A Comprehensive Ultrasound Evaluation Approach of Lower Facial Structure Before Masseter Muscle Botulinum Toxin Injection

Background Masseter reduction by botulinum neurotoxin A (BoNT-A) injection is a popular procedure in East Asia. Inappropriate injection can result in various complications.Objectives The aim of this study was to develop an approach for comprehensively evaluating the lower facial structure based on the shape of the mandible angle, the thickness of the masseter, and the thickness of subcutaneous fat.Methods Clinical profiles and standard ultrasound images were collected from patients seeking masseter BoNT-A injections. Lower facial structures were evaluated based on 3 aspects: mandible, masseter, and subcutaneous fat and skin. The mandibular angle was classified as extroverted, straight, or introverted. The starting point of the middle part of the masseter was recorded. The thickness of the deep inferior tendon, the superficial and deep belly of the superficial part of the masseter, and their contractility were measured. Overall thickness and the thickness of subcutaneous fat and skin were also measured.Results Eighty-four masseters from 42 patients were included. Straight mandibular angles were the most common type of angle (48.81%), followed by introverted (26.19%) and extroverted angles (25.00%). The middle part of the masseter starts from 2.33 [0.36] cm (mean [standard deviation]) above the mandible edge, which is above the usual injection points. The superficial belly was thicker than the deep belly in both static and clenching states (P = .048, P = .001) and had greater contractility. The average overall thickness was 1.87 [0.29] cm.Conclusions Comprehensive evaluation of the lower facial contour and structure based on ultrasound examination could be a reliable and replicable assessment approach.