the motor nerve entry point of the masseteric nerve into the masseter, and the relationship between the parotid gland and the marginal mandibular branch
of the facial nerve. However, the rare side effect of paradoxical masseteric bulging after BoNT-A treatment for masseteric hypertrophy has not been investigated in any recent anatomical or clinical studies.
Materials / method: Forty-four hemifaces from Korean (21 hemifaces, 10 left and 11 right; mean age 79.9 years) and Thai (23 hemifaces, 11 left and 12 right; mean age 68.6 years) embalmed cadavers were dissected to analyze the morphological patterns, with 30 of them used to measure the detailed location of the deep tendon structure of the masseter. The skin of the midface was carefully removed, and the superficial musculoaponeurotic system (SMAS), parotid gland, and masseteric fascia were dissected to reveal the entire masseter.
Results: The relationship between the masseter and DIT could be classified into three
types according to the coverage pattern: Type A, in which areas IV and V were covered by the DIT (27%, 12/44); Type B, in which areas V and VI were covered by the DIT (23%, 10/44); and Type C, in which areas IV, V, and VI were covered by the DIT (50%, 22/44). The superficial part of the masseter consists of not only the muscle belly but also the deep tendon structure.
Conclusion: Based on the results obtained in this morphological study, we recommend performing layer-by-layer retrograde injections into the superficial and deep muscle bellies of the masseter.
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