How ultrasound changes the way we inject the masseter

Objectives: To demonstrate how ultrasound assessment of the masseter muscle changes the conventional approach to botulinum toxin injection by enabling individualized needle length selection, functional muscle targeting, and identification of intramuscular structures such as the deep inferior tendon (DIT). This presentation aims to improve injection precision, safety, and treatment outcomes while moving beyond the traditional standardized two-bolus (bi-phasic) technique. Introduction: Conventional masseter toxin injection is commonly done using standardized injection technique. Two bolus technique targets both superficial and deep bellies of muscle. However, significant interindividual variation exists in muscle thickness, depth, functional anatomy, and intramuscular tendon distribution. Blind injection may lead to suboptimal toxin placement, reduced efficacy, or unintended diffusion into adjacent structures. Ultrasound allows real-time visualization of the muscle and surrounding anatomy, providing opportunities for a more personalized and anatomy-based injection approach Materials / method: Ultrasound examination of the masseter was performed prior to toxin injection(Ultrasound-mapping) or during the procedure(Ultrasound-guiding). Assessment included muscle thickness, depth from skin surface, localization of functional muscle portions, identification of intramuscular structures including the deep inferior tendon (DIT), and anatomy variation of surrounding structure such as the extention of parotid gland. Injection technique, needle length selection, and injection points were subsequently adjusted according to individual ultrasound findings rather than using the conventional one Results: Ultrasound evaluation show anatomical variation among patients in terms of muscle thickness, depth, and distribution of tendon. In some cases, the conventional two-point injection pattern does not adequately correspond to the most functional hypertrophic portion of the masseter. Ultrasound guidance enabled more precise needle length selection,individualized toxin placement while avoiding less functional regions, tendon-dominant areas, and avoid accidentally administer toxin into adjacent structure such as superficial fat pad and or parotid gland. Allow more precise treatment based on anatomy Conclusion: Ultrasound imaging significantly changes the conventional approach to masseter botulinum toxin injection by enabling personalized anatomical assessment prior to treatment. Individualized needle selection, functional muscle targeting, and recognition of deep inferior tendon anatomy may improve injection precision and optimize treatment outcomes. Ultrasound evaluation supports a shift away from standardized injection patterns toward a more patient-specific approach in masseter botulinum toxin treatment