Ultrasonographic characterization of the deep inferior tendon of the masseter muscle: morphology and bilateral distribution

BACKGROUND: The deep inferior tendon (DIT) represents a distinct intramuscular tendinous structure within the superficial layer of the masseter muscle and contributes to its internal compartmentalization. Although the presence of the DIT has been demonstrated in anatomical and ultrasonographic studies, detailed evaluations of its morphological variability and its association with patient-related factors remain limited. This study aimed to analyze ultrasonographically defined DIT types and investigate their relationship with demographic and structural variables, including muscle thickness, tooth loss, age, and sex. MATERIALS AND METHODS: This single-center retrospective cohort study included patients who underwent ultrasonographic evaluation of the masseter muscle. DIT morphology was classified into three types using ultrasonography: Type A, Type B (with subtypes B1 and B2), and Type C. Masseter muscle thickness at rest and during clenching, dental status (number of missing teeth), and demographic data (age and sex) were also recorded. Intra-observer reliability was evaluated using intraclass correlation coefficients (ICCs). Associations between DIT type and patient-related factors were analyzed statistically. RESULTS: A total of 90 patients were analyzed, comprising 53 females (58.8%) and 37 males (41.2%), with a mean age of 49.89 ± 9.72 years (range: 28-59). A statistically significant difference was found between right and left masseter muscle DIT distributions (p = 0.046). In addition, a significant association was observed between right-sided DIT type and sex (p = 0.016). No statistically significant differences were identified between DIT type and muscle thickness, tooth loss, or age. CONCLUSIONS: Ultrasonographic evaluation demonstrated variability in the morphology and bilateral distribution of the deep inferior tendon within the masseter muscle. Recognition of these anatomical variations may contribute to a more precise understanding of the internal architecture of the masseter and facilitate safer planning of intramuscular interventions.