(1) Background: With the increasing aesthetic pursuit of facial features, the clinical use of Botulinum Toxin Type A (BoNT-A) injections for masseter hypertrophy has been on the rise. However, due to variations in masseter muscle structure and differences in clinicians’ injection techniques, blind injections may lack precision, potentially compromising treatment accuracy and increasing the risk of complications. (2) Objectives: The study aims to use ultrasonography to detail the deep inferior tendon (DIT) within the masseter muscle in a young Chinese cohort, refine its classification, analyze muscle belly thickness and variations across groups, and propose a customized ultrasound-guided BoNT-A injection protocol. (3) Methods: Ultrasound imaging was used to observe the bilateral masseter muscles at rest and during clenching. The features of the DIT were classified from these images, and the thickness of the masseter’s distinct bellies associated with the DIT types was measured in both states. (4) Results: The study cohort included 103 participants (27 male, 76 female), with 30 muscles in the normal masseter group and 176 muscles in the hypertrophy group. The DIT was categorized as Type A, B (subtypes B1, B2), and C. The distribution of these types was consistent across normal, hypertrophic, and gender groups, all following the same trend (B > A > C). In hypertrophy, Type B1 showed uniform thickness across masseter bellies, B2 presented with a thinner intermediate belly, and Type C had mainly superficial muscle enlargement. Changes in muscle thickness during clenching were noted but not statistically significant among different bellies. (5) Conclusions: The study evidences individual variation in the DIT, highlighting the importance of precise DIT classification for effective BoNT-A injections. A tailored ultrasound-guided BoNT-A injection strategy based on this classification may enhance safety and efficacy of the therapy.

Objective: The objective of this study was to assess the diagnostic efficacy of dynamic high-resolution ultrasonography (HRUS) in detecting anterior disc displacement with reduction (ADDWR) and anterior disc displacement without reduction (ADDWoR) in the temporomandibular joint (TMJ). Methods: A total of 144 TMJs was categorized into three groups according to the magnetic resonance imaging (MRI) findings, which served as the reference standard: the normal disc position (NDP) group, the ADDWR group, and the ADDWoR group. Static images of the TMJ in full opening and maximum intercuspal positions, along with dynamic sequences during jaw opening, were obtained utilizing a 14 MHz L-shaped linear array transducer. The diagnostic efficacy of dynamic HRUS for identifying ADDWR and ADDWoR was evaluated in terms of accuracy, sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), diagnostic odds ratio (DOR), and the Youden index. Results: According to the MRI findings, the NDP, ADDWR, and ADDWoR groups consisted of 42 (29.2%), 47 (32.6%), and 55 (38.2%) TMJs, respectively. HRUS data revealed 54 TMJs (37.5%) in the NDP group, 26 TMJs (18.1%) in the ADDWR group, and 64 TMJs (44.4%) in the ADDWoR group. With MRI as the reference standard, HRUS exhibited a diagnostic accuracy of 71.4%, sensitivity of 51.4%, and specificity of 91.4% for ADDWR. For the ADDWoR, HRUS attained a diagnostic accuracy of 86.5%, sensitivity of 90.0%, and specificity of 82.1%. Conclusions: With MRI serving as the reference standard, dynamic HRUS has high diagnostic value for ADDWoR, with better diagnostic accuracy than ADDWR. Ultrasonography has the potential to be used as a highly effective and non-invasive imaging modality for the early screening of ADD in future clinical practice.