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


 
 
 Masseter reduction by botulinum neurotoxin A (BoNT-A) injection is a popular procedure in East Asia. Inappropriate injection could result in various complications.
 
 
 
 An approach for comprehensively evaluating the lower facial structure was proposed based on the shape of the mandible angle, the thickness of the masseter and the thickness of subcutaneous fat. This approach was validated by ultrasound examination to facilitate BoNT-A injection.
 
 
 
 Clinical profiles and standard ultrasound images were collected from patients seeking masseter BoNT-A injections. Lower facial structures were evaluated based on three aspects: mandible, masseter, and subcutaneous fat and skin. The mandibular angle was classified as extroverted, straight or introverted based on the degree. 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.
 
 
 
 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 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 = 0.048, P = 0.001) and had greater contractility. The average overall thickness was 1.87 ± 0.29 cm.
 
 
 
 Comprehensive evaluation of lower facial contour and structure based on ultrasound examination could be a reliable and replicable assessment approach.
